Acid-catalyzed rearrangements of flavan-4-phloroglucinol derivatives to novel 6-hydroxyphenyl-6a,11b-dihydro-6H-[1]benzofuro[2,3-c] chromenes and hydroxyphenyl-3,2′-spirobi[dihydro[1]benzofurans]

Abstract

Acetic acid-catalyzed cleavage of proanthocyanidins in the presence of phloroglucinol gives a series of 2R procyanidin- and prodelphinidin-phloroglucinol adducts together with a novel 2S all-cis derivative implicating cleavage of the pyran ring and subsequent inversion of stereochemistry at C-2_C. These flavan-4-phloroglucinol adducts also suffer dehydration to products with novel fused dihydrobenzopyran–dihydrobenzofuran rings. In addition, cleavage of the flavan C-10_A–C-4_C bond followed by dehydration results in unique 3,2′-spirobi[dihydro[1]benzofurans].