C14-Labeled Cardiac Glycosides
An essential structural and pharmacological characteristic of cardenolides is an a, ß-unsaturated γ-lactone ring (the butenolide ring) in the 17 β-position. Ruzicka, Reichstein, and Fürst  and others [2–5] first demonstrated the possibility of its synthesis from steroids other than cardenolides by means of the Reformatsky reaction [6,7]. By reaction of various 21-acyloxypregnanone-(20) derivatives with bromacetic acid ethyl ester and zinc, they obtained the hydroxylactone after acid cleavage of the organozinc compound and the corresponding ∆ 20,22-steroid lactone after subsequent dehydration. These studies were performed only with 14-deoxy derivatives because of the relatively strong nature of the reactions, but in 1947 Plattner and coworkers  achieved partial synthesis of a 14 β-OH compound for the first time. However, the yields were at most 20%, as was also the case for the partial synthesis of digitoxigenin-3-acetate-23-C14 later described by Turba and Scholtissek , probably because the lability of the 14 β -OH compounds is higher than usual for the Reformatsky reaction.
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