FACE SELECTIVITY OF THE NITRILE OXIDE CYCLO-ADDITION TO UNSATURATED SUGARS

Nitrile oxides cycloadd to unsaturated sugars 1a-h, to give the anti adduct with 73.5-96.8% π-facial stereoselectivity. The highest values of face selectivity were observed with dipolarophiles bearing an ether group in the homoallylic position. This finding was rationalized by considering the influence of the substituent lone pairs on the relative energies of the possible transition states. The minimum-energy conformations of the dipolarophiles and the relative energies of model transition states were evaluated by MM2 calculations. The structure of the major products, assigned by X-ray analysis, features a transition state that confirms the model proposed by Houk.