Resolution and rotational barriers of quinolinone and acridone sulfenamide derivatives: demonstration of the S-N chiral axis.

Achiral (8a) and chiral (8b) N-(2,4-dinitrobenzenesulfenyl)acridone derivatives were synthesized. Addition of the chiral solvating agent (S)- 2,2,2-trifluro-1-(anthryl)ethanol to 8a rendered the enantiotopic groups on the acridone ring diastereotopic and anisochronous, thus allowing the estimation of a lower limit for the rotational barrier about the S−N bond (18.7 kcal mol-1) by NMR spectroscopy. 8b and the previously reported chiral sulfenamide 5 (Raban, M.; Martin, V. A.; Craine, L. J. Org. Chem. 1990, 55, 4311) were resolved on a Chiracel OD HPLC column. This constitutes the first resolution of stereostable enantiomers of a compound in which the chirality is due only to the presence of the S−N chiral axis. The rotational barriers of both compounds are nearly equal (22.7−22.8 kcal mol-1 at 303.7 K) and are the largest determined to date for the rotation about the S−N bond in sulfenamides. The relatively high enantiomerization barrier for 8b is remarkable since the peri positions are unsubstituted.