Heterocyclic Deformations from Molecular Enlargement

Article abstract of DOI:10.1021/jo00141a017

The ease of distortion of saturated nitrogen heterocycles has been examined by progressively increasing the bulk of the substituent at nitrogen.The heterocycles included the pharmacophoric piperidine and morpholine six-membered rings, as well as the five-membered oxazolidine ring.Response to increased bulk of substitution was intended to be a crude model for distortions within the drug-receptor complex.Substitution at nitrogen included methyl, (1-adamantyl)methyl, and 6-substituted β-cyclodextrin within a tetrahedral series, and acetyl and (1-adamantyl)carbonyl within a trigonal series.With methyl and acetyl serving as standards for the undistorted rings, we have found that the NCH2CH2X dihedral angle within all three heterocycles is decreased anly by about 1 deg on introduction of adamantyl groups.In agreement with this flattening distortion, the barrier to ring reversal of the piperidine is decreased by 1.4 kcal/mol on replacement of N-methyl by N-adamantylmethyl.The β-cyclodextrin ring imposes a much more severe distortion, as this same dihedral angle in the piperidine and morpholine rings decreases 5-6 deg.The barrier to rotation about the amide bond decreases 5-6 kcal/mol in all three heterocycles on going from acetyl to adamantylcarbonyl.These studies show that the response of these heterocycles to incresed steric bulk of N substitution is a flatter and hence more flexible ring.