Crown ether tether

Importantly, the tethers in these bisadduds can be readily removed, making them true templating units. This point is illustrated by the cleavage of the crown ether tether in conjugate 24 to give untethered trans-1 fundionalized fullerenes that are of interest for use as further molecular scaffoldings (Scheme 5).156 60 611... 

Bourgeois, J.-P. Echegoyen. L. Fibbioli, M. Pretsch, E. Diederich. F. Regioselective synthesis of trans-l fuller-ene i.v-adducts directed by a crown ether tether Alkali metal cation modulated redox properties of fullerene-crown ether conjugates. Angew. Chem. Int. Ed. 1998. 37. 2118-2121. 

This route was also employed for the synthesis of macrocydic allenes 169 via intramolecular coupling of aromatic dialdehydes 168 [88]. Depending on the ring size, chemical yields of up to 90% of the desired products were achieved (Scheme 2.53). Furthermore, the introduction of additional oxygen atoms into the tether did not diminish the efficiency of the protocol and therefore allowed an access to allenic crown ethers of type 171. 

Transesterification of the fullerene crown ether conjugate (+)-89 allowed for the removal of the tether, leading to the D2fj-symmetrical bisadduct 94 (34%) [93], which is a very useful building block for further addition chemistry. 

The scope of the tether-directed remote functionalization has been expanded from Cgo to the higher fullerene C70, and the described reactions are completely regioselective, featuring, in the case of C70, the kinetically disfavored addition pattern. The crown ether is a real template, since it can be readily removed by transesterification, giving a much-improved access to certain bis-adducts that are not accessible by the direct route. Cation-binding studies by CV reveal that cyclophane-type crown ethers derived from C60 and C70 form stable complexes with metal cations, and a perturbation of the fullerene reduction potentials occurs because the cation is tightly held close to the fullerene surface. This conclusion is of great importance for future developments of fullerene-based electrochemical ion sensors. 

Aggregation of metallotriazole hemiporphyrazines possessing crown ethers was induced by alkali cations [95CC419] and the first examples of 2,2 -tethered bifluorenylidene crown ethers were synthesized using an oxidative photochemical cyclization process [95JOC7380]. 

In addition to crown-ethers, calix[4]resorcinarenes 19 have also been used as head groups to obtain azobenzene amphipbiles with sufficient photo-isomerization in LBK films. For this purpose, azobenzene moieties have been tethered to the lower rim of the crown conformer of the calixarene. O-octacarboxymethoxylated calix[4]resorcinarenes 19 (X = CH2-COOH) display efficient trans to cis photoisomerizability in densely packed mono-layers on a water surface, in LBK films, and in surface-adsorbed monolayers, whereas the noncarboxymetylated 19 (X = H) derivative gives films that are too densely packed. However, the aggregation is already suppressed efficiently compared to the azobenzene derivative without calixaren. ... 

Table 4 Summary of known compositions of LiX-, NaX-, and KX-crown ether complexes. Ratios are quoted as metal tether...

Three new azamacrocyclic-cyclophane, hybrid receptors have been created based on the incorporation of either 1,4,7,10-tetraazacyclododecane or 1,4,7-triazacyclononane units tethered by a short amidic spacer to an electron donor and a //-bonding crown ether. These direceptors are designed to act as hosts for biologically relevant guests <05JOC115>. 

Templates based on ether tethers also have been examined. Improvements in product yield may be anticipated from the reduced steric congestion in the cyclization step due to the substitution of an -O linkage for a -CH2 linkage [43]. In addition, such oxygenated tethers might serve as crown-ether type substrates, sequestering ions that in turn serve to promote macrocyclization. The beneficial effect on macrocyclization rates by complex-ation of this type has precedent in the work of Mandolini and Illuminati [44]. 

Additionally, functional groups (-RCOOH) were added to a crown ether to probe the influence of tethering the counterion to the macrocyclic host and to determine the optimum tether length for stable host-guest complex formation. At an elevated pH, the substituted crown ether serves as host and counterion. It was experimentally... 

With the development of 3-(co-bromoalkyl)thiophenes, several crown ether functionalized PTs have been synthesized by Bauerle and coworkers. Electropolymerization was performed on mono-, bi-, and terthiophene monomers 65-67 substituted with pendant 12-crown-4 receptor tethered with alkyl chains [175-177]. While electropolymerization failed with 65, compounds 66, 67, and 68 were easily electro-polymerized and the chemosensing properties of the polymers were analyzed. Cyclic voltammetric analysis showed that addition of increasing amounts of Li, Na, or produces a positive shift of the oxidation potential of poly(66, n=5), while this effect is less pronounced for poly(67). On the other hand, whereas the CV of poly(68, n=5) is strongly affected by the presence of alkali ions, the lengthening of the alkyl spacer in poly(68, n = 10) produces a complete loss of ion sensitivity. Optical and spectroelectrochemical experiments revealed that the changes in electronic properties were due to hindered diffusion of the counteranions into the film during polymer oxidation [177]. 

Many macrocyclic compounds have been prepared by the irradiation of oqto-dicinnamates and dicoumaryl derivatives (130, 130-16). > When the tether is a polyglycol unit, the photodimerization is sensitive to alkali metal ions, and crown-ether-Hke structures can be prepared. Intramolecular dimerization reactions can find preparative importance as, for instance, in the preparation of the P- and 8-truxinate dimers from the irradiation of propylene-1,3-dicinnamate and n-butylene-l,4-dicinnamate, respectively. The tether may also function as a chiral auxihary, such as in the irradiation of L-erythritol 1,4-dicinnamate or mannitol hexacinnamate. In the latter case, an excess of the (-l-)-8—truxinate isomer was observed, along with several other isomers. 

Scheme 5. Selective removal of the dibenzo[18]crown-6 methyl ether, pyridine, CH2CI2, 20 °C, 14 H 68 % from tether from conjugate ( )-24. a) TsOH, toluene, A, 3 h. b) ( )-24.

Translational isomerization of a series of [2]catenanes possessing an electron-rich dibenzo-34-crown-10 ether interlocked with rings containing an unsymmetrical 4-substituted resorcinol-based tether linking two electron-poor dipyridinyl groups was demonstrated and studied by VT NMR <07CC4289, 07JOC6454>. 

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