Synthesis, α-Glucosidase inhibition and molecular docking studies of tyrosol derivatives

Article abstract of DOI:10.1080/14786419.2019.1628750

To find a potent α-glucosidase inhibitor, 24 tyrosol derivatives with different substituents located at the meta, ortho, or para position of the phenyl group have been synthesised via the Mitsunobu reaction, characterised by ¹H NMR, ¹³

Full text of DOI:10.1080/14786419.2019.1628750

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
Synthesis, α-Glucosidase inhibition and molecular
docking studies of tyrosol derivatives
Luyun Zhang, Qian Xu, Junyi Zhu, Guangqing Xia & Hao Zang
To cite this article: Luyun Zhang, Qian Xu, Junyi Zhu, Guangqing Xia & Hao Zang (2019):
Synthesis, α-Glucosidase inhibition and molecular docking studies of tyrosol derivatives, Natural
Product Research
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2019.1628750
Synthesis, a-Glucosidase inhibition and molecular docking
studies of tyrosol derivatives
Luyun Zhanga,b, Qian Xua,c, Junyi Zhua, Guangqing Xiaa,b and Hao Zanga,c
aSchool of Pharmacy and Medicine, Green Medicinal Chemistry Laboratory, Tonghua Normal
b
University, Tonghua, China; Changchun University of Chinese Medicine, Jilin Ginseng Academy,
c
Changchun, China; College of Medicine, Yanbian University, Yanji, China
ABSTRACT
ARTICLE HISTORY
Received 29 March 2019
Accepted 31 May 2019
To find a potent a-glucosidase inhibitor, 24 tyrosol derivatives
with different substituents located at the meta, ortho, or para pos-
ition of the phenyl group have been synthesised via the
Mitsunobu reaction, characterised by 1H NMR, 13C NMR, ESI-MS
and IR and evaluated for inhibition. The derivatives possessed
varying degrees of in vitro inhibitory activity against a-glucosidase
and a relationship between the structure and activity was subse-
quently established for all compounds. Two of these compounds
with substituents at the para position showed significant inhibi-
tory effects surpassing that of the control standard acarbose.
Molecular docking studies performed to better understand the
binding interactions between the enzyme and the two most
active compounds showed substantial binding within the active
site of a-glucosidase. Taken together, these results indicate that
the position of the substituent plays a crucial role in this inhib-
ition and may facilitate the development of new a-glucosidase
inhibitors.
KEYWORDS
tyrosol; synthesis;
a-glucosidase; structure
activity relationship;
molecular docking
1. Introduction
Diabetes is a complicated chronic metabolic disorder of which there are two clinical
types. Type I diabetes is caused by an insufficient insulin secretion, whereas Type II
CONTACT Hao Zang
zanghao_1984@163.com; Junyi Zhu
swx0527@163.com;
Guangqing Xia
qingguangx@163.com
Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/14786419.2019.1628750.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
L. ZHANG ET AL.
diabetes is caused by an insufficient insulin utilisation (Wang et al. 2016). Postprandial
hyperglycaemia is a major feature in diabetes and causes many complications, includ-
ing kidney disease, retinopathy and cardiovascular disease (Han et al. 2017). One of
the most effective approaches to reducing postprandial blood glucose is to inhibit the
activity of carbohydrate hydrolases, especially a-glucosidase in the intestinal epithelial
cells (Schmidt et al. 2014). a-Glucosidase hydrolyses a-1,4 glycosidic bonds to release
a-glucose, which is the final step in carbohydrate digestion (Shimba et al. 2009;
Motabar et al. 2009). Therefore, a-glucosidase inhibitors can effectively control post-
prandial hyperglycaemia. Some a-glucosidase inhibitors, such as acarbose and vogli-
bose, have been clinically applied to treat diabetes. However, there are increasingly
more reports on their side effects, which include abdominal pain, diarrhoea and other
gastrointestinal diseases (Wei et al. 2017; Peng et al. 2016). Therefore, it is urgent to
find effective a-glucosidase inhibitors with fewer side effects.
Tyrosol is one of the major natural phenolic antioxidants contained in olive oil
(Servili et al. 2009) and has been reported to possess various physiological benefits
because of its potent antioxidant activity (Giovannini et al. 1999).
Recent studies have suggested that tyrosol possesses antigenotoxic activity and
may prevent apoptosis in keratinocytes (Salucci et al. 2015). Other studies indicate
that it may play a role in preventing Alzheimer’s disease and Parkinson’s disease by
antagonizing the b-amyloids and inhibiting apoptosis of dopaminergic neurons (St-
Laurent-Thibault et al. 2011; Dewapriya et al. 2013). In addition, there are claims that
tyrosol can effectively increase longevity (Canuelo et al. 2012), reduce the risk of heart
disease (Sun et al. 2015), and prevent cancer (Ahn et al. 2008). Tyrosol has been
shown to exhibit a-glucosidase inhibitory activity (Kwon et al. 2006), and plays a role
in the regulation of key enzymes in carbohydrate metabolism (Chandramohan et al.
2015; Chandramohan et al. 2017). Thus, tyrosol may be a promising candidate with
the potential to facilitate the prevention and treatment of diabetes.
The structural modification of tyrosol is a research area of growing interest in the
field of medicinal chemistry. In recent studies, tyrosol glucuronates have been pre-
pared as antitrypanosomal and antileishmanial agents (Belmonte-Reche et al. 2016).
Tyrosyl gallate has been prepared as potent melanin formation inhibitors (Lee et al.
2007). Other studies have focused on the synthesis of tyrosol ethers and esters and
evaluating their antioxidant activities (Madrona et al. 2011; Zang et al. 2018; Zhou
et al. 2017). In addition, tyrosol esters have been synthesised to investigate its role in
the control of metastatic breast cancer (Busnena et al. 2013). To our knowledge, there
have not been any studies on the hypoglycemic effect of tyrosol derivatives. Thus, we
report herein the design and synthesis of tyrosol derivatives, and the characterisation
of their a-glucosidase inhibitory activities in order to find highly active compounds.
2. Results and discussion
2.1. Chemistry
The 24 derivatives 3a-3x were synthesised from tyrosol via the Mitsunobu reaction
with substituted benzoic acid in tetrahydrofuran (THF) for 48 h to obtain overall high
yields of the target compounds. the synthetic routes are outlined in Scheme 1.

NATURAL PRODUCT RESEARCH
3
2.2. Biological activity
The inhibitory activity of compounds 3a-3x against a-glucosidase was then carried
out. The results summarized in Table 1. In addition to compound 3m, 3o, 3r and 3s,
other compounds showed a variable degree of a-glucosidase inhibition with IC50 val-
ues ranging between 15.2 and 730.5 lM in comparison with the positive control acar-
bose, which had an IC50 value of 60.9 lM. The structure and activity relationship was
investigated by adding different substituents to the phenyl group.
Compounds 3u displayed the most potent inhibitory activity among the series. The
greater potential shown by this compound might be due to the electron-withdrawing
cyano group. The most active compound 3u (IC50¼15.2 lM), a para-cyano substituted
derivative, was found to be superior in comparison with the compound
3s(IC50>800 lM) and 3t (IC50¼394.9 lM) containing ortho-cyano and meta-cyano sub-
stituents, respectively. These results indicate that the position of the substituent plays
a crucial role in this inhibition.
Table 1. a-Glucosidase inhibitory activity of compounds 3a-3x.
Compd.
Structure
IC50 (lM)
Compd.
Structure
IC50 (lM)
>800
3a
730.5 9.3
3m
3b
3c
619.1 7.6
594.3 5.7
3n
3o
450.8 3.8
>800
3d
309.2 4.0
3p
92.8 0.5
3e
3f
168.0 2.3
161.8 2.1
3q
3r
67.2 0.8
>800
3g
272.7 1.9
3s
>800
3h
3i
153.9 1.5
132.6 1.3
3t
394.9 3.4
15.2 0.1
3u
3j
375.8 2.5
3v
349.1 2.4
3k
3l
388.4 2.1
401.7 2.1
3w
3x
137.0 0.9
41.2 0.2
Acarbose
60.9 1.0

4
L. ZHANG ET AL.
A comparison of compound 3x (IC50 ¼ 41.2 lM) with 3v (IC50 ¼ 349.1 lM) and 3w
(IC50 ¼1 37.0 lM) shows that although these three derivatives have a hydroxyl group
on the phenyl ring, the position of the hydroxyl group is different in each and their
inhibitory activities vary. This confirms that the difference in the substituent position
affects the inhibitory potentials of the compounds. Compounds having the hydroxyl
substituent at the para position were able to efficiently inhibit a-glucosidase.
For compounds 3a-3c with fluoro substituents at the ortho, meta, and para posi-
tions of the phenyl ring, respectively, the inhibitory potential was lowest for the ortho-
fluoro substituent (3a) and highest for the para-fluoro substituent (3c). This further
supports that the position of the substituent is important in this inhibition. A similar
pattern was also observed for other derivatives in this series.
For electron-withdrawing and electron-donating groups on the phenyl ring, we
observed that the location of the substituent influenced the inhibitory properties of
the tyrosol derivatives. The compounds with electron-withdrawing substituents at the
para position were more active than their meta and ortho counterparts. In contrast,
the compounds containing electron-donating substituents at the meta position were
found to be more active than their para and ortho counterparts. To understand the
binding interaction of the most active derivatives, molecular docking studies
were performed.
2.4. Docking results
The molecular modelling simulation software AutoDock was used for the protein-lig-
and interaction study. The compound 3u and 3x, along with the control compound
acarbose, were docked. Our analysis showed that the active molecules of the tyrosol
derivatives displayed substantial binding within the active site of a-glucosidase.
Overall, the tyrosol derivatives with either a cyano or hydroxyl group at the para pos-
ition on the phenyl ring showed good activity.
The most active derivative among the series is compound 3u and is shown in
Figure 1. The hydroxyl group of the tyrosol formed H-bond with Tyr158 and Asn415,
whilst the carbonyl group of the para-cyanobenzoic acid formed H-bond with Arg315
and Thr310; these were the main interactions between 3u and a-glucosidase. Ser311
formed an amide-p stacking with the phenyl ring of the para-cyanobenzoic acid.
Figure 1. Compound 3u was docked to the binding pocket of the a-glucosidase.

NATURAL PRODUCT RESEARCH
5
Figure 2. Compound 3x was docked to the binding pocket of the a-glucosidase.
Figure 3. Acarbose was docked to the binding pocket of the a-glucosidase.
Additionally, the p-cation interactions and p-r interactions were observed
between the phenyl ring of the para-cyanobenzoic acid and the residues Asp307
and Pro312.
The second most active compound is compound 3x and is shown in Figure 2.
Similar to 3u, the docking studies showed that the hydroxyl group of the tyrosol in 3x
formed H-bond with Tyr158 and Asn415, whilst the carbonyl group of the para-cyano-
benzoic acid formed a H-bond with Arg315. For this derivative, the p-cation interac-
tions and p-r interactions were also observed between the phenyl ring of the para-
cyanobenzoic acid and the residue Asp307 and Pro312.
A molecular docking study of the standard drug acarbose with a-glucosidase was
also performed (Figure 3). The docking studies showed that the hydroxyl group of
acarbose formed H-bond with Tyr158, Asp242, Pro312, His280, Ser311, Asp307, Thr310
and Gly309. Compound 3u was the most active compound, followed by 3x and their
free binding energies were calculated to be ꢀ7.08 kcal/mol and -6.02 kcal/mol, respect-
ively, which are lower than acarbose (ꢀ5.67 kcal/mol).
Taken together, the a-glucosidase inhibition results and molecular interaction stud-
ies clearly suggest that the cyano group and hydroxyl group attached to the para pos-
ition of the phenyl ring from the tyrosol derivatives are responsible for influencing the
inhibitory activity. The presence of a hydrophilic group on the phenyl ring was
accountable for the improvement of activity, which provided valuable information for
the design of inhibitory compounds.

6
L. ZHANG ET AL.
3. Experimental
3.1. General information
Yeast a-Glucosidase (EC 3.2.1.20) was purchased from the supplier (Sigma-Aldrich).
Acarbose 98% from Ark Pharm. p-Nitrophenyl-a-D-glucopyranoside (pNPG, 99%) from
Acros. Tyrosol, 98%, Triphenylphosphine (TPP, 98%), Diisopropyl azodicarboxylate
(DIAD, 97%), dry THF, 99.5% were purchased from the supplier (Energy Chemical).
EtOAc, 99.5%, Methanol, 99.5% were used as received from the supplier
(Sinopharm Chemical).
Column chromatography was carried out on silica gel (200-300 mesh). Thin layer
chromatography (TLC) was performed using silica gel 60 F254 plates. 1H NMR and
13C NMR spectra were measured on an AV-600 Spectrometer (Bruker, Germany)
using tetramethylsilane as an internal standard. Electrospray ionization mass spec-
trometry (ESI-MS) were performed on an Aglient 6520 Q-TOF (Agilent, USA) in posi-
tive ionization mode. Epoch microplate spectrophotometer (BioTek, USA). Melting
points were determined in open capillary tubes and the temperature was
uncorrected.
3.2. Synthesis
A general experimental procedure for the synthesis of compounds (3a-3x)
The target compounds were synthesised as outlined in our previously published
work (Zang et al. 2018). Tyrosol 1 (0.4 mmol, 1.0 equiv), organic acids 2 (0.4 mmol,
1.0 equiv) and TPP (0.4 mmol, 1.0 equiv) were placed in a dry standard Schlenk
tube under N2. Dry THF (1.0 mL) was added, followed by the addition of DIAD
(0.4 mmol, 1.0 equiv) at 0 ꢁC. The reaction mixture was stirred at room
temperature for 48 h, and the reaction was monitored with thin-layer chromatog-
raphy. The crude reaction mixture was purified by flash silica gel column chroma-
tography
corresponding product.
(petroleum
ether:
ethyl
acetate
¼
4:1)
to
obtain
the
3.3. a-Glucosidase inhibition assay
a-Glucosidase was assayed according to the method with slight modifications (Yuan
et al. 2012). Each sample in DMSO solution(20 mL) (from 0.1 to 10 mM) was added to
100 mL of a-Glucosidase solution (pH 6.9, 0.1 U/mL, in 0.1 M phosphate buffer). The
reaction mixtures were incubated at 25 ꢁC for 10 min. Then, 50 lL pNPG solution (pH
6.9, 5 mM, in 0.1 M phosphate buffer) was added to each well, the reaction mixtures
were incubated at 25 ꢁC for 5 min. Before and after incubation, the absorbance was
recorded at 405 nm on a microplate spectrophotometer (BioTek). Acarbose was used
as a positive reference. The a-Glucosidase inhibition activity was expressed as % inhib-
ition and was calculated as follows:
ꢀ
ꢁ
DAsample
DAcontrol
%inhibition ¼ 1 ꢀ
ꢂ 100%

NATURAL PRODUCT RESEARCH
7
3.4. Statistical analysis
All the experiments were carried out in triplicate and the data were analyzed using
SPSS software (Version 22.0) and Origin software (Version 8.0).
3.5. Molecular docking studies
To investigate the binding modes of compound 3u and 3x, a docking simulation was
performed targeting the crystal structure of a-glucosidase. Because the crystal struc-
ture for a-glucosidase from Saccharomyces cerevisiae (S. cerevisiae) is still not available,
a docking study was conducted using a homology model for a-glucosidase.
Preliminary results on the sequence analysis of a-glucosidase from S. cerevisiae showed
that the most suitable template for homology modelling is isomaltase (EC 3.2.1.10,
oligo-1,6-glucosidase, MALX3) (protein data bank (PDB) ID: 3A4A) from baker’s yeast. It
shares 71% identity and 84% similarity with the target enzyme, a-glucosidase from S.
cerevisiae. The crystal structure was retrieved from the PDB, and the a-glycosidase was
pre-docked using the Autodock software. The 3D structures of 3u and 3x were built
with ChemBioDraw Ultra 14.0 and ChemBio3D Ultra 14.0, and then converted to
PDBQT coordinates using AutoDockTools 1.5.6. In AutoGrid, maps were calculated for
each atom type in the ligand with a 0.375 Å spacing between the grid points and the
centre of the grid box was placed at x ¼ 12.583, y ¼ ꢀ7.896, z ¼ 12.519. The dimen-
sions size was set at 40 Å ꢂ 40 Å ꢂ 40 Å. Flexible ligand dockings were accomplished
for the selected compounds. Each docked system was carried out by 50 runs of the
AutoDock search by the Lamarckian genetic algorithm. The best positions of the
selected compounds were chosen by analysing the interactions between the enzyme
and inhibitors. The best-scoring positions, as judged by the docking score, were then
selected and visually analysed using Discovery Studio Client 2017.
4. Conclusion
In summary, a series of tyrosol derivatives have been synthesised and screened for
a-glucosidase inhibitory activity. Compound 3u (IC50¼15.2lM) and 3x (IC50¼41.2 lM)
showed significant inhibitory effects, which surpassed that of the control standard acar-
bose (IC50¼60.9lM). The structure and activity relationship was examined by designing
tyrosol derivatives with various substituents at different positions on the phenyl group,
and evaluating how the position affected the inhibitory properties of the tyrosol deriva-
tives. The binding interactions of the most active compounds were confirmed through
molecular docking studies, which showed substantial binding within the active site of
a-glucosidase. The compounds synthesised in this study have demonstrated that the
position of the substituent plays a crucial role in affecting the inhibition activity, and
may be useful for the development of new a-glucosidase inhibitors.
Acknowledgement
We thank Rosalie Tran, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for
editing the English text of a draft of this manuscript.

8
L. ZHANG ET AL.
Disclosure statement
The authors declare no conflict of interest.
Funding
This work is supported by the Science and Technology Development Funds of Jilin Province
(No. 20160520044JH) and the Science and Technology Projects of Administration of Traditional
Chinese Medicine of Jilin Province (No. 2019145).
References
Ahn EY, Jiang Y, Zhang Y, Son EM, You S, Kang SW, Park JS, Jung JH, Lee BJ, Kim DK. 2008.
Cytotoxicity of p-tyrosol and its derivatives may correlate with the inhibition of DNA replica-
tion initiation. Oncol Rep. 19(2):527–534.
Belmonte-Reche E, Martinez-Garcia M, Penalver P, Gomez-Perez V, Lucas R, Gamarro F, Perez-
Victoria JM, Morales JC. 2016. Tyrosol and hydroxytyrosol derivatives as antitrypanosomal and
antileishmanial agents. Eur J Med Chem. 119:132–140.
Busnena BA, Foudah AI, Melancon T, El Sayed KA. 2013. Olive secoiridoids and semisynthetic
bioisostere analogues for the control of metastatic breast cancer. Bioorg Med Chem. 21(7):
2117–2127.
Canuelo A, Gilbert-Lopez B, Pacheco-Linan P, Martinez-Lara E, Siles E, Miranda-Vizuete A. 2012.
Tyrosol, a main phenol present in extra virgin olive oil, increases lifespan and stress resistance
in Caenorhabditis elegans. Mech Ageing Dev. 133(8):563–574.
Chandramohan R, Pari L, Rathinam A, Sheikh BA. 2015. Tyrosol, a phenolic compound, amelio-
rates hyperglycemia by regulating key enzymes of carbohydrate metabolism in streptozotocin
induced diabetic rats. Chem Biol Interact. 229:44–54.
Chandramohan R, Saravanan S, Pari L. 2017. Beneficial effects of tyrosol on altered glycoprotein
components in streptozotocin-induced diabetic rats. Pharm Biol. 55(1):1631–1637.
Dewapriya P, Himaya SW, Li YX, Kim SK. 2013. Tyrosol exerts a protective effect against dopa-
minergic neuronal cell death in in vitro model of Parkinson’s disease. Food Chem. 141(2):
1147–1157.
Giovannini C, Straface E, Modesti D, Coni E, Cantafora A, De Vincenzi M, Malorni W, Masella R.
1999. Tyrosol, the major olive oil biophenol, protects against oxidized-LDL-induced injury in
Caco-2 cells. J Nutr. 129(7):1269–1277.
Han L, Fang C, Zhu R, Peng Q, Li D, Wang M. 2017. Inhibitory effect of phloretin on a-glucosi-
dase: Kinetics, interaction mechanism and molecular docking. Int J Biol Macromol. 95:
520–527.
Kwon YI, Jang HD, Shetty K. 2006. Evaluation of Rhodiola crenulata and Rhodiola rosea for man-
agement of type II diabetes and hypertension. Asia Pac J Clin Nutr. 15(3):425–432.
Lee CW, Son EM, Kim HS, Xu P, Batmunkh T, Lee BJ, Koo KA. 2007. Synthetic tyrosyl gallate
derivatives as potent melanin formation inhibitors. Bioorg Med Chem Lett. 17(19):5462–5464.
Madrona A, Pereira-Caro G, Bravo L, Mateos R, Espartero JL. 2011. Preparation and antioxidant
activity of tyrosyl and homovanillyl ethers. Food Chem. 129(3):1169–1178.
Motabar O, Shi ZD, Goldin E, Liu K, Southall N, Sidransky E, Austin CP, Griffiths GL, Zheng W.
2009. A new resorufin-based a-glucosidase assay for high throughput screening. Anal
Biochem. 390(1):79–84.
Peng X, Zhang G, Liao Y, Gong D. 2016. Inhibitory kinetics and mechanism of kaempferol on
a-glucosidase. Food Chem. 190:207–215.
Salucci S, Burattini S, Battistelli M, Buontempo F, Canonico B, Martelli AM, Papa S, Falcieri E.
2015. Tyrosol prevents apoptosis in irradiated keratinocytes. J Dermatol Sci. 80(1):61–68.

NATURAL PRODUCT RESEARCH
9
Schmidt JS, Nyberg NT, Staerk D. 2014. Assessment of constituents in Allium by multivariate
data analysis, high-resolution a-glucosidase inhibition assay and HPLC-SPE-NMR. Food Chem.
161:192–198.
Servili M, Esposto S, Fabiani R, Urbani S, Taticchi A, Mariucci F, Selvaggini R, Montedoro GF.
2009. Phenolic compounds in olive oil: antioxidant, health and organoleptic activities accord-
ing to their chemical structure. Inflammopharmacol. 17(2):76–84.
Shimba N, Shinagawa M, Hoshino W, Yamaguchi H, Yamada N, Suzuki E. 2009. Monitoring the
hydrolysis and transglycosylation activity of alpha-glucosidase from Aspergillus niger by
nuclear magnetic resonance spectroscopy and mass spectrometry. Anal Biochem. 393(1):
23–28.
St-Laurent-Thibault C, Arseneault M, Longpre F, Ramassamy C. 2011. Tyrosol and hydroxytyrosol,
two main components of olive oil, protect N2a cells against amyloid-b-induced toxicity.
Involvement of the NF-jB signaling. Curr Alzheimer Res. 8(5):543–551.
Sun L, Fan H, Yang L, Shi L, Liu Y. 2015. Tyrosol prevents ischemia/reperfusion-induced cardiac
injury in H9c2 cells: involvement of ROS, Hsp70, JNK and ERK, and apoptosis. Molecules.
20(3):3758–3775.
Wang G, Wang J, He D, Li X, Li J, Peng Z. 2016. Synthesis and biological evaluation of novel
1,2,4-triazine derivatives bearing carbazole moiety as potent a-glucosidase inhibitors. Bioorg
Med Chem Lett. 26(12):2806–2809.
Wei M, Chai WM, Yang Q, Wang R, Peng Y. 2017. Novel insights into the inhibitory effect and
mechanism of proanthocyanidins from pyracantha fortuneana fruit on a-Glucosidase. J Food
Sci. 82(10):2260–2268.
Yuan T, Wan C, Liu K, Seeram NP. 2012. New maplexins F-I and phenolic glycosides from red
maple (Acer rubrum) bark. Tetrahedron. 68(4):959–964.
Zang H, Shen P, Wang EP, Xu Q, Zhang LY, Xia GQ, Zhu JY, Zhang H, Yang XH. 2018. Synthesis
and biological activity of tyrosol ester derivatives. Chem J Chinese Universities. 39:64–70.
Zhou D, Sun Y, Shahidi F. 2017. Preparation and antioxidant activity of tyrosol and hydroxytyro-
sol esters. J Funct Foods. 37:66–73.