4606
Finally, the optically active 2-acetoxy-1,1,1-tri¯uoro-3-(phenylthio)propane (2h) was shown to be
a useful intermediate for preparing the enantiomerically pure tri¯uorolactic acid (8) in Scheme 1.
The treatment of (S)-2h (89% ee) with SO2Cl2 and pyridine in CH2Cl2 at room temperature for 3
h aorded the a,a-dichlorosul®de 9 in quantitative yield.23 The hydrolysis of 9 by re¯uxing in a
mixture of 12 M HCl and MeOH (3:20) for 40 h and in 2 M HCl for 18 h gave the acid 8 in 80%
yield. A single recrystallization of this material from Et2O±CHCl3 provided a 61% recovery of
25
D
rt
D
the homochiral (R)-(+)-8 {ꢀ +21.1 (c 0.95, EtOH), lit.24 ꢀ +21.0 (c 1.98, EtOH)}.
Scheme 1. Preparation of enantiomerically pure (R)-(+)-tri¯uorolactic acid (8)
In conclusion, we have described the highly enantioselective hydrogenation of 1,1,1-tri-
¯uoroalkan-2-one enol acetates mediated by chiral ruthenium catalysts. Applications of this
method to the synthesis of versatile chiral ¯uorinated molecules are now being carried out.
References
1. (a) Bravo, P.; Resnati, G. Tetrahedron: Asymmetry 1990, 1, 661±692. (b) Enantiocontrolled Synthesis of Fluoro-
Organic Compounds; Hayashi, T.; Soloshonok, V. A., Eds. Tetrahedron: Asymmetry, Special Issue; Tetrahedron:
Asymmetry 1994, 5, issue N 6. (c) Iseki, K. Tetrahedron 1998, 54, 13887±13914. (d) Enantiocontrolled Synthesis of
Fluoro-Organic Compounds; Soloshonok, V. A., Ed. John Wiley: New York, 1999. (e) Asymmetric Fluoroorganic
Chemistry: Synthesis, Applications, and Future Directions (ACS Symposium Series 746); Ramachandran, P. V., Ed.
American Chemical Society: Washington DC, 2000.
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Bachman, G. L.; Vineyard, B. D. J. Org. Chem. 1980, 45, 2362±2365. (b) Burk, M. J. J. Am. Chem. Soc. 1991,
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4. For the enantioselective reduction of a,a,a-tri¯uoroacetophenones, see: (a) Corey, E. J.; Bakshi, R. K.
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