Enones organocatalytic asymmetric

The organocatalytic asymmetric intramolecular aldol reaction has also been used in the synthesis of a gibbane framework [117]. The proline-catalyzed aldol cycliza-tion of the triketone 104 into the tricyclic system 106 proceeds via the unstable ketol 105 (Scheme 6.47). For this reaction, which occurred at room temperature, a catalytic amount (10 mol%) of L-proline was used. The enone 106 was furnished in 92% yield and a single recrystallization resulted in an enantiomerically pure sample of 106. This aldol product 106 served as a useful intermediate in the synthesis of the desired gibbane framework. 

Halland N, Hazell RG, Jprgensen KA (2002) Organocatalytic asymmetric conjugate addition of nitroalkanes to alpha,beta-unsaturated enones using novel imidazoline catalysts. J Qrg Chem 67 8331-8338... 

Organocatalytic Asymmetric Epoxidation of Enones via Iminium Catalysis... 

Few examples have been reported for the organocatalytic asymmetric conjugate addition of sulfur nucleophiles other than thiols. The reaction of thiocarboxylic acids to cyclohex-2-enones [390] and a,p-nnsatnrated esters [391] was initially studied by Wynberg et al. employing Cinchona alkaloid catalysts with limited success in terms of selectivity (up to 54% ee). Slightly better enantioselectivities have been recently obtained by Wang et al. in the 1,4-addition of thioacetic acid to P-nitrostyrenes (up to 78% ee) [392] and trani-chalcones (up to 65% ee) [393], using Takemoto s thiourea 142 as catalyst (2-10 mol%). 

S.2.4.3. oL,p-Unsaturated Ketones as Acceptors. The earliest studies on the organocatalytic asymmetric Michael reactions were reported by the group headed by Wynberg [1,83]. The authors found that natural cinchona alkaloids could effectively promote the Michael addition of 1,3-dicarbonyl compounds to a,(3-unsaturated enones. In 2003, Jprgensen and co-workers [84] developed the first highly... 

An intriguing methodology for the organocatalytic asymmetric Michael addition of alkynyl borates to enones was reported by Chong in 2005 [99]. In this methodology BINOL derivatives are used as a catalyst and constitutes an example of a... 

Subsequently, organocatalytic asymmetric halogenation/semipinacol rearrangement sequences, both leading to chiral quaternary haloketones, have been reported by Tu and coworkers [61] and by Hermecke and coworkers [62]. The mechanism of these reactions probably involves an acid-catalyzed diastereoselective semipina-col rearrangement of a chiral halonium ion intermediate obtained by chiral tertiary amine-promoted asymmetric halogenation of an enone [63]. 

Organocatalytic Asymmetric Michael Addition of 4-Hydroxycoumarin to Enone... 

J0rgensen et al. s synthesis The important breakthrough in the field of organocatalytic asymmetric Michael reactions of 4-hydroxycoumarin to enone for... 

The asymmetric conjugate addition of activated methylenes is one of the most studied organocatalytic reactions. A wide variety of Michael acceptors such as enals, enones, a,P-unsaturated nitriles, nitroolefins, a,(i-unsaturated imides, and vinyl sulfones have been successfully employed as elechophiles with high degree of stereocontrol. 

A highly enantioselective organocatalytic Michael addition of 4-hydroxycouma-rines and related compounds to a,p-unsaturated ketones has been also achieved using imidazolidine catalyst 137 [213]. The reaction, which gives high yields and enantioselectivities for a wide range of cyclic 1,3-dicarbonyl compounds and enones, has been successfully employed for the asymmetric synthesis of the anticoagulant warfarin (Scheme 2.78) and derivatives [213], With respect to the reaction mechanism, very recent studies have demonstrated that the truly active catalyst in the process was the chiral diamine 138, which is formed in catalytic amounts under the reaction conditions by reaction with the hydroxycoumarine (Schane 2.79)... 

J0rgensen and Juhl reported the first organocatalytic enantioselective inverse-electron-demand hetero-Diels-Alder reaction of aldehydes (e.g., 71) and enones (e.g., 72) with excellent diastereo- and enantioselectivity. Scheme 3.26 [41], The reaction utilizes a chiral enamine intermediate as an alkene in catalytic asymmetric cycloaddition reactions. 

Very recently two comprehensive review articles dealing with organocatalytic carbon-sulfur bond-forming reactions and asymmetric sulfa-Michael additions have been published. Historically, the first catalytic enantio-selective sulfenylations of conjugated enones in the presence of Cinchona alkaloids were published in 1977 by Pracejus where acrylamides and... 

SCHEME 11 9. Asymmetric organocatalytic intramolecular aza-Michael addition of enone carbamates. 

Another example of organocatalytic Michael addition apphed in total synthesis is that J0rgensen and co-workers [38] developed an asymmetric total synthesis of (/ )-warfarin (104) in 2003 as shown in Scheme 17.17. The key Michael addition of enol 101 to enone 102 was catalyzed by chiral diamine 103 to afford (/ )-warfarin (104) in... 

Hack and Enders developed a novel asymmetric, organocatalytic Michael addition of pyrrole (80) to enone 81 via Cinchona alkaloids derivative 83. Pyrroles such as 82 were obtained in good to excellent yields with ee s from 75 to 93% (13S2904). 

Zhang, Y. Shao, Y.-L. Xu, H.-S. Wang, W. J. Organocatalytic Direct Asymmetric Vinylogous Michael Reaction of an a,P-Unsaturated y-Butyrolactam with Enones. Org. Chem. 2011,76,1472-1474. 

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