As part of an ongoing investigation into a possible linked origin of RNA and coded peptides, we have (further) investigated the potentially prebiotic synthesis of pyrimidine ribonucleosides (ribonucleotides), by stepwise nucleobase assembly on sugar (sugar phosphate) scaffolds. In this paper, we complete our description of this assembly process on aldopentose scaffolds. Treatment of D-ribose (14) and D-xylose (20) with cyanamide smoothly produces the furanosylamino-oxazolines 15 and 21, respectively (Schemes 1 and 3). Similar treatment of D-lyxose (26) also produces the furanosylamino-oxazoline 29, but as a minor component of an equilibrating mixture in which the pyranosylamino-oxazoline 28 predominates (Scheme 4). Treatment of the various amino-oxazolines with cyanoacetylene gives furanosylcytidines 16, 25, and 31, and the lyxopyranosylcytidine 30 (Schemes 1-4). Minor by-products of the cyanoacetylene reaction include imino lactone 17, acetals 19, 22, and 24, and cyanovinyl adducts 22 and 23 resulting from initial addition of nucleoside OH groups to the terminal acetylenic C-atom of cyanoacetylene (Schemes 2-4).