Our new approach to the synthesis of enantiomerically enriched 1,2,4-trioxanes is shown in Scheme 1. Studies by Ito et al. The presence of the MgCl 2 is crucial for the success of this reaction. We decided to apply this methodology to the synthesis of key nitrile 2.
Separation of the two geometric isomers was easily accomplished by column chromatography, and on the basis of previous work on the racemic synthesis of 4a , the Z -isomer was taken on to complete the synthesis. With the chiral precursor in hand, photo-oxygenation was carried out using methylene blue as sensitiser. X-ray cystallography confirmed the stereochemistry of 4a as shown in Figure 1. The results of these findings will be of fundamental importance to the future design and synthesis of synthetic 1,2,4-trioxane antimalarials. We have recently completed studies on a one-pot carbonyl oxide route to enantiomerically pure antimalarial endoperoxides.
This synthetic strategy has allowed efficient total synthesis of several members of cyclotryptamine alkaloids .
Series: Advances in Asymmetric Synthesis
As indicated in Scheme 3 , the core structure 24 could be prepared from diamide 25 via reductive cyclization. Obviously, the development of asymmetric catalytic reaction to generate compound 27 is the key point of the total synthesis . Thus, an asymmetric enantioselective substitution reaction of 3-hydroxyoxindoles 28 with enamide 29 was first investigated and found that chiral phosphoric acid 15a turned out to be the best catalyst. Since the Baeyer—Villiger oxidation of 30 failed to give desired products, we turned our attention to the Beckmann rearrangement, a reliable reaction to transform ketones into amides.
Fortunately, the ketoxime 31a could be completely converted into ketoxime 31b in the presence of p -toluenesulfonic acid. The removal of the p -methoxyphenyl group by means of ceric ammonium nitrate CAN furnished the compound Subsequently, Overman et al. The indoline aldehyde 38 Overman intermediate [20b] could be easily synthesized from biindolin-1,2-diol 39 , which would be accessed by reduction of the aldehyde Thus, we developed an enantioselective alkylation reaction of 3-hydroxindoles 28 with enalizable aldehydes 41 for the preparation of the key building block 40 .
The cinchona-based primary amines have been proved to be successful in either iminium or enamine catalyzed asymmetric transformations . Inspired by these achievements, we screened several families of chiral organocatalysts for the alkylation of 3-hydroxindoles 28a with 2-alkyloxy-acetaldehyde 41 and found that the combined use of cinchona alkaloid-based primary amine 42 and chiral phosphoric acid 15 provided the highest levels of stereoselectivity .
The expansion of the reaction conditions to other 3-hydroxindole derivatives was also successful to generate alkylation products in high yields and excellent stereoselectivities Scheme 7. The indoline aldehyde 48 Overman intermediate was successfully accessed by the oxidative cleavage of the diol 47 with NaIO 4.
Recently, the combination of metal complexes and organic molecules in relay and cooperative catalysis has gained increasing attention, because it could combine multiple transformations in one synthetic operation . More importantly, asymmetric relay catalysis ARC holds great potential in creation of new step economy transformations wherein either type of catalyst failed to afford alone.
The combination of transition metal and organocatalysts has enabled a diverse range of unprecedented transformations, providing efficient methods to access optically active heterocycles [6, 24]. In the field of organic synthesis, the development of new asymmetric transformations leading to novel scaffolds with unprecedented chemical and biological activities always hold great importance, but still represents a formidable challenge. Fattorusso, O. Taglialatela-Scafati Eds. Atwal, B.
- Debugging Linux Systems (Digital Short Cut).
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Swanson, S. Unger, D. Floyd, S. Moreland, A. Hedberg, B. Usami, J. Yamaguchi, A. For leading literatures of chiral phosphoric acid catalysis: a T. Akiyama, J. Itoh, K. Yokota, K.
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Uraguchi, M. Zamfir, S.
Schenker, M. Freund, S. Yu, F. Shi, L. For reviews, see: a Z. Shao, H. Asian J. Zhong, X. Hashmi, C. Sorimachi, M. For some reviews, see: a I. Coldham, R. Pandey, P.
The Asymmetric Synthesis of Furanoseterterpene from Marine Natural Products
Banerjee, S. For early reports, see: a J. Longmire, B. Wang, X. Gothelf, K. Gothelf, R. Hazell, K. Stanley, M. Adrio, J. Vicario, S. Reboredo, D. Ibrahem, R. Rios, J. Vesely, A. Tetrahedron Lett. Xue, X. Zhang, L. Synlett ; d Y. Liu, H.
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Liu, W. Du, L. Yue, Y. Chen, W. He, X.
Chen, D. Wang, S. Luo, W. Zhang, J. Yu, L. Ren, L. Chen, Q.
Asymmetric Synthesis of Natural Products
Wei, S. Luo, H. Xiao, L. Liu, X. Chen, L. Chen, J. Song, W.