Research Article

Decarboxylative borylation

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Science  09 Jun 2017:
Vol. 356, Issue 6342, eaam7355
DOI: 10.1126/science.aam7355

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Swapping boron acids for carbon acids

Carbon-bound boronic acids and their esters are widely used as coupling partners to make carbon-carbon bonds. More recently, these chemicals have garnered pharmaceutical interest in their own right. Li et al. report a versatile nickel-catalyzed process to replace carboxylic acids with boronate esters by using a phthalimide activator. The reaction is well suited to late-stage modification of complex molecules. The authors used the approach to produce a potent in vitro inhibitor of human neutrophil elastase, a target of interest in treating inflammatory lung diseases.

Science, this issue p. eaam7355

Structured Abstract

INTRODUCTION

The boronic acid is a functional group of enormous utility in materials science, chemosensor development, and drug discovery. In medicinal chemistry, boronic acids have been harnessed as a replacement for various structural motifs (a bioisostere) to improve the potency or pharmacokinetic profiles of lead compounds. However, the widespread incorporation of alkyl boronic acids has been largely hampered by the challenges associated with their preparation. Consequently, only two alkyl boronic acids are currently in clinical use, namely Velcade and Ninlaro. Few methods are capable of delivering alkyl boronates from readily available starting materials; most exhibit modest functional group compatibility. Indeed, boronate motifs are often installed at the early stage of a synthesis and thus consume disproportionate effort from the standpoint of planning and manipulation in multistep processes.

RATIONALE

Alkyl carboxylic acids, as the most variegated chemical building blocks on Earth, are present in a myriad of natural products and medicines. They represent an ideal precursor to boronic acids. Previous efforts from our laboratory revealed that, through the intermediacy of simple redox-active esters (RAEs, e.g., N-hydroxyphthalimide esters), alkyl carboxylic acids could be harnessed as convenient alkyl halide surrogates in metal-catalyzed decarboxylative cross-coupling reactions with carbon nucleophiles, using the same activating principles as amide bond formation. It was therefore surmised that such reactivity could be exploited in a decarboxylative borylation process wherein structurally diverse and ever-present carboxylic acids could be converted directly into high-value boronic acids.

RESULTS

Through the exclusive use of N-hydroxyphthalimide RAEs, a simple means to convert carboxylic acids into boronate esters was enabled with an inexpensive nickel catalyst. This reaction was broad in scope (>40 examples) and demonstrated excellent functional group compatibility (tolerating alkyl/aryl halides, amides/carbamates, alcohols, ketones, and olefins), and high levels of diastereoselectivity, allowing transformations of densely functionalized drug molecules (e.g., vancomycin and Lipitor) and natural products (e.g., enoxolone) into the analogous boronic acids. This method’s unique capacity to access α-amino boronic acids from native peptides not only allowed the concise syntheses of both Velcade and Ninlaro, it also enabled the expedient discovery of three highly potent human neutrophil elastase (HNE) inhibitors, the most potent of which has shown improved in vitro inhibitory activities (IC50 = 15 pM, Ki = 3.7 pM) relative to leading candidates previously tested in clinical trials. Enzymatic and pharmacokinetic studies indicated high functional stability in physiologically relevant media.

CONCLUSION

The nickel-catalyzed decarboxylative cross-coupling of RAEs enables substitution of ubiquitous alkyl carboxylic acids with boronate esters using an inexpensive boron source: B2pin2 (Bpin = pinacol boronate). This process provides simple and practical access to complex boronic acids that were heretofore difficult to prepare. The wide diversity of useful reactivity that is exclusive to boronic acids, such as cross-coupling, oxidation, amination, and homologation, will open distinct possibilities in retrosynthetic analysis. This work may also accelerate the discovery and development of new boron-containing therapeutics.

Decarboxylative borylation.

Decarboxylative borylation replaces alkyl carboxylic acids with boronate esters (top) across a broad range of substrates (middle left), enabling convenient disconnections for synthesis (middle right) and leading to the discovery of potent elastase inhibitors (bottom).

Abstract

The widespread use of alkyl boronic acids and esters is frequently hampered by the challenges associated with their preparation. We describe a simple and practical method to rapidly access densely functionalized alkyl boronate esters from abundant carboxylic substituents. This broad-scope nickel-catalyzed reaction uses the same activating principle as amide bond formation to replace a carboxylic acid moiety with a boronate ester. Application to peptides allowed expedient preparations of α-amino boronic acids, often with high stereoselectivity, thereby facilitating synthesis of the alkyl boronic acid drugs Velcade and Ninlaro as well as a boronic acid version of the iconic antibiotic vancomycin. The reaction also enabled the discovery and extensive biological characterization of potent human neutrophil elastase inhibitors, which offer reversible covalent binding properties.

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