Regiospecific Synthesis of 4-Fluoro-δ4,3-keto Steroids Using Caesium Fluoroxysulfate.

The regiospecific synthesis of two A-ring fluorinated steroids is described. Treatment of a 4-trimethylstannyl Δ4,3-keto steroid with caesium fluoroxysulfate gave the corresponding fluorinated steroid in good yield; use of the N-fluorodiazabicyclic reagent, Selectfluor, as the fluorine source provides the same compounds in lower yield. The fluorination was found to be sufficiently rapid to be potentially useful in the synthesis of 18F labelled steroids for PETT applications.