Nucleosides. 100. General synthesis of pyrimidine C-5 cucleosides related to pseudouridine. Synthesis of 5-(beta-D-ribofuranosyl) isocytosine (pseudoisocytidine),5-(beta-D-ribofuranosyl)-2-thiouracil(2-thiopseudouridine) and 5-(beta-D-ribofuranosyl)uracil(pseudouridine).