Synthesis of 15N-labelled 3,5-dimethylpyridine

Article abstract of DOI:10.1002/jlcr.3807

¹⁵N-labelled pyridines are liquid- and solid-state nuclear magnetic resonance (NMR) probes for chemical and biological environments because their ¹⁵N chemical shifts are sensitive to hydrogen-bond and protonation states. By variation of the type and number of substituents, different target pyridines can be synthesized exhibiting different pK_a values and molecular volumes. Various synthetic routes have been described in the literature, starting from different precursors or modification of other ¹⁵N-labelled pyridines. In this work, we have explored the synthesis of ¹⁵N ¹⁵N-labelled pyridines using a two-step process via the synthesis of alkoxy-3,4-dihydro-2H-pyran as precursor exhibiting already the desired pyridine substitution pattern. As an example, we have synthesized 3,5-dimethylpyridine-¹⁵N (lutidine-¹⁵N) as demonstrated by ¹⁵N-NMR spectroscopy. That synthesis starts from methacrolein, propenyl ether, and ¹⁵N-labelled NH₄Cl as nitrogen source.

Full text of DOI:10.1002/jlcr.3807

Schubert Mario (Orcid ID: 0000-0003-0278-4091)
Journal of Labelled Compounds and Radiopharmaceuticals
Synthesis of 15N labelled 3,5-dimethylpyridine
Mario Schubert1,2,3,*, Hans-Heinrich Limbach1, José Elguero2
1 Institute of Chemistry and Biochemistry, Free University of Berlin, Takustr. 3, 14195
Berlin, Germany
2 Instituto de Química Médica (IQM-CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain
3
Institute of Molecular Biology, University of Salzburg, Billrothstr. 11, 5020 Salzburg,
Austria
Correspondence: mario.schubert@sbg.ac.at
15N-labelled pyridines are liquid and solid state NMR probes for chemical and biological
environments because their 15N chemical shifts are sensitive to hydrogen-bond and
protonation states. By variation of the type and number of substituents different target
pyridines can be synthesized exhibiting different pKa values and molecular volumes. Various
synthetic routes have been described in the literature, starting from different precursors or
15
modification of other N-labelled pyridines. In this work we have explored the synthesis of
15N labelled pyridines using a two-step process via the synthesis of alkoxy-3,4-dihydro-2H-
pyran as precursor exhibiting already the desired pyridine substitution pattern. As an
example, we have synthesized 3,5-dimethylpyridine-15N (lutidine-15N) as demonstrated by
15N-NMR spectroscopy. That synthesis starts from methacrolein, propenyl ether and 15N-
labelled NH4Cl as nitrogen source.
KEYWORDS
pyridine, lutidine, 2H-pyran, nitrogen-15, isotope labelling, synthesis
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doi: 10.1002/jlcr.3807
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1 INTRODUCTION
15N-labelled pyridines and related heterocycles are important liquid and solid state NMR
probes for chemical and biological environments.1-6 That feature arises on one hand from the
basicity of pyridines and their ability to form hydrogen bonds. On the other hand 15N
15
chemical shifts are very sensitive to the N-1H distance and can be used to monitor the local
15
H-bond and protonation state.7-9 Therefore, N-labelled pyridines have been used to explore
the acidity of mesoporous surfaces or of biological environments using high-resolution solid
state NMR spectroscopy. Moreover, the mobility of pyridines to jump from one proton donor
to another allows one to obtain interesting information about local structures.2
15
15
So far, N pyridine is the only representative that is commercially available in a N-
labelled form. Thus, syntheses of various 15N labelled pyridine derivatives with a large range
of pKa values have been reported so far, applying several routes as illustrated in Scheme 1.
Route I starts from the appropriate pyrylium salt containing already the desired pyridine
substituents, using 15NH4Cl as nitrogen source. In route II alkoxy-3,4-dihydro-2H-pyrans
exhibiting the desired substituents are firstly synthesized as precursors via a Diels-Alder
addition of vinyl ethers to ,-unsaturated carbonyl compounds. The pyrans can then easily
be converted into the corresponding 15N labelled pyridines using 15NH4Cl. Finally, easily
available labelled pyridines can be converted into other derivatives (Route III). Some
examples are depicted in Scheme 2.
Up to date, most 15N-labelled pyridines have been synthesized following route I, namely
2,4.6-trimethyl-pyridine 4 also called collidine,10 2,6-di-tert-butyl-4-methyl-pyridine 5,11 4-
dimethylamino-2,6-dimethy-pyridine 6,12 and 4-diethylamino-2,6-di-tert-butyl-pyridine 7.11
However, route I is limited to pyridines with aliphatic substitutions at C2 and C6. Route II
15
12,13
has been used to synthesize N-labelelled plain pyridine 1
and 4-methyl-pyridine 2.9
Route III was used for the synthesis of 15N-labelled 2,4.6-trimethyl-3-nitro-pyridine 8, 2,4.6-
trimethyl-3-bromo-pyridine 9 and 4-N,N-dimethylamino-pyridine 10.12
As we wanted to obtain 3,5-dimethyl-pyridine-15N (3) as molecular sensor for comparison
with pyridine-15N (1) and collidine-15N (4) we explored the most suitable route to synthesize
3. We could not use route I as pyrylium salts without substituents in 2- and 6- position are
rare and not very stable.14 In addition, it is not possible to obtain 3 from 1 via route III.
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Therefore, we checked in more detail route II. That route had been used to synthesize
pyridine-15N (1). The required precursor 3,4-dihydro-2-methoxy-2H-pyran is commercially
available, and can be synthesized in solution at high pressures up to 15 000 bar15 or under
milder conditions using either dry state adsorption conditions16 or an ytterbium catalyst.17
The original synthesis of Longley et al.18 did not use a solvent or additives but only the neat
reactants, heating them up to about 200 °C in a normal laboratory autoclave. The pressure
achieved was not reported, but they probably did not exceed about 15 bar.18 Therefore, that
method seemed to us preferable as only small quantities of the pyran are needed. We found
that this method was suitable and succeeded to synthesize in a similar way also 4-methyl-
pyridine-15N (2).9 Therefore, we want to describe here in more detail how to prepare
pyridines for which commercial precursors are not available, using the example of 15N-
labelled 3,5-dimethylpyridine (3).
2 RESULTS AND DISCUSSION
In the first stage of this work, we checked out alternative routes starting from unlabelled 3,5-
dimethyl pyridine, but these efforts were not successful.
As precursor of the Diels-Alder reaction we used methacrolein 11 and ethyl 1-propenyl ether
12 leading to 2-ethoxy-3,4-dihydro-3,5-dimethyl-2H-pyran 13 (Scheme 3). Methacrolein was
stabilized with a small amount of hydroquinone to avoid polymerization. NMR spectroscopy
revealed a cis/trans mixture of compound 13 in the ratio of 2:3 (Figure 1), the chemical shifts
are listed and compared to literature values in Table 1). The chemical shifts and coupling
constants of the two 2H-pyran ring isomers fit very well to previous reports for 2-
benzoyloxy-3,4-dihydro-3,5-dimethyl-2H-pyran from Yamamoto et al.19 and 2-methoxy-3,4-
dihydro-3,5-dimethyl-2H-pyran from Descotes et al.20 Although the diastereomers could
potentially be separated by chromatography, a separation was not required, because both
diastereomers are an in situ source of 1,5-pentane-dial that is generated in the initial part of
the second step of the synthesis. A side product of the reaction was the Diels-Alder reaction
of methacrolein with itself forming 3,4-dihydro-2H-pyran-2-carbaldehyde, which was
however reduced by using an excess of the dienophile and was separated by distillation.
In the second step the dihydropyran mixture was converted to 3,5-dimethylpyridine
according to Scheme 4. 15N-labelled 3,5-dimethylpyridine was isolated as an aqueous
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azeotrope by steam distillation of the basified reaction mixture, after volatile substances were
initially removed by distillation of the acidic reaction mixture. Methylene chloride was used
to extract the product from the azeotrope with yields of ~ 55% relative to the amount of the
15N isotope used.
2.1 NMR characterization of 3,5-dimethylpyridine
1
15
The H NMR spectrum of N-labelled 3,5-dimethylpyridine was identical to the unlabelled
compound, except that the protons adjacent to the 15N nucleus show a splitting of 10.6 Hz due
2
to the JHN scalar coupling. The coupling is in agreement with previous reports measured in
1
the same solvent.21 The observed H and 13C resonances agree with previously reported
values measured in the same solvent,22 except that we assigned the 13C signal at 137.0 ppm to
13
C3/C5 and 132.4 ppm to C4 (swapped in Pazderski et al.). Our C assignment agrees also
with the data measured in DMSO-d6 and D2O despite small deviations due to the different
solvents.23,24 Although the 13C spectrum of labelled and unlabelled 3,5-dimethylpyridine
looked virtually identical, a closer interpretation revealed a small splitting of two signals due
n
to small JCN scalar couplings. Interestingly, no splitting was observed for the two carbons
directly adjacent to the nitrogen nucleus, but the signals of C3, C5 and C4 showed a splitting.
3
Values of 3.1 Hz for 2JCN and of 3.5 Hz for JCN were observed, which are in a similar range
as those observed in 15N-labelled pyridine (2JCN = –2.53 Hz and 3JCN = –3.85 Hz 25). 1JCN was
1
too small to be detectable in a splitting in agreement with a JCN of 0.67 Hz observed for 15N-
15
labelled pyridine.26 The observed N resonance of –69.7 ppm referenced to CH3NO2 agrees
well with previously reported chemical shifts measured at natural abundance.21,22
A
comparison with 15N chemical shifts of other methyl-substituted pyridine derivatives is given
in Supplementary Table 1.
In addition to NMR spectroscopy, mass spectrometry confirmed the chemical identity of
compound 3, whose mass spectrum differed from the unlabelled 3,5-dimethylpyridine,27 only
for the 15N-containing fragments.
3 CONCLUSION
15N-labelled 3,5-dimethylpyridine could be conveniently synthesized in two steps starting
from methacrolein, 1-ethoxypropene and 15NH4Cl.
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4 EXPERIMENTAL
15
Unlabelled reagents were purchased from Sigma-Aldrich. N-labelled NH4Cl was purchased
from Chemotrade Chemiehandelsgesellschaft (Leipzig, Germany).
4.1 NMR spectroscopy and mass spectrometry
Unless stated otherwise, NMR spectra were recorded either on a Bruker AMX 500 or a
Bruker AMSY 270 with CDCl3 as solvent at 298K. 1H and 13C chemical shifts were
referenced to TMS. The solvent signals of signals were set for DMSO-d6 to 2.49 ppm (1H)
and 39.51 ppm (13C) and for CDCl3 to 7.24 ppm (1H) and 77.2 ppm (13C). 15N resonances
15
were indirectly referenced to CH3NO2, using a saturated solution of NH4Cl in H2O (~5.64
M) with a chemical shift of –352.89 ppm.28 Mass spectra were recorded on a Varian MAT
711.
4.2 Synthesis of 2-ethoxy-3,4-dihydro-3,5-dimethyl-2H-pyran 13
14 g (0.2 mol) of methacrolein 11, 26 g (0.3 mol) 1-ethoxypropene 12 and 0.1 g
hydroquinone (0.25% of mixture) were heated in a 200 ml autoclave (high pressure
laboratory autoclave model II from Carl Roth, Germany) at 190 ºC for 16 h. During that time
the pressure first rose to 15 bar and then fell to 8 bar. After cooling, the reaction mixture was
distilled under reduced pressure of 48 mbar, yielding at 97ºC 18.8 g of a fruity smelling
colourless oil. The product 13 was further purified by column chromatography (Al2O3,
hexane/ethyl acetate 10:1, column dimensions 40  6 cm). Yield: 11.7 g (74.8 mmol; 37%).
20
1
nD = 1.4420. TLC (Al2O3 hexane/ethyl acetate 10:1): Rf = 0.727. H-NMR (DMSO-d6):
6.02 (s, 0.59H, H6trans), 5.99 (s, 0.41H, H6cis), 4.73 (d, 0.41H, H2cis, J=2.3 Hz), 4.52 (d,
0.59H, H2trans, J=4.6 Hz), 3.5-3.7 (m, 2H, CH2CH3), 1.5-2.6 (m, 3H, H3/H4), 1.5 (s, 3H, 5-
13
CH3), 1.12 (q, 3H, CH2CH3), 0.91 (2d, 3H, 3-CH3). C-NMR (DMSO-d6): 134.1 (C6),
108.1 (C5cis), 107.0 (C5trans), 100.0 (C2trans), 97.9 (C2cis), 62.9 (CH2CH3), 29.4-30.4 (C3
and C4), 17.9 (5-CH3), 16.2 (3-CH3 trans), 15.7 (3-CH3 cis), 15.0 CH2CH3). MS (EI): 156
(18, M+), 111 (19), 86 (100, retro-Diels-Alder), 58 (90).
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4.3 Synthesis of 3,5-dimethylpyridine 3
In a three-necked flask equipped with a reflux condenser, addition funnel and magnetic stirrer
150 ml deionized water were poured, followed by 4.4 ml concentrated H2SO4, 15 g (39.7
mmol) methylene blue and 2 g (36.7 mmol) 15NH4Cl. The solution was brought to reflux and
a solution of 5.78 g (37 mmol) 2-ethoxy-dihydro-3,5-dimethyl-2H-pyran 13 in 5 ml ethanol
was added dropwise over a period of 1 h and refluxed for 17 h. After cooling, 150 ml
deionized water were added and the mixture was distilled until the odour of glutaraldehyde
disappeared in the distillate (ca. 200 ml). After cooling of the remaining reaction mixture,
250 ml of 1.3 M NaOH were added gradually and distilled until ~200 ml of distillate were
collected. 0.1 g Na2CO3 was added and CH2Cl2 was used to extract the organic base. The
combined organic layers were dried with Na2SO4 and the solvent was removed with a rotary
1
2
evaporator. Yield: 2.18 g (20.2 mmol; 55%). H-NMR (CDCl3): 8.21 (d, 2H, H2/H6, JNH=
13
10.6 Hz), 7.26 (s, 1H, H4), 2.25 (s, 6H, CH3), C-NMR (CDCl3): 147.3 (C2/C6), 137.0 (d,
3JNC=3.5 Hz, C4), 132.4 (d, 2JNC=3.1 Hz, C3/C5), 18.1 (CH3). 15N-NMR (CDCl3): –69.7 ppm
15
referenced to CH3NO2. MS (EI): 108 (100, C7H9 N+), 93 (21), 79 (35), 77 (11).
ACKNOWLEDGMENTS
We thank Dr. Nadine Jagerovic, Dr. Christophe Dardonville and Dr. Ibon Alkorta, for their
advice and fruitful discussions, Reinhard Zander for his valuable support and organization of
the synthesis laboratory, Dr. Alžbeta Krutošiková for translating a Czech publication, Ilya
Shenderovich for his help accessing Russian references, Anja Peuker for providing details of
the used instruments and Dr. Chiara Cabrele for comments on the manuscript. We
acknowledge the European Human Capital & Mobility Network "Localization and Transfer
of Hydrogen", Network Reference no ERBCHRXCT940582.
ORCID
Mario Schubert http://orcid.org/0000-0003-0278-4091
Hans-Heinrich Limbach http://orcid.org/0000-0002-2084-6359
José Elguero http://orcid.org/0000-0002-9213-6858
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REFERENCES
1.
2.
Edgar M, Schubert M, Limbach HH, Goltner CG. NMR investigations of
nanostructured ceramics. Ber Bunsen Phys Chem. 1997;101(11):1769-1772.
Shenderovich IG, Buntkowsky G, Schreiber A, Gedat E, Sharif S, Albrecht J,
Golubev NS, Findenegg GH, Limbach HH. Pyridine-N-15 - A mobile NMR sensor
for surface acidity and surface defects of mesoporous silica. J Phys Chem B.
2003;107(43):11924-11939.
3.
4.
5.
Gurinov AA, Rozhkova YA, Zukal A, Cejka J, Shenderovich IG. Mutable Lewis and
Bronsted Acidity of Aluminated SBA-15 as Revealed by NMR of Adsorbed Pyridine-
N-15. Langmuir. 2011;27(19):12115-12123.
Limbach HH, Chan-Huot M, Sharif S, Tolstoy PM, Shenderovich IG, Denisov GS,
Toney MD. Critical hydrogen bonds and protonation states of pyridoxal 5 '-phosphate
revealed by NMR. Bba-Proteins Proteom. 2011;1814(11):1426-1437.
Chan-Huot M, Dos A, Zander R, Sharif S, Tolstoy PM, Compton S, Fogle E, Toney
MD, Shenderovich I, Denisov GS, Limbach HH. NMR Studies of Protonation and
Hydrogen Bond States of Internal Aldimines of Pyridoxal 5 '-Phosphate Acid-Base in
Alanine Racemase, Aspartate Aminotransferase, and Poly-L-lysine. J Am Chem Soc.
2013;135(48):18160-18175.
6.
7.
8.
9.
Shenderovich IG, Lesnichin SB, Tu CK, Silverman DN, Tolstoy PM, Denisov GS,
Limbach HH. NMR Studies of Active-Site Properties of Human Carbonic Anhydrase
II by Using N-15-Labeled 4-Methylimidazole as a Local Probe and Histidine
Hydrogen-Bond Correlations. Chem-Eur J. 2015;21(7):2915-2929.
Lorente P, Shenderovich IG, Golubev NS, Denisov GS, Buntkowsky G, Limbach HH.
H-1/N-15 NMR chemical shielding, dipolar N-15, H-2 coupling and hydrogen bond
geometry correlations in a novel series of hydrogen-bonded acid-base complexes of
collidine with carboxylic acids. Magn Reson Chem. 2001;39:S18-S29.
Tolstoy PM, Smirnov SN, Shenderovich IG, Golubev NS, Denisov GS, Limbach HH.
NMR studies of solid state - solvent and H/D isotope effects on hydrogen bond
geometries of 1 : 1 complexes of collidine with carboxylic acids. J Mol Struct.
2004;700(1-3):19-27.
Ip BCK, Shenderovich IG, Tostoy PM, Frydel J, Denisov GS, Buntkowsky G,
Limbach HH. NMR Studies of Solid Pentachlorophenol-4-Methylpyridine Complexes
Exhibiting Strong OHN Hydrogen Bonds: Geometric H/D Isotope Effects and
Hydrogen Bond Coupling Cause Isotopic Polymorphism. J Phys Chem A.
2012;116(46):11370-11387.
10.
Golubev NS, Smirnov SN, Shah-Mohammedi P, Shenderovich IG, Denisov GS,
Gindin VA, Limbach KK. Study of acid-base interaction by means of using NMR
low-temperature spectra. Structure of salicylic acid complexes. Russ J Gen Chem.
1997;67:1082-1087.
11.
12.
13.
Andreeva DV, Ip B, Gurinov AA, Tolstoy PM, Denisov GS, Shenderovich IG,
Limbach HH. Geometrical features of hydrogen bonded complexes involving
sterically hindered pyridines. J Phys Chem A. 2006;110(37):10872-10879.
Gurinov AA, Lesnichin SB, Limbach HH, Shenderovich IG. How Short is the
Strongest Hydrogen Bond in the Proton-Bound Homodimers of Pyridine Derivatives?
J Phys Chem A. 2014;118(45):10804-10812.
Whaley TW, Ott DG. Syntheses with Stable Isotopes - Pyridine-N-15. J Labelled
Compd. 1974;10(2):283-286.
This article is protected by copyright. All rights reserved.

14.
Taylor RJK, Hemming K, Demedeiros EF. Pyrylium-Salts in Polyene Natural Product
Synthesis - Organometallic Additions to 4-Methylpyrylium Tetrafluoroborate. J Chem
Soc Perk T 1. 1995(19):2385-2392.
15.
16.
Dauben WG, Krabbenhoft HO. Organic-Reactions at High-Pressure - Cycloadditions
with Enol and Dienol Derivatives. J Org Chem. 1977;42(2):282-287.
Veselovsky VV, Gybin AS, Lozanova AV, Moiseenkov AM, Smit WA, Caple R.
Dramatic Acceleration of the Diels-Alder Reaction by Adsorption on
Chromatography Adsorbents. Tetrahedron Lett. 1988;29(2):175-178.
Danishefsky S, Bednarski M. Lanthanide Catalysis of Cyclo-Additions of
Heterodienes with Enol Ethers. Tetrahedron Lett. 1984;25(7):721-724.
Longley RI, Emerson WS. The 1,4-Addition of Vinyl Ethers to Alpha,Beta-
Unsaturated Carbonyl Compounds. J Am Chem Soc. 1950;72(7):3079-3081.
Yamamoto Y, Suzuki H, Morooka Y. Moo2(Acac)2 Catalyzed Hetero-Diels-Alder
Reaction of Benzyl Prop-1-En-1-Yl Ether with Methacrolein. Chem Lett.
1986;15(1):73-76.
17.
18.
19.
20.
21.
Descotes G, Martin JC, Mathicol.N. Conformational-Analysis of 2-Alkoxy-2,3-
Dihydro-(2h)-Pyrans and 2-Alkoxy-Chromans. B Soc Chim Fr. 1972(3):1077-1084.
Holzer W, von Philipsborn W. Inter-Molecular and Intra-Molecular Hydrogen-
Bonding Effects on Geminal N-15, H-1 Spin Coupling and N-15 Chemical-Shifts in
Pyridine-Derivatives. Magn Reson Chem. 1989;27(6):511-514.
22.
23.
Pazderski L, Pawlak T, Sitkowski J, Kozerski L, Szlyk E. Structural correlations for
H-1, C-13 and N-15 NMR coordination shifts in Au(III), Pd(II) and Pt(II) chloride
complexes with lutidines and collidine. Magn Reson Chem. 2010;48(6):417-426.
Rochon FD, Beauchamp AL, Bensimon C. Multinuclear NMR spectra of [Pt(L)Cl-
3](-) (L equals pyridine derivatives) complexes and crystal structure of trans-Pt(2,6-
di(hydroxymethyl)pyridine)(2)Cl-2 center dot 2H2O. Can J Chem. 1996;74(11):2121-
2130.
24.
25.
Takeuchi Y. Long-Range C-13 - H-1 Coupling-Constants .3. Methylpyridines. Org
Magn Resonance. 1975;7(4):181-184.
Bundgaard T, Jakobsen HJ, Rahkamaa EJ. High-Resolution Investigation of Proton
Coupled and Decoupled C-13 Ft Nmr-Spectra of N-15-Pyrrole. J Magn Reson.
1975;19(3):345-356.
26.
27.
28.
Lichter RL, Roberts JD. N-15 Nuclear Magnetic Resonance Spectroscopy .13.
Pyridine-N-15. J Am Chem Soc. 1971;93(20):5218-&.
Grayson JI, Dinkel R. An Improved Liquid-Phase Synthesis of Simple
Alkylpyridines. Helv Chim Acta. 1984;67(8):2100-2110.
Witanowski M, Stefaniak L, Szymanski S, Januszewski H. External Neat
Nitromethane Scale for Nitrogen Chemical-Shifts. J Magn Reson. 1977;28(2):217-
226.
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Table 1: Chemical shifts of the mixture of 2-ethoxy-3,4-dihydro-3,5-dimethyl-2H-pyran
diastereomers and comparison with values of cis and trans 2-benzoyloxy-3,4-dihydro-3,5-
19
dimethyl-2H-pyran from Yamamoto et al.
and 2-methoxy-3,4-dihydro-3,5-dimethyl-2H-
20
pyran from Descotes et al.
.
Atom
H2
observed
(DMSO-d6)
trans
4.52, d,
J=4.6 Hz
1.5-2.2 ov a
1.5-2.2 ov a
1.5-2.2 ov a
6.02, d,
J=1.4 Hz
0.90, d,
J=6.9 Hz
1.50 ov
observed
(DMSO-d6)
cis
4.73, d,
J=2.3 Hz
1.5-2.2 ov a
1.5-2.2 ov a
1.5-2.2 ov a
5.99, d,
J=1.4 Hz
0.92, d,
J=6.5 Hz
1.50 ov
Yamamoto
(CDCl3)
trans
Yamamoto
(CDCl3)
cis
Descotes
(CCl4)
trans
4.35, d,
J=3.4 Hz
Descotes
(CDCl4)
cis
4.52, d,
J=1.7 Hz
4.60, d,
J=4.0 Hz
1.98 dddt
2.24 dd
1.53 dd
6.03, d,
J=1.2 Hz
0.96, d,
J=7.0 Hz
1.54, d,
J=1.2 Hz
4.79, d,
J=2.1 Hz
1.90 m
1.76 m
0.96 m
6.02, d,
J=1.2 Hz
1.00, d,
J=6.4 Hz
1.55, d,
J=1.2 Hz
H3
H41
H42
H6
5.91
5.91
H7
H8
0.91-1.51
0.91-1.51
0.96-1.52
0.96-1.52
3.33
H1’/H1” 3.50-3.70 ov
3.50-3.70 ov
1.13 ov
97.9
30.5
29.4
108.1
134.0
15.8
17.9
62.9
4.82 d, 4.57 d 4.78 d, 4.55 d 3.32
-
H2’
C2
C3
C4
C5
C6
C7
C8
C1’
C2’
1.13 ov
100.1
29.9 b
29.8 b
107.0
134.1
16.2
18.0
62.9
15.0
-
99.8
30.3
30.2
108.2
134.0
16.5
18.4
69.4
97.8
31.0
29.7
109.7
133.8
16.2
18.3
69.1
14.9
a individual assignment could not be achieved due to overlapping signals (ov: overlap)
b assignment might be swapped
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FIGURE 1. 1H NMR spectrum of 2-ethoxy-3,4-dihydro-3,5-dimethyl-2H-pyran 13
consisting of a 2:3 cis/trans mixture measured in DMSO-d6. For clarity, only one enantiomer
is shown for each diastereomer (2R,3R for trans and 2S,3R for cis). The dominating trans
form shows a larger 3JH2H3 scalar coupling. Signals between 1.5 and 2.2 ppm were only
tentatively assigned.
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Scheme 1: Synthetic routes to 15N-labelled pyridine derivatives.
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Scheme 2. Overview of so far reported 15N-labelled pyridine derivatives obtained either by
Route II (a), Route I (b) or Route III (c).
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Scheme 3: Synthetic route to 15N-labelled lutidine used in this work.
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Scheme 4. Proposed mechanism of the formation of 3,5-dimethylpyridine in analogy to
Whaley and Ott 1974.13
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TABLE OF CONTENTS FIGURE:
The synthesis of 15N labelled 3,5-dimethylpyridine is achieved in in two steps starting from
methacrolein, 1-propenyl ether and 15NH4Cl.
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