Lourdes E. Echegoyen

The first evidence for aggregation behavior in a lipophilic [2.2.2]-cryptand and in 18-membered ring steroidal lariat ethers

2-n-Tetradecyl-[2.2.2]-cryptand aggregates in aqueous solution to give micelles either in the presence or absence of Li+, Na+, or K+ but N-(3-cholesteryloxycarbonylmethyl)-aza-18-crown-6 shows unusual behavior that is interpreted in terms of micelle formation in the absence of cations and vesicle formation in their presence.

Aggregation of steroidal lariat ethers: the first example of nonionic liposomes (niosomes) formed from neutral crown ether compounds

N-(3-Dihydrocholesteryloxycarbonylmethyl)-(1) and N-(3-cholesteryloxycarbonylmethyl)-1-aza-4,7,10,13-tetraoxa-cyclopentadecane (2) aggregate in water to from the first examples of niosomes based on amphiphiles having an uncomplexed crown ether residue as the head group.

Regioselective One-Step Synthesis and Topological Chirality of trans-3, trans-3,trans-3 and e,e,e [60]Fullerene-Cyclotriveratrylene Tris-adducts: Discussion on a Topological meso-Form

The C3-symmetrical [60]fullerene-cyclotriveratrylene (CTV) tris-adducts (±)-1 (with a trans-3,trans-3,trans-3 addition pattern) and (±)-2 (with an e,e,e addition pattern) were prepared in 11 and 9% yield, respectively, by the regio- and diastereoselective tether-directed Bingel reaction of C60 with the tris-malonate-appended CTV derivative (±)-3 (Scheme). This is the first example for tris-adduct formation by a one-step tether-directed Bingel addition. Interchromophoric interactions between the electron-rich CTV cap and the electron-attracting fullerene moiety have a profound effect on the electrochemical behavior of the C-sphere (Fig. 4 and Table 1). The fullerene-centered first reduction potentials in compounds (±)-1 and (±)-2 are by 100 mV more negative than those of their corresponding tris[bis(ethoxycarbonyl)methano][60]fullerene analogs that lack the CTV cap. A particular interest in (±)-1 and (±)-2 arises from the topological chirality of these molecules. A complete topology study is presented, leading to the conclusion that the four possible classical stereoisomers of the e,e,e regioisomer are topologically different, and, therefore, there exist four different topological stereoisomers (Fig. 6). Interestingly, in the case of the trans-3,trans-3,trans-3 tris-adduct, there are four classical stereoisomers but only two topological stereoisomers (Fig. 7). An example of a target molecule representing a topological meso-form is also presented (Fig. 8).

Evidence from e.s.r. studies for virtual immobility in niosomes derived from steroidal lariat ethers

Nitroxide radical probes exhibit correlation times which are 300 times longer in steroidal lariat ether-derived niosomes than are observed in egg lecithin vesicles and the membranes are so rigid that the spectra are almost identical to spectra observed for frozen systems.

Electrochemical Switching in Reducible Lariat Ethers: from Cation Binding Enhancements to Electrochemically-Mediated Transport

The evolution of lariat ethers from relatively simple, substituted crown ethers into electrochemically sensitive ligands is presented. Although nitrogen-pivot lariats were observed to be better binders than the corresponding parent crowns and to retain considerable flexibility after complexation, overall stability constants were not favorable for cation transport applications. This led to the syntheses of nitrobenzene- and anthraquinone-substituted systems capable of reversible redox behavior and drastically enhanced cation binding abilities when reduced. Application of these in enhanced cation transport processes has been demonstrated.

Thermodynamics of Micellization for the K+ and Ba2+ Complexes of a Lipophilic Cryptand

Temperature variation of the critical micelle concentration (cmc) has been used to obtain the thermodynamic parameters for micellization of 2-n-C14H29-[2.2.2]cryptand 1, in the presence of one equivalent of K+ or Ba2+. ΔHmic for the K+ complex is 5.5 kJ/mol and 12.6 kJ/mol for the Ba2+ complex. ΔSmic was 0.1 kJ/molK for both complexes, K+ and Ba2+. The process of micellization is entropy driven, but the major thermodynamic difference is of enthalpic origin. This result is consistent with increased charge repulsion on the micellar surface in the case of Ba2+ when compared to K+.