Tetra- calix arene synthesis

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

Scheme 5.8 Synthesis of tetra-urea calix[4]arenes. (a) Alkylating reagents, NaH, DMF, rt (b) HN03/H0Ac, CH2CI2, rt (c) H2, Raney-Ni, toluene, rt (d) R-NCO, rt (e) p-nitrophenyl chloro-formate (f) R-NH2, rt.

Scheme 5.9 Synthesis of tetra-urea calix[4]arenes bearing two different urea residues in the sequence ABAB (a) and AABB (b), using the protection of amino functions by trityl- or Boc-groups. (Y = C5Hln).

Scheme 5.12 Selective synthesis of bis-, tris-, and tetraloop tetra-urea calix[4]arenes (a) metathesis and subsequent hydrogenation, (b) decomposition of the heterodimer (a pseudorotaxane) and isolation ofthe multiloop calixarene.

The inability of bis-, tris-, or tetraloop compounds to form homodimers and the general tendency of tetra-urea calix[4]arenes to dimerize can be further exploited for the synthesis of mechanically interlocked molecules. A 1 1 mixture of tetra-ureas 5 or 6 with bis- or tetraloop compounds 8 or 9 (in practice the non-reactive loop component is added in a small excess) contains exclusively heterodimers (e.g., 5-8, 5-9, or 6-9), since this is the only possibility, to have all the urea functions involved in the favorable hydrogen-bonded belt.5 Again this is easily evidenced by the complete absence of peaks for the homodimer 5-5, or 6-6 in the H NMR spectra (for an example see Figure 5.11). 

The synthesis of fourfold [2]rotaxanes by clipping requires the efficient formation of heterodimers between a tetra-urea substituted by bulky stopper groups and an octaalk-enyl urea 6. We initially hoped that the steric crowding in homodimers of a tetra-tritylphenylurea calix[4]arene would be sufficient to shift the equilibrium toward the heterodimers in a mixture with 6. However, the distribution of the dimers was close to the statistical ratio and the desired rotaxane could be obtained in only 5% yield [59]. 

In cooperation with the group of Dirk Volkmer (Ulm University), the synthetic procedure established for the preparation of the tetra-(4-(2,2 6, 2"-terpyridyl)-phenyl)-cavitand 49 was adapted for the synthesis of terpyridyl-substituted calix [n]arenes [77]. The tetra-(4-(2,2 6, 2"-terpyridyl)-phenyl) calix[4]arenes 54a,b and the penta-(4-(2, 2 6, 2"-terpyridyl)-phenyl)calix[5]arene 55 can be prepared from the boronic acid ester 48 and tetrabromocalix[4]arene 52a or b and pentabromocalix[5]arene 53, respectively (Fig. 21). 

In the bis-O-2-picolyl derivatives of calix[4]arene 26a-c, the two binding pendant functionalities can adopt syn-proximal (26a), syn-distal (26b) or flnh -distal (26c) relationships. On using lower proportions of NaH mainly st/n-proximal 26a and some syn-distal 26b products along with mono and tris derivatives were formed. 26a is a useful intermediate for further selective synthesis of tri and tetra alkylated cone conformers having the same or mixed pendants (92JOC2611). 

Synthesis of Calixarenes (1)-(4) Compounds p-5,ll,17,23-tetranitro-25,26,27,28-tetrahydroxy-calix[4]arene, (1), and 4,6,10,12,16,18,22,24-octahydroxy-2,8,14,20-tetraethyl-calix[4]resorcinarene, (2), were prepared in-house following procednres described elsewhere [37]. Compoimd 5,ll,17,23-tetra-A /s-(Af-methylmor-pholine)-25,26,27,28-tetrahydroxy-calix[4]arene, (3), was prepared by adapting the procednre described by Pandey [51]. Compoimd 5,ll,17,23-tetranitro-25,26,27,28-tetra(Af-(5-dimethylaminonaphthalene-l-sulfonyl))-cahx[4]arene, (4), was synthesized according to the reported literatnre procedure [52]. 

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