The expedient and regioselective metalation of unprotected biphenyl-2-carboxylic, biphenyl-3-carboxylic, and biphenyl-4-carboxylic acids

Unprotected biphenyl-2-carboxylic acid can be cleanly metalated with sec-butyllithium at the position adjacent to the carboxylate and can then be subjected to site-selective electrophilic substitution. The remote C2'-position is attacked by the superbasic mixture of n-butyllithium and potassium tert-butoxide (LICKOR, 3.5 equiv.) in THF or benzene at 20-60 degrees C. The resulting dianion cyclizes to give the fluorenone skeleton. The metalation reactions of biphenyl-3- and -4-carboxylic acids are also described. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).