Crown ether linked pyridines

The incorporation of two aryl bridgeheads, both linked in turn through the 2,6-substitution of a pyridine nucleus has provided an intermediate which has been converted to a crown ether. Thus, 2,6-bis(2, 6 -dimethoxyphenyl)pyridine after demethylation with boron tribromide or hydrogen bromide in 65% yield was reacted in dimethylsulphoxide containing potassium carbonate with the a,o)-diiodo derivative of triethylene glycol to afford a crown ether system in 22% yield (ref. 199). 

The combination of crown ether and porphyrin has recently been extended by Hamilton et al. The macrotetracyclic cryptand 226 was prepared by condensation of the biphenyl-linked bis-crown ether 224 with the di-p-nitrophenyl ester porphyrin 225a, b in pyridine at 55 °C under high dilution conditions (45-54% after chromatography). Reduction to the tetra-amine 227 was effected by treating the zinc complex with diborane... 

Assembling [2] rotaxanes (wheel and axle) can involve three basic processes (Scheme 13). One of these (93CC1269) involves slippage, in which the axle (115) is linked by a 4,4 -bipyridine, and the wheel (116) is a bisparaphenylene-34-crown-10 ether. Heating the two components in acetonitrile at 60°C yields the rotaxane, which can be characterized by FABMS and H and l3C NMR, but extrusion of the wheel occurs at 100°C. In other developments (94NAT(369)133), the pyridine component may be incorporated in the wheel, as in structure (117), where two bipyridinium units are connected by p-xylyl groups, and here the rotaxane acts as molecular switch. At room temperature in acetonitrile the wheel... 

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