. FOX. Can. J . Chem. 58, 1633 (1980). The three-dimensional structure of pseudoisocytidine hydrochloride was determined by X-ray crystallography. The crystals belong to the triclinic space group P I and the cell dimensions are a = 6.623(2), b = 8.053(2), c = 6.201(2) A, a = 108.35(2), = 101.36(2), y = 93.54(2)". Intensity data were measured with a diffractometer and the structure was solved by a combination of heavy-atom and direct methods. Least-squares refinement, which included hydrogen atoms, converged at R = 0.040. The conformation about the glycosyl bond is trrltr (xc( = 21.6"). the pucker of the furanose ring is C(l')eso, and the conformation of the -CH,OH side chain is gauche-trans (t). An examination of bond lengths indicates that of the three main resonance forms of the isocytosine cation the fully conjugated one contributes more to the structure than the cross-conjugated one. Bond angles in the sugar ring reflect its rare conformation. .6"), le plissement du cycle furannose est C(1')exo et la conformation de la chaine laterale CH,OH est gcrrlcl~e-trnns (t). Un examen des longueurs de liaison indique que parmi les trois formes principales de resonance du cation isocytosine, la forme completement conjuguee contribue plus a la structure que la forme ou la conjugaison est opposee. Les angles de liaison dans le cycle furannose refletent sa conformation rare.[Traduit par le journal]