Carbamates from azides

Jiao and co-workers reported in 2014 a simple and practical catalytic methodology for the direct formation and application of isocyanates for the synthesis of carbamates [48]. This chemistry provides an efficient and practical approach to synthesize carbamates from simple organic azides, CO atmosphere and alcohols. The broad scope, mild and neutral conditions, and only N2 as the byproduct make this transformation very useful. Moreover, simple examples of modilication of bioactive molecules and constmction of macrocycles were achieved through this protocol. By utilizing this protocol, the marketed dmgs Chlorzoxazone (a centrally acting muscle relaxant) and Chlorpropham (an important herbicide and sprout suppressant) were easily prepared from simple and readily available aromatic azides, CO, and alcohols in high yields (Scheme 5.17). The mechanism is proposed... 

Nitrogen nucleophiles used to diplace the 3 -acetoxy group include substituted pyridines, quinolines, pyrimidines, triazoles, pyrazoles, azide, and even aniline and methylaniline if the pH is controlled at 7.5. Sulfur nucleophiles include aLkylthiols, thiosulfate, thio and dithio acids, carbamates and carbonates, thioureas, thioamides, and most importandy, from a biological viewpoint, heterocycHc thiols. The yields of the displacement reactions vary widely. Two general approaches for improving 3 -acetoxy displacement have been reported. One approach involves initial, or in situ conversion of the acetoxy moiety to a more facile leaving group. The other approach utilizes Lewis or Brmnsted acid activation (87). 

The bicyclic meso-ionic 3-oxo-l,2,4-triazolo[4,5-a]pyridines (206) have been prepared by the following methods (i) the reaction of the hydrazines (207, X = H) with phosgene, (ii) heating the amide (207, X = CONH2) or the carbamate (207, X = COjEt), and (iii) alkylation or acylation of 3-oxo-l,2,4-triazolo[4,5-a]pyridine (208). The isomeric meso-ionic 2-oxo-l,3,4-triazolo[4,5-a]pyridines (209) are formed from the carbamoyl chlorides (210) and sodium azide. ... 

Synthesis of a reference sample of diol 20a (obtained from 19, see p 407) from (-)-dimethyl (7 ,7 )-tartiate 202 via O-protection to give 21, reduction to 22a, mesylation to 22b and azide substitution to give 22c followed by reduction and carbamation to give 20a. Compared were the Mosher esters 20b66. 

Support-bound isocyanates can be conveniently prepared from carboxylic acids by Curtius degradation. Because the reaction of the intermediate acyl azides with alcohols to yield esters is slow, Curtius degradation can be conducted in the presence of alcohols to yield carbamates directly (Entries 4 and 5, Table 14.8). 

An ester group at C-3 or C-5 is not different in function from any ester in aliphatic or aromatic compounds. The ester can be converted into an amide or a hydrazide, or it may be hydrolyzed. The hydrazide can be converted into an acid azide and rearranged to an isocyanate which in turn will form a carbamate or can be hydrolyzed to an amine. However, the amine group can be replaced with chlorine by diazotization in hydrochloric acid, not an ordinary pattern of behavior (76AHC(20)65) although similar reactions do occur in the 1,2,4-triazole ring system. 

Arylfurazancarboxamides on treatment with alkaline hypochlorite undergo Hofmann degradation to the amines likewise carbamates result from Curtius rearrangement of furazanylacyl azides in the presence of alcohols. 

Conversion of the corresponding isocyanates, resulting from the heating of the azides in benzene, with alcohols of general formula R OH, yielded the carbamic acid esters LXXVIb (150), and with amines R"R" NH, the urea derivatives LXXVIc and LXXVIIc (151, 152). 

On pyrolysis of methyl-i V-(2,4-diniIrophenyl)carbamate 5-nitro-2,l,3-benzoxadi-azole (5-nitrobenzofurazan) was isolated in a yield of 35% (Scheme 2.83) [519], The key product in this process is or/Zm-nitrozophenylnitrcnc from which ben-zofurazan is formed later. The reaction of 2-chloro-5-nitronitrozobenzene with sodium azide in an aqueous acetone medium is likely to follow a similar pathway. In this case the yield of 5-nitrobenzofurazan reaches 73% (Scheme 2.84) [520],... 

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