Structure-Activity Relationships of 2′-Deoxy-2′,2′-difluoro-L-erythro-pentofuranosyl Nucleosides Lakshmi P. Kotra, † Yuejun Xiang, † M. Gary Newton, ‡ Raymond F. Schinazi, § Yung-C. Cheng, | and Chung K. Chu* ,† Department of Medicinal Chemistry, College of Pharmacy, and Department of Chemistry, The University of Georgia, Athens, Georgia 30602-2352, Georgia Research Center for AIDS and HIV Infections, Veterans Affairs Medical Center, and Department of Pediatrics, Emory University School of Medicine, Decatur, Georgia 30033, and Department of Pharmacology, School of Medicine, Yale University, New Haven, Connecticut 06510 Received April 28, 1997 X Following the recent discoveries that some L-nucleosides are more or equal potent than their D-counterparts, we synthesized 2′-deoxy-2′,2′-difluoro-L-erythro-pentofuranosyl nucleosides as potential antiviral agents. The target compounds were synthesized via the key intermediates 7a or 7b from L-gulono γ-lactone. Compound 2 was oxidatively cleaved and coupled with ethyl bromodifluoroacetate in the presence of activated zinc under Reformatsky conditions to obtain a diastereomeric mixture of 4(R) and 4(S), in a 4:1 ratio. The major 4(R) isomer was cyclized and treated appropriately to obtain the mesylate 8a or 8b, which was condensed with various silyl-protected pyrimidines. Condensation of the alcohol 7a or 7b with 6-chloropurine under Mitsunobu conditions afforded the 6-chloropurine analogs 53a or 53b and 54a or 54b. Further treatment of the compounds 53a, 54a and 53b, 54b afforded the inosine and adenine derivatives 57-60, respectively. The condensation of 2-amino-6-chloropurine with compound 8a and subsequent treatment with 2-mercaptoethanol/sodium methoxide afforded the guanine analogs 63 and 64. All of the synthesized nucleosides 31-52, 57-60, 63, and 64 were evaluated for antiviral activity and for cellular toxicity. Adenine derivative 57 showed a moderate activity against HIV-1 in PBM cells (3.4 µM). None of the other compounds showed any significant activities against HIV-1, HBV, HSV-1, HSV-2, and toxicity in Vero, CEM, and PBM cell lines up to 100 µM. The X-ray structure of the 5-iodocytosine analog showed a 2′-exo/3′-endo conformation for the carbohydrate moiety, which is different from those of the biologically active compounds (-)-FTC and L-FMAU. Introduction Nucleoside analogs have played an important role in the treatment of various cancers and viral infections, including human immunodeficiency virus (HIV) infec- tion. 1,2 However, the toxicities associated with certain nucleoside analogs 3,4 and the emergence of resistant viral strains 5,6 warrant the search for further novel and structurally diverse compounds with minimally overlap- ping resistance profile and toxicity. Most of the cur- rently available nucleosides in the clinical use viz. AZT, ddI, ddC, and d4T have the same D-configuration as those of the natural nucleosides. Recently, the synthesis and antiviral activities of a new class of nucleosides viz. oxathiolanyl and dioxolanyl nucleosides have drawn a significant attention. 7,8 Among these classes of nucleo- sides, certain L-nucleosides like 3TC (Lamivudine) 9,10 and its 5-fluorocytosine analog, (-)-FTC, 11 exhibit either equal or more potent activities compared to their D-counterparts while exhibiting less toxicity. This sparked the interest to explore the L-nucleosides as potential antiviral agents, including -L-arabinofura- nosyl and -L-2′,3′-dideoxyribofuranosyl nucleosides 12-15 and 1-(2-deoxy-2-fluoro--L-arabinofuranosyl)pyrimidine nucleosides. 16 Among these compounds, 2′-fluoro-5- methyl--L-arabinofuranosyluracil (L-FMAU, Figure 1) was synthesized in our laboratories and has shown potent antihepatitis B virus (anti-HBV) and anti-Ep- stein-Barr virus (anti-EBV) activities 17 with a favorable toxicity profile. 18 Recently, L-FMAU has demonstrated outstanding in vivo efficacy in chronically infected woodchucks with woodchuck hepatitis virus. 19 L-FMAU is currently undergoing preclinical toxicological studies. Among the other classes of nucleosides, 2′-deoxy-2′,2′- difluoro-D-nucleosides have shown various antiviral and antineoplastic activities. 20,21 2′-Deoxy-2′,2′-difluorocy- tidine (Gemcitabine, Gemzar, Figure 1) was recently approved by FDA for inoperable pancreatic cancer and for 5-fluorouracil resistant pancreatic cancer. Gemcit- abine showed an interesting mechanism of action by inhibiting the DNA and RNA synthesis and/or by inhibiting ribonucleotide reductase. 22 Gemcitabine also showed activity against ovarian, small cell lung, breast cancer, and colon cancer after being activated to its triphosphate form. 23 The guanine derivative 2′-deoxy- 2′,2′-difluoroguanosine also exhibited a similar mecha- nism of action in the inhibition of DNA synthesis in Chinese hamster ovary cell line with a promising antimetabolite characteristics. 24 As part of our efforts to develop novel antiviral agents, recently we reported the preliminary accounts of the synthesis and anti-HIV activities of 2′-deoxy-2′,2′-dif- luoro--L-erythro-pentofuranosyl nucleosides. 25 These nucleosides were designed to take advantage of the characteristics of L-nucleosides and the potential activi- * Corresponding Author: Tel: (706) 542-5379. Fax: (706) 542-5381. E-mail: dchu@rx.uga.edu. † Department of Medicinal Chemistry, College of Pharmacy, The University of Georgia. ‡ Department of Chemistry, The University of Georgia. § Veterans Affairs Medical Center and Emory University School of Medicine. | Yale University. X Abstract published in Advance ACS Abstracts, October 1, 1997. 3635 J. Med. Chem. 1997, 40, 3635-3644 S0022-2623(97)00275-6 CCC: $14.00 © 1997 American Chemical Society