(a) The Robins reagent [2-acetamido-6-O-(diphenylcarbamoyl)purine] was utilized for glycosylation under Lewis acid conditions. Regioselectivity of glycosylation depends on the glycosyl donor and its 2-O- or 2-N-protecting group. Regioselective N9 glycosylation of 2-acetamido-6-O-(diphenylcarbamoyl)purine with problematic glucosamine has been accomplished by protecting the amino function as a phthalimido group with consequent stabilization of the oxocarbenium cation, and lowering the activation energy by introduction of trichloroacetimidate at the anomeric carbon. (b) 6-Heteroaryl functions [6-(1,2,4-triazol-4-yl) and 6-(imidazol-1-yl)] were introduced into purine derivatives for regioselective N9 alkylation. The regiospecificity of alkylation mainly results from steric effects due to the coplanar conformation of the two linked heterocyclic rings governed by conjugation. Several of the obtained acyclic derivatives showed antiviral and antitumor activities. (c) Glycosylation of purine derivatives with 2-deoxy-3,5-di-O-(p-toluoyl)-a-D-erythro-pentofuranosyl chloride using the sodium salt method usually gave a mixture of both anomers. Lipophilic groups were introduced into the imidazole ring of 6-(imidazol-1-yl)purine derivatives to increase the solubility of the sodium salts in moderately polar solvents. Differential solvation effects in binary solvent mixtures were utilized to improve the stereoselectivity of glycosylation. The stereoselectivity varied with the sizes of lipophilic groups and the polarity of solvents. With the propyl group, and in CH3CN/toluene (1:1) and/or CH3CN/CH2Cl2 (1:1), regiospecfic and highly stereoselective glycosylation of purines with 2-deoxy-3,5-di-O-(p-toluoyl)-a-D-erythro-pentofuranosyl chloride was achieved. (d) Using the above method, a low cost and efficient synthesis of 2-chloro-2'-deoxyadenosine (2-CdA, cladribine) was accomplished with an overall yield of 48% from inexpensive guanosine and 57% from 2,6-dichloropurine. 2-Chloro-6-(2-propylimidazol-1-yl)purine was prepared either from guanosine in a yield of 61% in 5 steps or from 2,6-dichloropurine in a yield of 72% in one step. Coupling of this 2-chloro-6-heteroarylpurine with 2-deoxy-3,5-di-O-(p-toluoyl)-α-D-erythro-pentofuranosyl chloride in binary solvent mixtures, followed by activation of imidazolyl as a better leaving group via benzylation at N3 and then ammonolysis gave cladribine in good yield (79%) for 3 steps. Analogs of purine derivatives with lipophilic groups (butyl, pentyl and 2-phenylpropyl) worked almost as well.