5-Alkoxy-4- oxazole

Finally, treatment of a-acidic isocyano esters (1) with base in the absence of other reagents is reported to furnish 5-alkoxy oxazoles (K) via ring closure [113, 114]. 

Electron-releasing substituents on the oxazoles increase the rate of reaction 5-alkoxy-oxazoles are comparable in reactivity to typical all-carbon dienes. 4-(Trimethylsilyloxy)-oxazoles react smoothly to produce furans at modest temperatures. ... 

The generation of a carbene (or when using a metal catalyst, a carbenoid) from an a-diazocarbonyl-componnd, in the presence of a nitrile, resnlts in overall cycloaddition and the formation of an oxazole. Both a-diazo-ketones and a-diazo-esters have been used, the examples in the seqnence below showing that the resnlt in the latter sitnation is the formation of a 5-alkoxy-oxazole. The exact sequence of events is not certain, bnt may involve a nitrile ylide, the result of electrophilic addition of the carbene to the nitrile nitrogen. 

Infrared absorption spectral data for several oxazole derivatives,86 90 including alkyl-98 186 and aryl-263 264 substituted oxazoles, 2-amino- and substituted-amino derivatives109 112 136, 5-amino179 198-201 and 5-alkoxy-oxazoles,66 carboxylic acids,112 147 esters,126 179 184 carboxamides,199 200 4-acetyloxazoles,147 halogenoalkyl oxazoles,91 oxazolines,262 and benzoxazoles262 have been reported. 

Oxazoles preferred in practice bear 5-ethoxy- or 5-cyanosubstituents. Other alkoxy (39—42), trimethylsilyloxy (43), alkythio (44), amino (45), and... 

FIGURE 1.18 2-AI koxy-5(4/7)-oxazol ones as intermediates in reactions of IV-alkoxycarbo-nylamino acids.22 After removal of the symmetrical anhydride from a reaction mixture containing Boc-valine and ethyl-(3-dimethylaminopropyl)-carbodiimide hydrochloride, the filtrate contained a novel activated form of Boc-valine (20% yield) that was established to be the 2-alkoxy-5(4H)-oxazolone. Slow addition of Boc-valine to ethyl-(3-dimethylamino-propyl)-carbodiimide hydrochloride in dilute solution gave a 55% yield. Petrol = petroleum ether, bp 40-60°. 

The thermally induced Cornforth rearrangement can be rationalized by postulating the dicarbonylnitrile ylid 103 as intermediate. It is especially the natures of the substituents R and R" that determine the direction of the rearrangement. For example, heating of 5-alkoxy-4-(aminocarbonyl)oxazole (102, R = OAlkyl, R" = NR2) readily gives in a good yield rearrangement into the isomeric 5-(substituted amino)-4-(alkoxycarbonyl) oxazole (104, R = OAlkyl, R" = NR2) (Scheme IV.40). 

In an attempt to investigate this theory further, the analogous alkoxy-substituted nitrile ylides 171 were investigated. Species of this type were previously unknown but were successfully generated for this work by the thermolysis of the 4-alkoxy substituted l,3-oxazol-5-ones 170 (89). In the absence of a dipolarophile, the nitrile ylides reacted via 1,5-electrocyclization to give the isoindoles 173 [see also... 

Substituted binaphthyl compounds can be synthesized in high optical yields using nucleophilic aromatic substitution reactions in which the chiral leaving groups are alkoxy moieties derived from naturally occurring alcohols28-29. For example, the condensation of 2-(l-alkoxynaphth-2-yl)-4.5-dihydro-4,4-dimethyl-l,3-oxazole with 1-naphthyllithium or 2-methoxy-l-naphthyl 2-magnesium bromide leads to (ft)- or (.S)-(l,T-binaphthyl-2-yl)-4,5-dihydro-4,4-dimethyl-l,3-oxazole derivatives. 

Dimethyl-3-(2-oxo-2-phenyl-ethyl--bromid lagert sich beim Kochen mit Triethylamin in Acetonitril in 2-(2-Imino-5-phenyl-2,3-dihydro-oxazol-3-yl)-4,6-dimethyl-pyrimidin um. Aus diesem werden durch Kochen in Alkoholen mit 4-Methyl-benzolsulfonsaure als Katalysator 2-Alkoxy-l-(4,6-dimethyl-2-pyrimidyl)-4-phenyl-imidazole oder durch Erhitzen mit primaren oder sekundaren Aminen ebenfalls in Gegenwart von 4-Methyl-benzolsulfonsaure 2-Alkylamino-l-(4,6-dimcthyl-2-pyrimidyl)-4-phe-nyl-imidazole erhalten374 ... 

Diels-Alder reactions of oxazoles afford useful syntheses of pyridines (Scheme 53) (74AHC( 17)99). A study of the effect of substituents on the Diels-Alder reactivity of oxazoles has indicated that rates decrease with the following substituents alkoxy > alkyl > acyl >> phenyl. The failure of 2- and 5-phenyl-substituted oxazoles to react with heterodienophiles is probably due to steric crowding. In certain cases, bicyclic adducts of type (359) have been isolated and even studied by an X-ray method (87BCJ432) they can also decompose to yield furans (Scheme 54). With benzyne, generated at 0°C from 1-aminobenzotriazole and lead tetraacetate under dilute conditions, oxazoles form cycloadducts (e.g. 360) in essentially quantitative yield (90JOC929). They can be handled at room temperature and are decomposed at elevated temperatures to isobenzofuran. 

Few reactions of the parent oxazole with the usual alkenic and alkynic dienophiles have been reported. Most oxazoles which yield Diels-Alder adducts contain electron-releasing substituents, the order of reactivity being alkoxy> alkyl 4-phenyl > acetyl > ethoxycarbonyl. This sequence suggests that the oxazole functions as the electron-rich component and that the reaction is governed by interaction of the highest occupied molecular orbital of the oxazole and the lowest unoccupied orbital of the dienophile. Cycloadditions with inverse electron demand of electron-deficient oxazoles with electron-rich dienophiles can be envisaged. 

The primary adducts (156) and (157) of oxazoles with alkenes and alkynes, respectively, are usually too unstable to be isolated. An exception is compound (158), obtained from 5-ethoxy-4-methyloxazole and 4,7-dihydro-l,3-dioxepin, which has been separated into its endo and exo components. If the dienophile is unsymmetrical the cycloaddition can take place in two senses. This is usually the case in the reactions of oxazoles with monosubstituted alkynes with alkenes on the other hand, regioselectivity is observed. Attempts to rationalize the orientation of the major adducts by the use of various MO indices, such as 7r-electron densities or localization energies and by Frontier MO theory (80KGS1255) have not been uniformly successful. A general rule for the reactions of alkyl- and alkoxy-substituted oxazoles is that in the adducts the more electronegative substituent R4 of the dienophile occupies the position shown in formula (156). The primary adducts undergo a spontaneous decomposition, whose outcome depends on the nature of the groups R and on whether alkenes or alkynes have been employed. 

Pathways C and D are less well documented, mainly because oxazoles lacking a 5-alkoxy substituent show reduced reactivity towards the usual dienophiles. An example of reaction C is the formation of 3-hydroxy-2-methylpyridine from 4-methyloxazole and acrylonitrile (equation 13). Dehydrogenation (path D) is rare but proceeds in the presence of a hydride acceptor. Thus 4-methyloxazole reacts with ethyl acrylate in the presence of hydrogen peroxide to give ethyl 3-hydroxy-2-methylpyridine-5-carboxylate in 27% yield (equation 14). 

The dye-sensitized photooxygenation of oxazoles with alkoxy substituents in positions 2, 4 or 5 gives 1,2,4-dioxazoles, e.g. (176). These novel heterocycles are believed to arise by addition of oxygen to the 4,5-double bond leading to peroxiranes, such as (175), as the primary intermediates (equation 20). 

Thermal or photochemical treatment of isoxazoies 851 has been found to result in a ring-contraction reaction to produce acyl 277-azirines 852, which sometimes rearrange to form other heterocycles like oxazoles 853. This ring-contraction reaction can also be promoted by iron(ii) catalysts. Thus, 5-alkoxy- and 5-aminoisoxazoles isomerize to 27/-azirine-2-carboxylic esters and 2/7-azirine-2-carboxamides, respectively, in nearly quantitative yield by reaction with catalytic FeCb (Scheme 212) <1997T10911>. 

Oxazoles can react at the C-4 position with aromatic aldehyde electrophiles under Friedel-Crafts conditions when the C-5 position is substituted with an alkoxy group. This feature has been exploited in a chiral Lewis acid-catalyzed formal [3-1-2] cycloaddition of aromatic aldehydes and 2-aryl-5-methoxyoxazoles 45 to generate enantiomerically enriched 2-oxazoline-4-carboxylates 46 (Scheme 4) <2001AGE1884>. These products can serve as masked /3-hydroxy a-amino acids, which are useful synthetic intermediates and have been found in peptide-based natural... 

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