Asymmetric Reactions - A Rich Reservoir

Even a fourth variant is feasible, namely the submission of meso-compounds -these contain stereogenic centers but feature internal elements of symmetry and, hence, are achiral - to a desymmetrization, which in principle generates only one stereoisomer. The classical case is the hydrolysis of a diester to form a half-acid/es-ter as a single antipode. Generation of both optical isomers of synthetically useful 1,2-diol products in good yield and stereoisomeric purity applying this methodology has recently been demonstrated (see Fig. 2.11) [65]. 

Elegant use of a desymmetrization protocol has recently been reported as a key step in a total synthesis of (-i-)-cocaine [66]. In this case, submitting a meso-dialde-hyde to a prohne-catalyzed (20 mol%) intramolecular enol-exo-aldol reaction afforded a 1 1 mixture of the axial-equatorial aldol product in excellent yield (91%). A few concluding final steps - amongst others a chromatographic separation of 

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