Synthesis of N-acetyl-1,3-dimethyltetrahydroisoquinolines by intramolecular amidomercuration:: Stereochemical aspects

Mercury(II)-mediated ring closure of N-[1-(2-allyl-3 -benzyloxy-4,6-dimethoxyphenyl)ethyl] acetamide 4 afforded N-acetyl-5-benzyloxy-6,8-dimethoxy-1,3-trans-dimethyl-1,2,3,4-tetrahydroisoquinoline 3. The product was shown to exist as a mixture of rotamers by NMR spectroscopy, since signals coalesced at higher temperatures. 2-[2-[1-(Acetylamino)ethyl]-6-(benzyloxy)3,5-dimethoxyphenyl]-1-methylethyI rnethanesulfonate 8 was also cyclized with sodium hydride to afford rotameric products with the same isoquinoline skeleton, but as a mixture of 1,3-cis- and trans-dimethyl isomers.