Enantioselective Enolate Protonation with Chiral Anilines: Scope, Structural Requirements, and Mechanistic Implications

Stereoselective deconjugation of macrocyclic α,β-unsaturated esters by sequential amidation and olefin transposition: application to enantioselective phase-transfer catalysis

The stereoselective synthesis of chiral macrocycles bearing two aliphatic amide functional groups is reported. After the amidation mediated by TBD, a guanidine derivative, the olefin transposition step is performed with a slight excess of t-BuOK. The products are afforded in moderate to good combined yields (up to 59%) and with an excellent syn diastereoselectivity (dr > 49 : 1). Introducing enantiopure α-branched substituents was possible and it resulted in mixtures of diastereomers, which could be tested as phase-transfer catalysts using the formation of a phenylalanine analog as a test reaction (up to 43% ee). A clear matched-mismatched situation was observed in the two diastereomeric series.

Recent Studies on the Regioselective C-Protonation of Enol Derivatives using Carbonyl-Containing Proton Donors

Enolate protonation can occur by two different pathways. With strong acids regioselective protonation generally occurs on the oxygen of the enolate to give an enol and then acid catalysed tautomerisation leads directly to the thermody-namically preferred carbonyl motif. However, with much weaker acids kinetic protonation on carbon can occur to give directly the carbonyl derivative. This review discuss recent developments into the use of chelating acids to promote this stereochemically important C-protonation and we comment on factors that appear to promote this selectivity.

Remote stereoselective deconjugation of α,β-unsaturated esters by simple amidation reactions

The thermodynamically disfavored isomerization of α,β-unsaturated esters to deconjugated β,γ-unsaturated analogues occurs readily when coupled to an amidation. Within the framework of macrocyclic derivatives, it is shown that 15, 16, and 18 membered macrocycles react with tBuOK and anilines to generate, in one-pot, β,γ-unsaturated amides (yields up to 88%). Importantly, single (chiral) diastereomers are isolated (d.r. > 49 : 1, ¹H NMR) irrespective of the size and nature of the rings, showing an effective transmission of remote stereochemistry during the isomerization process. CSP-chromatographic resolution and absolute configuration determination by VCD are achieved.

Asymmetric protonation of lithium enolates of alpha-amino acid derivatives with alpha-amino acid-based chiral Brønsted acids

The reaction of lithium enolates of alpha-amino acid derivatives with chiral amides, easily synthesized from L-tert-leucine, gives corresponding optically active unnatural alpha-amino acid derivatives with up to 87% ee.