Aldehydes 5-aryloxazoles

The tosyl compound reacts with aldehydes in the presence of potassium carbonate to yield 5-alkyl- or 5-aryl-oxazoles, the intermediate dihydrooxazoles (which can be isolated) eliminating toluene-p-sulfinic acid (Scheme 30). Use of acyl chlorides in place of aldehydes leads to 4-tosyloxazoles (288). Furthermore, alkylation of tosylmethyl isocyanide with an alkyl halide RfX, followed by treatment with an aldehyde R2CHO, yields a 4,5-disubstituted oxazole (289). A related reaction is that of A-tosylmethyl-iV -tritylcarbodiimide with aromatic aldehydes under phase-transfer catalysis to yield 2-tritylaminooxazoles which are readily converted into 2-amino-5-aryloxazoles (equation 117) (81JOC2069). 

Recently, a novel and efficient synthesis of oxazoles from tosylmethyl isocyanide and carbonyl compounds has been discovered by van Leusen et al.ai Thus, 5-aryloxazoles are prepared in 57-91% yield by refluxing tosylmethyl isocyanide (24) and aromatic aldehydes in methanol in the presence of potassium carbonate. 

Aryloxazole-4-aldehydes 59 (R = H, Me, Cl, Br or Ph) are formed in good yields in the reaction of 2-azidoacetophenones 58 with the Vilsmeier reagent dimethylformamide-phosphorus oxychloride <97TL6089>. A one-pot synthesis of the cyanooxazoleamines 60 is by the action of dicyclohexylcarbodiimide and pyridine on mixtures of aminonalononitrile tosylate and carboxylic acids <97S861>. 

TABLE 1.38. 4-AMINO-2.5-DIARYLOXAZOLES AND 2-ALKYL-4-AMINO-5-ARYLOXAZOLES FROM CONDENSATION OFAROYL CYANIDES, ALDEHYDES, AND AMMONIUM ACETATE"... 

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