THE PIANCATELLI REACTION

摘要

CVclopentenones are important intermediates for the synthesis of natural ducts and unnatural bioactive compounds. Classical methods for their syndesis involve the Pauson-Khand reaction,1-6 the Nazarov reaction,7-16 and the Rautenstrauch rearrangement.17-20 In both the Nazarov and the Rautenstrauch actions, the formation of the five-membered ring is achieved by a conrotatory 4-electrocyclization of a transient pentadienyl carbocation. In 1976, Piancatelli and coworkers reported a new method to access 4-hydroxycyclopent-2-enones that ajs0 relies on this type of stereoselective mechanistic step (see "Mechanism and Stereochemistry"). Hence, the treatment of 2-furylcarbinols with aqueous acids, such as formic acid, stereoselectively leads to the rrans-cyclopentenone product (Scheme l).21 This process is a rare type of reaction that directly transforms a heterocycle into a carbocycle. The same group later reported that substoichiometric amounts of zinc(II) chloride also promote the same rearrangement under milder conditions, which permits more reactive substrates that are prone to decomposition with Br0nsted acids to be employed.22 Although the original Br0nsted or Lewis acid promoted Piancatelli reaction has led to several applications in the synthesis of compounds of biological interest (see "Applications to Synthesis"), recent years have witnessed the development of new variations in which nucleophiles other than water are employed to trap one of the carbocationic intermediates. This approach permits the preparation of a variety of fran5-4-substituted cyclopent-2-enones and related compounds. Consequently, the combination of anilines as nucleophiles with dysprosium(III) trifluoromethanesulfonate as the catalyst provided the first catalytic aza-Piancatelli reactions.23 Subsequently, an intramolecular oxa-Piancatelli reaction was also described in which the nucleophile is a hydroxyalkyl substituent on the furan ring. The Piancatelli rearrangement was also extended to an intramolecular carba-Piancatelli reaction using aromatic tertiary amides as nucleophiles.24 Sulfur nucleophiles have not been reported in the Piancatelli reaction, but an alternative method for preparing mercaptocyclopentenones from furaldehydes is available (see "Comparison with Other Methods").