The Friedel-Crafts acylation of ethyl 1H-indole-2-carboxylate (1) with various acylating reagents having a functional group or hetero atom and the acylation of some derivatives (6, 7, 8, and 9) of ethyl 1H-indole-2-carboxylate (1) with simple acylating reagents are described. Acylation occurred at the C3-position or on the benzene moiety (mainly at the C5-position) of the indole nucleus. The regioselectivity of the above acylation of indoles (1, 6, 7, 8, and 9) is discussed.
Ethyl indole-2-carboxylate (1a) and its derivatives were reacted with various carboxylic acids by using trifluoroacetic anhydride and phosphoric acid (or polyphosphoric acid) to yield effectively the corresponding ethyl 3-acylindole-2-carboxylates (3). However, strongly acidic carboxylic acids and nitrogen-containing carboxylic acids were poor acylating agents. Ethyl 3-acylindole-2-carboxylate (3) could easily be converted to 3-acylindole (5)