反应详情

A more general access to 3-alkyl-4-formyl-benzoic acids is given in the following section: Commercially available 2-bromo-terephthalic acid or 2-bromo-terephthalic acid dimethyl ester is transformed to 2-bromo-terephthalaldehyde via a reduction-oxidation sequence in analogy to litereture procedures (e.g. K. R. Roesch, H. Zhang, R. C. Larock, J. Org. Chem. (2001), 66, 8042-8051 (supporting information); T. Ise, D. Shiomi, K. Sato, T. Takui, Chemistry of Materials (2005), 17, 4486-4492; S, Narsimhan, K. G. Prasad, S. Madhavan, Synthetic Commun. (1995), 25, 1689-1697). 2-Bromo-terephthalaldehyde is then subjected to a Cannizzaro reaction according to a literature procedure (S. E. Hazlet, G. Bosmajian, J. H. Estes, E. F. Tallyn, J. Org. Chem. (1964), 29, 2034-2036) to give 3-bromo-4-hydroxymethyl-benzoic acid which is then oxidised for instance by treatment with MnO2 (lit. e.g. T. Ise, D. Shiomi, K. Sato, T. Takui, Chemistry of Materials (2005), 17, 4486-4492; S. Goswami, et al. Chem. Letters (2005), 34, 194-195) to 3-bromo-4-formyl-benzoic acid. This compound is then elaborated to the desired 3-alkyl-4-formyl-benzoic acid by either treating it with the appropriate alkenyl boron derivative (e.g. 2,4,6-trivinyl-cyclotriboroxane) under Suzuki conditions (Lit.: e.g. F. Kerins, D. F. O'Shea, J. Org. Chem. (2002), 67, 4968-4971) followed by catalytic hydrogenation or by reacting it with the appropriate alkyl-Zn reagents under Negishi conditions (e.g. H. Matsushita, E. Negishi, J. Org. Chem. (1982), 47, 4161-4165). The 4-formyl-group and the carboxylic acid function may require protection e.g. as an acetal and ester, respectively, during these transformations.