Piancatelli rearrangement
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In 1976, the Italian chemist, Giovanni Piancatelli and coworkers developed a new method to synthesize 4-hydroxycyclopentenone derivatives from 2-furylcarbinols through an acid-catalyzed rearrangement.[1][2] This discovery occurred when Piancatelli was studying heterocyclic steroids and their reactive abilities in an acidic environment. As this rearrangement has continued to be studied, it has become a commonly used rearrangement in natural product synthesis because of the ability to create 4-hydroxy-5-substitutedcyclopent-2-enones.[2] Piancatelli’s motive for looking into this new rearrangement stemmed from the ever present 3-oxycyclopentene molecule, specifically its 5-hydroxy derivative, found in biologically active natural products.[3]
Piancatelli's proposed mechanism
The mechanism of this reaction is proposed to be a 4-π electrocyclization very much like the Nazarov cyclization reaction.[4] To obtain the 2-furyl carbinols, Piancatelli subjected furfural, an inedible biomass, to a Grignard reaction.[3] This is then submitted to acid-catalyzed hydrolysis to cause a molecular rearrangement and obtain the final 2-furyl carbinols.
It was proposed by Piancatelli that the reaction is a thermal electrocyclic reaction of a conrotatory 4π electron system while studying specifics of the mechanism conditions when synthesizing the 4-hydroxycyclopentenone derivatives. This mechanism was suggested when studying 1H NMR spectra as it became apparent that the final products exclusively delivered the trans isomer.[2]
In Piancatelli's proposed mechanism, the formation of the carbocation due to a protonation-dehydration sequence results in the two hydroxy groups being anti allowing for the trans-4hydroxy-5-substituted-cyclopent-2-enone from a 4π electrocylization ring closure.[2][3][5]
Alternative mechanisms
D'Auria proposed a possible mechanism that included zwitterionic intermediates as a way to form the cis isomer alongside the abundant trans isomer of the 2-furylcarbinol. D'Auria performed the rearrangement in boiling water without an acid catalyst.[2]
Another proposed mechanism is from Yin and co-workers that was studied while completing the rearrangement of 2-furylcarbinols with a hydroxyalkyl chain at the 5 position. Yin rationalized the mechanism by utilizing an aldol-type intramolecular addition.[2]
