Sugar Enolones, VII. Synthesis and γ‐Pyrone Formation of α,β‐Unsaturated Hexopyranosid‐4‐uloses

Dimethyl sulfoxide/acetic anhydride oxidation of methyl 2,3,6‐tri‐ O ‐benzoyl‐α‐ D ‐galactopyranoside ( 1 ) is accompanied by β‐elimination of benzoic acid to yield the 3,4‐enolone 4 . The intermediate 4‐uloside 2 , comprising about 65% of an 8 h oxidation mixture, could therefrom not be isolated in pure form because of its propensity towards enolone formation. In basic and acidic medium, the 3,4‐enolone structure in 4 is readily converted into the γ‐pyrone system by loss of the anomeric substituent to yield hydroxymaltol derivatives ( 13, 14 ). Some intermediates in this reaction, e. g. dihydropyranone 10 , can be isolated under mild conditions. The mechanistic implications of the 3,4‐enolone → γ‐pyrone conversion are discussed in relation to the formation of a pyrylium salt from the 3,4‐enaminone analogue of 4 (6 → 12 ) and to the γ‐pyrone formation from hexose‐3,2‐enolones.