Lewis base catalyzed asymmetric hydrosilylation of α-substituted β-enamino esters: Facile access to enantioenriched β2-amino esters via dynamic kinetic resolution

Article abstract of DOI:10.1055/s-0034-1378323

A chiral Lewis base organocatalyzed asymmetric hydrosilylation of α-substituted β-enamino esters is presented. The reactions proceeded through dynamic kinetic resolution to afford various enantioenriched β²-amino esters with high yields (up to

Full text of DOI:10.1055/s-0034-1378323

LETTER
▌1879
letter
Lewis Base Catalyzed Asymmetric Hydrosilylation of α-Substituted β-Enamino
Esters: Facile Access to Enantioenriched β2-Amino Esters via Dynamic Kinetic
Resolution
Hydrosilylation of α-Substituted β-Enamino Esters
Chang Shu,a,b Xiao-Yan Hu,a,b Shuai-Shuai Li,a,b Wei-Cheng Yuan,a Xiao-Mei Zhang*a
a
Key Laboratory for Asymmetric Synthesis & Chirotechnology of Sichuan Province, Chengdu Institute of Organic Chemistry, Chinese
Academy of Sciences, Chengdu 610041, P. R. of China
Fax +86(28)85257883; E-mail: xmzhang@cioc.ac.cn
b
Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. of China
Received: 13.04.2014; Accepted after revision: 19.05.2014
amine 1. Thus the DKR can be realized, and
Abstract: A chiral Lewis base organocatalyzed asymmetric hydro-
enantioenriched β2-amino ester 2 can be prepared.
silylation of α-substituted β-enamino esters is presented. The reac-
tions proceeded through dynamic kinetic resolution to afford
various enantioenriched β2-amino esters with high yields (up to
R2
R2
N
R2
N
98%) in moderate enantioselectivities (up to 77% ee).
Key words: asymmetric catalysis, dynamic kinetic resolution, β2-
amino acids, hydrosilylation, Lewis base
NH
+
R1
CO2R3
R1
CO2R3
R1
CO2R3
1
(S)-imine
(R)-imine
catalyst
HSiCl3
Enantioenriched β2-amino acids and derivatives are im-
portant building blocks in the synthesis of natural prod-
ucts, β-peptides, and pharmaceuticals.1 In contrast to β3-
homoamino acids, β2-amino acids cannot be obtained sim-
ply by homologation of the (natural) α-amino acids, but
have to be synthesized by asymmetric reactions. There-
fore, synthesis of enantioenriched β2-amino acids has at-
tracted increasing attention.2 Up to now, many efficient
strategies have been developed to prepare enantioenriched
β2-amino acid derivatives, such as Michael addition,3 C–
H insertions of carbenoids reaction,4 enantioselective H-
atom transfer reactions,5 asymmetric hydrogenation of
prochiral dehydro precursors of β2-amino acids deriva-
tives,6 and other protocols.7
H
N
R2
R1
CO2R3
2 (enantioenriched)
*
Scheme 1 Dynamic kinetic resolution in chiral Lewis base catalyzed
hydrosilylation of α-substituted β-enamino esters
Herein, we describe the chiral Lewis base catalyzed hy-
drosilylation of α-substituted β-enamino esters, through
which various enantioenriched β2-amino esters were syn-
thesized with good yields in moderate enantioselectivi-
ties.
Trichlorosilane is a cheap and readily available reducing
agent. Recently, chiral Lewis base catalyzed hydrosi-
lylation of C=N bond compounds with trichlorosilane has
drawn much attention,8,9 This mild and economical or-
ganocatalytic methodology has become a new approach in
the synthesis of chiral N-containing compounds. During
our continuing research on the asymmetric Lewis base
catalyzed hydrosilylation of C=N bond compounds,9q–t we
envisioned that chiral β2-amino esters could be accessed
by chiral Lewis base catalyzed hydrosilylation of α-sub-
stituted β-enamino esters through dynamic kinetic resolu-
tion (DKR).10,11 As illustrated in Scheme 1, α-substituted
β-enamino ester 1 isomerizes to generate (S)-imine and
(R)-imine. In the presence of a chiral Lewis base catalyst,
one of the enantiomers may be hydrosilylated preferen-
tially, and the other enantiomer isomerizes back to en-
First, various α-substituted β-enamino esters 113 were eas-
ily prepared according to the literature procedure12 with
minor modification (see the Supporting Information). Af-
terwards, several chiral Lewis base catalysts 3–7 (Figure
1) were tested in the hydrosilylation of α-phenyl-β-enami-
no ester 1a in dichloromethane at –10 °C. As can be seen
in Table 1, when trans-4-hydroxy-L-proline-derived cata-
lyst 3a was used, the reaction proceeded smoothly to give
β2-amino ester 2a in high yield with only 55% enantio-
meric excess (Table 1, entry 1). Several catalysts 3b–f
bearing an O-alkyl or O-phenyl group on C5 of the pridi-
nyl ring resulted in higher enantiomeric excesses (Table 1,
entries 2–6). Perhaps the electron-donating groups on C5
of pyridinyl ring increased the Lewis basicity of the cata-
lysts so that they exhibited stronger influence on the sub-
strate. 5-Phenyl catalyst 3g gave similar enantioselection
to 3a (Table 1, entry 7). When an electron-donation group
(methoxy) was introduced on C4 of the pridinyl ring, a
slightly lower enantiomeric excess was observed (Table 1,
entry 7). On the other hand, catalyst 3i bearing an elec-
SYNLETT 2014, 25, 1879–1882
Advanced online publication: 08.07.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0034-1378323; Art ID: st-2014-w0311-l
© Georg Thieme Verlag Stuttgart · New York

1880
C. Shu et al.
LETTER
tron-withdrawing group (chloro) delivered much better
enantioselectivity (Table 1, entry 8). Apparently, the
groups on C4 and C5 of the pyridinyl ring had opposite ef-
fects on the reaction. Several other chiral Lewis base cat-
alysts 4–7 with other chiral amine moieties were also
screened and very poor enantioselectivities were ob-
tained. Therefore 3i was determined as the optimal cata-
lysts and employed in subsequent investigations.
3a: R= H
3b: R= 5-OMe
3c: R= 5-Oi-Pr
3d: R= 5-OBn
3e: R= 5-OPh
3f: R= 5-O(CH2)2OMe
3g: R= 5-Ph
OPiv
R
N
N
O
Ph
Ph
3h: R= 4-OMe
3i: R= 4-Cl
HO
H
N
H
N
N
O
Table 1 Asymmetric Hydrosilylation of α-Substituted β-Enamino
Ester 1aa
N
O
O
O
O
Ph
O
H
H
Ph
N
N
catalyst (10 mol%), CH2Cl2
–10 °C, HSiCl3 (2.0 equiv)
5
PMP
PMP
O2N
4
Ph
CO2Et
1a
Ph
CO2Et
*
2a
Me
Me
N
N
Ph
Ph
Yield (%)b
ee (%)c
N
N
Entry
Catalyst
O
O
HO
HO
Ph
1
2
3a
3b
3c
3d
3e
3f
3g
3h
3i
4
93
93
94
96
96
98
95
93
98
70
90
92
95
55
70
72
72
66
65
58
52
74
20
39
27
17
6
7
Figure 1 Chiral Lewis base organocatalysts evaluated in this study
3
4
Table 2 Asymmetric Hydrosilylation of α-Substituted β-Enamino
Ester 1aa
5
6
Cl
OPiv
7
N
8
N
O
Ph
Ph
9
H
H
HO
N
N
PMP
PMP
3i (10 mol%)
10
11
12
13
HSiCl3, solvent
Ph
CO2Et
Ph
CO2Et
1a
*
5
2a
6
Entry
Solvent
Temp (°C)
Yield (%)b ee (%)c
7
1
2
CH2Cl2
CHCl3
DCE
–10
–10
–10
–10
–10
–10
0
98
95
92
91
88
88
94
97
96
97
74
11
47
3
a Reaction conditions: Unless otherwise specified, the reactions were
carried out on a 0.20 mmol scale with 2.0 equiv of HSiCl3 in 2.0 mL
of CH2Cl2.
b Isolated yield based on 1a.
3
c The ee values were determined using chiral HPLC.
4
MeCCl3
THF
5
29
11
53
65
65
72
Subsequent screening of the solvents revealed that dichlo-
romethane was the most favorable solvent in the reaction
(Table 2, entry 1). When the reaction was conducted at 0
°C, the enantiomeric excess dropped obviously (Table 2,
entry 7). When the temperature was lowered to –20 °C,
the enantiomeric excess also dropped obviously (Table 2,
entry 19). Addition of 4 Å molecular sieves in the reaction
system did not benefit the enantioselection (Table 2, entry
9). Increasing the catalyst loading to 20 mol% also did not
improve the enantioselection (Table 2, entry 10).
6
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
7
8
–20
–10
–10
9d
10e
a Reaction conditions: Unless otherwise specified, the reactions were
carried out on a 0.20 mmol scale with 2.0 equiv of HSiCl3 and 10
With the optimized conditions in hand, the scope of the mol% of 3i in 2.0 mL solvent.
b Isolated yield based on 1a.
chiral Lewis base organocatalyzed hydrosilylation of α-
c The ee values were determined using chiral HPLC.
substituted β-enamino esters was examined. Generally, α-
aryl substrates underwent the reactions smoothly to afford
the corresponding products with good yields in similar en-
d Conditions: 50 mg of 4 Å MS was added to the reaction mixture.
e Conditions: 20 mol% of 3i was used.
Synlett 2014, 25, 1879–1882
© Georg Thieme Verlag Stuttgart · New York

LETTER
Hydrosilylation of α-Substituted β-Enamino Esters
1881
and the corresponding product was obtained in good yield
with very poor enantioselection (Table 3, entry 18).
Table 3 Asymmetric Hydrosilylation of α-Substituted β-Enamino
Esters 1a–p with Trichlorosilane Catalyzed by 3ia
Cl
In conclusion, we have developed an asymmetric hydro-
silylation of α-substituted β-enamino esters promoted by
chiral Lewis base organocatalysts through dynamic kinet-
ic resolution. This transformation enables the straightfor-
ward and mild synthesis of various chiral β2-amino esters
with good yields in moderate enantioselectivities.
OPiv
N
N
O
Ph
Ph
HO
H
H
N
N
3i (10 mol%)
PMP
PMP
HSiCl3, CH2Cl2
–10 °C
R
CO2Et
1a–r
R
CO2Et
*
Acknowledgment
2a–r
We are grateful for financial support from the National Natural Sci-
ence Foundation of China (21172217 and 20972155) and National
Basic Research Program of China (973 Program) (2010CB833301).
Entry
2 R
Yield (%)b
ee (%)c
1
2
2a Ph
98
95
97
98
99
93
99
93
94
91
94
99
96
96
87
96
0
74
74
58
73
77
71
77
73
63
31
66
74
71
77
54
55
–
Supporting Information for this article is available online
2b 4-MeOC6H4
2c 2-MeOC6H4
2d 3-MeOC6H4
2e14 3,4-(MeO)2C6H3
2f piperonyl
2g 4-FC6H4
at
http://www.thieme-connect.com/products/ejournals/journal/
3
10.1055/s-00000083.SunogIpimrfiantoSuIpg
n
fonirtat
ori
4
References and Notes
5
(1) (a) Seebach, D.; Beck, A. K.; Capone, S.; Deniau, G.;
Groselj, U.; Zass, E. Synthesis 2009, 1. (b) Seebach, D.;
Gardiner, J. Acc. Chem. Res. 2008, 41, 1366. (c) Lelais, G.;
Seebach, D. Biopolymers 2004, 76, 206. (d) Cheng, R. P.;
Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101,
3219. (e) Subbaraju, G. V.; Golakoti, T.; Patterson, G. M. L.;
Moore, R. E. J. Nat. Prod. 1997, 60, 302. (f) Chaganty, S.;
Heltzel, C.; Golakoti, T.; Moore, R. E.; Yoshida, W. Y.
J. Nat. Prod. 2004, 67, 1403. (g) Seebach, D.; Kimmerlin,
T.; Šebesta, R.; Campo, M. A.; Beck, A. K. Tetrahedron
2004, 60, 7455.
(2) For reviews, see: (a) Sewald, N. Angew. Chem. Int. Ed.
2003, 42, 5794. (b) Weiner, B.; Szymanski, W.; Janssen, D.
B.; Minnaard, A. J.; Feringa, B. L. Chem. Soc. Rev. 2010, 39,
1656.
(3) (a) Duursma, A.; Minnaard, A. J.; Feringa, B. L. J. Am.
Chem. Soc. 2003, 125, 3700. (b) Sammis, G. M.; Jacobsen,
E. N. J. Am. Chem. Soc. 2003, 125, 4442. (c) Wang, J.; Li,
H.; Duan, W.; Zu, L.; Wang, W. Org. Lett. 2005, 7, 4713.
(d) Lee, H. S.; Park, J. S.; Kim, B. M.; Gellman, S. H. J. Org.
Chem. 2003, 68, 1575. (e) Lin, L.; Yin, W.; Fu, X.; Zhang,
J. L.; Ma, X. J.; Wang, R. Org. Biomol. Chem. 2012, 10, 83.
(4) Davies, H. M. L.; Venkataramani, C. Angew. Chem. Int. Ed.
2002, 41, 2197.
6
7
8
2h 2,4,5-F3C6H2
2i 4-ClC6H4
2j 2-ClC6H4
2k 3-ClC6H4
2l 4-BrC6H4
2m 4-PhC6H4
2n 2-naphthyl
2o 2-thienyl
2p Bn
9
10
11
12
13
14
15
16
17
18
2q i-Pr
2r PhCONH
88
19
a Unless otherwise specified, the reactions were carried out on a 0.20
mmol scale with 2.0 equiv of HSiCl3 and 10 mol% of 3i in 2.0 mL of
CH2Cl2.
(5) (a) Martin, N. J. A.; Cheng, X.; List, B. J. Am. Chem. Soc.
2008, 130, 13862. (b) Sibi, M. P.; Patil, K. Angew. Chem.
Int. Ed. 2004, 43, 1235.
b Isolated yield based on 1.
c The ee values were determined using chiral HPLC.
(6) (a) Elaridi, J.; Thaqi, A.; Prosser, A.; Jackson, W. R.;
Robinson, A. J. Tetrahedron: Asymmetry 2005, 16, 1309.
(b) Huang, H.; Liu, X.; Deng, J.; Qiu, M.; Zheng, Z. Org.
Lett. 2006, 8, 3359. (c) Qiu, L.; Prashad, M.; Hu, B.; Prasad,
K.; Repič, O.; Blacklock, T. J.; Kwong, F. Y.; Kok, S. H. L.;
Lee, H. W.; Chan, A. S. C. Proc. Natl. Acad. Sci. U.S.A.
2007, 104, 16787. (d) Deng, J.; Hu, X.; Huang, J.; Yu, S.;
Wang, D.; Duan, Z.; Zheng, Z. J. Org. Chem. 2008, 73,
2015. (e) Hoen, R.; Tiemersma-Wegman, T.; Procuranti, B.;
Lefort, L.; Vries, J. G. D.; Minnaard, A. J.; Feringa, B. L.
Org. Biomol. Chem. 2007, 5, 267. (f) Li, L. N.; Chen, B.; Ke,
Y. Y.; Li, Q.; Zhuang, Y.; Duan, K.; Huang, Y. C.; Pang, J.
Y.; Qiu, L. Q. Chem. Asian J. 2013, 8, 2167. (g) Luhr, S.;
Holz, J.; Zayas, O.; Seidelmann, O.; Domke, L.; Borner, A.
Tetrahedron: Asymmetry 2013, 24, 395.
antiomeric excesses, except those bearing bulky substitu-
ents on ortho position of the phenyl group (Table 3,
entries 3 and 10) and α-2-thienyl-β-enamino ester 2o (Ta-
ble 3, entry 15) which provided rather lower enantiomeric
excess. Moreover, reaction of α-benzyl-β-enamino ester
2p gave rise to good yield but poor enantiomeric excess
(Table 3, entry 16). Meanwhile, no desired product was
obtained with α-isopropyl-β-enamino ester 2q because 2q
was very unstable and decomposed quickly under the re-
action conditions (Table 3, entry 17). Finally, α-benzami-
do-β-enamino ester 2r was also subjected to the reaction,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1879–1882

1882
C. Shu et al.
LETTER
(7) For selected examples, see: (a) Chi, Y. G.; English, E. P.;
Pomerantz, W. C.; Horne, W. S.; Joyce, L. A.; Alexander, L.
R.; Fleming, W. S.; Hopkins, E. A.; Gellman, S. H. J. Am.
Chem. Soc. 2007, 129, 6050. (b) Chi, Y. G. Gellman S. H.
J. Am. Chem. Soc. 2006, 128, 6804. (c) Sibi, M. P.;
Tatamidani, H.; Patil, K. Org. Lett. 2005, 7, 2571.
Yuan, W. C.; Zhang, X. M. Adv. Synth. Catal. 2013, 355,
1931. (r) Chen, X.; Hu, X. Y.; Shu, C.; Zhang, Y. H.; Zheng,
Y. S.; Jiang, Y.; Yuan, W. C.; Liu, B.; Zhang, X. M. Org.
Biomol. Chem. 2013, 11, 3089. (s) Jiang, Y.; Chen, X.;
Zheng, Y. S.; Xue, Z. Y.; Shu, C.; Yuan, W. C.; Zhang, X.
M. Angew. Chem. Int. Ed. 2011, 50, 7304. (t) Zheng, H. J.;
Chen, W. B.; Wu, Z. J.; Deng, J. G.; Lin, W. Q.; Yuan, W.
C.; Zhang, X. M. Chem. Eur. J. 2008, 14, 9864.
(d) Beddow, J. E.; Davies, S. G.; Ling, K. B.; Roberts, P. M.;
Russell, A. J.; Smith, A. D.; Thomson, J. E. Org. Biomol.
Chem. 2007, 5, 2812.
(10) For reviews on DKR, see: (a) Zhang, B. L.; Qin, W.; Duan,
Y. C.; Yu, B.; Zhang, E.; Liu, H. M. Chin. J. Org. Chem.
2012, 32, 1359. (b) Pellissier, H. Tetrahedron 2011, 67,
3769. (c) Pellissier, H. Adv. Synth. Catal. 2011, 353, 659.
(d) Zhang, Z. H.; Liu, Q. B. Chin. J. Org. Chem. 2005, 25,
780.
(11) For recent selected examples, see: (a) Shen, X.; Miao, W. J.;
Ni, C. F.; Hu, J. B. Angew. Chem. Int. Ed. 2014, 53, 775.
(b) Nikitin, K.; Rajendran, K. V.; Müller-Bunz, H.;
Gilheany, D. G. Angew. Chem. Int. Ed. 2014, 53, 1906.
(c) Dornan, P. K.; Kou, K. G. M.; Houk, K. N.; Dong, V. M.
J. Am. Chem. Soc. 2014, 136, 291. (d) Yang, W.; Yang, Y.;
Du, D. M. Org. Lett. 2013, 15, 1190. (e) Metrano, A. J.;
Miller, S. J. J. Org. Chem. 2014, 79, 1542.
(8) For reviews on Lewis base catalysis and Lewis base catalytic
asymmetric hydrosilylations of C=N double bonds, see:
(a) Rendler, S.; Oestreich, M. Synthesis 2005, 1727.
(b) Orito, Y.; Nakajima, M. Synthesis 2006, 1391.
(c) Denmark, S. E.; Beutner, G. L. Angew. Chem. Int. Ed.
2008, 47, 1560; Angew. Chem. 2008, 120, 1584.
(d) Kočovský, P.; Malkov, A. V. Chiral Lewis Bases as
Catalysts, In Enantioselective Organocatalysis; Dalko, P. I.,
Ed.; Wiley-VCH: Weinheim, 2007, 255. (e) Kagan, H. B.
Organocatalytic Enantioselective Reduction of Olefins,
Ketones and Imines, In Enantioselective Organocatalysis;
Dalko, P. I., Ed.; Wiley-VCH: Weinheim, 2007, 391.
(f) Jones, S.; Warner, C. J. A. Org. Biomol. Chem. 2012, 10,
2189. (g) Guizzetti, S.; Benaglia, M. Eur. J. Org. Chem.
2010, 5529. (h) Weickgenannt, A.; Oestreich, M.
(12) Huang, L. J.; Hsieh, M. C.; Teng, C. M.; Lee, K. H.; Kuo, S.
C. Bioorg. Med. Chem. 1998, 6, 1657.
ChemCatChem 2011, 3, 1527.
(13) General Experimental Procedure for the Enantioselective
Hydrosilylation of α-Substituted β-Enamino Esters
A solution of trichlorosilane (41 μL, 0.3 mmol, 2.0 equiv) in
160 μL of CH2Cl2 was added to a stirred solution of the
corresponding α-substituted β-enamino ester (0.20 mmol)
and the catalyst 3i (0.020 mmol) in CH2Cl2 (2.0 mL) at –10
°C. The mixture was stirred at the same temperature until the
reaction reached completion. Then the reaction was
quenched with a sat. aq solution of NaHCO3 and was
extracted with CH2Cl2. The combined organic layer was
washed with brine, dried over anhydrous Na2SO4, and the
solvents were evaporated. Purification by column
(9) For representative examples, see: (a) Malkov, A. V.;
Stewart-Liddon, A. J. P.; McGeoch, G. D.; Ramirez-Lopez,
P.; Kočovský, P. Org. Biomol. Chem. 2012, 10, 4864.
(b) Malkov, A. V.; Stončius, S.; Vranková, K.; Arndt, M.;
Kočovský, P. Chem. Eur. J. 2008, 14, 8082. (c) Malkov, A.
V.; Stončius, S.; Kočovský, P. Angew. Chem. Int. Ed. 2007,
46, 3722. (d) Malkov, A. V.; Liddon, A. J. P. S.; Ramirez-
Lopez, P.; Bendova, L.; Haigh, D.; Kočovský, P. Angew.
Chem. Int. Ed. 2006, 45, 1432. (e) Wang, Z. Y.; Wang, C.;
Zhou, L.; Sun, J. Org. Biomol. Chem. 2013, 11, 787. (f) Liu,
X. W.; Yan, Y.; Wang, Y. Q.; Wang, C.; Sun, J. Chem. Eur.
J. 2012, 18, 9204. (g) Xiao, Y. C.; Wang, C.; Yao, Y.; Sun,
J.; Chen, Y. C. Angew. Chem. Int. Ed. 2011, 50, 10661.
(h) Wu, X. J.; Li, Y.; Wang, C.; Zhou, L.; Lu, X. X.; Sun, J.
Chem. Eur. J. 2011, 17, 2846. (i) Onomura, O.; Kouchi, Y.;
Iwasaki, F.; Matsumura, Y. Tetrahedron Lett. 2006, 47,
3751. (j) Genoni, A.; Benaglia, M.; Massolo, E.; Rossi, S.
Chem. Commun. 2013, 49, 8365. (k) Guizzetti, S.; Benaglia,
M.; Bonsignore, M.; Raimondi, L. Org. Biomol. Chem.
2011, 9, 739. (l) Guizzetti, S.; Benaglia, M.; Rossi, S. Org.
Lett. 2009, 11, 2928. (m) Sugiura, M.; Kumahara, M.;
Nakajima, M. Chem. Commun. 2009, 3585. (n) Jones, S.;
Zhao, P. C. Tetrahedron: Asymmetry 2014, 25, 238.
(o) Jones, S.; Li, X. F. Tetrahedron 2012, 68, 5522.
(p) Kashiwagi, T.; Kotani, S.; Nakajima, M.; Sugiura, M.
Tetrahedron Lett. 2014, 55, 1924. (q) Jiang, Y.; Chen, X.;
Hu, X. Y.; Shu, C.; Zhang, Y. H.; Zheng, Y. S.; Lian, C. X.;
chromatography (silica gel; hexane–EtOAc, 10:1) afforded
the products. The ee values were determined using
established HPLC techniques with chiral stationary phases.
(14) Ethyl 2-(3,4-Dimethoxyphenyl)-3-(4-methoxyphenyl-
amino)propanoate (2e)
Yield 99%; 77% ee; light yellow oil. 1H NMR (300 MHz,
CDCl3): δ = 6.76–6.84 (m, 5 H), 6.58 (d, J = 8.8 Hz, 2 H),
4.10–4.19 (m, 2 H), 3.86 (s, 6 H), 3.79–3.83 (m, 1 H), 3.72–
3.74 (m, 4 H), 3.59 (br s, 1 H), 1.18–1.28 (m, 3 H) ppm. 13
C
NMR (75 MHz, CDCl3): δ = 173.0, 152.3, 149.1, 148.4,
141.4, 129.2, 120.3, 114.9, 114.6, 114.3, 110.9, 60.9, 55.8,
55.7, 50.3, 47.9, 14.0 ppm. ESI-HRMS: m/z calcd for
[C20H25NO5 + H]+: 360.1811; found: 330.1804. [α]D20 +112
(c 0.50, CHCl3). AD-H column (n-hexane–2-PrOH, 80:20),
flow rate = 1.0 mL/min, tR = 16.7, 17.9 min.
Synlett 2014, 25, 1879–1882
© Georg Thieme Verlag Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not
be copied or emailed to multiple sites or posted to a listserv without the copyright holder's
express written permission. However, users may print, download, or email articles for
individual use.