Thiourea-catalyzed dearomatizing [4+2] cycloadditions of 3-nitroindole

We have developed two efficient thiourea promoted dearomatizing processes involving the cycloadditions of 3-nitroindoles. The C2=C3 double bond of the heteroarene can be involved as electron-poor 2 pi dienophile in [4+2] cycloadditions. While the uncatalyzed process requires harsh conditions, the organocatalyzed reaction takes place at room temperature and atmospheric pressure. The C2=C3 - N=O motif of the heteroarene can also react as an electron-poor 4 pi heterodiene in [4+2] / [3+2] cycloadditions cascades, under high pressure. In contrast to Lewis add activation, thiourea promotion thus proves efficient even under unconventional activation conditions and in the presence of acid sensitive reactants such as enol ethers. (C) 2014 Elsevier B.V. All rights reserved.