Synthesis of nucleoside analogs and new Tat protein inhibitors*

Two studies, concerning the synthesis of original nucleoside analogs regarded as an application of heterochemistry on thiaazaheterocycle systems from thiaazabutadienes are discussed. The preparation of new Nand C-nucleosides is presented. In the second part, the discovery of aromatic polycyclic derivatives as inhibitors of Tat protein is exposed. The work presented takes into account the participation of African partners in further synthetic research programs carried out in collaboration with the laboratory of Nantes. The HIV epidemic still requires considerable efforts to be contained, especially in Africa. Nucleoside analogs of reverse transcriptase (NARTIs) [1] occupied an essential position in the treatment of HIV infection and AIDS [2]. Even with the recent evolution of nonnucleoside analogs (NNRTIs) and the favored protease inhibitors, NARTIs still have a major role to play in combination therapy. There are different NARTIs currently available: Zidovudine (AZT, Retrovir), Didanosine (ddI, Videx), Zalcitabine (ddC, Hivid), Stavudine (d4T, Zerit), Lamivudine (3TC, Epivir), Adefovir (bis-POM PMEA), Abacavir (1592U89), FLT, and Carbovir. However, none of them are without problems of either viral resistance, tolerability, or ease of administration. Actually, the most popular two-drug combination cited as a first-line therapy was Zidovudine plus Lamivudine. Recent study of Stavudine with Didanosine suggest that this combinaition may also find favor in the clinic due to minimal dose required and lower cost. NNRTIs are a structurally diverse group of compounds that are potent inhibitors at nanomolar concentration. They do not require intracellular metabolism to become active and lack the hematological toxicity associated with the NARTIs. NNRTIs such as Nevirapine (Viramune), TIPO, HEPT, E-BPU, MKC-442 and Delavirdine (Rescriptor, U-90152), Atevirdine (U-87201), α-APA (R89439), TSAO, and Efavirenz (Sustina), are promising partners for polytherapy. Actually, Neviparine, Delavirdine, and Efavirenz are prone to be used in combination with nucleoside analogs due to limiting development of resistance and despite relatively mild side effects such as skin rash, elevated transaminase levels, and gastrointestinal disturbances. There are a few inhibitors on the market that target the HIV protease enzyme (pol protease), which stimulated the maturation. Protease inhibitors are mostly small peptide analogs: L’Indinavir *Lecture presented at the 8th International Chemistry Conference in Africa (8th ICCA), 30 July–4 August 2001, Dakar, Sénégal. Other presentations are published in this issue, pp. 1147–1223. †Corresponding author (Crixivan), Ritonavir (Norvir), Saquinavir (Invirase), and Nelfinavir (Viracept). More recently, 141W94 (VX-478), a second-generation nonpeptidic inhibitor, has progressed through development with remarkable speed. Since all protease drugs have been approved under accelerated process, the long-term safety of these agents remains unknown. The recommended treatment option for those initiating antiretroviral therapy is now recognized as triple combination therapy involving the use of protease inhibitors associated with NARTIs and NNRTs. However, many questions remain concerning their long-term efficacy. It is thought that in the future combinations of four or five drugs may emerge. Nucleotide analogs such as GS 840 (Adenovir dipivoxil), PMPA, and Zintevir seem to be characterized by their ability to inhibit viral replication for long periods. Recently, antisens technology, targeted at messenger RNA (mRNA) of conserved region of HIV genome, is regarded due to the advantage in avoiding the problem of resistance. The progress in the understanding of the viral events has aroused other promising targets to interact with the viral infection. Among the most advance research, we can cite: HIV integrase inhibitors, chemokine receptor inhibition, glycosylation inhibitors, ribozyme activators, and, more recently, immunotherapies. Another alternative concerning the inhibition of Tat or Rev proteins is actually under accelerated investigation. We present here our investigation on two of the above topics: • the synthesis of new Nand C-nucleoside analogs of reverse transcriptase and • the synthesis of new active triphenylene inhibitors of Tat protein. Our present purpose is mainly to illustrate the participation of African partners in the development of synthetic programs, in particular with the laboratories of organic synthesis of Abidjan (Ivory Coast) and Dakar (Sénégal). SYNTHESIS OF NUCLEOSIDE ANALOGS On the basis of the above considerations, original nucleoside analogs directed upon reverse transcriptase still aroused considerable interest [1]. In the hope of elucidating and/or finding better therapeutic agents, 3'-modified deoxynucleoside analogs seem to be one of the recommended targets [2]. They either act as enzyme inhibitors or as chain terminators of viral DNA-polymerization due to the lack or alteration of 3'-hydroxyl group. Modified Nor C-nucleosides tethered to original heterocycle at the anomeric sugar moiety, have also retained attention, and they have elicited a new class of heterocyle nucleoside analogs, which mostly behave like nonnucleoside RT substrates [3]. With respect to the previous studies carried out in our laboratory, N-thioacylamidine (1-thia-3aza-1,3-butadienes) and thiocarbamate precursors [4] have been regarded as promising intermediates to produce fiveand six-membered thiaazaheterocycles with some structural analogies with natural nucleobases. To illustrate our proposal, the synthesis of ribosides tethered to thiazolinone and thiazinone heterocycles, at 1'and 3'-positions is presented (Fig. 1). In continuation of this program, the access to new A. ANÉ et al. © 2001 IUPAC, Pure and Applied Chemistry 73, 1189–1196 1190