Synthesis of New Allocolchicinoids with Seven‐ and Eight‐Membered B‐Rings by Enyne Ring‐Closing Metathesis

Total syntheses of allocolchicines 4 and 5, with the ester functionality in the C-ring at the C10 or C11 positions, is reported. An asymmetric synthesis of (7S)-allocolchicine 5 is also described. The main features included the elaboration of a common intermediate, the AB bicyclic ring system, in which the construction of the seven-membered ring was achieved by an enyne ring-closing metathesis (RCM) reaction. A subsequent Diels–Alder/aromatization sequence afforded the set of functionalized ring-C allocolchicinoids with high regioselectivity. This strategy was also applied for the synthesis of allocolchicine 6, containing an eight-membered B-ring.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)