Allylic ethers, zincated

The mutual diastereoselection between the alkenyllithium and the zincated allylic ether turned out to be the same as for a crotylzinc species, and could be rationalized by kinetic addition of a cisoid -alkoxyallylic organozinc compound. 

The slow carbozincation, attributed to the presence of TMEDA, was more problematic when substrate-induced diastereoselection was also involved. Thus, addition of zincated allyl ethyl ether to the alkenyllithium derived from the secondary y-iodo allylic ether 209 afforded a 65/35 mixture of the diastereomers 228 and 229 in low yield after hydrolysis. The erosion of diastereoselectivity was not a consequence of a less efficient substrate-induced diastereoselection but rather of the fact that both the (E) and (Z) isomers of the zincated allyl ethyl ether had reacted. Although the use of a catalytic amount of TMEDA (10 mol%) in the metallation step substantially improved the yield, the diastereoselectivity remained low (equation 111)151. 

A stereoselective synthesis of substituted tetrahydrofiirans was achieved by a nickel catalyzed carbo-zincation of 2-halogenmethyIethyl allyl ethers <94TL8349>. 

Nevertheless, chiral propargylic amines remain interesting substrates for achieving the diastereoselective addition of substituted allylic organozinc compounds to metallated alkynes. Besides crotylzincation, one example of diastereoselective addition of zincated allyl ethyl ether to 328 has also been reported183. 

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