A Convenient Synthesis of 3-Acylindoles via Friedel-Crafts Acylation of 1-(Phenylsulfonyl)indole. A New Route to Pyridocarbazole-5,11-quinones and Ellipticine

Article abstract of DOI:10.1021/jo00350a001

A Friedel-Crafts acylation of 1-(phenylsulfonyl)indoles (1) with carboxylic acid anhydrides and acid chlorides in the presence of aluminum chloride gives 3-acyl-1-(phenylsulfonyl)indoles (2) in 81-99percent yields.Base hydrolysis converts 2 to 3-acylindoles (3) in 79-96percent yields.The reaction of 1-(phenylsulfonyl)indole (1a) with oxalyl chloride gives acid chloride 2h, which is converted to 3-cyanoindole (7) in three steps (75percent yield).Although a similar Friedel-Crafts alkylation of 1 was unsuccessful, in some cases the 3-acyl-1-(phenylsulfonyl)indoles 2a,e,f could be reduced to 3-alkyl-1-(phenylsulfonyl)indoles 8a,b,c in nearly quantitative yield with sodium borohydride in trifluoroacetic acid.The acid chloride derived from keto acid 9 did not cyclize to the desired pyridocarbazole-5,11-quinone 24 but rather to chloro keto lactam 10.However, acylation of 1a with acid chloride 22 followed by strong-base-mediated cyclization gives 24.Since quinone 24 has been previously converted to the alkaloid ellipticine 26, this route to 24 represents a new synthesis of ellipticine.Related synthetic schemes give rise to quinones 16 and 20.