Oxazoles review

Ring transformations heterocyclic compounds reviews, 1, 70 Ring-chain tautomerism polyheteroatom six-membered rings, 3, 1056 Ripariochromene A synthesis, 3, 751, 755 Robinson-Gabriel synthesis oxazoles, 6, 216... 

Although there are comprehensive reviews and various specialized publications available covering the field of oxazole and specially thiazole chemistry, this does not apply to the selenazoles. This class of compounds has been treated in a few paragraphs and then only in a few works on organic chemistry. The reason for this is, principally, because in this field so far there has been less work and, correspondingly, a relatively small number of substituted selenazoles are known. Thus the parent compound, selenazole itself, is still unknown, all attempts to synthesize it having failed. 

Progress in Heterocyclic Chemistiy (PHC) Volume 9 reviews critically the heterocyclic literature published mainly in 1996. The first two chapters are review articles. Chapter 1 by C.J. Moody and K.J. Doyle deals with "The Synthesis of Oxazoles from Diazocarbonyl Compounds," and Chapter 2 by J.A. Sikorski provides a detailed account of the heterocyclic chemistry suiTounding the remarkable herbicide glyphosate ("Roundup" ). This latter chapter illustrates the role that heterocyclic chemistry plays in other areas of modem chemistry, since glyphosate is a far cry from being heterocyclic ... 

The aromaticity of isoxazole has been reviewed in terms of theoretical and structural studies,138 139 and the conclusion is that it is slightly less aromatic than oxazole and furan. 

Sulfur. Thiophene and benzo[ >] thiophene are both aromatic heterocycles, as discussed earlier in this review. Isothiazole is a planar molecule with an aromaticity comparable with those of thiazole and pyrazole, and higher than those of isoxazole and oxazole,122 140 as evaluated on the basis of Bird s aromaticity index A, based upon the statistical degree of uniformity of the bond orders of the ring periphery. Theoretical calculations and experimental data in connection with the aromaticity of isothiazole have been reviewed.141 Thiazole is also viewed as an aromatic molecule, similar to thiophene. It lacks an experimental aromaticity value, but the heat of formation together with bond lengths and angles have been calculated by various computational meth-... 

Brooks, D. A. Fisher Oxazole Synthesis in Name Reactions in Heterocyclic Chemistry, Li, J. J. Corey, E. J., Eds. Wiley Sons Hoboken, NJ, 2005, 234-236. (Review). 

Hydroxylamines may be acyclic (1) with one or two substituents or cyclic (4). Cyclic compounds 5 where both the nitrogen and the oxygen atoms are ring members like oxaziridines (6), 1,2-oxazetidines (7), tetrahydro-l,2-oxazoles (8) (usually named isoxa-zolidines) and tetrahydro-l,2-oxazines (9) will not be considered in this chapter. Each of these compounds is regarded as a compound class on their own and their chemistry has usually been reviewed individually. 

The Neber rearrangement has been used as a valuable synthetic tool to introduce an a-amino group relative to a ketone and it has been used as a key step in the synthesis of a large array of heterocycles, including imidazoles, oxazoles, isoquinolines and pyrazines and has been reviewed long ago °° . ... 

New methods for the synthesis of 2,4-disubstitued oxazoles are summarized in a recent review. 2-Alkyl-1,3-oxazoles bearing alkyl, aryl, or acyl substitution at C4 are common substructures in natural products. Examples include macrolides such as rhizoxin (4), hennoxazole A (5), and phorboxazole A (6), ° as well as many cyclic peptides that incorporate an oxazole subunit presumably derived from serine. ... 

It would require a Herculean effort to prepare a complete discussion and review of every report related to the synthesis, reaction, or application of an oxazole while tabulating every oxazole, oxazolone, oxazoline, and chiral bis(oxazoline) prepared and evaluated during the period of 1983-2001. Such an undertaking is beyond the scope of this review. Furthermore, the ease with which electronic databases, including the patent literature, can be searched, the data retrieved, and the information tabulated would render such a project somewhat redundant. 

Rather, the intent of the current project is to provide the reader with a discussion and leading examples of significant advances made in the synthesis, reactions, and applications of mononuclear oxazoles, oxazolones, oxazolines, and chiral bis(oxazolines) during this time frame. The material focuses on the more recent literature, although an update of the older synthetic literature is included wherever possible. In an effort to be selective, references to relevant reviews of material, not discussed in a chapter, are provided. Completely reduced oxazoles, that is, oxazolidines as well as benzo-fused derivatives, are outside the scope of this review. 

The kinetics and product distributions of ozonolysis of vinylcyclohexane and methylene cyclohexane have been investigated.162 Steric hindrance of the cyclic substituent largely offsets electronic effects hi determining the rate of reaction. The main products of ozonation of catechols were quinones, while catechol acetals gave rise to compounds with an opened benzene ring.163 The ozonolysis of azoles such as pyrroles, oxazoles, and imidazoles has been reviewed.164... 

Lewis JR (2001) Amaryllidaceae, Sceletium, imidazole, oxazole, thiazole, peptide and miscellaneous alkaloids. Nat Prod Rep 18, 95-128. Earlier reviews 2000, 17, 57-84 1999,16, 389-416. 

Sulfur Heterocyclics. Sulfur containing compounds (thiols, thiophenes, thiazoles,. .. etc.) play a major role in the flavor of raw and processed foods. These compounds have characteristic flavor notes and the flavor thresholds are mostly low. Several reviews (ill, 112, 113) demonstrate the important role of sulfur compounds in food flavors. Organoleptic properties of these compounds may be pleasant, strong nut-like odor of U-methyl-5-vinylthiazole which is present in cocoa (llU) objectionable pyridine-like odor of thiazole (115) quinoline-like odor of benzothia-zole (ll6) strong tomato leaf-like odor of isobutylthiazole (117) and bread crust flavor of acetyl-2-thiazoline (ll8). A mixture of oxazoles, thiazoles, thiazolines, imidazoles, trithiolanes and... 

The physicochemical properties of oxazoles to 1972 have been comprehensively reviewed by Lakhan and Ternai (3) whose work constitutes a point of departure for this section. Mention is made here only of those properties relevant to the detection, isolation, structure elucidation, and behavior of the natural compounds. The oxazole moiety in nature is usually embedded in a variety of functionality, and the rather innocuous properties of the parent molecule do not dominate or influence the behavior of the oxazole alkaloids to the extent that these compounds can be collectively regarded as displaying any characteristic set of physicochemical properties. Table III lists the physical and spectral properties of the compounds covered in this chapter. 

The UV and fluorescence characteristics of simple substituted oxazoles have been discussed in the early review, which also made mention of the utility of 2,5-diaryl derivatives as scintillators (3). Among the natural products, the 2,5-diaryl compounds halfordinol (16), halfordine (17), O-isopentenylhalfordinol (19), balsoxin (25), O-methylhalfordinoI (22), compound 24, texamine (26), and texa-line (27) reportedly display a high intensity (log e 3.61-4.63) band in the range 323-354 nm (Table III). In the 2-pyridyl-5-phenyl derivatives this band undergoes a bathochromic shift of 17-23 nm on acidification (Table III), which may be rationalized by the formation of the pyridinium salt (e.g., 204) for O-isopentenylhalfordinol (19). In 204 the 2-pyridinium substituent is obviously... 

Historically, it is interesting to note that this was the first example reported of a catalytic asymmetric aldol reaction.31 The diastereomeric ratio of oxazole isomers was ca. 9 1 with an enantiomeric excess of up to 97%. This reaction has had a significant impact on organic chemistry, and already several summarising reviews have appeared on the reaction and its application in organic synthesis.32,33... 

The mass spectrometry of oxazole derivatives has been reviewed <1980H(14)847>. The main fragmentation path for most oxazole rings is shown in Scheme 8 <19920MS317>. 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

Multinuclear H, 13C, 14N, 15N, 170 NMR spectroscopy is widely used for the structural determination of nitroazole derivatives. Some NMR data on the nitroa-zoles have been published in monographs [217-219], thesis [220], and reviews dedicated to five-membered heterocycles [221, 222], the derivatives of pyrazole [223-225], isoxazole [226], oxazole [227,228], thiazole [229], 1,2,4-triazole [230], 1,2,3-triazole [231, 232], indazole [233], and our reports on trimethylsilylazoles [234], NMR of nitroazoles [235], etc. [236-240],... 

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