Towards a total synthesis of ulapualide A. Concise synthetic routes to the tris-oxazole ring system and tris-oxazole macrolide core in ulapualides, kabiramides, halichondramides, mycalolides and halishigamides

A range of methods for the synthesis of mono-, bis- and tris-2,4-disubstituted oxazoles were evaluated, which led ultimately to a concise synthesis of the three contiguous oxazole ring system 26 in the ulapualide family of 25-membered macrolides, e.g.1, found in marine organisms. The tris-oxazole macrolide core 30 in ulapualide A (1) was also synthesised based on a macrolactamisation strategy from the two functionalised mono-oxazole precursors 28 and 29, followed by oxazoline 45 and oxazole ring formation, exploiting the methodologies established in the synthesis of linear bis- and tris-oxazoles in the formation of 18 and 26. The tris-oxazole 26 was converted into the corresponding phosphonium salt 5 in readiness for elaboration to ulapualide A (1).