2D NMR structure elucidation of proanthocyanidins: The special case of the catechin‐(4α‐8)‐catechin‐(4α‐8)‐catechin trimer

A complete and unambiguous assignment of the 1 H and 13 C NMR spectra of peracetylated catechin‐(4α‐8)‐catechin‐(4α‐8)‐catechin procyandin trimer was accomplished by reverse two‐dimensional chemical shift correlation methods. An HMBC spectrum showed, for the first time, evidence of the sites of condensation between the interflavan units of a trimer. The 2,3‐ trans stereochemistry of the C‐ and F‐rings are easily deduced from measurement of 3 J coupling constants. The multiplicity analysis of the protons of the i ‐ring in the terminal unit does not lead to such an assignment. A ROESY experiment is needed to confirm the 2,3‐ trans relationship. Molecular mechanics calculations explain the unexpected coupling constants of these protons.