Stereoselective Construction of Pyrazinoindoles and Oxazinoindoles via Ring-Opening/Pictet-Spengler Reaction of Aziridines and Epoxides with 3-Methylindoles and Carbonyls

A highly efficient and stereoselective route to access 1,3-disubstituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles and 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indoles with excellent stereoselectivity (de, ee >99%)via base mediated ring opening of aziridines/epoxides with 3-methylindoles followed by BF3 center dot OEt2 catalyzed Pictet-Spengler reaction is accomplished. Interestingly, PTSA promoted cyclization led to the formation of oxidized 3,4-dihydropyrazino[1,2-a]indoles in excellent yields via an unprecedented Pictet-Spengler-detosylation cascade.