Condensed tannins. Circular dichroism method of assessing the absolute configuration at C-4 of 4-arylflavan-3-ols, and stereochemistry of their formation from flavan-3,4-diols

Abstract

Stereoselective condensation at C-4 of flavan-3,4-diols of known absolute configuration with phloroglucinol and resorcinol proceeds at ambient temperatures with either partial or complete retention of configuration for 2,3-trans-isomers and with inversion for 2,3-cis-isomers, phenomena that are considered to be of prime significance in condensed tannin formation; multiple Cotton effects contributed by aryl chromophores at C-4 dominate the c.d. spectra of the resultant 4-aryl-2,3-trans- and 2,3-cis-flavan-3-ols, enabling unambiguous determination of their absolute configurations at this chiral centre.