We describe the first example of the synthesis of a tetraoxa[8]circulene with only four substituents by means of BF 3 ·OEt 2 mediated tetramerisation of tert-butyl-1,4-benzoquinone. Two of the four regioisomers of tetra-tert-butyltetraoxa[8]circulene have been isolated and characterised by
In continuation of previous studies showing promising metal-molecule contact properties a variety of C(60) end-capped "molecular wires" for molecular electronics were prepared by variants of the Prato 1,3-dipolar cycloaddition reaction. Either benzene or fluorene was chosen as the central wire, and synthetic protocols for derivatives terminated with one or two fullero[c]pyrrolidine "electrode anchoring" groups were developed. An aryl-substituted aziridine could in some cases be employed directly as the azomethine ylide precursor for the Prato reaction without the need of having an electron-withdrawing ester group present. The effect of extending the π-system of the central wire from 1,4-phenylenediamine to 2,7-fluorenediamine was investigated by absorption, fluorescence, and electrochemical methods. The central wire and the C(60) end-groups were found not to electronically communicate in the ground state. However, the fluorescence of C(60) was quenched by charge transfer from the wire to C(60). Quantum chemical calculations predict and explain the collapse of coherent electronic transmission through one of the fulleropyrrolidine-terminated molecular wires.
The conformations of an acyclic, achiral enamide thymidine analogue 1 have been studied by model building and geometry calculations, as well as by NMR NOE and UV experiments. The results indicate that there are no significant barriers to rotation around any of the sigma bonds, in particular the N1-C1' enamide bond, and that the analogue should be able to accommodate conformations that mimic the conformations of natural nucleosides in A- and B-type helices quite well. For comparison the saturated analogue 2 has been prepared and built into oligonucleotides. It is shown that incorporation of 2 in oligonucleotides results in a much larger depression of the melting temperature (deltaTm -10 to -12.5 degrees C) than does incorporation of 1 (deltaTm -5 to -6.5 degrees C).
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