Enantioselective Conjugate Addition of Donor–Acceptor Hydrazones to α,β-Unsaturated Aldehydes through Formal Diaza–Ene Reaction: Access to 1,4-Dicarbonyl Compounds

Donor–acceptor monosubstituted hydrazones participate
as
suitable reagents able to undergo an enantioselective formal diaza–ene
reaction with α,β-unsaturated aldehydes under chiral secondary
amine catalysis. This constitutes a new approach for the enantioselective
conjugate addition of hydrazones to enals under metal-free conditions
and leads to the formation of γ-hydrazono carboxylic acids after
oxidation/[1,3]-H shift. The methodology is also useful for the synthesis
of enantioenriched β-substituted α-keto-1,5-diesters by
using the hydrazone moiety as a masked carbonyl group.