Asymmetric phase-transfer

Catalytic asymmetric synthesis with participation and formation of heterocycles (including asymmetric phase transfer reactions and asymmetric reactions with chiral Lewis catalysts) 93MI1. 

In contrast the progress of asymmetric synthesis by use of chiral non-racemic phase transfer catalysts had been slow compared to the ordinary phase transfer catalysis. However, recent achievements in this particular area are noteworthy and efficient asymmetric phase transfer catalysis has been increasingly explored.17 101... 

There are many examples on the asymmetric phase transfer catalysis, but highly efficient ones are not so many though they are increasing in recent years.17-101 This review will highlight the notable examples with emphasis on recent reports. 

Asymmetric Phase Transfer Catalysis Aldol Reactions... 

Epoxidation is another important area which has been actively investigated on asymmetric phase transfer catalysis. Especially, the epoxidation of various (i.)-a,p-unsaturated ketones 68 has been investigated in detail utilizing the ammonium salts derived from cinchonine and cinchonidine, and highly enantioselective and diastereoselective epoxidation has now been attained. When 30 % aqueons H202 was utilized in the epoxidation of various a, 3-unsaturated ketones 68, use of the 4-iodobenzyl cin-choninium bromide 7 (R=I, X=Br) together with LiOH in Bu20 afforded the a,p-epoxy ketones 88 up to 92% ee,1641 as shown in Table 5. The O-substituted... 

Excellent enantioselectivities have not been attained in the asymmetric phase transfer catalyzed epoxidation of (Z)-enones in contrast to that of (E)-enones. However, a few promising results168,691 have been reported on the epoxidation of 2-substituted... 

Numbers of asymmetric phase transfer catalysis can now be accomplished efficiently to give a variety of chiral non-racemic products with high enantiomeric excesses. Thus, asymmetric phase transfer catalysis has grown up into practical level in numbers of reactions and some optically pure compounds can be effectively produced on large scale by use of chiral phase transfer catalysts. 

In contrast to the maturity of asymmetric synthesis utilizing chiral transition metal catalysts, asymmetric phase transfer catalysis is still behind it and covers organic reactions to lesser extent. Thus, it is further necessary in wide range to explore efficient asymmetric phase transfer catalysis keeping its superiority of easy operation, mild reaction conditions, and environmental binignancy. 

The recent rapid progress in this area will definitely promise that asymmetric phase transfer catalysis is the reaction for the 21st century. 

M J. O Donnell, Asymmetric Phase Transfer Reactions , In Catalytic Asymmetric Synthesis (Ed. L Ojima), VCH, New York, 1993, Chapter 8 (pp. 390-411). 

K. Manabe, Asymmetric Phase-Transfer Alkylation Catalyzed by a Chiral Quaternary Phosphonium Salt with a Multiple Hydrogen-Bonding Site , Tetrahedron Lett. 1998, 39, 5807-5810. 

K. Manabe, Synthesis of Nobel Chiral Quaternary Phosphonium Salts with a Multiple Hydrogen-Bonding Site, and Their Application to Asymmetric Phase-Transfer Alkylation , Tetrahedron 1998, 54, 14465-14476. 

B. Lygo, P. G. Wainwright, A New Class of Asymmetric Phase-Transfer Catalysts Derived from Cinchona Alkaloids - Application in the Enantioselective Synthesis of a-Amino Acids , Tetrahedron Lett., 1997, 38, 8595-8598. 

R Lygo, J. Crosby, J. A. Peterson, Enantioselective Synthesis of Bis-a-Amino Add Esters via Asymmetric Phase-Transfer Catalysis , Tetrahedron Lett. 1999, 40, 1385-1388. 

C. Hofstetter, P. S. Wilkinson, T. C. Pochapsky, NMR Structure Determination of Ion Pairs Derived from Quinine A Model for Templating in Asymmetric Phase-Transfer Reductions by BH4" with Implications for Rational Design of Phase-Transfer Catalysts , J. Org. Chem 1999, 64, 8794-8800. 

Asymmetric phase-transfer catalysis using chiral nonra-cemic onium salts or crown ethers has now grown into a practical method whereby a large number of reactions can be performed and some optically pure compounds can be produced effectively on a large scale. 

Figure 16. Transition states for asymmetric phase-transfer-catalyzed epoxidation of cyclqhexenones (77).

Aqueous alkali hydroxides can be used to replace flammable bases of sodium metal, sodium hydride, sodamide, and other alkoxides. The reaction temperature is lowered while the reaction rate improves because the increased reactivity of anions in the nonpolar solvent (Goldberg, 1989 Dehmlow and Dehm-low, 1993 Starks et al., 1994) as have asymmetric phase-transfer reactions (O Donnell, 1993). 

This asymmetric phase-transfer method has been applied to enantio-selective Robinson annelation as shown in Scheme 14 (41). First, alkylation of a 1-indanone derivative with the Wichtetie reagent as a methyl vinyl ketone equivalent in the presence of p-CF3BCNB gives the S-alkylation product in 92% ee and 99% yield. With 1 -(p-trifluoro-methylbenzyl)cinchonidinium bromide, a pseudo-enantiomeric diaste-reomer of p-CF3BCNB, as catalyst, the -alkylation product is obtained in 78% ee and 99% yield. These products are readily convertible to the... 

Scheme 19 Synthesis of a Pyridoxal-Modified Alanine by Asymmetric Phase-Transfer Catalysis 53,541...

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