Cyclization of (+)- and (–)-S-(2-methylprop-1-enyl)-L-cysteine S-oxides

Abstract

S-(2-Methylprop-1-enyl)-L-cysteine (5) has been oxidized to two sulphoxides (7a), [α]_D²⁵+128°, and (7b), [α]_D²⁵–84.3°(in H₂O). Cyclization in base was stereospecific. The (+)-sulphoxide (7a) yielded the axial sulphoxide, (1S,3R)-5,5-dimethyltetrahydro-1,4-thiazine-3-carboxylic acid S-oxide (9), and the (–)-sulphoxide (7b) yielded the corresponding equatorial (1R) sulphoxide (10). This establishes the absolute configurations of the acyclic sulphoxides (7a and b) as R and S, respectively, at sulphur. Reduction of the cyclic sulphoxides to the corresponding cyclic sulphide, and reoxidation of this compound gave exclusively the equatorial sulphoxide (10), suggesting a large steric effect of the axial methyl group. Assignments of ring conformations and sulphoxide configurations are based on spectroscopic evidence.