Samarium(II) Iodide-Mediated Reductive Annulations of Ketones Bearing a Distal Vinyl Epoxide Moiety

Samarium(II) iodide in the presence of hexamethylphosphoramide (HMPA) efficiently promotes the intramolecular coupling of ketones with distal epoxy olefins. The reaction appears to proceed by a mechanism wherein a ketyl couples with the unsaturated epoxide. Subsequent fragmentation of the epoxide ring in compounds 1a−k yields carbocycles 2a−k with an allyl alcohol side chain in good yields, and often with high diastereoselectivity. When tetramethylguanidine was used as an additive instead of HMPA, the desired carbocycle was obtained in good yield, but the diastereoselectivity was diminished. A palladium(0)-catalyzed SmI₂ reaction provided the expected product in modest yield, but the sense of diastereoselectivity was reversed. In the latter case, a different reaction mechanism may be involved. Thus, formation of an allylsamarium species may be invoked, with nucleophilic carbonyl addition leading to the observed facial selectivity.