Synthesis of Deuterium-Labeled Ouabain - an Internal Standard in Toxicological and Pharmacological Studies

The efficiency of isotopic exchange using a homogeneous base (Et3N) and heterogeneous catalysts is compared. Deuterated water was used as the deuterium source. The incorporation of deuterium into ouabain reached on average about 3.26 atoms but the yield was limited to 15 - 20% if Et3N was used. It was found that 5% Pd/BaSO4 was best for obtaining preparative amounts of labeled ouabain. A preparation with a yield of 77% and an average content of 2.14 deuterium atoms per molecule was obtained when the reaction was carried out at 125oC for 30 min. The use of heterogeneous catalysts simplified the purification of the final product since the number of by-products was noticeably lower than when Et3N was used.