Reversal of the facial diastereoselectivity in the Paternò-Büchi reaction of silyl enol ethers carrying a chiral substituent in α-position

Abstract

The facial diastereoselectivity in the Paternò-Büchi reaction of benzaldehyde and silyl enol ethers 1 which bear a chiral substituent R∗ in α-position has been studied. Since the regioselectivity and the simple diastereoselectivity of the photocycloaddition is very good only two diastereoisomers are obtained whose ratio reflects the facial diastereoselectivity. With the silyloxy substituted substrates 1a-1c the selectivity is low (d.r. = <6040). The benzyloxy substitued silyl enol ether 1d (R∗ = CHOBni-Pr), however, exhibits already a significant degree of diastereofacial selection and yields preferentially the oxetane 2d (d.r. = 6733). If the benzyloxy substituent in the substrate is replaced by a chlorine atom (R∗ = CHCli-Pr) the direction of the facial diastereoselecitivity is reverted. Thus, the reaction of silyl enol ether 1e with benzaldehyde gives preferentially oxetane 3e (d.r. = 8515).