Ti-Crossed-Claisen Condensation between Carboxylic Esters and Acid Chlorides or Acids:  A Highly Selective and General Method for the Preparation of Various β-Keto Esters

Ti-crossed-Claisen condensation between a 1:1 mixture of carboxylic esters and acid chlorides promoted by TiCl4−Bu3N-N-methylimidazole proceeded successfully to give various β-keto esters in good yields with excellent selectivities (19 examples, ∼48−95% yield; cross/self-selectivity = ∼96/4−99/1). The present method was extended to the condensation between a 1:1 mixture of carboxylic acids and carboxylic esters (six examples, ∼70−92% yield; cross/self-selectivity = ∼91/9−99/1). To demonstrate the utility of the present two Ti-crossed-Claisen condensations, we performed a couple of efficient short-step syntheses of two natural, representative, and useful perfumes, cis-jasmone and (R)-muscone.