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DOI: 10.1055/s-0030-1264184
Proanthocyanidin/polyphenol research: trials and thrills
The 5-deoxyproanthocyanidin pools of plants are of exceptional complexity. This is mainly due to extensive variation in hydroxylation pattern as well as several regio-/stereo-chemical and conformational issues. Such structural complexities also complicate purification and structure elucidation especially via NMR techniques where 1H and 13C spin systems are often broadened and/or multiplied due to the restricted conformational mobility of the interflavanyl bond.
Our investigations to comprehend the intricate structural, configurational, and chemical features commenced in the late 1970's when we designed a synthesis protocol to unambiguously define the linkage mode and the absolute configuration of the constituent flavanyl moieties. We will discuss some key issues that emanated from these studies, e.g., (1) the principles that control the regio- and stereo-chemistry of the interflavanyl bond formation process, (2) the development of an electronic circular dichroism method to define the absolute configuration at C-4 of the chain extension unit and corroboration of the results via. theoretical calculation of ECD spectra, (3) the enantioselective total synthesis of potential monomeric proanthocyanidin precursors, (4) the chemical manipulation of some crucial bonds in the proanthocyanidin architecture, and (5) a unique fragmentation of the biaryl bond of Pomegranate ellagitannins.