Cinchona metal-catalyzed asymmetric oxidations

On the contrary, to achieve a successful cinchona-catalyzed asymmetric oxidation chemistry, cinchona-catalyzed asymmetric reduction has been explored very little despite the importance of this reaction. Previous reports on this subject are restricted to the reduction of aromatic ketones and, moreover, the enantioselectivities achieved to date remain far from satisfactory when compared with metal catalysis. 

Chiral sulfoxides have emerged as versatile building blocks and chiral auxiliaries in the asymmetric synthesis of pharmaceutical products. The asymmetric oxidation of prochiral sulfides with chiral metal complexes has become one of the most effective routes to obtain these chiral sulfoxides.We have recently developed a new heterogeneous catalytic system (WO3-30% H2O2) which efficiently catalyzes both the asymmetric oxidation of a variety of thioethers (1) and the kinetic resolution of racemic sulfoxides (3), when used in the presence of cinchona alkaloids such as hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQD)2-PYR], Optically active sulfoxides (2) are produced in high yields and with good enantioselectivities (Figure 9.3). ... 

In contrast to asymmetric oxidation chemistry, cinchona-catalyzed asymmetric reduction reactions have been explored very little, despite the importance of this reaction. Previous reports on this topic are restricted to the reduction of aromatic ketones, and the enantioselectivities achieved to date remain far from satisfactory when compared with metal catalysis. Moreover, Hantsch esters, another type of useful organic hydrides, have not yet been studied in combination with cinchona catalysts. However, as is well known, the structures of cinchona alkaloids are easily modifiable, thus permitting the easy tuning of the reaction course. The successful use of cinchona catalysts for this reaction will therefore likely be reported in the very near future. 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

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