Organocatalytic Diversity-Oriented Asymmetric Synthesis of Tricyclic Chroman Derivatives

The tandem oxo-Michael-IED/HDA
and oxo-Michael-IED/HDA-Michael-Aldol condensation
transformations between
(E)-2-hydroxyaryl-2-oxobut-3-enoate derivatives with
enals have been developed in the presence of (S)-diphenylprolinol
trimethylsilyl ether as an organocatalyst. Two types of tricyclic
chroman derivatives were, respectively, obtained, by adjusting the
reactant ratio and reaction temperature, in good yields (up to 96%)
with excellent enantioselectivities (up to >99%) and good diastereoselectivities
(up to >30/1). It should be noted that the divergent chiral chroman
derivatives were obtained by successive reaction of (E)-2-hydroxy­aryl-2-oxobut-3-enoate derivatives with two different
enal substrates in highly catalytic results.