Fluoro norcholesterol analogues. Synthesis of 6 beta-(2'-fluoro) ethyl-19-norcholest-5(10)-en-3 beta-ol.

A synthetic route for labelling 6 beta-(2'-fluoro)ethyl-19-norcholest-5(10)-en-3 beta-ol (VII) with fluorine-18 was developed to evaluate the potential utility of VII as an adrenal imaging agent. 6 beta-p-Toluene-sulphonyloxymethyl-19-norcholest-5(10)-en-3 beta-ol acetate (I) was converted to the 6 beta-(2'-hydroxy)ethyl (VI) in a five-step process. The treatment of VI with N-N-diethyl (2-chloro-1,1,2,-trifluoroethyl)amine in methylene chloride and subsequent hydrolysis with base gave the required VII. Iodination of VI with triphenoxymethylphosphonium iodide followed by alkaline hydrolysis gave the 6 beta-(2'-iodo)ethyl (IX) which, on treatment with silver fluoride in acetonitrile, furnished the 6 beta-(2'-fluoro)ethyl (VII). Excellent conversion of the 6-beta-(2'-p-toluenesulphonyloxy)-ethyl (X) into VII was also achieved by heating with potassium fluoride for 2 hr in diethylene glycol in 76% yield, which is a readily applicable method for labelling with fluorine-18.