It has been recognized in the past few years that a number of sesquiterpene lactones and other derivatives obtained from natural sources bearing a-methylene-γ-butyrolactone and related moieties have exhibited interesting biological activity and significant antitumor activity, with the O=C-C=CH2 moiety being responsible for this activity. The cytotoxic activity of a-methylene γ-lactones is attributed to their ability to act as alkylating agents via a Michael-type reaction with biological nucleophiles such as L-cysteine, glutathione, or thiol-rich enzymes. A large number of drug candidates with this functionality have been synthesized to develop effective clinical drugs, as natural derivatives have unfavorable therapeutic indices. As part of an anticancer drug development program, we aimed to synthesize suitably substituted nucleic acid bases bearing this moiety, given the scanty literature except for uracil and thymine derivatives with exocyclic double bonds and adeninyl/uracilylfuranones with endocyclic or fully substituted double bonds. Using the route described by Ohler et al.—involving formation of an organozinc intermediate from ethyl α-(bromomethyl)acrylate and zinc for a Reformatsky-type reaction with respective ketones—we prepared purinyl (2a, 3a) and pyrimidinyl (4a, 5a, 6a, 7a) lactone derivatives. In vivo antitumor screening against Ehrlich ascites carcinoma (EAC) in Swiss mice (25 and 12 mg/kg doses) showed good activity for 2a, 3a, and 7a, marginal to moderate activity for 4a, 5a, and 6a, and inactivity for ketones (2, 3, 5, 6) and 2c/3c (despite a 6-mercapto group). In vitro screening of 2a, 3a, and 4a against P-388 (PS) and human KB cells revealed significant inhibitory effects, with greater potency against PS cells. 2a and 3a showed comparable activity despite differing in chain length (propyl vs. pentyl). Further investigation is underway to establish structure-activity relationships for nucleic acid bases substituted with long-chain ketones convertible to a-methylene γ-lactones.