Masahito Yamagishi, Ken Nishigai, Azusa Ishii, Takeshi Hata and Hirokazu Urabe Angew. Chem. Int. Ed.
The addition of nucleophiles into electron deficient π-systems is a powerful and widely used concept in synthesis. Urabe and co-workers have shown that bromoalkynes can behave as competent electrophiles for such additions and demonstrate how the unique products of sulfonamide addition can act as precursors for heterocycle synthesis.
Addition under basic conditions with heating results in isolation of the cis-enamine as the only observed product. With an aryl substituent on the nitrogen, the adduct undergoes cyclisation by palladium catalysis to give the indole product. This process can be applied in a one-pot procedure, giving the indole scaffold in one step.
When an arylsulfonamide nucleophile does not contain an N-aryl substituent, an alternative heterocyclic system can be isolated from subjecting the addition product to palladium catalysis. 1,1-dioxobenzothiazines can be prepared using a 10 mol% loading of palladium acetate.
The higher catalyst loading is required for the less favourable cyclisation. When an N-aryl arylsulfonamide is subjected to cyclisation conditions only the indole product is observed.