Abstract
A stereospecific synthesis of substituted derivatives of 2,5-anhydro- d-allose via conformational control and diazotization is described. Diazotization of the conformationally mobile methyl 2-amino-2-deoxy-α- d-altropyranoside ( 7) and its 3,4,6-tri- O-benzoyl derivative ( 8) led to a selectively disfavored, ring-contraction reaction, but, for the conformationally rigid 2-amino-1,6-anhydro-2-deoxy-3- O-p-tolylsulfonyl-β- d-altropyranose ( 19), the rearrangement was unequivocally chanenlled stereospecifically, and almost quantitatively, to a 2,5-anhydro- d-alloseptanose derivative ( 21). Conformational assignments and rearrangement mechanisms are discussed. O-Detosylation was achieved by photolysis. The 1,3-dioxolane derivative used for the protection of the anomeric center was found very resistant toward acid hydrolysis, and disadvantageous in the regeneration of the free aldehyde group.
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