Diastereomerically Pure 3‐(Silyloxy)oxetanes by a Selective Paternò‐Büchi Reaction

Diastereomerically pure 3‐[(trimethylsilyl)oxy]oxetanes 3 were prepared in moderate to good yields (45–72%) by the Paternò‐Büchi reaction of silyl enol ethers 2 with benzaldehyde. The photocycloaddition exhibits a high degree of regio‐ and diastereoselectivity. The substituents R in the silyl enol ether have been varied [R = Me, Et, iPr, tBu, Ph, CH(OMe)₂, CH(OCH₂)₂, C(OCH₂)₂Me], and it was found that steric bulk is mainly responsible for enhanced selectivity (diastereoselectivity from 70:30 up to 95:5). The regiochemical control is perfect (regioselectivity >95:5) except for silyl enol ether 2a (R = Me) in the case of which a 90:10 ratio of regioisomers was observed. Irradiation of the reaction mixture at lower temperature (–25°C) led to a further improvement of diastereoselectivity. The relative configuration of the products obtained was elucidated both by ¹H‐NMR spectroscopy and by chemical degradation. As a mechanistic hypothesis to explain the high observed diastereoselectivity we propose that the steric environment in the intermediate diradical 11 determines the selectivity according to two possible reaction pathways, i.e. bond formation and retrocleavage.