Desymmetrization of Oxabicyclo Substrates

Lautens and Ma made use of Brown s asymmetric hydroboration reaction to afford optically enriched alcohol 104 in 83% ee,Eq. 78 [120,121]. 

Entry Product Substrate Reaction Yield ee Refer- 

Deprotonation of [3.2.1] oxabicyclic substrates by homochiral base 105 has been investigated by Simpkins [121]. Initial enantiomeric excesses of about 80% can be achieved which are improved by successive recrystallizations. Table 5 summarizes these transformations. 

Asymmetric hydroboration of substrates 106 a and 106b yielded exo alcohols with high enantioselectivities, Eq. 79 [120]. 

This section describes the most commonly used methods used to cleave one or more bonds within the oxabicydic framework. Aspects of this subject have been surveyed in other reviews describing the synthetic utility of [4+2] and [4+3] cycloaddition reactions [1,25,119,122,123]. The retro-Diels-Alder reactions of oxabicyclo[2.2.1] compounds under thermolytic conditions resulting in the extrusion of furan, acetylene or other stable species will not be covered, but the interested reader is directed to a recent review on this topic [122], 

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