Treatment of 1-[(1'R,2'S,3'S,4'S)-4'-methanesulfonyl-2',3'-O- isopropylidenedioxycyclopentan-1'-yl]-1-H-uracil with LiN3 gave 1-[(1'R,2'S,3'R)-2',3'-O-isopropylidenedioxy-4'- cyclopenten-1'-yl]-1-H-uracil. When KOBut was added instead of LiN3, 1-[(1'R,2'S)-2',3'-O- isopropylidenedioxy-3'-cyclopenten-1'-yl]-1-H-uracil was produced. An analogous treatment of the methanesulfonyl derivative of thymine was also carried out.