Syntheses of methylpyridine and methylpyridine N-oxide decorated benzoxazine and naphthoxazine platforms

Article abstract of DOI:10.1016/j.tetlet.2014.02.129

Simple, aqueous-based syntheses of methylpyridine and methylpyridine N-oxide decorated 3,4-dihydro-2H-naphthoxazine and 2,3-dihydro-1H-naphthoxazine monomers, as well as thermally promoted syntheses of 3,4-dihydro-2H-benzoxazine monomers and bisoxazine methylpyridine derivatives of substituted 1,5-, 2,6-, and 2,7-dihydroxynaphthalenes are described. The crystal structures of two derivatives are presented.

Full text of DOI:10.1016/j.tetlet.2014.02.129

Tetrahedron Letters 55 (2014) 2434–2437
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses of methylpyridine and methylpyridine N-oxide decorated
benzoxazine and naphthoxazine platforms
⇑
⇑
Lorraine M. Deck , Robert T. Paine , Elizabeth R. Bright, Sabrina Ouizem, Diane A. Dickie
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Simple, aqueous-based syntheses of methylpyridine and methylpyridine N-oxide decorated 3,4-dihydro-
Received 12 February 2014
Revised 26 February 2014
Accepted 27 February 2014
Available online 7 March 2014
2
H-naphthoxazine and 2,3-dihydro-1H-naphthoxazine monomers, as well as thermally promoted syn-
theses of 3,4-dihydro-2H-benzoxazine monomers and bisoxazine methylpyridine derivatives of substi-
tuted 1,5-, 2,6-, and 2,7-dihydroxynaphthalenes are described. The crystal structures of two derivatives
are presented.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Benzoxazines
Naphthoxazines
Bisnaphthoxazines
Mild conditions
Benzoxazines and naphthoxazines, originally proposed by Holly
groups. The favorable extraction properties of several of these re-
agents encouraged us to explore the formation of supramolecular
structures and solid-phase supported materials that contain do-
nor-decorated pyridine and pyridine N-oxide fragments. In this re-
gard, although there has been some attention given to the
synthesis of ring substituted benzoxazine and naphthoxazine
1
and Cope and subsequently elaboratedupon by Burke and co-work-
2
ers, are obtained via Mannich-type condensation-cyclization reac-
tions of phenols or naphthols with formaldehyde and primary
amines in a 1:2:1 ratio. The scope of this chemistry is extensive,
and the synthetic evolution has been reviewed by Synoradzki and
3
3,5
co-workers through 2005. The compounds display a host of inter-
monomers, there have been few reports of examples that carry
esting pharmacological properties that have been summarized by
Nath and co-workers, and Ishida and others have developed an
amide, phosphine oxide, pyridine, or pyridine N-oxide donor sub-
4
stituents suitable for coordination interactions with metal cat-
3
,11
intriguing family of robust phenolic resins that arise from thermally
promoted ring opening of selected benzoxazine and naphthoxazine
ions.
In order to eventually obtain multiply functionalized
oligomers of interest in our separations program, we first sought
to develop routes to simple, model monomers containing just the
substituent-free pyridine and pyridine N-oxide platforms. Initially,
5
monomers. In addition and pertinent to our own interests, Ishida
has noted the formation of supramolecular benzoxazine monomers
and oligomers that behave as ionophores toward alkali metal cat-
2
12
the Burke methods as well as a solvent-less procedure were ex-
plored for the synthesis of the benzoxazine monomers, but these
approaches produced inconsistent results. With several amines,
phenols, and naphthols, varying yields of impure products were ob-
tained. It became clear that for the target monomers, alternative
procedures were required in order to improve the yield and purity
of the products.
6
ions. Laobuthee has extended this area by exploring the formation
of alkali metal ion responsive supramolecular crown ethers contain-
7
ing ring opened benzoxazine fragments. The ion extraction proper-
0
ties of several model 3,4-dihydro-3-(2 -hydroxyethylene)-6-alkyl-
2H-benzoxazine monomers toward alkali metal cations and Ce(III)
have also been recently reported, and it was proposed that these
monomers complex Ce(III) through the ring ether O-atom.8
For the syntheses of the dihydronaphthoxazines 2a, 2b, and 3a–
3c, a modified one-pot, three-component procedure similar to that
9
In our own program, we have explored the metal binding and
9
q,r,10
4
extraction properties
of pyridine and pyridine N-oxide plat-
first reported by Mathew and Nath and summarized in Scheme 1,
forms decorated by methylphosphine oxide and alkyl amide donor
was utilized. This aqueous-based method avoids the use of expen-
sive, volatile organic solvents, and high reaction temperatures. In
fact for 2a, 2b, and 3a–3c, even modest reaction temperatures re-
sult in premature oxazine ring opening and the formation of unde-
sirable intermediates and side products.
⇑
Corresponding authors. Tel.: +1 (505) 277 5438 (L.M.D.), +1 (505) 277 1661
R.T.P.).
E-mail addresses: ldeck@unm.edu (L.M. Deck), rtpaine@unm.edu (R.T. Paine).
(
http://dx.doi.org/10.1016/j.tetlet.2014.02.129
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0

L. M. Deck et al. / Tetrahedron Letters 55 (2014) 2434–2437
2435
Scheme 1. Synthesis of dihydronaphthoxazines. Reagents and conditions: 2- or 4-aminomethylpyridine (1.0 equiv), H
2
O, 37% formaldehyde solution (4.0 equiv), 0.5 h, rt
then naphthol 1a or 1b (1.0 equiv), THF, rt, 14 h, 94%, 2a; 83%, 2b; 84%, 3a; 4-aminomethylpyridine-N-oxide dihydrochloride (1.0 equiv), H
formaldehyde solution (2.0 equiv) then 2-naphthol (1b, 1.0 equiv), THF, 14 h, rt, 84%, 3c.
2 2 3
O, saturated K CO , pH 7, 37%
The appropriate naphthol (1a or 1b) was added to a stirred
aqueous solution containing 4-aminomethylpyridine or 2-amino-
methylpyridine and fresh 37% aqueous formaldehyde solution. Be-
cause of the low solubility of the naphthols in this mixture, a small
amount of tetrahydrofuran was added dropwise until a homoge-
neous solution was obtained. After one half hour, additional form-
aldehyde solution was added which improved the yield. Without
the added THF, the reactions were slow and the yields were low.
The solution was stirred overnight at room temperature and
standard work-up afforded crude naphthoxazines, 3,4-dihydro-
The mixture was gently heated (80 °C), dissolved in dichlorometh-
ane, filtered, evaporated at room temperature, and chromato-
graphed to afford 5e and 5f in 51% and 55% yields respectively.
The syntheses of bisnaphthoxazines, 7a, 7b, 8a–8c, 9a, and 9b
from dihydroxynaphthalenes, 6a, 6b, and 6c are outlined in
Scheme 3. In these cases, the best results were obtained without
solvent. Pure, recrystallized dihydroxynaphthalenes were added
to a solution containing 2-aminomethylpyridine or 4-aminometh-
ylpyridine and 37% aqueous formaldehyde, and the mixture was
heated (80 °C) for two to three hours using the method described
1
5
3-(4-pyridinylmethyl)-2H-naphth[2,1-e][1.3]oxazine (2a), 3,4-dihy-
by Talele. The resulting solid was dissolved in dichloromethane,
filtered, and allowed to evaporate at room temperature. Pure bis-
naphthoxazines, 7a, 7b, 8a, 8b, 9a, and 9b were obtained in 53–
84% yield after recrystallization. Attempts to improve the yields
by using toluene as the solvent and heating the reaction mixture
dro-3-(2-pyridinylmethyl)-2H-naphth[2,1-e][1.3]oxazine (2b), 2,3-
dihydro-2-(4-pyridinylmethyl)-1H-naphth[2,1-e][1.3]oxazine (3a) and
2
,3-dihydro-2-(2-pyridinylmethyl)-1H-naphth[2,1-e][1.3]oxazine
(3b). Flash column chromatography using silica gel provided pure
1
6
compounds in 82–94% yields. Compounds 3a and 3b have been
at gentle reflux resulted in impure products. The N-oxide, com-
pound 8c, was synthesized using a procedure similar to that used
for the synthesis of compounds 5e and 5f. After heating the mix-
ture containing 4-aminomethylpyridine N-oxide dihydrochloride,
37% aqueous formaldehyde and 2,6-dihydroxynaphthalene (6b)
and cooling, the resulting solid was dissolved in dichloromethane,
dried, and filtered. Evaporation at room temperature afforded 8c in
51% yield after recrystallization.
previously reported in the literature but no spectral data were
1
1
given.
In a fashion similar to that described for 2a, 2b, 3a, and 3b, the
pyridine N-oxide derivative, 3c, was synthesized in 84% yield by
reaction of 4-aminomethylpyridine-N-oxide dihydrochloride,
naphthol (1b), and 37% formaldehyde solution in the presence of
a saturated aqueous potassium carbonate solution. The 4-amino-
methylpyridine-N-oxide dihydrochloride salt was synthesized
The structures of the naphthoxazines (2a, 2b, 3a–3c), benzoxa-
zines (5a–5f), and bisnaphthoxazines (7a, 7b, 8a–8c, 9a, 9b) mono-
mers were verified by proton and carbon NMR spectral data as well
13
from 4-pyridylacetamide followed by oxidation and hydrolysis
1
4
in hydrochloric acid.
6
,16–18
The synthesis of benzoxazines, 5a–5f, from p-cresol (4a) and
,3-dimethylphenol (4c), using similar reaction conditions, is de-
as infrared data,
which are presented in the Supplementary
2
information. The proton NMR spectra for the compounds contain
three singlets between 4.00 and 5.00 ppm, characteristic of the
scribed in Scheme 2. Phenol (4a or 4c) was added to an aqueous
solution containing 2-aminopyridine or 4-aminopyridine and fresh
5
methylene units in the oxazine ring and the methylene unit for
3
7% aqueous formaldehyde. In these cases THF was not required to
the pyridine ring. In the carbon NMR spectra the methylene reso-
nances appear at approximately 50 and 80 ppm for the oxazine
ring and at 55 ppm for the pyridine ring. The proton NMR spectra
solubilize the reagents. However, the formation of benzoxazines
did not occur readily at room temperature; therefore, the mixtures
were heated at 80 °C for two to three hours. After standard work-
up and chromatography, pure benzoxazines (5a–5d) were ob-
tained in 56–67% yield. The N-oxides, 5e and 5f, were synthesized
by combining 4-aminomethylpyridine N-oxide dihydrochloride, in
sufficient aqueous potassium carbonate solution to reach pH 7,
with 37% aqueous formaldehyde. After stirring for one half hour
at room temperature the appropriate phenol (4a or 4c) was added.
also display a downfield resonance that can be assigned to the pro-
ton(s) ortho to the nitrogen atom in the pyridine ring.1
1,17
The pro-
tons ortho to the nitrogen atom in pyridine N-oxides (3c, 5e, 5f, 8c)
are shifted upfield compared to the pyridine analogs. This upfield
shift also occurs for the ortho carbon resonances. Assignments
were made using DEPT, COSY, HMQC, and HMBC spectral data in
addition to routine proton and carbon NMR spectra. The infrared
2
Scheme 2. Synthesis of dihydronaphthoxazines. Reagents and conditions: 2- or 4-aminomethylpyridine (1.0 equiv), H O, 37% formaldehyde solution (2.0 equiv), 0.5 h, rt
then phenol 4a or 4c (1.0 equiv), 80 °C, 2–3 h, 67%, 5a; 55%, 5b; 61%, 5d; 4-aminomethylpyridine-N-oxide dihydrochloride (1.0 equiv), H
2 2 3
O, saturated k CO , pH 7, 37%
formaldehyde solution (2.0 equiv), 0.5 h, then naphthol 4a or 4b (1.0 equiv), 80 °C, 2–3 h, 51%, 5e; 55% 5f.

2
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L. M. Deck et al. / Tetrahedron Letters 55 (2014) 2434–2437
Scheme 3. Synthesis of bisdihydronaphthoxazines. Reagents and conditions: 2- or 4-aminomethylpyridine (2.0 equiv), 37% formaldehyde solution (4.0 equiv), 0.5 h, rt then
dihydroxynaphthalene 6a, 6b or 6c (1.0 equiv), 80 °C, 2–3 h, 69%, 7a; 53%, 7b; 83%, 8a; 76%, 8b; 84%, 9a; 59%, 9b; 4-aminomethylpyridine-N-oxide dihydrochloride
(
2 2 3
2.0 equiv), H O, saturated k CO , pH 7, 37% formaldehyde solution (4.0 equiv), 0.5 h, rt then dihydroxynaphthalene (4b, 1.0 equiv), 80 °C, 3 h, 51%, 8c.
ꢀ1
1
220 cm was observed for compounds 3c, 5e, 5f, and 8c. Further
verification of the structures of compounds 3c and 8a is provided
by single crystal X-ray diffraction analysis and views of the mole-
cules are shown in Figures 1 and 2. The bisnaphthoxazines exhibit
considerable symmetry as evidenced by the number of resonances
seen in the proton and carbon NMR spectra. In the IR spectra an
ꢀ1
additional absorption at 942–966 cm assigned to the C–H out
of plane deformation of disubstituted naphthalenes was
1
9
observed.
In conclusion, efficient syntheses have been developed for mod-
el methylpyridine and methylpyridine N-oxide decorated naph-
thoxazine, benzoxazine, and bisnaphthoxazine monomers. The
development of this chemistry will permit studies of the metal
ion coordination chemistry for ring-opened analogs which, in turn,
will guide the assembly of the benzoxazine and naphthoxazine
monomers and oligomers decorated with more complicated metal
chelating ligands such as described in our earlier work.9
Figure 1. ORTEP view of 3c with atom labeling scheme (50% probability ellipsoids,
H-atoms omitted for clarity). Selected bond lengths (Å): O1–C1 1.379(2), O1–C12
1
.455(2), N1–C11 1.471(2), N1–C12 1.425(2), N1–C13 1.464(2), N2–O2 1.315(2),
N2–C16 1.356(3), N2–C17 1.363(3).
Acknowledgments
Financial support for this study was provided by the Division of
Chemical Sciences, Geosciences and Biosciences, Office of Basic En-
ergy Sciences, U.S. Department of Energy (Grant DE-FG02-
0
3ER15419) (R.T.P.). In addition, funds from the National Science
Foundation, United States assisted with the purchases of the X-
ray diffractometer (CHE-0443580) and NMR spectrometers (CHE-
0
840523 and CHE-0946690).
Supplementary data
Supplementary data (experimental procedures, spectroscopic
1
13
and HRMS data and copies of H, C NMR, IR) associated with this
article can be found, in the online version, at http://dx.doi.org/
1
0.1016/j.tetlet.2014.02.129. Crystallographic data (excluding
Figure 2. ORTEP view of 8a with atom labeling scheme (50% probability ellipsoids,
H-atoms omitted for clarity). Selected bond lengths (Å): O1–C10 1.376(3), O1–C7
structure factors) for the structures in this Letter have been depos-
ited with the Cambridge Crystallographic Data Centre as supple-
mentary publication nos. CCDC-959251 (8a) and -959252 (3c).
Copies of the data can be obtained, free of charge, on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44
1
1
.449(3), N1–C3 1.325(3), N1–C2 1.328(3), N2–C7 1.425(3), N2–C8 1.474(3), N2–C6
.475(3).
(
0)1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
spectra for the compounds display the characteristic infrared
ꢀ1
absorptions at 1030–1040 cm for the C–O–C symmetric stretch,
References and notes
ꢀ
1
1
9
226–1237 cm for the Ar–O–C asymmetric stretch, and 930–
ꢀ1
4,18
45 cm for the oxazine ring.
An additional N–O stretch at
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