Enantioselective Synthesis of Spiropyrazolone-Fused Cyclopenta[<i>c</i>]chromen-4-ones Bearing Five Contiguous Stereocenters via (3+2) Cycloaddition

An enantioselective synthesis of
spiropyrazolone-fused cyclopenta­[c]­chromen-4-ones
is demonstrated via a (3+2) cycloaddition
reaction. The reactions of 3-homoacylcoumarins and α,β-unsaturated
pyrazolones in the presence of the cinchona-alkaloid derived hydrogen-bonding
catalyst provide aforementioned spiropyrazolone-chromenone adducts
bearing five contiguous stereocenters, of which one is the spiro all-carbon
quaternary stereocenter in high yields (up to 98%) with good to excellent
stereoselectivities (>25:1 dr and up to 99% ee). This one-pot methodology
could also be practically demonstrated on a gram-scale with similar
efficacy.