Catalytic nucleophilic acyl substitution of anhydrides by amphoteric vanadyl triflate

A Stoichiometric Solvent-Free Protocol for Acetylation Reactions

Considering the remarkable relevance of acetylated derivatives of phenols, alcohols, and aryl and alkyl thiols in different areas of biology, as well as in synthetic organic chemistry, a sustainable solvent-free approach to perform acetylation reactions is proposed here. Acetylation reactions are classically performed using excess of acetic anhydride (Ac₂O) in solvent-free conditions or by eventually working with stoichiometric amounts of Ac₂O in organic solvents; both methods require the addition of basic or acid catalysts to promote the esterification. Therefore, they usually lead to the generation of high amounts of wastes, which sensibly raise the E-factor of the process. With the aim to develop a more sustainable system, a solvent-free, stoichiometric acetylation protocol is, thus, proposed. The naturally occurring phenol, thymol, can be converted to the corresponding—biologically active—ester with good yields, in the presence of 1% of VOSO₄. Interestingly, the process can be efficiently adopted to synthesize other thymyl esters, as well as to perform acetylation of alcohols and aryl and alkyl thiols. Remarkably, a further improvement has been achieved replacing Ac₂O with its greener alternative, isopropenyl acetate (IPA).

An Approach to the Synthesis of Electron-Rich and Hindered Esters and Its Application to the Synthesis of Acteoside

Electron-rich esters are ubiquitously distributed in natural products and play a central role in bioactivities. Herein, we disclose an efficient, mild, and general esterification approach to the synthesis of these esters by employing gold(I)-catalyzed acylation reaction with alkyne-tethered mixed anhydrides and alcohols. This method can be applied to ester-bond formation in complex substrates and facilitates efficient synthesis of acteoside, which belongs to the family of phenylethanoid glycosides and possesses a broad range of bioactivities.

Multicatalytic Transformation of (Meth)acrylic Acids: a One-Pot Approach to Biobased Poly(meth)acrylates

Shifting from petrochemical feedstocks to renewable resources can address some of the environmental issues associated with petrochemical extraction and make plastics production sustainable. Therefore, there is a growing interest in selective methods for transforming abundant renewable feedstocks into monomers suitable for polymer production. Reported herein are one-pot catalytic systems, that are active, productive, and selective under mild conditions for the synthesis of copolymers from renewable materials. Each system allows for anhydride formation, alcohol acylation and/or acid esterification, as well as polymerization of the formed (meth)acrylates, providing direct access to a new library of unique poly(meth)acrylates.

Phosphotungstic acid as a novel acidic catalyst for carbohydrate protection and glycosylation

This work demonstrates the utilization of phosphotungstic acid (PTA) as a novel acidic catalyst for carbohydrate reactions, such as per-O-acetylation, regioselective O-4,6 benzylidene acetal formation, regioselective O-4 ring-opening, and glycosylation. These reactions are basic and salient during the synthesis of carbohydrate-based bioactive oligomers. Phosphotungstic acid's high acidity and eco-friendly character make it a tempting alternative to corrosive homogeneous acids. The various homogenous acid catalysts were replaced by the phosphotungstic acid solely for different carbohydrate reactions. It can be widely used as a catalyst for organic reactions as it is thermally stable and easy to handle. In our work, the reactions are operated smoothly under ambient conditions; the temperature varies from 0 °C to room temperature. Good to excellent yields were obtained in all four kinds of reactions.

This work demonstrates the utilization of PTA as an acidic catalyst in primary reactions of carbohydrate. PTA can be widely used as a catalyst for carbohydrate chemistry. The reactions are executed at 0 °C or room temperature in good yields.

P2O5 / SiO2 as a mild and efficient reagent for acylation of alcohols, phenols and amines under solvent-free conditions

P2O5 / SiO2 is a highly efficient reagent for the acetylations of a variety of alcohols, phenols and amines with acetic anhydride under solvent-free conditions. Primary, secondary, allylic and benzylic alcohols, diols and phenols with electron-donating or withdrawing substituents can be easily acetylated in good to excellent yield.

Sequential Dy(OTf)3 -Catalyzed Solvent-Free Per-O-Acetylation and Regioselective Anomeric De-O-Acetylation of Carbohydrates

Dysprosium(III) trifluoromethanesulfonate-catalyzed per-O-acetylation and regioselective anomeric de-O-acetylation of carbohydrates can be tuned by adjusting the reaction medium. In this study, the per-O-acetylation of unprotected sugars by using a near-stoichiometric amount of acetic anhydride under solvent-free conditions resulted in the exclusive formation of acetylated saccharides as anomeric mixtures, whereas anomeric de-O-acetylation in methanol resulted in a moderate-to-excellent yield. Reactions with various unprotected monosaccharides or disaccharides followed by a semi-one-pot sequential conversion into the corresponding acetylated glycosyl hemiacetal also resulted in high yields. Furthermore, the obtained hemiacetals could be successfully transformed into trichloroimidates after Dy(OTf)₃ -catalyzed glycosylation.

Evaluation of Maltose-Based Cationic Liposomes with Different Hydrophobic Tails for Plasmid DNA Delivery

In this paper, three cationic glycolipids with different hydrophobic chains Malt-DiC12MA (IX a), Malt-DiC14MA (IX b) and Malt-DiC16MA (IX c) were constructed by using maltose as starting material via peracetylation, selective 1-O-deacetylation, trichloroacetimidation, glycosylation, azidation, deacetylation, Staudinger reaction, tertiary amination and quaternization. Target compounds and some intermediates were characterized by ¹H-NMR, ¹³C-NMR, ¹H-¹H COSY and ¹H-¹³C HSQC. The results of gel electrophoresis assay, atomic force microscopy images (AFM) and dynamic light scattering (DLS) demonstrate that all the liposomes could efficiently bind and compact DNA (N/P ratio less than 2) into nanoparticles with proper size (88 nm–146 nm, PDI < 0.4) and zeta potential (+15 mV–+26 mV). The transfection efficiency and cellular uptake of glycolipids in HEK293 cell were evaluated through the enhanced green fluorescent protein (EGFP) expression and Cy3-labeled pEGFP-C1 (Enhanced Green Fluorescent Protein plasmid) images, respectively. Importantly, it indicated that Malt-DiC14MA exhibited high gene transfer efficiency and better uptake capability at N/P ratios of 8:1. Additionally, the result of cell viability showed glycolipids exhibited low biotoxicity and good biocompatibility by thiazolyl blue tetrazolium bromide (MTT) assay.

Exploring the catalytic activity of Lewis-acidic uranyl complexes in the nucleophilic acyl substitution of acid anhydrides

Uranyl(vi) ion is a strongly hard Lewis-acid and plays a catalytic role in the nucleophilic acyl substitution of acid anhydrides.

Tetranuclear zinc cluster: a dual purpose catalyst for per-O-acetylation and de-O-acetylation of carbohydrates

A tetranuclear Zn cluster-catalyzed per-O-acetylation and de-O-acetylation of carbohydrates has been reported.

Mechanistic studies of malonic acid-mediated in situ acylation

We have previously introduced an easy to perform, cost-effective and highly efficient acetylation technique for solid phase synthesis (SPPS). Malonic acid is used as a precursor and the reaction proceeds via a reactive ketene that acetylates the target amine. Here we present a detailed mechanistic study of the malonic acid-mediated acylation. The influence of reaction conditions, peptide sequence and reagents was systematically studied. Our results show that the methodology can be successfully applied to different types of peptides and nonpeptidic molecules irrespective of their structure, sequence, or conformation. Using alkyl, phenyl, and benzyl malonic acid, we synthesized various acyl peptides with almost quantitative yields. The ketenes obtained from the different malonic acid derived precursors were characterized by in situ (1) H-NMR. The reaction proceeded in short reaction times and resulted in excellent yields when using uronium-based coupling agents, DIPEA as a base, DMF/DMSO/NMP as solvents, Rink amide/Wang/Merrifield resins, temperature of 20°C, pH 8-12 and 5 min preactivation at inert atmosphere. The reaction was unaffected by Lewis acids, transition metal ions, surfactants, or salt. DFT studies support the kinetically favorable concerted mechanism for CO2 and ketene formation that leads to the thermodynamically stable acylated products. We conclude that the malonic acid-mediated acylation is a general method applicable to various target molecules.

Titanium-catalyzed esterification and transesterification reactions facilitated using microwave heating

Background: Esters can be biologically active against a range of potential targets. In addition, esterification has been used successfully to facilitate the penetration of polar compounds into cells, where the ester group can then be removed by nonspecific cellular esterases unmasking drug molecules. While direct esterification of carboxylic acids with alcohols using acid catalysts can prove effective in many cases, there are instances when substrates are acid sensitive or where the use of strong acids leads to side reactions and product decomposition. To overcome this, other methods are needed for the preparation of esters in the laboratory. Results: A methodology for titanium-catalyzed esterification and transesterification using microwave heating as a tool is presented. The reactions are complete within 1 h of heating at 160°C. The substrate scope of the methodology has been investigated. It is possible to use aromatic, aliphatic and heteroaromatic acids in the esterification protocol. Acid-sensitive alcohols, including furfuryl alcohol, are also suitable substrates. For transesterification, the reaction is again amenable to microwave heating. Conclusion: A range of esters can be prepared using a methodology based around the application of microwave heating and the use of a titanium catalyst.

Highly efficient chemoselective N-acylation with water microreaction system in the absence of catalyst

A high-speed, highly efficient chemoselective N-acylation by anhydride was achieved in the absence of catalyst for exothermic (DeltaH>0) and endothermic (DeltaH<0) acylation of various amines and anilines with the microreaction system of ambient water (micro-onH2O) and subcritical water (micro-subH2O), where the desired N-acylated products are chemoselectively obtained with high yield(s) and excellent selectivity (>95%).

Indium triflate catalyzed peracetylation of carbohydrates

Peracetylation is a very common protection strategy that is widely implemented in carbohydrate synthesis. Here, a method for the peracetylation of carbohydrates using catalytic In(OTf)(3) in neat acetic anhydride is reported. In(OTf)(3) has low toxicity and is mild and water tolerant, and the reactions are high yielding and efficient. Details regarding the scope and mechanism of the reaction are briefly discussed.

Direct and Remarkably Efficient Conversion of Methane into Acetic Acid Catalyzed by Amavadine and Related Vanadium Complexes. A Synthetic and a Theoretical DFT Mechanistic Study

Vanadium(IV or V) complexes with N,O- or O,O-ligands, i.e., [VO{N(CH2CH2O)3}], Ca[V(HIDPA)2] (synthetic amavadine), Ca[V(HIDA)2], or [Bu4N]2[V(HIDA)2] [HIDPA, HIDA = basic form of 2,2'-(hydroxyimino)dipropionic or -diacetic acid, respectively], [VO(CF3SO3)2], Ba[VO(nta)(H2O)]2 (nta = nitrilotriacetate), [VO(ada)(H2O)] (ada = N-2-acetamidoiminodiacetate), [VO(Hheida)(H2O)] (Hheida = 2-hydroxyethyliminodiacetate), [VO(bicine)] [bicine = basic form of N,N-bis(2-hydroxyethyl)glycine], and [VO(dipic)(OCH2CH3)] (dipic = pyridine-2,6-dicarboxylate), are catalyst precursors for the efficient single-pot conversion of methane into acetic acid, in trifluoroacetic acid (TFA) under moderate conditions, using peroxodisulfate as oxidant. Effects on the yields and TONs of various factors are reported. TFA acts as a carbonylating agent and CO is an inhibitor for some systems, although for others there is an optimum CO pressure. The most effective catalysts (as amavadine) bear triethanolaminate or (hydroxyimino)dicarboxylates and lead, in a single batch, to CH3COOH yields > 50% (based on CH4) or remarkably high TONs up to 5.6 x 103. The catalyst can remain active upon multiple recycling of its solution. Carboxylation proceeds via free radical mechanisms (CH3* can be trapped by CBrCl3), and theoretical calculations disclose a particularly favorable process involving the sequential formation of CH3*, CH3CO*, and CH3COO* which, upon H-abstraction (from TFA or CH4), yields acetic acid. The CH3COO* radical is formed by oxygenation of CH3CO* by a peroxo-V complex via a V{eta1-OOC(O)CH3} intermediate. Less favorable processes involve the oxidation of CH3CO* by the protonated (hydroperoxo) form of that peroxo-V complex or by peroxodisulfate. The calculations also indicate that (i) peroxodisulfate behaves as a source of sulfate radicals which are methane H-abstractors, as a peroxidative and oxidizing agent for vanadium, and as an oxidizing and coupling agent for CH3CO* and that (ii) TFA is involved in the formation of CH3COOH (by carbonylating CH3*, acting as an H-source to CH3COO*, and enhancing on protonation the oxidizing power of a peroxo-VV complex) and of CF3COOCH3 (minor product in the absence of CO).

O-Isopropylidenation of carbohydrates catalyzed by vanadyl triflate

Vanadyl triflate has been identified as a mild and efficient catalyst for the chemoselective O-isopropylidenation of functionalized carbohydrates with acetone and acetone equivalents. The current protocol is compatible with a diverse array of protecting groups and the products can be readily isolated by simple aqueous wash.

Chiral N-salicylidene vanadyl carboxylate-catalyzed enantioselective aerobic oxidation of alpha-hydroxy esters and amides

A series of chiral vanadyl carboxylates derived from N-salicylidene-L-alpha-amino acids and vanadyl sulfate has been developed. These configurationally well defined complexes were examined for the kinetic resolution of double- and mono-activated 2 degrees alcohols. The best chiral templates involve the combination of L-tert-leucine and 3,5-di-t-butyl-, 3,5-diphenyl-, or 3,4-dibromo-salicylaldehyde. The resulting vanadyl(V)-methoxide complexes after recrystallization from air-saturated methanol serve as highly enantioselective catalysts for asymmetric aerobic oxidation of alpha-hydroxyl-esters and amides with a diverse array of alpha-, O-, and N-substituents at ambient temperature in toluene. The asymmetric inductions of the oxidation process are in the range of 10 to >100 in terms of selectivity factors (k(rel)) in most instances. The previously undescribed aerobic oxidation protocol is also applicable to the kinetic resolution of C-13 taxol side chain with high selectivity factor (k(rel) = 35). X-ray crystallographic analysis of an adduct between a given vanadyl complex and N-benzyl-mandelamide allows for probing the stereochemical origin of the nearly exclusive asymmetric control in the oxidation process.

Direct Atom-Efficient Esterification between Carboxylic Acids and Alcohols Catalyzed by Amphoteric, Water-Tolerant TiO(acac)2

A diverse array of oxometallic species were examined as catalysts for a test direct condensation of benzoic acid and 2-phenylethanol in 1:1 stoichiometry. Besides group IVB MOCl2-xH2O and TiOX2-xH2O, group VB VOCl2-xTHF and group IVB TiO(acac)2 were found to be the most efficient and water-tolerant catalysts for the test reaction. The new neutral catalytic protocol with the optimal TiO(acac)2 tolerates many stereo/electronic structural variations in both (di)acid (1 degree -3 degrees alkyl and aryl) and (di)alcohol (1 degree, 2 degrees alkyl, and aryl) components with high chemoselectivity.

Stripping off Water at Ambient Temperature: Direct Atom-Efficient Acetal Formation between Aldehydes and Diols Catalyzed by Water-Tolerant and Recoverable Vanadyl Triflate

[reaction: see text]. Aromatic aldehydes can be readily protected as acetals with 1,2- and 1,3-diols by using vanadyl triflate as a catalyst in CH(3)CN at ambient temperature. Carbohydrate-based 1,2- and 1,3-diols can similarly be protected in good to excellent yields. The catalyst can be readily recovered from the aqueous layer. In combination with vanadyl triflate-catalyzed sequential regioselective, reductive acetal opening and chemoselective acylations, the title method allows for differential functionalization of all four hydroxyl units in a given glucopyranoside.

Nucleophilic acyl substitutions of anhydrides with protic nucleophiles catalyzed by amphoteric, oxomolybdenum species

[reaction: see text] Among six different group VIb oxometallic species examined, dioxomolybdenum dichloride and oxomolybdenum tetrachloride were the most efficient catalysts to facilitate nucleophilic acyl substitution (NAS) of anhydrides with a myriad array of alcohols, amines, and thiols in high yields and high chemoselectivity. In contrast to the well-recognized redox chemical behaviors associated with oxomolybdenum(VI) species, the catalytic NAS was unprecedented and tolerates virtually all kinds of functional groups. By using benzoic anhydride as a mediator for in situ generation of an incipient mixed anhydride-MoO(2)Cl(2) adduct with a given functional alkanoic acid, one can achieve oleate, dipeptide, diphenylmethyl, N-Fmoc-alpha-amino, pyruvic, and tert-butylthio ester, N-tert-butylamide, and trityl methacrylate syntheses with appropriate protic nucleophiles. The amphoteric character of the Mo=O unit in oxomolybdenum chlorides was found to be responsible for the catalytic NAS profile as supported by a control NAS reaction of using an authentic adduct-MoOCl(2)(O(2)CBu(t)())(2) between pivalic anhydride and MoO(2)Cl(2) as the catalyst.