Chemical Biology 3OA3, September 2009 Page 60 E XPERIMENT 8 C LAISEN C ONDENSATION : M ICROSCALE S YNTHESIS OF 4-H YDROXYCOUMARIN Lab by W. J. Leigh, edited by PH One of the important enolate condensation reactions is the Claisen condensation , which involves reaction of an enolate with the carbonyl compound of an ester. The enolate is generated by removal of a slightly acidic hydrogen from the α-carbon of a ketone, nitrile or ester using a relatively strong base (Step 1). The enolate then attacks the carbonyl carbon of an ester to yield an intermediate which undergoes rapid loss of the alkoxide leaving group to yield a β-dicarbonyl compound (Step 2). As usual, both steps in the sequence are completely reversible; the reaction is driven to the product side by deprotonation of the relatively acidic product by the alkoxide ion produced in the condensation. Claisen-like condensations are also found in vivo , e.g. the condensation of 2 molecules of acetyl-Coenzyme A to give acetoacetyl-CoA, a process catalyzed by acetoacetyl-CoA thiolase
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