Chiral Synthesis of 4-[1-(2-Deoxy--L-ribofuranosyl)] Derivatives of 2-Substituted 5-Fluoroaniline: “Cytosine Replacement” Analogues of Deoxy--L-cytidine Zhi-Xian Wang, † Leonard I. Wiebe, † Jan Balzarini, ‡ Erik De Clercq, ‡ and Edward E. Knaus* ,† Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2N8, and Rega Institute for Medical Research, Minderbroedersstraat 10, Leuven B-3000, Belgium Received April 4, 2000 Introduction L-Thymidine (L-TdR), a substrate for herpes simplex virus type 1 thymidine kinase (HSV-1 TK), reduces HSV-1 multiplication in HeLa cells. HSV-1 TK phosphor- ylates the L and D enantiomers of TdR to their corre- sponding monophosphates (MPs) with identical efficacy. 1 Similar results have been observed for the L-TdR ana- logues 5-iodo-2′-deoxyuridine (L-IUdR) and (E)-5-(2-bro- movinyl)-2′-deoxyuridine (L-BVUdR), whose D enanti- omers are potent, but cytotoxic, antiherpetic drugs. The approximately 1000-fold lower cytotoxicity of L-IUdR and L-BVUdR, relative to the D enantiomers, is due to the fact that L-IUdR lacks affinity for cellular TK and that L-IUdRMP and L-BVUdRMP, in contrast to their D enantiomers, do not inhibit thymidylate synthase (TS). 2 Thus, the viral TK enzyme, but not human cytosolic TK, lacks enantioselectivity for natural -D- and unnatural -L-nucleosides. Consequently, L-nucleosides have at- tracted the attention of medicinal chemists due to their unique potency, mechanism of action, and toxicity pro- file. 3 Some representative L-cytidine analogues such as 2′,3′-dideoxy-3′-thia--L-cytidine (3TC, Lamivudine), 4,5 2′,3′-dideoxy-3′-thia--L-5-fluorocytidine (L-FTC), 6 and 2′,3′-dideoxy--L-5-fluorocytidine (L-FddC) 7,8 have shown promising antiviral activity. 3TC and L-FTC exhibit more potent antiviral activity against human immunodefi- ciency virus (HIV) and hepatitis B virus (HBV), and lower toxicity, in comparison to the D enantiomers. 5,9 Nonpolar hydrophobic isosteres of -D-pyrimidine nu- cleosides which retain close structural, steric, and iso- electronic relationships to the natural base, which are not likely to form hydrogen bonds, have been reported by Kool et al. 10 In this regard, the 2,4-difluoro-5-meth- ylphenyl isostere (-D-1) was designed as an unnatural mimic of thymidine (-D-2). Furthermore, the 5′-tri- phosphate of -D-1 (-D-1-TP) was selectively inserted opposite adenine (A) into replicating DNA strands by the Klenow fragment (KF, exo - mutant) of Escherischia coli DNA polymerase 1 with an efficacy (V max /K m ) only 40- fold lower than that for -D-2-TP. 11 These results indi- cated that the 2,4-difluoro-5-methylphenyl moiety of -D-1 is isoelectronic with the thymine base which it replaces and is utilized by KF polymerase. 12-14 It was envisaged that the structurally related 4-(2-substituted 5-fluoroaniline)--L-nucleoside mimics (13a,b), which are hybrids of the C-aryl (-D-1) and deoxycytidine nucleo- sides (-L-3a,b), may have interesting biological activ- ity. 1,2 We now report the synthesis of the 4-[1-(2-deoxy- -L-ribofuranosyl)] derivatives of 2-substituted-5-fluoro- anilines (-L-13a,b), which were designed as unnatural C-aryl 2′-deoxy--L-cytidine mimics. Results and Discussion The Heck-type coupling reaction constitutes a simple, yet direct, method to form a C-C bond between a suitably protected glycal and an activated iodo- or trifluoro- methanesulfonate-substituted aryl (heteroaryl) reagent, to prepare the -anomer of nucleosides in reasonable yields. 15 In this study, the Heck coupling reaction is a key method for the synthesis of unnatural deoxy--L- cytidine mimics, as illustrated in Scheme 1. * To whom correspondence should be addressed. Tel: (780) 492- 5993. Fax: (780) 492-1217. E-mail: eknaus@pharmacy.ualberta.ca. † University of Alberta. ‡ Rega Institute for Medical Research. (1) Spadari, S.; Maga, G.; Focher, F.; Ciarrocchi, G.; Manservigi, R.; Arcamone, F.; Capobianco, M.; Carcuro, A.; Colonna, F. Iotti, S.; Garbesi, A. J. Med. Chem. 1992, 35, 4214-4220. (2) Spadari, S.; Ciarrocchi, G.; Focher, F.; Verri, A.; Maga, G.; Arcamone, F.; Iafrate, E.; Manzini, S.; Garbesi, A.; Tondelli, L. Mol. Pharmacol. 1995, 47, 1231-1238. 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