Oxazoline Chemistry. Part 11: Syntheses of natural and synthetic isoflavones, stilbenes and related species via C–C bond formation promoted by a Pd–oxazoline complex

Abstract

The complex trans-[PdCl 2(2-ethyl-2-oxazoline-κ 1 N) 2] ( 1 ) is shown to be an active and oxidatively robust catalyst for C–C bond forming reactions (Heck, Sonogashira, Ullmann, Miyaura–Suzuki, etc.). These reactions can be carried out in air without rigorous solvent/substrate purification and in the absence of additional free ligand. The general methodology described above has been applied to the high yield and regio-selective formation, via Miyaura–Suzuki coupling, of natural and synthetic isoflavones (i.e., isoflavone, 2′-methylisoflavone [ 7b ], 3′-methylisoflavone [ 7c ] and 3′,4′-benzoisoflavone: [ 7d ]). Compounds 7c and 7d are previously unknown. In addition, the synthesis of ( E)-tris- O-methylresveratrol and ( E)-3,5-dimethoxystilbene is also described; the former is a recognized anti-cancer agent while the latter is a biologically active extract from the bark of the conifer species Pinus armandii. Both of these latter products are produced as a result of a Heck coupling reaction promoted by 1 .