Synthesis of enantiomerically pure [14 C]-labelled morpholine derivatives for a class of trace amine-associate receptor 1 agonists.

Various agonists of the trace amine-associate receptor 1, under consideration as potential clinical development candidates, were labelled with carbon-14 for use in preclinical in vitro and in vivo drug metabolism studies. Herein, the [14 C]-radiosynthesis of 2-phenyl-substituted morpholines 1 is described. After evaluating and optimizing different synthetic routes, 4-iodonitrobenzene 3 was selected as starting material for the 14-step synthesis. Incorporation of carbon-14 into the acetyl moiety allowed a safe and efficient synthesis of [14 C]-labelled 4-nitroacetophenone 2 in five steps and 38% yield. Further transformation of 2 to the target compounds 1 was achieved in a 9-step synthesis. In a representative example, [14 C]-labelled 1 was obtained in an overall yield of 11% and was isolated in >99% radiochemical purity and a specific activity of 47 mCi/mmol.