In organic synthesis, the Piancatelli rearrangement is a strategy for the synthesis of 4-hydroxycyclopentenone derivatives from furfural developed by Italian chemist Giovanni Piancatelli and his coworkers in 1976.
Process The strategy is a Grignard addition to the aldehyde, followed by acid-catalyzed rearrangement: Lewis acids were discovered to help drive the second reaction to completion. Conversely, substituents stabilizing carbocations at the benzylic position impede the reaction.
Rearrangement mechanism The mechanism of the rearrangement is unclear. Piancatelli originally proposed that the rearrangment proceeds through a (conrotatory) 4π thermal electrocyclization, very much like the Nazarov cyclization reaction:
This mechanism was suggested when studying 1H NMR spectra as it became apparent that the final products exclusively delivered the trans isomer between the OH and the R (depicted).
D'Auria, who performed the rearrangement in boiling water without an acid catalyst, observed both trans and cis products. He proposed instead a zwitterionic pathway:
Another proposed mechanism is from Yin and co-workers, involving aldol/retro-aldol intramolecular addition:
Applications The Piancatelli strategy has been used extenively for prostaglandin synthesis:
See also Achmatowicz oxidation — another reaction of furancarbinols
References