Toward an artificial aldolase

Article abstract of DOI:10.1021/ol702901z

(Chemical Equation Presented) A new functional polymer where proline is bonded to polystyrene through a 1,2,3-triazole linker depicts characteristics targeted for an artificial aldolase. In spite of the hydrophobicity of the polymer backbone, the resin swells in water with building of an aqueous microenvironment. This property, arising from the formation of a hydrogen-bond network connecting the proline and 1,2,3-triazole fragments, is translated into a very high catalytic activity and enantioselectivity toward direct aldol reactions in water.

Full text of DOI:10.1021/ol702901z

ORGANIC
LETTERS
2008
Vol. 10, No. 2
337-340
Toward an Artificial Aldolase
Daniel Font,†,‡ Sonia Sayalero,† Amaia Bastero,† Ciril Jimeno,† and
Miquel A. Perica`s*,†,‡
Institute of Chemical Research of Catalonia (ICIQ), AV. Pa¨ısos Catalans, 16,
E-43007 Tarragona, Spain, and Departament de Qu´ımica Orga`nica,
UniVersitat de Barcelona (UB), E-08028 Barcelona, Spain
mapericas@iciq.es
Received December 4, 2007
ABSTRACT
A new functional polymer where proline is bonded to polystyrene through a 1,2,3-triazole linker depicts characteristics targeted for an artificial
aldolase. In spite of the hydrophobicity of the polymer backbone, the resin swells in water with building of an aqueous microenvironment.
This property, arising from the formation of a hydrogen-bond network connecting the proline and 1,2,3-triazole fragments, is translated into
a very high catalytic activity and enantioselectivity toward direct aldol reactions in water.
The aldol reaction, which is used by nature for the building
of carbohydrate molecules through the use of aldolase
enzymes,1 is a most useful synthetic procedure for the
construction of carbon-carbon bonds. The process can be
catalyzed by simple amino acids and some of their derivatives
which, in this context, have been considered as “micro-
aldolases”.2,3 Macromolecular catalytic systems, able to act
as artificial enzymes,4 are of even greater interest since they
can readily accommodate the structural information required
for substrate recognition and can present suitably placed
hydrophobic and hydrophilic regions, as true enzymes
operating in aqueous, biological systems also do.5
Among macromolecular enzyme mimics, catalytic anti-
bodies have been considered to date as the most advanced
synthetic aldolases.6 In addition to these, dendrimers7 and
polymers8 have gained in recent times considerable ac-
(5) Significant progress has been achieved in recent times on enzymelike
molecules promoting enantioselective aldol reactions in the presence of
water: (a) Mase, N.; Nakai, Y.; Ohara, N.; Yoda, H.; Takabe, K.; Tanaka,
F.; Barbas, C. F., III. J. Am. Chem. Soc. 2006, 128, 734. (b) Hayashi, Y.;
Sumiya, T.; Takahashi, J.; Gotoh, H.; Urushima, T.; Shoji, M. Angew.
Chem., Int. Ed. 2006, 45, 958. (c) Hayashi, Y.; Aratake, S.; Okano, T.;
Takahashi, J.; Sumiya, T.; Shoji, M. Angew. Chem., Int. Ed. 2006, 45, 5527.
(d) Guizzetti, S.; Benaglia, M.; Raimondi, L.; Celentano, G. Org. Lett. 2007,
9, 1247. Successful operation of these systems in purely aqueous media
remains challenging: (e) Aratake, S.; Itoh, T.; Okano, T.; Usui, T.; Shoji,
M.; Hayashi, Y. Chem. Commun. 2007, 2524.
(6) (a) Wagner, J.; Lerner, R. A.; Barbas, C. F., III. Science 1995, 270,
1797. (b) Zhong, G.; Hoffmann, T.; Lerner, R. A.; Danishefsky, S.; Barbas,
C. F., III. J. Am. Chem. Soc. 1997, 119, 8131. (c) Barbas, C. F., III; Heine,
A.; Zhong, G.; Hoffmann, T.; Gramatikova, S.; Bjornestedt, R.; List, B.;
Anderson, J.; Stura, E. A.; Wilson, I. A.; Lerner, R. A. Science 1997, 278,
2085.
† ICIQ.
‡ UB.
(1) Machajewski, T. D.; Wong, C. H. Angew. Chem., Int. Ed. 2000, 39,
1352.
(2) (a) List, B.; Lerner, R. A.; Barbas, C. F., III. J. Am. Chem. Soc. 2000,
122, 2395. (b) Gro¨ger, H.; Wilken, J. Angew. Chem., Int. Ed. 2001, 40,
529.
(3) For some relevant examples of simple chemical entities mimicking
the action mode of enzymes, see: (a) Corey, E. J.; Helal, C. J. Angew.
Chem., Int. Ed. 1998, 37, 1986. (b) Arp, F. O.; Fu, G. C. J. Am. Chem.
Soc. 2006, 128, 14264.
(4) (a) Breslow, R., Ed. Artificial Enzymes; Wiley-VCH: Weinheim,
Germany, 2005. (b) Breslow, R. Acc. Chem. Res. 1995, 28, 146. (c)
Murakami, Y.; Kikuchi, J.-I.; Hisaeda, Y.; Hayashida, O. Chem. ReV. 1996,
96, 721. (d) Motherwell, W. B.; Bingham, M. J.; Six, Y. Tetrahedron 2001,
57, 4663.
(7) (a) Kofoed, J.; Reymond, J.-L. Curr. Opin. Chem. Biol. 2005, 9, 656.
(b) Darbre, T.; Reymond, J.-L. Acc. Chem. Res. 2006, 39, 925.
10.1021/ol702901z CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/21/2007

ceptance as structural elements for the assembly of enzyme-
like systems with catalytic properties, since they allow the
multiple installment of catalytic sites and, to some extent,
control of the hydrophilic/hydrophobic nature of the en-
semble. However, performance of these catalytic systems in
asymmetric aldol reactions has remained low.9
In an attempt to develop a general and practical solution
to this problem, we have recently introduced resin 1a (Figure
1), prepared through “click-chemistry”,10 that efficiently
ary approach (resin 4). Resins 1a-4 were next evaluated in
the reaction of cyclohexanone with benzaldehyde in water,
at room temperature (Table 1). Neither the introduction of a
Table 1. Benchmark Aldol Reaction in Water Catalyzed by
Resins 1a-4
resin
t [h]
yield [%]a
anti:synb
ee [%]c
1a (1% DVB)
1a (2% DVB)
1c (1% DVB)
2
84
60
60
60
84
24
12
67
58
70
10
traces
74
70
95:5
94:6
93:7
n.d.
n.d.
96:4
93:7
95
92
40
91
70
98
97
3
4
4d
a Isolated yield. b Determined by 1H NMR. c Determined by HPLC.
d Reaction at 40 °C.
hydrophilic PEG region in 2, nor the use of the macroporous
resin 3 led to any improvement over 1a. Resin 1c, lacking
the triazole moiety, led to similar yields but much poorer
ee’s. Gratifyingly, resin 4 (see Supporting Information (SI)
for experimental details) exhibited an optimal catalytic per-
formance, with a notable rate acceleration and improved
stereoselectivity over those of the other studied resins. The
problem of low reaction rate, one of the most important lim-
itations of organocatalytic aldol reactions in water,5,11-13 finds
a quite satisfactory solution with resin 4. Thus, running the
reaction at 40 °C, a high acceleration was observed (70%
isolated yield in only 12 h) without deterioration in stereo-
control.
Noticeably, and in sharp contrast with 1a-3 and with other
polystyrene resins (1c), 4 perfectly swells in water. Indeed,
whereas the reaction mixtures with 1-3 were multiphase
systems, with resin 4 a gel-like single phase was formed (see
Figure 2). TGA analysis of the swollen resin showed a water
content up to 24% in weight. According to the resin
functionalization, this corresponds to ca. 36 water molecules
per proline unit.
Figure 1. Polymer-supported prolines tested in water. On Merri-
field resins (1a, 1c, 4); on PS-PEG NovaBioSyn resin (2); and on
Argopore resin (3). Reference monomer (1b).
catalyzes the asymmetric aldol reaction in water.11 Notice-
ably, resin 1a exhibited a much better catalytic performance
than its monomeric counterpart 1b, thus suggesting that the
polystyrene backbone in 1a can play a role similar to that
of the large hydrophobic pocket found in type I aldo-
lases.5a,6c,12-14 Moreover, since 1a was by far a superior
catalyst for aldol reactions than similar hydroxyprolines
directly supported on polystyrene, like 1c,15 it was also
suggested that the electron-rich 1,2,3-triazole fragment was
involved in some manner in the catalytic event.
To test these two hypotheses, natural 4-hydroxyproline was
grafted onto several resins using the same click-chemistry
strategy as for 1a (resins 2 and 3), or following a complement-
(8) (a) Suh, J. Synlett 2001, 1343. (b) Suh, J. Acc. Chem. Res. 2003, 36,
562.
(9) (a) Reymond, J. L.; Chen, Y. J. Org. Chem. 1995, 60, 6970. (b)
Kofoed, J.; Darbre, T.; Reymond, J.-L. Org. Biomol. Chem. 2006, 4, 3268.
(10) (a) Tornøe, C. W.; Meldal, M. In Proceedings of the Second
International and Seventeenth American Peptide Society Symposium; Lebl,
M., Houghten, R. A., Eds.; American Peptide Society and Kluwer Academic
Press: San Diego, 2001; pp 263-264. (b) Tornøe, C. W.; Christensen, C.;
Meldal, M. J. Org. Chem. 2002, 67, 3057. (c) Rostovtsed, V. V.; Green, L.
G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596.
For a review on the concept of click chemistry, see: (d) Kolb, H. C.; Finn,
M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004.
(11) Font, D.; Jimeno, C.; Perica`s, M. A. Org. Lett. 2006, 8, 4653.
(12) For recent alternative approaches to supported proline for aldol
reactions in the presence of water, see: (a) Giacalone, F.; Gruttadauria,
M.; Marculescu, A. M.; Noto, R. Tetrahedron Lett. 2007, 48, 255. (b)
Gruttadauria, M.; Giacalone, F.; Marculescu, A. M.; Lo Meo, P.; Riela, S.;
Noto, R. Eur. J. Org. Chem. 2007, 4688.
Figure 2. Phase behavior of resins 4 (left, gel) and 1a (right,
multiphase) under the benchmark reaction conditions in water.
(13) For a dendrimeric aldol catalyst operational in water, see: Wu, Y.;
Zhang, Y.; Yu, M.; Gang Zhao, G.; Wang, S. Org. Lett. 2006, 8, 4417.
338
Org. Lett., Vol. 10, No. 2, 2008

Table 2. Aldol Reactions in Water Mediated by Resin 4
Figure 3. ONIOM (MP2-FC/6-31G(d,p):PM3/ZDO) calculated
conformers of the hydrogen-bonded diaquo-triazolylproline model
system. High-layer atoms (MP2) are in ball-and-stick form. Low-
layer atoms (PM3) are in tube form. O, red; N, blue; C, gray; H,
white.
To understand the interaction of water with 4, a Monte-
Carlo simulation (MMFF) was performed on a catalyst model
in a cage containing 30 water molecules. Removing from
the model water not involved in hydrogen bonding with the
catalyst led ultimately to two distinct minima, both involving
1
a Isolated yield. b Determined by H NMR on a crude sample. c Deter-
mined by HPLC on a chiral stationary phase. d Isolated yield after 5 h.
only two water molecules connecting the amino acid and
the 1,2,3-triazole moieties. These minima were then re-
optimized with an ONIOM approach (Figure 3).16
The structure involving a dual role (donor and acceptor)
of the proline N-H moiety is predicted to be more stable
by 7.77 kcal‚mol-1. In any case, the important point is that
high-level theoretical calculations predict that a very small
amount of essential water is sufficient to establish a
hydrogen-bonding network between the triazole moiety and
the amine and carboxylic acid functions in proline. Thus,
the results of the theoretical calculations provide an explana-
tion for the ability of 4 to swell in water.
Scheme 1. Effect of Water in the Aldol Reaction Catalyzed by
4: (A) Activity of Water-Swollen vs Anhydrous Resin in
Organic Solvent; (B) Reaction in Water Using Water-Soluble
Reactants
To ascertain whether these water molecules can be
mechanistically involved in the aldol reaction catalyzed by
4, as occurs in type-I aldolases,17 the following comparative
experiment was performed: A sample of resin 4 was swollen
in water, excess water was then removed, and the resin was
transferred to dichloromethane, where the aldol reaction was
studied. In a parallel experiment, the same reaction was
performed with an anhydrous sample of 4. Interestingly, the
(14) For related backbone hydrophobic effects on asymmetric Michael
additions, see: Alza, E.; Cambeiro, X. C.; Jimeno, C.; Perica`s, M. A. Org.
Lett. 2007, 9, 3717.
(15) Kondo, K.; Yamano, T.; Takemoto, K. Makromol. Chem. 1985, 186,
1781.
(16) Dapprich, S.; Komiromi, I.; Byun, K. S.; Morokuma, K.; Frisch,
M. J. THEOCHEM 1999, 461-462, 1.
(17) Heine, A.; DeSantis, G.; Luz, J. G.; Mitchell, M.; Wong, C.-H.;
Wilson, I. A. Science 2001, 294, 369.
1
a Determined by H NMR on a crude sample of the 1,3-diol.
bDetermined by HPLC on a chiral stationary phase after conversion
to the monobenzoyl ester.
Org. Lett., Vol. 10, No. 2, 2008
339

reaction performed with water-swollen 4 was faster, more
diastereoselective, and more enantioselective than the cor-
responding one with anhydrous 4 (Scheme 1A). In sharp
contrast with these results, the related resin 1a behaves as a
much less active catalyst in organic media (DMF) when
dissolved amounts of water are present.11 Although the
elucidation of the exact role exerted by water in aldol
reactions mediated by 4 would require further investigation,
its unique behavior in this respect parallels the effect of
essential water on some natural enzymes used in organic
media.18
An additional experiment was performed to exclude the
possibility of the reaction taking place in a segregated organic
phase constituted by the reactants.5e The self-condensation
of propanal, which is readily soluble in water, was catalyzed
by resin 4 (Scheme 1B). As it can be seen, 1 mol % of 4 is
enough to induce the complete conversion of propanal in
24 h. The corresponding aldol product was formed with high
diastereoselectivity and enantiomeric purity.
performance of 4 in the studied process equals or surpasses
that of small molecule mimics previously used with the same
purpose. Furthermore, resin 4 could be recycled and reused
for at least five times without any apreciable loss in yield or
in stereoselectivity (see SI).
In summary, resin 4 represents a substantial progress
toward the conceptual development of an artificial aldolase.
The particular functional arrangement in the monomer plus
linker ensemble in the resin appears to facilitate the estab-
lishment of a hydrogen bond-based aqueous microphase
around the hydrophobic resin that appears to play a funda-
mental role in its catalytic activity. Thus, 4 is able to induce
fast and highly enantioselective aldol reactions in water or,
whenever the aqueous microenvironment is provided, also
in organic solvents.
Acknowledgment. This work was funded by MEC (Grant
CTQ2005-02193/BQU), DIUE (Grant 2005SGR225), Con-
solider Ingenio 2010 (Grant CSD2006-0003), and ICIQ
Foundation.
The scope of applicability of resin 4 in the direct aldol
reaction of a family of cyclic ketones in water was next tested
with a variety of aldehydes (Table 2). Excellent yields,
diastereoselectivities, and enantioselectivities were recorded
in all the studied cases. As a matter of fact, the catalytic
Supporting Information Available: Experimental pro-
cedures and characterization, and calculated atomic coordi-
nates. This material is available free of charge via the Internet
at http://pubs.acs.org.
(18) Zaks, A.; Klibanov, A. M. J. Biol. Chem. 1988, 263, 3194.
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