Journal of Organometallic Chemistry 581 (1999) 170–175 The synthesis of a carborane gadolinium–DTPA complex for boron neutron capture therapy

Five-carboxylate DTPA derivatives (5 and 10) were synthesized via the palladium-catalyzed reaction of allyl ethyl carbonate 9 with diethylenetriamine-N-ethoxycarbonyl acetic-N,N %,Nƒ,Nƒ-tetraacetic acid hexaethyl ester 7. This method can be applied to the synthesis of a Gd–carborane complex. The reaction of the hexaethyl ester 7 with carboranyl allyl carbonate 13a proceeded very smoothly under Pd(dba)2:2dppe catalyst (10 mol%) in THF at 50°C, giving the C–C bond formation product 14 in 74 % yield. Hydrolysis of the ethyl esters in 14 was carried out with LiOH in aqueous methanol followed by treatment with diluted hydrochloric acid (1N) to afford the corresponding pentaacid 15 in 68 % yield. Treatment of the carborane containing DTPA derivative 15 with gadolinium(III) chloride hexahydrate gave the desired Gd–DTPA carborane complex 16 in quantitative yield