Oxidation and Reduction Pathways in the Knowles Hydroamination via a Photoredox-Catalyzed Radical Reaction.

Systematic reaction path exploration revealed the entire mechanism of Knowles's light-promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non-adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield.

Synthesis of Bicyclo[1.1.1]pentane (BCP)-Based Straight-Shaped Diphosphine Ligands.

[1.1.1]Propellane, which is structurally simple and compact, exhibits promising potential for the synthesis of disubstituted straight-shaped bicyclo[1.1.1]pentane (BCP) compounds by manipulation of its highly reactive internal C-C bond. BCPs are considered to be isosteres of 1,4-disubstituted benzenes, which have found broad applications in the areas of functional molecules and drug discovery. The internal C-C single bond of [1.1.1]propellane is regarded as a charge-shift bond, which can be readily cleaved by radical means to construct BCPs. We herein report a novel synthetic method for (un)symmetric diphosphines based on the BCP motif, which can be interpreted as isosteres of 1,4-bis(diphenylphosphino)benzenes. The obtained BCP-diphosphine derivatives were used to generate a straight-shaped Au complex and an Eu-based coordination polymer.

Electrochemical Dearomative Dicarboxylation of Heterocycles with Highly Negative Reduction Potentials.

The dearomative dicarboxylation of stable heteroaromatics using CO2 is highly challenging but represents a very powerful method for producing synthetically useful dicarboxylic acids, which can potentially be employed as intermediates of biologically active molecules such as natural products and drug leads. However, these types of transformations are still underdeveloped, and concise methodologies with high efficiency (e.g., high yield and high selectivity for dicarboxylations) have not been reported. We herein describe a new electrochemical protocol using the CO2 radical anion (E1/2 of CO2 = -2.2 V in DMF and -2.3 V in CH3CN vs SCE) that produces unprecedented trans-oriented 2,3-dicarboxylic acids from N-Ac-, Boc-, and Ph-protected indoles that exhibit highly negative reduction potentials (-2.50 to -2.94 V). On the basis of the calculated reduction potentials, N-protected indoles with reduction potentials up to -3 V smoothly undergo the desired dicarboxylation. Other heteroaromatics, including benzofuran, benzothiophene, electron-deficient furans, thiophenes, 1,3-diphenylisobenzofuran, and N-Boc-pyrazole, also exhibit reduction potentials more positive than -3 V and served as effective substrates for such dicarboxylations. The dicarboxylated products thus obtained can be derivatized into useful synthetic intermediates for biologically active compounds in few steps. We also show how the dearomative monocarboxylation can be achieved selectively by choice of the electrolyte, solvent, and protic additive; this strategy was then applied to the synthesis of an octahydroindole-2-carboxylic acid (Oic) derivative, which is a useful proline analogue.

Prediction of High-Yielding Single-Step or Cascade Pericyclic Reactions for the Synthesis of Complex Synthetic Targets.

Pericyclic reactions, which involve cyclic concerted transition states without ionic or radical intermediates, have been extensively studied since their definition in the 1960s, and the famous Woodward-Hoffmann rules predict their stereoselectivity and chemoselectivity. Here, we describe the application of a fully automated reaction-path search method, that is, the artificial force induced reaction (AFIR), to trace an input compound back to reasonable starting materials through thermally allowed pericyclic reactions via product-based quantum-chemistry-aided retrosynthetic analysis (QCaRA) without using any a priori experimental knowledge. All categories of pericyclic reactions, including cycloadditions, ene reactions, group-transfer, cheletropic, electrocyclic, and sigmatropic reactions, were successfully traced back via concerted reaction pathways, and starting materials were computationally obtained with the correct stereochemistry. Furthermore, AFIR was used to predict whether the identified reaction pathway can be expected to occur in good yield relative to other possible reactions of the identified starting material. In order to showcase its practical utility, this state-of-the-art technology was also applied to the retrosynthetic analysis of a natural product with a relatively high number of atoms (52 atoms: endiandric acid C methyl ester), which was first synthesized by Nicolaou in 1982 and provided the corresponding starting polyenes with the correct stereospecificity via three pericyclic reaction cascades (one Diels-Alder reaction as well as 6π and 8π electrocyclic reactions). Moreover, not only systems that obey the Woodward-Hoffmann rules but also systems that violate these rules, such as those recently calculated by Houk, can be retrosynthesized accurately.

Experimental and Theoretical Evidence for Relativistic Catalytic Activity in C-H Activation of N-Phenylbenzamide Using a Cationic Iridium Complex.

This study elucidates that relativistic effect plays a key role in catalytic C-H activation using a cationic Ir complex. Experiments show that the cationic Ir(I)-diphosphine catalyst can be used for the deuterium substitution of N-phenylbenzamide, whereas a cationic Rh(I)-diphosphine catalyst is scarcely effective. Density functional theory calculations, including the relativistic effect, demonstrate a large difference in the reaction energy diagrams for the C-H activation of N-phenylbenzamide between the cationic Ir and Rh catalysts. In particular, the relatively low reaction barrier and considerably stabilized product obtained for the Ir catalysts are rationalized by strong Ir-C and Ir-H interactions, which originate from the relativistic self-consistent d-orbital expansion of Ir.

Synthesis of Difluoroglycine Derivatives from Amines, Difluorocarbene, and CO2 : Computational Design, Scope, and Applications.

Invited for the cover of this issue are Satoshi Maeda, Tsuyoshi Mita, and co-workers at ICReDD (Hokkaido University). The image depicts an Artificial Force Induced Reaction (AFIR) conducted on a supercomputer, which predicts a new chemical transformation and its application. Read the full text of the article at 10.1002/chem.202100812.

Synthesis of Difluoroglycine Derivatives from Amines, Difluorocarbene, and CO2 : Computational Design, Scope, and Applications.

A three-component reaction (3CR) for the synthesis of difluoroglycine derivatives has been achieved by using amines, difluorocarbene (generated in situ), and the abundant, inexpensive, and nontoxic C1 source CO2 . Various tert-amines and pyridine, (iso)quinoline, imidazole, thiazole, and pyrazole derivatives were incorporated, and the corresponding products were isolated in solid form without purification by column chromatography on silica gel. Detailed reaction profiles of the 3CR were obtained from computational analysis using DFT calculations, and the results critically suggest that simple ammonia is not applicable to this reaction. In addition, as strongly supported by computational predictions, a new reagent that can generate difluorocarbene at 0 °C without any additives was discovered. Finally, radical substitution reactions of the obtained difluoroglycine derivatives under photoredox conditions, as well as a synthetic application as an N-heterocyclic carbene ligand are shown.

Light Response of Pseudomonas putida KT2440 Mediated by Class II LitR, a Photosensor Homolog.

Pseudomonas putida KT2440 retains three homologs (PplR1 to PplR3) of the LitR/CarH family, an adenosyl B12-dependent light-sensitive MerR family transcriptional regulator. Transcriptome analysis revealed the existence of a number of photoinducible genes, including pplR1, phrB (encoding DNA photolyase), ufaM (furan-containing fatty acid synthase), folE (GTP cyclohydrolase I), cryB (cryptochrome-like protein), and multiple genes without annotated/known function. Transcriptional analysis by quantitative reverse transcription-PCR with knockout mutants of pplR1 to pplR3 showed that a triple knockout completely abolished the light-inducible transcription in P. putida, which indicates the occurrence of ternary regulation of PplR proteins. A DNase I footprint assay showed that PplR1 protein specifically binds to the promoter regions of light-inducible genes, suggesting a consensus PplR1-binding direct repeat, 5'-T(G/A)TACAN12TGTA(C/T)A-3'. The disruption of B12 biosynthesis cluster did not affect the light-inducible transcription; however, disruption of ppSB1-LOV (where LOV indicates "light, oxygen, or voltage") and ppSB2-LOV, encoding blue light photoreceptors adjacently located to pplR3 and pplR2, respectively, led to the complete loss of light-inducible transcription. Overall, the results suggest that the three PplRs and two PpSB-LOVs cooperatively regulate the light-inducible gene expression. The wide distribution of the pplR/ppSB-LOV cognate pair homologs in Pseudomonas spp. and related bacteria suggests that the response and adaptation to light are similarly regulated in the group of nonphototrophic bacteria.IMPORTANCE The LitR/CarH family is a new group of photosensor homologous to MerR-type transcriptional regulators. Proteins of this family are distributed to various nonphototrophic bacteria and grouped into at least five classes (I to V). Pseudomonas putida retaining three class II LitR proteins exhibited a genome-wide response to light. All three paralogs were functional and mediated photodependent activation of promoters directing the transcription of light-induced genes or operons. Two LOV (light, oxygen, or voltage) domain proteins, adjacently encoded by two litR genes, were also essential for the photodependent transcriptional control. Despite the difference in light-sensing mechanisms, the DNA binding consensus of class II LitR [T(G/A)TA(C/T)A] was the same as that of class I. This is the first study showing the actual involvement of class II LitR in light-induced transcription.

Catalytic Enantioselective Synthesis of Axially Chiral Polycyclic Aromatic Hydrocarbons (PAHs) via Regioselective C-C Bond Activation of Biphenylenes.

Chiral polycyclic aromatic hydrocarbons (PAHs) are expected to have unusual physical properties due to their chirality and expanded π-conjugated system. Indeed, PAHs are promising compounds as chiral recognizers and organic semiconductors. Therefore, an efficient method for the synthesis of chiral PAHs is strongly desired. In contrast to helically chiral PAHs, there are only limited examples of the enantioselective synthesis of axially chiral PAHs. Herein, we report the catalytic enantioselective synthesis of benzo[b]fluoranthene-based axially chiral PAHs in excellent yields and enantioselectivities (up to >99%, >99% ee) by regioselective cleavage of the sterically hindered C-C bond of biphenylenes. The consecutive cyclizations could provide polycyclic PAHs with two chiral axes. The obtained chiral PAHs have high ε values (up to ε = 8.9 × 104), quantum yields (up to Φ = 0.67), and circularly polarized luminescence (CPL) property (|glum| = up to 3.5 × 10-3).

Gold-catalyzed dual C-C bond cleavage of biphenylenes bearing a pendant alkyne at ambient temperature.

We report a catalytic skeletal rearrangement of biphenylenes with a pendant alkyne moiety at room temperature by a cationic gold catalyst, which involves the cleavage of two bonds: the C-C bond of biphenylene and the C(sp)-C(sp2 or sp3) bond. Experimental and theoretical studies revealed that the reaction mechanism included π-activation of the alkyne, ring expansion and 1,2-carbon shift.

Role and Function of Class III LitR, a Photosensor Homolog from Burkholderia multivorans.

The LitR/CarH protein family is an adenosyl B12 (AdoB12)-dependent photoreceptor family with DNA-binding activity, and its homologs are widely distributed in the genomes of diverse bacterial genera. In this investigation, we studied the role and functions of a LitR homolog from a Gram-negative soil bacterium, Burkholderia multivorans, which does not possess an AdoB12-binding domain. Transcriptome analysis indicated the existence of 19 light-induced genes, including folE2, cfaB, litS, photolyase gene phrB2, and cryB, located in the region flanking litR Disruption of litR caused constitutive expression of all the light-inducible genes, while mutation in the light-induced sigma factor gene, litS, abolished the transcription of the phrB2 operon and the cfa operon, indicating that LitR and LitS play a central role in light-inducible transcription. A gel shift assay showed that recombinant protein LitR specifically binds to the promoter regions of litR and the folE2 operon, and its binding was weakened by UV-A illumination. LitR absorbs light at maximally near 340 nm and exhibited a photocyclic response and light-dependent dissociation of multimer into tetramer. The litR mutant produced a 20-fold-higher intracellular level of folate than that of the wild-type strain. Thus, the evidence suggests that LitR light-dependently regulates the transcription of litR itself and the folE2 operon, resulting in the production of folate, and then the expressed RNA polymerase complex containing σLitS directs the transcription of the phrB2 operon and the cfa operon. These light-dependent characteristics suggest that class III LitR, in complex with a UV-A-absorbing molecule, follows a novel light-sensing mechanism.IMPORTANCE Members of the LitR/CarH family are adenosyl B12-based photosensory transcriptional regulator involved in light-inducible carotenoid production in nonphototrophic bacteria. Our study provides the first evidence of the involvement of a class III LitR, which lacks an adenosyl B12-binding domain in the light response of Burkholderia multivorans belonging to betaproteobacteria. Our biochemical analysis suggests that class III LitR protein exhibits features as a photosensor including absorption of light at the UV-A region (λmax = ca. 340 nm), photocyclic response, and light-dependent dissociation. This suggests that class III LitR associates with a UV-A-absorbing molecule, and it has a photosensing mechanism distinguishable from that of the B12-based type.

Extracytoplasmic diaphorase activity of Streptomyces coelicolor A3(2).

Metabolism and utilization of plant-derived aromatic substances are fundamental to the saprophytic growth of Streptomyces. Here, we studied an enzyme activity reducing 2,6-dichlorophenolindophenol and nitroblue tetrazolium in the culture supernatant of Streptomyces coelicolor A3(2). N-terminal amino acid sequencing of a nitroblue tetrazolium-reducing enzyme revealed that the enzyme corresponds to the SCO2180 product. The protein exhibited a marked similarity with dihydrolipoamide dehydrogenase, the E3 subunit of 2-oxo-acid dehydrogenase complex. A recombinant SCO2180 protein formed a homodimer and exhibited a diaphorase activity catalyzing NADH-dependent reduction of various quinonic substrates. Similar nitroblue tetrazolium-reducing activities were observed for other Streptomyces strains isolated from soil, implying that the diaphorase-catalyzed reduction of quinonic substances widely occurs in the extracytoplasmic space of Streptomyces.

Regioselective Activation of a Sterically More Hindered C-C Bond of Biphenylenes Using an Alkene as Both a Directing Group and a Reaction Moiety.

The Rh-catalyzed intramolecular reaction of 1-(2-vinylaryl)-substituted biphenylenes was used to construct a dihydrobenzo[b]fluoranthene skeleton. This transformation was achieved by regioselective C-C bond cleavage of a sterically more hindered biphenylene site by using alkene as both a directing group and a reaction moiety. Furthermore, we measured and analyzed the photophysical properties of the new multicyclic fused compounds.

Ir-Catalyzed Synthesis of Substituted Tribenzosilepins by Dehydrogenative C-H/Si-H Coupling.

The Ir-catalyzed intramolecular reaction of 2',6'-diaryl-2-(hydrosilyl)biphenyls gave substituted tribenzosilepins by direct dehydrogenative C-H/Si-H coupling. This is the first example of catalytic construction of the tribenzosilepin skeleton. Enantiomerically pure tribenzosilepin was prepared by optical resolution using chiral HPLC, and its inversion barrier was calculated by measurement of rate of racemization using the Eyring kinetic equation under heating conditions.

High production of a class III lantipeptide AmfS in Streptomyces griseus.

AmfS, a class III lantipeptide serves as a morphogen in Streptomyces griseus. Here, we constructed a high production system of AmfS in S. griseus. We isolated S. griseus Grd1 strain defective in glucose repression of aerial mycelium formation and found it suitable for the overproduction of AmfS. Two expression vectors carrying the strong and constitutive ermE2 promoter were constructed using a multicopy number plasmid, pIJ702. The use of the Grd1 strain combined with the expression vectors enabled high production of AmfS by S. griseus into its culture broth. The expression system was also effective for the generation of abundant AmfS derived from Streptomyces avermitilis. In addition, site-directed mutagenesis revealed the amino acid residues essential for the morphogen activity of AmfS. These results indicate that the constructed system enables efficient production of class III lantipeptides by Streptomyces.

Role of the semi-conserved histidine residue in the light-sensing domain of LitR, a MerR-type photosensory transcriptional regulator.

The LitR/CarH protein family transcriptional regulator is a new type of photoreceptor based on the function of adenosyl B12 (AdoB12) as a light-sensitive ligand. Here, we studied a semi-conserved histidine residue (His132) in the light-sensing (AdoB12-binding) domain at the C-terminus of LitR from a thermophilic Gram-negative bacterium, Thermus thermophilus HB27. The in vivo mutation of His132 within LitR caused a reduction in the rate of carotenoid production in response to illumination. BIAcore analysis revealed that the illuminated-LitRH132A possesses high DNA-binding activity compared to the wild-type protein. The subunit structure analysis showed that LitRH132A performed an incomplete subunit dissociation. The ability of LitRH132A to associate with AdoB12 was reduced compared with that of the wild-type protein in an equilibration dialysis experiment. Overall, these results suggest that His132 of LitR is involved in the association with AdoB12 as well as the light-sensitive DNA-binding activity based on oligomer dissociation.

Developmental defect of cytochrome oxidase mutants of Streptomyces coelicolor A3(2).

To study the link between energy metabolism and secondary metabolism/morphological development in Streptomyces, knockout mutants were generated with regard to the subunits of the cytochrome oxidase supercomplex (CcO) in Streptomyces coelicolor A3(2). All mutants exhibited an identical phenotype: viable but defective in antibiotic production and cell differentiation when grown in both complex and minimal media. The growth yield of the CcO mutant was about half of that of the WT strain on glucose medium while both strains grew similarly on maltose medium. Intracellular ATP measurement demonstrated that the CcO mutant exhibited high intracellular ATP level. A similar elevation of intracellular ATP level was observed with regard to the WT strain cultured in the presence of BCDA, a copper-chelating agent. Reverse transcriptase PCR analysis demonstrated that the transcription of ATP synthase operon is upregulated in the CcO mutant. Addition of carbonylcyanide m-chlorophenylhydrazone, an inhibitor of ATP synthesis, promoted antibiotic production and aerial mycelia formation in the CcO mutant and BCDA-treated WT cells. We hypothesize that the deficiency of CcO causes accumulation of intracellular ATP, and that the high ATP level inhibits the onset of development in S. coelicolor.

The regulatory mechanism underlying light-inducible production of carotenoids in nonphototrophic bacteria.

Light is a ubiquitous environmental factor serving as an energy source and external stimulus. Here, I review the conserved molecular mechanism of light-inducible production of carotenoids in three nonphototrophic bacteria: Streptomyces coelicolor A3(2), Thermus thermophilus HB27, and Bacillus megaterium QM B1551. A MerR family transcriptional regulator, LitR, commonly plays a central role in their light-inducible carotenoid production. Genetic and biochemical studies on LitR proteins revealed a conserved function: LitR in complex with adenosyl B12 (AdoB12) has a light-sensitive DNA-binding activity and thus suppresses the expression of the Crt biosynthesis gene cluster. The in vitro DNA-binding and transcription assays showed that the LitR-AdoB12 complex serves as a repressor allowing transcription initiation by RNA polymerase in response to illumination. The existence of novel light-inducible genes and the unique role of the megaplasmid were revealed by the transcriptomic analysis of T. thermophilus. The findings suggest that LitR is a general regulator responsible for the light-inducible carotenoid production in the phylogenetically divergent nonphototrophic bacteria, and that LitR performs diverse physiological functions in bacteria.

Streptomyces metabolites in divergent microbial interactions.

Streptomyces and related bacteria produce a wide variety of secondary metabolites. Of these, many compounds have industrial applications, but the question of why this group of microorganism produces such various kinds of biologically active substances has not yet been clearly answered. Here, we overview the results from our studies on the novel function and role of Streptomyces metabolites. The diverged action of negative and positive influences onto the physiology of various microorganisms infers the occurrence of complex microbial interactions due to the effect of small molecules produced by Streptomyces. The interactions may serve as a basis for the constitution of biological community.

Role and Function of LitR, an Adenosyl B12-Bound Light-Sensitive Regulator of Bacillus megaterium QM B1551, in Regulation of Carotenoid Production.

UNLABELLED: The LitR/CarH family of proteins is a light-sensitive MerR family of transcriptional regulators that contain an adenosyl B12 (coenzyme B12 or AdoB12)-binding domain at the C terminus. The genes encoding these proteins are found in phylogenetically diverse bacterial genera; however, the biochemical properties of these proteins from Gram-positive bacteria remain poorly understood. We performed genetic and biochemical analyses of a homolog of the LitR protein from Bacillus megaterium QM B1551, a Gram-positive endospore-forming soil bacterium. Carotenoid production was induced by illumination in this bacterium. In vivo analysis demonstrated that LitR plays a central role in light-inducible carotenoid production and serves as a negative regulator of the light-inducible transcription of crt and litR itself. Biochemical evidence showed that LitR in complex with AdoB12 binds to the promoter regions of litR and the crt operon in a light-sensitive manner. In vitro transcription experiments demonstrated that AdoB12-LitR inhibited the specific transcription of the crt promoter generated by a σ(A)-containing RNA polymerase holoenzyme under dark conditions. Collectively, these data indicate that the AdoB12-LitR complex serves as a photoreceptor with DNA-binding activity in B. megaterium QM B1551 and that its function as a transcriptional repressor is fundamental to the light-induced carotenoid production. IMPORTANCE: Members of the LitR/CarH family are AdoB12-based photosensors involved in light-inducible carotenoid production in nonphototrophic Gram-negative bacteria. Our study revealed that Bacillus LitR in complex with AdoB12 also serves as a transcriptional regulator with a photosensory function, which indicates that the LitR/CarH family is generally involved in the light-inducible carotenoid production of nonphototrophic bacteria.