Photochemical access to alkyl 3-deoxyglycopyranosid-4-uloses

Abstract

3-Deoxyaldos-4-uloses are suitable synthons in the synthesis of various natural products and analogues containing chiral diol subunits [l]. Moreover, 3,6-dideoxyhexoses are found in many biologically important compounds, for instance in lipopolysaccharides, conferring on them their serological specificity [2]. A general synthetic approach to these sugars involves the stereoselective reduction of aldose derived epoxides [3]. Alternatively they have been prepared from furfuryl aIcohols [4] by deamination of methyl 3-amino-3-deoxy-P-o-allopyranoside [5] or from methyl 4,6-0-benzylidene-3-deoxy-~-o-ti~o-hexopyranoside [6] in modest yields. In connection with our studies [7] on the chemistry of radical-anions produced photochemically by electron transfer from triethylamine onto carbonyl compounds, we have found that the photoreduction of @-epoxyketones, derived from carbohydrates, results in selective Ca-0 bond cleavage. This allows us to report here an efficient synthesis of alkyl 3-deoxyglycopyranosid-4-ulosides (Scheme 1). Furthermore, by using this methodology, a short synthesis of the methyl glycoside of cinerulose B, a rare sugar [8] present in the antibiotic Cinerubine B, has been achieved (Scheme 2). The epoxyketones ( >-2 and ( + j-2 were obtained by Swern oxidation of methyl 2,3-anhydro-p-L-ribopyranoside 191 ( j-1 and of methyl 2,3-anhydro-P-oribopyranoside [9] ( + >-1, respectively. Epoxyketone ( + )-5 was obtained by alkaline hydrogen peroxide oxidation of enone [lo] ( + )-4. The regiospecific ring opening of