Transglycosylation Reactions of Purine Nucleosides. A Review

N7‐ and N9‐substituted purine derivatives: a 15N NMR study

“…For example, we recently found that the alkylation of N 6 -[(N,Ndimethylamino)methylene]adenine with certain alkylating reagents leads selectively to the N 7 -substituted derivatives. 15 Alkylation of 2-aza-and 8-azaadenine produced the N 2 -and N 8 -substituted side products, respectively. 16,17 In addition to standard 1 H and 13 C NMR techniques, 1 H- 15 N NMR experiments proved to be very useful in determining the position of the alkylation in these nitrogen-rich compounds.…”

“…8 The standard method of choice for preparing N 9 -alkyladenines is the alkylation of the nucleobase in the presence of a base. The regioselectivity of this alkylation depends on the reaction conditions and on the nature of the purine base (e.g.…”

Reactions of Transglycosylation in the Nucleoside Chemistry

“…In line with this mechanism, the new glycosylic bond (N1-C1') is formed before the former one (N2-C1') is cleaved [14]. However, the intermediate (8) has never been isolated in the case of indazole nucleosides and, therefore, the mechanistic proposition presented here is just the author's suggestion based on the formation of analogical bis-ribosylated intermediates in the purine and pyrimidine series [6,7]. Another possible mechanism of transglycosylation, assuming a simultaneous cleavage of N2-C1' and formation of N1-C1', like in the reactions of the S N 2 type, cannot be excluded in this discussion.…”

“…This results in the formation of the kinetic product (2), which then undergoes an intermolecular rearrangement (transglycosylation) to regain the most stable aromatic system of the indazole portion. Thus, a second ribosylation at N1 gives the 1,2-bis-ribosyl intermediate (8), which decomposes to the stable final product, 1-regioisomer (3), with liberation of the 1,2-acyloxonium cation (7). In line with this mechanism, the new glycosylic bond (N1-C1') is formed before the former one (N2-C1') is cleaved [14].…”

“…Therefore, many examples of transglycosylation reactions have been reviewed by Watanabe, Hollenberg and Fox in their excellent article on mechanism of nucleoside synthesis [6]. More recently, I have published a review on transglycosylation reactions of purine nucleosides [7]. Because many new experimental facts have appeared since that time, I would like to comment on it and present a more general view on the mechanism and modern application of transglycosylation reactions.…”