Direct organocatalytic aldol reactions in buffered aqueous media

Article abstract of DOI:10.1039/b207664k

Organocatalytic cross-aldol reactions catalyzed by cyclic secondary amines in aqueous media provide a direct route to a variety of aldols including carbohydrate derivatives and may warrant consideration as a prebiotic route to sugars.

Full text of DOI:10.1039/b207664k

Direct organocatalytic aldol reactions in buffered aqueous media†
Armando Córdova, Wolfgang Notz 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, USA. E-mail: Carlos@scripps.edu;
Fax: +1-858-784-2583; Tel: +1-858-784-9098
Received (in Corvallis, OR, USA) 6th August 2002, Accepted 7th October 2002
First published as an Advance Article on the web 14th November 2002
Organocatalytic cross-aldol reactions catalyzed by cyclic
secondary amines in aqueous media provide a direct route to
a variety of aldols including carbohydrate derivatives and
may warrant consideration as a prebiotic route to sugars.
reactions in PBS buffer12 alone were significantly accelerated in
the presence or absence of proline (Table 1, entries 1,2, 4, and
5). We found that addition of sodium dodecyl sulfate (SDS, 0.1
equivalents) as an additive to PBS-buffer was beneficial. Under
these conditions, aldol product 1 was formed smoothly in a
clean reaction. The addition of SDS to the mixture completely
suppressed the uncatalyzed reaction (Table 1, entries 1 and 3)
The aldol reaction is a key carbon–carbon bond-forming
reaction in nature and in the repertoire of the synthetic chemist.
In nature, this reaction is typically catalyzed by enzymes that
utilize either an enamine mechanism (Class I aldolases) or a
zinc cofactor (Class II aldolases), to effect the coupling of
unmodified substrates in aqueous media while achieving
absolute stereocontrol. These features have been particularly
1
3
and also enhanced the solubility of hydrophobic substrates.
Under the conditions studied, minimal formation of aldol
condensation product 2 was observed.14 Further, the proline-
catalyzed reaction in PBS/SDS completely inhibited by addition
of sodium cyanide (Table 1, entry 7), indicating that proline is
the catalyst in this reaction and supporting the role of an imine
as an intermediate in the catalytic mechanism.9b
1
difficult to mimic chemically, despite the plethora of method-
ologies available for the stereoselective construction of virtually
2
any desired aldol product. In fact, synthetically useful aldol
We then turned our attention to further assessing the most
promising catalysts14 in cross-aldol reactions of a variety of
ketones with p-nitrobenzaldehyde (Table 2). Proline-catalysis
was effective for 2-butanone affording aldol adducts 3 and 4 in
good yield with modest regio- and diastereoselectivity (Entry 1)
but provided 5 with no diastereoselectivity when cyclohex-
anone was used as the donor ketone (Entry 2). Furthermore,
reactions in aqueous media are rare and mostly involve
3
4
enzymes or catalytic antibodies that exhibit broad substrate
specificity for the aldehyde acceptor substrate. The study of
amines, amino acids and small peptides as biomimetic catalysts
of a variety of reactions has long intrigued chemists.5 There
are, however, very few reports based on this concept that are of
,6
7
synthetic utility. Recently, it has been reported that primary
3-pentanone was not a donor under -proline catalysis but 7 was
L
and secondary amines are capable of catalyzing intermolecular
aldol reactions with acetone as the donor under physiological
conditions. Furthermore, we and others have reported that
could be obtained under catalysis with (S)-1-(2-pyrrolidinylme-
thyl)pyrrolidine 6 instead (Entry 3). The proline-catalyzed
reaction with hydroxyacetone as the donor afforded diol 8 in
high yield as the sole reaction product (Entry 4). This
transformation was also catalyzed by 6 and pyrrolidine itself to
provide 8 as the only observed regioisomer (Entries 5 and 6).
8
cyclic secondary amines catalyze direct intermolecular aldol
reactions with high regio-, diastereo- and enantioselectivities in
9
organic solvents. We therefore sought to extend this concept to
aqueous reaction media providing for the synthesis of poly-
hydroxylated products under environmentally benign reaction
Table 2 Amine-catalyzed aldol reactions of various ketones with p-
nitrobenzaldehyde in PBS-buffer
conditions.10 We were also interested in examining the
efficiency of this approach from the perspective of its potential
as a prebiotic route to carbohydrates.11
We initially studied the reaction of acetone with p-ni-
trobenzaldehyde (Table 1). As compared to water, aldol
Table 1 Direct catalytic aldol reaction of acetone with p-nitrobenzaldehyde
under various aqueous conditions
†
Electronic supplementary information (ESI) available: Experimental data
and full characterization of all new compounds: see http://www.rsc.org/
suppdata/cc/b2/b207664k
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024
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Hence, all the pyrrolidine-derived catalysts activate hydrox-
yacetone in a highly regioselective manner providing for a
facile entry to the synthesis of vicinal diols.15 None of the chiral
catalysts screened, however, provided product 8 in a diastereo-
or enantioselective manner in the PBS buffer. Next we
investigated whether the biologically significant substrate
dihydroxyacetone (DHA) could be used as a donor in reactions
catalyzed by pyrrolidine-based catalysts. Interestingly, proline
and nornicotine (9) provided only trace amounts of 10 (Entries
polyols under environmentally benign reaction conditions and
may warrant further attention as a prebiotic route to sugars.
This study was supported in part by the NIH (CA27489) and
the Skaggs Institute for Chemical Biology.
Notes and references
1 (a) Y. Mori, K. Manabe and S. Kobayashi, Angew. Chem., Int. Ed.,
2001, 40, 2815; (b) S. Kobayashi, T. Hamada, S. Nagayam and K.
Manabe, Org. Lett., 2001, 3, 165; (c) S. Kobayashi and H. Ishitani,
Chem. Commun., 1995, 1379; (d) S. Nagayama and S. Kobayashi, J.
Am. Chem. Soc., 2000, 122, 11531.
8
and 9). In contrast, 6, (S)-2-(methoxymethyl)pyrrolidine (11),
and pyrrolidine were highly efficient, with 6 being the best
catalyst affording polyol 10 in 90% yield within 2 h (Entries
10–14). The (S)-1-(2-pyrrolidinylmethyl)pyrrolidine-catalyzed
2
D. A. Evans, J. V. Nelson and T. R. Taber, Stereoselective Aldol
Condensationsin Topics in Stereochemistry, ed. E. L. Eliel and S. H.
Wilen, Wiley-Interscience, New York, 1982, Vol 13, p. 1; C. H.
Heathcock, The Aldol Addition Reactionin Asymmetric Synthesis, ed. J.
D. Morrison, Academic Press, New York, 1984, Vol. 3, Chapter 2, p.
111.
reaction with dihydroxyacetone was diastereoselective in
DMSO/PBS, providing 10 with a dr of 2+1 (Entry 11). This is
the first demonstration of the use of unprotected dihydrox-
yacetone as a donor in an organocatalytic cross-aldol reaction.
Because dihydroxyacetone dimerizes in organic solvent, none
of the catalysts tested provide 10 under non-aqueous reaction
conditions. The reactivity of the tested organocatalysts was in
the following order: 6 > 11 > pyrrolidine > proline > 9. As
a diamine, 6 can be viewed as a mimetic of the two lysines key
3 J. H. M. Gijsen, W. Fitz and C.-H. Wong, Chem. Rev., 1996, 443; C.-H.
Wong and G. Whitesides, Enzymes in Synthetic Organic Chemistry,
Pergamon Press, Oxford, 1994.
4
5
6
J. Wagner, R. A. Lerner and C. F. Barbas III, Science, 1995, 270,
797.
H. D. Dakin, J. Biol. Chem., 1910, 7, 49; H. S. Raper, J. Chem. Soc.,
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W. Langenbeck and G. Borth, Ber., 1942, 75, 951; T. A. Spencer, H. S.
Neel, T. W. Flechtner and R. A. Zayle, Tetrahedron Lett., 1965, 3889;
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F. Barbas III, Chem. Commun., 2001, 769.
1
16
to the mechanism of class I aldolases. In all the transforma-
tions reported in Table 2 only trace amounts of the desired
products were observed in the absence of organocatalyst.
1
L
-Proline and 6 were also efficient catalysts in aqueous media
of intermolecular aldol reactions involving nonactivated accep-
17,18
tors affording products 12–20 (Table 3).
Moreover, the
results presented in Table 3 demonstrate that catalysis by small
organic molecules provides a direct route to monosaccharides.
For example, catalyst 6 provided benzyl-protected pentulose 17.
This sugar was isolated as a single diastereomer possessing an
anti-configuration of the vicinal hydroxy groups (Entry 6).19 In
7
8
M. S. Sigman and E. N. Jacobsen, J. Am. Chem. Soc., 1998, 120, 4901;
T. E. Hortstmann, D. J. Guerin and S. J. Miller, Angew. Chem., Int. Ed.,
2
000, 39, 3635.
(a) J.-L. Reymond and Y. Chen, Tetrahedron Lett., 1995, 36, 2575; (b)
J.-L. Reymond and Y. Chen, J. Org. Chem., 1995, 60, 6970; (c) T. J.
Dickerson and K. D. Janda, J. Am. Chem. Soc., 2002, 124, 3220. For an
aldol reaction catalyzed by a Zn(II) complex and the ethyl ester of
tyrosine in water, see: M. Nakagawa, H. Nakao and K.-I. Watanabe,
Chem. Lett., 1985, 391.
addition, natural sugar derivatives such as protected
D-fructose
(1 of 4 sugars formed in this reaction) were obtained under
physiological conditions by enamine catalysis (Entry 9).
In conclusion, we have demonstrated that small organic
molecules can catalyze direct intermolecular aldol reactions
involving a variety of ketones, including dihydroxyacetone in
aqueous media. Reactions involving hydroxyacetone were
highly regioselective. Furthermore, our study suggests that
naturally occurring sec-amines with the pyrrolidine structural
motif can catalyze the synthesis of monosaccharides under
physiological conditions. Moreover, the use of organocatalysts
provides an inexpensive efficient route for the synthesis of
9
(a) B. List, R. A. Lerner and C. F. Barbas III, J. Am. Chem. Soc., 2000,
1
22, 2395; (b) W. Notz and B. List, J. Am. Chem. Soc., 2000, 122, 7386;
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1
0 C.-J. Li and T.-H. Chan, Organic reactions in aqueous media, John
Table 3 Amine-catalyzed aldol reactions between acetone or DHA and
Wiley & Sons, New York, 1997.
various aldehydes in aqueous media
11 R. M. Degraaf, J. Visscher, Y. Xu, G. Arrhenius and A. W. Schwartz, J.
Mol. Evol., 1998, 47, 501; R. Krishnamurthy, S. Pitsch and G.
Arrhenius, Orig. Life Evol. Biosph., 1999, 29, 139.
1
1
2 0.01M Phosphate buffer, 2.7 mM KCl, 137 mM NaCl, pH = 7.4.
3 Although generally formed as a racemate, aldol product 1 can also be
obtained enantiomerically enriched with ee’s of up to 63% by
performing the experiment in organic solvent containing 10 vol% of
water. Solvents used (ee): DMSO/H
2
O = 9+1 (40%); DMF/H
2
O =
O =
1
0+1 (35%); EtOH/H
2
O = 9+1 (16%); EtOH (19%), Dioxane/H
2
1
0+1 (63%).
1
1
4 See ESI†, Table 1S.
5 See also: S. Bahmanyar and K. N. Houk, J. Am. Chem. Soc., 2001, 45,
1
1
1273; J.-F. Lin, C.-C. Wu and M.-H. Lien, J. Phys. Chem., 1995, 99,
6903.
1
1
6 A. Heine, G. Desantis, J. G. Luz, M. Mitchell, C.-H. Wong and I. A.
Wilson, Science, 2001, 294, 369.
7 The reactions also proceeded in the following organic solvents: THF,
dioxane, EtOH and MeOH.
1
1
8 The products were obtained as racemates in DMSO/PBS 1+1.
1
13
9 The H and C NMR data of the isolated 5-O-benzyl-D-xylulose were
identical to that produced by rabbit muscle aldolase (RAMA)-catalysis.
See: M. D. Bednarski, E. S. Simon, N. Bischofberger, W.-D. Fessner,
M.-J. Kim, W. Lees, T. Saito, H. Waldmann and G. M. Whitesides, J.
Am. Chem. Soc., 1989, 111, 627.
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