3-Hydroxycyclopent-1-ene-1-carboxylic acid (HOCPCA (1)) is a potent ligand for high-affinity γ-hydroxybutyric acid binding sites in the central nervous system. Various approaches to the introduction of a hydrogen label onto the HOCPCA skeleton are reported. The outcomes of the feasible C─H activation of olefin carbon (C-2) by iridium catalyst are compared with the reduction of the carbonyl group (C-3) by freshly prepared borodeuterides. The most efficient iridium catalysts proved to be Kerr bulky phosphine N-heterocyclic species providing outstanding deuterium enrichment (up to 91%) in a short period of time. The highest deuterium enrichment (>99%) was achieved through the reduction of ketone precursor 2 by lithium trimethoxyborodeuteride. Hence, analogical conditions were used for the tritiation experiment. [3 H]-HOCPCA selectively labeled on the position C-3 was synthetized with radiochemical purity >99%, an isolated yield of 637 mCi and specific activity = 28.9 Ci/mmol.

Center for Nuclear Technologies DTU Nutech Hevesy Laboratory Technical University of Denmark Frederiksborgvej 399 Building 202 Roskilde 4000 Denmark

Department of Drug Design and Pharmacology Faculty of Health and Medical Sciences University of Copenhagen 2100 Copenhagen Denmark

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences v v i Flemingovo nám 2 Prague 6 16610 Czech Republic

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences v v i Flemingovo nám 2 Prague 6 16610 Czech Republic Center for Nuclear Technologies DTU Nutech Hevesy Laboratory Technical University of Denmark Frederiksborgvej 399 Building 202 Roskilde 4000 Denmark

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