不对称氢转移反应诱导动态动力学拆分
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摘要
动态动力学拆分是从外消旋的底物制备光学纯化合物的有效方法,从理论上讲可以获得定量的产率和接近100%的ee值,近年来引起了化学工作者的广泛关注。不对称氢转移反应是选择性地还原醛、酮及亚胺等化合物的重要方法之一,条件温和,操作简便。文献已有一些通过不对称氢转移反应来实现动态动力学拆分的报道。本论文中,我们在有关不对称氢转移反应诱导动态动力学拆分文献综述的基础上,设计合成了一系列β-羰基砜,对其进行不对称氢转移反应,实现了动态动力学拆分。
1.以较高收率地合成了一系列β-羰基砜,均通过核磁的鉴定。
2.利用[RuCl(p-cymene)](S,S)-TsDPEN为催化剂,HCOOH : Et3N = 5 : 2为氢源,不对称氢转移动态动力学拆分一系列β-羰基砜,以高产率、高dr值和ee值合成了相应的手性β-羟基砜。
Dynamic kinetic resolution (DKR) is an efficient method to obtain optically pure compounds from racemic substrates. Recently, increasing attention has been given to the discovery of DKR reactions, which can, theoretically, result in quantitative yield with enantiomeric excess (ee) approaching 100%. A few examples about DKRs have been reported via asymmetric transfer hydrogenation. After consulting a lot of literatures, this dissertation, we describe the design and synthesis of a series ofβ–keto sulfones, and the realization of DKR of these substrates via asymmetric transfer hydrogenation reaction.
1. The synthesis of a series ofβ–keto sulfones with good yield. All products was well confirmed with data of 1H NMR.
2. Dynamic kinetic resolution of a series ofβ–keto sulfones was achieved by their asymmetric transfer hydrogenation using HCOOH : Et3N = 5 : 2 as the hydrogen source and (S,S)-TsDPEN based Ru (II) catalyst, providing the corresponding chiralβ–hydroxyl sulfones with good yield, high dr and ee.
1.以较高收率地合成了一系列β-羰基砜,均通过核磁的鉴定。
2.利用[RuCl(p-cymene)](S,S)-TsDPEN为催化剂,HCOOH : Et3N = 5 : 2为氢源,不对称氢转移动态动力学拆分一系列β-羰基砜,以高产率、高dr值和ee值合成了相应的手性β-羟基砜。
Dynamic kinetic resolution (DKR) is an efficient method to obtain optically pure compounds from racemic substrates. Recently, increasing attention has been given to the discovery of DKR reactions, which can, theoretically, result in quantitative yield with enantiomeric excess (ee) approaching 100%. A few examples about DKRs have been reported via asymmetric transfer hydrogenation. After consulting a lot of literatures, this dissertation, we describe the design and synthesis of a series ofβ–keto sulfones, and the realization of DKR of these substrates via asymmetric transfer hydrogenation reaction.
1. The synthesis of a series ofβ–keto sulfones with good yield. All products was well confirmed with data of 1H NMR.
2. Dynamic kinetic resolution of a series ofβ–keto sulfones was achieved by their asymmetric transfer hydrogenation using HCOOH : Et3N = 5 : 2 as the hydrogen source and (S,S)-TsDPEN based Ru (II) catalyst, providing the corresponding chiralβ–hydroxyl sulfones with good yield, high dr and ee.
引文
1. Hélène, P. Dynamic kinetic resolution. Tetrahedron 2003, 59, 8291–8327.
2. Hélène, P. Recent developments in dynamic kinetic resolution. Tetrahedron 2008, 64, 1563–1601.
3. Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. The catalyst precursor, catalyst, and intermediate in the RuII-promoted asymmetric hydrogen transfer between alcohols and ketones. Angew. Chem. Int. Ed. Engl. 1997, 36, 285–288.
4. (a). Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. Asymmetric transfer hydrogenation of aromatic ketones catalyzed by chiral ruthenium(II) complexes. J. Am. Chem. Soc. 1995, 117, 7562–7563; (b). Fujii, A.; Hashiguchi, S.; Uematsu, N.;Ikariya, T.; Noyori, R. Ruthenium (II)-catalyzed asymmetric transfer hydrogenation of ketones using a formic acid-triethylamine mixture. J. Am. Chem. Soc. 1996, 118, 2521–2522; (c). Noyori, R.; Hashiguchi, S. Asymmetric transfer hydrogenation catalyzed by chiral ruthenium complexes. Acc. Chem. Res. 1997, 30, 97–102; (d) Everaere, K.; Carpentier, J.-F.; Mortreux, A.; Bulliard, M. Ruthenium catalyzed asymmetric transfer hydrogenation ofβ-ketoesters. Tetrahedron: Asymmetry 1998, 9, 2971–2974.
5. Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation ofα,β-Acetylenic Ketones. J. Am. Chem. Soc. 1997, 119, 8738–8739.
6. Noyori, R.; Ohkuma, T. Asymmetric catalysis by architectural and functional molecular engineering: practical chemo- and stereoselective hydrogenation of ketones. Angew. Chem. Int. Ed. 2001, 40, 40–73.
7. (a) Hélène, P. Chiral sulfur-containing ligands for asymmetric catalysis. Tetrahedron 2007, 63, 1297–1330. (b) Jung, W.; Benjamin, L. Catalytic Transfer Hydrogenation ofα-Ketoesters with Hantzsch Esters. Org. Lett. 2006, 8, 5653–5655. (c) Alexey, B. Z.; Hans, A. Enantioswitchable Catalysts for the Asymmetric Transfer Hydrogenation of Aryl Alkyl Ketones. Org. Lett. 2006, 8, 5129–5132. (d) Stephan, E.; Bernhard, H.; Santosh, B.; Gopinathan, A.; Man, K. T.; Bianaca, B.; Kathrin, J.; Giulia, E.; Matthias, B. New Ruthenium Catalysts for Asymmetric Transfer Hydrogenation of Prochiral Ketones. Adv. Synth. Catal. 2007, 349, 853–860.
8. Noyori, R. Asymmetric catalysis: science and opportunities (Nobel Lecture) Angew. Chem. Int. Ed. Engl. 2002, 41, 2008–2022.
9. Alcock, N. J.; Mann, I.; Peach, P.; Wills, M. Dynamic kinetic resolution–asymmetric transfer hydrogenation of 1-aryl-substituted cyclic ketones. Tetrahedron: Asymmetry 2002, 13, 2485–2490.
10. Peach, P.; Cross, D. J.; Kenny, J. A.; Mann, I.; Houson, I.; Campbell, L.; Walsgrove, T.; Wills, M. Asymmetric transfer hydrogenation ofα,β-unsaturated, α-tosyloxy andα-substituted ketones. Tetrahedron 2006, 62, 1864–1876.
11. Ros, A.; Magriz, A.; Dietrich, H.; Fernández, R.; Alvarez, E.; Lassaletta, J. M. Enantioselective synthesis of vicinal halohydrins via dynamic kinetic resolution. Org.Lett. 2006, 8, 127–130.
12. Fernández, R.; Ros, A.; Magriz, A.; Dietrich, H.; Lassaletta, J. M. Enantioselective synthesis of cis- -substituted cycloalkanols and trans-cycloalkyl amines thereof. Tetrahedron 2007, 63, 6755–6763
13. Eustache, F.; Dalko, P. I.; Cossy, J. Enantioselective monoreduction of 2-Alkyl-1,3-diketones mediated by chiral ruthenium catalysts. Dynamic kinetic resolution. Org. Lett. 2002, 4, 1263–1265.
14. Mohar, B.; Valleix, A.; Desmurs, J.-R.; Felemez, M.; Wagner, A.; Mioskowski, C. Highly enantioselective synthesis via dynamic kinetic resolution under transfer hydrogenation using Ru(-6-arene)-N-perfluorosulfonyl-1,2-diamine catalysts: a first insight into the relationship of the ligand’s pKa and the catalyst activity. Chem. Commun. 2001, 2572–2573.
15. (a) Eustache, F.; Dalko, P. I.; Cossy, J. Synthesis of the C14-C25 Subunit of Bafilomycin A1. J. Org. Chem. 2003, 68, 9994–10002. (b) Eustache, F.; Dalko, P. I.; Cossy, J. Enantioselective monoreduction of 2-alkyl 1,3-diketones using chiral ruthenium catalysts. Synthesis of the C14–C25 fragment of bafilomycin A1. Tetrahedron Lett. 2003, 44, 8823–8826.
16. terk, D.; Stephan, M.; Mohar, B. Highly enantioselective transfer hydrogenation of fluoroalkyl ketones. Org. Lett. 2006, 8, 5935–5938.
17. Ros, A.; Magriz, A.; Dietrich, H.; Lassaletta, J. M.; Fernández, R. Stereoselective synthesis of syn -hydroxy cycloalkane carboxylates: transfer hydrogenation of cyclicβ-ketoesters via dynamic kinetic resolution. Tetrahedron 2007, 63, 7532–7537.
18. Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation of Imines. J. Am. Chem. Soc. 1996, 118, 4916–4917.
19. Mao, J.; Baker, D. C. A chiral rhodium complex for rapid asymmetric transfer hydrogenation of imines with high enantioselectivity. Org. Lett. 1999, 1, 841–843.
20. Ros, A.; Magriz, A.; Dietrich, H.; Ford, M.; Fernández, R.; Lassaletta, J. M. Transfer hydrogenation ofα-Branched ketimines: enantioselective synthesis of cycloalkylamines via dynamic kinetic resolution. Adv. Synth. Catal. 2005, 347,1917–1921.
21. Wu, G.; Zhu, J.; Ding, Z.; Shen, Z.; Zhang, Y. Dynamic kinetic resolution of racemicα-sulfonylaldehydes via asymmetric transfer hydrogenation. Tetrahedron Lett. 2009, 50, 427–429.
1. Sato, T.; Okumura, Y.; Itai, J.; Fujisawa, T. (S)-β,ω-dihydroxyalkyl phenyl sulfones. synthesis by Bakers’Yeast reduction and use as precursors of optically active lactones. Chem. Lett. 1988, 1537-1540.
2. Tanikaga, R.; Hosoya, K.; Kaji, A. Synthesis of enantiomerically pure 2,5- disubstituted tetrahydrofurans using readily prepared (2S)-1-phenylsulphonyl- alkan-2-o1s J. Chem. Soc., Perkin Trans. 1 1987, 1799-1803.
3. (a) Servi, S. Baker’s Yeast as a reagent in oraganic synthesis. Synthesis 1990, 1-25. (b) Csuk, R.; Gl-nzer, B. I. Baker's Yeast mediated transformations in organic chemistry. Chem. Rev. 1991, 91, 49-97. (c) Robert, S. M. Preparative biotransformations. J. Chem. Soc. Perkin Trans. 1 1999, 1-21. (d) Crumbie, R. L.; Deol, B. S.; Nemorin, J. E.; Ridley, D. D. Asymmetric reduction of carbonyl compounds by yeast. IV* Preparation of optically activeβ-hydroxy sulfides, sulfoxides and sulfones. Aust. J. Chem. 1978, 31, 1965-1980.
4. Ohta, H.; Kato, Y.; Tsuchihashi, G. Switching of the direction of enzyme- mediated oxidation and reduction of sulfur-substituted 2-propanols and 2-propanones. J. Org. Chem. 1987, 52, 2735-2739.
5. Gotor, V.; Rebolledo, F.; Liz, R. Enantioselective bioreduction ofβ-keto sulfones with the fungus curvularia lunata. Tetrahedron: Asymmetry 2001, 12, 513-515.
6. Kie-basiński, P.; Rachwalski, M.; Miko-ajczyk, M.; Moelands, M. A. H.; Zwanenburg, B.; Rutjes, F. P. J. T. Lipase-promoted dynamic kinetic resolution of racemicβ-hydroxyalkyl sulfones. Tetrahedron: Asymmetry 2005, 16, 2157-2160.
7. Cho, B. T.; Kim, D. J. Efficient synthesis of optically activeβ-hydroxy p-tolylsulfones with very high enantiomeric excess via CBS-oxazaborolidine- catalyzed borane reduction. Tetrahedron: Asymmetry 2001, 12, 2043-2047.
8. Zhao, G.; Hu, J.; Qian, Z; Yin, W. Enantioselective reduction ofβ-keto sulfones using the NaBH4/Me3SiCl system catalyzed by polymer-supported chiral sulfonamide. Tetrahedron: Asymmetry 2002, 13, 2095-2098.
9. (a) Bertus, P.; Phansavath, P.; Ratovelomanana-Vidal, V.; Genêt, J.-P.; Touati, A. R.; Homri, T.; Ben Hassine, B. General synthesis of chiralβ-hydroxy sulfones via enantioselective ruthenium-catalyzed hydrogenation. Tetrahedron Lett. 1999, 40, 3175-3178; (b) Bertus, P.; Phansavath, P.; Ratovelomanana-Vidal, V.; Genêt, J. P.; Touati, A. R.; Homri, T.; Ben Hassine, B. Enantioselective hydrogenation of β-ketosulfones with chiral Ru(II)-catalysts: synthesis of enantiomerically pure butenolides andγ-butyrolactones. Tetrahedron: Asymmetry 1999, 10, 1369-1380.
10. Zhang, H.; Hou, X.; Dai, L.; Luo, Z. Synthesis of a biferrocene diphosphine ligand with only planar chirality and its application in the Rh-catalyzed asymmetric hydrogenation ofβ-keto sulfones. Tetrahedron: Asymmetry 2007, 18, 224-228.
11. Wan, X.; Meng, Q.; Zhang, H.; Sun, Y.; Fan, W.; Zhang, Z. An efficient synthesis of chiralβ-hydroxy sulfones via Ru-catalyzed enantioselective hydrogenation in the presence of iodine. Org. Lett. 2007, 9, 5613-5616.
2. Hélène, P. Recent developments in dynamic kinetic resolution. Tetrahedron 2008, 64, 1563–1601.
3. Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. The catalyst precursor, catalyst, and intermediate in the RuII-promoted asymmetric hydrogen transfer between alcohols and ketones. Angew. Chem. Int. Ed. Engl. 1997, 36, 285–288.
4. (a). Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. Asymmetric transfer hydrogenation of aromatic ketones catalyzed by chiral ruthenium(II) complexes. J. Am. Chem. Soc. 1995, 117, 7562–7563; (b). Fujii, A.; Hashiguchi, S.; Uematsu, N.;Ikariya, T.; Noyori, R. Ruthenium (II)-catalyzed asymmetric transfer hydrogenation of ketones using a formic acid-triethylamine mixture. J. Am. Chem. Soc. 1996, 118, 2521–2522; (c). Noyori, R.; Hashiguchi, S. Asymmetric transfer hydrogenation catalyzed by chiral ruthenium complexes. Acc. Chem. Res. 1997, 30, 97–102; (d) Everaere, K.; Carpentier, J.-F.; Mortreux, A.; Bulliard, M. Ruthenium catalyzed asymmetric transfer hydrogenation ofβ-ketoesters. Tetrahedron: Asymmetry 1998, 9, 2971–2974.
5. Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation ofα,β-Acetylenic Ketones. J. Am. Chem. Soc. 1997, 119, 8738–8739.
6. Noyori, R.; Ohkuma, T. Asymmetric catalysis by architectural and functional molecular engineering: practical chemo- and stereoselective hydrogenation of ketones. Angew. Chem. Int. Ed. 2001, 40, 40–73.
7. (a) Hélène, P. Chiral sulfur-containing ligands for asymmetric catalysis. Tetrahedron 2007, 63, 1297–1330. (b) Jung, W.; Benjamin, L. Catalytic Transfer Hydrogenation ofα-Ketoesters with Hantzsch Esters. Org. Lett. 2006, 8, 5653–5655. (c) Alexey, B. Z.; Hans, A. Enantioswitchable Catalysts for the Asymmetric Transfer Hydrogenation of Aryl Alkyl Ketones. Org. Lett. 2006, 8, 5129–5132. (d) Stephan, E.; Bernhard, H.; Santosh, B.; Gopinathan, A.; Man, K. T.; Bianaca, B.; Kathrin, J.; Giulia, E.; Matthias, B. New Ruthenium Catalysts for Asymmetric Transfer Hydrogenation of Prochiral Ketones. Adv. Synth. Catal. 2007, 349, 853–860.
8. Noyori, R. Asymmetric catalysis: science and opportunities (Nobel Lecture) Angew. Chem. Int. Ed. Engl. 2002, 41, 2008–2022.
9. Alcock, N. J.; Mann, I.; Peach, P.; Wills, M. Dynamic kinetic resolution–asymmetric transfer hydrogenation of 1-aryl-substituted cyclic ketones. Tetrahedron: Asymmetry 2002, 13, 2485–2490.
10. Peach, P.; Cross, D. J.; Kenny, J. A.; Mann, I.; Houson, I.; Campbell, L.; Walsgrove, T.; Wills, M. Asymmetric transfer hydrogenation ofα,β-unsaturated, α-tosyloxy andα-substituted ketones. Tetrahedron 2006, 62, 1864–1876.
11. Ros, A.; Magriz, A.; Dietrich, H.; Fernández, R.; Alvarez, E.; Lassaletta, J. M. Enantioselective synthesis of vicinal halohydrins via dynamic kinetic resolution. Org.Lett. 2006, 8, 127–130.
12. Fernández, R.; Ros, A.; Magriz, A.; Dietrich, H.; Lassaletta, J. M. Enantioselective synthesis of cis- -substituted cycloalkanols and trans-cycloalkyl amines thereof. Tetrahedron 2007, 63, 6755–6763
13. Eustache, F.; Dalko, P. I.; Cossy, J. Enantioselective monoreduction of 2-Alkyl-1,3-diketones mediated by chiral ruthenium catalysts. Dynamic kinetic resolution. Org. Lett. 2002, 4, 1263–1265.
14. Mohar, B.; Valleix, A.; Desmurs, J.-R.; Felemez, M.; Wagner, A.; Mioskowski, C. Highly enantioselective synthesis via dynamic kinetic resolution under transfer hydrogenation using Ru(-6-arene)-N-perfluorosulfonyl-1,2-diamine catalysts: a first insight into the relationship of the ligand’s pKa and the catalyst activity. Chem. Commun. 2001, 2572–2573.
15. (a) Eustache, F.; Dalko, P. I.; Cossy, J. Synthesis of the C14-C25 Subunit of Bafilomycin A1. J. Org. Chem. 2003, 68, 9994–10002. (b) Eustache, F.; Dalko, P. I.; Cossy, J. Enantioselective monoreduction of 2-alkyl 1,3-diketones using chiral ruthenium catalysts. Synthesis of the C14–C25 fragment of bafilomycin A1. Tetrahedron Lett. 2003, 44, 8823–8826.
16. terk, D.; Stephan, M.; Mohar, B. Highly enantioselective transfer hydrogenation of fluoroalkyl ketones. Org. Lett. 2006, 8, 5935–5938.
17. Ros, A.; Magriz, A.; Dietrich, H.; Lassaletta, J. M.; Fernández, R. Stereoselective synthesis of syn -hydroxy cycloalkane carboxylates: transfer hydrogenation of cyclicβ-ketoesters via dynamic kinetic resolution. Tetrahedron 2007, 63, 7532–7537.
18. Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation of Imines. J. Am. Chem. Soc. 1996, 118, 4916–4917.
19. Mao, J.; Baker, D. C. A chiral rhodium complex for rapid asymmetric transfer hydrogenation of imines with high enantioselectivity. Org. Lett. 1999, 1, 841–843.
20. Ros, A.; Magriz, A.; Dietrich, H.; Ford, M.; Fernández, R.; Lassaletta, J. M. Transfer hydrogenation ofα-Branched ketimines: enantioselective synthesis of cycloalkylamines via dynamic kinetic resolution. Adv. Synth. Catal. 2005, 347,1917–1921.
21. Wu, G.; Zhu, J.; Ding, Z.; Shen, Z.; Zhang, Y. Dynamic kinetic resolution of racemicα-sulfonylaldehydes via asymmetric transfer hydrogenation. Tetrahedron Lett. 2009, 50, 427–429.
1. Sato, T.; Okumura, Y.; Itai, J.; Fujisawa, T. (S)-β,ω-dihydroxyalkyl phenyl sulfones. synthesis by Bakers’Yeast reduction and use as precursors of optically active lactones. Chem. Lett. 1988, 1537-1540.
2. Tanikaga, R.; Hosoya, K.; Kaji, A. Synthesis of enantiomerically pure 2,5- disubstituted tetrahydrofurans using readily prepared (2S)-1-phenylsulphonyl- alkan-2-o1s J. Chem. Soc., Perkin Trans. 1 1987, 1799-1803.
3. (a) Servi, S. Baker’s Yeast as a reagent in oraganic synthesis. Synthesis 1990, 1-25. (b) Csuk, R.; Gl-nzer, B. I. Baker's Yeast mediated transformations in organic chemistry. Chem. Rev. 1991, 91, 49-97. (c) Robert, S. M. Preparative biotransformations. J. Chem. Soc. Perkin Trans. 1 1999, 1-21. (d) Crumbie, R. L.; Deol, B. S.; Nemorin, J. E.; Ridley, D. D. Asymmetric reduction of carbonyl compounds by yeast. IV* Preparation of optically activeβ-hydroxy sulfides, sulfoxides and sulfones. Aust. J. Chem. 1978, 31, 1965-1980.
4. Ohta, H.; Kato, Y.; Tsuchihashi, G. Switching of the direction of enzyme- mediated oxidation and reduction of sulfur-substituted 2-propanols and 2-propanones. J. Org. Chem. 1987, 52, 2735-2739.
5. Gotor, V.; Rebolledo, F.; Liz, R. Enantioselective bioreduction ofβ-keto sulfones with the fungus curvularia lunata. Tetrahedron: Asymmetry 2001, 12, 513-515.
6. Kie-basiński, P.; Rachwalski, M.; Miko-ajczyk, M.; Moelands, M. A. H.; Zwanenburg, B.; Rutjes, F. P. J. T. Lipase-promoted dynamic kinetic resolution of racemicβ-hydroxyalkyl sulfones. Tetrahedron: Asymmetry 2005, 16, 2157-2160.
7. Cho, B. T.; Kim, D. J. Efficient synthesis of optically activeβ-hydroxy p-tolylsulfones with very high enantiomeric excess via CBS-oxazaborolidine- catalyzed borane reduction. Tetrahedron: Asymmetry 2001, 12, 2043-2047.
8. Zhao, G.; Hu, J.; Qian, Z; Yin, W. Enantioselective reduction ofβ-keto sulfones using the NaBH4/Me3SiCl system catalyzed by polymer-supported chiral sulfonamide. Tetrahedron: Asymmetry 2002, 13, 2095-2098.
9. (a) Bertus, P.; Phansavath, P.; Ratovelomanana-Vidal, V.; Genêt, J.-P.; Touati, A. R.; Homri, T.; Ben Hassine, B. General synthesis of chiralβ-hydroxy sulfones via enantioselective ruthenium-catalyzed hydrogenation. Tetrahedron Lett. 1999, 40, 3175-3178; (b) Bertus, P.; Phansavath, P.; Ratovelomanana-Vidal, V.; Genêt, J. P.; Touati, A. R.; Homri, T.; Ben Hassine, B. Enantioselective hydrogenation of β-ketosulfones with chiral Ru(II)-catalysts: synthesis of enantiomerically pure butenolides andγ-butyrolactones. Tetrahedron: Asymmetry 1999, 10, 1369-1380.
10. Zhang, H.; Hou, X.; Dai, L.; Luo, Z. Synthesis of a biferrocene diphosphine ligand with only planar chirality and its application in the Rh-catalyzed asymmetric hydrogenation ofβ-keto sulfones. Tetrahedron: Asymmetry 2007, 18, 224-228.
11. Wan, X.; Meng, Q.; Zhang, H.; Sun, Y.; Fan, W.; Zhang, Z. An efficient synthesis of chiralβ-hydroxy sulfones via Ru-catalyzed enantioselective hydrogenation in the presence of iodine. Org. Lett. 2007, 9, 5613-5616.