Efficient Baylis−Hillman Reactions of Cyclic Enones in Methanol As Catalyzed by Methoxide Anion

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Abstract

The Baylis-Hillman reactions of cyclic enones with a variety of aldehydes were investigated. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was found to be a viable catalyst in promoting the reactions of sterically retarded substrates in methanol. The reactions showed clear solvent dependence and only occurred in hydroxylic solvents, especially in methanol. Further consideration on the steric character of DBU and its high basicity jointly with other experimental observations suggests that the methoxide anion should be the "true" Baylis-Hillman catalyst. This has been confirmed by the effectiveness of similar reactions directly employing methoxide as the catalyst. The reaction pathways of this type of catalysis are proposed to depend on the choice of substrates. Supporting experimental observations were demonstrated and discussed in relation to mechanistic considerations. This study also reveals that both DBU and sodium methoxide can be successfully applied as effective catalysts in methanol to promote the Baylis-Hillman reactions for a range of cyclic enones including cyclopent-2-enones, cyclohex-2-enones, gamma-pyrone, and 1-benzopyran-4(4H)-ones.

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