In our continuing search for alternatives to the 4,6-benzylidene protecting group, and for 2,3-protecting groups that will confer enhanced selectivity even in the 4,6-benzyl-idene series, we have encountered several systems, which, contrary to our initial expectations, were highly a-selective. Zongmin Dai, recalling the very early work of Perlin in which the 2,3-O-carbonate was found to be highly ^-selective when used in the mannosyl bromide/insoluble silver salt method [39], prepared the 2,3-O-car-bonate (77) of S-phenyl 4,6-benzylidene-a-d-mannothiopyranoside and investigated its reaction with 1,2;3,4-diacetone-d-galactose (78) by the benzenesulfenyl triflate method (Scheme 17). She found to her surprise that the only product isolated was the a-mannoside 79 [40]. Earlier Sanxing Sun had looked at the use of the 2,3;4,6-diacetonide 80 as glycosyl donor in conjunction with methyl 2,3,4-tri-O-acetyl-a-d-glucpyranoside as acceptor and had obtained a 1: 1 ratio of the a- and ^-mannosides [27]. Again, this result was surprising in view of the fact that Garegg had earlier successfully employed the 2,3;4,6-dicyclohexylidene protecting system in the synthesis of ^-mannosides by the mannosyl bromide/insoluble silver salt method [41].



Scheme 17

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