The photochemistry of ketones derived from carbohydrates. Part II. Evidence for a biradical mechanism in type i ring contraction of pyranosid-4-uloses to furanosides

Abstract

Photolysis of methyl 6-deoxy-2,3-O-isopropylidene-β-D-ribo-hexopyranosid-4-ulose (2) in pentane causes decarbonylation and gives the furanosides (7) and (8). These products are formed in the same ratio as that obtained from the photolysis of a solution of methyl 6-deoxy-2,3-O-isopropylidene-α-L-lyxo-hexopyranosid-4-ulose (1) in pentane. This result is consistent with the formation of a biradical intermediate in the photolysis. Pyranosiduloses (1) and (2) were not interconverted photochemically, which supports the earlier proposal that initial α cleavage occurs between C-3 and C-4. Methyl 2,3-O-isopropylidene-β-L-erythro-pentopyranosid-4-ulose (11) is also decarbonylated upon photolysis in pentane solution to afford methyl 2,3-O-isopropylidene-β-L-erythorfuranoside (12).