Microwave-Assisted Synthesis of 2-Aryl-2-oxazolines, 5,6-Dihydro-4H-1,3-oxazines, and 4,5,6,7-Tetrahydro-1,3-oxazepines

Article abstract of DOI:10.1021/acs.orglett.6b03122

The first general procedure for the synthesis of 5- to 7-membered cyclic iminoethers by microwave-assisted cyclization of ω-amido alcohols promoted by polyphosphoric acid (PPA) esters is presented. 2-Aryl-2-oxazolines and 5,6-dihydro-4H-1,3-oxazines were efficiently prepared using ethyl polyphosphate/CHCl₃. Trimethylsilyl polyphosphate in solvent-free conditions allowed for the synthesis of hitherto-unreported 4,5,6,7-tetrahydro-1,3-oxazepines. The method involves good to excellent yields and short reaction times.The reaction mechanism and the role of PPA esters were investigated in a chiral substrate.

Full text of DOI:10.1021/acs.orglett.6b03122

Letter
pubs.acs.org/OrgLett
Microwave-Assisted Synthesis of 2‑Aryl-2-oxazolines, 5,6-
Dihydro‑4H‑1,3-oxazines, and 4,5,6,7-Tetrahydro-1,3-oxazepines
María C. Mollo and Liliana R. Orelli*
́
Universidad de Buenos Aires, CONICET, Departamento de Química Organica, Facultad de Farmacia y Bioquímica, Junín 956, 1113
Buenos Aires, Argentina
S
* Supporting Information
ABSTRACT: The first general procedure for the synthesis of
5- to 7-membered cyclic iminoethers by microwave-assisted
cyclization of ω-amido alcohols promoted by polyphosphoric
acid (PPA) esters is presented. 2-Aryl-2-oxazolines and 5,6-
dihydro-4H-1,3-oxazines were efficiently prepared using ethyl
polyphosphate/CHCl3. Trimethylsilyl polyphosphate in sol-
vent-free conditions allowed for the synthesis of hitherto-
unreported 4,5,6,7-tetrahydro-1,3-oxazepines. The method
involves good to excellent yields and short reaction times.The reaction mechanism and the role of PPA esters were investigated
in a chiral substrate.
2-Oxazolines are present in natural and synthetic products with
biological activity as antibacterials,1 antifungals,2 antineo-
plastics,3 deferrating agents,4 antituberculosis agents,5 insecti-
cides,6 and anti-inflammatories (deflazacort),7 among others. In
organic synthesis, 2-oxazolines represent valuable synthetic
intermediates8 and can act as protecting groups for carboxylic
acids9 and as masked aldehydes.10 Some derivatives find
application in the industrial preparation of pharmaceuticals
such as cloramphenicol, thiamphenicol, and florphenicol.11 On
the other hand, chiral oxazolines and bisoxazolines are widely
employed as ligands in asymmetric catalysis.12 Other industrial
applications of 2-oxazolines include resin and ink modifiers and
polymer stabilizers.13 Because of the importance of 2-
oxazolines, an increasing number of methods have been
developed for their synthesis.14 Moreover, the presence of
this heterocyclic core in many bioactive natural products and
chiral catalysts has drawn considerable interest in the
development of stereoselective methodologies.15 One strategy
for the synthesis of 2-oxazolines involves the reaction of amino
alcohols with carboxylic acids or nitriles in the presence of
activating agents or catalysts.16,17 Alternatively, amino alcohols
can react with aldehydes in oxidizing media yielding 2-
oxazolines.18 Miscelaneous methods include MCRs, metal-
catalyzed reduction of oxazoles, and thionation followed by
cyclodesulfurization.19
Another classical synthetic approach is ring closure of β-
hydroxy amides. Several simple cyclization reagents have been
used for this transformation: SOCl2, PCl5, and some transition-
metal salts.11a,20 This transformation can also be achieved using
Ph3P/DEAD,21a,b Ph3P/DDQ,21c,d the Burgess reagent,22
DAST,23 and XtalFluor-E.24 The main drawbacks of the
reported procedures include long reaction times, drastic
conditions, partial epimerization of α-stereocenters, chemical
instability, and/or low functional group tolerance of the
reagents.
5,6-Dihydro-4H-1,3-oxazines have been reported as useful
synthons for selectively functionalized carboxylic acids,
aldehydes, ketones,25a,b and polymers25c or in the preparation
of natural products.25d,e This heterocyclic core is also present in
more complex molecules acting as acetylcholinesterase and
choline acetyl transferase inhibitors or antihypertensives.26 The
usual methods for their synthesis are an extension of those
developed for 2-oxazolines.
In contrast to their five- and six-membered homologues,
4,5,6,7-tetrahydro-1,3-oxazepines have remained almost unex-
plored, probably due to the lack of suitable methods for their
synthesis. Only one tetrahydro-1,3-oxazepine derivative has
been described, which was obtained with modest yields by ring
closure of the amido alcohol with XtalFluor-E (18 h, 55%)23a
and by tritylation followed by cyclization with phosphonium
anhydrides (>20 h, 31%).27 According to a recent publication,
such a compound shows activity as an inhibitor of glycogen
phosphorylase a, a target enzyme in type II diabetes mellitus.28
In this context, novel and expeditious procedures combining
easily available reagents, high yields, and general scope for 5- to
7-membered cyclic iminoethers are desired. Polyphosphoric
acid (PPA) esters are mild, irreversible dehydrating agents of
the Lewis acid type that activate oxygen and nitrogen
functionalities toward nucleophilic attack and, at the same
time, react chemically with water. Such reagents find
application in a variety of reactions including heterocycle
syntheses29 and are compatible with the use of microwave
irradiation.30 To our knowledge, however, PPA esters had not
been tested yet for the synthesis of cyclic iminoethers.
Received: October 17, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03122
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Organic Letters
Letter
a
The required ω-amido alcohols were easily synthesized in
high to quantitative yields by selective N-acylation of the
corresponding amino alcohols with acyl chlorides.
Scheme 1. Alternative Reaction Paths for the PPE/MW
Cyclization of 1h
We first examined the cyclization of N-benzoylaminoethanol
1a with PPE/CHCl3 (Table 1). The reaction, performed in an
Table 1. Synthesis of 2-Oxazolines 2
a
a
compd 2
R
R1
R2
time (min) yield (%)
MW irradiation: 4 min at 90 °C.
b
a
a
b
c
d
e
f
C6H5
C6H5
H
H
8
3
3
4
4
3
6
6
4
2
4
4
8
88
95
H
H
In order to widen the scope of the method, we next
examined the applicability of the PPE/MW system to the
synthesis of 5,6-dihydro-4H-1,3-oxazines. As shown in Table 2,
4-OCH3C6H4
4-CH3C6H4
2-CH3C6H4
4-ClC6H4
C6H5
H
H
96
H
H
88
H
H
90
H
H
85
Table 2. Synthesis of 5,6-Dihydro-4H-1,3-oxazines 4
CH3
CH3
H
H
91
g
h
i
4-OCH3C6H4
C6H5
H
90
CH3
CH3
H
88
4-ClC6H4
4-NO2C6H4
2,4-Cl2C6H3
C6H5CH2
H
85
c
j
H
40
c
k
l
H
H
10
a
H
H
traces
compd 4
R
reaction conditions
yield (%)
a
Unless otherwise indicated, reactions were performed in a closed-
vessel microwave reactor using a chloroformic solution of PPE (6 mL/
a
b
c
d
e
f
C6H5
5 min, 70 °C
5 min, 70 °C
5 min, 90 °C
5 min, 90 °C
5 min, 70 °C
5 min, 70 °C
5 min, 90 °C
3 min, 90 °C
3 min, 90 °C
10 min, 90 °C
85
96
84
92
91
85
78
65
4-OCH3C6H4
4-CH3C6H4
2-CH3C6H4
4-ClC6H4
b
mmol). The reaction was performed in an open-vessel microwave
reactor under reflux at 70 °C. Decomposes during workup.
c
4-NO2C6H4
2-NO2C6H4
2-FC6H4
open-vessel microwave reactor under reflux (8 min at 70 °C),
led to 2-phenyl-2-oxazoline in 88% yield. Using a closed-vessel
reactor (3 min at 85 °C), compound 2a was obtained in 95%
yield. No reaction occurred in the absence of PPE, while
classical Lewis acids (ZnCl2, AlCl3, BF3) in stoichiometric
amounts failed to promote the MW-assisted cyclization.31
Under the optimized experimental conditions, several 2-
oxazolines (2a−e) were synthesized in high to excellent yields
in short reaction times (Table 1). The cyclization also led to
satisfactory results for chiral substrates 2f−i. The procedure was
less efficient for derivatives with electron-withdrawing groups in
the aryl moiety (2j,k) and for 2-benzyl-2-oxazoline 2l, all of
which underwent partial hydrolysis during the workup
procedure. This is in line with previous literature reports, and
is due to their relatively higher instability in acidic medium.16b
To gain further insight into the reaction mechanism and the
role of PPA esters within it, we next examined the cyclization of
the chiral amido alcohol 1h (Scheme 1). On the basis of
previous literature data, two mechanisms are possible for the
cyclization: nucleophilic attack of the alcohol to the PPE/
PPSE-activated amide carbonyl followed by dehydration
(Scheme 1, path A) or OH activation followed by intra-
molecular SN2-like substitution (Scheme 1, path B). Cyclization
of (R)-N-benzoyl-1-amino-2-propanol 1h with PPE/MW led
exclusively to (S)-4-methyl-2-phenyloxazoline 2h. Analogous
results were obtained with neat PPSE/MW. This stereo-
chemical outcome supports an SN2-like mechanism (Scheme 1,
path B) for the reaction and suggests that PPA esters activate
the OH group, making it a better nucleofuge.
g
h
i
b
2,4-Cl2C6H3
C6H5CH2
27
b
j
35
a
Reactions were performed in a closed vessel microwave reactor using
a chloroformic solution of PPE (6 mL/mmol). Decomposes during
workup.
b
trimethylenic amido alcohols 3a−j were smoothly converted to
the corresponding cyclic imidates 4a−g in high yields, including
4-nitro derivative 4f. Ortho-substituted derivatives 4h,i as well
as 2-benzyldihydrooxazine 4j gave comparatively lower yields.
In view of these encouraging results, we decided to examine
the performance of the PPE/MW system in the cyclization
reaction leading to the more challenging 7-membered cyclic
iminoethers. As previously mentioned, the literature shows only
one example of such a heterocyclic system. Ring-closure
reactions leading to medium-sized rings usually require
comparatively longer reaction times and/or harsher conditions
than their five- and six-membered homologues. This is usually
attributed to the lower probability of both reacting sites to
interact as the length of the alkylene chain increases and to
unfavorable enthalpic and entropic factors.32 In fact, the
attempted cyclization of N-benzoyl-4-aminobutanol (5a) with
PPE/CHCl3 under microwave irradiation (reflux, 5 min at 90
°C) was not successful (Table 3).
Previous results showed that PPSE promotes the cyclization
of some substrates which are unreactive toward PPE.30b,33 This
B
DOI: 10.1021/acs.orglett.6b03122
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In conclusion, we have developed a novel and efficient
protocol for the synthesis of 2-aryl-2-oxazolines and their six-
and seven-membered homologues from amido alcohols. To our
knowledge, this is the first general method for the construction
of the 4,5,6,7-tetrahydro-1,3-oxazepine core reported in the
literature. The ring-closure reactions were conducted under
microwave irradiation and promoted by PPA esters in the
absence of metals, catalysts, or additives. Notably, classical
Lewis acids failed to promote this transformation.The method
involves inexpensive and easily available precursors and
reagents and leads to high yields of the heterocycles in short
reaction times together with controlled stereochemistry in the
case of chiral substrates.
Table 3. Synthesis of 4,5,6,7-Tetrahydro-1,3-oxazepines 6
a
compd 6
R
reaction conditions
PPE/CHCl3, 5 min
yield (%)
b
a
a
a
a
b
c
d
e
f
C6H5
C6H5
c
b
c
PPSE/CH2Cl2, 5 min
20
c
C6H5
10 min, conventional heating
30
C6H5
5 min
8 min
5 min
5 min
5 min
6 min
5 min
6 min
5 min
80
73
71
60
77
76
71
67
61
4-OCH3C6H4
4-CH3C6H4
2-CH3C6H4
4-ClC6H4
4-NO2C6H4
2-FC6H4
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03122.
g
h
i
2,4-Cl2C6H3
C6H5CH2
Experimental procedures, characterization of new com-
a
1
pounds, and H and 13C NMR spectra (PDF)
Unless otherwise indicated, reactions were performed in solvent-free
conditions in an open-vessel microwave reactor using neat PPSE (6 g/
b
mmol). The reaction was performed under reflux in an open-vessel
microwave reactor. Decomposes during workup.
c
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lorelli@ffyb.uba.ar.
ORCID
prompted us to investigate the use of PPSE for the synthesis of
the tetrahydrooxazepine under different experimental con-
ditions (Table 3). The reaction of 5a with PPSE/CH2Cl2 (5
min at 90 °C) led to low conversion of the precursor. A slightly
better yield was obtained by working under solvent-free
conditions under conventional heating (10 min at 90 °C),
although decomposition products were observed. Finally,
complete disappearance of the substrate was attained by
working under solvent-free conditions under MW irradiation (5
min at 90 °C), yielding 80% of compound 6a after purification.
Using the optimized reaction conditions, we synthesized a
series of novel tetrahydro-1,3-oxazepines 6b−i (Table 3). The
method led to the desired seven-membered cyclic imidates with
good to high yields in remarkably short reaction times and
compares favorably with the previously reported examples.23a,27
At variance with the lower homologues (Tables 1, 2), 2-ortho-
substituted aryl or 2-benzyl derivatives were easily synthesized
and gave only slightly lower yields (Table 3).
Liliana R. Orelli: 0000-0001-7654-7211
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the University of Buenos Aires
(20020130100466). We are grateful to Profs. A. Moglioni
(Universidad de Buenos Aires. CONICET, Departamento de
́
́
Farmacologia, Facultad de Farmacia y Bioquimica) and F.
Duran (Universidad de Buenos Aires, CONICET. Departa-
mento de Quimica Organ
́
́
́
ica, Facultad de Ciencias Exactas y
Naturales) for providing access to the microwave reactors.
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2j
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Org. Lett. XXXX, XXX, XXX−XXX

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