Ruthenium trichloride catalyzed synthesis of 2,3-unsaturated-N-glycosides via Ferrier azaglycosylation

An efficient, economical and mild protocol for the synthesis of 2,3-unsaturated-N-glycosides has been developed using ruthenium(III) chloride. The Ferrier azaglycosylation of glycals with various N-nucleophiles such as sulfonamides, benzamides, carbamates and N-substituted sulfonamides proceeded smoothly to afford the corresponding 2,3-unsaturated-N-glycosides or ‘N-pseudoglycals’ in good yields (64–98%). High α-anomeric selectivity was observed with N-substituted sulfonamides such as N-benzyl or N-phenyl sulfonamides under similar conditions.     

Gold(I)-catalyzed [4+2] cycloaddition of N-(hex-5-enynyl) tert-butyloxycarbamates

A study concerning the gold(I)-catalyzed transformation of N-(hex-5-enynyl) tert-butyloxycarbamates is described. The mild conditions employed allow the moderately efficient but stereoselective synthesis of a range of bicyclic carbamates following a formal [4+2] cycloaddition process.     

N-Alkylation of carbamates and amides with alcohols catalyzed by a CpIr complex

New atom-economical catalytic systems consisting of [Cp^∗IrCl₂]₂/NaOAc (Cp^∗=pentamethylcyclopentadienyl) for the N-alkylation of carbamates and amides using alcohols as alkylating agents under solvent-free conditions have been developed. For example, the reaction of n-butyl carbamate with benzyl alcohol in the presence of [Cp^∗IrCl₂]₂ (5.0 mol % Ir) and NaOAc (5.0 mol %) at 130 °C under the absence of solvent gives n-butyl N-benzylcarbamate in the yield of 94%. The present catalytic system is applicable to not only carbamates but also amides, and only harmless water is produced as co-product.     

Addition d’aminoalcools hautement fluorés sur les isocyanates d’alcoxy et d’aroxysulfonyle

The action of F-alkylated (alkylamino)ethanols on alkoxy- and aroxy-sulfonyl isocyanates allowed the preparation of the corresponding carbamates. The reaction occurred rapidly with good yields.     

Base catalyzed cyclization of N-aryl and N-alkyl-O-propargyl carbamates to 4-alkylidene-2-oxazolidinones

The base catalyzed cyclization of N-aryl and N-alkyl-O-propargyl carbamates is studied in detail. The effect of various bases and solvents on the efficacy of this cyclization reaction is analyzed and a new base-solvent system (LiOH in DMF) for effective cyclization of these carbamates is reported. A number of differentially substituted O-propargyl carbamates were cyclized to the corresponding 2-oxazolidinones under these conditions. The reaction conditions reported here are mild and no side reactions were observed in any of the substrates studied. A propargyl carbonate group was unaffected during the course of the cyclization of the O-propargyl carbamate group. The propargyl carbamates were prepared from the corresponding alkyl or aryl amines and the corresponding propargyl chloroformate, resulting in oxazolidinones diversely substituted at the nitrogen atom. N-Aryl-O-propargyl carbamates cyclized readily to the corresponding oxazolidinones with LiOH in DMF, whereas N-alkyl-O-propargyl carbamates reacted slowly under the same conditions. O-Propargyl carbamates substituted at the 1-position tend to cyclize faster whereas those substituted at 3-position cyclize considerably slower than the unsubstituted carbamates.     

Efficient and selective deprotection method for N-protected 2(3H)-benzoxazolones and 2(3H)-benzothiazolones

Cyclic carbamate flanked with heterocyclic or aliphatic moieties are frequently used in medicinal chemistry. The synthesis of derivatives bearing a free NH often requires the use of a protection method. A literature search reveals very few protection/deprotection methods for cyclic carbamates. In this paper, we described different methods applicable to 2(3H)-benzoxazolone and 2(3H)-benzothiazolone.     

Single-step conversion of N-benzyl, N-trityl and N-diphenylmethyl amines to t-butyl carbamates using polymethylhydrosiloxane

t-Butyl carbamates were obtained efficiently in high yields from the corresponding N-benzyl, N-trityl and N-diphenylmethyl precursors in a single-step reductive transformation employing polymethylhydrosiloxane and di-t-butyl dicarbonate under Pd(OH)₂/C catalysis.     

On-column liquid–liquid–liquid microextraction coupled with base stacking as a dual preconcentration method for capillary zone electrophoresis

A simple and efficient dual preconcentration method of on-column liquid–liquid–liquid microextraction (LLLME) coupled with base stacking was developed for capillary zone electrophoresis (CZE) in this paper. Four N-methyl carbamates were used as target compounds to evaluate the enrichment means. The carbamates in sample solutions (donor phase) were extracted into a dodecanol phase immobilized on a porous hollow fiber, hydrolyzed and back extracted into 0.20 μL running buffer (acceptor phase) of 30 mmol/L methylamine hydrochloride (pH 11.6) containing 0.5 mmol/L tetradecyltrimethylammonium bromide inside the hollow fiber, stacked further with 0.5 mol/L NaOH injected at −10 kV for 60 s, and separated by CZE. Analytical parameters affecting the LLLME, base stacking and CZE were investigated, including sample solution volume, pH and temperature, extraction time, stirring rate, buffer component, buffer pH, NaOH concentration, stacking time, etc. The enrichment factors of the carbamates were higher than 1100. The relative standard deviation (RSD) of peak height and limits of detection (LODs) were 4.5–5.5% (n = 6) and 2–4 ng/mL (S/N = 3) for standard solutions, respectively. The proposed method was applied to the analysis of vegetable and fruit samples with the RSD less than 6.0% (n = 3) and LODs of 6–10 ng/g (S/N = 3). The calibration solutions were prepared by diluting the stock solutions with blank sample solutions, and the calibration concentrations ranged from 0.012 to 1.0 μg/mL (r > 0.9951). The analytical results demonstrated that the LLLME coupled with base stacking was a simple, convenient and reliable on-column sample pretreatment method for the analysis of anionic analytes in CZE.     

A simple, two-step synthesis of 3-iodoindoles

2-Halo-anilines, protected as the corresponding sulfonamides or carbamates, can be converted very efficiently into 3-iodoindoles by sequential Sonogashira coupling with a 1-alkyne and 5-endo-dig iodocyclisation. Azaindoles can also be obtained using this methodology.     

Synthesis of cis-2,5-disubstituted pyrrolidines via diastereoselective reduction of N-acyl iminium ions

A new procedure for forming cis-2,5-disubstituted pyrrolidines having unsaturated side chains has been developed that features the diastereoselective reduction of N-acyl iminium ions, which were formed in situ by acid-catalyzed cyclizations of unsaturated γ-keto carbamates, with triphenylsilane. The sequence was applied to a very concise synthesis of 16, a subunit in the nonpeptide cholecystokinin antagonist (+)-RP-66803.     

Copper-catalyzed intermolecular C(sp3)–H bond functionalization towards the synthesis of tertiary carbamates

We describe the development of an intermolecular unactivated C(sp³)–H bond functionalization towards the direct synthesis of tertiary carbamates. The transformation proceeded using a readily available, abundant first-row transition metal catalyst (copper), and isocyanates as the source of the amide moiety. This is a novel strategy for direct transformation of a variety of unactivated hydrocarbon feedstocks to N-alkyl-N-aryl and N,N-dialkyl carbamates without pre-functionalization or installation of a directing group. The reaction had a broad substrate scope with 3° > 2° > 1° site selectivity. The reaction proceeded even on a gram scale, and a corresponding free amine was directly obtained when the reaction was performed at high temperature. Kinetic studies suggested that radical-mediated C(sp³)–H bond cleavage was the rate-determining step.     

Facile synthesis of 1-substituted 4,5-diaminopyrazoles and its application toward the synthesis of pyrazolo[3,4-b]pyrazines

1-Substituted 5-aminopyrazole-4-carbonylazides were prepared from the appropriate 5-aminopyrazole-4-carboxylates. The acyl azides undergo a Curtius rearrangement followed by quenching with alcohols to form the corresponding carbamates. The 1-substituted 5-amino-4-benzyloxycarbonylaminopyrazoles were unblocked by catalytic hydrogenolysis to give the desired 4,5-diaminopyrazoles. These 4,5-diaminopyrazoles were immediately condensed with glyoxal to afford 1-substituted pyrazolo[3,4-b]pyrazines.     

Iodine-catalyzed,one-pot,three-component aza-Friedel–Crafts reaction of electron-rich arenes with aldehyde/carbamate combinations

Iodine is shown to be an efficient catalyst for a one-step, three-component aza-Friedel–Crafts reaction of activated arenes or heteroarenes with benzyl or tert-butyl carbamates in combination with a wide variety of aldehydes in toluene under ‘open-flask’ and mild conditions. In the presence of 5 mol % of iodine in toluene at room temperature, the reaction gives the corresponding N-CBz or N-Boc protected α-branched amines, selectively, in good to excellent yields.     

Copper(II) tetrafluoroborate as a novel and highly efficient catalyst for N-tert-butoxycarbonylation of amines under solvent-free conditions at room temperature

Commercially available copper(II) tetrafluoroborate hydrate was found to be a highly efficient catalyst for chemoselective N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions and at room temperature. Various aromatic amines were protected as their N-tert-butyl carbamates in high yields and in short times. No competitive side reactions such as isocyanate, urea, and N,N-di-t-Boc formation was observed. Chemoselective N-tert-butoxycarbonylation was achieved with substrates bearing OH and SH groups. Chiral α-amino acid esters afforded the corresponding N-t-Boc derivatives in excellent yields.     

An efficient one-pot synthesis of N,N′-disubstituted phenylureas and N-aryl carbamates using hydroxylamine-O-sulfonic acid

We have developed an efficient one-pot method for the microwave-assisted synthesis of ureas and carbamates via a proposed Lossen rearrangement. Herein we report the first examples of the direct conversion of benzoyl chlorides into N,N′-disubstituted ureas and N-aryl carbamates using hydroxylamine-O-sulfonic acid as reagent. Using our general method, we have produced 11 examples of N,N′-disubstituted phenylureas in yields up to 95% using various substituted anilines, and primary and secondary amines. Additionally, we were able to generate a series of N-aryl carbamates in moderate yields using primary, secondary and tertiary alcohols.     

New opinions on the amidoalkylation of hydrophosphorylic compounds

A new and milder version of the procedure for the synthesis of N-protected α-aminoalkylphosphorylic compounds by reaction of alkyl carbamates, aldehydes and hydrophosphorylic compounds in acetic anhydride/acetyl chloride and a new mechanism for this type of reaction are described. The isolation, for the first time, of N,N′-benzylidene- and N,N′-alkylidenebiscarbamates as intermediates from the reaction medium and studies of the direct reaction of pre-obtained biscarbamates and hydrophosphorylic compounds in acetic anhydride are reported. A new version of the mechanism for this reaction which includes an Arbuzov-type reaction is proposed.     

Rapid access to 3-(N-substituted)-aminoquinolin-2(1H)-ones using palladium-catalyzed C-N bond coupling reaction

A series of 3-(N-substituted)-aminoquinolin-2(1H)-ones have been synthesized by the palladium-catalyzed C-N coupling reaction starting from 3-bromoquinolin-2-(1H)-ones. Various nucleophiles including amines, amides, sulfonamides, carbamates and ureas have been used successfully. In all the cases, the reactions take place rapidly in 1,4-dioxane and proceed in good to excellent yield using palladium acetate as a catalyst, Xantphos as a ligand and Cs₂CO₃ as a base.     

Identification of phenyl-N-methylcarbamates and their transformation products in Tunisian surface water by solid-phase extraction liquid chromatography-tandem mass spectrometry

Liquid chromatography-pneumatically assisted electrospray mass spectrometry with both negative and positive ionization has been used for the determination of carbamates pesticides and their transformation products in Tunisian surface water. Eight pesticides and four of their hydrolysis products were covered in this study.Optimization of electrospray inlet conditions is described as well as results from investigations of the linearity of the detector response. Conditions for tandem mass spectrometry (MS-MS) detection of characteristic daughter ions formed by collision induced dissociation (CID) of the parent ion are described. Detection limits using MS in the selected ion monitoring (SIM) mode were generally in the order of 0.5 μg L⁻¹ or below. A principle of analysis is proposed based on triple quadrupole MS as a method for quantitative determination followed by verification of positive findings by CID-MS-MS. Application of the method for detecting carbamates residues in surface water is demonstrated.     

Use of spermine and thiabendazole as analyte protectants to improve direct analysis of 16 carbamates by gas chromatography–mass spectrometry in green vegetable matrices

The carbamates are a well-known thermosensible pesticides class, which are highly prone to degradation via fragmentation and/or rearrangement mechanisms leading to a difficult direct gas chromatography (GC) analysis, i.e., without derivatization. In this paper, spermine and thiabendazole both at 1 mg/mL were highlighted as efficient analyte protectants to improve the direct and simultaneous analysis of 16 carbamates both in solvent and green vegetable matrices. These two molecules were compared in mixture or in combination with three well-known efficient analyte protectants 3-ethoxy-1,2-propanediol, d-sorbitol, and l-gulonic acid-γ-lactone. The potential benefits were investigated in GC hyphenated to mass spectrometry (GC–MS) with two injection modes: programmable temperature vaporizing injector in a solvent split mode (PTV-SSI) and on-column injection (OCI). It was shown that the combined effect of the five protective agents led to the best sensitivity improvement with limits of detection between 0.1–0.4 and 0.03–0.1 μg/kg and limits of quantification between 0.3–1.1 and 0.1–0.5 μg/kg for PTV-SSI and OCI mode, respectively. The correlation coefficients from the analyzed 1–500 μg/kg range were all >0.999 both in the solvent and matrices studied. The recoveries of carbamates from three spiked matrices over five replicates at 20 and 100 μg/kg were in the range 90–107% with relative standard deviation (RSD) equal to 2–7% for PTV-SSI and 92–107% with an RSD equal to 1–6% for OCI. The use of spermine and thiabendazole with other analyte protectants shows very efficient partial or total reduction of breakdown of the most sensitive carbamates such as the N-sulfenylated ones. An erratum to this article can be found at     

Enantioselective rhodium-catalyzed hydrogenation of enol carbamates in the presence of monodentate phosphines

The rhodium-catalyzed asymmetric hydrogenation of different acyclic and cyclic enol carbamates to give optically active carbamates has been examined in the presence of chiral monodentate ligands based on a 4,5-dihydro-3H-dinaphthophosphepine motif 4. The enantioselectivity is largely dependent upon the reaction conditions, the nature of substituents on the phosphorus ligand and structure of the enol carbamate. By applying the optimized reaction conditions, enantioselectivities of up to 96% ee have been achieved.