Synthesis of condensed tannins. Part 1. Stereoselective and stereospecific syntheses of optically pure 4-arylflavan-3-ols, and assessment of their absolute stereochemistry at C-4 by means of circular dichroism

Stereoselective and also stereospecific condensation at C-4 of flavan-3,4-diols of known absolute configuration with phloroglucinol and resorcinol in acid medium proceeds at ambient temperatures with partial retention of configuration for 2,3-trans-isomers and with inversion for 2,3-cis-analogues. Circular dichroism spectra of the resultant 4-arylflavan-3-ols all exhibit multiple Cotton effects. The sign of high intensity Cotton effects to low wavelength, contributed by aryl chromophores at C-4, may almost invariably be correlated with the absolute configuration at this chiral centre of 2,3-trans-3,4-trans, 2,3-trans-3,4-cis-, and 2,3-cis-3,4-trans-isomers.