Copper-catalyzed asymmetric synthesis and comparative aldose reductase inhibition activity of (+)/(-)-1,2-benzothiazine-1,1-dioxide acetic acid derivatives

Article abstract of DOI:10.1021/jo500338c

A copper catalyst system for the asymmetric 1,4-hydrosilylation of the α,β-unsaturated carboxylate class was developed by which synthesis of (+)- and (-)-enantiomers of 1,2-benzothiazine-1,1-dioxide acetates has been achieved with a good yield and an excellent level of enantioselectivity. A comparative structure-activity relationship study yielded the following order of aldose reductase inhibition activity: (-)-enantiomers > racemic > (+)-enantiomers. Further, a molecular docking study suggested that the (-)-enantiomer had significant binding affinity and thus increased inhibition activity.

Full text of DOI:10.1021/jo500338c

Article
pubs.acs.org/joc
Copper-Catalyzed Asymmetric Synthesis and Comparative Aldose
Reductase Inhibition Activity of (+)/(−)-1,2-Benzothiazine-1,1-dioxide
Acetic Acid Derivatives
Shagufta Parveen,† Saghir Hussain,† Xiangyu Qin,† Xin Hao,† Shaojuan Zhu,† Miao Rui,†
Shuzhen Zhang,† Fengyan Fu,† Bing Ma,† Qun Yu,‡ and Changjin Zhu*,†
†Department of Applied Chemistry, Beijing Institute of Technology, No. 5, Zhongguancun South Street, 100081 Beijing, China
‡Beijing Institute of Transfusion Medicine, 100850 Beijing, China
S
* Supporting Information
ABSTRACT: A copper catalyst system for the asymmetric 1,4-hydro-
silylation of the α,β-unsaturated carboxylate class was developed by which
synthesis of (+)- and (−)-enantiomers of 1,2-benzothiazine-1,1-dioxide
acetates has been achieved with a good yield and an excellent level of
enantioselectivity. A comparative structure−activity relationship study
yielded the following order of aldose reductase inhibition activity:
(−)-enantiomers > racemic > (+)-enantiomers. Further, a molecular
docking study suggested that the (−)-enantiomer had significant binding
affinity and thus increased inhibition activity.
mechanism of inhibitors bound to the enzyme. Therefore,
enantiomers of the 1,2-benzothiazine-1,1-dioxide-based class
provide valuable probes for justifying the configuration role of
the carboxylate head in enzyme inhibition.
INTRODUCTION
■
Aldose reductase (ALR2) is a cytosol enzyme, present in
several tissues of the human body. It plays a pivotal role in the
etiology of long-term diabetic complications such as neuro-
pathy, nephropathy, and retinopathy.1 It initializes the polyol
pathway under hyperglycemic conditions and, in the first rate-
limiting step, reduces glucose to sorbitol in the presence of
NADPH, leading to excessive osmotic and oxidative stress that
damages cells.2,3 Aldose reductase inhibitors have been
scrutinized as potential remedies for various diabetic
complications, and continuous efforts dedicated to its develop-
ment afforded two main chemical classes, carboxylic acid
derivatives (e.g., tolrestat4 and epalrestat5) and spirohydantoins
(e.g., fidarestat6 and sorbinil7). However, many ARIs have failed
to reach various clinical trial and have been withdrawn either
because of their cross reactivity with other analogous enzymes
or because of their adverse side effects and inefficacy.8
Therefore, the development of novel chemotypes with weaker
side effects and a rich pharmacokinetic profile is very
challenging.
Recently, we have designed a series of effective ARIs based
on the scaffolds of 1,2,4-benzothiadiazine-1,1-dioxide,9 pyrido-
[2,3-e][1,2,4]thiadiazine-1,1-dioxide,10,11 and 1,2-benzothia-
zine-1,1-dioxide.12 1,2-Benzothiazine-1,1-dioxide-based ARIs
contained a chiral center at position C4, to which the acetate
side chain is attached. The C4 carboxylate plays a key role for
this class of ARIs in binding within the active site of the enzyme
commonly known as the anion binding pocket, which is the
most important region for ligand binding.12 It is suggested that
the direction or orientation of the carboxylate head might have
a large impact on the inhibitory activity of ARIs, and studies of
this topic may then provide insight into the interaction
The demand for the enantiomerically pure compounds has
grown rapidly in recent years. It has become increasingly clear
that enantiomerically pure drugs have many advantages over
racemic drug mixtures because of fewer side effects and greater
potency in many cases.13 The catalytically asymmetric
reduction of α,β-unsaturated carbonyl compounds offers an
efficient and convenient method for the construction of tertiary
stereogenic centers in organic synthesis. Over the past few
decades, tremendous effort has been spent to achieve this
transformation with a substantial emphases on the use of
catalysts derived from various transition metals. Copper hydride
complexes with chiral ligands have emerged as powerful
reagents14,15 for the effective asymmetric reduction of a series
of α,β-unsaturated carbonyl compounds such as enones,16−19
α,β-unsaturated esters,20−24 α,β-unsaturated lactones and
lactams,25,26 nitroalkene,27,28 α,β-unsaturated sulfones,29 α,β-
unsaturated nitriles,30,31 and 2-alkenylheteroarenes.32 However,
it failed to convert members of the 1,2-benzothiazine-1,1-
dioxide-based α,β-unsaturated carboxylate class into their
corresponding acetates by using the reported catalytic methods
in our preliminary work, presumably because of the distinctive
substrate structures in which the α,β-double bond is attached to
the fused ring system.
This study focuses on the stereocontrolled synthesis of C4 of
1,2-benzothiazine-1,1-dioxide acetates by catalytic asymmetric
Received: March 6, 2014
Published: May 2, 2014
© 2014 American Chemical Society
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1,4-hydrosilylation of their respective α,β-unsaturated Z-esters
and the effect of the obtained (−)- and (+)-acid enantiomers
on the ALR2 inhibition to explore the structure−activity
relationship.
Fortunately, it was found that the reaction proceeded in 24 h
when NaOtBu was used as an additive instead of tBuOH,
producing the product in 10% yield and 36.4% ee (entry 2,
Table 1). Encouraged by this result, we conducted a systematic
exploration of different reaction conditions to optimize the
enantioselectivity of the reaction. A brief survey of the efficiency
of ligands for this process showed that p-tol-BINAP ligands L2
(S) and L3 (R) afforded high enantioslectivities for the
reaction, and opposite stereochemistry for the two products
(entries 3 and 4). However, this enantioselective reaction gave
only low yields, while the reaction using (R)-SEGPHOS (L4)
and (R)-DTBM-SEGPHOS (L5) failed to reduce the substrate
4 (entries 5 and 6). Then screening of copper salts (entries 7−
10) showed that the reaction could be improved significantly in
both yield and enantioselectivity with anhydrous Cu(OAc)2 as
the Cu precursor (entry 8). Further efforts aimed at
improvements include the screening of the hydrogen source
and the optimization of other reaction conditions. The effect of
silane reagents (entries 8 and 11−13) revealed that Ph2SiH2
offered best enantioselectivity and a moderate yield while
PhSiH3 could increase the yield but gave an obviously lower
enantioselectivity in comparison with those of other silane
reagents. By a solvent screening (entries 12 and 14−18), THF
was identified as being optimal. The reaction proceeded at a
higher rate in THF than in other solvents, and the reaction
conversion could be accomplished even at lower temperatures
(0−20 °C) and in a shorter time (18 h), presumably because of
the good solubility of the copper precatalyst. As a result, the
optimization of the solvent and temperature coefficient
provided a better yield and greater enantioselectivity for the
reaction (entry 14). These efforts eventually resulted in the
establishment of the catalyst system and reaction conditions for
1,4-hydrosilylation of 4.
We then investigated the influence of the substrate on the
catalytic 1,4-hydrosilyaltion of α,β-unsaturated benzothiazine-
1,1-dioxide ester derivatives 5−8 to further scrutinize the
efficiency of the catalyst system, although a moderate yield and
an excellent ee were achieved using methyl ester 4 (entry 1,
Table 2). Fortunately, both the reactivity and enantioselectivity
were significantly increased by using the isopropyl ester 5 in
place of the methyl ester 4, while the tert-butyl ester 6 was
found to be less selective, although it gave good yields (entries
1, 2, and 4, Table 2). In particular, compound 5 was found to
enhance greatly the reaction rate, and it subsequently provided
an excellent yield much higher than those seen with 4 and 6. In
addition, excellent yields and enantioselectivities were also
obtained from the hydrogenation of different isopropyl esters 7
and 8 (entries 5 and 7, Table 2). The positive substrate effect
was further confirmed by the reaction using this series of
isopropyl esters combined with the oppositely configured
ligand (S)-p-tol-BINAP L2 (entries 3, 6, and 8, Table 2).
Therefore, it indicated that bulkier ester groups were preferred
for the asymmetric 1,4-hydrosilylation over the methyl ester
group to achieve both a high yield and a high enantioselectivity.
Consequently, our studies resulted in a successful catalyst
system that is highly efficient for the asymmetric reduction of
α,β-unsaturated benzothiazine ester derivatives. In comparison
with conventional Cu-diphosphine catalyst systems,20,21 the
catalyst system presented here is more effective with respect to
reaction rate and enantioselectivity. The absolute configurations
of products were proposed by the sign of their optical rotation
compared with that of a reference compound.34
RESULTS AND DISCUSSION
Initially, we prepared the α,β-unsaturated (Z)-esters as the
precursor of targeted C4 enantiomers. As shown in Scheme 1,
■
Scheme 1. Synthesis of (Z)-Olifinic Isomers of 1,2-
a
Benzothiazine-1,1-dioxide Carboxylate
a
Ph3PCHCOOR (R = Me, iPr, or tBu), PhCH3, 40 °C.
ketones 1−3 were prepared according to the method reported
previously,12 and then coupling of 1−3 with alkyl bromoacetate
by the Wittig olefination reaction resulted in target substrates
4−8.33
To achieve further synthesis of C4 enantiomers, catalytically
asymmetric 1,4-hydrosilylation of the (Z)-isomer of α,β-
unsaturated 1,2-benzothiazine 1,1-dioxide carboxylate was
investigated. The chiral synthesis was started with ester 4 as
the substrate. The combinations of copper salts and
diphosphine ligands (Figure 1) were examined for the
establishment of a catalyst system in the reaction (Table 1).
Initially, the reported procedure21 that included the catalyst
complex of S-BINAP (L1) and Cu(OAc)2·H2O, the hydrogen
source of PMHS (polymethylhydrosiloxane), and tBuOH in
toluene was employed (entry 1, Table 1), but no reaction was
observed after 24 h at room temperature.
Figure 1. Chiral diphosphine ligands evaluated in the asymmetric 1,4-
reduction.
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Table 1. Asymmetric 1,4-Reduction of (Z)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-2H-1,2-benzothiazin-4(3H)-ylidene]acetic
a
Acid Methyl Ester (4)
b
c
entry
ligand
copper salt
hydrogen source
solvent
toluene
yield (%)
ee (%)
1
L1
L1
L2
L3
L4
L5
L3
L3
L3
L3
L3
L3
L3
L3
L3
L3
L3
L3
Cu(OAc)2·H2O/t-BuOH
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
CuCl
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
TMDS
Ph2SiH2
PhSiH3
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
−
−
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
10
28
30
−
36
86
86
−
3
4
5
6
−
−
7
25
43
29
39
42
45
51
57
30
15
21
−
80
91
85
89
91
92
77
8
Cu(OAc)2
9
[(Ph3P)CuH]6
Cu(Otf)2
10
11
12
13
14
15
16
17
18
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
d
Cu(OAc)2
90/94(+)
Cu(OAc)2
CH2Cl2
(C2H5)2O
1,4-dioxane
DMF
86
73
91
−
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
a
Substrate 4 (0.5 mmol), 0.1 equiv of chiral phosphine ligand (L), 0.05 equiv of Cu precursor, 0.1 equiv of NaOtBu, and 4 equiv of silane at room
b
c
d
temperature in 6 mL of solvent for 24 h. Isolated yields. Enantiomeric excesses were determined by chiral HPLC. Reaction conducted over the
temperature range of 0−20 °C for 18 h.
The enantiomers obtained from the asymmetric 1,4-hydro-
silylation were then converted into their respective acids, and
the inhibitory activity of aldose reductase was tested for all the
synthesized carboxylic acid enantiomers of 1,2-benzothiazine-
1,1-dioxide (Table 3).
The stereostructure−activity relationship study showed a
significant enhancement of ALR2 inhibition activity for the
(−)-14 enantiomer versus that of its corresponding (+)-14
enantiomer. Similar behavior was found in the case of 15.
Therefore, only the (−)-enantiomer of 16 was then
synthesized, and a significant increase in activity was found
for (−)-16 with respect to that of racemic ( )-16 (Table 3). It
was also observed that (−)-14 was the most active against
aldose reductase with an IC50 value of 0.5 μM among this series
of compounds, whereas (−)-16 was the least active.
binding pocket of the enzyme while the oxygen of the thionyl
group formed one hydrogen bond with Trp111 (3.24 Å) of the
pocket as shown in Figure 2A. By contrast, the carboxylate head
of (−)-14 was found to enter exactly into the anion pocket,
forming three strong hydrogen binding interactions with NE1
of the indole ring of Trp111 (2.89 Å), NE2 of the imidazole
ring of His110 (2.76 Å), and the OH of Tyr48 (2.68 Å) (Figure
2B). The N-benzyl ring of both enantiomers showed π-stacking
interaction with the indole ring of Trp111 (Figure 2A,C). The
protein surface presentation revealed that the carboxylic acid
head of (+)-14 (Figure 2B) resides away from the anion
binding pocket, while in the case of (−)-14 (Figure 2D), the
carboxylic acid head is deeply imbedded in the anion binding
pocket. Similar behavior was shown by the docking study of the
remaining enantiomers of this series. This made evident the fact
that the (−)-enantiomer had inhibitory activity against ALR2
higher than that of the (+)-enantiomer.
In summary, these studies developed an efficient copper
catalytic system for asymmetric 1,4-hydrosilylation of α,β-
unsaturated 1,2-benzothiazine-1,1-dioxide carboxylate ester
derivatives that led to the asymmetric synthesis of acetic acid
derivatives in good yield and excellent enantioselectivity. The
work also disclosed that isopropyl esters were the best
substrates for this catalytic system while substituents on the
N2-benzyl ring have no significant impact on selectivity.
Further, the SAR study suggested that the orientation of the C4
acetic acid group with levorotation or the (−)-configuration
played a vital role in the inhibition of ALR2, whereas
dextrorotation or the (+)-configuration led to a drastic loss of
inhibitory activity. It was also supported by molecular docking
Molecular docking was performed on human ALR2 (PDB
entry 1US0) complexed with NADP+ and ligand IDD 594 {[2-
(4-bromo-2-fluorobenzylthiocarbamoyl)-5-fluorophenoxy]-
acetic acid} to investigate the mechanism responsible for the
activity of the (−)-enantiomer being higher than that of the
(+)-enantiomer. The docking protocol was validated by the
ligand IDD 594, and it resulted in a MolDock score of
−148.133 kcal/mol. Then, the (−)- and (+)-enantiomers were
docked into the active site of ALR2, and it was determined that
the energy score for (−)-14 (MolDock score of −151.601 kcal/
mol) was better than those of (+)-14 (MolDock score of
−147.96 kcal/mol) and IDD 594.
It is also obvious from the docking results when comparing
the lowest-energy poses of the (−)- and (+)-enantiomers that
the carboxylic acid head of (+)-14 was not placed in the anion
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anion binding pocket while the (+)-enantiomer exhibited some
reverse behavior.
Table 2. Cu-Catalyzed Asymmetric 1,4-Reduction of (Z)-N-
Substituted Benzothiazine 1,1-Dioxide α,β-Unsaturated
Ester Derivatives (4−8)
a
EXPERIMENTAL SECTION
■
General Methods. All the solvents were dried and purified
according to standard procedures. All manipulations involving air
sensitive materials were conducted in a vacuum glovebox under an
atmosphere of argon unless stated otherwise. Yield refers to isolated
yields of >95% pure compounds as determined by NMR and HPLC.
Melting points were recorded and are uncorrected. Thin layer
chromatography (TLC) was performed on silica gel Merck 60F254
.
NMR spectra were recorded using a spectrometer (1H NMR at 400
MHz and 13C NMR at 100 MHz) in DMSO-d6 and CDCl3 using TMS
as an internal reference. 1H NMR data are reported as follows:
chemical shifts (δ), multiplicity (s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; q, quartet; m, multiplet; br, broad), integration, and
coupling constants. Data for 13CNMR are reported in terms of
chemical shifts (δ). Individual resonances were assigned on the basis of
their chemical shifts, signal intensities, multiplicities of resonances ,and
coupling constants. HPLC was conducted on an HPLC system
equipped with a Chiralpak ID column. MS and HRMS were
performed by the ESI and ESI-TOF methods, respectively. Optical
rotations were measured at the indicated concentration of grams per
100 mL.
The sodium saccharin, methyl bromoacetate, isopropyl bromoace-
tate, tert-butyl bromoacetate, 2,4,5-trifluorobenzyl bromide, 4-
fluorobenzyl bromide, and 4-(trifluoromethyl)benzyl bromide used
for the preparation of 1−3 according to reference methods and copper
salts, hydrosilane, ligands, and Naot-Bu/t-BuOH used for the copper-
catalyzed asymmetric reduction are commercially available.
Racemic products analogous to the enantiomerically pure
compounds described below were synthesized by reduction of the
olefin substrates catalyzed by Pd/C under a hydrogen atmosphere.
The HPLC retention times of enantiomerically enriched products
were the same as those of the racemic products.
General Synthetic Procedure for 1−3. Compounds 1−3 were
prepared according to the synthetic pathway depicted in Scheme 1.12
2-[4-(Fluoromethyl)benzyl]-2H-1,2-benzothiazin-4(3H)-one 1,1-
Dioxide (3). Yield: 77% (3.9 g). Pale yellow powder. mp: 107−110
b
c
d
entry
substrate; R1; R2
ligand yield (%)
ee (%) /configuration
e
1
2
3
4
5
6
7
8
4; 2,4,5-F3; Me
5; 2,4,5-F3; iPr
5; 2,4,5-F3; iPr
6; 2,4,5-F3; t-Bu
7; 4-CF3; iPr
7; 4-CF3; iPr
8; 4-F; iPr
L3
L3
L2
L2
L3
L2
L3
L2
57
85
89
61
84
86
87
85
94 (+)
96 (+)/S
97 (−)/R
93 (−)
91 (+)
96 (−)
96 (+)
8; 4-F; iPr
95 (−)
a
Substrates 4−8 (0.5 mmol), 0.1 equiv of chiral phosphine ligand (L),
0.05 equiv of Cu precursor, 0.1 equiv of NaOtBu, and 4 equiv of silane
b
at room temperature in 6 mL of solvent for 12 h. Isolated yields in
c
d
parentheses. Determined by chiral HPLC. Absolute configuration
determined by the sign of the specific rotation vs that of a reference
compound.34 Entry 14 of Table 1.
e
Table 3. Synthesis of ( )-N-Subsituted Benzothiazine-1,1-
dioxide Acetic Acid Derivatives and Biological Activity Data
1
°C. H NMR (400 MHz, DMSO-d6): δ 7.95−7.87 (m, 2H), 7.84−
7.76 (m, 2H), 7.28 (t, 2H, J = 6.0 Hz), 7.01−7.06 (m, 2H, J = 8.9, 2.3
Hz), 4.51 (d, 2H, J = 2.2 Hz), 4.46 (d, 2H, J = 1.6 Hz). 13C NMR (100
MHz, DMSO-d6): δ 189.5, 135.5, 133.4, 131.3, 131.1, 127.6, 123.9,
115.3, 115.1, 57.2, 53.0, 40., 39.8, 39.5, 39.3, 39.1, 38.9. MS (ESI): m/z
328.2 [M + Na]+.
a
b
c
d
product
yield (%)
ee (%) /configuration
ALR2 IC50 (μM)
(
)-14
(+)-14
(−)-14
−
89
85
−
90
80
−
−
0.74 (0.678−0.78)
4.15 (3.69−4.9)
96 (+)/S
97 (−)/R
−
93 (+)/S
96 (−)/R
−
0.50 (0.38−0.62)
23.9 (22.27−25.3)
31.4 (27.2−35.5)
20.9 (18.1−23.6)
85.7 (84.96−86.5)
55.9 (55.15−56.58)
0.12 (0.05−0.19)
General Synthetic Procedure for 4−8. To a stirred solution of
triphenylphosphine (26.2 g, 0.1 mol) in benzene (300 mL) was added
dropwise a solution with the appropriate amount of alkyl bromoacetate
(1.5 equiv) in benzene (100 mL) over 1 h. The solid was filtered and
washed with hexane to provide a white solid phosphonium product.
The crude product was dissolved in water (800 mL), and the mixture
was made alkaline via the dropwise addition of Et3N until the solution
became sufficiently basic. The solution was stirred for 1 h, and the
precipitates thus obtained were filtered and washed with water to
afford the product. Methyl 2-(triphenylphosphoranylidene) acetate,
isopropyl 2-(triphenylphosphoranylidene) acetate, and tert-butyl 2-
(triphenylphosphoranylidene) acetate were recrystallized from meth-
anol.
(
)-15
(+)-15
(−)-15
(
)-16
(−)-16
epalrestat
87
95 (−)/R
a
b
c
Isolated yield. Determined by HPLC. Absolute configuration
determined by comparing the sign of optical rotaion with that of a
reference compound.34 IC50 (95% CL) values represent the
d
concentration required to produce 50% enzyme inhibition.
A solution containing the appropriate choice of 1−3 (6 mmol) and
the appropriate amount of alkyl (triphenylphosphoranylidene) acetate
(9 mmol) in toluene (30 mL) was heated at 40 °C for 12 h. After
evaporation of the solvent, the residue was purified by column
results in which the (−)-enantiomer is tightly anchored in the
active site with its C4 acetic acid moiety deeply trapped in the
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Figure 2. Molecular docking of (+)- and (−)-enantiomers of inhibitor 14. Docking of (A and B) (+)-14 and (C and D) (−)-14 in cartoon diagrams
of protein backbones with selected and labeled protein residues presented in wireframe in medium yellow (C) and in surface presentation with
docked poses colored spring green (C), red (O), blue (N), dark yellow (S), and gray (F). Hydrogen bond interactions are shown as lime dotted
lines, while the NADP+ molecule is colored sky blue.
chromatography (1:15 to 1:20 ethyl acetate:petroleum ether) to afford
pure product.
White crystalline solid. mp: 172−176 °C. 1H NMR (400 MHz,
DMSO): δ 8.09−8.03 (m, 1H), 7.89−7.85 (m, 1H), 7.73−7.68 (m,
2H), 7.51 (dd, 1H, J = 13.6, 6.8 Hz), 7.48−7.42 (m, 1H), 6.71 (s, 1H),
5.00−4.95 (m, 1H), 4.91 (s, 2H), 4.19 (s, 2H), 1.24 (dd, 6H, J = 6.3,
1.7 Hz). 13C NMR (100 MHz, DMSO-d6): δ 164.6, 142.2, 134.8,
133.3, 132.1, 131.2, 126.6, 124.4, 119.0, 67.9, 49.3, 46.0, 39.9, 39.7,
39.5, 39.3, 39.1, 21.5. MS (ESI): m/z 424 [M − H]−. HRMS (ESI-
TOF): m/z [M + H]+ calcd for C20H19F3NO4S, 426.0981; found,
426.0984.
(Z)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-2H-1,2-benzothiazin-
4(3H)-ylidene]acetic Acid Methyl Ester (4).33 Yield: 66% (1.57 g).
(Z)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-2H-1,2-benzothiazin-
4(3H)-ylidene]acetic Acid tert-Butyl Ester (6). Yield: 72% (1.9 g).
White crystalline solid. mp: 132−136 °C. 1H NMR (400 MHz,
DMSO-d6): δ 8.07−7.98 (m, 1H), 7.92−7.82 (m, 1H), 7.71 (dd, 2H, J
= 6.3, 2.7 Hz), 7.48 (m, 2H), 6.75 (s, 1H), 4.93 (d, 2H, J = 11.4 Hz),
4.19 (s, 2H), 3.70 (s, 3H). 13C NMR (100 MHz, DMSO-d6): δ 165.2,
142.6, 134.8, 133.3, 132.1, 131.3, 128.8, 126.6, 124.4, 121.6, 119.2,
118.3, 106.7, 106.4, 106.1, 105.9, 51.6, 49.4, 46.0, 39.9, 39.7, 39.5, 39.3,
39.1. MS (ESI): m/z 398.1 [M + H]+.
(Z)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-2H-1,2-benzothiazin-
4(3H)-ylidene]acetic Acid Isopropyl Ester (5). Yield: 68% (1.7 g).
White crystalline solid. mp: 88−90 °C. 1H NMR (400 MHz, DMSO):
δ 8.06−8.02 (m, 1H), 7.90−7.82 (m, 1H), 7.71−7.68 (m, 2H), 7.56−
7.43 (m, 2H), 6.63 (s, 1H), 4.87 (s, 2H), 4.18 (s, 2H), 1.46 (s, 9H).
13C NMR (100 MHz, DMSO-d6): δ 164.5, 141.2, 134.8, 134.7, 133.3,
132.2, 131.1, 126.5, 124.4, 120.4, 119.2, 106.3, 81.0, 49.1, 46.0, 40.0,
39.7, 39.5, 39.3, 39.1, 38.9, 27.7. MS (ESI): m/z 438 [M − H]−.
HRMS (ESI-TOF): m/z [M + NH4]+ calcd for C21H20F3NO4SNH4,
457.1403; found, 457.1414.
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(dd, 2H, J = 17.6, 8.2 Hz), 4.51 (d, 1H, J = 14.6 Hz), 4.15 (d, 1H, J =
14.6 Hz), 3.72−3.64 (m, 2H), 3.61 (d, 1H, J = 6.3 Hz), 2.69−2.62 (m,
2H), 1.28 (d, 9H, J = 2.1 Hz). MS (ESI): m/z 440.1 [M − H]−.
HRMS (ESI-TOF): m/z [M + H]+ calcd for C21H23F3NO4S,
442.1294; found, 442.1301.
(Z)-2-[2-(4-Trifluoromethylbenzyl)-1,1-dioxido-2H-1,2-benzothia-
zin-4(3H)-ylidene]acetic Acid Isopropyl Butyl Ester (7). Yield: 61.5%
(1.6 g). White solid. mp: 116−119 °C. H NMR (400 MHz, DMSO-
1
d6): δ 8.09−8.05 (m, 1H), 7.91−7.87 (m, 1H), 7.73 (dd, 2H, J = 5.8,
2.1 Hz), 7.71 (d, 2H, J = 6.9 Hz), 7.53 (d, 2H, J = 7.7 Hz), 6.70 (s,
1H), 4.90 (t, 1H, J = 6.3 Hz), 4.85 (s, 2H), 4.24 (s, 2H), 1.25−1.13
(m, 6H). 13C NMR (100 MHz, DMSO-d6): δ 164.5, 142.2, 134.8,
133.3, 132.3, 131.3, 129.0, 126.8, 125.4, 124.3, 119.2, 67.8, 52.0, 49.5,
40.1, 39.9, 39.7, 39.5, 39.3, 39.1, 38.9, 21.5. MS (ESI): m/z 438 [M −
H]−. HRMS (ESI-TOF): m/z [M + H]+ calcd for C21H21F3NO4S,
440.1138; found, 440.1150
2-[2-(4-Trifluoromethylbenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Isopropyl Ester [( )-12]. Yield: 71.5%
(Z)-2-[2-(4-Fluorobenzyl)-1,1-dioxido-2H-1,2-benzothiazin-4(3H)-
ylidene]acetic Acid Isopropyl Ester (8). Yield: 65.2% (1.5 g). White
1
(0.158 g). White crystalline solid. mp: 98−100 °C. H NMR (400
MHz, DMSO): δ 7.82 (d, 1H, J = 7.9 Hz), 7.76 (d, 2H, J = 8.2 Hz),
7.64 (d, 2H, J = 5.8 Hz), 7.61 (s, 1H), 7.54−7.49 (m, 2H), 4.78−4.73
(m, 1H), 4.58 (d, 1H, J = 15.0 Hz), 4.27 (d, 1H, J = 15.1 Hz), 3.75−
3.65 (m, 2H), 3.62 (d, 1H, J = 3.3 Hz), 2.77−2.64 (m, 2H), 1.04−1.01
(m, 6H). 13C NMR (100 MHz, DMSO-d6): δ 170.4, 170.1, 141.1,
138.0, 137.9, 136.1, 135.6, 132.6, 129.0, 129.0, 127.7, 127.0, 125.4,
124.0, 123.9, 67.5, 49.3, 49.0, 39.9, 39.7, 39.5, 39.3, 39.1, 38.9, 37.3,
30.5, 21.3, 21.2. MS (ESI): m/z 440.1 [M − H]−. HRMS (ESI-TOF):
m/z [M + H]+ calcd for C21H23F3NO4S, 442.1294; found, 442.1295.
2-[2-(4-Fluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-benzo-
thiazinyl]acetic Acid Isopropyl Ester [( )-13]. Yield: 81.6% (0.16 g).
1
crystalline solid. mp: 109−112 °C. H NMR (400 MHz, DMSO): δ
8.05 (dd, 1H, J = 6.0, 3.1 Hz), 7.88 (dd, 1H, J = 6.0, 2.9 Hz), 7.75−
7.69 (m, 2H), 7.31 (dd, 2H, J = 7.2, 4.3 Hz), 7.15 (dd, 2H, J = 8.8, 5.8
Hz), 6.68 (s, 1H), 4.93 (dd, 1H, J = 7.7, 4.8 Hz), 4.81 (s, 2H), 4.12 (s,
2H), 1.22−1.18 (m, 6H). 13C NMR (100 MHz, DMSO-d6): δ 169.9,
147.7, 138.6, 137.6, 136.6, 135.9, 135.9, 132.1, 129.6, 124.4, 120.8,
120.6, 73.2, 57.0, 54.3, 45.5, 45.3, 45.1, 44.9, 44.7, 44.5, 26.9. MS
(ESI): m/z 388 [M − H]−. HRMS (ESI-TOF): m/z [M + H]+ calcd
for C20H21FNO4S, 390.1170; found, 390.1162.
General Method for Olefin Reduction. A solution of the
appropriate α,β-unsaturated ester of 4−8 (0.5 mmol) in EtOAc (5
mL) was added to a suspension of 10% palladium on carbon (0.16 g)
in methanol (5 mL). The mixture was allowed to stir at room
temperature for 12 h under a hydrogen atmosphere. The product
obtained was filtered through a Celite pad, and the filtrate was
concentrated over vacuum. Flash chromatography (SiO2, 3:1
EtOAc:petroleum ether) was performed to obtain pure product (9−
13).12
1
White powder. mp: 94−96 °C. H NMR (400 MHz, DMSO): δ 7.81
(d, 1H, J = 7.7 Hz), 7.60 (d, 1H, J = 7.1 Hz), 7.52−7.48 (m, 2H),
7.44−7.41 (m, 2H), 7.19 (d, 2H, J = 8.8 Hz), 4.79−4.73 (m, 1H), 4.48
(d, 1H, J = 14.3 Hz), 4.07 (s, 1H), 3.64−3.59 (m, 2H), 3.56 (d, 1H, J
= 3.3 Hz), 2.65 (dd, 2H, J = 23.2, 6.2 Hz), 1.05 (d, J = 1.5 Hz, 6H).
13C NMR (100 MHz, DMSO-d6): δ 170.2, 138.3, 138.2, 137.0, 136.3,
132.6, 130.7, 130.6, 129.0, 127.7, 123.8, 115.5, 115.3, 67.6, 48.9, 48.4,
40.2, 40.0, 39.8, 39.5, 39.3, 39.1, 38.9, 37.5, 30.6, 21.4. MS (ESI): m/z
390 [M − H]−. HRMS (ESI-TOF): m/z [M + H]+ calcd for
C20H23FNO4S, 392.1254; found, 392.1328.
2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Isopropyl Ester [( )-10]. Yield: 67%
General Method for Asymmetric 1,4-Reduction. In a flame-
dried Schlenk tube, the appropriate chiral ligand (0.05 mmol),
NaOtBu (0.05 mmol), and the appropriate Cu precusor (0.025 mmol)
were added sequentially under an argon-filled glovebox. The tube was
removed from the glovebox and degassed by briefly being subjected to
the vacuum and back-filled with argon (the degassing procedure was
repeated three times), and then solvent (6 mL) was added under
argon and the reaction mixture stirred for 1 h at 0−20 °C unless
otherwise stated. Hydrosilane (2 mmol) was then added followed by
the appropriate α,β-unsaturated ester of 4−8 (0.5 mmol), and the
reaction mixture was allowed to stir over the same temperature range
for 12−24 h. Ethanol was added dropwise to the reaction mixture after
the reaction had reached completion. The resulting solution was
diluted with petroleum ether, washed with water (5 mL) and brine (5
mL), and back-extracted with petroleum ether. The organic layer was
dried over MgSO4 and concentrated under reduced pressure. The
product was purified by silica column chromatography (15:1 to 20:1
petroleum ether:EtOAc).
(0.144 g). White crystalline solid. mp: 69−70 °C. 1H NMR (400
MHz, CDCl3): δ 7.90 (d, 1H, J = 7.8 Hz), 7.52−7.50 (m, 1H), 7.44 (t,
1H, J = 7.5 Hz), 7.35 (dd, 1H, J = 12.9, 5.7 Hz), 7.31 (d, 1H, J = 7.9
Hz), 6.98−6.92 (m, 1H), 4.99−4.93 (m, 1H), 4.55 (d, 1H, J = 15.0
Hz), 4.20 (d, 1H, J = 14.9 Hz), 3.82−3.77 (m, 1H), 3.70 (d, 1H, J =
7.7 Hz), 3.56 (d, 1H, J = 6.1 Hz), 2.64 (s, 2H), 1.19 (d, 3H, J = 1.3
Hz), 1.17 (d, 3H, J = 1.3 Hz). 13C NMR (100 MHz, CDCl3): δ 170.6,
137.8, 136.7, 135.7, 132.7, 128.7, 128.1, 126.9, 124.7, 118.9, 118.7,
106.0, 105.7, 77.5, 77.0, 76.8, 68.8, 49.7, 43.7, 39.4, 32.1, 21.8. MS
(ESI): m/z 426 [M − H]−. HRMS (ESI-TOF): m/z [M + H]+ calcd
for C20H21F3NO4S, 428.1138; found, 428.1141.
2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid tert-Butyl Ester [( )-11]. Yield: 67.7%
(0.15 g). Colorless oil. 1H NMR (400 MHz, DMSO): δ 7.79 (d, 1H, J
= 7.9 Hz), 7.64−7.59 (m, 2H), 7.57 (dd, 1H, J = 9.6, 2.1 Hz), 7.50
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1H, J = 5.0 Hz), 3.62−3.58 (m, 1H), 2.69−2.64 (m, 1H), 2.62−2.56
(m, 1H), 1.28 (s, 9H). 13C NMR (100 MHz, DMSO-d6): δ 170.0,
137.9, 132.5, 129.0, 127.7, 123.9, 119.1, 101.1, 80.4, 49.0, 43.5, 39.9,
39.7, 39.5, 39.3, 39.1, 38.9, 38.2, 30.5, 27.5. MS (ESI): m/z 440.1 [M
− H]−.
(+)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Methyl Ester [(+)-9]. Yield: 57% (0.114
g). Colorless sticky oil in 93.7% ee [Chiralpak ID, 10:90 ethanol:n-
hexane, 1.0 mL/min, 220 nm, tR(minor) = 16.63 min, tR(major) =
31
1
20.49 min]. [α]D = +5.7 (c 1.47, C2H5OH). H NMR (400 MHz,
CDCl3): δ 7.90 (d, 1H, J = 7.8 Hz), 7.53 (t, 1H, J = 8.1 Hz), 7.44 (d,
1H, J = 7.4 Hz), 7.33 (d, 1H, J = 9.0 Hz), 7.29 (s, 1H), 6.99−6.91 (m,
1H), 4.57 (s, 1H), 4.14 (s, 1H), 3.83 (d, 1H, J = 9.5 Hz), 3.63 (s, 3H),
3.54 (d, 2H, J = 5.4 Hz), 2.65 (d, 2H, J = 7.5 Hz). 13C NMR (100
MHz, CDCl3): δ 171.6, 137.7, 134.8, 132.8, 132.0, 128.7, 128.2, 124.7,
119.1, 109.8, 105.8, 77.4, 77.1, 76.8, 52.0, 49.5, 43.3, 38.8, 32.5. MS
(ESI): m/z 398 [M − H]−.
(+)-2-[2-(4-Trifluoromethylbenzyl)-1,1-dioxido-3,4-dihydro-2H-
1,2-benzothiazinyl]acetic Acid Isopropyl Ester [(+)-12]. Yield: 84%
(+)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Isopropyl Ester [(+)-10)]. Yield: 85%
(0.185 g). Colorless sticky oil in 91.2% ee [Chiralpak ID, 10:90
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(minor) = 5.91 min,
27
1
tR(major) = 6.76 min]. [α]D = +24.18 (c 0.43, C2H5OH). H NMR
(400 MHz, DMSO-d6): δ 7.82 (dd, 1H, J = 7.8, 1.1 Hz), 7.76 (d, 2H, J
= 8.2 Hz), 7.63 (s, 2H), 7.61 (s, 1H), 7.55−7.48 (m, 2H), 4.78−4.72
(m, 1H), 4.58 (d, 1H, J = 15.0 Hz), 4.26 (d, 1H, J = 15.1 Hz), 3.76−
3.64 (m, 2H), 3.62 (s, 1H), 2.74 (dd, 1H, J = 16.7, 4.2 Hz), 2.67 (dd,
1H, J = 16.6, 8.0 Hz), 1.02 (dd, 6H, J = 7.1, 6.5 Hz). 13C NMR (100
MHz, DMSO-d6): δ 170.2, 141.1, 138.0, 136.1, 132.6, 129.1, 128.9,
127.7, 127.6, 125.4, 123.9, 119.1, 109.2, 67.6, 49.4, 49.1, 40.1, 39.9,
39.7, 39.5, 39.3, 39.1, 38.9, 37.4, 30.5, 21.3, 21.2. MS (ESI): m/z 440.3
[M − H]−.
(0.181 g). Colorless sticky oil in 96% ee [Chiralpak ID, 10:90
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(minor) = 12.65 min,
31
1
tR(major) = 16.32 min]. [α]D = +7.97 (c 1.505, EtOAc). H NMR
(400 MHz, CDCl3): δ 7.89 (d, 1H, J = 7.8 Hz), 7.53 (t, 1H, J = 7.1
Hz), 7.43 (d, 1H, J = 7.5 Hz), 7.34 (d, 1H, J = 2.5 Hz), 7.32−7.27 (m,
1H), 6.98−6.91 (m, 1H), 4.99−4.92 (m, 1H), 4.55 (d, 1H, J = 15.0
Hz), 4.20 (d, 1H, J = 14.9 Hz), 3.82−3.76 (m, 1H), 3.70 (dd, 1H, J =
14.5, 6.8 Hz), 3.60−3.53 (m, 1H), 2.64 (t, 2H, J = 7.3 Hz), 1.19 (d,
3H, J = 1.7 Hz), 1.18 (d, 3H, J = 1.7 Hz). 13C NMR (100 MHz,
CDCl3): δ 170.6, 137.8, 136.7, 132.8, 128.7, 128.1, 124.7, 119.5, 119.4,
118.9, 118.7, 106.0, 105.8, 105.8, 105.5, 77.5, 77.2, 76.9, 68.8, 49.8,
43.7, 39.4, 32.1, 21.8. MS (ESI): m/z 450.3 [M + Na]+.
(−)-2-[2-(4-Trifluoromethylbenzyl)-1,1-dioxido-3,4-dihydro-2H-
1,2-benzothiazinyl]acetic Acid Isopropyl Ester [(−)-12]. Yield: 86%
(−)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Isopropyl Ester [(−)-10]. Yield: 89%
(0.189 g). Colorless sticky oil in 95.96% ee [Chiralpak ID, 10:90
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(major) = 5.91 min,
tR(minor) = 6.76 min]. [α]D27 = −21.78 (c 0.589, C2H5OH). 1H NMR
(400 MHz, DMSO-d6): δ 7.81 (d, 1H, J = 7.9 Hz), 7.75 (d, 2H, J = 8.2
Hz), 7.65−7.61 (m, 2H), 7.61−7.58 (m, 1H), 7.51 (dd, 2H, J = 12.8,
7.5 Hz), 4.78−4.72 (m, 1H), 4.57 (d, 1H, J = 15.0 Hz), 4.26 (d, 1H, J
= 15.1 Hz), 3.71 (dd, 1H, J = 15.1, 9.1 Hz), 3.64 (d, 1H, J = 4.9 Hz),
3.59 (d, 1H, J = 15.3 Hz), 2.74 (dd, 1H, J = 16.7, 4.1 Hz), 2.66 (dd,
1H, J = 16.7, 8.0 Hz), 1.06−0.97 (m, 6H). 13C NMR (100 MHz,
DMSO-d6): δ 170.2, 141.1, 138.0, 136.1, 132.6, 129.1, 128.9, 127.7,
125.5, 125.5, 125.4, 123.9, 118.8, 109.6, 67.5, 49.4, 49.1, 40.1, 39.9,
39.7, 39.5, 39.3, 39.1, 38.9, 37.4, 30.5, 21.3, 21.3. MS (ESI): m/z 440.3
[M − H]−.
(0.189 g). Colorless sticky oil in 97.2% ee [Chiralpak ID, 10:90
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(major) = 12.67 min,
31
1
tR(minor) = 16.33 min]. [α]D = −6.53 (c 1.5, EtOAc). H NMR
(400 MHz, CDCl3): δ 7.88 (d, 1H, J = 9.0 Hz), 7.43 (t, 1H, J = 7.4
Hz), 7.35 (d, 1H, J = 7.9 Hz), 7.30 (d, 1H, J = 8.0 Hz), 7.23 (dd, 1H, J
= 15.6, 7.7 Hz), 6.98−6.91 (m, 1H), 4.99−4.93 (m, 1H), 4.54 (d, 1H,
J = 15.0 Hz), 4.19 (d, 1H, J = 14.9 Hz), 3.82−3.76 (m, 1H), 3.70 (dd,
1H, J = 14.5, 6.8 Hz), 3.60−3.52 (m, 1H), 2.66−2.61 (m, 2H), 1.19
(d, 3H, J = 2.3 Hz), 1.17 (d, 3H, J = 2.2 Hz). 13C NMR (100 MHz,
DMSO-d6): δ 170.6, 137.8, 136.7, 134.4, 132.7, 128.7, 128.0, 124.6,
119.5, 118.9, 118.7, 106.0, 105.8, 105.7, 105.5, 77.5, 77.2, 76.7, 68.8,
49.8, 43.7, 39.3, 32.1, 21.7. MS (ESI): m/z 450.3 [M + Na]+.
(+)-2-[2-(4-Fluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-benzo-
thiazinyl]acetic Acid Isopropyl Ester [(+)-13]. Yield: 87% (0.170 g).
(−)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid tert-Butyl Ester [(−)-11]. Yield: 61%
(0.135 g). Colorless sticky oil in 93.3% ee [Chiralpak ID, 10:90
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(major) = 12.77 min,
tR(minor) = 18.52 min]. [α]D31 = −6.67 (c 0.33, C2H5OH). 1H NMR
(400 MHz, DMSO-d6): δ 7.79 (d, 1H, J = 7.9 Hz), 7.62 (d, 1H, J = 6.2
Hz), 7.60−7.55 (m, 2H), 7.50 (dd, 2H, J = 16.5, 8.1 Hz), 4.51 (d, 1H,
J = 14.6 Hz), 4.15 (d, 1H, J = 14.6 Hz), 3.75−3.68 (m, 1H), 3.66 (d,
Colorless sticky oil in 95.75% ee [Chiralpak ID, 10:90 ethanol:n-
hexane, 1.0 mL/min, 220 nm, tR(minor) = 7.31 min, tR(major) = 8.78
31
min]. [α]D = +37.57 (c 0.33, C2H5OH). 1H NMR (400 MHz,
DMSO-d6): δ 7.80 (d, 1H, J = 7.9 Hz), 7.63−7.58 (m, 1H), 7.49 (t,
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2H, J = 8.2 Hz), 7.42 (dd, 2H, J = 8.5, 5.6 Hz), 7.20 (t, 2H, J = 8.8
Hz), 4.79−4.73 (m, 1H), 4.48 (d, 1H, J = 14.3 Hz), 4.08 (d, 1H, J =
14.3 Hz), 3.65−3.59 (m, 2H), 3.56 (d, 1H, J = 3.4 Hz), 2.65 (dd, 2H, J
= 22.7, 6.2 Hz), 1.05 (dd, 6H, J = 8.1, 6.3 Hz). 13C NMR (100 MHz,
DMSO-d6): δ 170.2, 138.0, 136.3, 132.5, 132.1, 130.7, 130.6, 129.0,
127.7, 127.0, 123.8, 115.5, 115.3, 109.3, 67.6, 48.8, 48.4, 40.2, 39.9,
39.7, 39.5, 39.3, 39.1, 38.9, 37.5, 30.6, 21.7. MS (ESI): m/z 414.2 [M +
Na]+.
1H, J = 14.4 Hz), 4.14 (d, 1H, J = 14.5 Hz), 3.66 (d, 1H, J = 2.0 Hz),
3.58−3.53 (m, 1H), 3.16 (s, 1H), 2.72 (dd, 1H, J = 17.0, 4.1 Hz),
2.57−2.51 (m, 1H). 13C NMR (100 MHz, DMSO-d6): δ 172.4, 138.2,
136.0, 132.6, 132.1, 128.9, 127.7, 123.8, 119.4, 106.6, 106.3, 49.0, 43.5,
40.1, 39.9, 39.7, 39.5, 39.3, 39.1, 38.9, 37.3, 30.6, 20.6, 14.0. MS (ESI):
m/z 384 [M − H]−.
(−)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid [(−)-14]. Yield: 85% (0.181 g). Colorless
(−)-2-[2-(4-Fluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid Isopropyl Ester [(−)-13]. Yield: 85%
sticky oil in 97.2% ee [Chiralpak ID, 8:92:0.1 ethanol:n-hexane:TFA,
1.0 mL/min, 250 nm, tR(major) = 12.25 min, tR(minor) = 13.93 min].
31
1
(0.165 g). Colorless solid in 95.44% ee [Chiralpak ID, 10:90
[α]D = −9 (c 1.4, EtOAc). H NMR (400 MHz, CDCl3) δ 7.91 (d,
1H, J = 8.5 Hz), 7.55 (t, 1H, J = 7.0 Hz), 7.46 (t, 1H, J = 7.4 Hz),
7.36−7.31 (m, 1H), 7.28 (d, 1H, J = 10.8 Hz), 6.97−6.97 (m, 1H),
4.61 (d, 1H, J = 14.6 Hz), 4.15 (d, 1H, J = 14.6 Hz), 3.84 (d, 1H, J =
9.5 Hz), 3.64 (dd, 1H, J = 14.4, 6.1 Hz), 3.53 (d, 1H, J = 5.9 Hz), 3.49
(s, 1H), 2.72−2.69 (m, 1H). 13C NMR (100 MHz, CDCl3): δ 176.0,
137.4, 132.9, 128.6, 128.3, 124.7, 119.2, 119.0, 109.7, 105.9, 99.2, 77.5,
77.1, 76.8, 49.3, 49.3, 44.1, 43.4, 38.4, 32.3, 14.4. MS (ESI): m/z 384
[M − H]−.
ethanol:n-hexane, 1.0 mL/min, 220 nm, tR(major) = 7.37 min,
27
tR(minor) = 8.91 min]. mp: 57−59 °C. [α]D = −35.4 (c 0.53,
1
C2H5OH). H NMR (400 MHz, DMSO-d6): δ 7.81 (d, 1H, J = 7.9
Hz), 7.61 (dd, 1H, J = 10.9, 4.3 Hz), 7.55−7.47 (m, 2H), 7.42 (dd,
2H, J = 8.5, 5.6 Hz), 7.21 (t, 2H, J = 8.8 Hz), 4.78−4.71 (m, 1H, J =
12.5, 6.2 Hz), 4.48 (d, 1H, J = 14.3 Hz), 4.09 (d, 1H, J = 14.3 Hz),
3.69−3.58 (m, 2H), 3.57−3.52 (m, 1H), 2.70 (dd, 1H, J = 16.7, 3.9
Hz), 2.61 (dd, 1H, J = 16.7, 8.4 Hz), 1.05 (dd, 6H, J = 8.1, 6.3 Hz).
13C NMR (100 MHz, DMSO-d6): δ 170.2, 138.0, 136.2, 132.5, 132.1,
130.7, 130.6, 129.0, 127.7, 123.8, 115.5, 115.3, 109.2, 67.6, 48.8, 48.4,
40.1, 39.9, 39.7, 39.5, 39.3, 39.1, 37.4, 30.1, 21.3. MS (ESI): m/z 414.2
[M + Na]+.
2-{2-[4-(Trifluoromethyl)benzyl]-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl}acetic Acid [( )-15].12 Yield: 83% (0.183 g). White
General Method for Acid Hydrolysis. A mixture of the
appropriate ester of 10, 12, or 13 (0.5 mmol), 1,4-dioxane (5 mL),
and 10 M HCl (8 mL) was stirred at room temperature for 12 h unless
otherwise stated. After the reaction had reached completion, ice-cold
water (5 mL) was added to the reaction mixture and extracted 3-fold
with ethyl acetate (3 × 30 mL). The combined organic layers were
dried and concentrated under reduced pressure, and the crude product
was purified by silica gel chromatography (100:1 CH2Cl2:MeOH).
2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid [( )-14].12 Yield: 79% (0.168 g). White
1
powder. mp: 155−157 °C. H NMR (400 MHz, CDCl3): δ 7.93 (d,
1H, J = 8.0 Hz), 7.62 (d, 1H, J = 7.9 Hz), 7.53 (d, 2H, J = 6.5 Hz),
7.49−7.45 (m, 2H), 7.27 (d, 2H, J = 4.3 Hz), 4.79 (d, 1H, J = 14.5
Hz), 4.02 (d, 1H, J = 13.9 Hz), 3.79−3.73 (m, 1H), 3.55 (dd, 1H, J =
10.6, 4.2 Hz), 3.49 (s, 2H), 2.64 (d, J = 6.7 Hz, 1H). 13C NMR (100
MHz, DMSO-d6): δ 172.4, 141.2, 138.3, 136.1, 132.6, 129.0, 128.8,
127.6, 125.4, 123.8, 56.0, 49.7, 49.4, 39.7, 39.5, 39.3, 37.5, 30.4. MS
(ESI): m/z 398 [M − H]−.
(+)-2-{2-[4-(Trifluoromethyl)benzyl]-1,1-dioxido-3,4-dihydro-2H-
1,2-benzothiazinyl}acetic Acid [(+)-15]. Yield: 90% (0.199 g). White
1
crystalline solid. mp: 69−71 °C. H NMR (400 MHz, DMSO-d6): δ
12.36 (s, 1H), 7.79 (dd, 1H, J = 7.9, 1.2 Hz), 7.65−7.61 (m, 1H), 7.59
(dd, 1H, J = 4.6, 2.0 Hz), 7.56 (t, 1H, J = 3.8 Hz), 7.54−7.47 (m, 2H),
4.51 (d, 1H, J = 14.5 Hz), 4.15 (d, 1H, J = 14.5 Hz), 3.68−3.65 (m,
1H), 3.57 (d, 1H, J = 9.2 Hz), 3.17 (s, 1H), 2.72 (dd, 1H, J = 16.9, 4.1
Hz), 2.58−2.52 (m, 1H). MS (ESI): m/z 384 [M − H]−.
solid in 93.35% ee [Chiralpak ID, 8:92:0.1 ethanol:n-hexane:TFA, 1.0
mL/min, 250 nm, tR(minor) = 12.03 min, tR(major) = 13.04 min].
(+)-2-[2-(2,4,5-Trifluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-
benzothiazinyl]acetic Acid [(+)-14]. Yield: 89% (0.190 g). Colorless
31
1
mp: 117−119 °C. [α]D = +27.6 (c 0.55, EtOAc). H NMR (400
MHz, CDCl3) δ 7.93 (d, 1H, J = 7.8 Hz), 7.62 (d, 2H, J = 8.0 Hz),
7.53 (d, 2H, J = 4.5 Hz), 7.51 (s, 1H), 7.45 (d, 1H, J = 7.6 Hz), 7.29
(d, 1H, J = 7.6 Hz), 4.77 (d, 1H, J = 14.3 Hz), 4.05 (d, 1H, J = 14.4
Hz), 3.78−3.77 (m, 1H), 3.65 (d, 1H, J = 5.0 Hz), 3.49 (s, 2H), 2.64−
2.62 (m, 1H). 13C NMR (100 MHz, CDCl3): δ 174.3, 137.8, 136.8,
132.9, 132.9, 131.2, 129.4, 129.4, 128.7, 128.2, 128.2, 125.9, 124.7,
116.8, 112.7, 109.8, 77.5, 77.2, 76.9, 50.9, 50.0, 48.9, 38.4, 32.3. MS
(ESI): m/z 398.2 [M − H]−.
(−)-2-{2-[4-(Trifluoromethyl)benzyl]-1,1-dioxido-3,4-dihydro-2H-
1,2-benzothiazinyl}acetic Acid [(−)-15]. Yield: 80% (0.176 g).
Colorless sticky oil in 95.93% ee [Chiralpak ID, 8:92:0.1 ethanol:n-
hexane:TFA, 1.0 mL/min, 250 nm, tR(major) = 12.27 min, tR(minor)
= 13.31 min]. [α]D31 = −29.3 (c 0.375, EtOAc). 1H NMR (400 MHz,
sticky oil in 96% ee [Chiralpak ID, 8:92:0.1 ethanol:n-hexane:TFA, 1.0
mL/min, 250 nm, tR(minor) = 12.20 min, tR(major) = 13.73 min].
1
[α]D31 = +7.55 (c 1.325, EtOAc). H NMR (400 MHz, DMSO-d6): δ
12.37 (s, 1H), 7.78 (d, 1H, J = 7.1 Hz), 7.61 (s, 1H), 7.58 (d, 1H, J =
4.3 Hz), 7.53 (d, 2H, J = 7.7 Hz), 7.48 (d, 1H, J = 7.3 Hz), 4.51 (d,
4970
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Article
glyceraldehyde as a substrate in a final volume of 1 mL at 32 °C. The
reaction mixture excluding D,L-glyceraldehyde was incubated for 10
min, and then the substrate was added to start the reaction, which was
monitored for 4 min.
The inhibitory activity against ALR2 of the newly synthesized
compounds was assayed by adding 5 μL of the inhibitor, solution in
DMSO, to the reaction mixture described above. The inhibitory effect
was routinely estimated at a concentration of the compound in the
reaction mixture between 10−4 and 10−8 M. At least four
concentrations of the compound with three replicates at each
concentration with inhibitory activity between 20 and 80% was used
to generate each dose−response curve. The 95% confidence limits
(95% CL) were calculated from t values for n − 2, where n is the total
number of determinations.
Molecular Docking. Docking studies were executed using
Molegro Virtual Docker, version 5.0. The crystal structure of human
aldose reductase with bound inhibitor IDD 594 (PDB entry 1US0)
was used for docking, which was retrieved from the Protein Data Bank.
For the docking procedure, all solvent molecules within the protein
structure were removed, and to identify the potential binding site in
the protein, five possible binding cavities were detected. The cavity
around the anion binding site (volume of ∼124 Å3) was chosen for
docking calculations using the grid-based MolDock score (GRID)
function with a grid resolution of 0.30 Å, and each docking process
lasted between 4 and 6 min. The lowest-energy ligand poses were
chosen on the basis of the MolDock score and ReRank score. All
structural parameters of ligands such as bond orders, explicit hydrogen
atoms, hybridization, and charges were assigned when necessary in
Molegro Virtual Docker.
CDCl3): δ 7.94 (d, 1H, J = 7.5 Hz), 7.62 (d, 2H, J = 8.1 Hz), 7.53 (d,
2H, J = 7.0 Hz), 7.47 (dd, 1H, J = 10.2, 4.7 Hz), 7.28 (d, 2H, J = 7.9
Hz), 4.79 (d, 1H, J = 14.3 Hz), 4.03 (d, 1H, J = 14.2 Hz), 3.73 (t, 1H, J
= 5.9 Hz), 3.57−3.52 (m, 1H), 3.49−3.45 (m, 1H), 2.65 (d, 2H, J =
6.8 Hz). 13C NMR (100 MHz, DMSO): δ 172.4, 141.2, 138.4, 136.2,
132.6, 129.0, 128.8, 127.7, 125.4, 123.9, 49.7, 49.4, 39.9, 39.7, 39.5,
39.3, 39.1, 37.5, 30.4. MS (ESI): m/z 398.2 [M − H]−.
2-[2-(4-Fluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-benzo-
thiazinyl]acetic Acid [( )-16]. Yield: 80% (0.156 g). Colorless sticky
oil. 1H NMR (400 MHz, CDCl3): δ 7.92 (d, 1H, J = 7.8 Hz), 7.52 (d,
1H, J = 7.6 Hz), 7.46 (d, 1H, J = 7.6 Hz), 7.36−7.33 (m, 2H), 7.27 (d,
1H, J = 1.2 Hz), 7.02 (d, 2H, J = 8.5 Hz), 4.72 (d, 1H, J = 13.8 Hz),
3.92 (d, 1H, J = 13.9 Hz), 3.75−3.70 (m, 1H), 3.56−3.54 (m, 1H),
3.49 (s, 2H), 2.62 (d, 1H, J = 6.1 Hz). MS (ESI): m/z 348 [M − H]−.
HRMS (ESI-TOF): m/z [M − H]− calcd for C17H15FNO4S,
348.0711; found, 348.0710.
ASSOCIATED CONTENT
■
(−)-2-[2-(4-Fluorobenzyl)-1,1-dioxido-3,4-dihydro-2H-1,2-benzo-
S
* Supporting Information
thiazinyl]acetic Acid [(−)-16]. Yield: 87% (0.170 g). Colorless sticky
1
Copies of H NMR, 13C NMR, and NOE spectra and chiral
HPLC chromatograms of all chiral and racemic compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zcj@bit.edu.cn. Telephone and fax: +86 10 68918506.
■
oil in 95.44% ee [Chiralpak ID, 8:92:0.1 ethanol:n-hexane:TFA, 1.0
mL/min, 250 nm, tR(major) = 24.92 min, tR(minor) = 28.57 min].
Notes
31
1
[α]D = −34.6 (c 0.3, EtOAc). H NMR (400 MHz, CDCl3) δ 7.92
(d, 1H, J = 7.8 Hz), 7.52 (t, 1H, J = 7.6 Hz), 7.45 (t, 1H, J = 7.6 Hz),
7.38−7.32 (m, 2H), 7.28 (d, 1H, J = 7.8 Hz), 7.02 (t, 2H, J = 8.6 Hz),
4.70 (d, 1H, J = 13.9 Hz), 3.94 (d, 1H, J = 13.9 Hz), 3.74−3.69 (m,
1H), 3.57−3.51 (m, 1H), 3.46−3.45 (m, 2H), 2.60 (s, 1H). 13C NMR
(100 MHz, DMSO-d6): δ 172.4, 138.4, 136.3, 132.6, 132.1, 128.9,
127.7, 123.8, 115.5, 115.3, 49.1, 48.7, 40.0, 39.7, 39.5, 39.3, 39.1, 37.6,
30.6. MS (ESI): m/z 348 [M − H]−.
Enzymetic Assay. A partially purified ALR2 was obtained from
Wistar rats (200−250 g) supplied by Vital River (Beijing, China).
NADPH, D,L-glyceraldehyde, and all chemicals of reagent grade were
from Sigma-Aldrich.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant 21272025), the Research Fund for
the Doctoral Program of Higher Education of China (Grant
20111101110042), the Science and Technology Commission
of Beijing, China (Grant Z131100004013003), and the
Program for the 12th Five-year Plan (Grant
2012ZX10001003).
ALR2 of rat lens was prepared by following the method of Hayman
and Kinoshita35 and La Motta et al.36 Freshly removed rat lenses were
ground in a glass potter and homogenized in 3 volumes of deionized
cold water and then centrifuged for 30 min at 12000 rpm and 0−4 °C.
The supernatant was skimmed off and precipitated using (NH4)2SO4
at 40, 50, and 75% saturation. The precipitates obtained from the 75%
saturation fraction (possessing ALR2 activity) were dissolved in a 0.05
M NaCl solution, dialyzed overnight, stored at −20 °C, and then
further used for the enzymetic assay.
Enzyme inhibition activity was asssayed by measuring the decrease
in absorption of NADPH at 340 nm (λ), which escorts the oxidation
of NADPH catalyzed by ALR2. ALR2 activity was determined in a
reaction mixture containing 0.25 mL of 0.1 M sodium phosphate
buffer (pH 6.2), 0.15 mL of deionized water, 0.25 mL of 0.10 mM
NADPH, 0.1 mL of enzyme extract, and 0.25 mL of 10 mM D,L-
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