Asymmetric Formation and Isomerization of Three-Membered Rings : Catalyst Development and Evaluation
Sammanfattning: Enantiopure α-amino acid derivatives were prepared using a protocol which allows for highly controlled regio- and chemoselectivity in the hydrogenation/ hydrogenolysis of aza-Diels-Alder adducts. One of the resulting α-amino esters, (1S,3R,4R)-2-azabicyclo[2.2.1]heptane-3-carboxylic acid ethyl ester, was utilized further as a catalyst precursor. Significant improvements were achieved in the asymmetric lithium amide-mediated epoxide rearrangement reaction. An efficient catalytic system was developed, based on 5 mol% of (1S,3R,4R)-3-(amino)methyl-2-azabicyclo[2.2.1]heptanes and stoichiometric amounts of lithium diisopropylamide (LDA). With pyrrolidine or (2R,5R)-dimethylpyrrolidine as the amine moiety, the isomerization into allylic alcohols was accomplished for 5–7-membered cycloalkene oxides and (Z)-octene oxide with yields of >90% and enantioselectivities up to 99% ee. The studies indicate that the active catalyst species is a monomer, which is in equilibrium with inactive aggregates. The addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) led to a reaction rate acceleration. In the catalytic reaction, where there is a competing LDA-mediated rearrangement, the presence of DBU also improved the enantioselectivity. Furthermore, the asymmetric addition of carbenes and nitrenes to olefins was investigated. N,N'- and N,O-oxazolines were evaluated as ligands in the Cu-catalyzed aziridination of alkenes. Two of the oxazolines formed highly active catalysts (90% yield). However, the enantioselectivity was moderate (≤34% ee). Another catalyst, tetrakis-[(1S,3R,4R)-2-(p-tert-butylbenzene)sulphonyl-2-azabicyclo[2.2.1]heptane-3-carboxylate]dirhodium(II) was prepared and successfully applied in the reaction of styrenes and cyclic enol ethers with phenyl- and vinyldiazoacetates, affording cyclopropanes in up to 95% yield and 92% ee. In contrast to earlier observations, the best results were obtained with electron-rich olefins.
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