Yang M, Basada J, Gronert S, Erden I. An Oxy-anion Accelerated [1,5]-
o-Quinone Methide Shift During the Nucleophilic Epoxidation of Salicylfulvene.
European J Org Chem 2020;
2020:362-366. [PMID:
33071627 PMCID:
PMC7565285 DOI:
10.1002/ejoc.201901620]
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Abstract
A new facet of nucleophilic fulvene epoxidations has been uncovered. 6-Arylfulvenes containing an ortho or para hydroxyl group react with basic hydrogen peroxide in an unusual manner; the epoxidation of the fulvene exocyclic double bond is followed by a phenoxide ion initiated epoxide ring opening to form an o-quinone methide (o-QM) intermediate. The resulting cyclopentadienolate undergoes an unusual oxy-anion accelerated [1,5]-sigmatropic o-QM shift. Computational studies reveal that the activation energy for the [1,5]-QM-shift in the cyclopentadienolate intermediate is quite low, signifying the acceleration caused by the oxy-anion group. Placement of a second hydroxyl group in the 6-aryl ring at C5 epoxidation via electron donation to the o-QM carbon; instead, an intramolecular oxa-6-π-electrocyclization of the o-QM intermediate onto the cyclopentadiene is observed.
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