Esters, 1,3-keto intramolecular Barbier cyclization

Substituted -keto esters also provide excellent substrates for the intramolecular Barbier cyclization (eq 18). Diastereoselec-tivities are typically quite good but are highly dependent on substituent and solvent effects. 

Evidence for a radical coupling mechanism (as opposed to a carbanionic carbonyl addition mechanism) in the intramolecular Smh-promoted Barbier reactions has come from studies on appropriately functionalized substrates in the 3-keto ester series. It is well known that heterosubstituents are rapidly eliminated when they are adjacent to a carbanionic center. Indeed, treatment of a 3-methoxy organic halide (suitably functionalized for cyclization ) with an organolithium reagent leads only to alkene (equation 48). No cyclized material can be detected. On the other hand, treatment of the same substrate with Sml2 provides cyclized product and a small amount of reduced alcohol, with none of the alkene detected by gas chromatographic analysis (equation 49). ... 

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