Facile access to Fe(III)-complexing cyclic hydroxamic acids in a three-component format

Article abstract of DOI:10.3390/molecules24050864

Cyclic hydroxamic acids can be viewed as effective binders of soluble iron and can therefore be useful moieties for employing in compounds to treat iron overload disease. Alternatively, they are analogs of bacterial siderophores (iron-scavenging metabolites) and can find utility in designing antibiotic constructs for targeted delivery. An earlier described three-component variant of the Castagnoli—Cushman reaction of homophthalic acid (via in situ cyclodehydration to the respective anhydride) was extended to involve hydroxylamine in lieu of the amine component of the reaction. Using hydroxylamine acetate and O-benzylhydroxylamine was key to the success of this transformation due to greater solubility of the reagents in refluxing toluene (compared to hydrochloride salt). The developed protocol was found suitable for multigram-scale syntheses of N-hydroxy- and N-(benzyloxy)tetrahydroisoquinolonic acids. The cyclic hydroxamic acids synthesized in the newly developed format have been tested and shown to be efficient ligands for Fe³⁺, which makes them suitable candidates for the above-mentioned applications.

Full text of DOI:10.3390/molecules24050864

molecules
Article
Facile Access to Fe(III)-Complexing Cyclic
Hydroxamic Acids in a Three-Component Format
Evgeny Chupakhin 1,2 , Olga Bakulina , Dmitry Dar’in 1 and Mikhail Krasavin 1,2,
1
*
1
Institute of Chemistry, Saint Petersburg State University, 199034 Saint Petersburg, Russia;
chupakhinevgen@gmail.com (E.C.); o.bakulina@spbu.ru (O.B.); d.dariin@spbu.ru (D.D.)
Institute of Living Systems, Immanuel Kant Baltic Federal University, 236016 Kaliningrad, Russia
Correspondence: m.krasavin@spbu.ru; Tel.: +7-931-3617872; Fax: +7-812-428-6939
2
*
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Richard A. Bunce, Philippe Belmont, Wim Dehaen and Eugene Babaev
Received: 12 February 2019; Accepted: 25 February 2019; Published: 28 February 2019
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: Cyclic hydroxamic acids can be viewed as effective binders of soluble iron and can therefore
be useful moieties for employing in compounds to treat iron overload disease. Alternatively,
they are analogs of bacterial siderophores (iron-scavenging metabolites) and can find utility in
designing antibiotic constructs for targeted delivery. An earlier described three-component variant
of the Castagnoli—Cushman reaction of homophthalic acid (via in situ cyclodehydration to the
respective anhydride) was extended to involve hydroxylamine in lieu of the amine component of
the reaction. Using hydroxylamine acetate and O-benzylhydroxylamine was key to the success
of this transformation due to greater solubility of the reagents in refluxing toluene (compared to
hydrochloride salt). The developed protocol was found suitable for multigram-scale syntheses
of N-hydroxy- and N-(benzyloxy)tetrahydroisoquinolonic acids. The cyclic hydroxamic acids
synthesized in the newly developed format have been tested and shown to be efficient ligands
3+
for Fe , which makes them suitable candidates for the above-mentioned applications.
Keywords: iron complexation; cyclic hydroxamic acids; Castagnoli–Cushman reaction; homophthalic
acid; hydroxylamine acetate; azeotropic distillation; cyclodehydration
1
. Introduction
The ability of cyclic hydroxamic acids (N-hydroxylactams) to chelate metal ions in general—and
Fe3+ in particular—defines their utility in drug design [
1]. One principal avenue in this regard
is based on the recognition of the similarity of synthetic cyclic hydroxamic acids to bacterial
siderophores—special metabolites excreted by bacteria to scavenge and deliver iron for the
microorganism’s nutritional needs [
inside bacteria and thus overcome resistance of the latter to the antibacterial agent [
said application in resistance-free antibiotic design, selective, non-toxic iron chelators are needed as
treatments for hereditary iron overload disease [ ]. Unfortunately, streamlined and multicomponent
methods to prepare N-hydroxylactams are lacking. Among the existing synthetic methods for
hydroxamic acid, intramolecular nucleophilic cyclization onto O-protected acyclic hydroxamic acids [ ],
], and ring-closing methathesis of bis-olefinic hydroxamic
2
]. Conjugating such moieties to antibiotics helps shuttle those
3]. Besides
4
5
nitroso moiety insertion in cyclic ketones [
acids [7] can be mentioned.
6
Imines
polysubstituted lactams
as a name reaction—the Castagnoli–Cushman reaction (CCR) [
the research efforts of Neil Castagnoli and Mark Cushman over 40 years ago [10
anhydride (HPA, ) is one of the most frequently employed anhydrides in this reaction, which normally
1
are known to condense with
]. In the recent literature, this powerful reaction has been regarded
]—to acknowledge its origination from
11]. Homophthalic
α-C-H anhydrides of dicarboxylic acids 2 to give
3 [8
9
,
4
gives rise to trans-configured tetrahydroisoquinolonic (THIQ) acids 5 (Figure 1) [12,13]. These have
Molecules 2019, 24, 864; doi:10.3390/molecules24050864
www.mdpi.com/journal/molecules

Molecules 2019, 24, 864
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a documented utility in medicinal chemistry (in therapeutic areas such as cancer [14], malaria [15],
and diabetes [16]).
Figure 1. The Castagnoli–Cushman reaction (CCR).
Recently, we successfully replaced the imine component in the CCR of
obtained, after 24 h reflux in toluene, good to excellent yields of the respective N-hydroxy THIQ acids
17] which are representative of cyclic hydroxamic acids. The forcing conditions required in order to
obtain were in sharp contrast with ambient temperature normally sufficient for the preparation of 5.
4 with oximes 6 and
7
[
7
This was justified [17] by the need to re-generate HPA (4) from the initial, rapidly formed O-acylation
0
product 7 (Scheme 1).
Scheme 1. Reaction of oximes 6 with HPA (4) and its mechanistic interpretation [15].
More recently, we employed refluxing toluene for in situ dehydration of homophthalic acid which
allowed, for the first time, preparing THIQ acids
5 in a true multicomponent format (i.e., by mixing an
amine, an aldehyde and homophthalic acids) without a need for preliminary imine synthesis or the
use of hydrolytically prone HPA (Scheme 2) [18].
Scheme 2. Synthesis of THIQ acids 5 in a three-component format from homophthalic acid [18].
These findings encouraged us to investigate the preparation of N-hydroxy THIQ acids
7 from
homophthalic acid and oximes and possible applicability in this case of the same three-component
format as presented in Scheme 2. Herein, we present the results of these findings and present
characterization of compounds 7 with respect to their iron-binding properties, which validates them as
potential candidates for iron overload disease treatment or the design of siderophore-based constructs
for antibiotic delivery.

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2
. Results and Discussion
The initial reaction of oxime 6a with homophthalic acid in toluene proved rather encouraging
and yielded, after 16 h reflux and cooling to room temperature, 72% of the desired product 7a 17
Scheme 3). The reaction proceeded via formation of HPA ( ), which was supported by the results of a
[
]
(
4
separate experiment, where homophthalic acid was refluxed in toluene with azeotropic removal of
water to form corresponding anhydride in quantitative yield. This experiment can be also regarded as
a new method for the preparation of homophthalic anhydride.
Scheme 3. Two-component preparation of 7a from homophthalic acid.
An attempt to perform the same reaction by mixing homophthalic acid with p-anisaldehyde
and hydroxylamine hydrochloride in presence of pyridine (1 equiv.) did not lead to the formation
1
of 7a. Instead, the only product discernible by H-NMR (Nuclear Magnetic Resonance) analysis
of the reaction mixture was tetracyclic adduct formed, presumably, via the earlier described [19
]
Perkin-Michael domino transformation (Scheme 4).
Scheme 4. Attempted three-component preparation of 7a from hydroxylamine hydrochloride (see
ref. [19] for more mechanistic insight).
The observed complication is likely the result of the inefficient formation of the intermediate oxime
in the non-polar reaction medium (due to low solubility in it of hydroxylamine hydrochloride) and
6
the ready participation of free p-anisaldehyde in the route leading to tetracyclic adduct. To circumvent
this obstacle, we attempted to prepare hydroxylamine acetate and use it in the same transformation.
We anticipated that in addition to improved solubility of the acetate salt in toluene, this transformation
would not require the use of a base as acetic acid would be conveniently removed from the reaction
medium by azeotropic distillation with the solvent. Interestingly, hydroxylamine acetate has been
seldom [20] employed for the preparation of oximes despite the obvious advantage of not having
to use any base (as is the case with the usual hydroxylamine hydrochloride). The hydrochloride
salt of hydroxylamine was converted to its acetate by the action of sodium acetate and reacted with
p-anisaldehyde and homophthalic acid in refluxing toluene over 24 h. To our delight, on cooling to
r. t., a thick precipitate formed which was isolated by filtration to provide 65% yield of compound
7a. The same reaction was attempted at reflux in benzene and xylenes. While the former conditions

Molecules 2019, 24, 864
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led to <15% conversion over 24 h, the latter gave an appreciable amount of N-deoxygenation product.
Considering this and the results of the reaction monitoring at different time points, 24 h reflux in
toluene was considered to be optimal for the preparation of compound 7a. Thus, these conditions
were extended to the preparation of a new series of N-hydroxy THIQ acids 7a–m (Scheme 5).
O
OH
NH2OH*AcOH
CO2H
N
ArCHO +
CO2H
toluene, reflux
4 h
Ar
2
CO2H
7a-m
OMe
*
*
*
*
O
O
Ar =
MeO
OMe
OBn
7
a, 65%[15]
7b, 68%[15]
7c, 52%
OMe
7d, 61%
MeO
*
*
MeO
EtO
*
MeO
*
MeO
MeO
*
OMe
OMe
7f, 57%
7
e, 67%
7g, 45%
7h, 60%
MeO
*
MeO
*
*
OMe
*
Cl
OMe
t-Bu
2
MeO C
7i, 54%
7j, 72%
7k, 59%[15]
7l, 54%
7m, 51%
Scheme 5. Three-component synthesis of N-Hydroxy THIQ acids 7a–m.
An interesting result was obtained in an attempt to involve o-salicylaldehyde in the same reaction.
When all the aldehyde starting material was consumed (according to HPLC (High Performance Liquid
Chromatography) analysis of the reaction mixture), only a trace amount of anticipated compound
was detected. The major product identified and isolated from the reaction mixture was coumarin
8
. The same reaction run without hydroxylamine acetate gave identical yield of 8 (67%). It likely
that the reaction (described earlier but not interpreted from the mechanistic viewpoint [21]) involves
acylation of the phenolic hydroxy group followed by intramolecular Knoevenagel reaction (Scheme 6).
Involvement of O-acetyl salicylaldehyde in the same reaction led to de-acetylation and the formation
of 8 in 50% yield.
Scheme 6. Involvement of unprotected o-salicaldehyde in the reaction with homophthalic acid.
The reaction generally worked very well with electron-rich aromatic aldehydes and gave good
yields of respective N-hydroxy THIQ acids 7a
–k (Scheme 5). Substrates without alkoxy groups (such as
tert-butyl-, methoxycarbonyl-, fluoro-, bromo-, and nitro-benzaldehydes,) surprisingly, gave no desired

Molecules 2019, 24, 864
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product under these conditions. Increasing the concentration of reactants from 0.1 M to 1 M and
the reaction time from 24 to 48 h, allowed us to slightly increase the reaction scope by involving
4-(tert-butyl)benzaldehyde and 4-(methoxycarbonyl)benzaldehyde in the developed approach and to
prepare corresponding N-hydroxy THIQ acids 7l and 7m in good yields (54 and 51% respectively).
However, still the reaction was not applicable to more electron-poor substrates.
1
13
The H- and C-NMR analysis of corresponding reaction mixtures showed that in case of
electron-poor aldehydes the major reaction products are nitriles (formed by dehydration of oximes).
To prevent this side reaction, we replaced hydroxylamine acetate with O-benzylhydroxylamine
(
4
Scheme 6). To our delight this allowed to involve previously unreactive electron-poor aldehydes (even
-fluoro- and 4-nitro-benzaldehydes). Following this new modified protocol (also performed at 1 M
concentration of reactants) seven novel N-benzyloxy THIQ acid derivatives 9a
–g
have been prepared
in high and good yields (Scheme 7). For all prepared compounds 7a
–
m
and 9a
–
g
trans-configuration
3
was concluded from J
coupling constant values (~1–2 Hz) between protons at positions C3 and C4,
HH
which is consistent with our previous data [17].
Scheme 7. Three-component synthesis of N-Benzyloxy THIQ acids 9.
Additionally, two types of post-modifications have been investigated for the prepared compounds
and : decarboxylation (Scheme 8) and debenzylation (Scheme 9). Attempts to perform
7
9
decarboxylation of compound 7a under previously reported conditions [22] lead to formation of
complex mixture of products resulting from side reactions of oxidation and dehydration (supported
1
by HRMS (High Resolution Mass Spectrometry) and H-NMR data) (Scheme 8). At the same time,
decarboxylation can be smoothly conducted for O-benzylated compound 9b providing compound 10
in 77% yield, thus demonstrating another advantage of O-benzyloxime-based protocol for preparation
of N-hydroxy THIQ acids derivatives.
Scheme 8. Decarboxylation of compounds 9a and 9b.
Scheme 9. Debenzylation of compounds 9b, 10, and 7c.
We also have investigated the possibility of removal of benzyl protective group from prepared
compounds (Scheme 9). N-Benzyloxy THIQ acids 9b and 10 were successfully converted to

Molecules 2019, 24, 864
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corresponding OH-hydroxamic acids 7a and 11 via hydrogenolysis under standard conditions with
excellent yields. Surprisingly, attempted O-debenzylation of compound 7c only gave the product of
N-dehydroxylation 12 (Scheme 9).
Compounds
7 synthesized using the developed procedure are direct analogs of bacterial
siderophores [23] and, therefore, can be potentially regarded as new agents for iron overload disease
therapy and as precursors for the design of “Trojan horse” antibiotics [24] against drug resistant bacteria.
Therefore five selected compounds—7d,f–h,j which represent different substitution patterns in 3-aryl
moiety (4 types of di-substituted and one tri-substituted), were tested for iron(III) binding properties
using mole-ratio method to prove this assumption (Figure 2a,b). Upon addition of ferric nitrate to
solution of ligand
corresponds to formation of Fe(III)-7 complex, which is characterized by a new absorption band with
maximum around 500 nm (Figure 2b). All ligands in contrast to their complexes with iron do not
7 in aqueous methanol a color change from colorless to purple was observed. This
7
absorb in the visible region. Representative example of UV-Vis spectrum of free ligand
7 and its
changes upon addition of Fe3 ions is shown on Figure 2a for compound 7d. These spectra for other
+
tested compounds
7
are reported in ESI (Electronic Supporting Information, Figures S1–S3). In all
cases two isosbestic points were observed around 470 and 600 nm at CFe = 0.5-5 × 10− M.
4
−
5
Figure 2. (a) UV-Vis absorbance spectrum of compound 7d in methanol (C = 5 × 10 M); (b) UV-Vis
−
4
absorbance spectrum of compound 7d (C = 5
×
10 M, aq. MeOH 85%) in the presence of increasing
L
concentration of Fe3+ (CFe = 0.25-5 × 10 M).
−4
Applying the mole-ratio method allowed us to determine not only the stoichiometry of observed
complexes but also the associated binding constants [17 25]. Absorbance at characteristic wavelength
,
3+
was plotted as function of [Fe ]/[ligand] ratio to give the curves shown in Figure 3a. The binding
process involves two equilibria—the first one corresponding to 1:1 complex, and the second one to
:2 complex. Stepwise formation constants K and K were estimated in the range of 10
10 M , respectively. These values were calculated using computer nonlinear curve-fitting
of the absorbance values taken from experimental mole-ratio plots on Figure 3a to previously derived
5
7
−1
1
−
10 M
1
2
and 104
−
5
−1
Equations (1)–(3) describing formation of ML complex (See Material and Methods Section for details).
2
The example of such fitting (compound 7d) is presented on Figure 3b. The respective plots with
curve-fitting for other tested compounds 7f
–
h
,
j
are presented in ESI (Figures S4–S8) The results on
are summarized in
−1
M ) were found to be in accordance with our previous
spectrophotometric investigation of iron(III) binding properties for compounds
7
10
11
Table 1. The obtained Kf values (~10 –10
results [17] for similar compounds. No significant dependence of K values on substitution pattern
f
was found.

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(
a)
(b)
Figure 3.
(
a
) Absorbance vs. [Fe3+]/[ligand] mole-ratio plot for all Fe3+-7d
,
f
–
h
,
j
complexes;
7d complexation in aq. MeOH 85%
red asterisks—experimental data, black line—nonlinear curve-fitting according to eqn.3 from Materials
3+
3+
(
(
b) Absorbance vs. [Fe ]/[ligand] mole-ratio plot for Fe -
and Methods Section).
Table 1. Stoichiometry and Kf values for Fe3+ 7d, Fe3+ 7f, Fe3+ 7g, Fe3+ 7h and Fe3+
- - - - -7j complexes
determined by the mole-ratio method.
Ligand
M:L
K1,2
log K1,2
λ [nm] a
500
6
5
6
5
7
4
5
5
5
5
1:1
1:2
1:1
1:2
1:1
1:2
1.67
×
×
×
×
×
×
×
×
×
×
10
10
10
10
10
10
10
10
10
10
6.22
5.36
6.71
5.78
7.33
4.73
5.91
5.53
5.36
5.61
7
d
2.32
5.09
6.04
2.16
5.38
8.22
3.39
2.31
4.06
7
f
500
7
g
500
7
h
1:1
:2
1:1
:2
500, 550, 600
1
7
j
470
1
a
Characteristic absorbance wavelength at which absorbance of the complex was measured.
3
. Materials and Methods
3
.1. General Considerations
All commercial reagents were used without further purification. NMR spectra were acquired
1
13
using Bruker Avance III spectrometer (Billerica, MA, USA) ( H: 400.13 MHz; C: 100.61 MHz; chemical
shifts are reported as parts per million ( , ppm); solvents—DMSO-d or CDCl , the residual solvent
peaks were used as internal standards: 2.50 ppm for H and 39.52 ppm for C (DMSO-d ) or 7.26 ppm
δ
6
3
1
13
6
1
13
for H and 77.16 ppm for C (CDCl ); multiplicities are abbreviated as follows: s = singlet, d = doublet,
3
t = triplet, q = quartet, m = multiplet, br = broad; coupling constants, J, are reported in Hz. Mass spectra
were acquired using Bruker microTOF spectrometer (ionization by electrospray, positive ions detection;
Billerica, MA, USA). Melting points were determined in open capillary tubes on Stuart SMP50
Automatic Melting Point Apparatus (Stone, UK). Homophthalic acid, hydroxylamine hydrochloride,
O-benzylhydroxylamine, aldehydes, and Fe(NO3)3
All reactions were performed in air, unless otherwise noted. Analytical data obtained for compounds
a, 7b, 7k and 7m have been consistent with those reported in our previous work [17].
·
9H O were obtained from commercial sources.
2
7

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3
.2. Synthesis
3
.2.1. Homophthalic Anhydride 4 (Scheme 3)
Homophthalic acid (150 mg, 0.83 mmol) was refluxed in toluene (15 mL) in a flask equipped a
Dean-Stark distilling trap (ISOLAB Laborgeräte GmbH, Wertheim, Germany) for 16 h. After cooling to
room temperature, the reaction mixture was cooled to room temperature and concentrated in vacuo to
1
provide pure title compound (135 mg, 0.83 mmol, quant. yield). H-NMR (400 MHz, CDCl )
δ
8.21
3
(dd, J = 7.9, 1.4 Hz, 1H), 7.69 (td, J = 7.6, 1.4 Hz, 1H), 7.51 (td, J = 7.7, 1.0 Hz, 1H), 7.44–7.30 (m, 1H),
4
.14 (s, 2H) [26].
3
.2.2. Hydroxylamine Acetate
Hydroxylamine hydrochloride (10.0 g, 144 mmol) was dissolved in deionized water (5 mL).
The solution was added, on stirring, to a solution of sodium acetate (11.8 g, 144 mmol) in water
5 mL). The resulting clear solution was concentrated to dryness. The solid residue was suspended in
(
anhydrous methanol (20 mL) and filtered to remove sodium chloride. The filtrate was concentrated in
1
vacuo to give hydroxylamine acetate (13.0 g, 141 mmol, 98%). H-NMR (400 MHz, DMSO-d )
δ
8.20
6
(
s, 4H, NH OH, COOH), 1.86 (s, 3H, CH ) [27].
2
3
3
.2.3. General Procedure for the Preparation of N-Hydroxy THIQ Acids 7a–k
A mixture of homophthalic acid (180 mg, 1 mmol), hydroxylamine acetate (102 mg, 1.1 mmol) and
the respective aldehyde (1.1 mmol) in toluene (10 mL) was heated at reflux for 24 h in a flask equipped
a Dean-Stark distilling trap. A thick precipitate which formed on cooling the reaction mixture to
◦
−
20 C was isolated by filtration, washed with hexane, and crystallized from aqueous methanol to
provide pure title compounds.
(
3R/S,4R/S)-3-(2-(Benzyloxy)phenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7c).
◦
1
Yield 202 mg (52%); white powder, mp 214.3–214.7 C; H-NMR (400 MHz, DMSO-d )
δ
13.04 (br.s, 1H,
COOH), 10.23 (s, 1H, N-OH), 8.00–7.97 (m, 1H, CH(Ar)), 7.60 (d, J = 7.3 Hz, 2H, 2CH(Ar)), 7.46–7.33
m, 5H, 5CH(Ar)), 7.29–7.10 (m, 3H, 3CH(Ar)), 6.78 (t, J = 7.5 Hz, 1H, CH(Ar)), 6.66 (d, J = 7.5 Hz, 1H,
6
(
13
CH(Ar)), 5.89 (s, 1H, 3-H), 5.34–5.15 (m, 2H, CH ), 4.19 (s, 1H, 4-H). C-NMR (101 MHz, DMSO-d6)
δ
2
1
1
72.3, 161.1, 155.6, 137.5, 133.3, 132.1, 130.5, 129.3, 128.9, 128.8, 128.3, 128.2, 127.6, 127.0, 126.4, 126.2,
+
20.7, 113.0, 69.8, 61.0, 49.9. HRMS (ESI), m/z calcd for C H NO [M + H] 412.1155, found 412.1149.
23
20
5
(
3R/S,4R/S)-3-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-
◦
1
carboxylic acid (7d). Yield 208 mg (61%); white powder, mp 219.8–220.2 C. H-NMR (400 MHz,
DMSO-d6) 13.07 (br.s, 1H, COOH), 10.21 (s, 1H, N-OH), 7.97 (d, J = 7.0 Hz, 1H, CH(Ar)), 7.42 (p,
δ
J = 7.4, 7.0 Hz, 1H, CH(Ar)), 7.49–7.36 (m, 2H, 2CH(Ar)). 7.30 (d, J = 6.9 Hz, 1H, CH(Ar)), 6.73 (d,
J = 8.1 Hz, 1H, CH(Ar)), 6.61 (t, J = 7.9 Hz, 1H, CH(Ar)), 6.20 (d, J = 7.6 Hz, 1H, CH(Ar)), 5.72 (s, 1H,
13
3
-H), 4.46–4.31 (m, 2H, CH ), 4.27 (t, J = 4.0 Hz, 2H, CH ), 4.16 (s, 1H, CH, 4-H). C-NMR (101 MHz,
2 2
DMSO-d6)
δ
172.3, 161.1, 143.9, 141.0, 133.4, 132.1, 130.6, 128.9, 128.3, 126.9, 126.8, 120.6, 118.1, 116.9,
+
6
4.9, 64.3, 60.6, 49.8. HRMS (ESI), m/z calcd for C H NO [M + H] 342.0972, found 342.0970.
18
16
6
(
3R/S,4R/S)-3-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-
◦
1
carboxylic acid (7d). Yield 208 mg (61%); white powder, mp 219.8–220.2 C. H-NMR (400 MHz,
DMSO-d6) 13.07 (br.s, 1H, COOH), 10.21 (s, 1H, N-OH), 7.97 (d, J = 7.0 Hz, 1H, CH(Ar)), 7.42 (p,
δ
J = 7.4, 7.0 Hz, 1H, CH(Ar)), 7.49–7.36 (m, 2H, 2CH(Ar)). 7.30 (d, J = 6.9 Hz, 1H, CH(Ar)), 6.73 (d,
J = 8.1 Hz, 1H, CH(Ar)), 6.61 (t, J = 7.9 Hz, 1H, CH(Ar)), 6.20 (d, J = 7.6 Hz, 1H, CH(Ar)), 5.72 (s, 1H,
13
3
-H), 4.46–4.31 (m, 2H, CH ), 4.27 (t, J = 4.0 Hz, 2H, CH ), 4.16 (s, 1H, CH, 4-H). C-NMR (101 MHz,
2 2
DMSO-d6)
δ
172.3, 161.1, 143.9, 141.0, 133.4, 132.1, 130.6, 128.9, 128.3, 126.9, 126.8, 120.6, 118.1, 116.9,
+
6
4.9, 64.3, 60.6, 49.8. HRMS (ESI), m/z calcd for C H NO [M + H] 342.0972, found 342.0970.
18
16
6

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(
(
3R/S,4R/S)-3-(4-Ethoxy-3-methoxyphenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid
◦
1
7e). Yield 239 mg (67%); white powder, mp 218.7–218.9 C; H-NMR (400 MHz, DMSO-d )
δ
13.04 (s,
6
1H, COOH), 10.18 (s, 1H, N-OH), 7.94 (d, J = 7.3 Hz, 1H, CH(Ar)), 7.49–7.38 (m, 2H, 2CH(Ar)), 7.30
(d, J = 7.2 Hz, 1H, CH(Ar)), 6.80 (s, 1H, CH(Ar)), 6.78 (d, J = 8.4 Hz, 1H, CH(Ar)), 6.53 (d, J = 8.2 Hz,
1
H, CH(Ar)), 5.42 (s, 1H, 3-H), 4.28 (d, J = 1.8 Hz, 1H, 4-H), 3.91 (q, J = 6.9 Hz, 2H, CH ), 3.66 (s, 3H,
2
13
OCH ), 1.26 (t, J = 7.0 Hz, 3H, CH ). C-NMR (101 MHz, DMSO-d6)
δ 172.3, 160.7, 149.2, 147.8, 133.6,
3
3
1
32.1, 131.3, 130.2, 129.1, 128.2, 126.9, 118.3, 113.0, 110.7, 65.3, 64.1, 55.8, 51.9, 15.2. HRMS (ESI), m/z
+
calcd for C H NO [M + H] 358.1285, found 358.1300.
19
20
6
(
3R/S,4R/S)-2-Hydroxy-1-oxo-3-(3,4,5-trimethoxyphenyl)-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7f).
◦
1
Yield 212 mg (57%); white powder, mp 220.6–220.8 C; H-NMR (400 MHz, DMSO-d ) δ 13.09 (s, 1H,
6
COOH), 10.24 (s, 1H, N-OH), 7.95 (dd, J = 7.4, 1.3 Hz, 1H, CH(Ar)), 7.51–7.39 (m, 2H, 2CH(Ar)), 7.31 (d,
J = 7.1 Hz, 1H, CH(Ar)), 6.44 (s, 2H, CH(Ar)), 5.44 (s, 1H, 3-H), 4.33 (s, 1H, 4-H), 3.63 (s, 6H, 2OCH ),
3
3
1
.58 (s, 3H, OCH3). 13C-NMR (101 MHz, DMSO-d6)
δ
172.2, 160.7, 153.2, 137.2, 134.8, 133.7, 132.2, 130.2,
29.0, 128.3, 126.8, 104.1, 65.5, 60.3, 56.2, 51.8. HRMS (ESI), m/z calcd for C H NO [M + H] 374.1234,
+
19
20
7
found 374.1244
(
3R/S,4R/S)-3-(2,3-Dimethoxyphenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7g).
◦
1
Yield 154 mg (45%); white powder, mp 224.3–224.6 C; H-NMR (400 MHz, DMSO-d )
δ
13.04 (br.s,
H, COOH), 10.18 (s, 1H, N-OH), 7.94 (d, J = 7.1 Hz, 1H, CH(Ar)), 7.50–7.36 (m, 2H, 2CH(Ar)), 7.29 (d,
J = 7.1 Hz, 1H, CH(Ar)), 6.80 (s, 1H, CH(Ar)) 6.79 (d, J = 8.2 Hz, 1H, CH(Ar)), 6.54 (d, J = 9.6 Hz, 1H,
6
1
13
CH(Ar)), 5.42 (s, 1H, 4-H), 4.28 (s, 1H, 3-H), 3.66 (s, 3H, OCH ), 3.65 (s, 3H, OCH ). C-NMR (101 MHz,
3
3
DMSO-d6)
δ
177.1, 165.5, 153.9, 153.4, 138.4, 137.0, 136.1, 135.0, 133.8, 133.0, 131.66, 123.1, 116.7, 115.4,
+
7
0.0, 60.6, 60.6, 56.7. HRMS (ESI), m/z calcd for C H NO [M + H] 344.1129, found 344.1141.
18
18
6
(
3R/S,4R/S)-3-(2,4-Dimethoxyphenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7h).
◦
1
Yield 205 mg (60%); white powder, mp 208.2–208.7 C; H-NMR (400 MHz, DMSO-d )
δ
12.99 (br.s,
6
1
7
6
3
H, COOH), 10.14 (s, 1H, N-OH), 7.99–7.92 (m, 1H, CH(Ar)), 7.41 (td, J = 7.7, 6.6, 3.9 Hz, 2H, 2CH(Ar)),
.26 (dd, J = 5.8, 2.8 Hz, 1H, CH(Ar)), 6.59 (d, J = 2.2 Hz, 1H, CH(Ar)), 6.50 (d, J = 8.5 Hz, 1H, CH(Ar)),
.33 (dd, J = 8.5, 2.2 Hz, 1H, CH(Ar)), 5.66 (s, 1H, 4-H), 4.05 (s, 1H, 3-H), 3.86 (s, 3H, OCH ), 3.68 (s,
3
H, OCH3). 13C-NMR (101 MHz, DMSO-d6)
δ
172.5, 161.1, 160.6, 157.6, 133.5, 132.0, 130.4, 128.9, 128.2,
+
1
26.9, 126.7, 118.1, 104.7, 99.3, 60.7, 56.2, 55.6, 50.1. HRMS (ESI), m/z calcd for C H NO [M + H]
18 18 6
3
44.1129, found 344.1141.
(3R/S,4R/S)-3-(2,5-Dimethoxyphenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7i).
This product was isolated after washing the precipitate from r.m. with Et O and filtration. Yield
2
◦
35 mg (54%); white powder, mp 211.6–211.9 C; H-NMR (400 MHz, DMSO-d ) δ 13.06 (br.s, 1H,
6
1
2
COOH), 10.23 (s, 1H, N-OH), 8.00–7.94 (m, 1H, CH(Ar)), 7.47–7.38 (m, 1H, CH(Ar)), 7.29–7.25 (m, 1H,
CH(Ar)), 6.96 (d, J = 8.9 Hz, 1H, CH(Ar)), 6.77 (dd, J = 8.9, 3.0 Hz, 1H, CH(Ar)), 6.19 (d, J = 2.7 Hz,
13
1
H, CH(Ar)), 5.71 (s, 1H, 3-H), 4.08 (s, 1H, 4-H), 3.83 (s, 3H, OCH ), 3.51 (s, 3H, OCH ). C-NMR
3 3
(
1
101 MHz, DMSO-d6)
δ
172.3, 161.1, 153.2, 150.6, 133.4, 132.2, 130.5, 128.9, 128.3, 127.2, 126.9, 113.6,
+
12.5, 61.0, 56.5, 55.6, 49.8. HRMS (ESI), m/z calcd for C H NO [M + H] 344.1129, found 344.1143.
18
18
6
(
(
(
3R/S,4R/S)-3-(5-Chloro-2-methoxyphenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid
◦
1
7j). Yield 249 mg (72%); white powder, mp 229.2–229.5 C; H-NMR (400 MHz, DMSO-d )
δ
13.10
br.s, 1H, COOH), 10.35 (s, 1H, N-OH), 7.99 (d, J = 7.0 Hz, 1H, CH(Ar)), 7.48–7.40 (m, 2H, 2CH(Ar)),
.31–7.28 (m, 1H, CH(Ar)), 7.27 (d, J = 2.6 Hz, 1H, CH(Ar)), 7.08 (d, J = 8.8 Hz, 1H, CH(Ar)), 6.58 (s, 1H,
6
7
13
CH(Ar)), 5.72 (s, 1H, 3-H), 4.14 (s, 1H, 4-H), 3.89 (s, 3H, OCH3). C-NMR (101 MHz, DMSO-d6)
δ
1
6
72.15, 161.05, 155.59, 133.20, 132.35, 130.56, 129.08, 128.58, 128.50, 128.11, 127.02, 125.92, 124.34, 113.67,
+
0.91, 56.66, 49.35. HRMS (ESI), m/z calcd for C H ClNO [M + H] 348.0633, found 348.0621.
17
15
5

Molecules 2019, 24, 864
10 of 15
3
.2.4. General Procedure for the Preparation of N-Hydroxy THIQ Acids 7l,m
A mixture of homophthalic acid (3.6 g, 20 mmol), hydroxylamine acetate (1.86 g, 20 mmol) and
the respective aldehyde (20 mmol) in toluene (20 mL) was placed in a pre-heated oil bath and refluxed
for 48 h in a flask equipped a Dean-Stark distilling trap. A thick precipitate which formed on cooling
◦
the reaction mixture to
−
20 C was isolated by filtration, washed with hexane, and crystallized from
aqueous methanol to provide pure title compounds.
(
3R/S,4R/S)3-(4-(tert-Butyl)phenyl)-2-hydroxy-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (7l).
◦
1
Yield 3.67 g, 54%. White solid, mp 234–236 C; H-NMR (400 MHz, DMSO-d )
δ 13.06 (s, 1H),
6
1
5
0.18 (s, 1H), 7.95 (dd, J = 7.5, 1.3 Hz, 1H), 7.50–7.38 (m, 2H), 7.33–7.25 (m, 3H), 7.05 (d, J = 8.3 Hz, 2H),
.45 (d, J = 1.3 Hz, 1H), 4.27 (d, J = 1.3 Hz, 1H), 1.21 (s, 9H). 13C-NMR (101 MHz, DMSO-d6)
δ 172.4,
1
60.7, 150.4, 136.2, 133.5, 132.2, 130.4, 129.1, 128.3, 127.0, 126.3, 125.8, 65.4, 51.8, 34.6, 31.5. HRMS (ESI),
+
m/z calcd for C H NO [M + H] 340.1543, found 340.1553.
20
22
4
3
.2.5. 2-(2-Oxo-2H-chromen-3-yl)benzoic Acid 8
Homophthalic acid (180 mg, 1 mmol) and 2-hydroxybenzaldehyde (122 mg, 1 mmol) were heated
at reflux toluene (15 mL) in a flask equipped with a Dean-stark distilling trap for 24 h. On cooling
to r. t., the precipitate formed was separated by filtration, washed with hexane (5 mL), air-dried and
crystallized from MeOH: H O (4:2) to give 194 mg, 0.73 mmol (73%) of the title compound as white
2
◦
1
solid: mp 259.1
7
7
−
259 C; H-NMR (400 MHz, DMSO-d )
δ
12.85 (br.s, 1H, COOH), 8.00 (s, 1H, CH(Ar)),
.95 (d, J = 7.7 Hz, 1H, CH(Ar)), 7.79–7.73 (m, 1H, CH(Ar)), 7.69 (td, J = 7.5, 1.2 Hz, 1H, CH(Ar)),
.65–7.60 (m, 1H, CH(Ar)), 7.57 (t, J = 7.6 Hz, 1H, CH(Ar)), 7.50 (d, J = 7.5 Hz, 1H, CH(Ar)), 7.45
168.2,
6
(
d, J = 8.2 Hz, 1H, CH(Ar)), 7.39 (t, J = 7.9 Hz, 1H, CH(Ar)). 13C-NMR (101 MHz, DMSO-d6)
δ
1
60.3, 153.4, 139.0, 136.4, 132.6, 131.9, 131.8, 131.4, 130.7, 130.2, 129.3, 128.8, 125.1, 119.9, 116.4. HRMS
+
(ESI), m/z calcd for C H NaO [M + Na] 289.0471, found 289.0461.
16
10
4
3
.2.6. General Procedure for the Preparation of N-Benzyloxy THIQ Acids 9a–g
A mixture of homophthalic acid (3.6 g, 20 mmol), O-benzylhydroxylamine (2.5 g, 20 mmol) and
the respective aldehyde (20 mmol) in toluene (20 mL) was placed in a pre-heated oil bath and refluxed
for 48 h in a flask equipped a Dean-Stark distilling trap. A thick precipitate which formed on cooling
◦
the reaction mixture to
−
20 C was isolated by filtration, washed with hexane, and crystallized from
aqueous methanol to provide pure title compounds.
(
3S/R,4S/R)-2-(Benzyloxy)-3-(4-fluorophenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9a).
◦
1
Yield 5.7 g, 73%. White solid, mp 138–140 C; H-NMR (400 MHz, DMSO-d )
δ
13.21 (s, 1H), 7.98
dd, J = 7.6, 1.5 Hz, 1H), 7.51 (td, J = 7.5, 1.6 Hz, 1H), 7.45 (ddd, J = 6.7, 4.8, 1.6 Hz, 3H), 7.41–7.30
m, 4H), 7.24 (td, J = 8.5, 6.3 Hz, 2H), 7.10 (t, J = 8.8 Hz, 2H), 5.78 (d, J = 2.0 Hz, 1H), 5.07 (d, J = 9.9
171.5,
61.5 (d, J = 243.9 Hz), 160.9, 135.0, 134.6 (d, J = 3.0 Hz), 133.2, 132.5, 129.9, 129.2, 128.5, 128.3, 128.2
6
(
(
Hz, 1H), 5.00 (d, J = 10.0 Hz, 1H), 4.42 (d, J = 2.1 Hz, 1H). 13C-NMR (101 MHz, DMSO-d6)
δ
1
(
d, J = 8.2 Hz), 128.1, 128.0, 126.9, 115.4 (d, J = 21.6 Hz), 75.9, 62.7, 51.4. HRMS (ESI), m/z calcd for
+
C H FNO Na [M + Na] 414.1112, found 414.1113.
23
18
4
(
3S/R,4S/R)-2-(Benzyloxy)-3-(4-methoxyphenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9b).
◦
1
Yield 5.8 g, 54%. White solid, mp 176–178 C; H-NMR (400 MHz, DMSO-d )
δ
13.15 (s, 1H), 7.96 (dd,
J = 7.7, 1.5 Hz, 1H), 7.50 (dd, J = 7.4, 1.6 Hz, 1H), 7.44 (ddd, J = 7.1, 5.2, 1.7 Hz, 3H), 7.40–7.30 (m, 4H),
.12–7.03 (m, 2H), 6.89–6.73 (m, 2H), 5.68 (d, J = 2.1 Hz, 1H), 5.04 (d, J = 9.9 Hz, 1H), 4.98 (d, J = 10.0 Hz,
6
7
1
1
13
H), 4.36 (d, J = 2.1 Hz, 1H), 3.65 (s, 3H). C-NMR (101 MHz, DMSO-d6) δ 171.7, 161.0, 158.7, 135.1,
33.5, 132.4, 130.2, 129.9, 129.2, 128.5, 128.3, 128.2, 128.0, 127.3, 126.9, 113.9, 75.8, 63.0, 55.0, 51.6. HRMS
+
(
ESI), m/z calcd for C H NO Na [M + Na] 426.1312, found 426.1318.
24
21
5
(
3S/R,4S/R)-2-(Benzyloxy)-3-(4-(tert-butyl)phenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9c).
◦
1
Yield 6.0 g, 70%. White solid, mp 190–191 C; H-NMR (400 MHz, DMSO-d )
δ 13.19 (s, 1H), 7.99 (dd,
6

Molecules 2019, 24, 864
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J = 7.7, 1.5 Hz, 1H), 7.50 (td, J = 7.5, 1.6 Hz, 1H), 7.44 (ddd, J = 7.5, 5.7, 1.6 Hz, 3H), 7.39–7.31 (m, 4H),
.28 (d, J = 8.5 Hz, 2H), 7.11 (d, J = 8.5 Hz, 2H), 5.72 (d, J = 2.1 Hz, 1H), 5.05 (d, J = 9.9 Hz, 1H), 5.01
d, J = 9.9 Hz, 1H), 4.42 (d, J = 2.1 Hz, 1H), 1.18 (s, 9H). 13C-NMR (101 MHz, DMSO-d6)
171.7, 161.1,
7
(
δ
1
3
50.1, 135.4, 135.0, 133.4, 132.4, 130.0, 129.3, 128.5, 128.3, 128.2, 128.0, 126.9, 125.9, 125.4, 75.9, 63.2, 51.5,
+
4.1, 30.9. HRMS (ESI), m/z calcd for C H NO Na [M + Na] 452.1832, found 452.1836.
27
27
4
(
3S/R,4S/R)-2-(Benzyloxy)-3-(4-(methoxycarbonyl)phenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic
◦
1
acid (9d). Yield 5.6 g, 65%. White solid, mp 197–199 C; H-NMR (400 MHz, DMSO-d )
δ
13.26 (s,
6
1
7
H), 7.98 (dd, J = 7.6, 1.6 Hz, 1H), 7.92–7.77 (m, 2H), 7.49 (td, J = 7.5, 1.6 Hz, 1H), 7.46–7.40 (m, 3H),
.39–7.27 (m, 6H), 5.88 (d, J = 1.9 Hz, 1H), 5.09 (d, J = 10.0 Hz, 1H), 5.03 (d, J = 10.0 Hz, 1H), 4.48 (d,
13
J = 2.0 Hz, 1H), 3.78 (s, 3H). C-NMR (101 MHz, DMSO-d6)
δ 171.4, 165.7, 161.0, 143.8, 135.0, 133.0,
1
32.5, 129.9, 129.4, 129.2, 129.1, 128.5, 128.3, 128.1, 128.0, 127.0, 126.5, 76.0, 63.2, 52.1, 51.3. HRMS (ESI),
+
m/z calcd for C H NO Na [M + Na] 454.1261, found 454.1264.
25
21
6
(
3S/R,4S/R)-2-(Benzyloxy)-3-(4-bromophenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9e).
◦
1
Yield 5.1 g, 57%. White solid, mp 187–189 C; H-NMR (400 MHz, DMSO-d )
δ
13.16 (s, 1H), 7.97 (dd,
J = 7.6, 1.5 Hz, 1H), 7.58–7.40 (m, 6H), 7.40–7.26 (m, 4H), 7.24–7.08 (m, 2H), 5.76 (d, J = 2.0 Hz, 1H), 5.07
d, J = 9.9 Hz, 1H), 5.00 (d, J = 10.0 Hz, 1H), 4.42 (d, J = 2.0 Hz, 1H). 13C-NMR (101 MHz, DMSO-d6)
6
(
δ
1
7
71.4, 161.0, 137.9, 135.0, 133.1, 132.5, 131.5, 129.9, 129.2, 128.5, 128.4, 128.3, 128.1, 128.0, 127.0, 120.9,
+
5.9, 62.9, 51.3. HRMS (ESI), m/z calcd for C H BrNO Na [M + Na] 474.0311, found 4754.0305.
23
18
4
(
3S/R,4S/R)-2-(Benzyloxy)-3-(2-methylphenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9f).
◦
1
Yield 4.2 g, 54%. White solid, mp 176–178 C; H-NMR (400 MHz, DMSO-d )
δ 13.25 (s, 1H), 8.03 (dd,
6
J = 7.3, 1.9 Hz, 1H), 7.54–7.43 (m, 1H), 7.43–7.33 (m, 5H), 7.30 (dd, J = 6.9, 1.9 Hz, 1H), 7.22 (d, J = 7.5
Hz, 1H), 7.12 (td, J = 7.4, 1.3 Hz, 1H), 6.98 (td, J = 7.6, 1.4 Hz, 1H), 6.68 (dd, J = 7.9, 1.2 Hz, 1H), 5.85
(
3
d, J = 1.8 Hz, 1H), 5.01 (d, J = 10.1 Hz, 1H), 4.98 (d, J = 10.1 Hz, 1H), 4.25 (d, J = 1.8 Hz, 1H), 2.44 (s,
H). C-NMR (101 MHz, DMSO-d6) δ 171.8, 161.3, 135.8, 135.2, 135.0, 132.9, 132.3, 131.1, 130.1, 129.2,
13
1
28.5, 128.3, 128.2, 128.0, 127.6, 126.8, 125.8, 124.7, 76.0, 61.1, 49.9, 18.5. HRMS (ESI), m/z calcd for
+
C H NO Na [M + Na] 410.1363, found 410.1371.
24
21
4
(
3
3S/R,4S/R)-2-(Benzyloxy)-3-(4-nitrophenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (9g). Yield
◦
.3 g, 47%. Pale yellow solid, mp 190–191 C; H-NMR (400 MHz, DMSO-d ) δ 13.30 (s, 1H), 8.13
6
1
(
d, J = 8.8 Hz, 2H), 7.99 (dd, J = 7.6, 1.6 Hz, 1H), 7.58–7.41 (m, 6H), 7.40–7.30 (m, 4H), 5.97 (d, J = 1.8
Hz, 1H), 5.10 (d, J = 10.0 Hz, 1H), 5.04 (d, J = 10.0 Hz, 1H), 4.53 (d, J = 2.0 Hz, 1H). 13C-NMR (101
MHz, DMSO-d6)
δ
171.2, 161.0, 147.0, 146.0, 134.9, 132.9, 132.6, 130.0, 129.3, 128.6, 128.3, 128.2, 127.8,
+
127.5, 127.0, 123.7, 76.0, 62.9, 51.0. HRMS (ESI), m/z calcd for C H N O Na [M + Na] 441.1057,
23
18
2
6
found 441.1056.
3
.2.7. 2-(Benzyloxy)-3-(4-methoxyphenyl)-3,4-dihydroisoquinolin-1(2H)-one (10)
To a stirred solution of compound 9a (403 mg, 1 mmol) in DMSO (3 mL) in a screw-cap vial
◦
K CO (207 mg, 1.5 mmol) was added. The mixture was heated to 110 C and stirred overnight. After
2
3
cooling to room temperature, the reaction mixture was diluted with EtOAc (30 mL) and water (15 mL).
The organic layer was then separated and washed with water (3
and concentrated in vacuo to give pure title compound as beige foam. Yield 275 mg, 77%. H-NMR
×
15 mL), dried over Na SO , filtered,
2
4
1
(400 MHz, CDCl3)
δ 8.19 (dd, J = 7.6, 1.6 Hz, 1H), 7.44–7.35 (m, 2H), 7.33 (s, 5H), 7.17–7.10 (m, 2H),
7
.08–7.03 (m, 1H), 6.85–6.72 (m, 2H), 5.15 (d, J = 10.3 Hz, 1H), 4.91 (d, J = 10.3 Hz, 1H), 4.74 (t, J = 5.6
13
Hz, 1H), 3.75 (s, 3H), 3.41 (dd, J = 16.1, 6.1 Hz, 1H), 3.16 (dd, J = 16.1, 5.2 Hz, 1H). C-NMR (101
MHz, CDCl3)
δ
164.3, 159.4, 135.6 (d, J = 4.9 Hz), 132.6, 131.1, 129.8, 128.8, 128.8, 128.5, 128.2, 128.1,
+
127.6, 127.3, 114.1, 63.4, 55.4, 36.9. HRMS (ESI), m/z calcd for C H NO Na [M + Na] 382.1414,
23
21
3
found 382.1418.

Molecules 2019, 24, 864
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3
.2.8. General Procedure for O-debenzylation
Synthesis of Compounds 7a, 11, and 12
Compound 7a (100 mg, 0.25 mmol), 10 (50 mg, 0.14 mmol) or 7c (150 mg, 0.38 mmol) was dissolved
in MeOH (15 mL), 0.05 molar equiv. of 10% Pd/C was added, and the reaction was stirred under the
atmosphere of hydrogen (introduced via a balloon) for 16 h at room temperature. The catalyst was
removed by filtration of the reaction mixture through a pad of Celite and the filtrate was concentrated
in vacuo to give pure title compounds.
2
-Hydroxy-3-(4-methoxyphenyl)-3,4-dihydroisoquinolin-1(2H)-one (11). Yield 35 mg, 94%. White solid,
◦
1
mp 95–96 C (MeOH); H-NMR (400 MHz, DMSO-d )
7
6
δ
9.92 (s, 1H), 7.93 (dd, J = 7.6, 1.5 Hz, 1H),
.42 (td, J = 7.4, 1.5 Hz, 1H), 7.35 (t, J = 7.4 Hz, 1H), 7.20 (d, J = 7.4 Hz, 1H), 7.09 (d, J = 8.7 Hz, 2H),
.82 (d, J = 8.7 Hz, 2H), 5.07 (dd, J = 6.4, 3.8 Hz, 1H), 3.77–3.48 (m, 4H), 3.21–3.09 (m, 1H). 13C-NMR
6
(
3
101 MHz, DMSO-d6)
δ
161.4, 158.5, 135.4, 131.9, 131.8, 128.6, 127.8, 127.4, 126.8, 126.7, 113.7, 62.5, 55.0,
+
6.0. HRMS (ESI), m/z calcd for C H N O Na [2M + Na] 561.1996, found 561.1994.
32
30
2
6
(
9
1
3R/S,4R/S)3-(2-Hydroxyphenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic acid (12). Yield 100 mg,
◦
3%. White solid, mp 202.3 202.8 C; H-NMR (400 MHz, DMSO-d ) δ 12.79 (s, 1H, COOH), 9.85 (s,
6
1
−
H, OH), 8.24 (d, J = 4.9 Hz, 1H, NH), 7.91 (dd, J = 7.2, 1.9 Hz, 1H, CH(Ar)), 7.39 (td, J = 7.1, 1.7 Hz, 2H,
2
CH(Ar)), 7.22 (d, J = 7.0, 1H, CH(Ar)), 7.00 (t, J = 8.4 Hz, 1H, CH(Ar)). 6.81 (d, J = 8.1 Hz, 1H, CH(Ar)),
.71 (d, J = 7.5 Hz, 1H, CH(Ar)), 6.59 (t, J = 7.5 Hz, 1H, CH(Ar)), 5.39 (dd, J = 5.0, 1.7 Hz, 1H, 3-H), 4.07
6
(
s, 1H, 4-H). 13C-NMR (101 MHz, DMSO-d6)
δ
173.0, 164.8, 154.4, 135.4, 132.2, 130.1, 129.4, 128.5, 128.0,
+
1
27.7, 126.9, 126.9, 119.0, 115.6, 50.8, 48.6. HRMS (ESI), m/z calcd for C H NO4 [M + H] 284.0917,
16
14
found 284.0924.
3
.3. Spectrophotometric Investigation of Compounds 7d, 7f, 7g, 7h, 7j, and Their Complexation with
Iron(III) Nitrate
Spectrophotometric measurements were performed on a UV-1800 Shimadzu double beam
spectrophotometer (Kyoto, Japan) using 10.00 mm quartz cells. Methanol or methanol—water (85:15)
◦
mixture were used as solvent. All measurements were performed at room temperature (26 C) in
stoppered cells. For experiments with Fe3+ complexation, Fe(NO ) ·9H O was used as iron source.
3
3
2
UV-Vis spectra of compounds 7d, 7f, 7g, 7h, 7j (Figure 2a and ESI, Figures S1–S3) were recorded
−
5
for MeOH solutions (C = 5
×
10 M) prior to titration experiments. Sample preparation for UV-Vis
L aliquote of 0.001 M stock solution of compound
10 mol) in MeOH was placed in a 10.00 mm quartz cuvette equipped with
magnetic stir bar and diluted to 3 mL with 1050 L of MeOH and 450 L of water to obtain solution
10 M). Aliquotes, 5 or 10 L of 0.015 M aqueous Fe(NO ) solution,
titrations (Figure 3a and ESI, Figures S4–S8): a 1500
µ
−
6
7
d
,
7f
,
7g
,
7h
,
7j (1.5
×
µ
µ
−
4
in 85% aq. MeOH (C = 5
×
µ
L
3 3
−
4
were added to the cell with calibrated micropipette in a stepwise manner (CM = 0.25−5 × 10 M)
.
The solution was vigorously stirred after addition of each aliquote followed by registration of
absorbance spectrum over the wavelength range of 300–700 nm vs. 85% aq. MeOH (all measurements
were performed in stoppered cuvettes). Color of the solution was changed from colorless to purple,
which corresponds to appearance of a new absorption band with maximum at 500 nm. The absorbance
3+
at selected wavelength was plotted as a function of [Fe ]/[ligand] ratio to give binding isotherms
presented on Figure 3b and ESI, Figures S4–S8. The maxima of these curves correspond to the maximum
formation of complexes and indicate to the stoichiometry of the complexes. The average stoichiometry
of complex is estimated from the point where this curve changes its slope (this point is the intersect
point of bilinear fitting of experimental curve). 1:2 metal-to-ligand ratio was observed in all cases.
Formation constants (Table 1) were calculated from curves on Figure 3a using following equations:
[
MLx]
K =
(1)
[
M][L]x

Molecules 2019, 24, 864
13 of 15
3
2
K K [L] + K (1 + K (2C − C ))[L] + (1 + K (C − C ))[L] = 0
(2)
(3)
1
2
1
2
M
L
1
M
L
2
ε
(C )K [L] + 2ε
(C )K K [L]
∆ML2
M
1
2
∆
ML
M
1
∆
Aobs =
2
1
+ K [L] + K K [L]
1 1
2
Experimental curves were fitted to equation 3 corresponding to 1:2 metal-to-ligand complex
formation using nonlinear curve-fitting performed in ThordarsonFittingProgram [25]. The program
is based on the iterative adjustment of calculated values of absorbance (A) to observed values using
Equation (3) previously derived [25] from Equations (1) and (2), where K and K are stepwise
1
2
formation constants;
ε
=
ε
− ε and
ε
=
ε
− ε
where ε are molar absorptivities of
ML,
i
∆
ML
ML
L
∆ML2
ML2
3+
corresponding species; C and C are analytical concentrations of ligand and Fe respectively and
L
M
L is free ligand concentration.
4
. Conclusions
In summary, we have developed a practically convenient, three-component approach to
N-hydroxytetrahydroisoquinoline (THIQ) acids via a variant of the Castagnoli–Cushman reaction
involving in situ cyclodehydration of homophthalic acid with concomitant formation of an oxime
in refluxing toluene. Using hydroxylamine acetate or O-benzylhydroxylamine in lieu of the
hydroxylamine hydrochloride typically employed to prepare oximes was key to the success of the
reaction. For prepared N-bezyloxy THIQ acids decarboxylation and debenzylation reactions were
additionally investigated. Five selected cyclic hydroxamic acid compounds produced in the course of
3+
this study have been profiled and confirmed as ligands for Fe . Thus, a new, practically simple and
flexible approach to potential iron overload disease treatments or analogs of bacterial siderophores for
antibiotic delivery has been developed.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/24/5/864/s1,
1
13
copies of H and C spectra for all new compounds 7c–j
,l,m, 9a–g, 10-12, UV-Vis spectra for compounds 7d,f–h,j
(
Figures S1–S3), results of UV-Vis titration of compounds 7d,f–h,j with FeCl (Figure S4–S8).
3
Author Contributions: Organic synthesis and compound characterization, E.C. and O.B.; UV-Vis spectroscopy, O.B.;
conceptualization and project administration, M.K. and D.D.; methodology and validation, D.D., E.C. and O.B.;
Writing—Original Draft preparation, M.K. and O.B.; Writing—Review and Editing, D.D.; funding acquisition, O.B.
Funding: This research was funded by Russian Foundation for Basic Research (RFBR), grant number 18-33-00016.
Acknowledgments: We thank the Educational Resource Center of Chemistry, Research Center for Magnetic
Resonance and the Center for Chemical Analysis and Materials Research of Saint Petersburg State University
Research Park for obtaining the analytical data.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
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Sample Availability: Samples of the compounds 7a–m, 9a–g are available from the authors.
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(