NMR vs. molecular modeling Part IV - conformational analysis of menthone and isomenthone imines

The conformational analysis of a series of menthone and isomenthone imines was accomplished by (HNMR)-H-1 spectroscopy and was compared with molecular modeling computations. The (E) menthone derivatives contain about 20% of a 2,5-diaxial chair conformer at room temperature (22-23 degrees C), whereas in the (E)- and (Z)-isomenthone counterparts the major conformer has axial isopropyl and equatorial methyl groups. These peculiar conformations are a consequence of pseudo-allylic A((1,3)) interactions between the isopropyl group and the nonbonded pair of electrons of the nitrogen, which dominate the axial-axial interactions between the alkyl chains and the protons of the ring in the isomenthone imines and are of considerable importance in the menthone derivatives. Copyright (C) 1999 John Wiley & Sons, Ltd.