Diastereoselective Protonation of Chiral Enolates with Chelating Proton Donors under Reagent Control: Scope, Mechanism, and Applications

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Abstract
Endocyclic keto‐enolates of type 2 can be protonated under reagent control with high diastereoselectivities when chelating proton donors with a salicylate structure are used. The stereochemical course of these protonations is hardly affected by the ring size, the substitution pattern, and the presence of additional stereogenic centers in the enolate. The substrates can be prepared by conjugate cuprate addition as well as deprotonation; in the latter case, good diastereoselectivities are obtained by removal of competing proton sources and the inclusion of transmetalation steps. The chelate complex A serves as a mechanistic model that makes the usual trial and error search for stereoselective protonating agents unnecessary. The method is applied to stereoselective syntheses of a precursor for methyl epijasmonate and of the insect pheromone (2S,3S)‐diprionyl acetate.