Oxazolo[3,2-a]pyridinium and oxazolo[3,2-a]pyrimidinium salts in organic synthesis (original) (raw)
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Synthesis of 4-(4-pyridyl) oxazoles
Tetrahedron, 1994
Scheme 1 ;i:'ON&= Nu "-5" 2 3 loo0 RESULTS AND DISCUSSION Reaction of 5 with acetic anhydride at 100°C gave the dimer 6, but oxazole 7 was the exclusive product when the reaction was performed at reflux (140°C). However, when 5 bears a carbamate instead of an acylamine group, the only compound isolated was the dimer; its formation is independent of reaction temperature. This result could be due to the strong-1 effect of the alkoxy group. The structures of the dimeric products prepared are shown in Table 1. The dimerization could be explained through a mechanism that supposes the initial formation of the anhydrobase 8, which would be in equilibrium with the corresponding acylimine 9. The nucleophilic attack of 8 on 9 would give the Michael type adduct 10, which could lose a hydride ion, traped by electrophilic acetic anhydride of medium, to afford 6 (Scheme 2). The structure 6 was confirmed by unambiguous synthesis by methylation of 4 (R = OEt) to yield 6b. On the other hand, at higher temperatures, the anhydrobases 8 can be electrophilically attacked by the acetic anhydride to yield the cr-acetyl derivative 11, which undergoes a cyclization process usual in LX-acylamides4 to yield the corresponding oxazole 7 (Scheme 2).' This reaction is general, because when the propionic anhydride is used the corresponding 5-ethyloxazole 7c is obtained. Structures of the oxazoles which have been prepared are shown in Table 2. This hypothetic mechanism could explain the different behavior of pyridinium salts with temperature, because the dimerization products would be the kinetically controlled and the oxazoles the products of thermodynamic control.
Synthesis of 6-Nitroderivatives of Oxazolo[3,2-a]-pyridines and Their Reactions with Nucleophiles
Molecules, 2003
5-Nitro-2-pyridone can be selectively N-phenacylated, and the resulting phenacylpyridones I undergo cyclization to 6-nitrooxazolo[3,2-a]pyridinium salts II. These salts II readily react with ammonia and aliphatic amines leading to the products of pyridine ring opening -previously unknown 1-amino-2-nitro-4-(oxazole-2-yl)butadienes-1,3. Reaction of salts II with water lead to hydrolytic cleavage of the oxazole fragment.
New perspectives in oxazole chemistry
Tetrahedron, 1999
Addition -ring opening of the nitrooxazole 2a with diverse amino nucleophiles afforded directly the polfinctionalized nitroenamines 7a-g in good to excellent yields. At the same time, highly diastereoselective cascade reactions, easily performed both on 2a-c with the ynamine 8 and 2b with the en01 ether 13, led to the novel poly~yclic systems 12a-c and 15, respectively. 0
Synthetic Communications, 2019
The present study describes recent advances in the chemistry of heterocycles incorporated oxazolo[4,5-b]pyridine and oxazolo[5,4-b]pyridine skeletons. The main sections included the synthesis of the investigated compounds from readily accessible aminopyridinol derivatives or aminopyridines. The reactivity of substituents attached to ring carbon or nitrogen atoms were discussed. In addition, the synthetic and biological evaluation of the inspected oxazolopyridines were highlighted. The purpose of this review is to discuss the chemistry of the title so far. The present study will support researchers in the fields of organic and medicinal chemistry to design and develop new protocols for the construction of new biological components.
Organic letters, 2010
A variety of nucleophiles, thiolates, alkoxides, amines, iodide, and cyanide, react with oxazino-, oxazolino-, and benzoxazin[3,2-b]indazoles under microwave conditions to yield a diverse set of 2-substituted 1H-indazolones. The synthetic utility of these indazoles is further demonstrated by ANRORC (addition of the nucleophile, ring-opening, and ring closure) reactions to yield isomeric pyrazoloindazolones by a process wherein iodide acts first as a nucleophile and subsequently as a leaving group.
Ring transformation and complex formation of 3-acetyl-4,5-dihydro-1,2,4-triazole oximes
Inorganica Chimica Acta, 2005
Oximic 1,2,4-triazole ligands 2a-e were prepared from the reaction of 3-acetyl-4,5-dihydro-1H-1,2,4-triazoles 1a-e with hydroxylamine hydrochloride at room temperature. At higher temperatures, the reaction afforded, however, the novel ring transformation product 4-amino-2-(4-chlorophenyl)-5-methyl-2H-1,2,3,6-oxatriazine 3. The reaction of the ligands 2a-e with nickel (II) and palladium (II) acetates in ethanol at room temperature yielded the respective square planar complexes 5a-e, 6a,e. X-ray structure determination of one of these complexes (5a) revealed that metallation led to unexpected ring transformation of the triazole ligand. It is probable that such ring transformation generated the imidazole-N-oxide intermediate 4a which coordinated to Ni(II) ion, and the 4N-donor set comprises both imidazole nitrogen and arylhydrazone nitrogen. The whole process is associated with loss of one hydrogen molecule and formation of one new p-bond. The new compounds were characterized by elemental analysis, IR, 1 H NMR, 13 C NMR and HRMS.
Molecules
A simple and efficient synthetic route to the novel 3a,4-dihydro-3H,7H- and 4H,7H-pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazole ring systems from 3-(prop-2-en-1-yloxy)- or 3-(prop-2-yn-1-yloxy)-1H-pyrazole-4-carbaldehyde oximes has been developed by employing the intramolecular nitrile oxide cycloaddition (INOC) reaction as the key step. The configuration of intermediate aldoximes was unambiguously determined using NOESY experimental data and comparison of the magnitudes of 1JCH coupling constants of the iminyl moiety, which were greater by approximately 13 Hz for the predominant syn isomer. The structures of the obtained heterocyclic products were confirmed by detailed 1H, 13C and 15N NMR spectroscopic experiments and HRMS measurements.
Synthesis of (pyridinyl)-1,2,4-triazolo[4,3-a]pyridines
Journal of Heterocyclic Chemistry, 1986
Methods for the synthesis of (pyridinyl)1,2,4-triazolo[4,3-u]pyridines were developed. The principal route to the required intermediate 2-chloropyridines was based on rearrangements of mono N-oxides of 2,2'-bipyridine, 2,3'-bipyridine, 3,3'-bipyridine, 2,4'-bipyridine and 4,4'-bipyridine with phosphorus oxychloride. Reaction of 3,3'-bipyridine I-oxide or 2,2'-bipyridine 1-oxide with phosphorus oxychloride gave mixtures of chloro isomers. Reaction with acetic anhydride, 3,3'-bipyridine 1-oxide and 2,2'-bipyridine I-oxide gave exclusively [3,3'-bipyridine]-2(l m o n e and [2,2'-bipyridine]-6(1H)-one, respectively. 1,2,4-Triazolo[4,3-a]pyridines with pyridinyl groups at the 5,6,7 and 8 positions were synthesized.