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Oligomeric flavanoids. Part 3. Structure and synthesis of phlobatannins related to (–)-fisetinidol-(4α,6)- and (4α,8)-(+)-catechin profisetinidins

1988; Issue: 12 Linguagem: Inglês

10.1039/p19880003323

2050-8255

Jan P. Steynberg, Johann F. W. Burger, Desmond A. Young, E. Vincent Brandt, Jacobus A. Steenkamp, Daneel Ferreira,

Bioactive natural compounds

Several members of the novel class of natural 'phlobaphene' condensed tannins, representing the products of c-ring isomerization of 2,3-trans-3,4-trans-(–)-fisetinidol units present in (4,6)- and (4,8)-biflavanoid profisetinidins, have been characterized by 1H n.m.r. n.o.e. difference spectroscopy. These include the functionalized 8,9-trans-9,1 0-cis-tetrahydropyrano[2,3-h]chromenes (9) and (12), and the [2,3-f]- and [2,3-g] regioisomers (14) and (19). Since the (4α,8)-biflavanoid (1) is subject to extensive base-catalysed rearrangement and epimerization, the protected 4-O-methyl ethers (E-ring)(2) and (6) were utilized to confirm the proposed structures of phlobatannins by stereospecific C-ring isomerization of (2) and (6) under basic conditions.