Asymmetric Synthesis of α-Aminoboronates via Rhodium-Catalyzed Enantioselective C(sp~3)-H Borylation

摘要

α-Aminoboronic acids, isostructural boron analogues of α-amino acids, have received much attention because of the important biomedical applications implicated for compounds containing this structure. Additionally, the inherent versatility of α-aminoboronic acids as synthetic intermediates through diverse carbon—boron bond transformations makes the efficient synthesis of these compounds highly desirable. Here, we present a Rh-monophosphite chiral catalytic system that enables a highly efficient enantioselective borylation of N-adjacent C(sp~3)-H bonds for a range of substrate classes including 2-(N-allcylarnino)heteroaryls and N-alkanoyl- or aroyl-based secondary or tertiary amides, some of which are pharmaceutical agents or related compounds. Various stereospecific transformations of the enantioenriched α-aminoboronates, including Suzuki—Miyaura coupling with aryl halides and the Rh-catalyzed reaction with an isocyanate derivative of a-amino acid, affording a new peptide chain elongation method, have been demonstrated. As a highlight of this work, the borylation protocol was successfully applied to the catalyst-controlled site-selective and stereoselective C(sp~3)—H borylation of an unprotected dipeptidic compound, allowing remarkably streamlined synthesis of the anti-cancer drug molecule bortezomib and offering a straightforward route for the synthesis of privileged molecular architectures.