Use of diazophosphonates in the synthesis of cyclic ethers. Part 2. Synthesis of the pyranooxepane and oxepanooxepane subunits of marine polyether toxins

Abstract

3-Oxooxepan-2-ylphosphonates 3, prepared by rhodium carbenoid cyclisation of the diazophosphonates 1, are readily elaborated into the keto alcohols 6, by Wadsworth–Emmons reaction, hydrogenation and epimerisation. Cyclisation of 6 using trimethylsilyl triflate–triethylsilane results in formation of 7–6- and 7–7-bicyclic ethers 10, with trans selectivity.