1-Alkenylphosphonic acid derivatives of purines have been proven to exhibit significant antiviral activity among these series of compounds. Here we disclose the stereoselective synthesis of the constrained analogs of 1-alkenylphosphonate derivatives of purines via intramolecular epoxide opening reaction of gamma,delta-epoxyalkanephosphonates with subsequent Mitsunobu coupling reactions with purine bases. (C) 1999 Elsevier Science Ltd. All rights reserved.