Our new method for the generation of alkylidenecarbenes, based on the reaction of trimethylsilylazide/Bu(2)SnO with alpha-cyanomesylates, has been applied to the synthesis of enantiomerically pure polyhydroxylated cyclopentane derivatives from conveniently functionalized sugar intermediates prepared from D-mannose. The stereoselectivity of the 1,5 C-H insertion reaction leading to the major trans-isomers (8a,b) has been assigned by (1)H RMN spectroscopic data, and correctly rationalized by a computational analysis at DFT level. Compounds 8a and 8b have been designed as suitable intermediates for the synthesis of neplanocin A enantiomer.