P-Nitrophenyl groups

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

Completely stereospecific and regiospecific ring-opening reactions were observed with the just mentioned nucleophiles when the 3-aryl substituent is a phenyl or a p-nitrophenyl group (Scheme 20) [30]. 

The role of the serine residue in hydrolysis was further examined using pseudo-substrates, e.g. p-nitrophenylacetate—substrates which were only very slowly utilized by the enzyme. The p-nitrophenyl group was slowly released and the acyl group became attached to the same serine in hydrolases which had been detected by DIPF (Kilby and Youatt, 1954). Mechanisms for peptide and ester hydrolysis were therefore proposed in which the acyl group became transiently and covalently bound to serines in catalytic sites (see Hartley et al. 1969). 

X-Ray diffraction shows that the p- nitrophenyl group is equatorial and the morpholinocar-bonyl moiety axial in the isochroman-l-one cis-3,4-dihydro-4-morpholinocarbonyl-3-p-nitrophenyl-l//- 2-benzopyran-l-one (175) <78JCS(P1)1351). The pyranone ring has a distorted twist conformation. 

The basic structural unit needed for activity is R-pdT-R. R may be a free 3 -OH but a phosphate here markedly increases substrate binding. R may be a p-nitrophenyl group. The bond that is broken is the P-0 of the 5 -C—O-P ester linkage (60). 

The p-nitrophcnyl ester prepared in part (e) is an active ester. The p-nitrophenyl group is a good leaving group and can be displaced by the amino nitrogen of valine ethyl ester to form a new peptide bond. 

A mechanism that accounts for the influence of different substitution patterns (meso, dl) and explains the influence of a p-nitrophenyl group making a nucleophile more reactive (although not as the imine) includes the cyclization of a hemiacetal which is followed by different dehydration steps ... 

Additional examples of preparation of pyrroles from oxazolium oxides were reported. One example is the synthesis of the calcium channel activator FPL64176. The key feature of this synthesis was the prior rather than in situ generation of the mesoionic intermediate and the use of a p-nitrophenyl group for A -protection. <95JOC4947>... 

This antiparallel double helix has a central cavity which is filled by the polyiodide chain or, in other terms, the polyiodide chain serves as a matrix around which the p-nitrophenyl-a-maltohexaoside molecules are arranged. The complex is stabilized by the inter- and intramolecular hydrogen bonds, the coordination of Ba2+ to hydroxyl groups and 0(5) atoms, and by the stacking interactions between p-nitrophenyl groups [583, 583 a]. 

Spectrophotometric methods are, of course, not restricted to assays based on the NAD(P)+/NAD(P)H pair of coenzymes, and there are many natural and synthetic substrates whose reactions can be assayed in this way. The p-nitrophenyl group forms the basis of many convenient spectroscopic assays, for example for the enzyme glucuronidase ... 

For p-nitrobenzyl cyanide (pK° 13.4) [117] kinetic results were obtained [19] which are similar to the slow proton transfers described for nitroparaffins and ketones. In aqueous solution rate coefficients for amine catalysed detritiation give a Bronsted plot with slope 13 = 0.61 and the rates of the thermodynamically favourable recombination of the car-banion with ammonium ions vary between ca. 103 and 10s 1 mole-1 sec-1. In 80/20 (v/v) ethei ethanol at —77°, a value j3 = 0.49 was observed for catalysis by phenolate ions [12]. The p-nitrophenyl group in this cyanocarbon acid considerably alters the proton transfer behaviour. 

D. Horton and A. E. Luetzow. Migration of the O-p-nitrophenyl group. Mechanism whereby p-nitrophenyl ct-D-glycosides liberate p-nitrophenoxide in alkaline solution, J. C. S. Chem. Commun., (1971) 79-81. 

The enzyme SC was modified with methoxy PEG (molecular weight 5,000) activated with p-nitrophenyl carbonate (Sigma, M-3903). The enzyme (240 mg) was dissolved in 100 mM potassium phosphate buffer pH 7.8 and stirred with 480 mg ( 10 x molar equivalent) PEG for a period of 2 h. The reaction was monitored by release of the p-nitrophenyl group (absorbance at 405 nm). The modified enzyme was purified from the solution by washing with phosphate buffer in an ultrafiltration cell (Amicon, Model 8050) using a 30,000-Da molecular weight cut-off membrane, over several hours. The modified enzyme was then lyophiUzed as described below. 

Halogens (particularly chlorine) can be replaced by other electron-attracting functions, such as trifluoromethyl or cyano groups. In the antibiotic chloramphenicol, both the chlorine atoms of the dichloroacetic moiety and of the p-nitrophenyl group yielded productive isosteric replacements (Table 13.3). Many other examples of univalent atom or group replacements are found in the chapters dealing with substituent effects (Chapter 17) and with quantitative structure-activity relationships (Chapter 19). 

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