The Expedient and Regioselective Metalation of Unprotected Biphenyl‐2‐, ‐3‐, and ‐4‐carboxylic Acids

Abstract 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 °C. The resulting dianion cyclizes to give the fluorenone skeleton. The metalation reactions of biphenyl‐3‐ and ‐4‐carboxylic acids are also described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)