Synthesis and Properties of 5-Cyano-Substituted Nucleoside Analog with a Donor–Donor–Acceptor Hydrogen-Bonding Pattern

6-Aminopyridin-2-ones form Watson–Crick pairs with complementary purine analogues to add a third nucleobase pair to DNA and RNA, if an electron-withdrawing group at position 5 slows oxidation and epimerization. In previous work with a nucleoside analogue trivially named d<b>Z</b>, the electron withdrawing unit was a nitro group. Here, we describe an analogue of d<b>Z</b> (cyano-d<b>Z</b>) having a cyano group instead of a nitro group, including its synthesis, p<i>K</i><sub>a</sub>, rates of acid-catalyzed epimerization, and enzymatic incorporation.