Alanine and phenylalanine have been labelled in the 3-position, and 2-aminoadipic acid in the 6-position, with the short-lived positron-emitting radionuclide C-11 (t(1/2) = 20.3 min). (R)- and (S)-2-tert-butyl-1-tert-butyloxycarbonyl-3-methyl-4-imidazoli