A Short Diastereoselective Synthesis of the Putative Alkaloid Jamtine, Using a Tandem Pummerer/Mannich Cyclization Sequence

Treatment of 2-phenylhex-5-enal with benzylamine followed by sequential reaction with ethylthioacetyl chloride and sodium periodate oxidation afforded a E/Z mixture of α-sulfinylamides. As anticipated from a 4π-conrotatory mechanism, cyclization of each olefin afforded fused isoquinoline lactams as single diastereomers epimeric at the ethylthio position without any cross contamination. Some preliminary studies were directed toward the synthesis of mesembrine using a 3,4-dimethoxy aryl group. In this case, the Z-enamide prefers to undergo electrophilic aromatic substitution to give a substituted azepinone as the preferred product in 87% yield. In contrast, the E-enamide isomer provided the desired hydroindolone. The convergency and stereochemical control associated with the tandem Pummerer /Mannich cyclization make it particularly suited for the assembly of jamtine, a tetrahydroisoquinoline alkaloid reputed for its therapeutic properties. The key step in the synthesis involves a domino thionium/N-acylimini...