Intramolecular [3 + 2] cyclocondensations of alkenes with indolidenes and indolidenium cations.

C(2)-C(3) cyclopentannelated indole constructs are prepared by either (a) a cyclization cascade of an alkenyl sulfide tethered to a 2-azido-1-allenylbenzene core or (b) cationic cyclization of a tethered alkenyl sulfide with a putative 2-indolidenium cation. In both cases, issues of C-C versus C-N bond formation emerge, and the results indicate that the former is favored.