Carbamate //- Transformation

The same authors also published a simple one-pot protocol for the azide to carbamate transformation using a variety of chloroformates.21 This preparation is excellent for azide to carbamate transformations (such as Cbz, Troc, and Alloc) that would clearly not be feasible via the one-pot catalytic hydrogenation/protection preparations due to functional group incompatibility. Trimethylphosphine is the phosphine of choice once again. As the rapid and room temperature conversion of 33 to intermediate 34 illustrates, the use of trimethylphosphine allows for excellent yields under mild conditions. 

Conversion of the hexamethylenediamine to the carbamate transforms the diamine liquid to a more process-friendly solid. The carbamate also blocks the amine and adds scorch safety to the fast curing HMDC system. Bin stability is not usually a problem with HMDC but there are cases where the scorch safety may not be acceptable. In general, if the Mooney Scorch minimum is low, a compound... 

The presence of a tnalkylsUyl group in a fluonnated organic compound may be useful to direct further transformations of that matenal Yet m some instances it IS the fluonnated substituent that controls the reactions of the tnalkylsdyl group Contrary to predictions, treatment of tert-hnlyX 3-tnfluoromethyl-6-tnmethylsilyl-phenyl carbamate with rert-butyllithium results m metallation of one of the methyl groups attached to silicon rather than that of the aromatic nng [90] (equation 75)... 

FIGURE 3.5 Fourier Transform infrared (FTIR) spectra of acrylic rubber (ACM)-siUca hybrid nanocomposites. The numbers after ACM (10 and 50) indicate the wt% tetraethoxysilane (TEOS) concentration. The letters preceding the numbers indicate the ACM-silica samples cross-linked from benzoyl peroxide (B) and a mixed cross-linker hexamethylene diamine carbamate and ammonium benzoate (D). The numbers over the absorption peaks are the wave numbers corresponding to absorbance of those peaks. (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl. Polym. Sci., 93, 2579, 2004. Courtesy of Wiley InterScience.)... 

Oxidative carbonylation generates a number of important compounds and materials such as ureas, carbamates, 2-oxazolidinones, and aromatic polycarbonates. The [CuX(IPr)] complexes 38-X (X = Cl, Br, I) were tested as catalysts for the oxidative carbonylation of amino alcohols by Xia and co-workers [43]. Complex 38-1 is the first catalyst to selectively prepare ureas, carbamates, and 2-oxazolidinones without any additives. The important findings were the identity of the counterion and that the presence of the NHC ligand influenced the conversions. 2-Oxazohdinones were formed from primary amino alcohols in 86-96% yield. Complex 38-1 also catalysed the oxidative carbonylation of primary amines to ureas and carbamates. n-Propylamine, n-butylamine, and t-butylamine were transformed into the... 

Acidic compounds with N—H bonds such as amides, carbamates, and hydan-toins, may be transformed to /V-rnannich bases to form oral prodrugs [2], These prodrugs are generally made by reacting an amide, carbamate, or hydantoin with formaldehyde and a primary or secondary aliphatic or aromatic amine (Fig. 4). The (V-mannich prodrugs tend to have better physicochemical properties than the parent compounds. The derivatives may have increased water solubility, dissolution rate, and/or lipophilicity. 

Another microwave-mediated intramolecular SN2 reaction forms one of the key steps in a recent catalytic asymmetric synthesis of the cinchona alkaloid quinine by Jacobsen and coworkers [209]. The strategy to construct the crucial quinudidine core of the natural product relies on an intramolecular SN2 reaction/epoxide ringopening (Scheme 6.103). After removal of the benzyl carbamate (Cbz) protecting group with diethylaluminum chloride/thioanisole, microwave heating of the acetonitrile solution at 200 °C for 2 min provided a 68% isolated yield of the natural product as the final transformation in a 16-step total synthesis. 

Five approaches to the synthesis of 5-amino-4-unsubstituted imidazoles (96) have been described and are summarized in Scheme 9. These are (a) reduction of 5-nitroimidazoles (97), (b) hydrolysis of carbamates and amides (98), (c) decarboxylation of imidazole carboxylic acids (99), (,d) ring transformations of 5-aminothiazoles (100), and (e) cyclisation of nitrile derivatives (101). 

Some organochlorine, organophosphorus, and carbamate insecticides used after World War II (since 1945) were found to have various problems of adverse effects on mammals and environmental behavior and influences. The use of many industrial chemicals has been prohibited because those contained as impurities in minute quantities produced critical toxic substances by transformation and repeated chemical reactions in their environment. 

The stability of Molsidomine 84 and SIN-1 85 in aqueous systems under acidic, basic, thermic and photolytic conditions was investigated by Asahi et al. [92] (Scheme 6.17). In alkaline solutions the carbamate group is hydrolysed and decarboxylated to 85. This product immediately opens to SIN-1 A 105 that, in turn, is transformed into SIN-1C 106. The authors postulated that elimination of HNO underlies this last transformation. 

It is well known that oxidation of carbamates leads to the formation of N-acyliminium ions via dissociation of the C-H bond a. to nitrogen. The electrochemical,4 metal-catalyzed,5 and chemical methods6 have been reported in the literature to accomplish this transformation. The transformation serves as a useful tool for organic synthesis, although only compounds of high oxidation potentials such as methanol and cyanide ion can be used as nucleophile. It... 

In recent years, Hoppe s group has considerably extended the isomerization-addition methodology, especially for the highly regio- and stereoselective synthesis of 1,2-alkadienyl carbamates. It involves deprotonation of alkynyl carbamates, transme-talation into a titanium species and subsequent reaction with carbonyl compounds [26-30]. This group recently described the preparation of enantiomerically enriched 4-hydroxyallenyl carbamates 22 by sparteine-mediated lithiation of alkynyl carbamates 20 [29]. Impressive examples of these transformations are summarized in Scheme 8.8. 

Transformation of amines to aldehydes or ketones can be carried out via the anodic a-methoxylation of the corresponding carbamates, (4), followed by an acid-catalyzed conversion of the a-methoxylated... 

N-dealkylation results from an alkyl substitution on an aromatic molecule, which is one of the first places where microorganisms initiate catabolic transformation of atrazine, a xenobiotic molecule (Fig. 15.2). It is a typical example of a reaction leading to transformation of pesticides like phenyl ureas, acylanihdes, carbamates, s-tri-azines, and dinitranilines. The enzyme mediating the reaction is a mixed-function oxidase, requiring a reduced nicotinamide nucleotide as an H donor. 

---