Cyclopentadiene was converted in six steps to the key intermediate (±)-(1α,2β,4α)-4-amino-2- (benzyloxy)cyclopentanol (10), which in turn was converted to the carbocyclic nucleoside analogs 14 and 19 by standard procedures developed in these laboratories. Compounds 14 and 19 were then further converted to the target phosphonates 1b and 2b by modification of literature procedures. The phosphonate 1b was 40-fold more cytotoxic to HEp-2 cells than its parent, CDG, presumably after conversion to the diphosphoryl phosphonate. © 1994, American Chemical Society. All rights reserved.