The stereoselective synthesis of and unsaturated Wittig products derivated from 2,3--isopropylidene-5--trityl--ribofuranose (3) and 2,3:5,6-di--isopropylidene--mannofuranose (4) was described, as well as their stereo-specific cyclization induced by iodine, to C-glycosides. The absolute configurations at the anomeric center and at the C-α of the aglycone rest, were established by chemical and spectroscopic