Stereoselective synthesis of 2-O-MEM-2,3-unsaturated-β-O-glycosides and elaboration to useful synthetic tools

R = Me, Et, i-Pr, t-Bu, 6-(1,2:3,4-di-O-isopropylidene-α-d-galactopyranosyl). Glycosides 20-24β are transformed into corresponding 3-deoxy-glycosides, 3-deoxy-β-hexopyranosid-2-uloses, and 3,4-dideoxy-β-hex-3-enopyranosid-2-uloses.