Synthesis and reactions of 1,2-cyclopropanated sugars

Abstract

Diastereospecific cyclopropanation of glycals 1, 2, 3, 4, and 41 was carried out using either dihalocarbene addition or zinc-carbenoid addition to yield 1,2-cyclopropanated sugars. Dichloro- and dibromocarbenes added stereoselectively anti to the C₃ benzyloxy group, whereas (under Simmons−Smith conditions) the cyclopropanes were formed syn to the same substituent. The reactions of these 1,2-cyclopropanated sugars to provide either ring expanded glycosides or C₂-branched chain glycosides were explored. Solvolytic ring expansion of 1,2-dibromocyclopropanated sugars with K₂CO₃ in refluxing methanol yielded the corresponding chiral oxepins 20−22. Electrophilic ring openings of parent cyclopropanes (14 and 17 derived from glucal 1) were carried out with different electrophiles to give functionalized 2-deoxy-2-C-branched chain glycosides. The ring openings of 14 in different solvents resulted in both α- and β-anomers, whereas 17 gave exclusively the α-anomer.