Carbamate reagent

Formation of diastereomeric ureas from chiral carbamate reagents... 

Dlethylammonlum dlethyldithlocarbamate solution, 1 per cent (carbamate reagent). Dissolve 1 g of pure crystalline reagent In 100 ml of redistilled chloroform and store in dark bottle. This solution should be discarded after 1 week. Methyl red Indicator solution, 0.01 per cent. Warm 25 mg methyl red with 0.95 ml 0.05 M NaOH and 5 ml of 90j6 ethanol. When dissolution Is complete add 50)( ethanol to 250 ml. 

Add 10 ml of carbamate reagent and shake for 30 seconds. Transfer the chloroform, layer to a 100 ml flask. Wash the aqueous layer twice with small amounts of chloroform without shaking and add washings to the flask. 

Repeat the extraction with 10 ml of carbamate reagent, add to first extract. Reject aqueous layer. 

Extract the acid solution directly In the coldj first with 10 ml and then with 5 nil of carbamate reagent shaking for 30 seconds each time. Separate and discard the lower (chloroform) layer. Finally shake the acid with 5 ml of chloroform and discard the chloroform. 

Ill Carbamate reagent. Dissolve 1 g of pure crystalline diethylam-monium diethyldithiocarbamate in 100 ml of redistilled chloroform and store in an amber-coloured bottle. This solution is not stable and should be discarded after one week. 

Alcohols (but not ethers) also react with phenyl isocyanate or with the corre-sponding crystalline a-naphttiyl isocyanate to give carbamates or urethanes (see Section 111,27, ), but these substances are hardly suitable as class reagents. 

In each step of the usual C-to-N peptide synthesis the N-protecting group of the newly coupled amino acid must be selectively removed under conditions that leave all side-chain pro-teaing groups of the peptide intact. The most common protecting groups of side-chains (p. 229) are all stable towards 50% trifluoroacetic acid in dichloromethane, and this reagent is most commonly used for N -deprotection. Only /ert-butyl esters and carbamates ( = Boc) are solvolyzed in this mixture. 

Derivatization with Optically Active Reagents and Separation on Achiral Columns. This method has been reviewed (65) a great number of homochiral derivatizing agents (HD A) are described together with many appHcations. An important group is the chloroformate HD As. The reaction of chloroformate HD As with racemic, amino-containing compounds yields carbamates, which are easily separated on conventional hplc columns, eg (66),... 

Acylation. Reaction conditions employed to acylate an aminophenol (using acetic anhydride in alkaU or pyridine, acetyl chloride and pyridine in toluene, or ketene in ethanol) usually lead to involvement of the amino function. If an excess of reagent is used, however, especially with 2-aminophenol, 0,A/-diacylated products are formed. Aminophenol carboxylates (0-acylated aminophenols) normally are prepared by the reduction of the corresponding nitrophenyl carboxylates, which is of particular importance with the 4-aminophenol derivatives. A migration of the acyl group from the O to the N position is known to occur for some 2- and 4-aminophenol acylated products. Whereas ethyl 4-aminophenyl carbonate is relatively stable in dilute acid, the 2-derivative has been shown to rearrange slowly to give ethyl 2-hydroxyphenyl carbamate [35580-89-3] (26). 

Me3SiI, CCI4 or CHCI3, 25°, <0.1 h, 100% yield. Under suitable conditions this reagent also cleaves many other ethers, esters, ketals, and carbamates. ... 

Me3SiI, CH3CN, 25-50°, 100% yield. Selective removal of protective groups is possible with this reagent since a carbamate, =NCOOCMe3, is cleaved in 6 min at 25° an aryl benzyl ether is cleaved in 100% yield, with no formation of 3-benzyltyrosine, in 1 h at 50°, at which time a methyl ester begins to be cleaved. 

Benzyl carbamates of pyrrole-type nitrogens can be cleaVfed with nucleophilic reagents such as hydrazine hydrogenation and HF treatment are also effective. ... 

Carbonates, like esters, can be cleaved by basic hydrolysis, but generally are much less susceptible to hydrolysis because of the resonance effect of the second oxygen. In general, carbonates are cleaved by taking advantage of the properties of the second alkyl substituent (e.g., zinc reduction of the 2,2,2-trichloroethyl carbonate). The reagents used to introduce the carbonate onto alcohols react readily with amines as well. As expected, basic hydrolysis of the resulting carbamate is considerably more difficult than basic hydrolysis of a carbonate. 

Carbamates are formed from an amine with a wide variety of reagents, the chlo-roformate being the most common amides are formed from the acid chloride. n-Alkyl carbamates are cleaved by acid-catalyzed hydrolysis A-alkylamides are cleaved by acidic or basic hydrolysis at reflux and by ammonolysis, conditions that cleave peptide bonds. 

Benzyloxycarbonyloxy)phenyl]dimethylsulfonium methyl sulfate, NaOH, H20,51-95% yield. " This is a water-soluble reagent for benzyloxy carbamate formation. Analogous reagents for the introduction of BOC and Fmoc were also prepared and give the respective derivatives in similar high yields. 

Sm, I2, MeOH, reflux 24 h, 95% yield. This reagent also cleaves the Cbz group and other carbamates and esters. 

The most important group of derivatives for the amino function (Fig. 7-4) is the carbamate group, which can be formed by reactions with acids, acid chlorides or acid anhydrides. A series of chlorides as 2-chloroisovalerylchloride [1], chrysanthe-moylchloride [2] and especially chloride compounds of terpene derivatives (cam-phanic acid chloride [3], camphor-10-sulfonyl chloride [4]) are used. The a-methoxy-a-trifluoromethylphenylacetic acid or the corresponding acid chloride introduced by Mosher in the 1970s are very useful reagents for the derivatization of amines and alcohols [5]. 

In general, the reaction mechanism of elastomeric polymers with vulcanisation reagents is slow. Therefore, it is natural to add special accelerators to rubber compounds to speed the reaction. Accelerators are usually organic compounds such as amines, aldehyde-amines, thiazoles, thiurams or dithio-carbamates, either on their own or in various combinations. 

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