Regioselective Cleavage Research Articles

Synergistic [4 + 1] Spiroannulation and Selective Ring-Opening Strategy toward γ-Spirolactams.

A unique and propitious [4 + 1] spiroannulation of 2-aryl-4H-benzo[d][1,3]oxazine with maleimide has been delineated, furnishing diversely embellished γ-spirolactams featuring pendant benzyl alcohol via Rh(III)-catalyzed consecutive ring closing/ring opening followed by regioselective cleavage of the C4-O bond of 1,3-benzoxazine promoted by an in situ generated water maneuver constructing new C-C, C-N, and C-O bonds at a go. A detailed mechanistic study, including a thorough analysis of the incorporation of an extra oxygen source, has been showcased to make this strategy for structurally orchestrated isoindoline-1-one spirosuccinimides.

Regioselective Cleavage and Reconfiguration of C-S Bonds with Diazo Compounds.

A coupling reaction between diazo compounds and phenyl benzyl sulfide catalyzed by TfOH has been reported. This reaction can synthesize important α-arylthio carbonyl compounds via regioselective cleavage and reconfiguration of C-S bonds, and various functional groups were tolerant to the reaction conditions. Mechanistic studies have conclusively established that the pivotal intermediate in the reaction was meticulously investigated through spectroscopic evidence, complemented by rigorous control experiments.

K2S2O8-Mediated or Azobisisobutyronitrile-Catalyzed Regioselective Aerobic Oxidative Cleavage of 1-Arylbutadienes to Cinnamaldehydes.

This work reveals the regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes mediated by K2S2O8 or catalyzed by azobisisobutyronitrile, a very common free radical initiator, in an easy to handle, simple procedure and free of transition metals. This approach demonstrates excellent regioselectivity, mild reaction conditions, and compatibility with a broad range of functional groups (45 examples).

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis.

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

Asymmetric Molecular Adsorption and Regioselective Bond Cleavage on Chiral PdGa Crystals.

Homogenous enantioselective catalysis is nowadays the cornerstone in the manufacturing of enantiopure substances, but its technological implementation suffers from well-known impediments like the lack of endurable catalysts exhibiting long-term stability. The catalytically active intermetallic compound Palladium-Gallium (PdGa), conserving innate bulk chirality on its surfaces, represent a promising system to study asymmetric chemical reactions by heterogeneous catalysis, with prospective relevance for industrial processes. Here, this work investigates the adsorption of 10,10'-dibromo-9,9'-bianthracene (DBBA) on the PdGa:A( ) Pd3-terminated surface by means of scanning tunneling microscopy (STM) and spectroscopy (STS). A highly enantioselective adsorption of the molecule evolving into a near 100% enantiomeric excess below room temperature is observed. This exceptionally high enantiomeric excess is attributed to temperature activated conversion of the S to the R chiral conformer. Tip-induced bond cleavage of the R conformer shows a very high regioselectivity of the DBBA debromination. The experimental results are interpreted by density functional theory atomistic simulations. This work extends the knowledge of chirality transfer onto the enantioselective adsorption of non-planar molecules and manifests the ensemble effect of PdGa surfaces resulting in robust regioselective debromination.

Regioselective oxidative cleavage of conjugated dienes to access α,β-unsaturated nitriles

A highly regioselective oxidative cleavage method has been developed for the synthesis of α,β-unsaturated nitriles and N-15 atom labeled α,β-unsaturated nitriles from unsymmetric conjugated dienes and ammonium salts.

Visible-light-mediated metal-free regioselective oxidative C-C bond cleavage of lignin dimers to aromatic acids.

The upgrading of lignin is a sustainable and promising pathway for fossil-based aromatic compounds but always faces low selectivity. Herein, a metal-free photocatalyst, 2,4,6-triphenylpyrylium tetrafluoroborate (TPP), was illustrated to remarkably facilitate the regioselective oxidative Cα-Cβ bond cleavage of β-1 and β-O-4 lignin alcohol/ketone models into aromatic acids (92-99% yields) under visible-light irradiation at room temperature without any additive/co-catalyst, which was enabled by the synergistic effect of Cβ-H⋯C(TPP) interaction and·˙O2-/1O2 species. The synergy of the catalyst-substrate interaction and active species offers a reference for the enhancive and selective transformation of polymeric biomass and complex molecules.

Copper-Catalyzed Regioselective Cyanation of Indoles via C-H Bond Activation with α-Aminoacetonitriles

AbstractA metal-free and organic cyanating agent, 2-(diisopropylamino)acetonitrile is disclosed for the regioselective synthesis of cyanated indoles such as 2-cyanoindole and 3-cyanoindole derivatives. This method can tolerate a variety of functional groups and can furnish the corresponding cyanated products in moderate to high yields. Moreover, Cu/PhSiH3 system has been identified for the regioselective cleavage of C–H bonds of indoles by the use of metal-free CN sources via a one-pot sequential iodination/cyanation. This cost effective, homogeneous and metal-free CN source system represents the best reaction efficiency in these methodologies.

Dinuclear platinum(II) complexes with 1,5-nphe bridging ligand: Spectroscopic and molecular docking study of the interactions with N-acetylated L-methionylglycine and human serum albumin

The hydrolysis of N-acetylated L-methionylglycine dipeptide (Ac-L-Met-Gly) in the presence of dinuclear platinum(II)-aqua complexes with general formula [{Pt(L)(H2O)}2(μ-1,5-nphe)]4+ (1,5-nphe is 1,5-naphthyridine; L is bidentately coordinated ethylenediamine (1w), (±)-1,2-propylenediamine (2w) and 1,3-propylenediamine (3w)) is described in detail. The hydrolytic activity of these complexes was monitored by 1H NMR spectroscopy, which confirmed the regioselective cleavage of the Met-Gly-amide bond in this peptide. Quantum-chemical calculations revealed the primary reaction path with coordination of dipeptide via the terminal amide nitrogen of methionine followed by its coordination for the sulphur atom, in which the decomposition of the resulted platinum(II)-dipeptide complex and the coordination of its glycine residue precedes the hydrolytic cleavage of the Met-Gly amide bond. In the secondary reaction path, in which Ac-L-Met-Gly is coordinated via the methionine sulphur atom, the decomposition of the starting [{Pt(L)(H2O)}2(μ-1,5-nphe)]4+ complex is followed by the regioselective hydrolysis of the investigated dipeptide. The binding affinity of the chloride derivatives of the corresponding dinuclear platinum(II)-aqua complexes, [{Pt(L)Cl}2(μ-1,5-nphe)]2+ (1–3) towards the human serum albumin (HSA), a transport protein, was investigated using electronic absorption and fluorescence emission measurements, and results indicated the static interactions between these complexes and HSA. A docking study revealed a unique binding site on HSA, based on the electrostatic and the hydrogen bonding, which does not have a methionine residue nearby, therefore does not exist danger of HSA hydrolysis by these complexes.

Mechanism of Radical Initiation in the Radical SAM Enzyme Superfamily.

Radical S-adenosylmethionine (SAM) enzymes use a site-differentiated [4Fe-4S] cluster and SAM to initiate radical reactions through liberation of the 5'-deoxyadenosyl (5'-dAdo•) radical. They form the largest enzyme superfamily, with more than 700,000 unique sequences currently, and their numbers continue to grow as a result of ongoing bioinformatics efforts. The range of extremely diverse, highly regio- and stereo-specific reactions known to be catalyzed by radical SAM superfamily members is remarkable. The common mechanism of radical initiation in the radical SAM superfamily is the focus of this review. Most surprising is the presence of an organometallic intermediate, Ω, exhibiting an Fe-C5'-adenosyl bond. Regioselective reductive cleavage of the SAM S-C5' bond produces 5'-dAdo• to form Ω, with the regioselectivity originating in the Jahn-Teller effect. Ω liberates the free 5'-dAdo• as the catalytically active intermediate through homolysis of the Fe-C5' bond, in analogy to Co-C5' bond homolysis in B12, which was once viewed as biology's choice of radical generator.

Epoxide-Based Synthetic Approaches toward Polypropionates and Related Bioactive Natural Products.

Polypropionate units are a common structural feature of many of the natural products in polyketides, some of which have shown a broad range of antimicrobial and therapeutic potential. Polypropionates are composed of a carbon skeleton with alternating methyl and hydroxy groups with a specific configuration. Different approaches have been developed for the synthesis of polypropionates and herein we include, for the first time, all of the epoxide-based methodologies that have been reported over the years by several research groups such as Kishi, Katsuki, Marashall, Miyashita, Prieto, Sarabia, Jung, McDonald, etc. Several syntheses of polypropionate fragments and natural products that employed epoxides as key intermediates have been described and summarized in this review. These synthetic approaches involve enatio- and diastereoselective synthesis of epoxides (epoxy-alcohols, epoxy-amides, and epoxy-esters) and their regioselective cleavage with carbon and/or hydride nucleophiles. In addition, we included a description of the isolation and biological activities of the polypropionates and related natural products that have been synthetized using epoxide-based approaches. In conclusion, the epoxide-based methodologies are a non-aldol alternative approach for the construction of polypropionate.

Regioselective protein oxidative cleavage enabled by enzyme-like recognition of an inorganic metal oxo cluster ligand

Oxidative modifications of proteins are key to many applications in biotechnology. Metal-catalyzed oxidation reactions efficiently oxidize proteins but with low selectivity, and are highly dependent on the protein surface residues to direct the reaction. Herein, we demonstrate that discrete inorganic ligands such as polyoxometalates enable an efficient and selective protein oxidative cleavage. In the presence of ascorbate (1 mM), the Cu-substituted polyoxometalate K8[Cu2+(H2O)(α2-P2W17O61)], (CuIIWD, 0.05 mM) selectively cleave hen egg white lysozyme under physiological conditions (pH =7.5, 37 °C) producing only four bands in the gel electropherogram (12.7, 11, 10, and 5 kDa). Liquid chromatography/mass spectrometry analysis reveals a regioselective cleavage in the vicinity of crystallographic CuIIWD/lysozyme interaction sites. Mechanistically, polyoxometalate is critical to position the Cu at the protein surface and limit the generation of oxidative species to the proximity of binding sites. Ultimately, this study outlines the potential of discrete, designable metal oxo clusters as catalysts for the selective modification of proteins through radical mechanisms under non-denaturing conditions.

Synthesis of Bicyclo[3.2.1]octanones by Regioselective Cyclopropyl Ring Cleavage of Tricyclo[3.3.0.02,8]octanones in an Ionic Liquid

AbstractThe synthesis of bicyclo[3.2.1]octanones by the regioselective cyclopropyl ring cleavage of bicyclo[3.3.0.02,8]octanes using hydrochloric acid in an ionic liquid BMImCl is reported herein. The reaction provides the products regioselectively in very good yields in much faster times compared to such reactions in traditional solvents. Direct head to head comparison of the ring cleavage reaction in BMImCl with other conventional organic solvents clearly demonstrate the advantages of the former as the medium for the ring cleavage reactions.

Reductive elimination of alkoxy group in anthraquinone derivatives

Anthraquinone derivatives are constantly in a focus of synthetic chemists due to a diversity of valuable biological, photochemical and redox properties. However, quinone core limits applicability of chemical transformations making a search for new methods of modification of anthraquinones highly demanded. A convenient approach of regioselective reductive α-alkoxy group cleavage of anthraquinone derivatives was developed and evaluated. Most effectively transformation goes via a zinc reduction of quinone moiety and prototropic rearrangement in acidic media followed by the alkoxy group elimination. Strong donors in β-position of anthraquinone core are crucial for transformation and provide the elimination exclusively from α-position. The method simplifies an access to some bioactive 1,3-substituted anthraquinones and was applied for synthesis of natural products, e.g., rubiadin 1-methyl ether, anthragallol 1,2-dimethyl ether, and damnacanthol.

Cobalt-Catalyzed Fluoroallyllation of Carbonyls via C-C Activation of gem-Difluorocyclopropanes.

A new Co-catalyzed sequential C-C and C-F activation of gem-difluorinated cyclopropanes (gem-FCPs) to form nucleophilic fluoroallylcobalt, followed by addition to aldehydes, is reported. The protocol features the regioselective cleavage of dual chemical bonds of readily available gem-FCPs to prepare easily separable linear (Z)- and (E)-fluorinated homoallylic alcohols with a broad scope. This discovery established a new strategy for the efficient transformation of gem-FCPs as well as the synthesis of challenging fluorinated homoallylic alcohols.

Regioselective Disulfide-Catalyzed Photocatalytic Oxidative Cleavage of 1-Arylbutadienes to Cinnamaldehydes.

This work discloses a simple, efficient, and environmentally benevolent disulfide-catalyzed photocatalytic regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes. This methodology illustrates mild reaction conditions, ambient temperature, excellent regioselectivity, and compatibility with wide range of functional groups (38 examples). The method gains significance, as few reports with limited substrate scope are available for such excellent photocatalytic oxidative cleavage of conjugated dienes.

Gold(I)-Catalyzed Cycloisomerization/Hetero-Diels-Alder Reaction/Ring Opening Cascade to Functionalized Cyclopentadienes.

Six- and seven-membered ring-fused, functionalized cyclopentadienes can be obtained in moderate to excellent yields by a cascade process entailing the Au(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization of propargyl vinyl ethers, the hetero-Diels-Alder reaction with dialkylazodicarboxylates, and the spontaneous conversion of cycloaddition products into cyclopentadienes by a highly regioselective cleavage of a C-N bond. Depending on the treatment of the crude reaction mixtures, two types of products can be obtained: cyclopentadienes with pendant hydrazine and aldehyde moieties that intramolecularly react to form hemiaminals are obtained in 43-52% overall yields when the crude reaction mixtures are left over K2CO3 in a DCM solution. Instead, by reducing in situ the aldehyde group just after addition of the heterodienophile, the regioselective C-N bond cleavage generates the corresponding cyclopentadienes bearing a hydrazine and an alcohol appendage in excellent yields (66-82%) over four steps, all in one pot. Two examples from the latter class of compounds were also converted into ring-fused, functionalized cyclopentadienes, bearing a protected amino group, by the selective N-N cleavage of the hydrazine moiety.

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.

One Pot Catalyst‐free Synthesis of Substituted Di‐amino N‐tosyl Benzoyl Thiazoles byRegioselective C−N Bond Cleavage and Its Anticancer Activity

AbstractThe synthesis of hitherto unknown thiazole derivatives have been disclosed via purely green approach. This proposed work is novel in terms of C−C bond formation at guanidino carbon in 2‐amino benzimidazole moiety. The unstable benzimidazol‐hemiaminals have been used as a source of reactive intermediates.The catalyst‐free synthesis of diamino benzoyl N‐tosyl thiazole governed bythe sequential chemical conversions, which was steered by simple N‐tosylation of benzimidazol‐hemiaminal. The reaction was internally edge forward by thiazolidine ring opening, N‐tosylation, C−C bond formation and regioselective cleavage of C−N bondat guanidino carbon in a single step. Oxidation of tertiary alcohol to ketone with ring opening by C−N bond cleavage and N‐tosylation are the valuable conversions to accomplish the regioselectivity in the development of new desired thiazole ring. Moreover benzimidazol‐hemiaminal used as a storage of 2‐oxo‐2‐phenylethyl N′‐(1H‐benzo[d]imidazol‐2‐yl)‐N‐phenylcarbamimidothioate which is again highly unstable intermediate. The reverse back step mechanism was developed for the synthesis of thiazoles unlike other hemiaminals. The protocol is free of catalyst and toxic solvents.Use of ionic liquid, shorter reaction time, high yielding and easy isolation of products makes this strategy more environmental‐friendly. The importance of products attributed to the presence of active benzoyl and amino groups. Product thiazoles (T3) (IC50=7.906 μM), (T4) (IC50=5.250 μM) and (T7) (IC50=4.709 μM) were found potent anticancer agents against lung cancer with reference to doxorubicin as standard.

Toward the stereoselective synthesis of zaragozic acid framework: A desilylation-aldol reaction approach

A convergent synthesis of the C3-C8 fragment of zaragozic acids is described. The key reactions include desilylation-aldol reaction, rearrangement induced by regioselective reductive cleavage, BAIB/TEMPO-Pinnick oxidation, esterification, silylation, and hydrogenolysis.