Antibody-catalyzed enantioselective Robinson annulation

Full text of DOI:10.1021/ja970944x

J. Am. Chem. Soc. 1997, 119, 8131-8132
8131
Previously, two antibodies 33F12 and 38C2, capable of
catalyzing a broad range of intermolecular aldol reactions
between ketone donors and aldehyde acceptor substrates, were
produced using the strategy of reactive immunization7,8 with
the â-diketone hapten 1. A carbonyl group of 1 reacts with
the ꢀ-amino group of the lysine residue within the active site
of the antibody to form a tetrahedral hemiaminal which
subsequently dehydrates and then tautomerizes to the stable
vinylogous amide 2 (eq 1).
Antibody-Catalyzed Enantioselective Robinson
Annulation
Guofu Zhong,† Torsten Hoffmann,† Richard A. Lerner,*,†
Samuel Danishefsky,*,‡ and Carlos F. Barbas III*,†
The Skaggs Institute for Chemical Biology
and the Department of Molecular Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, California 92037
Laboratory for Bioorganic Chemistry
The Sloan-Kettering Institute for Cancer Research
1275 York AVenue, New York, New York 10021
ReceiVed March 25, 1997
We report an antibody that is remarkable in that it catalyzes
both steps of an important synthetic transformation, the Rob-
inson annulation. The Robinson annulation which accomplishes,
in net terms, the conversion of a f c occupies a key role in
organic synthesis.1 In most instances, the overall annulation is
comprised of an alkylation (or Michael addition) step leading
to b followed by a cyclodehydration step to give a cycloalkenone
b f c.
We first describe an experiment which probed the feasibility
of antibody catalysis of the cyclodehydration step while
exploring the possibility of kinetic resolution at this step. For
these purposes, we prepared the (S)- and (R)-versions of
compound 3 by known chemistry.9 As shown in Table 1, both
antipodes (S)- and (R)-3 were efficient substrates for the
annulation reaction with Ab38C2, leading to (S)- and (R)-4,
respectively (eqs 2 and 3). We took this powerful catalysis of
A particularly well-known synthetic intermediate, which is
assembled through a Robinson annulation sequence, is the
Wieland-Miescher (WM) ketone 6.2 The Robinson annulation
reaction, in general, and the WM ketone, in particular, have
been employed on countless occasions in the synthesis of natural
products, notably steroids and terpenoids.1a-c For instance, a
recent total synthesis of taxol started with the (S)-antipode of
the WM ketone.3 For this, as well as other applications,4,5 the
availability of the enantiopure version of the WM ketone is
enormously helpful. In practice, however, the enantioselection
in the cyclization step of prochiral 5 en route to 6 is on the
order of 70% enantiomeric excess (ee).2,6 Fractional crystal-
lization, with attendant losses, is necessary to attain acceptable
homogeneity of the desired (R)- or (S)-antipode of 6 or
derivatives thereof. The goals of the research described herein
were to explore the possibility of utilizing catalytic antibodies
to achieve the entire Robinson annulation sequence (a f b f
c) and the cyclodehydration step (b f c). For the case of the
synthesis of the WM ketone, we explore whether enantioselec-
tion in the last step (cf. 5 f 6) could be achieved under
governance by the catalytic antibody.
the cyclodehydration step to be a positive omen for the
feasibility of at least the cyclodehydration phase of the Robinson
annulation under mediation by Ab38C2. Surprisingly, the
values of kcat for the cyclization of each enantiomer are quite
comparable. Hence, the catalytic antibody is not kinetically
responsive to the enantiosense of the remote C2-stereogenic
center of the cyclohexanone moiety.
From this perspective, we addressed the WM ketone problem.
Here, three issues were under consideration. First, the gross
feasibility of cyclodehydration step had to be determined. Thus,
the vulnerability of the WM ketone to retro-Claisen type
cleavages10 is well-known. Second was the critical issue as to
whether enamine produced by the chiral catalytic antibody could
differentiate the two ring ketones flanking the prochiral qua-
ternary carbon at C2. Finally, the possibility of mediation by
Ab38C2 of the Michael phase (cf. a f b) would be determined.
Antibody catalysis of the formation of the annulation product
6 and consumption of the triketone 5 were monitored by
reversed-phase high-performance liquid chromatography (RP-
HPLC). The intramolecular aldol condensation catalyzed by
both antibodies followed Michaelis-Menten kinetics. We chose
† The Skaggs Institute for Chemical Biology and the Scripps Research
Institute.
‡ The Sloan-Kettering Institute for Cancer Research.
(1) (a) Rapson, W. S.; Robinson, R. J. Chem. Soc. 1935, 1285. (b)
Friedman, C. A.; Robinson, R. Chem. Ind. (London) 1951, 777. (c) Cocker,
J. D.; Halsall, T. G. J. Chem. Soc. 1957, 3441. (d) Sondheimer, F.; Elad,
D. J. Am. Chem. Soc. 1957, 79, 5542. (e) Jung, M. E. Tetrahedron 1976,
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Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl. 1971, 10,
496. (c) Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615.
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L. B.; Magee, T. V.; Jung, D. K.; Isaacs, R. C. A.; Bornmann, W. G.;
Alaimo, C. A.; Coburn, C. A.; Di Grandi, M. J. J. Am. Chem. Soc. 1996,
118, 2843.
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(8) Bjo¨rnestedt, R.; Zhong, G.; Lerner, R. A.; Barbas, C. F., III J. Am.
Chem. Soc. 1996, 118, 11720.
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(10) Wendler, N. L.; Slates, H. L.; Tishler, M. J. Am. Chem. Soc. 1951,
73, 3816.
(11) (a) Kikuchi, K.; Thorn, S. N.; Hilvert, D. J. Am. Chem. Soc. 1996,
118, 8184. (b) Hollfelder, Fl.; Kirby, A. J.; Tawfik, D. S. Nature 1996,
383, 60.
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Org. Chem. 1988, 53, 93.
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Chem. Soc. 1985, 107, 423.
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S0002-7863(97)00944-X CCC: $14.00 © 1997 American Chemical Society

8132 J. Am. Chem. Soc., Vol. 119, No. 34, 1997
Communications to the Editor
Table 1. Kinetic Parameters for Antibody-Catalyzed Annulation
Scheme 1
Reactionsa
substrate
kcat (min-1
)
Km (mM)
kun (min-1
)
kcat/kun
nd
(S)-3
(R)-3
5
0.186
0.126
0.086
12.4
2.45
2.34
nd
nd
nd
2.4 × 10-8
3.6 × 106
a Conditions: pH 6.5 in the presence of 10 µM active sites of 38C2.
to study catalysis by the antibody Ab38C2 in detail. The
Michaelis constant Km and catalytic rate constant kcat values were
determined to be 2.34 mM and 8.6 × 10-2 min-1, per active
site, respectively. The background rate of this annulation
reaction in the absence of antibody was 2.4 × 10-8 min-1
providing a rate enhancement of 3.6 × 106 (kcat/kun). Other
antibodies that bound hapten 5 with high affinity albeit in a
noncovalent fashion were also studied and were found not to
catalyze this reaction. Since bovine serum albumin, BSA, is
known to possess a lysine residue within a permissive hydro-
phobic binding pocket and is also known to catalyze a variety
of reactions, we studied catalysis of this reaction by BSA. No
catalysis above background was observed when the antibody
was substituted by a molar equivalent of BSA. Traditionally,
the WM ketone 6 is synthesized using optically active proline
in organic solvent.2,6,12 No significant catalysis was observed
when L-proline substituted for antibody in our aqueous reaction,
even when provided at a concentration equimolar with substrate.
To confirm that catalysis proceeded as designed, we studied
the reaction in the presence of the mechanism-based inhibitor
2,4-pentanedione. When 1 equiv of 2,4-pentanedione was
incubated with Ab38C2 prior to the aldol condensation assay,
the catalytic activity was inhibited 71% even though 5000 equiv
of triketone 5 was employed. This result demonstrates that the
intramolecular aldol condensation takes place at the active site
of the antibody with the essential participation of the ꢀ-amino
group of the lysine residue. The proposed mechanism for this
reaction is shown (Scheme 1). We failed to observe accumula-
tion of an aldol addition intermediate suggesting that the
elimination step occurs within the active site of the catalyst,
perhaps with assistance of the enamine intermediate shown.
The enantioselectivity of this antibody-catalyzed cyclization
(the final step of the Robinson annulation) was studied by chiral
normal-phase HPLC analysis, chemical shift reagent (europium
tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphor-
the presence of Ab38C2. Indeed, the one-flask annulation could
be conducted. Catalysis of the Michael addition step was
modest, kcat/kun ) 125, and was not inhibited by the addition of
2,4-pentanedione.
In summary, we have successfully used reactive immunization
to generate catalysts to achieve the cyclodehydration step of
the Robinson annulation reaction and, indeed, the entire process.
The catalyst, which uses the enamine mechanism of natural class
I aldolase enzymes, demonstrates an extremely high rate
enhancement and control over prochiral discrimination. These
results underscore the ability of the process of reactive im-
munization to generate efficient biocatalysts. It seems likely
that catalysts generated using the strategy of reactive immuniza-
tion will have advantages in terms of mechanistic programming
and scope over antibody catalysts generated through the
traditional transition-state analog approach alone. Since the
selective pressure for antibody induction in the reactive im-
munization strategy is focused on chemical reactivity, the
catalysts may not be highly refined with respect to noncovalent
contacts which do not contribute much to the chemical reactivity
of the antibody pocket. The end result can be, as is the case
for the catalysts here, catalytic activity coupled with a permissive
binding pocketsthe key components of useful synthetic cata-
lysts. Having demonstrated the ability of this catalyst to
accomplish complex transformations in series, we expect it to
be used to prepare even less accessible compounds. Toward
this end, this catalyst will be commercially available from
Aldrich Chemical Company, thus allowing the chemical com-
munity to further define its scope.
1
ate]) H NMR analysis,12 and optical measurement of product
obtained from a 100 mg scale reaction. Only a single enanti-
omer was detected in HPLC and NMR studies, suggesting 6
was formed in >95% ee. The product demonstrated a 96%
optical purity by polarimetry. The enantioselectivity of the
antibody-catalyzed Robinson annulation is much higher than
that obtained with optically active proline induced annulation
in organic solvent (ca. 70% ee).2,6
Acknowledgment. This study was supported by the NIH [CA27489
and AI16943 (S.J.D.)] and the Skaggs Institute for Chemical Biology.
T.H. thanks the Alexander von Humbol Foundation, Germany, for a
Feodor Lynen Fellowship. C.F.B. acknowledges an Investigator Award
from the Cancer Research Institute.
To investigate the possibility of conducting the entire
Robinson annulation transformation through catalytic antibody
mediation, we studied the reaction of 2-methyl-1,3-cyclo-
hexanedione with methyl vinyl ketone in aqueous medium in
Supporting Information Available: Experimental details for the
preparation, HPLC purification, and determination of optical purity of
6 and Michaelis-Menten plots for substrates (R and S)-3 and -6 (4
pages). See any current masthead page for ordering and Internet access
instructions.
(12) Buchschacher, P.; Fu¨rst, A.; Gutzwiller, J. Org. Synth. 1984, 63,
368.
JA970944X