Organocatalytic anti-Mannich reactions with dihydroxyacetone and acyclic dihydroxyacetone derivatives: A facile route to amino sugars

Article abstract of DOI:10.1002/adsc.200800069

The first primary amine-containing amino acid-catalyzed anti-Mannich reactions of dihydroxyacetone and acyclic dihydroxyacetone derivatives with a variety ofimines have been developed. This approach complements proline-based strategies in the preparation ofamino sugars.

Full text of DOI:10.1002/adsc.200800069

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DOI: 10.1002/adsc.200800069
Organocatalytic anti-Mannich Reactions with Dihydroxyacetone
and Acyclic Dihydroxyacetone Derivatives: A Facile Route to
Amino Sugars
Haile Zhang,a S. S. V. Ramasastry,a Fujie Tanaka,a and Carlos F. Barbas IIIa,*
a
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, USA
Fax : (+1)-858-784-2583; e-mail: carlos@scripps.edu
Received: January 30, 2008; Published online: March 31, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
[4]
Abstract: The first primary amine-containing amino
acid-catalyzed anti-Mannich reactions ofdihydroxy-
actions ofDHA and DHA derivatives,
these sub-
strates are not well explored in organocatalytic Man-
nich and Mannich-type reactions. Highly enantioselec-
tive Mannich reactions ofdioxanone catalyzed by
proline or its derivative were recently achieved; how-
ever, only syn-Mannich products were produced in
those reactions (Scheme 1).[5,6] In addition, these reac-
tions are limited in scope to the single DHA deriva-
tive 2,2-dimethyl-1,3-dioxane-5-one. Herein, we dis-
close a significant elaboration of our organocatalytic
carbohydrate strategy as we describe the first organo-
catalytic anti-Mannich reactions ofprotected and un-
protected DHA derivatives with a variety ofaromatic
and aliphatic aldehyde-derived imines.
A
with a variety ofimines have been developed. This
approach complements proline-based strategies in
the preparation ofamino sugars.
Keywords: acyclic dihydroxyacetone derivatives;
anti-Mannich reaction; dihydroxyacetone; primary
amine-containing amino acids
Amino and imino sugars are ofsignificant interest not
As one part ofour ongoing program to meet the
only for use as tools in glycobiology but also for the stereochemical challenges in Mannich chemistry that
treatment ofa range ofhuman diseases ranging rfom are not addressed with proline and its derivatives, we
HIV-1 and hepatitis C infections to genetic disorders have recently developed efficient anti-Mannich strat-
like Gaucherꢀs disease and other inborn errors ofme-
egies.[7,8] In our previous anti-Mannich studies, we
tabolism. Consequently, there has been significant in- demonstrated an efficient route to highly enantiomer-
terest in the development of efficient syntheses of ically enriched anti-1,2-amino alcohols by employing
[1,2]
these types ofcompounds.
Typically, these sugars unmodified hydroxyacetone and proposed the inter-
require multistep syntheses.[3] The most expedient de mediacy ofa hydrogen bond-stabilized ( Z)-enamine
novo syntheses use aldolase enzymes or organocata-
lytic aldol reactions involving dihydroxyacetone
(DHA) derivatives.[4] Although aldol-based DHA
routes are efficient, this approach is limited with re-
spect to the placement ofthe amino ufnctionality
within the target sugar. Enzyme-based strategies
based on DHA are limited to the aldol reaction since
the enzymes do not accept Mannich imine acceptors.
Direct catalytic asymmetric Mannich reactions have
À
emerged as a powerful class of C C bond-forming re-
actions that provide access to optically enriched nitro-
gen-containing compounds. The use ofDHA and
DHA derivatives as C3 synthons in organocatalytic
Mannich reactions promise efficient syntheses of
amine-containing sugars.[5,6] Despite considerable at-
tention afforded to the catalytic asymmetric aldol re- Scheme 1.
Adv. Synth. Catal. 2008, 350, 791 – 796
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Haile Zhang et al.
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in the transition state (Scheme 2).[7] In accord with
this design principle, we reasoned that the anti-Man-
nich product ofunprotected DHA should be selec-
tively formed under primary amine-based amino acid
catalysis (Scheme 3). The DHA derivative 2,2-dimeth-
yl-1,3-dioxane-5-one can only form an (E)-enamine,
which leads to the syn-Mannich product (Scheme 1).
A strategy based on unprotected DHA as a Mannich
donor would be a more economical route to amino
sugars than that using 2,2-dimethyl-1,3-dioxane-5-one
as a donor.
We first investigated the Mannich reaction between
unprotected DHA and a preformed imine of nitro-
benzaldehyde in the presence ofthe amino acid l-
tryptophan (l-Trp) or O-t-Bu-l-threonine (O-t-Bu-l-
Thr) (Table 1). In the presence of20 mol% O- t-Bu-l-
Thr in N-methylpyrrolidinone (NMP) at ambient tem-
peratue, we obtained the anti-Mannich product 1 with
a dr of4:1 and 85% ee (Table 1, entry 1). We found
that the dr and ee of 1 could be significantly increased
to 99:1 and 97%, respectively, by simple recrystalliza-
tion. Further optimization revealed that the addition
of5-methyl-1 H-tetrazole[9] enhanced both reaction
rate and enantioselectivity (compare entries 1 and 2).
In contrast to our syn-aldol reaction with Bn-protect-
ed DHA, addition of3 vol% ofwater signiifcantly
slowed the reaction although the ee increased (com-
Scheme 2.
Scheme 3.
Table 1. Mannich reaction ofunprotected DHA and p-nitrobenzaldehyde preformed imine.[a]
Entry
Catalyst
Time [days]
Yield [%][b]
dr (anti:syn)[c]
ee (anti/syn) [%][d]
1
A
A
A
A
B
1
0.8
1
3
1
68
77
23
55
50
73
4:1 (99:1)[i]
4:1
2:1
3:1
3:1
85/- (97)[i]
90/64
89/67
92/-
79/30
51/49
2[e]
3[f]
4[e,g]
5[e]
6[e,h]
C
0.8
1:1.8
[a]
See Supporting Information.
Isolated yields.
Determined by HPLC or NMR.
Determined by chiral-phase HPLC.
With additive 5-methyl-1H-tetrazole.
3 vol% H2O.
At 48C.
40 mol% of l-proline.
After recrystallization.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
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Organocatalytic anti-Mannich Reactions with Dihydroxyacetone
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[a]
Table 2. Scope ofthe anti-Mannich reaction ofunprotected DHA with various acceptors.
Entry
R
Product
Time [h]
Yield [%][b]
dr (anti:syn)[c]
ee (anti/syn) [%][d]
1
2
3
p-NO2C6H4
p-CNC6H4
p-BrC6H4
p-CF3C6H4
Cyclohexyl
CH2CH2Ph
1
2
3
4
5
6
7
8
21
24
24
24
72
60
18
17
77
75
78
71
70
65
72
53
4:1
3:1
2:1
3:1
1.5:1
2:1
1:1
90
88
78
81
66
72
71
28
4[e]
5[f]
6[f]
7[e]
8[g]
CH
A
CO2Et
1:1
[a]
See Supporting Information.
[b]
[c]
[d]
[e]
Isolated yield containing anti and syn diastereomers.
1
Determined by H NMR or HPLC.
Determined by chiral-phase HPLC.
Three-component reaction: ketone (1 mmol, 2.0 equiv.), aldehyde (0.5 mmol, 1.0 equiv.), PMPNH2 (0.55 mmol,
1.1 equiv.), catalyst (0.1 mmol, 0.2 equiv., 20 mol% to aldehyde), and 5-methyl-1H-tetrazole (0.05 mmol, 0.1 equiv., 10
mol% to aldehyde).
Three-component reaction: ketone (1 mmol, 2.0 equiv.), aldehyde (0.5 mmol, 1.0 equiv.), PMPNH2 (0.5 mmol, 1.0 equiv.).
Without additive 5-methyl-1H-tetrazole.
[f]
[g]
pare entries 1 and 3). Lowering the reaction tempera- hyde-derived imines is ofconsiderable importance.
ture was not a benefit (entry 4). O-t-Bu-l-Thr cataly- The a-oxygen-bearing acceptors and glyoxylate imine
sis proved to be superior to l-Trp catalysis with re- are poor substrates for this reaction. In previous stud-
spect to both dr and ee (entry 5). Interestingly, when ies, we found that the Mannich reaction of protected
using 40 mol% proline and 5-methyl-1H-tetrazole, 1 monohydroxyacetone with p-nitrobenzaldehyde af-
could be prepared in good yield, albeit with moderate forded the anti-Mannich product in high yield and
ee and syn-favored selectivity (entry 6). We believe high enantioselectivity under primary amine-contain-
this to be the only known proline-catalyzed Mannich ing amino acid catalysis.[12] Thus, we reasoned that
reaction involving unprotected DHA.
protected forms of DHA should also form a (Z)-en-
amine in the transition state (Scheme 4).
The scope ofthe reaction was then studied using
unprotected DHA and various aromatic and aliphatic
Thus, we studied the Mannich reaction ofBn-pro-
acceptors under the optimized conditions (Table 2). tected DHA with ethyl glyoxylate imine.[13] To our de-
Results ofthe reaction with preofrmed imines as ac-
light, O-t-Bu-l-Thr catalyzed the Mannich-type reac-
ceptors are shown in entries 1–3 and 8 and those from tion ofBn-protected DHA with ethyl glyoxylate
the traditional three-component Mannich process in imine, affording high enantioselectivity (Table 3,
entries 4–7. Electron-deficient aromatic imines gener-
ally gave good selectivities (entries 1–4), whereas ali-
phatic imines gave lower diastereo- and enantioselec-
tivities (entries 5 and 6). Aryl imines bearing elec-
tron-donating groups were also substrates for this re-
action, albeit with less favorable outcomes. The O-t-
Bu-l-Thr catalyzed three-component Mannich reac-
tion ofDHA with dimethoxyacetaldehyde and p-ani-
sidine afforded Mannich product 7 with the dr of
almost 1:1. The reaction ofDHA and glyoxylate
imine proceeded smoothly without additive;[10] how-
ever, the product was obtained in poor ee and dr
(entry 8).
Mannich products 7 and 8 are very useful synthons
for the synthesis of aza-sugars[1,2,5,6] and polyoxamic
acid[11], respectively. Development of anti-Mannich re-
actions with imino esters or a-oxygen-bearing alde- Scheme 4.
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Haile Zhang et al.
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Table 3. Bn-protected DHA as donor in the anti-Mannich reactions.[a]
Entry
R
Product
Catalyst
Yield [%][b]
dr (anti:syn)[c]
ee (anti/syn) [%][d]
1
CO2Et
9
A
A
B
A
B
A
B
A
A
65
78
76
68
73
91
78
77
85
-
2:1
2:1
3:1
2.3:1
2:1
4:1
4:1
4:1
4:1
3:1
2:1
87/37
89/68
91/63
81/19
79/21
97/77
92/54
94/84
88/57
85/38
73/7
2[e,f]
3[e,f]
4[g]
5
CH
A
10
11
6
p-NO2C6H4
7[f,h]
8
p-CF3C6H4
p-BrC6H4
12
13
9
10[f]
11[g]
CH
A
14
A
73
[a]
See Supporting Information.
[b]
[c]
[d]
[e]
[f]
Isolated yield containing anti and syn diastereomers.
1
Determined by H NMR or HPLC.
Determined by chiral-phase HPLC.
2-PrOH as solvent.
Without additive 5-methyl-1H-tetrazole.
[g]
Three-component reaction: ketone (1 mmol, 2.0 equiv.), aldehyde (0.5 mmol, 1.0 equiv.), PMPNH2 (0.55 mmol,
1.1 equiv.), catalyst (0.1 mmol, 0.2 equiv., 20 mol% to aldehyde), and 5-methyl-1H-tetrazole (0.05 mmol, 0.1 equiv., 10
mol% to aldehyde).
30 mol% of l-Trp.
[h]
entry 1). In the absence ofadditive, higher diastereo-
anti-selective Mannich products in good yields and
and enantioselectivity were observed when l-Trp was with high enantioselectivities (Table 3). To briefly ex-
employed as catalyst rather than O-t-Bu-l-Thr amine protecting group tolerability ofthe reaction,
(entry 3). The three-component Mannich reaction of we studied methoxymethyl (MOM) and TBS since
Bn-protected DHA with dimethoxyacetaldehyde and these protecting groups are readily removed under
p-anisidine afforded the anti-isomer as the major mild conditions. As expected, reactions involving
product with 81% ee in the presence ofO- t-Bu-l-Thr. MOM-protected DHA furnished the desired anti-
Bn-protected DHA was also a suitable donor for aro- major Mannich products in good yield (Table 4). Sig-
matic and aliphatic aldehyde-derived imines, giving nificantly, the three-component Mannich reaction of
Table 4. MOM-protected DHA as donor in the anti-Mannich reactions.[a]
Entry
R
Product
Yield [%][b]
dr (anti:syn)[c]
ee [%] anti[d]
1
p-NO2C6H4
p-CF3C6H4
CH2OBn
15
16
17
75
71
61
2.5:1
2:1
1.5:1
90
87
72
2
3[e]
[a]
See Supporting Information.
Isolated yield containing anti and syn diastereomers.
Determined by HPLC or NMR.
[b]
[c]
[d]
[e]
Determined by chiral-phase HPLC.
Three-component reaction: ketone (0.5 mmol, 2.0 equiv.), aldehyde (0.25 mmol, 1.0 equiv.), PMPNH2 (0.275 mmol,
1.1 equiv.), catalyst (0.05 mmol, 0.2 equiv., 20 mol% to aldehyde), and 5-methyl-1H-tetrazole (0.025 mmol, 0.1 equiv., 10
mol% to aldehyde).
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Organocatalytic anti-Mannich Reactions with Dihydroxyacetone
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Scheme 5. Synthesis ofprotected 4-amino-4-deoxy- d-psicose 18.
TBS-protected DHA with the acetonide of d-glyceral- Acknowledgements
dehyde and p-anisidine under our standard conditions
This study was supported in part by the Skaggs Institute for
Chemical Biology.
successfully afforded the protected 4-amino-4-deoxy-
d-psicose 18 in virtually stereoisomerically pure form
(>98:2 dr, 98% ee) (Scheme 5). Since protecting
groups have considerable strategic value in synthetic
chemistry, our strategy ofemploying acyclic protected
DHA as donors should facilitate the development of
anti-selective direct asymmetric Mannich reaction of
DHA derivatives for the synthesis of amino sugars.
The absolute configuration of anti-1 was deter-
mined to be 3R,4R (see Supporting Information);
these results are in accord with our previous results in
References
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catalytic asymmetric Mannich reactions that involve
unprotected dihydroxyacetone and acyclic protected
dihydroxyacetones with a variety ofaromatic and ali-
phatic aldehyde-derived imines. These reactions gen-
erated anti-polyhydroxylated amino alcohols and
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Experimental Section
General Procedure for Two-Component Mannich-
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a solution of N-PMP-protected preformed imine
(0.5 mmol, 1 equiv.) in anhydrous 1-methyl-2-pyrrolidinone
(NMP, 0.5 mL) was added 1,3-dihydroxyacetone dimer
(0.5 mmol, 1 equiv.; 2 equiv. as monomer) followed by O-t-
Bu-l-Thr (0.1 mmol, 0.2 equiv.) and 5-methyl-1H-tetrazole
(0.05 mmol, 0.1 equiv.) at room temperature. The reaction
was stirred at room temperature for the time as indicated in
Table 1 and Table 2. The reaction mixture was worked up by
addition ofsaturated ammonium chloride, and extracted
with AcOEt. The organic layers were washed with water
and brine, dried over Na2SO4, filtered, concentrated under
vacuum, and purified by flash column chromatography
(AcOEt/hexane) to afford the corresponding Mannich ad-
ducts as an anti/syn mixture. Racemic standards were pre-
pared using (Æ)-tryptophan and phenylphosphinic acid.
Adv. Synth. Catal. 2008, 350, 791 – 796
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[10] According to Westermann, no reaction could be ob-
served with DHA and glyoxylate imine under the con-
ditions: 30 mol% proline, DMSO, room temperature.
See ref.[5]
[11] A. Tarrade, D. Dauban, R. H. Dodd, J. Org. Chem.
2003, 68, 9521, and references cited therein.
[12] Mannich reaction ofmethoxyacetone to p-nitrobenzal-
dehyde in the presence of30 mol% l-Trp in DMF at
room temperature affording the desired anti-major
product in 93% yield; 4.4:1 dr (anti:syn), 93% ee for
anti, and 83% ee for syn. Similar work was done by Lu
et al., see ref.[8e]
[13] Poor enantioselectivity was observed in the b-proline
catalyzed Mannich reaction with Bn-protected DHA
and ethyl glyoxylate imine; dr 1.4:1 (anti:syn) and 45%
ee (anti); see ref.[8c]
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