(Z) and (E)-2-(purin-9-yl/pyrimidin-1-yl)ethylen-1-ylphosphonic acid 10-18 were synthetized by Michael addition of heterocyclic base with the diethylethynylphosphonate and deprotection of the acyclic nucleoside phosphonate with bromotrimethylsilane. (C) 1998 Elsevier Science Ltd. All rights reserved.