Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 136, Issue 51, Pages 17869-17881Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja510603w
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Funding
- University of the Basque Country UPV/EHU [UFI 11/22]
- Basque Government [IT-628-13]
- Ministerio de Economia y Competitividad, Spain [CTQ2013-47925-C2]
- Ministerio de Educacion y Ciencia
- Gobierno Vasco
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Catalytic and asymmetric Michael reactions constitute very powerful tools for the construction of new C-C bonds in synthesis, but most of the reports claiming high selectivity are limited to some specific combinations of nucleophile/electrophile compound types, and only few successful methods deal with the generation of all-carbon quaternary stereocenters. A contribution to solve this gap is presented here based on chiral bifunctional Bronsted base (BB) catalysis and the use of alpha'-oxy enones as enabling Michael acceptors with ambivalent H-bond acceptor/donor character, a yet unreported design element for bidentate enoate equivalents. It is found that the Michael addition of a range of enolizable carbonyl compounds that have previously demonstrated challenging (i.e., alpha-substituted 2-oxindoles, cyanoesters, oxazolones, thiazolones, and azlactones) to alpha'-oxy enones can afford the corresponding tetrasubstituted carbon stereocenters in high diastereo- and enantioselectivity in the presence of standard BB catalysts. Experiments show that the alpha'-oxy ketone moiety plays a key role in the above realizations, as parallel reactions under identical conditions but using the parent alpha,beta-unsaturated ketones or esters instead proceed sluggish and/or with poor stereoselectivity. A series of trivial chemical manipulations of the ketol moiety in adducts can produce the corresponding carboxy, aldehyde, and ketone compounds under very mild conditions, giving access to a variety of enantioenriched densely functionalized building blocks containing a fully substituted carbon stereocenter. A computational investigation to rationalize the mode of substrate activation and the reaction stereochemistry is also provided, and the proposed models are compared with related systems in the literature.
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