NUCLEOSIDES AND NUCLEOTIDES .151. CONVERSION OF (Z)-2'-(CYANOMETHYLENE)-2'-DEOXYURIDINES INTO THEIR (E)-ISOMERS VIA ADDITION OF THIOPHENOL TO THE CYANOMETHYLENE MOIETY FOLLOWED BY OXIDATIVE SYN-ELIMINATION REACTIONS

摘要

The Wittig reaction of 1-[3,5-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-beta-D-erythro-pentof uranos-2-ulosyl]uracil (6) with Ph(3)P=CHCN afforded (Z)-2'-cyanomethylene derivative 7 exclusively. The (E)-isomer was accessed from its (Z)-isomer through a sequence of addition of thiophenol to the 2'-cyanomethylene moiety of the (Z)-isomer from the alpha-face, selectively, and stereoselective oxidative syn-elimination of the resulting adduct. The diastereofacial selectivity of the benzenethiolate addition to the cyanomethylene moiety was found to be influenced by participation of the 2-carbonyl group at the base moiety and steric hindrance of the sugar protecting groups. Although nucleophilic addition reactions at the 2'-position of 6 have been well-known to occur from the alpha-face selectively, treatment of 7 with LiSPh in THF unexpectedly afforded a mixture of alpha- and beta-phenylthio derivatives 8 and 9 in almost equal ratio. Furthermore, an unusual beta-facial selective addition was observed on treatments of 7 with PhSAlMe(2) in CH2Cl2 or wit LiSPh in the presence of Mg(ClO4)(2) in THF. On the other hand, when (Z)-2'-(cyanomethylene)-5'-O-triisopropylsilyl derivative 10 was treated with LiSPh, the alpha-phenylthio derivative 13 was obtained predominantly (89:11). Oxidation of 8 with m-chloroperbenzoic acid (m-CPBA) in CH2Cl2 and pyrolysis of the resulting sulfoxides afforded the (Z)-isomer 7 exclusively. Treatment of 13 with m-CPBA under the same conditions afforded the desired (E)-cyanomethylene derivatives 18 as a major product (E:Z = 14:1) in good yield. Deprotection of 18 by the standard procedures furnished (E)-2'-(cyanomethylene)-2'-deoxyuridine (5). [References: 47]