Tandem Pummerer/Mannich cyclization cascade of α-sulfinylamides as a method to prepare aza-heterocycles

A series of alpha-sulfinylenamides was conveniently prepared by the condensation of a primary amine with a ketone, followed by reaction of the resulting imine with ethylsulfenylacetyl chloride and subsequent oxidation with sodium periodate. When treated with p-TsOH, cyclization occurred to produce fused isoquinoline lactams by a mechanism that involves an initial Pummerer reaction followed by a subsequent cyclization of the resulting N-acyliminium ion onto the tethered aromatic ring. The isolation of a single diastereomer was rationalized in terms of a Nazarov-type 4pi-electrocyclic reaction followed by pi-cyclization onto the least hindered side of the N-acyliminium ion. Another method that was used to generate the alpha-acylthionium ion intermediate involved the reaction of bis(ethylsulfenylacetyl)acetamides with dimethyl(methyl)thiosulfonium tetrafluoroborate (DMTSF). Treatment of several bis-ethylsulfenylenamides with DMTSF delivered novel spiroheterocycles as single diastereomers in good yield by a related process. The convergency and stereochemical control associated with this cascade sequence make it particularly suited for the assembly of natural product scaffolds. Some preliminary studies were directed toward both mesembrine and deethylibophyllidine. When the model Z-enamido sulfoxide 33 was heated with p-TsOH, a 80% yield of tosylate 34 was obtained as a single diastereomer. In this case, the carbocation intermediate derived from cyclization onto the terminal pi-bond was trapped with p-TsOH from the least hindered face, opposite the angular carbomethoxy and methyl groups. Related cyclization cascades were also found to occur with systems containing tethered indole rings.