Carbamoylation effect

The preparation of the selenazolyl-pyrazolones was then effected by a second method, in which first the pyrazolone ring and afterward the selenazole ring was formed. For this purpose -ketoester seleno-semicarbazones were first converted to the corresponding 1-seleno-carbamoyl-3-aIkylpyrazol-5-one. These, by condensation with a-halo-genocarbonyl compounds according to the Hantzsch synthesis, formed the selenazole ring as a second step (17). 

Angelici and Brink (40) have found that in the reactions of amine with trans-M(CO),(PPhMe2)2+ (M = Mn or Re), the rate of carbamoyl formation follows the order, n-butylamine > cyclohexyl-amine >, isopropylamine > sec-butylamine >> tert-butylamine, implying a strong steric effect in carbamoyl formation. A similar order has been observed in the rate of reaction of organic esters with amines to form amides (41). The data in Table III indicate that a steric effect may be operative in the Ru (CO) /NR3-catalyzed WGSR, since with tertiary amines the rate follows the order, NMeQ > MeNC.H > NEt > NBu0, which does not reflect the basicity of these amines. 

Guan X, Davis MR, Tang C, et al. Identification of S-(n-butylcarbamoyl)glutathione, a reactive carbamoylating metabolite of tolbutamide in the rat, and evaluation of its inhibitory effects on glutathione reductase in vitro. Chem Res Toxicol 1999 12(12) 1138-1143. 

Symmetrical and unsymmetrical carbonates have also been obtained from the reaction of chloroformates with alcohols under soliddiquid conditions [55], and the reaction of carbamoyl fluorides with alcohols produces alkyl carbamates [58]. r-Butyloxycarbonylation of alcohols and phenols is effected by trans-esterification of di-r-butyl carbonate under basic phase-transfer catalysed conditions [59]. Yields tend to be high for the reaction with the phenols (>85%), but only moderate with the alcohols (30-81%). 

FIGURE 1.6 Effect of organic modifier (methanol) percentage in the elnent on the retention factors (a) and observed enantioselectivities (b) of Af-(2,4-dinitrophenyl)-a-(2-chlorobenzyl)-proline employing an 0-9-[(2,6-diisopropylphenyl)carbamoyl]quinine-based CSP. Experimental conditions Elnent, ammonium acetate buffer-methanol (total ionic strength = 25 mM pHj, = 6.5), methanol content varied between 60 and 90%, while ionic strength and apparent pH were kept constant temperature, 40 C flow rate, 0.8 mLmin . (Reproduced from A. Peter et al., J. Sep. ScL, 26 1125 (2003). With permission.)... 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

The concentration of ammonia in the liver is not saturating for carbamoyl phosphate synthetase, so that the greater the flux of ammonia into or within the liver, the higher the concentration of ammonia and the higher the activity of the synthetase. The effect of ammonia concentration is, therefore, a mass-action effect. 

Arginine stimulates carbamoyl-phosphate synthetase (an allosteric effect). 

Carbamoyl phosphate synthetase formation in liver taken from tadpoles treated with thyroxine is enhanced by the addition of orotic acid, uracil or uridine (cytosine and adenosine had no effect). The synthesis of this enzyme is not affected by these pyrimidines in untreated animals. This indicates that there is a relative pyrimidine deficiency during thyroxine-induced metamorphosis [140]. 

The carbamoyl group is transferred to the serine hydroxyl in the enzyme, but the resultant carbamoyl-enzyme intermediate then hydrolyses only very slowly (minutes rather than microseconds), effectively blocking the active site for most of the time. The slower rate of hydrolysis of the serine carbamate ester is a consequence of decreased carbonyl character resulting from resonance stabilization, as shown. 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

The generation of a nitrile oxide bearing a carbamoyl group (i.e., 16) was effected by treating 4-nitro-3-isoxazoline-5-one (15) with a mixture of acetonitrile and water (Scheme 6.5). Although the mechanism of this reaction is not clear, the method allows for the formation of a functionalized nitrile oxide (16) and subsequent cycloaddition under mild conditions (91). 

Several lines of investigation assert to the inability of canal ine to function as an effective ornithine antagonist. Ornithine interaction with canaline has been evaluated with the ornithine carbamoyl transferase (EC 2.1.3.3) of human liver. Neither canal ine nor ornithine inhibited this enzyme when the other member of this set served as the carbamoyl group recipient (29). The ornithine antagonist, 2,4-diaminobutyric acid drastically reduced urea production in the rat this reflected curtailment of the ornithine carbamoyl transferase-mediated conversion of ornithine to citrulline. Yet, canaline had no such effect on urea formation in this mammal (30). 

Canaline is a potent inhibitor of all seven pyridoxal phosphate-containing enzymes studied by Rahiala et (27) but it lacks adverse effects on three ornithine-utiTTzing enzymes lacking a Bg cofactor. Finally, in jack bean, Canavalia ensiformis, ornithine carbamoyl transferase can form 0-ureido-L-homoserine from canaline and carbamoyl phosphate as it does citrulline from ornithine and carbamoyl phosphate. Nevertheless, neither compound inhibited formation of the reaction products (31). 

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