New building blocks for fluorinated bioimidazole derivatives II: preparation of beta-fluorourocanic acids.

Replacement of vinyl hydrogen with fluorine is based on addition of an FBr equivalent to a double bond followed by HBr elimination. This sequence has been adapted to prepare 3-fluoro-3-imidazolyl-propenoic acids (beta-fluorourocanic acids), and the related fluorinated imidazolyl propenals and prop-2-en-1-ols, from urocanic acid. Tritylation of the imidazole nitrogen was necessary for successful addition of "FBr" to the double bond, and prior reduction of the carboxyl group to the alcohol was required to provide the desired chemoselectivity in the elimination of HX. Reoxidation and deprotection produced the fluorinated urocanic acids.