The Michael Type Ring Closure Leading to the Formation of Nirangenin

Introduction

The Michael type ring closure involves the addition of the carbon nucleophilic to the electrophile in the carbon double bond; the reaction is catalyzed by acids and bases and the bromide group is displaced by an E1CB mechanism. The hydroxylation process occurs after the Michael ring closure.

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The bibenzyl bromide undergoes Michael ring closure reaction to form nirangenin

The nucleophilic attack by the gallic acid nucleophile leading to the formation of Epigallocatechin(()-cis-2-(3,4,5-Trihydroxyphenyl)-3,4-dihydro-1(2H)-benzopyran-3,5,7-triol, ()-cis-3,3,4,5,5,7-Hexahydroxyflavane)2

Protection Process

After the cyclization, the resulting product is then protected to form (-)-Penta-O-benzylepigallocatechin. The reaction is as follows

Kindly note that thatthere was an omission in drawing the epigallectechn structure but it should be as corrected below. Thr mechansmn remin s explined above.

The figure above indicates step 3 and 4.

The steps involved in the formation of the new product include polymerization and then cyclization. The synthesis in this process involves simple compounds which are combined in a direct to form the complex compound in the final product. The benzene groups undergo polymerization to form a phenyl group. The final step in the diagram above involves the addition of the benzyls to increase the branching of the molecule to form the final compound. The product is then cyclized directly into the product in step 15 of the old synthesis proposal. The formation of the initial reactants is also simple since it only involves substitution and alkalization of the benzene structures; though, the process is omitted in this synthesis.

The benzyl groups can be synthesized using the Williamson Esther Synthesis process which involves the deprotonation. The subsequent reaction with the benzyl bromide delivers the protected alcohol. The deprotonation is done using sodium hydroxide which creates a conducive environment for the substitution process. The other alternative to using in the cleavage of the benzyl group is the use of the strong acid; though, the method is limited due to the acid-sensitivity to the substrates (Kabalka, & Yao, 2014).

The benzyl group is commonly used in the protection of the phenol groups in the stereo-selective synthesis of the organic chemical compounds. The displacement or substitution reaction leading to the formation of the final product is based on the orthogonal strategy for the stereo-selective synthesis (Teixeira, Mateus, & de Freitas, 2016). The older methodology involved a coupling of the O-benzyl protected phloroglucinol (7) and (E)-cinnamyl alcohol to form (E)-1, 3-diarylpropene. From the series of reactions, it is apparent that a long way is involved as compared to the newly proposed method. On the other hand, the processes involved at each reaction stage are similar consider that they are majorly substitution and deprotection reactions.

Further Discussion

The old synthesis involved a series of eight steps beginning with the O-benzyl protected phloroglucinol to the formation of the (-)-Penta-O-benzylepigallocatechin. The processes eventually lead to the formation of the EGCG compound; however, the target, in this case, is to produce the (-)-Penta-O-benzylepigallocatechin. Various approaches have been developed to produce organic compounds with a series of transformational processes being involved (Teixeira, Mateus, & de Freitas, 2016). The explanation of the specific reactions and chemical transformations taking place at each stage of the synthesis is described below:

The E-retro-2-methoxymethylchacone methyl ether is transformed through a series of reduction and dehydration of the alcohol leading to the development of the E-1, 3-diarylpropene. The subsequent reactions involve simultaneous cyclization of the diols in the earlier products through the use of concentrated hydrochloric acid in methanol. The process will yield 2, 3-trans-catechin derivatives. The various derivatives can be transformed to the desired analogs using the AD-mix-beta rather than the alpha. The process represents a simpler method of preparing the (-)-epigallocatechin-3-gallate and its enantiomers. The targeted chemical structure of the final compound in the synthesis process is (-)-Penta-O-benzylepigallocatechin.

The earlier synthesis proposal involved a series of steps and compounds that were transformed to form the final target product which is (-)-Penta-O-benzylepigallocatechin. The new proposal is simpler compared to the older one because of the fewer number of steps involved in the synthesis process. The coupling of the O-benzyl protected phloroglucinol and the E-cinnamyl alcohol were used to synthesize the E-1, 3-diarylpropene which then undergoes dihydroxylation. On the other hand, the new method uses the E-rector-2-methoxymetylchacone methyl ether as the original derivative which is then transformed through a series of events to produce the final desired product (Teixeira, Mateus, & de Freitas, 2016).

Conclusion

The proposal, in this case, involves the production of (-)-Penta-O-benzylepigallocatechin using the 2,3-cis-flavan-3-ol methyl ether acetate which is transformed to (E)-retro-2-methoxymethylchacone methyl ether as the derivative. The process involves a series of deprotection and a substitution reaction which eventually leads to the production of the desired product. The process involved in the synthesis of the (-)-Penta-O-benzylepigallocatechin, in this case, is simpler compared to the old synthesis model which involved more steps compared to this. The process in terms of the number of steps involved and the reactions involved in the synthesis process can be used to determine the differences in terms of simplicity in the two methods. On the other hand, the comparison is based on the immediate derivatives used in the synthesis of the organic compounds; though, the real processes involved in the synthesis from the scratch may be complex in either case. The ease of the substitution of the hydroxyl group with the benzyl group in the preparation of (-)-Penta-O-benzylepigallocatechin is easier as compared to the processes involved in the old version.

References

Teixeira, N., Mateus, N., & de Freitas, V. (2016). Experimental data for the synthesis of a new dimeric prodelphinidin gallate. Data in brief, 8, 631-636.

Kabalka, G. W., & Yao, M. L. (2014). Direct propargylic substitution of hydroxyl group in propargylic alcohols. Current Organic Synthesis, 11(5), 28-32.