One-pot enantioselective syntheses of iminosugar derivatives using organocatalytic anti -michael- anti -aza-henry reactions

Article abstract of DOI:10.1021/ol102292a

Organocatalyst-controlled asymmetric anti-Michael reactions of (tert-butyldimethylsilyloxy)acetaldehyde with a range of nitroolefins, followed by an intermolecular aza-Henry reaction with imine, provided iminosugar derivatives with five contiguous stereoc

Full text of DOI:10.1021/ol102292a

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5250-5253
One-Pot Enantioselective Syntheses of
Iminosugar Derivatives Using Organocatalytic
anti-Michael-anti-Aza-Henry Reactions
Ritsuo Imashiro, Hisatoshi Uehara, 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, United States
carlos@scripps.edu
Received September 23, 2010
ABSTRACT
Organocatalyst-controlled asymmetric anti-Michael reactions of (tert-butyldimethylsilyloxy)acetaldehyde with a range of nitroolefins, followed by an
intermolecular aza-Henry reaction with imine, provided iminosugar derivatives with five contiguous stereocenters in very high enantiomeric excess
in one pot. The stereochemistry of the aza-Henry reaction was substrate controlled and is explained by a six-membered cyclic transition-state model.
Iminosugars (azasugars), which are carbohydrate analogues
having nitrogen rather than endocyclic oxygen, are known to be
glucosidase inhibitors.1 Two N-alkylated piperidinol compounds
that inhibit glucosidases are used clinically: miglitol (Glyset) in
treatment of type II diabetes and miclustat (Zavesca) to treat patients
with Gaucher’s disease. Iminosugars are also inhibitors of glycosyl
transferases, glycogen phosphorylases, nucleoside-processing en-
zymes, and metalloproteinases.2 2-Aryl-3-nitrogen-substituted pi-
peridine derivatives inhibit farnesyltransferase3 and dipeptidyl
peptidase4 and have potential as anticancer and antidiabetic agents,
respectively. The synthesis of iminosugars is generally performed
using the chiral pool method from monosaccarides.5 Recently,
catalytic enantioselective syntheses of iminosugar derivatives have
been reported.6 However, both methods require multiple steps, and
development of a more efficient method is desirable.
syntheses of a number of compound classes using organoca-
talysis have been developed.9 Furthermore, development of
organocatalytic asymmetric assembly reactions by our group10
and others11 has made synthesis of complex molecules with
multiple stereocenters more convenient.
Recently, we reported a simple and robust methodology for
the asymmetric synthesis of pyranose derivatives with talo- and
manno-configurations from simple achiral precursors through
organocatalytic asymmetric intermolecular Michael-Henry
reactions.12b We proposed that the substrate-controlled syn-
(4) Pei, Z.; Li, X.; von Geldern, T. W.; Longenecker, K.; Pireh, D.;
Stewart, K. D.; Backes, B. J.; Lai, C.; Lubben, T. H.; Ballaron, S. J.; Beno,
D. W. A.; Kempf-Grote, A. J.; Sham, H. L.; Trevillyan, J. M. J. Med. Chem.
2007, 50, 1983.
(5) For reviews, see: (a) Cippolla, L.; Ferla, B. L.; Nicotra, F. Curr.
Top. Med. Chem. 2003, 3, 485. (b) Pearson, M. S. M.; Mathe-Allainmat,
M.; Fargeas, V.; Lebreton, J. Eur. J. Org. Chem. 2005, 11, 2159.
(6) (a) Johnson, C. R.; Johns, B. A. J. Org. Chem. 1997, 62, 6046. (b)
Marin, R.; Moyano, A.; Pericas, M. A.; Riera, A. Org. Lett. 2000, 2, 93.
(c) Haukaas, M. H.; O’Doherty, G. A. Org. Lett. 2001, 3, 401. (d) Singh,
O. V.; Han, H. Tetrahedron Lett. 2003, 44, 2387. (e) Somfai, P.; Marchand,
P.; Torsell, P. S.; Lindstro¨m, U. M. Tetrahedron 2003, 59, 1293.
(7) (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.
Since our discovery of the proline-catalyzed intermolecular
aldol reaction7 and other related reactions,8 enantioselective
(1) Stu¨tz, A. E. Iminosugars as Glycosidase Inhibitors: Nojirimycin and
Beyond; Wiley-VCH: Weinheim, Germany, 1999.
(2) (a) Compain, P.; Martin, O. R. Curr. Top. Med. Chem. 2003, 3,
471. (b) Iminosugars: From Synthesis to Therapeutic Applications; Wiley-
VCH: West Sussex, England, 2007.
(3) Nara, S.; Tanaka, R.; Eishima, J.; Hara, M.; Takahashi, Y.; Otaki,
S.; Foglesong, R. J.; Hughes, P. F.; Turkington, S.; Kanada, Y. J. Med.
Chem. 2003, 46, 2467.
(8) (a) Notz, W.; Tanaka, F.; Barbas, C. F., III. Acc. Chem. Res. 2004,
37, 580. (b) Barbas, C. F., III. Angew. Chem., Int. Ed. 2008, 47, 42.
(9) Bertelsen, S.; Jørgensen, K. A. Chem. Soc. ReV. 2009, 38, 2178.
10.1021/ol102292a  2010 American Chemical Society
Published on Web 10/29/2010

Henry reaction proceeded via a six-membered cyclic transition
state as shown in Scheme 1, in which the side chain of the
controlled aza-Henry reaction, and provide a model to explain
the observed stereoinduction in the aza-Henry reactions.
The Michael reaction of aldehyde 1 with ꢀ-nitrostyrene 2a
in the presence of 20 mol % of primary amine-thiourea catalyst
3 proceeded to give 4a in high anti-selectivity (anti/syn )
98/2) as reported previously (Table 1).12a Without isolation of 2a,
Scheme 1. Predicted Stereoselectivity of the Aza-Henry Reaction
Table 1. Optimization of Reaction Conditions of the One-Pot
anti-Michael-Aza-Henry Reactiona
aldehyde occupies the equatorial position. The success of this
method prompted us to use an imine as an electrophile to
produce iminosugar derivatives. We hypothesized that the
limited number of coordination sites of the nitrogen in the
protected imine would ensure that the side chain would be in
an axial orientation in the transition state to give an anti-aza-
Henry product with opposite stereochemistry. Recently, ste-
reoselective synthesis of multisubstituted piperidine derivatives
using stepwise13 or one-pot Michael-aza-Henry reactions was
reported.14 The stereochemistry of these products vary, and the
mechanism of stereoinduction was not explained. Herein, we
describe the highly enantioselective one-pot syntheses of
iminosugar derivatives utilizing an organocatalyst-controlled
asymmetric anti-Michael reaction, followed by substrate-
base
AcOH temp time
yield (%)
drc eed
entry
(equiv)
(equiv) (°C) (h) 6a + 7ab 8a 6a:7a (%)
1
Et3N (0.5)
DBU (0.5)
Cs2CO2 (1)
Cs2CO2 (1)
K2CO2 (1)
t-BuOK (1)
i-Pr2EtN (1)
DABCO (1)
DBU (1)
-
-
-
-
-
-
-
rt
rt
rt
0
0
0
3.5
1
1
0.5
0.5
0.5
3
16
37
43
61
59
36
30
18
52
58
49
64
68
25
22
n.d.f
6
14
13
15
18
17
15
6
>10:1 98
1:10 99
2e
3
2:1
5:1
6:1
3:1
>10:1
2:1
>10:1
5:1
6:1
-
-
-
-
-
-
-
-
-
-
4
5
6
7
8
9
10
11
12
0
-
-
-
0
0
0
0
0
0
3
0.5
0.5
0.5
0.5
0.5
TMG (1)
TMG (1)
0.5
0.5
0.5
TMG (1.5)
6
3
7:1
13g TMG (1.5)
7:1 99
(10) For early studies, see: (a) Bui, T.; Barbas, C. F., III. Tetrahedron
Lett. 2000, 41, 6951. (b) Chowdari, N. S.; Ramachary, D. B.; Co´rdova, A.;
Barbas, C. F., III. Tetrahedron Lett. 2002, 43, 9591. (c) Chowdari, N. S.;
Ramachary, D. B.; Barbas, C. F., III. Org. Lett. 2003, 5, 1685. (d)
Ramachary, D. B.; Co´rdova, A.; Barbas, C. F., III. Angew. Chem., Int. Ed.
2003, 42, 4233. (e) Ramachary, D. B.; Barbas, C. F., III. Chem.sEur. J.
2004, 10, 5323. (f) Ramachary, D. B.; Anebouselvy, K.; Barbas, C. F., III.
J. Org. Chem. 2004, 69, 5838.
a 2a (0.2 mmol) was reacted with commercially available 1 (0.6 mmol) in
the presence of 20 mol % of 3 (0.04 mmol) in CH2Cl2 at rt; 5 (0.3 mmol),
AcOH, and base were then added and reacted for time (h) at temp °C. b Isolated
c
1
yield of a mixture of 6a and 7a. Determined by H NMR analysis of an
isolated mixture of 6a and 7a. d Determined by chiral phase HPLC analysis of
the major diastereomer. e 1.5 equiv of imine 5 was used. f n.d.: not detected.
g 2 equiv of 1 M CH2Cl2 solution of 1 (0.4 mmol), which was purified by
column chromatography, was used.
(11) For reviews, see: (a) Enders, D.; Grondal, C.; Hu¨ttl, M. R. M.
Angew. Chem., Int. Ed. 2007, 46, 1570. (b) MacMillan, D. W. C.; Walji,
A. M. Synlett 2007, 1477. (c) Grondal, C.; Jeanty, M.; Enders, D. Nature
Chem. 2010, 2, 167. For recent examples, see: (d) Enders, D.; Hu¨ttl,
M. R. M.; Runsink, J.; Raabe, G.; Wendt, B. Angew. Chem., Int. Ed. 2007,
46, 467. (e) Aroyan, C. E.; Miller, S. J. J. Am. Chem. Soc. 2007, 129, 256.
(f) Wang, J.; Li, H.; Xie, H.; Zu, L.; Shen, X.; Wang, W. Angew. Chem.,
Int. Ed. 2007, 46, 9050. (g) Carlone, A.; Cabrera, S.; Marigo, M.; Jørgensen,
K. A. Angew. Chem., Int. Ed. 2007, 46, 1101. (h) Hayashi, Y.; Okano, T.;
Aratake, S.; Hazelard, D. Angew. Chem., Int. Ed. 2007, 46, 4922. (i) Reyes,
E.; Jiang, H.; Milelli, A.; Elsner, P.; Hazell, R. G.; Jørgensen, K. A. Angew.
Chem., Int. Ed. 2007, 46, 9202. (j) Zhou, J.; List, B. J. Am. Chem. Soc.
2007, 129, 7498. (k) Vicario, J. L.; Reboredo, S.; Bad´ıa, D.; Carrillo, L.
Angew. Chem., Int. Ed. 2007, 46, 5168. (l) Ramachary, D. B.; Kishor, M.
J. Org. Chem. 2007, 72, 5056. (m) Ramachary, D. B.; Kishor, M.; Reddy,
Y. V. Eur. J. Org. Chem. 2008, 6, 975. (n) Cabrera, S.; Aleman, J.; Bolze,
P.; Bertelsen, S.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2008, 47, 121.
(o) Penon, O.; Carlone, A.; Mazzanti, A.; Locatelli, M.; Sambri, L.; Bartoli,
G.; Melchiorre, P. Chem.sEur. J. 2008, 14, 4788. (p) Enders, D.; Wang,
C.; Bats, J. W. Angew. Chem., Int. Ed. 2008, 47, 7539. (q) Lu, M.; Zhu,
D.; Lu, Y. P.; Hou, Y. X.; Tan, B.; Zhong, G. F. Angew. Chem., Int. Ed.
2008, 47, 10187. (r) Han, R.-G.; Wang, Y.; Li, Y.-Y.; Xu, P.-F. AdV. Synth.
Catal. 2008, 350, 1474. (s) Ramachary, D. B.; Reddy, Y. V.; Prakash, B. V.
Org. Biomol. Chem. 2008, 6, 719. (t) Rueping, M.; Kuenkel, A.; Tato, F.;
Bats, J. W. Angew. Chem., Int. Ed. 2009, 48, 3699. (u) Enders, D.; Kru¨ll,
R.; Bettray, W. Synthesis 2010, 4, 567. (v) Jiang, K.; Jia, Z.-J.; Wu, L.;
Chen, Y.-C. Org. Lett. 2010, 12, 2766. (w) Ramachary, D. B.; Mondal, R.;
Venkaiah, C. Org. Biomol. Chem. 2010, 8, 321. (x) Ramachary, D. B.;
Mondal, R.; Venkaiah, C. Eur. J. Org. Chem. 2010, 17, 3205.
p-toluenesulfonyl imine 5 and Et3N were added to the mixture,
and the reaction was allowed to proceed at room temperature.
Although the yield was low, iminosugar derivative 6a was obtained
with ee (98% ee) comparable to that of the Michael reaction (entry
1). The stereocenters at the 3 and 4 positions of 6a were presumably
fixed during the asymmetric anti-Michael reaction. Although eight
possible diastereomers could have been produced at 2, 5, and 6
positions, the diastereomer 6a was predominantly obtained after
column chromatography. The high selectivity of the aza-Henry
reaction in the presence of achiral base indicated that the stereo-
chemistry of the product was induced by the chirality of the
Michael adduct 4a.12b,13,14 The configuration of 6a was
determined as (2R,3S,4S,5R,6S) by X-ray crystallographic
analysis (Figure 1a).
(12) (a) Uehara, H.; Barbas, C. F., III. Angew. Chem., Int. Ed. 2009,
48, 9848. (b) Uehara, H.; Imashiro, R.; Herna´ndez-Torres, G.; Barbas, C. F.,
III. Proc. Natl. Acad. Sci. U.S.A. 2010 (Epub Aug 3).
Org. Lett., Vol. 12, No. 22, 2010
5251

used successfully in aza-Henry reactions,14a provided better yield
(entry 10) than DBU. We expected that a Brønsted acid would
selectively activate the imine. Therefore, combinations of TMG
and AcOH were examined. As indicated by entry 11, this
combination provided 6a in good yield (64%) and suppressed
formation of 8a. An excess amount of aldehyde 1 (3 equiv) was
needed to drive the Michael reaction to completion and to ensure
high selectivity.12a Commercially available 1 exists as a mixture
of monomer and oligomer; the oligomer converted into monomeric
1 under the basic conditions of the aza-Henry reaction, and this
might cause production of 8a. The oligomer of 1 was removed by
column chromatography to give monomeric 1.16 Use of 2 equiv
of purified 1 resulted in complete reaction with high selectivity.
Under these conditions, production of 8a was inhibited, and the
yield of 6a and 7a was improved to 68% (entry 13).
Figure 1. (a) X-ray structure of 6a. (b) X-ray structure of 7a.
Under these optimized conditions, we studied the substrate
scope of this reaction (Table 2). Nitrostyrenes with both
Compound 6a was rapidly converted into diastereomer 7a
when DBU was used as a base in the aza-Henry reaction (entry
2). Compound 7a was shown to be an epimer at the 5 position
of 6a by X-ray crystallographic analysis (Figure 1b). To clarify
the mechanism of formation of the epimer 7a, an epimerization
study was carried out (Scheme 2). 6a was converted into the
Table 2. Substrate Scope of the One-Pot anti-Michael-Aza-Henry
Reactiona
Scheme 2. Epimerization of 9a (Acetate of 6a)
time
(h)
yieldd
dre
eef
entry
2
R
6 + 7 (%)
6:7
(%)
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5
4
8
4
68
64
69
59
65
46
44
18
16
7:1
10:1
4:1
4:1
0:1
6:1
1:3
1:0
0:1
99
99
99
99
99
98
99
98
99
4-MeOC6H4
4-Br-C6H4
3-Br-C6H4
2-CF3-C6H4
2-thienyl
3-pyridyl
n-C7H15
phthalimido
corresponding acetylated compound 9a, which was treated with
DBU at room temperature for 10 min to give a 1:2 mixture of
9a and its 5-epimer 10a. These data suggested that thermody-
namically stable 7a formed by direct epimerization at the 5
position rather than by a retro-aza-Henry/aza-Henry process.12b
Compound 8a was obtained as a side product presumably
through Henry reaction of 4a with excess aldehyde 1.12b To
improve the yield of 6a and 7a and inhibit production of 8a,
additional reaction conditions were examined.
The side product 8a was not detected when Cs2CO3 was used
as base (entry 3).15 When the reaction was performed at 0 °C, the
yields of 6a and 7a were improved compared to the reaction at
room temperature, and kinetic product 6a was favored (entry 4).
Use of K2CO3 (entry 5) gave results similar to Cs2CO3, but yields
were lower in t-BuOK (entry 6). Although reaction in the presence
of sterically hindered i-Pr2EtN resulted in high diastereoselectivity,
the yield was not improved. Use of the less hindered DABCO
resulted in lower yield and lower diastereoselectivity (entry 8).
When DBU was used as the base, reaction at 0 °C suppressed
epimerization compared to the reaction at room temperature (entries
2 and 9). N,N,N′,N′-tetramethylguanidine (TMG), which has been
4
24
30
3
4
10
7
8b
9c
a 2 (0.2 mmol) was reacted with a 1 M CH2Cl2 solution of 1 (0.6 mmol)
in 20 mol % of 3 (0.04 mmol) in CH2Cl2 at rt for time (h); 5 (0.3 mmol),
AcOH (0.1 mmol), and TMG (0.3 mmol) were added at 0 °C and reacted for
0.5 h at 0 °C. b 50 mol % of 3 was used. c 2.5 equiv of 1 (0.5 mmol) was used.
d Isolated yield of mixture of 6 and 7. e Determined by 1H NMR analysis of a
mixture of 6 and 7. f Determined by chiral phase HPLC analysis of the major
diastereomer.
electron-donating and electron-withdrawing groups on the
aromatic ring gave iminosugar derivatives in good yield with
excellent ee (entries 2-5). Only epimerized product 7e was
isolated in the case of 2-trifluoromethyl-ꢀ-nitrostyrene 2e (entry
5). Iminosugar derivatives with heteroaromatic rings at the
4-position could be obtained in moderate yield with excellent
ee (entries 6 and 7). Less reactive alkyl-substituted nitroolefin
2h predominantly provided 6h despite low yield (entry 8). The
anti-Michael reaction of a nitroolefin with a phthalimido group17
(13) Han, B.; Xiao, Y.-C.; He, Z.-Q.; Chen, Y.-C. Org. Lett. 2009, 11,
4640.
(15) Takeda, K.; Nagasawa, K. AdV. Synth. Catal. 2009, 531, 345.
(16) Although concentrated purified 1 oligomerized when stored at -20
°C, a solution of 1 in CH2Cl2 was stable as a monomer at -20 °C.
(17) Zhu, S.; Yu, S.; Wang, Y.; Ma, D. Angew. Chem., Int. Ed. 2010,
49, 4656.
(14) (a) Jakubec, P.; Helliwell, M.; Dixon, D. J. Org. Lett. 2008, 10,
4267. (b) Wang, Y.; Yu, D.-F.; Liu, Y.-Z.; Wei, H.; Luo, Y.-C.; Dixon,
D. J.; Xu, P.-F. Chem.sEur. J. 2010, 16, 3922. (c) Urushima, T.; Sakamoto,
D.; Ishikawa, H.; Hayashi, Y. Org. Lett. 2010, 12, 4588.
5252
Org. Lett., Vol. 12, No. 22, 2010

Scheme 4. Plausible Transition States of the aza-Henry Reaction
Figure 2. X-ray crystal structure of 12.
also proceeded stereoselectively; the subsequent aza-Henry reaction
provided epimerized 7i (entry 9), a precursor of a 4-aminoimino-
sugar derivative. On the basis of these results, we suggest that the
tendency for epimerization at the 5 position is affected by the
electronegativity of the substituent at the 4 position.
reaction described previously.12b A similar six-membered cyclic
transition model was proposed for an asymmetric anti-aza-Henry
reaction21 and the syn-Henry reaction22 catalyzed by phosphoric
acid and a transition metal complex. However, to the best of our
knowledge, this six-membered transition state bridged by a
protonated base has not been proposed for stereoselective aza-
Henry and Henry reactions. This transition state model explains
the high selectivity of both anti-aza-Henry and syn-Henry reactions
and may prove a general feature with nitronate anions generated
with organic base. After construction of a piperidine ring, epimer-
ization occurs, especially in the substrates with electron-deficient
substituents at position 4, to give the thermodynamically stable
(5S,6S)-product 7. In contrast, as shown in Scheme 4B, the 2,6-
dichlorophenyl group was recognized as bulkier than R′; the imine
attacks from the Re face of the nitronate anion; and the nitronate
anion attacks from the Si face of the imine. This results in the
predominant production of (6R)-isomer 11. As shown in Scheme
4, differences in steric bulk between R and R′ seem to be a critical
factor in controlling facial selectivity. The aza-Henry reaction
proceeds in an anti-fashion regardless of facial selectivity via a
six-membered cyclic transition state to provide (5R,6S)-6 or
(5S,6R)-product 11.
The Michael-aza-Henry reaction of 2,6-dichloro-ꢀ-nitrostyrene
2j was then carried out (Scheme 3). Compounds 7j and 11 were
Scheme 3. Michael-aza-Henry Reaction of
(E)-2,6-Dichrolo-ꢀ-nitrostyrene 2j
isolated as closed forms of the Michael-aza-Henry product, in
contrast with the open form observed in the Michael-Henry
reaction.12b Compound 11 was an anomeric mixture; anomer 12
was isolated after column chromatography.18 X-ray crystallographic
analysis of 12 showed that its configuration was 5S,6R (Figure 2).
It should be emphasized that only in reaction of 2,6-dichloro-ꢀ-
nitrostyrene 2j did we observe a reversal of stereoinduction at the
6 position.
The presumed mechanism of stereoinduction at the 5 and 6
positions is illustrated in Scheme 4. It is known that allylic 1,3-
strain restricts rotation of the σ-bond connected to enolates and
nitronates.19 For substitutents R that are less hindered than branched
group substrates R′ (Table 2), the imine should approach from the
Si face of nitronate anion as shown in Scheme 4A.12b,20 Further-
more, the nitronate anion should approach from the Re face of
imine to coordinate the lone pair on nitrogen of imine to the
protonated base via a six-membered cyclic transition state. As a
result, the aza-Henry reaction occurrs anti-selectively to afford
(5R,6S)-iminosugar derivative 6. This is in contrast to the syn-Henry
In conclusion, we established a methodology for the highly
stereoselective synthesis of iminosugar derivatives through one-
pot anti-Michael-anti-aza-Henry reactions. A six-membered
cyclic transition state model that is also consistent with our early
work on the anti-Michael-syn-Henry reaction explains the
stereoinduction in the anti-aza-Henry reaction.
Acknowledgment. This research was supported by the
Skaggs Institute for Chemical Biology.
Supporting Information Available: Experimental pro-
cedures, spectral data, and X-ray crystallographic analysis
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL102292A
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Org. Lett., Vol. 12, No. 22, 2010
5253