Redox-triggered dearomative [5 + 1] annulation of indoles with O-alkyl ortho-oxybenzaldehydes for the synthesis of spirochromanes

The dearomative [5 + 1] annulation of 2-methylindoles with new five-membered synthons O-alkyl ortho-oxybenzaldehydes was developed unprecedentedly through cascade [1,5]-hydride transfer/dearomative cyclization in HFIP for the synthesis of spirochromanes bearing the 2-methylindolenine skeleton. In addition, the dual alkylation of the methyl group of 2-methylindolenines was achieved by sequential operation through the redox neutral [5 + 1] annulation with the second five-membered synthon N-alkyl ortho-aminobenzaldehyde, providing the chromane and tetrahydroquinoline fused spiroindolenines in good yields. Furthermore, the auxiliary group that facilitates the hydride transfer process could be simply removed.