The X-ray crystal structure of 4-diphenylcarbamyl-N-methyl-piperidine methobromide (the carbamate analogue of 4-DAMP methiodide).

1. Crystals of 4-diphenylcarbamyl-N-methylpiperidine methobromide contain 2 conformers, one with the ester group equatorial and the other with the ester group axial to the piperidine ring. 2. In both conformers and diphenylcarbamyl nitrogen is trigonally substituted and flat compared with the tetrahedral substitution of the corresponding carbon atom in 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide. 3. The low activity and selectivity of the carbamate at muscarinic receptors is comparable with that of compounds containing only one phenyl ring, e.g. phenylacetyl-N-methyl piperidine methiodide or phenylacetylcholine. It is suggested that the high affinity of 4-DAMP metho-salts and their selectivity depend upon interactions with groups arranged in three dimensions and that the trigonal arrangement about the nitrogen atom in the carbamate makes it impossible for both benzene rings to interact at once.