Nucleosides. 100. General synthesis of pyrimidine C-5 nucleosides 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)