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Compounds with bridgehead nitrogen 52 —NMR spectra and stereochemistry of the 2‐alkylperhydroimidazolo[3,4‐a]pyridines

In contrast to perhydro‐oxazolo[3,4‐a]pyridine and perhydrothiazolo[3,4‐a]pyridine, which adopt equilibria in CDCl3 solution at room temperature containing ca 70% trans‐fused conformers in equilibria with O‐ or S‐inside cis‐fused conformers, 2‐alkylperhydroimidazolo[3,4‐a]pyridines are found to adopt equilibria containing >98% trans‐fused conformers. Comparison of NMR parameters of 2‐methylperhydroimidazolo[3,4‐a]pyridine with those of the two isomers of 1,2‐dimethylperhydroimidazolo[3,4‐a]pyridine indicates an equilibrium for the former compound between the two trans‐fused conformers, with ca 83% of that conformation containing a trans arrangement of nitrogen lone pairs of electrons. These observations are explained in terms of the generalized anomeric effect.