T. Koch et al., ANTIBODY CATALYSIS OF MULTISTEP REACTIONS - AN ALDOL ADDITION FOLLOWED BY A DISFAVORED ELIMINATION, Journal of the American Chemical Society, 117(37), 1995, pp. 9383-9387
The intramolecular aldol condensation of keto-aldehyde 1 yields a subs
tituted 2-benzyl-3-hydroxy-cyclohexanone 2 and subsequently 2-benzyl-2
-cyclohexenone (3). The sequence involves four individual reaction ste
ps, Three of these steps can be accelerated using general acid-base ca
talysis to effect proton transfer at or near the alpha-carbon of the k
etone involved in the condensation, which is at the homobenzylic posit
ion relative to the aromatic group of the substrate (Ar). An antibody
to the corresponding N-benzyl-N-methylpiperidinium hapten 5 was found
to catalyze the entire reaction sequence. This antibody seems to act p
urely as a general base and does not catalyze the carbon-carbon bond f
orming step. Catalysis of the aldol elimination is selective for the d
isfavored trans-elimination with a single enantiomer of stereoisomer 2
a. Catalysis is suppressed by incubating the antibody with a carboxyl-
specific reagent, suggesting that a carboxyl group acts as a general b
ase to catalyze the sequence. The antibody is approximately 2.0 x 10(5
) times more reactive than acetate for catalysis of the sequence. Thes
e experiments demonstrate that catalysis of reactions with several con
secutive transition states is possible using catalytic antibodies.