2- Cyclopentene-1-malonic acid, diethyl

DIETHYL A2-CYCLOPENTENYLMALONATE (2-Cyclopentene-l-malonic acid, diethyl ester)... 

The preparation described here of 3-cyclopentene-1-carboxylic acid from dimethyl malonate and cis-1,4-dichloro-2-butene is an optimized version of a method reported earlier3 for obtaining this often used and versatile building block.6 The procedure is simple and efficient and requires only standard laboratory equipment. 3-Cyclopentene-1-carboxylic acid has previously been prepared through reaction of diethyl malonate with cis-1,4-dichloro(or dibromo)-2-butene in the presence of ethanolic sodium ethoxide, followed by hydrolysis of the isolated diethyl 3-cyclopentene-1,1-dicarboxylate intermediate, fractional recrystallization of the resultant diacid to remove the unwanted vinylcyclopropyl isomer, and finally decarboxylation.2>7 Alternatively, this compound can be obtained from the vinylcyclopropyl isomer (prepared from diethyl malonate and trans-1,4-dichloro-2-butene)8 or from cyclopentadiene9 or cyclopentene.10 In comparison with the present procedure, however, all these methods suffer from poor selectivity, low yields, length, or need of special equipment or reagents, if not a combination of these drawbacks. 

Scheme 7. RCM of diethyl-diallyl-malonate to give the diethyl ester of 3-cyclopentene-l,l-dicarboxylic acid.

---