A substrate controlled, very highly diastereoselective Morita-Baylis-Hillman reaction: a remote activation of the diastereofacial selectivity in the synthesis of C-3-branched deoxysugars

The Morita-Baylis-Hillman (MBH) reaction of p-nitrobenzaldehyde with C (6) acyl protected enuloside 1 in the presence of TiCl₄/TBAI yielded highly diastereoenriched C-3-branched deoxysugar derivative or MBH adduct 1′a in high yield, while reactions of unprotected enuloside 2a and C (6) alkyl protected enulosides 2d-e with p-nitrobenzaldehyde under the same conditions afforded the adducts 2′a and 2′d-e, respectively, in low yield with moderate selectivity. Several representative aromatic and aliphatic aldehydes were selected to undergo MBH reaction with 1 to give their respective adducts in very good yield with a very high diastereoselectivity. A plausible mechanism based on the assumption of a Zimmerman-Traxler-type transition state was proposed to explain the excellent selectivity observed with adducts derived from 1. The synthetic application of these adducts were shown by their stereoselective reduction to corresponding threo isomers in very good yield.