N,N′-Dicyclohexyl-N-[(1RS,2RS,6SR)-4-(4-methoxyphenyl)-2,6-dimethyl-3-cyclohexene-1-carbonyl]urea:: an unusual crystalline enantiomeric pairing

The synthesis of (1SR,2SR,5RS,6RS)-5-ethyl-4-(4-methoxyphenyl)-2,6-dimethyl-3-cyclohexenecarboxylic acid was accompanied by the formation of a by-product shown to differ only by the absence of the 5-ethyl group. That mixture was subjected to an enantiomer resolution process utilizing (-)-menthol and dicyclohexylcarbodiimide (DCC), and the expected diastereomers of both compounds were formed along with their anhydrides and an unexpected, but related, compound in both cases. The unexpected compound formed from the substrate lacking the 5-ethyl group was unequivocally identified by X-ray analysis as the title compound, C29H42N2O3, (I), a result that, together with the NMR spectra, similarly identified the corresponding product from the compound possessing the 5-ethyl substituent. The two enantionters of (I) appear to be disordered and superimposed in the asymmetric unit, differing only in the position of the double bond, a rare phenomenon made possible by the unusual stereochemistry of (I).