3- Piperideines, conformations

In contrast to the five-membered ring, conformational factors would be expected to influence the equilibrium between the imine and enamine forms in the case of the six-membered-ring piperideine derivatives (154). 

A situation analogous to that of the pyrroline derivatives also exists, according to spectroscopic data, with N-unsubstituted piperideine compounds. There is little experimental data because A 2-piperideines have not been studied as extensively as the analogous pyrrolines. The A structure has been established for some aliphatically substituted piperideines, e.g., J1(8)-hexahydropyrindene,12,13 J1(10)-octahydro-quinoline,13 and the alkaloid y-coniceine.14,15 According to conformational considerations, structures other than A piperideine could be expected more frequently in the piperideine series. The thia analog16 3 occurs in the amino form as shown by infrared spectral data and the estimation of active hydrogen. 

The conformational preferences in alkyl- and phenyl-substituted 3-piperideines (1,2,3,6-tetrahydropyridines) were theoretically calculated by ab initio and molecular mechanics calculations (01CEJ4715). A set of rules for the estimation of the position of conformational equilibria were derived that are very reliable as indicated by comparing the results with previous stereochemical assignments. 

A -Piperideine — see Pyridine, 2,3,4,5-tetrahydro-A2-Piperideine — see Pyridine, 1,2,3,4-tetrahydro-A3-Piperideine — see Pyridine, 1,2,3,6-tetrahydro-Piperideines — see Pyridines, tetrahydro-Piperidine, 1-acryloyl-polymers, I, 284 Piperidine, N-acyl-IJC chemical shifts, 2, 15 Piperidine, 2-alkyl- N-nitro-conformation, 2, 160 Piperidine, 4-amino-synthesis... 

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