An Efficient Asymmetric Approach to Carbocyclic Nucleosides: Asymmetric Synthesis of 1592U89, a Potent Inhibitor of HIV Reverse Transcriptase.

Carbocyclic nucleosides have been the focus of much recent attention in the development of new antitumor and antiviral therapeutic agents.1 The search for antiviral agents, particularly for the treatment of human immunodeficiency virus (HIV) and hepatitis B virus (HBV), resulted in the discovery of carbovir (1), which has been shown to possess significant in vitro activity as an inhibitor of HIV reverse transcriptase (Scheme 1).2,3 More recently, a new reverse transcriptase inhibitor 1592U89 (2), which is currently in phase II clinical trials, has been discovered and reported to hold remarkable promise for the treatment of HIV.4 Reduction of viral load of greater than 99% as well as significant improvements in CD4 counts for HIV-infected patients have been observed after dosing for 12 weeks with 1592U89 (2).5 Continuous improvement in the enantioselective syntheses of carbocyclic nucleosides is required due to their therapeutic significance. An efficient and general approach to the asymmetric synthesis of carbocyclic nucleosides such as carbovir (1) and 1592U89 (2) is described here. An attractive convergent approach for the enantioselective synthesis of carbocyclic nucleosides involves Trost’s6 palladium-catalyzed coupling of purine or pyrimidine bases with carbocyclic allylic carbonates or acetates.7 This strategy requires ready access to enantiomerically pure 5-(hydroxymethyl)-2-cyclopenten-1-ol (3) or the isomeric 4-(hydroxymethyl)-2-cyclopenten-1-ol (4), each of which gives access to the same π-allyl palladium intermediate for coupling to the nucleoside base. Our approach to the synthesis of 3 relies on the realization that combination of an asymmetric aldol condensation8 with a ring closure metathesis reaction9 can provide rapid entry into functionalized, enantiomerically pure carbocycles. Condensation of the lithiated (S)-4-benzyl-2-oxazolidinone with the pentenoic pivalic mixed anhydride 5 provided the pentenoyloxazolidinone 610 in near-quantitative yield (Scheme 2). Use of the Evans’ dialkylboron triflate protocol8a for diastereoselective syn aldol condensation with acrolein produced the aldol product 710 in 82% yield [(>99% de [R]D +50.6° (c ) 0.89, CHCl3)]. The critical ring closure metathesis was accomplished in 97% yield by exposure of a dichloromethane solution of diene 7 to 1% of the Grubbs9a catalyst for 30 min to form the cyclopentenol 810 [[R]D -92.5° (c ) 0.795, CHCl3)]. The chiral auxiliary was reductively removed with lithium borohydride11 to provide the required diol 3 in 78% yield [[R]D -125.1° (c ) 0.47, CHCl3), >99.6% ee by chiral HPLC of the dip-toluate]. Diol 3 was converted to cyclic carbonate 9 (52%),3b dicarbonate 10 (90%),3a,f,k and diacetate 1110 (90%) for evaluation of the palladium-catalyzed coupling with 2-amino-6-chloropurine (12) and 2-amino-6-(cyclopropylamino)purine (13). Reaction of diacetate 11 with 2-amino6-chloropurine (12) in the presence of tetrakis(triphenylphosphine)palladium(0) and sodium hydride gave an 86:14 mixture of the carbocyclic nucleoside 14a10 and the corresponding N7 coupling product 15a (65% yield of 14a after chromatography) (Scheme 3).12 The problem of N9N7 regioselectivity is a common problem in classic Vorbruggen coupling of purines with sugars,13 but has * To whom correspondence should be addressed. Phone: (919) 9665177. FAX: (919) 962-2388. E-mail: [email protected]. (1) For recent reviews on the synthesis of carbocyclic nucleosides, see: (a) Agrofoglio, L.; Suhas, E.; Farese, A.; Condom, R.; Challand, S. R.; Earl, R. A.; Guedj, R. Tetrahedron 1994, 50, 10611-10670. (b) Borthwick, A. D.; Biggadike, K. Tetrahedron 1992, 48, 571-623. Huryn, D. M.; Okabe, M. Chem. Rev. 1992, 92, 1745-1768. (2) Vince, R.; Hua, M. J. Med. Chem. 1990, 33, 17. (3) For some recent carbovir syntheses, see: (a) Berranger, T.; Langlois, Y. Tetrahedron Lett. 1995, 36, 5523-5526. (b) Hildebrand, S.; Troxler, T.; Scheffold, R. Helv. Chim. Acta 1994, 77, 1236-1240. (c) Legraverend, M.; Aubertin, A. M.; Obert, G.; Huel, C.; Bisagni, E. Nucleosides Nucleotides 1994, 13, 915-923. (d) Diaz, M.; Ibarzo, J.; Jimenez, J. M.; Ortuno, R. M. Tetrahedron: Asymmetry 1994, 5, 129140. (e) Jung, M. E.; Rhee, H. J. Org. Chem. 1994, 59, 4719-4720. (f) Nokami, J.; Matsuura, H.; Nakasima, K.; Shibata, S. Chem. Lett. 1994, 1071-1074. (g) Asami, M.; Takahashi, J.; Inoue, S. Tetrahedron: Asymmetry 1994, 5, 1649-1652. (h) MacKeith, R. A.; McCague, R.; Olivo, H. F.; Palmer, C. F.; Roberts, S. M. J. Chem. Soc., Perkin Trans. 1 1993, 313-314. (i) Jung, M. E.; Rhee, H. Tetrahedron Lett. 1993, 34, 4449-4452. (j) Evans, C. T.; Robert, S. M.; Shoberu, K. A.; Sutherland, A. G., J. Chem. Soc., Perkin Trans. 1 1992, 589-592. (k) Trost, B. M.; Li, L.; Guile, S. D. J. Am. Chem. Soc. 1992, 114, 87458747. (l) Peel, M. R.; Sternbach, D. D.; Johnson, M. R. J. Org. Chem. 1991, 56, 4990-4993. (m) Hodgson, D. M.; Witherington, J.; Moloney, B. A. J. Chem. Soc., Perkin Trans. 1 1994, 3373-3378. (4) Daluge, S. M. U.S. Patent 5,034,394, 1991. (5) (a) Saag, M. Abstracts from The Third Conference on Retroviruses and Opportunistic Infections, 1996, abstract 195. (b) Torres, G.GMHC Treatment Issues Feb 1996, 10, 8. (6) Trost, B. M.; Kuo, G.-H.; Benneche, T. J. Am. Chem. Soc. 1988, 110, 621-622. (7) Trost, B. M.; Verhoeven, T. R. Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1982; Vol. 8, p 799. (8) (a) Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127-2129. (b) Evans, D. A.; Rieger, D. L.; Bilodeau, M. T.; Urpi, F. J. Am. Chem. Soc. 1991, 113, 1047-1049. (c) Yan, T.-Y.; Tan, C.W.; Lee, H.-C.; Lo, H.-C.; Huang, T.-Y. J. Am. Chem. Soc. 1993, 115, 2613-2621. (d) Ahn, K. H.; Lee, S.; Lim, A. J. Org. Chem. 1992, 57, 5065-5066. (e) Bonner, M. P.; Thornton, E. R. J. Am. Chem. Soc. 1991, 113, 1299-1308. (f) Oppolzer, W.; Lienard, P. Tetrahedron Lett. 1993, 34, 4321-4324. (9) Schwab, P.; Grubbs, R. H.; Ziller, J. J. Am. Chem. Soc. 1996, 118, 100-110. Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem. Res. 1995, 28, 446-452. (10) All new compounds gave satisfactory combustion analyses and consistent 1H, 13C, and IR spectra. Yields are for isolated, chromatographically purified material. Yields are not fully optimized. (11) Evans, D. A.; Ng, H. P.; Rieger, D. L. J. Am. Chem. Soc. 1993, 115, 11446-11459. (12) (a) Gundersen, L.-L.; Benneche, T.; Rise, F.; Gogoll, A.; Undheim, K. Acta Chem. Scand. 1992, 46, 761-771. (b) Gosh, A.; Ritter, A. R.; Miller, M. J. J. Org. Chem. 1995, 60, 5808-5813. (13) (a) Vorbruggen, H.; Krolikiewicz, K.; Bennua, B. 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