Carbamates, addition

Nitrophenyl)ethyl Carbamate. The photolytic removal of this group occurs twice as fast as does the 2-nitrobenzyl carbamate. Additionally, substitution at the alpha carbon increases the rate of cleavage even more. 

Carbamates (substituted urethanes) are prepared when isocyanates are treated with alcohols. This is an excellent reaction, of wide scope, and gives good yields. Isocyanic acid HNCO gives unsubstituted carbamates. Addition of a second mole of HNCO gives allophanates. 

Later, several mononuclear Ru complexes were shown to be better catalysts for the activation of terminal alkynes towards ammonium N,N-dialkyl carbamates [90]. The formation of vinyl carbamates was, in general, highly regioselective with mononuclear complexes as compared to Ru3(CO)12, as the carbamate addition took place mainly at the unsubshtuted carbon atom leading to the Z and E isomers. 

Compound 26 containing an oxime moiety saw an increase in rat bioavailability and AUC of two-fold and approximately four-fold, respectively, when compared to azithromycin [82]. Other cores not depicted here but that can be incorporated into the series include telithromycin-type carbamates, additional oxime variants, as well as any of the common 14- and 15-membered macrolides. 

The Peterson reactions of a-sUylbenzyl carbamates provide a method for the preparation of aromatic alkenyl carbamates. Addition of carbonyl compounds to the carbanions derived from the a-sUylbenzyl carbamates 64 with Bu Li leads directly to the corresponding alkenyl carbamates 65 with Z-selectivity (Scheme 2.41)... 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

In this condenser, part of the stripper off-gases are condensed (the heat of condensation is used to generate low pressure steam). The carbamate formed and noncondensed NH and CO2 are put into the reactor bottom and conversion of the carbamate into urea takes place. The reactor is sized to allow enough residence time for the reaction to approach equiUbrium. The heat required for the urea reaction and for heating the solution is suppHed by additional condensation of NH and CO2. The reactor which is lined with 316 L stainless steel, contains sieve trays to provide good contact between the gas and Hquid phases and to prevent back-mixing. The stripper tubes are 25-22-2 stainless steel. Some strippers are still in service after almost 30 years of operation. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

An hplc assay was developed suitable for the analysis of enantiomers of ketoprofen (KT), a 2-arylpropionic acid nonsteroidal antiinflammatory dmg (NSAID), in plasma and urine (59). Following the addition of racemic fenprofen as internal standard (IS), plasma containing the KT enantiomers and IS was extracted by Hquid-Hquid extraction at an acidic pH. After evaporation of the organic layer, the dmg and IS were reconstituted in the mobile phase and injected onto the hplc column. The enantiomers were separated at ambient temperature on a commercially available 250 x 4.6 mm amylose carbamate-packed chiral column (chiral AD) with hexane—isopropyl alcohol—trifluoroacetic acid (80 19.9 0.1) as the mobile phase pumped at 1.0 mL/min. The enantiomers of KT were quantified by uv detection with the wavelength set at 254 nm. The assay allows direct quantitation of KT enantiomers in clinical studies in human plasma and urine after adrninistration of therapeutic doses. 

Carbamates. Lower alkyl A/-halo- and W,W-dihalocarbamates are distillable Hquids (70,112). A/-Halo-A/-metallocarbamates are crystalline hygroscopic soHds. A/-Chloro-A/-sodiourethane [17510-52-0] C2H OCONQNa, does not decompose on heating to 250°C (113), but violent decompositions have occurred at room temperature (114). A/-Halocarbamates react with a variety of organic substrates, eg, the free-radical addition of W-chlorourethane [16844-21 -6] C2H OCONHCl, and A,A-dichlorourethane [15698-16-5], C2H OCONCl2, to olefins provides a convenient route to... 

This ammonia is recycled to the reactor via a compressor and a heater. Liquid ammonia is used as reflux on the top of the absorber. The net amount of carbon dioxide formed in the reactor is removed as bottom product from the absorber in the form of a weak ammonium carbamate solution, which is concentrated in a desorber-washing column system. The bottom product of this washing column is a concentrated ammonium carbamate solution which is reprocessed in a urea plant. The top product, pure ammonia, is Hquefted and used as reflux together with Hquid makeup ammonia. The desorber bottom product, practically pure water, is used in the quench system in addition to the recycled mother Hquor. 

Benzyl carbamates are readily cleaved under strongly acidic conditions HBr, AcOH 50% CF3COOH (25°, 14 days, partially cleaved) - 70% HF, pyridine CF3S03H FSOaH, or CHjSO.H.- In cleaving benzyl carbamates from peptides, 0.5 M 4-(methylmercapto)phenol in CF3CO2H has been recommended to suppress Bn additions to aromatic amino acids. To achieve deprotection via an Sn2 mechanism that also reduces the problem of Bn addition, HF-Me2S-p-cresol (25 65 10, v/v) has been recommended for peptide deprotection. 

The only practical method for preparing alkylene thioureas is by the action of the diamines upon carbon disulfide in aqueous alcohol. The final heating is essential to convert the thio-carbamic acid into the cyclic compound, the addition of hydrochloric acid being beneficial. ... 

A. Ethyl N- p-tolylsulfonylmethyl)carhamate,[Carbamic acid, (4 -methyl-phenylsulfonylmethyl)-, ethyl ester]. A solution of 178 g. (1.0 mole) of sodium p-toluenesulfinate (Note 1) in 1 1. of water is placed in a 3-1., three-necked daak, equipped with a condenser, an efficient mechanical stirrer, and a thermometer. After addition of 100 ml. (108 g.) of a 34—37% solution of formaldehyde ca. 1.2-1.4 moles) (Note 2), 107 g. (1.2 moles) of ethyl carbamate (Note 3), and 250 ml. of formic acid (Note 4), the stirred solution is heated to 70°. Soon after this temperature is reached, the reaction mixture becomes turbid by separation of the... 

In addition, some COg is directly transported by hemoglobin in the form of carbamate (—NHCOO ). Free a-amino groups of Hb react with COg reversibly ... 

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