Addition and cycloaddition to 2- and 8-vinylpurines

The reactivity of purines carrying an alkenyl substituent in the 2- or 8-position in nucleophilic addition and cycloaddition reactions has been studied. A vinyl substituent situated at C-8 is highly electrophilic and readily participates as a Michael acceptor in nucleophilic additions and as a dienophile in Diels-Alder reactions. 2-Vinylpurines may also give addition and cycloaddition products. When benzenethiol was added to 9-benzyl-2-vinylpurine, both the simple adduct as well as 9-benzyl-2-(2-phenylthio-1-hydroxyethyl)-9H-purine were formed. The structure of the latter compound was determined by single-crystal X-ray methods.