The evaluation of 1-tetralone and 4-chromanone derivatives as inhibitors of monoamine oxidase

Article abstract of DOI:10.1007/s11030-020-10143-w

Abstract: Monoamine oxidase (MAO) is of much clinical relevance, and inhibitors of this enzyme are used in the treatment for neuropsychiatric and neurodegenerative disorders such as depression and Parkinson’s disease. The present study synthesises and evaluates the MAO inhibition properties of a series of 33 1-tetralone and 4-chromanone derivatives in an attempt to discover high-potency compounds and to expand on the structure–activity relationships of MAO inhibition by these classes. Among these series, eight submicromolar MAO-A inhibitors and 28 submicromolar MAO-B inhibitors are reported, with all compounds acting as specific inhibitors of the MAO-B isoform. The most potent inhibitor was a 1-tetralone derivative (1h) with IC₅₀ values of 0.036 and 0.0011?μM for MAO-A and MAO-B, respectively. Interestingly, with the reduction of 1-tetralones to the corresponding alcohols, a decrease in MAO inhibition potency is observed. Among these 1-tetralol derivatives, 1p (IC₅₀ = 0.785?μM) and 1o (IC₅₀ = 0.0075?μM) were identified as particularly potent inhibitors of MAO-A and MAO-B, respectively. Potent compounds such as those reported here may act as leads for the future development of MAO-B specific inhibitors. Graphic abstract: The present study describes the MAO inhibitory activities of a series of 1-tetralone and 4-chromanone derivatives. Numerous high-potency MAO-B specific inhibitors were identified.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s11030-020-10143-w

Molecular Diversity
https://doi.org/10.1007/s11030-020-10143-w
ORIGINAL ARTICLE
The evaluation of 1‑tetralone and 4‑chromanone derivatives
as inhibitors of monoamine oxidase
Stephanus J. Cloete1 · Clarina I. N’Da1 · Lesetja J. Legoabe1 · Anél Petzer1,2 · Jacobus P. Petzer1,2
Received: 27 May 2020 / Accepted: 11 September 2020
© Springer Nature Switzerland AG 2020
Abstract
Monoamine oxidase (MAO) is of much clinical relevance, and inhibitors of this enzyme are used in the treatment for neu-
ropsychiatric and neurodegenerative disorders such as depression and Parkinson’s disease. The present study synthesises
and evaluates the MAO inhibition properties of a series of 33 1-tetralone and 4-chromanone derivatives in an attempt to
discover high-potency compounds and to expand on the structure–activity relationships of MAO inhibition by these classes.
Among these series, eight submicromolar MAO-A inhibitors and 28 submicromolar MAO-B inhibitors are reported, with
all compounds acting as specifc inhibitors of the MAO-B isoform. The most potent inhibitor was a 1-tetralone derivative
(1h) with IC50 values of 0.036 and 0.0011 µM for MAO-A and MAO-B, respectively. Interestingly, with the reduction of
1-tetralones to the corresponding alcohols, a decrease in MAO inhibition potency is observed. Among these 1-tetralol
derivatives, 1p (IC50 =0.785 μM) and 1o (IC50 =0.0075 μM) were identifed as particularly potent inhibitors of MAO-A
and MAO-B, respectively. Potent compounds such as those reported here may act as leads for the future development of
MAO-B specifc inhibitors.
Graphic abstract
The present study describes the MAO inhibitory activities of a series of 1-tetralone and 4-chromanone derivatives. Numer-
ous high-potency MAO-B specifc inhibitors were identifed.
Keywords 1-Tetralone · 4-Chromanone · Monoamine oxidase · Inhibition · Parkinson’s disease
Introduction
Electronic supplementary material The online version of this
The monoamine oxidase (MAO) enzymes are of much clini-
article (https://doi.org/10.1007/s11030-020-10143-w) contains
cal interest since they metabolise neurotransmitter and die-
tary amines in the peripheral and central tissues [1]. Amine
substrates undergo MAO-catalysed deamination to yield an
aldehyde, ammonia (or a substituted amine) and hydrogen
supplementary material, which is available to authorised users.
* Jacobus P. Petzer
jacques.petzer@nwu.ac.za
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Molecular Diversity
peroxide as products. Two MAO isoforms which are prod-
ucts of distinct genes are expressed in mammalian tissues,
MAO-A and MAO-B. These two isoforms are approximately
70% similar on the amino acid sequence level [2, 3]. Con-
siderable overlap in substrate specifcities occurs between
MAO-A and MAO-B, and dopamine, adrenaline, noradrena-
line and tyramine are substrates for both isoforms [4]. In
contrast, serotonin is a MAO-A specifc substrate, while the
arylalkylamines, benzylamine and 2-phenylethylamine are
specifc substrates for MAO-B.
shown to yield good MAO inhibition when substituted on
C6 and C7 of 1-tetralone [15–18].
4-Chromanone compounds are the corresponding pyra-
none analogues of 1-tetralone and with appropriate substi-
tution, may be expected to act as suitable scafolds for the
design of MAO inhibitors. In accordance with this, a recent
study shows that a series of 4-chromanones acts as potent
MAO inhibitors [19]. This study expands on the SARs of
MAO inhibition by 4-chromanone compounds by investigat-
ing a series substituted on C7 with a variety of substituents
(2a–q) (Table 2). In this respect, benzyloxy, 2-phenyleth-
oxy, 3-phenylpropoxy and 2-phenoxyethoxy substitution of
4-chromanone was considered. In particular, substitution
with halogens (Cl, Br and F) on the phenyl rings of these
moieties was investigated since such substitution has been
shown to be benefcial for MAO-B inhibition [19]. The C7
position was selected since substitution of 1-tetralones on
C7 yields signifcantly better MAO inhibition than substi-
tution on C6 [15, 16]. Examples of 1-tetralone (3–4) and
4-chromanone (5–6) compounds that have been reported to
be potent MAO inhibitors are given in Fig. 1.
Since inhibitors of MAO-A reduce the central catabo-
lism of serotonin (and to a lesser extent noradrenaline), these
compounds have been used as antidepressant and anxiolytic
agents for many decades [5]. MAO-B inhibitors, in turn,
block the central catabolism of dopamine and are established
treatment for symptomatic Parkinson’s disease [6]. In Par-
kinson’s disease, MAO-B inhibitors are combined with levo-
dopa, the metabolic precursor of dopamine, for the treatment
for motor fuctuations. MAO inhibitors may be of further
therapeutic relevance by reducing the formation of hydrogen
peroxide that is produced by the MAO catalytic cycle [7]. In
this regard, MAO inhibitors may protect susceptible neurons
in the brain from oxidative damage mediated by reactive
oxygen species (produced from hydrogen peroxide) and thus
act as potential neuroprotective agents in Parkinson’s disease
[1]. Similarly, MAO-A is a source of hydrogen peroxide in
the heart, and inhibitors may thus potentially protect against
the development and progression of heart disease by reduc-
ing hydrogen peroxide production and intracellular levels of
reactive oxygen species [8, 9]. Interestingly, MAO inhibitors
may have roles in the treatment for other disease states, most
notably prostate cancer, cardiovascular disease and infam-
matory diseases [10–12]. Furthermore, MAO inhibitors such
as phenelzine and tranylcypromine have been shown to lead
to disease remission in patients sufering from Crohn’s dis-
ease or rheumatoid arthritis [13, 14].
Results and discussion
Chemistry
In the frst step of the synthesis of the 1-tetralone derivatives,
the key starting materials, namely 5-, 6- and 7-hydroxy-
1-tetralone (8a–c), were synthesised from the correspond-
ing methoxy-1-tetralone derivatives (7a–c) by reaction with
aluminium chloride (AlCl3) (Fig. 2) [15, 16]. In the sub-
sequent step, the hydroxy derivatives were alkylated with
a benzyl bromide or 2-phenoxyethyl bromide to yield the
substituted 1-tetralone derivatives 1a–h [15, 16]. Reduction
with sodium borohydride (NaBH4) yielded the alcohol deriv-
atives 1i–p. The reduction of achiral carbonyl compounds
with sodium borohydride yields the racemic alcohols [20].
The 4-chromanone derivatives (2a–q) were synthesised
by alkylation of 7-hydroxy-4-chromanone (12) with an
appropriately substituted arylalkyl bromide in the presence
of potassium carbonate (Fig. 3). 7-Hydroxy-4-chromanone
(12), in turn, was obtained from reaction of resorcinol (9)
and 3-chloropropionic acid (10) in the presence of trifuo-
romethanesulfonic acid to give 2′,4′-dihydroxy-3-chloropro-
piophenone (11), which was further cyclised in NaOH to
give 12 [21].
Based on these considerations, much interest in the dis-
covery and development of MAO inhibitors exists in aca-
demia and industry [12]. Based on reports that 1-tetralone
derivatives act as MAO inhibitors, this study synthesised a
series of 1-tetralone derivatives in an efort to expand on the
structure–activity relationships (SARs) of MAO inhibition
by this class of compounds [15, 16]. In particular, the efect
on MAO inhibition of the reduction of the 1-tetralone car-
bonyl group to the corresponding alcohol was determined.
For this purpose, this study synthesises the derivatives 1a–p
shown in Table 1. The chemical modifcations that were
made to 1-tetralone included substitution on C5, C6 and C7
and reduction of the carbonyl to the corresponding alcohol.
The substituents considered were the benzyloxy and 2-phe-
noxyethoxy moieties. Both these substituents have been
shown to enhance the MAO inhibition potencies of cafeine
derivatives, while the benzyloxy side chain has also been
MAO inhibition studies
The series of tetralone and chromanone derivatives were
investigated as potential inhibitors of recombinant human
MAO-A and MAO-B [22]. For this purpose, kynuramine
1 3

Molecular Diversity
Table 1 IC50 values for the inhibition of recombinant human MAO-A and MAO-B by compounds 1a–p
IC50 (µM)a
MAO-A
R
Position
SIb
MAO-B
1a
1b
C6H5CH2O
C6H5CH2O
5
6
53.9 7.02
3.62 0.34
15
56
1.39 0.30
0.025 0.0029
(0.792d)
(0.063d)
1c
C6H5CH2O
7
0.623 0.021
(0.443d)
0.0024 0.0004
(0.0072d)
3.26 1.86
0.033 0.0053
0.0012 0.0003
260
1d
1e
1f
C6H5OCH2CH2O
C6H5OCH2CH2O
C6H5OCH2CH2O
4-ClC6H4CH2O
5
6
7
6
NIc
>31
78
1592
105
2.58 0.30
1.91 0.37
1g
0.575 0.018
0.0055 0.0006
(0.238d)
(0.007d)
1h
4-ClC6H4CH2O
7
0.036 0.0059
(0.033d)
0.0011 0.0002
(0.0041d)
33
1i
C6H5CH2O
C6H5CH2O
C6H5CH2O
C6H5OCH2CH2O
C6H5OCH2CH2O
C6H5OCH2CH2O
4-ClC6H4CH2O
4-ClC6H4CH2O
5
6
7
5
6
7
6
7
9.27 0.83
35.5 5.60
34.6 3.80
NIc
5.05 0.42
1.8
449
139
>60
>1471
>529
145
1j
0.079 0.012
0.249 0.061
1.66 0.14
0.068 0.0034
0.189 0.013
0.0075 0.0011
0.039 0.013
1k
1l
1m
1n
1o
1p
NIc
NIc
1.086 0.12
0.785 0.047
20
aAll values are expressed as the mean SD of triplicate determinations
bSelectivity index (SI)=IC50(MAO-A)/IC50(MAO-B)
cNo inhibition observed at maximum tested concentration of 100 µM
dValues from references [15, 16]
was used as enzyme–substrate for both MAO isoforms.
Kynuramine is oxidised by MAO to yield 4-hydroxyqui-
noline, which may be accurately measured and quanti-
tated by fuorescence spectrophotometry, as it fuoresces
in alkaline media [23, 24]. This procedure represents a
swift and dependable assay for the evaluation of MAO
activity. Typical enzyme reactions contained kynuramine
(50 µM), the test inhibitor (0.0003–100 µM) and DMSO as
co-solvent. The reactions were initiated with the addition
of MAO, incubated for 20 min and terminated with the
addition of sodium hydroxide. At endpoint, 4-hydroxyqui-
noline concentrations in the incubations were measured
and enzyme catalytic rates were calculated. Sigmoidal
curves of enzyme catalytic rate versus the logarithm of
inhibitor concentration were constructed from which the
IC50 values were determined (Fig. 4). The IC50 values for
the inhibition of MAO are presented in Tables 1 and 2.
MAO inhibition by the tetralone derivatives
It is clear that both the 1-tetralone (1a–h) and alcohol
(1i–p) derivatives are selective inhibitors of MAO-B over
the MAO-A isoform (Table 1). In this respect, the SI values
were > 1.8, indicating selectivity for MAO-B in all cases.
Among the compounds evaluated, 1d and 1l–n are particu-
larly selective and do not display inhibition of MAO-A at
a maximal tested concentration of 100 µM. Although the
1-tetralone and alcohol derivatives are MAO-B selective,
some compounds were found to be good-potency MAO-A
inhibitors. Compound 1h, in particular, is a high-potency
MAO-A inhibitor with an IC50 value of 0.036 µM. This is
108-fold more potent than the reference MAO-A inhibitor,
toloxatone (IC50 = 3.92 µM), which was evaluated under
identical experimental conditions [25]. This study also iden-
tifes three other compounds with submicromolar MAO-A
1 3

Molecular Diversity
Table 2 IC50 values for the
inhibition of recombinant
human MAO-A and MAO-B by
compounds 2a–q and 12
IC50 (µM)a
MAO-A
R
SIb
MAO-B
2a
C6H5CH2O
NIc
0.015 0.003
>6666
(18%e)
(0.057d)
2b
2c
2d
2e
C6H5(CH2)2O
C6H5(CH2)3O
C6H11(CH2)2O
4-BrC6H4CH2O
2.49 0.965
6.40 0.581
7.33 0.222
0.018 0.003
0.019 0.006
0.015 0.003
138
337
489
56
0.332 0.026
0.0059 0.002
(19%e)
(0.022d)
2f
4-ClC6H4CH2O
4-CH3C6H4CH2O
4-FC6H4CH2O
0.410 0.044
(37%e)
0.004 0.0008
(0.033d)
103
2g
2h
NIc
0.012 0.001
>8333
155
(26%e)
(0.091d)
1.04 0.107
0.0067 0.0001
(0.0086d)
(43%e)
2i
4-CF3C6H4CH2O
C6H5O(CH2)2O
4-ClC6H4O(CH2)2O
4-BrC6H4O(CH2)2O
4-CH3OC6H4(CH2)2O
4-ClC6H4(CH2)2O
4-FC6H4O(CH2)2O
4-CH3C6H4(CH2)2O
Isoamyl-O
3.33 0.101
NIc
0.0038 0.0001
0.014 0.003
0.008 0.002
0.030 0.002
1.06 0.078
0.043 0.00002
0.010 0.002
0.356 0.017
0.006 0.001
34.2 3.21
876
2j
>7142
>12,500
>3333
13
6.7
574
27
141
>2.9
2k
2l
NIc
NIc
2m
2n
2o
2p
2q
12
13.6 0.261
0.286 0.036
5.74 1.15
9.66 0.345
0.844 0.088
NIc
OH
a–cSee Table 1 for footnotes
dValues from Ref. [19]
ePercentage (%) inhibition at 100 µM [19]
inhibition potencies, compounds 1c (IC50 =0.623 µM), 1g
(IC50 =0.575 µM) and 1p (IC50 =0.785 µM). As mentioned,
the 1-tetralone and alcohol derivatives are MAO-B selec-
tive inhibitors. This study discovers several high-potency
MAO-B inhibitors. Among the 16 compounds evaluated,
ten compounds display IC50 <0.1 µM, while fve compounds
display IC50 < 0.01 µM. The most potent MAO-B inhibi-
tors are compounds 1f and 1h with IC50 values of 0.0012
and 0.0011 µM, respectively. These compounds are 75-fold
more potent than the reference MAO-B inhibitor, lazabe-
mide (IC50 =0.091 µM), which was evaluated under identical
experimental conditions [25].
Substitution on C6 in turn leads to more potent inhibi-
tion than substitution on C5 (e.g. 1b vs. 1a). This trend is
observed for all the 1-tetralone derivatives evaluated. For the
alcohol derivatives, a clear trend with regard to the position
of the substituent was not observed. It is, however, note-
worthy that the C7-substituted derivative 1p is more potent
than the corresponding C6-substituted derivative 1o. It is
also noteworthy that the derivatives containing chlorine are
more potent MAO-A inhibitors compared to the correspond-
ing unsubstituted homologues (e.g. 1h vs. 1c). This trend is
observed for all compounds evaluated, including the alcohol
derivatives (e.g. 1p vs. 1k).
From the MAO-A inhibition data, the following struc-
ture–activity relationships (SARs) may be derived. Among
the 1-tetralone derivatives, the position of the substituent
on the 1-tetralone moiety has a pronounced efect on the
inhibition potency with substitution on C7 yielding more
potent inhibition than substitution on C6 (e.g. 1c vs. 1b).
From the MAO-B inhibition data, the following SARs
may be derived. As for MAO-A, substitution on C7 yields
more potent inhibition than substitution on C6 (e.g. 1c vs.
1b). Substitution on C6 in turn leads to more potent inhi-
bition than substitution on C5 (e.g. 1b vs. 1a). This trend
is observed for all the 1-tetralone derivatives evaluated.
1 3

Molecular Diversity
For the alcohol derivatives, a clear trend with regard to the
position of the substituent was not observed. Interestingly,
among the alcohol derivatives, a C6-substituted derivative
1o (IC50 =0.0075 µM) is the most potent MAO-B inhibitor.
For MAO-B inhibition, the chlorine-containing compounds
also are more potent inhibitors compared to the unsubsti-
tuted derivatives (e.g. 1g vs. 1b, and 1o vs. 1j). This trend
is observed for both the 1-tetralone and alcohol derivatives.
A further SAR is that among the 1-tetralone derivatives,
the 2-phenoxyethoxy-substituted derivatives are weaker
MAO-A and MAO-B inhibitors than the corresponding
benzyloxy-substituted derivatives (e.g. 1d–f vs. 1a–c). The
exception here is that 1f is a higher potency MAO-B inhibi-
tor than 1c, while MAO-B inhibition by 1a and 1d is similar.
Among the alcohol derivatives, this trend is also observed
for MAO-A, but not for MAO-B inhibition (e.g. 1l–n vs.
1i–k). It is noteworthy that the alcohol derivatives substi-
tuted with the 2-phenoxyethoxy are not MAO-A inhibitors
with IC50 >100 µM. This group of compounds is thus highly
selective for MAO-B with 1m representing a particularly
potent MAO-B inhibitor.
Fig. 1 1-Tetralone (3–4) and 4-chromanone (5–6) compounds that
have been reported to act as MAO inhibitors [15, 16, 19]
Finally, it should be kept in mind that the 1-tetralol deriv-
atives are the racemic mixtures of two enantiomers. Consid-
ering that two enantiomers may exhibit diferent inhibition
potencies, the IC50 values given in Table 1 are the combina-
tion of the individual enantiomeric potencies. The higher
potency enantiomer may thus exhibit a much higher degree
of inhibition than that given in the results table. For this
reason, clear SARs and trends are not as readily observed for
the alcohol derivatives compared to the 1-tetralone deriva-
tives. Considering this, it is not surprising that with the
exception of 1a vs. 1i (MAO-A) and 1d vs. 1l (MAO-B), the
1-tetralone derivatives are in all instances more potent MAO
inhibitors than the corresponding alcohol derivatives. The
enantiomers of a number of inhibitors exhibit diferent inhi-
bition potencies towards the MAOs and include compounds
such as (R)-deprenyl, rasagiline, safnamide and a variety
of experimental compounds such as pyrazoline derivatives
[4, 26, 27].
Fig. 2 Reaction pathway for the synthesis of 1-tetralone derivatives
(1a–h), and subsequent reduction to the alcohol derivatives 1i–p.
Key: a AlCl3, toluene, refux, 1–2 h; b RBr, K2CO3, acetone, refux,
20 h; c NaBH4, ethanol, refux, 18 h
Fig. 3 Reaction pathway for
the synthesis of 4-chromanone
derivatives (2a–q). Key: a
CF3SO3H, 80 °C, 1.5 h; b
2 N NaOH, 0 °C, 4 h; c RBr,
K2CO3, acetone, refux, 20 h
1 3

Molecular Diversity
2-Phenoxyethoxy substitution on both C6 and C7 yields
similar MAO-B inhibition potencies compared to benzy-
loxy substitution.
100
75
50
25
0
MAO inhibition by chromanone derivatives
Table 2 gives the IC50 values for the inhibition of human
MAO-A and MAO-B by the 4-chromanone derivatives.
The 4-chromanones are specifc inhibitors of MAO-B with
15 compounds (of 17) displaying IC50 <0.1 µM. Only four
compounds have IC50 <1 µM for the inhibition of MAO-A,
with the most potent compound (2n) exhibiting an IC50 of
0.286 µM. While clear SARs for the inhibition of MAO-B
are not apparent, a variety of C7 substituents yield good-
potency inhibition, e.g. benzyloxy (2a, 2e–i), 2-phenyleth-
oxy (2b, 2m, 2n, 2p), 3-phenylpropoxy (2c), 2-cyclohexy-
lethoxy (2d), 2-phenoxyethoxy (2j–l, 2o) and isoamyloxy
(2q). This demonstrates that C7-substituted 4-chromanones
are a class of high-potency MAO-B specifc inhibitors.
The C7 substituent is a requirement for inhibition since
7-hydroxy-4-chromanone (12, IC50 = 34.2 µM) is a weak
MAO-B inhibitor. For the benzyloxy-substituted homo-
logues (2a, 2e–i), the presence of a halogen (e.g. 2e, 2f, 2h,
2i) leads to higher MAO-B inhibition potency compared to
the unsubstituted homologue (2a) and the methyl-substituted
compound (2g). This trend is not as apparent for the other
substituents considered, although the methoxy- and methyl-
substituted compounds are the weakest MAO-B inhibi-
tors among the 2-phenylethoxy (e.g. 2m, 2p)-substituted
4-chromanones.
-3
-2
-1
Log[I]
0
1
2
Fig. 4 Examples of sigmoidal plots obtained for the inhibition of
MAO-A by 1p (open circles) and MAO-B by 1o (flled circles)
Literature reports the MAO inhibition potencies of a
number of 1-tetralone derivatives that have been substi-
tuted on C6 and C7 with the benzyloxy side chain [15, 16].
When considering these as well as the 1-tetralones of the
current study, the following observations may be made:
(1) In most instances, substitution on the benzyloxy ring
with both halogens (F, Cl, Br, I) and alkyl groups (CH3,
CN, CF3) enhances MAO-B inhibition compared to the
unsubstituted derivative. A clear trend regarding prefer-
ence for meta vs. para substitution is not apparent, with
very similar inhibition potencies recorded for these two
positions. (2) For MAO-A inhibition, substitution on the
benzyloxy ring with both halogens (F, Cl, Br) and alkyl
groups (CN, CF3) also enhances potency compared to the
unsubstituted derivative. The notable exception is a signif-
cant reduction in MAO-A inhibition potency observed for
meta and para substitution with methyl and iodo groups
(with benzyloxy ring on C6), and para CF3 substitution
(with benzyloxy ring on C7). Again, no apparent trend
exists regarding preference for meta vs. para substitu-
tion. (3) In general, benzyloxy substitution on C7 results
in more potent MAO-A and MAO-B inhibition compared
to substitution on C6, which in turn is more benefcial
than substitution on C5. (4) While it would be speculative
to identify a group and position (meta vs. para) that is
most benefcial for MAO-B inhibition, bromo substitution
consistently leads to compounds with IC50 < 0.009 µM for
both positions. For MAO-A, substitution with the cyano
group on the meta position has been particularly benef-
cial. (5) It is clear that benzyloxy substitution on both
C6 and C7 is preferable for MAO-A inhibition compared
to substitution with the 2-phenylethoxy, 3-phenylpropoxy
and 2-phenoxyethoxy moieties. The only exception is
2-phenylethoxy substitution on C7. For MAO-B, 2-phe-
nylethoxy and 3-phenylpropoxy substitution on C6 yields
slightly better inhibition compared to benzyloxy substi-
tution, while on C7, the benzyloxy moiety is preferred.
The MAO inhibition potencies of a number of 4-chro-
manone derivatives have also been reported, with most being
substituted on C7 with the benzyloxy moiety [19]. When
considering these 4-chromanone derivatives and those of the
current study, the following observations may be made: (1)
Halogen (F, Cl, Br) substitution on both the meta and para
positions of the benzyloxy ring enhances MAO-B inhibi-
tion compared to the unsubstituted derivative. Here, para
substitution may be slightly more preferred than meta sub-
stitution. The efect of alkyl (CH3 and CN) substitution is
much smaller and in some instances even reduces MAO-B
inhibition potency (e.g. 4-CH3). The ortho fuorine-substi-
tuted compound is equipotent to the unsubstituted deriva-
tive. (2) In general, the 4-chromanones are weak MAO-A
inhibitors. Chloro and bromo substitution in the para posi-
tion, however, resulted in submicromolar inhibition. (3) In
an attempt to identify a group and position (meta vs. para)
that is most benefcial for MAO-B inhibition, halogen (F,
Cl, Br) substitution on the para position yielded compounds
with IC50 < 0.035 µM. As mentioned above, for MAO-A,
chloro and bromo substitution in the para position yielded
the best results. (4) It is interesting to note that besides ben-
zyloxy substitution, 2-phenylethoxy, 3-phenylpropoxy and
1 3

Molecular Diversity
2-phenoxyethoxy substitution also leads to high-potency
MAO-B inhibition, with these compounds exhibiting almost
equipotent activities. With the exception of para chloro and
fuoro substitution, further substitution with halogens and
the methyl group does not signifcantly enhance MAO-B
inhibition by the 2-phenoxyethoxy-substituted compounds.
Para methyl substitution leads to a notable reduction in
MAO-B inhibition. It is noteworthy that substitution with
the isoamyloxy moiety (2q) results in particularly potent
MAO-B inhibition, while it also yields good-potency
MAO-A inhibition. (5) 2-Phenylethoxy and 3-phenylpro-
poxy substitution resulted in improved MAO-A inhibition
compared to benzyloxy and 2-phenoxyethoxy substitution. It
is particularly noteworthy that para chloro substitution of the
2-phenylethoxy-substituted compound 2b yielded the most
potent MAO-A inhibitor among the 4-chromanones of the
current study, compound 2n (IC50 =0.286 µM).
inhibition, enzyme activity is quickly recovered after drug
withdrawal. Reversibility is thus a desired property in the
design of MAO inhibitors. Reversibility is often evaluated
by measuring the degree of recovery of enzyme activity
after dialysis of mixtures containing the test inhibitor and
enzyme [23]. In this study, 1-tetralol compounds 1p and 1o
were selected as representative inhibitors for the evalua-
tion of the reversibility of MAO-A and MAO-B inhibition,
respectively.
MAO-A and MAO-B were frstly incubated with 1p and
1o, respectively, at concentrations equal to 4 × IC50. The
samples were subsequently dialysed for 24 h and diluted
twofold to yield an inhibitor concentration of 2×IC50, and
the residual enzyme activity was measured. As positive con-
trols, MAO-A and MAO-B were similarly incubated and dia-
lysed in the presence of the irreversible inhibitors, pargyline
and (R)-deprenyl, respectively, while for the negative con-
trol, this experiment was carried out in the absence of inhibi-
tor. For comparison, undialysed mixtures of the enzyme and
test inhibitors were maintained over the same 24 h period
and diluted and assayed as above.
Reversibility of MAO inhibition
To determine the reversibility of MAO inhibition, the
most potent MAO inhibitors among the 1-tetralol deriva-
tives were selected as representative inhibitors. Alcohol
derivatives were selected since the reversibility of inhi-
bition of 1-tetralone and 4-chromanone derivatives has
been investigated before [16, 19]. Reversibility of MAO
inhibition is an important consideration since compounds
which are reversible inhibitors, especially of MAO-A,
have a reduced risk for causing the cheese reaction com-
pared to irreversible inhibitors. Furthermore, for reversible
After dialysis of mixtures of 1p and MAO-A, an almost
complete recovery of MAO-A activity could be observed
with the activity of MAO-A recovering to 81% of the control
value (recorded in the absence of inhibitor) (Fig. 5). MAO-A
inhibition by compound 1p is therefore reversible. In con-
trast, in undialysed mixtures of 1p and MAO-A, the residual
activity was only 34%. Dialysis of mixtures of pargyline and
MAO-A resulted in no recovery of enzyme activity with
residual activity at 1.9%.
(b) MAO-B
(a) MAO-A
100
100
75
50
25
0
75
50
25
0
NI
1o depr
dialysed
1o
NI
1p parg
dialysed
1p
undialysed
undialysed
Fig. 5 Reversibility of inhibition of MAO-A (a) and MAO-B (b) by
compounds 1p and 1o, respectively. MAO-A was preincubated in the
absence of inhibitor (NI—dialysed) and the presence of 1p (1p—
dialysed) and pargyline (parg—dialysed), and MAO-B was preincu-
bated in the absence of inhibitor (NI—dialysed) and presence of 1o
(1o—dialysed) and (R)-deprenyl (depr—dialysed). After dialysis, the
residual enzyme activities were measured. For comparison, the MAO
activities of undialysed mixtures of the MAOs and the test inhibitors
were also measured (1p/1o—undialysed)
1 3

Molecular Diversity
The inhibition of MAO-B by 1o also was reversible since
dialysis of mixtures of 1o and MAO-B resulted in recovery
of enzyme activity to 76% of the control value (Fig. 5). For
undialysed mixtures of 1o and MAO-B, the residual enzyme
activity was only 48%. Dialysis of mixtures of (R)-deprenyl
and MAO-B did not lead to recovery of enzyme activity with
residual enzyme activity at only 3.5%.
(a)
Competitive mode of MAO inhibition
The modes of inhibition (e.g. competitive or noncompeti-
tive) of compounds 1p and 1o were examined using sets
of Lineweaver–Burk plots. Compounds 1p and 1o were
selected as representative inhibitors since they were the most
potent MAO-A and MAO-B inhibitors, respectively, among
the 1-tetralol derivatives, while the modes of inhibition have
been investigated for tetralones [16]. The catalytic rates of
MAO-A and MAO-B were measured at eight diferent kynu-
ramine concentrations (15–250 µM) in the absence and pres-
ence of fve diferent concentrations of the test inhibitor (e.g.
¼×IC50, ½×IC50, ¾×IC50, 1×IC50 and 1¼×IC50).
Examination of the Lineweaver–Burk plots shows that
for both 1p and 1o, the lines are linear and intersect on the
y-axis (Fig. 6). This is indicative of competitive inhibition
and supports the fnding that 1p and 1o are reversible inhibi-
tors of MAO-A and MAO-B, respectively. From the replots
of the slopes of the Lineweaver–Burk plots versus inhibitor
concentration, Ki values of 1.0 µM and 0.0065 µM are esti-
mated for the inhibition of MAO-A and MAO-B by 1p and
1o, respectively.
(b)
Fig. 6 Lineweaver–Burk plots for the activities of MAO-A (a) and
MAO-B (b) in the absence and presence of 1p and 1o, respectively.
The insets are replots of the slopes of the Lineweaver-Burk plots ver-
sus inhibitor concentration
Conclusion
This study investigates the MAO inhibition properties of
a series of 1-tetralone and 4-chromanone derivatives and
fnds that these compounds are specifc inhibitors of the
MAO-B isoform. Among these series, 28 MAO-B inhibi-
tors with submicromolar inhibition potencies are reported.
Good-potency MAO-A inhibitors have also been identifed,
with eight compounds possessing submicromolar IC50 val-
ues. Highly potent MAO-B inhibitors were present among
both series with 11 compounds exhibiting IC50 <0.01 µM.
Reduction of 1-tetralones to the corresponding alco-
hols also yielded potent inhibition with, for example, 1p
(IC50 =0.785 µM) and 1o (IC50 =0.0075 µM) being good-
potency inhibitors of MAO-A and MAO-B, respectively.
These two compounds are reversible and competitive MAO
inhibitors with Ki values of 1.0 µM and 0.0065 µM, respec-
tively. Interestingly, the small molecule inhibitor 12 is a
weak MAO inhibitor, which demonstrates the requirement
for a substituent (e.g. benzyloxy, 2-phenylethoxy, 2-phe-
noxyethoxy) for MAO inhibition.
Considering the MAO inhibition data of the 1-tetralone
compounds of this study and those that have been reported
in the literature, the following key conclusions may be made:
(1) It is clear that substitution on C7 yields more potent inhi-
bition of both MAO-A and MAO-B than substitution on C6,
which in turn leads to more potent inhibition than substitu-
tion on C5. From a potency point of view, C7-substitution
is thus most advantageous for MAO inhibition. (2) For both
MAO-A and MAO-B, the chlorine-containing compounds
are more potent inhibitors compared to the unsubstituted
derivatives. This is observed for both the 1-tetralone and
alcohol derivatives. (3) It is clear that both the 1-tetralone
and alcohol derivatives are selective inhibitors of MAO-B
over the MAO-A isoform.
Although the specifc molecular interactions between the
MAOs and the 1-tetralone (and alcohol) derivatives are not
1 3

Molecular Diversity
known, the reported crystal structures of the enzymes may
provide some insight [28–30]. The active site of MAO-B is
composed of an entrance cavity that leads to the substrate
binding cavity where the favin co-factor is located. While
smaller substrates and inhibitors bind within the substrate
cavity, larger compounds exhibit cavity-spanning binding
modes and bind to both cavities. For example, safnamide
binds with the polar amide in the substrate cavity, while the
fuorobenzyloxy side chain extends into the entrance cav-
ity [31]. The inhibitor therefore forms some polar interac-
tions (e.g. hydrogen bonding) in the substrate cavity, while
interactions in the entrance cavity are largely van der Waals
interactions. Based on this, it seems likely that the inhibitors
of the current study exhibit a similar binding mode to MAO-
B. The polar moiety (e.g. 1-tetralone, 1-tetralol and 4-chro-
manone) thus may bind within the substrate cavity, and the
substituent (e.g. benzyloxy, 2-phenylethoxy, 2-phenoxyeth-
oxy) extends into the entrance cavity. It could be proposed
that C7 substitution yields cavity-spanning inhibitors that
are better accommodated by the MAO-B active site and it
forms more productive interactions compared to C6 and C5
substitution. For this reason, C7 substitution of 1-tetralone is
more optimal for MAO-B inhibition. It may also be expected
that the addition of a chlorine to the side chain will increase
MAO-B inhibition potency since this will lead to additional
van der Waals interactions. Similarly, the good MAO-B inhi-
bition observed with the 4-chromanone derivatives may be
dependent on the enhanced polarity of this moiety (com-
pared to 1-tetralone) and thus the increased potential for
polar interactions in the MAO-B substrate cavity.
It must be acknowledged that while the above consid-
eration of the MAO active sites does provide some insight
into the specificities and potencies of the study com-
pounds, it cannot explain all the inhibition data presented
here. For example, small structural modifcations (e.g. 2b
vs. 2m) may result in a large efect on MAO-B inhibition
potency. For 2b vs. 2m, with the addition of the methoxy
group, it may have been expected that MAO-B inhibition
potency should increase. In contrast, a large reduction in
MAO-B inhibition was observed. Similarly, some com-
pounds exhibit good MAO-A inhibition (e.g. 1h, 2n and
others) although being relatively large molecules. Consid-
ering the limited size of the active site of MAO-A, it may
have been expected that such compounds would not act as
MAO-A inhibitors. These unexpected fndings are prob-
ably the result of complex processes that involve induced
ft and multiple interactions that cannot be predicted.
Experimental section
Materials and methods
Starting materials (reagents and solvents) that were used
in this study were obtained from Sigma-Aldrich and were
used without further purifcation. TLC was carried out to
determine whether the reactions proceeded to completion
and were carried out using silica gel sheets containing
UV254 fuorescent indicator. The developed TLC sheets
were observed under an UV lamp at a wavelength of
254 nm. A Büchi B-545 melting-point apparatus was used
to measure the melting points of all of the synthesised
compounds. High-resolution mass spectra (HRMS) were
obtained with a Bruker micrOTOF-Q II mass spectrom-
eter in atmospheric-pressure chemical ionisation (APCI)
mode. A Bruker Avance III 600 spectrometer was used
to record proton (1H) and carbon (13C) NMR spectra at
frequencies of 600 MHz and 150 MHz, respectively. NMR
measurements were conducted in CDCl3 or DMSO-d6, and
the chemical shifts are reported in parts per million (δ).
Chemical shifts were referenced to the residual solvent
signals for CDCl3 at 7.26 ppm and 77.16 ppm, and for
DMSO-d6 at 2.50 and 39.52 ppm for 1H and 13C, respec-
tively. Spin multiplicities are given as s (singlet), d (dou-
blet), dd (doublet of doublets), dt (doublet of triplets), t
(triplet) or m (multiplet). The coupling constants (J) are
given in Hz. Microsomes from insect cells containing
recombinant human MAO-A and MAO-B (5 mg protein/
mL), kynuramine dihydrobromide, (R)-deprenyl and par-
gyline were obtained from Sigma-Aldrich. A Varian Cary
Eclipse fuorescence spectrophotometer was used to deter-
mine fuorescence intensities.
In contrast to MAO-B, the active site of MAO-A is com-
posed of a single cavity and accommodates smaller inhibi-
tors better than larger compounds [28]. This is due to restric-
tions imposed by Phe-208 on the binding of larger inhibitors
to MAO-A. It may thus be expected that the relatively larger
compounds of this study will be less well accommodated
in the MAO-A active site compared to MAO-B, which
explains the specifcity of MAO-B inhibition by the study
compounds.
The large degree of MAO-B specifcity of compounds
2j, 2k and 2l cannot be explained by only considering the
dimensions of the MAO-A vs. MAO-B active sites. Based
on the limited space available in the MAO-A active site,
it could have been anticipated that these larger inhibitors
would not be able to bind to MAO-A and therefore act as
specifc MAO-B inhibitors. As pointed out above, some
compounds such as 1h and 2n exhibit good MAO-A inhibi-
tion although being relatively large molecules. While gener-
alisations regarding optimal structural features for inhibitor
binding can be useful, the complexities of ligand interac-
tions, induced ft and the similarity between the MAO-A and
MAO-B active sites could have unpredictable consequences
on inhibitor binding and interaction.
1 3

Molecular Diversity
Purity determination of synthesised compounds
Synthesis of 1‑tetralol derivatives (1i–p)
HPLC analyses were conducted with an Agilent 1200 HPLC
system equipped with a quaternary pump and an Agilent
1200 series diode array detector. The HPLC analyses were
used to determine the purities of the synthesised compounds.
HPLC-grade acetonitrile (Merck) and Milli-Q water (Mil-
lipore) were used for the chromatography. A Venusil XBP
C18 column (4.60×150 mm, 5 μm) was used for the sepa-
ration, and the mobile phase consisted at the start of each
run of 30% acetonitrile and 70% Milli-Q water. The fow
rate was set to 1 ml/min. At the start of each run, a solvent
gradient program was initiated. The composition of the ace-
tonitrile in the mobile phase was increased linearly to 80%
over a period of 5 min. Each HPLC run lasted 15 min, and a
time period of 5 min was allowed for equilibration between
runs. A volume of 20 µL of solutions of the test compounds
(1 mM dissolved in acetonitrile) was injected into the HPLC
system, and the eluent was monitored at a wavelength of
254 nm.
In a round-bottom flask, 1-tetralone derivatives 1a–h
(10 mmol) were dissolved in 100 mL ethanol. Sodium
borohydride (20 mmol) was added, and the reaction was
heated under refux for 18 h. Silica gel TLC was used to
determine whether the reactants have been consumed indi-
cating completion of the reaction [ethyl acetate (30%),
petroleum ether (70%)]. The solvent was removed under
reduced pressure, and water (100 mL) was cautiously
added to the reaction. The resulting mixture was extracted
to dichloromethane (3 × 75 mL), the organic phase was
dried over anhydrous MgSO4 and the organic phase was
evaporated under reduced pressure. The residue was sub-
sequently purifed by recrystallisation from cyclohexane
to give the target alcohols 1i–p.
Synthesis of 7‑hydroxy‑4‑chromanone (12)
To a mixture of resorcinol (9, 10.0 mmol) and 3-chloro-
propionic acid (10, 10.1 mmol) was slowly added trifuo-
romethanesulfonic acid (5 mL). The solution was stirred at
80 °C for 1.5 h and poured into dichloromethane (100 mL).
The resulting solution was poured into water (100 mL),
and the aqueous layer was extracted to dichloromethane
(2×100 mL). The combined organic layers were dried over
anhydrous MgSO4 and evaporated under reduced pressure.
The resulting product (11) was used directly in the next reac-
tion without further purifcation. NaOH (2 N; 400 mL) was
added to the crude product, and the resulting mixture was
stirred at 0 °C for 4 h. After the reaction has completed, the
pH was adjusted to 2 with concentrated HCl. The reaction
was extracted to ethyl acetate (3× 100 mL), washed with
brine, dried over anhydrous MgSO4 and concentrated under
reduced pressure. The desired product, 7-hydroxy-4-chro-
manone (12), was obtained by recrystallisation from ethyl
acetate/n-hexane (1:5) [21].
Synthesis of 5‑, 6‑ and 7‑hydroxy‑1‑tetralones (8a–c)
The key starting materials, 5-, 6- and 7-hydroxy-1-tetralone
(8a–c), were synthesised as reported in the literature [15].
The corresponding methoxy-1-tetralone derivatives 7a–c
(10 mmol) were added to aluminium chloride (AlCl3)
(25 mmol) and toluene (50 mL) in a round-bottom fask.
The reaction was heated under refux for 1 h. Silica gel
TLC was used to determine whether the reaction has pro-
ceeded to completion [ethyl acetate (30%), petroleum ether
(70%)]. The reaction was placed on ice for 30 min, and water
(75 mL) was cautiously added to the reaction. The mixture
was subsequently extracted to ethyl acetate (3× 75 mL).
The combined organic phases were dried over anhydrous
MgSO4, the organic phase was evaporated under reduced
pressure and the crude product was purifed by recrystallisa-
tion from ethyl acetate.
Synthesis of substituted 1‑tetralone derivatives
(1a–h)
Synthesis of substituted 4‑chromanone derivatives
(2a–q)
5-, 6- or 7-Hydroxy-1-tetralone (8a–c) (10 mmol) was
added to a round-bottom fask containing 100 mL acetone.
Anhydrous potassium carbonate (K2CO3) (20 mmol) and
the appropriate substituted alkyl bromide (11.2 mmol) were
added. The reaction was heated under refux for 20 h. Sil-
ica gel TLC was used to determine whether the reactants
have been consumed indicating completion of the reaction
[ethyl acetate (30%), petroleum ether (70%)]. The remain-
ing K2CO3 was removed from the reaction by fltration. The
organic phase was evaporated under reduced pressure, and
the residue was recrystallised from cyclohexane [15].
A mixture of 7-hydroxy-4-chromanone (12, 11.3 mmol), an
appropriately substituted arylalkyl bromide (11.6 mmol),
and K2CO3 (15.2 mmol) in acetone (110 mL) was heated
under refux for 20 h. The reaction was cooled and fltered
through a pad of Celite. The fltrate was evaporated under
reduced pressure, and the residue was dissolved in ethyl ace-
tate. The organic phase was washed with water and brine,
dried over MgSO4, concentrated under reduced pressure and
dried at 40 °C. Recrystallisation from ethylacetate/n-hexane
(1:5) yielded the product.
1 3

Molecular Diversity
5‑Benzyloxy‑1‑tetralone (1a)
22.90, 22.50. APCI-HRMS m/z: calculated for C18H19O3
(MH+), 283.1329, found 283.1349.
Melting point: oil; purity: 98.5%; yield: 38.3% (final
1
step). H NMR (600 MHz, chloroform-d) δ 7.60 (dd,
6‑(2‑Phenoxyethoxy)‑1‑tetralone (1e)
J = 7.9, 1.1 Hz, 1H), 7.39–7.35 (m, 2H), 7.35–7.31 (m,
2H), 7.31–7.24 (m, 1H), 7.21–7.15 (m, 1H), 7.01 (dd,
J = 8.1, 1.1 Hz, 1H), 5.03 (s, 2H), 2.90 (t, J = 6.2 Hz,
2H), 2.59–2.53 (m, 2H), 2.08–2.01 (m, 2H). 13C NMR
(151 MHz, CDCl3) δ 198.65, 155.90, 136.81, 133.95,
133.79, 128.63, 128.04, 127.20, 126.73, 119.11, 115.76,
70.33, 38.86, 23.04, 22.54. APCI-HRMS m/z: calculated
for C17H17O2 (MH+), 253.1223, found 253.1216.
Melting point: 107.0–128.1 °C; purity: 98.2%; yield (fnal
1
step): 60.2%. H NMR (600 MHz, CDCl3) δ 7.94 (d,
J = 8.7 Hz, 1H), 7.27–7.20 (m, 2H), 6.96–6.85 (m, 3H),
6.80 (dd, J = 8.7, 2.6 Hz, 1H), 6.69 (d, J = 2.5 Hz, 1H),
4.33–4.24 (m, 4H), 2.85 (t, J=6.1 Hz, 2H), 2.57–2.51 (m,
2H), 2.08–2.00 (m, 2H). 13C NMR (151 MHz, CDCl3) δ
197.20, 162.56, 158.46, 146.96, 129.68, 129.55, 126.62,
121.25, 114.66, 113.47, 113.42, 66.61, 66.20, 38.92, 38.16,
31.26, 30.15, 29.71, 23.37, 14.14. APCI-HRMS m/z: calcu-
lated for C18H19O3 (MH+), 283.1329, found 283.1313.
6‑Benzyloxy‑1‑tetralone (1b)
Melting point: 100.6–106.2 °C, lit. 100.9–102.1 °C
7‑(2‑Phenoxyethoxy)‑1‑tetralone (1f)
1
[15]; purity: 99.8%; yield (fnal step): 76.3%. H NMR
(600 MHz, CDCl3) δ 7.93 (d, J = 8.8 Hz, 1H), 7.37–7.27
(m, 4H), 7.30–7.24 (m, 1H), 6.82 (dd, J =8.7, 2.6 Hz, 1H),
6.71 (d, J = 2.4 Hz, 1H), 5.03 (s, 2H), 2.84 (t, J = 6.1 Hz,
2H), 2.53 (t, J=7.0 Hz, 2H), 2.07–1.99 (m, 2H). 13C NMR
(151 MHz, CDCl3) δ 197.18, 162.69, 146.96, 136.23,
129.68, 128.70, 128.23, 127.48, 126.53, 113.70, 113.59,
70.06, 38.92, 30.17, 23.37. APCI-HRMS m/z: calculated
for C17H17O2 (MH+), 253.1223, found 253.1202.
Melting point: 115.6–118.4 °C; purity: 99.7%; yield (fnal
1
step): 83.7%. H NMR (600 MHz, CDCl3) δ 7.49 (d,
J=2.8 Hz, 1H), 7.25–7.18 (m, 2H), 7.10 (d, J=8.4 Hz, 1H),
7.04 (dd, J=8.4, 2.8 Hz, 1H), 6.92–6.85 (m, 3H), 4.31–4.27
(m, 2H), 4.27–4.22 (m, 2H), 2.83 (t, J=6.1 Hz, 2H), 2.56 (t,
J=6.8 Hz, 2H), 2.11–2.00 (m, 2H). 13C NMR (151 MHz,
CDCl3) δ 198.31, 158.57, 157.37, 137.57, 133.35, 130.11,
129.51, 122.48, 121.13, 114.70, 109.84, 66.79, 66.37, 39.00,
28.91, 23.49. APCI-HRMS m/z: calculated for C18H19O3
(MH+), 283.1329, found 283.1339.
7‑Benzyloxy‑1‑tetralone (1c)
6‑(4‑Chlorobenzyloxy)‑1‑tetralone (1g)
Melting point: 87.6–88.3 °C, lit. 86.4–87.1 °C [16]; purity:
1
99.7%; yield (fnal step): 77.2%. H NMR (600 MHz,
Melting point: 96.1–98.2 °C, lit. 96.6–98.7 °C [15]; purity:
1
CDCl3) δ 7.54 (d, J=2.8 Hz, 1H), 7.39–7.34 (m, 2H), 7.31
(t, J = 7.8 Hz, 2H), 7.28–7.23 (m, 1H), 7.09 (d, J= 8.4 Hz,
1H), 7.05 (dd, J = 8.4, 2.8 Hz, 1H), 5.01 (s, 2H), 2.82 (t,
J = 6.1 Hz, 2H), 2.56 (t, J = 6.9 Hz, 2H), 2.07–2.00 (m,
2H). 13C NMR (151 MHz, CDCl3) δ 198.31, 157.48,
137.41, 136.68, 133.39, 130.07, 128.60, 128.04, 127.58,
122.36, 110.27, 70.17, 39.01, 28.92, 23.49. APCI-HRMS
m/z: calculated for C17H17O2 (MH+), 253.1223, found
253.1243.
99.7%; yield (final step): 48.2%. H NMR (600 MHz,
CDCl3) δ 7.94 (d, J=8.7 Hz, 1H), 7.32–7.26 (m, 4H), 6.80
(dd, J = 8.7, 2.6 Hz, 1H), 6.69 (d, 1H), 5.00 (s, 2H), 2.84
(t, J=6.1 Hz, 2H), 2.53 (t, J=6.8 Hz, 2H), 2.08–2.00 (m,
2H). 13C NMR (151 MHz, CDCl3) δ 197.13, 162.38, 146.98,
134.73, 134.05, 129.72, 128.88, 128.76, 126.69, 113.63,
113.59, 69.24, 38.90, 30.15, 23.34. APCI-HRMS m/z: cal-
culated for C17H16ClO2 (MH+), 287.0833, found 287.0857.
7‑(4‑Chlorobenzyloxy)‑1‑tetralone (1h)
5‑(2‑Phenoxyethoxy)‑1‑tetralone (1d)
Melting point: 93.6–95.8 °C, lit. 92.7–93.6 °C [16]; purity:
1
99.8%; yield (final step): 73.4%. H NMR (600 MHz,
Melting point: 135.6–144.0 °C; purity: 99.8%; yield (fnal
CDCl3) δ 7.51 (d, J=2.8 Hz, 1H), 7.32–7.25 (m, 4H), 7.10
(d, J = 8.3 Hz, 1H), 7.03 (dd, J = 8.4, 2.8 Hz, 1H), 4.98
(s, 2H), 2.83 (t, J = 6.1 Hz, 2H), 2.56 (t, J= 6.5 Hz, 2H),
2.08–2.00 (m, 2H). 13C NMR (151 MHz, CDCl3) δ 198.24,
157.19, 137.60, 135.20, 133.82, 133.40, 130.15, 128.85,
128.77, 122.32, 110.24, 69.34, 38.98, 28.91, 23.46. APCI-
HRMS m/z: calculated for C17H16ClO2 (MH+), 287.0833,
found 287.0839.
1
step): 66.6%. H NMR (600 MHz, CDCl3) δ 7.60 (d,
J = 7.9 Hz, 1H), 7.26–7.21 (m, 2H), 7.21–7.17 (m, 1H),
6.99 (d, J=8.0 Hz, 1H), 6.92 (d, J=7.1 Hz, 1H), 6.89 (d,
J=8.4 Hz, 2H), 4.28 (s, 4H), 2.82 (t, J=6.2 Hz, 2H), 2.54
(t, J=6.4 Hz, 2H), 2.05–1.98 (m, 2H). 13C NMR (151 MHz,
CDCl3) δ 198.64, 158.63, 155.91, 134.05, 133.79, 129.57,
126.71, 121.21, 119.32, 115.68, 114.69, 67.29, 66.46, 38.85,
1 3

Molecular Diversity
5‑Benzyloxy‑1‑tetralol (1i)
6‑(2‑Phenoxyethoxy)‑1‑tetralol (1m)
Melting point: 86.1–89.9 °C; purity: 99.6%; yield (fnal
Melting point: 116.8–117.7 °C; purity: 98.3%; yield (fnal
1
1
step): 43.8%. H NMR (600 MHz, CDCl3) δ 7.35 (d,
step): 44.9%. H NMR (600 MHz, CDCl3) δ 7.26 (d,
J=7.2 Hz, 2H), 7.30 (t, J=7.3 Hz, 1H), 7.28–7.20 (m, 1H),
7.09 (t, J=7.9 Hz, 1H), 7.00 (d, J=7.7 Hz, 1H), 6.73 (dd,
J=8.0, 1.1 Hz, 1H), 4.99 (s, 2H), 4.69 (s, 1H), 2.81–2.73
(m, 1H), 2.60–2.51 (m, 1H), 1.93–1.82 (m, 2H), 1.82–1.76
(m, 1H), 1.75–1.66 (m, 2H). 13C NMR (151 MHz, CDCl3)
δ 156.15, 140.20, 137.38, 128.54, 127.79, 127.08, 126.53,
126.51, 120.84, 110.08, 69.82, 68.20, 31.76, 23.24, 18.08.
APCI-HRMS m/z: calculated for C17H18O2 (M+), 254.1307,
found 254.1295.
J = 8.5 Hz, 1H), 7.26–7.18 (m, 2H), 6.92–6.84 (m, 3H),
6.74 (dd, J = 8.5, 2.7 Hz, 1H), 6.60 (d, J = 2.5 Hz, 1H),
4.69–4.64 (m, 1H), 4.23 (s, 4H), 2.71 (dt, J=17.3, 4.3 Hz,
1H), 2.66–2.57 (m, 1H), 1.93–1.79 (m, 2H), 1.73–1.64 (m,
1H), 1.61 (s, 1H). 13C NMR (151 MHz, CDCl3) δ 158.60,
157.94, 138.69, 131.70, 130.12, 129.51, 121.10, 114.70,
114.31, 113.15, 67.65, 66.51, 66.42, 32.40, 29.61, 18.59.
APCI-HRMS m/z: calculated for C18H20O3 (M+), 284.1412,
found 284.1384.
6‑Benzyloxy‑1‑tetralol (1j)
7‑(2‑Phenoxyethoxy)‑1‑tetralol (1n)
Melting point: 59.5–93.5 °C; purity: 98.9%; yield (fnal
Melting point: 103.4–106.5 °C; purity: 98.6%; yield (fnal
1
1
step): 70.5%. H NMR (600 MHz, DCl3) δ 7.36–7.31 (m,
step): 50.5%. H NMR (600 MHz, CDCl3) δ 7.27–7.19
2H), 7.29 (t, J=8.2 Hz, 2H), 7.25–7.21 (m, 2H), 6.75 (dd,
J=8.5, 2.7 Hz, 1H), 6.62 (d, J=2.7 Hz, 1H), 4.95 (s, 2H),
4.64 (t, J=4.3 Hz, 1H), 2.74–2.66 (m, 1H), 2.65–2.55 (m,
1H), 1.91–1.77 (m, 2H), 1.73–1.61 (m, 2H). 13C NMR
(151 MHz, CDCl3) δ 158.11, 138.68, 137.08, 131.55,
130.13, 128.60, 127.95, 127.45, 114.45, 113.26, 69.95,
67.65, 32.40, 29.63, 18.61. APCI-HRMS m/z: calculated
for C17H18O2 (M+), 254.1307, found 254.1277.
(m, 2H), 6.99–6.92 (m, 2H), 6.92–6.83 (m, 3H), 6.75 (dd,
J=8.4, 2.7 Hz, 1H), 4.69–4.62 (m, 1H), 4.24 (d, J=2.2 Hz,
4H), 2.68 (dt, J = 16.4, 5.9 Hz, 1H), 2.63–2.53 (m, 1H),
1.99–1.82 (m, 2H), 1.82–1.73 (m, 1H), 1.72–1.62 (m, 2H).
13C NMR (151 MHz, CDCl3) δ 158.63, 157.04, 139.91,
130.01, 129.60, 129.50, 121.07, 115.08, 114.71, 113.63,
68.48, 66.67, 66.48, 32.42, 28.46, 19.20. APCI-HRMS m/z:
calculated for C18H20O3 (M+), 284.1412, found 284.1410.
7‑Benzyloxy‑1‑tetralol (1k)
6‑(4‑Chlorobenzyloxy)‑1‑tetralol (1o)
Melting point: 57.2–59.9 °C; purity: 99.9%; yield (fnal
Melting point: 91.6–97.0 °C; purity: 99.5%; yield (fnal
step): 30.4%. 1H NMR (600 MHz, CDCl3) δ 7.30–7.25 (m,
5H), 6.74 (dd, J=8.5, 2.7 Hz, 1H), 6.61 (d, J=2.5 Hz, 1H),
4.93 (s, 2H), 4.70–4.64 (m, 1H), 2.71 (dt, J=17.2, 5.3 Hz,
1H), 2.65–2.57 (m, 1H), 1.91–1.85 (m, 1H), 1.88–1.79
(m, 1H), 1.73–1.64 (m, 1H), 1.57 (d, J =5.7 Hz, 1H). 13C
NMR (151 MHz, CDCl3) δ 157.82, 138.74, 135.58, 133.70,
131.75, 130.14, 128.75, 128.71, 114.45, 113.21, 69.15,
67.64, 32.39, 29.61, 18.58. APCI-HRMS m/z: calculated
for C17H17ClO2 (M+), 288.0917, found 288.0925.
1
step): 79.9%. H NMR (600 MHz, CDCl3) δ 7.38–7.33
(m, 2H), 7.33–7.27 (m, 1H), 7.27–7.20 (m, 1H), 7.00 (d,
J=2.7 Hz, 1H), 6.94 (d, J=8.6 Hz, 1H), 6.77 (dd, J=8.4,
2.7 Hz, 1H), 4.97 (d, J=1.8 Hz, 2H), 4.65 (t, J=5.3 Hz, 1H),
2.71–2.63 (m, 1H), 2.62–2.54 (m, 1H), 1.96–1.81 (m, 2H),
1.82–1.74 (m, 1H), 1.72–1.63 (m, 1H). 13C NMR (151 MHz,
CDCl3) δ 157.20, 139.89, 137.15, 129.99, 129.44, 128.57,
127.91, 127.50, 115.11, 113.80, 70.08, 68.48, 32.41, 28.47,
19.20. APCI-HRMS m/z: calculated for C17H18O2 (M+),
254.1307, found 254.1305.
7‑(4‑Chlorobenzyloxy)‑1‑tetralol (1p)
5‑(2‑Phenoxyethoxy)‑1‑tetralol (1l)
Melting point: 95.4–98.2 °C; purity: 99.6%; yield (fnal
step): 54.7%. 1H NMR (600 MHz, chloroform-d) δ 7.31–7.24
(m, 4H), 6.98 (d, J = 2.7 Hz, 1H), 6.94 (d, J = 8.4 Hz,
1H), 6.74 (dd, J = 8.4, 2.8 Hz, 1H), 4.94 (d, J = 2.0 Hz,
2H), 4.68–4.61 (m, 1H), 2.67 (dt, J = 16.5, 6.0 Hz, 1H),
2.62–2.54 (m, 1H), 1.97–1.89 (m, 1H), 1.91–1.81 (m, 1H),
1.81–1.73 (m, 1H), 1.71–1.64 (m, 1H).13C NMR (151 MHz,
CDCl3) δ 156.92, 139.97, 135.68, 133.66, 130.03, 129.66,
128.76, 128.73, 115.05, 113.77, 69.27, 68.48, 32.44, 28.46,
19.21. APCI-HRMS m/z: calculated for C17H17ClO2 (M+),
288.0917, found 288.0938.
Melting point: 102.3–103.4 °C; purity: 99.6%; yield (fnal
1
step): 90.8%. H NMR (600 MHz, CDCl3) δ 7.26–7.16
(m, 2H), 7.11 (t, J=7.9 Hz, 1H), 7.01 (d, J=7.7 Hz, 1H),
6.93–6.85 (m, 3H), 6.72 (dd, J=8.1, 1.1 Hz, 1H), 4.72–4.66
(m, 1H), 4.29–4.20 (m, 4H), 2.70 (dt, J=17.4, 5.4 Hz, 1H),
2.53–2.44 (m, 1H), 1.91–1.74 (m, 3H), 1.74–1.65 (m, 1H).
13C NMR (151 MHz, CDCl3) δ 158.72, 156.09, 140.24,
129.52, 126.64, 126.50, 121.08, 121.02, 114.73, 109.98,
68.19, 66.80, 66.54, 31.74, 23.06, 18.03. APCI-HRMS m/z:
calculated for C18H20O3 (M+), 284.1412, found 284.1416.
1 3

Molecular Diversity
7‑Hydroxy‑4‑chromanone (12)
(dd, J=8.8, 2.4 Hz, 1H), 6.37 (d, J =2.4 Hz, 1H), 4.48 (t,
J=6.4 Hz, 2H), 4.00 (t, J=6.6 Hz, 2H), 2.74 (t, J=6.4 Hz,
2H), 1.81–1.54 (m, 6H), 1.53–1.40 (m, 1H), 1.34–1.07 (m,
4H), 1.01–0.76 (m, 2H). 13C NMR (151 MHz, CDCl3) δ
190.56, 165.58, 163.77, 128.79, 115.03, 110.30, 101.14,
67.32, 66.43, 37.41, 36.27, 34.44, 33.21, 29.68, 26.45,
26.17. APCI-HRMS m/z: calculated for C17H23O3 (MH+),
275.1642, found 275.1653.
Melting point: 143.6–143.8%; purity: 98.0%; yield: 75.1%.
1H NMR (600 MHz, DMSO-d6) δ 10.56 (s, 1H), 7.60 (d,
J = 8.7 Hz, 1H), 6.47 (dd, J = 8.7, 2.3 Hz, 1H), 6.30 (d,
J=2.3 Hz, 1H), 4.42 (t, J=6.4 Hz, 2H), 2.63 (t, J=6.4 Hz,
2H). 13C NMR (151 MHz, DMSO-d6) δ 189.87, 164.48,
163.45, 128.62, 114.02, 110.47, 102.45, 66.99, 36.97. APCI-
HRMS m/z: calculated for C9H9O3 (MH+), 165.0546, found
165.0552.
7‑(4‑Bromobenzyloxy)‑4‑chromanone (2e)
7‑Benzyloxy‑4‑chromanone (2a)
Melting point: 144.8–144.9%; purity: 97.5%; yield: 68.3%.
1H NMR (600 MHz, CDCl3) δ 7.82 (d, J = 8.8 Hz, 1H),
7.50 (d, J = 8.3 Hz, 2H), 7.27 (d, J = 8.4 Hz, 2H), 6.61
(dd, J=8.8, 2.4 Hz, 1H), 6.43 (d, J=2.4 Hz, 1H), 5.02 (s,
2H), 4.49 (t, J=6.5 Hz, 2H), 2.73 (t, J=6.5 Hz, 2H). 13C
NMR (151 MHz, CDCl3) δ 190.42, 164.64, 163.66, 134.89,
131.83, 129.02, 128.98, 122.23, 115.58, 110.31, 101.75,
69.43, 67.36, 37.39. APCI-HRMS m/z: calculated for
C16H14BrO3 (MH+), 333.0121, found 333.0089.
Melting point: 101.2–101.4%; purity: 98.8%; Yield: 70.6%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.45–7.36 (m, 4H), 7.36–7.30 (m, 1H), 6.64 (dd, J = 8.8,
2.4 Hz, 1H), 6.47 (d, J = 2.4 Hz, 1H), 5.07 (s, 2H), 4.49
(t, J = 6.3 Hz, 2H), 2.73 (t, J = 6.3 Hz, 2H). 13C NMR
(151 MHz, CDCl3) δ 190.47, 165.01, 163.70, 135.86,
128.92, 128.69, 128.29, 127.47, 115.44, 110.42, 101.74,
70.25, 67.35, 37.42. APCI-HRMS m/z: calculated for
C16H15O3 (MH+), 255.1016, found 255.1028.
7‑(4‑Chlorobenzyloxy)‑4‑chromanone (2f)
7‑(2‑Phenylethoxy)‑4‑chromanone (2b)
Melting point: 121.3–121.5%; purity: 99.5%; yield: 69.6%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.39–7.30 (m, 4H), 6.62 (dd, J=8.8, 2.4 Hz, 1H), 6.43 (d,
J=2.4 Hz, 1H), 5.04 (s, 2H), 4.50 (t, J=6.4 Hz, 2H), 2.74
(t, J=6.4 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 190.43,
164.69, 163.68, 134.38, 134.14, 129.00, 128.90, 128.76,
115.59, 110.33, 101.77, 69.44, 67.38, 37.41. APCI-HRMS
m/z: calculated for C16H14ClO3 (MH+), 289.0626, found
289.0611.
Melting point: 77.8–77.9%; purity: 96.4%; yield: 70.4%.
1H NMR (600 MHz, CDCl3) δ 7.81 (d, J = 8.8 Hz, 1H),
7.35–7.28 (m, 2H), 7.28–7.21 (m, 3H), 6.55 (dd, J = 8.8,
2.4 Hz, 1H), 6.38 (d, J= 2.4 Hz, 1H), 4.49 (t, J= 6.5 Hz,
2H), 4.18 (t, J=7.1 Hz, 2H), 3.09 (t, J=7.1 Hz, 2H), 2.73
(t, J=6.3 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 190.52,
165.16, 163.75, 137.66, 128.94, 128.87, 128.55, 126.67,
115.26, 110.21, 101.31, 68.97, 67.34, 37.41, 35.47. APCI-
HRMS m/z: calculated for C17H17O3 (MH+), 269.1172,
found 269.1179.
7‑(4‑Methylbenzyloxy)‑4‑chromanone (2g)
Melting point: 132.4–133.5%; purity: 97.6%; yield: 74.3%.
1H NMR (600 MHz, CDCl3) δ 7.81 (d, J = 8.8 Hz, 1H),
7.28 (d, J=8.0 Hz, 2H), 7.18 (d, J=7.8 Hz, 2H), 6.62 (dd,
J=8.8, 2.4 Hz, 1H), 6.46 (d, J=2.4 Hz, 1H), 5.02 (s, 2H),
4.49 (t, J = 6.3 Hz, 2H), 2.73 (t, J = 6.2 Hz, 2H), 2.34 (s,
3H). 13C NMR (151 MHz, CDCl3) δ 190.69, 165.16, 163.74,
138.17, 132.77, 129.37, 128.89, 127.63, 115.30, 110.49,
101.70, 70.23, 67.31, 37.37, 21.20. APCI-HRMS m/z: cal-
culated for C17H17O3 (MH+), 269.1172, found 269.1176.
7‑(3‑Phenylpropoxy)‑4‑chromanone (2c)
Melting point: 78.5–78.6%; purity: 99.5%; yield: 73.1%.
1H NMR (600 MHz, CDCl3) δ 7.81 (d, J = 8.8 Hz, 1H),
7.30–7.25 (m, 2H), 7.22–7.15 (m, 3H), 6.56 (dd, J = 8.8,
2.4 Hz, 1H), 6.36 (d, J= 2.4 Hz, 1H), 4.49 (t, J= 6.5 Hz,
2H), 3.96 (t, J=6.3 Hz, 2H), 2.79 (t, J=7.7 Hz, 2H), 2.72
(t, J=6.3 Hz, 2H), 2.15–2.05 (m, 2H). 13C NMR (151 MHz,
CDCl3) δ 190.47, 165.39, 163.73, 141.06, 128.83, 128.45,
126.03, 115.17, 110.22, 101.19, 67.33, 67.23, 37.41, 31.96,
30.47. APCI-HRMS m/z: calculated for C18H19O3 (MH+),
283.13229, found 283.1351.
7‑(4‑Fluorobenzyloxy)‑4‑chromanone (2h)
Melting point: 92.8–94.6%; purity: 99.4%; yield: 75.3%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.41–7.33 (m, 2H), 7.06 (t, J = 8.6 Hz, 2H), 6.62 (dd,
J=8.8, 2.4 Hz, 1H), 6.45 (d, J=2.4 Hz, 1H), 5.02 (s, 2H),
4.49 (t, J=6.4 Hz, 2H), 2.73 (t, J=6.2 Hz, 2H). 13C NMR
(151 MHz, CDCl3) δ 190.45, 164.77, 163.68, 163.43,
7‑(2‑Cyclohexylethoxy)‑4‑chromanone (2d)
Melting point: 88.5–88.8%; purity: 97.5%; yield: 73.0%. 1H
NMR (600 MHz, CDCl3) δ 7.80 (d, J = 8.8 Hz, 1H), 6.54
1 3

Molecular Diversity
161.80, 131.62, 131.60, 129.40, 129.35, 128.96, 115.71,
115.57, 115.51, 110.33, 101.70, 69.55, 67.36, 37.39. APCI-
HRMS m/z: calculated for C16H14FO3 (MH+), 273.0921,
found 273.0918.
113.46, 110.10, 101.49, 67.36, 66.64, 66.36, 37.39. APCI-
HRMS m/z: calculated for C17H16BrO4 (MH+), 363.0226,
found 363.0191.
7‑[2‑(4‑Methoxyphenyl)ethoxy]‑4‑chromanone (2m)
7‑[(4‑Trifuoromethyl)benzyloxy]‑4‑chromanone (2i)
Melting point: 87.5–88.4%; purity: 96.1%; yield: 78.0%. 1H
NMR (600 MHz, CDCl3) δ 7.80 (d, J=8.8 Hz, 1H), 7.17 (d,
J=8.6 Hz, 2H), 6.84 (d, J=8.7 Hz, 2H), 6.55 (dd, J=8.8,
2.4 Hz, 1H), 6.37 (d, J= 2.4 Hz, 1H), 4.48 (t, J = 6.4 Hz,
2H), 4.13 (t, J=7.0 Hz, 2H), 3.78 (s, 3H), 3.02 (t, J=7.0 Hz,
2H), 2.73 (t, J=6.4 Hz, 2H). 13C NMR (151 MHz, CDCl3)
δ 190.50, 165.19, 163.74, 158.37, 129.92, 129.66, 128.86,
115.23, 113.96, 110.22, 101.29, 69.23, 67.34, 55.26, 37.42,
34.58. APCI-HRMS m/z: calculated for C18H19O4 (MH+),
299.1278, found 299.1258.
Melting point: 129.7–130.3%; purity: 99.5%; yield: 66.7%.
1H NMR (600 MHz, CDCl3) δ 7.84 (d, J = 8.8 Hz, 1H),
7.64 (d, J = 8.1 Hz, 2H), 7.52 (d, J = 7.9 Hz, 2H), 6.63
(dd, J=8.8, 2.4 Hz, 1H), 6.44 (d, J=2.4 Hz, 1H), 5.13 (s,
2H), 4.49 (t, J=6.4 Hz, 2H), 2.74 (t, J=6.4 Hz, 2H). 13C
NMR (151 MHz, CDCl3) δ 190.42, 164.50, 163.69, 139.93,
130.54, 130.32, 129.08, 127.36, 125.71, 125.68, 125.65,
125.63, 123.05, 115.71, 110.25, 101.80, 69.28, 67.39,
37.40. APCI-HRMS m/z: calculated for C17H14F3O3 (MH+),
323.0890, found 323.0895.
7‑[2‑(4‑Chlorophenyl)ethoxy)‑4‑chromanone (2n)
7‑(2‑Phenoxyethoxy)‑4‑chromanone (2j)
Melting point: 86.5–88.3%; purity: 95.7%; yield: 72.1%. 1H
NMR (600 MHz, CDCl3) δ 7.81 (d, J = 8.8 Hz, 1H), 7.27
(d, J=7.8 Hz, 2H), 7.25–7.22 (m, 1H), 7.18 (d, J=8.1 Hz,
2H), 6.54 (dd, J = 8.7, 2.2 Hz, 1H), 6.36 (d, J = 2.3 Hz,
1H), 4.49 (t, J=6.6 Hz, 2H), 4.15 (t, J=6.9 Hz, 2H), 3.05
(t, J = 6.7 Hz, 2H), 2.73 (t, J= 6.7 Hz, 2H). APCI-HRMS
m/z: calculated for C17H16ClO3 (MH+), 3.3.0782, found
303.0805.
Melting point: 125.0–125.2%; purity: 99.5%; yield: 71.0%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.32–7.25 (m, 2H), 6.96 (t, J = 7.4 Hz, 1H), 6.92 (d,
J = 7.6 Hz, 2H), 6.61 (dd, J = 8.8, 2.4 Hz, 1H), 6.44 (d,
J=2.4 Hz, 1H), 4.50 (t, J=6.7 Hz, 2H), 4.37–4.26 (m, 4H),
2.74 (t, J = 6.3 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ
190.49, 164.89, 163.67, 158.37, 129.52, 128.93, 121.25,
115.51, 114.60, 110.17, 101.50, 67.36, 66.80, 66.00,
37.40. APCI-HRMS m/z: calculated for C17H17O4 (MH+),
285.1121, found 285.1090.
7‑[2‑(4‑Fluorophenoxy)ethoxy]‑4‑chromanone (2o)
Melting point: 114.9–115.2%; purity: 96.6%; yield: 75.1%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.02–6.93 (m, 2H), 6.90–6.82 (m, 2H), 6.61 (dd, J = 8.8,
2.4 Hz, 1H), 6.44 (d, J=2.4 Hz, 1H), 4.50 (t, J=6.4 Hz, 2H),
4.37–4.29 (m, 2H), 4.29–4.22 (m, 2H), 2.74 (t, J=6.4 Hz,
2H). 13C NMR (151 MHz, CDCl3) δ 190.49, 164.84, 163.69,
154.54, 128.99, 115.98, 115.83, 115.78, 115.73, 115.57,
110.15, 101.51, 67.38, 66.85, 66.79, 37.42. APCI-HRMS
m/z: calculated for C17H16FO4 (MH+), 303.1027, found
303.1003.
7‑[2‑(4‑Chlorophenoxy)ethoxy]‑4‑chromanone (2k)
Melting point: 145.9–146.2%; purity: 94.9%; yield: 77.6%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.27–7.21 (m, 2H), 6.91–6.81 (m, 2H), 6.61 (dd, J = 8.8,
2.4 Hz, 1H), 6.44 (d, J= 2.4 Hz, 1H), 4.50 (t, J= 6.5 Hz,
2H), 4.35–4.30 (m, 2H), 4.30–4.25 (m, 2H), 2.74 (t,
J = 6.4 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 190.52,
164.78, 163.69, 157.02, 129.40, 128.99, 126.17, 115.92,
115.58, 110.12, 101.50, 67.37, 66.67, 66.44, 37.40. APCI-
HRMS m/z: calculated for C17H16ClO4 (MH+), 319.0732,
found 319.0692.
7‑[2‑(4‑Methylphenyl)ethoxy]‑4‑chromanone (2p)
Melting point: 87.9–88.8%; purity: 96.4%; yield: 66.3%. 1H
NMR (600 MHz, CDCl3) δ 7.80 (d, J=8.8 Hz, 1H), 7.14 (d,
J=8.1 Hz, 2H), 7.11 (d, J=7.9 Hz, 2H), 6.55 (dd, J=8.8,
2.4 Hz, 1H), 6.37 (d, J= 2.4 Hz, 1H), 4.48 (t, J = 6.4 Hz,
2H), 4.15 (t, J=7.2 Hz, 2H), 3.04 (t, J=7.0 Hz, 2H), 2.73 (t,
J=6.3 Hz, 2H), 2.31 (s, 3H). 13C NMR (151 MHz, CDCl3)
δ 190.53, 165.20, 163.74, 136.24, 134.51, 129.23, 128.85,
128.82, 115.22, 110.23, 101.28, 69.12, 67.33, 37.41, 35.01,
21.03. APCI-HRMS m/z: calculated for C18H19O3 (MH+),
283.1329, found 283.1330.
7‑[2‑(4‑Bromophenoxy)ethoxy]‑4‑chromanone (2l)
Melting point: 163.3–164.1%; purity: 89.2%; yield: 78.2%.
1H NMR (600 MHz, CDCl3) δ 7.83 (d, J = 8.8 Hz, 1H),
7.36 (d, J = 9.0 Hz, 2H), 6.79 (d, J = 8.9 Hz, 2H), 6.60
(dd, J=8.8, 2.4 Hz, 1H), 6.43 (d, J =2.4 Hz, 1H), 4.50 (t,
J=6.5 Hz, 2H), 4.34–4.29 (m, 2H), 4.29–4.25 (m, 2H), 2.74
(t, J=6.5 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ 190.48,
164.75, 163.67, 157.52, 132.32, 128.97, 116.42, 115.57,
1 3

Molecular Diversity
7‑Isoamyloxy‑4‑chromanone (2q)
at 4 °C for 24 h. Stock solutions of the inhibitors were pre-
pared in DMSO and added to the dialysis samples to yield
a fnal concentration of 4% DMSO. The dialysis bufer was
replaced at 3 h and 7 h after the start of dialysis. As posi-
tive controls, dialysis of MAO-A and MAO-B was carried
out in the presence of the irreversible inhibitors, pargyline
(IC50 =13 μM) and (R)-deprenyl (IC50 =0.079 μM), respec-
tively [18, 32]. As negative control, dialysis was carried out
in the absence of inhibitor. After dialysis, the samples (250
µL) were diluted twofold with the addition of kynuramine
(250 μL) and incubated at 37 °C for 20 min. This gave a fnal
kynuramine concentration of 50 µM and an inhibitor concen-
tration of 2×IC50. The reactions were terminated with the
addition of NaOH (2 N, 400 µL) and water (1000 μL), and
4-hydroxyquinoline was determined as described for the IC50
measurements. For comparison, undialysed samples contain-
ing the MAO enzyme and test inhibitors were maintained at
4 °C for 24 h and diluted and assayed as described above.
All the dialysis reactions were carried out in triplicate, and
the residual enzyme activities are given as the mean SD.
Melting point: 80.5–81.9%; purity: 99.5%; yield: 64.4%.
1H NMR (600 MHz, CDCl3) δ 7.80 (d, J = 8.9 Hz, 1H),
6.54 (dd, J = 8.9, 2.4 Hz, 1H), 6.37 (d, J = 2.4 Hz, 1H),
4.49 (t, J = 6.3 Hz, 2H), 3.99 (t, J = 6.6 Hz, 2H), 2.73 (t,
J = 6.4 Hz, 2H), 1.85–1.75 (m, 1H), 1.70–1.63 (m, 2H),
0.94 (d, J = 6.5 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ
190.49, 165.54, 163.75, 128.79, 115.06, 110.27, 101.12,
67.33, 66.81, 37.61, 37.42, 24.97, 22.50. APCI-HRMS m/z:
calculated for C14H19O3 (MH+), 235.1350, found 235.1329.
IC50 determinations
IC50 values for the inhibition of the MAOs were deter-
mined using the method reported in the literature [22,
23]. Recombinant human MAO-A and MAO-B served as
enzyme sources, and all reactions were carried out in potas-
sium phosphate bufer (pH 7.4, 100 mM, made isotonic
with KCl). The reactions contained kynuramine (50 μM),
MAO-A (0.0075 mg protein/mL) or MAO-B (0.015 mg
protein/mL) and the test inhibitor (0.0003–100 μM) and
were prepared to a fnal volume of 200 μL. Dimethyl sul-
foxide (DMSO) was used to prepare stock solutions of the
test inhibitors, which were added to the reactions to yield a
DMSO concentration of 4%. The reactions were initiated
with the addition of the MAO the enzymes, incubated at
37 °C for 20 min and subsequently terminated with the addi-
tion NaOH (2 N, 80 µL). At endpoint, the MAO-generated
4-hyroxyquinoline was measured by fuorescence spectro-
photometry (λex = 310 nm; λem = 400 nm) and quantitated
with a linear calibration curve constructed with authentic
4-hydroxyquinoline (0.047–1.56 μM). The rates of MAO
catalysis were calculated, and the rate data were ftted to the
one-site competition model incorporated into the Prism 5
software package (GraphPad). From the resulting sigmoi-
dal plots of enzyme catalytic rate versus the logarithm of
inhibitor concentration, IC50 values were determined. The
IC50 values were measured in triplicate and are given as the
mean standard deviation (SD).
Lineweaver–Burk plots
For each test inhibitor, six Lineweaver–Burk plots were
constructed in the absence of inhibitor and presence of
fve diferent inhibitor concentrations (¼× IC50, ½ × IC50,
¾×IC50, 1×IC50 and 1¼×IC50). Eight diferent concentra-
tions of kynuramine (15–250 μM) were used for each plot,
while the concentration of MAO was 0.015 mg protein/mL.
The concentration of 4-hydroxyquinoline was measured as
described above for the IC50 measurements with the excep-
tion that the reactions were carried out to a volume of 500
µL and were terminated with the addition of NaOH (2 N,
400 µL) and water (1000 μL).
Electronic supplementary material
1H NMR and 13C NMR spectra for the synthesised
compounds.
Reversibility of inhibition by dialysis
Acknowledgements The NMR and MS spectra were recorded by
André Joubert and Johan Jordaan of the Laboratory for Analytical
Services (LAS) in the focus area Chemical Resource Benefciation,
while HPLC analysis was carried out by Jan du Preez of the Analytical
Technology Laboratory (ATL) at the North-West University (NWU).
The reversibility of inhibition was determined by dialysis
according to the method reported in the literature [23]. Slide-
A-Lyzer dialysis cassettes (Thermo Fisher Scientifc) with a
molecular cutof of 10,000 and a 0.5–3 mL sample volume
capacity were used for dialysis, and potassium phosphate
bufer (100 mM, pH 7.4) containing 5% sucrose served as
dialysis bufer. The test inhibitor, at a concentration equal
to 4×IC50, and MAO (0.03 mg protein/mL) were dissolved
in the dialysis bufer (0.8 mL), preincubated at 37 °C for
15 min and subsequently dialysed in 80 mL dialysis bufer
Funding The fnancial assistance of the National Research Founda-
tion (NRF) of South Africa [Grant specifc unique Reference Numbers
(UID) 85642, 96180] and NRF Innovation Fellowship (85189) towards
this research is hereby acknowledged. The grant holders acknowledge
that opinions, fndings and conclusions or recommendations expressed
in any publication generated by the NRF-supported research are those
of the authors and that the NRF accepts no liability whatsoever in this
regard.
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Molecular Diversity
16. Legoabe LJ, Petzer A, Petzer JP (2015) The synthesis and evalu-
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Compliance with ethical standards
Conflict of interest The authors declare that they have no confict of
interest.
17. Strydom B, Malan SF, Castagnoli N Jr, Bergh JJ, Petzer JP
(2010) Inhibition of monoamine oxidase by 8-benzyloxycaf-
feine analogues. Bioorg Med Chem 18:1018–1028. https://doi.
org/10.1016/j.bmc.2009.12.064
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Afliations
Stephanus J. Cloete1 · Clarina I. N’Da1 · Lesetja J. Legoabe1 · Anél Petzer1,2 · Jacobus P. Petzer1,2
1
2
Centre of Excellence for Pharmaceutical Sciences, North-
West University, Private Bag X6001, Potchefstroom 2520,
South Africa
Pharmaceutical Chemistry, School of Pharmacy, North-West
University, Private Bag X6001, Potchefstroom 2520,
South Africa
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