Use of Hypervalent Iodine in the Synthesis of Isomeric Dihydrooxazoles

Article abstract of DOI:10.1007/s10593-018-2288-9

[Figure not available: see fulltext.] A convenient synthesis of 2- and/or 3-oxazolines has been developed depending on the structure and stereochemistry of the starting amino alcohol. PhI(OAc)₂ acted as oxidant on the intermediate imine, as sup

Full text of DOI:10.1007/s10593-018-2288-9

DOI 10.1007/s10593-018-2288-9
Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
Use of hypervalent iodine in the synthesis
of isomeric dihydrooxazoles
Claudia Carlucci1*, Arianna Tota1, Marco Colella1, Giuseppe Ronamazzi2,
Guy J. Clarkson3, Renzo Luisi1*, Leonardo Degennaro1
1 Flow Chemistry and Microreactor Technology Laboratory (FLAME-Lab),
Department of Pharmacy – Pharmaceutical Sciences, University of Bari Aldo Moro,
Via E. Orabona 4, Bari 70125, Italy; е-mail: claudia.carlucci@uniba.itt
2 DICATECh, Politecnico di Bari,
Via E. Orabona 4, Bari 70125, Italy; e-mail: g.romanazzi@poliba.it
3 Department of Chemistry, University of Warwick,
Gibbet Hill Road, Coventry CV4 7AL, UK; е-mail: g.clarkson@warwick.ac.uk
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2018, 54(4), 428–436
Submitted January 29, 2018
Accepted after revision May 25, 2018
Isomeric dihydrooxazoles
 One-pot operation
 Enantioenriched products
 NMR monitoring

NH2
OH
R
R
R
O
MeOH
PhI(OAc)2
N
O
N
N
+
and/or
R
*
Ar
Ar
*
Ar
H
OH
O
Ar
A convenient synthesis of 2- and/or 3-oxazolines has been developed depending on the structure and stereochemistry of the starting
amino alcohol. PhI(OAc)2 acted as oxidant on the intermediate imine, as supported by NMR investigation. The findings demonstrate a
new route providing access to unusual 3-oxazolines.
Keywords: hypervalent iodine, 2-oxazolines, 3-oxazolines, NMR studies, stereoselective synthesis.
Compounds of hypervalent iodine have found broad
practical applications in organic synthesis due to their
diverse reactivity combined with safety and commercial
availability.1 The organic derivatives of 3- and 5-iodanes,
commonly known as hypervalent iodine reagents, have
proved particularly useful. Compared with classical heavy
metal-containing oxidants, hypervalent iodine reagents can
be characterized by ready availability, low toxicity, and
environmentally benign nature.
Such iodine(III) derivatives include phenyliodine(III)
diacetate (PhI(OAc)2, PIDA, or DIB), phenyliodine(III) bis
(trifluoroacetate) (PIFA), and iodosobenzene (PhIO),
which have been widely applied to form C–C, C–X, and X
–Y bonds leading to an extensive collection of new
heterocyclic compounds (Scheme 1).2–7
In addition to the formation of biologically relevant
heterocycles, such transformations represent an attractive
alternative to traditional transition metal-catalyzed
oxidative heterocyclizations. For example, PhI(OAc)2 has
been used as oxidant for the synthesis of such heterocycles
as 2-oxazolines.8–10 Ranjith and coworkers developed a
facile synthesis of 2,5-disubstituted 2-oxazolines by using
PhI(OAc)2 and pyridinium fluoride (HF·Py) in THF/
CH2Cl2 at room temperature.8 The same author reported
Scheme 1
R1
R1
O
O
O
R1
R1
N
R2
N
R3
N
H
R2
R
R2
Ar
N
O
Ar
I
R1
O
R
O
O
R
N
R2
R = Me, CF3
Ar = Ph
R1
O
R1
O
R
R2
N
N
R1
R2
O
N
R1
O
that N-allylamides bearing a ring at the distal position of
the olefin provided an oxazine, instead of the expected
2-oxazoline. A mechanism involving two different routes,
exo or endo cyclization, has been proposed, where HF·Py
acted as a promoter and PhI(OAc)2 was consumed as
oxidant.8 Liu and coworkers reported a similar synthetic
0009-3122/18/54(4)-0428©2018 Springer Science+Business Media, LLC
428

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
pathway where the reaction was carried out using N-allyl
the expected oxazolidine intermediate and imine 4a have
carboxamide as starting material, PhI(OAc)2 as oxidant,
the same molecular weight.
and trimethyl silyl iodide as promoter.9b
In order to shed light on this novel and direct conversion
of 1,2-amino alcohols into 2-oxazolines, we decided to
monitor the reaction by NMR. Similarly to the report by
Karade,11 we considered that a mixture of aldehyde and
amino alcohol upon treatment with PhI(OAc)2 could be a
converted to 2-oxazolines. In fact, when 2-amino-1-phenyl-
ethanol (1a) was mixed to an equimolar amount of
p-chlorobenzaldehyde (3b) in an NMR tube, using CD3OD
as solvent, a quick and clean formation of the imine 4b was
observed (Fig. 1). 1H NMR analysis revealed the
characteristic singlet peak at approximately 8 ppm,
belonging to the =C–H proton of the imine intermediate
4b. After the addition of PhI(OAc)2 and immediate
The reactivity of the oxygen atom of the carboxamide
functional group might be increased after the formation of
N-TMS intermediate. Karade and coworkers reported a
straightforward synthesis of 2-oxazolines starting from
amino alcohols and aldehydes in the presence of PhI(OAc)2.
The authors proposed an oxazolidine as intermediate, based
on GC-MS analysis.11 Fujioka and coworkers reported the
synthesis of 3-oxazoline-4-carboxylates as precursors of
oxazoles, starting from amino alcohols and aliphatic
aldehydes.12 Xiao and coworkers demonstrated the use of
3-oxazolines as intermediates in photochemical syntheses
of oxazoles.13 As the synthesis of 3-oxazolines has been
seldom documented in the literature, compared to
2-oxazolines, we believe that this approach could also
provide useful preparative routes to 3-oxazolines.
1
recording of H NMR spectrum only the formation of
oxazoline 2b was observed (Fig. 1).
According to our NMR evidence, we deduced that the
imine acted as intermediate in the synthesis of 2-oxazolines.
However, even though NMR analysis did not provide any
evidence for the formation of oxazolidine (i.e., a signal
around 5 ppm for the 2-CH proton), we cannot rule out its
involvement in the reaction. To further support the
proposed route to 2-oxazolines, imines 4b–j were prepared
simply by mixing equimolar amounts of amino alcohols 1a–d
and aldehydes 3a–g (Scheme 4). The reaction occurred
quickly in methanol solution.
In the present work we revisited the Karade's protocol
for the synthesis of oxazolines by investigating the reaction
through 1H NMR spectroscopy and highlighting an unusual
reaction pathway leading to 3-oxazolines. Our investigation
started from the observation that ( )-2-amino-1-phenyl-
ethanol (1a) reacted with PhI(OAc)2, furnishing 2,4-di-
phenyl-2-oxazoline (2a) in moderate yield. The reaction
was carried out in MeOH at room temperature (Scheme 2).
Scheme 2
The imines 4a–j thus obtained were subsequently
converted into the respective 2-oxazolines 2a–i and
PhI(OAc)2
(1 equiv)
N
OH
Ph
+ PhI + PhCHO
Scheme 4
O
(40%)
NH2
Ph
MeOH
rt, 1 h
R2
Ph
3a
2a
OH
1a
O
MeOH
R1
OH
R2
R1
+
N
rt, 5 min
Ar
3a g
H
NH2
–
Ar
It was observed that the reaction leading to 2-oxazoline
2a and iodobenzene was accompanied by the formation of
benzaldehyde (3a). On the basis of these preliminary
observations, we considered a PhI(OAc)2-promoted cleavage
of the amino alcohol producing benzaldehyde (3a),
methanimine, and iodobenzene as a plausible mechanism
(Scheme 3).
1a–d
4a–j
Ph
OH
Ph
Ph
OH
N
N
N
OH
4a
(>99%)
4b
(>99%)
4c
(90%)
Cl
Scheme 3
Ph
OH
Ph
Ph
OH
N
N
OCH3
N
OH
4d
4e
4f
N
O2N
O
(51%)
(>99%)
(>99%)
Me
N
Me
OMe
OH
OH
N
4g
(>99%)
4h
(87%)
Cl
Cl
We anticipated that the aldehyde formed in situ could
react with the amino alcohol 1a, producing the respective
imine 4a that underwent cyclization promoted by PhI(OAc)2,
resulting in the 2-oxazoline 2a. Nevertheless, our hypo-
thesis did not consider the role of oxazolidine intermediate,
as reported by Karade.11 It is worth pointing out that both
Me Me
N
HO
N
Cl
OH
4i
(87%)
4j
(>99%)
Cl
429

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
Figure 1. NMR monitoring of the conversion of imine 4b into oxazoline 2b.
benzoxazole 2j by reaction with PhI(OAc)2 (Scheme 5).
The process also can be conducted by one-pot procedure,
with essentially the same yields as over two separate steps,
as long as the oxidant is added only after complete forma-
tion of the imine (NMR or GCMS control). The reaction
did not proceed smoothly with an aliphatic aldehyde, likely
because the formation of the respective imine proved
problematic. However, several 2,5-disubstituted 2-oxazolines
2a–f were prepared using aromatic, heteroaromatic, and
unsaturated aldehydes. Moreover, the use of 2-methyl-
2-aminopropanol provided 2,4,4-trisubstituted 2-oxazolines
2g,h, while the use of racemic serine methyl ester gave
access to 2-aryl-substituted 2-oxazoline-4-carboxylate 2i in
a very good yield (Scheme 5).
Scheme 5
With the aim of preparing enantioenriched oxazolines, a
surprising and almost unexpected result was obtained when
the readily available chiral enantioenriched (er 98:2) amino
alcohol (R)-1e was employed for this reaction. In fact,
when the imine 4k was reacted with PhI(OAc)2 under
optimized conditions, two different products were isolated
in almost 1:1 ratio. After isolation of the two products, one
was identified as the expected 2-oxazoline 2k, while the
other product was found to be the isomeric 3-oxazoline 5a.
In an attempt to optimize the reaction, NMR monitoring
was performed using imine 4k as the starting material.
After the addition of PhI(OAc)2, rapid formation of com-
pounds 2k and 5a was observed by NMR (Fig. 2). In
accordance to what was reported above for imine 4b
(Fig. 1), the reaction was fast even with imine 4k, giving
430

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
Figure 2. NMR monitoring of the conversion of imine 4k into the isomeric oxazolines 2k and 5a.
the expected 2-oxazoline 2k. Nevertheless, the presence of
a phenyl substituent at a position relative to the nitrogen
atom affected the selectivity of the reaction, leading to a
mixture of two isomeric oxazolines.
The scope of the reaction was explored using three
different aromatic aldehydes 3a–c, which gave similar
outcomes (Scheme 6). Two isomeric oxazolines could be
isolated in each case. The structure of the 3-oxazolines 5a,c
was unequivocally established by single crystal X-ray
analysis (Fig. 3). As expected, after HPLC analysis using
chiral stationary phase only the 2-oxazolines 2k–m were
optically active (see the Supplementary material), whereas
the 3-oxazolines 5a,b were found to be racemic.
The mechanistic rationale for the competing formation
of 2- and 3-oxazolines is reported in Scheme 7. Even if
NMR analysis did not provide evidence on the involvement
of the oxazolidine, it is likely that the chiral imine reacted
with the oxidant PhI(OAc)2, giving complex I by ligand
exchange. The latter underwent a fast intramolecular
cyclization producing two diastereomeric complexes
Scheme 6
Ar
Ph
(R)
Ph
NH2
O
MeOH
5 min
PhI(OAc)2
MeOH, 1 h
N
N
+
+
N
OH
Ar
Ar
Ph
Ar
3a–c
H
OH
O
2k–m
O
5a–c
Ph
(R)-1e
4k–m
Ph
(R)
Ph
(R)
Ph
(R)
N
O
N
O
N
Cl
O
2k
2l
2m
(39%, er >98:2)
(30%, er >98:2)
(45%, er >98:2)
Ph
Ph
N
Ph
N
N
O
Cl
O
O
5a
5b
(45%)
5c
(30%)
(39%)
431

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
Figure 3. The molecular structure of 3-oxazolines 5a,c with atoms represented by thermal vibration ellipsoids of 50% probability.
Scheme 7
AcO
Ph
AcO–
I
Ph
Ph
(R)
N
O
down
Ar
(R) N
O
(R,R)-6
Ar
(R)
H
Ph
Ar
H
Ar
AcO–
2-Oxazoline 2
AcO
PhI(OAc)2
I
down
AcO–
Ph
N
N
er >98:2
top
(R)
(R)
OH
OH
AcO
Ph
Ph
I
Ph
H
4
Ph
N
H
I
(R) N
O
(R,S)-
er >98:2
Ar
O
(S)
top
Ar
AcO–
6
3
( )-3-Oxazoline
(R,R)-6 and (R,S)-6,14 following two pathways (top and
down in Scheme 7). Under the reaction conditions, such
diastereomeric complexes can undergo a very fast b-
elimination involving the proton at C-2 atom of both
epimers (R,R)-6, and (R,S)-6, leading to enantioenriched 2-
oxazolines. The removal of proton at the C-4 atom of both
epimers (R,R)-6, and (R,S)-6 led to a racemic 3-oxazoline
(Scheme 6). Nevertheless, alternative pathways cannot be
ruled out.15
In conclusion, this work demonstrated that hypervalent
iodine can be used as a convenient reagent for the
preparation of heterocycles such as 2-oxazolines and/or 3-
oxazolines depending on the structure and stereochemistry
of the starting 1,2-amino alcohol. NMR investigation
supported the hypothesis that PhI(OAc)2 acted as oxidant
towards imine intermediates. In addition, a new route was
developed that opened access to unusual 3-oxazolines.
spectrometer equipped with an electrospray ion source
operating in positive ion mode. Column chromatography
was performed on neutral alumina (150 mesh, Brockmann
I) with the composition of eluent reported separately for
each experiment. The TLC analyses were performed using
Merck F-254 silica gel plates with fluorescent indicator and
the visualization was performed by UV light (254 nm). The
optical rotation values ([α]D20) were measured by using a
PerkinElmer 341 polarimeter with 1 dm cell path length
and the concentration (c) was expressed in g/100 ml.
Synthesis of oxazoline 2a starting from amino alcohol
1a. Amino alcohol 1a (0.14 g, 1.0 mmol) and methanol
(3.6 ml) were placed in a round-bottom flask. Then
PhI(OAc)2 (0.32 g, 1.0 mmol) was added. The mixture was
stirred for 1 h and the reaction progress was monitored by
TLC (hexane–EtOAc, 1:1). The solvent was removed
under vacuum, the crude mixture was purified on neutral
alumina (hexane–EtOAc, 8:2), yielding oxazoline 2a as
yellow oil. Yield 89 mg (40%).
Experimental
IR spectra were recorded on a PerkinElmer 283 spectro-
Synthesis of imines 4a–m from amino alcohols and
aldehydes (General method). Benzaldehyde (3a) (0.11 g,
1.0 mmol) and ( )-2-amino-1-phenylethanol (1a) (0.14 g,
1.0 mmol) were dissolved in MeOH (3.6 ml) in a round-
bottom flask and stirred for 5 min. After monitoring the
imine formation by GC or TLC the solvent was removed
under vacuum. The product 4a was characterized and used
without further purification. Compounds 4b–m were
synthesized analogously.
1
photometer in thin film. H (300 and 500 MHz) and 13C
(126 MHz) NMR spectra were recorded on Varian Inova-300
and Agilent VNMRS 500 spectrometers, using CD3OD or
CDCl3 as solvents. The chemical shifts were reported in
ppm using the residual protonated solvent as internal
1
standard for the scale (CDCl3: 7.26 ppm for H nuclei and
1
77.0 ppm for 13C nuclei; CD3OD: 4.87 ppm for H nuclei
and 49.0 ppm for 13C nuclei). The NMR yields were
calculated using an internal standard. High-resolution mass
spectra were recorded using a Bruker micrOTOF-Q II mass
Synthesis of oxazolines 2a–m starting from imines
4a–m (General method). Imine 4a (0.22 g, 1.0 mmol) was
432

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
dissolved in MeOH (3.6 ml) in a round-bottom flask. Then
120.9; 115.7; 111.2; 80.8; 63.3; 55.6. Found, m/z: 280.1335
+
PhI(OAc)2 (0.32 g, 1.0 mmol) was added and the reaction
mixture was stirred for 1 h at room temperature. After
disappearance of the imine (TLC or GC monitoring) the
solvent was removed under vacuum and oxazoline 2a was
purified on neutral alumina (hexane–EtOAc, 8:2). Com-
pounds 2b–m were synthesized analogously.
[M+H]+. C18H18NO2 . Calculated, m/z: 280.1332.
2-(4-Nitrophenyl)-5-phenyl-4,5-dihydro-1,3-oxazole (2e).17
Yield 206 mg (77%, from imine), white solid, mp 143–
1
145°C (mp 153–156°C17). H NMR spectrum (500 MHz,
CD3OD), δ, ppm (J, Hz): 8.36 (2H, d, J = 9.0, H Ar); 8.21
(2H, d, J = 9.0, H Ar); 7.41–7.36 (5H, m, H Ar); 5.86 (1H,
dd, J = 10.4, J = 8.2, CH); 4.55 (1H, dd, J = 15.0, J = 10.2,
CH2); 3.99 (1H, dd, J = 15.0, J = 8.2, CH2). Found, m/z:
269.0922 [M+H]+. C15H13N2O3. Calculated, m/z: 269.0921.
5-Phenyl-2-(pyridin-3-yl)-4,5-dihydro-1,3-oxazole (2f).
Yield 202 mg (90%, from imine), yellow solid, mp 57–59°C.
IR spectrum, ν, cm–1: 2928, 2872, 1655 (C=N), 1592, 1415,
One-pot synthesis of oxazolines 2b,k–m, 5a–c (General
method). The preparation of oxazoline 2b is described.
Compounds 2k–m and 5a–c were synthesized analogously.
p-Chlorobenzaldehyde (3b) (0.14 g, 1.0 mmol) and
( )-2-amino-1-phenylethanol (1a) (0.14 g, 1.0 mmol) were
dissolved in MeOH (3.6 ml) in a round-bottom flask and
stirred for 5 min. After monitoring the imine formation by
GC or TLC, PhI(OAc)2 (0.32 g, 1.0 mmol) was added to
the mixture. The reaction was stirred for 1 h at room
temperature and the progress of the reaction was monitored
by TLC or GC. After full conversion the solvent was
removed under vacuum and the crude residue was purified
on neutral alumina (hexane–EtOAc, 8:2).
1
1340, 1259, 1079, 1022, 700. H NMR spectrum (500 MHz,
CD3OD), δ, ppm (J, Hz): 9.10 (1H, s, H Ar); 8.67 (1H, d,
J = 4.9, H Ar); 8.33 (1H, d, J = 8.1, H Ar); 7.52 (1H, dd,
J = 8.0, J = 6.9, H Ar); 7.38–7.32 (5H, m, H Ar); 5.78 (1H,
dd, J = 10.5, J = 8.1, CH); 4.48 (1H, dd, J = 15.1, J = 10.1, CH2);
3.93 (1H, dd, J = 14.9, J = 8.0, CH2). 13C NMR spectrum
(CD3OD), δ, ppm: 163.7; 153.0; 149.7; 141.7 (2C); 137.4;
129.9 (2C); 129.6; 126.8 (2C); 125.2; 83.0; 63.4. Found, m/z:
225.1022 [M+H]+. C14H13N2O. Calculated, m/z: 225.1022.
4,4-Dimethyl-2-styryl-4,5-dihydro-1,3-oxazole (2g).18
2,5-Diphenyl-4,5-dihydro-1,3-oxazole (2a).16 Yield
1
178 mg (80%, from imine), yellow oil. H NMR spectrum
(500 MHz, CDCl3), δ, ppm: 8.03 (2H, d, J = 6.6, H Ar);
7.50–7.30 (8H, m, H Ar); 5.69 (1H, dd, J = 10.0, J = 7.9,
CH); 4.51 (1H, dd, J = 14.7, J = 10.0, CH2); 4.03 (1H, dd,
J = 14.7, J = 7.9, CH2). Found, m/z: 224.1073 [M+H]+.
C15H14NO. Calculated, m/z: 224.1070.
1
Yield 181 mg (90%, from imine), yellow oil. H NMR
spectrum (500 MHz, CD3OD), δ, ppm (J, Hz): 7.56 (2H, d,
J = 6.9, H Ar); 7.44–7.36 (4H, m, H Ar, CH=); 6.59 (1H,
d, J = 15.6, CH=); 4.12 (2H, s, CH2); 1.33 (6H, s, CH3).
Found, 202.1230 m/z: [M+H]+. [C13H16NO]+. Calculated,
m/z: 202.1226
2-(4-Chlorophenyl)-5-phenyl-4,5-dihydro-1,3-oxazole
(2b).15 Yield 128 mg (50%, from imine, 50%, one-pot
method), yellow oil. 1H NMR spectrum (500 MHz,
CDCl3), δ, ppm (J, Hz): 7.95 (2H, d, J = 8.7, H Ar); 7.42–
7.33 (7H, m, H Ar); 5.66 (1H, dd, J = 9.8, J = 8.2, CH);
4.48 (1H, dd, J = 15.0, J = 9.8, CH2); 4.00 (1H, dd,
J = 15.0, J = 8.0, CH2). Found, m/z: 280.0500 [M+Na]+.
C15H12ClNNaO. Calculated, m/z: 280.0500.
2-(Naphthalen-1-yl)-5-phenyl-4,5-dihydro-1,3-oxazole
(2c). Yield 246 mg (90%, from imine), yellow oil.
IR spectrum, ν, cm–1: 2529, 1719, 1643 (C=N), 1449, 1242,
1121, 778. 1H NMR spectrum (500 MHz, CD3OD), δ, ppm
(J, Hz): 8.85 (1H, d, J = 8.6, H Ar); 8.08 (2H, t, J = 9.4,
H Ar); 7.96 (1H, d, J = 7.7, H Ar); 7.59–7.43 (8H, m,
H Ar); 5.86 (1H, dd, J = 10.3, J = 7.5, CH); 4.63 (1H, dd,
J = 14.7, J = 10.3, CH2); 4.08 (1H, dd, J = 14.7, J = 7.5,
CH2). 13C NMR spectrum (CD3OD), δ, ppm: 166.6;
142.4; 135.3; 133.2; 132.3; 130.0 (3C); 129.6; 129.5;
128.3; 127.4; 127.0; 126.9 (2C); 125.9; 125.8; 82.2; 63.8.
Found, m/z: 274.1227 [M+H]+. C19H16NO. Calculated, m/z:
274.1226.
2-(2-Methoxystyryl)-5-phenyl-4,5-dihydro-1,3-oxazole
(2d). Yield 178 mg (64%, from imine), yellow oil.
IR spectrum, ν, cm–1: 1719, 1647 (C=N), 1489, 1249, 1105.
1H NMR spectrum (500 MHz, CDCl3), δ, ppm (J, Hz): 7.76
(1H, d, J = 16.5, CH=); 7.52 (1H, d, J = 7.5, H Ar); 7.42–
7.30 (6H, m, H Ar); 6.97 (1H, t, J = 7.5, H Ar); 6.91 (1H,
d, J = 8.4, H Ar); 6.82 (1H, d, J = 16.5, CH=); 5.57 (1H, t,
J = 8.9, CH); 4.41 (1H, dd, J = 14.8, J = 10.1, CH2); 3.91
(1H, dd, J = 15.2, J = 7.8, CH2); 3.87 (3H, s, OCH3).
13C NMR spectrum (CDCl3), δ, ppm: 164.7; 158.0; 141.3;
135.7; 130.8; 128.9 (2C); 128.4 (2C); 125.9 (2C); 124.3;
2-(4-Chlorophenyl)-4,4-dimethyl-4,5-dihydro-1,3-
oxazole (2h).19 Yield 188 mg (90%, from imine), white
solid, mp 65–67°C (mp 31–33°C19). IR spectrum, ν, cm–1:
2926, 1724, 1651 (C=N), 1492, 1312, 1262, 1092, 1015,
1
731. H NMR spectrum (500 MHz, CD3OD), δ, ppm
(J, Hz): 7.88 (2H, d, J = 8.5, H Ar); 7.48 (2H, d, J = 8.5,
H Ar); 4.21 (2H, s, CH2); 1.37 (6H, s, CH3). 13C NMR
spectrum (CD3OD), δ, ppm: 163.7; 138.9; 130.7; 129.8;
127.4; 80.5; 68.5; 28.3. Found, m/z: 210.0684 [M+H]+.
C11H13ClNO+. Calculated, m/z: 210.0680.
Methyl-2-(4-chlorophenyl)-4,5-dihydro-1,3-oxazole-4-
carboxylate (2i).20 Yield 215 mg (90%, from imine), white
solid, mp 85–87°C. IR spectrum, ν, cm–1: 2925, 2254,
1
1725, 1645 (C=N), 1492, 1405, 1265, 1093, 913. H NMR
spectrum (500 MHz, CDCl3), δ, ppm (J, Hz): 7.92 (2H, d,
J = 9.2, H Ar); 7.39 (2H, d, J = 8.5, H Ar); 4.95 (1H, t, J = 9.4,
CH); 4.70 (1H, t, J = 8.5, CH2); 4.60–4.58 (1H, m, CH2);
3.83 (3H, s, CH3). 13C NMR spectrum (CDCl3), δ, ppm:
171.6; 138.3; 130.1 (2C); 128.9 (2C); 125.6; 69.9; 68.8;
66.0; 53.0. Found, m/z: 262.0242 [M+Na]+. C11H10ClNNaO3.
Calculated, m/z: 262.0241.
5-Chloro-2-(4-chlorophenyl)-1,3-benzoxazole (2j).21
Yield 152 mg (58%, from imine), white solid, mp 148–
149°C (mp 114°C21). 1H NMR spectrum (300 MHz,
CDCl3), δ, ppm (J, Hz): 8.18–8.15 (2H, m, H Ar); 7.74–
7.73 (1H, d, J = 1.9, H Ar); 7.52–7.48 (3H, m, H Ar); 7.35–
7.31 (1H, m, H Ar). Found, m/z: 263.9988 [M+Na]+.
C13H8Cl2NO3. Calculated, m/z: 263.9978.
(R)-2,4-Diphenyl-4,5-dihydro-1,3-oxazole (2k).22 Yield
70 mg (30%, one-pot procedure), white solid, mp 32–33°C.
433

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
[α]D20 +21° (c 1.04, CHCl3) ([α]D20 +36.4° (c 1.04, CHCl3)22).
(500 MHz, CD3OD), δ, ppm (J, Hz): 8.94 (1H, s, H–C=N);
8.57 (1H, d, J = 7.7, H Ar); 7.97–7.86 (3H, m, H Ar); 7.56–
7.46 (5H, m, H Ar); 7.37–7.26 (4H, m, H Ar); 5.06 (1H,
dd, J = 7.6, J = 5.4, CH); 4.02 (1H, dd, J = 11.5, J = 5.0,
CH2); 3.49 (1H, dd, J = 11.5, J = 7.6, CH2). 13C NMR
spectrum (CD3OD), δ, ppm: 165.0; 144.4; 135.2; 132.9;
132.6; 132.2; 129.6; 129.3; 128.8; 128.5; 128.0; 127.4;
127.1; 126.2; 125.0; 74.5; 70.4. Found, m/z: 276.1386
[M+H]+. C19H18NO. Calculated, m/z: 276.1383.
1-Phenyl-2-[(pyridin-3-ylmethylene)amino]ethanol (4d).
Yield 115 mg (51%), yellow oil. IR spectrum, ν, cm–1:
3232 (O‒H), 2848, 1706, 1648 (C=N), 1589, 1421, 1246,
1021, 702. 1H NMR spectrum (500 MHz, CD3OD), δ, ppm
(J, Hz): 8.84 (1H, s, H Ar); 8.59 (1H, d, J = 4.9, H Ar);
8.33 (1H, s, H–C=N); 8.20 (1H, d, J = 8.1, H Ar); 7.49
(1H, dd, J = 8.1, J = 4.9, H Ar); 7.50–7.24 (5H, m, H Ar);
5.01 (1H, dd, J = 7.9, J = 4.5, CH); 3.94 (1H, dd, J = 11.9,
J = 4.3, CH2); 3.81 (1H, dd, J = 11.9, J = 7.8, CH2).
13C NMR spectrum (CD3OD), δ, ppm: 162.3; 151.9; 150.4;
144.2; 136.9; 133.5; 129.3 (2C); 128.5; 127.3 (2C); 125.4;
74.3; 70.0. Found, m/z: 227.1186 [M+H]+. C14H15N2O.
Calculated, m/z: 227.1184.
1H NMR spectrum (500 MHz, CD3OD), δ, ppm (J, Hz):
8.06–8.02 (2H, m, H Ar); 7.53–7.49 (1H, m, H Ar); 7.42 (2H,
m, H Ar); 7.38–7.26 (5H, m, H Ar); 5.39 (1H, dd, J = 10.3,
J = 8.3, CH); 4.80 (1H, dd, J = 10.3, J = 8.3, CH2); 4.28
(1H, t, J = 8.3, CH2). Found, m/z: 224.1077 [M+H]+.
C15H14NO. Calculated, m/z: 224.1070.
(R)-2-(4-Chlorophenyl)-4-phenyl-4,5-dihydro-1,3-oxazole
(2l). Yield 100 mg (45%, one-pot procedure), white solid,
20
mp 58–60°C. [α]D +38° (c 1, CHCl3). IR spectrum, ν, cm–1:
1
2918, 1722, 1642 (C=N), 1265, 1095, 728, 665. H NMR
spectrum (500 MHz, CD3OD), δ, ppm (J, Hz): 7.98 (2H, d,
J = 8.3, H Ar); 7.51 (2H, d, J = 8.3, H Ar); 7.38–7.31 (5H,
m, H Ar); 5.41 (1H, dd, J = 9.9, J = 8.1, CH); 4.90 (1H, dd,
J = 10.2, J = 8.6, CH2); 4.31 (1H, t, J = 8.2, CH2). 13C
NMR spectrum (CD3OD), δ, ppm: 166.2; 143.3; 139.3;
131.0 (2C); 130.0 (2C); 129.9 (2C); 128.9; 127.9 (2C);
127.0; 76.6; 70.9. Found, m/z: 258.0683 [M+H]+.
C15H13ClNO. Calculated, m/z: 258.0680.
(R)-2-(Naphthalen-1-yl)-4-phenyl-4,5-dihydro-1,3-oxazole
(2m). Yield 106 mg (39%, one-pot procedure), yellow oil.
[α]D20 –10.7° (c 1, CHCl3). IR spectrum, ν, cm–1: 2918, 1638,
1510, 1351, 1124, 807, 776. 1H NMR spectrum (500 MHz,
CD3OD), δ, ppm (J, Hz): 8.84 (1H, d, J = 7.9, H Ar); 8.08–
8.06 (2H, m, H Ar); 7.96–7.94 (1H, m, H Ar); 7.61–7.54
(4H, m, H Ar); 7.42–7.41 (3H, m, H Ar); 7.34–7.31 (1H,
m, H Ar); 5.54 (1H, dd, J = 10.2, J = 8.3, CH); 4.96 (1H,
dd, J = 10.2, J = 8.3, CH2); 4.37 (1H, t, J = 8.3, CH2).
13C NMR spectrum (CD3OD), δ, ppm: 167.6; 143.6;
133.3; 130.1 (2C); 129.9 (2C); 129.6 (2C); 128.9;
128.3; 127.8; 127.4 (2C); 126.9 (2C); 125.9; 75.9; 71.4.
Found, m/z: 274.1226 [M+H]+. C19H16NO. Calculated, m/z:
274.1228.
2-[(4-Nitrobenzylidene)amino]-1-phenylethan-1-ol (4e).25
Yield 243 mg (90%), white solid, mp 180–185°C. 1H NMR
spectrum (300 MHz, CD3OD), δ, ppm (J, Hz): 8.37 (1H, s,
H–C=N); 8.28 (2H, d, J = 8.9, H Ar); 7.97 (2H, d, J = 8.9,
H Ar); 7.30–7.18 (5H, m, H Ar); 5.01 (1H, dd, J = 7.8,
J = 4.3, CH); 3.97 (1H, dd, J = 11.8, J = 4.3, CH2); 3.83
(1H, dd, J = 11.9, J = 7.9, CH2). Found, m/z: 271.1074
[M+H]+. C15H15N2O3. Calculated, m/z: 271.1077.
2-{[3-(2-Methoxyphenyl)prop-2-en-1-ylidene]amino}-
1-phenylethan-1-ol (4f). Yield 252 mg (99%), yellow oil.
IR spectrum, ν, cm–1: 3429 (O‒H), 2529, 1633 (C=N),
1449, 1244, 1212, 1026, 728. 1H NMR spectrum
(500 MHz, CD3OD), δ, ppm (J, Hz): 7.98 (1H, d, J = 9.1,
CH=); 7.54 (1H, d, J = 7.7, CH=); 7.40–7.20 (7H, m,
H Ar); 7.00 (1H, d, J = 8.5, CH=); 6.95 (2H, m, H Ar);
4.90 (1H, dd, J = 7.7, J = 4.4, CH); 3.88 (3H, s, OCH3);
3.78 (1H, dd, J = 11.9, J = 4.4, CH2); 3.66 (1H, dd,
J = 11.9, J = 7.9, CH2). 13C NMR spectrum (CD3OD),
δ, ppm: 168.4; 159.1; 144.2; 140.2; 132.0; 129.3 (2C);
128.9; 128.5; 128.1; 127.2 (2C); 125.4; 121.8; 112.3; 74.5;
69.2; 56.0. Found, m/z: 282.1490 [M+H]+. C18H20NO2.
Calculated, m/z: 282.1489.
2-(Benzylideneamino)-1-phenylethan-1-ol (4a).23 Yield
223 mg (99%), white solid, mp 113–115°C (mp 114°C23).
1H NMR spectrum (500 MHz, CD3OD), δ, ppm (J, Hz):
8.31 (1H, s, HC=N); 7.80–7.77 (2H, m, H Ar); 7.49–7.41
(5H, m, H Ar); 7.36–7.31 (2H, m, H Ar); 7.26–7.21 (1H,
m, H Ar); 5.05–5.03 (1H, m, CH); 3.93 (1H, dd, J = 12.0,
J = 3.3, CH2); 3.73 (1H, dd, J = 12.0, J = 9.0, CH2).
Found, m/z: 226.1237 [M+H]+. C15H16NO. Calculated, m/z:
226.1226.
2-{[(4-Chlorophenyl)methylidene]amino}-1-phenylethan-
1-ol (4b).24 Yield 256 mg (99%), clear crystalline solid, mp
153–156°C (mp 160–163°C (THF)24). IR spectrum, ν, cm–1:
3410 (O‒H), 2518, 1647 (C=N), 1446, 1116, 1062.
1H NMR spectrum (500 MHz, CD3OD), δ, ppm (J, Hz):
8.24 (1H, s, H–C=N); 7.73 (2H, d, J = 9.0, H Ar); 7.45
(2H, d, J = 8.4, H Ar); 7.42–7.24 (5H, m, H Ar); 4.97 (1H,
dd, J = 8.1, J = 4.2, CH); 3.89 (1H, dd, J = 12.0, J = 4.3,
CH2); 3.76 (1H, dd, J = 12.0, J = 8.2, CH2). 13C NMR
spectrum (CD3OD), δ, ppm: 164.4; 144.3; 138.0; 135.9;
130.8 (2C); 129.9 (2C); 129.3 (2C); 128.5; 127.3 (2C);
74.4; 69.8. Found, m/z: 260.0838 [M+H]+. C15H15ClNO.
Calculated, m/z: 260.0837.
Methyl 2-(4-chlorobenzylideneamino)-3-hydroxy-
propanoate (4g).26 Yield 178 mg (74%), yellow oil.
1H NMR spectrum (300 MHz, CD3OD), δ, ppm (J, Hz):
8.38 (1H, s, H–C=N); 7.80 (2H, d, J = 8.4, H Ar); 7.43
(2H, d, J = 8.6, H Ar); 4.23 (1H, dd, J = 7.2, J = 4.9, CH);
4.03 (1H, dd, J = 11.1, J = 4.9, CH2); 3.83 (1H, dd,
J = 11.1, J = 4.6, CH2); 3.74 (3H, s, CH3). Found, m/z:
242.0590 [M+H]+. C11H13ClNO3. Calculated, m/z:
242.0579.
2-[(4-Chlorobenzylidene)amino]-2-methylpropan-1-ol
(4h).27 Yield 183 mg (87%), yellow solid, mp 60–63°C
1
2-[(Naphthalen-1-ylmethylene)amino]-1-phenylethan-
1-ol (4c). Yield 247 mg (90%), straw-yellow solid, mp 126–
131°C. IR spectrum, ν, cm–1: 3429 (O‒H), 2914, 2376,
(mp 61°C27). H NMR spectrum (500 MHz, CD3OD),
δ, ppm (J, Hz): 8.36 (1H, s, H–C=N); 7.77 (2H, d, J = 8.3,
H Ar); 7.43 (2H, d, J = 8.3, H Ar); 3.52 (2H, s, CH2); 1.26
(6H, s, CH3). 13C NMR spectrum (CD3OD), δ, ppm: 158.8;
1
1723, 1643 (C=N), 1249, 1063, 704. H NMR spectrum
434

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
130.6 (2C); 129.8 (2C); 129.5; 129.0; 71.2; 62.5; 24.2 (2C).
CD3OD), δ, ppm (J, Hz): 7.87–7.85 (2H, m, H Ar); 7.59–
7.56 (1H, m, H Ar); 7.53–7.50 (2H, m, H Ar); 7.46–7.44
(2H, m, H Ar); 7.43–7.35 (3H, m, H Ar); 6.76 (1H, dd,
J = 5.2, J = 4.3, CH); 5.34 (1H, dd, J = 14.0, J = 5.4, CH2);
5.16 (1H, dd, J = 14.0, J = 4.1, CH2). 13C NMR spectrum
(CD3OD), δ, ppm: 170.7; 141.1; 133.1; 131.7; 130.0 (2C);
129.8; 129.5 (2C); 129.1 (2C); 127.6 (2C); 108.4; 75.4.
Found, m/z: 224.1069 [M+H]+. C15H14NO. Calculated, m/z:
224.1070.
Found, m/z: 212.0841 [M+H]+. C11H15ClNO. Calculated,
m/z: 212.0837.
2-Methyl-2-[(3-phenylallylidene)amino]propan-1-ol
(4i).28 Yield 177 mg (87%), yellow solid, mp 104–106°C.
1H NMR spectrum (500 MHz, CD3OD), δ, ppm (J, Hz):
8.14 (1H, d, J = 8.8, CH=); 7.54 (2H, d, J = 7.3, H Ar);
7.35 (3H, m, H Ar); 7.15 (1H, d, J = 16.0, CH=); 6.92
(1H, dd, J = 16.0, J = 8.8, CH=); 3.48 (2H, s, CH2); 1.23
(6H, s, CH3). 13C NMR spectrum (CD3OD), δ, ppm: 162.5;
144.4; 137.0; 130.4; 129.9; 128.4; 71.0; 62.2; 24.2.
Found, m/z: 204.1392 [M+H]+. C13H18NO. Calculated, m/z:
204.1383.
2-(4-Chlorophenyl)-4-phenyl-2,5-dihydro-1,3-oxazole
(5b). Yield 100 mg (45%, one-pot procedure), white solid,
mp 92–97°C. IR spectrum, ν, cm–1: 2851, 1630 (C=N),
1490, 1290, 1067, 759, 691. 1H NMR spectrum (500 MHz,
CD3OD), δ, ppm (J, Hz): 7.83–7.82 (2H, m, H Ar); 7.56–
7.53 (1H, m, H Ar); 7.50–7.47 (2H, m, H Ar); 7.43–7.37
(4H, m, H Ar); 6.74–6.71 (1H, m, CH); 5.33–5.27 (1H, m,
CH2); 5.15–5.11 (1H, m, CH2). 13C NMR spectrum (CD3OD),
δ, ppm: 169.5; 138.6; 131.7; 128.6 (2C); 128.2 (2C); 128.1;
127.8 (2C); 127.7 (2C); 126.1; 106.1; 74.1. Found, m/z:
280.0504 [M+Na]+. C15H13ClNNaO. Calculated, m/z: 280.0500.
2-(Naphthalen-1-yl)-4-phenyl-2,5-dihydro-1,3-oxazole
(5c). Yield 106 mg (39%, one-pot procedure), white solid,
mp 92–97°C. IR spectrum, ν, cm–1: 3052, 2852, 1635
(C=N), 1449, 1265, 1088, 788, 738, 692. 1H NMR
spectrum (500 MHz, CD3OD), δ, ppm (J, Hz): 8.38 (1H, d,
J = 8.5, H Ar); 7.95–7.90 (4H, m, H Ar); 7.63–7.48 (8H,
m, Ar, CH); 5.37 (1H, dd, J = 14.0, J = 5.6, CH2); 5.31
(1H, dd, J = 14.0, J = 4.1, CH2). 13C NMR spectrum
(CD3OD), δ, ppm: 173.0; 136.6 (2C); 135.4; 133.0 (2C);
132.4; 132.0; 130.2; 129.6 (2C); 129.2; 127.3; 126.8; 126.3
(2C); 125.1; 105.8; 75.1. Found, m/z: 274.1225 [M+H]+.
C19H16NO. Calculated, m/z: 274.1226.
X-ray structural study of compounds 5a,c. Single
crystals of compound 5a were grown from CH2Cl2 – petrol
ether, while single crystals of compound 5c were grown
from diethyl ether. A suitable crystal was selected and
mounted on a glass fibre with Fomblin oil and placed on a
Rigaku Oxford Diffraction SuperNova diffractometer with
a dual source equipped with an Atlas S2 CCD area
detector. CuKα radiation (1.54184 Å) was used. The crystal
was kept at 150(2) K during data collection. The structure
was solved with Olex234 and ShelXT35 software using
intrinsic phasing and refined with the ShelXL36 refinement
package using least squares minimization. The crystallo-
graphic data sets were deposited at the Cambridge Crystallo-
graphic Data Center (for compound 5a deposit CCDC
1846701, for compound 5c deposit CCDC 1819791).
4-Chloro-2-{[(4-chlorophenyl)methylidene]amino}-
phenol (4j).29 Yield 263 mg (99%), yellow solid, mp 113–
118°C. IR spectrum, ν, cm–1: 3378 (O‒H), 3054, 2306,
1626 (C=N), 1492, 1265, 738. 1H NMR spectrum
(300 MHz, CD3OD), δ, ppm (J, Hz): 8.67 (1H, s, H–C=N);
8.01–7.98 (2H, m, H Ar); 7.51–7.48 (2H, m, H Ar); 7.24
(1H, d, J = 2.5, H Ar); 7.11–7.07 (1H, m, H Ar); 6.86 (1H,
d, J = 6.9, H Ar). 13C NMR spectrum (CD3OD), δ, ppm:
160.7; 131.6 (2C); 130.1; 128.5; 118.6 (2C); 118.0 (2C);
116.3 (2C); 116.1 (2C). Found, m/z: 266.0131 [M+H]+.
C13H10Cl2NO. Calculated, m/z: 266.0134.
(R)-2-(Benzylideneamino]-2-phenylethan-1-ol (4k).30
Yield 130 mg (58%), colorless needles, mp 77–79°C
20
([α]D –115° (c 0.48, CH2Cl2))30. 1H NMR spectrum
(500 MHz, CD3OD), δ, ppm (J, Hz): 8.40 (1H, s, H–C=N);
7.82–7.24 (10H, m, H Ar); 4.50 (1H, dd, J = 7.9, J = 4.3,
CH); 3.97 (1H, dd, J = 11.0, J = 4.3, CH2); 3.90 (1H, dd,
J = 11.0, J = 7.9, CH2). Found, m/z: 226.1230 [M+H]+.
C15H16NO. Calculated, m/z: 226.1226.
(R)-2-[(4-Chlorobenzylidene)amino]-2-phenylethan-
20
1-ol (4l).31 Yield 233 mg (90%), yellow oil, [α]D +38°
(c 1, CHCl3). IR spectrum, ν, cm–1: 3350 (O‒H), 2862,
1
1722, 1640 (C=N), 1489, 1088, 1013, 820, 699. H NMR
spectrum (500 MHz, CD3OD), δ, ppm (J, Hz): 8.39 (1H, s,
H–C=N); 7.78 (2H, d, J = 8.2, H Ar); 7.44–7.23 (7H, m,
H Ar); 4.42 (1H, t, J = 7.5, CH); 3.85–3.83 (2H, m, CH2).
13C NMR spectrum (CD3OD), δ, ppm: 162.3; 142.2; 137.7;
136.2; 130.9 (2C); 129.8 (2C); 129.5 (2C); 128.4; 128.4
(2C); 78.2; 67.8. Found, m/z: 260.0840 [M+H]+. C15H15ClNO.
Calculated, m/z: 260.0837.
(R)-2-[(Naphthalen-1-ylmethylene)amino]-2-phenyl-
ethan-1-ol (4m).32 Yield 273 mg (99%), yellow oil.
[α]D20 +24.2° (c 1, CHCl3). IR spectrum, ν, cm–1: 3343 (O‒H),
1
2484, 2215, 2071, 1642 (C=N), 1123, 976, 824. H NMR
spectrum (500 MHz, CD3OD), δ, ppm (J, Hz): 9.09 (1H, s,
H–C=N); 8.94 (1H, d, J = 7.9, H Ar); 7.95–7.89 (3H, m,
H Ar); 7.60–7.50 (5H, m, H Ar); 7.38–7.25 (4H, m, H Ar);
4.56–4.53 (1H, m, CH); 3.94–3.90 (2H, m, CH2). 13C NMR
spectrum (CD3OD), δ, ppm: 163.4; 142.5; 135.3 (2C);
133.0; 132.7 (2C); 132.2 (2C); 129.9; 129.6 (3C); 128.1
127.1; 126.2; 125.5; 79.3; 68.1. Found, m/z: 276.1380
[M+H]+. C19H18NO. Calculated, m/z: 276.1383.
Supplementary information file, containing NMR spectra,
chiral HPLC data, and X-ray structural study parameters, is
available at the journal website at http://link.springer.com/
journal/10593.
We thank the University of Bari Aldo Moro, the
Polytechnic of Bari, the project Laboratorio SISTEMA
(Code PONa300369) financed by Italian Ministry of
Education, Universities, and Research.
We are also grateful to Chiara Claudio and Valeria
Decaro for their precious contribution.
2,4-Diphenyl-2,5-dihydro-1,3-oxazole (5a).33 Yield 70 mg
(30%, one-pot procedure), white solid, mp 91–94°C (mp 85–
87°C33). IR spectrum, ν, cm–1: 3430, 1629 (C=N), 1450,
1
1266, 1064, 759, 692. H NMR spectrum (500 MHz,
435

Chemistry of Heterocyclic Compounds 2018, 54(4), 428–436
17. Minakata, S.; Morino, Y.; Ide, T.; Oderaotoshi, Y.; Komatsu, M.
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