Chiral Induction in the Methylation of N-Ethoxycarbonylmethyl Sulfoximines

Abstract

Chiral inductions in the C- methylations of four optically active N-ethoxycarbonylmethyl-S-p-tolylsulfoximines were explored by using the reagents lithium diisopropylamide (in tetrahydrofuran at -78°) then methyl iodide. When the fourth substituent at sulfur was a methyl group, complex mixtures of products were formed. However, when the fourth substituent was an aryl group, moderate yields of methylated compounds were formed and the ratio of diastereomers produced were 50 : 50 ( Ar = phenyl), 76 : 24 ( Ar = o- methoxyphenyl ) and 100 : 0 [ Ar = o-(2-methoxyethoxy)phenyl]. The diastereoselectivities are accounted for by intramolecular chelation of the aryl-oxygen substituents with the intermediate lithiated carbanion.

The structure and absolute configuration where Ar = o-(2-methoxyethoxy )phenyl were confirmed by X-ray crystallographic methods. Space group P 2₁2₁2₁, orthorhombic, a 10.2500(7), b 13.281(2), c 16.526(2) Ǻ; final R 0.064 (on 924F). The configuration at the sulfur atom is (S) and that induced at the carbon atom is (R).