Regio- and stereoselective 5-exo radical cyclizations on a chiral perhydro-1,3-benzoxazine moiety.: An access to enantiopure 3-alkylpyrrolidines

Both enantiomers of chiral, nonracemic S-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.