Synthesis of the α-tocopheramine-15N model compound 2,2,5,7,8- pentamethyl-6-chromanylamine-15N

Article abstract of DOI:10.2174/157017810791130612

2,2,5,7,8-Pentamethyl-6-chromanylamine-¹⁵N was prepared by condensation of ZnCl2-catalyzed condensation of 2,3,5-trimethylphenol with isoprene, followed by conversion of the resulting chroman into the 6-nitro-derivative and further reduction to the title compound, which possesses a 2-methyl group instead of the isoprenoid side chain of tocopherols and tocopheramines. Tocopheramines and their short-chain analogues are current targets in medicinal chemistry and anti-cancer research.

Full text of DOI:10.2174/157017810791130612

Letters in Organic Chemistry, 2010, 7, 335-337
335
Synthesis of the ꢀ-Tocopheramine-15N Model Compound 2,2,5,7,8-
Pentamethyl-6-chromanylamine-15N
Stefan Böhmdorfera, Lars Gilleb and Thomas Rosenau*,a
aDepartment of Chemistry, University of Natural Resources and Applied Life Sciences (BOKU Vienna), Muthgasse 18,
A-1190 Vienna, Austria
bMolecular Pharmacology and Toxicology Unit, Department of Biomedical Sciences, University of Veterinary Medicine
Vienna, Veterinärplatz 1, A-1210 Vienna, Austria
Received January 13, 2010: Revised March 03, 2010: Accepted March 13, 2010
Abstract: 2,2,5,7,8-Pentamethyl-6-chromanylamine-15N was prepared by condensation of ZnCl2-catalyzed condensation
of 2,3,5-trimethylphenol with isoprene, followed by conversion of the resulting chroman into the 6-nitro-derivative and
further reduction to the title compound, which possesses a 2-methyl group instead of the isoprenoid side chain of
tocopherols and tocopheramines. Tocopheramines and their short-chain analogues are current targets in medicinal
chemistry and anti-cancer research.
Keywords: Tocopherols, tocopheramine, vitamin E, isotopic labeling, chromans.
INTRODUCTION
utilizes H15NO3 to introduce the label as 6-nitro-derivative 2.
Direct nitration of 1 was unsuccessful and only the use of the
HOAc / HNO3 couple forming intermediate acetyl nitrate
proved viable. Subsequent reduction to the amino
functionality (compound 3) was done by LiAlH4 according
to standard procedures, deamination and re-formation of 1
was a major side reaction in this final step of the sequence.
Tocopheramines [1] represent aza-analogues of
tocopherols, which are the most potent lipophilic
antioxidants in mammalian tissue [2]. Commonly referred to
as vitamin E, the tocopherols – with ꢀ-tocopherol as the
most prominent congener – are phenolic, radical chain-
breaking antioxidants, but possess also several non-
antioxidant functions, e.g. in cell signaling [3].
Tocopheramines are not only of academic interest with
regard to their oxidation behavior in one-electron and two-
electron processes and possible differences and similarities
relative to the tocopherol counterparts, but have recently also
been found effective as cancerostatic agents [4]. Analytical
studies of oxidation behavior by electron paramagnetic
resonance (EPR) and nuclear magnetic resonance (NMR) as
well as metabolic studies required 15N-labeled ꢀ-
tocopheramine in the form of the less lipophilic truncated
model compound 2,2,5,7,8-pentamethyl-6-chromanylamine
in which the isoprenoid side chain is replaced by a methyl
group.
i
+
OH
O
1
H2*N
O2*N
iii
ii
O
O
3
2
Scheme 1. Synthesis of 2,2,5,7,8-pentamethyl-6-chromanylamine-
15N: i) ZnCl2, AcOH; 13.7 %; ii) H15NO3, AcOH; iii) LAH, THF,
26.7 % over two steps, * = 15N.
RESULTS AND DISCUSSION
Recently, an elegant approach to tocopheramines by a
simultaneous, Pd-catalyzed deoxygenative amination
approach of the corresponding tocopherols has been
presented [5]. For introduction of the 15N-label it was less
suitable, however, as the corresponding amine components
were rather costly. The alternative approach as shown in
Scheme 1 builds the chroman skeleton of 1 by ZnCl2-
catalyzed Friedel-Crafts type condensation, and subsequently
CONCLUSION
The present synthesis of 2,2,5,7,8-pentamethyl-6-
chromanylamine-15N offers the advantages of facile
preparation also of gram-amounts and uses one of the
cheapest
15N-starting
materials
available.
This
counterbalances the drawback of rather moderate yields. The
15N-label in tocopheramines and their model compounds
have the advantage of being detectable in EPR, NMR and
mass spectrometry as well, thus being a quite versatile and
useful probe.
*Address correspondence to this author at the Department of Chemistry,
University of Natural Resources and Applied Life Sciences (BOKU
Vienna), Muthgasse 18, A-1190 Vienna, Austria; Tel: +43-1-36006 6071;
Fax: +43-1-36006 6059; E-mail: thomas.rosenau@boku.ac.at
1570-1786/10 $55.00+.00
© 2010 Bentham Science Publishers Ltd.

336 Letters in Organic Chemistry, 2010, Vol. 7, No. 4
Böhmdorfer et al.
EXPERIMENTAL
General
0.3 ml of glacial acetic acid, which was added to the
solution. The greenish-brown solution was stirred for 5 hours
and then added carefully to a beaker containing 75 ml of a
saturated aqueous sodium bicarbonate solution and 5 ml of
ethyl acetate. The aqueous solution was extracted four times
with ethyl acetate (25 ml each), and the combined organic
layers were washed with saturated sodium bicarbonate
solution and brine, dried over MgSO4, filtered and
evaporated to give a brown oil (725 mg) which was used
without purification in the next step. 10% of the crude
intermediate were kept for analytical studies, the pure
compound 2 being isolated by flash chromatography (n-
Commercial chemicals were of the highest grade
available and were used without further purification.
Reagent-grade solvents were used for all extractions and
workup procedures. Distilled water was used for all aqueous
extractions and for all aqueous solutions. The organic
solvents used were distilled before use. TLC was performed
using Merck silica gel 60 F254 pre-coated plates. All given
yields refer to isolated, pure products. Melting points,
determined on a Kofler-type micro hot stage with Reichert-
Biovar microscope, are uncorrected. 15N-Labeled nitric acid
(H15NO3) was obtained from Sigma-Aldrich, Austria.
1
hexane / ethyl acetate, v/v = 10:1), m.p. 126°C. H NMR: ꢀ
2.62 (2H, t, 3J = 6.80 Hz, 4-CH2), 2.14 (s, 3H, 8b-CH3), 2.11
1H NMR spectra were recorded at 400.13 MHz for H,
1
3
(s, 6H, 5a-CH3, 7a-CH3), 1.82 (t, 2H, J = 6.80 Hz, 3-CH2),
100.41 MHz for 13C, and 40.56 MHz for 15N NMR with
CDCl3 as the solvent if not otherwise stated. Chemical shifts,
relative to tetramethylsilan (TMS) for 1H and 13C and
nitromethane for 15N, are given in ꢀ values, coupling
constants in Hz.
1.32 (s, 6H, 2a-CH3). 13C NMR: ꢀ 153.0 (C-8a), 146.6 (d, C-
6, JC,N = 14.2 Hz), 126.5 (C-7), 125.4 (C-5), 124.1 (C-8),
117.8 (C-4a), 74.4 (C-2), 32.5 (C-3), 27.1 (C-2a), 21.2 (C-4),
14.9 (C-7a), 14.1 (C-5a), 12.1 (C-8b). EI-MS (70 eV): 195
(100 %), 250 (64 %), 233 (30 %), 177 (23 %), 178 (22 %),
91 (14 %), 196 (11 %),251 (10 %), 77 (10 %). Rf (n-hexane /
ethyl acetate, v/v = 5:1): 0.63. Microanalysis calcd. for
C14H1915NO3 (250.31): C 67.14, H 7.65, N 5.62; found C
67.12, H 7.79, N 5.39.
2,2,5,7,8-Pentamethylchroman (1) [6]
A 500 ml round-bottom flask equipped with a magnetic
stirrer, a reflux condenser and a septum was flame dried,
purged with Ar and charged with 2,3,5-trimethylphenol
(45.40 g, 333 mmol) and 40 mL of anhydrous glacial acetic
acid, which failed to dissolve the phenol completely.
Anhydrous zinc chloride (5.00 g, 36.7 mmol) was added
directly from the ampoule. After the addition of isoprene
(33.40 ml, 22.70 g, 333 mmol), the mixture was stirred
overnight. After 17 h the solution was heated to reflux for
another 8 h (changing its color from brown to black),
allowed to cool down again and was stirred at r.t. for another
three days. After that, water was added and the solution was
extracted three times with n-hexane. The combined organic
layers were washed three times with a saturated solution of
sodium bicarbonate, twice with distilled water and once with
brine. The organic solution was dried over MgSO4, filtered
and evaporated. The resulting brown oil was purified by
Kugelrohr distillation to provide 1 as a white solid (9.34 g,
13.7 %). 1H NMR: ꢀ 6.68 (s, 1H, 6-CH), 2.73 (t, 2H, 3J= 6.7
Hz, 4-CH2), 2.34; (s, 3H, 8b-CH33), 2.30 (s, 3H, 7a-CH3),
2.22 (s, 3H, 5a-CH3), 1.92 (t, 2H, J= 6.6 Hz, 3-CH2), 1.44
(s, 6H, 2a-CH3). 13C NMR: ꢀ 152.2 (C-8a), 135.1 (C-7),
133.9 (C-5), 122.8 (HC-6), 122.4 (C-8), 117.1 (C-4a), 73.5
(C-2), 33.3 (C-3), 27.4 (C-2a), 21.3 (C-4), 20.3 (C-7a), 19.3
(C-5a), 11.9 (C-8b). EI-MS (70 eV): 149 (100%), 204
(66%), 148 (24%), 105 (20%), 150 (14%), 189 (10%), 205
(10%). Rf (n-hexane / ethyl acetate, v/v = 5:1): 0.75.
Microanalysis calcd. for C14H20O (204.31): C 82.30, H 9.87;
found C 82.27, H 9.94.
2,2,5,7,8-Pentamethyl-6-chromanylamine-15N
hydrochloride (3)
The crude 2 as obtained above was dissolved in THF (6
ml, dried over sodium and distilled) in a 10 ml round bottom
flask, and the solution was cooled with an ice/water bath.
Lithium aluminum hydride (200 mg, 5.29 mmol) was added
in small quantities. The reaction mixture was allowed to
warm to r.t. and was stirred overnight. The resulting
suspension was slowly poured into a beaker with ice cold
water. The mixture was extracted four times with ethyl
acetate, and the combined organic layers were washed with
brine, dried over MgSO4, filtered and the solvent evaporated
to afford pure 3 (see analytical data) as a brownish oil
(26.7%) that was used for NMR analysis. For storage, the
product was dissolved in diethyl ether, and the amine was
precipitated as the hydrochloride by adding HCl in diethyl
ether (1 M) to afford 201 mg of an off-white powder that
was storable for prolonged times without decomposition,
1
3
m.p. 40°C. H NMR: ꢀ 2.66 (2H, t, J = 6.80 Hz, 4-CH2),
2.14 (s, 3H, 8b-CH3), 2.12 (s, 3H, 7a-CH3), 2.06 (s, 3H, 5a-
CH3), 1.79 (t, 2H, J = 6.80 Hz, 3-CH2), 1.29 (s, 6H, 2a-
3
CH3). 13C NMR: ꢀ 145.5 (C-8a), 134.7 (d, JC,N = 9.4 Hz, C-
6), 122.6 (C-8), 121.1 (C-7), 118.4 (C-4a), 117.3 (C-5), 72.6
(C-2), 33.2 (C-3), 27.0 (C-2a), 21.7 (C-4), 13.9 (C-7a), 12.9
(C-5a), 12.2 (C-8b). 15N NMR: ꢀ –331.3. EI-MS (70 eV):
164 (100 %), 220 (58 %), 165 (37 %), 136 (19 %), 121 (15
%), 135 (11 %), 221 (10 %). Rf (n-hexane / ethyl acetate, v/v
= 5:1): 0.30. Microanalysis calcd. for C14H2115NO (220.33):
C 76.32, H 9.61, N 6.39; found C 76.31, H 9.68, N 6.33.
6-Nitro-2,2,5,7,8-pentamethylchromane-15N (2)
(Caution! Explosive acetyl nitrate is
a
possible
intermediate / byproduct of this reaction. Failing to remove
this compound before work-up and solvent evaporation can
result in an explosion.) 2,2,5,7,8-Pentamethylchroman (600
mg (2,94 mmol) was dissolved in glacial acetic acid (3 ml) in
a 10 ml round bottom flask, and the solution was kept at
14°C with a cold water bath. 15N-Nitric acid (10 N, 1 ml)
was added slowly, and the acid ampoule was rinsed with
ACKNOWLEDGEMENT
The financial support by the Austrian Fonds zur
Förderung der wissenschaftlichen Forschung (FWF, project
P-19081) is gratefully acknowledged.

Synthesis of the ꢀ-Tocopheramine-15N Model
Letters in Organic Chemistry, 2010, Vol. 7, No. 4
337
[4]
Tomic-Vatic, A.; EyTina, J.; Chapman, J.; Mahdavian, E.; Neuzil,
J.; Salvatore, B. A. Vitamin E amides, a new class of vitamin E
analogues with enhanced proapoptotic activity. Int. J. Cancer,
2005, 188.
Mazzini, F.; Netscher, T.; Salvadori, P. Efficient synthesis of
vitamin E amines. Eur. J. Org. Chem., 2009, 2063.
Ramage, R.; Green, J.; Blake, A. J. An acid labile arginine
derivative for peptide synthesis: N-2,2,5,7,8-pentamethylchroman-
6-sulphonyl-L-arginine. Tetrahedron, 1991, 6353.
REFERENCES
[1]
[2]
[3]
Schwieter, U.; Tamm, R.; Weise, H.; Wiss, O. Zur Synthese und
Vitamin-E-Wirksamkeit von Tocopheraminen und ihren N-Alkyl-
Derivaten. Helv. Chim. Acta, 1966, 2297.
Lucarini, M.; Pedulli, G.F. in The Encyclopedia of Vitamin E;
Preedy, V.R.; Watson, R.R., Eds.; CAB International: Wallingford,
2007, pp. 3-10.
Pfluger, P.; Kluth, D.; Landes, N.; Bumke-Vagt, C.; Brigelius-
Flohe, R. Vitamin E: underestimated as an antioxidant. Redox
Report, 2004, 249.
[5]
[6]