Improved Synthesis of Methyl 4,6-O-Benzylidene-3-cyano-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside and Methyl 4,6-O-Benzylidene-2,3-dideoxy-3-C-formyl-α-D-erythro-hex-2-enopyranoside