Enantioselective Syntheses of Dopaminergic (R)- and (S)-Benzyltetrahydroisoquinolines

Optically pure (1S,R)- and (1R,S)-benzyltetrahydroisoquinolines (BTHIQs), 12a,b as the major diastereomers, were prepared by stereoselective reduction of the isoquinolinium salt possessing (R)- and (S)-phenylglycinol as the chiral auxiliary, respectively. The absolute configurations of (1S,R)-13a hydrochloride (O-debenzoylated derivative from 12a) and (1R,S)-12b diastereomers were unambiguously determined by single-crystal X-ray analysis. Reductive removal of the chiral auxiliary group, subsequent N-propylation, and cleavage of the methylenedioxy group furnished the optically active catecholamines (1S)-16a and (1R)-16b in good overall yield. We have separately prepared for the first time pairs of dopaminergic 1-BTHIQs enantiomers through a classical methodology in asymmetric synthesis. The (1S)-enantiomers (14a-16a) bind to D1 and D2 dopamine receptors with affinities 5-15 times higher than those of the corresponding (1R)-enantiomers (14b-16b). Moreover, (1S)-14a inhibits [3H]dopamine uptake with high affinity. It appears that synthesis and testing of (S)-enantiomers of BTHIQ are very important for the search for new active drugs at dopamine receptors.