A high-yield route to synthesize the P-glycoprotein radioligand [11C]N-desmethyl-loperamide and its parent radioligand [11C]loperamide

N-Desmethyl-loperamide and loperamide were synthesized from α,α-diphenyl-γ-butyrolactone and 4-(4-chlorophenyl)-4-hydroxypiperidine in five and four steps with 8% and 16% overall yield, respectively. The amide precursor was synthesized from 4-bromo-2,2-diphenylbutyronitrile and 4-(4-chlorophenyl)-4-hydroxypiperidine in 2 steps with 21-57% overall yield. [11C]N-Desmethyl-loperamide and [11C]loperamide were prepared from their corresponding amide precursor and N-desmethyl-loperamide with [11C]CH3OTf through N-[11C]methylation and isolated by HPLC combined with solid-phase extraction (SPE) in 20-30% and 10-15% radiochemical yields, respectively, based on [11C]CO2 and decay corrected to end of bombardment (EOB), with 370-740 GBq/μmol specific activity at EOB.