Halogen effects in Robinson–Gabriel type reaction of cyclopropanecarboxylic acid N′-substituted-hydrazides with PPh3/CX4

We found that the reaction of cyclopropanecarboxylic acid N′-substituted-hydrazides with PPh3/CCl4 proceeded smoothly to give the corresponding normal Robinson–Gabriel type product 2-cyclopropyl-5-substituted-[1,3,4]-oxadiazoles in good yields. Using CBr4 or CI4 instead of CCl4 in the above system, the ring opening of cyclopropane occurred after dehydration to give the corresponding 2-(3-halopropyl)-5-substituted-[1,3,4]-oxadiazoles (4 or 5) in good yields.