Synthesis, Chiral Resolution and Enantiomers Absolute Configuration of 4-Nitropropranolol and 7-Nitropropranolol - MDPI
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molecules
Article
Synthesis, Chiral Resolution and Enantiomers Absolute
Configuration of 4-Nitropropranolol and 7-Nitropropranolol
Rosa Sparaco 1 , Antonia Scognamiglio 1 , Angela Corvino 1 , Giuseppe Caliendo 1 , Ferdinando Fiorino 1 ,
Elisa Magli 2 , Elisa Perissutti 1 , Vincenzo Santagada 1 , Beatrice Severino 1 , Paolo Luciano 1 ,
Marcello Casertano 1 , Anna Aiello 1 , Gilberto De Nucci 3, * and Francesco Frecentese 1
1 Department of Pharmacy, University of Naples Federico II, Via D. Montesano 49, 80131 Naples, Italy
2 Department of Public Health, University of Naples Federico II, Via Pansini 5, 80131 Naples, Italy
3 Department of Pharmacology, Faculty of Medical Sciences, State University of Campinas (UNICAMP),
Campinas 13083-970, SP, Brazil
* Correspondence: denucci@unicamp.br; Tel.: +55-19-991788879; Fax: +55-19-32521516
Abstract: We recently identified 6-nitrodopamine and other nitro-catecholamines (6-nitrodopa,
6-nitroadrenaline), indicating that the endothelium has the ability to nitrate the classical catecholamines
(dopamine, noradrenaline, and adrenaline). In order to investigate whether drugs could be subject to
the same nitration process, we synthesized 4-nitro- and 7-nitropropranolol as probes to evaluate the
possible nitration of the propranolol by the endothelium. The separation of the enantiomers in very
high yields and excellent enantiopurity was achieved by chiral HPLC. Finally, we used Riguera’s
method to determine the absolute configuration of the enantiomers, through double derivatization
with MPA and NMR studies.
Keywords: 4-nitropropranolol; 7-nitropropranolol; chiral HPLC; NMR absolute configuration assign-
ment; Riguera’s method
Citation: Sparaco, R.; Scognamiglio,
A.; Corvino, A.; Caliendo, G.; Fiorino,
F.; Magli, E.; Perissutti, E.; Santagada,
1. Introduction
V.; Severino, B.; Luciano, P.; et al. Propranolol is a beta blocker used to treat cardiovascular disorders, tremors, hyper-
Synthesis, Chiral Resolution and tension, and angina. Propranolol also is an effective and safe drug for treating migraine
Enantiomers Absolute Configuration headaches, anxiety disorders, and infantile hemangiomas. Moreover, several studies have
of 4-Nitropropranolol and recently reported that patients receiving propranolol reduced their risk of neck and head,
7-Nitropropranolol. Molecules 2023, colon, stomach, and prostate cancers [1,2].
28, 57. https://doi.org/10.3390/ Nitroaromatic compounds are relatively rare in nature and have been introduced
molecules28010057 into the environment mainly by human activities. The nitro group provides chemical and
Academic Editor: Antonio Zucca functional diversity in these molecules due to its electron-withdrawing nature that, in
concert with the stability of the benzene ring, makes nitroaromatic compounds resistant to
Received: 3 November 2022 oxidative degradation [3].
Revised: 6 December 2022
6-Nitrocatecholamines are a wide family of neurotransmitters; 6-nitroepinephrine and
Accepted: 15 December 2022
6-nitrodopamine belong to this family and have been shown to have several biological activ-
Published: 21 December 2022
ities, such as inhibition of neuronal norepinephrine uptake, catechol-O-methyltransferase
activity, neuronal nitric oxide synthase activity, and contractile capacity at vascular adreno-
ceptors [4].
Copyright: © 2022 by the authors. 6-Nitrodopamine, in particular, has been recently identified by us from vascular tissues,
Licensee MDPI, Basel, Switzerland. such as human umbilical vessels [5], and its release proved to be substantially reduced
This article is an open access article when the endothelium was mechanically removed from these vessels.
distributed under the terms and 6-Nitrodopamine is one hundred times more potent than dopamine, noradrenaline,
conditions of the Creative Commons and adrenaline as a positive chronotropic agent [6]. The basal release of 6-nitrodopamine
Attribution (CC BY) license (https:// was detected in Chelonoidis carbonarius aortic rings [7], as well as in rat [8] and human vas
creativecommons.org/licenses/by/ deferens, too [9].
4.0/).
Molecules 2023, 28, 57. https://doi.org/10.3390/molecules28010057 https://www.mdpi.com/journal/molecules6-Nitrodopamine is one hundred times more potent than dopamine, noradrenaline,
and adrenaline as a positive chronotropic agent [6]. The basal release of 6-nitrodopamine
was detected in Chelonoidis carbonarius aortic rings [7], as well as in rat [8] and human vas
deferens, too [9].
Molecules 2023, 28, 57 2 of 15
We have now identified other catecholamines (6-nitrodopa and 6-nitroadrenaline),
indicating that the endothelium has the ability to nitrate the classical catecholamines (do-
pamine, noradrenaline, and adrenaline). Thus, it is very possible that not only endogenous
We havebut
compounds nowalsoidentified other
drugs could becatecholamines (6-nitrodopa
subject to the same process. and 6-nitroadrenaline),
indicating that the endothelium has the ability to nitrate
Propranolol belongs to the 3-(aryloxy)-1-(alkylamino)-β-blockerthe classical catecholamines
family, a class of
(dopamine, noradrenaline, and adrenaline). Thus, it is very possible that not only endoge-
molecules with activity residing mainly in the S isomers [10]. Indeed, (S)-(−)-propranolol
nous compounds
is 98 times but also
more active drugs
than could be subjectMethods
(R)-(+)-enantiomer. to the same process.
reported for the synthesis of (S)-
Propranolol belongs to the 3-(aryloxy)-1-(alkylamino)-β-blocker
propranolol have been reported in the literature, ranging from the resolution family, ofa class
a chiralof
molecules with activity residing mainly in the S isomers [10]. Indeed, (S)-(−)-propranolol
mixture of a final compound, or of intermediates, to asymmetric synthesis [11–15]. More-
is 98 times more active than (R)-(+)-enantiomer. Methods reported for the synthesis of
over, several examples of propranolol derivatives, designed and synthesized for variable
(S)-propranolol have been reported in the literature, ranging from the resolution of a
purposes, have been reported in the literature [16–19].
chiral mixture of a final compound, or of intermediates, to asymmetric synthesis [11–15].
In order to investigate whether drugs could be subject to the same nitration process
Moreover, several examples of propranolol derivatives, designed and synthesized for
discovered for endogenous catecholamines, we synthesized 4-nitro- and 7-nitropropran-
variable purposes, have been reported in the literature [16–19].
olol. These propranolol derivatives will be used as probes to evaluate the possible nitra-
In order to investigate whether drugs could be subject to the same nitration process
tion of the propranolol by the endothelium. Because enantiomers may not share the same
discovered for endogenous catecholamines, we synthesized 4-nitro- and 7-nitropropranolol.
pharmacologic profile, in order to discover differences in nitration by the endothelium of
These propranolol derivatives will be used as probes to evaluate the possible nitration
different enantiomers of the same compound, we focused our attention on chiral resolu-
of the propranolol by the endothelium. Because enantiomers may not share the same
tion.
pharmacologic profile, in order to discover differences in nitration by the endothelium of
The enantiomers
different pure enantiomers werecompound,
of the same obtained bywechiral
focused chromatography,
our attention onand theresolution.
chiral absolute
configuration of secondary alcohol was obtained by applying Riguera’s
The pure enantiomers were obtained by chiral chromatography, and the absolutemethod based on
the sign distribution
configuration [20,21].was obtained by applying Riguera’s method based on
of ΔδHalcohol
of secondary
the sign distribution of ∆δH [20,21].
2. Results
2. Results
Our study commenced with the preparation of (±)-4-nitropropranolol (2a) and (±)-7-
Our study commenced
nitropropranolol (2b). with the preparation of (±)-4-nitropropranolol (2a) and (±)-7-
nitropropranolol (2b).
The nitrate derivatives of propranolol were obtained following the procedure de-
The nitrate derivatives
scribed in Scheme 1. of propranolol were obtained following the procedure de-
scribed in Scheme 1.
Scheme 1.1. Synthetic procedure
Synthetic for for
procedure the synthesis of (±)-4-nitropropranolol
the synthesis (2a) and(2a)
of (±)-4-nitropropranolol (±)-7-nitropro-
and (±)-7-
pranolol (2b).
nitropropranolol (2b).
particular,(±
In particular, )-4-nitropropranolol (2a)
(±)-4-nitropropranolol and(±(±)-7-nitropropranolol
(2a)and )-7-nitropropranolol (2b) were
(2b) synthe-
were syn-
sized from
thesized the the
from reaction between
reaction 4-nitro-1-naphthol
between or 7-nitro-1-naphthol
4-nitro-1-naphthol and and
or 7-nitro-1-naphthol epichlorohy-
epichlo-
drin; the glycidyl
rohydrin; naphthyl
the glycidyl ethersethers
naphthyl (1a and andthus
(1a1b) 1b) obtained were reacted
thus obtained with isopropy-
were reacted with iso-
lamine providing the racemic mixtures of the nitrate derivatives of propranolol.
propylamine providing the racemic mixtures of the nitrate derivatives of propranolol.
The four enantiomers (two for each racemic mixture) mixture) were then
then obtained
obtained by
by resolu-
resolu-
tion of the corresponding
corresponding racemic
racemic mixture
mixtureby bychiral
chiralHPLC.
HPLC.Chromatograms
Chromatogramsofofprepara-
prepar-
ative purificationsare
tive purifications arereported
reportedininFigure
Figure1 1(Panels
(PanelsAA and
and BBforfor 4-nitropropranolol
4-nitropropranolol and
and 7-
7-nitropropranolol, respectively).
nitropropranolol, respectively).
At the end of racemic resolutions, each enantiomer was further analyzed to determine
the purity, which was generally >98% with an enantiomeric excess >98%.
The purity of (+) and (−) enantiomers was assessed by chiral HPLC with the same
solvents and conditions as for preparative purification, but with a flow rate of 1 mL/min.
The chromatograms obtained for each single enantiomer are reported in the Supplementary
Materials (Figures S1–S4).
The optical rotation values for each enantiomer were then determined by optical
polarimetry.Molecules 2022, 28,
Molecules 2023, 27, 57
x FOR PEER REVIEW 33 of
of 15
15
Figure 1.
Figure 1. Chiral resolution
Chiral chiralchiral
resolution resolution of (±)-4-nitropropranolol
resolution 2a (Panel2a
of (±)-4-nitropropranolol A) (A)
and and
(±)-7-nitro-
(±)-7-
propranolol 2b (Panel B).
nitropropranolol 2b (B).
At the
The end ofcorresponding
isomers racemic resolutions,
to Peak each1 gaveenantiomer
positive [α] was further analyzed to deter-
D values for both 4-nitro- and
for 7-nitropropranolol. The optical evaluation of a solution of eachexcess
mine the purity, which was generally >98% with an enantiomeric isomer >98%.
corresponding
The2 purity
to Peak confirmed of (+)
thatand (−) were
these enantiomers was assessed
characterized by chiral
by negative HPLC with the same
[α]D values.
solvents andthe
Finally, conditions
absolute as for preparative
configuration (AC)purification, but with
of the synthesized a flow rate
compounds wasofdetermined
1 mL/min.
Thethe
by chromatograms obtained
double derivatization offor
each each single
chiral enantiomer
compound withare
an reported
adequate in the Supplemen-
chiral derivatizing
tary Materials
agent represented (Figures S1–S4).
by α-methoxyphenylacetic acid (MPA) followed by an NMR analysis of
The optical rotation
the resulting derivatives. values for each enantiomer were then determined by optical po-
larimetry.
Defining the absolute stereochemistry is essential to understanding many of the chem-
The isomers
ical, biological, andcorresponding
pharmaceutical to Peak 1 gave of
properties positive [α]D values
new chiral for both
compounds, and4-nitro- and
therefore
for 7-nitropropranolol.
developing easy-to-use The optical
methods forevaluation of a solution
its determination of eachisisomer
in solution corresponding
of utmost interest for
to Peak 2 confirmed
researchers involved that these were
in bioactive characterized
compounds. by negative
Nowadays, [α] D values.
the method of choice for the AC
assignment
Finally,tothechiral secondary
absolute alcohols (AC)
configuration by NMR is Mosher’s
of the synthesizedmethod, first described
compounds was deter- in
1973, and since then, it has become increasingly popular, thanks to its
mined by the double derivatization of each chiral compound with an adequate chiral deri- demonstrated relia-
bility [22].
vatizing The represented
agent method has been extensively studied and
by α-methoxyphenylacetic further
acid (MPA) optimized
followedby byRiguera’s
an NMR
research
analysis ofgroup [20].
the resulting derivatives.
Each optically
Defining pure derivative
the absolute stereochemistry is essential to (−
(+)-4-nitropropranolol, )-4-nitropropranolol,
understanding many of (+)-7-
the
nitropropranolol,
chemical, biological, andand(−)-7-nitropropranolol),
pharmaceutical properties respectively
of newnamed
chiral as (+)-2a, (−)-2a,
compounds, and(+)-2b,
there-
and −)-2b, was easy-to-use
fore (developing reacted bothmethods
with (R)-( for−its
)-α-methoxyphenylacetic
determination in solution acidis and (R)-MPA
of utmost and
interest
with (S)-(+)-α-methoxyphenylacetic acid and (S)-MPA, which has been
for researchers involved in bioactive compounds. Nowadays, the method of choice for the proven to be a chiral
derivatizing
AC assignment agent (CDA)secondary
to chiral of choice for alcohols
alcohols by [20,21].
NMR is Mosher’s method, first described
The and
in 1973, synthetic
since procedure
then, it hasisbecome
reportedincreasingly
in Scheme 2.popular, thanks to its demonstrated
reliability [22]. The method has been extensively studied and further optimized by Ri-
guera’s research group [20].
Each optically pure derivative (+)-4-nitropropranolol, (−)-4-nitropropranolol, (+)-7-
nitropropranolol, and (−)-7-nitropropranolol), respectively named as (+)-2a, (−)-2a, (+)-2b,and (−)-2b, was reacted both with (R)-(−)-α-methoxyphenylacetic acid and (R)-MPA and
with (S)-(+)-α-methoxyphenylacetic acid and (S)-MPA, which has been proven to be a chi-
Molecules 2023, 28, 57 4 of 15
ral derivatizing agent (CDA) of choice for alcohols [20,21].
The synthetic procedure is reported in Scheme 2.
Scheme 2. Synthetic
Syntheticprocedure
procedureforfor
thethe
synthesis of bis-MPA
synthesis derivatives
of bis-MPA of propranolol
derivatives nitro-ana-
of propranolol nitro-
logues.
analogues.
The assignment of the absolute configuration of secondary alcohol was based on the
sign distribution
distribution ofof ∆δ
ΔδHH (calculated
(calculated as as δδHHRR −
− δδHHSS),),obtained
obtainedfrom from the
the comparison
comparison of the
NMR spectra of the different MPA-derived
MPA-derived compounds.
compounds. In that regard, the NMR signals
were identified, paying
paying special
specialattention
attentiontotothe theprotons
protonslocatedlocatedatat
the the
twotwo substituents
substituents of
of the chiral carbon, which constitute the L1 and L2 groups
the chiral carbon, which constitute the L1 and L2 groups (Figures 4, 7, 10, and (Figures 4, 7, 10, and 13).
13). We
ΔδRS ∆δ
We interpreted the experimental RS
interpreted the experimental data data
as theasresult
the result
of theofshielding/deshielding
the shielding/deshielding aniso-
anisotropic
tropic effects effects exerted
exerted by theby MPA
the MPAunitsunits around
around thethe stereogenic
stereogenic center,
center, which
which showeda
showed
adifferent
differentand
andopposite
oppositebehavior
behaviorwhenwhen(R)- (R)-oror(S)-MPA
(S)-MPA was was used.
used. This
This effect
effect isis moved
moved inin
consistent chemical shift differences that allowed to analyze the ∆δ
consistent chemical shift differences that allowed to analyze the Δδ pattern according to
RS pattern according to
RS
the∆δ
RS R S
Riguera’s
Riguera’s proposed
proposed model
model [20,21].
[20,21]. Indeed,
Indeed, the the analysis
analysisof ofthe ΔδRS(δ(δR −− δδS)) parameters,
parameters,
obtained
obtained from
from the
the comparison
comparison of of the
the spectra,
spectra, showed
showed aa consistent
consistent distribution
distribution of of positive
positive
and negative ∆δ values around the stereogenic carbon and
and negative Δδ values around the stereogenic carbon and allowed us to deduce: allowed us to deduce:
- the S configuration
the S configuration forfor (+)-4-NO -propranolol and
(+)-4-NO22-propranolol and (+)-7-NO
(+)-7-NO22-propranolol;
-propranolol;
- the R configuration for ( − )-4-NO -propranolol and ( −
the R configuration for (−)-4-NO22-propranolol and (−)-7-NO2-propranolol.)-7-NO 2 -propranolol.
3.
3. Materials
Materials and
and Methods
Methods
3.1. Chemistry
3.1. Chemistry
All commercial reagents and solvents were purchased from Merck(Darmstadt, Ger-
many),AllTCI
commercial reagents andBelgium)
Europe (Zwijndrecht, solvents were purchased
and Enamine from
(Kyiv, Merck(Darmstadt,
Ukraine). Ger-
Reactions were
many), TCI Europe (Zwijndrecht, Belgium) and Enamine (Kyiv, Ukraine).
stirred at 400 rpm by a Heidolph MR Hei-Standard magnetic stirrer. Solutions were con-Reactions were
stirred at 400
centrated withrpm by a R-114
a Buchi Heidolph MR
rotary Hei-Standard
evaporator magnetic
(Flawil, stirrer. Solutions
Switzerland) were con-
at low pressure. All
reactions were followed by TLC carried out on Merck silica gel 60 F254 plates withAll
centrated with a Buchi R-114 rotary evaporator (Flawil, Switzerland) at low pressure. a
reactions were
fluorescent followed
indicator by TLC
on the carried
plates out on
and were Merck silica
visualized withgel
UV60light
F254(254
plates
nm).with a flu-
Prepara-
orescent
tive indicator on the
chromatographic plates andwere
purifications wereperformed
visualizedusing
with silica
UV light (254 nm).
gel column Preparative
(Kieselgel 60).
chromatographic purifications were performed using silica gel column
Chiral HPLC resolution was performed using a WATERS (Milford, MA, USA) Quaternary (Kieselgel 60). Chi-
ral HPLCMobile
Gradient resolution
2535 was performed
instrument usingwith
equipped a WATERS
WATERS(Milford,
UV/VisibleMA,Detector
USA) Quaternary
2489 set to
Gradient Mobile 2535 instrument equipped with WATERS UV/Visible
a dual-wavelength UV detection at 254 and 280 nm. The chiral resolutions Detector 2489 set
were achieved
on the Kromasil 5-Amycoat column Phenomenex (150 mm × 21.2 mm, 5 µm particle were
to a dual-wavelength UV detection at 254 and 280 nm. The chiral resolutions size).
achieved
LC-MS on the Kromasil
analysis was5-Amycoat
performedcolumn
using aPhenomenex
VANQUISH(150 FLEX mmmodule
× 21.2 mm, 5 μm par-
comprising a
ticle size). pump with degasser, an autosampler, a column oven (set at 40 ◦ C), a diode-
quaternary
array detector DAD, and a column as specified in the respective methods below. The
MS detector (ISQ Thermo Fisher Scientific, Waltham, MA, USA) was configured with an
electrospray ionization source. Mass spectra were acquired by scanning from 100 to 700
in 0.2 s. The capillary needle voltage was 3 kV in positive and 2 kV in negative ionization
mode, and the source temperature was maintained at 250 ◦ C. Nitrogen was used as theMolecules 2023, 28, 57 5 of 15
nebulizer gas. Reversed phase UHPLC was carried out on a Luna Omega-C18 column
Phenomenex (3 µm, 50 × 2.1 mm) with a flow rate of 0.600 mL/min. Two mobile phases
were used, mobile phase A: water with 0.1% formic acid and mobile phase B: acetonitrile
(LiChrosolv for LC-MS Merck), and they were employed to run gradient conditions from
15% B for 0.20 min, from 10% to 95% for 1.60 min, 95% B for 0.80 min, and 15% B for
0.10 min, and these conditions were held for 1.05 min in order to re-equilibrate the column
(Total Run Time 3.55 min). An injection volume of 0.8 µL was used. Data acquisition was
performed with Chromeleon 7. HPLC on silica gel was carried out on a Luna SiO2 column
(Phenomenex, 3 µM, 50 × 4.6 mm) using a Knauer K-501 apparatus equipped with a Knauer
K-2301 RI detector (LabService Analytica s.r.l., Anzola dell’Emilia, Italy). Melting points,
determined using a Buchi Melting Point B-540 instrument, are uncorrected and represent
values obtained on recrystallized or chromatographically purified material. Mass spectra of
final products were performed on LTQ Orbitrap XL™ Fourier transform mass spectrometer
(FTMS) equipped with an ESI ION MAX™ (Thermo Fisher Scientific, Waltham, MA, USA)
source operating in positive mode. MeOH was used as solvent for compound infusion
into LTQ Orbitrap source. NMR experiments were recorded on Varian Mercury Plus
700 MHz instrument. All spectra were recorded in CDCl3 . Chemical shifts were reported
in ppm and referenced to the residual solvent signal (CDCl3 : δH = 7.26, δC = 77.0). The
following abbreviations are used to describe peak patterns when appropriate: s (singlet),
d (doublet), dd (double doublet), t (triplet), m (multiplet), and bs (broad singlet). NMR
signals and monodimensional and bidimensional NMR spectra of 4-nitropropranolol 2a
and 7-nitropropranolol 2b are reported in Supplementary Materials (Figures S5–S14). NMR
signals, 1 H-NMR spectra, 13 C-NMR spectra, and HRMS spectra for bis-MPA derivatives of
4-nitropropranolol and 7-nitropropranolol are reported in the Supplementary Materials
(Figures S15–S46). Optical rotation values were determined using a JACSCO P-2000 series
polarimeter (Tokyo, Japan).
3.1.1. 2-(((4-Nitronaphthalen-1-yl)oxy)methyl)oxirane (1a)
4-Nitro-1-naphthol (0.6 g, 3.18 mmol) was dissolved in 10 mL of ethanol:water (9:1, v/v)
followed by adding 0.18 g of KOH into the mixture under stirring for 30 min. Successively,
1 mL of epichlorohydrin (12.7 mmol) was added until the color of the mixture changed
to orange-yellow. The mixture was heated under reflux for 4 h, and the formation of
the intermediate 1a was monitored by TLC (dichloromethane). Then, the solvent of the
reaction mixture was evaporated, and the residue was taken up in dichloromethane (90 mL)
extracted with brine (3 × 30mL). The organic layer was dried on anhydrous Na2 SO4 ,
concentrated, and chromatographed on a silica gel column (dichloromethane) to provide
the intermediate 1a as a yellow powder (298 mg). Yield: 38%. m.p. 80–81 ◦ C. 1 H NMR
(CDCl3 ) δ 8.75 (d, J = 8.6, 1H), 8.41 (d, J = 8.6, 1H), 8.35 (d, J = 8.2, 1H), 7.74 (t, J = 8.6, 1H),
7.60 (t, J = 8.6, 1H), 6.81 (d, J = 8.6, 1H), 4.56 (dd, J = 11.1, 1.9, 1H), 4.18 (dd, J = 11.1, 6.0,
1H), 3.53 (m, 1H), 3.01 (t, J = 4.0, 1H), and 2.86 (dd, J = 4.0, 2.6, 1H). 13 C NMR (CDCl3 ) δ
44.5, 49.7, 69.8, 102.8, 122.8, 123.5, 125.6, 126.6, 126.7, 126.9, 130.1, 139.7, and 159.2. ESI-MS
[M + H]+ m/z calc. 245.23 for C13 H11 NO4 found 246.2.
3.1.2. 2-(((7-Nitronaphthalen-1-yl)oxy)methyl)oxirane (1b)
Compound 1b was obtained following the same procedure reported for 1a starting
from 7-nitro-1- naphthol (0.6 g, 3.18 mmol). Yellow powder. 430 mg. Yield: 55%. m.p.
103–104 ◦ C. 1H NMR (CDCl3 ) δ 9.23 (d, J = 2.0, 1H), 8.23 (dd, J = 9.0, 2.0, 1H), 7.89 (d, J = 9.0,
1H), 7.58 (t, J = 8.0, 1H), 7.51 (d, J = 8.2, 1H), 6.95 (d, J = 7.7, 1H), 4.46 (dd, J = 10.9, 3.0, 1H),
4.18 (dd, J = 10.9, 5.9, 1H), 3.53 (m, 1H), 3.03 (t, J = 4.4, 1H), and 2.85 (dd, J = 4.4, 2.6, 1H).
13 C NMR (CDCl ) δ 44.8, 50.0, 69.6, 106.6, 119.7, 120.0, 120.5, 124.3, 129.5, 130.2, 136.9, 145.1,
3
and 155.7. ESI-MS [M + H]+ m/z calc. 245.23 for C13 H11 NO4 found 246.2.Molecules 2023, 28, 57 6 of 15
3.1.3. (±)-1-(Isopropylamino)-3-((4-nitronaphthalen-1-yl)oxy)propan-2-ol (2a)
The glycidyl naphthyl ether 1a (0.25 g, 1.02 mmol) was dissolved in 30 mL of iso-
propylamine, and the stirred mixture was heated to 30 ◦ C for 1 h. Then, the solvent of
the reaction mixture was evaporated, and the residue was chromatographed on a silica
gel column (dichloromethane/methanol 8:2, v/v) to provide ±-4-nitropropranolol 2a as
a yellow powder (85 mg). Yield: 28%. m.p. 98–99 ◦ C. 1 H NMR (CDCl3 ) δ 8.78 (d, J = 8.6,
1H), 8.38 (d, J = 8.6, 1H), 8.36 (d, J = 8.2, 1H), 7.74 (t, J = 8.6, 1H), 7.60 (t, J = 8.6, 1H), 6.84 (d,
J = 8.6, 1H), 4.28 (m, 1H), 4.24 (m, 1H), 4.17 (m, 1H), 3.03 (dd, J = 12.2, 4.0, 1H), 2.87–2.83
(m, overlapped, 2H), 1.12 (d, J = 3.4, 3H), and 1.11 (d, J = 3.4, 3H). 13 C NMR (CDCl3 ) δ 23.1,
23.2, 49.0, 49.1, 68.3, 71.4, 102.8, 122.6, 123.5, 125.5, 126.5, 126.9, 127.0, 130.1, 139.4, and 159.5.
ESI-MS [M + H]+ m/z calc. 304.34 for C16 H20 N2 O4 found 305.2.
3.1.4. (±)-1-(Isopropylamino)-3-((7-nitronaphthalen-1-yl)oxy)propan-2-ol (2b)
Compound 2b was obtained following the same procedure reported for 2a starting
from glycidyl naphthyl ether 1b (0.25 g, 1.02 mmol). Yellow powder. 96 mg. Yield: 31%.
m.p. 139–140 ◦ C. 1 H NMR (CDCl3 ) δ 9.20 (d, J = 2.0, 1H), 8.22 (dd, J = 9.0, 2.0, 1H), 7.88
(d, J = 9.0, 1H), 7.58 (t, J = 8.0, 1H), 7.49 (d, J = 8.2, 1H), 6.96 (d, J = 7.7, 1H), 4.24–4.19
(m, overlapped, 2H), 4.18 (m, overlapped, 1H), 3.03 (dd, J = 12.3, 3.6, 1H), 2.88–2.85 (m,
overlapped, 2H), 1.13 (d, J = 3.4, 3H), and 1.12 (d, J = 3.4, 3H). 13 C NMR (CDCl3 ) δ 23.0,
23.3, 48.9, 49.3, 68.1, 71.1, 106.5, 119.6, 119.9, 120.2, 124.3, 129.0, 136.8, and 156.0. ESI-MS
[M + H]+ m/z calc. 304.34 for C16 H20 N2 O4 found 305.2.
3.2. Chiral Resolution
Resolution of the racemic mixtures containing (±)-4-nitropropranolol 2a or (±)-7-
nitropropranolol 2b was performed using a Waters 2535 Quaternary Gradient Mobile
equipped with Waters 2489 UV/Visible Detector set to a dual-wavelength UV detection at
254 and 280 nm. The chiral resolutions were achieved on the Kromasil 5-Amycoat column
Phenomenex (150 mm × 21.2 mm, 5 µm particle size). Two mobile phases, in isocratic
condition, were used. Mobile phase A: n-Hexane (Chromasolv Sigma-Aldrich) and mobile
phase B: Isopropanol (Chromasolv Sigma-Aldrich) + 0.1% Diethylamine. Ratio of the mobile
phases was 86(A):14(B). The racemic mixtures were dissolved in ethanol at concentration of
75 mg/mL, and the injection volume was 200 µL (repeated 5 times); the sample was eluted
from the column at a flow rate of 15.0 mL/min at room temperature (pressure: ≈500 psi).
At the end of racemic resolutions, 30 mg of each enantiomer was collected. Purity of (+)
and (−) enantiomers was assessed by chiral HPLC using the Kromasil 5-Amycoat column
Phenomenex (150 mm × 4.6 mm, 5 µm particle size) with the same solvents and conditions
as for preparative purification, but with a flow rate of 1 mL/min.
3.3. Determination of Optical Rotation Values
Optical rotation values (Table 1) were determined in a JASCO P-2000 series polarimeter
for each single enantiomer.
Table 1. [α]D values of the propranolol nitro-derivatives.
Peak 1 (+)-4-Nitropropranolol [α]D = +9.25 (C = 1; MeOH)
Peak 2 (−)-4-Nitropropranolol [α]D = −9.25 (C = 1; MeOH)
Peak 1 (+)-7-Nitropropranolol [α]D = +7.41 (C = 1; MeOH)
Peak 2 (−)-7-Nitropropranolol [α]D = −7.41 (C = 1; MeOH)
3.4. Absolute Configuration Assignment
General synthetic procedure for the obtaining of the bis MPA derivatives of (±)-4-
nitropropranolol (2a) and (±)-7-nitropropranolol (2b):Molecules 2023, 28, 57 7 of 15
Each pure enantiomer was converted into the relevant MPA derivative by applying
the double derivatization method proposed by Riguera and coworkers, which is based on
the use of both R and S enantiomer of the α-methoxyphenylacetic acid [10,11]. In detail, a
portion of the pure nitro propranolol compounds ((+)-2a, (−)-2a, (+)-2b, and (−)-2b) was
dissolved in dry dichloromethane (1mL). To this solution, 2.2 equivalent (eq.) of either
(R)-(−)-α-methoxyphenylacetic acid or (S)-(+)-α-methoxyphenylacetic acid, 2.2 eq. of 1-
ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), and a catalytic amount of DMAP
were added. The mixture was stirred overnight at room temperature (rt) before removing
the solvent under vacuum. Each residue was separately purified by HPLC on silica gel
(Luna 3-µM SiO2 column, flow rate 0.5 mL/min, mobile phase n-hexane/ethyl acetate 6:4
v/v) providing the desired compounds in pure form. The reaction conditions and yields
are summarized in Table 2. 1 H NMR and 13 C NMR shifts were assigned by HSQC and
HMBC experiments and are reported in Figures 2–13.
Table 2. Description of the reaction conditions for the synthesis of MPA-derived compounds.
Reagent (R) or
Compounds (mg) EDC (mg) tR (min.)
(mg) (S)-MPA (mg)
bis-(R)-MPA derivative of
2.4 3 4 23.9
(+)-4-NO2 -propranolol (2.2 mg)
bis-(S)-MPA derivative of
2.3 3 4 26.7
(+)-4-NO2 -propranolol (2.4 mg)
bis-(R)-MPA derivative of
2.4 3 4 30.7
(+)-7-NO2 -propranolol (1.5 mg)
bis-(S)-MPA derivative of
2.6 4 5 32.5
(+)-7-NO2 -propranolol (2.5 mg)
bis-(R)-MPA derivative of
2.7 4 4 25.9
(−)-4-NO2 -propranolol (2.7 mg)
bis-(S)-MPA derivative of
2.2 3 3 23.2
(−)-4-NO2 -propranolol (2.1 mg)
bis-(R)-MPA derivative of
2.5 4 4 32.5
(−)-7-NO2 -propranolol (2.1 mg)
bis-(R)-MPA derivative of
2.5 4 4 30.7
(−)-7-NO2 -propranolol (1.8 mg)
Bis-(R)-MPA derivative of (+)-4-NO2 -propranolol: the analysis of the key correlations
1 H-13 C (2,3J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon resonances for the synthesized compound (Figure 2). HRESI-MS: m/z 601.2541
[M + H]+ (calculated for C34 H37 O8 N2 : 601.2544); HRESI-MS m/z 623.2359 [M+Na]+ (calcu-
lated for C34 H36 O8 N2 Na: 623.2364, Figure S17).
Bis-(S)-MPA derivative of (+)-4-NO2 -propranolol: all proton and carbon resonances
for the synthesized compound are reported in Figure 3. HRESI-MS: m/z 601.2549 [M + H]+
(calculated for C34 H37 O8 N2 : 601.2544, Figure S20).
Applying Riguera’s method [20,21], by double derivatization procedure, for assigning
the absolute configuration of secondary alcohol based on the sign distribution of ∆δH
(calculated as δH R − δH S ) allowed assigning S configuration to the (+)-4-NO2 -propranolol
(Figure 4).
Bis-(R)-MPA derivative of (+)-7-NO2 -propranolol: the analysis of the key correla-
tions 1 H-13 C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all
proton and carbon resonances for the synthesized compound (Figure 5). HRESI-MS:
m/z 601.2539 [M + H]+ (calculated for C34 H37 O8 N2 : 601.2544); HRESI-MS m/z 623.2358
[M+Na]+ (calculated for C34 H36 O8 N2 Na: 623.2364, Figure S23).Molecules 2022, 27, x FOR PEER REVIEW 8 of 15
Molecules2023,
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Figure 2. NMR signals of bis-(R)-MPA derivative of (+)-4-NO2-propranolol.
Bis-(S)-MPA derivative of (+)-4-NO2-propranolol: all proton and carbon resonances
for the synthesized compound are reported in Figure 3. HRESI-MS: m/z 601.2549 [M+H]+
Figure
Figure 2.2.NMR
NMR
(calculated signals
forsignals of
C34H37of bis-(R)-MPAderivative
Obis-(R)-MPA derivative
8N2: 601.2544,
of(+)-4-NO
Figure of (+)-4-NO22-propranolol.
S20). -propranolol.
Bis-(S)-MPA derivative of (+)-4-NO2-propranolol: all proton and carbon resonances
for the synthesized compound are reported in Figure 3. HRESI-MS: m/z 601.2549 [M+H]+
(calculated for C34H37O8N2: 601.2544, Figure S20).
Figure3.3.NMR
Figure NMRsignals
signalsof
ofbis-(S)-MPA
bis-(S)-MPAderivative
derivativeofof(+)-4-NO
(+)-4-NO22-propranolol.
-propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
Figure 3. NMR
(calculated as signals
δHR − δof bis-(S)-MPA derivative of (+)-4-NO2-propranolol.
HS) allowed assigning S configuration to the (+)-4-NO2-propranolol
(Figure 4).
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
(calculated as δHR − δHS) allowed assigning S configuration to the (+)-4-NO2-propranolol
(Figure 4).Molecules2023,
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15
Figure 4. (S)-(+)-4-NO2-propranolol absolute configuration assignment.
Bis-(R)-MPA derivative of (+)-7-NO2-propranolol: the analysis of the key correlations
1H-13C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon resonances for the synthesized compound (Figure 5). HRESI-MS: m/z 601.2539
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2358 [M+Na]+ (calculated
Figure
for C344.4.
Figure (S)-(+)-4-NO22-propranolol
H(S)-(+)-4-NO -propranolol
36O8N2Na: 623.2364,
absolute
absolute
Figure configuration assignment.
S23).configuration assignment.
Bis-(R)-MPA derivative of (+)-7-NO2-propranolol: the analysis of the key correlations
1H-13C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon resonances for the synthesized compound (Figure 5). HRESI-MS: m/z 601.2539
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2358 [M+Na]+ (calculated
for C34H36O8N2Na: 623.2364, Figure S23).
Figure5.5.NMR
Figure NMRsignals
signalsofofbis-(R)-MPA
bis-(R)-MPAderivative
derivativeofof(+)-7-NO
(+)-7-NO2 -propranolol.
2-propranolol.
Bis-(S)-MPA
Bis-(S)-MPAderivative
derivativeofof(+)-7-NO
(+)-7-NO2 -propranolol:
2-propranolol: all allproton
protonandandcarbon
carbonresonances
resonances
for
forthethesynthesized
synthesizedcompound
compoundare reported
are reportedin in
Figure
Figure6. HRESI-MS:
6. HRESI-MS: m/z 601.2550
m/z 601.2550 + H]+ +
[M[M+H]
+
(calculated
(calculatedfor forC34
C34HH3737O
O88N
N22: 601.2544); HRESI-MSm/z
601.2544); HRESI-MS m/z 623.2368
623.2368[M+Na]
[M+Na]+ (calculated
(calculatedfor
for
CC3434H
H36 O N Na: 623.2364, Figure
36O8N22Na: 623.2364, Figure S26). S26).
Figure 5. NMR Riguera’s
Applying signals of bis-(R)-MPA derivative
method [20,21], of (+)-7-NO
by double 2-propranolol.
derivatization procedure, for assigning
the absolute configuration of secondary alcohol based on the sign distribution of ∆δH
Bis-(S)-MPA
(calculated as δH R −derivative of (+)-7-NO
δH S ) allowed 2-propranolol:
assigning S configuration all proton and carbon
to the (+)-7-NO resonances
2 -propranolol
for the
(Figure 7).synthesized compound are reported in Figure 6. HRESI-MS: m/z 601.2550 [M+H]+
(calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2368 [M+Na] (calculated for +
C34H36O8N2Na: 623.2364, Figure S26).Molecules2023,
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Figure 6. NMR signals of bis-(S)-MPA derivative of (+)-7-NO2-propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
(calculated as δHR − δHS) allowed assigning S configuration to the (+)-7-NO2-propranolol
(Figure 7).
Figure6.6.NMR
Figure NMRsignals
signalsof
ofbis-(S)-MPA
bis-(S)-MPAderivative
derivativeofof(+)-7-NO
(+)-7-NO22-propranolol.
-propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
(calculated as δHR − δHS) allowed assigning S configuration to the (+)-7-NO2-propranolol
(Figure 7).
Figure 7.
Figure 7. (S)-(+)-7-NO
(S)-(+)-7-NO22-propranolol
-propranolol absolute
absolute configuration assignment.
Bis-(R)-MPA
Bis-(R)-MPAderivative
derivativeof −)-4-NO22-propranolol:
of((−)-4-NO -propranolol: the
the analysis
analysis of the key
of the key correlations
correlations
11H-13 2,3
H- C ( J)J)recorded
13 C (2,3 recordedby bygradient
gradient2D 2D NMRNMR experiments
experiments allowed
allowed toto assign
assign all
all proton
proton and
and
carbon
carbon resonances
resonances for for the
the synthesized
synthesized compound
compound (Figure
(Figure8). HRESI-MS:m/z
8). HRESI-MS: m/z 601.2537
601.2537
[M + H]+ +(calculated
[M+H] (calculatedfor forCC34H
HO378N
3437 O82:N601.2544);
2 : 601.2544); HRESI-MS
HRESI-MS m/zm/z 623.2356
623.2356 [M+Na]
[M+Na]
+ (calcu-
+ (calculated
lated
for C34for
H36CO H236Na:
348N O8 N 2 Na: 623.2364,
623.2364, Figure Figure
S29). S29).
Figure 7. (S)-(+)-7-NO
Bis-(S)-MPA derivative of (−absolute
2-propranolol )-4-NO2configuration
-propranolol:assignment.
the analysis of the key correlations
1 H-13 C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon Bis-(R)-MPA
resonancesderivative of (−)-4-NOcompound
for the synthesized 2-propranolol: the analysis
(Figure of the key
9). HRESI-MS m/zcorrelations
623.2365
1H-13C (+ 2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
[M+Na] (calculated for C34 H36 O8 N2 Na: 623.2364, Figure S32).
carbon resonances for the synthesized compound (Figure 8). HRESI-MS: m/z 601.2537
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2356 [M+Na]+ (calculated
for C34H36O8N2Na: 623.2364, Figure S29).Molecules 2022, 27, x FOR PEER REVIEW 11 of 15
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15
Figure 8. NMR signals of bis-(R)-MPA derivative of (−)-4-NO2-propranolol.
Bis-(S)-MPA derivative of (−)-4-NO2-propranolol: the analysis of the key correlations
1H- C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
13
carbon resonances for the synthesized compound (Figure 9). HRESI-MS m/z 623.2365
Figure8.8.+NMR
Figure
[M+Na] NMR signalsfor
signals
(calculated ofbis-(R)-MPA
of bis-(R)-MPA derivative
C34H36O8N2derivative
Na: ofof(−
623.2364, (−)-4-NO
Figure22-propranolol.
)-4-NO S32).
Bis-(S)-MPA derivative of (−)-4-NO2-propranolol: the analysis of the key correlations
1H-13C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon resonances for the synthesized compound (Figure 9). HRESI-MS m/z 623.2365
[M+Na]+ (calculated for C34H36O8N2Na: 623.2364, Figure S32).
Figure 9.
Figure 9. NMR
NMR signals
signals of
of bis-(S)-MPA
bis-(S)-MPAderivative
derivativeof
of((−)-4-NO
−)-4-NO22-propranolol.
-propranolol.
Applying
ApplyingRiguera’s
Riguera’smethod
method[20,21],
[20,21],by
bydouble
doublederivatization procedure,
derivatization forfor
procedure, assigning
assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of∆δ
the absolute configuration of secondary alcohol based on the sign distribution of ΔδHH
R S allowed assigning R configuration to the (−)-4-NO -propranolol
(calculated
(calculatedas asδδHHR −
− δδHHS) )allowed assigning R configuration to the (−)-4-NO22-propranolol
Figure 9.10).
(Figure
(Figure NMR signals of bis-(S)-MPA derivative of (−)-4-NO2-propranolol.
10).
Bis-(R)-MPA derivative of (−)-7-NO2 -propranolol: the analysis of the key correlations
Applying
1 H-13 C Riguera’s
(2,3 J) recorded method [20,21],
by gradient 2D NMR byexperiments
double derivatization
allowed toprocedure, for assign-
assign all proton and
ing the absolute configuration of secondary alcohol based on the sign
carbon resonances for the synthesized compound (Figure 11). HRESI-MS: m/z 601.2542 distribution of ΔδH
(calculated
[M as δHR − δfor
+ H]+ (calculated HS)C allowed assigning R configuration to the (−)-4-NO2-propranolol
+
34 H37 O8 N2 : 601.2544); HRESI-MS m/z 623.2360 [M+Na] (calcu-
(Figure
lated for10).
C34 H36 O8 N2 Na: 623.2364, Figure S35).Molecules2023,
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15
Figure 10. (R)-(−)-4-NO2-propranolol absolute configuration assignment.
Bis-(R)-MPA derivative of (−)-7-NO2-propranolol: the analysis of the key correlations
H-13C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
1
carbon resonances for the synthesized compound (Figure 11). HRESI-MS: m/z 601.2542
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2360 [M+Na]+ (calculated
Figure
for C3410.
Figure H 8N−
10.36(R)-( )-4-NO
(R)-(−)-4-NO
O 2Na: -propranolol
22-propranolol
623.2364, Figure absolute
absolute configuration assignment.
S35). configuration assignment.
Bis-(R)-MPA derivative of (−)-7-NO2-propranolol: the analysis of the key correlations
1H-13C (2,3 J) recorded by gradient 2D NMR experiments allowed to assign all proton and
carbon resonances for the synthesized compound (Figure 11). HRESI-MS: m/z 601.2542
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2360 [M+Na]+ (calculated
for C34H36O8N2Na: 623.2364, Figure S35).
Figure 11.
Figure 11. NMR
NMR signals
signals of
of bis-(R)-MPA
bis-(R)-MPAderivative
derivativeof −)-7-NO22-propranolol.
of((−)-7-NO -propranolol.
Bis-(S)-MPA derivativeofof(−
Bis-(S)-MPAderivative )-7-NO22-propranolol: the analysis of the key correlations
(−)-7-NO correlations
11H-13 2,3
H- C ( J)J)recorded
13 C (2,3 recordedby bygradient
gradient 2D
2D NMR
NMR experiments
experiments allowed
allowed toto assign
assign all
all proton
proton and
and
carbon
carbon resonances
resonances for for the
thesynthesized
synthesizedcompound
compound(Figure
(Figure12). HRESI-MS:m/z
12).HRESI-MS: m/z 601.2542
601.2542
[M + H]+ (calculated for C34 H37 O8 N2 : 601.2544); HRESI-MS m/z 623.2361 [M+Na]+ (calcu-
lated
Figurefor
11.CNMR
34 H36signals
O8 N2 Na: 623.2364, Figure
of bis-(R)-MPA S38).
derivative of (−)-7-NO2-propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assigning
Bis-(S)-MPA
the absolute derivativeofofsecondary
configuration (−)-7-NO2-propranolol:
alcohol basedthe onanalysis
the signofdistribution of ∆δH
the key correlations
1H-13C (2,3 J)as
(calculated δH R − δH
recorded byS )gradient
allowed assigning R configuration
2D NMR experiments allowed (−assign
to the to )-7-NOall
2 proton and
-propranolol
(Figure
carbon 13).
resonances for the synthesized compound (Figure 12). HRESI-MS: m/z 601.2542Molecules 2022, 27, x FOR PEER REVIEW 13 of 15
Molecules 2023, 28, 57 13 of 15
[M+H]+ (calculated for C34H37O8N2: 601.2544); HRESI-MS m/z 623.2361 [M+Na]+ (calculated
for C34H36O8N2Na: 623.2364, Figure S38).
Figure 12. NMR signals of bis-(S)-MPA derivative of (−)-7-NO2-propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
(calculated as δHR − δHS) allowed assigning R configuration to the (−)-7-NO2-propranolol
(Figure 13).
Figure 12.
Figure 12. NMR
NMR signals
signals of
of bis-(S)-MPA
bis-(S)-MPAderivative
derivativeof −)-7-NO22-propranolol.
of((−)-7-NO -propranolol.
Applying Riguera’s method [20,21], by double derivatization procedure, for assign-
ing the absolute configuration of secondary alcohol based on the sign distribution of ΔδH
(calculated as δHR − δHS) allowed assigning R configuration to the (−)-7-NO2-propranolol
(Figure 13).
Figure13.
Figure 13.(R)-( −)-7-NO22-propranolol
(R)-(−)-7-NO -propranolol absolute
absolute configuration
configuration assignment.
4. Conclusions
In summary, we have reported a practical synthesis of 4-nitro- and 7-nitropropranolol
as racemic mixtures. Moreover, we disclosed a very efficient chiral HPLC method for
separating the enantiomers that allowed very high yields and enantiopurity. Finally, we
used Riguera’s method to determine the absolute configuration of the enantiomers through
double derivatization
Figure 13. with MPA and
(R)-(−)-7-NO2-propranolol NMRconfiguration
absolute studies. assignment.
We believe that these synthetic procedures will be useful for preparing various enan-
tiopure biologically active products and drugs containing chiral secondary alcohols.Molecules 2023, 28, 57 14 of 15
Supplementary Materials: The following supporting information can be downloaded at: https://www.
mdpi.com/xxx/s1, Figures S1–S4: Stereochemical purity and LC/MS analyses for 4-nitropropranolol
and 7-nitropropranolol enantiomers; Figures S5–S14: Mono- and bidimensional NMR spectra of
4-nitropropranolol and 7-nitropropranolol; Figures S15–S38: 1 H-NMR, 13 C-NMR, and HRMS spectra
for bis-MPA derivatives of 4-nitropropranolol and 7-nitropropranolol.
Author Contributions: F.F. (Francesco Frecentese), G.D.N., and G.C. planned the study and coor-
dinated the project (Conceptualization and Methodology); R.S., A.S., and E.M. synthesized all the
compounds (Investigation); F.F. (Ferdinando Fiorino), B.S., and A.C. analyzed and discussed all
the chemical data (Validation and Formal Analysis); P.L., M.C., and A.A. performed the structural
characterization of the compounds (Investigation and Formal Analysis). F.F. (Francesco Frecentese),
R.S., G.C., V.S., E.P., and G.D.N. drafted and revised the manuscript (Supervision, Writing—original
draft, and Writing—review and editing). All authors have read and agreed to the published version
of the manuscript.
Funding: This research was partially funded by State of São Paulo Research Foundation (FAPESP)
grant 2019/16805-4 (G.D.N).
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of all compounds are available from the authors.
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