Synthesis of condensed tannis. Part 8. The first ‘Branched’[4,6 : 4,8 : 4,6]-tetraflavanoid. Coupling sequence and absolute configuration

Abstract

Condensation of (+)-(2R)-2,3-trans-3,4-trans-flavan-3,3′,4,4′,7-pentaol [(+)-mollisacacidin] with excess of (–)-(2R)-2,3-trans-flavan-3,3′,4′,7-tetraol [(–)-fisetinidol] proceeds beyond the expected biflavanoid range to generate significant yields of both the ‘linear’[4,6 : 4,6]-2,3-trans-3,4-cis : 2′,3′-trans-3′,4′-trans : 2″,3″-trans-trifisetinidol and the first ‘branched’[4,6 : 4,8 : 4,6]-2,3-trans-3,4-cis : 2′,3′-trans-3′,4′-trans : 2″,3″-trans-3″,4″-trans : 2‴,3‴-trans-tetrafisetinidol.

The products indicate a selective condensation sequence due to differing steric constraints, operative at competing nucleophilic centres in each intermediate substrate, assisted by hyperconjugative effects.