A microwave-assisted acid and base co-catalyzed strategy shows high efficiency in the synthesis of 3-arylindoles through tandem nucleophilic ring-opening and Fischer indolization of aryloxiranecarbonitriles and arylhydrazine hydrochlorides.

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The formation of highly strained 6,7-dihydroazeto[2,1-b]oxazol-3-ium derivatives (4-oxa-1azabicyclo[3.2.0]hepta-1(5),2-dien-1-ium) as reactive intermediates was observed through the treatment of N-2aryl-2-oxoethylazetidin-2-ones with phosphorus oxychloride as a dehydrating agent, affording 2-vinyloxazoles as major products through elimination and 2-(2-chloroethyl)oxazoles as byproducts through nucleophilic substitution with the chloride anion. The current results provide important and useful information on the formation and existence of highly strained 6,7-dihydroazeto[2,1-b]oxazol-3-ium derivatives, which can be generated in situ and further undergo elimination and nucleophilic displacement immediately due to their high strain. 6,7-Dihydroazeto[2,1-b]oxazol-3-ium species are unstable and cannot be isolated from the reaction mixtures. They also provide an efficient method for the synthesis of 2-vinyloxazole derivatives.