A strained aza-dibenzocyclooctyne was prepared via a high-yielding synthetic route. Copper-free, strain-promoted click reaction with azides showed excellent kinetics, and a functionalised aza-cyclooctyne was applied in fast and efficient PEGylation of enzymes.

The structural complexity of molecules isolated from biological sources has always served as an inspiration for organic chemists. Since the first synthesis of a natural product, urea, chemists have been challenged to prepare exact copies of natural structures in the laboratory. As a result, a broad repertoire of synthetic transformations has been developed over the years. It is now feasible to synthesize organic molecules of enormous complexity, and also molecules with less structural complexity but prodigious societal impact, such as nylon, TNT, polystyrene, statins, estradiol, XTC, and many more. Unfortunately, only a few chemical transformations are so mild and precise that they can be used to selectively modify biochemical structures, such as proteins or nucleic acids; these are the so-called bioconjugation strategies. Even more challenging is to apply a chemical reaction on or in living cells or whole organisms; these are the so-called bioorthogonal reactions. These fields of research are of particular importance because they not only pose a worthy challenge for chemists but also offer unprecedented possibilities for studying biological systems, especially in areas in which traditional biochemistry and molecular biology tools fall short. Recent years have seen tremendous growth in the chemical biology toolbox. In particular, a rapidly increasing number of bioorthogonal reactions has been developed based on chemistry involving strained alkenes or strained alkynes. Such strained unsaturated systems have the unique ability to undergo (3 + 2) and (4 + 2) cycloadditions with a diverse set of complementary reaction partners. Accordingly, chemistry centered around strain-promoted cycloadditions has been exploited to precisely modify biopolymers, ranging from nucleic acids to proteins to glycans. In this Account, we describe progress in bioconjugation centered around cycloadditions of these strained unsaturated systems. Being among the first to recognize the utility of strain-promoted cycloadditions between alkenes and dipoles, we highlight our report in 2007 of the reaction of oxanobornadienes with azides, which occurs through a sequential cycloaddition and retro Diels-Alder reaction. We further consider the subsequent refinement of this reaction as a valuable tool in chemical biology. We also examine the development of the reaction of cyclooctyne, the smallest isolable cyclic alkyne, with a range of substrates. Owing to severe deformation of the triple bond from ideal linear geometry, the cyclooctynes show high reactivity toward dienes, 1,3-dipoles, and other molecular systems. In the search for bioorthogonal reactions, cycloadditions of cyclic alkenes and alkynes have now established themselves as powerful tools in reagent-free bioconjugations.

β-Lactams are the most successful antibacterials, but their effectiveness is threatened by resistance, most importantly by production of serine- and metallo-β-lactamases (MBLs). MBLs are of increasing concern because they catalyze the hydrolysis of almost all β-lactam antibiotics, including recent-generation carbapenems. Clinically useful serine-β-lactamase inhibitors have been developed, but such inhibitors are not available for MBLs. l-Captopril, which is used to treat hypertension via angiotensin-converting enzyme inhibition, has been reported to inhibit MBLs by chelating the active site zinc ions via its thiol(ate). We report systematic studies on B1 MBL inhibition by all four captopril stereoisomers. High-resolution crystal structures of three MBLs (IMP-1, BcII, and VIM-2) in complex with either the l- or d-captopril stereoisomer reveal correlations between the binding mode and inhibition potency. The results will be useful in the design of MBL inhibitors with the breadth of selectivity required for clinical application against carbapenem-resistant Enterobacteriaceae and other organisms causing MBL-mediated resistant infections.

Multicomponent reactions (MCRs) are by far the most successful class of reactions leading to high structural diversity and molecular complexity through a single transformation. As part of the ongoing search for pharmacologically active lead structures, the obtained structural diversity allows for the fast exploration of a large chemical space. Not surprisingly, the development of MCRs, leading to new structural frameworks or serving as key transformations in the total synthesis of natural products, has expanded rapidly over the last few decades. To date a multitude of new three- and four-component reactions have already been described; however, examples of "higher-order" MCRs where five or even more components are combined in a single reaction vessel are remarkably scarce. This tutorial review aims to critically describe the developments achieved in recent years, charting the ideas, challenges, and milestone reactions that were essential for the progress of this field.

A robust, generally applicable, nongenetic technology is presented to convert monoclonal antibodies into stable and homogeneous ADCs. Starting from a native (nonengineered) mAb, a chemoenzymatic protocol allows for the highly controlled attachment of any given payload to the N-glycan residing at asparagine-297, based on a two-stage process: first, enzymatic remodeling (trimming and tagging with azide), followed by ligation of the payload based on copper-free click chemistry. The technology, termed GlycoConnect, is applicable to any IgG isotype irrespective of glycosylation profile. Application to trastuzumab and maytansine, both components of the marketed ADC Kadcyla, demonstrate a favorable in vitro and in vivo efficacy for GlycoConnect ADC. Moreover, the superiority of the native glycan as attachment site was demonstrated by in vivo comparison to a range of trastuzumab-based glycosylation mutants. A side-by-side comparison of the copper-free click probes bicyclononyne (BCN) and a dibenzoannulated cyclooctyne (DBCO) showed a surprising difference in conjugation efficiency in favor of BCN, which could be even further enhanced by introduction of electron-withdrawing fluoride substitutions onto the azide. The resulting mAb-conjugates were in all cases found to be highly stable, which in combination with the demonstrated efficacy warrants ADCs with a superior therapeutic index.