Thr reactions of 3-O-benzyl-6-deoxy-1, 2-O-isopropylidene-5-O-methanesulfonyl-α-D-gluco- and β-L-idofurnoses (1 and 2) with tetrabutylammonium fluoride in N, N-dimethylformamide gave (E)- and (Z)-3-O-benzyl-5, 6-dideoxy-1, 2-O-isopropylidene-β-L-threo-hex-4-enofuranoses (3 and 4), respectively, as the major products. The structure of the (E)-isomer 3 was determined by single-crystal X-ray analysis. 3-O-Benzyl-5, 6-dideoxy-5-fluoro-1, 2-O-isopropylidene-β-L-ido- and α-D-glucofuranoses (6 and 7) were formed as minor products in the respective reactions.