3′,4′,7,8-tetrahydroxyflavonoids from the heartwood of Acacia nigrescens and their conversion products

Article abstract of DOI:10.1016/0031-9422(72)85034-9

Nigrescin, the first optically active 2-hydroxy-2-benzylcoumaranone, and a (+)-2,3-trans-flavan-3,4-cis-diol, represent new members of the natural 3′,4′,7,8-tetrahydroxyflavonoid group. These accompany chlacone, flavanone, flavonol and dihydroflavonol analogues, the flavan-3,4-diols (-)-melacacidin and (-)-isomelacacidin, and protocatechuic acid in the black heartwood of Acacia nigrescens. Polyflavonoid tannins are significantly absent. Synthetic nigrescin tetramethyl ether together with its flavonol analogue is obtained from (±)-3′,4′,7,8-tetra-O-methyldihydroflavonol by brief treatment with alkali. Reduction of the flavanone tetramethyl ether with LiAlH₄ affords new flavan-4α-ol and flavan-4β-ol analogues. Treatment of 2R : 3R : 4R-flavan-3,3′,4,4′,7,8-hexaol [(-)-melacacidin : 2,3-cis-3,4-cis, or its methyl ether with thioglycollic acid yields, after methylation, the anticipated methyl (-)-(2,3-cis-3,4-trans-3-hydroxy-3′,4′,7,8-tetramethoxyflavan-4-ylthio) acetate, together with optically pure 2S-flavanone and 2R: 3R-dihydroflavanol (2,3-trans) analogues for the first time. The same reaction applied to (±)-2,3,-trans-[4-²H]-3′,4′,7,8-tetramethoxyflavan-3,4-trans-diol does not give the flavanone analogue. The relative chemical shifts of 8-methoxy proton resonances of flavan-3,4-diol, flavanone, coumaranone and flavone analogues obtained by progressive addition of C₆D₆ to CDCl₃ solutions are of diagnostic value in polyflavanoid chemistry.