The Hydrolysis of Ethyl 1‐Methyl‐2,4‐cyclopentadiene‐1‐carboxylate by Nonenzymatic and Enzymatic Methods. Carbon‐Carbon vs. Carbon‐Oxygen Bond Cleavage

The title ester 5 is shown to undergo C–C bond cleavage under the conditions of basic ester hydrolysis (KOH/EtOH) with formation of potassium ethyl carbonate ( 6 ) and the tautomeric methylcyclopentadienes 7 and 8 . In contrast, porcine liver esterase (PLE, EC 3.1.1.1 ) cleanly hydrolyses 5 to give the isolable 1‐methyl‐2,4‐cyclopentadiene‐1‐carboxylic acid ( 13 ). The latter undergoes thermal dimerization with conservation of the geminal‐substitution pattern. The configuration of the Diels‐Alder adduct 17 is ascertained by it photochemical transformation into bishomocubane dicarboxylic acid 12 , easily distinguished by its C 2 symmetry. Under the conditions of acid‐catalyzed hydrolysis, dimerization of ester 5 and polymerization prevail, unless low acid concentration is used. The dimer 9 of 5 has one ester function that is reluctant to undergo basic hydrolysis.