Synthesis and Spectral Properties of Amphiphilic Lipids with Linear Conjugated Polyene and Phenylpolyene Fluorescent Groups

Abstract

Lipophilic fluorescent groups with a chain-like linear conformation emitting in the visible range and with high photostability are presently unavailable. These structures would be of great interest as labels for long-chain fatty acids and phospholipids lacking intrinsic fluorescent groups. With this aim in mind, we report the synthesis and the spectroscopic characterization of a series of emitting amphiphilic lipids that may approach that ideal fluorescent tag. Each lipid was constructed by attaching a linear, all-(E) conjugated pentaene, tetraenyne, -phenyltetraene, or -phenyltrienyne chromophore to a hydrophilic head-group through a polymethylene chain spacer. The key steps of the synthesis were the Pd0-mediated cross-coupling reaction between bromopolyenes and terminal acetylenic compounds, yielding tetraenynes or -phenyltrienynes, and the subsequent triple-bond partial reduction, producing the corresponding pentaenes or -phenyltetraenes in good overall yields. This method represents a further successful example of the so-called acetylenic approach to the indirect high-yield synthesis of polyene systems. In the case of -phenyltrienynes, a higher proportion of the all-(E) isomer was obtained using an alternative method based on the reaction of an -phenyldienylphosphonate with an -acetylenic aldehyde. Some of the resulting compounds exhibit spectral and photochemical properties that warrant their use as emitting lipophilic tags. Thus, the -phenyltetraene and -phenyltrienyne members of the series show intense absorption bands in the 320-370 nm range with fluorescence emission centered at 475 nm and quantum yields up to 0.25. These parameters are appropriate for the applications noted above.