The synthesis and bioactivity of dimethyl (2,3-dihydrobenzo[b][1,4]dioxin- 6-yl)(aryl amino)methylphosphonates

Article abstract of DOI:10.1080/10426507.2013.842998

A new series of α-aminophosphonates have been synthesized by a one-pot three-component reaction of 2,3-dihydrobenzo[b][1,4]dioxine-6- carbaldehyde, various amines, and dimethyl phosphite by using nano-TiO ₂ as a catalyst under solvent-free conditions at 50°C. The major advantages of the present method are high yields, short reaction times, recyclable catalyst, and solvent-free reaction conditions. Among these new structurally diversified set of α-aminophosphonates, dimethyl (2,3-dihydrobenzo[b][1,4]dioxin-6-yl)(3-nitrophenylamino) methylphosphonate and dimethyl (2,3-dihydrobenzo[b][1,4]dioxin-6-yl)(4-fluoro-3-nitro-phenyl-amino) methylphosphonate have shown higher antioxidant activity in diphenyl picryl hydrazyl (DPPH) scavenging, reducing power assay, and lipid peroxidation methods.

Full text of DOI:10.1080/10426507.2013.842998

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The Synthesis and Bioactivity of
Dimethyl (2,3- Dihydrobenzo[b]
[1,4]Dioxin-6-Yl)(Aryl
Amino)Methylphosphonates
Ch. Syama Sundar a , N. Bakthavatchala Reddy a , S. Siva Prasad a ,
K. Uma Maheswara Rao a , S. H. Jaya Prakash a & C. Suresh Reddy a
a Department of Chemistry , Sri Venkateswara University , Tirupati ,
Andhra Pradesh , India
Accepted author version posted online: 23 Sep 2013.Published
Click for updates
online: 12 Mar 2014.
To cite this article: Ch. Syama Sundar , N. Bakthavatchala Reddy , S. Siva Prasad , K. Uma Maheswara
Rao , S. H. Jaya Prakash & C. Suresh Reddy (2014) The Synthesis and Bioactivity of Dimethyl (2,3-
Dihydrobenzo[b][1,4]Dioxin-6-Yl)(Aryl Amino)Methylphosphonates, Phosphorus, Sulfur, and Silicon and
the Related Elements, 189:4, 551-557, DOI: 10.1080/10426507.2013.842998
To link to this article: http://dx.doi.org/10.1080/10426507.2013.842998
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Phosphorus, Sulfur, and Silicon, 189:551–557, 2014
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.842998
THE SYNTHESIS AND BIOACTIVITY OF DIMETHYL (2,3-
DIHYDROBENZO[B][1,4]DIOXIN-6-YL)(ARYL
AMINO)METHYLPHOSPHONATES
Ch. Syama Sundar, N. Bakthavatchala Reddy, S. Siva Prasad,
K. Uma Maheswara Rao, S. H. Jaya Prakash,
and C. Suresh Reddy
Department of Chemistry, Sri Venkateswara University, Tirupati, Andhra Pradesh,
India
GRAPHICAL ABSTRACT
Abstract A new series of α-aminophosphonates have been synthesized by a one-pot three-
component reaction of 2,3-dihydrobenzo[b][1,4]dioxine-6-carbaldehyde, various amines, and
dimethyl phosphite by using nano-TiO2 as a catalyst under solvent-free conditions at 50◦C.
The major advantages of the present method are high yields, short reaction times, recyclable
catalyst, and solvent-free reaction conditions. Among these new structurally diversified set of
α-aminophosphonates, dimethyl (2,3-dihydrobenzo[b][1,4]dioxin-6-yl)(3-nitrophenylamino)
methylphosphonate and dimethyl (2,3-dihydrobenzo[b][1,4]dioxin-6-yl)(4-fluoro-3-nitro-
phenyl-amino) methylphosphonate have shown higher antioxidant activity in diphenyl picryl
hydrazyl (DPPH) scavenging, reducing power assay, and lipid peroxidation methods.
[Supplementary materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental
files: Additional text and figures.]
Keywords α-Aminophosphonates; nano-TiO2; solvent-free; antioxidant activity
INTRODUCTION
The synthesis of α-aminophosphonates has gained much interest in recent times due
to their structural similarity to the corresponding α-amino acids and role played as transition
Received 10 April 2013; accepted 16 August 2013.
The authors express their grateful thanks to Prof. C. D. Reddy, Department of Chemistry, Sri Venkateswara
University, Tirupati, for his helpful discussions and also thank DRDO, New Delhi, for providing financial assistance
(ERIP/ER/1103894M/ 01/1346). The authors also express their thanks to Prof. Ch. Appa Rao and S. Swapna,
Department of Bio-chemistry, S.V. University, Tirupati for conducting antioxidant activity.
Address correspondence to C. Suresh Reddy, Department of Chemistry, Sri Venkateswara University,
Tirupati, Andhra Pradesh, India. Tel.: +91-9849694958; Fax: +91877-2289555; E-mail: csrsvu@gmail.com
551

552
Ch. S. SUNDAR ET AL.
state mimics in peptide hydrolysis.1,2 Many natural and synthetic α-aminophosphonates and
their derivatives have potential applications as anti-HIV, anticancer, and antibiotic agents.3–8
Furthermore, they are used in agriculture as fungicides, herbicides, and plant growth regula-
tors.9–11 Consequently different methods have been developed for their synthesis. Of these,
nucleophilic addition of phosphites to imines is an established and useful method in which
catalysts like InCl3, SmI2, ZnCl2, SnCl4, In(OTf)3, Cu(OTf)2, MgClO4, and BF3.Et2O are
conveniently used.12–19 However, some of these methods are associated with one or more
shortcomings such as longer reaction time, use of hazardous and expensive catalysts, low
product yields, use of solvents, and harsh reaction conditions. Hence, they do not comply
with green chemistry protocols. Due to their valuable role of α-aminophosphonates as ver-
satile building blocks in the synthesis of biologically important compounds, development
of an eco-friendly synthesis for them is desirable.
Multicomponent, one-pot synthesis has received considerable attention because of
its wide range of applications in pharmaceutical chemistry for the creation of structural
diverse combinatorial libraries in drug discovery.20 Multicomponent reactions are extremely
convergent, producing remarkably high increase of molecular complexity in a single step
operation.21 In recent years, much attention has been focused on the design and use of
environment-friendly catalysts to increase the product yields and to reduce the amount of
toxic waste.22 Nano-scale metal oxides are found to be highly effective as catalysts for
many organic transformations and for the destruction of hazardous chemicals.23 As a part
of our continued interest in the development of synthetic methods for the phosphonate
derivatives,24–27 we found that nano-TiO2 can be readily used as efficient catalyst in the
Kabachnik–Fields reaction for the preparation of α-aminophosphonates under solvent-free
conditions. Even though Hosseini–Sarvari reported synthesis of α-aminophosphonates with
TiO2 as catalyst,28 this reaction requires a large amount of catalyst and long reaction time.
Use of nano-TiO2 reduces this reaction time and catalyst amount. All the synthesized novel
α-aminophosphonates (4a–o) were screened for antioxidant activity by diphenyl picryl
hydrazyl (DPPH) scavenging, reducing power assay, and lipid peroxidation methods.
RESULTS AND DISCUSSION
An efficient and environmentally benign one-pot method was developed for the
synthesis of α-aminophosphonates (4a–o) by reaction of 2,3-dihydrobenzo[b][1,4]di-oxine-
6-carbaldehyde (1), various amines (2), and dimethyl phosphite (3) in the presence of
nano-TiO2 as catalyst under solvent-free conditions at 50ºC (Scheme 1).
In order to standardize the operating reaction conditions, the reaction among 2,3-
dihydrobenzo[b][1,4]dioxine-6-carbaldehyde, aniline, and dimethyl phosphite was run
as a model. At room temperature there is minimal formation of the corresponding α-
aminophosphonates in the presence of nano-TiO2 catalyst even after 6 h of reaction time
under solvent-free conditions (Table 1, Entry 1). Increase of the reaction temperature from
30ºC to 50ºC led to the formation of α-aminophosphonates up to 94% yield. Further in-
crease in the temperature did not show any improvement in the yield. No formation of
product was observed in the absence of nano-TiO2 even after 6 h of the reaction time
(Table 1, Entry 6). This shows that nano-TiO2 is absolutely essential to drive the reaction
to completion. The reusability of the nano-TiO2 as a catalyst was also examined. After
each run, the product was filtered and the catalyst residue was washed with CH2Cl2, dried,
and reused. It was found that the catalyst can be used for three cycles in the synthesis of

DIHYDROBENZO[B][1,4] DIOXINYL AMINOPHOSPHONATES AS ANTIOXIDANTS
553
Scheme 1
compound 4a and in each case it gave 94%, 94%, 93% yield of the product, respectively
(Table 1, Entry 3).
After optimization of experimental conditions with aniline, we extended
this three-component coupling reaction with several other amines keeping 2,3-
dihydrobenzo[b][1,4]dioxine-6-carbaldehyde and dimethyl phosphite as constant
substrates and nano-TiO2 as the catalyst.
In all the cases, the reactions were completed within 10–15 min and gave the cor-
responding α-aminophosphonates in 86%–96% yield. In these reactions, initial formation
of imine intermediate is the key step for which further addition of dimethyl phosphite
Table 1 Optimization of reaction conditions for synthesis of 4a
Entry
Temperature (◦C)
Nano TiO2 (mol%)
Time (min/h)
Yield (%)a
1
30/RT
40
50
60
70
5
5
5
5
5
0
6 h
48
72
94,94,93
94
94
—
2
15 min
10 min
10 min
10 min
6 h
3b
4
5
6
50
aIsolated yield.
bCatalyst was reused three times.

554
Ch. S. SUNDAR ET AL.
Table 2 Physical characteristics of α-aminophosphonates (4a–o)
Product (4)
Time (min)
Yield (%)a
mpb (◦C)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
10
12
12
10
12
15
15
12
14
10
12
14
12
15
15
94
90
89
96
95
86
87
91
89
91
87
88
90
89
90
184–186
171–173
175–176
142–144
197–199
131–133
142–144
128–130
101–103
125–127
135–137
138–140
161–163
125–127
141–143
aIsolated yield.
bMelting point.
occurs to form α-aminophosphonates. Analysis of the data (Table 2) reveals that the amine
substrates with electron donating groups (-Me, -OMe) gave high yields when compared
with those having electron withdrawing groups (-NO2, -F, -Cl). Electron donating group
increases the nucleophilicity of N-atom facilitates its bonding with electrophilic carbonyl
carbon of the aldehyde during the formation of imine intermediate. The proposed chemical
1
structures for the synthesized compounds 4a–o were confirmed by IR, H, 13C, 31P, and
mass spectrometry and elemental analyses.
CONCLUSION
An efficient one-pot synthesis of α-aminophosphonates by the reaction of aldehyde,
amine, and dimethyl phosphite in the presence of nano-TiO2 as catalyst under solvent-
free conditions in excellent yields is accomplished. The advantages of this method are
mild reaction conditions, use of eco-friendly reusable catalyst, easy workup, and high
yields. Being an effective and green procedure, this may serve as the method of choice for
commercial production of α-aminophosphonates. Their antioxidant activity as evaluated by
DPPH scavenging, reducing power assay, and lipid peroxidation methods for the compounds
4f, 4g, 4i, and 4k shown them to have higher antioxidant activity when compared with that
of the standard Ascorbic acid antioxidant.
EXPERIMENTAL
All reagents were purchased from Sigma-Aldrich, Hyderabad, India, and used with-
out further purification. IR spectra were recorded in KBr pellets on a Perkin-Elmer 683
spectrophotometer (California, USA). 1H, 13C, 31P NMR spectra were taken on Jeol JNM
ECP 400 NMR instrument (Tokyo) at room temperature in DMSO-d6 or CDCl3 using
tetramethylsilane (TMS) as internal standard. 31P NMR (161.7 MHz) was taken in DMSO-
d6 using 85% H3PO4 as external standard with broadband 1H decoupling. EI-Mass spectra

DIHYDROBENZO[B][1,4] DIOXINYL AMINOPHOSPHONATES AS ANTIOXIDANTS
555
were obtained on JEOL GCMATE II GC-MS spectrometer (Tokyo) at SAIF IIT-Madras,
Chennai. Melting points were determined on Guna Mel-Temp apparatus (Tempo Instru-
ments and Equip., Mumbai, India) and were uncorrected.
General Procedure for α-Aminophosphonates (4a–o)
A mixture of aldehyde (1 mmol), amine (1 mmol), dimethylphosphite (1 mmol), and
nano-TiO2 (5 mol%) was stirred at 50◦C. The completion of the reaction was confirmed
by thin layer chromatography (TLC). CH2Cl2 (10 mL) was added, stirred and filtered the
mixture to separate the catalyst that was dried under vacuum and reused. The filtrate was
dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product
obtained was subsequently purified by silica gel column chromatography using 30% ethyl
acetate: n-hexane as an eluent. This procedure was successfully applied for the preparation
of all title products (Table 2). The supplemental material contains complete characterization
data for the new compounds and selected spectra for 4a, 4b, and 4l (Figures S4–S12).
Physical, Analytical, and Spectral Data for Selected Compounds 4a, 4b,
and 4l
Dimethyl (2,3-Dihydrobenzo[b][1,4]Dioxin-6-yl)(Phenylamino) Methyl
Phosphonate (4a). Brown solid, Yield: 94%, mp: 184◦C–186◦C. IR (KBr) (νmax,
cm−1): 3296 (NH), 1235 (P = O), 751 (P-Caliphatic). 1H NMR (400 MHz, DMSO-d6) δ: 3.53
(3H, d, J = 12 Hz, P-OCH3), 3.77 (3H, d, J = 8 Hz, P-OCH3), 4.21 (4H, s, OCH2CH2O),
4.68 (1H, d, J = 20 Hz, P-CH), 4.73 (1H, s, NH), 6.58–7.13 (8H, m, Ar-H). 13C NMR
(100.5 MHz, DMSO-d6) δ: 53.8 (d, J = 12 Hz, 2 X P-OCH3), 54.9 (d, J = 152 Hz, P-CH),
64.2 (C-4 & C-5), 113.8 (C-21 & C-61), 116.6 (C-6), 117.5 (C-3), 118.5 (C-2), 120.7
(C-41), 128.5 (C-1), 129.1 (C-31 & C-51), 143.3 (C-7), 143.7 (C-8), 146.0 (C-11). 31P NMR
(161.7 MHz, DMSO-d6) δ: 20.0. EI-MS (m/z, %): 350 (M+1, 20), 349 (M+•, 100). Anal.
Calcd. for C17H20NO5P: C, 58.45; H, 5.77; N, 4.01. Found: C, 58.35; H, 5.67; N, 3.95.
Dimethyl (2,3-Dihydrobenzo[b][1,4]Dioxin-6-yl)(4-Fluorophenylamino)
Methyl Phosphonate (4b). Red solid, Yield: 90%, mp: 171◦C–173◦C. IR (KBr) (νmax,
cm−1): 3298 (NH), 1246 (P = O), 749 (P-Caliphatic). 1H NMR (400 MHz, DMSO-d6) δ: 3.48
(3H, d, J = 12 Hz, P-OCH3), 3.66 (3H, d, J = 12 Hz, P-OCH3), 4.19 (4H, s, OCH2CH2O),
4.95 (1H, dd, J = 12 Hz, J = 28 Hz, P-CH), 6.23–6.26 (1H, m, NH), 6.76–7.02 (7H, m,
Ar-H). 13C NMR (100.5 MHz, DMSO-d6) δ: 52.9 (d, J = 7 Hz, P-OCH3), 53.3 (d, J =
6 Hz, P-OCH3), 52.9 (d, J = 153 Hz, P-CH), 63.9 (C-4 & C-5), 114.4 (C-31 & C-51), 114.8
(C-6), 115.0 (C-21 & C-61), 116.8 (C-3), 121.2 (C-2), 129.3 (C-1), 142.7 (C-7), 142.9
(C-8), 143.6 (C-11), 154.8 (C-F, d, J = 203 Hz,). 31P NMR (161.7 MHz, DMSO-d6) δ:
22.2. EI-MS (m/z, %): 368 (M+1, 18), 367 (M+•, 100). Anal. Calcd. for C17H19FNO5P:
C, 55.59; H, 5.21; N, 3.81. Found: C, 55.49; H, 5.16; N, 3.75.
Dimethyl ((2,3-Dihydrobenzo[b][1,4]Dioxin-6-yl)(Pyridin-4-Ylamino)Met
hyl) Phosphonate (4l). Green solid, Yield: 88%, mp: 138◦C–140◦C. IR (KBr) (νmax,
cm−1): 3301 (NH), 1262 (P=O), 749 (P-Caliphatic). 1H NMR (400 MHz, DMSO-d6) δ: 3.75
(3H, d, J = 8 Hz, P-OCH3), 3.91 (3H, d, J = 8 Hz, P-OCH3), 4.20 (4H, s, OCH2CH2O),
4.63 (1H, d, J = 24, P-CH), 6.89–6.93 (1H, m, NH), 7.26–8.20 (7H, m, Ar-H). 13C NMR
(100.5 MHz, CDCl3) δ: 52.9 (d, J = 6.2 Hz, P-OCH3), 53.3 (d, J = 6.2 Hz, P-OCH3), 54.9
(d, J = 154.3 Hz, P-CH), 63.4 (C-4 & C-5), 114.5 (C-6), 114.7 (C-21 & C-51), 116.5 (C-3),
121.0 (C-2), 129.9 (C-1), 142.6 (C-7), 142.8 (C-8), 143.4 (C-31 & C-41), 155.8 (C-11). 31
P

556
Ch. S. SUNDAR ET AL.
NMR (161.7 MHz, DMSO-d6) δ: 20.5. EI-MS (m/z, %): 351 (M+1, 21), 350 (M+•, 100).
Anal. Calcd. for C16H19N2O5P: C, 54.86; H, 5.47; N, 8.00. Found: C, 54.76; H, 5.41; N,
7.94.
Biology
Antioxidant Activity. According to our prediction, the α-aminophosphonates
(4a–o) have the ability to scavenge the DPPH radical by donating one electron. Since
phosphorus has affinity toward oxygen, it can easily bind and scavenge reactive oxygen
species (ROS) effectively. The compounds (4a–o) containing phosphorus, oxygen, and
nitrogen atoms are expected to be more active due to the presence of nonbonded electron
pairs containing hetero atoms that serve as binding sites in the bio-matrix. 4f, 4g, 4i, and
4k displayed appreciable antioxidant activity. 4i showed the highest activity because both
fluorine and –NO2 substituents helps homolytic cleavage of α-methylene C-H bond to form
corresponding free radicals which in turn reacts with ROS radical and scavenge them. The
α-C-H bond in 4i has to take place since the resulting phosphoryl free radical is more
stable due to the extensive delocalization of the free radicals over the conjugated aromatic
phosphoryl system.
Diphenyl Picryl Hydrazyl (DPPH) Radical Scavenging Activity. The free
radical scavenging activity of α-aminophosphonates against DPPH radical was performed
in accordance with Choi et al.29 and results are summarized in Figure S1.
Reducing Power Assay. The reducing power of synthesized compounds was
determined according to the method of Oyaizu et al.30 and results are summarized in Figure
S2 (supplemental materials).
Lipid Peroxidation Assay. It was determined using a modified method of Ohkawa
et al.31 and results are summarized in Figure S3 (supplemental materials).
REFERENCES
1. Kafarski, P.; Lejczak, B. Phosphorus, Sulfur Silicon Relat. Elem. 1991, 63, 193-215.
2. Thirumurugan, P.; Nandakumar, A.; Priya, N. S.; Muralidaran, D.; Perumal, P. T. Tetrahedron
Lett. 2010, 51, 5708-5712.
3. Tibhe, G. D.; Lagunas-Rivera, S.; Vargas-Diaz, E.; Garcia-Barradas, O.; Ordonez, M. Eur. J.
Org. Chem. 2010, 2010, 6573-6581.
4. Boukallaba, K.; Elachqar, A.; Hallaoui, A. E.; Alami, A.; Hajji, S. E.; Labriti, B.; Martinez, J.;
Rolland, V. Phosphorus, Sulfur Silicon Relat. Elem. 2006, 181, 819-823.
5. Ravinder, K.; Reddy, A. V.; Krishnaiah, P.; Venkataramana, G.; Reddy, V. L. N.; Venkateswarlu,
Y. Synth. Commun. 2004, 34, 1677-1683.
6. Dake, S. A.; Raut, D. S.; Kharat, K. R.; Mhaske, R. S.; Deshmukh, S. U.; Pawar, R. P. Bioorg.
Med. Chem. Lett. 2011, 21, 2527-2532.
7. Lavielle, G.; Hautefaye, P.; Schaeffer, C.; Boutin, J. A.; Cudennec, C. A.; Pierre, A. J. Med.
Chem. 1991, 34, 1998-2003.
8. Xu, Y.; Yan, K.; Song, B.; Xu, G.; Yang, S.; Xue, W.; Hu, D.; Lu, P.; Ouyang, G.; Jin, L.; Chen,
Z. Molecules 2006, 11, 666-676.
9. Maier, L; Diel, P. J. Phosphorus, Sulfur Silicon Relat. Elem. 1991, 57, 57-64.
10. Kozolowski, H.; Pusino, A.; Swiatek, J.; Spychala, J.; Glowiak, T.; Micera, G.; Gessa, C. J.
Agric. Food Chem. 1990, 38, 1989-1992.
11. Nska, H.; Bonarska, D.; Bielecki, K., Sarapuk, J. Cell. Mol. Biol. Lett. 2002, 7, 929-935.
12. Ranu, B. C.; Hajra, A.; Jana, U. Org. Lett. 1999, 1, 1141-1143.

DIHYDROBENZO[B][1,4] DIOXINYL AMINOPHOSPHONATES AS ANTIOXIDANTS
557
13. Xu, F.; Luo, Y. Q.; Deng, M. Y.; Shen, Q. Eur. J. Org. Chem. 2003, 4728-4730.
14. Zon, J. Pol. J. Chem. 1981, 55, 643-646.
15. Laschat, S.; Kunz, H. Synthesis. 1992, 90-94.
16. Ghosh, R.; Maiti, S.; Chakraborty, A.; Maiti, D. K. J. Mol. Catal. A: Chem. 2004, 210, 53-57.
17. Paraskar, A. S.; Sudalai, A. Arkivoc. 2006, x, 183-189.
18. Bhagat, S.; Chakraborti, A. K. J. Org. Chem. 2007, 72, 1263-1270.
19. Ha, H. J.; Nam, G. S. Synth. Commun. 1992, 1143-1148.
20. Subba Reddy, B. V.; Reddy, M. R.; Narasimhulu, G.; Yadav, J. S. Tetrahedron Lett. 2010, 51,
5677-5679.
21. Jiang, B.; Rajale, T.; Wever, W.; Tu, S. J.; Li, G. Chem. Asian J. 2010, 5, 2318-2335.
22. Mukhopadhyay, C.; Ray, S. Tetrahedron 2011, 67, 7936-7945.
23. Karmakar, B.; Banerji, J. Tetrahedron Lett. 2011, 52, 4957-4960.
24. Syama Sundar, Ch.; Srinivasulu, D.; Nayak S. K.; Suresh Reddy, C. Phosphorus, Sulfur Silicon
Relat. Elem. 2012, 187, 523-534.
25. Bakthavatchala Reddy, N.; Uma Maheswara Rao, K.; Syama Sundar, Ch.; Siva Prasad, S.; Nayak,
S. K.; Suresh Reddy, C. Org. Commun. 2012, 5, 171-178.
26. Santhi Sudha, S.; Syama Sundar, Ch.; Bakthavatchala Reddy, N.; Uma Maheswara Rao, K.; Jaya
Prakash, S. H.; Suresh Reddy, C. Phosphorus, Sulfur Silicon Relat. Elem. 2012, 188, 1402-1411.
27. Narayana Reddy, M. V.; Siva Kumar, B.; Balakrishna, A.; Suresh Reddy, C.; Nayak, S. K.;
Reddy, C. D. Arkivoc. 2007, xv, 246-254.
28. Hosseini–Sarvari, M. Tetrahedron 2008, 64, 5459-5466.
29. Choi, C. W.; Kim, S. C.; Hwang, S. S.; Choi, B. K.; Ahn, H. J.; Lee, M. Y.; Park, S. H.; Kim, S.
K.; Plant Sci. 2002, 153, 1161-1168.
30. Oyaizu, M. Jpn. J. Nutr. 1986, 44, 307-315.
31. Ohkawa, N.; Ohishi, K.; Yagi, K. Anal. Biochem. 1979, 95, 351-358.