Utility of 4,6-dichloro-2-(methylthio)-5-nitropyrimidine. Part 3: Regioselective solid-phase synthesis of a 2,6,8,9-tetrasubstituted purine library

The first regiocontrolled solid-phase synthesis of a 2,6,8,9-tetrasubstituted purine library was performed through on-resin elaboration of 4,6-dichloro-2-(methylthio)-5-nitropyrimidine. A series of primary amines were loaded on ArgoGel-MB-CHO resin via reductive amination to yield secondary amines. Subsequent attachment of the starting pyrimidine core unit and C6-chloride substitution by primary amines yielded the resin-bound 4,6-disubstituted-2-methylthio-5-nitropyrimidines. Oxone^® mediated oxidation of the 2-methylthio moiety to the corresponding sulfone allowed facile substitution at the 2-position. CrCl₂ assisted reduction of the nitro group, followed by acid catalyzed orthoester cyclization and finally TFA mediated cleavage provided the tetrasubstituted purine final products. Most of the final purines were cleaved in good to excellent yield and purity, however, it was found that bulky groups at N9 hindered cyclization in C8-substituted derivatives. For these systems, LC purification of the crude cleavage products provided the target purines in high purity.