Catalytic oxidations of steroid substrates by artificial cytochrome p-450 enzymes

Remote functionalization reactions in steroids: discovery and application

Research published between 2001 and 2022 on the functionalization of remote positions of steroids, as well as the use of this technique in the generation of biologically active compounds has been reviewed. In the first section of the analysis established and novel methods for activation of sites deemed to be remote were reported. A series of manganese- (mainly), rhodium-, ruthenium- and osmium-centered porphyrins as catalysts in the presence of PIDA as oxidant have effected hydroxylation at C-1, -5, -6, -7, -11, -14, -15, -16, -17, -20, -24 and -25. Dioxiranes have been utilized in inserting hydroxyl groups at the 5, 12, 14, 15, 16, 17, 20, 24 and 25 positions (tertiary centers for the most part). Alcohols at C-12 and -16 were oxidized further to ketones. The Schönecker oxidation, discovered and developed during the period, has revolutionized the selective functionalization at C-12 of steroids possessing a 17-keto group. In the presence of iron-centered PDP- and MCP-based catalysts, hydrogen peroxide and acetic acid, substrates tended to be hydroxylated at C-6 and -12, with further oxidation to ketones often accompanying this reaction. The hypohalite reaction, utilizing the more modern Suarez conditions (irradiation in the presence of iodine and PIDA), was reported to facilitate the insertion of a hydroxyl moiety five atoms away from an existing alcohol oxygen. Steroidal-3β-diazoacetates tend to decompose on heating with di-rhodium-centered catalysts while activating carbons four or five atoms away. Chromium- and iron-based acetates were observed to functionalize C-5 and -25. Other reactions involving ring cleavage and halogenation, ketone irradiation and α-hydroxylation of ethers were also covered. The syntheses of compounds with marked biological activity from readily available steroids is described in the second section of the study. Cyclopamine, cephalostatin-1, ritterazine B and three polyhydroxypregnanaes (pergularin, utendin and tomentogenin) were generated in sequences in which a key step required hydroxylation at C-12 using the Schönecker reaction. A crucial stage in the preparation of cortistatin A, the saundersioside core, eurysterol A, 5,6-dihydroglaucogenin C, as well as clinostatins A and B involved the functionalization of C-18 or -19 utilizing hypohalite chemistry. The synthetic route to xestobergsterol A, pavonin-4-aglycone and ouagabagenin included a transformation where ketone irradiation played a part in either producing a Δ14 or a C-19 activated steroid. The radical relay reaction, where a 17α-chloro-steroid was formed, was central in the generation of pythocholic acid. The lead tetraacetate reaction was pivotal in the functionalization of C-19 during the synthesis of cyclocitrinol.

Biotransformation of metenolone acetate and epiandrosterone by fungi and evaluation of resulting metabolites for aromatase inhibition

The present study describes the microbial transformation of anabolic drugs, metenolone acetate (1), and epiandrosterone (6). Three new metabolites, 6β,17β-dihydroxy-1-methyl-3-oxo-5α-androst-1-en (2), 5α,15α-dihydroxy-1-methyl-3-oxo-1-en-17-yl acetate (3), 15β-hydroxy-1-methyl-3-oxo-5α-androst-1,4-dien-17-yl acetate (4), and a known metabolite, 17β-hydroxy-1-methyl-4-androstadiene-3-one (5) were obtained by biotransformation of metenolone acetate (1) via Trametes hirsuta mushroom. Metabolites 7, and 8 were obtained from the incubation of epiandrosterone (6) with Cunninghamella blakesleeana. While bioconversion of compound 6 with Aspergillus alliaceus yielded seven known metabolites 9-15. Modern spectroscopic techniques were employed for the structure elucidation of biotransformed products. All compounds were evaluated for their aromatase inhibitory activity. Among them, new metabolite 3 exhibited a significant human placental aromatase activity with an IC50 = 19.602 ± 0.47 µM, as compared to standard anti-cancer drug exemestane (IC50 = 0.232 ± 0.031 µM), whereas, metabolite 5 (IC50 = 0.0049 ± 0.0032 µM) exhibited a very potent activity. While substrate 6, and metabolites 2, 7, and 9 were found inactive. Aromatase plays a key role in the biosynthesis of estrogen hormone, responsible for cancer cell proliferation. Its inhibition is therefore targeted for the treatment of ER + breast cancer. Further structural modifications (lead optimization) of compound 3 can lead to more potent aromatase inhibition for possible treatment of ER + breast cancer.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Site-selective methylene C-H oxidation of an alkyl diamine enabled by supramolecular recognition using a bioinspired manganese catalyst

Site-selective oxidation of aliphatic C-H bonds is a powerful synthetic tool because it enables rapid build-up of product complexity and diversity from simple precursors. Besides the poor reactivity of alkyl C-H bonds, the main challenge in this reaction consists in differentiating between the multiple similar sites present in most organic molecules. Herein, a manganese oxidation catalyst equipped with two 18-benzo-6-crown ether receptors has been employed in the oxidation of the long chain tetradecane-1,14-diamine. 1H-NMR studies evidence simultaneous binding of the two protonated amine moieties to the crown ether receptors. This recognition has been used to pursue site-selective oxidation of a methylenic site, using hydrogen peroxide as oxidant in the presence of carboxylic acids as co-ligands. Excellent site-selectivity towards the central methylenic sites (C6 and C7) is observed, overcoming selectivity parameters derived from polar deactivation by simple amine protonation and selectivity observed in the oxidation of related monoprotonated amines.

Tweezer-Based C-H Oxidation Catalysts Overriding the Intrinsic Reactivity of Aliphatic Ammonium Substrates

The site-selective C-H oxygenation of alkyl chains as well as deactivated positions remains a great challenge for chemists. Here, we report the synthesis and application of four new supramolecular tweezer-based oxidation catalysts. They consist of the well-explored M(pdp/mcp) oxidation moiety and a molecular tweezer capable of binding ammonium salts. All catalysts display preferential oxidation of the strongly deactivated C3/C4 positions, however to different degrees. Furthermore, the best performing catalyst Fe(pdp)Twe was explored with an expanded substrate scope. It was demonstrated that the deactivated positions C3/C4 are also preferentially oxidized in these cases.

Three-in-one: Miniature Models of Natural Acyl-transfer Systems Enable Vector-selective Reaction on the Primary Side of Cyclodextrins

Miniature models of acyl-transfer systems in cells, which were composed by replacing the protein, coenzyme and substrate with CD, functional group, and CD, respectively, and combining them all together in one, displayed definite role-sharing and exact cooperation of the functional groups and hydrophobic cavity, and thus enabled the regio-specific reaction.

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

Detection of anabolic steroids via cyclodextrin-promoted fluorescence modulation

Reported herein is the detection of anabolic steroids through the use of cyclodextrin-promoted interactions between the analyte of interest and a high quantum yield fluorophore, which lead to measurable, analyte-specific changes in the fluorophore emission signal. By using a variety of β-cyclodextrin derivatives (unmodified β-cyclodextrin, methyl-β-cyclodextrin, and 2-hydroxypropyl-β-cyclodextrin) in combination with high quantum yield fluorophore rhodamine 6G, we detected five anabolic steroid analytes with 100% differentiation between structurally similar analytes and micromolar level limits of detection. Overall, these results show significant potential in the development of practical, fluorescence-based steroid detection devices.

Porphyrinoid-Cyclodextrin Assemblies in Biomedical Research: An Update

Porphyrinoids, well-known cofactors in fundamental processes of life, have stimulated interest as synthetic models of natural systems and integral components of photodynamic therapy, but their utilization is compromised by self-aggregation in aqueous media. The capacity of cyclodextrins to include hydrophobic molecules in their cavity provides porphyrinoids with a protective environment against oxidation and the ability to disperse efficiently in biological fluids. Moreover, engineered cyclodextrin-porphyrinoid assemblies enhance the photodynamic abilities of porphyrinoids, can carry chemotherapeutics for synergistic modalities, and can be enriched with functions including cell recognition, tissue penetration, and imaging. This Perspective includes synthetic porphyrinoid-cyclodextrin models of proteins participating in fundamental processes, such as enzymatic catalysis, respiration, and electron transfer. In addition, since porphyrinoid-cyclodextrin systems comprise third generation photosensitizers, recent developments for their utilization in photomedicine, that is, multimodal therapy for cancer (e.g., PDT, PTT) and antimicrobial treatment, and eventually in biocompatible therapeutic or diagnostic platforms for next-generation nanomedicine and theranostics are discussed.

Bacterial terpene biosynthesis: challenges and opportunities for pathway engineering

Terpenoids are the largest and structurally most diverse class of natural products. They possess potent and specific biological activity in multiple assays and against diseases, including cancer and malaria as notable examples. Although the number of characterized terpenoid molecules is huge, our knowledge of how they are biosynthesized is limited, particularly when compared to the well-studied thiotemplate assembly lines. Bacteria have only recently been recognized as having the genetic potential to biosynthesize a large number of complex terpenoids, but our current ability to associate genetic potential with molecular structure is severely restricted. The canonical terpene biosynthetic pathway uses a single enzyme to form a cyclized hydrocarbon backbone followed by modifications with a suite of tailoring enzymes that can generate dozens of different products from a single backbone. This functional promiscuity of terpene biosynthetic pathways renders terpene biosynthesis susceptible to rational pathway engineering using the latest developments in the field of synthetic biology. These engineered pathways will not only facilitate the rational creation of both known and novel terpenoids, their development will deepen our understanding of a significant branch of biosynthesis. The biosynthetic insights gained will likely empower a greater degree of engineering proficiency for non-natural terpene biosynthetic pathways and pave the way towards the biotechnological production of high value terpenoids.

Continued Progress towards Efficient Functionalization of Natural and Non-natural Targets under Mild Conditions: Oxygenation by C-H Bond Activation with Dioxirane

The successful isolation and characterization of a dioxirane species in 1988 opened up one of the most attractive methods for the efficient oxidation of simple and/or structurally complex molecules. Dioxirane today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. They were quickly recognized as efficient oxygen transfer agents, especially for epoxidations and for a wide range of O-insertion reactions into C-H bonds. Dioxirane possess catalytic activity and appear as highly (chemo-, regio-, and stereo-) selective oxidants, despite their reactivity under mild and strictly neutral conditions being controlled by a combination of steric and electronic factors. In this review, we discuss some of the most recent and significant developments in the selective homogeneous and heterogeneous oxyfunctionalization of non-activated C-H bonds in hydrocarbons of natural and non-natural targets by using isolated dioxirane or, more generally, by using the ketones (i.e., the dioxirane precursors) as organocatalysts.

Methylene chain ruler for evaluating the regioselectivity of a substrate-recognising oxidation catalyst

Regioselective C-H oxidation of aliphatic molecules with synthetic catalysts is challenging. We incorporated substrate-recognition sites into a ruthenium porphyrin-heteroaromatic N-oxide catalytic system in order to characterise its regioselectivity for the oxidation of alkanes. This substrate-recognition catalytic reaction exhibits high regioselectivity and high reaction efficiency.

Arylation Chemistry for Bioconjugation

Bioconjugation chemistry has been used to prepare modified biomolecules with functions beyond what nature intended. Central to these techniques is the development of highly efficient and selective bioconjugation reactions that operate under mild, biomolecule compatible conditions. Methods that form a nucleophile-sp2 carbon bond show promise for creating bioconjugates with new modifications, sometimes resulting in molecules with unparalleled functions. Here we outline and review sulfur, nitrogen, selenium, oxygen, and carbon arylative bioconjugation strategies and their applications to modify peptides, proteins, sugars, and nucleic acids.

Beyond the Second Coordination Sphere: Engineering Dirhodium Artificial Metalloenzymes To Enable Protein Control of Transition Metal Catalysis

Transition metal catalysis is a powerful tool for chemical synthesis, a standard by which understanding of elementary chemical processes can be measured, and a source of awe for those who simply appreciate the difficulty of cleaving and forming chemical bonds. Each of these statements is amplified in cases where the transition metal catalyst controls the selectivity of a chemical reaction. Enantioselective catalysis is a challenging but well-established phenomenon, and regio- or site-selective catalysis is increasingly common. On the other hand, transition-metal-catalyzed reactions are typically conducted under highly optimized conditions. Rigorous exclusion of air and water is common, and it is taken for granted that only a single substrate (of a particular class) will be present in a reaction, a desired site selectivity can be achieved by installing a directing group, and undesired reactivity can be blocked with protecting groups. These are all reasonable synthetic strategies, but they also highlight limits to catalyst control. The utility of transition metal catalysis could be greatly expanded if catalysts possessed the ability to regulate which molecules they encounter and the relative orientation of those molecules. The rapid and widespread adoption of stoichiometric bioorthogonal reactions illustrates the utility of robust reactions that proceed with high selectivity and specificity under mild reaction conditions. Expanding this capability beyond preprogrammed substrate pairs via catalyst control could therefore have an enormous impact on molecular science. Many metalloenzymes exhibit this level of catalyst control, and directed evolution can be used to rapidly improve the catalytic properties of these systems. On the other hand, the range of reactions catalyzed by enzymes is limited relative to that developed by chemists. The possibility of imparting enzyme-like activity, selectivity, and evolvability to reactions catalyzed by synthetic transition metal complexes has inspired the creation of artificial metalloenzymes (ArMs). The increasing levels of catalyst control exhibited by ArMs developed to date suggest that these systems could constitute a powerful platform for bioorthogonal transition metal catalysis and for selective catalysis in general. This Account outlines the development of a new class of ArMs based on a prolyl oligopeptidase (POP) scaffold. Studies conducted on POP ArMs containing a covalently linked dirhodium cofactor have shown that POP can impart enantioselectivity to a range of dirhodium-catalyzed reactions, increase reaction rates, and improve the specificity for reaction of dirhodium carbene intermediates with targeted organic substrates over components of cell lysate, including bulk water. Several design features of these ArMs enabled their evolution via random mutagenesis, which revealed that mutations throughout the POP scaffold, beyond the second sphere of the dirhodium cofactor, were important for ArM activity and selectivity. While it was anticipated that the POP scaffold would be capable of encapsulating and thus controlling the selectivity of bulky cofactors, molecular dynamics studies also suggest that POP conformational dynamics plays a role in its unique efficacy. These advances in scaffold selection, bioconjugation, and evolution form the basis of our ongoing efforts to control transition metal reactivity using protein scaffolds with the goal of enabling unique synthetic capabilities, including bioorthogonal catalysis.

Mechanistic Insight into Catalytic Redox-Neutral C-H Bond Activation Involving Manganese(I) Carbonyls: Catalyst Activation, Turnover, and Deactivation Pathways Reveal an Intricate Network of Steps

Manganese(I) carbonyl-catalyzed C-H bond functionalization of 2-phenylpyridine and related compounds containing suitable metal directing groups has recently emerged as a potentially useful synthetic methodology for the introduction of various groups to the ortho position of a benzene ring. Preliminary mechanistic studies have highlighted that these reactions could proceed via numerous different species and steps and, moreover, potentially different catalytic cycles. The primary requirement for typically 10 mol % catalyst, oftentimes the ubiquitous precursor catalyst, BrMn(CO)₅, has not yet been questioned nor significantly improved upon, suggesting catalytic deactivation may be a serious issue to be understood and resolved. Several critical questions are further raised by the species responsible for providing a source of protons in the protonation of vinyl-manganese(I) carbonyl intermediates. In this study, using a combination of experimental and theoretical methods, we provide comprehensive answers to the key mechanistic questions concerning the Mn(I) carbonyl-catalyzed C-H bond functionalization of 2-phenylpyridine and related compounds. Our results enable the explanation of alkyne substrate dependencies, i.e., internal versus terminal alkynes. We found that there are different catalyst activation pathways for BrMn(CO)₅, e.g., terminal alkynes lead to the generation of Mn^I-acetylide species, whose formation is reminiscent of Cu^I-acetylide species proposed to be of critical importance in Sonogashira cross-coupling processes. We have unequivocally established that alkyne, 2-phenylpyridine, and water can facilitate hydrogen transfer in the protonation step, leading to the liberation of protonated alkene products.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

A manganese porphyrin–α-cyclodextrin conjugate as an artificial enzyme for the catalytic epoxidation of polybutadiene

A manganese porphyrin–α-cyclodextrin conjugate was designed as an artificial clamp-like enzyme to catalyze the epoxidation of cis-polybutadiene with trans-epoxide preference.

Ir-Catalyzed ortho-Borylation of Phenols Directed by Substrate-Ligand Electrostatic Interactions: A Combined Experimental/in Silico Strategy for Optimizing Weak Interactions

A strategy for affecting ortho versus meta/para selectivity in Ir-catalyzed C-H borylations (CHBs) of phenols is described. From selectivity observations with ArylOBpin (pin = pinacolate), it is hypothesized that an electrostatic interaction between the partial negatively charged OBpin group and the partial positively charged bipyridine ligand of the catalyst favors ortho selectivity. Experimental and computational studies designed to test this hypothesis support it. From further computational work a second generation, in silico designed catalyst emerged, where replacing Bpin with Beg (eg = ethylene glycolate) was predicted to significantly improve ortho selectivity. Experimentally, reactions employing B₂eg₂ gave ortho selectivities > 99%. Adding triethylamine significantly improved conversions. This ligand-substrate electrostatic interaction provides a unique control element for selective C-H functionalization.

Harnessing non-covalent interactions to exert control over regioselectivity and site-selectivity in catalytic reactions

Asymmetric catalysis has been revolutionised by the realisation that attractive non-covalent interactions such as hydrogen bonds and ion pairs can act as powerful controllers of enantioselectivity when incorporated into appropriate small molecule chiral scaffolds. Given these tremendous advances it is surprising that there are still a relatively limited number of examples of non-covalent interactions being harnessed for control of regioselectivity or site-selectivity in catalysis, two other fundamental selectivity aspects facing the synthetic chemist. This perspective examines the progress that has been made in this area thus far using non-covalent interactions in conjunction with transition metal catalysis as well as in the context of purely organic catalysts. We hope this will highlight the great potential in this approach for designing selective catalytic reactions.

Selective C–H bond hydroxylation of cyclohexanes in water by supramolecular control

A new approach for selective hydroxylation of non-activated cyclohexanes using dioxirane generated in situ in water through supramolecular control has been developed.

Immobilized Lignin Peroxidase-Like Metalloporphyrins as Reusable Catalysts in Oxidative Bleaching of Industrial Dyes

Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.

Benzothiazole Amphiphiles Promote the Formation of Dendritic Spines in Primary Hippocampal Neurons

The majority of excitatory synapses in the brain exist on dendritic spines. Accordingly, the regulation of dendritic spine density in the hippocampus is thought to play a central role in learning and memory. The development of novel methods to control spine density could, therefore, have important implications for treatment of a host of neurodegenerative and developmental cognitive disorders. Herein, we report the design and evaluation of a new class of benzothiazole amphiphiles that exhibit a dose-dependent response leading to an increase in dendritic spine density in primary hippocampal neurons. Cell exposure studies reveal that the increase in spine density can persist for days in the presence of these compounds, but returns to normal spine density levels within 24 h when the compounds are removed, demonstrating the capability to reversibly control spinogenic activity. Time-lapse imaging of dissociated hippocampal neuronal cultures shows that these compounds promote a net increase in spine density through the formation of new spines. Biochemical studies support that promotion of spine formation by these compounds is accompanied by Ras activation. These spinogenic molecules were also capable of inhibiting a suspected mechanism for dendritic spine loss induced by Alzheimer-related aggregated amyloid-β peptides in primary neurons. Evaluation of this new group of spinogenic agents reveals that they also exhibit relatively low toxicity at concentrations displaying activity. Collectively, these results suggest that small molecules that promote spine formation could be potentially useful for ameliorating cognitive deficiencies associated with spine loss in neurodegenerative diseases such as Alzheimer disease, and may also find use as general cognitive enhancers.

Directed Evolution of RebH for Catalyst-Controlled Halogenation of Indole C-H Bonds

RebH variants capable of chlorinating substituted indoles ortho-, meta-, and para- to the indole nitrogen were evolved by directly screening for altered selectivity on deuterium-substituted probe substrates using mass spectrometry. This systematic approach allowed for rapid accumulation of beneficial mutations using simple adaptive walks and should prove generally useful for altering and optimizing the selectivity of C-H functionalization catalysts. Analysis of the beneficial mutations showed that structure-guided selection of active site residues for targeted mutagenesis can be complicated either by activity/selectivity tradeoffs that reduce the possibility of detecting such mutations or by epistatic effects that actually eliminate the benefits of a mutation in certain contexts. As a corollary to this finding, the precise manner in which the beneficial mutations identified led to the observed changes in RebH selectivity is not clear. Docking simulations suggest that tryptamine binds to these variants as tryptophan does to native halogenases, but structural studies will be required to confirm these models and shed light on how particular mutations impact tryptamine binding. Similar directed evolution efforts on other enzymes or artificial metalloenzymes could enable a wide range of C-H functionalization reactions.

Fluorinated porphyrinoids as efficient platforms for new photonic materials, sensors, and therapeutics

Porphyrinoids are robust heterocyclic dyes studied extensively for their applications in medicine and as photonic materials because of their tunable photophysical properties, diverse means of modifying the periphery, and the ability to chelate most transition metals. Commercial applications include their use as phthalocyanine dyes in optical discs, porphyrins in photodynamic therapy, and as oxygen sensors. Most applications of these dyes require exocyclic moieties to improve solubility, target diseases, modulate photophysical properties, or direct the self-organization into architectures with desired photonic properties. The synthesis of the porphyrinoid depends on the desired application, but the de novo synthesis often involves several steps, is time consuming, and results in low isolated yields. Thus, the application of core porphyrinoid platforms that can be rapidly and efficiently modified to evaluate new molecular architectures allows researchers to focus on the design concepts rather than the synthesis methods, and opens porphyrinoid chemistry to a broader scientific community. We have focused on several widely available, commercially viable porphyrinoids as platforms: meso-perfluorophenylporphyrin, perfluorophthalocyanine, and meso-perfluorophenylcorrole. The perfluorophenylporphyrin is readily converted to the chlorin, bacteriochlorin, and isobacteriochlorin. Derivatives of all six of these core platforms can be efficiently and controllably made via mild nucleophilic aromatic substitution reactions using primary S, N, and O nucleophiles bearing a wide variety of functional groups. The remaining fluoro groups enhance the photo and oxidative stability of the dyes and can serve as spectroscopic signatures to characterize the compounds or in imaging applications using (19)F NMR. This review provides an overview of the chemistry of fluorinated porphyrinoids that are being used as a platform to create libraries of photo-active compounds for applications in medicine and materials.

Enzymatic hydroxylation of an unactivated methylene C-H bond guided by molecular dynamics simulations

The hallmark of enzymes from secondary metabolic pathways is the pairing of powerful reactivity with exquisite site selectivity. The application of these biocatalytic tools in organic synthesis, however, remains under-utilized due to limitations in substrate scope and scalability. Here, we report how the reactivity of a monooxygenase (PikC) from the pikromycin pathway is modified through computationally guided protein and substrate engineering, and applied to the oxidation of unactivated methylene C-H bonds. Molecular dynamics and quantum mechanical calculations were used to develop a predictive model for substrate scope, site selectivity and stereoselectivity of PikC-mediated C-H oxidation. A suite of menthol derivatives was screened computationally and evaluated through in vitro reactions, where each substrate adhered to the predicted models for selectivity and conversion to product. This platform was also expanded beyond menthol-based substrates to the selective hydroxylation of a variety of substrate cores ranging from cyclic to fused bicyclic and bridged bicyclic compounds.

Galactodendritic porphyrinic conjugates as new biomimetic catalysts for oxidation reactions

This work employed [5,10,15,20-tetrakis(pentafluorophenyl)porphyrin] ([H2(TPPF20)], H2P1) as the platform to prepare a tetrasubstituted galactodendritic conjugate porphyrin (H2P3). After metalation with excess copper(II) acetate, H2P3 afforded a new solid porphyrin material, Cu4CuP3S. This work also assessed the ability of the copper(II) complex (CuP3) of H2P3 to coordinate with zinc(II) acetate, to yield the new material Zn4CuP3S. UV-visible, Fourier transform infrared, and electron paramagnetic resonance spectroscopies aided full characterization of the synthesized solids. (Z)-Cyclooctene epoxidation under heterogeneous conditions helped to evaluate the catalytic activity of Cu4CuP3S and Zn4CuP3S. The efficiency of Cu4CuP3S in the oxidation of another organic substrate, catechol, was also investigated. According to the results obtained in the heterogeneous process, Cu4CuP3S mimicked the activity of cytochrome P-450 and catecholase. In addition, Cu4CuP3S was reusable after recovery and reactivation. The data obtained herein were compared with the results achieved for the copper complex (CuP1) of [H2(TPPF20)] and for CuP3 under homogeneous conditions.

Supramolecular nanoassemblies of an amphiphilic porphyrin-cyclodextrin conjugate and their morphological transition from vesicle to network

An amphiphilic compound, 5-(4'-dodecyloxyphenyl)-10,15,20-tri(permethyl-β-CD)-modified Zn(II)-porphyrin (1; β-CD = β-cyclodextrin), was synthesized by means of the click reaction of an alkylated Zn-porphyrin derivative with 6-deoxy-6-azidopermethyl-β-CD. The complexation between 1 and tetrasodium tetraphenylporphyrintetrasulfonate (5) with different molar ratios led to the formation of two distinctly different nanoarchitectures, which were proven to be vesicle and network aggregates, respectively, by using dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. On the basis of the results of the time-dependent TEM studies, fluorescence, and NMR spectroscopic measurements, we have determined that the mechanism of the morphology transition from vesicles to networks is controlled by the stepwise complexation of 1 with 5. Furthermore, these supramolecular nanoarchitectures show the controlled- release property of doxorubicin as potential candidates for drug delivery.

Molecular recognition in protein modification with rhodium metallopeptides

Chemical manipulation of natural, unengineered proteins is a daunting challenge which tests the limits of reaction design. By combining transition-metal or other catalysts with molecular recognition ideas, it is possible to achieve site-selective protein reactivity without the need for engineered recognition sequences or reactive sites. Some recent examples in this area have used ruthenium photocatalysis, pyridine organocatalysis, and rhodium(II) metallocarbene catalysis, indicating that the fundamental ideas provide opportunities for using diverse reactivity on complex protein substrates and in complex cell-like environments.

Toward a tunable synthetic [FeFe] hydrogenase mimic: single-chain nanoparticles functionalized with a single diiron cluster

We report two novel “clickable” [(μ-S₂C₂H₄NR)Fe₂(CO)₆] complexes and their incorporation into single-chain nanoparticles.

Supramolecular control of selectivity in transition-metal catalysis through substrate preorganization

The Perspective highlights possibilities to use supramolecular interactions between a substrate molecule and a (bifunctional) catalyst as a powerful tool to control the selectivity in transition-metal catalysis.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.