Semiempirical MNDO/H calculations of opiates. Part 1. Building blocks: conformations of piperidine derivatives and the effect of hydrogen bonding

Conformations of piperidine, N-methylpiperidine, N-methyl-3-piperidinol and N-methyl-4-piperidinol were studied by MNDO/H in the gas phase. A few representative confonners were considered for each compound, and both the enthalpy and entropy were calculated and compared with experimental values, where possible. The results obtained indicate that, for each compound, several conformers are expected to coexist at room temperature in various amounts determined by the free energy Boltzmann distribution. Hydrogen bonding contributes to the stabilization of some piperidinol conformations with axial substituents as well as some boat or flexible forms. In general, protonation increases the stabilization of the chair compared with the boat or flexible conformers, except for 4-hydroxy-N-methylpiperidine-H⁺, where the boat form predominates. However, the maximum stabilization by internal hydrogen bonding is 4-5 kcal mol^-1, so that hydration can break these bonds. In _N-methylpiperidine stabilization of the N-Meeq conformer with respect to the axial methyl group increases to 1.4 kcal mol^-1 from the value of 0.4 kcal mol^-1 for the same equilibrium of N-H in piperidine. The theoretical distribution of conformers in N-methyl-3-piperidinol is consistent with the known experimental results.