On the origins of diastereoselectivity in the alkylation of enolates derived from N-1-(1'-naphthyl)ethyl-O-tert-butylhydroxamates: chiral Weinreb amide equivalents.
Davies SG., Goodwin CJ., Hepworth D., Roberts PM., Thomson JE.
The stereochemical outcome observed upon alkylation of enolates derived from N-1-(1'-naphthyl)ethyl-O-tert-butylhydroxamates (chiral Weinreb amide equivalents) may be rationalized by a chiral relay mechanism. Deprotonation with KHMDS leads to a nonchelated (Z)-enolate in which the oxygen atoms adopt an anti-periplanar conformation. The configuration of the N-1-(1'-naphthyl)ethyl group dictates the conformation of the O-tert-butyl group and the configuration adopted by the adjacent pyramidal nitrogen atom. Highly diastereoselective enolate alkylation then proceeds anti to both the bulky tert-butyl group (sterically driven) and the N-lone pair (stereoelectronically driven).