Triflic Acid Mediated Dealkylative Lactonisation via NMR-Observable Alkyloxonium Intermediates

M. Paz Munoz, Guy C. Lloyd-Jones

Research output: Contribution to journalArticle (Academic Journal)

16 Citations (Scopus)

Abstract

Trifluoromethanesulfonic acid (TfOH) efficiently induces the dealkylative cyclisation of pent-4-enoates to generate gamma-lactones with high selectivity. For primary alkyl esters bearing an additional alkene, only monolactonisation occurs, even in the presence of excess TfOH. The kinetics of the reaction have been studied by H-1 NMR spectroscopy, which reveal that the TfOH acid undergoes a self-catalysed reaction with the pent-4-enoate to generate an oxonium triflate intermediate (rate approximate to k(obsd.)[TfOH](2)[ester](1)), possibly via the dimer (TfOH)(2). The oxonium triflate intermediate then evolves to the gamma-lactone according to unimolecular kinetics, liberating MeOTf in an S(N)i reaction. H-2-labelling experiments with TfOD suggest that the acid protonates the carbonyl moiety of the ester, with subsequent intramolecular delivery of D+ to the alkene. The resulting carbocation. is transient, being rapidly captured intramolecularly to generate the oxonium species. Reversibility in this step mediates equilibration of diastereomeric oxonium intermediates via the carbocation. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Original languageEnglish
Pages (from-to)516-524
Number of pages9
JournalEuropean Journal of Organic Chemistry
Issue number4
DOIs
Publication statusPublished - Feb 2009

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