New methods and reagents in organic synthesis. 35. A new synthesis of some non-steroidal anti-inflammatory agents with the 2-arylpropionic acid skeleton by the use of diphenyl phosphorazidate (DPPA) as a 1,3-dipole

Article abstract of DOI:10.1248/cpb.31.3139

Reaction of diphenylphosphonic azide (8) with the pyrrolidine enamine 13 of 4-isobutylpropiophenone afforded two amidines 15a and 16a, which were derived from the 1,3-dipolar cycloadduct 14a by the expulsion of nitrogen followed by 1,2-aryl migration (path a) and by 1,3-dipolar elimination (path b), respectively. Although diphenylthiophosphinic azide (9) and ethyl phenylthiophosphonoazidate (10) also gave similar results, diphenylphosphorazidate (DPPA, (C₆H₅O)₂P(O)N₃) furnished the amidine 15d formed via path a as the sole isolable product. Hydrolysis of 15d with potassium hydroxide afforded ibuprofen (3) in good yield. Other nonsteroidal antinflammatory agents, naproxene (4), ketoprofen (5), and flurbiprofen (6), were analogously and conveniently prepared from the ketones 17, 22, and 25, respectively, by a similar three-step operation using pyrrolidine, DPPA, and potassium hydroxide. 2-(2-Dibenzofuranyl)-propionic acid (7) was also prepared from the ketone 28 by the three-step procedure.