Abstract
A novel high yielding, enantio- and diastereoselective protocol for the synthesis of α-allylated highly functionalised β-amino acid derivatives by adding isoxazolidin-5-ones to MBH carbonates under asymmetric phase-transfer catalysis has been developed.
Highlights
Chiral phase-transfer catalysis is one of the most powerful strategies to achieve the catalytic non-covalent asymmetric control of prochiral nucleophiles in stereoselective transformations (i.e. C–C bond forming reactions via alkylations or conjugate additions and heterofunctionalization reactions).[1]
The synthesis of chiral β-amino acids under chiral phase-transfer catalysis has so far mainly been associated with Mannich type approaches,[1] while the use of simple isoxazolidin-5-ones 2 as starting materials for the asymmetric syntheses of α,α-difunctionalized β-amino acids emerged only recently as a powerful and complementary strategy.[5,6,7]
We started our screening by investigating the reaction between the benzyl-substituted isoxazolidin-5-one 2a and the parent MBH carbonate 3a (Table 1 gives an overview of the most significant results obtained using the chiral phase-transfer catalysts (PTCs) A–D under a variety of different conditions)
Summary
Chiral phase-transfer catalysis is one of the most powerful strategies to achieve the catalytic non-covalent asymmetric control of prochiral nucleophiles in stereoselective transformations (i.e. C–C bond forming reactions via alkylations or conjugate additions and heterofunctionalization reactions).[1]. More than 10 years ago O’Donnell’s group already demonstrated that the use of chiral Cinchona alkaloid-based PTCs allows for the highly enantioselective β-addition of glycine Schiff bases 1 to allylic acetates 3′ (Scheme 1A).[9,10] Besides this inspiring initial report,[9] it was impressively shown that, based on the nature of the employed catalyst, the nucleophilic attack on allylic substrates 3 can either occur in the β- or in the γ-position.[11,12]
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