C(2)-Methylation abolishes DA1 dopamine agonist activity of 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (6,7-ADTN): steric intolerance by the receptor

High-field 1H NMR studies of the O,O-dimethyl ethers for both compounds as their free bases in chloroform-d revealed that the 2-methyl homologue probably exists as a rapidly equilibrating mixture of conformers; it seems likely that it can adopt the active conformation proposed to be required by the dopamine receptor.

Abstract

The synthesis of 2-amino-2-methyl-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene is reported. This compound did not produce vasodilation in the dog renal artery and was inactive as a DA1-type dopamine agonist. This is in contrast to the 2-nonmethylated homologue 6,7-ADTN, which is a potent DA1 agonist. High-field 1H NMR studies of the O,O-dimethyl ethers for both compounds as their free bases in chloroform-d revealed that the 2-methyl homologue probably exists as a rapidly equilibrating mixture of conformers; it seems likely that it can adopt the active conformation proposed to be required by the dopamine receptor. The lack of activity is therefore attributed to the steric effect of the 2-methyl group, consistent with explanations offered by others that the dopamine receptor cannot tolerate alkylation at the alpha side-chain carbon.