We have previously shown that macrotricyclic host 1a is a powerful receptor for glucopyranosyl units in the nonpolar medium of chloroform. However, the solubility properties of 1a did not permit studies of the extraction of carbohydrates from aqueous solution. This paper describes the synthesis of the new variant 1b, furnished with a highly lipophilic exterior array of 12 benzyloxy substituents. In homogeneous solution, 1b behaves much as 1a, binding n-octyl -D-glucoside with Ka ؍ 720 M ؊1 in CD3OH͞CDCl3 (8:92). In twophase experiments, the improved solubility of 1b allows carbohydrate extraction to be observed. Three hexoses (glucose, galactose, and mannose), two pentoses (ribose and xylose), and the two methyl glucosides are all extracted substantially into chloroform from 1 M aqueous solutions. Among the hexoses, 1b shows notable affinity and selectivity for glucose, extracting detectable amounts even from 0.1 M aqueous solutions. C arbohydrate recognition is an active area of supramolecular chemistry, motivated by the biological importance of saccharides and also by the unusual challenge represented by these complex substrates (1-13). On the one hand, carbohydrates must be recognized and processed during metabolism, whereas saccharide motifs are known to mediate cell-cell recognition, the infection of cells by pathogens, and many aspects of the immune response (14-17). On the other, carbohydrates possess structures that are fairly large, irregular, and multivalent, while being quite similar to clusters of water molecules. Even in nonpolar solvents, selective carbohydrate receptors require extended, sophisticated architectures (1, 3-6). In the presence of liquid water, the problem becomes yet more difficult. Recognition by means of boronate formation has been relatively successful (2, 7-10), but there are few effective systems for binding saccharides from water by using noncovalent bonds (1, 11-13).A receptor can be shown to discriminate between saccharide and water in one of two ways. Firstly (and most obviously), it can be studied in homogeneous aqueous solution. Secondly, it can be designed for solubility in nonpolar media and studied in a two-phase system, demonstrating the ability to extract or transport the carbohydrate (18)(19)(20)(21)(22)(23)(24). This latter approach is directly relevant to the transport of carbohydrates across biological membranes. Boron-based systems have again proved quite effective (22-24). However, for those receptors operating through noncovalent interactions, success has been limited thus far. Positive results have been obtained with the less hydrophilic saccharides at high aqueous concentrations (18, 20), but not in general with the more hydrophilic substrates such as glucose. § Recently we described the tricyclic tetraester 1a (Fig. 1), a novel receptor that showed an exceptional affinity for n-octyl -D-glucoside, remarkable  vs. ␣ selectivity, and the unusual ability to dissolve solid glucose in CHCl 3 (25). Although we hoped to demonstrate the extraction of glucose ...
Symmetrical and unsymmetrical archaeal tetraether glycolipid analogues have been prepared. The syntheses are based upon the elaboration of lipid cores from versatile chiral starting materials followed by simultaneous or sequential introduction of polar headgroups. Three pathways (A-C) were elaborated for the synthesis of stereochemically defined lipids 14-16 characterized by a straight bridging spacer and two dihydrocitronellyl chains attached to glycerol units at the sn-3 and sn-2 positions, respectively. Pathway C appeared to be particularly advantageous for the synthesis of tetraether 9, which possesses a cyclopentane unit as found in thermoacidophilic lipids. Diglycosylated lipids 4-6 were produced in 49-53% yields by reaction of diols 14-16 with beta-D-galactofuranosyl donor 31, whereas unsymmetrical lipids possessing either two different carbohydrate units 7 or a saccharidic moiety and a phosphate group 8 were efficiently prepared from monoprotected diol 35. These compounds represent the first examples of tetraether-type analogues containing a phosphate unit and/or glycosyl moieties.
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