An efficient synthesis of benzofurans and their application in the preparation of natural products of the genus Calea

Abstract

The intramolecular cyclization of the b-substituted olefins methyl 2-aryloxy-3-dimethylaminopropenoates 3a–3f catalyzed by Lewis acids leads to a short and novel synthesis of benzofurans 2a–2f. When the olefins 4-dimethylamino-3-aryloxy-3-buten-2-ones 4a–4f were used, the cyclization process was faster and provided the corresponding substituted 2-acetylbenzofurans 1a–1f. Among the latter, naturally occurring compounds calebertin (1a), caleprunin A (1b), and caleprunin B (1c) were prepared in good overall yields. These benzofurans were also obtained by direct treatment under MW irradiation of the precursors 1-aryloxypropan-2-ones 7a–7c with DMFDMA, followed by addition of the catalyst, resulting in a route that was one step shorter.