Benzaldehyde derivatives with functional propargyl groups as α-glucosidase inhibitors

Article abstract of DOI:10.1016/j.molstruc.2020.127780

The benzaldehyde derivatives (1–6) containing one and/or two propargyl arm(s) attached to the phenolic hydroxyl oxygen atom on the ortho-, meta- or para-positions of the aromatic ring were synthesized and characterized by spectral and microanalytical meth

Full text of DOI:10.1016/j.molstruc.2020.127780

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Benzaldehyde derivatives with functional propargyl groups as α-glucosidase inhibitors
Seyit Ali Güngör, Mehmet Tümer, Muhammet Köse, Sultan Erkan
PII:
S0022-2860(20)30104-6
DOI:
https://doi.org/10.1016/j.molstruc.2020.127780
MOLSTR 127780
Reference:
To appear in: Journal of Molecular Structure
Received Date: 25 November 2019
Revised Date: 14 January 2020
Accepted Date: 21 January 2020
Please cite this article as: S.A. Güngör, M. Tümer, M. Köse, S. Erkan, Benzaldehyde derivatives with
functional propargyl groups as α-glucosidase inhibitors, Journal of Molecular Structure (2020), doi:
https://doi.org/10.1016/j.molstruc.2020.127780.
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Credit author statement
Seyit Ali Güngör: Synthetic part of the work.
Mehmet Tümer: Supervision of the work
Muhammet Köse: X-ray crystallography
Sultan Erkan: Computational work

Benzaldehyde Derivatives with Functional Propargyl Groups as α-
Glucosidase Inhibitors
Seyit Ali Güngöra*, Mehmet Tümera, Muhammet Kösea and Sultan Erkanb
aChemistry Department, K.Maras Sütcü Imam University, 46100, K.Maras, Turkey.
bSivas Cumhuriyet University Yıldızeli Vocational School, Department of Chemistry and
Chemical Processing Technologies, 58140 Sivas, Turkey
Abstract
The benzaldehyde derivatives (1-6) containing one and/or two propargyl arm(s) attached to
the phenolic hydroxyl oxygen atom on the ortho-, meta- or para-positions of the aromatic ring
were synthesized and characterized by spectral and microanalytical methods. The molecular
structures were of 1, 4-6 determined by the single crystal X-ray crystallography. The
benzaldehyde derivatives containing free phenolic groups involved intra- and inter-molecular
hydrogen bonding interactions. The electrochemical properties of the compounds were
investigated at the different scan rates and it was found that the compounds show different
electrochemical behaviors. α-Glucosidase inhibitor properties of the carbonyl compounds
were investigated and it has been determined that some of them show high activity.
Compounds with a free phenolic group (-OH) exhibit better inhibition activity than
compounds with no free phenolic group. It was also found that the position of propargyl
group played an important role in the α-glucosidase inhibition activity. Molecular docking
studies were performed to investigate the binding interactions and interaction energies of the
(1-6) compounds with the α-glucosidase inhibitor.
Keywords: Hydroxy Benzaldehyde; X-ray; Electrochemistry; α-Glucosidase, Propargyl;
Molecular Docking
*Corresponding author: Tel.: +90 344 300 1444; fax: +90 344 300 1352.
E-mail address: sagungor@ksu.edu.tr
1

1. Introduction
The benzaldehyde group containing organic compound occurs in many plants in nature
in combined and/or non-combined forms. The natural source of benzaldehyde is amygdaline
which is found in bitter almonds. Due to the emergence in bitter almonds, the aldehyde
compound was formerly known as “bitter almond oil”. Benzaldehyde is the main component
of the essential oils obtained by pressing the seeds of peach group fruit. Benzaldehyde may be
miscible at the room temperature with many organic and inorganic solvents. These are
concentrated H2SO4, liquid CO2, liquid NH3, CH3NH2, and (C2H5)2NH. As the benzaldehyde
compound has many substitutable groups on the aromatic ring, there are a lot of benzaldehyde
derivatives. Some of these derivatives are chloro-, nitro-, hydroxyl-, amino-, alkyl
benzaldehydes etc [1–5].
The propargyl functional group has a significant role in neuroprotection[6] and
possesses peerless inhibitory properties toward flavin-linked oxidases[7]. As this group has
the reactive property for many chemical reactions, it can easily involve in click reaction. This
reaction has a great deal of applications containing drug exploration[8], sciences of the
materials[9] and biology[10]. Moreover, it is used for preparing a large number of novel
compounds which contain glyco-based materials such as, glycodendrimers, glycopolymers
and glycopeptides[11]. The compounds containing propargyl group passes high anticancer
activity, because they produce the benzene-1,4-diradical which cleaves the DNA [12].
In this paper, we report the synthesis, structural characterization and α-glucosidase
activity of the hydroxy benzaldehyde derivatives (2,3-dihydroxy, 2,4-dihydroxy, 2,5-
dihydroxy benzaldehyde and 2,3,4-trihydroxy benzaldehyde) containing propargyl group. The
benzaldehydes having propargyl arms were characterized by microanalytical, spectroscopic
and single crystal X-ray diffraction techniques. The electrochemical behaviors of the carbonyl
compounds were investigated by cyclic voltammetry. In addition, α-glucosidase inhibitory
effect of compounds was investigated and interaction modes energies were determined by
molecular docking studies.
2. Experimental
2.1. General Methods
All reagents and solvents were of reagent-grade quality and obtained from commercial
suppliers (Aldrich or Merck). Column chromatography was performed on silica gel (100–200
mesh). Elemental analyses (C,H,N) were performed using a LECO CHNS 932. Infrared
2

spectra were obtained using KBr disc (4000-400 cm-1) on a Perkin Elmer Spectrum 100 FT-
IR. Mass spectra of the ligands were recorded on a LC/MS APCI AGILENT 1100 MSD
13
spectrophotometer. 1H and C NMR spectra of the carbonyl compounds (1-6) were recorded
on a Bruker 400 MHz instrument. TMS was used as internal standard and d6-DMSO and
CDCl3 as solvent. The electronic spectra in the 200–900 nm range of the benzaldehyde
derivatives with propargyl arm(s) were investigated by Perkin Elmer Lambda 45
spectrophotometer. All samples were prepared in spectrophotometric grade solvents and
analysed in a 1 cm optical path quartz cuvette. The solutions of the compounds (1.0 x 10-3 mol
L-1) were prepared in MeCN, EtOH, toluene and xylene solvents.
A stock solution in the 1x10–3 M concentration of the propargyl based compounds (1-
6) was prepared in DMF for electrochemical studies. Cyclic voltammograms were recorded
on a Iviumstat Electrochemical workstation equipped with a low current module (BAS PA–1)
recorder. The electrochemical cell was equipped with a BAS glassy carbon working electrode
(area 4.6 mm2), a platinum coil auxiliary electrode and a Ag+/AgCl reference electrode filled
with tetrabutylammonium tetrafluoroborate (TBABF4) solution and adjusted to 0.00 V vs
SCE. The buffer solution (tetrabutylammonium tetrafluoroborate) used in the study was
prepared in water as 1.0 M, it was diluted to the lower concentrations (0.1 M) with DMF.
Cyclic voltammetric measurements were made at room temperature in an undivided cell
(BAS model C–3 cell stand) with a platinum counter electrode and an Ag+/AgCl reference
electrode (BAS). All potentials are reported with respect to Ag+/AgCl. The solutions were
deoxygenated by passing dry nitrogen through the solution for 30 min prior to the
experiments, and during the experiments the flow was maintained over the solution. Digital
simulations were performed using DigiSim 3.0 for windows (BAS, Inc.). Experimental cyclic
voltammograms used for the fitting process had the background subtracted and were corrected
electronically for ohmic drop. Mettler Toledo MP 220 pH meters was used for the pH
measurements using a combined electrode (glass electrode reference electrode) with an
accuracy of ±0.05 pH.
2.2. X-ray Crystallography for Compounds (1-6)
Data collection for X-ray crystallography were collected by using a Bruker D8 QUEST
diffractometer (Mo-Kα radiation (λ= 0.71073 Å) and data reduction was performed using
Bruker SAINT[13]. SHELXS97 was used to solve and SHELXL2014/6 to refine the
structure[14]. The structures were solved by direct methods and refined on F2 using all the
reflections [15]. All the non-hydrogen atoms were refined using anisotropic atomic
3

displacement parameters and hydrogen atoms bonded to carbon atoms were inserted at
calculated positions using a riding model. Details of the crystal data and refinement are given
in Table 1. Bond distances and angles for compounds are given in the supplementary
documents (Table S1 and S2).
2.3. Synthesis of 1, 2 and 4
The compounds 1, 2 and 4 were synthesized according to the reported procedure [16].
Propargyl bromide (15 mmol), hydroxybenzaldehyde derivatives [2,3-dihydroxybenzaldeyhde
for (1), 2,4-dihydroxybenzaldeyhde for (2) an 2,5-dihydroxybenzaldeyhde for (4)] (10 mmol)
and potassium bicarbonate (10 mmol) were suspended in acetonitrile (30 ml) and refluxed for
30 h in presence of KI (0.1 g) as a promoter. The reaction mixture was filtered while hot to
remove insoluble impurities, neutralized with dilute HCl (3 M), extracted with
dichloromethane and dried over Na2SO4. The extracts were concentrated to obtain a brown
solid which was then purified by column chromatography over SiO2 by eluting with a mixture
of 10% ethyl acetate in n-hexane. Evaporation of the purified extract yielded prop-2-ynyloxy
benzaldehyde derivatives. Single crystals of the compounds suitable for X-ray diffraction
analysis were obtained from slow evaporation in methanol.
3-Hydroxy-2-(prop-2-yn-1-yloxy)benzaldehyde (1): C10H8O3. Yield: 55%, color: light
brown, melting point: 110 °C. Elemental analysis; found (calcd.) %; C, 68.24 (68.18); H, 4.64
1
(4.58). HNMR (δ, CDCl3): 10.33 (s, 1H, HC=O), 10.21 (s, 1H, OH), 7.22-7.09 (m, 3H,
aromatic ring-H), 4.89 (s, 2H, ortho-OCH2), 3.58 (s, 1H, -C≡CH). 13CNMR (δ, CDCl3):
185.13 (HC=O), 145.25-117.42 (aromatic ring C-atoms), 73.32 (β-C), 72.83 (α-C) [-O-
(β)C≡CH(α)], 58.18 (OCH2). Mass spectrum (LC/MS, m/z): Calcd.: 176.17, found: 176.59
[M]+. FTIR (KBr, ν, cm-1): 3245 ν(O–H), 2935 ν(CH2)alph, 2118 terminal ν(C≡CH), 1660
ν(C=O), 1471 ν(C=C)aromatic, 1358 ν(C–O)phenolic
.
2-Hydroxy-4-(prop-2-yn-1-yloxy)benzaldehyde (2): C10H8O3. Yield: 85%, color: white,
melting point: 84 °C. Elemental analysis; found (calcd.) %; C, 68.22 (68.18); H, 4.65 (4.58).
1H NMR (δ, CDCl3): 11.47 (s, 1H, OH), 9.76 (s, 1H, HC=O), 7.49-6.54 (m, 3H, aromatic
ring-H), 4.76 (s, 2H, para-OCH2), 2.61 (s, 1H, -C≡CH). 13CNMR (δ, CDCl3): 194.48
(HC=O), 164.55-115.75 (aromatic ring C-atoms), 108.64 (β-C), 101.92 (α-C) [-O-
(β)C≡CH(α)], 56.08 (OCH2). Mass spectrum (LC/MS, m/z): Calcd.: 176.17, found: 176.29
[M]+. FTIR (KBr, ν, cm-1): 3239 ν(O–H), 2923 ν(CH2)alph, 2127 terminal ν(C≡CH), 1634
ν(C=O), 1454 ν(C=C)aromatic, 1360 ν(C–O)phenolic
.
4

2-Hydroxy-5-(prop-2-yn-1-yloxy)benzaldehyde (4): C10H8O3. Yield: 20%, color: white,
melting point: 105 °C. Elemental analysis; found (calcd.) %; C, 68.25 (68.18); H, 4.61 (4.58).
1HNMR (δ, CDCl3): 10.39 (s, 1H, OH), 10.25 (s, 1H, HC=O), 7.22-6.95 (m, 3H, aromatic
ring-H), 4.83 (s, 2H, meta-OCH2), 3.56 (s, 1H, -C≡CH). 13CNMR (δ, CDCl3): 191.26
(HC=O), 152.02-112.74 (aromatic ring C-atoms), 73.48 (β-C), 71.29 (α-C) [-O-(β)C≡CH(α)],
52.10 (OCH2). Mass spectrum (LC/MS, m/z): Calcd.: 176.17, found: 176.81 [M]+. FTIR
(KBr, ν, cm-1): 3262 ν(O–H), 2918 ν(CH2)alph, 2127 terminal ν(C≡CH), 1667 ν(C=O), 1486
ν(C=C)aromatic, 1372 ν(C–O)phenolic
.
2.4. Synthesis of 3, 5 and 6
The procedure used for the synthesis 1, 2 and 4 were followed for the synthesis of 3, 5 and 6
except using double amount of propargyl bromide (30 mmol) and potassium carbonate (10
mmol) as base.
2,4-bis(prop-2-yn-1-yloxy)benzaldehyde (3): C13H10O3. Yield: 95%, color: white, melting
point: 110 °C. Elemental analysis; found (calcd.) %; C, 72.92 (72.89); H, 4.74 (4.71). 1HNMR
(δ, d6-DMSO): 10.19 (s, 1H, HC=O), 7.73-6.76 (d, 3H, aromatic ring-H), 4.94 (s, 2H, ortho-
OCH2), 5.00 (s, 2H, para-OCH2), 3.33 (s, 1H, ortho-C≡CH), 3.65 (s, 1H, para-C≡CH).
13CNMR (δ, d6-DMSO): 187.67 (HC=O), 164.01-119.72 (aromatic ring C-atoms), 79.62
(ortho-β-C), 78.87 ((ortho-α-C) [(-O-β)C≡CH(α)], 108.37 (para-β-C), 101.56 (ortho-α-C) [(-
O-β)C≡CH(α)], 56.56 (para-OCH2), 57.07 (ortho-OCH2). Mass spectrum (LC/MS, m/z):
Calcd.: 214.21, found: 214.78 [M]+. FTIR (KBr, ν, cm-1): 2917 ν(CH2)alph, 2127 terminal
ν(C≡CH), 1670 ν(C=O), 1491 ν(C=C)aromatic, 1373 ν(C–O)phenolic.
2,5-bis(prop-2-yn-1-yloxy)benzaldehyde (5): C13H10O3. Yield: 93%, color: white, melting
point: 86 °C. Elemental analysis; found (calcd.) %; C, 72.94 (72.89); H, 4.75 (4.71). 1HNMR
(δ, d6-DMSO-): 10.32 (s, 1H, HC=O), 7.36-7.14 (d, 3H, aromatic ring-H), 4.96 (s, 2H, meta-
OCH2), 4.83 (s, 2H, para-OCH2), 3.33 (s, 1H, para-C≡CH), 3.57 (s, 1H, meta-C≡CH).
13CNMR (δ, d6-DMSO): 189.13 (HC=O), 154.98-112.35 (aromatic ring C-atoms), 79.24
(ortho-β-C), 78.96 (ortho-α-C) [(-O-β)C≡CH(α)], 79.50 (meta-β-C), 79.45 ((meta-α-C) [(-O-
β)C≡CH(α)], 56.49 (meta-OCH2), 57.72 (ortho-OCH2). Mass spectrum (LC/MS, m/z):
Calcd.: 214.21, found: 214.92 [M]+. FTIR (KBr, ν, cm-1): 2918 ν(CH2)alph, 2119 terminal
ν(C≡CH), 1666 ν(C=O), 1488 ν(C=C)aromatic, 1377 ν(C–O)phenolic.
2-hydroxy-3,4-bis(prop-2-yn-1-yloxy)benzaldehyde (6): C13H10O4. Yield: 75%, color:
white, melting point: 108 °C. Elemental analysis; found (calcd.) %; C, 67.85 (67.82); H, 4.42
5

1
(4.38). H-NMR (δ, d6-DMSO): 10.75 (s, 1H, OH), 10.18 (s, 1H, HC=O), 7.58-7.11 (d, 2H,
aromatic ring-H), 4.99 (s, 2H, meta-OCH2), 4.79 (s, 2H, para-OCH2), 3.63 (s, 1H, para-
13
C≡CH), 3.57 (s, 1H, meta-C≡CH). CNMR (δ, d6-DMSO): 188.78 (HC=O), 157.22-110.85
(aromatic ring C-atoms), 78.79 (meta-β-C), 61.94 (meta-α-C) [(-O-β)C≡CH(α)], 79.29 (para-
β-C), 79.02 (para-α-C) [(-O-β)C≡CH(α)], 60.45 (meta-OCH2), 57.12 (para-OCH2). Mass
spectrum (LC/MS, m/z): Calcd.: 230.22, found: 230.69 [M]+. FTIR (KBr, ν, cm-1): 3292
ν(OH), 2917 ν(CH2)alph, 2121 terminal ν(C≡CH), 1673 ν(C=O), 1490 ν(C=C)aromatic, 1372
ν(C–O)phenolic.
2.5. X-ray Structure Solution and Refinement
The structures of 1, (4-6) were solved by direct methods and refined on F2 using all the
reflections. All the non-hydrogen atoms were refined using anisotropic atomic displacement
parameters and hydrogen atoms bonded to carbon and oxygen atoms were inserted at
calculated positions using a riding model. The X-ray crystallographic data of the structures
are listed in Table 1. Bond lengths and angles for compounds are listed in the supplementary
documents.
2.6. α-Glucosidase Inhibition Assay Method
The α-glucosidase inhibition assay was performed according to the method [17] with slight
change. Compounds were solved in EtOH at a concentration of 20 mg/ml. A series of
dilutions for each compound were prepared in 100 mM EtOH to final concentrations of 32, 16
and 8 ꢀg/mL. In a 96-well plate, a reaction mixture containing 10 µL of compounds of
changeable concentrations, 40 µL of 100 mM phosphate buffer (pH 6.8), 25 µL of α-
glucosidase (0.4 U/mL, Sigma G0660) were pre-incubated for 10 min at 37 °C. EtOH was
added to the respective control wells to a 0.64% final concentration. Then 25 µL of a 2.5 mM
p-nitrophenyl-α-D-glucopyranoside (pNPG, Sigma N1377) solution was added to the mixture
as a substrate. After further incubation at 37 °C for 30 min, the reaction was stopped by
addition of 100 µL of 0.2 M sodium carbonate. The amount of discharged p-nitrophenol from
pNPG was measured at 405 nm with microplate reader equipped spectrophotometer.
Acarbose was used as a positive control.
The % inhibition was obtained using the formula:
%Inhibition= 100% x (ODcontrol – ODsample)/ODcontrol
6

2.7. Molecular Docking
Docking studies were performed against α-glucosidase for (1-6) compounds, which are
hydroxy benzaldehyde analogs. The crystallographic structure of Saccharomyces cerevisiae α-
glucosidase is not yet available. Although only a few homology models have been reported,
the target protein (PDB ID: 4J5T) corresponding to Sigma G0660 was identified[18,19]. The
molecular docking environment was provided using Virtua Drug research and development
company's Docking Server service.
3. Results and Discussion
In this paper, the propargyl group substituted benzaldehydes were prepared by the
reaction of 2,3-dihydroxy, 2,4-dihydroxy, 2,5-dihydroxy benzaldehyde and 2,3,4-trihydroxy
benzaldehyde derivatives and propargyl bromide in acetonitrile (Fig. 1). The reaction of
propargyl bromide with 2,3-dihydroxybenzaldeyhde, 2,4-dihydroxybenzaldeyhde or 2,5-
dihydroxybenzaldeyhde (1.5 : 1 ratio) in the presence of potassium bicarbonate resulted in
mono-substituted propargyl derivatives 1, 2 and 4 were obtained. The yield in 1 and 4
compounds are lower due to the formation side products in the reactions. While the compound
1 containing single propargyl arm on the ortho position of the 2,3-dihydroxy benzaldehyde
was obtained, the carbonyl compound 6 containing propargyl arms at the meta and para
positions of the 2,3,4-trihydroxy benzaldehyde was obtained. On the other hand, the carbonyl
compounds involving the one and two propargyl arms of the 2,4-dihydroxy benzaldehyde [2
(one arm), 3 (two arms)] and 2,5-dihydroxy benzaldehyde [4 (one arm), 5 (two arms)] were
synthesized. All the compounds were characterized by the microanalytical analysis, FTIR,
UV-vis, photoluminescence (emission and excitation), LC/MS, 1H and 13C NMR spectra. The
single crystals of 1, (4-6) were obtained and their molecular structures were investigated by
the X-ray crystallography.
3.1. Characterization by the spectroscopic methods of the propargyl based carbonyl
compounds
The obtained microanalytical and spectroscopic data of the compounds (1-6) were given
in the experimental section. In the FTIR spectra of the compounds 1, 2, 4 and 6, the vibration
band ν(OH) of the free hydroxy group is shown in the 3292-3245 cm-1 range. In the spectra of
all compounds, the vibration bands in the 2935-2917 cm-1 range can be attributed to the
ν(CH2) group. In the spectra of the compounds, the weak vibration band in the range of 2127-
7

2118 cm-1 belongs to the characteristic terminal ν(C≡C) group. The strong and sharp vibration
bands in the range of 1673-1634 cm-1 belong to the carbonyl group ν(C=O).
The UV-vis absorption spectral properties of the compounds were investigated in the
1.0x10-3 M polar and apolar organic solvents such as, MeCN, EtOH, toluene and xylene. The
UV-vis spectra of the compounds were given in the Fig. 2 and Fig. S1. Polar and apolar
solvents were used in the spectra uptake to examine the interactions between solvents and the
carbonyl compounds (1-6). The compounds 1 and 2 showed a band at 315 nm and this band
comes from the π-π* transitions. The band at 315 nm shifted to the shorter wavelengths (300,
305 nm) in the spectrum of the compound 3. On the other hand, in the spectra of compounds 4
and 5, this band shifted to longer wavelengths. The π-π* transition bands with the highest
absorbance in the 300-360 nm region of the UV spectra of the compounds were observed
toluene and xylene. In the spectra of all compounds, the absorption bands in the 295-220 nm
range can be assigned to the π-π* and δ-π* transitions. In general, the electronic transitions of
the compounds shift to longer wavelengths in apolar solvents (toluene and xylene) due to the
keto-enol tautomeric transformation [20,21]. In the apolar solvents, the compounds with polar
character prefer the forms such as keto-, lactim- or hydrazo. In this type solvent, there is no
possible hydrogen bond interactions between solvent and solute. Since compounds 1 and 4
show the keto-enol tautomer in the toluene and xylene, the n-π* transitions shifted to longer
wavelengths (Fig. 3). On the other hand, in the MeCN and EtOH, since there are hydrogen
bond interactions between the polar ends (oxygen atoms) of the compounds and the solvent
molecules, the electronic transitions shifted to the shorter wavelengths. In the spectra of all
compounds, the absorbance intensity of
(hyperchromic effect) in toluene or xylene.
π-π* electronic transitions increased
1H and 13C NMR spectra of the carbonyl compounds (1-6) were recorded and the
spectra of the compounds (1-6) were given in the supplementary file as Fig. S2. While the
carbonyl compounds 1, 2 and 4 have only one propargyl group, the other compounds have
two propargyl groups on the different positions (ortho-, meta- or para-) of the phenyl ring. In
the 1H NMR spectra of the compounds 1, 2, 4 and 6, the broad signals at 10.21, 11.47, 10.39
and 10.75 ppm can be attributed to the hydrogen atom of the phenolic hydroxy group (Ph-
OH). The hydrogen atom of the carbonyl group on the compounds were observed in the
10.33-9.76 ppm range. The hydrogen atoms of aromatic ring were seen in the 7.73-6.54 ppm
range as multiples. Hydrogen atoms of the -OCH2- group bound to the aromatic ring are
shown in the 5.00-4.76 ppm range. The chemical shift values of OCH2 hydrogen atoms in the
para-position have been shifted to upfield due to electron delocalization with respect to the
8

the other OCH2- groups. The hydrogen atom of the -C≡CH group of the carbonyl compounds
was seen in the 3.65-2.61 ppm range. In the 13C NMR spectra of the propargyl based
compounds, the carbon atoms of the carbonyl groups were observed in the 194.48-185.13
ppm range. The signals in the 164.55-110.85 ppm range may be assigned to the aromatic ring
carbon atoms. The compounds 1, 2 and 4 have the terminal acetylenic moiety on the ortho-,
para- and meta-positions of the aromatic ring, respectively. In the spectra of the compounds 1
and 4, the (β) C≡ and ≡CH (α) atoms of the acetylenic group are shown in the 73.48-71.29
ppm range. But, as the acetylenic group in the compound 2 is on the para-position of the
aromatic ring, the chemical shifting values of carbon atoms of the acetylenic group shifted to
the upfield (108.64 and 101.92 ppm). Similar chemical shifting values for the acetylenic
carbon atoms were also observed in the other compounds. The signals in the 52.10-60.45 ppm
range can be attributed to the carbon atom of the –OCH2- groups.
3.2. Redox properties of the carbonyl compounds (1-6) with propargyl arm(s)
The cyclic voltammograms of the compounds and buffer solution were given in Fig. 4
and Fig. S3. The electrochemical properties were performed in the 100-1000 mVs-1 scan rate
range to investigate the effect of the scan rate on the redox properties of the carbonyl
compounds having propargyl arm(s). The compounds 1, 2 and 4 have only one acetylenic
group on the ortho-, para- and meta-positions, respectively. At all scan rates, the compound 2
shows only one oxidation and one reduction peak in the 0.62-0.74 V (Epa) and 0.59-0.73 V
(Epc), respectively. These redox processes are reversible (Fig. 5a). The compound 1 has one
cathodic and two anodic peak potentials at the 100, 250 and 500 mVs-1 scan rates. But, at 750
and 1000 mVs-1 scan rates, the 1 shows two cathodic and two anodic redox couples. In the
0.40-0.49 V range (positive region and all scan rates), the compound has reversible redox
properties. But, at 750 and 1000 mVs-1 scan rates, the compound indicates the irreversible
redox properties in the -0.21-(-0.41) V range (Fig. 5b). In this step, it can be stated that the
compound is hydrolyzed and the acetylenic group is removed irreversibly from the
compound. In the positive region and all scan rates, the compound 4 has reversible redox
couples in the 0.33-0.44 V range (Ipa/Ipc= 1.0). In the negative region and 100, 250 and 500
mVs-1 scan rates, the compound show the reversible redox processes in the -0.31-(-0.32) V
range. However, at the other scan rates, the compound has the irreversible process in the -
0.29-(-0.37) V. Although the carbonyl compounds 3, 5 and 6 contain two acetylenic groups,
they show different electrochemical behaviors. The compounds with two acetylenic groups
have two cathodic and two anodic peak potentials in the -0.47-0.57 V (Epc) and -0.36-0.62 V
9

(Epa). The compound 3 has reversible redox properties (Ipa/Ipc= 1.0) in the positive region
(0.58-0.44 V range) and all scan rates. In the negative region, at 1000 mVs-1 scan rate, the
compound 3 shows the reversible redox couple (Epa/Epc, -0.36/-0.37). But, at the other scan
rates, it shows the irreversible processes (Fig. 5c). The compound 5 has the similar
electrochemical properties to the 3. But, while the 5 show the reversible electrochemical
behaviors in the negative region (-0.34-0.31 V range and 500, 750 and 1000 mVs-1 scan
rates), in the positive region, all redox processes are irreversible. Unlike compounds 3 and 5,
the compound 6 exhibits irreversible redox behavior in all potentials. When the anodic and
cathodic peak potentials of all compounds are examined at different scanning rates, it is
observed that both electrochemical data shift towards the positive region.
3.3. X-ray Structures of 1, (4-6)
X-ray quality crystals for the diffraction experiments were grown from re-crystallization
in MeOH. The structures of the compounds are shown in Fig. 6. The molecules are all
benzaldehyde derivatives differ in the number and position of oxyprop-2-ynyl group on the
phenyl rings. The C≡C and C=O bond distances are within the normal range and are
comparable with those found in similar structures [16,22,23]. There are two independent
molecules in the asymmetric unit of 1 while the asymmetric unit of compounds (4-6) has only
one molecule. The two independent molecules of 1 have comparable bond lengths and angles
and differing in the conformation of the prop-2-ynyl group. In all four compounds, the
aldehyde group and benzene ring are almost co-planar with slight deviations. The propyne
group in each compound is almost linear and located in the flagpole position on the oxygen
atom. The oxyprop-2-ynyl group on the ortho position of the phenyl ring with respect to the
aldehyde group considerably twisted from the benzene ring (C2/C7). In compound 4, the
phenyl ring and oxyprop-2-ynyl group (O3-C8-C9-C10) are almost co-planar with the
dihedral angle of 4.08(16)°. For compound (6), the phenyl (C2/C7) and oxyprop-2-ynyl group
(O3-C8-C9-C10) on the meta position are almost perpendicular with the dihedral angle of
89.46(22)° while the phenyl (C2/C7) and the other oxyprop-2-ynyl group (O4-C11-C12-C13)
on the para position are almost in the same plane with the twist angle of 5.37(22)°. The huge
difference in the dihedral angle between the phenyl ring and oxyprop-2-ynyl group may be
due to the intermolecular contacts in which oxyprop-2-ynyl groups involve in weak hydrogen
bond contacts (C–H····O).
The intermolecular contacts between neighbouring molecules were investigated by
using Hirschfeld analysis for the compounds. The 2D fingerprint plots of compound 4
10

showing the contribution of different intermolecular interactions are given in Fig. 7 and rest
of the fingerprint plots are given in the supplementary documents (Figs. S4-S6). The 2D
fingerprint plot of the compounds showed two sharp peaks due to the H····O contacts. These
short contacts were seen as intensive red spots in the dn surface. The compounds 4 and 6 have
free phenolic groups (-OH) on the ortho position with respect to the aldehyde group and these
two molecules make intramolecular phenol-aldehyde (OH····O) hydrogen bond. Compound 1
has a free phenolic group on the metal position of the phenyl ring with respect to the aldehyde
group and this group makes an intermolecular hydrogen bond with the aldehyde oxygen atom
of a neighboring molecule resulting in a 1D hydrogen bond chain. The chains are interlinked
by C-C≡C-H····Oaldehyde contacts as shown in Fig. 8. Moreover, molecules also show π-π edge
to edge stacks stabilizing the structure of the compound. In the structure of 4, two molecules
are dimerized by two concordant hydrogen bonds between the phenolic group and aldehyde
group of an adjacent molecule (O2-H····O1) under symmetry operation of -x+1,-y,-z+1. The
hydrogen bonded dimers are linked by C-C≡C-H····Oaldehyde contacts forming a hydrogen
bond chain (Fig. 9). The chains are further conntected by π-π stackings between the phenyl
ring of the molecules. In (5), molecules form 2D sheets by C-C≡C-H····Oaldehyde, C-
Hphenyl····Ooxyprop-2-ynyl and C-Haldeyhde····π oxyprop-2-ynyl contacts. Packing diagram of 5 showing
these interactions are given in Fig. 10. Molecules also involve in π-π edge to edge interactions
between the phenyl rings. Molecules of 6 also makes similar intermolecular contacts to 5
forming 2D sheets. Packing diagram of 6 is given in the supplementary file. The hydrogen
bond parameters are also included in the supplementary documents (Fig. S7).
3.4. α-Glucosidase Inhibition
Type 2 diabetes mellitus (T2DM) is most common form of diabetes and is associated
with low production of insulin or resistance to insulin. One of the strategies to control T2DM
is to inhibit the activity of α-glucosidase that acts as catalyst for the digestion of
carbohydrates in the intestine. The α-glucosidase inhibitors have a crucial role to prevent or
delay the digestion or absorption of carbohydrates and thus overcoming postprandial
hyperglycemia. Acarbose, miglitol, and voglibose are clinically approved α-glucosidase
inhibitors[24]. Due to the side effects (such as pain, diarrhea and abdominal discomfort) of
these inhibitors, the design of new α-glucosidase inhibitors is off interest[25]. In the course of
this, the benzaldehyde derivatives having oxyprop-2-ynyl group (1-6) were screened for α-
glucosidase inhibitory activity. The synthesized benzaldehyde derivatives (1-6) exhibited
considerable inhibitory activity with IC50 values ranged from 221.88 µM to 439.45 µM
11

(Table 2). Compounds except 3 were found to be almost two-fold and four-fold more active
than metformin (IC50: 439.45 µM and acorbase (IC50: 906 µM), respectively. Compounds 2, 4
and 6 showed similar inhibitory effect and this may be attributed to free phenolic groups on
the ortho-position of the phenyl ring with respect to the aldehyde group. The extra oxyprop-2-
ynyl group in 6 did not cause a considerable change in the activity. Compounds 3 and 5 have
two oxyprop-2-ynyl groups one of which located on the different positions (para for 3 and
meta for 5 with respect to the aldehyde group). The second oxyprop-2-ynyl group on the para
position 3 declined the activity of the compound. The oxyprop-2-ynyl group on the meta
position of 3 may cause steric effect resulting in lower binding affinity of the enzyme towards
the molecule. Although compounds 1, 2, 4 and 6 have a free phenolic group (-OH), compound
1 showed a slightly better inhibition activity than the others. The higher activity of 1 can be
assigned the position of the phenolic groups which can involve in intermolecular hydrogen
bonding interactions with the enzyme while in 2, 4 and 6, the phenolic group makes
intramolecular rather than intermolecular hydrogen bonds.
3.5. Docking study
The benzaldehyde derivatives examined (1-6) contain similar but with different
orientations oxyprop-2-ynyl group groups. Molecular docking was performed to investigate
the inhibitory activities of these compounds in detail. In docking studies, the target protein
was identified as the homology model of α-glucosidase from Saccharomyces cerevisiae
(Sigma G0660). The docking poses obtained from the simulations are given in Fig. 11.
It can be clearly seen from Fig. 11 that the interaction of studied ligands with active
residues differs from each other. The interaction results obtained from the docking results are
highly consistent with the experimental half-maximal inhibitory concentration (IC50).
Docking interaction types show that all compounds (1-6) involve in polar, hydrophobic and
π−π interactions as well as H-bonds with the target protein. According to the binding mode
analysis the most active compound is 1 among the compounds studied, the 3-hydroxy group
formed a hydrogen bond with the nitrogen of LYS514. The benzaldehyde oxygen, 2-(prop-2-
yn-1-yloxy) and 3-hydroxy group oxygen of compound 1 contains polar interaction with the
backbone of Thr307 andGLU417, respectively. The phenyl ring in 1 exhibit π−π interaction
with TYR507 amino acid residues. In addition, benzaldehyde and 2- (prop-2-yn-1-yloxy)
carbons AL(4)13 amino acid residues of the compound 1 exhibit hydrophobic interaction. The
benzaldehyde oxygen of 5 with high inhibition efficiency formed the H-bond with the amino
acid residue ARG28. The polar interaction of ASN9 with oxyprop-2-ynyl oxygen of 5 is
12

remarkable. In addition, the phenyl ring carbons of 5 exhibits π−π interactions with TYR29
while the other carbons of the compound perform hydrophobic interactions with LEU13,
TYR29, VAL30 and LEU159. Compound 3 exhibits minimal interaction with the amino acid
residues of the target protein. Therefore, both experimental and calculated inhibition efficacy
results follow similar activity trend. Benzaldehyde's oxygen atoms of the compound’s forms
H- bond with the NH2 group of ARG28. In addition, the carbons of the compound generally
exhibit hydrophobic and π−π interactions. In 2, 4 and 6, phenolic groups exhibited polar
interaction with the target protein, while the carbon atoms of these compounds interacted
hydrophobic with amino acid residues. The estimated free binding energy (Eest. binding),
inhibition constant (Ki), secondary interaction (van der Waals forces + hydrogen bond +
disolve) energy (Esecondary) of (1-6) against α-glucosidase are given in Table 3.
The molecular simulation results given in Table 3 may provide important information
for further development of α-glucosidase inhibitors. According to the estimated free binding
energy values, the interaction between α-glucosidase and 1 is the highest and the interaction
with 3 is the lowest. These energy values are almost identical for 2 and 6. As different from
the structure of 2, compound 6 contains one more oxyprop-2-ynyl group located at the meta
position with respect to the aldehyde group and the presence of this group did not affect the
estimated binding energy. In addition, the estimated binding energies are consistent with the
results of the α-glucosidase assay. As the van der Waals forces, hydrogen bond and dissolved
energy values obtained from docking data are more negative, as binding of the ligands to the
the target enzyme is higher. Inhibition constant (Ki) provides information about the binding
ability of a compound to inhibit enzyme function and Ki is also a measure of ligand binding
affinity to the protein [26]. The results show that the inhibitory activity of 1 is higher than the
other compounds.
4. Conclusions
The benzaldehyde derivatives with propargyl arm(s) were synthesized and their
molecular structures were determined by single crystal X-ray crystallography. The propargyl
groups play an important role in the crystal packing of the molecules. The electrochemical
properties of the compounds were investigated at the different scan rates and it was found that
the compounds showed reversible and irreversible redox processes. Compounds were also
screened for their α-glucosidase inhibitor properties. It was found that compounds with free
phenolic group (-OH) exhibit better inhibition activity and the position of oxy propargyl
group have an effect on the α-glucosidase inhibition activity. All compounds showed higher
13

α-glucosidase inhibitory properties then metformin and acarbose with IC50 values of 221-412
µM. The binding interaction of these active compounds with α-glucosidase was supported in
detail using molecular insertion. Eest, Ki (inhibition constant) and Esecondary values from
molecular docking are consisted with the experimented α-glucosidase IC50 values.
14

Acknowledgments
We thank to Kahramanmaras Sutcu Imam University Science Project unit for the financial
support for this study (Project Number: 2019/1‐17D and 2019/5-20M).
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17

Fig. 1. General reaction method for the synthesized propargyl based carbonyl compounds.

2
1.5
1
1
MeCN
EtOH
Xylene
Toluene
0.5
0
200
230
260
290
320
350
380
410
nm
2
5
MeCN
EtOH
Xylene
Toluene
1.5
1
0.5
0
200
250
300
350
400
450
nm
Fig. 2. The electronic spectra of 1 and 5 compounds in the MeCN, EtOH, toluene and xylene
solutions.

Fig. 3. Enol-Keto tautomerism in 2.

16
12
8
1
1000 mV/s
750 mV/s
500 mV/s
250 mV/s
100 mV/s
4
Epc
0
Epa
-4
-8
-12
-16
-0.68
-0.34
0
0.34
Potential (V)
0.68
1.02
15
10
5
5
1000 mV/s
750 mV/s
500 mV/s
250 mV/s
100 mV/s
Epc
0
Epa
-5
-10
-15
-20
-0.9
-0.65
-0.4
-0.15
0.1
0.35
0.6
0.85
1.1
Potential (V)
Fig. 4. Cyclic voltammograms of 1 and 5 compounds in the 1.0 × 10-3 M DMF solution and
different scan rates.

Fig. 5. The proposed electrochemical mechanism for the carbonyl compounds.

4
1

5
6
Fig. 6 Molecular structures of the compounds with atom numbering. Intrmolecular hydrogen bonds in 4 and 6 are shown as dashed lines.

Fig. 7 2D fingerprint plots for 4.

Fig. 8 The crystal packing of the 1 molecules. The O-H····O and C-H····O contacts are shown as dashed lines.

Fig. 9 Packing diagram of the 4 showing O-H····O and C-H····O contacts (dashed lines).