Products Cited in Publications Worldwide Synthetic Elaboration of β-Carbonyl Alkylboronic Esters

Synthetic Elaboration of β-Carbonyl Alkylboronic Esters

Hamilton, Mason D

Abstract

compounds are some of the most synthetically versatile compounds in organic chemistry due to the many valuable transformations of the C-B bond. This synthetic versatility combined with the pharmacophoric nature of has led to an increased interest in the one-pot difunctionalization of vinyl arenes using CO2 and . Recently, much progress has been made to improve the scope and versatility of boracarboxylation reactions to now include electron-deficient and α-methyl substituted vinyl arenes. However, the potential transformations of boracarboxylated products have remained unexplored. Here, methodologies to transform the β-aryl alkylboronic ester into new C-C, C-N, and C-X bonds will be described. Medically relevant 2,3-diarylpropionic acids can now be accessed via a two-step protocol consisting of boracarboxylation of a vinyl arene followed by a palladium(0)-catalyzed . This methodology provides access to both the α- and β-regioisomers independently whereas traditional strategies to access these compounds afford only one regioisomer, and in most cases, a mixture of regioisomers. Interesting biaryl and heterocyclic products can be accessed and to demonstrate the synthetic utility of this protocol, a glucagon receptor antagonist was synthesized in 4 less steps than the previously reported method while maintaining similar yields. The transformative capability of boracarboxylated products is further demonstrated through a base-_x005f_x0002_and external oxidant-free copper(II)-catalyzed amination to generate β2-amino acid derivatives. While the β-carboxylic acid was intolerable to the conditions, protection via esterification or amidation allowed for successful amination of the alkylboronic ester to occur. Amination of two bora-NSAIDs, bora-ibuprofen and bora-naproxen, was successful and a number of cyclic and acyclic amines are suitable for the transformation. Preliminary mechanistic work suggests that this amination does not proceed through a free-radical intermediate but rather a two-electron pathway. Finally, a novel halogenation of boracarboxylated products is achieved to generate the corresponding β-aryl alkyl halides. This methodology is performed in a base, metal, and additive free manner that utilizes cheap and readily available sources of electrophilic halide. Both bromination and iodination are demonstrated and can be achieved on a variety of electron-rich and electron-poor boracarboxylated products and can subsequently undergo amination to provide an alternative route to β2-amino acid derivatives. Mechanistic experiments suggest that the β-carboxylic acid is required to achieve the activation of the C-B bond. Radical trapping experiments also indicate that this transformation may occur through the formation of an alkyl radical although this is unlikely.

Keywords

Boracarboxylation ; alkylboronic ester ; ; oxidative amination ; 2,3-diarylpropionic acid ; β2-amino acid

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