• The mechanism starts with amine (1) attacking the β- carbon of β-
keto esters (2) & forms an enamine(3).The enamine attacks the
carbonyl carbon of α-ketones(4).This is followed by the loss of
H2O,giving the imine(5).This intermediate undergoes an
intramolecular nucleophilic attack forming 5-membered ring
(6).Hydrogen is eliminated and the π-bonds are rearranged in the
ring ,yielding the final product.
• The other mechanism includes the enamines (3) attacks the α-
carbon of the α-halo ketone(4) as a part of a nucleophilic
substitution instead of carbonyl carbon.
A 2007 study revised the reaction mechanism and based on NMR spectroscopy
and theoretical calculations favors an O,O-dicationic intermediate (a super
electrophile) over the N,O dicationic intermediate . For preparative purposes
triflic acid is recommended: