Study of the N7 Regioselective Glycosylation of 6-Chloropurine and 2,6-Dichloropurine with Tin and Titanium Tetrachloride

6-Chloropurine and 2,6-dichloropurine were regioselectively glycosylated at position 7 to give the corresponding peracetylated N7-nucleosides, which can be suitable for other purine transformations. In this work, we study the distribution of N7/N9-isomers produced via the Vorbrüggen method under different conditions, using an N-trimethylsilylated purine derivative and SnCl4 or TiCl4 as a catalyst. The main effort is devoted to reversing the disadvantageous predominant selectivity of most glycosylation reactions at the N9 position and thus to determining conditions that maximize the regioselectivity of glycosylation toward the desired N7-isomer.