Abstract
The product (III) was obtained by Diels-Alder reaction of 16, 17-dehydrodigitoxigenin 3-acetate (I) with ethyl acrylate on heating. The structure of III was determined by X-ray analysis, and III was found to be a head to head adduct of ethyl acrylate on the β-side of the 16, 20 (22)-diene moiety of I. The analogous reaction of I with dimethyl maleate gave two products (VI and VII) in similar yields. The structure of VI was also determined by X-ray analysis, and VII was considered to be the 24-epimer of VI, based on the spectral data and interconversion between VI and VII upon alumina treatment. In VI and VII, the double bond was found to be shifted towards the conjugated position from the original position in the diene synthetic product. Dimethyl fumarate gave a normal product (X) on addition to I on the β-side of the diene system. Upon alumina treatment, X was transformed into VI and VII, confirming the structures of VII and X. By analogy with the β-side addition of the above dienophiles, the configurations of 16-H and 22-H of the dimer (II) were both proposed to be α.
