Premium
Triflic Acid Mediated Dealkylative Lactonisation via NMR‐Observable Alkyloxonium Intermediates
Author(s) -
Muñoz M. Paz,
LloydJones Guy C.
Publication year - 2009
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200800970
Subject(s) - oxonium ion , chemistry , alkene , trifluoromethanesulfonate , carbocation , triflic acid , moiety , medicinal chemistry , intramolecular force , stereochemistry , photochemistry , organic chemistry , catalysis , ion
Trifluoromethanesulfonic acid (TfOH) efficiently induces the dealkylative cyclisation of pent‐4‐enoates to generate γ‐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 1 H 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 ≈ k obsd. [TfOH] 2 [ester] 1 ), possibly via the dimer (TfOH) 2 . The oxonium triflate intermediate then evolves to the γ‐lactone according to unimolecular kinetics, liberating MeOTf in an S N i reaction. 2 H‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)