Epoxidation with organocatalysts

The use of a-fluoroketone derivatives, as catalysts for the aforementioned epoxidation, has been less effective. So, the epoxidation of methyl cinnamate using the tropinone derivative 63 [69] gave the expected epoxide with modest results (33%, 64% ee). The use of organocatalysts 64 [70] did not improve the enantioselectivity but did improve chemical yields (up to 97%). 

Researchers at Degussa AG focused on an alternative solution with respect to a technical application of the Julia-Colonna epoxidation (Tso-goeva et al. 2002). Successful process development is based on the design of a continuously operated process in a chemzyme membrane reactor (CMR reactor). Therein, the epoxide and unconverted chalcone pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The setup for this type of continuous epoxidation reaction is shown in Scheme 9. 

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... 

All these studies set up the basis to apply this class of organocatalysts in MIRC-type processes for the synthesis of cyclopropane-containing molecules. Indeed, these compounds are, together with epoxides and aziridines, among the most useful cyclic scaffolds, present in several biological relevant products and pharmaceuticals. 

To date, many electrophilic reagents, such as alkyl halides, alkenes, alkynes, carbonyl compounds, epoxides, alcohols, and ethers, have been investigated in AFC alkylation reactions. On the other hand, the reactive 5-membered heteroaromatic compounds, such as indole, pyrrole, furan, and thiophene derivatives, and electron-rich benzene derivatives have been successfully applied in AFC alkylation reactions. Indole and pyrrole derivatives are most popular substrates due to their high reactivity and account for almost 80% of the published methodologies. A variety of chiral organometal-lic catalysts and organocatalysts are employed in the catalytic AFC alkylation reactions with high enantiomeric control. 

An asymmetric epoxidation using a fructose-derived chiral ketone. It is an organocatalyst with Oxone typically used as the primary oxidant. 

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