Investigations of acetonitrile solvent cluster formation in supercritical carbon dioxide, and its impact on microscale syntheses of carbon-11-labeled radiotracers for PET

A new strategy has been developed for synthesizing positron emission tomography (PET) radiotracers using [11C]methyl iodide. This strategy relies on the ability of organic co-solvents to cluster within mixtures of supercritical fluids resulting in localized regions of high density which can serve as microscopic pockets for reaction. We've shown that acetonitrile will cluster about dilute solutes when mixtures of this co-solvent with carbon dioxide are forced to behave as a homogeneous fluid at the critical point. We applied this strategy in a systematic investigation of the conditions for optimized reaction between methyl iodide and L-α-methyl-N-2-propynyl phenethylamine (nordeprenyl) to yield L-deprenyl. Variables such as temperature, ultraviolet light exposure, co-solvent concentration, system pressure, and methyl iodide concentration were explored. The synthesis of radioactive [11C]-L-deprenyl using no-carrier-added concentrations of [11C]methyl iodide was also tested. Results showed that greater than 90% radiochemical yield of the desired product could be attained using 40 times less labeling substrate than in conventional PET tracer syntheses