The protected 2'-deoxyguanosine derivatives 5a ± c undergo N 9 3 N 7 isomerization in the melt and in solution. The rate of isomerization is much faster than in the case of the corresponding ribonucleosides and occurs even in the absence of a catalyst. In the melt (1958, 2 min), the N 2 ,3'-O,5'-O-tris(4-toluoyl) derivative 5b and the N 2 -acetyl-3',5'-bis-O-[(tert-butyl)dimethylsilyl] derivative 5c gave anomeric mixtures of the N 7 -isomers 9b/10b (43%) and 9c/10c (55%), respectively. In addition, the N 9 -a-d-anomers 8b and 8c were obtained. Different from 5b, the isomerization of peracetylated 5a resulted in low yields. Compound 5b was also prone to isomerization performed in solution (toluene, 1008, 5 min; chlorobenzene, 1208, 5 min), furnishing the N 7 -regioisomers in 24 ± 53% yield. The highest yield of the N 9 3 N 7 isomerization occurred in the presence of 2-. When the glycosylation reaction is performed in MeCN with SnCl 4 as catalyst, the protected N 7 -nucleosides are formed predominantly [2]. This indicates that the formation of regioisomeric glycosylation products depends on the catalyst, the solvent, and the temperature. The outcome of the reaction is also influenced by the structure of the base and the activated sugar [2]. The mechanism of the glycosylation reaction has been discussed by Garner and Ramakanth [3] and also recently by Robins and co-workers [4]. The isomerization of glycosylated intermediates is described for purine N 3 -nucleosides [5]. Little attention has been paid to the fact that preformed purine N 7 -or N 9 -nucleosides can isomerize at elevated temperature. Miyaki and Shimizu have noted a reversible conversion of the N 2 -acetyl-N 7 -(2',3',5'-tri-O-benzoylribosyl)guanine to the corresponding N 9 -nucleoside [5]. Later, it was observed that N 9 -ribosylated 6-oxopurines or corresponding acyclo derivatives, which are fully acylated undergo thermal isomerization to N 7 /N 9 -mixtures [6 ± 8]. The reaction takes place even in the absence of a catalyst, when the starting material is kept in a melt for 5 ± 10 min at a temperature exceeding 1908. While dependent on the reaction temperature and time, the ratio of regioisomers (N 7 vs. N 9 ) is also influenced by the structures of the base and the substituents, including protecting groups. Thus, elevated temperatures, applied to an isomerically pure N 9 -b-d-ribonucleoside, can lead to the isomerization of the starting material under the formation of an N 7 -regioisomer.So far, these isomerization reactions have been performed with purine ribonucleosides and were not studied with purine 2'-deoxyribonucleosides. As the purine N 7 -2'-