Structural assignment of 6-oxy purine derivatives through computational modeling, synthesis, X-ray diffraction, and spectroscopic analysis

Abstract

6-Oxy purine derivatives have been considered as potential therapeutic agents in various drug discovery efforts reported in the literature. However, the structural assignment of this important class of compounds has been controversial concerning the specific position of a hydrogen atom in the structure. To theoretically determine the most favorable type of tautomeric form of 6-oxy purine derivatives, we have carried out first-principles electronic structure calculations on the possible tautomeric forms (A, B, and C) and their relative stability of four representative 6-oxy purine derivatives (compounds 1-4). The computational results in both the gas phase and aqueous solution clearly reveal that the most favorable type of tautomeric form of these compounds is A, in which a hydrogen atom bonds with the N1 atom on the purine ring. To examine the computational results, one of the 6-oxy purine derivatives (i.e., compound 4) has been synthesized and its structure has been characterized by X-ray diffraction and spectroscopic analysis. All of the obtained computational and experimental data are consistent with the conclusion that the 6-oxy purine derivative exists in tautomer A. The conclusive structural assignment reported here is expected to be valuable for future computational studies on 6-oxy purine derivative binding with proteins and for computational drug design involving this type of compounds.

Figure 1

Geometries optimized at the B3LYP/6-31G* level for the three possible types of tautomeric forms of 6-oxy purin derivatives (compounds 1 to 4). Color codes: red, oxygen; blue, nitrogen; gray, carbon; white, hydrogen.

Figure 2

Intermolecular interaction in the X-ray crystal structure of compound 4. Color codes: red, oxygen; blue, nitrogen; gray, carbon.

Scheme 1

Three possible types of tautomeric forms of 6-oxy purine derivatives; tautomer B may take two different types of conformations (B-1 and B-2).

Scheme 2

Synthesis of compound 4. Reagents and conditions: (a) pyridine, acetonitrile, reflux 1 h and then 15 min; (b) methyl 3-methoxyphenyl acetate, CH₃ONa, reflux.