Palladium Promoted Allylic Rearrangement Route to 6-Substitutbd 2-O-Acetyl-Hex-3-Enopyranosides

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Abstract
Readily available 6-substituted 3,4-di-O-acetyl-1,2-glycals have been converted to their 2-O-acetyl-3,4-dideoxy-hex-3-enopyranosides by a stereoselective alkoxypalladation followed by addition of sodium cyanoborohydride which effected a stereoselective and regioselective allylic rearrangement with chirality transfer. Using this reaction methyl 2,6-di-O-acetyl-3,4-dideoxy-α-D-erythro-hex-3-enopyranoside (13). ethyl 2,6-di-O-acetyl-3-4-dideoxy-α-D-erythro-hex-3-enopyranoside (15), β-(trimethylsilyl) ethyl 2,6-di-O-acetyl-3,4,-dideoxy-α-D-erythro-hex-3-enopyranoside (17), methyl 2-O-acetyl-3,4-dideoxy-α-D-erythro-hex-3-enopyranoside (19), methyl 2-O-acetyl-6-azido-3,4,6-trideoxy-α-D-erythro-hex-3-enopyranoside (21), methyl 2-O-acetyl-6-O-methyl-3,4,-dideoxy-α-D-erythro-hex-3-enopyranostde (20), methyl 2,6-di-O-acetyl-3,4,-dideoxy-α-D-threo-hex-3-enopyranoside (22), methyl 4-O-acetyl-6-cyano-2,3,6-trideoxy-α-D-erythro-hex-2-enopyranoside (24), methyl 4-O-acetyl-2,3-dideoxy-α-D-glycero-pent-2-enopyranoside (25), and methyl-4-O-acetyl-2,3,6-trideoxy-α-L-erythro-hex-2-enopyranostde (26) were synthesized in excellent yield from their corresponding 1,2-glycals. In this reaction it was found that both sodium cyanoborohydride and a coordinating group at C-6 were necessary to effect the regioselective allylic rearrangement. These rearrangements can be rationalized on the basis that following transalkoxypalladation the resulting 2,3-palladium π complex is reduced to a Pd(0) complex by sodium cyanoborohydride which then undergoes allylic rearrangement assisted by coordination of the group at C-6 to control the regioselectivity.