Chiroptical properties of 2-substituted piperidines : A simple helicity rule

The CD spectra of fifteen 2-substituted piperidines 2 and their N-methylated derivatives 1 have been determined. The sign of the Cotton effect observed for the n→σ^* transition of nitrogen is shown to be correlated, for a given absolute configuration, with the axial or equatorial orientation of the nitrogen lone pair, and to be determined by the screw sense of the helicity between the nitrogen lone pair and the 2-substituent R in 1 and 2: for compounds of the absolute D configuration, a positive helicity gives a positive CD and vice versa.