STEREOSELECTIVE ADDITIONS OF CHIRAL ALPHA-SULFINYL KETIMINE ANIONS TO ENE ESTERS - ASYMMETRIC SYNTHESES OF INDOLO[2,3-A]QUINOLIZIDINE AND YOHIMBAN ALKALOIDS

The in-situ 1,4-addition/ring-closure reactions of chiral alpha-sulfinyl ketimine anions with acyclic and cyclic ene esters offer a simple, convenient route for the construction of chiral cyclic alkaloids having a nitrogen-atom ring juncture. Asymmetric induction in the conjugate-addition reaction of the carbanions derived from alpha-sulfinyl ketimines possessing chiral sulfur with various cyclic and acyclic ene esters, subsequent ring-closure reaction, and reduction of the resulting beta-sulfinyl enamides were utilized in the syntheses of (-)-1,2,3,4,6,7,12,12b-octahydroindolo[2,3-a] quinolizine [(-)-1], (-)-alloyohimban [(-)-2], (+)-3-epi-alloyohimban [(+)-3], and (-)-yohimban [(-)-4].