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 4,5-dihydrofuran. Additionally, it was demonstrated that the decarboxylation and rearrangement are simultaneous and not a stepwise decarboxylation followed by rearrangement and ring closure.
Herein, we report a versatile approach for the
endocyclic ring-opening of bicyclic vinylcyclopropanes triggered by Heck
arylations. Key step for this transformation is a [1,3]-migratory shift of Pd
allowing the ring expansion of cyclopropanated pyrroles, piperidines, furans as
well as cyclopentadienes to grant access to the corresponding
1,2-dihydropyridines, 2<i>H</i>-pyrans,
2,3-dihydro-1<i>H</i>-azepines and
1,4-cyclohexadienes, respectively. Additionally, <i>gem</i>-disubstituted
cyclopropanated furans showed unexpected behavior by giving
diastereoselectively asymmetrically substituted dienes. Mechanistic studies and
theoretical calculations point towards a facile [1,3]-migratory shift
of Pd along the cyclopropane moiety, which can successfully compete with the usual
termination step of a Heck reaction via a <i>syn</i>-b-hydride elimination.<br>
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