New spiropiperidines as potent and selective non-peptide tachykinin NK2 receptor antagonists

The synthesis of a series of 2-(5-fluoro-1H-indol-3-yl)ethyl spiropiperidines is described together with their tachykinin NK2 receptor affinities measured in a rat colon binding assay. Equivalent NK2 receptor binding affinity was observed for the spirooxazolidinone 3-benzyl-8-[2-(5-fluoro-1H-indol-3-yl)ethyl]-1-oxa-3,8-diazaspiro[4.5] decan-2-one (3a), the imidazolidinone 3-benzyl-8-[2-(5-fluoro-1H-indol-3-yl)ethyl]-1,3,8-triazaspiro[4.5 ] decan-2-one (3s), and the pyrrolidinone 2-benzyl-8-[2-(5-fluoro-1H-indol-3-yl)ethyl]-2,8-diazaspiro[4.5]decan -3 - one (3t). Substitution in the phenyl ring of compound 3a produced no significant enhancement in NK2 binding affinity. Replacement of the phenyl ring in 3a with other aromatic rings resulted in a significant loss in binding affinity. Compound 3a was shown to be a potent NK2 receptor antagonist in guinea pig trachea where it also demonstrated 1000-fold selectivity for NK2 receptors over NK1. In the anesthetized guinea pig, compound 3a administered by the intravenous or oral route displayed potent and long-lasting antagonist activity against NK2 receptor agonist induced bronchoconstriction.