Enantiospecific synthesis of a glycoside of -epi-purpurosamine

2-Propyl -epi-purpurosaminide dihydrochloride 14 and its di-N-acetylated derivative 15 were synthesized by an enantiospecific sequence which involves the 2-propyl 6-O-acetyl-3,4-dideoxy-α- -erythro-hex-3-enopyranosid-2-ulose 2 as the key precursor. The first approach through the saturated diol 4, prepared by reduction of the enone system of 2, was unsuccessful as the C-2 position of 2,6-di-O-sulfonyl derivatives 5 and 6 resisted substitution by azide. Therefore, an alternative sequence starting from the allylic alcohol 3, also derived from 2, was developed. In this case, the 2,6-di-O-tosyl derivative 9 gave the expected 2,6-diazide 10 with additional unwanted rearrangement of the double bond to the 2-propyl 4,6-diazido-2,3,4,6-tetradeoxy-α- -threo-hex-2-enopyranoside 11 isomer. However, the ditriflate derivative 13, analogous to 9, underwent substitution to afford the diazide 10 in good yield. Upon reduction of the azide functions and saturation of the double bond of 10 by catalytic hydrogenation under acidic conditions, the dihydrochloride salt 14 was obtained as a crystalline product (43% overall yield from 3).