Ah. Schmidt et al., OXOCARBONS AND RELATED-COMPOUNDS .24. CHLOROSQUARYLATION OF INDOLES, Journal of the Chemical Society. Perkin transactions. I, (6), 1996, pp. 495-496
Chlorosquarylated indoles 3 and 4 have been synthesized from indoles 2
and squaric dichloride 1 and have been treated with nitrogen- and oxy
gen-nucleophiles.