The neplanocin A analogue 3-deazaneplanocin A (2b) has been synthesized. A direct SN2 displacement on the cyclopentenyl mesylate 3 by the sodium salt of 6-chloro-3-deazapurine afforded the desired regioisomer 4 as the major product. After deprotection, this material was converted to 3-deazaneplanocin A in two steps. X-ray crystallographic analysis confirmed the assigned structure. Consistent with its potent inhibition of S-adenosylhomocysteine hydrolase, 3-deazaneplanocin A displayed excellent antiviral activity in cell culture against vesicular stomatitis, parainfluenza type 3, yellow fever, and vaccinia viruses. Antiviral activity was also displayed in vivo against vaccinia virus by using a mouse tailpox assay. The significantly lower cytotoxicity of 3-deazaneplanocin A, relative to its parent compound neplanocin A, may be due to its lack of conversion to 5'-triphosphate and S-adenosylmethionine metabolites.