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Oxazoles addition reactions

The Michael addition reaction of the serine-derived oxazolidine 326 with ethyl acrylate gave two products. The major product of the reaction was found to be the bicyclic compound 327, which was formed in 27% yield, accompanied by the unsaturated ester 328. The Dess-Martin oxidation of 327 resulted only in formation of the elimination product, the 7,7a-dihydro-177, 377-pyrrolo[l,2-r ]oxazole 328 (Scheme 46) <2001JOC7555>. [Pg.87]

The carbon atoms of azole rings can be attacked by nucleophilic (Section 3.4.1.6), electrophilic (Section 3.4.1.4) and free radical reagents (Section 3.4.1.9.2). Some system, for example the thiazole, imidazole and pyrazole nuclei, show a high degree of aromatic character and usually revert to type if the aromatic sextet is involved in a reaction. Others such as the isoxazole and oxazole nuclei are less aromatic, and hence more prone to addition reactions. [Pg.367]

The photochemical addition of both aliphatic and aromatic aldehydes to o-quinones monoimines has been widely used in the preparation of oxazoles [Eq. (92)].340 An intermediate amide has been isolated in a number of cases, and can be thermally converted into the oxazole. The reaction, therefore, does not appear to be a cycloaddition. An analogous addition occurs between o-quinones and aldehydes, and the photoproducts have been shown to have an acyclic structure341 rather than the previously assigned 1,3-dioxole structure. [Pg.86]

Although electrophilic substitution reactions are possible with oxazoles, they are frequently accompanied by addition reactions, as in furan. The bromination of 4-methyl-2-phenyloxazole with bromine or A/-bromosuccinimide yields 5-bromo-4-methyl-2-phenyloxazole, and that of 2-methyl-5-phenyloxazole gives 4-bromo-2-methyl-5-phenyloxazole. Mercury(II) acetate in acetic acid acetoxymercurates 4-sub-stituted oxazoles in the 5-position, 5-substituted oxazoles in the 4-position, and 4,5-disubstituted oxazoles in the 2-position. The acetoxymercury group can be substituted by electrophiles, e.g. ... [Pg.124]

Reactions of Oxazoles. The reaction of 4-phenyloxazole with di(acetoxy-methyl)acetylene gives the furan (531 R = CH2OAC) by extrusion of benzo-nitrile from the intermediate Diels-Alder adduct (530 R = CH20Ac). Similarly, 5-cyano-4-methyloxazole and hept-l-yne yield mainly the furan (532) and methyl cyanide a minor product, the 4-substituted isomer (533), arises from the alternative mode of addition of the acetylene. The sensitized photooxidation of 2,4-diphenyl-2-oxazolin-5-one (534) affords, inter alia, the coupled product (535). ... [Pg.192]

Alternatively, initial nucleophilic addition of 384 to 385 gave rise to 386, which underwent internal deprotonation to generate a nitrile ylide 387. Conversion of 387 to 390 could occur via either of two pathways. In the first case, the nitrile ylide cyclized to a 5-(dialkylamino)-2-substimted oxazole 388. Reaction of 388 with a second equivalent of 385 would then afford 390. The authors showed that there was no reaction of an analog of 388 with a second equivalent of 385, thereby discounting this proposal. These results led the authors to conclude that 387... [Pg.84]

The reader is directed to several excellent reviews for further details. Hassner and Fischer s general review of oxazoles covers both electrophilic aromatic substitution (EAS) reactions and addition reactions. Belen kii and Chuvylkin surveyed EAS reactions of oxazoles as part of a larger review for azoles. Larina and co-workers published two extensive reviews of nitration of azoles, including oxazoles. The articles cover kinetics and the mechanism of nitrations as well as the synthesis of nitroazoles via heterocyclization and ring transformations and direct methods of nitration. In light of these reviews, only a few selected examples of EAS reactions of oxazoles are described in this section. [Pg.128]

Quaternization of the nitrogen atom in a heterocyclic system can activate the ring towards addition reactions. An oxazoline moiety was converted to an aldehyde in 97% overall yield by alkylation of oxazoline with MeOTf, reduction with sodium boro-hydride, and hydrolysis of the resulting aminal (eq 10). Alternatively, treatment of an TV-methylated oxazolium triflate with a Grignard reagent followed by aqueous acid produced a ketone. Reaction of alkenyl oxazoles with MeOTf induced spontaneous intramolecular [4 -i- 2] cycloaddition at room temperature leading to a hydroindole or a hydroisoquinoline after reduction by sodium borohydride (eq 11). ... [Pg.403]

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... [Pg.375]

It is presumed [306-308] that addition across the C=C bond generates a zwit-terion, in which intramolecular proton transfer occurs through the five-membered intermediate to give nitrone. The latter enters the 1,3-dipolar addition reaction with the second molecule of dimethyl acetylenedicarboxylate affording oxazole derivative. [Pg.120]

In addition to the reactions described in the preceding section, alkyl groups in the 2-positions of imidazole, oxazole and thiazole rings show reactions which result from the easy loss of a proton from the carbon atom of the alkyl group which is adjacent to the ring (see Section 4.02.3.1.2). [Pg.88]

Williams and McClymont have observed that acylation reactions of the dianion of 2-(5-oxazolyl)-l,3-dithiane (15) lead to formation of 4,5-disubstituted oxazole products through a Comforth rearrangement pathway under base-induced, low-temperature conditions. For example, deprotonation of 15 with LiHMDS (3.0 equivalents) at -78°C, followed by addition of benzoyl chloride or p-chlorobenzoyl chloride and warming to 0°C, provided 16 in 74% and 47% yield, respectively. [Pg.227]

Ingham proposed the following sequence to explain the formation of oxazole products following his study of the reaction of benzaldehyde with mandelonitrile and hydrogen chloride. In the event, addition of hydrogen chloride to the cyanide is the first step providing the intermediate iminochloride 5 (Ari = Ph), which upon reaction with benzaldehyde affords oxazole 2 (Ari, Ar2 = Ph) via intermediate 6 (Ari, Ar2 = Ph). [Pg.234]

In 1972, van Leusen, Hoogenboom and Siderius introduced the utility of TosMIC for the synthesis of azoles (pyrroles, oxazoles, imidazoles, thiazoles, etc.) by delivering a C-N-C fragment to polarized double bonds. In addition to the synthesis of 5-phenyloxazole, they also described reaction of TosMIC with /7-nitro- and /7-chloro-benzaldehyde (3) to provide analogous oxazoles 4 in 91% and 57% yield, respectively. Reaction of TosMIC with acid chlorides, anhydrides, or esters leads to oxazoles in which the tosyl group is retained. For example, reaction of acetic anhydride and TosMIC furnish oxazole 5 in 73% yield. ... [Pg.254]

Van Leusen and co-workers also demonstrated the utility of dilithio-tosylmethyl isocyanide (dilithio-TosMIC) to extend the scope of the application. Dilithio-TosMIC is readily formed from TosMIC and two equivalents of n-butyllithium (BuLi) in THF at -70"C. Dilithio-TosMIC converts ethyl benzoate to oxazole 14 in 70% yield whereas TosMIC monoanion does not react. In addition, unsaturated, conjugated esters (15) react with dilithio-TosMIC exclusively through the ester carbonyl to provide oxazoles (16). On the other hand, use of the softer TosMIC-monoanion provides pyrroles through reaction of the carbon-carbon double bond in the Michael acceptor. [Pg.256]

Kondrat eva pyridine synthesis. This methodology to pyridine rings continues to be applied in total synthesis. An approach to the antitumor compound ellipticine 34 ° makes use of a Diels-Alder reaction of acrylonitrile and oxazole 32 to form pyridiyl derivative 33. Addition of methyllithium and hydrolysis transforms 33 into 34. [Pg.329]

More recently, Williams has described the one pot synthesis of 2-substituted oxazoles 11 by the thermolysis of triazole amides 9 the reaction does not proceed photo-chemically.<92TL1033> Although the reaction does not involve addition to a nitrile, it is an interesting application of a diazo compound since the proposed zwitterionic intermediate 10 is a resonance form of a diazo imine, so formally the reaction may be thought of as a thermal decomposition of a diazo imine (Scheme 6). [Pg.3]

The base-catalysed reaction of a-bromo-a,P-unsaturated ketones with aliphatic nitro compounds leads to 2-isoxazoline A-oxides by tandem conjugate addition-ring closure (Scheme 5) <95JOC6624>. A -Acyl-3-isoxazolin-5-ones are transformed into oxazoles by photolysis or by flash vacuum pyrolysis (Scheme 6) <96TL675>. [Pg.209]

Ru, Os, and Ir carbene complexes have been prepared from reactions of anionic or low-valent metal complexes with some organic salts or neutral compounds with highly ionic bonds. Oxidative addition of halothiazole and -oxazole species to IrCl(CO)(PMe2Ph)2 affords Ir(III) complexes which on protonation yield cationic carbenes (69), e.g.,... [Pg.141]


See other pages where Oxazoles addition reactions is mentioned: [Pg.171]    [Pg.217]    [Pg.661]    [Pg.225]    [Pg.155]    [Pg.299]    [Pg.545]    [Pg.106]    [Pg.171]    [Pg.55]    [Pg.661]    [Pg.232]    [Pg.167]    [Pg.128]    [Pg.274]    [Pg.62]    [Pg.104]    [Pg.344]    [Pg.71]    [Pg.1029]    [Pg.429]    [Pg.106]    [Pg.150]   
See also in sourсe #XX -- [ Pg.471 ]

See also in sourсe #XX -- [ Pg.471 ]

See also in sourсe #XX -- [ Pg.471 ]

See also in sourсe #XX -- [ Pg.471 ]

See also in sourсe #XX -- [ Pg.471 ]




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Oxazole reactions

Oxazoles reactions

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