Carbocupration ethyl propiolate

This method can be extended to the preparation of alkenylcopper compounds. Thus, treatment of the iodoalkenyl azide 10 with nBuLi at —100 °C (Scheme 2.3), followed by transmetalation with Znl2 in THF and then by a second transmetalation with CuCN-2LiCl, produces the copper species 11. This then effects a cis-selective carbocupration of ethyl propiolate to furnish the ( , ) diene 12 in 81% yield. 

Various functionalized alkynes can be submitted to carbocupration reactions, such as alkoxyalkynes,150 alkynyl carbamates,151 acetylenic orthoesters,152 and thioalkynes.153 The carbocupration of orthoesters, for example, 204, has been used to prepare a-substituted esters of the type 206 by acidic hydrolysis of the adduct 205 (Scheme 51).152 This allows the formation of regioisomers that are not accessible by copper-mediated addition to acetylenic esters. A stereoselective synthesis of trisubstituted alkenes has been described by Normant et al.lSd> starting from phenylthio-acetylene 207. Carbocupration with lithium di- -butylcuprate affords the intermediate 208 which, upon addition of /z-butyllithium, undergoes a 1,2-metalate rearrangement to the vinylcuprate 209. The latter can be trapped with various electrophiles, for example, ethyl propiolate, providing product 210 with complete regio- and stereocontrol. 

The cyclization of 4-substituted 5-hexenyl iodides proceeds well, leading to cyclopentylz-inc iodides that can be trapped with various electrophiles such as 3-iodo-2-cyclohexenone, iodine, acid chlorides, allylic halides, and ethyl propiolate (carbocupration) (Scheme 1) Various substitution patterns allow a successful cyclization. Also, a range of functional groups like esters or nitriles are tolerated in the ring closure (Scheme 8). 

A one-pot synthesis of y-butyrolactones such as 85 can be realized via an intermediate aHylic zinc species 86 generated by the homologation of an alkenylcopper reagent obtained by the carbocupration of ethyl propiolate (Scheme 7.30). 

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