Asymmetric Diels-Alder Reactions of Sulfonyl-Functionalized α,β-Unsaturated Ketones with Cyclopentadiene Catalyzed by Chiral Lewis Acid

Abstract

Asymmetric Diels-Alder reactions of (E)-1-substituted sulfonyl-3-penten- 2-ones with cyclopentadiene catalyzed by a chiral titanium reagent were investigated. The enantioselectivity was studied with different substituents in the position of sulfonyl moiety. 6-Methyl-5-(substituted-sulfonylacetyl)bicyclo[2.2.1]hept-2-enes (4) were prepared in high yield with high optical purity. The absolute configuration of the cycloadducts was determined by an alternative synthetic route to compound 4, which was prepared by an asymmetric Diels-Alder reaction of the acyloxazolidinone 6 with cyclopentadiene, followed by nucleophilic addition of substituted sulfonylmethyl lithium to the product. Crystals of (5R,6S)-6-methyl-5-(phenylsulfonylacetyl)bicyclo-[2.2.1]hept-2-ene (4a) with >99% ee was prepared and studied. A mechanism of asymmetric reaction of sulfonyl-functionalized α,β-unsaturated ketone with cyclopentadiene is presented.