Regioselective Methylation Research Articles

Microwave-Assisted Ru(II)-Catalyzed Regioselective Methyl Acylation of 2-Arylbenzoazoles: Synthesis of Benzofuran Conjugates via C-H Activation/Annulation.

An efficient Ru(II)-catalyzed C-H functionalization protocol for 2-arylbenzoazoles as the directing group and sulfoxonium ylide has been developed. Gratifyingly, concomitant annulation was observed when 3-(benzo[d]azol-2-yl) phenol was used, enabling the construction of benzofuran conjugates. Notably, the utilization of water as the solvent and an energy efficient approach makes the reaction greener, contributing to overall sustainability. This protocol exhibits excellent scalability up to the gram scale with a diverse array of substitutions. Furthermore, the mechanism was examined by ESI-MS, and photophysical studies were also performed.

A Universal, Continuous Assay for SAM-dependent Methyltransferases.

Enzyme-catalyzed late-stage functionalization (LSF), such as methylation of drug molecules and lead structures, enables direct access to more potent active pharmaceutical ingredients (API). S-adenosyl-l-methionine-dependent methyltransferases (MTs) can play a key role in the development of new APIs, as they catalyze the chemo- and regioselective methylation of O-, N-, S- and C-atoms, being superior to traditional chemical routes. To identify suitable MTs, we developed a continuous fluorescence-based, high-throughput assay for SAM-dependent methyltransferases, which facilitates screening using E. coli cell lysates. This assay involves two enzymatic steps for the conversion of S-adenosyl-l-homocysteine into H2 S to result in a selective fluorescence readout via reduction of an azidocoumarin sulfide probe. Investigation of two O-MTs and an N-MT confirmed that this assay is suitable for the determination of methyltransferase activity in E. coli cell lysates.

Ein universeller, kontinuierlicher Assay für SAM‐abhängige Methyltransferasen

AbstractDie enzymkatalysierte Funktionalisierung (engl.: late‐stage functionalization (LSF)), wie z. B. die Methylierung von Arzneimittelmolekülen und Leitstrukturen, ermöglicht den direkten Zugang zu wirksameren pharmazeutischen Wirkstoffen (API). S‐Adenosyl‐l‐Methionin‐abhängige Methyltransferasen (MTs) können eine Schlüsselrolle bei der Entwicklung neuer Wirkstoffe spielen, da sie die chemo‐ und regioselektive Methylierung von O‐, N‐, S‐ und C‐Atomen katalysieren und damit den herkömmlichen chemischen Wegen überlegen sind. Um geeignete MTs zu identifizieren, haben wir einen kontinuierlichen fluoreszenzbasierten Hochdurchsatzassay für SAM‐abhängige Methyltransferasen entwickelt, der das Screening unter Verwendung von E. coli Zelllysaten erleichtert. Dieser Assay besteht aus zwei enzymatischen Schritten für die Umwandlung von S‐Adenosyl‐l‐Homocystein zu H2S, was zu einer selektiven Fluoreszenzmessung durch Reduktion eines Azidocoumarin‐Sulfidsensors führt. Die Untersuchung von zwei O‐MTs und einer N‐MT bestätigte, dass dieser Assay für die Bestimmung der Methyltransferase‐Aktivität in E. coli Zelllysaten geeignet ist.

Biocatalytic C3-Indole Methylation-A Useful Tool for the Natural-Product-Inspired Stereoselective Synthesis of Pyrroloindoles.

Enantioselective synthesis of bioactive compounds bearing a pyrroloindole framework is often laborious. In contrast, there are several S‐adenosyl methionine (SAM)‐dependent methyl transferases known for stereo‐ and regioselective methylation at the C3 position of various indoles, directly leading to the formation of the desired pyrroloindole moiety. Herein, the SAM‐dependent methyl transferase PsmD from Streptomyces griseofuscus, a key enzyme in the biosynthesis of physostigmine, is characterized in detail. The biochemical properties of PsmD and its substrate scope were demonstrated. Preparative scale enzymatic methylation including SAM regeneration was achieved for three selected substrates after a design‐of‐experiment optimization.

Efficient Chemical Synthesis of (Epi)catechin Glucuronides: Brain-Targeted Metabolites for Treatment of Alzheimer's Disease and Other Neurological Disorders.

Grape seed extract (GSE) is rich in flavonoids and has been recognized to possess human health benefits. Our group and others have demonstrated that GSE is able to attenuate the development of Alzheimer’s disease (AD). Moreover, our results have disclosed that the anti-Alzheimer’s benefits are not directly/solely related to the dietary flavonoids themselves, but rather to their metabolites, particularly to the glucuronidated ones. To facilitate the understanding of regioisomer/stereoisomer-specific biological effects of (epi)catechin glucuronides, we here describe a concise chemical synthesis of authentic standards of catechin and epicatechin metabolites 3–12. The synthesis of glucuronides 9 and 12 is described here for the first time. The key reactions employed in the synthesis of the novel glucuronides 9 and 12 include the regioselective methylation of the 4′-hydroxyl group of (epi)catechin (≤1.0/99.0%; 3′-OMe/4′-OMe) and the regioselective deprotection of the tert-butyldimethylsilyl (TBS) group at position 5 (yielding up to 79%) over the others (3, 7 and 3′ or 4′).

Regioselectivity of Cobalamin-Dependent Methyltransferase Can Be Tuned by Reaction Conditions and Substrate.

Regioselective reactions represent a significant challenge for organic chemistry. Here the regioselective methylation of a single hydroxy group of 4‐substituted catechols was investigated employing the cobalamin‐dependent methyltransferase from Desulfitobacterium hafniense. Catechols substituted in position four were methylated either in meta‐ or para‐position to the substituent depending whether the substituent was polar or apolar. While the biocatalytic cobalamin dependent methylation was meta‐selective with 4‐substituted catechols bearing hydrophilic groups, it was para‐selective for hydrophobic substituents. Furthermore, the presence of water miscible co‐solvents had a clear improving influence, whereby THF turned out to enable the formation of a single regioisomer in selected cases. Finally, it was found that also the pH led to an enhancement of regioselectivity for the cases investigated.

Highly regioselective methylation of inosine nucleotide: an efficient synthesis of 7-methylinosine nucleotide

A facile, straightforward, reliable, and an efficient chemical synthesis of inosine nucleotides such as 7-methylinosine 5′-O-monophosphate, 7-methylinosine 5′-O-diphosphate, and 7-methylinosine 5′-O-triphosphate, starting from the corresponding inosine nucleotide is delineated. The present methylation reaction of inosine nucleotide utilizes dimethyl sulfate as a methylating agent and water as a solvent at room temperature. It is noteworthy that the present methylation reaction proceeds smoothly under aqueous conditions that is highly regioselective to afford exclusive 7-methylinosine nucleotide in good yields with high purity (>99.5%).

Cp*Rh(III)-Catalyzed Regioselective C(sp3)-H Methylation of 8-Methylquinolines with Organoborons.

Rh(III)-catalyzed highly regioselective methylation of the unactivated C(sp3)-H bond of 8-methylquinolines with bench stable organoboron reagents is described. A variety of substituted 8-methylquinolines provided the highly regioselective monomethylated products with potassium methyltrifluoroborates/methylboronic acid through primary C(sp3)-H bond activation. Complete chemoselectivity and regioselectivity were observed in all cases as methylation at the C2 position or dimethylation of the C(sp3)-H bond of 8-methylquinoline was not detected. The mechanistic study uncovered the fact that the reaction may proceed through the five-membered rhodacycle intermediate.

Highly Regioselective Methylation of Guanosine Nucleotides: An Efficient Synthesis of 7-Methylguanosine Nucleotides.

This article describes a simple, reliable, efficient, and general method for the synthesis of 7-methylguanosine nucleotides such as 7-methylguanosine 5'-O-monophosphate (m7 GMP), 7-methylguanosine 5'-O-diphosphate (m7 GDP), 7-methyl-2'-deoxyguanosine 5'-O-triphosphate (m7 2'dGTP), and 7-methylguanosine 5'-O-triphosphate (m7 GTP) starting from the corresponding guanosine nucleotide is described. The present protocol involves methylation reaction of guanosine nucleotide using dimethyl sulfate as a methylating agent and water as a solvent at room temperature to provide the corresponding 7-methylguanosine nucleotide in good yields with high purity (>99.5%). It is noteworthy that the present methylation reaction proceeds smoothly under aqueous conditions that is highly regioselective to afford exclusive 7-methylguanosine nucleotide. © 2019 by John Wiley & Sons, Inc. Basic Protocol: Synthesis of 7-methylguanosine nucleotides.

Genome mining and biosynthesis of a polyketide from a biofertilizer fungus that can facilitate reductive iron assimilation in plant

Fungi have the potential to produce a large repertoire of bioactive molecules, many of which can affect the growth and development of plants. Genomic survey of sequenced biofertilizer fungi showed many secondary metabolite gene clusters are anchored by iterative polyketide synthases (IPKSs), which are multidomain enzymes noted for generating diverse small molecules. Focusing on the biofertilizer Trichoderma harzianum t-22, we identified and characterized a cryptic IPKS-containing cluster that synthesizes tricholignan A, a redox-active ortho-hydroquinone. Tricholignan A is shown to reduce Fe(III) and may play a role in promoting plant growth under iron-deficient conditions. The construction of tricholignan by a pair of collaborating IPKSs was investigated using heterologous reconstitution and biochemical studies. A regioselective methylation step is shown to be a key step in formation of the ortho-hydroquinone. The responsible methyltransferase (MT) is fused with an N-terminal pseudo-acyl carrier protein (ψACP), in which the apo state of the ACP is essential for methylation of the growing polyketide chain. The ψACP is proposed to bind to the IPKS and enable the trans MT to access the growing polyketide. Our studies show that a genome-driven approach to discovering bioactive natural products from biofertilizer fungi can lead to unique compounds and biosynthetic knowledge.

Divergent total syntheses of five illudalane sesquiterpenes and assignment of the absolute configuration.

Concise, divergent total syntheses of five bioactive illudalane sesquiterpenes have been achieved. Our synthesis features an intermolecular [2+2+2] cycloaddition, and a lactone-directed aromatic C-H oxygenation to generate a temporary phenolic hydroxyl group which enables regioselective methylation. Furthermore, the absolute configuration of radulactone was assigned by chemical synthesis.

In vitro evaluation of the effect of C-4 substitution on methylation of 7,8-dihydroxycoumarin: metabolic profile and catalytic kinetics.

Daphnetin (7,8-dihydroxycoumarin (7,8-DHC)) and its C-4 derivatives have multiple pharmacological activities, but the poor metabolic stability of these catechols has severely restricted their application in the clinic. Methylation plays important roles in catechol elimination, although thus far the effects of structural modifications on the metabolic selectivity and the catalytic efficacy of human catechol-O-methyltransferase (COMT) remain unclear. This study was aimed at exploring the structure–methylation relationship of daphnetin and its C-4 derivatives, including 4-methyl, 4-phenyl and 4-acetic acid daphnetin. It was achieved by identifying the methylated products generated and by careful characterization of the reaction kinetics. These catechols are selectively metabolized to the corresponding 8-O-methyl conjugates, and this regioselective methylation could be elucidated by flexible docking, in which all the 8-OH groups of these catechols are much closer than the 7-OH groups to catalytic residue LYS144 and methyl donor AdoMet. The results of the kinetic analyses revealed that the Clint values of the compounds could be strongly affected by the C-4 substitutions, which could be partially explained by the electronic effects of the C-4 substituents and the coordination modes of 7,8- dihydroxycoumarins in the active site of COMT. These findings provide helpful guidance for further structural modification of 7,8-DHCs to improve metabolic stability.

An expedient method for regioselective methylation of catechol coumarins

An expedient and efficient method for selective methylation of catechol coumarins by working with different alkalis under appropriate reaction conditions is reported. Esculetins were selectively methylated at position 6 and position 7 in good yield using CH3I in the presence of NaH and Na2CO3, respectively. However, daphnetins showed less regioselectivity under the same reaction conditions. Furthermore, the site preference for the methylation reaction was interpreted by the density functional theory at B3LYP/6-31+G(d) level.

Copper mediated cyclization of 1-substituted enamides, dienamides and trienamides: regiochemistry, indigoid formation and methyl migration-aromatization

Copper mediated cyclization of activated 1-substituted enamides occurs via a 5-endo radical-polar crossover process. Trichloroacetyl derivatives can undergo further reactions post cyclization (elimination of HCl or dimerization potentially via copper carbenoid intermediates). Reaction of α-halo trienamides derived from β-ionone furnish either β- or γ-lactams via 4-exo or 5-exo cyclizations respectively depending upon the enamide tautomer undergoing reaction. For the less reactive dichloroacetamide derivative a competing regioselective methyl migration-aromatization prior to cyclization is observed.

ChemInform Abstract: A Carbohydrate Approach for the First Total Synthesis of Cochliomycin C (I): Stereoselective Total Synthesis of Paecilomycin E, Paecilomycin F and 6′‐epi‐Cochliomycin C.

AbstractStarting from readily available sugar D‐lyxose, a unique strategy for the total synthesis of natural resorcylic acid lactones involving an Ohira—Bestmann alkynylation, ring closing metathesis, and regioselective methylation under Mitsunobu conditions as key steps is accomplished.

Synthesis of icariin from kaempferol through regioselective methylation and para-Claisen-Cope rearrangement.

The hemisynthesis of the naturally occurring bioactive flavonoid glycoside icariin (1) has been accomplished in eleven steps with 7% overall yield from kaempferol. The 4′-OH methylation of kaempferol, the 8-prenylation of 3-O-methoxymethyl-4′-O-methyl-5-O-prenyl-7-O-benzylkaempferol (8) via para-Claisen–Cope rearrangement catalyzed by Eu(fod)3 in the presence of NaHCO3, and the glycosylation of icaritin (3) are the key steps.

A Carbohydrate Approach for the First Total Synthesis of Cochliomycin C: Stereoselective Total Synthesis of Paecilomycin E, Paecilomycin F and 6′‐epi‐Cochliomycin C

AbstractThe first total synthesis of the chlorinated resorcylic acid lactone cochliomycin C is achieved starting from the readily available sugar D‐lyxose. The strategy has also lead to the total synthesis of natural resorcylic acid lactones paecilomycin E, paecilomycin F and a synthetic analogue 6‐epi‐cochliomycin C. The key reactions include Ohira–Bestmann alkynylation, ring closing metathesis and regioselective methylation under Mitsunobu conditions.

ChemInform Abstract: Total Synthesis of the Congested Propellane Alkaloid (‐)‐Acutumine

AbstractReview: [57 refs.

Total synthesis of the congested propellane alkaloid (-)-acutumine.

The enantioselective total synthesis of (-)-acutumine is described. The synthetic strategy was inspired by the premise that the cyclohexenone ring could be derived from an aromatic precursor. After successful construction of a propellane model system, an initial attempt to prepare the spirocyclic subunit was thwarted by incorrect regioselectivity in a radical cyclization. A second-generation approach involving a radical-polar crossover reaction was successful, and the chemistry developed in the aforementioned model system was then applied to synthesize the natural product. Key reactions included a phenolic oxidation, a diastereoselective ketone allylation utilizing Nakamura's chiral allylzinc reagent, an anionic oxy-Cope rearrangement, an acid-promoted cyclization of a secondary amine onto an α,β-unsaturated ketal, and a regioselective methyl enol etherification of a 1,3-diketone.

Total Synthesis of Carbazomycin G by a Thermal Ring Expansion/Self‐Redox Reaction Cascade

AbstractThe total synthesis of carbazomycin G (7) has been accomplished in six steps and 26 % overall yield starting from commercially available materials. As an efficient access to the tricyclic skeleton of carbazomycins A–H, an interesting thermal ring expansion/self‐redox reaction cascade of intermediate 11 has been developed in our study. The self‐redox reaction of intermediate 10 can be readily accelerated in the presence of triethylamine, and a plausible reaction mechanism for this process has been proposed. Finally, the synthesis of carbazomycin G was completed by the widely applied regioselective methylation reaction.