Enantioselective Synthesis of α-Alkyl-β-ketoesters: Asymmetric Roskamp Reaction Catalyzed by an Oxazaborolidinium Ion

Catalytic Asymmetric Homologation of 4-Substituted Cyclohexanones with CF3 CHN2 : Enantioselective Synthesis of α-Trifluoromethyl Cycloheptanones

Introduction of the trifluoromethyl group (CF₃ ) into organic molecules in an enantioselective manner has attracted significant attention, but still remains a challenging problem. We herein report a catalytic asymmetric trifluoromethylation of cyclic ketones via a Sc^III /chiral bisoxazoline-catalyzed homologation reaction by employing 2,2,2-trifluorodiazoethane (CF₃ CHN₂ ) as the CF₃ source. This desymmetrization process is highly efficient and generates two chiral centers with excellent diastereoselectivity and enantioselectivity, affording chiral α-trifluoromethyl cyclic ketones in a straightforward manner.

Catalytic Asymmetric Darzens-Type Epoxidation of Diazoesters: Highly Enantioselective Synthesis of Trisubstituted Epoxides

Highly enantioselective Darzens-type epoxidation of diazoesters with glyoxal derivatives was accomplished using a chiral boron-Lewis acid catalyst, which facilitated asymmetric synthesis of trisubstituted α,β-epoxy esters. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction proceeded in high yield (up to 99 %) with excellent enantio- and diastereoselectivity (up to >99 % ee and >20:1 dr, respectively). The synthetic potential of this method was illustrated by conversion of the products to various compounds such as epoxy γ-butyrolactone, tertiary β-hydroxy ketone and epoxy diester.

Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yield (up to 98%) with high to excellent enantioselectivity (up to 99% ee). A rational mechanistic model for the complex of COBI and aldimine is provided to account for these enantioselective reactions.

Asymmetric Synthesis of Enantioenriched 2-Aryl-2,3-Dihydrobenzofurans by a Lewis Acid Catalyzed Cyclopropanation/Intramolecular Rearrangement Sequence

A cyclopropanation/intramolecular rearrangement initiated by the Michael addition of in situ generated ortho-quinone methides (o-QMs) has been developed for the enantioselective synthesis of 2-aryl-2,3-dihydrobenzofurans containing two consecutive stereogenic centers, including a quaternary carbon atom. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction proceeded in excellent yields (up to 95 %) with excellent stereoselectivity (up to >99 ee, up to >20:1 d.r.).

Asymmetric Synthesis of Cyclobutanone via Lewis Acid Catalyzed Tandem Cyclopropanation/Semipinacol Rearrangement

Chiral Lewis acid catalyzed asymmetric formation of cyclobutanones from α-silyloxyacroleins and α-alkyl or α-aryl diazoesters has been developed. In the presence of a chiral oxazaborolidinium ion catalyst, various α-silyloxycyclobutanones possessing a chiral β-quaternary center were synthesized in high yield (up to 91%) with excellent enantio- and diastereoselectivity (up to 98% ee and up to >20:1 dr) through tandem cyclopropanation/semipinacol rearrangement. The synthetic potential of this method was illustrated by conversion of the product to various cyclic compounds such as γ-lactone, cyclobutanol, and cyclopentanone.