Synthesis of 6-substituted uridines. synthesis of (R or S)-6-(3-amino-2-carboxypropyl)uridine

Addition of 5-bromo-2′,3′-O-isopropylidene-5′-O-trityluridine (2) in pyridine to an excess of 2-lithio-1,3-dithiane (3) in oxolane at 78° gave (6R)-5,6-dihydro-(1,3-dithian-2-yl)-2′,3′-O-isopropylidene -5′-O-trityluridine (4), (5S,6S)-5-bromo-5,6-dihydro-(1,3-dithian-2-yl)-2′,3′-O-isopropylidene-5′-O-trityluridine (5), and its (5R) isomer 6 in yields of 37, 35, and 10%, respectively. The structure of 4 was proved by Raney nickel desulphurization to (6S)-5,6-dihydro-2′,3′-O-isopropylidene-6-methyl-5′-O-trityluridine (7) and by acid hydrolysis to give D-ribose and (6R)-5,6-dihydro-6-(1,3-dithian-2-yl)uracil (9). Treatment of 4 with methyl iodide in aqueous acetone gave a 30&%; yield of (R,S)-5,6-dihydro-6-formyl-2′,3′-O-isopropylidene-5′-O-trityl-uridine (10), characterized as its semicarbazone 11. Both 5 and 6 gave 4 upon brief treatment with Raney nickel. Both 5 and 6 also gave 6-formyl-2′,3′-O-isopropylidene-5′- O-trityluridine (12) in ～41%; yield when treated with methyl iodide in aqueous acetone containin- 10%; dimethyl sulfoxide. A by-product, identified as the N-methyl derivative (13) of 12 was also formed in yields which varied with the amount of dimethyl sulfoxide used. Reduction of 12 with sodium borohydride, followed by deprotection, afforded 6-(hydroxymethyl)uridine (17), characterized by hydrolysis to the known 6-(hydroxymethyl)uracil (18). Knoevenagel condensation of a mixture of the aldehydes 12 and 13 with ethyl cyanoacetate yielded 38%; of E- (or Z-)6-[(2-cyano-2-ethoxycarbonyl)ethylidene]-2′,3′-O-isopropylidene-5′-O-trityluridine (19) and 10%; of its N-methyl derivative 20. Hydrogenation of 19 over platinum oxide in acetic anhydride followed by deprotection gave R (or S)-6-(3-amino-2-carboxypropyl)uridine (23).