Keto-ylids

Phosphorus ylids are quite common (see 16-47) and keto-phosphorus ylids (RCOCH=PPh3) are also known. When these compounds are heating (flash vacuum pyrolysis, FVP) to great than >500°C, alkynes are formed. Simple alkynes can... 

The obvious Vfittig disconnection gives stabilised ylid (5fi) and keto-aldehyde (57). We have used many such long-chain dicarbonyl compounds in this Chapter and they are mostly produced from available alkenes by oxidative cleavage (e.g. ozonolysis). In this case, cyclic alkene (58) is the right starting material, and this can be made from alcohol (59) by elimination,... 

Decarboxylation of an a-keto acid like pyruvate is a difficult reaction for the same reason as are the ketol condensations (see fig. 12.33) Both kinds of reactions require the participation of an intermediate in which the carbonyl carbon carries a negative charge. In all such reactions that occur in metabolism, the intermediate is stabilized by prior condensation of the carbonyl group with thiamine pyrophosphate. In figure 13.5 thiamine pyrophosphate and its hydroxyethyl derivative are written in the doubly ionized ylid form rather than the neutral form because this is the form that actually participates in the reaction even though it is present in much smaller amounts. 

Selective reactions. Wittig reactions of 1 with an aldehyde are possible in the presence of keto, ester, and amino groups.2 Wittig reagents react under normal conditions with acid chlorides. The stabilized ylid I reacts preferentially with the acid chloride group of 4-lormylhcnzoyl chloride (2) lo give 3 as the major product. ... 

We have shown that the ylids 3 react with a number of nucleophiles (3). In particular 3a and 3b react quantitatively with the alkylidene phosphoranes 5 to give 3-pyrrolines 6a and 6b as a mixture of diastereoisomers. The cyclisation occurs via an intramolecular Wittig reaction on the carbonyl of an ester group (M. In the same conditions, 2c reacts with 5 to give quantitatively the pyrrolines 7c (two diastereoisomers) as a result of a Wittig reaction on the carbonyl of the keto group in 3c. 

Decomposition of this product with the release of the thiazolium ylid also releases the product of coupling between the two keto-acids a l-hydroxy-2-keto-acid (in green). The original keto group of... 

The full structures of thiamine and of Stetter s catalyst appear in the chapter (pp. 1392-7). We use an abbreviated structure for the mechanism of conjugate addition. The essential features of this catalyst are that it is biomimetic and a d reagent that does conjugate additions. The first stages are the formation of the ylid, direct attack on the most reactive carbonyl group, the ketone of the a-keto-acid, and loss of CO2 to give the key intermediate. 

Thenappan, A, and Burton, D.J., Acylation of lluorocarbethoxy-substituted ylids. A simple and general route to a-fluoro P-keto esters, J. Org. Chem.. 56, 273, 1991. 

Keto-thiols add to alkenylphosphonium ions, affording ylids which then ring close by Wittig reaction and give 2,5-dihydrothiophenes, which can be dehydrogenated. ... 

A similar sequence to those described above converted phenylalanine 5.76) to N-tritylcyclohexyl derivative 5.94. This reaction sequence took a novel turn when 5.94 was converted to a phosphite ester (5.95). Treatment with sodium hydride gave a very novel, highly functionalized ylid that reacted with the keto-ester shown to give 5.96. Basic hydrolysis led to 5-(N-tritylamino)-6-cyclohexyl-2-dimethyl-methylene-4-oxohexanoic acid, 5.97. 

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