Configuration and conformation of substituted azetidines

Studies of compounds such as 1‐cyclohexyl‐2‐carbomethoxy‐4‐methylazetidine by H‐1 nmr coupling constants, nuclear Overhauser effects, C‐13 steric shifts, and N‐15 nmr support the revised assignment of configuration for cis/trans isomers in a series of N‐alkyl substituted azetidines. The trans isomer displayed spectral characteristics consistent with a planar or nearly planar ring. The cis isomer favors a puckered ring, with major substituents at C‐2 and C‐4 equatorial. Compounds lacking the C‐4 methyl are also puckered. Puckering angles are estimated. In contrast, trans‐1‐cyclohexyl‐2‐carboxy‐4‐methylazetidine (a zwitterion) prefers a puckered ring, possibly implicating the time averaged effect of nitrogen inversion on ring shape. The N‐15 spectra show a large difference in chemical shift between cis and trans isomers of the azetidines, although analogous aziridines show an even larger difference.