CHIRALITY IN UNSYMMETRICALLY SUBSTITUTED BENZOPENTATHIEPINS - THE RESULT OF A HIGH BARRIER TO RING INVERSION

The H-1 NMR spectrum of varacin (1), the first naturally-occurring com pound determined to bear a 1,2,3,4,5-benzopentathiepin ring, shows une xpectedly complex signals for the side chain methylene protons. Eviden ce is presented which indicates that unsymmetrically substituted benzo pentathiepins are asymmetric molecules as a result of a high energy ba rrier to inversion of the low energy chair conformations of the polysu lfide ring. Derivatization of varacin with a chiral auxiliary provides diastereomeric products which can be separated by fractional recrysta llization.