An operationally simple and scalable synthesis of enantiomerically pure bicyclo[2.2.2]octadiene (bod*) ligands relying on an organocatalytic one-pot Michael addition-aldol reaction with cheap 2-cyclohexenone and phenylacetaldehyde is presented. The crystalline bicyclic product 4a (6-hydroxy-5-phenylbicyclo[2.2.2]octan-2-one) is transformed into phenylbicyclo[2.2.2]oct-5-en-2-one 2, a versatile starting material for the 2-step synthesis of both symmetrical, such as Hayashi's Ph-bod* ligand, as well as novel unsymmetrical chiral dienes.
A practical enantioselective route to chiral 5-phenylbicyclo[2.2.2]oct-5-en-2-one 1 has been designed and developed. The target compound has been obtained as colorless crystals in 22% yield from 2-cyclohexenone, with an enantiomeric ratio higher than 99.5:0.5 and notably high chemical purity (> 99%). Three intermediates out of nine chemical steps are isolated. It is noteworthy that this process is devoid of any chromatography or distillation although all but one intermediate are oils. Key to success was the optimization of an intramolecular aldol reaction of an in situ prepared ketone aldehyde leading to the solid intermediate (1R,4R,4S,6S)-6-hydroxybicyclo[2.2.2]octan-2-one 9a that is isolated in very high chemical and chiral purity. This is an example of an intramolecular crystallization-induced diastereomer transformation (CIDT). The dehydration of this secondary alcohol to 1 required an extensive screen of reaction conditions to secure an excellent purity, essential for crystallization of this low-melting compound. The final process is simple and concentrated as demonstrated by an expeditious synthesis of 1 kg of 1 in a 30-L reactor in 10 working days.
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