A one-pot bicycloannulation method for the synthesis of tetrahydroisoquinoline systems

A highly effective method for the synthesis of the core indolo[2,3-alpha]quinolizidine skeleton found in yohimbine is described. The reaction of N-monosubstituted thioamides with bromoalkenoyl chlorides furnishes thioisomünchnones as transient 1,3-dipoles that undergo ready intramolecular cycloaddition across the tethered pi-bond to give thio-bicycloannulated products in a one-pot operation. The stereochemical outcome of the intramolecular reaction is the consequence of an endo cycloaddition of the neighboring pi-bond across the transient thioisomünchnone dipole. A major limitation of the method is that when a hydrogen is present in the alpha-position of the thioamide the initially formed thio-N-acyliminium ion undergoes proton loss to produce a S,N-ketene acetal at a faster rate than dipole formation. Treatment of tetrahydro-beta-carboline-1-thione with 2-bromooct-7-enoyl chloride followed by reductive removal of sulfur from the cycloadduct resulted in the formation of (+/-)-alloyohimbanone. Attempts to cycloadd the thioisomünchnone dipole across several nucleophilic pi-bonds failed, and instead, products derived from cyclization of the pi-bond onto the initially formed thio-N-acyliminium ion were formed. The resulting N,S-ketals were further converted into several tetrahydroisoquinoline alkaloids in good yield.