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13.6.1.1.3.2.3 Variation 3: From an Amidoxime and an Anhydride

DOI: 10.1055/sos-SD-013-00175

Hemming, K.Science of Synthesis, (200413141.

The use of carboxylic acid anhydrides to provide the single carbon for reaction with the amidoxime NCNO unit is very well established,[‌1‌‌6‌,‌26‌,‌27‌,‌81‌,‌99‌‌104‌] and the method is noteworthy for allowing access to the parent unsubstituted 1,2,4-oxadiazole.[‌13‌,‌99‌] The reaction proceeds via an intermediate O-acyl amidoxime 29, which cyclizes upon heating in a suitable solvent (Table 3). The mechanism of the cyclization step has been studied,[‌105‌] but the intermediate is not normally isolated. A wide range of 1,2,4-oxadiazoles can be prepared using this chemistry, including dipeptidomimetic oxadiazoles.[‌26‌] A selection of 1,2,4-oxadiazoles that are available by this route is shown in Table 3. A solid-phase synthesis of 1,2,4-oxadiazoles by the reaction of an anhydride with a polymer-supported amidoxime has been developed (see Section 13.6.5.2).

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