A new type of NADH model compound: Synthesis and enantioselective reduction of benzoylformates to the corresponding mandelates

Article abstract of DOI:10.3390/12050979

A new type of NADH model compound with good reactivity and enantioselectivity has been synthesized in good yields by an efficient and convenient synthetic method. The structures of these model compounds were confirmed by ¹H- and ¹³C-

Full text of DOI:10.3390/12050979

Molecules 2007, 12, 979-987
molecules
ISSN 1420-3049
© 2007 by MDPI
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Communication
A New Type of NADH Model Compound: Synthesis and
Enantioselective Reduction of Benzoylformates to the
Corresponding Mandelates
Jia Zhao 1, Nai-Xing Wang 1,2,*, Wu-Wei Wang 1, Gui-Xia Wang 1, Yan-Hong Liu 1, Li Li 1,
Jin-Lan Yu 2 and Xin-Liang Tang 2
1
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100080,
P.R. China; E-mails: jetjoo@hotmail.com, wwwang2007@yahoo.com.cn, hoppy@sohu.com,
liuyanhong-2000@163.com, dogli2@163.com
2 Beijing Insititute of Technology, Beijing 100081, P. R. China; E-mail: yujinlan@sogou.com,
txliang@163.com
* Author to whom correspondence should be addressed; E-mail: nxwang@mail.ipc.ac.cn
Received: 23 March 2007; in revised form: 17 April 2007 / Accepted: 19 April 2007 /
Published: 11 May 2007
Abstract: A new type of NADH model compound with good reactivity and
enantioselectivity has been synthesized in good yields by an efficient and convenient
1
synthetic method. The structures of these model compounds were confirmed by H- and
13C-NMR and MS.
Keywords: NADH model, C2 symmetry, chirality, synthesis.
Introduction
Since Ohno and coworkers [1] reported the first NADH model compound, a large number of such
mimics have been developed [2]. In particular, chiral NADH model compounds have been extensively
studied with the aim of developing new enantioselective reducing agents [2]. Generally, highly
stereoselective chiral NADH models have been designed by incorporating remote sterically-
demanding side chains [3] or a substituent at the reaction centre: the C4 position of the
dihydropyridine ring [4]. However, the former approach implies a significant modification of the

Molecules 2007, 12
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dihydropyridine ring and the latter one resulted in a loss of chirality at the C4 position during the
course of the model reaction. Recently, several NADH models with specific conformations which
could steroselectively reduce pyruvate mimics were reported [5], and a few such models have been
studied so far. Herein we report a novel chiral NADH model compounds 1a,b, possessing a specific
C2-symmetric conformation.
Figure 1. The new NADH model compounds 1a,b.
O
O
O
O
NH
HN
R N
N R
NH HN
1
1a: R= -CH3
1b: R= -CH2Ph
In our model compounds 1a,b four chiral carbon centers were introduced by the presence of
(1R,2R)-diaminocyclohexane [6]. Two identical pyridine-3,5-dicarbonyl units were connected by two
identical (1R,2R)-diaminocyclohexane units into a large ring. The C2-symmetric structure has the
following advantages: (i) we can take advantage of an efficient bi-directional synthetic method to
prepare 1a,b; (ii) it simplified the analysis of the reduction reactions using our model compounds 1a,b.
Results and Discussion
Generally, the single-protection method was used to synthesize macrocyclic compounds such as
our models 1a,b (Scheme 1). One of amino groups of (1R,2R)-diaminocyclohexane was protected and
the resulting compound 6 was reacted with pyridine-3,5-dicarbonyl dichloride to obtain compound 7.
After deprotection of 7, the resulting amide 8 was converted to macrocyclic compound 4 by treatment
with another portion of pyridine-3,5-dicarbonyl dichloride. However, this traditional method suffered
from low yields and a long synthetic route. Herein, we would like to report a novel practical and
efficient method as outlined in Scheme 2.
Although the pentafluorophenoxy is a good leaving group, bis(pentafluorophenyl)pyridine-3,5-
dicarboxylate (3) is not so active as pyridine-3,5-dicarbonyl dichloride in the reaction with (1R,2R)-
diaminocyclohexane. As a result, the key intermediate 4 was obtained conveniently in only one step
from (1R,2R)-diaminocyclohexane, avoiding the complicated single-protected method. When an
appropriate halide was added to a DMF solution of 4 and the resulting reaction mixture was heated at
80 ºC ~90 ºC for 12 h, then 5a,b were obtained. Crude 5a,b, used directly without further purification,

Molecules 2007, 12
981
was reduced by sodium dithionite to afford crude compounds 1a,b. After separation on Sephadex LH-
20 with methanol as the eluent, pure model compounds 1a,b were obtained.
Scheme 1. Single-protection route to the key intermediate 4.
O
O
O
O
Cl
Cl
NH NH protected group
NH HN protected group
N
N
H2N HN protected group
6
7
Deprotection
O
O
O
O
O
O
O
Cl
Cl
NH HN
NH NH2
N
N
N
N
NH HN
NH NH2
O
8
4
Scheme 2. Preparation of NADH model compound 1.
F
F
F
F
F
F
F
F
O
O
HOOC
COOH
b
a
O
O
N
2
F
F
N
3
O
O
O
O
O
O
O
O
O
O
O
NH HN
NH HN
NH HN
NH HN
d
c
R N
N R
R N
N R
N
N
NH
HN
NH HN
O
4
5
1
5a: R= -CH3
5b: R= -CH2Ph
1a: R= -CH3
1b: R= -CH2Ph
Reagents and conditions: a) pentafluorophenol, DMF, RT, 6 h, 92%; b) (1R,2R)-diamino-
cyclohexane, THF, RT, 4 h, 62%; c) 5a: CH3I, 80 ºC ~90 ºC, 12 h, 72%; 5b: BrCH2Ph, 80 ºC ~90
ºC, 12 h, 64%; d) Na2S2O4, Na2CO3, H2O, 12 h, 1a: 71%, 1b: 41%

Molecules 2007, 12
982
The C2-symmetric NADH models 1a,b could enantioselectivly reduce the pyruvate mimic methyl
benzoylformate in acetonitrile in the presence of magnesium perchlorate (Scheme 3). The results
summarized in Table 1 shows that NADH models 1a and 1b are similar as reducing agents. The
resulting reduction products were the same enantiomer (R-mandelate) and they dispalyed similar
enantiomeric excess values, so we believe that the reductibility and the enantioselectivity of model
compounds 1a,b is related to their specific C2-symmetric conformation, rather than the substituent at
the N1 position of the dihydropyridine rings.
Scheme 3. Asymmetric reduction of methyl benzoylformate with 1.
OH
CO2Me
O
1, Mg(ClO4)2
CO2Me
CH3CN
9
Table 1 Asymmetric reduction of methyl benzoylformate with 1.
9
Entry
T (°C)
t
Ee (%)a
68
Config.
Yield (%)b
1a
1b
RT
RT
3 d
3 d
R
98
96
70
R
a Determined by HPLC, see Experimental; b isolated yields.
Conclusions
In summary, a new type of NADH model compound with good reactivity and enantioselectivity has
been synthesized in good yields by an efficient and convenient synthetic method.
Experimental Section
General
All starting materials were commercially available. DMF was dried with CaSO4 and distilled under
reduced pressure. THF was distilled from CaH2. Melting points were measured on an X-4 digital
microscope melting point apparatus (Beijing Tech Instrument Co., Ltd.). IR spectra (KBr disks) were
1
recorded on a Perkin-Elmer Nicol FT-50X spectrometer. H-NMR and 13C-NMR were recorded on
Bruker AC-300 FT or AV-400 FT instruments. The chemical shifts are reported in parts per million
(ppm) downfield from internal tetramethylsilane (TMS). Electron impact MS spectra were obtained on
a JEOL JMS-HX 100 instrument. High resolution mass spectra (HRMS) were measured in negative
ion mode by Electrospray (ESI) on APEX-Qe 94 instrument. Chiralcel OD-H columns were purchased
from Daicel Chemical Industries. Column chromatography was carried with silica gel (200-300 mesh),
and HF254 silica gel for TLC was obtained from Qingdao Marine Chemistry Co. Ltd., Qingdao, China.
Sephadex LH-20 (18-110 µm) was provided by H&E Co., Ltd.

Molecules 2007, 12
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bis(Pentafluorophenyl) pyridine-3,5-dicarboxylate (3)
1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) (6.4 g, 33.2 mmol) was
added to a solution of pyridine-3,5-dicarboxylic acid (2.8 g, 16.8 mmol) and pentafluorophenol (6.2 g,
33.2 mmol) in 150 mL of dry DMF. The resulting solution was stirred at room temperature for 6 h,
water (200 mL) was added and a white precipitate formed. The product was collected by filtration and
purified by flash column chromatography (silica gel, eluent CH2Cl2) to give compound 3 as a white
1
solid (7.80 g, 93 %); m.p. 154~157 °C; H-NMR (300 MHz, CDCl3): δ=9.20 (s, 1H; pyr-4-CH), 9.68
ppm (s, 2H; pyr-2,6-CH); IR (KBr): v=3061, 2670, 2465, 1773, 1655, 1602, 1516, 1471, 1430, 1321,
1294, 1223, 1154, 1095, 996, 731 cm-1; EI-MS m/z (%): 316 (100), 260 (23.8), 183 (6.6), 105 (10.8).
(4R,9R,19R,24R)-3,10,14,18,25,29-Hexaazapentacyclo[25.3.1.112,16.04,9.019,24]dotriacontane-1(31),12
(32),13,15,27,29-hexaene-2,11,17,26-tetrone (4)
A solution of (1R,2R)-diaminocyclohexane (1.1 g, 9.6 mmol) in tetrahydrofuran (30 mL) was
added to a solution of bis(pentafluorophenyl)pyridine-3,5-dicarboxylate (5.0 g, 10.0 mmol) in tetra-
hydrofuran at 0 ºC (ice bath). Upon complete addition and then the solution was stirred at room
temperature for 3 h. The solvent was removed to give the crude product. The product was purified by
column chromatography (eluent: 30 % ethyl acetate-hexane) to give 4 as a white solid (1.5 g, yield 62
%); 1H- NMR (400 MHz, (CD3)2SO): δ=1.25-1.34 (m, 4H; CHH’CH2CHNH), 1.48-1.50 (m, 4H;
CHH’CH2CHN-H), 1.71-1.73 (m, 4H; CHH’CHNH), 1.84-1.91 (m, 4H; CHH’CHNH), 3.94-3.99 (m,
13
4H; CHNH), 8.38 (s, 2H; pyr-4-CH), 8.57 (d, J=7.76, 4H; NH), 8.92 ppm (s, 4H; pyr-2,6-CH); C-
NMR (100 MHz, (CD3)2SO): δ=24.9 (CH2CH2CHNH), 31.5 (CH2CHNH), 53.5 (CHNH), 130.3 (pyr-
3,5-C), 134.8 (pyr-4-CH), 150.1 (pyr-2,6-CH), 165.0, 165.1 ppm (C=O); ESI-MS m/z=491.4 [M+H]+.
(4R,9R,19R,24R)-14,29-Dimethyl-3,10,14,18,25,29-hexaazapentacyclo[25.3.1.112,16.04,9.019,24]-dotria-
conane-1(31),12(32),13,15,27,29-hexaene-2,11,17,26-tetrone iodide (5a)
Methyl iodide (5 mL) was added to a dry DMF (5 mL) solution of compound 4 (1.0 g, 4.1 mmol).
The mixture was stirred under nitrogen at 80 ºC ~90 ºC for 12 h, then cooled and ether was added. The
precipitate formed was collected by filtration and washed with ether to give the crude product 5a as a
yellow solid (1.1 g, yield 72 %), which can be used without further purification.
(4R,9R,19R,24R)-14,29-Dibenzyl-3,10,14,18,25,29-hexaazapentacyclo[25.3.1.112,16.04,9.019,24]dotria-
contane-1(31),12(32),13,15,27,29-hexaene-2,11,17,26-tetrone bromide (5b)
This preparation was carried out in a similar fashion to that of 5a, and the product was obtained as
a yellow solid (1.1 g, yield 64 %).

Molecules 2007, 12
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(4R,9R,19R,24R)-14,29-Dimethyl-3,10,14,18,25,29-hexaazapentacyclo[25.3.1.112,16.04,9.019,24]dotria-
contane-1,12,15,27-tetraene-2,11,17,26-tetrone (1a)
An aqueous solution (20 mL) of sodium dithionite (1.8 g, 10.3 mmol) and sodium carbonate (0.68
g, 6.5 mmol) was added dropwise at room temperature under nitrogen to an aqueous solution (10 mL)
of compound 5 (0.5 g, 1.3 mmol). The mixture was left to stir over night at room temperature, during
which time a yellow precipitate formed. The product was collected by filtration and purified by gel
chromatography (Sephadex LH-20, eluent CH3OH) to give 1a as a yellow solid (0.24 g, yield 71 %);
1H-NMR (400 MHz, (CD3)2SO): δ=1.23-1.25 (m, 8H; CH2CH2CHNH), 1.66 (m, 4H; CHH’CHNH),
1.88 (m, 4H; CHH’CHNH), 2.90 (s, 4H; pyr-4-CH), 3.03 (s, 6H; CH3N), 3.54 (m, 4H; CHNH), 6.80
(s, 4H; pyr-2,6-CH), 7.27-7.28 ppm (m, 4H; NH); 13C-NMR (100 MHz, (CD3)2SO): δ=21.5
(CH2CH2CHNH), 24.6 (CH2CHNH), 31.8 (CH3N), 40.4 (pyr-4-CH2), 53.9 (CHNH), 104.1 (pyr-3,5-
C), 136.6 (pyr-2,6-CH), 167.1 ppm (C=O); ESI-MS m/z=523.5 [M+H]+, 545.5 [M+Na]+.
(4R,9R,19R,24R)-14,29-Dibenzyl-3,10,14,18,25,29-hexaazapentacyclo[25.3.1.112,16.04,9.019,24]dotria-
contane-1,12,15,27-tetraene -2,11,17,26-tetrone (1b)
This preparation was carried out in a similar fashion to that of 6a. The product was obtained as a
yellow solid (0.17g, yield 41 %); 1H-NMR (400 MHz, (CD3)2SO): δ=1.24-1.29 (m, 8H;
CH2CH2CHNH), 1.65 (m, 4H; CHH’CHNH), 1.88-1.90 (m, 4H; CHH’CHNH), 2.95 (s, 4H; pyr-4-
CH), 3.54 (m, 4H; CHNH), 4.42 (d, J=15.66, 2H; ArCHH’), 4.58 (d, J=15.66, 2H; ArCHH’), 6.92 (s,
4H; pyr-2,6-CH), 7.27 (m, 4H; NH), 7.29-7.39 ppm (m, 10H; C6H5); 13C-NMR (100 MHz, (CD3)2SO):
δ=21.9 (CH2CH2CHNH), 24.7 (CH2CHNH), 31.8 (pyr-4-CH2), 54.0 (CHNH), 56.4 (ArCH2), 104.8
(pyr-3,5-C), 127.4 (Ar-4-CH), 127.7 (Ar-2,6-CH), 128.9 (Ar-3,5-CH) 138.0 (Ar-1-C) 167.2 ppm
(C=O); HR-MS(ESI) m/z=[M-H]- calcd. 673.35078, found 673.34907.
General Procedure for the asymmetric reduction of methyl benzoylformate with NADH model
compounds 1a-b [2c]
The NADH model 1a-b (1 mmol), methyl benzoylformate (2 mmol) and magnesium perchlorate (2
mmol) were dissolved in acetonitrile (5 mL). The resulting solution was stirred in the dark under
nitrogen at room temperature for 3 days. Water (0.5 mL) was then added and the product extracted
with ether. The organic phase was dried and the solvent evaporated. The crude methyl mandelate was
purified by chromatography, using 1:5 ethyl acetate/petroleum ether as eluent, to give a white solid.
Product identity and enantiomeric excess were determined by HPLC analysis using a Chiracel OD-H
column (0.46 cm I.D. x 25 cm L). Chromatographic conditions: injection: 10 µL; eluent: n-hexane/2-
propanol = 85:15; flow rate: 1.0 mL/min; temperature: 35 °C. For 1a (Figure 2): UV detection: λ= 220
nm; tR = 5.819 min [(S)-enantiomer] and 8.495 min [(R)-enantiomer]. For 1b (Figure 3): UV
detection: λ= 220 nm; tR = 5.814 min [(S)-enantiomer] and 8.464 min [(R)-enantiomer].

Molecules 2007, 12
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Figure 2. Resolution of methyl mandelate produced with 1a.
Figure 3. Resolution of methyl mandelate produced with 1b.
Acknowledgements
Support for this research was provided by the Nature Science Foundation of China (grants
20472090 and 10576034).
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Sample Availability: Samples of the compounds described in this paper are available from the authors.
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