Organocopper compounds enolates

During the last decade, a substantial number of novel (sometimes even stereoselective) strategies for the preparation of allenic prostaglandins have been devised. The approach used by Patterson involves a three-component coupling via a 1,4-addi-tion of the organocopper compound 121 to the enone 120, followed by alkylation of the enolate formed with the bromide 122 (Scheme 18.40) [121]. However, due to the notoriously low reactivity in the alkylation of the mixed copper-lithium enolate formed during the Michael addition [122], the desired product 123 was obtained with only 28% chemical yield (the alkylation was not even stereoselective, giving 123 as a 1 1 mixture of diastereomers). 

As is implicit in the fact that the products of the (stoichiometric) 1,6-cuprate addition - the lithium allenyl enolate and the organocopper compound - are formed as independent species, it is also possible to conduct the reaction catalytically through in situ regeneration of the cuprate. The reaction can thus be run in a continuous mode, with only catalytic amounts of the preformed cuprate being necessary (with simultaneous addition of the substrate and the organolithium compound) enabling the desired allenes to be prepared even on larger scales (Eq. 4.17) [3oj. 

The chemistry of Metal Enolates The chemistry of Organocopper Compounds... 

In this chapter, reagents that transfer a carbanion (in contrast to an enolate ion) to the C atom of a C=0 double bond are referred to as C nucleophiles. The most important nucleophiles of this kind are organolithium compounds and Grignard reagents. Organocopper compounds transfer their carbanion moieties to the carbonyl carbon far less easily and usually not at all. 

Chemists soon showed that it was an easy matter to synthesize a related ester by a conjugate addition of an organocopper derivative (Chapter 10) and then the alkylation of an ester enolate (Chapter 26). The enolate reacts with Mel on the face opposite the propenyl side chain—a good example of stereochemical control with cyclic compounds (Chapter 33). 

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