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Synthesis of [11C]ureas by [11C]CO2-fixation. (A) Synthesis of parent [11C]urea using LHMDS and aqueous ammonium chloride. (B) Synthesis of symmetrical and unsymmetrical substituted [11C]ureas by activation of an ammonium carbamate intermediate with POCl3. The utility of this reaction was greatly expanded by judicious control of reaction conditions to favour unsymmetrical products. TEA: triethylamine; TMS: trimethylsilyl; asterisk denotes 11C.
