Alternative synthesis for the preparation of 16α-[18F]fluoroestradiol

We have developed a new precursor, 3,17-O-bis(methoxymethyl)-16-O-p-nitrobenzenesulfonylestriol (14c) of 16-[F-18]fluoroestradiol ([F-18]FES). Although we could not selectively protect the C17 alcohol in the presence of the C16 alcohol, we were able to prepare and chromatographically isolate the desired C16 TBDMS, C17,C3-dimethoxymethyl (diMOM) protected estriol derivative and convert into the ultimate fluorination precursor. The MOM protective group proved to be more quickly removed than the cyclic sulfate group. The di-MOM protective precursor at the C3 and C17 alcohols instead of a cyclic sulfate group shortened hydrolysis time. We prepared three different sulfonate precursors at C16 alcohol. After checking their reactivity in the [F-18]fluorination step and considering the stability of the precursors, we obtained the best results with nosylate precursor 14c.