NEW STEREOSPECIFIC REARRANGEMENTS OF PYRANOSIDES

Methyl beta-D-arabinopyranoside (1), phenyl 1-thio-beta-D-galactopyran oside (3), methyl alpha-L-fucopyranoside (7), methyl beta-1-fucopyrano side (9), 1,6-anhydro-beta-D-galactopyranoside (D-galactosan; 11), and 1,6-anhydro-beta-D-mannopyranoside (D-mannosan; 14) were stereospecif ically converted in moderate up to good yields into methyl -(2,2,2-tri chloroethylidene)-beta-D-lyxopyranoside (2), phenyl -trichloroethylide ne)-1-thio-beta-D-gulopyranoside (4) / phenyl -trichloroethylidene)-1- thio-beta-D-gulopyranoside (5), methyl richloroethylidene)-6-deoxy-alp ha-L-gulopyranoside (8), methyl -cyclohexylcarbamoyl-6-deoxy-beta-L-gu lopyranoside (10), -O-(2,2,2-trichloroethylide)-beta-D-gulopyranoside (12), and (2,2,2-trichloroehtylidene)-beta-D-altropyranoside (15), res pectively, using a nonclassic pathway of chloral acetalisation with di cyclohexylcarbodiimide (DCC) as coagent. In the case of 1, 3, and 9, c hloral acetalisatiens yielded diastereomeric mixtures, e.g., the aceta ls 2, 4, 5, and 10 consist of endo-H/exo-H dioxolane type acetals with preference of the endo-H form. In contrast to this, the compounds 7, 11, and 14 gave exclusively the endo-H diastereomers 8, 12, and 15. Ad ditionally, the structure of the anhydro compound 15 was confirmed by intramolecular glycosylation of the (2,2,2-trichloroethylidene)-alpha- D-altropyranosyl fluoride (17). Finally, the 6-O-formyl-beta-D-gulopyr anoside 4 was alternatively deformylated by methanol/triethylamine giv ing 5 and methanol/sodium methoxide yielding phenyl -trichloroethylide ne)-1-thio-beta-D-gulopyranoside (6). The carbamoyl protecting group o f 12 was cleaved by refluxing with methanolic sodium methoxide solutio n giving -(2,2,2-trichloroethylidene)-beta-D-gulopyranoside (13).