Cyclobutanone Approach to the Synthesis of Cardenolides

17β-(3-Oxocyclobutyl)androstane, prepared by the thermal [2 + 2] cycloaddition of dichloroketene to 3β-acetoxypregna-5,20-diene, is the key intermediate in the new, efficient synthesis of steroids bearing the 17β-butenolide fragment that characterizes cardenolides. The six-step synthesis of 3β-tert-butyldimethylsilyloxy-14α-carda-5,20(22)-dienolide was achieved with a total yield of 32%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)