 The epoxy ring of the "β"-epoxide (1.19) is protonated to give an oxironium ion intermediate.In a concerted process, fluoride attacks the ring's "α"-face from below, as one of the two oxygen-carbon bonds is broken on the opposite face; hence regenerating the 11"α"-hydroxyl trans to the fluorine substituent.The resulting structure (1.20) is the androgenic steroid, fluoxymesterone.