Pyrroline Ring Research Articles

Heme d formation in a Shewanella benthica hemoglobin

In our continued investigations of microbial globins, we solved the structure of a truncated hemoglobin from Shewanella benthica, an obligate psychropiezophilic bacterium. The distal side of the heme active site is lined mostly with hydrophobic residues, with the exception of a tyrosine, Tyr34 (CD1) and a histidine, His24 (B13). We found that purified SbHbN, when crystallized in the ferric form with polyethylene glycol as precipitant, turned into a green color over weeks. The electron density obtained from the green crystals accommodated a trans heme d, a chlorin-type derivative featuring a γ-spirolactone and a vicinal hydroxyl group on a pyrroline ring. In solution, exposure of the protein to one equivalent of hydrogen peroxide resulted in a similar green color change, but caused by the formation of multiple products. These were oxidation species released on protein denaturation, likely including heme d, and a species with heme covalently attached to the polypeptide. The Tyr34Phe replacement prevented the formation of both heme d and the covalent linkage. The ready modification of heme b by SbHbN expands the range of chemistries supported by the globin fold and offers a route to a novel heme cofactor.

A Quantum Chemical Study on the Mechanisms of Acylpyrrolylacetylenes Reactions With 1‐Pyrrolines

AbstractThe mechanisms of reactions between electron‐deficient acetylenes, specifically acylpyrrolylacetylenes, and 1‐pyrrolines were examined using a DFT B2PLYP‐D3/6‐311++G**//B3LYP/6‐31+G* approach. The study was focused on the interaction of 1‐phenyl‐3‐(1H‐pyrrol‐2‐yl) prop‐2‐yn‐1‐one with 5‐methyl‐3,4‐dihydro‐2H‐pyrrole, and was aimed at a clearer understanding of the reaction progress. The reaction pathway for the synthesis of a dipyrroloimidazole derivative comprising three five‐membered nitrogen‐containing heterocycles is compared to one typical for N‐substituted acylethynylpyrroles, which involves opening the pyrroline ring to form δ‐keto aminoenones. An explanation is presented for the high level of deuterium exchange during the reaction in deuterated methanol. Also, the impact of trace water in acetonitrile was assessed. A ring‐opening mechanism is proposed, involving the 1‐pyrroline molecule as a base. The photochemical isomerization process of the end product is also examined.

Spatial structure regulation towards armor-clad five-membered pyrroline nitroxides catholyte for long-life aqueous organic redox flow batteries

Five-membered pyrroline nitroxides with high-potential is fascinating as catholyte for aqueous organic redox flow batteries (AORFBs), however, it suffers from a primary deficiency of insufficient stability due to ring-opening side reaction. Herein we report a spatial structure regulation strategy by host-guest chemistry, encapsulating 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (CPL) into hydrosoluble cyclodextrins (CDs) with an inclusion structure of N–O· head towards cavity bottom, to boost the solubility and cyclability of pyrroline nitroxides significantly. The armor-clad CPL (CPL⊂HP-β-CD) catholyte in 0.05–0.5 ​M presents a battery capacity fade rate as low as 0.002 ​%/cycle (0.233 ​%/day) compared to the sole CPL in 0.05 ​M (0.039 ​%/cycle or 5.23 ​%/day) over 500 cycles in assembled AORFBs. The optimized reclining spatial structure with N–O· head towards CD cavity bottom effectively inhibits the attack of Lewis base species on the hydrogen abstraction site in pyrroline ring, and thus avoids the ring-opening side reaction of pyrroline nitroxides.

Bacteriochlorin syntheses - Status, problems, and exploration

Bacteriochlorins – Nature’s near-infrared (NIR) chromophores – are distinguished by an intense ([Formula: see text] ∼;105 M[Formula: see text]cm[Formula: see text] long-wavelength absorption band in the ∼;700–1000 nm. The development of routes to prepare synthetic, tailorable bacteriochlorins holds promise for multiple disciplines where NIR-light-promoted photoactivity is of interest. A de novo route to bacteriochlorins equipped with a stabilizing gem-dimethyl group in each pyrroline ring was discovered in 2003. Continued development in this arena over 20 years has led to additional routes as well as methods to install substituents at selected positions about the perimeter of the macrocycle. The present paper reports studies that highlight substantial limitations of existing synthetic routes, including stymied access to multi-bacteriochlorin arrays and the inability to install (in a rational way) distinct groups at opposite sides of the macrocycle. The origins of the limitations are traced to particular stages of the chemistry ranging from derivatizing pyrroles, creating pyrrolines, constructing and elaborating dihydrodipyrrins, coupling dihydrodipyrrins, and forming macrocycles. Through exploration of a dozen aspects of bacteriochlorin syntheses, 60 new compounds (and nine known compounds via improved syntheses) have been prepared and characterized; the data include 20 single-crystal X-ray diffraction analyses. The research taken together points to areas of focus to fulfill the promise of this fascinating class of compounds.

Tolyporphins-Exotic Tetrapyrrole Pigments in a Cyanobacterium-A Review.

Tolyporphins were discovered some 30 years ago as part of a global search for antineoplastic compounds from cyanobacteria. To date, the culture HT-58-2, comprised of a cyanobacterium-microbial consortium, is the sole known producer of tolyporphins. Eighteen tolyporphins are now known-each is a free base tetrapyrrole macrocycle with a dioxobacteriochlorin (14), oxochlorin (3), or porphyrin (1) chromophore. Each compound displays two, three, or four open β-pyrrole positions and two, one, or zero appended C-glycoside (or -OH or -OAc) groups, respectively; the appended groups form part of a geminal disubstitution motif flanking the oxo moiety in the pyrroline ring. The distinct structures and repertoire of tolyporphins stand alone in the large pigments-of-life family. Efforts to understand the cyanobacterial origin, biosynthetic pathways, structural diversity, physiological roles, and potential pharmacological properties of tolyporphins have attracted a broad spectrum of researchers from diverse scientific areas. The identification of putative biosynthetic gene clusters in the HT-58-2 cyanobacterial genome and accompanying studies suggest a new biosynthetic paradigm in the tetrapyrrole arena. The present review provides a comprehensive treatment of the rich science concerning tolyporphins.

Synthesis of a BC-Dihydrodipyrrin Building Block of Bacteriochlorophyll a.

A strategy for the synthesis of bacteriochlorophyll a relies on joining AD and BC halves that contain the requisite stereochemical configurations of the target macrocycle. The BC half (1) is a dihydrodipyrrin bearing a dimethoxymethyl group at the 1-position, a β-ketoester at the 8-position, and (R)-2-methyl and (R)-3-ethyl substituents in the pyrroline ring. An established route to AD-dihydrodipyrrins (Pd-mediated coupling of a 2-halopyrrole with a chiral 4-pentynoic acid followed by Petasis methenylation, acidic hydrolysis, Paal-Knorr ring closure, and Riley oxidation) proved to be unviable for BC-dihydrodipyrrins given the presence of the β-ketoester unit. A route presented here entails Pd-mediated coupling of a 2-halopyrrole (2) with (3R,4R)-4-ethyl-1,1-dimethoxy-3-methylhex-5-yn-2-one (3), anti-Markovnikov hydration of the alkyne to give the 1,4-diketone, and Paal-Knorr ring closure. Compound 3 was prepared by Schreiber-modified Nicholas reaction beginning with (S)-4-isopropyl-3-propionyloxazolidin-2-one and the hexacarbonyldicobalt complex of (±) 3-methoxy-1-(trimethylsilyl)pentyne followed by transformation of the aldehyde derived therefrom to the 1,1-dimethoxymethylcarbonyl motif. The absolute stereochemical configuration of the Schreiber-Nicholas alkylation product was confirmed by single-crystal X-ray diffraction, whereas the BC half (1) by 1H NMR spectroscopy showed a J value of 2.9 Hz consistent with the trans-configuration. Taken together, the route provides a key chiral building block for the synthesis of photosynthetic tetrapyrroles and analogues.

Unveiling the structural features that regulate carbapenem deacylation in KPC-2 through QM/MM and interpretable machine learning.

Resistance to carbapenem β-lactams presents major clinical and economical challenges for the treatment of pathogen infections. The fast hydrolysis of carbapenems by carbapenemase-producing bacterial strains enables the effective deactivation of carbapenem antibiotics. In this study, we aim to unravel the structural features that distinguish the notable deacylation activity of carbapenemases. The deacylation reactions between imipenem (IPM) and the KPC-2 class A serine-based β-lactamases (ASβLs) are modeled with combined quantum mechanical/molecular mechanical (QM/MM) minimum energy pathway (MEP) calculations and interpretable machine-learning (ML) methods. We first applied a dual-level computational protocol to achieve fast sampling of QM/MM MEPs. A tree-based ensemble ML model was employed to learn the MEP activation barriers from the conformational features of the KPC-2/IPM active site. The barrier-predicting model was then unboxed using the Shapley additive explanation (SHAP) importance attribution methods to derive mechanistic insights, which were also verified by additional QM/MM wavefunction analysis. Essentially, we show that potential hydrogen bonding interactions of the general base and the tautomerization states of the carbapenem pyrroline ring could concertedly regulate the activation barrier of KPC-2/IPM deacylation. Nonetheless, we demonstrate the efficacy of interpretable ML to assist the analysis of QM/MM simulation data for robust extraction of human-interpretable mechanistic insights.

Functionalized 10-Membered Aza- and Oxaenediynes through the Nicholas Reaction.

The scope and limitations of the Nicholas-type cyclization for the synthesis of 10-membered benzothiophene-fused heterocyclic enediynes with different functionalities were investigated. Although the Nicholas cyclization through oxygen could be carried out in the presence of an ester group, the final oxaenediyne was unstable under storage. Among the N-type Nicholas reactions, cyclization via an arenesulfonamide functional group followed by mild Co-deprotection was found to be the most promising, yielding 10-membered azaendiynes in high overall yields. By contrast, the Nicholas cyclization through the acylated nitrogen atom did not give the desired 10-membered cycle. It resulted in the formation of a pyrroline ring, whereas cyclization via an alkylated amino group resulted in a poor yield of the target 10-membered enediyne. The acylated 4-aminobenzenesulfonamide nucleophilic group was found to be the most convenient for the synthesis of functionalized 10-membered enediynes bearing a clickable function, such as a terminal triple bond. All the synthesized cyclic enediynes exhibited moderate activity against lung carcinoma NCI-H460 cells and had a minimal effect on lung epithelial-like WI-26 VA4 cells and are therefore promising compounds in the search for novel antitumor agents that can be converted into conjugates with tumor-targeting ligands.

Titania-mediated photoinduced fluorination of nitrone spin traps in acetonitrile (an EPR study)

Cyclic nitrone spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and its derivatives were applied in an EPR study of paramagnetic intermediates generated by in situ photoexcitation of TiO2 nanoparticles dispersed in acetonitrile with fluorinated ions. The UVA irradiation of deoxygenated acetonitrile titania suspensions containing the spin traps in the presence of fluoride or tetrafluoroborate, and Ag(I) or Cu(II) as photoelectron acceptors, resulted in the generation of the corresponding fluorine spin-adducts with well-resolved EPR signals. The EPR spectra of β-fluorine spin-adducts of the DMPO analogs, featuring ethoxycarbonyl (EMPO), diethoxyphosphoryl (DEPMPO), or diisopropoxyphosphoryl (DIPPMPO) groups at position 5 of the pyrroline ring, are reported for the first time. The reaction mechanisms leading to the β-fluorine spin-adduct formation in the given photocatalytic system are discussed.

Interplay between the Enamine and Imine Forms of the Hydrolyzed Imipenem in the Active Sites of Metallo-β-lactamases and in Water Solution.

Deactivation of the β-lactam antibiotics in the active sites of the β-lactamases is among the main mechanisms of bacterial antibiotic resistance. As drugs of last resort, carbapenems are efficiently hydrolyzed by metallo-β-lactamases, presenting a serious threat to human health. Our study reveals mechanistic aspects of the imipenem hydrolysis by bizinc metallo-β-lactamases, NDM-1 and L1, belonging to the B1 and the B3 subclasses, respectively. The results of QM(PBE0-D3/6-31G**)/MM simulations show that the enamine product with the protonated nitrogen atom is formed as the major product in NDM-1 and as the only product in the L1 active site. In NDM-1, there is also another reaction pathway that leads to the formation of the (S)-enantiomer of the imine form of the hydrolyzed imipenem; this process occurs with the higher energy barriers. The absence of the second pathway in L1 is due to the different amino acid composition of the active site loop. In L1, the hydrophobic Pro226 residue is located above the pyrroline ring of imipenem that blocks protonation of the carbon atom. Electron density analysis is performed at the stationary points to compare reaction pathways in L1 and NDM-1. Tautomerization from the enamine to the imine form likely happens in solution after the dissociation of the hydrolyzed imipenem from the active site of the enzyme. Classical molecular dynamics simulations of the hydrolyzed imipenem in solution, both with the neutral enamine and the negatively charged N-C2-C3 fragment, demonstrate a huge diversity of conformations. The vast majority of conformations blocks the C3-atom from the side required for the (S)-imine formation upon tautomerization. Thus, according to our calculations, formation of the (R)-imine is more likely. QM(PBE0-D3/6-31G**)/MM molecular dynamics simulations of the hydrolyzed imipenem with the negatively charged N-C2-C3 fragment followed by the Laplacian bond order analysis demonstrate that the N═C2-C3- resonance structure is the most pronounced that facilitates formation of the imine form. The proposed mechanism of the enzymatic enamine formation and its subsequent tautomerization to the imine form in solution is in agreement with the recent spectroscopic and NMR studies.

New Trends in 1,4-Dipolar Cycloaddition Reactions. Thermodynamic Control Synthesis of Model 2'-(isoquinolin-1-yl)-spiro[oxindole-3,3'-pyrrolines

Background: A direct synthesis of functionalized spiro[oxindole-3,3'-pyrrolines] is achieved via thermodynamic control (~60 oC), three-component 1,4-dipolar cycloaddition reaction involving 3-phenylimidazo[5,1-a]isoquinoline, dimethyl acetylenedicarboxylate, and N-alkylisatins. Methods: Conversely, this one-pot reaction furnished, upon conduction at 25–38 oC, the expected 1,3-oxazepino[7,6-b]indoles as the main kinetic control products. The calculated energy of the optimized molecular structures of model spiro-oxindole and its isomeric oxazepinoindole indicate that spiro-oxindole is more stable by 76.1 kJ/mol. Results: The structures of the synthesized spiro adducts were evidenced from NMR and MS spectral data and further confirmed by single-crystal X-ray diffraction. Characteristic features of the spiro-oxindoles are displaced in their 13C-spectra as diagnostic signals at ~53 and ~70 ppm assigned, respectively, to the spiro carbon-3 and sp3 CH-2' of the pyrroline ring. Conclusion: This unprecedented thermally induced pathway in 1,4-dipolar cycloaddition, utilizing imidazo[5,1- a]isoquinoline and related congeners, would serve as a new route towards the synthesis of spiro[oxindole-3,3'- pyrrolines], a class of diverse biological activities. An insight into the thermodynamic control pathway is presented.

De Novo Synthesis of Bacteriochlorins Bearing Four Trideuteriomethyl Groups

Site-specific introduction of isotopes in tetrapyrrole macrocycles provides the foundation for probing physicochemical features germane to photosynthetic energy-transduction processes, but has chiefly been done with porphyrins rather than the more biologically relevant hydroporphyrin analogues of native photosynthetic pigments. A prior study incorporated pairwise 13C or 15N atoms in the skeleton of a bacteriochlorin containing a gem-dimethyl group in each pyrroline ring. Here, a complementary effort is reported that installs deuterium atoms in substituents at the perimeter of a bacteriochlorin. Thus, perdeuteriated 3-methyl-2,4-pentanedione was converted in an 8-step synthesis via the intermediacy of tert-butyl 5-formyl-3,4-bis(trideuteriomethyl)pyrrole-2-carboxylate to the 2,3,12,13-tetrakis(trideuteriomethyl)-8,8,18,18-tetramethylbacteriochlorin (BC-2). The fidelity of isotope substitution was maintained throughout the synthesis. Resonance Raman spectroscopy of the copper chelate (CuBC-2) revealed that addition of the four β-pyrrolic substituents alone is not sufficient to account for the vibronic complexity observed for the copper chelate of bacteriochlorophyll a (CuBChl a). The increased vibronic activity exhibited by the natural pigments and CuBChl a must arise from the increased structural complexity of the macrocycle.

Synthesis of AD-Dihydrodipyrrins Equipped with Latent Substituents of Native Chlorophylls and Bacteriochlorophylls.

Native chlorophylls and bacteriochlorophylls share a common trans-substituted pyrroline ring D (17-propionic acid, 18-methyl), whereas diversity occurs in ring A particularly at the 3-position. Two dihydrodipyrrins equipped with native-like D-ring substituents and tailorable A-ring substituents have been synthesized. The synthesis relies on a Schreiber-modified Nicholas reaction to construct the stereochemically defined precursor to ring D, a dialkyl-substituted pent-4-ynoic acid. The carboxylic acid group of the intact propionic acid proved unworkable, whereupon protected propionate (-CO2tBu) and several latent propyl ethers were examined. The tert-butyldiphenylsilyl-protected propanol substituent proved satisfactory for reaction of the chiral N-acylated oxazolidinone, affording (2S,3S)-2-(3-((tert-butyldiphenylsilyl)oxy)propyl)-3-methylpent-4-ynoic acid in ∼30% yield over 8 steps. Two variants for ring A, 2-tert-butoxycarbonyl-3-Br/H-5-iodo-4-methylpyrrole, were prepared via the Barton-Zard route. Dihydrodipyrrin formation from the pyrrole and pentynoic acid entailed Jacobi Pd-mediated lactone formation, Petasis methenylation, and Paal-Knorr-type pyrroline formation. The two AD-dihydrodipyrrins bear the D-ring methyl and protected propanol groups with a stereochemical configuration identical to that of native (bacterio)chlorophylls, and a bromine or no substitution in ring A corresponding to the 3-position of (bacterio)chlorophylls. The analogous β-position of a lactone-pyrrole intermediate on the path to the dihydrodipyrrin also was successfully brominated, opening opportunities for late-stage diversification in the synthesis of (bacterio)chlorophylls.

Molecular encapsulation of a key odor-active 2-acetyl-1-pyrroline in aromatic rice with β-cyclodextrin derivatives

2-Acetyl-1-pyrroline (2AP) is a key volatile compound contributing to a characteristic popcorn-like odor in several Asian aromatic rice varieties. Even though a demand for 2AP has been continuously increasing in global markets, its instability is a significant problem for commercial and food applications. Beta-cyclodextrin (βCD) has been extensively used to improve the stability, solubility, and sensorial properties of many hydrophobic and volatile compounds via an encapsulation process into its hydrophobic inner cavity. In this work, molecular complexation between 2AP and βCDs (hydroxypropyl (HP) and methyl (M) analogs) was theoretically and experimentally investigated using molecular dynamics (MD) simulation, molecular mechanics/generalized Born surface area-based free energy calculation, and gas chromatography–mass spectrometry (GC–MS). Triplicate 500 ns MD simulations revealed that, among six studied βCDs, only 6-HPβCD and 2,6-DMβCD derivatives could stably form inclusion complexes with 2AP, driven mainly by van der Waals interactions. The pyrroline ring of 2AP was preferentially embedded in the hydrophobic interior of 2,6-DMβCD, whereas the pyrroline and acetyl moieties of 2AP were freely rotated inside 6-HPβCD along the simulation times. The binding affinity, the compactness, and the number of atomic contacts of 2AP/2,6-DMβCD complex were higher than those of 2AP/6-HPβCD system, in good agreement with the highest remaining 2AP in the 2AP/2,6-DMβCD complex from GC–MS result. Altogether, the obtained theoretical and experimental data demonstrated the good potentiality of 2,6-DMβCD as suitable host molecule for 2AP encapsulation.

Novel inhibition mechanism of carbapenems on the ACC-1 class C β-lactamase

The hydrolysis of β-lactam antibiotics by class C β-lactamases proceeds through the acylation and the rate-determining deacylation steps mediated by the nucleophilic serine and the deacylation water, respectively. The pose of poor substrates such as carbapenems in the acylated enzyme is responsible for the low efficient deacylation reaction. Here we present the crystal structures of the Y150F variant of the ACC-1 class C β-lactamase in the apo and acylated states. In the acylated enzyme complexed with two carbapenems, imipenem and meropenem, the lactam carbonyl oxygen is located in the oxyanion hole. However, the five-membered pyrroline ring displays a novel orientation that has not been reported so far. The ring is rotated such that its C3 carboxylate makes salt bridges with Lys67 and Ly315, which is accompanied by the side-chain rotamer change of Phe150. The C3 carboxylate is placed where the deacylation water occupies in the apo-enzyme, which, together with the displacement of the catalytic base residue at position 150, explains why carbapenems are poor substrates of ACC-1.

Asymmetric Synthesis of a Bacteriochlorophyll Model Compound Containing trans-Dialkyl Substituents in Ring D

Challenges to the de novo synthesis of bacteriochlorophyll a (BChl a), the chief pigment for anoxygenic bacterial photosynthesis, include creating the macrocycle along with the trans-dialkyl substituents in both pyrroline rings (B and D). A known route to a model bacteriochlorophyll with a gem-dimethyl group in each pyrroline ring has been probed for utility in the synthesis of BChl a by preparation of a hybrid macrocycle (BC-1), which contains a trans-dialkyl group in ring D and a gem-dimethyl group in ring B. Stereochemical definition began with the synthesis of (2S,3S)-2-ethyl-3-methylpent-4-ynoic acid, a precursor to the trans-dialkyl-substituted AD dihydrodipyrrin. Knoevenagel condensation of the latter and a gem-dimethyl, β-ketoester-substituted BC dihydrodipyrrin afforded the enone (E, 70%; Z, 3%); subsequent double-ring cyclization of the E-enone (via Nazarov, electrophilic aromatic substitution, and elimination reactions) gave BC-1 (53% yield) along with a trace of chlorin byproduct (1.4% relative to BC-1 upon fluorescence assay). BC-1 exhibited the desired trans-dialkyl stereochemistry in ring D and was obtained as a 7:1 mixture of (expected) epimers owing to the configuration of the 132-carbomethoxy substituent. The strategy wherein trans-dialkyl substituents are installed very early and carried through to completion, as validated herein, potentially opens a synthetic path to native photosynthetic pigments.

Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives.

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D2 symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting "densely" intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig-Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a-i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D2 symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C2v symmetric pyrroline fused COT derivatives.

Mechanistic Insights into \u03b2-Lactamase-Catalysed Carbapenem Degradation Through Product Characterisation

β-Lactamases are a major threat to the clinical use of carbapenems, which are often antibiotics of last resort. Despite this, the reaction outcomes and mechanisms by which β-lactamases degrade carbapenems are still not fully understood. The carbapenem bicyclic core consists of a β-lactam ring fused to a pyrroline ring. Following β-lactamase-mediated opening of the β-lactam, the pyrroline may interconvert between an enamine (2-pyrroline) form and two epimeric imine (1-pyrroline) forms; previous crystallographic and spectroscopic studies have reported all three of these forms in the contexts of hydrolysis by different β-lactamases. As we show by NMR spectroscopy, the serine β-lactamases (KPC-2, SFC-1, CMY-10, OXA-23, and OXA-48) and metallo-β-lactamases (NDM-1, VIM-1, BcII, CphA, and L1) tested all degrade carbapenems to preferentially give the Δ2 (enamine) and/or (R)-Δ1 (imine) products. Rapid non-enzymatic tautomerisation of the Δ2 product to the (R)-Δ1 product prevents assignment of the nascent enzymatic product by NMR. The observed stereoselectivity implies that carbapenemases control the form of their pyrroline ring intermediate(s)/product(s), thereby preventing pyrroline tautomerisation from inhibiting catalysis.

Metal-mediated synthesis of pyrrolines.

The five-membered, nitrogen-containing pyrroline ring is a privileged structure. This ring is present in many bioactive compounds from natural sources. Pyrrolines—the dihydro derivatives of pyrroles—have three structural isomer classes, depending on the location of the double bond: 1-pyrrolines (3,4-dihydro-2H-pyrroles), 2-pyrrolines (2,3-dihydro-1H-pyrroles) and 3-pyrrolines (2,5-dihydro-1H-pyrroles). This review aims to describe the latest advances for the synthesis of pyrrolines by transition metal-catalyzed cyclizations. Only reactions in which the pyrroline ring is formed by metal promotion are described. Transformations of the pyrroline ring in other heterocycles, and the structural manipulations of the pyrroline itself are not discussed. The review is organized into three parts, each covering the metal-mediated synthesis of the three pyrroline isomers. Each part is subdivided according to the metal involved, and concludes with a brief description of notable biological activities within the class.

Semiexperimental equilibrium structure of 1-methylisatin from gas-phase electron diffraction data and structural changes in isatin due to 1-methyl and 5-fluoro substituents as predicted by coupled cluster computations

ABSTRACTThe molecular structure of 1-methylisatin (1) has been studied by gas-phase electron diffraction (GED) and quantum chemical computation up to the coupled cluster (CCSD(T)) level of theory. The semiexperimental equilibrium structure (Cs point group symmetry) has been determined from the GED data taking into account anharmonic vibrational corrections calculated in curvilinear coordinates with the B2PLYP force field. To observe fine structural effects due to the presence of different kinds of substituents, the accurate molecular structures of isatin (2) and 5-fluoroisatin (3) were also computed at the CCSD(T) level. The (O=)C–C(=O) carbon–carbon bonds of the pyrroline moiety in 1–3 are found to be remarkably longer than the typical single C–C bond. The electron donating methyl group causes a decrease of the C−N−C angle and an elongation of the N−C bond lengths in the pyrroline ring by 0.7° and up to 0.008 Å, respectively, whereas the electron withdrawing fluorine atom increases the ipso CCC angle by 2.5° in comparison to that in unsubstituted isatin.