To a stirred suspension of 10 g of ethyl 3,3-bis(4-fluorophenyl)-2-(1H-tetrazol-5-yl)-2-propenoate in 250 mL dry diethyl ether cooled at -50° C. was slowly added 60 mL of liquid isobutylene (previously condensed from gaseous material in a dry ice-alcohol bath). With continued stirring and cooling, 50 mL of concentrated H2SO4 was added slowly and carefully. The mixture was then sealed in a stainless steel Parr container and stirred at -30° C. for 40 hours. After releasing the pressure, the mixture was added slowly and carefully to excess saturated NaHCO3 solution. The aqueous mixture was extracted with diethyl ether, dried over Na2SO4 and concentrated in vacuo to give 7.8 g (67.8%) of the title compound, m.p. 143°-144° C.
