The unexpected formation of trisaccharide nucleosides protected purine nucleosides in the presence of Lewis acids. The formation of these by-products can be avoided by using during synthesis of disaccharide nucleosides with a purine base moiety has been observed. The occurrence of this side the 2,3-O-isopropylidene group instead of acyl groups for the blocking of the secondary hydroxyl groups of the nucleoside. reaction can be explained in terms of the instability of the
We have recently developed a general route for the prep-biotics have this type of structure [5] . This reaction was further extended to the synthesis of 3Ј-O-β--ribofurano-aration of 2Ј-O-β--ribofuranosyl nucleosides [1] [2] by condensation of partially protected nucleosides with 1-O-ace-syl 2Ј-deoxypyrimidine nucleosides [6] and 5Ј-O-β--ribofuranosyl ribo (and 2Ј-deoxy) pyrimidine nucleosides [7] . It tyl-2,3,5-tri-O-benzoyl-β--ribofuranose in the presence of tin tetrachloride. Purine nucleosides having a -ribofurwas shown that application of the reaction to the preparation of purine nucleosides in the latter two cases gave anosyl substituent bonded to the 2Ј-hydroxy function of the nucleoside sugar moiety have recently been isolated complex mixtures of products [6] [7] . Therefore, we set about a more detailed investigation of this condensation reac-from t-RNA [3] [4] . Other types of disaccharide nucleosides have also shown biological activities, and several anti-tion.