Phosphonate carbanions acyl anion equivalents

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

Z4.8 a-HETEROSUBSTITUTED PHOSPHONATE CARBANIONS AS ACYL ANION EQUIVALENTS... 

A system analogous to a cyanohydrin is a siloxyalkylphosphonate. Thus, the diethyl 1-phenyl-1-tri-methylsiloxymethylphosphonate carbanion derived from (61) on treatment with LDA acts as an effective acyl anion equivalent in the preparation of a-hydroxy ketones and ketones. The phosphonate (61) is formed from triethyl phosphite, benzaldehyde and TMS-Cl in excellent yield. 

The most systematically investigated acyl anion equivalents have been the IMS ethers of aromatic and heteroaromatic aldehyde cyanohydrins, TBDMS-protected cyanohydrins, benzoyl-protected cyanohydrins, alkoxycarbonyl-protected cyanohydrins, THP-protected cyanohydrins, ethoxyethyl-protected cyanohydrins, a-(dialkylamino)nitriles, cyanophosphates, diethyl l-(trimethylsiloxy)-phenylmethyl phosphonate and dithioacetals. Deprotonation of these masked acyl anions under the action of strong base, usually LDA, followed by treatment with a wide variety of electrophiles is of great synthetic value. If the electrophile is another aldehyde, a-hydroxy ketones or benzoins are formed. More recently, the acyl carbanion equivalents formed by electroreduction of oxazolium salts were found to be useful for the formation of ketones, aldehydes or a-hydroxy ketones (Scheme 4). a-Methoxyvinyl-lithium also can act as an acyl anion equivalent and can be used for the formation of a-hydroxy ketones, a-diketones, ketones, y-diketones and silyl ketones. - ... 

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