RESUMEN
A color change from purple to green takes place on addition of tetrathiafulvalene (TTF) to the macrobicyclic receptor 14+ , which is composed of a cyclobis(paraquat-p-phenylene) tetracation that shares one of its paraphenylene rings with a 1,5-naphthoparaphenylene-[36]crown-10 macrocycle. The TTF molecule forces the macrobicycle to turn inside out (see schematic drawing below) and displaces the self-complexed 1,5-dioxynaphthalene ring system from the center of the tetracationic cyclophane.
RESUMEN
Reaction of three different 1-(dialkylamino) derivatives of 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid) with lead(II) nitrate in aqueous solution yielded polymeric complexes which have been characterized by single-crystal X-ray analysis. Pb[(CH(3))(2)NC(4)O(3)](2)(OH(2))(2).H(2)O (1) produced from 1-(dimethylamino)-2-methoxycyclobutenedione crystallizes in the triclinic space group P&onemacr;, with a = 8.591(5) Å,b = 9.308(4) Å, c = 10.571(4) Å, alpha = 95.58(2) degrees, beta = 94.96(3) degrees, gamma = 95.78(4) degrees, and Z = 2. The complex of formula Pb[(C(2)H(5))(2)NC(4)O(3)](2)(H(2)O) (2) formed from the corresponding 1-(diethylamino) analogue of the ligand also crystallizes in space group P&onemacr;, with a = 7.521(2) Å, b = 10.928(4) Å, c = 11.749(4) Å, alpha = 90.10(2) degrees, beta = 97.84(2) degrees, gamma = 95.12(2) degrees, and Z = 2. In both of these complexes, the lead atom is eight-coordinate with the ligands being bound in bidentate and binucleating fashions. The di-n-propylamino substituent in the di-n-propyl homologue hydrolyzed during crystallization, and a lead(II) squarate, Pb(C(4)O(4))(H(2)O)(4) (3), was produced: monoclinic space group P2(1), with a = 6.466(2) Å, b = 7.898(3) Å, c= 8.307(3) Å, beta = 92.88(3) degrees, and Z = 2. The lead atom is again eight-coordinate, and the squarate moiety, bidentate and binucleating. The respective coordination polyhedra for the complexes are square antiprismatic (1), bicapped trigonal prismatic (2), and distorted mono faced-capped and edge-capped trigonal prismatic (3).
RESUMEN
The macrocyclic "cholaphanes" 3a-c were synthesized from the inexpensive steroid cholic acid. Like earlier relatives they feature substantial cavities with inward-directed hydroxyl groups, suitable for binding polar molecules such as carbohydrates in nonpolar media. New features are the externally directed alkyl chains, promoting solubility in organic solvents, and (in the case of 3b/c) reduced conformational freedom resulting from truncation of the steroidal side-chain. In particular, modeling shows that the smallest macrocycle 3c possesses very little flexibility, preferring an open conformation which is also revealed in the X-ray crystal structure of its pentahydrate. NMR studies indicated that all three cholaphanes form 1:1 complexes with octyl beta-D-glucoside in CDCl(3), with K(a) = 600-1560 M(-)(1). Cholaphanes 3b/c proved able to extract methyl beta-D-glucoside from aqueous solutions into CHCl(3). The transport of methyl beta-D-glucoside across a chloroform barrier was also demonstrated for 3c.
RESUMEN
An efficient polycondensation-cyclization approach to the synthesis of cyclodextrin analogues is demonstrated by the preparation of cyclohexaoside 1 and cyclooctaoside 2. The key intermediate, disaccharide 3, bearing the cyanoethylidene group as a glycosyl donor function and the trityloxy group as a glycosyl acceptor function was prepared in 15 steps starting from L-rhamnose and D-mannose. The crucial cyclooligomerization of the disaccharide monomer 3 was carried out in the presence of TrClO4 as a promoter with the use of ultra-dry conditions at normal concentrations. This reaction led to formation of the cyclic oligosaccharides 28 and 29 (in 34 and 31% yield, respectively), which were deprotected to afford 1 and 2, respectively. The X-ray crystal structural analysis of the cyclooctaoside 2 reveals a cylindrical shape for the cyclic oligosaccharide with C4 symmetry. Individual molecules of 2 are arranged perfectly in stacks that form nanotubes in the solid state.