Organocatalytic Enantioselective Vinylogous Aldol Reaction of 5‑Alkyl-4-Nitroisoxazoles to Paraformaldehyde

An
organocatalytic asymmetric aldol reaction of 5-alkyl-4-nitroisoxazoles
to paraformaldehyde has been developed. By using a chiral dipeptide-based
urea–amide–guanidinium as the phase-transfer catalyst,
the transformations were promoted by sodium acetate, leading to a
range of enantio-enriched isoxazole derivatives in high yields with
moderate to excellent enantioselectivities. This work represents the
first example of constructing chiral C–C bonds at the α-position
of 5-alkyl-4-nitroisoxazoles. The viability of readily accessing chiral
fungicides from the obtained aldol products demonstrates the important
utility of this method.