Phase cinchona-derived

While the oxygenation of alkenes represents the lion s share of methodology development, new and improved protocols for the [C=0 + C] approach to epoxides have also been reported. For example, the phase-transfer-catalyzed asymmetric Daizens reaction using the chiral Cinchona derivative 41 afforded spirocyclic epoxide 52 in good to excellent yields and fair to good ee s <99T6375>. 

A number of other types of compounds have been used as chiral catalysts in phase-transfer reactions. Many of these compounds embody the key structural component, a P-hydroxyam-monium salt-type structure, which has been shown to be crucial to the success of the above described cinchona-derived quats. Although they have not been as successful as the cinchona catalysts, the ephedra-alkaloid derived catalysts (see 20, 22, 23 and 25 in Charts 3 and 4) have been used effectively in several reactions. In general, quats with chirality derived only from a single chiral center, which cannot participate in a multipoint interaction with other reaction species, have not been effective catalysts [80]. 

Lygo and Wainwright recently reported a detailed study of the asymmetric phase-transfer mediated epoxidation of a variety of acyclic a,P-unsaturated ketones of the chalcone type. The third-generation cinchona-derived quats (8c and 7c), related to those discussed earlier in the alkylation section and Scheme 10.4, gave the best inductions (89% ee, 88 to 89, Scheme 10.13 and 86% ee for the pseudoenantiomeric catalyst 7c to give, as product, the enantiomer of 89). 

Cinchona-Derived Chiral Phase-Transfer Catalysts for Amino Acid Synthesis... 

In particular, it is not only the cinchona alkaloids that are suitable chiral sources for asymmetric organocatalysis [6], but also the corresponding ammonium salts. Indeed, the latter are particularly useful for chiral PTCs because (1) both pseudo enantiomers of the starting amines are inexpensive and available commercially (2) various quaternary ammonium salts can be easily prepared by the use of alkyl halides in a single step and (3) the olefin and hydroxyl functions are beneficial for further modification of the catalyst. In this chapter, the details of recent progress on asymmetric phase-transfer catalysis are described, with special focus on cinchona-derived ammonium salts, except for asymmetric alkylation in a-amino acid synthesis. 

Cinchona-Derived Chiral Poly(Phase-Transfer Catalysts) for Asymmetric Synthesis... 

Michael additions of C-nudeophiles such as the indanone 1 have been the subject of numerous further studies For example, the reaction between the indanone 1 and methyl vinyl ketone was effected by a solid-phase-bound quinine derivative in 85% yield and with remarkable 87% ee by d Angelo, Cave et al. [5], Co-polymers of cinchona alkaloids with acrylonitrile effected the same transformation Kobaya-shi and Iwai [6a] achieved 92% yield and 42% ee and Oda et al. [6b] achieved almost quantitative yield and up to 65% ee. Similarly, partially resolved 2-(hydroxy-methyl)quinudidine was found to catalyze the reaction between 1 and acrolein and a-isopropyl acrolein with induction of asymmetry, but no enantiomeric excesses were determined [7]. As shown in Scheme 4.4, the indanone 7 could be added to MVK with up to 80% ee under phase-transfer conditions, by use of the Cinchona-derived PT-catalysts 9a and 9b, affording the Michael-product 8 or enf-8, respectively [8]. The adducts 8 or ent-8 were intermediates in the stereoselective Robinson anellation of a cydohexenone ring to the indanone 7 [8],... 

Asymmetric Weitz-Scheffer epoxidation is commonly used for the epoxidation of electron-poor alkenes. Cinchona-derived phase-transfer catalysts, initially used... 

In the presence of cinchona derivatives as catalysts, peroxides or hypochlorites as Michael donors react with electron-deficient olefins to give epoxides via conjugate addition-intramolecular cyclization sequence reactions. Two complementary methodologies have been developed for the asymmetric epoxidation of electron-poor olefins, in which either cinchona-based phase-transfer catalysts or 9-amino-9(deoxy)-epi-dnchona alkaloids are used as organocatalysts. Mechanistically, in these two... 

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