Chiral Calix[3]pyrrole Derivatives: Synthesis, Racemization Kinetics, and Ring Expansion to Calix[9]- and Calix[12]pyrrole Analogues.

Chiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole-based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene (1), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N-methylation suppressed racemization and permitted chiral resolution. Enantioselective N-methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2-4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole-like species to be structurally characterized.

Modulations of a Metal-Ligand Interaction and Photophysical Behaviors by Hückel-Möbius Aromatic Switching.

In organometallic complexes containing π-conjugated macrocyclic chelate ligands, conformational change significantly affects metal-ligand electronic interactions, hence tuning properties of the complexes. In this regard, we investigated the metal-ligand interactions in hexaphyrin mono-Pd(II) complexes Pd[28]M and Pd[26]H, which exhibit a redox-induced switching of Hückel-Möbius aromaticity and subsequent molecular conformation, and their effect on the electronic structure and photophysical behaviors. In Möbius aromatic Pd[28]M, the weak metal-ligand interaction leads to the π electronic structure of the hexaphyrin ligand remaining almost intact, which undergoes efficient intersystem crossing (ISC) assisted by the heavy-atom effect of the Pd metal. In Hückel aromatic Pd[26]H, the significant metal-ligand interaction results in ligand-to-metal charge-transfer (LMCT) in the excited-state dynamics. These contrasting metal-ligand electronic interactions have been revealed by time-resolved electronic and vibrational spectroscopies and time-dependent DFT calculations. This work indicates that the conspicuous modulation of metal-ligand interaction by Hückel-Möbius aromaticity switching is an appealing approach to manipulate molecular properties of metal complexes, further enabling the fine-tuning of metal-ligand interactions and the novel design of functional organometallic materials.

Strain-Induced Ring Expansion Reactions of Calix[3]pyrrole-Related Macrocycles.

The recent discovery of calix[3]pyrrole, a porphyrinogen-like tripyrrolic macrocycle, has provided an unprecedented strain-induced ring expansion reaction into calix[6]pyrrole. Here, we synthesized calix[n]furan[3-n]pyrrole (n=1∼3) macrocycles to investigate the reaction scope and mechanism of the ring expansion. Single crystal X-ray analysis and theoretical calculations revealed that macrocyclic ring strain increases as the number of inner NH sites increases. While calix[1]furan[2]pyrrole exhibited almost quantitative conversion into calix[2]furan[4]pyrrole within 5 minutes, less-strained calix[2]furan[1]pyrrole and calix[3]furan were inert. However, N-methylation of calix[2]furan[1]pyrrole induced a ring-expansion reaction that enabled the isolation of a linear reaction intermediate. The mechanism analysis revealed that the ring expansion consists of regioselective ring cleavage and subsequent cyclodimerization. This reaction was further utilized for synthesis of calix[6]-type macrocycles.

Calix[3]pyrrole: A Missing Link in Porphyrin-Related Chemistry.

A long-standing question in porphyrin chemistry is why pyrrole monomers selectively form tetrapyrrolic macrocycles, whereas the corresponding tripyrrolic macrocycles are never observed. Calix[3]pyrrole, a tripyrrolic porphyrinogen-like macrocycle bearing three sp3-carbon linkages, is a missing link molecule that might hold the key to this enigma; however, it has remained elusive. Here we report the synthesis and strain-induced transformations of calix[3]pyrrole and its furan analogue, calix[3]furan. These macrocycles are readily accessed from cyclic oligoketones. Crystallographic and theoretical analyses reveal that these three-subunit systems possess the largest strain energy among known calix[n]-type macrocycles. The ring-strain triggers transformation of calix[3]pyrrole into first calix[6]pyrrole and then calix[4]pyrrole under porphyrin cyclization conditions. The present results help explain the absence of naturally occurring three-pyrrole macrocycles and the fact that they are not observed as products or intermediate during classic porphyrin syntheses.

Hybrid EuIII Coordination Luminophore Standing on Two Legs on Silica Nanoparticles for Enhanced Luminescence.

In this study, we have demonstrated a two-legged, upright molecular design method for monochromatic and bright red luminescent LnIII -silica nanomaterials. A novel EuIII -silica hybrid nanoparticle was developed by using a doubly binding TPPO-Si(OEt)3 (TPPO: triphenyl phosphine oxide) linker. The TPPO-Si(OEt)3 was confirmed by 1 H, 31 P, 29 Si NMR spectroscopy and single-crystal X-ray analysis. Luminescent Eu(hfa)3 and Eu(tfc)3 moieties (hfa: hexafluoroacetylacetonate, tfc: 3-(trifluoromethylhydroxymethylene)camphorate) were fixed onto TPPO-Si(OEt)3 -modified silica nanoparticles, producing Eu(hfa)3 (TPPO-Si)2 -SiO2 and Eu(tfc)3 (TPPO-Si)2 -SiO2 , respectively. Eu(hfa)3 (TPPO-Si)2 -SiO2 exhibited the higher intrinsic luminescence quantum yield (93 %) and longer emission lifetime (0.98 ms), which is much larger than those of previously reported EuIII -based hybrid materials. Eu(tfc)3 (TPPO-Si)2 -SiO2 showed an extra-large intrinsic emission quantum yield (54 %), although the emission quantum yield for the precursor Eu(tfc)3 (TPPO-Si(OEt)3 )2 was found to be 39 %. These results confirmed that the TPPO-Si(OEt)3 linker is a promising candidate for development of EuIII -based luminescent materials.

Identification of the Inhibitory Compounds for Metallo-ß-lactamases and Structural Analysis of the Binding Modes.

Metallo-ß-lactamases (MBLs) are significant threats to humans because they deteriorate many kinds of ß-lactam antibiotics and are key enzymes responsible for multi-drug resistance of bacterial pathogens. As a result of in vitro screening, two compounds were identified as potent inhibitors of two kinds of MBLs: imipenemase (IMP-1) and New Delhi metallo-ß-lactamase (NDM-1). The binding structure of one of the identified compounds was clarified by an X-ray crystal analysis in complex with IMP-1, in which two possible binding poses were observed. Molecular dynamics (MD) simulations were performed by building two calculation models from the respective binding poses. The compound was stably bound to the catalytic site during the simulation in one pose. The binding model between NDM-1 and the compound was constructed for MD simulation. Calculation results for NDM-1 were similar to those of IMP-1. The simulation suggested that the binding of the identified inhibitory compound was also durable in the catalytic site of NDM-1. The compound will be a sound basis for the development of the inhibitors for MBLs.

Modular synthesis of oligoacetylacetones via site-selective silylation of acetylacetone derivatives.

Oligoacetylacetones consisting of 3,3-disubstituted pentane-2,4-diones were synthesized through a terminal silylation and oxidative coupling protocol. Highly selective formation of mono-enol silyl ethers of 3,3-disubstituted acetylacetones was achieved using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base. Subsequent silver(i) oxide mediated coupling reactions provided tetraketones. Unique substituent dependence was found for the terminal-selective silylation of tetraketones. Finally, octaketones (tetramers of acetylacetone derivatives) with three types of monomer sequences were prepared in their discrete forms. Single crystal X-ray analysis revealed that the solid-state conformations of oligoketone chains were predominantly governed by the ketone sequence rather than substituents. However, differences in the packing structures induced by alkyl substituents led to significant differences in melting points for the structural isomers of octaketones.

Deprotection of a benzyl unit induces a 22π aromatic macrocycle of 3-oxypyripentaphyrin(0.1.1.1.0) with strong NIR absorption.

We report aromaticity switching from a 6π pyridine ring to a 22π macrocyclic ring of 3-oxypyripentaphyrin(0.1.1.1.0). This system has potential applications in photodynamic therapy owing to macrocyclic aromaticity being selectively induced by protecting group removal and strong absorption bands produced in the NIR region especially in methanol.

Adjunctive application of solid-state culture products and its freeze-dried powder from Aspergillus sojae for semi-hard cheese.

BACKGROUND: Species belonging to the genus Aspergillus have been used in traditional Japanese fermented foods. Aspergillus sojae is a species responsible for strong proteolytic activity. Freeze-drying treatments followed by physical disruption enables the pulverization of the mycelia of A. sojae RIB 1045 grown in whey protein-base solid media. Intracellular proteases were extracted using this protocol to compare extracellular protease activity in terms of the reaction's pH dependence in the presence or absence of inhibitors. RESULT: With different sensitivities to inhibitors, intracellular and extracellular proteases showed the strongest activity under acidic conditions, which were considered suitable for cheese application. The raw culture product (CP) and its freeze-dried product (FDP) were mixed with cheese curds, prepared according to Gouda-type cheese-making methods, and were allowed to ripen for 3 months. Chemical analysis of the products showed 13.3% water-soluble nitrogen (WSN) in the control, which had received noncultured media, whereas 20.0% and 21.1% WSN was found in the CP and FDP experimental cheeses, respectively. Although these adjuncts significantly increased WSN, an insignificant difference was found between CP and FDP. Free fatty acids in all experimental cheeses were similar, showing that CP and FDP caused no rancid defects. CONCLUSION: The introduction of freeze-drying treatments accompanied by cell disruption resulted in a negligible effect in terms of WSN. However, the application of A. sojae can be beneficial when it comes to increasing the level of WSN compared with A. oryzae, as shown in our previous study. © 2020 Society of Chemical Industry.

Excited-State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time-Resolved Electronic and Vibrational Spectroscopy.

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited-state aromaticity. The coordinated metal allows the excited-state aromaticity in the triplet state to be detected through the heavy-atom effect, but other metalation effects on the excited-state aromaticity were ambiguous. Herein, the excited-state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The SQ states of gold [26]- and [28]-hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground-state, indicating aromaticity reversal. Furthermore, while the T1 states of gold [28]-hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand-to-metal charge-transfer state of gold [26]-hexaphyrins contributed by the gold metal showed non-aromatic features arising from the odd-number of π-electrons.

Two-Step Transformation of Aliphatic Polyketones into π-Conjugated Polyimines.

The chemo- and stereo-selective two-step transformation of aliphatic polyketones composed of 3,3-dimethylpentane-2,4-dione units was achieved to generate π-conjugated polyimines. Upon treatment with hydrazine, discrete oligoketones with 4-8 carbonyl groups afforded ethylene-bridged oligoisopyrazoles in 80-89% yields. These oligoisopyrazoles underwent stereoselective oxidation at the ethylene bridge to give fully π-conjugated oligo(isopyrazole-3,5-diyl-trans-vinylene)s in 73-87% yields. Oxidation of the oligoimines drastically changed their absorption and metal-coordination behaviors. Finally, this two-step transformation was applied to polydisperse polymers. Imine formation proceeded almost quantitatively, even for longer polyketones, including docosamer. Subsequent oxidation of the polyimines furnished a virtually insoluble material that showed broad and red-shifted solid-state absorption over the whole visible region resulting from extended π-conjugation.

Utility of heme analogues to intentionally modify heme-globin interactions in myoglobin.

Myoglobin reconstitution with various synthetic heme analogues was reviewed to follow the consequences of modified heme-globin interactions. Utility of dimethyl sulfoxide as the solvent for water-insoluble hemes was emphasized. Proton NMR spectroscopy revealed that loose heme-globin contacts in the heme pocket eventually caused the dynamic heme rotation around the iron-histidine bond. The full rotational rate was estimated to be about 1400 s(-1) at room temperature for 1,4,5,8-tetramethylhemin. The X-ray analysis of the myoglobin containing iron porphine, the smallest heme without any side chains, showed that the original globin fold was well conserved despite the serious disruption of native heme-globin contacts. Comparison between the two myoglobins with static and rotatory prosthetic groups indicated that the oxygen and carbon monoxide binding profiles were almost unaffected by the heme motion. On the other hand, altered tetrapyrrole array of porphyrin dramatically changed the dissociation constant of oxygen from 0.0005 mm Hg of porphycene-myoglobin to ∞ in oxypyriporphyrin-myoglobin. Heme-globin interactions in myoglobin were also monitored with circular dichroism spectroscopy. The observation on several reconstituted protein revealed an unrecognized role of the propionate groups in protoheme. Shortening of heme 6,7-propionates to carboxylates resulted in almost complete disappearance of the positive circular dichroism band in the Soret region. The theoretical analysis suggested that the disappeared circular dichroism band reflected the cancellation effects between different conformers of the carboxyl groups directly attached to heme periphery. The above techniques were proposed to be applicable to other hemoproteins to create new biocatalysts. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

[24]Pentaphyrin(2.1.1.1.1): A Strongly Antiaromatic Pentaphyrin.

[24]Pentaphyrin(2.1.1.1.1) 1 was synthesized by dehydrogenation of dihydropentaphyrin(2.1.1.1.1) 2 as the first example of vinylogous pentaphyrin. Pentaphyrin 1 takes a roughly planar structure and shows strong antiaromatic character, reflecting a 24π-conjugated circuit. In spite of the antiaromatic character and the relatively small circuit, 1 is stable under ambient conditions.

[62]Tetradecaphyrin and Its Mono- and Bis-Zn(II) Complexes.

A coiled structure of meso-pentafluorophenyl-substituted [62]tetradecaphyrin 1 was revealed by X-ray structural analysis. Synthetic protocols were devised to form mono- and bis-Zn(II) complexes, 1 Zn and 1 Zn2 , selectively. The former displayed a trigonal-bipyramidal pentacoordinated Zn(II) ion as a rare case and a cyclic voltammogram exhibiting eleven reversible redox waves. The latter showed a Ci-symmetric structure with modest Hückel aromaticity owing to a 62 π-electronic circuit as the largest aromatic molecule to date.

Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps.

A rectangular [28]hexaphyrin bearing outer straps at the long side has been synthesized by SN Ar reaction of [26]hexaphyrin with allyl alcohol, intramolecular olefin metathesis by using Hoveyda-Grubbs second-generation catalyst, and reduction with NaBH4 . The peripheral straps enforce a rectangular conformation for the [28]hexaphyrin, which shows Hückel antiaromatic character, as confirmed by its planar X-ray structure, a strong paratropic ring current, characteristic UV/Vis/NIR absorption features, small electrochemical HOMO-LUMO gap, and very fast S1 decay.

meso-Free Corroles: Syntheses, Structures, Properties, and Chemical Reactivities.

5,10-Bis(pentafluorophenyl)corrole (5) and 5,15-bis(pentafluorophenyl)corrole (9) have been synthesized as meso-free corroles by rational synthetic routes. Both the structures of these corroles have been unambiguously revealed by X-ray diffraction analysis and their optical and electrochemical properties have been studied. Chlorination and oxidative dimerization of 5 and 9 have been explored, which revealed a marked different reactivity of the free meso-positions in 5 and 9. 10-Chlorinated corrole 11 was effectively prepared by the reaction of 9 with Palau'chlor in the presence of 1 % pyridine whereas 5-chlorinated corrole 12 was obtained in a trace amount from similar chlorination of 5. 5,5'-Linked corrole dimer 13 was produced by reaction of 5 with AgNO2 in a good yield, whereas 10,10'-linked corrole dimer 14 was formed in a moderate yield by the reaction of 9 with [bis(trifluoroacetoxy)iodo]benzene. Observed large electronic interaction between the two corroles in 13 as compared with that in 14 has been ascribed mainly to conformational flexibility of the former, which allows more coplanar conformation.

A Stable Organic π-Radical of a Zinc(II)-Copper(I)-Zinc(II) Complex of Decaphyrin.

A Zn(II) -Cu(I) -Zn(II) heterotrimetal complex of decaphyrin was synthesized by stepwise metalations: metalation of a [46]decaphyrin with Zn(II) ions to produce a 46π decaphyrin bis(Zn(II) ) complex and its subsequent metalation with Cu(II) ion. In the second metalation step, it has been shown that Cu(II) ion is reduced to a Cu(I) ion in the complex and a dianionic bis(Zn(II) ) containing [46]decaphyrin ligand is oxidized to the corresponding monoanionic [45]decaphyrin ligand, indicating a non-innocent nature of the decaphyrin ligand. Despite the radical nature, the heterotrimetal complex is fairly stable under ambient conditions and exhibits almost no intermolecular magnetic interaction, owing to extensive delocalization of an unpaired electron in the large π-conjugated circuit of decaphyrin moiety.

Synthesis and catalytic activities of porphyrin-based PCP pincer complexes.

2,18-Bis(diphenylphosphino)porphyrins undergo peripheral cyclometalation with group 10 transition-metal salts to afford the corresponding porphyrin-based PCP pincer complexes. The porphyrinic plane and the PCP-pincer unit are apparently coplanar, with small strain. The catalytic activities of the porphyrin-based pincer complexes at the periphery were investigated in the allylation of benzaldehyde with allylstannane and in the 1,4-reduction of chalcone to discover the electronic interplay between the inner metal and the outer metal in catalysis.

Pd(II) complexes of [44]- and [46]decaphyrins: the largest Hückel aromatic and antiaromatic, and Möbius aromatic macrocycles.

Reductive metalation of [44]decaphyrin with [Pd2(dba)3] provided a Hückel aromatic [46]decaphyrin Pd(II) complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd(II) complex. In CH2Cl2 solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd(II) complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd(II) complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.

Synthesis of a [26]hexaphyrin bis-Pd(II) complex with a characteristic aromatic circuit.

Not one, but two: Metalation of a mono-Pd(II) hexaphyrin (1) with [Pd(OCOCF3)2] gave a bis-Pd(II) complex (2), which possesses a characteristic 26 π-aromatic circuit with two outer amino-pyrroles within a rectangular molecular framework. Complex 2 was readily deprotonated to afford a dianion, which was regioselectively methylated to give a methyl derivative. A similar methylation reaction of 1 produced a skeletally rearranged product that contained an N-confused pyrrole.