Novel Flexible and Rigid Tetraether Acyclic and Macrocyclic Bisphosphocholines:  Synthesis and Monolayer Properties

A related series of flexible and rigid C 16 - and C 20 -tetraether bolaamphiphiles containing two phosphocholine headgroups and either acyclic or macrocyclic side chains were synthesized starting from 2-phenyl-5-hydroxy-1,3-dioxane. Two different synthetic strategies were developed. The saturated acyclic and macrocyclic derivatives were prepared via olefin metathesis using a [(PCy 3 ) 2 Ru CHPh]Cl 2 catalyst in the key homocoupling step. Acyclic and macrocyclic tetrayne analogues were prepared using high-temperature Glaser oxidation in the dimerization step. Monolayer experiments of these materials suggest that the flexible lipids assume a U-shaped conformation; however, the rigid derivatives appear to adopt an upright conformation at the air−water interface in agreement with the predictions of Diamant and Andelman. We infer from these studies that bolaamphiphiles bearing macrocyclic transmembrane C20 hydrophobic chains are sufficiently rigid that they sacrifice contact of a headgroup with the aqueous subphase to accommodate an extended molecular orientation at the air−water interface.