Ylids from phosphine oxides

Stereoselective alkenylation reactions have been achieved using chiral addi-tives " or auxiliaries. Ylids formed from phosphine oxides,... 

From Azides and x-Acylphosphorus ylids Addition of azides to a-acylphosphorus ylids takes place at room temperature in dichloromethane or at 80°C in benzene, giving triazolines from which a phosphine oxide is spontaneously eliminated. " The ylids exist almost exclusively in the cis-enolate configuration, and a mechanism involving concerted 1,3-dipolar addition has been proposed (Scheme 12) on the basis that there is a low entropy of activation for the reaction, and that the reaction rate is insensitive to changes in solvent polarity. ... 

When the ylid derived from the oxide 7 was treated with benzaldehyde, a 2-methoxy-l,3-diene was formed. This led us to attempt a synthesis of (i) ar-turmerone by a sequence (Scheme 3) in which 7 effectively acts as an acyl anion equivalent. The required aldehyde 8 was synthesized in good yield using initially diphenyl(methoxymethyl)phosphine oxide (3), and then chlorotri-methylsilane-sodium iodide (12) to cleave the enol ether. 

A.iu. Phosphine Oxides and Phosphonate Esters. Many extensions of the Wittig reaction have been introduced that improve or modify the reactivity and/or stereoselectivity of the ylid. Horner et al. showed that a-lithiophosphine oxides such as that derived from 552 react with aldehydes or ketones to give a p-hydroxy phosphine oxide (553) as an isolable species. Subsequent treatment with base liberates the alkene, (554). Wadsworth and Emmons modified the Horner reaction to use phosphonate ester derivatives such as... 

Monophosphazenes are more acidic than phosphonium ylids. Amino triphenylphosphonium bromide is converted by triethylamine (7.475), but the corresponding phosphonium ylid requires metal alkyls (7.476). Phosphine oxides can be obtained from monophosphazenes by the action of carbon dioxide, sulphur dioxide, alkyl isocyanates or ketones ((7.477) through (7.480)). 

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