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The synthesis of polysubstituted spirocyclopropyl oxindoles using a series of rare-earth metal (REM) salts is reported. REMs, in particular Sc(OTf)3, allowed access to the target compounds by a multicomponent reaction with high diastereoselectivity (≤94:6:0:0). Density functional theory calculations on the model reaction are consistent with the observed selectivity and revealed that the special coordinating capabilities and the oxophilicity of the metal are key factors in inducing the formation of one main diastereoisomer. The authors thank Diana Cabrera, Sebastiaan van Liempd, and Juan M. Falcon-Perez from the CIC bioGUNE Metabolomics Platform for performing the UPLC-MS/MS analyses. The authors also acknowledge the Italian Ministry of University and Research (MUR) for Project SI.F.I.PA.CRO.DE. - Sviluppo e industrializzazione farmaci innovativi per terapia molecolare personalizzata PA.CRO.DE. (PON ARS01_00568, CUP: B29C20000360005) and for two doctoral grants. Moreover, the authors thank the University of Calabria and Calabria Region (PAC CALABRIA 2014–2020-Asse Prioritario 12, Azione B 10.5.12 CUP: H28D19000040006) for financial support. This research was also funded by MCIN/AEI/10.13039/501100011033 (Grant PID2021-125946OB-I00 to G.J.-O. and Severo Ochoa Excellence Accreditation CEX2021-001136-S to CIC bioGUNE).

This article presents an overview of recent computational studies dedicated to the analysis of binding between galectins and small-molecule ligands. We first present a summary of the most popular simulation techniques adopted for calculating binding poses and binding energies and then discuss relevant examples reported in the literature for the three main classes of galectins (dimeric, tandem, and chimera). We show that simulation of galectin-ligand interactions is a mature field that has proven invaluable for completing and unraveling experimental observations. Future perspectives to further improve the accuracy and cost-effectiveness of existing computational approaches will involve the development of new schemes to account for solvation and entropy effects, which represent the main current limitations to the accuracy of computational results.

Engineering of donor–acceptor light-harvesting systems based on geometrical features: applications in energy transfer & energy/photon upconversion.

A multidisciplinary study is presented to shed light on how pyrylium frameworks, as π–hole donors, establish π–π interactions. The combination of CSD analysis, computational modelling (ab intitio, DFT and MD simulations) and experimental NMR spectroscopy data provides essential information on the key parameters that characterize these intereactions, opening new avenues for further applications of this versatile heterocycle.

Lanthipeptides belong to the family of ribosomally synthesized and post-translationally modified peptides (RiPPs). The (methyl)lanthionine cross-links characteristic to lanthipeptides are essential for their stability and bioactivities. In most bacteria, lanthipeptides are maturated from single precursor peptides encoded in the corresponding biosynthetic gene clusters. However, cyanobacteria engage in combinatorial biosynthesis and encode as many as 80 substrate peptides with highly diverse sequences that are modified by a single lanthionine synthetase into lanthipeptides of different lengths and ring patterns. It is puzzling how a single enzyme could exert control over the cyclization processes of such a wide range of substrates. Here, we used a library of ProcA3.3 precursor peptide variants and show that it is not the enzyme ProcM but rather its substrate sequences that determine the regioselectivity of lanthionine formation. We also demonstrate the utility of trapped ion mobility spectrometry-tandem mass spectrometry (TIMS-MS/MS) as a fast and convenient method to efficiently separate lanthipeptide constitutional isomers, particularly in cases where the isomers cannot be resolved by conventional liquid chromatography. Our data allowed identification of factors that are important for the cyclization outcome, but also showed that there are no easily identifiable predictive rules for all sequences. Our findings provide a platform for future deep learning approaches to allow such prediction of ring patterns of products of combinatorial biosynthesis.

[Image: see text] Accurately determining the acid dissociation constants (K(a) or their logarithmic form, pK(a)) of small molecules and large biomolecules has proven to be pivotal for the study different biological processes and developing new drugs. This Viewpoint summarizes some of the most common methodologies and recent advances described for pK(a) prediction using computational techniques when experimental values are not easily accessible such as in proteins and/or for the screening of large libraries of new compounds.

Peptides containing variations of the β-amyloid hydrophobic core and five-membered sulfamidates derived from β-amino acid α-methylisoserine have been synthesized and fully characterized in the gas phase, solid state and in aqueous solution by a combination of experimental and computational techniques. The cyclic sulfamidate group effectively locks the secondary structure at the N-terminus of such hybrid peptides imposing a conformational restriction and stabilizing non-extended structures. This conformational bias, which is maintained in the gas phase, solid state and aqueous solution, is shown to be resistant to structure templating through assays of in vitro β-amyloid aggregation, acting as β-sheet breaker peptides with moderate activity.

Strained alkenes and alkynes are the predominant dienophiles used in inverse electron demand Diels–Alder (IEDDA) reactions. However, their instability, cross-reactivity, and accessibility are problematic. Unstrained dienophiles, although physiologically stable and synthetically accessible, react with tetrazines significantly slower relative to strained variants. Here we report the development of potassium arylethynyltrifluoroborates as unstrained dienophiles for fast, chemically triggered IEDDA reactions. By varying the substituents on the tetrazine (e.g., pyridyl- to benzyl-substituents), cycloaddition kinetics can vary from fast (k2 = 21 M–1 s–1) to no reaction with an alkyne-BF3 dienophile. The reported system was applied to protein labeling both in the test tube and fixed cells and even enabled mutually orthogonal labeling of two distinct proteins.

Chiral bicyclic

Protein conjugates are valuable tools for studying biological processes or producing therapeutics, such as antibody-drug conjugates. Despite the development of several protein conjugation strategies in recent years, the ability to modify one specific amino acid residue on a protein in the presence of other reactive side chains remains a challenge. We show that monosubstituted cyclopropenone (CPO) reagents react selectively with the 1,2-aminothiol groups of N-terminal cysteine residues to give a stable 1,4-thiazepan-5-one linkage under mild, biocompatible conditions. The CPO-based reagents, all accessible from a common activated ester CPO-pentafluorophenol (

New enzyme catalysts are usually engineered by repurposing the active sites of natural proteins. Here we show that design and directed evolution can be used to transform a non-natural, functionally naive zinc-binding protein into a highly active catalyst for an abiological hetero-Diels–Alder reaction. The artificial metalloenzyme achieves >104 turnovers per active site, exerts absolute control over reaction pathway and product stereochemistry, and displays a catalytic proficiency (1/KTS = 2.9 × 1010 M−1) that exceeds all previously characterized Diels–Alderases. These properties capitalize on effective Lewis acid catalysis, a chemical strategy for accelerating Diels–Alder reactions common in the laboratory but so far unknown in nature. Extension of this approach to other metal ions and other de novo scaffolds may propel the design field in exciting new directions. A de novo designed zinc-binding protein has been converted into a highly active, stereoselective catalyst for a hetero-Diels–Alder reaction. Design and directed evolution were used to effectively harness Lewis acid catalysis and create an enzyme more proficient than other reported Diels–Alderases.

A biotinylated phenazine compound as a phenosafranin conjugate (Biot-PSF) was synthesized and reported for the first time. The new conjugate was found to be useful as a chemotherapeutic agent with a cancer targeting unit for imaging. We conjugated a molecule recognition moiety with the photosensitive phenosafranin (PSF) to apply the innovative compound in antineoplastic therapy and diagnostics. A new Biot-PSF sensing (bio)probe was established as a biotin-avidin ligand receptor. Thus, Biot-PSF can be an efficient pre-targeting agent for therapeutic and imaging applications. Moreover, the Biot-PSF conjugate exhibits good activity against colorectal carcinoma (HCT116) and adenocarcinoma (COLO 205, SW620) cancer cell lines.

Glycosylations promoted by triflate-generating reagents are widespread synthetic methods for the construction of glycosidic scaffolds and glycoconjugates of biological and chemical interest. These processes are thought to proceed with the participation of a plethora of activated high energy intermediates such as the α- and β-glycosyl triflates, or even increasingly unstable glycosyl oxocarbenium-like species, among which only α-glycosyl triflates have been well characterized under representative reaction conditions. Interestingly, the remaining less accessible intermediates, yet to be experimentally described, seem to be particularly relevant in α-selective processes, involving weak acceptors. Herein, we report a detailed analysis of several paradigmatic and illustrative examples of such reactions, employing a combination of chemical, NMR, kinetic and theoretical approaches, culminating in the unprecedented detection and quantification of the true β-glycosyl triflate intermediates within activated donor mixtures. This achievement was further employed as a stepping-stone for the characterization of the triflate anomerization dynamics, which along with the acceptor substitutions, govern the stereochemical outcome of the reaction. The obtained data conclusively show that, even for highly dissociative reactions involving β-close ion pair (β-CIP) species, the formation of the α-glycoside is necessarily preceded by a bimolecular α → β triflate interconversion, which under certain circumstances becomes the rate-limiting step. Overall, our results rule out the prevalence of the Curtin-Hammett fast-exchange assumption for most glycosylations and highlight the distinct reactivity properties of α- and β-glycosyl triflates against neutral and anionic acceptors., A.G.S. would like to thank the EU commission for a MSCA-IF postdoctoral fellowship. L.M.-J and L.D.-C thank the Ministerio de Economía , Industria y Competitividad and the Ministerio de Ciencia, Innovación y Universidades the for their FPI fellowships (BES-2014-070232 and BES-2017-080618). This investigation was supported by research grants from the Spanish Ministry of Science and Innovation (CTQ2016−79255-P, PID2019−107476GB-I00, BFU2017−89707-P, RTI2018−094862-B-100, RTI2018−099592−B-C21, PID2019−104090RB-100, RTI2018−094751−B-C22, and RTI2018−099592−B-C22), the Mizutani Foundation forGlycoscience (200077 and 170045), the Aragon Government (Groups 17R-34) and the computational resources from BIFI-ZCAM (Universidad de Zaragoza, Spain).

Selenium plays a crucial role in different biological processes, being necessary for theproper functioning of the human body. Therefore, selenium compounds have becomemolecules of great interest due to their antiviral and anticancer activities and their use as naturalfood supplements.Selenocysteine (Sec) is the 21st genetically encoded amino acid and plays an importantrole in protein folding and stability, conferring interesting redox properties. In addition, protectedselenocysteine derivatives are used as precursors of dehydroalanine, which allows theintroduction of various post-translational modifications. On the other hand, some aryl derivativesof Se serve as chemical models for the inhibition of selenoenzymes, which has implications forcancer therapy. Beyond applications in bioconjugation, selenoamino acids are especiallyrelevant in Native Chemical Ligation (NCL).[1]Although the nucleophilic substitution reaction has been widely employed for thesynthesis of these Sec derivatives, 1,4-conjugate addition has been less explored, especiallythe stereoselective 1,4-conjugate addition of Se-nucleophiles to chiral Michael acceptors.Therefore, in this work, we aim to extend the methodology established by our research group[2]to the synthesis of enantiomerically pure selenoamino acids. The key step of such synthesis isthe attack of different Se-nucleophiles to a chiral dehydroalanine, to obtain a singlediastereoisomer. The methodology is simple and does not require the use of any catalyst,providing excellent yields at room temperature. Subsequent facile acid hydrolysis of the abovediastereoisomers leads to the corresponding selenoamino acids

A multidisciplinary study is presented to shed light on how pyrylium frameworks, as π-hole donors, establish π-π interactions. The combination of CSD analysis, computational modelling (

An azanorbornadiene bromovinyl sulfone reagent for cysteine-selective bioconjugation has been developed. Subsequent reaction with dipyridyl tetrazine leads to bond cleavage and formation of a pyrrole-linked conjugate. The latter involves ligation of the tetrazine to the azanorbornadiene-tagged protein through inverse electron demand Diels-Alder cycloaddition with subsequent double retro-Diels-Alder reactions to form a stable pyrrole linkage. The sequence of site-selective bioconjugation followed by bioorthogonal bond cleavage was efficiently employed for the labelling of three different proteins. This method benefits from easy preparation of these reagents, selectivity for cysteine, and stability after reaction with a commercial tetrazine, which has potential for the routine preparation of protein conjugates for chemical biology studies.

An azanorbornadiene bromovinyl sulfone reagent for cysteine‐selective bioconjugation has been developed. Subsequent reaction with dipyridyl tetrazine leads to bond cleavage and formation of a pyrrole‐linked conjugate. The latter involves ligation of the tetrazine to the azanorbornadiene‐tagged protein through inverse electron demand Diels–Alder cycloaddition with subsequent double retro‐Diels–Alder reactions to form a stable pyrrole linkage. The sequence of site‐selective bioconjugation followed by bioorthogonal bond cleavage was efficiently employed for the labelling of three different proteins. This method benefits from easy preparation of these reagents, selectivity for cysteine, and stability after reaction with a commercial tetrazine, which has potential for the routine preparation of protein conjugates for chemical biology studies.

The highly diastereoselective 1,4-conjugate additions of several nitrogen nucleophiles to chiral bicyclic dehydroalanines have been assessed effectively at room temperature in good to excellent yields without needing any catalyst or additional base. This methodology is general, simple, oxygen and moisture tolerant, high-yielding, totally chemo- and stereoselective. This procedure offers an efficient and practical approach for the synthesis of Nβ-substituted α,β-diamino acids, such as 1-isohistidine, τ-histidinoalanine, β-benzylaminoalanine, β-(piperidin-1-yl)alanine, β-(azepan-1-yl)alanine, and fluorescent and ciprofloxacin-containing amino acid derivatives.

The interaction of human galectin-8 and its two separate N-terminal and C-terminal carbohydrate recognition domains (CRD) to their natural ligands has been analysed using a synergistic combination of experimental NMR and ITC methods, and molecular dynamics simulations. Both domains bind the minimal epitopes N-acetyllactosamine (1) and Galβ1–3GalNAc (2) in a similar manner. However, the N-terminal and C-terminal domains show exquisite and opposing specificity to bind either Neu5Ac- or Fuc-containing ligands, respectively. Moreover, the addition of the high-affinity ligands specific for one of the CRDs does not make any effect on the binding at the alternative one. Thus, the two CRDs behave independently and may simultaneously target different molecular entities to promote clustering through the generation of supramolecular assemblies., NMR, ITC, and MD data show that the two domains of human galectin-8 independently recognize sialyl- and fucosyl-containing glycans.

Cysteine plays a major role in the redox homeostasis and antioxidative defense mechanisms of many parasites of the phylum Apicomplexa. Of relevance to human health is Toxoplasma gondii, the causative agent of toxoplasmosis. A major route of cysteine biosynthesis in this parasite is the reverse transsulfuration pathway involving two key enzymes cystathionine b-synthase (CBS) and cystathionine c-lyase (CGL). CBS from T. gondii (TgCBS) catalyzes the pyridoxal-5´ -phosphate-dependent condensation of homocysteine with either serine or O-acetylserine to produce cystathionine. The enzyme can perform alternative reactions that use homocysteine and cysteine as substrates leading to the endogenous biosynthesis of hydrogen sulfide, another key element in maintaining the intracellular redox equilibrium. In contrast with human CBS, TgCBS lacks the N-terminal heme binding domain and is not responsive to Sadenosylmethionine. Herein, we describe the structure of a TgCBS construct that lacks amino acid residues 466-491 and shows the same activity of the native protein. TgCBS D466-491 was determined alone and in complex with reaction intermediates. A complementary molecular dynamics analysis revealed a unique domain organization, similar to the pathogenic mutant D444N of human CBS. Our data provides one missing piece in the structural diversity of CBSs by revealing the so far unknown three-dimensional arrangement of the CBS-type of Apicomplexa. This domain distribution is also detected in yeast and bacteria like Pseudomonas aeruginosa. These results pave the way for understanding the mechanisms by which TgCBS regulates the intracellular redox of the parasite, and have far-reaching consequences for the functional understanding of CBSs with similar domain distribution. 2021 The Authors. Published by Elsevier B.V. on behalf of Research Network of Computational and Structural Biotechnology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/

A cytochrome c heme protein was recently engineered to catalyze the formation of carbon–silicon bonds via carbene insertion into Si–H bonds, a reaction that was not previously known to be catalyzed by a protein. High chemoselectivity towards C–Si bond formation over competing C–N bond formation was achieved, although this trait was not screened for during directed evolution. Using computational and experimental tools, we now establish that activity and chemoselectivity are modulated by conformational dynamics of a protein loop that covers the substrate access to the iron-carbene active species. Mutagenesis of residues computationally predicted to control the loop conformation altered the protein’s chemoselectivity from preferred silylation to preferred amination of a substrate containing both N–H and Si–H functionalities. We demonstrate that information on protein structure and conformational dynamics, combined with knowledge of mechanism, leads to understanding of how non-natural and selective chemical transformations can be introduced into the biological world.

Colorectal cancer pathogenesis and progression isassociated with the presence of Fusobacteriumnucleatum and the reduction of acetylated deriva-tives of spermidine, as well as dietary componentssuch as tannin-rich foods. We show that a new tan-nase orthologue of F. nucleatum (TanBFnn) has sig-nicant structural differences with its Lactobacillusplantarum counterpart affecting the ap covering theactive site and the accessibility of substrates. Crys-tallographic and molecular dynamics analysisrevealed binding of polyamines to a small cavity thatconnects the active site with the bulk solvent whichinteract with catalytically indispensable residues. Asa result, spermidine and its derivatives, particularlyN8-acetylated spermidine, inhibit the hydrolytic activ-ity of TanBFnnand increase the toxicity of gallotan-nins to F. nucleatum. Our results support a model inwhich the balance between the detoxicant activity ofTanBFnnand the presence of metabolic inhibitorscan dictate either conducive or unfavourable condi-tions for the survival of F. nucleatum., Supported by grants from the Spanish Ministry of Science and Innovation (MCI) cofinanced with FEDER funds (SAF2015‐65327‐R and RTI2018‐096494‐B‐100 to JA; AGL2017‐86757‐R to LA, SAF2015‐73549‐JIN to HR; RTI2018‐099592‐B‐C22 to GJO) and the Mizutani Foundation for Glycoscience (200077 to GJO). LA and GJO are supported by the Ramon y Cajal program (RYC‐2013‐13666 and RYC‐2013‐14706 respectively). JTC and AP are the recipients postdoctoral fellowships from the Basque Government. DB is the recipient of a MCI FPI fellowship. APC is the recipient of a fellowship from the University of the Basque Country. We thank the MCI for the Severo Ochoa Excellence accreditation (SEV‐2016‐0644) and the Basque Department of Industry, Tourism and Trade (Etortek and Elkartek programs). JMM thanks the ALBA synchrotron for providing access time to the BL‐13 XALOC beamline. This work is supported by grants from the Jesús de Gangoiti Barrera Foundation

Cystathionine β-synthase (CBS), the pivotal enzyme of the reverse transsulfuration pathway, catalyzes the pyridoxal-5’-phosphate-dependent condensation of serine with homocysteine to form cystathionine. Additionally, CBS performs alternative reactions that use homocysteine and cysteine as substrates leading to the endogenous biosynthesis of hydrogen sulfide (H2S), an important signal transducer in many physiological and pathological processes. Toxoplasma gondii, the causative agent of toxoplasmosis, encodes a functional CBS (TgCBS) that contrary to human CBS, is not allosterically regulated by S-adenosylmethionine and can use both, Ser and O-acetylserine (OAS) as substrates. TgCBS is also strongly implicated in the production of H2S, and thus involved in redox homeostasis of the parasite. Here, we report its crystal structure, the first CBS from a protozoan described so far. Our data reveals a basal-like fold that unexpectedly differs from the active conformations found in other organisms, but structurally similar to the pathogenic activated mutant D444N of the human enzyme.

The first totally chemo- and diastereoselective 1,4-conjugate additions of

Protein conjugates are valuable tools to create therapeutics, such as antibody-drug conjugates, or to study biological processes. Despite a number of protein conjugation strategies having been developed over recent years, the ability to modify one specific amino acid on a protein in the presence of other side chains with similar reactivity remains a challenge. We used the reaction between a monosubstituted cyclopropenone (CPO) probe and the 1,2-aminothiol of an N-terminal cysteine to give a stable 1,4-thiazepa-5-none linkage under mild, biocompatible conditions. The method relies on the ability of cyclopropenones to ring-open after sequential sulfhydryl and α-amine conjugation to be truly specific for N-terminal cysteine. We show that our CPO probes selectively label N-terminal cysteine containing peptides and proteins even in the presence of internal, solvent-exposed cysteines, which can be subsequently modified by using conventional cysteine modification reagents. The ability to distinguish and specifically target N-terminal cysteine residues on a protein will facilitate the construction of elaborate multi-labelled bioconjugates.

Galactosaminogalactan (GAG) is a prominent cell wall component of Aspergillus fumigatus, an opportunistic fungal pathogen, and plays an important role in host infection and invasion. GAG is a heteropolysaccharide composed of a-1,4-linked galactose, galactosamine and N-acetylgalactosamine residues. To probe the biosynthesis of this complex biopolymer and generate structures for diagnostic and therapeutic applications, a library of GAG-fragments was constructed featuring specimens containing a-galactose-, a-galactosamine and a-N-acetyl galactosamine linkages. Key features of the synthetic strategy include the use of di-tert-butylsilylidene directed a-galactosylation methodology and regioselective benzoylation reactions using benzoyl-hydroxybenzotriazole (Bz-OBt). Structural analysis of the Gal, GalN and GalNAc oligomers by a combination of NMR and MD approaches revealed that the oligomers adopt an elongated, almost straight, structure, stabilized by inter-residue H-bonds, one of which is a non-conventional C-H····O hydrogen bond between H5 of the residue (i+1) and O3 of the residue (i). The structures position the C-2 substituents almost perpendicular to the oligosaccharide main chain axis, pointing to the bulk solvent and available for interactions with antibodies or other binding partners.

Duramycin is a heavily post-translationally modified peptide that binds phosphatidylethanolamine. It has been investigated as an antibiotic, an inhibitor of viral entry, a therapeutic for cystic fibrosis, and a tumor and vasculature imaging agent. Duramycin contains a β-hydroxylated Asp (Hya) and four macrocycles, including an essential lysinoalanine (Lal) cross-link. The mechanism of Lal formation is not known. Here we show that Lal is installed stereospecifically by DurN via addition of Lys19 to a dehydroalanine. The structure of DurN reveals an unusual dimer with a new fold. Surprisingly, in the structure of duramycin bound to DurN, no residues of the enzyme are near the Lal cross-link. Instead, Hya15 of the substrate makes interactions with Lal, suggesting it acts as a base to deprotonate Lys19 during catalysis. Biochemical data suggest that DurN preorganizes the reactive conformation of the substrate, such that the Hya15 of the substrate can serve as the catalytic base for Lal formation. During the biosynthesis of the lanthipeptide duramycin, DurN catalyzes stereospecific lysinoalanine formation by preorganizing the reactive conformation of the substrate, such that one of the substrate’s own residues serves as the catalytic base.

Neisseria meningitidis serogroup A (MenA) is an aerobic diplococcal Gram-negative bacterium responsible for epidemic meningitis disease. Its capsular polysaccharide (CPS) has been identified as the primary virulence factor of MenA. This polysaccharide suffers from chemical lability in water. Thus, the design and synthesis of novel and hydrolytically stable structural analogues of MenA CPS may provide additional tools for the development of therapies against this disease. In this context, the structural features of the natural phosphorylated monomer have been analyzed and compared to those of its carba-analogue, where the endocyclic oxygen has been replaced by a methylene moiety. The lowest energy geometries of the different molecules have been calculated using a combination of quantum mechanical techniques and molecular dynamics simulations. The predicted results have been compared and validated using NMR experiments. The results indicate that the more stable designed glycomimetics may adopt the conformation adopted by the natural monomer, although they display a wider flexibility around the torsional degrees of freedom.

The thermal decompositions of metallaisoxazolin-5-ones containing Ir, Rh, or Co are investigated using density functional theory. The experimentally observed decarboxylations of these molecules are found to proceed through retro-(3+2)-cycloaddition reactions, generating the experimentally reported η2 side-bonded nitrile complexes. These intermediates can isomerize in situ to yield a η1 nitrile complex. A competitive alternative pathway is also found where the decarboxylation happens concertedly with an aryl migration process, producing a η1 isonitrile complex. Despite their comparable stability, these η1 bonded species were not detected experimentally. The experimentally detected η2 side bound species are likely involved in the subsequent C–H activation reactions with hydrocarbon solvents reported for some of these metallaisoxazolin-5-ones.

The reactions between low-valent Rh(I) and Ir(I) metal-carbonyl complexes and arylnitrile oxides possess the electronic and structural features of 1,3-dipolar cycloadditions. Density functional theory (DFT) calculations on these reactions, involving both cyclopentadienyl and carboranyl ligands on the metal carbonyl, explain the ease of the chemical processes and the stabilities of the resulting metallaisoxazolin-5-ones. The metal-carbonyl bond has partial double bond character according to the Wiberg index calculated through NBO analysis, and so the reaction can be considered a normal 1,3-dipolar cycloaddition involving M═C bonds. The rates of formation of the metallacycloadducts are controlled by distortion energy, analogous to their organic counterparts. The superior ability of anionic Ir complexes to share their electron density and accommodate higher oxidation states explains their calculated higher reactivity toward cycloaddition, as compared to Rh analogues.