Regio- and Stereoselective 5-exo Radical Cyclizations on a Chiral Perhydro-1,3-benzoxazine Moiety. An Access to Enantiopure 3-Alkylpyrrolidines

Abstract

Both enantiomers of chiral, nonracemic 3-alkyl-substituted pyrolidines are prepared by diastereoselective 5-exo-trig cyclization on (−)-8-aminomenthol-derived perhydro-1,3-benzoxazines used as chiral auxiliaries, followed by elimination of the menthol appendage. The diastereoselective radical cyclization is promoted by tributyltin hydride and occurs on a 3-aza-5-hexenyl-type radical, leading to five-membered rings in high yield. The stereocontrol of the cyclization is strongly influenced by 1,3-allylic strain so that an appropriate substitution pattern on the olefin-acceptor and the presence of a vicinal stereocenter are crucial for achieving good diastereoselectivity. The enantiopure pyrrolidines are obtained in three steps with concomitant recovering of the starting (+)-pulegone auxiliary.