Radiolabelled [14C]entecavir, (1), was prepared in 12 steps from (1S,2R,3S,5R)-3-(benzyloxy)-2-(benzyloxymethyl)-6-oxa-bicyclo[3.1.0]hexane 2. The chemical yield of [14C]entecavir was 14% from the epoxide 2. Introduction of [14C] radiolabel was achieved by elaboration of 4,5-diaminopyrimidine 8 with triethyl[14C]orthoformate to purine derivative 9. The radiochemical yield of [14C]entecavir from triethyl[14C]orthoformate was 11.3%. Radiochemical purity of [14C]entecavir determined by HPLC was 99.8%. The specific activity of [14C]entecavir was 108 µCi/mg (29.9 mCi/mmol). Copyright © 2005 John Wiley & Sons, Ltd.