Other heterocyclic nitrogen centres

The hydrolysis of acetyl- (150 R = MeCO) and benzoyl-imidazoles (150 R = PhCO) involves the reversible addition of water to the imidazole to give the T1. At low acidities the decomposition of this is the slow step but, as the acidity increases and water activity decreases, its formation becomes rate determining.128 By contrast, the hydrolysis reactions of the /V-acyl derivatives of 2,4,5-triphenylimidazole proceed in a concerted manner and do not involve tetrahedral intermediates.129 

Some interesting reactions of the super-electrophile 4,6-dinitrobenzofuroxan (151) with aryloxide ions have been reported 130 (151) with phenoxide ion gives the O-bonded cr-adduct (152) which has been fully characterized for the first time. On 

The acid-catalysed hydrolysis of the cyclic diazothiolactone, 4-diazoisochroman-3-one (159) to a ring-contracted product involves protonation of the diazo carbon in the slow step.133 A variable-temperature NMR study has been undertaken to study the reaction of the pyrazine (160) giving (161a R = OH, Me, H). Spectral evidence has been obtained supporting the involvement of the dihydroimidazopyrazine ring system (161b).134 Work by Edward and co-workers on the alkaline hydrolysis of strychnine and some of its derivatives was dealt with earlier.21 

The mechanism of 2-phenylbenzoxazole (162) formation from benzoic acid and o-aminophenol in polyphosphoric acid has been studied by NMR and chemical analysis.135 Initially benzoic acid reacts with (162) to form benzoic phosphoric anhydride and benzoic polyphosphoric anhydride. When o-aminophenol dissolves in 

5-Dioxopiperazines are products of sometimes unwanted cyclizations of the N-terminal residues of di- and oligo-peptides and proteins. As a model for this process. 

The general chemistry of acylpyridinium salts (181) and their role in the nucleophilic catalysis by pyridine of carbonyl substitution reactions have been reviewed and compared with the role of acylammonium salts (182).  

The rates of alkaline hydrolysis of a series of amino derivatives of 4-fluoro-(183 X = NH2) and 2-fluoro-pyrimidines (184 X = NH2) have been compared, revealing that the former hydrolyse more rapidly. Some difluoro analogues (184 X = F) with amino and alkyl substituents were also studied, the 4-fluoro substituent departing the more readily. 

Kinetic evidence has been obtained for ion-pair formation when the effects of inorganic salts on the alkaline hydrolysis of A(-phthaloylglycine (188) were investigated. Kinetic studies have been reported of acetyl transfer in acetonitrile from N-acetyloxypyridinium cations (189) to 4-(4 -A(,A(-dimethylaminostyryl)pyridine A(-oxide (190), pyridine A(-oxide (191) and 4-dimethylaminopyridine (192). In a follow-up 

Solvent effects have been investigated in isatin (193) hydrolysis. Results from ethanol-water and acetonitrile-water mixtures revealed that for alkaline hydrolysis log k was correlated with the reciprocal of the dielectric constant. A tetrahedral intermediate (194) is involved, which breaks down to yield the ring-opened amino acid (195). A comparison has been made of the lability of isatin (193) towards diethyl-amine and hydroxide ion, the latter showing the greater effect.  

Base-promoted hydrolysis kinetics for A-(4-nitrobenzoyl)pyrrole (98) have been measured as a function of buffer concentration at several pH values at 25 °C. In addition, carbonyl- exchange kinetics have been determined at a single pH value (9.48) as a function of l,4-diazobicyclo[2.2.2]octane (DABCO) concentration. The results are consistent with the buffer acting as a general base to catalyse both the attack of water to generate an anionic tetrahedral intermediate (To-) and the breakdown of To- to give hydrolysis products.  

The expected rate-enhancing effects of gem-dimethyl substituents were not seen in the base-catalysed cyclizations of ethyl A-(4-nitrophenyl)hydantoate (106 R, = Me, R = 4-O2NC6H4) and of ethyl A-phenylhydantoate (106 R R = Me, R° = Ph). Solvent kinetic isotope effects and other data led to the conclusion that the loss of the gem-dimethyl effects was due to steric hindrance to proton transfer of 

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