Synthesis of isotopically labelled [14C]ZT-1, [d3]ZT-1 & (-)-[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, [14C]ZT-1 (Debio-9902) and [d3]ZT-1, were used in clinical studies to evaluate a potential treatment for Alzheimer’s disease. The pro-drug [14C]ZT-1 was isolated with a radiochemical purity of >98% and a gravimetric specific activity of 129mCi/mg in a seven-step synthesis starting from [U-14C]phenol in 7% yield.
Subsequently, the deuterium labelled target (
)-[d3]huperzine A was achieved in six steps with an overall yield of 15% and gave an isotopic distribution of d2 (1.65% huperzine A) and d3 (97.93% huperzine A) with a chemical purity of 98.5%. Condensation of the substrate (
)-[d3]huperzine A with 5-chloro-o-vanillin gave the Schiff base [d3]ZT-1 in a chemical yield of 80%. Reduction of the Schiff base gave reduced-[d3]ZT-1, which was converted into the hydrochloride salt with an isotopic distribution of d2 (1.60%) and d3 (98.02%).