Synthesis of [15N]t-butylamine hydrochloride

Article abstract of DOI:10.1002/jlcr.1747

This report presents an efficient synthesis of [¹⁵N]t-butylamine hydrochloride. Acylation of [¹⁵N]ammonia with pivaloyl chloride provided [¹⁵N]pivalamide, this was converted to benzyl [ ¹⁵N]N-t-butylcarbamate th

Full text of DOI:10.1002/jlcr.1747

Research Article
Received 9 October 2009,
Revised 11 January 2010,
Accepted 15 January 2010
Published online 17 February 2010 in Wiley Interscience
(www.interscience.wiley.com) DOI: 10.1002/jlcr.1747
Synthesis of [15N]t-butylamine hydrochloride
Yingdan Zhang,a Chaojie Lin,a Zhan Li,b Liqiong Qin,a and Hongliang Wena
Ã
This report presents an efficient synthesis of [15N]t-butylamine hydrochloride. Acylation of [15N]ammonia with pivaloyl
chloride provided [15N]pivalamide, this was converted to benzyl [15N]N-t-butylcarbamate through a Hofmann rearrange-
ment. Hydrogenolysis of benzyl [15N]N-t-butylcarbamate and acidification afforded [15N]t-butylamine hydrochloride in an
overall yield of 79.2% in four steps.
Keywords: 15N-labelling synthesis; [15N]t-butylamine hydrochloride; Hofmann rearrangement
converted to the t-butyl [15N]isocyanate 6 with the aid of
Hg(OAc)2. Normally alkyl isocyanates reacted only slowly with
Introduction
t-Butylamine is a versatile and important building block for
compounds with biological activities. Finasteride, a 5a-reductase
inhibitor for use in treating acne, female hirsutism and benign
prostatic hyperplasia, was prepared by the reaction of the
carboxylate of 4-aza-5a-androst -1-ene-3-one with t-butylamine.1
It was also built into an apoptosis inhibitor,2 an HIV protease
inhibitor,3 various antibotics4 and nucleosides.5 Moreover t-butyl-
amine residues are commonly in adrenergic drugs to increase
affinity for b-adrenoceptors.6–8
benzyl alcohols at room temperature and did not react
completely;11 however, t-butyl [15N]isocyanate 6 reacted with
benzyl alcohol smoothly at room temperature and gave an
excellent yield in this reaction. It is conceivable that the acetic
acid produced in situ catalyzed the transformation. Finally
catalytic hydrogenolysis of benzyl [15N]N-t-butylcarbamate 3
and acidification with HCl gas gave [15N]t-butylamine hydro-
chloride 4 in 79.2% overall yield in four steps.
Here we describe an efficient synthesis of [15N]t-butylamine
hydrochloride, which can be used as biomarker for biologically
active compounds containing this motif or incorporated in the
synthesis of drug candidates for use as analytical internal
standards or for metabolic studies.
Experimental
Materials and instruments
[15N]Ammonium chloride (99.5 atom% 15N) was purchased from
Shanghai Engineering Research Center of Stable Isotope.
Pivaloyl chloride was distilled before use. All other chemicals
were of analytical grade. Melting points were determined on
XT4A microscopic digital melting-point apparatus and are
uncorrected. 1H-NMR (400 MHz) and 13C-NMR (100 MHz) spectra
were recorded on a JNM-ECA-400 NMR spectrometer in CDCl3 or
DMSO-d6 (TMS as internal standard). FT-IR spectra were recorded
on a Nicolet FT-IR 5700 spectrometer using KBr pellets. EI-MS
Spectra were obtained with ZAB-HS spectrometer.
Results and discussion
Reports of the preparation of [15N]t-butylamine are very rare.
Previously Glueck et al. described the preparation of [15N]t-
butylamine hydrochloride.9 Pivaloyl chloride reacted with
[15N]ammonia to give [15N]pivalamide, which was converted to
t-butyl [15N]isocyanate with KOBr via a Hofmann rearrangement.
t-Butyl [15N]isocyanate was then reacted with HCl to give [15N]t-
butylamine hydrochloride in moderate yield. In this method
[15N]t-butylamine hydrochloride and its intermediate were
insufficiently characterized. In order to improve the synthesis
of [15N]t-butylamine, we developed a new synthetic method
with a higher labelling yield.
As shown in Scheme 1, acylation of [15N]NH3, which was
derived from [15N]NH4Cl 1, with pivaloyl chloride in Et2O-H2O
gave [15N]pivalamide 2 in 93.8% yield. [15N]Pivalamide 2 was
converted to benzyl [15N]N-t-butylcarbamate 3 through a
Hofmann rearrangement with N-bromosuccinimide, mercuric
acetate and benzyl alcohol in DMF under nitrogen at room
temperature in 90.5% yield. Jew et al. discussed the reaction
mechanism in detail (Scheme 2).10 When NBS was added to the
solution of pivalamide, Hg(OAc)2 and benzyl alcohol in DMF,
the N-Br bond cleaved with the help of DMF and reacted
with pivalamide to give [15N]N-bromopivalamide 5, which was
[
15N]Pivalamide (2)
Pivaloyl chloride (3.0 mL, 24 mmol) in Et2O (10 mL) was layered
onto a solution of 15NH4Cl (1.0 g, 18.3 mmol) in H2O (4 mL) in a
25 mL flask so the layers did not mix. The flask was cooled to 01C
and NaOH (4.4 g, 110 mmol) in H2O (6 mL) was added slowly by
aSchool of Chemical Engineering and Environment, Beijing Institute of
Technology, Beijing 100081, People’s Republic of China
bInstitute of Medicinal Plant, Chinese Academy of Medical Science & Peking
Union Medical College, Beijing 100094, People’s Republic of China
*Correspondence to: Hongliang Wen, School of Chemical Engineering and
Environment, Beijing Institute of Technology, No. 5 Zhongguancun South Street,
Haidian District, Beijing 100081, People’s Republic of China.
E-mail: hongliang_wen@hotmail.com
J. Label Compd. Radiopharm 2010, 53 183–185
Copyright r 2010 John Wiley & Sons, Ltd.

Y. Zhang et al.
O
O
NaOH
NBS-Hg(OAc)2-BzOH
DMF
15NH4Cl
+
15NH2
Cl
Et2O-H2O
1
2
O
1) H2/Pd-C, MeOH
2) HCl
15NH
15NH2 HCl
O
4
3
Scheme 1.
O
O
O
O
O
DMF
H
Br
+
N
+
H
15N
N
15NH2
Br
5
2
O
O
H
15N
AcOH
HgBr(OAc) +
+
C
O
15N
+ Hg(OAc)2
Br
6
5
O
O
3
H+
15NH
15N
C
O
+
CH2OH
6
Scheme 2.
3
pipette to the aqueous layer with slow stirring to avoid mixing 90.5%). 1H-NMR (CDCl3)d: 1.18(d, 9H, J1H–15N = 2.8 Hz), 4.83(d,
of the layers. A white precipitate formed during addition. The 1H, J1H–15N = 88.9 Hz), 4.91(s, 2H), 7.14–7.19(m, 5 H); 13C-NMR
flask was warmed to room temperature and stirred for 15 min, (CDCl3)d: 28.6(s), 50.0(d, J13C–15N = 9.3 Hz), 65.6(s), 127.6(s),
and then the mixture was stirred violently for additional 10 min. 127.7(s), 128.1(s), 136.6(s), 154.4(d, J13C–15N = 27 Hz); FT-IR (KBr)
The white solid was collected by filtration and washed with cmÀ1: 3341, 2969, 1713, 1498, 1454, 1394, 1365, 1266, 1210,
diethyl ether. After drying under vacuum, the white solid was 1072, 914, 776, 738, 696; EI-MS m/z : 208 (M1, 3), 193(4), 149(9),
purified by flash chromatography (petroleum ether: ethyl 108(47), 91(100), 89(3), 79(5), 57(11).
acetate = 1:1and 5% methanol was added) to afford [15N]pival-
amide (2) (1.75 g, 93.8%), m.p.: 154–1551C (Lit. 155–1561C for
the unlabelled analogue12). 1H-NMR (DMSO-d6)d: 1.06 (s, 9H);
13C-NMR (DMSO-d6)d: 27.5 (s), 37.8(d, 2J13C–15N = 6.8 Hz), 179.8(d,
[
15N]t-Buytlamine hydrochloride (4)
Benzyl [15N]N-t-butylcarbamate (1.04 g, 5 mmol) and 10% Pd-C
(1.0g) were dissolved in absolute methanol (10 mL) at room
temperature. A gentle stream of hydrogen was passed through
the reaction mixture. The reaction proceeded for 4 h and the
mixture was then filtered and washed with methanol. The filtrate
was cooled to 01C and gaseous HCl was bubbled slowly through
the solution. The reaction mixture was evaporated to give [15N]t-
butylamine hydrochloride (4) (0.515g, 93.3%), m.p.: 230–2401C
J
13C–15N = 13.8 Hz); FT-IR (KBr) cmÀ1: 3390, 3199, 2961, 2923,
2756, 1648, 1485, 1455, 1408, 1376, 1356, 1221, 1103, 863, 816,
732, 615; EI-MS m/z : 102 (M1, 18), 87(10), 69(5), 57(100), 47(18),
45(23), 41(66), 29(43).
Benzyl [15N]N-t-butylcarbamate (3)
Benzyl alcohol (1.107 g, 10.25 mmol) was added to a solution (sublimation)(Lit. 275–2801C for the unlabelled analogue13).
of [15N]pivalamide (0.209 g, 2.05 mmol) and Hg(OAc)2 (0.795 g, 1H-NMR (DMSO-d6)d: 1.25(d, 9 H, J1H–15N = 2.8 Hz), 7.93(d, 3 H,
3
2.45 mmol) in DMF (8 mL), followed by the addition of a solution
J =
1H–15N = 71.6Hz); 13C-NMR (DMSO-d6)d: 27.1(s), 50.9(d, J13C–15N
of NBS (0.48 g, 2.7 mmol) in DMF (2 mL) at room temperature 4.2 Hz); FT-IR (KBr) cmÀ1: 2977, 2887, 2789, 2696, 2583, 2493, 2078,
under nitrogen. After reaction at this temperature for 30 h, the 1661, 1503, 1401, 1376, 1298, 1215, 994; EI-MS m/z : 59 (M-15,
reaction mixture was poured into water and extracted with 100), 57(13), 53(3), 43(51), 41(61), 36(100), 31(33), 27(15).
CH2Cl2 (15 mL Â 3). The combined extracts were washed with
water, saturated NaHCO3, dried over anhydrous magnesium Acknowledgement
sulfate and evaporated. The crude product was purified by
flash chromatography (petroleum ether: ethyl acetate = 40:1) to
We would like to thank Peking University Analytical Instrumentation
Center for spectral data.
give yellow oil benzyl [15N]N-t-butylcarbamate (3) (0.385 g,
www.jlcr.org
Copyright r 2010 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2010, 53 183–185

Y. Zhang et al.
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