Structure–activity studies on [4-(phenylsulfonyl) phenyl] methylpiperazine led to the discovery of 4-cyclohexyl-α-[4-[[4-methoxyphenyl](S)-sulfinyl] phenyl]-1- piperazineacetonitrile, 1, an M2 selective muscarinic antagonist. Affinity at the cloned human M2 receptor was 2.7 nM; the M1/M2 selectivity is 40-fold.