“…Prolonged reaction of pennogenin 3,17-diacetate (212) with boron trifluoride etherate is now found to give the diene (214) as well as the spiroketalorthoester (21 3). 134 Both of these products are probably formed via the common intermediate (215), which arises by neigh-bouring group participation of the 17a-acetoxyl group. The structures ( 213) and ( 214) are supported by detailed spectroscopic studies.…”