Alkaloids Darzens-reaction

Table 1.12 Cinchona alkaloid-derived phase-transfer catalysts for asymmetric Darzens reactions.

The asymmetric Darzens reaction of a-chlorocycloalkanones using quatemized cinchona alkaloids as phase-transfer catalyst and LiOH as base in BU2O at room temperature is usually high yielding. However, the moderate ee is disappointing. 

Synthesis of Optically Active Epoxides. Alkaloids and alkaloid salts have been successfully used as catalysts for the asymmetric synthesis of epoxides. The use of chiral catalysts such as quinine or quinium benzylchloride (QUIBEC) have allowed access to optically active epoxides through a variety of reaction conditions, including oxidation using Hydrogen Peroxide (eq 5), Darzens condensations (eq 6), epoxidation of ketones by Sodium Hypochlorite (eq 7), halohydrin ring closure (eq 8), and cyanide addition to a-halo ketones (eq 9). Although the relative stereochemistry of most of the products has not been determined, enan-tiomerically enriched materials have been isolated. A more recent example has been published in which optically active 2,3-epoxycyclohexanone has been synthesized by oxidation with t-Butyl Hydroperoxide in the presence of QUIBEC and the absolute stereochemistry of the product established (eq 10). ... 

Since this pioneering study, a breathtaking number of naturally occurring alkaloid derivatives have emerged, finding application in a wide range of processes such as alkylations, Michael additions, aldol reactions, Darzens... 

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