The photochemistry of ketones derived from carbohydrates. Part III. Type II reactions of some pyranosid-2- and -3-ulose derivatives

Abstract

Solutions of α- and β-anomers of methyl 3,4-O-isopropylidene-L-erythro-pentopyranosid-2-ulose [(2) and (4)] undergo type II elimination upon photolysis, giving 1,5-anhydro-3,4-O-isopropylidene-L-erythro-pent-2-ulose (3) and formaldehyde. The α-isopropyl analogue (1) behaves similarly, affording compound (3) and acetone.

The type II reaction is shown to be strongly dependent upon the stereochemistry of the ring substituent which contains the hydrogen atom γ to the keto-group by the photolytic behaviour of the C-2 epimeric pair of methyl 4,6-O-benzylidene-2-O-methyl-α-D-hexopyranosid-3-uloses [(8) and (13)]. The arabino-isomer (8) eliminates the axial methoxy-group to give the 3-ulose derivatives (11) and (1 2) and formaldehyde, whereas the ribo-isomer (13) gives a 2,3-fused oxetanol derivative (15). Reductive cleavage of the oxetanol (15) with lithium aluminium hydride afforded the branched chain sugar derivative methyl 4,6-O-benzylidene-3-C-methyl-α-D-allopyranoside (21).