ASYMMETRIC PHASE-TRANSFER CATALYSIS - Modern Tools for the Synthesis of Complex Bioactive Molecules - page 237

O O

( S )- 46b
(2 mol%)

F
CO 2 t -Bu

CO 2 t -Bu

PhO 2 S SO 2 Ph

+

N

K 2 CO 3 aq
Et 2 O
-20°C, 1 h
99%

F

(ee = 96%)

Ar Ar

Br -

OH

CF 3

+

N S

Ar =

CF 3

OH

Ar Ar

( S )- 46b

SCHEME 7.29

compounds with a fluorine atom is now a common strategy in the field of medicinal
chemistry. For this reason, an efficient method for direct enantioselective construction
of fluorinated stereogenic carbon centers is extremely important [72]. We developed
highly enantioselective fluorination of -keto esters under mild phase-transfer

conditions employing chiral bifunctional ammonium bromide 46b as a key catalyst
(Scheme 7.29) [73].

b

7.10. CONCLUSION

The development of various types of chiral phase-transfer catalysts largely relies on
the molecular design of both natural product-derived and purely synthetic chiral
quaternary onium salts, which often delivers not only higher reactivity and stereo-
selectivity but also new synthetic opportunities, expanding the applicability of
asymmetric catalysis in modern organic synthesis. Continuous efforts should be
made toward the understanding of the relationship between the structure of the
catalyst and its activity and the stereocontrolling ability. Systematic accumulation of
such knowledge would allow us to conduct even more rational catalyst design for
pursuing selective chemical synthesis in a reliable and practical manner, thereby
allowing to establish genuinely sustainable chemical processes within the context of
forthcoming paradigm shift in worldwide production of highly valuable substances
in this century.

REFERENCES

1. (a) E. V. Dehmlow, S. S. Dehmlow, Phase Transfer Catalysis , 3rd ed., VCH,Weinheim, 1993; (b) C. M.
Starks, C. L. Liotta, M. Halpern, Phase Transfer Catalysis , Chapman & Hall, New York, 1994; (c) Y.
Sasson, R. Neumann (Eds), Handbook of Phase Transfer Catalysis , Blackie Academic &Professional,
London, 1997; (d) M. E. Halpern (Ed.), Phase Transfer Catalysis , ACS Symposium Series 659,
American Chemical Society, Washington, DC, 1997.