The photochemistry of ketones derived from carbohydrates. Part I. Type I cleavage of a pyranosid-4-ulose

Abstract

U.v. irradiation of methyl 6-deoxy-2,3-O-isopropylidene-α-L-lyxo-hexopyranosid-4-ulose (1) in various solvents has been found to give carbon monoxide and the furanosides (2) and (3). The ring contraction occurs stereoselectively, affording the 5-deoxyriboside (3) as the major product. The predominance of a decarbonylation reaction in solution has been ascribed to the ease of formation of a biradical (7) which arises from the stabilising influence of the oxygen atoms.

In t-butyl alcohol, diastereoisomeric esters are also formed, to a small extent; their structures [(4) and (4′)] suggest that the initial α cleavage occurs preferentially between C-3 and C-4 rather than between C-4 and C-5.