1.1.2 Michel Addition
Michel addition of silyl ketene acetals to cyclic and acyclic α,β -unsaturated ketones has been studied. In these reactions, the addition of catalytic amount of Ph3PO increases the enantioselectivity because it could trap Me3Si species that could form during the reaction. For example, BLA 1b has been used for the Michel addition of cyclo hexenone with silyl ketene acetal to afford key intermediate for the enantioselective synthesis of caryophyllene (Scheme 7). The absolute stereochemical course of the reaction can be rationalized by the above proposed transition states.
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Examples:
Scheme 7
1.1.3 [3+2] Cycloaddition
Several benzoquinones proceed reactions with 2,3-dihydrofuran in the presence of BLA 1b to afford a variety of chiral phenolic tricycles with high enantioselectivities. The application of this reaction has been demonstrated in the total synthesis of aflatoxin B2. The reaction pathway has been elucidated by performing the reaction in the presence of excess of 2,3-dihydrofuran (Scheme 8).
Scheme 8