Synthesis of isotopically labelled [14C]ZT-1 (Debio-9902), [d3]ZT-1 and (-)-[d3]huperzine A, a new generation of acetylcholinesterase inhibitors

A method has been developed for the synthesis of two isotopically labelled forms of a pro-drug of the acetylcholinesterase inhibitor (-)-huperzine A. These labelled compounds, [C-14]ZT-1 (Debio-9902) and [d(3)]ZT-1, were used in clinical studies to evaluate a potential treatment for Alzheimer's disease. The pro-drug [C-14]ZT-1 was isolated with a radiochemical purity of >98% and a gravimetric specific activity of 129 mu Ci/mg in a seven-step synthesis starting from [U-C-14]phenol in 7% yield. Subsequently, the deuterium labelled target (-)-[d(3)] huperzine A was achieved in six steps with an overall yield of 15% and gave an isotopic distribution of d(2) (1.65% huperzine A) and d(3) (97.93% huperzine A) with a chemical purity of 98.5%. Condensation of the substrate (-)-[d(3)] huperzine A with 5-chloro-o-vanillin gave the Schiff base [d(3)]ZT-1 in a chemical yield of 80%. Reduction of the Schiff base gave reduced-[d(3)]ZT-1, which was converted into the hydrochloride salt with an isotopic distribution of d(2) (1.60%) and d(3) (98.02%).