Synthesis of CH2-linked alpha(2,3)sialylgalactose analogue: on the stereoselectivity of the key Ireland-Claisen rearrangement.

A CH 2 -linked alpha(2,3)sialylgalactose analogue was efficiently synthesized using an Ireland-Claisen rearrangement, which was developed recently by our group for constructing a CF 2 -sialoside. The reaction conditions of the rearrangement were optimized for alpha-stereoselective formation of the CH 2 -sialoside. On the basis of the observed temperature effects, the origin of the stereoselectivity of the Ireland-Claisen rearrangement is discussed. Moreover, reconstruction of the 2alpha-hydroxyl group on the galactose unit of the rearrangement product was achieved by means of stereoselective dihydroxylation and deoxygenation.