2- pyridinium bromide, ring

Intramolecular nucleophilic displacement of the bromo group by an azine-nitrogen occurs in the cyclization of A-2-quinaldyl-2-bromo-pyridinium bromide (248) to give the naphthoimidazopyridinium ring system. The reaction of 2-bromopyridine and pyridine 1-oxide yields l-(2-pyridoxy)pyridinium bromide (249) which readily undergoes an intramolecular nucleophilic substitution in which departure of hydrogen as a proton presumably facilitates the formation of 250 by loss of the JV-oxypyridyl moiety. 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

Fused dihydrooxazolium compounds react readily by nucleophilic displacement of the oxygen from the azine ring. Thus, 2-carboxy-2,3-dihydrooxazolo[2,3-a]pyridinium bromide reacts with oxygen or nitrogen nucleophiles to form 2-oxo- or 2-imino-l(2ff)-pyridinelactic acids (204) (52JA4906). Similarly, the pyridinium salt (205) reacts by substitution in potassium hydroxide solution, whereas potassium t-butoxide causes proton abstraction and formation of the JV-vinyl derivative (206) (79JA3607). 

These enones reacted with substituted phenacyl pyridinium bromides and ammonium acetate under pyridine ring formation. Cleavage was performed with 25 % TFA/CH2CI2. Twelve compounds were synthesized, with yields of the crude products ranging from 50 % to 85% (HPLC-purities 76-97%). 

The benzene-fused analogue (189) has also been prepared in 14% yield by a Wittig reaction <72JA7087>. This could be oxidized to the sulfoxide and sulfone by H2O2 in acetic acid. Neither the sulfoxide nor the sulfone showed any tendency to dimerize by a Diels-Alder mode. This is in contrast to simple thiophene 5-oxides, and is probably because any such dimerization would result in the formation of a benzocyclobutadiene structure. Photolysis of the sulfone leads to a head-to-head dimer. The thiophene ring in (189) is very reactive. Bromination with pyridinium bromide... 

MSubstituted-2-pyridones can be used as substrates for ring closure to a number of polyheterocyclic systems, as an alternate to the Pschorr cycliza-tion Of to the cyclizations of the corresponding At-alkyl-2-halopyridinium halides. 2-Substituted pyridines (MI-625 X =C1, Br, OC Hj) and 3-(2-bromo-ethyl)indole give -[2-(3-indolyl)ethyl]-2-substituted pyridinium bromide... 

Indolizines, formation from pyridine acetates, 355-357 formation from pyridine side-chain carboxylic acids, 355-357, 481-488 5-methyl, synthesis, 352 N-(2- (3-Indolyl)ethyl]-2-halo (or ethoxy) pyridinium bromide, hydrolysis, 787 ring closure by fusion, 787... 

The formation of imidazopyridines is known to be a two-step process. It was shown earlier [46] that the first step in the reaction - the alkylation of 2-aminopyridine with bromomethyl (adamantan-l-yl) ketone was successfully carried out with a high yield of l-[3-adamantan-l-yl)-2-oxoethyl]-2-amino-pyridinium bromide 29a by boiling the reagents for 1 h in ethanol or ethyl acetate. They have established that the introduction of an electron-acceptor substituent into the pyridine ring, considera-bly hinders the reaction. When 2-aminopyridine, 2-amino-4-methylpyridine, or 2-amino-3-methylpyridine were heated with bromomethyl(adamantan-l-yl) ketone in ethyl acetate for 1 h, the yields of compounds 29a, 52a, 56a varied within the limits of 80-95% and, whereas with 2-amino-6-bromo and 2-amino-5-chloropyridine the corresponding alkylation products - compounds 49a, 57a were obtained in yields of only 50%. In the case of 2-amino-5-bromo-3-trifluoromethylpyridine, the formation of only a negligible amount of compound 58a was observed after 1 h and the yield reached 42% only after 5 h... 

Among other condensed thiazole ring systems, imidazo[2,l-6]thiazoles (tetramisole is a member of this class), thiazolo-pyrimidines, and thiazolo-pyridines continue to attract the most interest. Attention is concentrated on synthesis and biological activity. Unexpectedly, dihydrothiazolo[3,2-a]pyridinium bromide is readily brominated when the pyridine ring has a 3-nitro-substituent but not when it has a 3-amino-substituent. 

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