For the electrophilic preparation of 6-[18F]fluoro-L-m-tyrosine ([18F]FMT), a PET tracer for measuring changes in dopaminergic function in movement disorders, a novel precursor, N-(tert-butoxycarbonyl)-3-(tert-butoxycarbonyloxy)-6-trimethylstannnyl-L-phenylalanine ethyl ester, was synthesized in four steps and 26% yield starting from L-m-tyrosine. [18F]FMT produced by two methods at two institutions was comparable in both radiochemical yield, 25-26%, and quality (chemical, enantiomeric, and radiochemical purity and specific activity) as that obtained with the original N-trifluoroacetyl-3-acetyl-6-trimethylstannyl-L-m-tyrosine ethyl ester [18F]FMT precursor.
Treatment of solutions of 1,3‐thiazole‐5(4H)‐thiones 1 in CH2Cl2 at room temperature with BF3⋅Et2O and 1,2‐epoxycyclohexane (7‐oxabicyclo[4.1.0]heptane; 2a) or 1,2‐epoxycyclopentane (6‐oxabicyclo[3.1.0]hexane; 2b) yielded mixtures of diastereoisomeric spirocyclic 1,3‐oxathiolanes (3/4 and 8/9, respectively). In addition, the corresponding 1,3‐dithiolane 6 was formed as a minor product in the reaction of 4,4‐dimethyl‐2‐phenyl‐1,3‐thiazole‐5(4H)‐thione (1a) with 2a. The structures of the different types of products have been established by X‐ray crystal‐structure analysis. An ionic two‐step mechanism via nucleophilic ring‐opening of the complexed oxirane by the attack of the thiocarbonyl S‐atom is proposed. This proposal is supported by the formation of the propellane 10 with a Wagner‐Meerwein rearrangement as the key step.
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