Cardiotonic steroids with additional lactone rings

Ring A in A/B-trans steroids becomes fluxional and can convert from the chair to the boat conformation. This causes the 3-OH group and the 19-C functional group to approach each other. We used this feature to react the 19-COOH and the 3-OH to form a -lactone. The starting compound for this experiment was 8(14)-anhydrobovogenin-19-carboxylic acid (1), which was prepared via hydrolysis of bovoside-A-19-carboxylic acid. The reaction was carried out under rather forcing conditions (using HCl, see Experimental) because of the difficulty of hydrolyzing -L-tevetosides such as bovoside A and bovoside-A-19-carboxylic acid. This also eliminated the 14-OH group in the aglycon. The lactone ring closed to form bufadienolide 2 upon formation of compound 1 by the hydrolysis. Column chromatography produced pure 2, the properties of which were investigated. The IR spectrum of 2 showed clearly two lactone rings. The bufadienolide doubly unsaturated six-membered lactone ring was characterized by a strong