N′-Acylation of (3,2′)-indole dimers

Noland, W. E.; Kumar, H. V.; Lu, C.; Brown, C. D.; Wiley-Schaber, E.; Johansson, A.; LaBelle, E. V.; O'Brian, N. C.; Jensen, R. C.; Tritch, K. J. Tetrahedron Letters. 2016, 57, 2158

N-Acylated indoles are abundant in nature, and have much biological importance. In the present investigation, we synthesize homo- and mixed-indole dimers (1, 5) derived from indole and 2-methylindole. N-Acylation of the indoline unit was then performed using triethylamine with acetyl chloride or benzoyl chloride or vinylogues thereof (9a–f). The resulting amides, (10a–f) and (11a–f), 10 of which are novel, could be of synthetic and biological interest. Acylation yields ranged from 50% to 91%, with most examples above 80%. The best results were obtained when base was slowly added to a mixture of indole dimer hydrochloride and acyl chlorides. When crotonoyl or sorboyl chloride was added to a basic solution of indole dimer, the product mixtures included small amounts of isomeric amides. These probably form by ketenes produced by conjugate elimination of HCl from the acyl chloride. No process was found by which the ketene-derived isomers could be obtained in good yield, or quantitatively separated from amides (10) or (11).