Section 2 Alkaloids

## **Chapter 5**

## Alkaloids as New Leads for Neurodegenerative Diseases

*Farah Al-Mamoori and Ashraf M.A. Qasem*

## **Abstract**

Conventionally, diseases involving the selective loss of neurons are referred to as neurodegenerative diseases. Traditional and more recent compounds have been explored, but they only provide symptomatic benefits and have a large number of negative effects. It will be regarded as a modern vision if stronger molecules are found that can stop the pathophysiology of these diseases. In order to replace existing medications, natural compounds are being developed from plants and other sources. Natural products, including alkaloids that originate from plants, have emerged as potential protective agents against neurodegenerative disorders (e.g., Alzheimer's and Parkinson's), psychiatric conditions, and many more. They provided unique lead compounds for medicine. Alkaloids could be exploited as starting materials for novel drug synthesis or, to a lesser extent, used to manage neurodegenerative-related complications due to their diverse mechanistic effects. This chapter aims to highlight the importance of alkaloids as new leads for the development of potential clinical drug candidates for the management and treatment of neurodegenerative diseases.

**Keywords:** alkaloids, leads, neurodegenerative diseases, Parkinson's disease, Alzheimer's disease

## **1. Introduction**

Neurodegenerative diseases are debilitating conditions that affect memory, cognition, mobility, and overall functioning. Although these diseases have diverse patterns of signs and symptoms, they have several characteristics: A high correlation with age, protein aggregation that is abnormal, and a natural history that is gradual and relentless. This group of illnesses is likewise distinguished by a slow beginning, with neuropathological alterations developing years before clinical manifestation [1]. Instances of neurodegenerative diseases include: Alzheimer's disease and Parkinson's disease.

Worldwide, dementia affects more than 25 million people, the majority of whom have Alzheimer's disease. It has had a significant influence on affected individuals, carers, and society in both developed and developing countries [2]. The abundance of experimental and clinical evidence suggests that Alzheimer's disease is a complicated disorder characterized by extensive neurodegeneration of the central nervous system with significant involvement of the cholinergic system, resulting in gradual cognitive

decline and dementia [3]. New approaches, such as the detection of amyloid-beta (Aβ) and tumor necrosis factors (NFTs), lead to the amyloid and tau theories as potential causes of Alzheimer's development. Multi-target compounds that inhibit cholinesterases while also interfering with Aβ- aggregation and/or tau protein neuroinflammation may be effective in the treatment of Alzheimer's disease. Natural compounds, particularly plant alkaloids, have been a steady supply of innovative choices for the treatment of Alzheimer's disease. For example, the prototype of rivastigmine, physostigmine (*Physostigma venenosum*), is an cholinesterase enzyme inhibitors (IChE) inhibitor and allosteric modulator of the central nicotinic receptor. Galanthamine (*Galanthus woronowii*) is a selective acetylcholinesterase inhibitor, an allosteric modulator of the central nicotinic receptor, an inhibitor of Aβ aggregation, and an inducer of hippocampus neurogenesis [4]. Alkaloids have been one of the most appealing classes for searching for novel medications since the release of the Amaryllidaceae alkaloid as a drug in 2001 [3].

Parkinson's disease is the neurodegenerative disease with the second-highest prevalence. The age-adjusted prevalence was 205.89 per 100,000 people. The prevalence of advanced Parkinson's disease increased with age, from 3.77% in the 40–49 year age group to 25.86% in those over 89 years [5]. The development of pharmacotherapy for Parkinson's disease in terms of historically significant plant-derived substances, *Atropa belladonna* (deadly nightshade), *Hyoscyamus niger* (henbane), and *Datura stramonium* (thorn apple or jimsonweed), includes large amounts of pharmacologically potent anticholinergic tropane alkaloids (atropine, hyoscyamine, and hyoscine) [6].

Alkaloids, the main natural medicinal source, are a little-explored component of plant chemistry. They are cyclic organic compounds with at least one nitrogen atom [7]. The majority are biologically active. Alkaloids can be classified based on their ring chemistry or the amino acid from which they are formed [8]. The richness of alkaloid content varies between plant species, but most include a variety of these substances that are different in both their molecular structure and the biology of their effects. Individual plant alkaloid levels vary by component, life cycle, and season [9]. According to a review, 84% of medications licensed for central nervous system indications are derived from naturally occurring compounds [10].

Historically, the pharmaceutical industry originated from traditional plant medical knowledge. Natural scaffolds share molecular characteristics that can improve affinity with receptor binding sites. Natural structures confer more chirality (resulting in unique D- and L- stereoisomers) and more rigidity (due to ring conformations) than fully synthesized agents [10]. These chemicals can also go through the bloodbrain barrier more easily. Because of evolutionary links between plants and mammals, natural products and natural-inspired medications may have a favorable influence on neurotransmitter systems [6]. This chapter summarizes the role of alkaloids in the management and treatment of neurodegenerative diseases and identifies them as lead compounds in the development of potential clinical drug candidates (**Table 1**)**.**

## **2. Plant alkaloids as new leads for neurodegenerative diseases**

## **2.1 Purine alkaloids: Theacrine**

Some plants, like tea, coffee, chocolate, and mate, make purine alkaloids like caffeine, theobromine, theophylline, 7-methylxanthosine, and theacrine. The alkaloid theacrine is found in *Camellia kucha*, which is in the family Theaceae.

*Alkaloids as New Leads for Neurodegenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.112584*



**Table 1.**

*Summarize classes of alkaloids, their source, and neurodegenerative diseases they target.*

*Alkaloids as New Leads for Neurodegenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.112584*

The leaves of *C. kucha* contain both theacrine **(1)** and caffeine. Theacrine **(1)** is thought to be made from caffeine through an N-methyltransferase process that uses *S*-adenosyl-L-methionine (SAM) as a methyl donor. Theacrine **(1)** is an adenosine receptor blocker that speeds up movement and makes people feel less tired. In animal models of Parkinson's disease, theacrine **(1)** stops the loss of dopaminergic cells and changes in behavior by reducing oxidative damage and mitochondrial dysfunction (**Figure 1**) [11].

## **2.2 Isoquinoline alkaloids: Berberine, avicine, chelerythrine, sanguinarine, and aromoline**

The isoquinoline alkaloids include, most famously, the opiates morphine and codeine, as well as berberine. Berberine **(2)** is an alkaloid found in the roots, rhizomes, stems, and bark of several medicinal plants, including *Berberis*, *Coptis chinensis*, *Phellodendron amurense*, and *Hydrastis canadensis*. Berberine significantly reduced nucleotide-binding domain, leucine-rich–containing family, pyrin domain– containing-3(NLRP3) inflammasome levels in Parkinson's disease mic [27].

Moreover, avicine **(3)** is an alkaloid isolated from *Zanthoxylum rigidum* (Rutaceae family). It is the most effective dual cholinesterase inhibitor, with IC50 values of 0.15 and 0.88 M for both AChE and BuChE, respectively [12]. Chelerythrine **(4)** and sanguinarine **(5)** are the main active ingredients of *Macleaya cordata* (Papaveraceae family) [28]. These bioactives inhibit cholinesterase activity extremely well [13]. Aromoline **(6)**, an isoquinoline alkaloid, was extracted from the root bark of *Berberis vulgaris* (Berberidaceae family), where it showed a significant inhibitory activity against human Butyrylcholinesterase (BuChE) with an IC50 = 0.82 ± 0.10 μM [14]. The therapeutic potentiality of aromoline **(6)** is worthy of further investigation, as the computational analysis supports its high affinity and selectivity for the active site of human BuChE (**Figure 2**).

### **2.3 Indole alkaloids: Geissoschizoline and conophylline**

Indole alkaloids have been found in many well-known plant groups, such as Apocynaceae, Rubiaceae, Nyssaceae, and Loganiaceae. Researchers think that indole alkaloids may have brain effects because they have the same structure as endogenous amines and neurotransmitters. Several substances with an indole group have been shown to bind to different serotonin receptors [29, 30].

Geissoschizoline **(7)** is an alkaloid isolated from *Geissospermum vellosii*emerges (Apocynaceae family) as a possible multi-target prototype that can be very useful in preventing neurodegeneration and restoring neurotransmission [15].

**Figure 3.** *Geissoschizoline (7) and Conophylline (8).*

Neurodegenerative diseases are caused by nerve cell degeneration or death, and it was reported that autophagy is crucial for the prevention of such diseases. Conophylline **(8)**, isolated from *Ervatamia microphylla* (Apocynaceae family*)* leaves, was found to activate autophagy and suppress protein aggregation to protect the neural cells from cell death (**Figure 3**) [16].

#### **2.4 Quinazoline alkaloids: Vasicinone and dehydroevodiamine**

Quinazoline alkaloids belong to the N-based heterocyclic chemical class. To date, around 150 naturally occurring quinazoline alkaloids have been isolated from various plant species as well as animals and microbes; many are biogenetically generated from anthranilic acid. Vasicine was the first quinazoline alkaloid discovered, isolated from *Adhatoda vasica* (Acanthaceae family) and later from additional species [31]. Vasicinone **(9)**, is a vasicine autooxidation product. It demonstrates a neuroprotective mechanism in paraquat-induced Parkinsonian modalities in SH-SY5Y cells [17].

Dehydroevodiamine **(10)** is one of the bioactive components of *Evodiae Fructus* (Rutaceae family), which is widely used in traditional Chinese medicine. *Evodiae fructus* (Wuzhuyu in Chinese) is traditionally used for the treatment of various conditions, including migraine and central nervous system diseases [18]. Dehydroevodiamine **(10)** is the main component of *Evodiae fructus* for its neuroprotective action. Dehydroevodiamine **(10)** is highly permeable through the blood brain barrier and has a protective effect on Alzheimer's disease through its inhibitory effect on acetylcholine esterase (AChE). Clinical results on dehydroevodiamine **(10)** suggest that it's a potential drug candidate for stress-induced depression, neuronal death, and memory impairment [18].

In addition, chemical modification of dehydroevodiamine **(10)** results in carboxydehydroevodiamine. HCl (cx-DHE), which has a better water solubility, bioavailability, and effect on memory impairment. Through several clinical models in mice, the results suggested that cx-DHE is a promising drug candidate that could prevent the progression of Alzheimer's disease pathology (**Figure 4**) [18].

### **2.5 Protoalkaloids: Capsaicin**

Capsaicn **(11)** is a pungent and irritant alkaloid isolated from *Capsicum annuum* (Solanaceae family). It's considered a protoalkaloid as it has

**Figure 4.** *Vasicinone (9) and Dehydroevodiamine (10).*

non-hetercyclic nitrogen that comes from the amino acid precursors (phenylalanine and valine) [19].

It was previously reported that consumption of a capsaicin-rich diet was associated with better cognition. A recent study found that capsaicin **(11)** has a preventive effect on Alzheimer's disease by promoting the maturation of disintegrin and metalloproteinase 10 and also alleviating other Alzheimer's disease-type pathologies, such as neurodegeneration, tau hyperphosphorylation, and neuroinflammation [19]. These results suggest that supplementation with capsaicin **(11)** and chili peppers could be useful for the prevention and treatment of Alzheimer's disease. In addition, capsaicin **(11)** was found to protect the neural cells and reduce apoptosis by down-regulating Actg1 and up-regulating Gsta2 in the 6-6-hydroxydopamine (6-OHDA)-induced Parkinson's disease cell model (**Figure 5**) [20].

#### **2.6 Carbazole alkaloids: Clauselansiumines A and B**

Clauselansiumines A **(12)** and B **(13)** are two new geranylated carbazole alkaloids found in the stem and leaves of *Clausena lansium* (family Rutaceae). The alkaloids were unambiguously determined by spectral analysis, and their neuroprotective effect for Parkinson disease was tested against 6-hydroxydopamine induced cell death in human neuroblastoma and compared with curcumin as a positive control [21].

Clauselansiumines A **(12)** and B **(13)** displayed significant neuroprotective activity with an EC50 equal to 0.48 ± 0.04 μM and 0.98 ± 0.08 μM, respectively, which is more potent than the positive control that possessed an EC50 value of 6.03 ± 0.10 μM.

The structure activity relationship studies of clauselansiumines A **(12)** and B **(13)** and other geranylated carbazole alkaloids highlight the importance of the isopentenyl group at C-2′ and the methoxy group at positions 7 and 8 for the neuroptotective

**Figure 5.** *Capsaicin (11).*

**Figure 6.** *Clauselansiumines A (12) and B (13).* activity [21]. In conclusion, the geranylated carbazole alkaloids separated from *C. lansium* could be considered promising candidates for therapeutic purposes in Parkinson's disease and other neural degenerative diseases (**Figure 6**).

## **2.7 Aporphine alkaloids**

Aporphine alkaloids are a group of naturally occurring compounds with an aporphine nucleus isolated from several plant families such as Annonaceae, Papaveraceae, Ranunculaceae, and others. Recent work on the roots of *Artabotrys spinosus* (Annonaceae family) yielded the isolation of several aporphine alkaloids, of which two compounds **(14)** and **(15)** showed promising inhibitory activity towards AChE and BChE [22]. The *in silico* study confirmed the experimental results and supported the idea that compounds **(14)** and **(15)** are potential candidates for the treatment of Alzheimer's disease (**Figure 7**) [22].

## **2.8 Norditerpenoid alkaloids (C18 and C19): Lappaconitine, 3-O-acetylaconitine, bulleyaconitine A, and methyllycaconitine**

The majority of norditerpenoid alkaloids (NDAs) are isolated from the genera Delphinium and Aconitum, and they are of pharmacological importance. NDAs have a complex hexacyclic structure (A, B, C, D, E, and F). Despite the chemical similarity between NDAs, they display various pharmacological actions, and such variety encourages researchers to work on their structure activity relationship [23]. Lappaconitine **(16)** is the first C18 NDA to be reported from *Aconitum septentrionale* Koelle in 1895 and the most successful NDA in terms of clinical application [23]. Lappaconitine **(16)** acts as a voltage gated sodium channel blocker and has a potent non-addictive analgesic effect that is comparable to morphine with an ED50 = 3.5 mg/ kg [24]. The structure activity relationship studies highlight the importance of the benzoyl ester moiety and the amide group for the activity [23]. Based on the lappaconitine structure activity relationship, several lappaconitine **(16)** analogues were synthesized by replacing the amide moiety with different amides and sulphonamides; this strategy was successful in getting lead compounds as potential analgesics with comparable activity and lower toxicity to lappaconitine **(16)** [25].

3-*O*-acetylaconitine **(17)** and crassicauline A **(18)** are C19 NDAs isolated from *Aconitum* spp. They have a similar chemical structure to lappaconitine **(16)** but display an opposite pharmacological action as they keep VGSCs in their open state

**Figure 7.** *Aporphine alkaloids (14) and (15).*

#### **Figure 8.**

*Lappaconitine (16), 3-O-Acetylaconitine (17), Crassicauline A (18), and Methyllycaconitine (19).*

conformation, and they exhibit a non-addictive potent analgesic activity that is comparable to morphine, where 3-*O*-acetylaconitine **(17)** and crassicauline A **(18)** have ED50 values of 0.16 and 0.087 mg/kg, respectively. 3-O-acetylaconitine **(17)** and crassicauline A **(18)** were introduced in China into clinical use in the 1980s as analgesic agents [23].

Methyllycaconitine (MLA) **(19)** is a C19 alkaloid that was first reported from *Delphinium brownii* Rydb by Manske in 1938. Methyllycaconitine **(19)** is one of the most potent competitive antagonists of α7-nicotinic acetylcholine receptors (nAChRs) with an IC50 value of 2 nM [23]. As the total synthesis of MLA **(19)** has not been achieved yet, the synthesis of simple small analogues could be useful to achieve better structure activity relationship understanding and possibly to identify potential candidates for the treatment of several neurodegenerative diseases, including Alzheimer's disease.

Structure activity relationship studies on MLA **(19)** showed that the neopentyl ester side-chain and the piperidine ring *N*-side chain are important features in MLA **(19)** activity [26]. The synthesis of several AE-bicyclic analogues of MLA **(19)** was reported recently, possessing different nitrogen and ester side chains. The antagonist effects of these analogues on human α7 nAChRs showed promising results that

suggest that further optimization and research may enhance the activity of this analogue model (**Figure 8**) [26].

## **3. Conclusion and future perspective**

There are many drugs that have been used to treat neurodegenerative illnesses, but none of them have been able to prevent the disease from getting worse. Instead, they have caused a lot of side effects. Several neurodegenerative illnesses can be treated with natural alkaloids that continue to grow stronger. Analysis of the physicochemical properties of alkaloids showed that most of them follow the Lipinski rules of drug likeness. But only a few alkaloids are widely used in clinical practice. Because natural alkaloids give patients hope that neurodegenerative diseases can be slowed down, it is very important to plan clinical trials for these kinds of compounds that have not even been tried in clinical trials yet. Also, the blood-brain barrier is a key component of keeping substances from going into the brain, and it needs more attention. Meanwhile, it's easy to see why the development of possible candidates into therapeutic leads has stalled because of problems with compounds that come from nature, such as low extraction yields and safety profiles. More studies have to be done on them before they can be used as therapeutics.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Farah Al-Mamoori\* and Ashraf M.A. Qasem Faculty of Pharmacy, Department of Pharmaceutical Sciences, Zarqa University, Zarqa, Jordan

\*Address all correspondence to: fmamoori@zu.edu.jo

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Kromer R, Serbecic N, Hausner L, Froelich L, Aboul-Enein F, Beutelspacher SC. Detection of retinal nerve fiber layer defects in Alzheimer's disease using SD-OCT. Frontiers in Psychiatry. 2014;**5**:22. DOI: 10.3389/ fpsyt.2014.00022

[2] Qiu C, Kivipelto M, von Strauss E. Epidemiology of Alzheimer's disease: Occurrence, determinants, and strategies toward intervention. Dialogues in Clinical Neuroscience. 2009;**11**(2):111- 128. DOI: 10.31887/DCNS.2009.11.2/cqiu

[3] Vrabec R, Blunden G, Cahlíková L. Natural alkaloids as multi-target compounds towards factors implicated in Alzheimer's disease. International Journal of Molecular Sciences. 2023;**24**(5):4399. DOI: 10.3390/ijms24054399

[4] Lima JA, Hamerski L. Alkaloids as potential multi-target drugs to treat Alzheimer's disease. Studies in Natural Products Chemistry. 2019;**61**:301-334. DOI: 10.1016/ B978-0-444-64183-0.00008-7

[5] Orozco JL, Valderrama-Chaparro JA, Pinilla-Monsalve GD, Molina-Echeverry MI, Pérez Castaño AM, Ariza-Araújo Y, et al. Parkinson's disease prevalence, age distribution and staging in Colombia. Neurology International. 2020;**12**(1):8401. DOI: 10.4081/ ni.2020.8401

[6] Kempster P, Ma A. Parkinson's disease, dopaminergic drugs and the plant world. Fronties in Pharmacology. 2022;**13**:970714. DOI: 10.3389/ fphar.2022.970714

[7] Pelletier SW. The nature and definition of an alkaloid. In: Pelletier SW, editor. Alkaloids: Chemical and Biological Perspectives. Vol. 1. New York: Wiley; 1983. pp. 1-31

[8] Aniszewski T. Alkaloids-Secrets of Life: Aklaloid Chemistry, Biological Significance, Applications and Ecological Role. Amsterdam: Elsevier; 2007

[9] Waller GR, Nowacki EK. Alkaloid Biology and Metabolism in Plants. London & New York: Plenum Press; 1978. pp. 121-141. DOI: 10.1007/978-1-4684-0772-3\_4

[10] Bharate SS, Mignani S, Vishwakarma RA. Why are the majority of active compounds in the CNS domain natural products? A critical analysis. Journal of Medicinal Chemistry. 2018;**61**(23):10345-10374. DOI: 10.1021/ acs.jmedchem.7b01922

[11] Duan WJ, Liang L, Pan MH, Lu DH, Wang TM, Li SB, et al. Theacrine, a purine alkaloid from kucha, protects against Parkinson's disease through SIRT3 activation. Phytomedicine. 2020;**77**:153281

[12] Plazas E, Hagenow S, Murillo MA, Stark H, Cuca LE, Plazas E, et al. Isoquinoline alkaloids from the roots of *Zanthoxylum rigidum* as multi-target inhibitors of cholinesterase, monoamine oxidase A and Aβ1-42 aggregation. Bioorganic Chemistry. 2020;**98**:103722. DOI: 10.1016/j.bioorg.2020.103722

[13] Tuzimski T, Petruczynik A, Szultka-Młyńska M, Sugajski M, Buszewski B. Isoquinoline alkaloid contents in *Macleaya cordata* extracts and their acetylcholinesterase and Butyrylcholinesterase inhibition. Molecules. 2022;**27**(11):3606. DOI: 10.3390/molecules27113606

*Alkaloids as New Leads for Neurodegenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.112584*

[14] Hostalkova A, Marikova J, Opletal L, Korabecny J, Hulcova D, Kunes J, et al. Isoquinoline alkaloids from *Berberis vulgaris* as potential lead compounds for the treatment of Alzheimer's disease. Journal of Natural Products. 2019;**82**(2):239-248

[15] Lima JA, Costa R, et al. Geissoschizoline, a promising alkaloid for Alzheimer's disease: Inhibition of human cholinesterases, anti-inflammatory effects and molecular docking. Bioorganic Chemistry. 2020;**104**:104215. DOI: 10.1016/j.bioorg.2020.104215

[16] Umezawa K, Kojima I, Simizu S, Lin Y, Fukatsu H, Koide N, et al. Therapeutic activity of plant-derived alkaloid conophylline on metabolic syndrome and neurodegenerative disease models. Human Cell. 2018;**31**:95-101

[17] Huang CY, Sivalingam K, Shibu MA, Liao PH, Ho TJ, Kuo WW, et al. Induction of autophagy by vasicinone protects neural cells from mitochondrial dysfunction and attenuates paraquatmediated Parkinson's disease associated α-synuclein levels. Nutrients. 2020;**12**(6):1707. DOI: 10.3390/ nu12061707

[18] Fu S, Liao L, Yang Y, Bai Y, Zeng Y, Wang H, et al. The pharmacokinetics profiles, pharmacological properties, and toxicological risks of dehydroevodiamine: A review. Frontiers in Pharmacology. 2022;**13**:1040154

[19] Wang J, Sun BL, Xiang Y, Tian DY, Zhu C, Li WW, et al. Capsaicin consumption reduces brain amyloid-beta generation and attenuates Alzheimer's disease-type pathology and cognitive deficits in APP/PS1 mice. Translational Psychiatry. 2020;**10**(1):230

[20] Liu J, Liu H, Zhao Z, Wang J, Guo D, Liu Y. Regulation of Actg1 and Gsta2 is possible mechanism by which capsaicin alleviates apoptosis in cell model of 6-OHDA-induced Parkinson's disease. Bioscience Reports. 2020;**40**(6):BSR20191796

[21] Liu YP, Guo JM, Wang XP, Liu YY, Zhang W, Wang T, et al. Geranylated carbazole alkaloids with potential neuroprotective activities from the stems and leaves of *Clausena lansium*. Bioorganic Chemistry. 2019;**92**:103278

[22] Sichaem J, Tip-pyang S, Lugsanangarm K. Bioactive aporphine alkaloids from the Roots of *Artabotrys spinosus*: Cholinesterase inhibitory activity and molecular docking studies. Natural Product Communications. 2018;**13**(10):1934578X1801301011

[23] Qasem AMA, Zeng Z, Rowan MG, Blagbrough IS. Norditerpenoid alkaloids from *Aconitum* and *Delphinium*: Structural relevance in medicine, toxicology, and metabolism. Natural Product Reports. 2022;**39**(3):460-473

[24] Salehi A, Ghanadian M, Zolfaghari B, Jassbi AR, Fattahian M, Reisi P, et al. Neuropharmacological potential of diterpenoid alkaloids. Pharmaceuticals. 2023;**16**(5):747

[25] Li Y, Shang Y, Li X, Zhang Y, Xie J, Chen L, et al. Design, synthesis, and biological evaluation of low-toxic lappaconitine derivatives as potential analgesics. European Journal of Medicinal Chemistry. 2022;**243**:114776

[26] Qasem AMA, Rowan MG, Sanders VR, Millar NS, Blagbrough IS. Synthesis and antagonist activity of methyllycaconitine analogues on human α7 nicotinic acetylcholine receptors. ACS Biology & Medicinal Chemistry. 2023;**3**(2):147-157

[27] Huang S, Liu H, Lin Y, Liu M, Li Y, Mao H, et al. Berberine protects against NLRP3 inflammasome via ameliorating autophagic impairment in MPTP-induced Parkinson's disease model. Frontiers in Pharmacology. 2021;**11**:618787

[28] Lin L, Liu YC, Huang JL, Liu XB, Qing ZX, Zeng JG, et al. Medicinal plants of the genus Macleaya (*Macleaya cordata*, Macleayamicrocarpa): A review of their phytochemistry, pharmacology, and toxicology. Phytothery Research. 2018;**32**(1):19-48. DOI: 10.1002/ptr.5952

[29] Kochanowska-Karamyan AJ, Hamann MT. Marine indole alkaloids: Potential new drug leads for the control of depression and anxiety. American Chemical Society. 2010;**110**(8):4489- 4497. DOI: 10.1021/cr900211p

[30] Omar F, Tareq AM, Alqahtani AM, Dhama K, Sayeed MA, Emran TB, et al. Plant-based indole alkaloids: A comprehensive overview from a pharmacological perspective. Molecules. 2021;**26**(8):2297. DOI: 10.3390/ molecules26082297

[31] Roja G, Vikrant BH, Sandur SK, Sharma A, Pushpa KK. Accumulation of vasicine and vasicinone in tissue cultures of *Adhatoda vasica* and evaluation of the free radical-scavenging activities of the various crude extracts. Food Chemistry. 2011;**126**(3):10338. DOI: 10.1016/j. foodchem.2010.11.115

## **Chapter 6** Methods of Alkaloids Synthesis

*Nitin Dumore, Namita Girhepunje, Monali Dumore and Kishor Danao*

## **Abstract**

The investigation of plants used in traditional medicine in the early nineteenth century found alkaloids have developed into a group of natural products with exceptional structural and taxonomic diversity, as well as important chemical, biological, and medicinal importance. Since the early twentieth century, only a few routes have been thoroughly explored, and researchers have struggled to grasp their biogenesis and biosynthesis. Even for many pharmaceutically important alkaloids, there is still much to learn about how alkaloids are generated in nature, despite recent enzymatic efforts that have significantly advanced our understanding of this process. Certain aspects of empirically determined or speculated mechanistic routes of alkaloids creation are explored, with an emphasis on clinically relevant alkaloids.

**Keywords:** alkaloids, synthesis, plants, therapeutic, natural products, traditional medicines

## **1. Introduction**

Alkaloids are organic compounds that are naturally occurring and are primarily found in plant sources, such as marine algae, and rarely in animals (e.g., in the toxic secretions of fire ants, ladybugs and toads). They are predominantly located in berries, bark, fruits, roots, and leaves of plants that produce seeds. Alkaloids often have a heterocyclic ring with at least one nitrogen atom [1].

Since their discovery and early isolation in the nineteenth century, alkaloids have attracted the attention of chemists' imaginations, creative energies, and very souls. They are still actively sought after because of their astounding and seemingly unlimited structural variety, the challenges their synthesis poses to even the most skilled and knowledgeable organic chemists, the range of biological reactions they supply, and the profusion of innovation and acrobatics in the routes of biosynthetic creation. In the past 200 years, no other group of natural compounds has stimulated both chemists and biologists [2]. The "principium somniferum" that Serturner isolated from opium and published in the Journal für Pharmazie in 1805 was most likely the first semi-purified alkaloid. However, it was because to the work of French scientists Pelletier and Caventou that alkaloids truly came of age. Following their successful separation of emetine in 1817, they isolated brucine, quinine, and strychnine between 1819 and 1821. Piperine, atropine, caffeine, solanine, chelidonine, coniine, nicotine, aconitine, and colchicine were all identified before 1833 as a result of other chemists

taking on the challenge of researching the components of physiologically relevant plants [3]. By the time Berzelius' Lehrbuch der Chemie was published in 1837, the Swedish chemist had identified thirteen "Pflanzenbasen." Coke was discovered in 1860, and spartine in 1851. Thirteen "Pflanzenbasen" had already been recognised by Berzelius by the time his Lehrbuch der Chemie was published in 1837. However, it has remained difficult to define what an alkaloid is, thus no attempt will be made here to close the apparent gap. Despite not being polypeptides, proteins, or nucleic acids, they do contain nitrogen [4, 5]. A common reason why most alkaloids are optically active is that they contain tertiary nitrogen in their structural makeup. This leads to varied physical, chemical, and pharmacological properties in the various isomeric forms. For example, (+)-tubocurarine, which was isolated from *Chondrodendron tomentosum* (Bisset, 1992), has muscle-relaxant activity, whereas its leavo isomer has less activity [2].

It has given a very helpful overview of alkaloids and their role in biology and medicine [6]. Higher plants, particularly those with medical purposes or a reputation for being exceedingly deadly, provided the first isolates of alkaloids. In the late twentieth century, "alkaloids" were isolated from a wide variety of terrestrial and marine sources, including frogs, arthropods, mammals, insects, sponges, fish, fungus, and bacteria, as well as, of course, *Homo sapiens*, as the natural world was being chemically explored. The number of known alkaloids from higher plants alone has increased to at least 22,000, meaning that the sum from all sources is currently likely to be more than 30,000 [7]. Alkaloid isolations from the beginning were done before stereochemical concepts and the three-dimensional character of compounds were formed, before there was an understanding of the intricacy of molecular structure. The development of methods for figuring out the precise structures of these alkaloids—first via chemical analysis, then spectroscopy—posed one of the main difficulties over the following 160 years [8, 9]. In 1882, the structure of the first known alkaloid, xanthine, was discovered. The initial synthesis of alkaloids was published by Ladenburg, (+)-coniine, in 1886. Chemical deterioration under difficult circumstances was frequently engaged in structure determination. The result is, the core heterocyclic nucleus was occasionally the only one to survive, and this provided the foundation for the new field of organic chemistry known as heterocyclic nuclei. For instance, isolating quinoline from quinine by distilling it with KOH and indigo provided indole [10, 11]. Many of the alkaloids were extremely complicated structurally and stereochemically, making it difficult to determine their structure or synthesise them. Notwithstanding the fact that the indole alkaloid strychnine was discovered in 1818, it was not completely understood until 1947, and its synthesis was initially described by Woodward in a 1954 publication. On the other hand, Robinson successfully synthesised the crucial tropane nucleus in 1917 following what turned out to be biogenetic lines. However, it took a long time before this philosophical idea inspired the biogenetically-patterned synthesis of a wide variety of alkaloids [12, 13]. Synthesis and biogenesis advanced hand in hand in this setting, strengthened at key points by biosynthetic experiments, beginning with radioactive isotopes and afterwards the use of stable isotopes. Following these discoveries, an effort was made to identify, describe, and determine the genes and enzymes in charge of producing alkaloids inside the organism and within its cellular structure. However, testing is still far behind the ideas of spontaneous creation [14]. As a result, it is quite evident that "biogenesis" refers to theoretical ideas about the method of development of a natural product, whereas "biosynthesis" refers to the experimental confirmation of such pathway (feeding experiments with precursors, enzyme isolations and

#### *Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

characterizations, etc.) [15]. For all but a few classes of alkaloid, experimentation has not yet surpassed theory, as will be shown in this chapter. As more and more alkaloid structures were revealed, it became necessary to divide "alkaloids" into different subgroups and analyse each of these groups separately. The "The Alkaloids, Chemistry and Pharmacology" series of publications, edited by R.H.F. Manske, was first released in 1950 and is still in print today. Alkaloids were categorised on the basis of their structural makeup, and groups of alkaloids were given names based on the heterocyclic nucleus of their parent compound, such as tropanes, indole alkaloids, isoquinoline alkaloids, benzylisoquinoline alkaloids, acridine alkaloids, steroidal alkaloids, etc. [16]. As it turned out, these categories also often—though not always—reflected a shared biosynthetic origin. For instance, the amino acid tryptophan, which has an indole nucleus, would provide an indole alkaloid. These alkaloid group names often still reflect a fundamental structural component as well as a shared biosynthetic source [17]. However, it is not suitable to designate them as "piperidine" or "quinoline" alkaloids because a large number of heterocyclic nuclei, such as the piperidine and quinoline nuclei, are known to have numerous biosynthetic origins. On the other hand, higher plants contain alkaloids at a rate of 14.2%, as indicated by plant genera. The 83 higher plant orders identified by Cronquist were examined by Cordell, Quinn, and Farnsworth and found to be the case (1730 of 7231). Though none have been isolated as of yet, alkaloids have been discovered in over 35 higher plant groupings. In addition, the alkaloids of 153 plant groups have never been examined [18]. Over 1870 alkaloid skeletal were recognised at that time, and over 21,120 alkaloids had their structural makeup established. These are the twenty most important: Apocynaceae, Asteraceae, Berberidaceae, Boraginaceae, Buxaceae, Celastraceae, Fabaceae, Lauraceae, Liliaceae, Loganiaceae, Menispermaceae, Papaveraceae, Piperaceae, Poaceae, Ranunculaceae, Rubiaceae, Rutaceae, and Solanace. There was a great deal of conjecture about the formation of alkaloids and the interactions between alkaloid groups before there was any actual evidence that alkaloids were derived from amino acids. Organic chemists took the lead when biosynthetic experimentation started in the early 1950s following the introduction of radio-labelled precursors, with groups led by Birch, Barton, Battersby, Arigoni, Scott, Spencer, and Leete who clarified many crucial fundamental alkaloid biosynthetic pathway elements [19]. When it became apparent, roughly 20 years ago, that research at the cellular and then enzyme levels was required, and from there to the cloning and expression of systems that could generate alkaloids ex situ, the groups of Zenk, Stöckigt, Kutchan, Robins, Yamada, and Verpoorte took the lead. The study of alkaloid biosynthesis from a regulatory and metabolic engineering approach has now entered a new phase. Leaders in this circumstance have included Kutchan, Facchini, and Yamada. Up until recently, Richard Herbert's heroic efforts allowed The Royal Society of Chemistry to publish Natural Products Reports, a review magazine that provided excellent coverage of this area of natural product chemistry and biology. Three significant reactions serve as the foundation for the biosynthesis of alkaloids: the Pictet-Spengler condensation, the Mannich reaction of a Schiff base with a nucleophile, and the phenolic coupling reaction. Before going over some of the amazing pathways that lead to the diversity of alkaloids, it is important to review these three reactions [20]. Alkaloids have long been thought to be crucial for humans, despite the fact that they are secondary metabolites, which might imply that they are useless. In very little quantities, alkaloids exhibited potent biological influences on human and animal species. Alkaloids are found in food and drink used by humans every day, as well as in several stimulant medications (**Figure 1**) [21].

**Figure 1.** *Structure of atropine.*

## **2. Occurrence**

While alkaloids often present in all sections of a plant, they occasionally concentrate solely in one organ, leaving other parts of the plant free of them. For example, The potato plant's edible tubers are free of alkaloids, but its green parts are poisonous because they contain solanine. Alkaloids are not always synthesised in the organ in which they collect; for example, Tobacco's roots are where nicotine is produced before being carried to the leaves, where it is subsequently found. (Harborne and Herbert, 1995). In the epidermis of a human, about 300 alkaloids from over 24 classes have been discovered. The Phyllobates genus of frogs'skin was the source for the lethal neurotoxic alkaloids. Daly, (1993) separated different antibacterial alkaloids from reptile skin. Some isoquinoline and indole alkaloids were includes mammalian morphine but is segregated from them. The human diet includes several alkaloids in both food and beverages. The plants in the human diet in which alkaloids are present are not only coffee seeds, caffeine (**Figure 2**) [22].

## **3. Medicinal significance**

Alkaloids have a variety of medicinal uses. Despite the fact that many of them exhibit local anaesthetic properties, they rarely have therapeutic uses. One of the most

**Figure 2.** *Structure of caffeine.*

*Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

**Figure 3.** *Structure of ergotamine.*

well-known alkaloids that has been utilised for medical purposes both historically and currently is morphine. This alkaloid is a strong narcotic that can only be used in small doses to treat pain due to its addictive properties [1].

Medicine has used alkaloids having hallucinogenic, narcotic, or analgesic characteristics, such as morphine, atropine, and quinine. While many alkaloids are abused as illegal substances, such as cocaine, For modern synthesised medications, several alkaloids served as model substances. Some alkaloids, such as strychnine and coniine, are too poisonous for any medicinal use. Additionally, new biologically active chemicals are continually being discovered in the plant. The drug atropine (Mann et al., 1994) relaxes smooth muscles and expands the eyeballs' pupils. Papaverine, a compound isolated from *Papaver somniferum*, has been shown by Pictet and Gums (1909) to have relaxing effects on blood vessel smooth muscle as well as intestinal and bronchial smooth muscle. Strong painkiller morphine is frequently prescribed to individuals with terminal illnesses. Although less strong, codeine performs similar pharmacological processes as morphine. Heroin is a highly addictive synthetic morphine derivative. As a particular analgesic, ergotamine (**Figure 3**) in the form of ergotamine tartrate coupled with caffeine is used to treat migraines [1].

## **4. Quinolines alkaloid**

This particular type of quinolone-nucleus carrying alkaloid is only found in the bark of the Cinchona plant. However, a number of simple heteroaromatic quinolines have also been discovered in a number of marine sources. (2-heptyl-4 hydroxyquinoline from a marine pseudomonad and 4, 8-quinolinediol from cephalopod ink 2-heptyl-4-hydroxyquinoline from a marine pseudomonad). The primary alkaloids in this group include cinchonine, cinchonidine, quinine, and quinidine (**Figure 4**).

#### **4.1 Quinoline alkaloid synthesis**

The cross coupling of phenyl magnesium bromide with 2-chloroquinoline product I, which was catalysed by cobalt (II) acetylacetonate in dioxane at 50°C, resulted in the alkaloid in a 74% yield. By heating acetophenone with 2- aminobenzyl alcohol product (II) in dioxane in the presence of the catalyst for oxidation [Ru(DMSO)4]Cl2

(2 mol%) and benzophenone as a hydrogen scavenger, the product was produced in 94% yield. In toluene at 100°C, molecular oxygen and hydrotalcite with ruthenium grafts, a multifunctional heterogeneous catalyst, have also been used to achieve this oxidative cyclisation in 89% yield. Reducing the titanium tetrachloride, zinc, and malononitrile-2-nitrochalcone adduct iii in boiling tetrahydrofuran resulted in the production of 2- phenylquinoline in a 78% yield. Finally, ytterbium (III) triflate in dichloromethane was used to achieve the three-component condensation of aniline with benzaldehyde and phenyl vinyl sulphide to yield 2-phenyl-4-phenylthio-1,2,3,4 tetrahydroquinoline 40 at room temperature .32 The alkaloid was subsequently produced in an overall yield of 23% by oxidising product iv with solid-supported periodate and thermolyzing the intermediate sulfoxide. These five adaptable methods were also used to create a variety of synthetic 2-phenylquinoline analogues, so they ought to work just as well when creating additional straightforward 2-arylquinoline alkaloids of related interest is the oxidation of 2-aryl-2,3-dihydroquinolin-4(1H)-ones such as product v with ferric chloride hexahydrate in boiling Methanol is used to produce 2-aryl-4-methoxyquinolines, including the naturally occurring compound 2-phenyl-4-methoxyquinoline vi, which was produced with a 78% yield [23].

#### **4.2 Reagents and conditions in Quinoline alkaloid synthesis**


*Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

**Figure 5.** *Quinoline alkaloid synthesis.*

## **5. Isoquinoline alkaloids**

There are a few categories of isoquinolinoid marine alkaloids, however the majority of higher plants, isoquinoline alkaloids are present. The fundamental structural element is the isoquinoline nucleus. Numerous therapeutic effects, these particular alkaloids contain substances that have antiviral, antifungal, anticancer, antioxidant, antispasmodic, and enzyme inhibitory properties. Morphine and codeine are the two most significant and extensively studied isoquinoline alkaloids. They originate from tyrosine or phenylalanine.

They are produced using a ketone or an aldehyde and the precursor of dopamine (3, 4-dihydroxytryptamine). These alkaloids are further divided into the following categories: Simple isoquinoline alkaloids, such as salsoline and mimosamycin; benzylisoquinoline alkaloids, such as reticuline and imbricatine; bisbenzylisoquinoline alkaloids, such as fumaricine; manzamine alkaloids, such as manzamine a; pseudobenzylisoquinoline alkaloids, such as polycarpine and ledecorine; and secobisbenzylisoquinoline.

A vast family of naturally occurring substances known as isoquinoline alkaloids exhibits a wide variety of structural variation as well as biological and pharmacological activity. Much work has been done over the past 10 years to develop efficient synthetic methods to obtain these alkaloids in chiral nonracemic form. There have been a variety of approaches used that rely on diastereoselective or enantioselective catalytic processes For a very long time, isoquinoline alkaloids have been important targets for chemical synthesis, both as a source of intellectual challenge and as substances with potential medical value.

Recently, chiral N-sulfinyl β-arylethylamines have been employed as substrates for the asymmetric synthesis of isoquinoline alkaloids.72,73 The MexicanSpanish team72 in a short and efficient synthesis of (+)-crispine A (177) applied sulfinamide

193, which was prepared from β-3,4-dimethoxyphenylethylamine and (S)-menthyl ptoluene sulfinate as the starting compounds [1].

## **5.1 Using chiral Oxazoloisoquinoline 794 and Arylmagnesium bromide as a substrate, (S)-(+)-Cryptostyline II (793) is synthesised**

By cyclocondensing 4,5-dimethoxy-2-vinylbenzaldehyde with (R) phenylglycinol in Asao's method, the main substrate, the oxazoloisoquinoline, was produced in 72% yield as a 93:7 combination of diastereomers and used for the synthesis of (S)-(+)-cryptostyline II (**Figure 5**). Thus, the reaction of 794 with 3, 4-dimethoxyphenylmagnesium bromide produced addition product 797, from which, after the chiral inductor was removed and N-methylation was performed, the target alkaloid 793 was recovered with 96% in 57% overall yield (**Figure 6**).

Amat's group employed it as a substrate for the synthesis of C-1-substituted tetrahydroisoquinoline derivatives (**Figure 6**), including alkaloids, starting with oxazolotetrahydroisoquinolone 795 made from aldehyde ester 796 and (R)-phenylglycinol in 58% yield (**Figure 5**). As a result, the reaction of 795 with the proper Grignard reagents produced addition products 798, which were separated as single isomers in a yield of 49–63%. (**Figure 7**) Removal of the N-chiral auxiliary led to lactam 799, in which reduction of the lactam carbonyl fulfilled the synthesis of five alkaloids: ()-salsolidine (559), ()-O,O-dimethylcoclaurine (ent-581), ()-norcryptostyline II (ent-245), norcryptostyline III (558), and ()-crispine A (ent-177) [24].

## **6. Indole alkaloids**

Terpenoid indole alkaloids are present in a large number of plant species from the families Apocynaceae, Loganiaceae, Rubiaceae, and Nyssaceae (TIAs). TIAs are a broad class of structurally varied molecules that include substances with interesting pharmacological properties. The anti-neoplastic drugs vincristine and vinblastine, the anti-hypertensive drugs reserpine and ajmalicine, as well as the anti-arrhythmic drug ajmaline, are only a few of the terpenoid indole alkaloids used in modern medicine. The intermediate strictosidine, which is created by combining the amino acids

**Figure 7.**

*Synthesis of Norcryptostyline. Grignard reagent reaction of Oxazolotetrahydroisoquinolone 795 with a series of 1-substituted Isoquinoline Alkaloids.*

tryptamine and secologanin, which are respectively generated directly from the amino acid tryptophan and indirectly from (10-hydroxy-) geraniol, is essential to the biosynthesis of all terpenoid indole alkaloids [25].

## **7. Retrosynthetic analysis of arbornamine (monoterpene indole alkaloid)**

Arbornamine (1) is a monoterpene indole alkaloid that was discovered in 2016 by Kam and co-unique from a Malayan Kopsia arborea. It has a unique 6/5/6/5/6 "arbornane" skeleton that is different from those of the eburnane and tacaman classes families (**Figure 8**).

Arbornamine's (1) retrosynthesis calls for the worldwide Pentacyclic lactam 4 is reduced and produce the amino moiety and hydro Xymethine group. It was believed that the pentacyclic lactam 4 would result from a reductive Heck cyclization of vinyl iodide 5.4. The tetracyclic-lactam 6 would result from a The tryptamine derivative 75 and the dimethyl ester 8 of 2-ketoglutaric acid undergo Pictet-Spengler cyclization/ intramolecular ammonolysis. The unsaturated tetracyclic-lactam 5 may then created from there. The tryptamine Nb-nitrogen atom has a protecting group called a benzyl group. An undesirable regioisomeric -lactam 9 makes this design strategically important. In the foregoing one-pot reaction, it might have been produced if the free tryptamine had been employed.

We started our synthesis with a key cascade cyclization. According to **Figure 6**, The tetracyclic -lactam 6 was isolated in a yield of 73% by heating benzyl tryptamine 7 with dimethyl ester 8 and 2 equiv. of TFA in refluxing toluene. Using Pearlman's catalyst, lactam 6 was first hydrogenolyzed under atmospheric pressure to produce vinyl iodide under the circumstances required for the Nb-nitrogen atom's

**Figure 8.** *Synthesis of monoterpene indole alkaloid.*

benzyl protective group to be removed and transform it into 11. Thereafter, easily reachable The product was alkylated with (Z)-1- bromo-2-iodobut-2-ene7 to create vinyl iodide 11.

To produce arbornamine, it was globally reduced using lithium aluminium hydride (1). Both the methyl ester and the -lactam have been decreased during this process. The great facial selectivity of -lactam reduction was most likely related to the C-3 methyl ester's top-side shielding action, which allowed the hydride to approach from the bottom side. The synthetic sample's NMR results match those from the literature. As a result, we have created a brief initial method for arbornamine (1), a recently identified monoterpene indole alkaloid, to be completely synthesised. Six steps were all that it took to complete the synthesis, which had a 31% total yield. With the exception of the phase where the N-benzyl protecting group is cleaved, which is used deliberately to prevent the creation of the undesirable product, each step is effective in increasing molecular complexity.

#### *Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

Our synthesis got underway with a critical cascade cyclization. According to above Synthetic pathway, the tetracyclic -lactam 6 was isolated with a yield of 73% the benzyl tryptamine 7 is heated along with the dimethyl ester 8 and two equivalents of TFA in refluxing toluene. At room temperature and pressure, lactam 6 was first hydrogenolyzed with Pearlman's catalyst to produce vinyl iodide, which was then applied to remove the benzyl protecting group from the Nb-nitrogen atom and transform it into 11. The result was then alkylated with easily available (Z)-1- bromo-2 iodobut-2-ene7 to create vinyl iodide 11. Combining vinyl iodide 11 with a common selenenylation/elimination procedure produced a conjugated tetracyclic -lactam 5. The scene was prepared for the last ring's completion with - lactam 5 in hand. Using a reductive Heck cyclization, Ni(cod)2 was used to mediate the creation of the necessary pentacyclic product 4 in 91% yield. The next product was a pentacyclic -lactam 4. Remaining solvent signals for CDCl3 were detected by 1H NMR (7.26) and 13C NMR (77.0). The following peak multiplicities were noted: Brs for electrospray ionisation (ESI), high resolution mass spectral (HRMS) data were acquired, and There were reported mass-to-charge ratios (m/z). Melting points were determined on a WRX- 5A melting point apparatus [25].

## **8. Tropane alkaloid**

In general, but not always, the roots are where tropane alkaloid production takes place. Translocation then takes place through the aerial sections' xylem, where limited further metabolism may occur. For instance, many Datura species only convert hyoscyamine to hyoscine in the roots, so some synthesis may possibly take place in the aerial parts. For instance, despite the fact that A. Concentrations of mydriatic alkaloids are present in belladonna scions grown on foreign stocks and detached leaves of *A. belladonna* were discovered to have an increase in alkaloid content after 5 days, which was associated with a decrease in protein nitrogen (171).

From grafting studies, which typically used stocks and scions from plants of various solanaceous genera containing distinct alkaloids, such as Datura and Nicotine, it was possible to infer the root origins and subsequent migrations of alkaloids in a number of species (**Figure 9**).

### **8.1 Biosynthetic pathway of tropane alkaloid**

Arginase is aAR. Ornithine decarboxylase, abbreviated ODC. Putrescine N-methyltransferase, or PMT. N-methylputrescine oxidase, or MPO. Tropineforming Reductase, abbreviated TRI. littorine mutase, CYP80F1. The hydroxylase of hyoscyamine 6 (H6H). Transferase for aromatic amino acids, AT4. Phenylpyruvic acid reductase, or PPAR.

### **8.2 Tropate ester biosynthesis**

The (S)-tropic acid 7 ester moiety is a structural component of the alkaloids hyoscyamine 1 and scopolamine 2. Since even before the year 2000, there has been active discussion about the The biosynthesis of tropic acid 7, and the matter is still unresolved, at least in the finer points. We need to go back and look at Robinson's work from the years 1927 and 1955, when he first brought the topic of tropic acid biosynthesis to light and then went into greater detail about it in his book on the

#### *Medicinal Plants – Chemical, Biochemical, and Pharmacological Approaches*

**Figure 9.** *Tropane alkaloid synthesis. Tropane alkaloids' proposed biosynthetic pathway in the Solanaceae.*

**Figure 10.** *Ester alkaloid synthesis.*

structural relationships of natural products. The fact that tropic acid 7 has a branching carbon skeleton and must originate either from the isomerization of a phenylpropionoid moiety or through a unique synthesis was acknowledged. After feeding (1, 3-C2) phenylalanine to Datura innoxia, Leete was able to conclusively demonstrate that it derived by isomerization. As indicated in the following scheme, the resulting hyoscyamine 1C now has the isotopes close to one another in positions C-1 and C-2 of the alkaloid (**Figure 10**) [26].

## **9. Xanthine alkaloids**

Purine alkaloids, commonly referred to as xanthine alkaloids, consist of methylxanthines and methyluric acids and their structures are based on the xanthine and uric acid skeletons. Coffee (Coffea arabica), tea (*Camellia sinensis*), mate (*Ilex paraguariensis*), cocoa (*Theobroma cacao*), and guarana (*Paullinia cupana*), which are used to make popular non-alcoholic beverages, all contain caffeine *Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

(1, 3, 7-trimethylxanthine) and theobromine (3,7-dimethylxanthine). The isolation Caffeine of from coffee seeds was first reported independently in 1820 by the German researchers, Runge and von Giese. Caffeine was found as "thein" in tea

leaves by Oudry in 1827. Daniell discovered it in kola nuts (*Cola acuminata*) in 1865, while Stenhouse discovered it in mate' in 1843. Woskresensky identified theobromine in cacao seeds in 1842. Salomon discovered paraxanthine (1,7-dimethylxanthine) from human urine in 1883, but Chou and Waller did not find it in coffee seeds until 1980. Fischer and Ach published the complete chemical synthesis of Caffeine in 1895. Studies on caffeine biosynthesis were initiated in the 1960s, while highly purified caffeine synthase was isolated by Kato et al. (**Figure 11**) [27].

Solid arrows represent the four steps that make up the main pathway (steps1–4). Three different N-methyltransferases are shown as I, II, and III: caffeine synthase, theobromine synthase, and 7-methylxanthosine synthase. N-methylnucleosidase is responsible for catalysing the second step, which converts 7-methylxanthosine to 7-methylxanthine. I, III. Due to the broad substrate specificities of caffeine synthase, minor routes, denoted by dotted arrows, may take place (III). The production of 7-methylxanthosine from XMP via 7-methyl-XMP (steps 7–8) was suggested by Schulthess et al., but no recombinant N-methyltransferases have been found to catalyse these conversions [27].

## **10. Pyrrole-imidazole alkaloids**

As an example, consider the pyrrole-imidazole alkaloids (**Figure 1A**). Sponge natural products classified as pyrrole-imidazole alkaloids have approximately 150 congeners and are a broad and highly intricate class. Because of their chemical complexity, pyrrole-imidazole alkaloids have undergone a number of structural revisions, provided title compounds for organic synthesis, and maintained pharmaceutical interest due to their attractive bioactivity profiles. The enantioselective dimerization of three important monomeric building blocks, oroidin (1), hymenidin (2), and clathrodin (3), is suggested by retrosynthetic pathways for pyrrole-imidazole alkaloids. In fact, in vitro biomimetic research, these monomeric building blocks were dimerized utilising enzymes isolated from sponges that contained pyrrole-imidazole alkaloids. But there have not been many insights into the biosynthesis of 1–3 themselves; the only information we presently have came from observing the incorporation of radiolabeled amino acid precursors into 1 product (**Figure 12**) [28].

## **11. Synthesis of pyrrole-imidazole alkaloids is proposed**

Retrobiosynthetic plan explaining how product 1 is produced from the building blocks of amino acids. 8 and 9 are hypothesised to be directly connected with pyrrole carboxylic acids in the chemical structures of pyrrole-imidazole alkaloids (B-E) EICs and MS<sup>2</sup> spectra mirror plots comparing the *Stylissa* metabolome's 7–9 and 11 identified metabolites to synthetic standards. We have highlighted important MS<sup>2</sup> ions (**Figure 13**).

Barbaleucamides A–B are shown in the metabolome BPC, which has superimposed EICs showing their existence. (B) EICs, produced within 1 ppm error tolerance for 7–12 across *Stylissa, Axinella, Agelas*, and *Dysidea* polar metabolomics LC/MS datasets. With the exception of EIC 7, which has the high abundance structural annotations of fragment ions, all EIC y-axes are similar as indicated. Each structural annotation's

*Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

**Figure 12.** *Pyrrole imidazole alkaloid synthesis.*

related ppm mistake is listed. The MS<sup>2</sup> spectra displayed here were obtained using an orbitrap mass spectrometer and extremely precise Fourier transform mass spectrometric fragmentation.

## **12. Piperidine alkaloids**

This class of alkaloids' primary ring system is the piperidine nucleus. Monocycle molecules with the C5N nucleus are the primary defining feature of true piperidine alkaloids. The odour of piperidine alkaloids is one of their shared characteristics. They lead to long-term neurotoxicity. The majority of them come from plants. Even though the piperidine alkaloid is made from lysine, some piperidine alkaloids, like the straightforward pyrrolidine alkaloids, are also made from acetoacetate. Lobeline is a major alkaloid in this group [22].

## **13. Imidazole alkaloid**

The imidazole ring structure of this form of alkaloid is what makes it unique. Since the imidazole ring was already formed during the precursor step, these alkaloids constitute an exception to the structure-transformation process. This type of structurally diverse alkaloids occurs in a variety of situations, particularly in marine and microbial alkaloids. They have a significant potential for therapeutic use and demonstrate a wide spectrum of biological activities [29].

#### **Figure 13.**

*List of pyrrole-imidazole alkaloid biosynthesis intermediates that have been rationalised. A* Dysidea *species.*

## **14. Pyrrolizidine alkaloids**

The pyrrolizidine nucleus is the defining characteristic of this class of alkaloids. Plants from the Fabaceae and Asteraceae families contain them. The bulk of

*Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

pyrrolizidine alkaloids are present in plants as N-oxides, but when they are separated, they lose their functionality. A lot of research has been done on alkaloids because of their potentially harmful side effects, particularly liver damage. The animals that consume these alkaloids become antifeedants when they enter the food chain [30].

## **Acknowledgements**

All authors are thankful to management and principal of Dadasaheb Balpande College of diploma in Pharmacy, Nagpur.

## **Author details**

Nitin Dumore1†, Namita Girhepunje<sup>1</sup> \*†, Monali Dumore2† and Kishor Danao2†

1 Dadasaheb Balpande College of Diploma in Pharmacy, Nagpur, Maharashtra, India


© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Kurek J. Introductory Chapter: Alkaloids-their Importance in Nature and for Human Life. London, UK: InTech; 2019

[2] Hesse M. Alkaloids, Nature's Curse or Blessing, Weinheim, Germany: Wiley-VCH, A very interesting book of the history, biological significance, and synthesis of alkaloids 2003, 42, 40, 4852-4854

[3] Geoffrey A. Cordell, Alkaloids and their Biosynthesis - Introduction to Alkaloids. A Biogenetic Approach, Phytochemistry and Pharmacognosy, Natural Products Inc., Evanston, 1981, 21, 3, 1055

[4] Dewick PM. Medicinal Natural Products. A Biosynthetic Approach. Second ed. Chichester, UK: John Wiley & Sons. [The alkaloid chapter in this book offers a useful and succinct overview of alkaloids as medicinal agents]; 1997. p. 466

[5] Cordell GA, Quinn-Beattie ML, Farnsworth NR. The potential of alkaloids in drug discovery. Phytotherapy Research [A review of the occurrence of alkaloids and their biological potential]. 2001;**15**:183-205

[6] Wink M. Molecular modes of action of cytotoxic alkaloids: From DNA intercalation, spindle poisoning, topoisomerase inhibition to apoptosis and multiple drug resistance. In: The Alkaloids, Chemistry and Biology. Vol. 64. Elsevier Publishers, A review of the relationships of the structure of alkaloids and their interactions with cell systems; 2007. pp. 1-47

[7] Herbert RB. The biosynthesis of plant alkaloids and nitrogenous microbial metabolites. Natural Product Reports, A

series of reviews on the biosynthesis of alkaloids in plants, fungi, bacteria, and marine organisms. 2003;**20**(5):494-508

[8] Kutchan TM. Alkaloid biosynthesis – The basis for metabolic engineering of medicinal plants. The Plant Cell, American Society of Plant Physiologists, A good introduction to the relevance of metabolic engineering in developing alkaloids. 1995;**7**(7):1059-1070

[9] Zenk MH, Juenger M. Evolution and current status of the phytochemistry of nitrogenous compounds. Phytochemistry, Why the study and the continuous development of alkaloid biosynthesis using metabolic engineering is important. 2007;**68**(22–24):2757-2772

[10] Usera AR, O'Connor SE. Mechanistic advances in plant natural product enzymes. Current Opinion in Chemical Biology, A good introduction to the relevance of metabolic engineering in developing alkaloids. 2009;**13**:492-498

[11] Roberts M, Strack D, Wink M. Biosynthesis of alkaloids and betains. Annual Plant Reviews, An overview of alkaloid biosynthesis from chemical, enzymatic and gene perspectives. 2010; **40**:20-91

[12] Suzuki K-I, Yamada Y, Hashimoto T. Expression of Atropa belladonna putrescine N-methyl transferase gene in root pericycle. Plant Cell Physiology, The cDNAs for putrescine N-methyl transferase are described. 1999;**40**: 289-297

[13] Heim WG, Sykes KA, Hildreth SB, Sun J, Lu RH, Jelesko JG. Cloning and characterization of a Nicotiana tabacum methyl putrescine oxidase transcript. Phytochemistry, One of the key enzymes *Methods of Alkaloids Synthesis DOI: http://dx.doi.org/10.5772/intechopen.111785*

in tropane alkaloid biosynthesis is described. 2007;**68**:454-463

[14] Lounasmaa M, Tamminen T. The tropane alkaloids. In: Cordell GA, editor. The Alkaloids, Chemistry and Pharmacology. Vol. 44. San Diego, California: Academic Press, A review of the tropane alkaloids; 1993. pp. 1-114

[15] Robins RJ, Walton NJ. The biosynthesis of tropane alkaloids. In: Cordell GA, editor. The Alkaloids, Chemistry and Pharmacology. Vol. 44. San Diego, California: Academic Press. An overview of tropane alkaloid biosynthesis; 1993. pp. 115-187

[16] Robins RJ, Abraham TW, Parr AJ, Eagles J, Walton NJ. The biosynthesis of tropane alkaloids in Datura stramonium: The identity of the intermediates between N-methylpyrrolinium salt and tropinine. Journal of the American Chemical Society, Acetoacetate is incorporated intact into the tropane nucleus. 1997;**119**:10929-10934

[17] Sandala GM, Smith DM, Radom L. The carbon skeleton rearrangement in tropane alkaloid biosynthesis. Journal of the American Chemical Society, Quantum chemistry calculations suggest a concerted carbocation rearrangement in hyoscyamine biosynthesis. 2008;**130**: 10684-10690

[18] Humphrey A.J. and O'Hagan D. Tropane alkaloid biosynthesis. A century old problem unresolved. Natural Product Reports, An historical overview of the complexities of tropane alkaloid biosynthesis, 2001, 18, 494-502

[19] Stöckigt J, Panjikar S. Structural biology in plant natural product biosynthesis – Architecture of enzymes from monoterpenoid alkaloid and tropane alkaloid biosynthesis. Natural Product Reports, A contemporary view of the importance of studying the enzymes of tropane and indole alkaloid biosynthesis. 2007;**24**:1382-1400

[20] Khadem S, Marles RJ. Chromone and flavonoid Alkaloids: Occurrence and bioactivity. Molecules. 2012;**17**(12): 191-206

[21] Dey P, Kundu A, Kumar A, Gupta M, Lee BM, Bhakta T, et al. Recent Advances in Natural Products Analysis. Elsevier; 2020

[22] Dey P, Kundu A, Kumar A, Gupta M, Lee BM, Bhakta T, et al. Analysis of alkaloids (indole alkaloids, isoquinoline alkaloids, tropane alkaloids). In: Recent Advances In Natural Products Analysis. Elsevier; 2020. pp. 505-567

[23] Michael OP. Quinoline, quinazoline and acridone alkaloids. Natural Product Reports. 2007;**24**(1):223

[24] Chrzanowska M, Grajewska A, Rozwadowska MD. Asymmetric synthesis of Isoquinoline alkaloids. Chemical Reviews. 2016;**116**(19): 12369-12465

[25] Collu G, Unver N, Peltenburg-Looman AMG, van der Heijden R, Verpoorte R, Memelink J, et al. Geraniol 10-hydroxylase1, a cytochrome P450 enzyme involved in terpenoid indole alkaloid biosynthesis. FEBS Letters. 2001;**508**(2):215-220

[26] Andrew J. Humphreya, David O'Haganb, a century old problem unresolved. Natural Product Reports. 2001;**18**(5):494-502

[27] Zheng Y, Yue B-B, Wei K, Yang Y-R. Total synthesis of ()-Geissoschizol through Ir-catalyzed allylic Amidation as the key step. Organic Letters. 2017; **19**(23):6460-6462

[28] Mohanty I, Moore SG, Yi D, Biggs JS, Gaul DA, Garg N. Vinayak Agarwal precursor-guided mining of marine sponge metabolomes lends insight into biosynthesis of pyrrole-imidazole alkaloids. ACS Chemical Biology. 2020; **15**(8):2185-2194

[29] Kohnen-Johannsen KL, Kayser O. The imidazole alkaloid with the greatest medicinal significance is pilocarpine. Tropane Alkaloids: Chemistry, Pharmacology, Biosynthesis and Production, National Liabrary of Medicine. 2019;**24**(4):796

[30] Schramm S, Köhler N, Rozhon W, Alkaloids P. Biosynthesis, biological activities and occurrence in crop plants. Molecules. 2019;**24**(3):498

## **Chapter 7**
