NHC-catalysed enantioselectiveintramolecular formal [4+2] cycloadditions using carboxylic acids as azolium enolate precursors
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An NHC-catalysed intramolecular formal [4 + 2] cycloaddition protocol using carboxylic acid starting materials containing a tethered enone is reported. Optimisation studies using a model substrate showed that the use of the Yamaguchi reagent (2,4,6-trichlorobenzoyl chloride) for in situ preparation of a mixed anhydride was necessary for effective catalysis, leading to highest product yield and stereoselectivity. Using this protocol, a range of dihydrobenzofurans were accessed in moderate to high yield, and with high to excellent diastereo- and enantioselectivity (up to > 95:5 dr and > 99:1 er). This methodology was extended to the synthesis of syn-dihydrochromenone and syn-dihydropyranone derivatives in moderate to high yield and with excellent diastereo- and enantioselectivity.
Attaba , N & Smith , A D 2019 , ' NHC-catalysed enantioselectiveintramolecular formal [4+2] cycloadditions using carboxylic acids as azolium enolate precursors ' , Tetrahedron , vol. In press . https://doi.org/10.1016/j.tet.2019.130835
Copyright © 2019 Published by Elsevier Ltd. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1016/j.tet.2019.130835
DescriptionWe thank the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) ERC grant agreement no 279850 (NA) and the Royal Society (Wolfson Merit Award to ADS).
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