Optical rotatory dispersion and circular dichroism of 8-cyclonucleosides derived from adenosine were measured. <8,3'-<8,5'-cyclonucleoside both in the case of O-and S-cyclonucleosides. All of these cyclonucleosides had positive Cotton curve at absorption maxima. The synthesis and properties of various cyclonucleosides were reported. 3, 4) Cyclonucleo sides are unique in the respect of the structure, possessing anhydro linkages, which fix the furanose and the base moiety at certain angle. This

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... tuation may serve as a good model for the extreme conformation of ordinary nucleosides, in which the base is rather freely rotatable despite barriers caused by the interference of adjacent atoms. 5) Especially in the purine 1) Part XLII: M. Ikehara and M. Kaneko, Chem.