A CONVENIENT SYNTHESIS OF 4-HYDROXY[1-C-13]BENZOIC ACID AND RELATED RING-LABELED PHENOLIC-COMPOUNDS

4-Hydroxy[1-C-13]benzoic acid (5) was synthesized starting from commer cially available sodium [2-C-13]acetate (1) by a 4-step sequence in an overall yield of 55-65%. Key step in this synthesis is the formation of the benzene ring by base-catalyzed condensation of diethyl [2-C-13] malonate (3) with 4H-pyran-4-one. Acid 5 or its ethyl ester could be c onverted into 3,4-dihydroxy[1-C-13]benzoic acid (9), D,L-[1'-C-13]tyro sine (14) and 1,2,4-trihydroxy[4-C-13]benzene (19) with high efficienc y.