Synthesis asymmetric, general principles

Photochemical asymmetric synthesis with native and modified CDx s is certainly one of the most potentially successful examples of supramolecular photochirogen-esis, which is applicable in principle to most photoreactive substrates of appropriate size and shape. Despite this wide applicability, the enantiodifferentiating ability of CDx and hence the product ee obtained are not sufficiently high in many cases. Furthermore, the elucidation of the chiral discrimination mechanism and the rationalization of the product chirality and ee are generally more difficult in photochemical asymmetric induction. For more simple and well-defined host-guest interactions (in the ground as well as excited states), several approaches to photochemical asymmetric induction with newly designed synthetic chiral hosts have been reported. 

The preparation of new asymmetric organosilicon compounds using the principles of asymmetric synthesis so widely employed in organic chemistry (65) has also been envisaged. Kinetic resolution, as depicted in Scheme 8, is of general... 

According to the general principles of asymmetric synthesis, chiral induction can be effected via substrate, reagent, or external (catalyst) control. Effective substrate control in the sense of induced diastereoselectivity requires a preformed stereogenic center within the substrate. For organometallic catalytic conversions a stereospecific reaction course and simple diastereoselectivity, as outlined above, is prerequisite. 

These transformations serve to illustrate the principles involved in asymmetric synthesis. The requirements for efficient synthetic utilization are (a) an easily available optically active reagent that can carry out the desired transformation, and (b) reaction conditions that lead to a high percentage of enantiomeric preference. In general, it is also desirable to be able to recover the optically active reagent. The Diels-Alder example is a case where this can be accomplished. Hydrolysis or lithium aluminum hydride reduction gives the product and also returns the original alcohol, which can be reused. Similarly, in the synthesis of dialkylacetic acids, the optically active amino alcohol can be recovered by hydrolysis. 

Asymmetric synthesis has been extensively developed during the last few decades. Steric approach control is a reliable and generally applicable principle for the development of asymmetric synthesis. Highly enantioselective reaction is expected if one of the two potentially reactive enantiofaces is effectively shielded by steric interaction (Fig. la decelerative selectivity). On the other hand, steric approach control may not be an effective strategy for site-selective functionalization... 

General Synthesis.- The principle of steric shielding has been applied by a number of research groups to the elaboration of 3-substituted carboxylic acids, with enantiomeric enrichments of 94-98% one of the variants of these asymmetric Michael additions is outlined in Scheme 1. Overall yields are generally excellent, the chiral auxiliary can easily be recovered, and the sense of the chirality in the final product can in principle be varied simply by changing the order of introduction of the two substituents, given that both organo-copper species are available. An entirely different approach to chiral carboxylic... 

Synthetic methods for enantiomers in a drug discovery programme differ appreciably from those used in their manufacture but the same basic principles are useful in both, for example, resolution by physical, chemical, or biocatalytic means, asymmetric synthesis, or use of the chirality pool. Generally, the method of choice ultimately used in manufacture of a pharmaceutical will be quite different from that first used to obtain the single-enantiomer material in a drug-discovery programme. 

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