Asymmetric catalytic 1,3-dipolar cycloaddition of α-diazoesters for synthesis of 1-pyrazoline-based spirochromanones and beyond

A highly enantioselective 1,3-dipolar cycloaddition of α-substituted diazoesters with exocyclic enones was achieved with chiral scandium(III)/N,N′-dioxide complex as the catalyst. This protocol provided a facile and efficient route to optically active 1-pyrazoline-based spirochromanones and others with good outcomes(up to 97% yield, 98% ee with >95:5 dr). Moreover,enantioenriched 2-pyrazoline-based spirochromanones were also accessible by switching α-substituted diazoesters to α-diazoacetates. The further specific transformations of chiral pyrazoline-based spiro-compounds to spirocyclopropane derivatives were disclosed as well.