Highly diastereoselective and enantioselective synthesis of α-hydroxy β-amino acid derivatives: Lewis base catalyzed hydrosilylation of α-acetoxy β-enamino esters

Article abstract of DOI:10.1002/anie.201102150

By design: A series of α-acetoxy-β-enamino esters 1 were synthesized and then subjected to catalytic asymmetric hydrosilylation. In the presence of a chiral Lewis base catalyst, the reactions proceeded smoothly to provide a wide range of chiral α-acetoxy β-amino acid derivatives in high yields with good diastereoselectivities and enantioselectivities. Copyright

Full text of DOI:10.1002/anie.201102150

Communications
DOI: 10.1002/anie.201102150
Asymmetric Hydrosilylation
Highly Diastereoselective and Enantioselective Synthesis of a-Hydroxy
b-Amino Acid Derivatives: Lewis Base Catalyzed Hydrosilylation of
a-Acetoxy b-Enamino Esters**
Yan Jiang, Xing Chen, Yongsheng Zheng, Zhouyang Xue, Chang Shu, Weicheng Yuan, and
Xiaomei Zhang*
Chiral a-hydroxy b-amino acid moieties are important
structural components in a wide variety of biologically
active compounds as well as natural products, of which the
side chains of Taxol and its analogues are the most famous
examples.[1] Consequently, the synthesis of chiral a-hydroxy
b-amino acid derivatives has attracted considerable atten-
tion.[2] Some syntheses include the Sharpless asymmetric
aminohydroxylation,[3] asymmetric dihydroxylation,[4] ring
opening of chiral epoxides,[5] asymmetric nitroaldol reac-
tions,[6] asymmetric Mannich reaction,[7] asymmetric 1,3-
dipolar cycloaddition,[8] and other transformations.[9] Among
the successful strategies developed for obtaining optically
active a-hydroxy b-amino acid derivatives, those that lead to
substrates containing certain functional groups are of great
significance. Accordingly, it can be reasoned that direct
Therefore, we first tried to introduce an
acetoxy group to the a position of b-enam-
ino esters so as to generate a-acetoxy b-
enamino ester 1 in which every functional
group was finely assembled as depicted in
Figure 1.
(1S,2S)-2-Amino-1-(4-nitrophenyl)pro-
pane-1,3-diol (2) is the intermediate of
chloramphenicol. It is very cheap and
easily accessible. The two hydroxy groups
Figure 1. a-Ace-
toxy b-enamino
esters 1 that
were subjected
to hydrosilyla-
tion in this
can undergo condensation with a ketone or study.
an aldehyde to generate a six-membered
ring.[13] Thus it occurred to us that we could
make a novel chiral, rigid picolinamide Lewis base catalyst
through the same transformation. We reasoned that this rigid
catalyst might be highly selective in promoting asymmetric
=
asymmetric reduction of the corresponding C N double bond
=
in substrates would be the most straightforward way to
construct chiral a-hydroxy b-amino acid derivatives. How-
ever, to the best of our knowledge, the research on the above-
mentioned reaction has not yet been reported.
Recently, asymmetric reactions involving the Lewis base
activation of Lewis acids has attracted much attention.[10]
Among these reactions, chiral Lewis base catalyzed asym-
hydrosilylation of C N double bonds. Hence we synthesized
catalysts through two facile steps. As can be seen in Scheme 1,
2 was condensed with picolinic acid to give amide 3. The two
hydroxy groups of amide 3 were then condensed with
formaldehyde or a ketone to generate the cyclic catalysts
4a–4d.
=
metric hydrosilylation of C N double bonds has become an
important approach to chiral nitrogen-containing compounds
because of the mild reaction conditions, cheap reagents, and
the environmentally benign nature of this transformation.[11]
Several groups have achieved impressive progress in this
field.[12] During our ongoing studies on chiral Lewis base
=
catalyzed asymmetric hydrosilylation of C N double bond
compounds such as b-enamino esters,[12m] we envisioned that
the design and synthesis of b-enamino esters bearing various
functional groups on the a position would provide a wide
range of precursors to a-substituted b-amino acid derivatives.
[*] Y. Jiang, X. Chen, Y. Zheng, Z. Xue, C. Shu, W. Yuan, X. Zhang
Key Laboratory for Asymmetric Synthesis and Chiraltechnology of
Sichuan Province, Chengdu Institute of Organic Chemistry
Chinese Academy of Sciences, Chengdu, 610041 (China)
E-mail: xmzhang@cioc.ac.cn
Scheme 1. Synthesis of the novel chiral Lewis base catalysts 4a–4d.
First we initiated the hydrosilylation of a-acetoxy b-
enamino ester 1a by employing 4a as the catalyst. Gratify-
ingly, the reaction proceeded smoothly in 1,2-dichloroethane
at À108C for 40 hours to generate the desired product in
almost quantitative yield with a high diastereoselectivity of
91:9 (syn/anti), as well as a high enantioselectivity of 94%
(Table 1, entry 1). Encouraged by this result, we then used the
bulkier catalysts 4b–4d to promote the hydrosilylation of 1a.
Y. Jiang, X. Chen, Y. Zheng, Z. Xue, C. Shu
Graduate School of Chinese Academy of Sciences
Beijing, 100049 (China)
[**] This work was supported by the National Sciences Foundation of
China (20972155) and the National Basic Research Program of
China (973 Program) (2010CB833301).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102150.
7304
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Angew. Chem. Int. Ed. 2011, 50, 7304 –7307

Table 1: Asymmetric hydrosilylation of a-acetoxy b-enamino ester 1a
promoted by 4.
10 mol% of catalyst 4a, a wide variety of a-acetoxy b-
enamino esters were hydrosilylated in 1,2-dichloroethane
at À108C using trichlorosilane. The results are summarized
in Table 2. Generally, most of the b-aryl-a-acetoxy b-
enamino esters underwent the reaction smoothly to give
the desired products in high yields with good diastereose-
lectivities and enantioselectivities (Table 2, entries 1–9, 11
and 15). The electronic nature of the substituents on the
Entry[a] Cat*
(mol%)
Solvent
T
[8C]
t
Yield syn/
ee
[h] [%][b] anti[c] [%][d]
Table 2: Asymmetric hydrosilylation of a-acetoxy b-enamino esters 1a–
1n promoted by 4a.
1
2
3
4
5
6
7
8
4a (10%) ClCH2CH2Cl À10 40
4b (10%) ClCH2CH2Cl À10 40
4c (10%) ClCH2CH2Cl À10 40
4d (10%) ClCH2CH2Cl À10 40
95
99
96
93
90
47
54
98
98
99
97
96
91:9
94
60:40 80
63:37 87
62:38 80
84:16 86
77:23 88
89:11 84
82:18 91
91:9
91:9
91:9
91:9
Entry[a]
t
[h]
Yield
[%][b]
syn/
ee
anti[c]
[%][d]
4a (10%) CH2Cl2
À10 30
4a (10%) Cl2CHCHCl2 À10 60
4a (10%) CHCl3
4a (10%) THF
À10 40
À10 15
9
4a (15%) ClCH2CH2Cl À10 24
4a (20%) ClCH2CH2Cl À10 24
93
93
90
93
1
2
3
4
5
6
7
8
1a: R1 =Ph
40
40
40
42
40
40
40
40
40
30
24
40
95
90
94
91
98
96
93
97
92
96
98
93
91:9[e]
93:7
90:10
92:8
91:9
94[f]
92
89
93
93
87
91
93
93
77
96
41
10
11
12
1b: R1 =4-BrC6H4
1c: R1 =4-FC6H4
1d: R1 =4-MeOC6H4
1e: R1 =4-MeC6H4
1 f: R1 =3-MeOC6H4
1g: R1 =3-ClC6H4
1h: R1 =3, 4-MeO2C6H3
1i: R1 =2-naphthyl
1j: R1 =2-thienyl
1k: R1 =2-furanyl
1l: R1 =Bn
4a (10%) ClCH2CH2Cl
0
24
4a (10%) ClCH2CH2Cl À20 60
[a] Unless otherwise specified, the reactions were carried out on a
0.15 mmol scale with 2.0 equiv of HSiCl3 in 1.5 mL of solvent. [b] Yield of
isolated 5a. [c] The d.r. values of 6a were determined by HPLC analysis
91:9
92:8
92:8
92:8
95:5
88:12
99:1
using
a chiral stationary phase. [d] The ee values of the major
diastereomer of 6a were determined by HPLC analysis using a chiral
stationary phase.
9
10
11
12
These catalysts proved to be highly active, however, the
diastereoselectivity and enantioselectivity were lower
(Table 1, entries 2–4).
13
14
1m: R2 =Me
1n: R2 =tBu
40
40
97
93
91:9
93:7
93
93
Therefore, 4a was determined to be the optimal catalyst
and was employed in the subsequent investigations. Several
solvents were tested for the reaction. Dichloromethane gave a
good yield but lower diastereoselectivity and enantioselectiv-
ity (Table 1, entry 5). To our surprise, 1,1,2,2-tetrachloro-
ethane and chloroform resulted in very poor yields (Table 1,
entries 6 and 7). The reaction in THF was complete in only
15 hours; however, it suffered from an evident decrease in
diastereoselectivity and a little erosion in enantioselectivity
(Table 1, entry 8). Hence, 1,2-dichloroethane was determined
to be the most appropriate solvent for this study. In addition,
we found that the reaction proceeded sluggishly in dehy-
drated solvent to give very little product, whereas the reaction
proceeded smoothly in commercially available solvent with-
out any pretreatment. We reasoned that trace acid arising
from the reaction of trichlorosilane with traces of water in the
solvent promoted the isomerization of the substrate from the
enamine tautomer to the imine tautomer, which is the species
involved in the hydrosilylation. Further attempts to enhance
the d.r. and ee values by increasing the catalyst loading did not
prove to be successful (Table 1, entries 9 and 10). When the
reaction was conducted at a higher temperature (08C), the
diastereoselectivity remained at the same level and the
ee value decreased slightly (Table 1, entry 11). However,
when the temperature fell to À208C, no improvement in
either the d.r. or ee values were observed (Table 1, entry 12).
Having established the optimal reaction conditions, the
generality of this reaction was examined. In the presence of
15
1o
42
90
95:5
93
[a] Unless otherwise specified, the reactions were carried out on a
0.15 mmol scale with 2.0 equiv of HSiCl3 in 1.5 mL of solvent. [b] Yield of
isoalted 5. [c] The d.r. values of 5 were determined by HPLC analysis
using
a chiral stationary phase. [d] The ee values of the major
diastereomers of 5 were determined by HPLC analysis using a chiral
stationary phase. [e] The d.r. value of 6a was determined by HPLC
analysis using a chiral stationary phase. [f] The ee value of the major
diastereomer of 6a was determined by HPLC analysis using a chiral
stationary phase.
phenyl group showed very little influence on the results of the
reaction. b-Heterocyclic aryl substrates 1j and 1k were found
to undergo the reaction more quickly. Notably, the thienyl-
derived 1j gave a much lower ee value (Table 2, entry 10),
whereas furanyl-derived 1k resulted in excellent enantiose-
lectivity (96% ee) albeit with a slightly lower diastereoselec-
tivity (Table 2, entry 11). The reaction of b-benzyl-a-acetoxy
b-enamino ester 1l proceeded smoothly to generate the
corresponding product in good yield with excellent diaste-
reoselectivity (99:1), but, unfortunately with a poor ee value
Angew. Chem. Int. Ed. 2011, 50, 7304 –7307
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Communications
(Table 2, entry 12). It is clear that the b-
aryl group plays a key role in enantio-
selection in our asymmetric catalytic
system. Variation of the ester group of
the substrates led to little change in d.r.
and ee values (Table 2, entries 13 and
14).
To further illustrate the synthetic
potential of this methodology, the
formal synthesis of the C13 side chain
of taxol was performed. As shown in
Scheme 2, the asymmetric hydrosilyla-
tion product 5a was treated with K2CO3
to generate 6a. Then the PMP group
was removed with CAN to afford
amino alcohol 7. Finally, alcohol 7 was
Scheme 2. Synthesis of the taxol C13 side chain. PMP=para-methoxyphenyl, CAN=ceric
ammonium nitrate.
benzoylated to give 8. Compound 8 was identical with the
reported compound, which can be hydrolyzed to provide the
C13 side chain of taxol (9) according to the literature.[8a]
Furthermore, product 5o was converted into oxazolidi-
none 12, which shows promising activity in the inhibition of
cholesterol transporter NPC1L1 and is thus, a potent
hypocholesterolemic agent.[14] As seen in Scheme 3, the
asymmetric hydrosilylation product 5o was treated with
K2CO3 to make 6o, which was then cyclized with triphosgene
to afford oxazolidinone 10. Subsequent reaction of 10 with
benzyl amine in dichloromethane, followed by hydrogenation
with Pd/C and hydrogen yielded oxazolidinone 12. Notably,
the N-aryl group is an exact fragment found in the final
product and therfore does not need to be removed in this case.
In conclusion, various a-acetoxy b-enamino esters were
designed and synthesized, and a set of novel chiral Lewis base
catalysts were prepared from readily available chiral sources.
One of the catalysts was found to accelerate the asymmetric
hydrosilylation of a-acetoxy b-enamino esters efficiently. This
methodology was used to prepare a wide range of chiral a-
acetoxy b-amino acid derivatives in high yields with good
diastereoselectivities and enantioselectivities. The reactions
were conducted under very mild reaction conditions and the
removal of water and oxygen from the reaction system was
not necessary. This methodology was additionally applied
successfully in synthesis of the taxol C13 side chain and an
oxazolidinone which is a potent hypocholesterolemic agent.
Investigations of the mechanism and further application of
this transformation to the construction of other complex
natural products or pharmaceutically active substances are in
progress. In addition, the catalytic asymmetric hydrosilylation
of other a-substituted b-enamino esters is in progress.
Scheme 3. Synthesis of oxazolidinone 12. Bn=benzyl.
brine and dried over anhydrous Na2SO4. The solvents were removed
under vacuum. Purification of the reaction mixture by column
chromatography (silica gel, hexanes/EtOAc = 10:1) afforded pure
product. The ee values were determined using established HPLC
techniques with chiral stationary phases.
Received: March 28, 2011
Revised: May 18, 2011
Published online: June 17, 2011
Keywords: amino acids · asymmetric catalysis · Lewis bases ·
organocatalysis · synthetic methods
.
Experimental Section
General procedure for asymmetric hydrosilylation of a-acetoxy b-
enamino esters: A solution of trichlorosilane (31 mL, 0.3 mmol,
2.0 equiv) in 120 mL of ClCH2CH2Cl was added to a stirred solution of
the corresponding a-acetoxy b-enamino ester (0.15 mmol) and the
catalyst (0.015 mmol) in ClCH2CH2Cl (1.5 mL) at À108C. The
mixture was stirred at the same temperature until the reaction was
complete as determined by TLC. The reaction mixture was then
quenched with a saturated aqueous solution of NaHCO3 and was
extracted with EtOAc. The combined extracts were washed with
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