Stereospecific entry to [4.5]spiroketal glycosides using alkylidenecarbene C-H insertion

A novel method for the stereospecific preparation of [4.5]spiroketal glycosides utilizing the 1,5 C-H bond insertion of alkylidenecarbenes is described. Treatment of 2-oxopropyl beta-pyranosides A with lithium (trimethylsilyl)diazomethane in THF at -78 degreesC afforded 1,6-dioxaspiro[4,5]decenes B in good yield. Submission of the corresponding a-glycosides C to the same reagent gave the isomeric insertion products D in moderate to high yield.