Preparative methods were developed for the synthesis of various compounds of the benzofurooxazulene series. It was established that the formation of an oxazulenium salt and a dihydrooxazulene derivative during the treatment of 3-(5-methyl-2-furyl)-2-(3-oxobutyl)benzofurans with perchloric acid results from disproportionation of the intermediate compound - an oxazulene derivative. ©1999 Kluwer Academic/Plenum Publishers.