large strong-field increment for 2-CH~ was observed here, and the chemical shift for this carbon atom is 14.5 ppm, whereas it was more than 19.8 ppm in all other cases. A "~ effect" in this case is clearly observed not only for C(s ) but also for C(~) and C(6 ) (Table 3).
The substantial dependence of the vicinai SSCC on the solvent in isomer IIIC (Table 2) nevertheless indicates that, despite the clear preponderance of the 2a,4e,5a conformation, equilibrium 2a4e5a ~ 2e4a5e exists and is shifted to favor the diequatorial form on passing from C6D6 to the polar solvent CDCI~.
In principle, the low barrier to inversion of the pyramidal ring nitrogen atom in N-alkylpiperidines [3] makes it possible to consider both equatorial and axial orientations of the N-methyl group as possible in all of the investigated configurational isomers. However, in no case did we obtain NMIR data that would enable us to reliably establish the orientation, although, as we have previously pointed out [i], its equatorial orientation in isomers A and B finds confirmation in the IH chemical shifts of the protons of the piperidine ring.
EXPERIMENTAL
The iH NMR spectra of 5% solutions in CDCI 3 or C6H 6 (with tetramethylsilane as the internal standard) were obtained with a WM-400 spectrometer (400 MHz). The method of double homonuclear resonance was used for assignment of the signals~ The IHNMR spectra of isomer IIIB on heating to 120~ in d6-DMSO were recorded with a WP-80 spectrometer. The 13C NMR spectra of isomer IIIB were obtained at 100.6 MHz with a WM-400 spectrometer. The 13C NMR spectra of the remaining compounds were recorded with a WP-80 spectrometer (20 MHz).
The authors thank I. I. Chervin for assistance in recording the NMRspectra with a WM-400 spectrometer. I, 2.