Some novel photochemical and related aryl couplings and migrations in flavonoid synthesis

Abstract

2′-Methoxymethoxy-4,4′,6′-trimethoxychalcone epoxide couples at the β-position with 3,5-dimethoxyphenol under photolytic conditions to form isomeric 1,3,3-triaryl-2-hydroxypropiophenones. These propiophenones are subject to photo-induced α-ketol rearrangements yielding isomeric 1-hydroxypropan-2-ones. Together these serve as useful synthetic intermediates for 4-arylflavan-3-ones and novel 2-hydroxy-2-arylbenzylbenzo[b]furan-3(2H)-ones and 4-aryl-3-hydroxy-3,4-cis-dihydrocoumarins. The same epoxide reacts ionically under ambient conditions with 2,4,6-trihydroxybenzoic acid to afford a 3-O-benzoylpropiophenone intermediate, which provides novel access to isoflavones in high overall yield. Analogous coupling of phloroglucinol to epoxycinnamates gives diastereoisomeric 3,3-diaryl-2-hydroxypropionates which serve as precursors for 3,4-trans- and 3,4-cis-4-aryl-3-hydroxydihydrocoumarins and thence for 3-aryl- and 3-hydroxycoumarins.