To carbamate

Na2C03, H2O, dioxane, warm, 97% yield. Phenols can be protected under similar conditions. Amines are converted by these conditions to carbamates... 

Na2C03, H2O, dioxane, warm, 97% yield. Phenols can be protected under similar conditions. Amines are converted by these conditions to carbamates that are stable to alkaline hydrolysis with sodium carbonate. Carbamates are cleaved by acidic hydrolysis (HBr, MeOH, CH2CI2, 8 h), conditions that do not cleave alkyl or aryl vinyl carbonates. 

In a variation of this reaction, alkyl halides can be converted to carbamates, by treatment with a secondary amine and K2CO3 under phase-transfer conditions. ... 

Au Amines (oxidative carbonylation with CO/O2 to carbamates and symmetric ureas)... 

Phenyliodonium diacetate,274 275 and phenyliodonium to-trifluoroacetate, 276 are also useful oxidants for converting amides to carbamates. 

The reactor residence time is about 45 minutes, a 95 per cent approach to equilibrium being achieved in this time. The ammonia is fed directly to the reactor, but the carbon dioxide is fed to the reactor upwardly through a stripper, down which flows the product stream from the reactor. The carbon dioxide decomposes some of the carbamate in the product stream, and takes ammonia and water to a high-pressure condenser. The stripper is steam heated and operates at 180°C, whilst the high-pressure condenser is at 170°C and the heat released in it by recombination of ammonia and carbon dioxide to carbamate is used to raise steam. Additional recycled carbamate solution is added to the stream in the high-pressure condenser, and the combined flow goes to the reactor. 

Occupational exposure to carbamate insecticides may be monitored by measuring RBC-ACHE and/or PCHE. However, given the low cholinesterase inhibition levels and the short time duration of this effect, ACHE inhibition can generally be used as a biomarker of exposure only when exposure levels are high. Three sequential samples are recommended to establish an individual baseline before exposure. In exposed workers, blood sampling and analysis should be carried out soon after the end of exposure (WHO, 1986). 

Table 5 Biomarkers Used in Biological Monitoring of Human Exposure to Carbamates...

Rhodium compounds have also been used as catalysts since the late 1960s and mechanistic studies date from the 1970s.534,578-582 The binuclear rhodium complex [(Ph3P)4Rh2(//-OH)2] was found to be an effective catalyst for the reductive carbonylation of nitrobenzenes to carbamate esters. Electron-withdrawing groups at the para-position enhance the reactivity of the substrate.583... 

Allylic carbamates have also been cyclized to carbamate-linked fused-ring aziridines. The cyclization of homoallylic carbamates to the corresponding aziridines has not been successful until a recent report <06CC4501>. The reaction of homoallylic carbamate 63 with a rhodium catalyst and iodosobenzene provides moderate yields of the fused-ring aziridine 64. The major byproduct of this reaction is the C-H insertion product 65. The relative amounts of the aziridine to the C-H insertion product could be modulated by the choice of rhodium catalyst. The use of Rh2(OAc)4 provides a 68 14 ratio of aziridine C-H insertion product, while Rh2(oct)4 provides a slightly better 71 6 ratio. 

Plants, fruits, vegetables, and grains exposed to carbamate nerve agents should be quarantined until tested. 

Currently, there is no information on performance testing of chemical protective clothing against carbamate nerve agents. Evaluation of fabrics used to prevent exposure to carbamate pesticides may provide guidance on selection of appropriate protective clothing. 

Conversion to carbamates. Cyclic carbamates derived from sugars and related compounds have repeatedly been encountered, the first being an inosamine derivative, obtained by Curtius degradation of dihydroshikimic acid.82,83... 

If we consider the generation of alkoxycarbenium ions by C-H bond dissociation, ethers should be of our first choice as precursors of alkoxycarbenium ions by analogy to carbamates. The oxidation potentials of ethers, especially aliphatic ethers, however, are very positive, and therefore, it is rather difficult to oxidize ethers selectively under usual conditions. The regioselectivity is also a problem. Usually a mixture of two regioisomers of products is obtained because two regioisomeric alkoxycarbenium ions are generated. 

Unfortunately, the presence of the benzylic alcohol moiety at the focal point of the dendrimer, along with the catalyst (used in the urethane formation) led to the formation of undesired side products, presumably due to carbamate interchange. These side reactions were avoided by switching to monomer 19, methyl-3,5-dihydroxybenzoate. While the carbamate linkages of dendrons 53 and 54 were too unstable under the alkylation conditions required to afford larger dendrons, the merits of the concept was adequately demonstrated for this accelerated synthesis of [G-3] dendrons. 

Oxidative carbonylation is not necessarily associated with C - C bond formation. Indeed, heteroatom carbonylation may occur exclusively, as in the oxidative carbonylation of alcohols or phenols to carbonates, of alcohols and amines to carbamates, of aminoalcohols to cyclic carbamates, and of amines to ureas. All these reactions are of particular significance, in view of the possibility to prepare these very important classes of carbonyl compounds through a phosgene-free approach. These carbonylations are usually carried out in the presence of an appropriate oxidant under catalytic conditions (Eqs. 31-33), and in some cases can be promoted not only by transition metals but also by... 

Fig. 7.3. Hydrolysis of carbamate esters (7.2) to carbamic acid (7.3), and spontaneous breakdown of the latter...

Scheme 6.24. Diastereoselective cuprate addition to -carbamate-functionalized enoate 113.

For resin-bound ureas (22-25), Indole and Rink linkers, generally ranked 1 and 2 in cleavage kinetics, were still the most acid-labile linker for this class of compounds. Resins (24) and (25) were cleaved with 0.5% TFA in 2 and 23 min, respectively. Resins (22) and (23) were cleaved with 1% TFA in more than 10 h. The order of cleavage rates is similar to carbamate compounds. 

For synthetic purposes the cyanate ion is a promising reagent leading to amines in water and to carbamates in dcohol (Hartsuiker et al., 1971). Alkyl-lithium reagents may also function in nucleophilic photosubstitutions with aromatic partners containing suitable leaving groups (Shapiro and Tomer, 1968). 

In his meticulous exploration of methods for his thesis research, Proctor ( ) noted that the toxicity (MIP) of cell free extracts of Gonyaulax increased following heating at low pH. Hall (1,22) further explored this increase in potency, referring to it as Proctor enhancement, and demonstrated that it was primarily due to the hydrolysis of sulfamate to carbamate toxins. It was found that heating at 100 C for 5 minutes in aqueous HCl with a free acid concentration of 0.1 M is sufficient to insure complete hydrolysis and attain maximum MIP. 

A subsequent process, still reliant on salt resolution, is described in Scheme 14.8. In this case, (3-keto amine 34 was converted to carbamate 35, which was then transformed into the desired amino alcohol intermediate 36. The resolution was performed in three distinct stages (1) initial resolution of the racemate, (2) racemization of the (/J)-enriched mixture, and (3) second-order asymmetrically induced crystallization of the (5)-salt. 

Thus, treatment of the benzamide (35-1) from 2-phenethylamine with phosphorus oxychloride probably results in an initial formation of a transient enol chloride this then cycUzes to (35-2) under reaction conditions. The imine is then reduced with sodium borohydride. Resolution by means of the tartrate salt affords (35-3) in optically pure form. Acylation of that intermediate with ethyl chloroformate leads to carbamate (35-4). Reaction of this last with the anion from chiral quiniclidol (35-5) interestingly results in the equivalent of an ester interchange. There is thus obtained the anticholinergic agent solifenacin (35-6) [40]. 

Pillai, R.P. Raushel, F.M. Villafranca, J.J. Stereochemistry of binding of thiophosphate analogs of ATP and ADP to carbamate kinase, glutamine synthetase, and carbamoyl-phosphate synthetase. Arch. Biochem. Biophys., 199, 7-15 (1980)... 

K. Buch measured the partial press, of ammonia and carbon dioxide in mixtures of ammonium carbonate and carbamate, and from the results calculated the cone, of the free and bound ammonia and higher carbonate, and of the carbamate and carbonic acid. The hydrolysis and equilibrium constants were then calculated. The hydrolysis constants Kx and K% and the equilibrium constant K3 of the hydrocarbonate to carbamate, were ... 

If ammonium carbamate be treated with less than its eq. of calcium chloride, it soon gives a precipitate, and the filtered soln. gives no further precipitation when heated. The reaction is the same as when calcium chloride is added in excess. An ammoniacal soln. of calcium chloride gives an instantaneous but slight precipitate when treated with an excess of a soln. of ammonium carbamate. The precipitate increases slowly on standing, and next day, when boiled, the supernatant liquor gives a copious precipitate. The immediate precipitation is presumably due to the presence of ammonium carbonate, and the subsequent precipitation is due to the slow conversion of carbonate to carbamate in the presence of free ammonia. Hence, an ammoniacal soln. of calcium chloride gives an immediate precipitation with an excess of ammonium carbonate and a very slow precipitation with an excess of the carbamate. 

The NH3 and the C02 gas, recovered from the second decomposition stage, is cooled in the second absorption stage where all the C02 is reacted to carbamate and dissolved in water along with some of the excess NH3. The solution thus obtained is pumped back to the first absorption stage and thence to the reactor for total recovery. Only part of the excess ammonia is dissolved in the solution recovered from the second absorption stage. The remaining portion of the excess ammonia gas is separated from the aqueous carbamate solution, purified from the last traces of C02, in a bubble cap column, compressed to about 20 atm, condensed to... 

The Curtius rearrangement provides a route to carbamates of a-amino sulfonamides (Scheme 25). 108 Reaction of the ethyl ester of an a-bromo acid 52 with Na2S03 yielded the sodium salt of the corresponding sulfonic acid 53. Treatment of 53 with PQ5 afforded the sulfonyl chloride 54 which on reaction with an amine gave the sulfonamide 55. The latter... 

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