The reactions of 3-alkylindoles with cyclopropanes: An unusual rearrangement leading to 2,3-disubstitution

Indoles that bear an alkyl substituent in the 3-position, when treated with cyclopropanediesters, typically undergo a [3 + 2]-annulation reaction in a kinetically controlled process (1-->4). If the reaction is performed at elevated temperatures for a longer period of time, a rearrangement of the putative intermediate 3 occurs in which the alkylating species undergoes a migration to the 2-position followed by loss of a proton to reform the benzopyrrole ring. The yields range from 78 to 10%. If a 3-allylindole is used in combination with a cyclopropanediester, which is further substituted with an alkenyl moiety, the product is an effective ring closing metathesis substrate and can be converted to the 1,3,3a,4,5,6-hexahydro-1H-pyrido[3,2,1-jk]carbazole system. A mechanistic discussion of the rearrangement process is presented.