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