Synthesis of iso-epoxy-amphidinolide N and des-epoxy-caribenolide I structures. Revised strategy and final stages

A general and highly convergent synthetic route to the macrocyclic core structures of the antitumour agents amphidinolide N ( 1) and caribenolide I ( 2) has been developed, and the total synthesis of iso-epoxy-amphidinolide N and des-epoxy- caribenolide I structures is described. Central to the revised strategy was the use of a Horner - Wadsworth - Emmons olefination between beta-ketophosphonate 51 and aldehyde 14 to construct the C1 - C13 sector common to both 1 and 2. Stereoselective alkylation of hydrazone 11 with iodide 65 and then with bromide 56 allowed for the rapid assembly of the complete caribenolide I carbon skeleton. Key steps in the completion of the synthesis of des-epoxy- caribenolide I structure 78 included hydrolysis of a sensitive methyl ester using Me3SnOH, followed by regioselective macrolactonisation of the resulting diol seco-acid and global deprotection. Coupling of hydrazone 11, bromide 56 and iodide 64 was followed by an analogous sequence of late-stage manoeuvres to arrive at the fully deprotected des-epoxy- amphidinolide N framework, obtained as a mixture of hemiacetal 83 and bicyclic acetal 84. Regio- and diastereo-selective epoxidation of the C6 methylene group in bicyclic acetal 84 provided access to iso-epoxy-amphidinolide N stereoisomer 89.