Benzothiazolium N-phenacylide reacted with methylenecyclopropenes having an acyl group on the 4-position to give 3a,11a-dihydro-5aH-furo[3′,2′:2,3]pyrido[6,1-b]benzothiazole derivative via an intermediary 3-butadienylbenzothiazolium betaine. The reaction of the ylide with a methylenecyclopropene bearing two cyano groups on the 4-position in THF gave a cyclobutane together with benzothiazole, whereas