In-Cell Engineering of Protein Crystals into Hybrid Solid Catalysts for Artificial Photosynthesis.

The emergence of protein-based crystalline materials offers promising opportunities in enzyme immobilization. However, the current systems used for encapsulation of protein crystals are limited to either exogenous small molecules or monomeric proteins. In this work, polyhedra crystals were used to simultaneously encapsulate the foreign enzymes FDH and the organic photocatalyst eosin Y. These hybrid protein crystals are prepared easily by cocrystallization within a cell without a requirement for complex purification processes because they spontaneously form 1 µm scale solid particles. After immobilization within protein crystals, the recombinant FDH is recyclable and thermally stable and maintains 94.4% activity compared to the free enzyme. In addition, the incorporation of eosin Y endows the solid catalyst with CO2-formate conversion activity based on a cascade reaction. This work indicates that engineering protein crystals by both in vivo and in vitro strategies will provide robust and environmentally friendly solid catalysts for artificial photosynthesis.

Cross-Linked Crystals of Dirhodium Tetraacetate/RNase A Adduct Can Be Used as Heterogeneous Catalysts.

Due to their unique coordination structure, dirhodium paddlewheel complexes are of interest in several research fields, like medicinal chemistry, catalysis, etc. Previously, these complexes were conjugated to proteins and peptides for developing artificial metalloenzymes as homogeneous catalysts. Fixation of dirhodium complexes into protein crystals is interesting to develop heterogeneous catalysts. Porous solvent channels present in protein crystals can benefit the activity by increasing the probability of substrate collisions at the catalytic Rh binding sites. Toward this goal, the present work describes the use of bovine pancreatic ribonuclease (RNase A) crystals with a pore size of 4 nm (P3221 space group) for fixing [Rh2(OAc)4] and developing a heterogeneous catalyst to perform reactions in an aqueous medium. The structure of the [Rh2(OAc)4]/RNase A adduct was investigated by X-ray crystallography: the metal complex structure remains unperturbed upon protein binding. Using a number of crystal structures, metal complex accumulation over time, within the RNase A crystals, and structures at variable temperatures were evaluated. We also report the large-scale preparation of microcrystals (∼10-20 µm) of the [Rh2(OAc)4]/RNase A adduct and cross-linking reaction with glutaraldehyde. The catalytic olefin cyclopropanation reaction and self-coupling of diazo compounds by these cross-linked [Rh2(OAc)4]/RNase A crystals were demonstrated. The results of this work reveal that these systems can be used as heterogeneous catalysts to promote reactions in aqueous solution. Overall, our findings demonstrate that the dirhodium paddlewheel complexes can be fixed in porous biomolecule crystals, like those of RNase A, to prepare biohybrid materials for catalytic applications.

Synthesis of the diazatricycloundecane scaffold via gold(I)-catalysed Conia-ene-type 5-exo-dig cyclization and stepwise substituent assembly for the construction of an sp3-rich compound library.

The bridged diazatricycloundecane sp3-rich scaffold was synthesised via the gold(I)-catalysed Conia-ene reaction. The electron-donating property of the siloxymethyl group on alkyne 1 enabled 6-endo-dig cyclization, whereas the ethoxy carbonyl group on alkyne 4 led to 5-exo-dig cyclization with complete regioselectivity in the Conia-ene reaction. The resulting bridged diazatricycloundecane scaffold 5 allowed the construction of a library of sp3-rich compounds. Among the compounds synthesised, compounds 6e and 6f inhibited the hypoxia inducible factor 1 (HIF-1) downstream signaling pathway without affecting HIF-1α mRNA expression.

Discovery of three-dimensional bicyclo[3.3.1]nonanols as novel heat shock protein 90 inhibitors.

We developed an efficient synthetic method for constructing bicyclo[3.3.1]nonane, an sp3-carbon-rich three-dimensional scaffold consisting of two fused six-membered rings. Among the bicyclo[3.3.1]nonanols synthesized, several bicyclo[3.3.1]nonane derivatives were found to inhibit gene transcription by hypoxia-inducible factor-1 (HIF-1). The structure-activity relationship study revealed that the number of hydrophobic functional groups and a carboxylic acid moiety in the bicyclo[3.3.1]nonanols are important for inhibitory activities of both gene transcription by HIF-1 and cell growth. Bicyclo[3.3.1]nonanols fluctuated the amounts of client proteins of heat shock protein (HSP) 90 without inducing a heat shock response in cells and specifically inhibited the ATPase activity of HSP90. These results indicate that bicyclo[3.3.1]nonanols are novel HSP90 inhibitors with a different mechanism of action from conventional HSP90 inhibitors.

Iodophenyl-conjugated closo-dodecaborate as a promising small boron molecule that binds to serum albumin and accumulates in tumor.

The development of novel boron carriers applicable to various cancers is required for further expansion of boron neutron capture therapy (BNCT). In this study, we took advantage of the fact that serum albumin accumulates in tumors and developed a boron compound that interacts non-covalently with the serum albumin. 4-Iodophenylbutanamide was chosen as an albumin ligand and conjugated with closo-dodecaborate (boron-conjugated 4-iodophenylbutanamide: BC-IP). BC-IP was found to be water soluble with low cytotoxicity. The IC50 values of BC-IP were 475 µM for U87MG cells, 738 µM for HeLa cells, and > 1000 µM for A549 cells. The dissociation constant (Kd) value of BC-IP to HSA was 148 ± 8 µM, while that of disodium closo-dodecaborate (4) was > 1000 µM. Significant tumor accumulation was observed in the U87MG tumor mouse model 3 h after injection. The boron concentration in the tumor reached a maximum of 11 µgB/g at 3 h and gradually decreased to 2.4 and 2.3 µgB/g at 12 and 24 h, respectively.

Enantioselective Synthesis of Oxazaborolidines by Palladium-Catalyzed N-H/B-H Double Activation of 1,2-Azaborines.

A palladium-catalyzed N-H/B-H double activation of 1,2-dihydro-1,2-benzazaborines proceeded via cycloaddition with vinyl ethylene carbonate to produce polycyclic oxazaborolidines in 31-96 % yield. The key step in this process is the release of molecular hydrogen from a borate intermediate. Using a SPINOL-derived phosphoramidite as a chiral ligand, chiral oxazaborolidines were synthesized in good to high yields with excellent enantioselectivity (up to 95 % ee). The vinyl group of the resulting oxazaborolidine underwent metathesis, Heck reaction, and Wacker oxidation without affecting the oxazaborolidine framework.

Synthesis of Three-Dimensional (Di)Azatricyclododecene Scaffold and Its Application to Peptidomimetics.

A novel sp3 carbon-rich tricyclic 3D scaffold-based peptide mimetic compound library was constructed to target protein-protein interactions. Tricyclic framework 7 was synthesized from 9-azabicyclo[3,3,1]nonan-3-one (11) via a gold(I)-catalyzed Conia-ene reaction. The electron-donating group on the pendant alkyne of cyclization precursor 12 b-e was the key to forming 6-endo-dig cyclized product 7 with complete regioselectivity. Using the synthetic strategy for regioselective construction of bridged tricyclic framework 7, a diazatricyclododecene 3D-scaffold 8 a, which enables the introduction of substituents into the scaffold to mimic amino acid side chains, was designed and synthesized. The peptide mimetics 21 a-u were synthesized via step-by-step installation of three substituents on diazatricyclododecene scaffold 8 a. Compounds 21 a-h were synthesized as α-helix peptide mimics of hydrophobic ZZxxZ and ZxxZZ sequences (Z=Leu or Phe) and subjected to cell-based assays: antiproliferative activity, HIF-1 transcriptional activity which is considered to affect cancer malignancy, and antiviral activity against rabies virus. Compound 21 a showed the strongest inhibitory activity of HIF-1 transcriptional activity (IC50 =4.1±0.8 µM), whereas compounds 21 a-g showed antiviral activity with IC50 values of 4.2-12.4 µM, suggesting that the 3D-scaffold 8 a has potential as a versatile peptide mimic skeleton.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement.

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

Copper(I)-catalysed intramolecular hydroarylation-redox cross-dehydrogenative coupling of N-propargylanilines with phosphites.

Intramolecular hydroarylation-redox cross-dehydrogenative coupling of N-propargylanilines with phosphite diesters proceeded in the presence of Cu(I)-catalysts (20 mol%) to selectively give 2-phosphono-1,2,3,4-tetrahydroquinolines in good yields with 100% atomic utilization. P-H and two C-H bonds are activated at once and these hydrogen atoms are trapped by a propargylic triple bond in the molecule.

Rhodium(III)-catalysed decarboxylative C-H functionalization of isoxazoles with alkenes and sulfoxonium ylides.

Decarboxylative C-H functionalization of isoxazoles with electron-deficient alkenes and sulfoxonium ylides at the C5 position was achieved in the presence of rhodium(iii) catalysts to give the corresponding alkenylation and acylmethylation products, respectively.

Aliphatic Radical Relay Heck Reaction at Unactivated C(sp3 )-H Sites of Alcohols.

The Mizoroki-Heck reaction is one of the most efficient methods for alkenylation of aryl, vinyl, and alkyl halides. Given its innate nature, this protocol requires the employment of compounds possessing a halogen atom at the site of functionalization. However, the accessibility of organic molecules possessing a halogen atom at a particular site in aliphatic systems is extremely limited. Thus, a protocol that allows a Heck reaction to occur at a specific nonfunctionalized C(sp3 )-H site is desirable. Reported here is a radical relay Heck reaction which allows selective remote alkenylation of aliphatic alcohols at unactivated ß-, γ-, and δ-C(sp3 )-H sites. The use of an easily installed/removed Si-based auxiliary enables selective I-atom/radical translocation events at remote C-H sites followed by the Heck reaction. Notably, the reaction proceeds smoothly under mild visible-light-mediated conditions at room temperature, producing highly modifiable and valuable alkenol products from readily available alcohols feedstocks.

Generation of an 4-Isoxazolyl Anion Species: Facile Access to Multifunctionalized Isoxazoles.

A direct functionalization of unsubstituted isoxazole (1) was achieved by generation of 4-isoxazolyl anion species (3). An efficient 4-iodination of isoxazole and halogen-metal exchange reaction using a turbo Grignard reagent (iPrMgCl⋅ LiCl) were essential for the generation of 3, which reacted with various electrophiles to give 4-functionalized isoxazoles in good to high yields. Isoxazolyl boronate, boronic acid, and stannane were also synthesized as useful building blocks from 1. The current methods enabled us to synthesize multi-functionalized isoxazoles by introducing each substituent into the desired positions. Furthermore, total synthesis of triumferol, which was isolated from Triumfetta rhomboidea, was achieved from 1 in only three steps.

Directing/protecting-group-free synthesis of tetraaryl-substituted pyrazoles through four direct arylations on an unsubstituted pyrazole scaffold.

A directing/protecting-group-free synthesis of 1,3,4,5-tetraaryl-substituted pyrazoles was achieved through four transition metal-catalyzed direct arylations. Various pyrazoles with four different aryl rings were obtained using readily available reagents from an unsubstituted pyrazole. Two aryl-substituted pyrazoles showed intense violet fluorescence, high quantum yields (Φf =0.68, 0.64), and large Stokes shifts (19000, 15200 cm(-1) ).

Methylene Insertion into Nitrogen-Heteroatom Single Bonds of 1,2-Azoles via a Zinc Carbenoid: An Alternative Tool for Skeletal Editing.

The nitrogen-heteroatom single bonds of 1,2-azoles and isoxazolines underwent methylene insertion in the presence of CH2 I2 (6 equiv.) and diethylzinc (3 equiv.) to produce a wide variety of the ring-expanded six-membered heterocycles. Density functional theory calculations suggest that the methylene insertion proceeds via cleavage of nitrogen-heteroatom single bonds followed by ring closure.

Discovery of Disubstituted Carboranes as Inhibitors of Heat Shock Protein 90-Heat Shock Factor 1 Interaction.

Efficient synthesis of disubstituted para- and ortho-carboranes (2 and 3, respectively) was achieved. Among the compounds synthesized, 3e showed potent suppression of hypoxia-inducible factor 1 (HIF-1) transcriptional activity under hypoxia by a cell-based reporter gene assay. Detailed mechanism-of-action studies revealed that 3e reduced the stability of heat shock protein (HSP) 90 client proteins such as CDK4, AKT, and cyclin D1 by inhibiting HSP90 chaperone activity but did not induce a heat shock response (HSR), which may cause drug resistance. Furthermore, 3e inhibited the interaction between HSP90 and heat shock factor 1 (HSF1), resulting in reducing HSF1 protein stability and thereby suppressing the transcription of heat shock proteins.

Asymmetric Synthesis of Bicyclic Isoxazolines via Dearomative Cycloaddition of 4-Nitroisoxazoles with Zwitterionic π-Allyl Palladium Species.

Enantioselective dearomative cycloadditions of 4-nitroisoxazoles with vinylethylene carbonate (2) proceeded in the presence of Pd2(dba)3 and (S)-DTBM-SEGPHOS to give the corresponding bicyclic isoxazolines 3 and 4 in good to high yields with excellent enantioselectivities (≤99% ee). This synthetic approach could be applied to N-tosyl vinyl aziridine and 2-methylidenetrimethylene carbonate. Further transformations of the resulting cycloadducts 4a and 4i yielded not only its derivatives 10 and 11 but also the novel tetracyclic skeleton 12.

Chemoselective α-Trifluoroacetylation of Amides Using Highly Electrophilic Trifluoroacetic Anhydrides and 2,4,6-Collidine.

Chemoselective α-acylation of tertiary amides proceeded with highly electrophilic acid anhydrides and weak bases under mild conditions. ß-Ketoamides containing trifluoroacetyl or perfluoroacyl groups were selectively obtained even in the presence of other functional groups such as ketone, ester, etc. Density functional theory calculations suggest that 1-acyloxyenamine is the key intermediate for the chemoselective α-acylation.

Efficient neutron capture therapy of glioblastoma with pteroyl-closo-dodecaborate-conjugated 4-(p-iodophenyl)butyric acid (PBC-IP).

Boron neutron capture therapy (BNCT) has been applied for clinical trials on glioblastoma patients since 1950s, however, the low survival rate under the treatments has hampered the widespread use of BNCT. In this study, we developed a novel boron agent, PBC-IP, which consists of three functional groups: FRα-targeting, 10B resource (twelve 10B atoms in the molecule), and albumin-binding moieties. PBC-IP was selectively taken up by glioma cell lines such as C6, F98, and U87MG cells and accumulated 10- to 20-fold higher than L-4­boronophenylalanine (BPA). PBC-IP administrated intravenously to the human glioblastoma (U87MG) xenograft model showed higher boron accumulation in tumors (29.8 µg [10B]/g at 6 h) than BPA (9.6 µg [10B]/g at 3 h) at a 25 mg [10B]/kg dose, effectively suppressing tumor growth after thermal neutron irradiation. PBC-IP administrated via convection-enhanced delivery (CED) accumulated in the F98 glioma orthotopic rat model, achieving 26.5 µg [10B]/g in tumors with tumor/normal (T/N) brain and tumor/blood (T/B) boron ratios of 37.8 and 94.6, respectively, 3 h after CED. Survival at 180 days after BNCT was 50% in the PBC-IP group and 70% in the combined BPA and PBC-IP groups, with no residual brain tumors.

Synthesis of isoxazoloazaborines via gold(I)-catalyzed propargyl aza-Claisen rearrangement/borylative cyclization cascade.

Isoxazoloazaborines 3 and 5 have been synthesized from 4-N-propargylaminoisoxazole 1via gold(I)-catalyzed propargyl aza-Claisen rearrangement followed by electrophilic borylative cyclization in 27-86% yields. In situ generation of isoxazole 2 having an amino group and allenyl functionality is essential to give highly substituted isoxazoloazaborines 3 and 5, although the conventional propargyl aza-Claisen rearrangement readily affords the corresponding nitrogen-containing heterocycles, such as pyridines and pyrroles. The resulting isoxazoloazaborine 5a underwent the N-O bond insertion of zinc carbenoid to give oxazine-fused azaborine 6 in 48% yield.

Asymmetric construction of densely functionalized three-dimensional aza-tetracyclic scaffolds for drug discovery.

Asymmetric construction of densely functionalized three-dimensional aza-tetracyclic scaffolds were achieved by intramolecular Diels-Alder reaction of bicyclic precursors derived from L-tyrosine. Three substituents were systematically introduced into the developed scaffolds in various combinations, demonstrating their high utility in drug discovery.