Enantioselective Construction of Spirooxindole-Fused Cyclopenta[c]chromen-4-ones Bearing Five Contiguous Stereocenters via a Stepwise (3+2) Cycloaddition

The bifunctional quinine-catalyzed stepwise (3+2) cycloaddition for the enantioselective construction of spirooxindole-fused cyclopenta[c]chromen-4-ones is developed. The reactions of 3-homoacylcoumarins and alkylidene oxindole electrophiles generate aforementioned spirooxindole-chromenone adducts bearing five contiguous stereocenters, of which one is the spiro all-carbon quaternary stereocenter in high yields (up to 99%) with excellent stereoselectivities (up to >20:1 dr and 99% ee). This methodology was investigated for three different alkylidene oxindole electrophiles and could also be practically demonstrated on a gram scale. Mechanistic investigations revealed that the (3+2) cycloaddition for the enantioselective synthesis of spirooxindole-fused cyclopenta[c]chromen-4-ones is proceeding via a stepwise reaction pathway.