Synthetic challenge to ubiquitous natural products from plant origin: flavan-derived polyphenols.

Flavan-derived polyphenols (catechin derivatives), widely destributed in the plant kingdom, have been given much considerable attention owing to their significant bioactivitites coupled with their extreme structural diversity. However, biochemical functions of this class of molecules are still not well understood because of the limited availavility of natural samples in sufficient quantity and quality. Here we report our synthetic challenges toward flavan-derived polyphenols, based on the flavonoid-sugar analogy. The key for success was the C(4)-elaboration of the flavan skeleton, which posed an important relevance to their structure diversification both in terms of the biogenetic origin of flavan derivatives as well as chemical synthesis of this class of compounds. Various nucleophilic units could be introduced to the C(4) position via the S(N) 1-type substitution, and orthogonal activation of two distinct flavan units enabled block assembly of linear catechin oligomers. These methodologies would serve as a reliable way to supply valuable, homogeneuous samples for biological testing.