Synthesis of precursors 13 and 15 and reference compound 1. SnCl2, tin(II) chloride; PFA, paraformaldehyde; MeI, methyl iodide; 4-DMAP, 4-dimethylaminopyridine; XtalFluor-E®, diethylaminodifluorosulfonium tetrafluoroborate; NEt3-3HF, triethylamine trihydrofluoride.