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

Abstract

AbstractLipophilic 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, ω‐phenyltetraene, or ω‐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 ω‐phenyltrienynes, and the subsequent triple‐bond partial reduction, producing the corresponding pentaenes or ω‐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 ω‐phenyltrienynes, a higher proportion of the all‐(E) isomer was obtained using an alternative method based on the reaction of an ω‐phenyldienylphosphonate with an α‐acetylenic aldehyde. Some of the resulting compounds exhibit spectral and photochemical properties that warrant their use as emitting lipophilic tags. Thus, the ω‐phenyltetraene and ω‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)