Synthesis of Enantiopure Carbohydrate Mimetics by Lewis Acid Catalyzed Rearrangement of 1,3-Dioxolanyl-Substituted l,2-Oxazines

摘要

Enantiomerically pure 3,6-dihydro-2H-1,2-oxazines,which are easily available by [3+3] cyclization of lithiated alkoxyallenes with aldonitrones,'1' are versatile intermediates for the stereoselective synthesis of a range of highly function-alized compounds.All these products,including polyhydrox-ylated pyrrolidines,stereodefined amino polyols,and substituted tetrahydrofuran derivatives,are interesting because of their potential biological activities,for example,as glycosi-dase inhibitors.While trying to deprotect 1,2-oxazine syn-3 with Lewis acids we found that a rearrangement to tetrahy-dropyran-bridged bicyclic 1,2-oxazine 4 occurred in moderate yield (Scheme l).