Manganese ester enolates

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

The isomeric 1-suhstituted imidazole-5-carboxylates are made by cyclization of 3-amino-2-alkylaminopropanoic acids with triethyl orthoformate followed by active manganese dioxide oxidation of the imidazoline product (see Section 3.1.1), or from IV-substituted glycine esters, which are formylated, converted into the enolates and then condensed with potassium thiocyanate... 

Compound 136 can be transformed to the 3,4-trans isomer 138, which has the same configuration as ( + ) thienamycin, according to the procedure described by Chiba and coworkers in related structures [74]. The method involved oxidation of the hydroxyl group in 136 with activated manganese dioxide with concomitant isomerization at C-3, followed by K-selectride reduction of the resulting methyl ketone 137. Zinc enolates of iV,iV-disubstituted a-aminoacid esters have also been recently utilized for the synthesis of 3-amino P-lactams [75]. 

Using the manganese enolates, the chiraUty is transfered from the chiral ester moiety towards the peptide backbone, but not inside the peptide backbone. Therefore, the influence of the chelating metal salt in the rearrangement of several dipeptide crotyl esters was investigated. 

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