The two new crown ethers presented in this study were synthesized in order to investigate two important features of ionophores, namely metal cation complexation and interfacial properties, and the way in which they interrelate. The two derivatives were conceived as analogs of membrane phospholipids with respect to their amphiphilicity and geometry. They contain a hydrophilic 1,1'-dioxo-3,3'-dithio-14-crown ether headgroup and bear two myristoyl or stearoyl lateral chains. The length of the myristoyl and stearoyl derivatives in an extended conformation is comparable with the thickness of the individual leaflets of cell membranes. The membrane-related and complexation properties of the two crown ether derivatives were studied in monomolecular films spread on pure water and on aqueous solutions of mono-, di-, and trivalent metal salts. The properties of the monolayers are described quantitatively using thermodynamic models. The compression isotherms of the monolayers formed on different subphases show a clear-cut differentiation of the monovalent and di- or trivalent cations with both ligands. This differentiation was interpreted in terms of conformational changes occurring in the crown ether derivatives upon complexation. Molecular modeling indicates that the mono- and divalent cations are coordinated differently by the ligands, yielding complexes with different conformations. The differences of the conformations of the mono- and di- or trivalent cation complexes may be important from the point of view of the interactions with lipid membranes and the biological activity of these potential ionophores.
A one‐pot, two‐step synthesis of chiral 7‐oxa‐1‐azabicyclo[2.2.1]heptane derivatives from unprotected D‐xylose was achieved by an intramolecular 1,3‐dipolar cycloaddition of the N‐(γ‐alkenyl)‐substituted nitrone intermediate. The stereochemical course of the cycloaddition was altered by the addition of an achiral Lewis acid. The bicyclic byproducts were employed in a concise synthesis of two new enantiomeric piperidine‐type iminosugars.
A novel protocol for preparation of tegafur (a prodrug of 5-fluorouracil) is reported. The process involves the 1,8-diazabicycloundec-7-ene-mediated alkylation of 5-fluorouracil with 2-acetoxytetrahydrofuran at 90 °C, followed by treatment of the prepurified mixture of the alkylation products with aqueous ethanol at 70 °C. The yield of the two-step process is 72%.
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