Silica gel-catalysed addition of methyl nitroacetate to 1,2: 3,4-di-O-isopropylidene-α-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-α-d-galacto-octopyranuronate