Oxazoles 4-acyl-2-phenyl

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. 

Oxazol-4(5ff)-one, 5-acetyl-5-methyl-synthesis, 6, 225 Oxazol-4(5ff)-one, 2-phenyl-photorearrangement, 6, 200 synthesis, 6, 225 Oxazol-5(2ff)-one, 2-acyl-2,4-disubstituted pyrolysis, 6, 200 Oxazol-5(2H)-one, allyl-photochemical rearrangement, 6, 200 Oxazol-5(2ff)-one, 2-arylmethylene-synthesis, 6, 227... 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

Trifluoromethyl-l, 3-oxazoles are formed on heating 2-trifluoromethyl-2-acyl-2//-oxazol-5-ones (71CB1408) as well as 3-trifluoromethyl-3-alkox-ycarbonyl-2,2,2-trimethoxy-5-phenyl-2,3-dihydro-l,4,2-oxazaphospholes (89CZ243). Both reaction sequences include a thermally induced [3 + 2] cycloreversion reaction and a 1,5-electrocyclization of the conjugated 1,3-dipolar species initially formed (Scheme 93). 

Control in the selection of ring substituents is an important element of oxazole synthesis. This is a feature of a new route that employs 0-trimethylsilyl acyltrimethylsilane cyanohydrins (148), which are obtained from aldehydes or acyl silanes <92JOC333l>. These intermediates, which provide the C-5 substituent and four of the five ring atoms, are reacted sequentially with organolithium reagents (C-4 substituent) and acyl chlorides or anhydrides (C-2 substituent) to furnish, -bis(trimethylsilyl) enamines (149), which cyclize under thermal conditions or upon treatment with trimethylsilyl trilluoromethanesulfonate (Scheme 68). The range of oxazoles accessible by this method includes those with alkyl, alkenyl, phenyl, and functionalized substituents at C-2, alkyl, alkenyl, and phenyl substituents at C-4, and alkyl and phenyl substituents at C-5. The rare 4-(, -dialkylamino)oxazoles (150) may also be prepared. 

Die aus N-Acyl-a-aminosauren leicht zuganglichen 5-Oxo-2-phenyl-4,5-dihydro-1,3-oxazole (Azlactone) reagieren mit Oximen zu O-Acyl-oximen ... 

Haufig wird diese Methode in ihrer Anwendbarkeit durch schwer abzutrennende Neben-produkte eingeschrankt. Eine elegante Variante, bei der die Nebenprodukte leicht abzu-trennen sind, nutzt die gemischten Carbonsaure-Kohlensaure-Anhydride, die aus Chlor-ameisensaure-ethylester und der N-Acyl-a-aminosaure gut zuganglich sind. So wird das 5-Oxo-2-phenyl-4,5-dihydro-l, 3-oxazol (80%) nach dieser Methode aus Benzoylamino-es-sigsaure (Hippursaure) hergestellt69. Besonders empfohlen wird diese Methode, wenn die Azlactone nur in situ prapariert warden so lien. 

TABLE 1.53. iV-PHENYL-5-ACYL-A -l,2,4-TRIAZOLINE-l,2-DICARBOXIMIDES FROM SUBSTITUTED OXAZOLES AND PTAD... 

In this reaction, initially, acetophenone 36 was converted to a-iodo acetophenone 39 by iodine, which was further transformed into phenyl glyoxal 40 by DMSO (dimethyl sulfoxide). The formed aldehyde then reacted with 2-aminothiophenol 37 to provide intermediate 41, which was further converted to 42 through an intramolecular 1,2-addition. Then, 42 underwent iodine-catalyzed oxidative dehydrogenation to afford the desired product 2-acyl benzothiazole 38 (Scheme 9.9). Very recently, the same authors reported the synthesis of 2,5-disubstituted oxazoles using a similar domino process [17]. 

While the cycloaddition of oxazole with acetylene is a well-established method for furan synthesis [36], the corresponding reaction of 5-amino oxazole was unknown at the outset of this work. As shown in Eq. (3) of Scheme 15.19, the reaction of 24b with 3-phenyl-2-propynoyl chloridephenylpropioloyl chloride (55a) proceeded smoothly to provide the 5,6-dihydro-furo[2,3-c]pyrrol-4-one (56) in greater than 95% yield. A triple domino sequence involving acylation/intramolecular DA cycloaddition/retro DA could explain the reaction outcome. That the domino process was initiated by acylation was evidenced by the fact that 24b failed to react with dimethyl acetylenedicarboxylate (DMAD) under the identical conditions. 

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