DOPAMINERGIC 2-AMINOTETRALINS - AFFINITIES FOR DOPAMINE D-2-RECEPTORS, MOLECULAR-STRUCTURES, AND CONFORMATIONAL PREFERENCES

Abstract

A combination of X-ray crystallography, NMR spectroscopy, and molecular mechanics (MMP2) calculations was used to determine the three-dimensional structures and conformational preferences of the enantiomers of 5-hydroxy-2-(di-n-propylamino)tetralin and their C1-methylated derivatives. In addition, the affinities of the compounds for striatal 3H-spiroperidol- and 3H-N-n-propylnorapomorphine-binding sites were determined. In the present series, the dopamine D2-receptor agonists have the S-configuration at the nitrogen-bearing carbon (C2), whereas the only established D2-receptor antagonist, 1S,2R-5-hydroxy-1-methyl-2-(di-n-propylamino)-tetralin (1S,2R-UH-242), has the opposite absolute configuration at C2. Two conformational parameters, the tetralin inversion angle (.vphi.) and the dihedral angle .tau.(C1, C2, N, N-H or electron pair) (.tau.N), are shown to be critical for D2-receptor agonism; .vphi. values aroude 0.degree. and .tau.N values around 60.degree. appear to be optimal. The low D2-affinity of 1S,2S-5-hydroxy-1-methyl-2-(di-n-propylamino)tetralin seems to be related to its inability to assume a low-energy "D2-receptor agonistic conformation". It is noted that the common structural denominator between the D2-receptor antagonists 1S,2R-UH-242 and 6aS-apomorphine is their ability to assume "dopamine D2-receptor agonistic nitrogen electron pair orientations".