Scheme 7.
A) Retrosynthetic analysis of 5’-phosphonate and 5’-methyl phosphonates of (N)-methanocarba adenine or 2-Cl adenine derivatives. Use of route B, in which the phosphonate was introduced prior to the nucleobase, proved successful. B) Retrosynthetic analysis of saturated and unsaturated long chain 5’-phosphonates of (N)-methanocarba adenine or 2-Cl adenine derivatives. Use of the shorter synthetic route C, in which the nucleobase was introduced prior to the phosphonate, proved successful.