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Conjugate bifunctional cinchona catalysts

A practical and highly enantioselective Michael addition of malonates to enones catalysed by bifunctional primary amine-thiourea (5) derived from 1,2-diaminocyclohexane has been reported. The addition of weak acids and elevated temperature improved the efficiency of the Michael reaction. This approach enables the efficient synthesis of 1,5-ketoesters with good yields, excellent enantioselectivities (up to 99% ee), and low loading (0.5-5 mol%) of catalysts. A related bifunctional cinchona-thiourea catalyst has been involved in asymmetric organocatalysed conjugate addition reactions of monoth-iomalonates to nitroolefins. ... [Pg.309]

Despite the importance of the Michael addition in organic synthesis, the tandem conjugate addition/enantioselective protonation has been little explored [14] and only a few publications have involved cinchona alkaloids as bifunctional catalysts B for controlling the configuration of the chiral carbon created during protonation (Scheme 7.9). [Pg.178]

The synthetic utility of the bifunctional catalysts in various organic transformations with chiral cyclohexane-diamine derived thioureas was estabhshed through the works of Jacobsen, Takemoto, Johnston, Li, Wang, and Tsogoeva. In the last decade, asymmetric conjugate-type reactions have become popular with cinchona alkaloid derived thioureas. The next section presents non-traditional asymmetric reactions of nitroolefins, enones, imines, and cycloadditions to highhght the role of chiral Br0nsted base derived thiourea catalysts. [Pg.352]

In 1981, Wynberg and Hiemstra already identified the unmodified cinchona alkaloids as chiral bifunctional catalysts for enantioselective conjugate additions to cycloalkenones [3]. They proposed that the OH group of cinchonine would act as a hydrogen bond donor site and stabilize the enolate-Uke transition state of the conjugate addition reaction (Scheme 6.1). [Pg.186]

The extension of the thiourea-tertiary amine concept to the cinchona alkaloids was reported by several groups independently in 2005. The general usefulness of cinchona alkaloids as bifunctional catalysts was recognized by several groups even before this date [9]. The focus herein is on (thio)urea and squaramide catalysts since they are prototypes of more efficient catalysts. The Chen and the Dixon groups screened cinchonine and cinchonidine-derived thiourea catalysts for conjugate... [Pg.187]


See other pages where Conjugate bifunctional cinchona catalysts is mentioned: [Pg.106]    [Pg.194]    [Pg.110]    [Pg.318]    [Pg.1146]    [Pg.318]    [Pg.147]    [Pg.173]    [Pg.193]    [Pg.195]    [Pg.328]    [Pg.317]    [Pg.319]    [Pg.6]    [Pg.123]    [Pg.149]    [Pg.153]    [Pg.179]    [Pg.301]    [Pg.95]    [Pg.115]    [Pg.161]    [Pg.331]    [Pg.346]    [Pg.23]    [Pg.130]    [Pg.493]    [Pg.60]    [Pg.70]    [Pg.217]    [Pg.329]    [Pg.350]    [Pg.350]    [Pg.1146]   
See also in sourсe #XX -- [ Pg.193 , Pg.199 ]




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