Asymmetric Synthesis of Spiropyrazolone-Fused Dihydrofuran-naphthoquinones Bearing Contiguous Stereocenters via a Michael Addition/Chlorination/Nucleophilic Substitution Sequence

An enantioselective synthesis of spiropyrazolone-fused
dihydrofuran-naphthoquinones
is first demonstrated via a Michael addition/Chlorination/Nucleophilic
substitution sequence. The reactions of 2-hydroxy-1,4-naphthoquinone
and α,β-unsaturated pyrazolones in the presence of the
cinchona alkaloid derived hydrogen-bonding catalyst and NCS provide
spiropyrazolone-fused 2,3-dihydronaphtho[2,3-b]furan-4,9-diones
bearing contiguous stereocenters, of which one is the spiro quaternary
stereocenter in high yields with exclusive diastereoselectivity and
good to excellent enantioselectivities.