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Malaria is a serious infectious disease caused by protozoan parasites in tropical and subtropical regions. Even inhabitants of temperate zones are exposed to the danger of malaria infection because of travel and global warming. Novel, effective, safe, and inexpensive drugs are required to treat malaria and contribute to the global goal of eradication. A search for new antimalarial agents has been performed by the synthesis of new benzo[a]phenoxazines, followed by biological evaluations. The derivative SSJ-183 (5), having a 4-aminopyridine group, showed an IC 50 value against Plasmodium falciparum of 7.6 nM and a selectivity index of >7300. Cure was achieved by three oral doses of 5 at 100 mg/kg to mice infected with the Plasmodium berghei ANKA strain. The safety of 5 was supported by acute toxicity testing in mice with single doses up to 2000 mg/kg po, chromosome aberration test, in vitro as well as in vivo micronucleus tests, and phototoxicity studies in mice. Thus, 5 is a promising candidate as a new antimalarial agent.

A productive synthesis of benzo[a]phenoxazine derivative SSJ-183 (1), a promising lead for antimalarial agents, was developed using a one pot procedure. Furthermore, N-deethylated metabolite 3 and bis-N,N-deethylated metabolite 4 were synthesized by the application of the method. The metabolites 3 and 4 showed comparable and ∼2-fold increased activities against drug-sensitive and drug-resistant Plasmodium falciparum parasites.

Anti-Leishmania in vitro and in vivo activities of various rhodacyanine derivatives have been examined. Among them, the fluorinatied variant SJL-01 (8) showed IC(50) of 0.011 microM against Leishmania donovani strain MHOM/ET/67/L82 (selective index of >15000) and 95-97% inhibition against L. donovani strain MHOM/ET/67/HU in female BALB/c mice by 1.3-12.5 mg/kg x 5 iv administrations. Negative results on chromosomal aberration test and in vitro micronucleus test suggest that compound 8 is a hopeful candidate for visceral leishmaniasis (VL).

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N-Heterocyclic carbenes (NHCs) are strong s-donating species, showing coordination properties similar to those of phosphines.1,2) Initially, the widespread use of catalysts containing carbene ligands was limited due to their relatively difficult preparation. Since the discovery of stable carbenes by Arduengo 3) in 1991, interest in the use of N-heterocyclic carbene-metal complexes have increased, and it has been demonstrated that they are efficient catalysts in important chemical transformations such as Pd-catalyzed coupling reactions, 4) Ru-catalyzed olefin metathesis, 5) and Rh-catalyzed hydrosilylations. [6][7][8] NHCs have also attracted great attention as organocatalysts in several reactions (e.g., benzoin condensation, [9][10][11][12][13][14] Stetter reaction, [15][16][17] transesterification/acylation reactions, 18,19) and nucleophilic substitution reactions). [20][21][22] Recently, our research group as well as other groups have reported the cyanosilylation of aldehydes catalyzed by NHCs. [23][24][25][26] In the course of our studies on the use of NHCs in organic synthesis, we have found that copper-NHC complexes exhibit the ligand acceleration effect (LAE) in the addition of diethylzinc to N-sulfonylimines. The LAE of copper-NHC complexes was first reported in the addition of diethylzinc to cyclohexenone, 27) and later the asymmetric version of these conjugate addition reactions was accomplished by using chiral NHC ligands. 28,29) In this study, we report our results on the addition of diethylzinc to N-sulfonylimines by using catalytic amounts of copper-NHC complexes. Results and DiscussionIn order to examine the catalytic abilities of the copper-NHC complexes, an addition reaction of diethylzinc to N-(benzylidene)-p-methylbenzenesulfonamide (N-sulfonylimine) 1a 30) was carried out in the presence of 5 mol% of imidazolium salt 2a as a ligand source, 5 mol% of CuI, and 5 mol% of t-BuOK in toluene (Table 1). After reacting at room temperature for 1.5 h, the addition product 3a (50%) and the reduction product 4a (28%) were obtained (Table 1, entry 1). The formation of 4a could be due to the transfer of the b-hydrogen of the ethyl functionality to the CϭN bond of the N-sulfonylimine 1a. In the absence of 2a, the reaction afforded the desired product 3a in only 15% yield and afforded 4a as the major product (entry 2). The reaction without CuI at room temperature for 6 h afforded 3a and 4a in 31% and 22% yields, respectively (entry 3). These results indicate that the LAE of NHC is involved in the addition reaction. To increase the yield of 3a and to suppress the byproduct formation, the reaction was carried out at 0°C. It was observed that though the yield of 3a was increased to 65%, byproduct 4a (12%) was still obtained. Further lowering of the reaction temperature to Ϫ5°C led to the exclusive formation of 3a (entry 5).At Ϫ5°C, the addition reactions of diethylzinc to 1a were carried out by using imidazolium and imidazolinium salts 2b-d as ligand precursors, and the addition products 3a were isolated in go...

973The synthesis, isolation, and characterization of stable Nheterocyclic carbenes (NHCs) first reported by Arduengo et al. 1) in 1991, have attracted attention for the use of NHCs as ancillary ligands for transition metal complexes. Due to their strong s-donating properties, NHCs can form metal complexes that have high stabilities toward heat, moisture, and air, and they have higher catalytic abilities than their phosphine counterparts. [2][3][4][5][6] NHCs are now used as ligands for transition metals in many important chemical transformations such as Pd-catalyzed coupling reactions, 7) Ru-catalyzed olefin metathesis, 8) Rh-catalyzed hydrosilylations, [9][10][11] and Cu-catalyzed conjugate addition reactions. [12][13][14] Moreover, NHCs have attracted considerable attention as organocatalysts in several reactions such as benzoin condensation, [15][16][17][18][19][20] Stetter reaction, [21][22][23] transesterification/acylation reactions, 24,25) and nucleophilic substitution reactions. [26][27][28] In our study on the use of NHCs as ligands in organic transformations, we have recently reported the addition of diethylzinc to N-sulfonylarylimines catalyzed by 29) where NHCs exhibit a strong ligand acceleration effect (LAE). This finding has prompted us to investigate the applicability of a similar methodology to the Rh-NHC complex-catalyzed addition of phenylboronic acid to imines.The addition reaction of organometallic reagents to imines is one of the most efficient procedures for the synthesis of diarylmethylamines, which are important subunits of biologically significant compounds. [30][31][32][33] Despite many studies on the Rh-phosphine complex-catalyzed addition of arylboronic acids to imines, 34,35) there is only one study, i.e., that by Charette and coworkers, in which the Rh-NHC complex is used.36) Here, we report our results on the addition of arylboronic acid to both N-sulfonylarylimines and N-phosphinoylarylimines by using a catalytic amount of Rh-NHC complexes. Results and DiscussionIn the study by Charette and coworkers, the Rh-NHC complex was prepared in situ using the NHC-transfer reagent Ag-NHC and used as a catalyst in the arylation of pmethylphenyl-N-phosphinoylimine with phenylboronic acid. 36) They showed only one reaction example. We examined the catalytic ability of the Rh-NHC complex generated in situ 37) from azolium salts and [RhCl(cod)] 2 for the arylation of imines. The Rh-NHC complex was prepared from [RhCl(cod)] 2 , 1,3-diadamantylimidazolium chloride 2a, and t-BuOK; then, N-sulfonylphenylimine 1a 38) and phenylboronic acid were added to the mixture. The reaction in dioxane at 70°C for 5 h afforded N-sulfonyldiarylmethylamine 3a in 91% yield (Table 1, entry 1). The reaction without 2a in dioxane afforded the desired product 3a in only 62% yield after 10 h at 70°C (entry 2). These results showed and indicated that the NHC ligand accelerated the addition reaction. The reaction in THF afforded 3a in 48% yield, whereas in toluene, the reaction was very sluggish and gave no adducts (Ta...