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Notes and references
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4 C. A. Bunton and J. H. Fendler, J. Org. Chem., 1967, 33, 1547.
5 This hypothesis was strengthened by the observation of a much less
negative entropy of activation for the fluoride-catalysed hydrolysis
than either the corresponding spontaneous hydrolysis or general
base-catalysed processes.
Scheme 1 Observation of an acyl fluoride intermediate.
6 K. K. Oglive and S. L. Beaucage, J. Chem. Soc., Chem. Commun., 1976,
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´ ´
´
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9 J. M. W. Chan, H. Sardon, A. C. Engler, J. M. Garcıa and J. L. Hedrick,
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Fig. 2 Proposed catalytic cycle.
12 For seminal reports see: (a) J. Hiratake, Y. Yamamoto and J. Oda,
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13 For selected reviews detailing anhydride desymmetrisation by alco-
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14 Selected examples of anhydride desymmetrisation by thiolysis:
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of 51 we could detect the presence of 53 by 19F NMR spectroscopy.
In the absence of the PTC, 53 is not formed; while the levels of 53
detected were higher in the absence of MeOH than in its presence.
In the absence of methanol, we could (using the catalyst’s CF3
resonances as an internal standard) track the increase in concen-
tration of 53 over time. After 5 h, all KF is converted to 53. These
observations do not exclude a competing general base catalysed
pathway, however, it is seems likely that nucleophilic catalysis
(Fig. 2) is responsible for much of the observed activity.
In summary, the first asymmetric reaction involving nucleo-
philic catalysis by fluoride ion has been developed. A library of
quinine-derived PTCs capable of forming ammonium fluorides
in situ were evaluated in the desymmetrisation of a meso-
anhydride by methanolysis. The presence of a strong H-bond
donating unit at C-9 is essential from an enantioselectivity
standpoint. The optimum catalyst designed was capable of
promoting the desymmetrisation of 14 with 61% ee. Experi-
ments involving the fluoride-catalysed methanolysis of benzoic
anhydride unambiguously demonstrated the formation of the
corresponding acyl fluoride, and in the absence of methanol,
quantitative formation of 53 was observed.
16 We have found ethereal solvents to be superior in reactions invol-
ving amine catalysts where the product is a carboxylic acid, for
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17 For examples of these types of catalysts see: (a) P. Bernal,
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This publication has emanated from research conducted
with the financial support of the Synthesis and Solid State
Pharmaceutical Centre (SSPC), funded by Science Foundation
Ireland (SFI) under grant numbers 12\RC\2275 and 12-IA-1645
and the European Regional Development Fund (EDRF). Finan-
cial support from the Irish Research council is also gratefully
acknowledged.
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18 Pyrrolidine is added after 48 h to destroy remaining anhydride.
Chem. Commun.
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