[reaction: see text] The first total synthesis of (+/-)-jamtine (4), a tetrahydroisoquinoline alkaloid reputed for its therapeutic properties, is described. The key step involves a tandem thionium/N-acyliminium ion cyclization from enamido sulfoxide 13. The cascade process takes place with high diastereoselectivity and in excellent yield.

The α-thiocarbocation generated from the Pummerer reaction of N-methyl-N-phenyl-2-[2-(toluene-4-sulfinyl)phenyl]acetamide undergoes Friedel−Crafts reaction at the γ-carbon with the tethered aromatic ring. Reductive removal of the phenylthio group from the resulting product using Raney nickel occurs in high yield, and the overall reaction represents a new method for the synthesis of a variety of 3-phenyl-substituted oxindoles. Treatment of the related N-benzyl-N-alkyl amido sulfoxide system with trifluoroacetic anhydride affords tetrahydroisoquinolone derivatives. The product distribution encountered coincides with the rotamer population of the starting amide. When the N-benzyl-N-methyl amide is used, only the normal Pummerer product is formed. In this case, the thionium ion is generated in the wrong conformation for π-cyclization to occur. The corresponding N-tert-butyl amido system, however, exists in a geometric orientation which places the benzylic group in the crucial conformation necessary for π-cyclization, and consequently, the reaction proceeds smoothly. Related cyclization reactions occur in good yield with the corresponding furanyl and cyclohexenyl N-tert-butyl amido sulfoxides. The additive Pummerer reaction of 3-phenylsulfinyl-N-benzyl-N-tert-butylacrylamide gave products derived from both 5- and 6-exo trig cyclizations. Intramolecular electrophilic aromatic substitution via six-membered ring closure ultimately afforded a dihydropyridone. The competitive process involving ipso attack of the aromatic ring on the thionium ion generates a spiro cyclohexadienyl cation that undergoes fragmentation of the adjacent σ-bond. The resulting acyl iminium ion is converted to N-tert-butyl-2-phenyl-3-phenylsulfinylacrylamide upon aqueous workup. Only cyclizations leading to five-membered rings occur with the corresponding indolyl and alkenyl N-tert-butyl amido sulfoxides.

Albert Padwa* and Alex G. Waterson. Studies Dealing with Thionim Ion Promoted Mannich Cyclization Reactions. Page 238. The transformation of structures 29/30 to 31/32 in Scheme 9 was incorrectly drawn. This scheme should be modified as indicated below. We thank Professor Stephen Martin, University of Texas, for pointing this out to us.