Dimethyl 3-cyclopentene-1,1 -dicarboxylate

The submitters report that dimethyl 3-cyclopentene-1,1-dicarboxylate (with <1% of the vinyl isomer)2 3 can be isolated at this stage in 92% yield and then transformed to methyl 3-cyclopentene-1-carboxylate4 with lithium chloride in wet dimethyl sulfoxide (DMSO)5 in 85% yield. 

Dimethyl carbonate Carbonic acid, dimethyl ester (616-38-6), 76, 170 Dimethyl 3-cyclopentene-1,1-dicarboxylate, 75, 197... 

When (67) was treated with a wide variety of cycloaddition reagents under various conditions, it behaved as a diene or a dienophile but not as a 1,3-dipole. As a dienophile it reacted with 2,3-dimethyl-1,3-butadiene to give (70) and with cyclopentadiene to give an analogous product. As a diene it reacted with [2.2.1] bicycloheptene to give (72), presummably via (71), by loss of carbon monoxide and hydrogen. No products were isolated when (67) was treated with maleic anhydride, dimethyl acetylene-dicarboxylate, diphenylacetylene, dimethyl fumarate, carbon disulfide, isobutyl vinyl ether, cyclohexene, and cyclopentene. 

B. 3-Cyclopentene-1 -carboxylic acid. A 250-mL, one-necked, round-bottomed flask is charged with 35.8 g of 3-cyclopentene-1,1-dicarboxylic acid and then fitted with a reflux condenser capped with a rubber septum and connected to a Nujol-filled bubbler by means of a syringe needle. The contents of the flask are heated in an oil bath at 170-175°C until carbon dioxide evolution is complete (ca. 2 hr) and then allowed to cool to room temperature. The resulting oil is transferred to a 50-mL flask and vacuum distilled without fractionation to provide 23.0 g (89% or 82% overall from dimethyl malonate) of 3-cyclopentene-1-carboxylic acid as a clear, colorless oil, bp 88°C (2 mm) (Note 7). 

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. 

Cyclopentene dimethyl m-l,2-cyclopentane-dicarboxylate dimethyl ds-1,3-cyclopentane- 1 1.5 60... 

Dimethyl maleate, dimethyl fumarate, (Z)- and (Z)-dibenzoylethylene, maleic anhydride, methyl cinnamate, styrene, methylstyrene, norbornenes and norbornadiene, cyclopentene, tetramethyl- and tetracyanoethylene, and dimethyl tricyclo [4.2.2.0 ]deca-3,7,9-triene-7,8-dicarboxylate react similarly with 2,5-diaryl-l,3-dithiolium-4-olates. In many cases the stereochemistry of the products has been elucidated. 

Similar reactions have been reported for acenaphthylenes, cyclopentenes, p-benzoquinone, nor-bornadiene, cyclopentadiene, 1,3-cyclohexadiene, and 1,5-cyclooctadiene <78CB3037>. The reaction of dimethyl-7-oxabicyclo[2.2. l]hepta-2,5-diene-2,3-dicarboxylate (120) with the 1,3-dithiolone (113) proceeds with the formation of the cycloadduct (121) (Scheme 28) pyrolysis or photolysis of this adduct (121) resulted in a double fragmentation to give the thiophene (122) and the furan (123) <75CC840>. 

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