Urethane group directed reductive couplings mediated by SmI2

Abstract

The SmI₂-induced ketone–olefin coupling reactions of α-(alkoxycarbonyl)amino ketones 1 and 3 with methyl, ethyl, isopropyl, and tert-butyl crotonate took place with high stereocontrol about the new chiral centers providing the syn-1,2-amino alcohol products, syn-trans-γ-lactones 2 and 4, in excellent yields. Apparently, the stereochemical course of these reductive couplings is stereocontrolled by chelation of the Sm(III) cations attached to the resulting ketyl radicals with the urethane groups. Stereoselectivity increased as the size of the alkyl group of the esters of crotonic acid increased. In particular, 2 and 4 were almost exclusively obtained when the SmI₂-induced couplings of 1 and 3 were carried out with tert-butyl crotonate. Interestingly, the hydroxy group-directed couplings induced by SmI₂ of the α-hydroxy ketone 11 with methyl, ethyl, and isopropyl crotonate proceeded with a complete reversal of diastereoselectivity, almost exclusively providing the syn-1,2-diol product, syn-cis-γ-lactone 12.