Chiral calixarenes synthesis

Chiral groups may be attached to both the narrow and the wide rims of the calixarene skeleton I (or even to the bridges14) and this may be done by chemical modification of the calixarene or by the use of appropriate chiral phenols for the calixarene synthesis. With resorcarenes II chiral groups may be introduced at the hydroxy groups or at the 2-position between the OH-functions. Their attachment at the bridges (CHR ) was also realized. 

Synthesis of Dissymmetric and Asymmetric Calixarenes Chiral Calixarenes... 

Calixarenes Synthesis and Historical Perspectives, p. 153 Chiral Guest Recognition, p. 236... 

Attachment of chiral substituents to calixarene skeleton is the first, the most straightforward and still the most popular way to constmct chiral calixarenes. In 1979 Gutsche reported the synthesis of the first chiral calixarene by attaching camphorosulfonyl group to p-tBu-calix[8]arene [1]. Nowadays, attachment of virtually any chiral moiety at the selected position is synthetically feasible. However, some chiral groups, due to their availability and versatility, have been particularly widely exploited, for example amino acid derivatives, peptides, carbohydrates, chiral amines and axially chiral groups. 

The synthesis of 8 is a very rare example that utilizes diastereoselective pathways to achieve chiral calixarenes. In most cases chiral groups are simply attached to the macrocyclic ring. Here, the stereogenic centers were introduced on an imine-calixarene scaffold using a chiral auxiliary strategy. 

With the increasing demand for chiral nonracemic compounds, stereoselective methods for the synthesis of 1,3-oxazine derivatives and applications of enantiopure 1,3-oxazines in asymmetric transformations have gained in importance in the past decade, as reflected by the increasing trend in the number of publications on this topic, and accordingly by the share of this topic in the present compilation. The limited size of this survey and the scope of this chapter do not allow a discussion here of the applications of 1,3-oxazines in polymer chemistry and the synthesis and properties of 1,3-benzoxazine-containing hetero-calixarenes. 

Recent examples of calixarenes having chiral substituents attached to the wide rim are calix[n]arenes la and lb which were obtained by one-pot synthesis from the respective p-substituted phenol in moderate15 to low yield.16... 

The cyclic phosphinate (96) has been isolated from the reaction of dichloro(methyl)phosphine with the ethoxycarbonylimine derived from hexafluoroacetone. Treatment of trichloro(organo)phosphonium-hexafluorophosphate salts with dichloro(diethylamino)phosphine results in the halophosphonium salts (97). Some reactions of dichloro(-)menthylphosphine have been reported.As usual, nucleophilic displacement reactions of halogenophosphines have received attention as routes to new systems of interest as ligands.Of particular interest in this connection is a report of the synthesis of the phosphorus-functionalised calixarenes (98). Only one chlorine atom of dichloro(phenyl)-phosphine is replaced on treatment with an excess of dicyclohexylamine, enabling the stepwise synthesis of the chiral aminophosphines (99), described as air-stable solids. 

A complete survey on calixarenes was far beyond the scope of this chapter. We therefore concentrated mainly on the chemistry, namely the synthesis and the (basic) chemical modification of calixarenes, which form the basis for aU fnrther stndies. Fnrther interesting aspects, such as inherent chirality of calixarenes, catalytic or biomimetic effects , larger structures formed via self-assembly in solution or in mono- and multilayers could be stressed only shortly. Important properties snch as complexation of cations , anions and of nentral gnests , including fnUerenes , conld not be treated at all. The same is tme for applications arising from these properties in snch different areas as sensor techniqnes , chromatographic separations or treatment of nuclear wastes . In all these cases, the reader is referred to special reviews. 

Despite the overall decline in output, the year has produced ftirther interesting developments in the fteld of hypervalent phosphorus chemistry. These include phosphoranes containing acetylenic links, the synthesis and enantiomeric separation of a bicyclic phosphorane with chirality only at phosphorus, and a wide range of phosphoranes incorporated within macrocyclic ring systems (Houlla et al). In addition Lattman has reported on phosphoranes bound to transition metals or enclosed within calixarene skeletons and Holmes and his co-workers have made further substantial contributions on the X-ray crystal structures of phosphoranes containing eight-membered rings. 

Sugars, due to their chiral and polyhydroxylated nature, when coupled on either rim of a calixarene platform could constitute a unique class of water-soluble molecular receptors, which offer various opportunities for molecular recognition of chiral highly polar organic molecules in water and under physiological conditions. The synthesis of some such hybrids and their interactions are described. 

Synthesis of Chirally Modified Calixarenes and Resorci-narenes. (l5)-Camphorsulfonyl chloride was used to introduce camphorsulfonate groups to form the chirally modified calixarene 63 (eq 21) and the diastereomeric tetraalkoxyresorcm[4]arenes 65 from racemic 64 (eq 22). Diastereomers 65a and 65b can be separated chromatographicaUy. This allowed 64 to be resolved into chiral nonracemic (/ ,5)-(+)-64 and (M,R)-(—)-64 after cleavage of the sulfonate ester groups. The absolute stereochemistry of (i, 5)-(+)-64 and (M,R)-(—)-64 was obtained from an X-ray structure analysis of 65a. 

Limitations on the application of hydrogen bonded capsular assemblies come from the poor stability of these assemblies in polar media compared to those held by covalent bonds. In an attempt to overcome these Umitations, Bohmer described the synthesis of a mechanically locked chiral bis-catenane based on calixarene capsules [55]. Inspired by this pioneering work, our group produced a mechanically locked hybrid tetraurea calix[4]arene-calix[4]pyrrole capsule 30 (Fig. 32.14) [56]. This catenated capsule displayed reversible encapsulation (i.e. reversible assembly of the urea belt). The two hemispheres of the capsule never dissociate completely due to the bis-catenated topology. 

Although chiral calix[4]arenes can be generated by simply attaching chiral residues at the upper [40] or lower [41-43] rim of the calixarene skeleton, recent interest has been focused on the possibility of synthesizing inherently chiral calix[4]arenes, which are build up of nonchiral subunits and consequently owe their chirality to the fact that the calixarene molecule is nonplanar. Molecular asymmetry can arise from the substitution pattern at the lower rim and/or conformation. In this respect, Shinkai has recently reported a systematic classification of all possible chiral isomers derivable from calix[4]arene, and delineated some basic concepts for the design and synthesis of chiral derivatives [44]. 

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