Marlo К., S. M. Roberts, R. Storer, R.C. Bethell, Synthesis and Biological Activity of the Diphosphorylphosphonate Derivatives of (+) and -is-9-(4'-hydroxylcyclopent-2'-enyl)guanine, J. Chem. Soc. Perkin Trans. 1, 1994, 1477-1481.