Catalytic Asymmetric Synthesis of Unprotected β²-Amino Acids

We report here a scalable, catalytic one-pot approach to enantiopure and unmodified β2-amino acids. A newly developed confined imidodiphosphorimidate (IDPi) catalyzes a broadly applicable reaction of diverse bis-silyl ketene acetals with a silylated aminomethyl ether, followed by hydrolytic workup, to give free β2-amino acids in high yields, purity, and enantioselectivity. Importantly, both aromatic and aliphatic β2-amino acids can be obtained using this method. Mechanistic studies are consistent with the aminomethylation to proceed via silylium-based asymmetric counteranion-directed catalysis (Si-ACDC) and a transition state to explain the enantioselectivity is suggested on the basis of density functional theory calculation

Asymmetric Organocatalysis with bis-Silyl Ketene Acetals

Enantiomerically pure α-stereogenic carboxylic acids are encountered in a variety of natural products and pharmaceuticals and are useful substrates for various transformations. The direct synthesis of such motifs via catalytic asymmetric α-functionalization remains challenging in both metal- and organocatalysis. A general approach toward α-functionalization can be envisioned via formation of bis-silyl ketene acetal intermediates followed by functionalization with an electrophilic counterpart. This thesis focuses on the development of enantioselective transformations with bis-silyl ketene acetals, exploring the generality of this strategy within Brønsted and Lewis acid catalysis for a variety of enantioselective C–H and C–C bond forming reactions, using simple and unactivated substrates. This strategy was successfully applied to the deracemization of α-branched aryl carboxylic acids via catalytic asymmetric protonation of bis-silyl ketene acetals with water or methanol as a proton source, delivering valuable products with high enantiomeric purity and high yields, including non-steroidal anti-inflammatory arylpropionic acids, such as Ibuprofen. Furthermore, this strategy showed great potential under Lewis acidic conditions for a direct aminomethylation, allowing the first asymmetric organocatalytic synthesis of β2-amino acids from aliphatic and aromatic unactivated substrates, in very good enantioselectivities and excellent yields. This work opens the field of catalytic asymmetric transformations with bis-silyl ketene acetals for the direct access of enantioenriched α-branched carboxylic acids

Deracemizing α‐Branched Carboxylic Acids by Catalytic Asymmetric Protonation of bis‐Silyl Ketene Acetals with Water or Methanol

We report a highly enantioselective catalytic protonation of bis‐silyl ketene acetals. Our method delivers α‐branched carboxylic acids, including nonsteroidal anti‐inflammatory arylpropionic acids such as Ibuprofen, in high enantiomeric purity and high yields. The process can be incorporated in an overall deracemization of α‐branched carboxylic acids, involving a double deprotonation and silylation followed by the catalytic asymmetric protonation

Flow Chemistry Approaches Applied to the Synthesis of Saturated Heterocycles

Continuous-flow processing approaches are having a significant impact on the way we devise and perform chemical synthesis. Flow chemistry has repeatedly demonstrated numerous improvements with respect to synthesis efficiency, process safety and ease of reaction scale-up. In recent years flow chemistry has been applied with remarkable success to the generation of valuable target structures across a range of industries from basic bulk chemical manufacture and materials development to flavours, food and cosmetic applications. However, due to its earlier implementation, it has found so far many more advocates in areas of medicinal and agrochemical research and manufacture. In this review article, we summarise the key developments that continuous-flow synthesis has had in the area of saturated heterocycles, specifically focusing on approaches that generate these important entities from acyclic precursors