**1.1 Terpenoids, thymol and biosynthesis of thymol**

The term terpenoid, isoprenoid and terpenes are frequently used interchangeably [11]. Terpenes are hydrocarbons built from isoprene units joined in head-to-tail fashion, while the terpenoids are the oxygen-containing analogues of the terpenes. They are the most diversified group of plant-derived natural products, having a broad range of biological functions. Different types of terpenoids have been isolated from plants and animals [11, 12]. Terpenoids have also been classified based on the functional groups they contain, such as hydrocabons (limonene, Myrecene, α-pinene and β-pinene), aldehydes (geranial, neral), esters (bornile acetate, methyl salicylate), alcohols (menthol, bisabolol), ketones (camphor, thujone), lactones (aesculatine, citroptene), Phenols (limol, carvacrol), ethers (1, 8-cineol) and peroxides (ascaridol).

Thymol is a naturally occurring monoterpenoid (2-isopropyl-5-methylphenol). The primary active compounds of essential identified from *Thymus vulgaris* (Lamiaceae) is an isomer of carvacrol. Thymol is a potent antiseptic that have a wide range of applications in home/personal home care products such as mouthwash, hand sanitizer, and acne medicines [13]. Treatment of gastrointestinal and respiratory problems [14–17]. *Ocimum gratissimum*, *Trachyspermum ammi*, *Carum copticum*, *Oliveria decumbens*, and *Anemopsis californica* are among the plants from which thymol has been extracted [18–22]. Thymol is suggested as a potential natural remedy in these various categories of recognized pharmaceutical uses [23]. Thymol has been used in formulations as an insecticide, fungicide, and medicinal disinfectant, among other non-pharmacological uses [24, 25]. Isopentenyl-pyrophosphate (IPP) and its isomer dimethylallyl-pyrophosphate (DMAPP), both of which are produced by the

*Thymol and Its Derivatives Rich Essential Oils from* Asparagus flagellaris *and Therapeutic… DOI: http://dx.doi.org/10.5772/intechopen.113197*

plastidic methyerythritol pathway and the cytosolic mevalonic pathway which are the five-carbon building blocks from which terpenes are biosynthesized.

The biosynthetic precursor of terpenes in the Mevalonic Acid (MVA) pathway of thymol biosynthesis (**Figure 1**) is acetyl-Coenzyme A, commonly known as activated acetic acid. It is similar to Claisen condensation and involves the coupling of two acetyl-CoAs to produce acetoacetyl-CoA, a biological equivalent of acetoacetate.

**Figure 1.** *Biosynthesis of thymol via the mevalonate pathway.*

In a process that resembles an aldol reaction, acetoacetyl-CoA joins with another equivalent of acetyl-CoA as a carbon nucleophile to produce -hydroxy-methylglutaryl-CoA (HMG-CoA). The next step is an enzymatic reduction of HMG-CoA with nicotinamide adenine dinucleotide in the presence of water, which produces mevalonic acid [26]. Mevalonic acid monophosphate is subsequently phosphorylated by adenosine triphosphate (ATP) to produce mevalonic acid diphosphate. Isopentenylpyrophosphate (IPP), a five-carbon atom intermediate product, is then produced by decarboxylation and drying out. Isomerase, an enzyme with SH group, reacts with the IPP to produce dimethylallylpyrophosphate (DMAPP), another intermediate molecule with five carbon atoms. Geranylpyrophosphate (GPP), a precursor to monoterpenes, is produced when the electrophilic allylic CH2 group of dimethylallylpyrophosphate (DMAPP) and the nucleophlic methylene group of isopentenylpyrophosphate (IPP) mix. Thymol and its isomer, carvacol, are produced by further reactions and rearrangements inside geranylpyrophosphate (GPP) [26].

Throughout Northern and Southern Nigeria, *A. flagellaris* (Kunth) Bak., an aromatic herb, is found [27]. Asparagaceae is its family. It has arching spiny branchlets and grows to a height of roughly 1 m as seen in **Figure 1** [28]. It is ascendant, generally erect plant. Africa as a whole uses it for a wide range of medical purposes. In both the guinea worm remedy and the ointment for hair development, the branchlets (cladodes) comprise the major component [27]. Gonorrhea, syphilis and other sexually transmitted diseases (STDs), as well as diarrhea and urinary infections, are all treated using the stem and leaves [27]. The roots are employed in Unani medicine as laxatives, tonics, aphrodisiacs, galactagogues, and treatments for renal and liver problems [29]. This herb was used to treat cholera, rheumatism and diuresis by the ancient Greeks, Romans, Indians and Chinese [30]. Indians utilized it to enhance fertility, relieve menstruation cramps, and boost nursing mothers' milk production. It works to boost kidney cellular activity, which in turn speeds up urine production. Chinese pharmacists store sparagus roots for their loved ones because they think it will make them more compassionate and loving [30]. *A. flagellaris'* stem bark and leaves were subjected to a phytochemical screening, which revealed a sizable quantity of flavonoid and a modest amount of carbohydrate, as well as trace amounts of ketones, pentose, and cardiac glycoside [28]. Mshelia *et al*. [30] reported the inhibition of six microorganisms viz. *Corynebacteria, Escherichia coli*, *Klebsiella*, *Shigella dysenteriae, Neisseria gonorrheae* and *Candida albicans* at different molarity of *A. flagellaris* ethanol extract, while the aqueous extract was susceptible to five organisms namely *Streptococcus pyogenes*, *Corynebacteria*, *Neisseria gonorrhoeae Proteus* spp. and *Treponema pallidum*.
