REDUCTIVE CLEAVAGE WITH METAL IN LIQUID AMMONIA: I. THE SELECTIVE CLEAVAGE OF THE BENZYLTHIO C—S BOND VERSUS THE BENZYLIDENEDIOXY C—O BOND IN METHYL S-BENZYL-4,6-O-BENZYLIDENE-2-THIO-α-D-ALTROPYRANOSIDES; THE INFLUENCE OF THE COSOLVENT 1,2-DIMETHOXYETHANE ON THE COURSE OF THE REACTION

The improved solubility of methyl S-benzyl-4,6-O-benzylidene-2-thio-α-D-altropyranoside (1) in liquid ammonia diluted with 1,2-dimethoxyethane has permitted the selective cleavage by metallic sodium or lithium of the C—S bond to give methyl 4,6-O-benzylidene-2-thio-α-D-altropyranoside in 70–75% yield. On the other hand, the slight solubility of I in liquid ammonia alone results only in the completely hydrogenolyzed material, methyl 2-thio-α-D-altropyranoside, along with unchanged I.Generally, in liquid ammonia alone, reductive cleavage is rapid (15–20 min) and the benzylidene and benzyl groups are converted largely into toluene accompanied by a small amount of bibenzyl. In liquid ammonia – 1,2-dimethoxyethane mixtures the reaction is much slower (≥ 1.5 h); under these conditions the benzylidene and benzyl groups are converted to a larger extent into bibenzyl, the rest becoming toluene.The two strong infrared absorption bands (in Nujol) in the region of 766 to 778 cm−1 and 706 to 718 cm−1 have been assigned to the phenyl moiety of the benzylidene group, and the one strong band in the region of 702 cm−1 to the phenyl moiety of the S-benzyl group.