Structure and synthesis of flavan oligomers from Cassia petersiana

English: The bark of Cassia petersiana afforded the known flavan-f-ols (+)-catechin, (-)- epicatechin, (+)-gallocatechin and epigallocatechin which co-occured with three new procassinidins namely cassiaflavan-( 4ß-8)-gallocatechin, cassiaflavan-( 4a.-8)- epigallocatechin and cassiaflavan-( 4a-8)-epigallocatechin as well as four novel probutinidins 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 chromatography procedures were used to isolate and purify the compounds mentioned. Structure elucidation was done using high resolution IH NMR spectroscopy which included NOE and COSY experiments in conjunction with FAB-MS. Due to a high degree of rotational isomerism structural assignments of the following compounds cassiaflavan-( 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 obtained by tedious NMR experiments and CD data hence recourse to synthesis was absolutely essential to confirm the proposed structures beyond any doubt. Biomimetic synthesis of the procassinidin dimers via reduction of the racerrue flavanones, (±)-4',7-di-O-methyIcassinidin to the diastereomeric flavan-e-ols and condensation with 3',4',S',S,7-penta-O-methylepigallocatechin usmg titanium tetrachloride 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 by condensation with 3',4',5,7 -tetra-O-methylepicatechin and 3',4',5',5,7 -penta-Omethylepigallocatechin using titanium tetrachloride as Lewis acid was also employed to confirm the proposed structures. The biornimetic synthesis of probutinidins yielded two other novel compounds which were not obtained from natural source viz. butiniflavan-(4a.~8)-epigallocatechin and ent- butiniflavan-( 413~8)-epigalloca techin. The isolation and identification of the three new procassinidins and four probutinidins from Cassia petersiana represent the first report of dimeric compounds in this rare class of the proanthocyanidins. This study also represents the first report of the synthesis of the new dimers butiniflavan-( 4a.~8)-epicatechin, butiniflavan-( 413~8)-epicatechin, butiniflavan- (413~8)-epigallocatechin and ent-butiniflavan-( 413~8)-epicatechin as well as butiniflavan-( 4a.~8)-epigallocatechin and ent-butiniflavan-( 413~8)-epigallocatechin.