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DOI: 10.1055/sos-SD-026-01067

Chataigner, I.; Harrison-Marchand, A.; Maddaluno, J.Science of Synthesis, (2005261174.

An example of the synthesis of a 3-oxo ester by a [3,3]-sigmatropic rearrangement involving an allylic and vinylic ether was described in Ludwig Claisen's original paper on what has since become known as the Claisen rearrangement.[‌454‌] An intramolecular mechanism has been proposed for the thermal rearrangement of 3-(alk-2-enyl)oxybut-2-enoates into 2-alkenyl-3-oxobutanoates via a cyclic transition state.[‌455‌] Alkylidenetetrahydro­furans (e.g., 149) rearrange either into cycloheptanones, with allyl inversion (under thermal conditions), or into cyclopentanones (e.g., 150) with no allyl inversion in the presence of a catalytic amount of palladium(0) (Scheme 89).[‌456‌,‌457‌] The stereo- and regiochemistry of the 3-oxo esters obtained has been studied in detail.[‌458‌] As well as vinyl and allyl ethers, vinyl, and propargyl ethers can also be involved in this thermal rearrangement. In the latter case, fragile skeletons such as cephems bearing allenyl 3-oxo esters can be obtained.[‌459‌] The Claisen rearrangement is also a synthetically useful method that can be applied to an advanced bicyclic 3-oxo ester intermediate en route to bruceantin,[‌460‌] various α-allylated tetronic esters,[‌461‌] or the bicyclic 5-oxolactone 151 present in the acetoxycrenulide structure (Scheme 89).[‌462‌] Note also that the same electrocyclic transformation can be used to convert fused polycyclic enol ethers into polyfunctionalized bridged bicyclic [4.n.1]-ring systems.[‌463‌]

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