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·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.