Cinchona-derived catalysts

TABLE 10.2. Reactions with First- and Second-Generation Cinchona-Derived Catalysts... 

A caution has been noted for chiral PTC alkylations involving alkyl halides that can be easily reduced. Attempted alkylation of 35 with (bromomethyl)cyclooctatetraene with a chiral cinchona-derived catalyst gave only racemic product [20]. 

Scheme 1.7 Some cinchona alkaloids and cinchona-derived catalysts.

In 2004, Fioravanti, Pellacani, and Tardella reported the asymmetric aza-Michael-initiated ring closure additions of ethyl nosyloxycarbamate to 2-(phenylsulfanyl)-2-cycloalkenones using the cinchona-based PTCs 11 or 29, affording the corresponding aziridines with moderate ee values (Scheme 5.25) [31]. Interestingly, this reaction afforded the same enantiomer, regardless of which pseudoenantiomer of the cinchona-derived catalyst was employed. However, the absolute configuration of the products was not determined in this study. 

In 2008, Dixon and coworkers reported catalytic enantioselective a-alkylations of [i-keto carbonyl derivatives with aziridines by using a cinchona-derived catalyst 79 (Scheme 6.23) [48]. [3-Keto carbonyl substrates 76 were reacted with N-protected... 

The asymmetric aldol reaction represents the most versatile protocol for the preparation of optically enriched (5-hydroxy ketones. During the last two decades, a number of observations have been made regarding asymmetric aldol and related reactions mediated by a cinchona-derived catalyst that affords high stereoselectivity. 

The NeuviUe/Zhu synthesis of the alstoscholarines began with desym-metrization of meso-anhydride 287 employing the cinchona derived catalyst 288 and methanol in ether to furnish carboxyhc acid 289 (Scheme 40). The carboxyhc acid moiety was then activated as a 2-pyridylthioester employing... 

To realize an efficient catalytic asymmetric MBH reaction, wherein a high level of asymmetric induction as well as desired rate acceleration is obtained, the appropriate combination of chiral amine catalyst and suitably activated alkene is required. Besides cinchona derivative catalysts, more chiral catalysts have been developed from achiral molecules such as DABCO, quinuclidine, indoli-zine or pyrrolizine-derived catalysts by introducing asymmetric functions. Hirama and co-workers have examined chiral C2-symmetric 2,3-disubstituted 1,4-diazabicyclo[2.2.2]octanes such as 2,3-(dibenzoxymethyl)-DABCO (162) as a catalyst for the asymmetric MBH reaction between 4-nitrobenzaldehyde and MVK (Scheme 2.79). Compared with achiral DABCO, chiral catalyst 162 showed diminished reactivity and the reaction required high pressure to ensure a reasonable reaction rate. The corresponding adduct was obtained in up to 47% ee. 

To improve the enantioselectivities, we then reinvestigated Hatakeyama s cinchona-derived catalyst, p-ICD, promoted MBH reaction of aldehydes with methyl vinyl ketone or (ot)-naphthyl acrylate in the presence of co-catalyst. The enantioselectivities increased in some cases, and the absolute configuration of the MBH adducts can be inverted by the use of proline or lithium... 

Table 29.2 Asymmetric Henry reaction of ketones catalyzed by cinchona-derived catalysts.

Inspired by the positive effect of dimeric cinchona alkaloid ligands in the Sharpless asymmetric dihydroxylation [55], Jew, Park, and coworkers developed a new family of dimeric cinchona-derived catalysts. The authors first prepared a series of dimeric cinchonidinium salts 24, 25a, and 26 using a phenyl spacer (Figure 12.7)... 

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