Water-compatible organocatalysts for direct asymmetric syn-aldol reactions of dihydroxyacetone and aldehydes

Article abstract of DOI:10.1021/ol8002833

A novel organocatalyst was developed that effectively catalyzed the reactions of unprotected or protected dihydroxyacetone with a variety of aldehydes to provide syn-aldol products with good yields and ee values up to >99%. Significantly, this amide catalyst was effective with a variety of nonaromatic aldehyde acceptors that had proven difficult in the presence of other catalysts. Reactions of protected dihydroxyacetone proceeded in aqueous media without addition of organic solvents.

Full text of DOI:10.1021/ol8002833

ORGANIC
LETTERS
2008
Vol. 10, No. 8
1621-1624
Water-Compatible Organocatalysts for
Direct Asymmetric syn-Aldol Reactions
of Dihydroxyacetone and Aldehydes
S. S. V. Ramasastry, Klaus Albertshofer, Naoto Utsumi, and Carlos F. Barbas III*
The Skaggs Institute for Chemical Biology and the Departments of Chemistry and
Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road,
La Jolla, California 92037
carlos@scripps.edu
Received February 7, 2008
ABSTRACT
A novel organocatalyst was developed that effectively catalyzed the reactions of unprotected or protected dihydroxyacetone with a variety of
aldehydes to provide syn-aldol products with good yields and ee values up to >99%. Significantly, this amide catalyst was effective with a
variety of nonaromatic aldehyde acceptors that had proven difficult in the presence of other catalysts. Reactions of protected dihydroxyacetone
proceeded in aqueous media without addition of organic solvents.
Dihydroxyacetone-based aldol reactions are of considerable
importance because they provide expedient access to both
natural carbohydrates and unnatural molecules of significance
in medicine and materials and food sciences. Prior to the
emergence of organocatalysis as a broadly applicable ap-
proach to asymmetric synthesis, direct asymmetric dihy-
droxyacetone-based aldol reactions were the sole purview
of aldolase enzymes.1 Recently, direct enantioselective
organocatalytic dihydroxyacetone aldol reactions that provide
both anti- and syn-configured 1,2-diols have been devel-
oped.2,3 The anti-configured 1,2-diols are directly accessible
by using proline/pyrrolidine catalysis as we originally
(2) for anti-dihydroxyacetone based organocatalytic aldol reactions see:
(a) Suri, J. T.; Ramachary, D. B.; Barbas, C. F., III Org. Lett. 2005, 7,
1383. (b) Suri, J. T.; Mitsumori, S.; Albertshofer, K.; Tanaka, F.; Barbas,
C. F., III J. Org. Chem. 2006, 71, 3822. (c) Enders, D.; Grondal, C. Angew.
Chem., Int. Ed. 2005, 44, 1210. (d) Enders, D.; Grondal, C.; Vrettou, M.;
Raabe, G. Angew. Chem., Int. Ed. 2005, 44, 4079. (e) Enders, D.; Palecek,
J.; Grondal, C. Chem. Commun. 2006, 655. (f) Enders, D.; Vrettou, M.
Synthesis 2006, 2155. (g) Grondal, C.; Enders, D. Tetrahedron 2006, 62,
329. (h) Grondal, C.; Enders, D. AdV. Synth. Catal. 2007, 349, 694. (i)
Enders, D.; Gasperi, T. Chem. Commun. 2007, 88.
(3) for syn-dihydroxyacetone-based organocatalytic aldol reactions see:
(a) Ramasastry, S. S. V.; Albertshofer, K.; Utsumi, N.; Tanaka, F.; Barbas,
C. F., III Angew. Chem., Int. Ed. 2007, 46, 5572. (b) Utsumi, N.; Imai, M.;
Tanaka, F.; Ramasastry, S. S. V.; Barbas, C. F., III Org. Lett. 2007, 9,
3445. (c) Markert, M.; Mulzer, M.; Schetter, B.; Mahrwald, R. J. Am. Chem.
Soc. 2007, 129, 7285. (d) Markert, M.; Mahrwald, R. Chem. Eur. J. 2008,
14, 40. (e) Luo, S.; Xu, H.; Zhang, L.; Li, J.; Cheng, J. P. Org. Lett. 2008,
10, 653.
(1) (a) Wong, C.-H.; Whitesides, G. M. Enzymes in Synthetic Organic
Chemistry; Pergamon: Oxford, UK, 1994. (b) Takayama, S.; McGarvey,
G. J.; Wong, C.-H. Chem. Soc. ReV. 1997, 26, 407. (c) Machajewski, T.
D.; Wong, C.-H. Angew. Chem., Int. Ed. 2000, 39, 1352. (d) Wymer, N.;
Toone, E. J. Curr. Opin. Chem. Biol. 2000, 4, 110. (e) Wong, C.-H.; Bryan,
M. C.; Nyffeler, P. T.; Liu, H.; Chapman, E. Pure Appl. Chem. 2003, 75,
179. (f) Fessner, W. D.; Helaine, V. Curr. Opin. Biotechnol. 2001, 12, 574.
For a recent study see: Sugiyama, M.; Hong, Z.; Liang, P.-H.; Dean, S.
M.; Whalen, L. J.; Greenberg, W. A.; Wong, C.-H. J. Am. Chem. Soc. 2008,
129, 14811.
10.1021/ol8002833 CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/20/2008

described.4 Enantioselective approaches to the syn-configured
products came some years later with our development of
primary amino acid catalysts that provide anti-Mannich and
syn-aldol products with hydroxyketone donors.3,5 These
studies indicated that O-tBu-L-Thr and O-tBu-L-Thr(NHTf)
were promising catalysts for the direct, syn-selective, asym-
metric dihydroxyacetone aldol reaction.3a,b A distinct limita-
tion to these reactions was their relative inefficiencies with
nonaromatic aldehyde acceptors, particularly with unfunc-
tionalized aliphatic aldehydes. Additionally, these reactions
were optimal in organic reaction media. Other syn-aldol
catalysts have similar limitations: for example, a recently
described diamine acid catalyst is largely restricted to
aromatic acceptor aldehydes.3e As illustrated in Scheme 1,
that function efficiently in reaction media consisting largely
or completely of water. Indeed, water is proving to be an
effective medium for a range of organocatalytic reactions.9
Given the success of the amide catalyst O-tBu-L-Thr-
(NHTf) in our earlier dihydroxyacetone studies and advances
in proline-based anti-aldol reactions obtained through the
preparation of prolylamide derivatives pioneered by Gong
and others,10 we sought to design a family of O-tBu-L-Thr-
based amide catalysts for direct asymmetric aldol reactions.
As shown in Table 1, we prepared three O-tBu-L-Thr-based
Table 1. Catalyst Screening for the Aldol Reaction of
Dihydroxyacetone with p-Trifluoromethylbenzaldehydea
Scheme 1. The syn-1,2-Diol is a Common Structural Motif1,6
syn-configured 1,2-diols are structural motifs common to
diverse compounds of medicinal significance and thus
expansion of the scope and efficiency of this reaction is of
importance.1,6 We were also interested in developing orga-
nocatalytic approaches that use water as a reaction medium
since it is an environmentally friendly, safe, and cost-effective
medium.7 In previous studies,8 we have successfully gener-
ated catalysts for a variety of organocatalytic reactions
(including aldol,8a Michael,8b Mannich,8c and Diels-Alder8d)
a See the Supporting Information for reaction conditions. b Isolated yield.
c Determined by chiral phase HPLC analysis. d No reaction observed under
aqueous conditions. e p-Nitrobenzaldehyde was used as acceptor. f 10 mol
% trifluoroacetic acid (TFA) was used as additive. 5-Me-tet: 5-methyl-
1H-tetrazole
amide derivatives and studied them in the reaction of free
dihydroxyacetone and p-trifluoromethylbenzaldehyde (Table
1, entries 1-4). This reaction was chosen for screening since
it proved to be one of the lower yielding reactions in our
studies of O-tBu-L-Thr catalysis.3a Catalysts 1-3 were
synthesized by using â-amino alcohols based on the Gong
(4) For select references see: (a) List, B.; Lerner, R. A.; Barbas, C. F.,
III J. Am. Chem. Soc. 2000, 122, 2395. (b) Sakthivel, K.; Notz, W.; Bui,
T.; Barbas, C. F., III J. Am. Chem. Soc. 2001, 123, 5260. (c) Chowdari, N.
S.; Ramachary, D. B.; Cordova, A.; Barbas, C. F., III Tetrahedron Lett.
2002, 43, 9591. (d) Chowdari, N. S.; Ramachary, D. B.; Barbas, C. F., III
Org. Lett. 2003, 5, 1685. (e) Thayumanavan, R.; Tanaka, F.; Barbas, C. F.,
III Org. Lett. 2004, 6, 3541. (f) Mase, N.; Tanaka, F.; Barbas, C. F., III
Angew. Chem., Int. Ed. 2004, 43, 2420. (g) Notz, W.; Tanaka, F.; Barbas,
C. F., III Acc. Chem. Res. 2004, 37, 580. (h) Mase, N.; Nakai, Y.; Ohara,
N.; Yoda, H.; Takabe, K.; Tanaka, F.; Barbas, C. F., III J. Am. Chem. Soc.
2006, 128, 734. (i) For a comprehensive review that considers the
contributions of the many other laboratories that have studied the organo-
catalytic aldol reaction, see: Tanaka, F.; Barbas, C. F., III In Enantiose-
lectiVe Organocatalysis, Reactions and Experimental Procedures; Dalko,
P. I., Ed.; Weiley-VCH: Weinheim, Germany, 2007; pp 19-55.
(5) (a) Ramasastry, S. S. V.; Zhang, H.; Tanaka, F.; Barbas, C. F., III J.
Am. Chem. Soc. 2007, 129, 288. For other syn-aldol studies see: (b) Kano,
T.; Yamaguchi, Y.; Tanaka, Y.; Maruoka, K. Angew. Chem., Int. Ed. 2007,
46, 1738. (c) Luo, S.; Xu, H.; Li, J.; Zhang, L.; Cheng, J. P. J. Am. Chem.
Soc. 2007, 129, 3074. (d) Wu, X.; Jiang, Z.; Shen, H.-M.; Lu, Y. AdV.
Synth., Catal. 2007, 349, 812. (e) Xu, Y.-Y.; Wang, Y.-Z.; Gong, L.-Z.
Org. Lett. 2007, 9, 4247.
(6) (a) Auge, C.; Bouxom, B.; Cavaye, B.; Gautheron, C. Tetrahedron
Lett. 1989, 30, 2217. (b) Auge, C.; Gautheron, C.; David, S.; Malleron, A.;
Cavaye, B.; Bouxom, B. Tetrahedron 1990, 46, 201. (c) Australine:
Molyneux, R. J.; Benson, M.; Wong, R. Y.; Tropea, J. E.; Elbein, A. D. J.
Nat. Prod. 1988, 51, 1198. (d) Alexine: Nash, R. J.; Fellows, L. E.; Dring,
J. V.; Fleet, G. W. J.; Derome, A. E.; Hamor, T. A.; Scofield, A. M.; Watkin,
D. J. Tetrahedron Lett. 1988, 29, 2487-2490. (e) Khafrefungin: Mandala,
S. M.; Harris, G. H. Methods Enzymol. 2000, 311 (Sphingolipid Metabolism
and Cell Signalling, Part A), 335. (f) Bengamides: Madeline, A.; Emilio,
Q.; Phillip, C. J. Am. Chem. Soc. 1989, 111, 647.
(7) For reviews on organic synthesis in water, see: (a) Lindstrom, U.
M. Chem. ReV. 2002, 102, 2751. (b) Kobayashi, S.; Manabe, K. Acc. Chem.
Res. 2002, 35, 209. (c) Li, C.-J.; Chan, T.-H. Organic Reactions in Aqueous
Media; Wiley: New York, 1997. For a recent theoretical discussion
concerning the acceleration of organic reactions on water see: (d) Jung,
Y.; Marcus, R. A. J. Am. Chem. Soc. 2007, 129, 5492.
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Org. Lett., Vol. 10, No. 8, 2008

hypothesis10 that such amides activate the electrophilic
aldehyde component of the aldol reaction by forming two
hydrogen bonds with its carbonyl with use of both the amide
and the hydroxy functionalities. All three were found to be
syn-selective catalysts in the presence of 5-methyl-1H-
tetrazole as an acid additive, providing the desired aldol
product with ee up to 96%. Catalyst 2 was the superior amide
with respect to yield and stereochemical control. This catalyst
incorporates the phenylalanine-derived â-amino alcohol
component identified by Singh et al. in studies of prolylamide
aldol reactions.9i,p Catalyst 4 was designed to incorporate a
carboxylate, like that of proline, for activation of the
electrophilic aldehyde. This catalyst was efficient with respect
to enantioselectivity but gave a lower chemical yield than
catalyst 2 and provided no diastereoselectivity. Catalyst 5
was designed based on our diamine/acid strategy4b,f,g,8a proven
effective in pyrrolidine-based anti-aldol catalysts and in
recent3e,5c syn-aldol catalyst designs. Catalyst 5, together with
trifluoroacetic acid or 5-methyl-1H-tetrazole as cocatalysts,
was less optimal than catalyst 2 with DMF as a solvent and
was not reactive with free dihydroxyacetone under aqueous
conditions.
Table 2. Scope of syn-Aldol Reactions of Dihydroxyacetone
with Various Aldehydesa
a See the Supporting Information for the general procedure. b Isolated
yield. c Determined by chiral-phase HPLC analysis. d 3 vol % water used
as additive instead of 5-methyl-1H-tetrazole (5-Me-tet).
We then studied the scope of the free dihydroxyacetone
aldol reaction using catalyst 2 in DMF with 5-methyl-1H-
tetrazole as an additive (Table 2). The results obtained with
aromatic acceptors (entries 1 and 2) were similar with respect
to yield and enantioselectivity to those obtained with the
parent catalyst O-tBu-L-Thr; however, the diastereoselectivity
was reduced.3a Catalyst 2 differentiated itself from O-tBu-
L-Thr catalysis in reactions with nonaromatic aldehydes
(entries 3-6). We previously showed that O-tBu-L-Thr
catalysis provided the product of entry 3 with a maximum
yield of 28%; here we obtained 68% yield with catalyst 2.
Yields with hexanal and cyclohexane carboxaldehyde were
70% and 74%, respectively (entries 5 and 6), with excellent
ee. Product 11 is a precursor for the synthesis of L-xylose11
previously synthesized by using organocatalysis only through
the use of protected dihydroxyacetone.3b Earlier attempts3a
to synthesize 11 with free dihydroxyacetone and O-tBu-L-
Thr catalysis failed to provide significant amounts of clean
compound. Amide 2, however, provided 11 in 62% yield
and 94% ee.
A goal of this study was to establish aqueous conditions
for these reactions. Studies with free dihydroxyacetone in
aqueous media failed to provide promising results. However,
since protective groups can have considerable value in
multistep syntheses, we were compelled to study TBS-
protected dihydroxyacetone as a donor under aqueous condi-
tions. Our results are shown in Table 3. Entry 1 demonstrates
that our original catalyst O-tBu-L-Thr is active under brine
solvent conditions but suffers significantly with respect to
ee as compared to our original study of this catalyst in
N-methylpyrrolidone (NMP), which provided product 12 in
93% ee. Entries 2 and 3 compare the NMP/3 vol % water
conditions3b originally established for O-tBu-L-Thr catalysis
with the use of brine as solvent with catalyst 2. In brine,
catalyst 2 provided aldol product 12 with enhanced chemical
yield and diastereoselectivity maintaining essentially the same
enantioselectivity compared to the original conditions. In
brine, the reactions with the simple aliphatic acceptor
aldehydes dihydrocinnamaldehyde and hexanal (entries 4 and
5) provided excellent results (up to 11:1 syn-favored dr, and
96% ee). Next we studied functionalized nonaromatic
aldehydes as acceptors since their products are related to
(8) (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) Mase, N.;
Watanabe, K.; Yoda, H.; Takabe, K.; Tanaka, F.; Barbas, C. F., III J. Am.
Chem. Soc. 2006, 128, 4966. (c) Notz, W.; Tanaka, F.; Watanabe, S.;
Chowdari, N. S.; Turner, J. M.; Thayumanuvan, R.; Barbas, C. F., III J.
Org. Chem. 2003, 68, 9624. (d) Ramachary, D. B.; Chowdari, N. S.; Barbas,
C. F., III Tetrahedron Lett. 2002, 43, 6743.
(9) For other studies concerning organocatalytic aldol reactions in aqueous
media see: (a) Hayashi, Y.; Sumiya, T.; Takahashi, J.; Gotoh, H.; Urushima,
T.; Shoji, M. Angew. Chem., Int. Ed. 2006, 45, 958. (b) Hayashi, Y.; Aratake,
S.; Okano, T.; Takahashi, J.; Sumiya, T.; Shoji, M. Angew. Chem., Int. Ed.
2006, 45, 5527. (c) Jiang, Z.; Liang, Z.; Wu, X.; Lu, Y. Chem. Commun.
2006, 2801. (d) Font, D.; Jimeno, C.; Pericas, M. A. Org. Lett. 2006, 8,
4653. (e) Guillena, G.; Hita, M. C.; Najera, C. Tetrahedron: Asymmetry
2006, 17, 1493. (f) Wu, Y.; Zhang, Y.; Yu, M.; Zhao, G.; Wang, S. Org.
Lett. 2006, 8, 4417. (g) Wu, X.; Jiang, Z.; Shen, H.-M.; Lu, Y. AdV. Synth.
Catal. 2007, 349, 812. (h) Aratake, S.; Itoh, T.; Okano, T.; Usui, T.; Shoji,
M.; Hayashi, Y. Chem. Commun. 2007, 2524. (i) Maya, V.; Raj, M.; Singh,
V. K. Org. Lett. 2007, 9, 2593. (j) Zu, L.; Xie, H.; Li, H.; Wang, J.; Wang,
W. Org. Lett., published online Feb.14, http://dx.doi.org/10.1021/ol800074z.
For representative studies concerning the use of aqueous media in other
organocatalytic reactions see: (k) Zu, L.-S.; Wang, J.; Li, H.; Yu, X-H.;
Wang, W. Org. Lett. 2006, 8, 3077. (l) Ramachary, B. D.; Kishor, M.;
Ramakumar, K. Tetrahedron Lett. 2006, 47, 651. (m) Ramachary, D. B.;
Reddy, G. B. Org. Biomol. Chem. 2006, 4, 4463. (n) Cao, Y.-J.; Lai, Y.-
Y.; Wang, X.; Li, Y.-J.; Xiao, W.-J. Tetrahedron Lett. 2007, 48, 21. (o)
Palomo, C.; Landa, A.; Mielgo, A.; Oiarbide, M.; Puente, A.; Vera, S.
Angew. Chem., Int. Ed. 2007, 46, 8431. (p) Ramachary, D. B.; Kishor, M.
J. Org. Chem. 2007, 72, 5056. (q) Singh, V.; Singh, V. K. Org. Lett. 2007,
9, 1117.
(10) (a) Tang, Z.; Jiang, F.; Yu, L.-T.; Cui, X.; Gong, L.-Z.; Mi, A.-Q.;
Jiang, Y.-Z.; Wu, Y.-D. J. Am. Chem. Soc. 2003, 125, 5262. (b) Tang, Z.;
Jiang, F.; Cui, X.; Gong, L.-Z.; Mi, A.-Q.; Jiang, Y.-Z.; Wu, Y.-D. Proc.
Natl. Acad. Sci. U.S.A. 2004, 101, 5755. (c) Tang, Z.; Yang, Z.-H.; Chen,
X.-H.; Cun, L.-F.; Mi, A.-Q.; Jiang, Y.-Z.; Gong, L.-Z. J. Am. Chem. Soc.
2005, 127, 9285. (d) Raj, M.; Vishnumaya; Ginotra, S. K.; Singh, V. K.
Org. Lett. 2006, 8, 4097.
(11) Borysenko, C. W.; Spaltenstein, A.; Straub, J. A.; Whitesides, G.
M. J. Am. Chem. Soc. 1989, 111, 9275.
Org. Lett., Vol. 10, No. 8, 2008
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favorably with our O-tBu-D-Thr-catalyzed synthesis of
protected D-fructose (note: D-Thr not L-Thr was used), which
was prepared in 68% yield and 98% ee. No bis-aldol products
were noted in our studies.
Table 3. Scope of syn-Aldol Reactions of TBS-Protected
Dihydroxyacetone with Various Aldehydesa
As shown in entries 7 and 8, we also explored the synthesis
of glyoxalic acid derivates that had proven challenging3b
under O-tBu-L-Thr catalysis. Under O-tBu-L-Thr catalysis,
product 18 was provided in 36% yield and 26% ee with no
diastereoselection and product 19 was provided in 29% yield
and 24% ee with only a 1.2:1 dr.3b Amide catalyst 2,
however, provided product 18 in 77% yield, 3:1 syn-favored
dr, and 96% ee and product 19 in 55% yield, 3:1 syn-favored
dr, and 94% ee. Thus catalyst 2 provided dramatic improve-
ment in both chemical and optical yields of polyol products
obtained through direct asymmetric aldol reactions of
protected dihydroxyacetone and nonaromatic aldehydes rela-
tive to previously employed catalysts.
In summary, amide 2 is an effective organocatalytic
catalyst for the synthesis of syn-aldol products from free
dihydroxyacetone in organic solvent and from protected
dihydroxyacetone in aqueous medium. This catalyst provided
substantial improvements in this important class of syn-aldol
reactions, providing for the first time effective syntheses
based on nonaromatic aldehyde acceptors, in particular
simple aliphatic aldehyde acceptors that did not yield desired
products or were low yielding in other catalyst systems.3a,b,e
This methodology provides a direct route to aldol products
of the type synthesized with the DHAP aldolase enzymes
L-rhamnulose 1-phosphate and D-fructose 1,6-diphosphate
aldolase and expands the scope of simplified and effective
organocatalytic routes to a variety of carbohydrates and their
derivatives.
a See the Supporting Information for reaction conditions. b Isolated yield.
c Determined by chiral phase HPLC analysis. d Reaction performed with
20 mol % tBu-L-Thr as catalyst. e Reaction performed3b in NMP, water (3
vol %), rt. f ee determined after TBS deprotection3b and acetylation. g ee
determined after TBS deprotection3b and benzoylation. h 10 mol % 5-methyl-
1H-tetrazole was used as additive. i ee determined after benzoylation.
naturally occurring carbohydrates. The methoxyacetal of
glyoxal was also an excellent substrate, furnishing the
L-xylose precursor in quantitative yield, 9:1 syn-favored dr,
and 97% ee (entry 7). Our earlier studies3b of O-tBu-L-Thr
catalysis of this reaction provided this product with the same
ee but with a chemical yield of 71% and dr of 5:1. Reaction
of the acetonide of D-glyceraldehyde was also efficient,
providing the protected D-sorbose derivative 17 in 86% yield,
3:1 syn-favored dr, and 98% ee. This result compares
Acknowledgment. This study was supported by The
Skaggs Institute for Chemical Biology.
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL8002833
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