Intramolecularly Bridged Calix[4]arenes with Pronounced Complexation Ability toward Neutral Compounds

Upper rim-bridged calixarenes

Calix[n]arenes represent a very attractive family of macrocyclic compounds with many potential supramolecular applications. Due to their well-established chemistry and many different synthetic strategies, enabling practically any derivatization of the basic skeleton, calixarenes are among the very popular building blocks used for the design and construction of various receptors, sensors and other sophisticated supramolecular systems. Regio- and/or stereo-selective derivatization of calixarenes currently represents a very extensive set of reactions, the overview of which would fill many thick books. Therefore, this review deals with only a small part of the above-mentioned reactions, specifically describes possible ways of bridging the upper rim of calixarenes, often leading to interesting rigidified structures, and also briefly mentions the potential use of these compounds.

Gold(I)-catalysed hydroarylations of alkynes for the synthesis of inherently chiral calix[4]arenes

We describe the first gold(I)-catalysed intramolecular hydroarylation of alkynes for the straightforward synthesis of inherently chiral calix[4]arenes. This step- and atom-economical approach, which exploits a formal meta-functionalisation of the calix[4]arene macrocycle, is able to deliver an ample family of N-heterocyclic, chiral compounds in high yields and functional group tolerance.

Direct meta substitution of calix[4]arenes

Calixarenes represent very popular building blocks in supramolecular chemistry. Compared to other macrocyclic families, they exhibit an almost infinite possibility of derivatization of the basic skeleton, which makes them ideal candidates for the design of new receptors or other functional systems. Although the chemistry of calixarenes is well established, there are still some substitution patterns that are unavailable or require a very lengthy synthetic approach. Among such synthetic challenges is the meta substitution of the aromatic skeleton (relative to phenolic oxygen), which, in conjunction with the 3D structure of calixarenes, leads to the inherent chirality and enables the synthesis of derivatives with a hitherto undescribed topology. This review deals with the current achievements in the meta substitution of calixarenes.

The atropisomeric dynamic behaviour of symmetrical N-acylated amine-bridged calix[4]arenes

N-acylated amine-bridged calix[4]arenes exhibit unusual dynamic behaviour which can be attributed to the restricted rotation around the amide C–N bond.

Ketone transformation as a pathway to inherently chiral rigidified calix[4]arenes

The preparation of inherently chiral rigidified calix[4]arenes with an intact cavity is a synthetic challenge due to the complicated synthesis of the starting compounds. Here, we report on a novel strategy for bridging the upper rim of calix[4]arenes consisting in carbonyl group formation and subsequent "extension" into a two-atom bridge using corresponding rearrangement reactions (Baeyer-Villiger, Beckmann). The resulting inherently chiral compounds are potentially applicable in the design of novel receptors. The complexation properties towards selected quaternary ammonium salts were studied by 1H NMR titration. Unusual complexation stoichiometries can be observed depending on the enantiomeric purity (racemic mixture versus pure enantiomers) of an amidic receptor 7 used.

Rearrangement of meta-Bridged Calix[4]arenes Promoted by Internal Strain

The meta-bridged calixarenes possess a rigidified and highly distorted cavity, where the additional single-bond bridge imposes an extreme internal strain on the whole system. As a consequence, these compounds exhibit a reasonably amended reactivity, compared with common calix[4]arene derivatives, which is governed by the release of internal strain. This can be documented by the reaction of the bridged calix[4]arene with P₂O₅ or Nafion-H, leading (apart from polymers) to a macrocyclic product with a rearranged basic skeleton. The methylene bridge next to the fluorene moiety is intramolecularly shifted from position 2 to position 4 of the phenolic subunit to minimize the tension. As revealed by single-crystal X-ray analysis and by application of the residual dipolar coupling method, the rearrangement occurs without altering the original conformation.

Synthesis of upper rim-double-bridged calix[4]arenes bearing seven membered rings and related compounds

Meta/meta- and meta/para-disubstituted organomercury calix[4]arenes in the cone conformation were transformed into corresponding amino derivatives. Acylation and subsequent intramolecular cyclization using the Bischler–Napieralski reaction provided, in the case of the meta/meta-series, double bridged calixarenes possessing seven membered rings on the upper rim. A similar synthetic strategy applied to meta/para-isomers allowed for the isolation of monobridged compounds bearing an additional trifluoroacetamido group located distally to seven-membered rings. Both series represent inherently chiral systems, which were successfully resolved using preparative chiral HPLC. The pure enantiomers exhibited a recognition ability towards selected chiral guest molecules as documented by the ¹H NMR titration experiments. The absolute configuration of the phenyl-substituted enantiomer (meta/meta-) was confirmed by single crystal structure determination (X-ray).

Calix[4]arenes bearing one or two bridges on the upper rim were prepared as novel inherently chiral derivatives potentially capable of chiral recognition.

meta-Bridged calix[4]arenes with the methylene moiety possessing in/out stereochemistry of substituents

Calix[4]arenes possessing two different substituents on an additional methylene bridge were prepared and the relationship between the in/out positions of these substituents and the complexation properties was studied.

meta-Bridged calix[4]arenes prepared by Friedel–Crafts alkylation

meta-Mercuration allowed the introduction of substituted hydroxymethyl groups that were intramolecularly cyclized to form a novel type of bridged calix[4]arenes.

Synthesis of inherently chiral calixarenes via direct mercuration of the partial cone conformation

Regioselective mercuration of calix[4]arene in the partial cone conformation enabled a straightforward access to a novel type of inherently chiral calixarenes with a highly distorted cavity.