Crystal and molecular structures of two unsaturated monosaccharides: methyl 6-(N-benzenesulfonylamino)-N,O-isopropylidene-2,3,6,8-tetradeoxy-α-D-erythrooct-2-enopyranos-4-uloside, and 1,1-O,O-ethylidene-3,4-dideoxy-D,L-hex-3-enoz-2-ulopyranos-5-ulose