Glycocalix[4]arenes and their affinity to a library of galectins: the linker matters.

Galectins are lectins that bind ß-galactosides. They are involved in important extra- and intracellular biological processes such as apoptosis, and regulation of the immune system or the cell cycle. High-affinity ligands of galectins may introduce new therapeutic approaches or become new tools for biomedical research. One way of increasing the low affinity of ß-galactoside ligands to galectins is their multivalent presentation, e.g., using calixarenes. We report on the synthesis of glycocalix[4]arenes in cone, partial cone, 1,2-alternate, and 1,3-alternate conformations carrying a lactosyl ligand on three different linkers. The affinity of the prepared compounds to a library of human galectins was determined using competitive ELISA assay and biolayer interferometry. Structure-affinity relationships regarding the influence of the linker and the core structure were formulated. Substantial differences were found between various linker lengths and the position of the triazole unit. The formation of supramolecular clusters was detected by atomic force microscopy. The present work gives a systematic insight into prospective galectin ligands based on the calix[4]arene core.

Chemistry of 2,14-Dithiacalix[4]arene: Searching for the Missing Fifth Conformer.

Contrary to theoretical predictions, direct alkylation of 2,14-dithiacalix[4]arene provides only four out of five basic conformers (atropisomers). To prepare the missing 1,2 (C)-alternate conformer, the indirect alkylation strategy was applied using 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane as a protective agent. As proved by the combination of NMR and X-ray crystallography, the position of the disiloxane bridge on the macrocycle is not fixed and can be changed under basic conditions, representing thus so far unknown rearrangement of the siloxane moiety. The subsequent dialkylation/deprotection and dialkylation enabled the synthesis of the last missing conformer. As demonstrated by several examples, the mixed-bridge macrocycle (with both CH2 and S bridging units) enables preparation of unusual conformers or substitution patterns, which are difficult to obtain, if at all, in classical calixarene chemistry. This feature makes 2,14-dithiacalix[4]arene a very promising candidate for the role of molecular scaffold or platform in various supramolecular applications.

Chemistry of 2,14-Dithiacalix[4]arene: Alkylation and Conformational Behavior of Peralkylated Products.

2,14-Dithiacalix[4]arene, prepared on a multigram scale, was alkylated using the reaction conditions well known from the chemistry of classical calixarenes or thiacalixarenes to study the specific conformational preferences and dynamic behavior of the corresponding tetraalkylated derivatives. As proved by the combination of the X-ray crystallography and dynamic NMR techniques, the presence of mixed bridges (-CH2- and -S- groups) within the basic skeleton brings about considerable changes in the mutual ratios of the individual conformers compared to the parent macrocycles. Interestingly, certain conformers, hardly accessible for common calixarenes/thiacalixarenes (e.g., 1,2-alternates) are easily prepared in very good yields in the case of 2,14-dithiacalix[4]arene, which makes this mixed-bridge system attractive as molecular scaffold for supramolecular applications.

Galectin-targeting glycocalix[4]arenes can enter the cells.

Elevated levels of galectin-3 are associated with tumorigenesis. Its inhibition with high-affinity carbohydrate ligands opens new therapeutic routes. Targeting of intracellular galectin-3 is challenging for polar inhibitors like carbohydrates. We demonstrate the potential of novel biomedical research tools, glycocalix[4]arenes, to enter epithelial cells, which may allow their interaction with galectin-3.

S-alkylation of thiacalixarenes: how the regio- and stereoselectivities depend on the starting conformation.

S-alkylation of all four thiacalix[4]arene conformations was accomplished using alkyl triflates. The corresponding sulfonium salts are formed in a highly regio- and stereoselective manner depending on the conformation used. Interestingly, only mono- or disubstituted sulfonium salts can be prepared. Although many regio- and stereoisomers are theoretically possible, only one dialkylated cone and 1,2-alternate derivatives were formed, while only a single isomer of monoalkylated partial cone and 1,3-alternate were isolated. The combination of experimental results with the quantum-chemical approach using the B3LYP/6-311G(d,p) method resulted in the elucidation of the rules governing the regio- and stereochemical outcomes of the alkylation reactions. All S-alkylated compounds represent a novel type of substitution pattern in calixarene chemistry showing the wide-ranging possibility of thiacalixarene skeleton modifications.

Uncommon regioselectivity in the thiacalix[4]arene series: gross formylation of the cone conformer.

Thiacalix[4]arene immobilized in the cone conformation undergoes a direct Gross formylation reaction (Cl(2)CH-O-CH(3)/SnCl(4)/CH(2)Cl(2)) to give the upper-rim formylated thiacalixarene. Albeit using excess of the formylation agent and various reaction temperatures, only one formyl group is introduced into the meta position of the thiacalixarene skeleton. The surprising regioselectivity indicates dramatically different reactivity of the thiacalix[4]arene system when compared with a classical calix[4]arene analogue, which yields exclusively para isomers. The introduction of functional groups into the meta position represents an exceptional substitution pattern in thiacalixarene chemistry, which imparts an interesting conformational behavior to these compounds. The systematic NMR study revealed that the pinched cone-pinched cone equilibrium is remarkably shifted toward one pinched cone structure depending on the substitution.

Meta nitration of thiacalixarenes.

Nitration of thiacalix[4]arene, immobilized in the 1,3-alternate conformation, leads regioselectively to meta-substituted products. Depending on the reaction conditions, mono- and dinitro-derivatives can be isolated in acceptable yields. This unique substitution pattern is inaccessible in classical calixarene chemistry, and yields inherently chiral compounds, which makes thiacalixarenes very attractive as building blocks or molecular scaffolds.

Uncommon regioselectivity in thiacalix[4]arene formylation.

To reveal the alternative ways for upper-rim thiacalixarene derivatization, the formylation reactions (Gross and/or Duff conditions) of the corresponding tetrapropoxythiacalix[4]arene immobilized in the 1,3-alternate conformation were systematically studied. Surprisingly, albeit using an excess of the formylation agent, only two formyl groups were introduced exclusively into the meta positions of thiacalixarene skeleton. Unexpected regioselectivity of these reactions opens the door for a unique substitution pattern in thiacalixarene chemistry. The formation of meta-substituted aldehydes is another illustration showing remarkably different reactivity of the thiacalix[4]arene system compared with that of a classical calyx[4]arene analogue.

S-alkylation of thiacalixarenes: a long-neglected possibility in the calixarene family.

Despite the high nucleophilicity of sulfur atoms, thiacalixarenes have been alkylated only on oxygen atoms thus far. Using strong alkylating agents (triflates, trialkyloxonium salts), the substitution of the sulfur bridges has been successfully accomplished. The corresponding sulfonium salts of thiacalix[4]arene are formed regio- and stereoselectively as a completely new type of substitution pattern in thiacalixarene chemistry. These compounds possess interesting conformational behavior and could be used as unusual alkylating agents with uncommon selectivity.

Unusual intramolecular bridging reaction in thiacalix[4]arene series.

Thiacalix[4]arene immobilized in the cone conformation undergoes a direct formylation reaction giving unusual products in high yields. The Duff reaction (urotropine/TFA) leads to unprecedented intramolecularly bridged compounds possessing two formyl groups on the opposite para- or para-/meta-positions. The comparison with the corresponding classical calix[4]arene analogues indicates an amazingly different reactivity of the thiacalix[4]arene system.