Selective Methylation Research Articles

Unexpected Selective Methyl Group Abstractions from SiMe3 Moieties of CH2 SiMe3 Ligands To Give New Cationic Titanium Complexes.

The synthesis of the alkylated pentafulvene titanium complex Cp*Ti(CH2 SiMe3 )(π-η5 :σ-η1 -C5 H4 =CR2 ) (CR2 =adamantylidene) is described. The coordination mode of the pentafulvene ligand in this complex allowed for the subsequent synthesis of a series of nitrile-insertion products. By reacting these neutral compounds with the strong Lewis acid B(C6 F5 )3 , unprecedented selective methyl-group abstractions from the SiMe3 moieties under formation of novel cationic titanium complexes with the corresponding MeB(C6 F5 )3 - anion are demonstrated. The proposed silylium ion intermediates of these reactions are directly stabilized by either the exocyclic carbon atom of the pentafulvene ligand or by the lone pairs of the nitrogen atoms of the chelating Cp,N-ligands. In addition, a titanium enamine complex and its reaction with B(C6 F5 )3 is reported, which results in the formation of a betaine.

Mn-Catalyzed Selective Double and Mono-N-Formylation and N-Methylation of Amines by using CO2.

Functionalization of amines by using CO2 is of fundamental importance considering the abundance of amines and CO2 . In this context, the catalytic formylation and methylation of amines represent convenient and successful protocols for selective CO2 utilization as a C1 building block. This study represents the first example of selective catalytic double N-formylation of aryl amines by using a dinuclear Mn complex in the presence of phenylsilane. This robust system also allows for selective formylation and methylation of amines under a range of conditions.

UV Light-mediated regioselective methylsulfanyl discrimination via Pd-catalyzed cross-coupling reactions of 2,4-dimethylsulfanylpyrido[2,3-d]pyrimidines

ABSTRACTAn effective and novel photochemical method for the preparation of 2,4-disubstituted pyrido[2,3-d]pyrimidines is reported from 2,4-dimethylsulfanyl-pyrido[2,3-d]pyrimidines through a Liebeskind-Sroglcoupling reaction involving an original selective methyl sulfanyl discrimination using UV light as a source of energy. Our strategies involve a two-step and a one-pot approach with reaction times of 11-18 h and yields ranging between 72 and 94%. The two strategies are compared.

Toward the elucidation of the mechanism for passive membrane permeability of cyclic peptides.

Toward the elucidation of the mechanism for passive membrane permeability of cyclic peptides.

Regiospecific methylation of all the hydroxyls in (+)-catechin by a stepwise differentiation strategy.

Methylated derivatives of catechins have received great attention for health beneficial effects, especially antiallergic activity. However, the scarce natural abundance of methylated catechins limits further bioactive studies. The objective of this work was to investigate regiospecific methylation of the hydroxyls of (+)-catechin through a stepwise differentiation strategy based on electronic difference between the hydroxyl groups. Selective methylation of the hydroxyls on different rings was realized by employing Meerwein salt as the methylation reagent. Preferential acylation of the phenolic hydroxyls on A and B rings allowed selective exposure of the aliphatic hydroxyl on C ring to methylation. The vicinal phenolic hydroxyls on B ring were preferentially methylated under mild basic condition due to the acidic properties. Methylation of the phenolic hydroxyls on A ring was achieved by sequential protection and deprotection operations. Finally, antioxidant activities of all the individual methylated (+)-catechins were explored by 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) assay. Regiospecific methylation of the phenolic and aliphatic hydroxyls was systematically achieved under mild conditions. Preparation of all the individual methylated (+)-catechins was accomplished with a greener methylation reagent: nonvolatile Meerwein salt. This work laid a solid foundation for preparation of diverse O-methylated catechins for bioactivity studies. © 2019 Society of Chemical Industry.

Selective Methylation of Sulfides in Petroleum for Electrospray Ionization Mass Spectrometry Analysis

A method for selective methylation of sulfides (particularly aliphatic sulfides) but not thiophenes in petroleum has been developed for positive-ion electrospray ionization (ESI) mass spectrometry. The new method uses only CH3I without AgBF4 to produce methyl sulfonium ions of sulfides for molecular composition analysis. It provides simplicity and saves time compared to a previously developed methylation method using CH3I/AgBF4 to methylate all sulfur compounds into ionic species analyzable by positive-ion ESI. The composition of sulfides by this method agrees with that of the sulfide fraction obtained from CH3I/AgBF4 methylation/demethylation. Hence, the separation of sulfides from thiophenes through elaborate procedures is eliminated. The new method was validated by model compounds and has been successfully applied to the analysis of sulfides in diesel, vacuum gas oil, crude oil, and atmospheric pressure distillation residue. When combined with the previously developed CH3I/AgBF4 method, the new method may have the potential to differentiate reactive sulfur, such as sulfides, from nonreactive sulfur, such as thiophenes. This could provide a means to develop correlations between sulfur types and corrosion, and possibly for other studies.

Selective formylation or methylation of amines using carbon dioxide catalysed by a rhodium perimidine-based NHC complex

Selective formylation and methylation of amines using low pressure CO2 as a carbon source; mechanistic studies, time course and reaction scope.

Trimethyl orthoacetate as a convenient reagent for selective methylation of β-OH groups of (poly)hydroxynaphthazarins

Trimethyl orthoacetate was found to be a convenient reagent for methylation of the β-OH groups of (poly)hydroxynaphthazarins. The substrates bearing one β-OH group react with MeC(OMe)3 to give the corresponding methoxy derivatives in 79–89% yields. Depending on the reaction conditions, methylation of substrates with two β-OH groups on the different and the same rings affords either the corresponding mono-O-methylated (43–70%) or di-Omethylated (71–78%) derivatives. Trimethyl orthoacetate offers a good alternative to CH2N2 in the preparative synthesis of O-methylated (poly)hydroxynaphthazarin derivatives.

Protein engineering strategies for improving the selective methylation of target CpG sites by a dCas9-directed cytosine methyltransferase in bacteria.

Mammalian gene expression is a complex process regulated in part by CpG methylation. The ability to target methylation for de novo gene regulation could have therapeutic and research applications. We have previously developed a dCas9-MC/MN protein for targeting CpG methylation. dCas9-MC/MN is composed of an artificially split M.SssI methyltransferase (MC/MN), with the MC fragment fused to a nuclease-null CRISPR/Cas9 (dCas9). Guide RNAs directed dCas9-MC/MN to methylate target sites in E. coli and human cells but also caused some low-level off-target methylation. Here, in E. coli, we show that shortening the dCas9-MC linker increases methylation of CpG sites located at select distances from the dCas9 binding site. Although a shortened linker decreased methylation of other CpGs proximal to the target site, it did not reduce off-target methylation of more distant CpG sites. Instead, targeted mutagenesis of the methyltransferase’s DNA binding domain, designed to reduce DNA affinity, significantly and preferentially reduced methylation of such sites.

Selective Catalytic Methylation of Phloroglucinol with Dimethyl Carbonate in the Presence of Heterogeneous Acids

A facile process for the methylation of phloroglucinol using environmentally friendly dimethyl carbonate as methylating agent and solvent is described. Widely available and inexpensive solid acids have been used as catalysts to promote this transformation. H‐Y zeolite proved particularly selective for monomethylation, hence it has been used for the convenient preparation of methoxyresorcinol (MR), with only limited co‐production of dimethoxyphenol (DMP). Conversely, the use of tungstosilicic acid on silica easily afforded DMP in excellent isolated yield.

The Use of Trimethyl Orthoacetate for the Conversion of Natural Mono- and Dihydroxynaphthazarins and Related Compounds to Their Methyl Ethers

Trimethyl orthoacetate can be used as an alternative to CH2N2 on the methylation of β-OH groups of (poly)hydroxynaphthazarins. In contrast to CH2N2 it can be applied both to complete and to selective methylation of these groups.

Toolkit for NMR Studies of Methyl-Labeled Proteins.

Selective methyl labeling is an extremely powerful approach to study the structure, dynamics, and explore mechanistic insights of large biomolecules by solution NMR. Methyls are relatively insensitive to chemical exchange-induced depolarization and provide superior probes of supramolecular interactions and allostery in such systems. In this chapter, we describe our systematic approach and contributions in the areas of sample preparation, data collection, and data analysis that streamline the application of methyl labeling in solution NMR studies of large proteins. We focus our effort on the initial and often onerous task of methyl resonance assignment and here we detail our approaches to simplify the process. We produce new methyl labeling combinations using Escherichia coli auxotrophs, increase speed, sensitivity, and resolution of NOESY experiments by employing 3D SOFAST-NOESY, and assign methyl resonances from raw data with spectral simulation tools and(or) automatically with minimal expert supervision using the MAGIC algorithm.

Catalytic conversion of duckweed to methyl levulinate in the presence of acidic ionic liquids

In this study, an efficient strategy is proposed for selective methyl levulinate production from duckweed, a typical fast-growing aquatic microalgae in warm and humid regions, in the presence of acidic ionic liquids (ILs). The results show that IL structure has a significant effect on its acidic strength, which finally determines the process efficiency for levulinate methyl generation. With the optimized catalyst of [C3H6SO3HPy]HSO4, 88.0% duckweed is consumed, resulting in a comparable methyl levulinate yield of 73.7% and a process efficiency of 81.8% at 170 °C for 5 h. Furthermore, this process is substantially influenced by the reaction condition, particularly, it is significantly temperature-dependent. In addition, solvent has a remarkable intensified effect on the process efficiency, which dramatically decreases from 81.8 to 53.7% when methanol is replaced by water.

Synthesis of 9-Substituted Berberine Derivatives with Microwave Irradiation

Hyperglycemia is frequently accompanying with hyperlipidemia. To explore the potent drugs with dual-activity and dual-site effects that could reduce blood glucose and blood lipid at the same time, fibrate group with lipid-lowering effect on the 9th position of berberine(BBR) was introduced using the drug design combination principle and the multitarget collaborative treatment method. Moreover, the molecular structure of BBR was modified, and six 9-substituted derivatives of BBR were designed and synthesized, among which, five compounds have never been reported before. In addition, the molecular structures of these derivatives were identified using liquid chromatography-mass spectrometry(LC-MS), 1H nuclear magnetic resonance(1H NMR) and 13C NMR, respectively. Furthermore, the microwave irradiation experimental technique was applied in the synthesis reaction using the novel microwave synthesizer, which accelerated the reaction rate, enhanced the reaction yield, reduced the reaction by-products, and simplified the post-processing steps. In the meantime, the 9-position regioselective demethylation of BBR was explored through quantum chemical calculation during the synthesis of berberrubine. The computations were consistent with the experimental results, which contributed to deducing the mechanism of its selective methylation.

Selective Methylation of Arenes: A Radical C−H Functionalization/Cross‐Coupling Sequence

A selective, nonchelation-assisted methylation of arenes has been developed. The overall transformation, which combines a C-H functionalization reaction with a nickel-catalyzed cross-coupling, offers rapid access to methylated arenes with high para selectivity. The reaction is amenable to late-stage methylation of small-molecule pharmaceuticals.

Selective Methylation of Arenes: A Radical C−H Functionalization/Cross‐Coupling Sequence

AbstractA selective, nonchelation‐assisted methylation of arenes has been developed. The overall transformation, which combines a C−H functionalization reaction with a nickel‐catalyzed cross‐coupling, offers rapid access to methylated arenes with high para selectivity. The reaction is amenable to late‐stage methylation of small‐molecule pharmaceuticals.

N-Monomethylation of Aromatic Amines with Methanol via PNHP-Pincer Ru Catalysts.

The use of methanol for the selective methylation of aromatic amines with RuHCl(CO)(PNHP) (PNHP = bis(2-diphenylphosphinoethyl)amine) is reported. Various aromatic amines were transformed into their corresponding monomethylated secondary amines in high yields at 150 °C with a very low catalyst loading (0.02-0.1 mol %) in the presence of KO tBu (20-60 mol %). The catalyst precursor, RuHCl(CO)(PNHP), was converted to [RuH(CO)2(PNHP)]+ under the catalytic conditions and also serves as a highly effective catalyst. The robustness of this catalyst contributes to its outstanding catalytic activity, even under reaction conditions, in which CO is liberated from methanol.

Precursor-Based Selective Methyl Labeling of Cell-Free Synthesized Proteins.

NMR studies of large proteins are complicated by pronounced spectral overlap and large line width. Reducing complexity by [13C, 1H] selective labeling of l-Val, l-Leu, and/or l-Ile residues in combination with optional perdeuteration is therefore commonly approached by supplying labeled amino acid precursors into bacterial expression cultures, although often compromised by high label costs, precursor instability, and label scrambling. Cell-free expression combines efficient production of membrane proteins with significant advantages for protein labeling such as small reaction volumes, defined amino acid pools, and reliable label incorporation. While amino acid specific isotopic labeling of proteins is routine application, the amino acid methyl side-chain labeling was so far difficult as appropriately labeled amino acids are hardly available. On the basis of recent proteome analyses of cell-free lysates, we have developed a competitive strategy for efficient methyl labeling of proteins based on conversion of supplied precursors. Pathway complexity of methyl side-chain labeling was reduced by implementing the promiscuous aminotransferase IlvE catalyzing the selective l-Leu, l-Val, or l-Ile biosynthesis from specific ketoacid precursors. Precursor-based l-Leu and l-Val synthesis was demonstrated with the cell-free labeling of peptidyl-prolyl cis/trans isomerase cyclophilin D and of the proton pump proteorhodopsin. The strategy is fast and cost-effective and enables the straightforward methyl side-chain labeling of individual amino acid types. It can easily be applied to any cell-free synthesized protein.

Identification of multiple isomeric core chitobiose–modified high-mannose and paucimannose N-glycans in the planarian Schmidtea mediterranea

Cell surface–associated glycans mediate many cellular processes, including adhesion, migration, signaling, and extracellular matrix organization. The galactosylation of core fucose (GalFuc epitope) in paucimannose and complex-type N-glycans is characteristic of protostome organisms, including flatworms (planarians). Although uninvestigated, the structures of these glycans may play a role in planarian regeneration. Whole-organism MALDI-MS analysis of N-linked oligosaccharides from the planarian Schmidtea mediterranea revealed the presence of multiple isomeric high-mannose and paucimannose structures with unusual mono-, di-, and polygalactosylated (n = 3–5) core fucose structures; the latter structures have not been reported in other systems. Di- and trigalactosylated core fucoses were the most dominant glycomers. N-Glycans showed extensive, yet selective, methylation patterns, ranging from non-methylated to polymethylated glycoforms. Although the majority of glycoforms were polymethylated, a small fraction also consisted of non-methylated glycans. Remarkably, monogalactosylated core fucose remained unmethylated, whereas its polygalactosylated forms were methylated, indicating structurally selective methylation. Using database searches, we identified two potential homologs of the Galβ1–4Fuc–synthesizing enzyme from nematodes (GALT-1) that were expressed in the prepharyngeal, pharyngeal, and mesenchymal regions in S. mediterranea. The presence of two GALT-1 homologs suggests different requirements for mono- and polygalactosylation of core fucose for the formation of multiple isomers. Furthermore, we observed variations in core fucose glycosylation patterns in different planarian strains, suggesting evolutionary adaptation in fucose glycosylation. The various core chitobiose modifications and methylations create >60 different glycoforms in S. mediterranea. These results contribute greatly to our understanding of N-glycan biosynthesis and suggest the presence of a GlcNAc-independent biosynthetic pathway in S. mediterranea.

Selective terminal methylation of a tRNA wobble base.

Active tRNAs are extensively post-transcriptionally modified, particularly at the wobble position 34 and the position 37 on the 3′-side of the anticodon. The 5-carboxy-methoxy modification of U34 (cmo5U34) is present in Gram-negative tRNAs for six amino acids (Ala, Ser, Pro, Thr, Leu and Val), four of which (Ala, Ser, Pro and Thr) have a terminal methyl group to form 5-methoxy-carbonyl-methoxy-uridine (mcmo5U34) for higher reading-frame accuracy. The molecular basis for the selective terminal methylation is not understood. Many cmo5U34-tRNAs are essential for growth and cannot be substituted for mutational analysis. We show here that, with a novel genetic approach, we have created and isolated mutants of Escherichia coli tRNAPro and tRNAVal for analysis of the selective terminal methylation. We show that substitution of G35 in the anticodon of tRNAPro inactivates the terminal methylation, whereas introduction of G35 to tRNAVal confers it, indicating that G35 is a major determinant for the selectivity. We also show that, in tRNAPro, the terminal methylation at U34 is dependent on the primary m1G methylation at position 37 but not vice versa, indicating a hierarchical ranking of modifications between positions 34 and 37. We suggest that this hierarchy provides a mechanism to ensure top performance of a tRNA inside of cells.