Alkenylmetals diastereoselective

For alkenylmetals bearing two stereocenters at both the allylic and homoallylic positions, two situations have to be distinguished depending on their relative influence (matched or mismatched) with respect to the stereochemical outcome of the allylzincation. Not surprisingly, in the matched case, as illustrated for substrate 232, the diastereoselectivity was excellent and 233 was obtained as a single diastereomer. The allyl moiety was delivered anti to both the allylic and homoallylic substituents in the chelated allyl alkenylzinc species (equation 113). 

Although substrate-induced diastereoselection can be conveniently achieved when the alkenylmetal was coordinated by an oxygen atom, other heteroatoms such as sulfur and nitrogen resulted in high inductions as illustrated by the crotylzincation of the organolithium reagents derived from the allylic sulfide 238 or the tertiary amine 239 (equation 117)154. 

Substrate-induced diastereoselective allylzincations essentially rely on coordination of the alkenylmetal by an appropriately located heteroatom, but a zinc-alkene -interaction23 27 can also exert a remarkable stereodirecting effect. Indeed, the alkenyllithium derived from 248, bearing an appropriately located carbon—carbon double bond, underwent highly diastereoselective allyl- and crotylzincations which led after hydrolysis to the corresponding 1,6-dienes 249 and 250. The stereochemical outcome... 

As mutual face selectivity between an alkenylmetal and a substituted ally lie organozinc reagent, combined with the alkenylmetal-induced diastereoselection, enabled the creation of two stereocenters, turning the sp3 dimetallated carbon into an additional asymmetric carbon has also been examined. 

Allylzincation of alkenylmetals provides a useful entry to the diastereoselective synthesis of sp3 yem-dirnclallic species that can react with two different electrophiles in a one-pot protocol, leading to elaborated acyclic structures with control of the configuration of up to three adjacent stereocenters, as well as to cyclopropanes bearing various substitution patterns126,163. 

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