Cinchona diimides

Cinchona diimides have also been used to develop a simple colorimetric indicator displacement assay for the determination and chiral discrimination of tartaric acid. Thus, conjugation of 72 with bromophenol blue 73 resulted in the formation of a dark blue complex (A,m lx at 597 nm) that after treatment by an appropriate acid released the protonated form of the indicator with a well-separated absorption maximum at 426 nm. This assay allowed to determine (R,R)-tartaric acid with the detection limit as low as 0.02 mg/ml and efficiently discriminated the enantiomers of tartaric adds,... 

Aromatic diimides 71 and 72 derived from 9-epi-9-amino cinchona alkaloids and pyromellitic or 1,4,5,8-naphthalenetetracarboxylic anhydride have been designed by Gawronski and Kacprzak as novel chiral receptors. These triads are present at room temperature as equimolar mixture of syn and anti conformers, resulting from restricted rotation around the imide C—N bonds. These triads show high affinity toward carboxylic adds. The syn conformer binds preferentially 1,2-dicarboxylates in a 1 1 molecular ratio whereas the anti conformer is selective toward monocarbox-ylates forming a complex in a ratio 1 2 (triad acid). The response is sensitive and could be observed by either 1H NMR or by CD spectroscopy even when equimolar amounts of the adds are present in the solution. The competition experiments have shown a higher selectivity of triads toward 1,2-dicarboxylates [140],... 

In addition to metal-based catalysts, organocatalysts are also selective promoters of asymmetric Diels-Alder reactions. Several groups reported the use of cinchona alkaloid catalysts in standard Diels-Alder reactions. Deng combined 2-pyrones with a,P unsaturated ketones, while Bernard and Ricci focused on the reactions of vinylindoles with quinones and maleimides. Lectka reported enantioselective inverse electron demand hetero Diels-Alder reactions of ketene enolates and o-benzoquininone diimides catalyzed by a combination of benzoylquinidine and zinc triflate. For example, subjecting diimide 51 to the standard reaction conditions yields cycloadduct 52 as a single stereoisomer, which can be easily converted to... 

In 2006, Lectka and coworkers reported an asymmetric inverse electron demand hetero-Diels-Alder reaction (HDAR) of acyl chlorides (Bekele et al. 2006 Wolfer et al. 2006) and o-benzoquinone diimides to deliver chiral quinoxalinones (Abraham et al. 2006). In spite of perfect ee values observed by the catalysis of Lewis bases derived from cinchona alkaloids, the reaction conditions were somewhat harsh and metal triflates had to be used as co-catalysts to activate the electrophilic o-benzoquinone diimides (Abraham et al. 2006 Pauli et al. 2(X)8). 

Cinchona alkaloids have also heen used to catalyse the asymmetric functionalisation of anions, such as amination with diimides. Such diimide amination reactions can he used to functionalise the a-position or the y-position of a stabilised anion to yield functionalised chiral amines (331) (Scheme 14.100). This is a metal-free alternative to palladium catalysed allylation chemistry. 

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