Abstract. M r = 448.94, monoclinic, P21, a = 9.207 (1), b= 10.503 (2), c= 22.208 (3),/k, fl= 101.4 (1) °, V = 2105.2 (9) •3, Z = 4, D x = 1.416 gcm -3, 2(Cu Ka) = 1-5418 A, ~t = 9.3 cm -1, F(000) = 942, T= 293 K. Final R =0.053 for 4132 observed intensities. Both independent molecules in the asymmetric unit are zwitterions. One molecule has an extended conformation with the base anti and the sugar C(2') endo (2E); the other molecule has a closed conformation with the base syn and the sugar C(3') endo (3E).