Absolute configuration of the alpha (1B)-adrenoceptor antagonist (+)-cyclazosin.

(+)-Cyclazosin is a quinazoline derivative pharmacologically characterized as alpha(1b)-adrenoceptor selective ligand in binding assays and alpha(1B)-selective antagonist in isolated tissues. In this work, we determined the absolute configuration of (+)-cyclazosin by X-ray crystallographic analysis applied to the (R,R)-(+)-tartrate of (-)-2, which is the enantiomer of the specific synthetic precursor of (+)-cyclazosin. Results indicated that (+)-cyclazosin has an R and S configuration at its C4a and C8a atoms, respectively.