Carbamates, preparation

The diphenylmethyl carbamate, prepared from the azidoformate, is readily cleaved by mild acid hydrolysis (1.7 N HCl, THF, 65°, 10 min, 100% yield). ... 

The carbamate, prepared from the 4-nitrophenyl carbonate, is cleaved by reduction with dithiothreitol (DTT) and TEA to give the aniline, which triggers fragmentation, releasing the amine. ... 

The direct addition of nonprostereogenic ketones, however, is preparatively useful. Reaction of (S)-( )-(l-lithioalkenyl) carbamates, prepared in toluene according to Section 1.3.3.3.1.2., with acetone achieves the y-adduct with 84% ee and 61 % yield122. The (S)-(Z)-reagents lead to the opposite enantiomers122. 

Alkyl carbamates prepared from the carbamyl chlorides also react with amines to give semicarbazides [15, 17] (Eq. 25). 

Table 3-27 Carbamates prepared from 1-chloroethyl carbonates. ...

Note also that reaction of methyl-N-chlorocarbonyl-N-methyl carbamate (preparation through phogenation given in section 3-3-4, this volume) is very suitable for the preparation of numerous urazoles. Some examples are presented in table 3-30 (Ref. 265). 

Regioselective C-2 amination of 2,3-epoxy alcohols can be realized by intramolecular oxirane ring opening of the carbamates prepared by the reaction of the epoxy alcohols with isocyanates." -" In some cases, however, acyl transfer is a serious side reaction (Scheme 59)." ... 

N-AUyl carbamates. Preparation of the carbamates from allyl ethers may incur 1,3-transposition. 

The thermal decomposition of urethanes can be catalyzed by tertiary amines. This was shown by the reduced thermal stability of carbamates prepared from primary alcohols under nitrogen in the presence of a catalytic amount of amine. ... 

The chiral A7sulfinyl carbamate prepared from (+)-camphor (eq 13) displayed high enantloselectlvlty in the tltanlum(IV) chloride catalyzed [4 + 2] cycloaddltlon with 1,3-cyclohexadlene, affording only one cycloadduct. The stereochemistry of compound (12) can be rationalized by a cycloaddltlon occurring through either conformer (10) via an endo transition state or... 

The newest variation of the HR is an electrochemical method e.g., 46- 48, which does not appear to have been explored beyond its discoverers/ " The method is particularly useful for the synthesis of alkyl carbamates from primary alcohols containing sensitive functional groups since this electrochemical HR operates under neutral conditions. Some of the alkyl carbamates prepared using this HR are shown (202-205). The yields of the same methyl carbamates are uniformly higher with this electrochemical HR than with the classical bromine and sodium methoxide HR. Furthermore, little or no urea products are formed in the electrochemical HR. ... 

Many procedures for carbamate preparation have been developed in connection with the high demand for amine group protection of amino acids in peptide synthesis [155, 365]. Some examples of reported procedures employing phosgene, alkyl- or alkylene chloroformates are given below. 

Carbamates Prepared with Isocyanates or Carbamoyl Chlorides... 

Chloroalkyl carbamates, prepared with phosgene (see Section 4.3.2 Carbamates ), react with hard nucleophiles such as methoxymethyl amine to afford known ureas such as the herbicide Linuron [219]. 

A large-scale cyclization reaction of carbamates with functionalized butenoates, starting from carbamates prepared without phosgene, for the efEdent preparation of 3-(substituted phenyl)-5-isopropylidene-l,3-oxazolidine-2,4-dione derivatives (azaladones) having potent herbicidal adivity, has been described in a patent application [240]. By reacting an N-(substituted phenyl)carbamate 312 with a 2-hydroxy-3-alkenoic acid ester (for example, ethyl 2-hydroxy-3-methyl-3-butenoate) or a 3-alkoxy-2-hydroxyalkanoic add ester, at 210 °C for 15 h, azaladone 313 was formed in 71.5% yield. 

Finally, a few references have appeared in which bidentate compounds have been cyclized using well understood chemistry to produce some novel heterocycles. There are numerous examples of 5- and 6-membered diazaphos-phorus heterocycles but there are far fewer with smaller or larger rings. Dichlorophosphinyl carbamates, prepared from isocyanates, have been cyclized with diamines to afford the novel benzodiazaphosphepin system (395). Oxadiazepins (396) have been prepared by reaction of the amino-oximes with formaldehyde and 1,3,4,6-thiatriazepins (397) were obtained by Moss and Taylor using cyclic thioureas. 

N-Benzyl carbamates prepared as outlined above can also serve as useful intermediates in the overall A-methylation of secondary... 

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