Synthesis of (Carbo)nucleoside Analogues via [3+2] Annulation of Aminocyclopropanes**

(Carbo)nucleoside derivatives constitute an important class of pharmaceuticals, yet there are only few convergent methods to access new analogues. In this communication, we report the first synthesis of thymine, uracil and 5-fluorouracil substituted diester donor-acceptor cyclopropanes and their use in the indiumand tin-catalyzed [3+2] annulations with aldehydes, ketones and enol ethers. The obtained diester products could be easily decarboxylated and reduced to the corresponding alcohols. The method gives access to a broad range of new (carbo)nucleoside analogues in only four-five steps and will be highly useful for the synthesis of libraries of bioactive compounds. The natural nucleosides constitute the building blocks of DNA and RNA. The interaction of enzymes and other biomolecules with nucleosides is essential for the regulation of genetic expression and cell replication. Therefore, the nucleoside scaffold constitutes a privileged structure in medicinal chemistry (Figure 1). In addition to bioactive natural products, such as the antiviral and antibiotic aristeromycin (1), more than 45 FDA approved drugs are nucleoside analogues. Besides only slightly modified analogues, such as cytarabine (2) and telbivudine (3), more elaborated compounds derived from thymine have been successful, such as the carbonucleoside stavudine (4), the anti-HIV front drug azidothymidine (5) or the fluorinated floxuridine (6). Nevertheless, resistances are emerging in viral infections, and less toxic anticancer agents would be highly desirable, asking for the development of new bioactive nucleoside analogues. The synthesis of nucleoside analogues has been the focus of intensive effort since several decades. Nevertheless, most methods are based on a linear approach involving first the synthesis of a ribose analogue followed by introduction of the nucleobase, either via formation of the C-N bond using a substitution reaction from an acetate I (Vorbrüggen reaction) or a condensation reaction from an aminoglycoside II (Scheme 1, A). This approach is efficient if the targeted analogue is similar to a natural ribose derivative, but can involve a long multi-step sequence if a more elaborate scaffold is desired. This is particularly true for carbonucleoside analogues, for which elegant synthetic approaches involving ring-closing metathesis, Pauson-Khand or desymmetrization starting from cyclopentadiene and proceeding via diols, [3c-e] Vince’s lactam or nitroso cycloaddition reactions have been developed. Figure 1. Natural and synthetic bioactive nucleoside analogues. Our group has introduced the use of imide-substituted diester cyclopropanes in [3+2] annulation reactions. With this new class of donor-acceptor cyclopropanes, access to intermediates of type II became possible (Scheme 2, B). Nevertheless, the efficiency of the annulation process was mitigated by the necessary removal of the phthalimide group followed by DNA-base construction, which would add several steps to the synthetic sequence. Furthermore, the deprotection of the pththalimide group could not be achieved on the tetrahydrofurylamines. If a DNA-base could be used as amino substituent on the cyclopropane, a more efficient synthesis would become possible (Scheme 1, C). Herein, we would like to report the successful [] Sophie Racine, F. de Nanteuil, Eloisa Serrano and Prof. Dr. J. Waser Laboratory of Catalysis and Organic Synthesis Ecole Polytechnique Fédérale de Lausanne EPFL SB ISIC LCSO, BCH 4306, 1015 Lausanne (CH) Fax: (+)41 21 693 97 00 E-mail: [email protected] Homepage: http://lcso.epfl.ch/ [] EPFL, F. Hoffmann-La Roche Ltd, SNF (grant number 200021_129874) and the NCCR chemical biology are acknowledged for financial support. Dr. Rosario Scopelliti (EPFL) is acknowledged for the X-ray studies. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201xxxxxx