An X-Ray Analysis on Restricted Rotational Isomers of 2-Thioxo-4-oxazolidinone

The thermodynamically more stable diastereomeric rotational isomer I, of 3-(o-tolyl)-5-methyl-2-thioxo-4-oxazolidinone, which is first crystallized out in solution, shows that the ortho methyl substituent is cisoid to the methyl group of the hetero ring, and the thermodynamically less stable isomer II, crystallized later, has both methyl groups in transoid conformation, as is found in X-ray studies. The carbonyl and thiocarbonyl bonds are found to be longer in isomer II than in isomer I, which is attributed to the favored conjugation interaction of carbonyl, thiocarbonyl groups with the lone pair electrons of nitrogen, that may cause the lessened stability of isomer II.