The formation of branched-chain deoxypentofuranosides by ring contraction in the reductive desulfonyloxylation of hexopyranoside p-toluenesulfonates

Abstract

Several methyl 6-deoxy-, 6-deoxy-6-halo- and 6-O-p-tolylsulfonylhexopyranoside 2- and 4-p-toluenesulfonates reacted readily with lithium triethylborohydride in boiling tetrahydrofuran under reductive desulfonyloxylation at C-2 and C-4, respectively, and with reduction at C-6 where applicable. The desulfonyloxylations proceeded with ring contraction to furnish, in 40-60% isolated yields, new branched-chain pentofuranoside derivatives, namely, methyl 2,5-dideoxy-2-C-(hydroxymethyl)pentofuranosides having the .alpha.-L-xylo, .alpha.-D-xylo, and .alpha.-D-ribo configurations (from 2-tosylates) and methyl 3,5-dideoxy-3-C-(hydroxymethyl)pentofuranosides having the .alpha.-D-arabino and .alpha.-D-ribo configurations (from 4-tosylates). In some of the reactions, reductively desulfonyloxylated but unrearranged products were formed in addition. Methyl 4,6-O-benzylidene-2,3-di-O-p-tolylsulfonyl-.alpha.-D-galactopyranoside reacted by partial de-O-sulfonylation and partial desulfonyloxylation, with and without ring contraction, to give methyl 3,5-O-benzylidene-2-deoxy-2-C-(hydroxymethyl)-.alpha.-D-lyxo-pentofuranoside, methyl 4,6-O-benzylidene-3-deoxy-.alpha.-D-lyxo-hexopyranoside, and the parent 2,3-diol.