Catalyst-Dependent Stereodivergent and Regioselective Synthesis of Indole-Fused Heterocycles through Formal Cycloadditions of Indolyl-Allenes

Stereo- and regioselective construction
of poly-heterocycles, especially
those with several contiguous stereocenters, is still a challenge.
In this paper, catalyst-dependent stereodivergent and regioselective
synthesis of indole-fused heterocycles through formal cycloadditions
of indolyl-allenes has been developed. The reaction features total
reversion of an all-carbon quaternary stereocenter when a gold or
platinum complex was employed as the catalyst through [3 + 2] cycloaddition
of allene with indole, affording different diazabenzo­[a]­cyclopenta­[cd]­azulenes as epimers, respectively.
In addition, in the presence of IPrAuCl and AgNTf2, highly
regioselective exo-type [2 + 2] cycloaddition was
observed, in which allene served as a 2C synthon. This methodology
provides a simple and straightforward approach for the construction
of indole-fused tricyclic systems under mild conditions in an atom-economical
way.