Lithium dialkenylcuprates

The more reactive lithium dialkenylcuprates (R2CuLi) cannot be directly acylated even at low temperature, because further addition to the product takes place. These reagents can be modified in situ so that the actual species undergoing reaction is an organozinc (requires palladium catalysis) however, other methods for generating organozincs are available that do not involve prior organocopper formation (Section 1.13.4.4). Stoichiometric vinylcopper and other copper-mediated acylations were not directly applicable to the case shown in equation (58) (however, see Section 1.13.2.1). 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

The stereoselective conjugate addition of lithium (Z)-dialkenylcuprates to vinyl sulfones gives (Z)-olefms in the range of 70-80% overall yield and no ( )-isomer is detected (equation 66)56. The degree of stereoselectivity is higher than 90%. 

Secondary and tertiary dialkylcuprates, lithium dialkenyl-, and even diphenyl-cuprates, add in very good yields to the reactive propionaldehyde diethyl acetal. The syn addition products may be trapped with a variety of electrophiles such as alkyl, alkenyl, alkynyl and aryl halides. The method has been used for the synthesis of several natural products. Substituted alkynic acetals also react with lithium dialkylcuprates in ether to furnish stable dialkenylcuprates of type (128) which do not eliminate to the corresponding alkoxy allenes (129) if the temperature is maintained below -20 C.164-179... 

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