News and Publications
The Skaggs Institute For Chemical Biology
Scientific Report 1997-1998
Antibody-Catalyzed Organic and Organometallic Transformations and Chemical
Libraries of Annonaceous Acetogenins
E. Keinan, S.C. Sinha,* F. Grynszpan,* P. Neogi, H. Avedissian, A. Yazbak,
S.C. Sinha, D. Shabat, H. Itzhaky, A. Haskel, S. Nimri, O. Levy, S. Saphier,
H. Shulman, A. Shulman, A. Brik
* The Scripps Research Institute
Antibody-Catalyzed Organic and Organometallic Transformations
Catalytic antibodies offer unique opportunities to examine mechanistic hypotheses
and the relative importance of individual design elements in catalysis. We showed
that antibodies elicited against the cyclopropenone hapten 1 (in Fig.
1) catalyze ester hydrolysis with a 1000-fold enhancement in rate. Because hapten 1 does
not mimic the shape of the transition state, yet elicits efficient catalysts,
it most likely generates the necessary charge complementarity in the active site.
Thus, haptenic tetrahedral geometry is a desirable feature but not a prerequisite
for generating hydrolytic antibodies.
This study, together with information reported about a cyclopropenone-containing
protease inhibitor, highlights the potential applications of the rarely used
cyclopropenone functionality in the future design of both haptens and enzyme
inhibitors. The versatility of the antibodies to hapten 1 was indicated
by their ability to catalyze the ring opening of epoxides to the corresponding
One obvious need in antibody catalysis, particularly for practical applications
in organic synthesis, is an increase in the cost-effectiveness of the antibodies.
We reported the first successful noncovalent entrapment of catalytic antibodies
in a sol-gel matrix. Antibodies entrapped directly within a tetramethoxysilane-derived
glass retained their activity for long periods. This finding suggests that this
continuous-flow-reactor approach is the method of choice for preparative-scale
organic synthesis that uses catalytic antibodies. We envisage that the catalytic
reactor will allow convenient changes in reaction conditions, the substrate,
and even the reaction type.
We showed that catalytic antibodies elicited against a metalloporphyrin hapten
mimic some of the enzymatic features of cytochrome P-450. We improved the haptens
designed to elicit substrate-selective catalytic antibodies that function as
P-450 analogs. For example, a water-stable porphyrin hapten (2 in Fig.
1) presents the metalloporphyrin cofactor together with the organic substrate
in the correct orientation to mimic the transition state of several oxygenation
reactions, including epoxidation, sulfoxidation, and hydroxylation.
The importance of selective isotopic labeling of organic compounds stems
not only from the value of such labeling in elucidating chemical mechanisms and
biosynthetic pathways but also from the usefulness of labeled compounds in biomedical
applications. Deuteration of ketones at the α-position with deuterium oxide
requires strong basic conditions. Moreover, with polyfunctional ketones, the
lack of selectivity and the occurrence of side reactions are serious drawbacks.
The mechanism by which catalytic lysine antibodies, such as the antibody
38C2, promote the aldol condensation involves activation of the carbonyl donor
in the enamine form. In the absence of a proper carbonyl acceptor, this enamine
can only hydrolyze back to its ketone precursor. This equilibrium provides the
means for efficient exchange of the α-hydrogens with the water hydrogens
within the binding site of the antibody. Lysine antibodies can be highly selective
catalysts for α-deuteration of ketones and aldehydes, allowing chemoselective,
regioselective, and enantioselective reactions to be carried out under neutral
aqueous conditions at room temperature. Preliminary experiments with the antibody
38C2 indicated that deuterium labeling of ketones with this catalyst occurs with
rate enhancements that rival those of enzyme catalysis.
Chemical Libraries of Annonaceous Acetogenins
Annonaceous acetogenins, particularly those with adjacent bis-tetrahydrofuran
(THF) rings, have remarkable cytotoxic, antitumor, antimalarial, immunosuppressive,
pesticidal, and antifeedant activities. Many of these fatty acid derivatives
have similar carbon skeletons; their striking diversity originates mainly from
the relative and absolute configuration of their various stereogenic oxygen functions.
We used our "naked carbon skeleton" strategy to synthesize many naturally
occurring and nonnatural analogs of bis-THF acetogenins. In this approach, enantioselective
olefin oxidation methods are combined with ligand-assisted chirality transfer
techniques based on the chemistry of rhenium(VII) oxides. A set of empirical
rules has been deduced to predict the stereochemistry of the THF rings on the
basis of the behavior of 4 possible isomeric dienol substrates. These rules are
valid for polycyclization with trifluoroacetylperrhenate of polyenols in a single
step producing a single isolatable compound. The acetogenins synthesized include
mono-THF structures, such as solamin and reticulatacin; bis-THF compounds, such
as asimicin, bullatacin, trilobacin, trilobin, uvaricin, squamotacin, and rolliniastatin;
and tris-THF molecules, such as goniocin and its isomers.
Grynszpan, F., Keinan, E. Use of antibodies to dissect the components
of a catalytic event: The cyclopropenone hapten. Chem. Commun. 865, 1998.
Neogi, P., Doundoulakis, T., Yazbak, A., Sinha, S.C., Keinan, E. Total
synthesis of mucocin. J. Am. Chem. Soc., in press.
Shabat, D., Shulman, H., Itzhaky, H., Reymond, J.L., Keinan, E. Enantioselectivity
vs kinetic resolution in antibody catalysis: Formation of the (S) product
despite preferential binding of the (R) intermediate. Chem. Commun. 1759,
Shulman, A., Shabat, D., Barbas, C.F. III, Keinan, E. Teaching catalytic
antibodies to undergraduate students: An organic chemistry lab experiment. J.
Chem. Educ., in press.
Sinha, S.C., Keinan, E., Sinha, S.C. Rules of stereoselectivity in
the tandem oxidative cyclization reaction with rhenium(VII) oxides. J. Am. Chem.
Soc. 120:9076, 1998.
Sinha, S.C., Sinha, A., Sinha, S.C., Keinan, E. Tandem oxidative cyclization
with rhenium oxide: Total synthesis of 17,18-bisepi-goniocin. J. Am. Chem. Soc.
Sinha, S.C., Sinha, A., Sinha, S.C., Keinan, E. Total synthesis of
goniocin and cyclogoniodenin T: Unique biosynthetic implications. J. Am. Chem.
Soc. 120:4017, 1998.
Yazbak, A., Sinha, S.C., Keinan, E. Total synthesis of uvaricin. J.
Org. Chem. 63:5863, 1998.