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P. Kumar et al. / Tetrahedron Letters 55 (2014) 7172–7176
imide, ester group, and Boc groups were found to be compatible
under the reaction condition employed (Table 4, entries 7–8). Thus
this method could synthetically be useful in the selective deprotec-
tion of the N-allyl group over the sec-O-allyl group and several
other functionalities and thus may find application in the design
and synthesis of complex natural products.
A plausible mechanism analogous to those reported for the
cleavage of prenyl ethers, cinnamyl ethers, or OPMB8a is shown
in Scheme 2. The reaction proceeds by the hydride abstraction
from the activated methylene of allylic amine 1 by DDQ followed
by trapping the iminium ion 4 by water giving a hemiaminal 5
which decomposes to give a secondary amine 2, DDQH2, and acro-
lein 6. During the cleavage process the heteroatom nitrogen
located at the
a
-position activates the adjacent allylic sp3 C–H
bond and further stabilizes the in situ formed intermediate. The
oxidative cation formation appears to proceed through a sequence
of radical cation formations followed by hydrogen atom abstrac-
tion. DDQ is a well-known electron acceptor and forms charge
transfer (CT) complexes with a variety of donors indicated usually
by an immediate color change in the reaction mixture.13 In conclu-
sion, we have demonstrated an alternative, convenient, and gen-
eral method for the chemoselective cleavage of allylic amines14
using DDQ-CH2Cl2–H2O as a mild and efficient reagent.
12. General experimental procedure: To a stirred solution of the N-allyllic amine
(1 mmol) dissolved in 9 mL of CH2Cl2 and 1 mL of water, DDQ (1.2 equiv) was
added intermittently in 3–4 portions.
Reaction progress was monitored by TLC. Upon consumption of the starting
material, 2,3-dichloro-5,6-dicyanohydroquinone (DDQH2) was removed by
filtration. A saturated NaHCO3 solution was added to the filtrate and the
aqueous phase was extracted twice with CH2Cl2 (2 Â 15 mL). Drying over
Na2SO4, evaporation to dryness gave a material that was purified by flash
column chromatography using neutral or basic alumina as stationary phase
and hexane–CH2Cl2 (3:7) as eluent. The compound was characterized by
comparison with authentic samples and by spectral data.
Acknowledgments
14. Spectral data for selected compounds:
R.J. thanks CSIR New Delhi for the CSIR-Nehru Postdoctoral
fellowship and S.K.C. thanks CSIR, New Delhi for a research fellow-
ship. The authors thank the Council of Scientific and Industrial
Research (CSIR), New Delhi for financial support as part of XII five
year plan under title ORIGIN (CSC0108).
N-Benzylcyclohexanamine (2a): Yellow liquid; IR (CHCl3, cmÀ1): mmax 3435,
3071, 3065, 2980, 2933, 2854, 1580, 1455, 1372, 1264, 1212, 1119, 758. 1H
NMR (200 MHz, CDCl3) d 1.10–1.23 (m, 5H), 1.57–1.60 (m, 1H), 1.69–1.72 (m,
2H), 1.89–1.92 (d, J = 11.8 Hz, 2H), 2.17 (brs, 1H), 2.46–2.50 (m, 1H), 3.79
(s, 2H), 7.21–7.23 (m, 1H), 7.27–7.30 (m, 4H) ppm.13C NMR (50 MHz, CDCl3):
d 24.9, 26.0, 33.2, 50.7, 56.0, 126.8, 128.1, 128.3, 140.3 ppm.
N-(4-Methoxybenzyl)cyclohexanamine (2e): Yellow liquid; IR (CHCl3, cmÀ1):
m
max 3435, 3073, 3001, 2931, 2854, 1700, 1612, 1508, 1450, 1245, 1169, 1037,
Supplementary data
819, 753. 1H NMR (200 MHz, CDCl3): d 0.97–1.26 (m, 5H), 1.35–1.42 (m, 1H),
1.52–1.80 (m, 4H), 2.44–2.56 (m, 1H), 3.52 (s, 2H), 3.73 (s, 3H), 4.16 (brs, 1H),
6.75–6.80 (d, J = 8.6 Hz, 2H), 7.20–7.23 (m, 2H) ppm. 13C NMR (50 MHz, CDCl3):
d 23.2, 25.5, 29.6, 49.9, 55.6, 57.5, 112.9, 126.5, 130.2, 157.8 ppm
Supplementary data associated with this article can be found, in
N-(4-Methoxybenzyl)hexadecan-1-amine (2f): Pale yellow viscous oil; IR (CHCl3,
cmÀ1):
mmax 3478, 3071, 2975, 2852, 1728, 1612, 1511, 1645, 1248, 1171, 1039,
817, 760. 1H NMR (200 MHz, CDCl3): d 0.87–0.93 (t, 3H), 1.37–1.35 (m, 29H)
2.49–2.57 (m, 2H), 3.61 (s, 2H), 3.82 (s, 3H), 6.84–6.88 (m, 2H), 7.23–7.28 (m,
2H) ppm. 13C NMR (50 MHz, CDCl3): d 12.3, 20.9, 25.1, 25.6, 27.6, 27.8, 27.9,
30.2, 51.5, 53.4, 55.5, 111.7, 128.2, 129.9, 156.7 ppm. Anal. Calcd for C24H43NO:
C, 79.72; H, 11.99; N, 3.87%; Found: C, 79.74; H, 11.95; N, 3.92%.
References and notes
tert-Butylcyclohexylcarbamate (2n): White solid; mp 74–76 °C; IR (CHCl3,
cmÀ1): mmax 3356, 2984, 2934, 2822, 2713, 1720, 1658, 1452, 1416, 1263,
830. 1H NMR (200 MHz, CDCl3): d 1.05–1.36 (m, 5H), 1.44 (s, 9H), 1.58–1.71 (m,
3H), 1.90–1.95 (m, 2H), 3.39–3.42 (m, 1H), 4.43 (brs, 1H) ppm.13
C NMR
(50 MHz, CDCl3): d 24.8, 25.4, 28.3, 33.4, 49.3, 78.8, 155.1 ppm.