Synthetic studies toward the total synthesis of swinholide.: 1.: Stereoselective construction of the C19-C35 subunit

The development of an approach directed at the total synthesis of the complex cytotoxic marine macrodiolide swinholide is described. The present study focuses on the development of a synthetic route far the preparation of the C-19-C-35 segment of the structure, which is composed of a trisubstituted pyran moiety with a contrathermodynamic anti arrangement of the C-2 and C-6 pyran substituents (swinholide C-27 and C-31) which is joined by an ethano linker to an acyclic array containing five contiguous stereocenters. The pyran subunit was constructed using a stereoselective allylation of a beta-alkoxy aldehyde with 1,3-asymmetric induction and a second stereoselective allylation to prepare the C-glycosidic type of linkage. Use of the Hafner-Duthaler reagent was investigated as a potential means of constructing the anti vicinal hydroxyl-methyl relationships found in the C-19-C-24 Segment but was found not to be practical in this instance. The Evans bis propionate methodology was used to introduce a four-carbon unit, and a Mukaiyama aldol was used for chain extension to incorporate the remaining two carbons and two stereocenters of this segment. Attempted use of the Hanessian benzylidene acetal fragmentation reaction in this sequence was thwarted by neighboring group participation of an oxazolidinone in one case and an unexpected regiochemical outcome in another. The approach developed affords the C19-C35 substructure in 18 steps overall from ethyl acetoacetate and in adequate quantities (10% overall yield) to support the projected total synthesis.