A novel metal-free protocol for the construction of sulfonylated indeno1,2-bindoles with a highly decorated quaternary carbon stereocenter was developed through the TMSCl-promoted cascade sulfonylation/cyclization of indole-linked propargylic alcohols and sodium sulfinates. The novel transformation involved allenyl sulfones as the key intermediates, followed by 5-exo-trig cyclization to achieve the desired products. Notably, this cascade reaction proceeded via a new pathway with high regioselectivity of the allenyl sulfone intermediate compared with the conventional cyclization pathway.
Liang et al. (2025) studied this question.