SILICA GEL-CATALYZED ADDITION OF METHYL NITROACETATE TO 1,2-3,4-DI-O-ISOPROPYLIDENE-ALPHA-D-GALACTO-HEXODIALDO-1,5-PYRANOSE AND 2,3-O-ISOPROPYLIDENE-D-GLYCERALDEHYDE - CRYSTAL-STRUCTURE OF METHYL 7-ACETAMIDO-7-DEOXY-1,2-3,4-DI-O-ISOPROPYLIDENE-L-THREO-ALPHA-D-GALACTO-OCTOPYRANURONATE

The addition of methyl nitroacetate to 1,2: 3,4-di-O-isopropylidene-alpha-D-galacto-hexodialdo-1,5-pyranose (5) and to 2,3-O-isopropylidene-D-glyceraldehyde (6) in the presence of silica gel led stereoselectively to the formation of only (from 5) or mainly (from 6) two of the four possible nitroaldols in high yield. The catalytic action of silica gel was required in the reaction of 6, but not in that of 5. Isolation of a crystalline, reduced and N-acetylated derivative from the mixture of nitroaldols (7 + 8) obtained from 5 allowed for the determination of its crystal structure, showing that it has the 6R, 7R absolute configuration (one of the two possible 6,7-threo configurations) by X-ray diffraction methods. Therefore, the same 6R configuration was also assigned to the nitroaldols 7 and 8, which on the basis of the easy alpha-epimerisation of the alpha-nitro esters should differ only in the configuration at C-7. A pair of amino alcohols obtained from the same mixture of nitroaldols were separately transformed into a 4,5-cis- and a 4,5-trans-oxazoline, the configurations of which were correlated with the C-6,7 relative configuration of the respective amino alcohols and also the respective nitroaldol. The Felkin model explained the high stereoselectivity observed. The configurations of the two predominant nitroaldols obtained from 6 were tentatively assigned by applying the Felkin model.