This; is the first report on the Ferrier rearrangement of 2-halo-glycals with; S-nucleophiles. We present herein the selective synthesis of new; 2-iodo-2,3-unsaturated O- and S-glycosides. We obtained in good yields and high; anomeric selectivity α-glycosydes. As a particular behaviour, we also; describe heteroaromatic thio sugar derivatives were the C3 addition was; performed in an alternative mechanism. A complete structure and conformation analysis; by NMR was also presented.Centro de Estudios de Compuestos OrgánicosCentro de Investigación y Desarrollo en Ciencias Aplicadas