Structure and synthesis of flavan oligomersfrom Cassia petersiana

[摘要] English: The bark of Cassia petersiana afforded the known flavan-f-ols (+)-catechin, (-)-epicatechin, (+)-gallocatechin and epigallocatechin which co-occured with three newprocassinidins namely cassiaflavan-( 4ß-8)-gallocatechin, cassiaflavan-( 4a.-8)-epigallocatechin and cassiaflavan-( 4a-8)-epigallocatechin as well as four novelprobutinidins namely butiniflavan-( 4a.-8)-epicatechin, butiniflavan-( 4ß~8)-epicatechin, butiniflavan-( 4~8)-epigallocatechin and ent-butiniflavan-( 4J3~8)-epicatechin.A combination of solvent extraction, column (LH20) and thin layer chromatographyprocedures were used to isolate and purify the compounds mentioned.Structure elucidation was done using high resolution IH NMR spectroscopy whichincluded NOE and COSY experiments in conjunction with FAB-MS. Due to a highdegree of rotational isomerism structural assignments of the following compoundscassiaflavan-( 4J3~8)-epigallocatechin, cassiaflavan-( 4a.~8)-epigallocatechin,butiniflavan-( 4a.~8)-epicatechin, butiniflavan-( 4J3~8)-epicatechin, butiniflavan-(4J3~8)-epigallocatechin and ent-butiniflavan-( 4J3~8)-epicatechin were obtainedby tedious NMR experiments and CD data hence recourse to synthesis was absolutelyessential to confirm the proposed structures beyond any doubt.Biomimetic synthesis of the procassinidin dimers via reduction of the racerrueflavanones, (±)-4',7-di-O-methyIcassinidin to the diastereomeric flavan-e-ols andcondensation with 3',4',S',S,7-penta-O-methylepigallocatechin usmg titaniumtetrachloride as Lewis acid was used to confirm the structures.A similar biomimetic synthesis of probutinidins via reduction of the racemic flavanones,(±)-3',4',7-tri-O-methylbutin to the diastereomeric flavan-t-ols followed bycondensation with 3',4',5,7 -tetra-O-methylepicatechin and 3',4',5',5,7 -penta-Omethylepigallocatechinusing titanium tetrachloride as Lewis acid was also employed toconfirm the proposed structures.The biornimetic synthesis of probutinidins yielded two other novel compounds whichwere not obtained from natural source viz. butiniflavan-(4a.~8)-epigallocatechin andent- butiniflavan-( 413~8)-epigalloca techin.The isolation and identification of the three new procassinidins and four probutinidinsfrom Cassia petersiana represent the first report of dimeric compounds in this rare classof the proanthocyanidins.This study also represents the first report of the synthesis of the new dimersbutiniflavan-( 4a.~8)-epicatechin, butiniflavan-( 413~8)-epicatechin, butiniflavan-(413~8)-epigallocatechin and ent-butiniflavan-( 413~8)-epicatechin as well asbutiniflavan-( 4a.~8)-epigallocatechin and ent-butiniflavan-( 413~8)-epigallocatechin.

[发布机构] University of the Free State