Chirality research overview

During our research in this field of small-ring heterocycles we found that functionahzed aziridines are attractive chiral catalysts, e.g., in the diethylzinc addition to aldehydes. Aspects of such uses of aziridines will be discussed as well. This overview does not pretend to be an exhaustive coverage of all existing literature on small-ring aza-heterocycles as that would require a separate monograph. Instead, emphasis is put on functionahzed three-membered aza-heterocycles, that were investigated in the author s laboratory [1], and relevant related literature. The older literature on these heterocycles is adequately summarized in some extensive reviews [2]. Chiral aziridines have been reviewed recently by Tanner [3], by Osborn and Sweeney [4], and by McCoull and Davis [5]. 

Asymmetric catalysis is a vital and rapidly growing branch of modern organic chemistry. Within this context, Ti- and Zr-based chiral catalysts have played a pivotal role in the emergence of a myriad of efficient and enantioselective protocols for asymmetric synthesis. In this chapter, a critical overview of enantioselective reactions promoted by chiral Zr-based catalysts is provided. Since an account of this type is most valuable when it provides a context for advances made in a particular area of research, when appropriate, a brief discussion of related catalytic asymmetric reactions promoted by non-Zr-based catalysts is presented as well. 

Enantioselective synthesis is a topic of undisputable importance in current chemical research and there is a steady flow of articles, reviews and books on almost every aspect involved. The present overview will concentrate on the application of solid chiral catalysts for the enantioselective synthesis of chiral molecules which are a special class of fine chemicals. Included is an account on our own work with the cinchona-modified Pt catalysts. Excluded is the wide field of immobilized versions of active homogeneous complexes or of bio-catalysts. During the preparation of this survey, several reviews have been found to be very informative [1-14]. 

Since much of the knowledge about chiral lithium amides has been obtained from research on achiral amides, this section will give a short overview of the field of lithium amides. Furthermore, without the development of NMR techniques in the last two decades the structural knowledge of organolithium compounds would still be in its infancy. 

Another approach towards asymmetric heterogeneous catalysts is the immobilization of chiral homogeneous complexes via different methods. In this way the advantages of homogeneous catalysts (high activity and selectivity) and heterogeneous catalysts (easy recovery) can be combined. For a complete overview of this active research field the reader is referred to several reviews on this topic [50, 51]. The practical applicability of these catalysts is hampered by the fact that severe demands of recyclability and stability need to be obeyed. In certain cases promising results have been obtained as outlined here. 

During the past decade, there has been an increased emphasis on new approaches to chiral compoimds and asymmetric syntheses. This focus has been particularly pronounced in medicinal chemistry, where a specific enantiomer or diastereomer often exhibits enhanced therapeutic potency compared with the racemate. Organofluorine compounds have played a significant role in these advances. An earlier report emphasized a range of methods for the synthesis of chiral bioactive fluoroorganic compounds (1). Since the intent of this paper is to provide an overview which captures the scope and flavor of these recent developments, it seems quite appropriate to briefly cite the fascinating range of research studies of the other contributors to this book. 

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