Selective esterification, substitutions (SN1), and transformations of flavan-3,3′,4,4′,7-pentaols
The 3- and 4-monoacetates of (+)-3′,4′,7-trimethoxy-2,3-trans-flavan-3,4-trans-diol have been prepared for the first time. These compounds show different mass spectral fragmentation patterns.
Substitutions by primary alcohols result in mixtures (1 : 2) of 2,3-trans-3,4-trans- and 2,3-trans-3,4-cis-4-alkyl ethers irrespective of whether the trimethyl ethers of the 3,4-trans- or the 3,4-cis-flavan-3,4-diols are used as reactants. The 3-acetates of 2,3-trans-3,4-cis-4-alkyl ethers have an unusual twist-boat conformation.
Treatment of (+)-flavan-3,3′,4,4′,7-pentaol with mercaptoacetic acid results in similar substitution restricted to the 4-position, without heterocyclic ring fission. When access of air is permitted, optically pure dihydroflavonol and flavanone analogues are formed in prominent side reactions. The mechanism of their formation is discussed.