448
N. Paul
starting with (E,S)-allyl alcohol, Re-facial attack is recorded to yield 62% (S,S)
isomer while Si-facial approach also left a considerable contribution like 38% for
(R,S) isomer making product only 24% optically pure.
4 Conclusion
The model study states the origin of enantioselectivity in Sharpless epoxidation
has been linked to geometrical constraint noticed in the aggregate of homogeneous
catalyst, substrate and epoxidating reagent in transition structure. The analysis also
points out best synthetic strategy for producing enantiomerically pure epoxide must
start with an optically pure R-allyl alcohol and to direct predominating Re-facial
epoxidation, tartrate-based chiral auxiliary must contain a more space-filling and
more electron-releasing moiety. These epoxides appear as potential synthons for
many drug molecules. Occasionally this is noticed that an in vitro active substance
is almost inactive in in vivo atmosphere. Laborious attempts of synthetic chemists
clinch enantiomerically pure substances in a substantial amount. The flavour of asymmetric synthesis is realized by performing resolution. The loss of more reactive half
of enantiomers may often turn beneficial. Exclusion of one enantiomer from racemic
mixture is a cherished target in synthetic chemistry. In this article, model based on
geometry of structures meticulously illustrates dynamicity of the homogeneous catalysis leading to peerless resolution of enantiomerically pure epoxides in Sharpless
asymmetric epoxidation. It is this enantioselectivity that plays a key role in providing
characteristic features within molecules to attain bioselectivity.
Acknowledgements Authoress is indebted to Dr. Santanu Chakraborti, Dr. Sanjay Roy and Dr.
Anjan Das for their constant encouragement.
References
Gao Y, Hanson RM, Klunder JM, Masamune SY, Sharpless KB (1987) Catalytic asymmetric epoxidation and kinetic resolution: modified procedures including in situ derivatization. J Am Chem
Soc 109(19):5765–5780
Katsuki T, Sharpless KB (1980) The first practical method for asymmetric epoxidation. J Am Chem
Soc 102(18):5974–5976
Katsuki T, Sharpless KB (1992) The nature of the Katsuki-Sharpless asymmetric epoxidation
catalyst. J Org Chem 57(24):6629–6635
Martin V, Woodward S, Katsuki T, Yamada Y, Ikeda M, Sharpless KB (1981) Kinetic resolution
of racemic allylic alcohols by enantioselective epoxidation. A route to substances of absolute
enantiomeric purity? J Am Chem Soc 103(20):6237–6240
Takano S, Iwabuchi Y, Ogasawara K (1991) Inversion of enantioselectivity in the kinetic resolution
mode of the Katsuki-sharpless asymmetric epoxidation reaction. J Am Chem Soc 113(7):2786–
2787
N. Paul
starting with (E,S)-allyl alcohol, Re-facial attack is recorded to yield 62% (S,S)
isomer while Si-facial approach also left a considerable contribution like 38% for
(R,S) isomer making product only 24% optically pure.
4 Conclusion
The model study states the origin of enantioselectivity in Sharpless epoxidation
has been linked to geometrical constraint noticed in the aggregate of homogeneous
catalyst, substrate and epoxidating reagent in transition structure. The analysis also
points out best synthetic strategy for producing enantiomerically pure epoxide must
start with an optically pure R-allyl alcohol and to direct predominating Re-facial
epoxidation, tartrate-based chiral auxiliary must contain a more space-filling and
more electron-releasing moiety. These epoxides appear as potential synthons for
many drug molecules. Occasionally this is noticed that an in vitro active substance
is almost inactive in in vivo atmosphere. Laborious attempts of synthetic chemists
clinch enantiomerically pure substances in a substantial amount. The flavour of asymmetric synthesis is realized by performing resolution. The loss of more reactive half
of enantiomers may often turn beneficial. Exclusion of one enantiomer from racemic
mixture is a cherished target in synthetic chemistry. In this article, model based on
geometry of structures meticulously illustrates dynamicity of the homogeneous catalysis leading to peerless resolution of enantiomerically pure epoxides in Sharpless
asymmetric epoxidation. It is this enantioselectivity that plays a key role in providing
characteristic features within molecules to attain bioselectivity.
Acknowledgements Authoress is indebted to Dr. Santanu Chakraborti, Dr. Sanjay Roy and Dr.
Anjan Das for their constant encouragement.
References
Gao Y, Hanson RM, Klunder JM, Masamune SY, Sharpless KB (1987) Catalytic asymmetric epoxidation and kinetic resolution: modified procedures including in situ derivatization. J Am Chem
Soc 109(19):5765–5780
Katsuki T, Sharpless KB (1980) The first practical method for asymmetric epoxidation. J Am Chem
Soc 102(18):5974–5976
Katsuki T, Sharpless KB (1992) The nature of the Katsuki-Sharpless asymmetric epoxidation
catalyst. J Org Chem 57(24):6629–6635
Martin V, Woodward S, Katsuki T, Yamada Y, Ikeda M, Sharpless KB (1981) Kinetic resolution
of racemic allylic alcohols by enantioselective epoxidation. A route to substances of absolute
enantiomeric purity? J Am Chem Soc 103(20):6237–6240
Takano S, Iwabuchi Y, Ogasawara K (1991) Inversion of enantioselectivity in the kinetic resolution
mode of the Katsuki-sharpless asymmetric epoxidation reaction. J Am Chem Soc 113(7):2786–
2787