Base-Catalyzed Pyran Ring Rearrangement of Oligomeric Proanthocyanidins

Base-Catalyzed Pyran Ring Rearrangement of Oligomeric Proanthocyanidins 

The natural occurrence and synthesis of a novel class of C-ring isomerized oligomeric flavanoids, termed phlobatannins, was demonstrated some twelve year ago (111, 112). These 3,4,9,10-tetrahydro-2if,8//-pyrano[2,3-/]chromenes, e.g. (104), are characterized by the liberated resorcinol moieties from the A/C-ring arrangement of the parent biflavanoid, e.g. (24), and by the conspicuous absence of the effects of dynamic rotational isomerism in the NMR spectra of their permethylaryl ether diacetates at ambient temperatures. 

3- trans-3,A-cis flavanyl constituent units which result in exceptionally complex reaction mixtures, the stepwise construction of the dipyrano-chromene framework via sequential interflavanyl bond formation and pyran ring rearrangement permitted concise synthetic access to phlobatannins at the trimeric level. 

The susceptibility of the constituent flavanyl units of proanthocya-nidins to intramolecular rearrangement via B-ring quinone methides 

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