Chiral Bicyclic Guanidine-Catalyzed Enantioselective Reactions of Anthrones

Shen, Juan; Nguyen, Thanh Truc; Goh, Yong-Peng; Ye, Weiping; Xiao, Fu; Xu, Junye; Tan, Choon‐Hong

doi:10.1021/ja064636n

Cited by 193 publications

(87 citation statements)

References 29 publications

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“…As reported by Tan, [11] dithranol afforded exclusively the Michael adducts, in high yields (91-94%) and with excellent enantioselectivities (up to 99% ee; entry 1). [18] The absolute configuration of the Michael adducts was ascertained by chemical correlation of product 4a with that previously obtained by Tan.…”

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confidence: 70%

“…Both the NMR and polarimetric data for this compound were in accordance with those previously reported in the literature. [10,11] …”

Section: Experimental Section Typical Experimental Procedures For the mentioning

confidence: 99%

“…The enantioselective Diels-Alder cycloaddition of substituted anthrones with maleimides catalyzed by chiral bicyclic guanidines, which takes place with excellent yields and enantioselectivities (85-99% ee) when run in dichloromethane solution at À20 8C, was reported in 2006 by Tan and co-workers. [11] This reaction showed, however, a very strong temperature dependence, and the enantiomeric purity of the Diels-Alder adduct was much lower when performed at room temperature. In 2008, Gçbel and co-workers developed an addition of anthrones to maleimides catalyzed by metal-free bis(oxazolines), with moderate enantioselectivities (39-70% ee).…”

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confidence: 98%

“…[18] The absolute configuration of the Michael adducts was ascertained by chemical correlation of product 4a with that previously obtained by Tan. [11] When catalyst (R,R)-I was used, the resulting Michael adduct had an (S) configuration.…”

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confidence: 99%

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Bifunctional Thiourea‐Catalyzed Asymmetric Addition of Anthrones to Maleimides

et al. 2010

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For the first time, the addition of anthrones to maleimides catalyzed by bifunctional thiourea catalysts is reported. The thiourea moiety is able to activate the maleimide and the tertiary amine deprotonates the anthrone, furnishing the final DielsAlder or Michael adducts in excellent yields and enantioselectivities.Keywords: anthrones; Diels-Alder cycloaddition; enantioselective; maleimides; Michael addition; organocatalysis Anthrones are important compounds in natural products and in medicinal chemistry.[1] They are isolated, either in the free form or as O-or C-glycosides, from a wide variety of natural sources (plants, shrubs), such as rhubarb, Cassia, and "Cµscara sagrada" bark, among others.[2] From the chemical standpoint, anthrones and their enol tautomers, the 9-anthrols, play a central role in the chemistry of anthracenes, because by oxidation of the central ring they afford 9,10-anthraquinones, extremely valuable compounds as pigments or anticancer agents.[3] On the other hand, the reduction of anthrones provides anthracenes, useful in the preparation of dyestuffs and of optoelectronic materials.[4]The parent anthrone, namely 9A C H T U N G T R E N N U N G (10H)-anthracenone, has been classically used as a reagent for the analytical determination of sugars and many of its derivatives have interesting biological properties: antimicrobial, laxative, antipsoriatic or as telomerase inhibitors.[5] Recently, it has been found that some anthrone derivatives may display potent and selective antitumour activity. [6] Anthrone has been used as the diene component in Diels-Alder cycloadditions with a variety of dienophiles. [7,8] Kinetic studies performed by Koerner and Rickborn [8] on the reaction between anthrone and Nmethylmaleimide showed that a substantial acceleration took place in basic solvents such as DMF, pyridine or triethylamine, or upon the addition of a small quantity of triethylamine to a chloroform or THF solution of the reactants.[8] These results were interpreted by a catalytic oxyanion acceleration of the process, probably via the anthrone enolate (Scheme 1).Building upon these results, Riant and Kagan investigated the effect of chiral bases in the same reaction, [9] finding that quinidine and prolinol were the most enantioselective catalysts (up to 61% ee was achieved with a 10 mol% loading of quinidine in chloroform at À50 8C). Later on, Yamamoto and coworkers [10] described the asymmetric cycloaddition of anthrone and maleimides catalyzed by C 2 -chiral 2,5-bis(hydroxymethyl)pyrrolidines, obtaining good enantioselectivities in some instances (up to 87% ee). The enantioselective Diels-Alder cycloaddition of substituted anthrones with maleimides catalyzed by chiral bicyclic guanidines, which takes place with excellent yields and enantioselectivities (85-99% ee) when run in dichloromethane solution at À20 8C, was reported in 2006 by Tan and co-workers.[11] This reaction showed, however, a very strong temperature dependence, and the enantiomeric purity of the Diels-Alder adduct was much...

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confidence: 70%

“…Both the NMR and polarimetric data for this compound were in accordance with those previously reported in the literature. [10,11] …”

Section: Experimental Section Typical Experimental Procedures For the mentioning

confidence: 99%

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confidence: 98%

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Bifunctional Thiourea‐Catalyzed Asymmetric Addition of Anthrones to Maleimides

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“…[1] We have also reported a chiral bicyclic guanidine-catalyzed anthrone Diels-Alder reaction. [12] Herein, we report the development of chiral bicyclic guanidine 1 as a versatile Brønsted base catalyst for the enantioselective Michael reactions of dithiomalonates and b-keto thioesters using a range of acceptors, including maleimides, cyclic enones, furanone and acyclic 1,4-dioxobutenes.…”

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Chiral Bicyclic Guanidine as a Versatile Brønsted Base Catalyst for the Enantioselective Michael Reactions of Dithiomalonates and β‐Keto Thioesters

et al. 2007

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A chiral bicyclic guanidine was developed as a versatile Brønsted base catalyst for the enantioselective Michael reactions of dithiomalonates and b-keto thioesters using a range of acceptors including maleimides, cyclic enones, furanone and acyclic 1,4-dicarbonylbutenes.Keywords: asymmetric catalysis; dithiomalonates; guanidines; Michael addition; organic catalysis Catalytic enantioselective Michael reactions are widely used for the construction of C À C bonds.[1] Tremendous advances have been achieved using Lewis acidic, metal-based catalysts for the Michael reaction of 1,3-dicarbonyl compounds.[2] Organocatalytic approaches have also gained wide popularity in recent years.[3] Successful organocatalysts include imidazolidines, [4] bifunctional thioureas, [5] Cinchona alkaloids and derivatives, [6] quaternary ammonium salts [7] and a proline-tetrazole.[8] These catalysts are typically reported as highly effective for a particular type of Michael acceptor. Catalysts that are applicable for a range of substrates are still of high interest.Guanidines have been shown to catalyze the Strecker reaction with high enantioselectivity.[9] Chiral guanidines and guanidinium salts were also found to be useful for several other base-catalyzed reactions. [10] Recently, highly efficient axially chiral guanidine catalysts were reported.[1] We have also reported a chiral bicyclic guanidine-catalyzed anthrone Diels-Alder reaction.[12] Herein, we report the development of chiral bicyclic guanidine 1 as a versatile Brønsted base catalyst for the enantioselective Michael reactions of dithiomalonates and b-keto thioesters using a range of acceptors, including maleimides, cyclic enones, furanone and acyclic 1,4-dioxobutenes.Our initial investigation revealed that with 20 mol % of guanidine 1, dimethyl malonate 3 underwent a Michael addition to N-ethyl maleimide 2a [Eq.(1)]. The reaction was relatively slow, giving the Michael adduct 4 in only 20 % yield after 96 h and with 47 % ee. The a-proton acidity of thioesters is usually higher than that of their corresponding esters due to the poor overlap of the C(2p) and S(3p) orbitals. This enhanced acidity makes thioesters useful enol equivalents in the laboratory as well as in nature (e.g., as acyl coenzyme A demonstrates). Seeking a more active donor, we prepared a series of S,S'-dialkyl dithiomalonates and they were tested in place of the dialkyl malonates. Significant improvements in both reaction rates and ee values were achieved [Eq. (2)].With 10 mol % of catalyst 1, the reaction between S,S'-di-tert-butyl dithiomalonate 5a and maleimide 2a was completed within 1 h at room temperature, giving

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2,3,5,6-Tetrahydro-2,6-bis(phenylmethyl)-1H-imidazo[1,2-a]imidazole

Tan

Shen

2008

Encyclopedia of Reagents for Organic Synthesis

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Chiral Bicyclic Guanidine-Catalyzed Enantioselective Reactions of Anthrones

Cited by 193 publications

References 29 publications

Bifunctional Thiourea‐Catalyzed Asymmetric Addition of Anthrones to Maleimides

Bifunctional Thiourea‐Catalyzed Asymmetric Addition of Anthrones to Maleimides

Chiral Bicyclic Guanidine as a Versatile Brønsted Base Catalyst for the Enantioselective Michael Reactions of Dithiomalonates and β‐Keto Thioesters

2,3,5,6-Tetrahydro-2,6-bis(phenylmethyl)-1H-imidazo[1,2-a]imidazole

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