Abstract
Previously we described the thermal decarboxylation of 1-(cyclopropylcarbonyl) cyclopropane carboxylic acid at 120 o C yielded 2-cyclopropyl-4,5-dihydrofuran instead of the expected dicyclopropyl ketone. The proposed mechanism proceeded through an initial ring opening of the 1,2-disubstitued cyclopropyl moiety leading to an α-allyl-β-keto acid system, specifically in this case α-(cyclopropylcarbonyl)-3-butenoic acid. In this paper we demonstrate that an α(carbonyl)-3-butenoic acid when thermally decarboxylated rearranges to the substituted 4,5dihydrofuran. Additionally, it was demonstrated that the decarboxylation and rearrangement are simultaneous and not a stepwise decarboxylation followed by rearrangement and ring closure.
Highlights
In a previous report studying the reactions of the dianion of cyclopropane carboxylic acid it was observed that at higher temperatures the acid dimerized to α-[cyclopropylcarbonyl] cyclopropane carboxylic acid
At that time we proposed a mechanism in which the initial step of the sequence was a ring opening of the α-[cyclopropylcarbonyl] cyclopropane carboxylic acid to α(cyclopropylcarbonyl)-3-butenoic acid
While there is a possibility that the entire decarboxylationrearrangement occurs simultanelously, the possibility of an initial decarboxylation of the β-keto acid leaving an intact cyclopropyl ring was discounted since our previous studies on the decarboxylation of α-[cyclopropylcarbonyl] cyclopropane carboxylic acid or α
Summary
In a previous report studying the reactions of the dianion of cyclopropane carboxylic acid it was observed that at higher temperatures the acid dimerized to α-[cyclopropylcarbonyl] cyclopropane carboxylic acid. At that time we proposed a mechanism in which the initial step of the sequence was a ring opening of the α-[cyclopropylcarbonyl] cyclopropane carboxylic acid to α(cyclopropylcarbonyl)-3-butenoic acid This would be followed by decarboxylation and ring closure to the 2-cyclopropyl-4,5-dihydrofuran. The diradical mechanism was demonstrated through the regio-specificity of the ring expansion that lead to the dihydrofuran, the stereochemistry of the reaction lead to isomerization of stereochemical centers This is a phenomenon of ring opening prior to ring closure is shared by the rearrangements of vinylcyclopropane and cyclopropylimines to the corresponding cyclopentenes and dihydropyrroles respectively[2,3,8]. It has been shown that the stereochemistry of the ring expansion could be controlled by the addition of Al2O3, which presumably acts as a template for the reaction[3] Another interesting reaction is the ring of opening cyclopropane carboxylic acid to the β,γunsaturated acid prior to decarboxylation[9]. In addition to a number of α-(carbonyl) cyclopropane carboxylic acids to demonstrate the generality of the reaction, we have synthesized the corresponding putative intermediate, β,γ-unsaturated ketones and the α-allyl-β-keto acids
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