An unprecedented enantioselective and general olefin isomerization was realized via biomimetic proton transfer catalysis with a new chiral organic catalyst. A broad range of mono- and di-substituted β,γ-unsaturated butenolides were transformed into the corresponding chiral α,β-unsaturated butenolides in high enantioselectivity and yield in the presence of as low as 0.5 mol% catalyst. Mechanistic studies have revealed the protonation as the rate-determining step.

The simple α,β-unsaturated ketones and 2-pyrones are readily available and synthetically important dienophiles and dienes, respectively, for Diels−Alder reactions. However, both prove to be challenging substrates for catalytic asymmetric Diels−Alder reactions. By exploring a new catalysis strategy featuring cooperative catalysis with readily available cinchona catalysts, an unprecedented asymmetric Diels−Alder reaction of simple α,β-unsaturated ketones with 2-pyrones has been successfully developed. With broad scopes for both reactants, the reaction provides a direct and versatile asymmetric access to a wide range of structurally novel bicyclic chiral building blocks amenable for further synthetic elaborations.

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Despite their synthetic significance there is a general lack of asymmetric vinylogous aldol reactions that tolerate variations of both the silyloxy furans and aldehydes. We have developed a new chiral organic catalyst based on a carboxylate-ammonium salt prepared form a thiourea-amine and a carboxylic acid. This new catalyst enabled us to develop an efficient asymmetric vinylogous aldol reaction of unprecedented scope with respect to both 2-trimethylsilyloxy furans and aldehydes.Chiral butenolides are common structural subunit in natural products1 and provide valuable chiral building block for the asymmetric synthesis of biologically active compound.2 Owing to their synthetic significance, the development of efficient catalytic methods for the generation of optically active butenolides has attracted considerable attention.3 In particular asymmetric vinylogous aldol reactions of 2-silyloxy furan and prochiral carbonyl compounds have been investigated with both chiral metal and organic catalysts.4 Significant progress has also been made in the promotion of catalytic asymmetric vinylogous aldol reactions between dihydrofuranone and aldehydes.5 Nonetheless, the development of such asymmetric vinylogous aldol reactions that afford useful level of enantioselectivity and diastereoselectivity with both aryl and alkyl aldehydes still represents a challenging task. Moreover there is a general lack of catalytic methods that tolerate variations of both the silyloxy furans and aldehydes. For example, there are only two examples of anti-selective, highly enantioselective vinylogous aldol reactions of silyloxy furans and aliphatic aldehydes, which are achieved with a chiral Cu-bisoxazoline complex and chiral phase transfer catalyst, respectively.4b , 4g However, the former affords useful enantioselectivity and diastereoselectivity for only benzyloxyacetaldehyde while the latter for only the 4-methyl 2-silyloxy furan. Herein, we report the design and development of a chiral organic catalyst system that effectively promotes anti-selective asymmetric vinylogous aldol reactions of various 2-trimethylsilyloxyfurans with aryl, alkenyl and alkyl aldehydes.Earlier we reported a highly enantioselective addition of TMSCN to α-acetal ketones with cinchona alkaloids as monofunctional chiral Lewis base catalysts.6 Recently, modified cinchona alkaloids bearing various hydrogen bond donors have been shown by us and others as efficient bifunctional chiral organic catalysts with a broad range of asymmetric reactions, including 1,2-additions to carbonyls.7 These precedents prompted us to explore 6 ′-OH, 6′-and 9-thiourea cinchona alkaloids as bifunctional catalysts for the vinylogous aldol reactions Correspondence to: Li Deng, deng@brandeis.edu. † This paper is dedicated to Professor Eric N. Jacobsen on the occasion of his 50th birthday. SUPPORTING INFORMATION:Experimental procedures and characterization of the products. This material is available free of charge via the Internet at http://pubs.acs.org. of 2-trimethylsilyloxyfuran (6a) a...

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A metal-free and mild, photoinduced decarboxylative 4-position alkylation of coumarins has been reported. Photoinduced single electron transfer has been initiated by utilizing the visible-light absorptivity of Eosin Y for a reductive generation of alkyl radicals from N-(acyloxy)­phthalimide esters. Depending on the nature of N-(acyloxy)­phthalimide esters (primary, secondary, and tertiary carboxylic acid derived), several saturated and unsaturated C-4 alkylated coumarins were synthesized. Both control experiments and photophysical studies supported a radical based mechanism for the selective alkylation.