Dihydroxyacetone in amino acid catalyzed mannich-type reactions

Article abstract of DOI:10.1002/anie.200500297

The C₃ equivalent dihydroxyacetone was used in its protected form in proline-catalyzed Mannich reactions (see scheme). As little as 5 mol % catalyst is required when using 2,2,2-trifluoroethanol as solvent. The reaction is improved further (rea

Full text of DOI:10.1002/anie.200500297

Angewandte
Chemie
Organocatalysis
of considerable biological interest owing to their glycosidase-
inhibiting and aminoglycoside-mimicking properties.[7] As a
multistep sequence is generally required in the synthesis of
these compounds, a simple approach is highly desirable.
Herein we report the use of protected dihydroxyacetone
and various imines in Mannich reactions. In this context, we
present 2,2,2-trifluoroethanol (TFE) as a highly suitable
solvent and describe the application of microwave irradiation
to accelerate organocatalytic reactions.[8]
Dihydroxyacetone in Amino Acid Catalyzed
Mannich-Type Reactions**
Bernhard Westermann* and Christiane Neuhaus
After lying in obscurity for more than 40 years, proline-
catalyzed carbonyl reactions have been studied intensively in
recent years.[1] Although Martens and co-workers pointed out
the enormous potential of these reactions in a Review in 1982,
they did not attract significant interest until the studies of List,
Barbas, MacMillan, Jørgensen, and others were published.[2]
Proline-catalyzed reactions could possibly explain the origin
of prebiotic aldol and Mannich products and thus the
stereoselective synthesis of carbohydrates. During glycolysis
and gluconeogenesis, an aldolase-catalyzed aldol reaction of
two C3 building blocks, 3-glyceraldehyde phosphate (GAP),
and dihydroxyacetone phosphate (DHAP) takes place
(Scheme 1).[3]
In contrast to hydroxyacetone no reaction could be
observed with DHA (1) under the conditions provided by
List and by Cordova and co-workers (l-proline (30 mol%),
DMSO, room temperature).[9] Therefore, we used phosphate-
protected dihydroxyacetone derivatives 2 and 3 in prelimi-
nary reactions.[10] Unfortunately, no reaction occurred
between imine 6 and the methylene compound (Schemes 1
and 2).[11] Subsequently, we used the acetonide 5 of dihydroxy-
acetone which can easily be synthesized in large amounts in
two steps. Acetonide 5 was previous employed in diastereo-
and enantioselective aldol reactions as a DHA equivalent.[12]
Besides the distinct reactivity, we assumed that a very
compact, bicyclic transition state would result with this
cyclic ketone, thus leading to high diastereoselectivities.
In the first experiment it could be shown that the desired
products 10 (syn) und 11 (anti) were formed by using
protected DHA 5 and imine 6 in the presence of racemic
proline (30 mol%) (Scheme 2; Table 1, entry 1 (91:9 d.r.)).
Further experiments with l-proline led to identical diaster-
eomeric ratios; the enantioselectivity (82% ee) was satisfy-
ing.[13] Whereas solvents such as acetonitrile, toluene, and
ionic liquids (Table 1, entries 4–6) did not result in notable
improvements in stereoselectivites, very polar solvents such
as formamide and TFE led to excellent diastereo- and
enantioselectivities (Table 1, entries 7, 8). The use of TFE
Scheme 1. Dihydroxyacetone and phosphorylated derivatives in aldol
and Mannich reactions.
Despite the enormous number of publications dealing
with organocatalytic carbonyl reactions, only a few examples
can be found in which dihydroxyacetone (DHA, 1) was used
as the C3 methylene compound in aldol reactions towards
carbohydrate precursors.[4,5] Mannich and Mannich-type
reactions of DHA are, to the best of our knowledge,
unknown.[6] These reactions would offer a very simple
approach to azasugar and aminosugar derivatives, which are
[*] Prof. Dr. B. Westermann, Dipl.-Chem.-Ing. C. Neuhaus
Leibniz Institute of Plant Biochemistry
Department of Bioorganic Chemistry
Weinberg 3, 06120 Halle (Saale) (Germany)
Fax: (+49)345-5582-1309
E-mail: Bernhard.Westermann@ipb-halle.de
[**] The authors are grateful to Dr. M. Lange, Girindus AG (Halle) for
helpful discussions.
Scheme 2. Catalyzed Mannich reactions. The reaction conditions are
given in Table 1; PMP=p-methoxyphenyl.
Angew. Chem. Int. Ed. 2005, 44, 4077 –4079
DOI: 10.1002/anie.200500297
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Table 1: Results of the Mannich reaction with 5 and 6 (see Scheme 2).
results, TFE was once again the solvent of
choice. After only 10 min 10 was obtained in
72% yield with high diastereo- (90:10 d.r.)
Entry Catalyst
Conc.
Solvent[a]
t [h] T [8C] Yield [%][b] d.r.[c]
ee [%][d]
[mol%]
and enantioselectivities (95% ee).
A
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
d,l-proline
30
30
30
30
30
30
30
30
30
30
20
10
5
DMSO
DMSO
H2O
acetonitrile
toluene
BMIMBF4
formamide
TFE
TFE/H2O (95:5)
TFE/H2O (90:10) 20
TFE
TFE
TFE
TFE
TFE
TFE
TFE
20
20
96
40
40
40
20
20
20
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
46
37
34
52
52
39
54
72
76
76
68
75
70
44
36
28
91:9
91:9
93:7
93:7
92:8
–
decrease in the irradiating power led to a
lower yield, although the selectivities
remained the same. The use of DMSO or
formamide as solvent led to a significant
decrease in the yield and diastereoselectiv-
ity.[15]
The Mannich-type reaction could also
be carried out as a three-component process
(Scheme 3). The reaction of glyoxylate 14,
p-anisidine (15), and 5 yielded 10 in 30%
yield with selectivities comparable with
those of the two-component reaction.
The relative configuration of all the
products was determined by NMR spec-
troscopy. The syn conformation of the main
diastereomer can be concluded based on the
3J coupling constant (CHO-CHN) of 2.3 Hz
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
l-proline
DMTC (12)
82
34
80
42
89:11 n.d.
95:5
97:3
95:5
95:5
96:4
95:5
93:7
97:3
96:4
96
99
96
95
99
95
91
97
97
20
20
20
20
20
40
30
30
30
À20
RT
86:14 n.d.
diamine (13) 30
no reaction
[a] BMIMBF4 =1-butyl-3-methylimidazolium tetrafluoroborate; TFE=2,2,2-trifluoroethanol. [b] Yields of
isolated products. [c] d.r. determined by GC–MS. [d] ee determined by HPLC (Chiralcel OD-H); n.d.: not
determined.
allowed the isolation of the product in 72% yield with
reproducible diastereoselectivity (97:3 d.r.) and enantioselec-
tivity (99% ee). A slight decrease in the selectivities (95:5 d.r.
and 96/95% ee (Table 1, entries 9, 10) was observed in TFE/
H2O solvent mixtures (95:5; 90:10). To decrease the amount
of proline, we ran further experiments in TFE with decreasing
concentrations of catalyst (Table 1, entries 11–13). We proved
that the amount of catalyst could be decreased significantly.
Only at 5 mol% of l-proline did the stereoselectivity
decrease quite drastically. Temperature effects were not
observed (Table 1, entries 14, 15).
Other proline-derived catalysts proved to be unsuitable.
l-5,5-Dimethylthiazolidine-4-carboxylic acid (DMTC; 12)
resulted in poor selectivities. No reaction occurred in the
presence of (S)-1-(2-pyrrolidinylmethyl)pyrroline triflate (13)
(Table 1, entries 16, 17). Disappointingly, imines 7 and 8 and
hydrazone 9[14] did not react with 5.
Scheme 3. Three-component reaction to 10.
and on NOE interactions.[16] This is consistent with results
from other Mannich-type reactions. Therefore, it is possible to
assume the same transition state proposed by Cordova.[6]
In conclusion nitrogen-containing carbohydrate deriva-
tives were obtained with very good stereoselectivities from
DHA derivatives. Furthermore it is possible to improve
existing protocols (shorter reaction times, less catalyst) by
using 2,2,2-trifluoroethanol as solvent and microwave-
assisted procedure.[17]
One disadvantage of many proline-catalyzed reactions is
the long reaction time. We therefore tried to accelerate the
reaction under the action of microwaves. The results of the
microwave-assisted organocatalytic Mannich reactions are
listed in Table 2. Consistent with the previously observed
Experimental Section
Solvent (1 mL) and l-proline (35 mg, 0.3 mmol) were stirred at room
temperature for 3 min, before 5 (130 mg, 1.0 mmol) was added. After
15 min 6 (207 mg, 1.0 mmol) was added. The reaction mixture was
stirred for 20 h. A saturated solution of NH4Cl (1.0 mL) was added,
and the mixture was extracted with ethyl acetate (2 ꢀ 15 mL). The
combined organic extracts were dried with Na2SO4, filtered, and
concentrated. The residue was purified by column chromatography
through silica gel (petroleum ether/ethyl acetate 3:1) to give the
product 10 as an oil. Rf = 0.8 (petroleum ether/ethyl acetate 3:2);
HPLC (Daicel Chiralcel OD-H, hexane/2-propanol 90:10,
1.0 mLminÀ1, l = 254 nm); [a]2D0 = À107.0 (c = 0.2, ethyl acetate);
1H NMR (CDCl3, 300 MHz): d = 1.24 (t, 3J = 7.1 Hz, 3H; Me), 1.44 (s,
3H; Me), 1.49 (s, 3H; Me), 3.73 (s, 3H; Me), 4.02 (d, 2J = 16.5 Hz, 1H;
CHH, 5-H), 4.13 (dq, J = 7.1, 10.8 Hz, 1H; OCH2CH3), 4.24 (dq, J =
7.1, 10.8 Hz, 1H; OCH2CH3), 4.30 (dd, J1 = 1.6 Hz, J = 16.5 Hz, 1H;
CHH, 5-H), 4.59 (d, J = 2,3 Hz, 1H; 2-H), 4.74 (dd, J = 1.6, 2.3 Hz,
1H; 3-H), 6.7–6.8 ppm (m, 4H; Ar-H); 13C NMR (CDCl3, 75.5 MHz):
d = 14.2 (q), 23.3 (q), 24.2 (q), 55.5 (q), 58.8 (d), 61.5 (t), 67.0 (t), 76.4
Table 2: Results of the microwave-assisted Mannich reaction of 5 and 6
in the presence of l-proline (30 mol%).[a]
Solvent
t [min]
E [W][b]
Yield [%][c]
d.r.[d]
ee [%][e]
TFE
TFE
TFE
DMSO
formamide
formamide
5
10
10
10
5
300
300
100
300
300
300
63
72
64
18
40
38
89:11
90:10
90:10
92:8
83:17
80:20
94
94
95
n.d.
n.d.
n.d.
10
[a] All reactions were carried out in sealed 10-mL tubes. [b] Maximal
irradiated power. [c] Yields of isolated products. [d] Determined by GC–
MS. [e] Determined by HPLC (Chiralcel OD-H).
4078
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Chemie
(d), 100.7 (s), 114.5 (d, 2C), 116.8 (d, 2C), 140.7 (s), 153.3 (s), 171.1 (s),
206.0 ppm (s); HRMS [M+Na+]: 337.1423 (calcd), 337.1427 (found).
Received: January 26, 2005
Published online: May 24, 2005
Keywords: asymmetric synthesis · Mannich reaction ·
.
microwave irradiation · organocatalysis · proline
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[11] Strictly speaking it is a Mannich-type reaction.
[12] For a review, see: D. Enders, M. Voith, A. Lenzen, Angew.
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[13] The diastereoselectivities were determined by GC–MS, the
enantioselectivities by HPLC: Daicel Chiralcel OD-H.
[14] K. L. Yamada, M. Shibasaki, S. J. Harwood, H. Grꢂger, Angew.
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[15] A comparative experiment at 608C (oil bath, atmospheric
pressure, 30 mol% l-proline, TFE) afforded 10 with a complete
turnover after 30 min (67% yield, 90:10 d.r., 91% ee).
[16] T. Murakami, K. Taguchi, Tetrahedron 1999, 55, 989.
[17] After submission of the manuscript, the use of 5 in proline-
catalyzed aldol reactions was described: D. Enders, C. Grondal,
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1210.
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