NUCLEOSIDES AND NUCLEOTIDES .163. SYNTHESIS OF 3'-BETA-BRANCHED URIDINE DERIVATIVES VIA INTRAMOLECULAR REFORMATSKY-TYPE REACTION PROMOTED BY SAMARIUM DIIODIDE

A novel efficient method for the synthesis of 3'-beta-branched uridine s starting from uridine was developed, in which a SmI2-promoted intram olecular Reformatsky-type reaction was effectively used. 5'-O-(Bromoac etyl)-3'-ketouridine derivatives 12, 26, and 27 were synthesized from uridine and were subjected to an intramolecular Reformatsky-type react ion. When 12, 26, and 27 were treated with 2.0 equiv of SmI2 in THF at -78 degrees C, intramolecular carbon-carbon bond formation at the 3'- beta-position proceeded smoothly to give the corresponding 3',5'-lacto nes 14, 28, and 29 in high yields, respectively. Treatment of 28 with NH3/MeOH gave the 3'-beta-branched uridine derivative 32 quantitativel y, which was then deprotected to give 3'-C-(carbamoylmethyl)uridine (3 3).