Expanded calixpyrroles

Today it is possible to synthesize calix[ ]arenes with 4 n 11However, this is not the case for calixpyrroles. Indeed, before recent work in this area by Sessler et al caIix[n]pyrroles with n 4 were (apparently) unknown. It was quickly found that such species were often byproducts of condensations leading to the generation of calix[4)pyrroles, and that they were difficult to isolate cleanly. Therefore a template-type. strategy was explored that might allow the synthesis and isolation of a higher-order calixpyrrole. To test this strategy, p-fcrf-butylcalix[4]arene tetramethyl ketone was condensed with pyrrole in the presence of methanesulfonic acid. This afforded the cylindrical calix[4]arene-calix[4]pyrrole pseudo dimer 18 in 32% yield. 

The hydrogen bonding between the pyrrole-NH groups and phenolic oxygen atoms is maintained in solution as 

BF3 OEt2 catalyzed condensation. Compound 19 constitutes the first example of a higher- order calixpyrrole to be fully characterized.  

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Sessler and coworkers have prepared fluorinated calix[4]pyrroles (97 = 1) and expanded calixpyrroles (98 n = 2-5) by the condensation of 3,4-difluoropyrrole with acetone under acid catalysis (Fig. 3.47). The fluorinated dipyrrolylqumoxaline 99 was also synthesized from 3,4-difluoropyrrole. These were studied as neutral anion receptors that had higher affinity and greater selectivity relative to their nonfluorinated congeners. 

FIGURE 3.47 Fluorinated calix[4]pyrroles and expanded calixpyrroles and a fluorinated dipyrroly Iquinoxaline. 

Sessler and co-workers have incorporated two carbazole subunits into expanded calixpyrrole-type macrocycle 53 [71]. Fluorescence titration experiments in dichloromethane at 0.5 iM concentration of host revealed that compound 53 shows selectivity for acetate (fCa = 229 000 M ) over a number of other carboxylate-type anions (benzoate, oxalate and succinate). 

The synthesis and biological evaluation of pyrrole macrocycles (i.e., porphyrins, expanded porphyrins, and calixpyrroles) and linear oligopyrroles comprise perhaps the largest body of work published each year that can be classified as pyrrole chemistry. Unfortunately, due to space limitations, the discussion of selected advances in this area could not be included. 

A number of N-confused systems are known with macrocyclic conjugation interrupted by sp3 meso bridges. These systems, which include analogs of calixpyrroles [252] and calixphyrins [253], fall outside the scope of the present review. A number of fully conjugated N-confused expanded porphyrins have been obtained by use of predesigned N-confused substrates. The most representative examples of such structures are collected in Fig. 32. 

Sessler and coworkers expanded the calix[4]pyrrole binding cavity by employing bipyrrole in place of pyrrole, yielding new calixpyrrole analogs such as calix[n]bipyrroles" (n = 3, 4), a calix[2]bipyrrole[2]furan, and a calix[2]bipyr-role[2]thiophene. The carbazole moiety was also an attractive linker to include as the fluorescent nature of... 

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