Symmetry and strategic disconnections

Structural symmetry, either in a target molecule or in a subunit derived from it by antithetic dissection, can usually be exploited to reduce the length or complexity of a synthesis. 

A molecule which is not itself symmetrical may still be cleavable to identical precursors. Examples of such cases are carpanone (123)34a n j usnic acid. h 

The antibiotic rifamycin provides an example of a different and more common situation in which a target structure which has no overall symmetry has imbedded within it a Ci-symmetrical or nearly symmetrical substructure that, in turn, can be converted retrosynthetically to either a C2-symmetrical precursor or a pair of precursors available from a common intermediate. 6 

The direct goal of stereochemical strategies is the reduction of stereochemical complexity by the retrosynthetic elimination of the stereocenters in a target molecule. The greater the number and density of stereocenters in a TGT, the more influential such strategies will be. The selective removal of stereocenters depends on the availability of stereosimplifying transforms, the establishment of the required retrons (complete with defined stereocenter relationships), and the presence of a favorable spatial environment in the precursor generated by application of such a transform. The last factor, which is of crucial importance to stereoselectivity, mandates a bidirectional approach to stereosimplification which takes into account not only the TGT but also the retrosynthetic precursor, or reaction substrate. Thus both retrosynthetic and synthetic analyses are considered in the discussion which follows. 

References are located on pages 92-95. A glossary of terms appears on pages 96-98. 

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