**2. Syntheses and stereochemical characteristics of peridinin derivatives**

#### **2.1 Design of peridinin derivatives and synthetic strategy**

We have focused on the subjects of why peridinin possesses a unique allene group, a ylidenebutenolide ring and the irregular C37 carbon not to be usual C40 skeleton, how these functional groups play a role in the exceptionally high energy transfer, and how they affect the ICT state. In order to solve these questions, we designed and began to synthesize allenemodified, ylidenebutenolide-modified and conjugated chain-modified derivatives of peridinin (Fig. 2). For example, in order to understand the exact roles of the allene group, we designed following three peridinin derivatives. Acetylene derivative **2** possesses an epoxy-acetylene, olefin derivative **3** has an epoxy-olefin, and diolefin derivative **4** has a conjugating olefin group instead of the hydroxy-allene group. Next, in order to understand why peridinin possesses the irregular C37 skeleton, we designed three peridinin derivatives as a series of different – electron chain length compounds. These are C33 derivative **5** which has two fewer double bonds than peridinin, C35 derivative **6** which has one less double bond, and C39 derivative **7** which has one more double bond. On the other hand, in order to understand the role of the ylidenebutenolide group, we designed the open-ring peridinin derivatives **8** and **9**. Derivative **8** possesses a triple bond and a methyl ester group, and derivative **9** has a double bond and also a methyl ester group instead of the -ylidenebutenolide group. These derivatives would provide useful information on the roles of these unique functional groups by comparing their data on the spectroscopies and energy transfer efficiencies.

According to the stereocontrolled synthesis of peridinin, which we previously established and the strategy is shown in Fig. 3 (Furuichi et al., 2002, 2004), we planned to synthesize these peridinin derivatives by a coupling between the pairs of C17-allenic segment **10** and the corresponding ylidenebutenolide-modified half-segments **15**-**17** or C20 ylidenebutenolide half-segment **14** and the corresponding allene-modified half-segments **11**- **13** using the modified Julia olefination reaction (Baudin et al., 1991, 1993) (Fig. 3). Namely, we planned to synthesize the allene modified derivatives **2** and **3** by a coupling between the corresponding allene modified half-segment **12** and **13** and C20-ylidenebutenolide halfsegment **14**. Next, the syntheses of both C35 and C39 peridinin derivatives **6** and **7** would be possibly synthesized by utilizing the modified Julia olefination of the appropriate allenic

half segments such as **10** and **11** with the suitable -ylidenebutenolide half segments such as **14** and **15**, respectively. Thus, a coupling between C15-allenic segment **11** and C20 ylidenebutenolide segment **14** would produce C35 peridinin derivative **6**. Meanwhile, applying the same method to the coupling between C17-allenic segment **10** and C22 ylidenebutenolide segment **15** might produce the desired C39 peridinin derivative **7**, if the coupling product would be enough stable to be handled. On the other hand, we planned to synthesize acetylene ester derivative **8** and olefin ester derivative **9** by a coupling between the C17-allenic half-segment **10** and the corresponding ylidenebutenolide-modified halfsegments **16** and **17**. We have really achieved the synthesis of these complex peridinin derivatives **2**-**9** by this efficient strategy.

136 Artificial Photosynthesis

subjects of why peridinin possesses a unique allene group, a ylidenebutneolide ring and an irregular C37 skeleton, and how these functions play a role in the exceptionally high energy

AcO OH

Fig. 1. (A) Crystal structure of PCP complex and (B) the structure of peridinin

**2.1 Design of peridinin derivatives and synthetic strategy** 

data on the spectroscopies and energy transfer efficiencies.

**2. Syntheses and stereochemical characteristics of peridinin derivatives** 

We have focused on the subjects of why peridinin possesses a unique allene group, a ylidenebutenolide ring and the irregular C37 carbon not to be usual C40 skeleton, how these functional groups play a role in the exceptionally high energy transfer, and how they affect the ICT state. In order to solve these questions, we designed and began to synthesize allenemodified, ylidenebutenolide-modified and conjugated chain-modified derivatives of peridinin (Fig. 2). For example, in order to understand the exact roles of the allene group, we designed following three peridinin derivatives. Acetylene derivative **2** possesses an epoxy-acetylene, olefin derivative **3** has an epoxy-olefin, and diolefin derivative **4** has a conjugating olefin group instead of the hydroxy-allene group. Next, in order to understand why peridinin possesses the irregular C37 skeleton, we designed three peridinin derivatives as a series of different – electron chain length compounds. These are C33 derivative **5** which has two fewer double bonds than peridinin, C35 derivative **6** which has one less double bond, and C39 derivative **7** which has one more double bond. On the other hand, in order to understand the role of the ylidenebutenolide group, we designed the open-ring peridinin derivatives **8** and **9**. Derivative **8** possesses a triple bond and a methyl ester group, and derivative **9** has a double bond and also a methyl ester group instead of the -ylidenebutenolide group. These derivatives would provide useful information on the roles of these unique functional groups by comparing their

According to the stereocontrolled synthesis of peridinin, which we previously established and the strategy is shown in Fig. 3 (Furuichi et al., 2002, 2004), we planned to synthesize these peridinin derivatives by a coupling between the pairs of C17-allenic segment **10** and the corresponding ylidenebutenolide-modified half-segments **15**-**17** or C20 ylidenebutenolide half-segment **14** and the corresponding allene-modified half-segments **11**- **13** using the modified Julia olefination reaction (Baudin et al., 1991, 1993) (Fig. 3). Namely, we planned to synthesize the allene modified derivatives **2** and **3** by a coupling between the corresponding allene modified half-segment **12** and **13** and C20-ylidenebutenolide halfsegment **14**. Next, the syntheses of both C35 and C39 peridinin derivatives **6** and **7** would be possibly synthesized by utilizing the modified Julia olefination of the appropriate allenic

*Allene function*

•

OH

O

*Ylidenebutenolide ring*

O O

*C37*-Peridinin (**1**)

transfer and in the special excited state, ICT state.

(A) (B)

Allene-modified Derivatives Polyene chain-modified Derivatives Ylidenebutenolide-modified Derivatives

Fig. 3. Synthetic strategy

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

acid in CDCl3.

*i*

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 139

consideration; this reaction proceeded by intramolecular SN2' hydride reduction resulting from the coordination between the oxygen atom of the epoxide in **21** and the aluminum atom of DIBAL, as shown in **22**. The obtained acetyl diol **24** was transformed into the C17 allenic segment **10** using the Mitsunobu reaction with 2-mercaptobenzothiazole, followed

The terminal segments **26**, **30** and **34** were led to the each half-segment **12**, **13** and **37** shown in Fig. 4, respectively. The Sonogashira cross-coupling between **26** and vinyl iodide **27** under the same reaction condition produced the desired alcohol **28** in 80% yield. The obtained alcohol **28** was transformed into the acetylene segment **12** using the Mitsunobu reaction with 2-mercaptobenzothiazole, followed by oxidation of the resulting sulfide with aqueous 30% H2O2 and Na2WO4・2H2O, which was milder than (NH4)6Mo7O24 (Schulz et al., 1963). On the other hand, the olefin segment **13** would be obtained by a coupling between vinyl iodide **30** and vinylstannane **31**. The Stille cross-coupling reaction of **30** with vinyl stannane **31** in the presence of PdCl2(CH3CN)2 and LiCl gave the desired alcohol **32** in exellent yield as a single isomer. The Stille cross-coupling of the opposite conbination between the corresponding stannane and iodide did not afford the desired result. The alcohol **32** was transformed into sulfide **33** by the same procedure. Oxidation of **32** under the same reagent as that for the preparation of **12** gave the desired **13**. However, the use of 30 % H2O2 and (NH4)6Mo7O24, which is a little strict condition, gave a mixture of the desired **13** and the isomerized **13'** in low yield, and the ratio of **13** and **13'** was not reproducible (1: 4 to 1: 1). It is noteworthy that sulfone **13** was easily isomerized to **13'** by a trace amount of hydrochloric

Next was the synthesis of diolefin segment **37**. The Stille cross-coupling of **34** with vinyl stannane **31**, which was used in the synthesis of **13**, afforded tetraene alcohol **35** as a single isomer. In this coupling, the reaction smoothly proceeded at room temperature, and when

Pr2NEt was not used, a mixture of **35** and its 9Z-isomer was obtained in a ratio of eight to one by NMR. The amount of 9Z-isomer seemed to increase at higher reaction temperature, for instance, 9E/9Z = 3/1 at 60 oC, which is the same condition to that of the synthesis of **13**. The desired sulfone **37** was obtained from **35** by the Mitsunobu reaction with 2 mercaptobenzothiazole, followed by oxidation of the resulting sulfide with aqueous 30% H2O2 and Na2WO4・2H2O, as a mixture of 9E/9Z = 10/1 in 31% yield. The use of 30% H2O2 with (NH4)6Mo7O24 and mCPBA gave a complex mixture. Oxidation of the allylic sulfide to

We chose the modified Julia olefination as the final C-C coupling reaction, because most of this olefination proceeded even at -78 oC. Such low temperature reaction was well suitable for the construction of the poly functionalized polyene chain such as peridinin. For instance, the crucial modified Julia olefination was explored as the final key step in the synthesis of acetylene derivative **2**. The reaction of an anion derived from **12** with **14** at –78 oC smoothly proceeded within 5 min in the dark to produce the peridinin derivatives in 42% amount as a mixture of stereoisomers. Due to the previous experiments in our carotenoid syntheses and the reports of the Brückner's and de Lera's groups that the modified Julia olefination of polyene compounds generally produced the Z-isomer at the connected double bond (Bruckner et al., 2005; Vaz et al., 2005), we tried to isomerize the connected double bond monitoring by HPLC as shown in Fig. 5. The resulting mixture was allowed to stand in benzene at room temperature under fluorescent light in an argon atomosphere. The isomerization under fluorescent light was faster than that in the dark. After 2 days, we

the corresponding sulfone in longer conjugated polyenes was still problematic.

by oxidation of the resulting sulfide **25** with aqueous 30% H2O2 and (NH4)6Mo7O24.

#### **2.2 Syntheses of allene modified derivaitves**

First, the synthetic studies of the allene modified derivatives **2**-**4** are described. The synthesis of the longer conjugated half-segments was more difficult and needed the milder reaction conditions than those of the peridinin synthesis. Under stereospecific manner for the construction of the desired conjugated chains, palladium catalyzed sp-sp2 and sp2-sp2 couplings were very effective. Meanwhile, the synthesis of the allenic functional group has already been established as follows: acetylene derivative **19** was prepared starting from (-) actinol **18** as shown in Fig. 4. The Sonogashira cross-coupling between **19** and vinyl iodide **20** in the presence of catalytic amounts of Pd(PPh3)4 and CuI in diisopropylamine produced the desired ester **21** in 84% yield. In the case of using organic solvents, such as THF and CH2Cl2, the yield was lower. The conjugated diene ester **21** thus obtained was transformed into allenic triol **23** by the stereospecific hydride redution in 80% yield, whose method was already established and generally used for the synthesis of the allenic carotenoids. The stereochemistry of the obtained allenic triol **23** was well explained by the following

Fig. 4. Synthesis of allenic and allene modified half-segments

First, the synthetic studies of the allene modified derivatives **2**-**4** are described. The synthesis of the longer conjugated half-segments was more difficult and needed the milder reaction conditions than those of the peridinin synthesis. Under stereospecific manner for the construction of the desired conjugated chains, palladium catalyzed sp-sp2 and sp2-sp2 couplings were very effective. Meanwhile, the synthesis of the allenic functional group has already been established as follows: acetylene derivative **19** was prepared starting from (-) actinol **18** as shown in Fig. 4. The Sonogashira cross-coupling between **19** and vinyl iodide **20** in the presence of catalytic amounts of Pd(PPh3)4 and CuI in diisopropylamine produced the desired ester **21** in 84% yield. In the case of using organic solvents, such as THF and CH2Cl2, the yield was lower. The conjugated diene ester **21** thus obtained was transformed into allenic triol **23** by the stereospecific hydride redution in 80% yield, whose method was already established and generally used for the synthesis of the allenic carotenoids. The stereochemistry of the obtained allenic triol **23** was well explained by the following

**2.2 Syntheses of allene modified derivaitves** 

Fig. 4. Synthesis of allenic and allene modified half-segments

consideration; this reaction proceeded by intramolecular SN2' hydride reduction resulting from the coordination between the oxygen atom of the epoxide in **21** and the aluminum atom of DIBAL, as shown in **22**. The obtained acetyl diol **24** was transformed into the C17 allenic segment **10** using the Mitsunobu reaction with 2-mercaptobenzothiazole, followed by oxidation of the resulting sulfide **25** with aqueous 30% H2O2 and (NH4)6Mo7O24.

The terminal segments **26**, **30** and **34** were led to the each half-segment **12**, **13** and **37** shown in Fig. 4, respectively. The Sonogashira cross-coupling between **26** and vinyl iodide **27** under the same reaction condition produced the desired alcohol **28** in 80% yield. The obtained alcohol **28** was transformed into the acetylene segment **12** using the Mitsunobu reaction with 2-mercaptobenzothiazole, followed by oxidation of the resulting sulfide with aqueous 30% H2O2 and Na2WO4・2H2O, which was milder than (NH4)6Mo7O24 (Schulz et al., 1963).

On the other hand, the olefin segment **13** would be obtained by a coupling between vinyl iodide **30** and vinylstannane **31**. The Stille cross-coupling reaction of **30** with vinyl stannane **31** in the presence of PdCl2(CH3CN)2 and LiCl gave the desired alcohol **32** in exellent yield as a single isomer. The Stille cross-coupling of the opposite conbination between the corresponding stannane and iodide did not afford the desired result. The alcohol **32** was transformed into sulfide **33** by the same procedure. Oxidation of **32** under the same reagent as that for the preparation of **12** gave the desired **13**. However, the use of 30 % H2O2 and (NH4)6Mo7O24, which is a little strict condition, gave a mixture of the desired **13** and the isomerized **13'** in low yield, and the ratio of **13** and **13'** was not reproducible (1: 4 to 1: 1). It is noteworthy that sulfone **13** was easily isomerized to **13'** by a trace amount of hydrochloric acid in CDCl3.

Next was the synthesis of diolefin segment **37**. The Stille cross-coupling of **34** with vinyl stannane **31**, which was used in the synthesis of **13**, afforded tetraene alcohol **35** as a single isomer. In this coupling, the reaction smoothly proceeded at room temperature, and when *i* Pr2NEt was not used, a mixture of **35** and its 9Z-isomer was obtained in a ratio of eight to one by NMR. The amount of 9Z-isomer seemed to increase at higher reaction temperature, for instance, 9E/9Z = 3/1 at 60 oC, which is the same condition to that of the synthesis of **13**. The desired sulfone **37** was obtained from **35** by the Mitsunobu reaction with 2 mercaptobenzothiazole, followed by oxidation of the resulting sulfide with aqueous 30% H2O2 and Na2WO4・2H2O, as a mixture of 9E/9Z = 10/1 in 31% yield. The use of 30% H2O2 with (NH4)6Mo7O24 and mCPBA gave a complex mixture. Oxidation of the allylic sulfide to the corresponding sulfone in longer conjugated polyenes was still problematic.

We chose the modified Julia olefination as the final C-C coupling reaction, because most of this olefination proceeded even at -78 oC. Such low temperature reaction was well suitable for the construction of the poly functionalized polyene chain such as peridinin. For instance, the crucial modified Julia olefination was explored as the final key step in the synthesis of acetylene derivative **2**. The reaction of an anion derived from **12** with **14** at –78 oC smoothly proceeded within 5 min in the dark to produce the peridinin derivatives in 42% amount as a mixture of stereoisomers. Due to the previous experiments in our carotenoid syntheses and the reports of the Brückner's and de Lera's groups that the modified Julia olefination of polyene compounds generally produced the Z-isomer at the connected double bond (Bruckner et al., 2005; Vaz et al., 2005), we tried to isomerize the connected double bond monitoring by HPLC as shown in Fig. 5. The resulting mixture was allowed to stand in benzene at room temperature under fluorescent light in an argon atomosphere. The isomerization under fluorescent light was faster than that in the dark. After 2 days, we

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

of **45** gave the stereocontrolled ylidenebutenolide segment **14**.

derivative of **47** did not give the desired result because of its instability.

HO

TBSO

HO

• HO OAc

(11E/11Z = 1/3)

 Pd(PPh3)4, CuI, Et3N ; then HCO2H, 35% 2) Isomerization by fluorescent light

HCO2H 49%

11

**46**

<sup>I</sup> OTES 1)

**47**

Fig. 6. Synthesis of stereocontrolled ylidenebutenolide moiety

2) MnO2

Pd2(dba)3, TFP, CuI, Et3N ; then HCO2H, 40%

1)

I

O

O

O

O O

O OH

O

Pd

1) *<sup>i</sup>* Pr2NEt, 2) CBr4, PPh3, Et3N 53% for 4 steps Br

O O

**45**

HO

HO

We thus successfully synthesized the C20- and C22-ylidenebutenolide half-segments **14** and **15**, and C33 peridinin derivative **5** by the same way. The isomerization of C33 peridinin

OH

**39** TBSO

OH

MnO2

**43 44**

O O

O

O

**15**: C22-Ylidenebutenolide Segment (13'E/ 13'Z = 10/ 1)

**5**: C33-Peridinin Derivative

11

HO

HO

O

O

O O

**40**

O

O O

O

**14**: C20-Ylidenebutenolide Segment

O

Pd

Br Br

OH

TBAF 81%

CHO

CHO 13'

O

O

• OAc HO

HO

TBSO

O

O O

CHO

1)

O TBS P+Ph3Br-

2) O2, TPP, h

O **38**

> I OH Pd(PPh3)4 CuI, Et3N

, *n*BuLi

**42**

**41**

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 141

Next, the intermediary **43** underwent -allylalkenylpalladium(II)-assisted regio- and stereoselective intramolecular cyclization to form the -allylalkenylpalladium lactone intermediate **44**. In the final step, the -allylalkenylpalladium moiety was removed by hydrogenolysis with formic acid to give the desired ylidenebutenolide **45**. MnO2 oxidation

The crucial one-pot ylidenebutenolide formation from **41** and vinyl iodide **46**, which was previously synthesized by us, was explored as the key step in the synthesis of C33 peridinin derivative **5**. Thus, a mixture of **41** and **46** was stirred in the presence of catalytic amounts of Pd(PPh3)4 and cuprous iodide in triethylamine at 45 oC for 10 min. After the complete consumption of **41** was ascertained by TLC, formic acid was added to the reaction mixture and then the mixture was stirred at 45 oC for 10 min to produce the desired C33 peridinin derivative in 35% yield as a mixture of stereoisomers in one-pot. The undesired 11Z-isomer **46** resulted in the undesired 11Z-isomer of the compound **5**. The resulting mixture was then allowed to isomerize in benzene at room temperature under fluorescent light in an argon atmosphere to successfully produce the desired **5** as a mixture of stereoisomers (Fig. 7). Next, the stereocontrolled preparation of C22-ylidenebutenolide segment **15** from alkyne **41** was fortunately successful by the same procedure; a mixture of **41** and vinyl iodide **47** was stirred at 45 oC for 10 min to produce the desired ylidenebutenolide compound **15** in 40% yield as the 13'E/ 13'Z mixture (10/ 1). The reaction with the corresponding hydroxy

Fig. 5. Isomerization and structure of acetylene derivative

observed that the initially generated major peak (peak 1 in the immediate situation) changed to another major peak (peak 2). After 11 days, while the peak 2 gradually decreased, the peak 3 increased. After 14 days, the peak 2 became the major peak in an equilibrium state. We isolated all peaks by both the mobile-phase and the reverse-phase HPLC, and elucidated their structures by NMR (400 and 750 MHz). Thus, we clarified that the peak 1 was (9E, 13Z)-isomer **2**, the peak 2 was (9E, 13E)-all-*trans* acetylene derivatve **2**, and peak 3 was (9Z, 13E)-isomer **2**. All-*trans* derivative **2** did not isomerize to the 9Z-isomer at –20 oC but gradually isomerized at room temperature in the dark. Obviously, all-*trans* isomer was unstable at room temperature and easily isomerized to the 9Z-isomer (Vaz et al., 2006), which was the most stable isomer. In addition, olefin derivative **3** and diolefin derivative **4** were synthesized by the same procedure (Kajikawa et al., 2009a).

### **2.3 Syntheses of polyene-chain modified derivaitves**

Next, we synthesized polyene chain modified peridinin derivatives by using a stereocontorolled domino one-pot formation of the ylidenebutenolide as a key step. First, dibromide **40** was obtained by a sequence of the Wittig reaction, 1O2 oxygenaiton followed by a treatment with diisopropylethylamine in the presence of allyl bromide, and the Corey's dibromination from aldehyde **38**, which was prepared from (-)-actinol in 53% for 4 steps. Dibromide **40** was successfully transformed into alkyne **41** by the treatment with TBAF in 81% yield (Tanaka et al., 1980). Next was the key stereocontrolled preparation of the ylidenebutenolide segment **14** from alkyne **41** (Fig. 6). Thus, alkyne **41** was treated with vinyl iodide **42** and cuprous iodide in triethylamine for 1 h followed by an addition of formic acid after confirming the consumption of the starting **41** by TLC analysis. The mixture was then further stirred at room temperature for overnight to produce the desired ylidenebutenolide **45** in 49% yield under the stereocontrolled fashion in one-pot. This threestep domino one-pot reaction to prepare the ylidenebutenolide **45** could be explained in detail by the possible mechanism shown in Fig. 6. At first, Sonogashira coupling of **41** and iodide **42** proceeded to afford the desired coupling product, in which the -allylpalladium generated from the allylester group was formed and coordinated to the alkyne such as **43**.

observed that the initially generated major peak (peak 1 in the immediate situation) changed to another major peak (peak 2). After 11 days, while the peak 2 gradually decreased, the peak 3 increased. After 14 days, the peak 2 became the major peak in an equilibrium state. We isolated all peaks by both the mobile-phase and the reverse-phase HPLC, and elucidated their structures by NMR (400 and 750 MHz). Thus, we clarified that the peak 1 was (9E, 13Z)-isomer **2**, the peak 2 was (9E, 13E)-all-*trans* acetylene derivatve **2**, and peak 3 was (9Z, 13E)-isomer **2**. All-*trans* derivative **2** did not isomerize to the 9Z-isomer at –20 oC but gradually isomerized at room temperature in the dark. Obviously, all-*trans* isomer was unstable at room temperature and easily isomerized to the 9Z-isomer (Vaz et al., 2006), which was the most stable isomer. In addition, olefin derivative **3** and diolefin derivative

Next, we synthesized polyene chain modified peridinin derivatives by using a stereocontorolled domino one-pot formation of the ylidenebutenolide as a key step. First, dibromide **40** was obtained by a sequence of the Wittig reaction, 1O2 oxygenaiton followed by a treatment with diisopropylethylamine in the presence of allyl bromide, and the Corey's dibromination from aldehyde **38**, which was prepared from (-)-actinol in 53% for 4 steps. Dibromide **40** was successfully transformed into alkyne **41** by the treatment with TBAF in 81% yield (Tanaka et al., 1980). Next was the key stereocontrolled preparation of the ylidenebutenolide segment **14** from alkyne **41** (Fig. 6). Thus, alkyne **41** was treated with vinyl iodide **42** and cuprous iodide in triethylamine for 1 h followed by an addition of formic acid after confirming the consumption of the starting **41** by TLC analysis. The mixture was then further stirred at room temperature for overnight to produce the desired ylidenebutenolide **45** in 49% yield under the stereocontrolled fashion in one-pot. This threestep domino one-pot reaction to prepare the ylidenebutenolide **45** could be explained in detail by the possible mechanism shown in Fig. 6. At first, Sonogashira coupling of **41** and iodide **42** proceeded to afford the desired coupling product, in which the -allylpalladium generated from the allylester group was formed and coordinated to the alkyne such as **43**.

Fig. 5. Isomerization and structure of acetylene derivative

**4** were synthesized by the same procedure (Kajikawa et al., 2009a).

**2.3 Syntheses of polyene-chain modified derivaitves** 

Next, the intermediary **43** underwent -allylalkenylpalladium(II)-assisted regio- and stereoselective intramolecular cyclization to form the -allylalkenylpalladium lactone intermediate **44**. In the final step, the -allylalkenylpalladium moiety was removed by hydrogenolysis with formic acid to give the desired ylidenebutenolide **45**. MnO2 oxidation of **45** gave the stereocontrolled ylidenebutenolide segment **14**.

The crucial one-pot ylidenebutenolide formation from **41** and vinyl iodide **46**, which was previously synthesized by us, was explored as the key step in the synthesis of C33 peridinin derivative **5**. Thus, a mixture of **41** and **46** was stirred in the presence of catalytic amounts of Pd(PPh3)4 and cuprous iodide in triethylamine at 45 oC for 10 min. After the complete consumption of **41** was ascertained by TLC, formic acid was added to the reaction mixture and then the mixture was stirred at 45 oC for 10 min to produce the desired C33 peridinin derivative in 35% yield as a mixture of stereoisomers in one-pot. The undesired 11Z-isomer **46** resulted in the undesired 11Z-isomer of the compound **5**. The resulting mixture was then allowed to isomerize in benzene at room temperature under fluorescent light in an argon atmosphere to successfully produce the desired **5** as a mixture of stereoisomers (Fig. 7).

Next, the stereocontrolled preparation of C22-ylidenebutenolide segment **15** from alkyne **41** was fortunately successful by the same procedure; a mixture of **41** and vinyl iodide **47** was stirred at 45 oC for 10 min to produce the desired ylidenebutenolide compound **15** in 40% yield as the 13'E/ 13'Z mixture (10/ 1). The reaction with the corresponding hydroxy derivative of **47** did not give the desired result because of its instability.

Fig. 6. Synthesis of stereocontrolled ylidenebutenolide moiety

We thus successfully synthesized the C20- and C22-ylidenebutenolide half-segments **14** and **15**, and C33 peridinin derivative **5** by the same way. The isomerization of C33 peridinin

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

**spectral characteristics of the synthesized derivatives** 

procedure (Kajikawa et al., 2009b).

(A)) (Kajikawa et al., 2010).

case of the 9'E-olefin ester derivative **9-2**.

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 143

case of peridinin, which could be stored without any remarkable decomposition under the same conditions. Meanwhile, C35 peridinin derivative was also synthesized by the same

**2.4 Syntheses of ylidenebutenolide modified derivatives and the stereochemical and** 

Finally, the synthesis of ylidenebutenolide modified peridinin derivatives is described. The acetylene ester derivative was synthesized by the same protocol. Namely, the coupling between C17-allenic segment **10** and C20-acetylene methyl ester segment **16** by using the modified Julia olefination, which was followed by the isomerization gave all-*trans* **8** (Fig. 8

On the other hand, there were some difficulities in the synthesis of the olefin ester derivative (Fig. 8). We obtained the only 9'E-olefin ester segment **17** by the similar synthetic process, which was not the desired 9'Z half-segment resulting from the contribution of the carbonyl group of the methyl ester. We then tried to connect the segments **10** and **17** by the modified Julia olefination and to obtain all-*trans* **9-3** by the isomerization. The anion derived from **10** was stirred with a mixture of stereoisomers of **17** under the same conditions used for the coupling of the previous peridinin derivatives. The reaction was completed within 5 min in the dark to produce the coupling products as a mixture of the stereoisomers in 46% amount, whose HPLC is shown in Fig. 8 (B). The major peak (peak 1) was estimated to be 45% of the mixture by HPLC analysis (other isomers were 16%, 14%, 5%, 5%, and others). Isomerization to the desired all-*trans* **9-3** was attempted under the same conditions previously used. After 5 days, the initially generated major peak (peak 1) changed to another peak (peak 2; 44% based on HPLC analysis) in an equilibrium state. We then isolated both compounds and elucidated their structures by NMR (400 and 750 MHz), and clarified that peak 1 was (13Z, 9'E)-isomer **9-1** and peak 2 was (13E, 9'E)-isomer **9-2**.

Unfortunately, we could not obtain the desired all-*trans* (13E, 9'Z)- isomer **9-3**.

We investigated the stability of the synthesized ring opened derivatives **8** and **9**, and found the isolated all-*trans* acetylene ester derivative **8-1** was more labile than **8-2** (13Z-isomer). For instance, the isolated all-*trans* derivative **8-1** (13E-isomer) isomerized to the dihydrofuran derivative **8'** by a trace amount of hydrogen chloride in CDCl3, but the corresponding isomerization of **8-2** (13Z-isomer) was not observed (Fig. 8). In addition, the all-*trans* derivative **8-1** rapidly isomerized to give a mixture of Z-isomers upon illumination. This might occur due to the contribution of the carbonyl group of the methyl ester similar to the

In the PCP complex, peridinin exhibits an exceptionally high efficiency of energy transfer to Chl a. In order to make clear the effect of the ylidenebutenolide, we needed to measure the energy transfer efficiencies in peridinin derivatives. Futhermore, it was tried to construct the corresponding PCP derivatives using the synthesized peridinin derivatives **8** and **9** to compare with the energy transfer efficiencies of peridinin (private information from Dr. H. A. Frank). First, it was attempted to reconstitute the PCP apoprotein using the 9'E-olefin ester derivative **9-2** under the same conditions that were successful for peridinin (Ilagan et al., 2006), but the reconstitution was not observed. It was also tried to reconstitute the PCP apoprotein using the 13Z-isomer **8-2**, but it did not bind the protein either. The reason might be that these compounds were bent into a *cis* configuration, and hence they might not fit properly into the protein binding site. These results apparently showed that the ylidenbutenolide of peridinin at least contributes to the stereochemical stability of the

derivative was shown in Fig. 7 (A). The resulting mixture was then allowed to isomerize in benzene at room temperature under fluorescent light in an argon atmosphere. After 2 days, we observed that the initially generated major peak (peak 2) in Fig. 7 (A) changed into another major peak (peak 1) in the HPLC. In addition, peak 3 became larger after 2 days, when the situation would be an equilibrium state. We isolated all peaks by the mobile-phase HPLC and elucidated their structures by NMR (400 MHz), and we elucidated that peak 1 was fortunately (11E, 11'Z)-all*-trans* C33 peridinin **5**, peak 2 was (11Z, 11'Z)-isomer **5'** and peak 3 was (11E, 11'E)-isomer **5''**, respectively. Interestingly, (11E, 11'E)-isomer **5''** was the secondarily larger isomer in the equilibrium state.

Fig. 7. Structure and HPLC analysis of (A) C33 and (B) C39 peridinin derivatives

Furthermore, relatively unstable C39 peridinin derivative **7** was synthesized by the same protocol as shown in Fig. 7 (B). Thus, the anion derived from **10**, which was the allenic halfsegment of the established peridinin synthesis (Fig. 3), was stirred with **15** under the same condition. Fortunately, the reaction completed within 5 min in the dark to produce the coupling products as a mixture of the stereoisomers in almost 35% amount, in which the 13Z-isomer (peak 1) was estimated to be 48% of the mixture by HPLC analysis (13E-isomer: peak 2 was 19%). Isomerization to the desired **7** was again attempted by the same method. After 2 days, a large amount of the 13Z-isomer **7'** (peak 1) changed to the all-*trans* C39 peridinin derivative **7** (peak 2) (57% based on HPLC analysis) in an equilibrium state. We then isolated both compounds, and confirmed their structures by NMR (400 and 750 MHz). The synthesized all-*trans* C39 peridinin derivative gradually decomposed within one month under an argon gas atmosphere at around –20 oC. This instability was in good contrast to the procedure (Kajikawa et al., 2009b).

142 Artificial Photosynthesis

derivative was shown in Fig. 7 (A). The resulting mixture was then allowed to isomerize in benzene at room temperature under fluorescent light in an argon atmosphere. After 2 days, we observed that the initially generated major peak (peak 2) in Fig. 7 (A) changed into another major peak (peak 1) in the HPLC. In addition, peak 3 became larger after 2 days, when the situation would be an equilibrium state. We isolated all peaks by the mobile-phase HPLC and elucidated their structures by NMR (400 MHz), and we elucidated that peak 1 was fortunately (11E, 11'Z)-all*-trans* C33 peridinin **5**, peak 2 was (11Z, 11'Z)-isomer **5'** and peak 3 was (11E, 11'E)-isomer **5''**, respectively. Interestingly, (11E, 11'E)-isomer **5''** was the

Fig. 7. Structure and HPLC analysis of (A) C33 and (B) C39 peridinin derivatives

Furthermore, relatively unstable C39 peridinin derivative **7** was synthesized by the same protocol as shown in Fig. 7 (B). Thus, the anion derived from **10**, which was the allenic halfsegment of the established peridinin synthesis (Fig. 3), was stirred with **15** under the same condition. Fortunately, the reaction completed within 5 min in the dark to produce the coupling products as a mixture of the stereoisomers in almost 35% amount, in which the 13Z-isomer (peak 1) was estimated to be 48% of the mixture by HPLC analysis (13E-isomer: peak 2 was 19%). Isomerization to the desired **7** was again attempted by the same method. After 2 days, a large amount of the 13Z-isomer **7'** (peak 1) changed to the all-*trans* C39 peridinin derivative **7** (peak 2) (57% based on HPLC analysis) in an equilibrium state. We then isolated both compounds, and confirmed their structures by NMR (400 and 750 MHz). The synthesized all-*trans* C39 peridinin derivative gradually decomposed within one month under an argon gas atmosphere at around –20 oC. This instability was in good contrast to the

secondarily larger isomer in the equilibrium state.
