Potential and perspectives of cyclonucleosides.

Cyclonucleosides are defined as analogs of natural nucleosides with an additional covalent bond between the nucleobase and the sugar moiety. They differ from classical nucleosides in more rigid structure and fixed conformation, which are responsible for unique properties and further applications. For instance, rigid structure can determine better interaction of the molecule with the acceptor, which is important in the design new bioactive of compounds. This class of nucleosides is known from the early fifties, when Todd et al. obtained cyclic salts of nucleosides. Although the formation of cyclic salts by purine nucleosides is quite common, the variety of cyclonucleosides is not only limited to this group. Up to now, various miscellaneous purine and pyrimidine cyclonucleosides and their analogs with great structural diversity were obtained; they differ from each other in position, length and type of linkage. Purine cyclonucleosides form a large group of artificially obtained derivatives. However, recently turned out cyclonucleosides also exist in nature. In fact, cyclopurine N(3),5-cycloxanthine was isolated from a marine sponge of genus Eryus sp. The aim of this review is to give an overview of the synthesis of some cyclonucleosides according to their structural types and to underline their biological activities. The article also refers to other relevant review articles that have covered particular areas of investigation or have dealt in depth with a single compound.