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-3-01s 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. * This also gave low yields of full C-3 O-methyl ethers [( 10, (13), (17), and (26)] as side-reactions. t Magnetic non-equivalence of H-3 and H-5 of the phloroglucinol D-rings of the 2,3-trans-3,4-trans-analogues (7) and ( 15) at ambient temperatures is due t o chirality at C-4 coupled with restricted rotation, as evidenced by their coalescence at higher temperatures.

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The generation of flavanyl-4-carbo-cations from flavan-3,4-diols and their condensation with nucleophilic flavan-3-01s to form [4,6]-and [4,8]-biflavanoids at ambient temperatures and under mildly acidic aqueous conditions apparently simulates the initial step in condensed tannin formation in a number of natural sources. The stereospecificity (or stereoselectivity) of the reaction is conditioned mainly by the 2,3-cis or 2,3-trans stereochemistry of the parent flavan-3,4-diol, but also by the nucleophilicity of the flavan-3-01, and its regiospecific (or regioselective) course by steric factors arising from variation in substitution of the receptive A-ring of the flavan-3-01.

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