Chemoselective Transformation Research Articles

Boosting the activity of BiVOx via vanadium-promotion for highly selective oxidation of biomass-derived xylose toward formic acid

Formic acid is a versatile and promising value-added chemical derived from the chemoselective transformation of renewable biomass resources. However, achieving a high production rate of formic acid remains a pivotal and enormous challenge. Herein, we synthesized a sequence of BiVOx catalysts based on vanadium-promotion by a facile and cost-effective strategy for selective oxidation of biomass-derived xylose in water under O2 atmosphere. Particularly, the resultant Bi1V2Ox exhibited a remarkably enhanced catalytic activity, higher selectivity toward formic acid, and excellent recyclability, giving a yield of 75.8 % and a production rate of formic acid (9.22 mol FA molV−1h−1) at 170 °C within only 20 min. Such pronounced improved performance was largely attributed to the synergistic effect between V and Bi species, with higher surface area, abundant surface acidity, as well as stronger adsorption energy, meanwhile efficiently suppressing the formation of undesired by-products. This study demonstrates that the promotion of V in BiVOx leads to more efficient catalytic oxidation of xylose to formic acid, and with further driven research that proved great potential for generating formic acid from renewable biomass resources for future industrial application.

NQNHC Ligand-Enabled Cu(I)-Catalyzed Double Hydroamination: A Regio- and Chemoselective Bicyclization of o-Amino 1,6-Diyne.

In this work, we have developed an efficient method for the intramolecular double hydroamination of aniline by employing o-amino 1,6-diyne as a potential starting material. This protocol enables easy access to bioactive motif 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole through an intramolecular cascade bicyclization and concomitant isomerization pathway in one pot. This transformation has been effectively achieved by utilizing a stereoelectronically tuned, π-accepting NHC-supported copper(I) system. During ligand optimization trials, naphthoquinone-annulated N-heterocyclic carbene, Nq(IDipp) [1,3-bis(2,6-diisopropylphenyl)-4,5-naphthoquino-imidazolidene]-supported copper(I) complexes of the type Nq(IDipp)CuX (X = Cl or I) were synthesized and fully characterized using various spectroscopic techniques. For this conversion, NHC plays a crucial role in providing the optimum electron density around the metal center. It is a highly regio- and chemoselective transformation with a high atom economy and uses cheap, environmentally benign copper-based catalysts. Furthermore, a plausible mechanism has been proposed on the basis of experimental observations and literature support.

Impact of Ceria Support Morphology on Au Single‐Atom Catalysts for Benzyl Alcohol Selective Oxidation

AbstractAlcohol oxidations are a key industrial chemical transformation, with aldehydes and ketones finding use in an array of applications. Nobel metals are known for their activity towards this chemoselective transformation, however, sustainable catalyst synthesis requires optimal utilisation of these scarce elements. Here, we report Au catalytic systems based on the deposition of isolated Au sites on different morphologies of ceria in which different surface facets of the support are exposed. Through tailoring the support morphology and from extensive catalyst characterisation, it is shown that the exposed facet is critical for controlling the formation (or not) of isolated Au sites. Both the 110 and 111 facets are capable of this feat, yielding single‐atom sites for rod, octahedron, and polyhedron morphologies. In contrast, the 100 facet is not, resulting in Au nanoparticles on cubic ceria. This dictation over Au species is critical to benzyl alcohol oxidation capacity at mild conditions and in the absence of a soluble base, with only single‐atom catalyst (SAC) systems demonstrating activity. Furthermore, the exposed surface facet also governs the degree of surface oxygen vacancies, which is critical to catalyst activity due to their control over substrate adsorption strength, as revealed through T1/T2 NMR relaxation measurements.

Late-Stage Derivatization of Oleanolic Acid-Based Anti-HIV-1 Compounds.

A 12-keto-type oleanolic acid derivative (4) has been identified as a potent anti-human immunodeficiency virus type-1 (HIV-1) compound that demonstrates synergistic effects with several types of HIV-1 neutralizing antibodies. In the present study, we used a common key synthetic intermediate to carry out the late-stage derivatization of an anti-HIV compound based on the chemical structure of a 12-keto-type oleanolic acid derivative. To execute this strategy, we designed a diketo-type oleanolic acid derivative (5) for chemoselective transformation, targeting the carboxy group and the hydroxyl group on the statine unit, as well as the 3-carbonyl group on the oleanolic acid unit, as orthogonal synthetic handles. We carried out four types of chemoselective transformations, leading to identification of the indole-type derivative (16) as a novel potent anti-HIV compound. In addition, further optimization of the β-hydroxyl group on the statine unit provided the R-4-isobutyl γ-amino acid-type derivative (6), which exhibited potent anti-HIV activity comparable to that of 4 but with reduced cytotoxicity.

Phosphazene-Catalyzed Regioselective Condensation of Allyl Thioethers with Aldehydes: A Rapid Approach to 1,3-Dienyl Sulfides, -Sulfoxides and -Sulfones

AbstractThe phosphazene-catalyzed regioselective condensation of allyl thioethers with aldehydes is investigated. Upon treatment of allyl thioethers with P4-t-Bu, allyl thioether anions are generated and rapidly react with aromatic aldehydes, leading to 1,3-dienyl sulfides in good yields with high regioselectivity. This finding provides an alternative approach for the preparation of allyl thioether anions in a regioselective manner. In addition, chemoselective transformations of 1,3-dienyl sulfides to provide the corresponding 1,3-dienyl sulfoxides or 1,3-dienyl sulfones are also demonstrated.

Direct assembly of N-sulfamoyl lactam scaffolds bearing a zinc-binding group for inhibiting metalloenzymes based on desymmetrization of sulfamide and the Castagnoli-Cushman reaction

Sulfamide was desymmetrized by a reaction with aldehydes to give N-sulfamoylimines. The latter reagents were successfully introduced into diastereo- and chemoselective transformation with cyclic anhydride using the Castagnoli-Cushman reaction to give unprotected N-sulfamoyl tetrahydroisoquinolonic (THIQ) acids under simple metal-free protocol. Thereby, the first general approach to the direct assembly of six-membered N-sulfamoyl lactams was developed. The synthesized compounds belong to representative, drug-like chemotypes with their well-defined three-dimensional structures and tunable physicochemical properties. In an attempt to probe their pharmacological potential, the newly synthesized lactams were screened against therapeutically relevant human and bacterial carbonic anhydrases, with some derivatives showing low micromolar enzyme inhibitory profiles.

Synthesis of Chroman-2,4-diones via Ring-Opening/Ring-Closing Reaction Involving Palladium-Catalyzed Intramolecular Aryloxycarbonylation.

Palladium-catalyzed aminocarbonylation of 3-iodochromone was studied in the presence of primary and secondary amines using atmospheric pressure of carbon monoxide as a carbonyl source. This procedure successfully provided a library of chromone-3-carboxamides and 3-substituted chroman-2,4-diones in 40 to 92% isolated yields. The reaction proceeded via highly chemoselective aminocarbonylation (up to 100%) in the presence of secondary amines by using monodentate or bidentate phosphine ligands. The tendency of 3-iodochromone substrate to undergo ANRORC rearrangement with N-nucleophiles was crucial to shift the reaction toward an unprecedented chemoselective carbonylative transformation, where a late-stage carbonyl insertion is favored concomitantly to the last ring-closure step. The proposed aza-Michael addition/ring-opening/intramolecular aryloxycarbonylation sequence showed compatibility, uniquely, to primary amines when XantPhos was used as a ligand. The solid-state structures of chromone-3-carboxamide (2a) and chroman-2,4-dione (3s) were undoubtedly established by single-crystal XRD analysis. A catalytic cycle was proposed to rationalize the formation of the two types of carbonylated compounds.

Designing Cobalt(II) Complexes for Tandem Dehydrogenative Synthesis of Quinoline and Quinazoline Derivatives.

In this work, we have constructed three new Co(II) complexes in which steric features govern their structural geometry. The metal ligand-cooperation behavior of the alkoxy arm is utilized to explore the catalytic activities of these complexes with respect to dehydrogenation. A wide range of C-3-substituted quinoline and quinazoline derivatives were synthesized in high yields. The developed protocol's usefulness is enhanced by the chemoselective transformation of different fatty alcohols to synthesize heterocycles having distal unsaturation. Various kinetic, mechanistic, and control studies were conducted to comprehend the reaction route.

Base-controlled NHC-Ru-catalyzed transfer hydrogenation and α-methylation/transfer hydrogenation of ketones using methanol

Herein, we report the NHC-Ru catalyst system that realizes the chemo-selective transformation of ketones with methanol. By simply changing the base, a broad range of structurally diverse ketones, could be selectively and efficiently converted to the corresponding β-methylated secondary alcohols or secondary alcohols. Remarkably, this catalytic system was very effective for the synthesis of bio-related molecules and deuterated alcohols, as well as the three-component coupling between methyl ketones, primary alcohols, and methanol. The reaction mechanism was further revealed by experiment and DFT mechanistic investigations.

Easy Access to Tertiary Amines from Carbonyl Compounds with Substituted Amine-Boranes: A Substrate, Catalyst, and Additive Free Approach Under Mild Conditions.

Tertiary amines are ubiquitous and play an essential role in organocatalysis, pharmaceuticals, and fine chemicals. Amongst various synthetic procedures known for their synthesis, the reductive amination of carbonyl compounds has been found to be a proficient method. Over the past few decades, different synthetic strategies for reductive amination have been developed. Most of them suffer from the use of transition metals and/or harsh reaction conditions. Herein, we present an efficient, operationally simple protocol for the chemoselective transformation of carbonyl compounds to tertiary amines under benign conditions. The strategy encompasses broad substrate scope under the metal-free condition at room temperature and does not require any solvent. A detailed mechanistic investigation was performed with the aid of control experiments and computational study to shed light on the reaction pathway.

Chemoselective Transformations of Amides: An Approach to Quinolones from β-Amido Ynones.

An efficient and chemoselective transformation of β-amido ynones to 3-acyl-substituted quinolones 2 and 3-H-quinolones 4 has been developed. In this reaction, β-cyclic amido ynones can be selectively transformed into quinolones 2 in anhydrous EG via a selective C═O bond cleavage, 1,5-O migration, and C═C bond recombination process. The practical approach of this reaction renders it a viable alternative for the construction of various quinolones.

Anthraquinone-Catalyzed Photooxidation of Boronic Acids in a Bio-Based Solvent (2-Me-THF)

AbstractThe phenol moiety appears in a wide variety of natural products, exhibiting biological activity, and in numerous active pharmaceutical compounds. Boronic acids are potential precursors of the phenol scaffold, and a plethora of efforts has been focused in developing novel and green protocols, targeting their chemoselective transformation into phenols. Photochemistry is a rapidly expanding research field converting light energy into chemical potential. Photochemical aerobic processes possess additional advantages to photochemistry and may find applications in chemical industries. Herein, a low-catalyst-loading anthraquinone-catalyzed photochemical process is demonstrated, under CFL lamp irradiation, while exploiting 2-Me-THF as the reaction medium for the conversion of boronic acids into phenols. Furthermore, a broad substrate scope was employed.

A Metal‐Free C−H Amination‐Based Strategy for N‐Amino Indole Synthesis

AbstractAn oxidative cyclization of electron‐rich α‐arylhydrazones promoted by phenyliodine bis(trifluoroacetate) (PIFA) has been accomplished. This metal‐free, chemoselective transformation allows to obtain synthetically and medicinally important N‐amino‐1H‐indoles, obviating the need for pre‐functionalization of substrates.

An Iron-Catalyzed Protein Desulfurization Method Reminiscent of Aquatic Chemistry.

One pillar of protein chemical synthesis based on the application of ligation chemistries to cysteine is the group of reactions enabling the selective desulfurization of cysteine residues into alanines. Modern desulfurization reactions use a phosphine as a sink for sulfur under activation conditions involving the generation of sulfur-centered radicals. Here we show that cysteine desulfurization by a phosphine can be effected efficiently by micromolar concentrations of iron under aerobic conditions in hydrogen carbonate buffer, that is using conditions that are reminiscent of iron-catalyzed oxidation phenomena occurring in natural waters. Therefore, our work shows that chemical processes taking place in aquatic systems can be adapted to a chemical reactor for triggering a complex chemoselective transformation at the protein level, while minimizing the resort to harmful chemicals.

An Iron‐Catalyzed Protein Desulfurization Method Reminiscent of Aquatic Chemistry

AbstractOne pillar of protein chemical synthesis based on the application of ligation chemistries to cysteine is the group of reactions enabling the selective desulfurization of cysteine residues into alanines. Modern desulfurization reactions use a phosphine as a sink for sulfur under activation conditions involving the generation of sulfur‐centered radicals. Here we show that cysteine desulfurization by a phosphine can be effected efficiently by micromolar concentrations of iron under aerobic conditions in hydrogen carbonate buffer, that is using conditions that are reminiscent of iron‐catalyzed oxidation phenomena occurring in natural waters. Therefore, our work shows that chemical processes taking place in aquatic systems can be adapted to a chemical reactor for triggering a complex chemoselective transformation at the protein level, while minimizing the resort to harmful chemicals.

A Simple Four-Step Synthesis of the Potato Alkaloid Demissidine from the Common Sapogenin Tigogenin.

A simple synthesis method of solanidane alkaloids from common steroidal sapogenins was developed. Previously described multi-step transformations of tigogenin to demissidine (8-12 steps) were shortened to four steps only. The key-step of the present synthesis was the epimerization at C25 of the lactam intermediate. Different approaches to this reaction, i. e., a classical one via enolate, and a chemoselective umpolung transformation, were thoroughly investigated. The epimerization step is unnecessary if the starting sapogenin has the same configuration at C25 as the target alkaloid because the configuration at C25 (either R or S) remains intact throughout the synthesis. Thus, the related solanidane alkaloids, 12β-hydroxy-25-epi-demissidine and 5-epi-demissidine, were synthesized in the three-step procedure with retention of configuration at this stereogenic center from rockogenin (25R-5α-sapogenin) or sarsasapogenin (25S-5β-sapogenin), respectively.

Ru(II) catalyzed chelation assisted C(sp2)-H bond functionalization along with concomitant (4 + 2) annulation.

Efficacious protocols have been established to synthesize a structurally privileged Π-extended coumarin-fused pyridone nucleus by activating the vinylic C(sp2)-H bond of coumarin-3-carboxamide under the influence of inexpensive Ru(II)-metal. Here an N-methoxy carboxamide entity has been exploited as the chelating fragment to manifest C(sp2)-H bond functionalization with a concomitant (4 + 2) annulation reaction, resulting in heterocyclic ring-forming protocols along with sulfoxonium ylide and iodonium ylide as representative bench-stable carbene surrogates. This diverse heterocycle formation via carbene insertion strategies, is further expanded to activate the ortho-C(sp2)-H bonds of different heterocycles by employing the sp2-N moiety as the directing group to develop acyl-alkylated/alkenylated quinazolines, isoxazoles and highly fluorescent pyridone-N-oxides. Intriguingly, during an evaluation of the versatility of the current protocols, a one-pot double C-H activation has been rationalized in the presence of iodonium ylide, which results in biologically potent benzimidazole-fused coumarin-centered bridge-headed polycyclic heteroarenes. Furthermore, a chemo-selective late-stage synthetic transformation is being designed to develop differently substituted pyridone analogues by switching the nature of the reducing agent. In addition, a photophysical experiment was done on one pyridine-N-oxide compound (7e) and delightfully it exhibited fluorescence quenching activity selectively in the presence of Al3+ ions, which appears to be a unique feature of our methodology. Finally, upon correlation of the merit of the developed pathways, the iodonium ylide mediated strategy appears to be superior.

Chemoselective derivatisation and ultrahigh resolution mass spectrometry for the determination of hydroxyl functional groups within complex bio-oils.

Bio-oils are a renewable alternative resource for the production of fine chemicals and fuels. Bio-oils are characterised by a high content of oxygenated compounds with a diverse array of different chemical functionalities. Here, we performed a chemical reaction to transform the hydroxyl group of the various components in a bio-oil prior to characterisation with ultrahigh resolution mass spectrometry (UHRMS). The derivatisations were first evaluated using twenty lignin-representative standards with different structural features. Our results indicate a highly chemoselective transformation of the hydroxyl group despite the presence of other functional groups. Mono- and di-acetate products were observed in acetone-acetic anhydride (acetone-Ac2O) mixtures for non-sterically hindered phenols, catechols and benzene diols. Dimethyl sulfoxide-Ac2O (DMSO-Ac2O) reactions favoured the oxidation of primary and secondary alcohols and the formation of methylthiomethyl (MTM) products of phenols. The derivatisations were then performed in a complex bio-oil sample to gain insights into the hydroxyl group profile of the bio-oil. Our results indicate that the bio-oil before derivatisation is composed of 4500 elemental compositions containing 1-12 oxygen atoms. After the derivatisation in DMSO-Ac2O mixtures, the total number of compositions increased approximately five-fold. The reaction was indicative of the variety of hydroxyl group profiles within the sample in particular the presence of phenols that were ortho and para substituted, non-hindered phenols (about 34%), aromatic alcohols (including benzylic and other non-phenolic alcohols) (25%), and aliphatic alcohols (6.3%) could be inferred. Phenolic compositions are known as coke precursors in catalytic pyrolysis and upgrading processes. Thus, the combination of chemoselective derivatisations in conjunction with UHRMS can be a valuable resource to outline the hydroxyl group profile in elemental chemical compositions in complex mixtures.

Fast Access to 5-Hydroxymethyl Derivatives of 2′-Deoxyuridine Promoted by Acidic Amberlyst 15 Resin

Abstract5-Hydroxymethyl derivatives of pyrimidine nucleosides are an important class of biologically relevant compounds. In addition, such derivatives and related compounds can be functionalized for various applications. To enable fast, economical, and efficient access to 5-hydroxymethylated derivatives of 2′-deoxyuridine, we report a method for the O-5 chemoselective transformation of unprotected 5-(hydroxymethyl)-2′-deoxyuridine through selective etherification in the presence of an alcohol promoted by acidic Amberlyst 15 resin at room temperature. These mild conditions constitute a significant improvement compared with the harsh conditions previously described. Applied to various primary or secondary alcohols, the reaction showed a broad substrate scope, and 24 C(5)-modified derivatives of 5hmdU were synthesized with good isolated yields. Notably, this efficient procedure represents a straightforward method for preparing (i) several useful building blocks for subsequent chemical ligation by using CuAAC reactions; (ii) natural hypermodified thymidines and analogues, including glycosylated derivatives; and (iii) cyanoethyl-protected 5hmdU, useful for solid-phase oligonucleotide syntheses.

Visible-Light-Mediated Organophotocatalyzed C(sp3)–H Activation and Intramolecular Cyclization

AbstractA metal-free approach for C(sp3)–H activation followed by an intramolecular Giese reaction to construct a wide range of cyclic ether scaffolds of various ring sizes under environmentally benign and straightforward conditions is reported. An easily prepared pyrylium salt is employed as an organophotocatalyst for this visible-light-driven, highly atom-economical (PMI = 64.34 g/g for a 0.2 mmol scale), cost-effective, and chemoselective transformation. The reported method has a broad functional-group tolerance, resulting in good-quality products. Furthermore, downstream functionalizations of a product and a gram-scale synthesis (PMI = 17.41 g/g for a 10 mmol scale) are demonstrated, highlighting our method’s advantages.