Synthesis of condensed tannins. Part 6. The sequence of units, coupling positions and absolute configuration of the first linear [4,6 : 4,6]-triflavanoid with terminal 3,4-diol function

Circular Dichroic Properties of Flavan-3,4-diols

“…Comparison of the CD data of epioritin-4α-ol ( 11 ) with those of its permethylaryl ether diacetate 12 , and of those of the fisetinidol-4-ols 3 and 4 with those of their corresponding derivatives, indicates that the sign and the wavelength of the CEs are not significantly changed by derivatization of the hydroxyl functionalities. It is important to point out that sample concentrations are vitally important for spectral reproducibility.…”

“…Except for the catechin-4β-ol 2 , the signs of these CEs seem to be opposite for the 2,3- trans -3,4- cis -flavan-3,4-diols, e.g., the ent -fisetinidol-4α-ol 6 and the ent -oritin derivative 16 , respectively. Screening of the signs of the 1 L a CEs in Table , however, clearly indicates that these CEs do not consistently obey the De Angelis−Wildman aromatic quadrant rule for correlating the 1 L a CE sign with the absolute C-4 configuration as was proposed occasionally for some flavan-3,4-diol derivatives. ,− In all these cases, the more reliable CEs of the long-wavelength 1 L b transition were completely ignored.…”

Synthesis of condensed tannins. Part 12. Direct access to [4,6]- and [4,8]-all-2,3-cis-procyanidin derivatives from (–)-epicatechin : assessment of bonding positions in oligomeric analogues from Crataegus oxyacantha L.

“…The suggested use of this shift effect, previously recorded by Hemingway and coworkers ' for procyanidin peracetates, in differentiating between [4,8] and [4,6] isomers is mainly restricted to the ' upper ' terminal units of 2,3-cis-3,4-truns configuration, since it is much less obvious for their 2,3-truns-3,4-trans analogues (cf. An associated phenomenon is the selective and pronounced broadening of 6-H (A-ring) doublets attributable to all [4,8]linked ' upper ' terminal units of the methyl ether acetates of the bi-(16), tri-(18) and tetra-flavanoid (20) analogues (cf. Figure 1).…”

“…Tetra-acetate (20).-Fractions 207-236 (260 mg) were methylated and the product purified by p.1.c. [benzeneacetone (7 : 3, v/v)], RF 0.40 (20 mg).…”