Spirans. XXII. Spiroconjugation in heteraspirans studied by UV and 13C NMR spectra.

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  • Smolinski Stefan
    Stereochemical Laboratory, Institute of Chemistry, Jagellonian University
  • Balazy Michal
    Stereochemical Laboratory, Institute of Chemistry, Jagellonian University
  • Iwamura Hiizu
    Division of Applied Molecular Science, The Institute for Molecular Science
  • Sugawara Tadashi
    Stereochemical Laboratory, Institute of Chemistry, Jagellonian University
  • Kawada Yuzo
    Division of Applied Molecular Science, The Institute for Molecular Science
  • Iwamura Michiko
    Department of Chemistry, Faculty of Science, Toho University

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UV and 13C NMR spectra of spirocyclic orthocarbonates, a spirobi(benzodithiepin) and a spirobi(naphthopyran) have been measured and compared with those of the “half compounds” to see the spiroconjugation effect. Whereas batho- and hyperchromic shifts are usually brought about by spiroconjugation, hypso- and hypochromic effects have been observed for 2,2′-spirobi[1,3-benzodioxole]. Through-bond interaction similar to the mechanism for the anomeric effect is considered to be operative in attenuating the electron release from the orthocarbonate moiety to the benzene chromophore. The downfield shifts of 20–25 and ca. 40 ppm were noted for the spirocarbons of the conformationally flexible and rigid orthocarbonates, respectively, as compared to the corresponding methylene carbons of the half compounds. The largest paramagnetic shift of 66.3 ppm is observed for the orthothiocarbonate and attributed to the presence of the low-lying d-orbitals due to through-space interaction of the four sulfur atoms.

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