Reaction of tributylstannyl, radical generated in situ with AIBN, with (2',5'-di-O-tert-butyldimethylsilyl-3'-C-ethynyl-beta-D-ribo-furanosyl)adenine (5) gave in 690% yield the new conformationally locked nucleosides 6 (EIZ). (C) 2004 Elsevier Ltd. All rights reserved.