Total Synthesis of Archaeal 72-Membered Macrocyclic Tetraether Lipids

Total synthesis of archaeal 72-membered macrocyclic tetraether lipids 3a and 3b is reported. The synthesis was principally composed of preparation of the functionalized half-sized diether compounds 11 and 15 first followed by appropriate dimerization through Julia coupling and final macrocyclization of the crucial dialdehydes 23 and 31 by McMurry coupling. This strategy appeared to be advantageous for the stereoselective synthesis of both natural 72-membered tetraether lipids 3a and 3b using common synthetic intermediates. In addition, this approach was so designed that its synthetic flexibility would allow construction of unnatural structural variants for physicochemical studies. Also described are the results of differential scanning calorimetric analysis of the synthesized lipids 3a and 3b. Both 3a and 3b showed almost the same phase behavior with the broad endothermic phase transition at -53 degrees C. The enthalpy of the phase transition, DeltaH, was estimated to be 1.8 and 1.9 kcal/mol for 3a and 3b, respectively. The physicochemical as well as polymorphismic properties of 3a and 3b turned out to be indistinguishable despite of their regioisomeric structures. The physical structure of the phases in terms of the chemical structure is also discussed.