Reaction of benzenesulfonyl chloride (I) with methylamine afforded sulfonamide (II). Regioselective ortho lithiation of (II) with n-butyllithium, followed by condensation with 3-methylbenzaldehyde (III) gave carbinol (IV), which was cyclized to the sultam (V) by treatment with concentrated H2SO4. The desired (S)-enantiomer was finally isolated by chiral HPLC.
An enantioselective synthesis of the desired (S)-isomer was further developed starting from saccharin (VI). After conversion to the magnesium amide enolate (VII) by means of phenylmagnesium bromide, addition of Grignard reagent (VIII) produced the aryl sulfonimide (IX). Enantioselective reduction to the (S)-sulfonamide (X) was then accomplished using a chiral rhodium catalyst. Then, N-methylation of (X) with yodomethane in the presence of Cs2CO3 provided the title compound.