Imine acylation via benzotriazole derivatives: The preparation of enaminones

Article abstract of DOI:10.1055/s-2000-8723

Metalated ketimines 2a-c are converted into enaminones 3a-p in good to excellent yields by acylbenzotriazoles 1a-f.

Full text of DOI:10.1055/s-2000-8723

PAPER
2029
Imine Acylation via Benzotriazole Derivatives: The Preparation of
Enaminones
Alan R. Katritzky,* Yunfeng Fang, Augustine Donkor, Jiaye Xu
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
Fax +1(352)3929199; E-mail: katritzky@chem.ufl.edu
Received 26 June 2000; revised 18 August 2000
ketimines 2a-c23 (1.0 equivalent) with LDA (2.0 equiva-
lents) in THF at 0 ∞C, followed by the addition, at -78 ∞C,
of a THF solution of a N-acylbenzotriazole 1a-f (1.0
equivalent). The solution was allowed to warm up to room
temperature overnight. After aqueous workup, enamino-
nes 3a-p were isolated as the only products in good to ex-
cellent yields (Scheme, Table).
Abstract: Metalated ketimines 2a-c are converted into enaminones
3a-p in good to excellent yields by acylbenzotriazoles 1a-f.
Key words: imine, acylation, lithiation, benzotriazole, enaminone
Azolides in general and acylbenzotriazoles in particular
are powerful acylating reagents.1 1-(Trifluoroacetyl)ben-
zotriazole is a convenient and effective trifluoroacetylat-
ing reagent for amines and alcohols.2 N-
Acetylbenzotriazole is superior in many ways to other re-
agents for the acetylation of proteins.3 Stable N-formyl-
benzotriazole is convenient for N- and O- formylation of
amines and alcohols, respectively.4 1,1¢-(1,2-Dioxo-
ethane-1,2-diyl)bis-1H-benzotriazole provides asymmet-
rical oxamides.5 A 1-benzotriazolylcarbonyl group offers
convenient protection of a primary amine in the synthesis
of terminal peptides.6 Oligomeric N-acylbenzotriazoles
are used in medicinal chemistry as matrices for binding
pharmacologically active substrates with hydroxyl or
amino groups.7 Poly(1-acroylbenzotriazole) with amines,
alcohols, phenols or hydrazines give the corresponding
polyesters and polyamides.8a-c Asymmetrical ketones are
formed from acylbenzotriazoles with alkylaluminum
chlorides9 or organozinc agents.10
Scheme
The structures of compounds 3a-p were supported by
NMR spectroscopy, elemental analysis and/or high-reso-
lution mass spectrometry. For example, for compound 3a,
Enaminones are important synthetic intermediates, partic-
ularly in heterocyclic chemistry.11 Heterocycles prepared
from enaminones include carbazolequinone alkaloids,12
tricyclic benzo[a]quinolizines,13 pyrroles,14 benzodi-
azepines,15 pyrimidines,16a,b pyrazoles,17a,b isoxazoles18
and quinolines.19 The preparation of enaminones is well
documented.11,20,21 A common method for the synthesis of
enaminones not involving C-C bond formation is to react
ammonia or a primary or secondary amine with a 1,3-
diketone.11 Alternatively imine anion can be acylated at
the b-carbon by esters to give enaminones:21 in the pub-
lished procedure,21 1.5 moles of imine was reacted with
one mole of ester to give enaminones (over 11 examples)
in 70% yield based on esters, or 47% yield based on imine
(no imine is reported to be recovered). We now report that
the analogous acylation by N-acylbenzotriazoles produces
enaminones from imines in significantly higher conver-
sions.
1
characteristic signals in the H NMR spectrum were ob-
served at d 1.25 (s, 9H), 5.61 (s, 1H), 7.34-7.53 (m, 8H),
7.80-7.98 (m, 2H) and 11.85 (br s, 1H) which were as-
signed to the t-Bu protons, the vinylic proton, the protons
from the two phenyl groups and NH group, respectively.
The carbonyl group appears at d 187.5 (CO) in the 13C
NMR spectrum. These signals along with the downfield
shift of the NH group suggest six-membered intramolecu-
lar hydrogen bonding -N-H---O = C-, indicating that
the compounds 3a-p exist in the stable Z configuration
enamino tautomeric form 3b (Figure). A characteristic
feature of this procedure is the total C-C versus C-N regi-
oselectivity: the enaminones 3a-p were the sole products
isolated; no compounds corresponding to the N-acylation
have been detected in the GC/MS spectra, the single peak
had the corresponding mass.
The N-acylbenzotriazoles 1a-f were prepared from the
corresponding acyl chloride and benzotriazole following
literature procedures in 80-95% yields.22a,b The acylation
reactions were accomplished by treatment of the
In conclusion, enaminones 3a-p are prepared via acyl-
ation of metalated ketimines 2a-c using acylbenzotri-
azoles 1a-f as acylating agents. With the exception of
enaminones 3l-n for which the chloromethyl group at-
Synthesis 2000, No. 14, 2029–2032 ISSN 0039-7881 © Thieme Stuttgart · New York

2030
A. R. Katritzky et al.
PAPER
Table Preparation of Enaminones 3a−p
Compd. SM 1 Imine 2 R1
by the addition of sat. NH4Cl, and extracted with EtOAc. The organ-
ic extracts were combined, washed with sat. Na2CO3 and brine, and
dried (Na2SO4). After evaporation under vacuum, the residue was
purified by flash chromatography (hexanes/EtOAc, 1:10) to afford
the desired product 3a-p.
R2
R3
Yielda
(%)
3a
3b
3c
3d
3e
3f
a
a
a
b
b
b
c
c
c
d
d
e
e
e
f
b
a
c
Ph
Ph
t-Bu
85
65
(Z)-3-(tert-Butylamino)-1,3-diphenylprop-2-en-1-one (3a)
Yield: 85%; mp: 117-118 ∞C.
1H NMR: d = 1.25 (s, 9H), 5.61 (s, 1H), 7.34-7.52 (m, 8H), 7.80-
7.98 (m, 2H), 11.85 (br s, 1H).
13C NMR: d = 31.8, 54.0, 95.1, 127.0, 127.8, 128.1, 128.2, 128.9,
130.6, 137.5, 140.3, 167.0, 187.5.
Ph
i-Pr
c-Pr
i-Pr
Ph
n-Bu
Ph
c-hexyl 90
a
b
c
t-Bu
t-Bu
t-Bu
n-Bu
t-Bu
73
89
Anal. Calcd for C19H21NO (279.39): C, 81.68; H, 7.58; N, 5.01.
Found: C, 81.76; H, 7.70; N, 5.34.
c-Pr
c-hexyl 90
3g
3h
3i
a
b
c
p-MeOC6H4 i-Pr
p-MeOC6H4 Ph
p-MeOC6H4 c-Pr
n-Bu
t-Bu
70
71
(Z)-3-(Butylamino)-4-methyl-1-phenylpent-2-en-1-one (3b)
Yield: 65%; oil.
1H NMR: d = 0.99 (t, J = 7.2 Hz, 3H), 1.24 (d, J = 6.7 Hz, 6H),
1.44-1.60 (m, 2H), 1.60-1.78 (m, 2H), 2.78-2.89 (m, 1H), 3.34-
3.41 (m, 2H), 5.73 (s, 1H), 7.35-7.42 (m, 3H), 7.84-7.89 (m, 2H),
11.74 (br s, 1H).
c-hexyl 85
3j
a
b
a
b
c
p-MeC6H4
p-MeC6H4
ClCH2
i-Pr
Ph
n-Bu
t-Bu
n-Bu
t-Bu
78
66
35
33
3k
3l
13C NMR: d = 13.8, 20.1, 21.3, 28.8, 32.3, 42.1, 86.6, 126.8, 128.1,
130.2, 140.8, 174.5, 188.0.
i-Pr
Ph
HRMS (CI): m/z calcd for C16H24NO (M+1): 246.1858. Found:
246.1856.
3m
3n
3o
3p
ClCH2
ClCH2
c-Pr
i-Pr
c-Pr
c-hexyl 29
n-Bu 61
c-hexyl 75
(Z)-3-Cyclohexylamino-3-cyclopropyl-1-phenylprop-2-en-1-
one (3c)
Yield: 90%; mp: 78-80 ∞C.
a
c
styryl
f
styryl
1H NMR: d = 0.90-0.99 (m, 2H), 0.99-1.05 (m, 2H), 1.20-1.60
(m, 5H), 1.60-1.80 (m, 2H), 1.80-1.90 (m, 2H), 1.90-2.10 (m,
2H), 3.75-3.90 (m, 1H), 5.35 (s, 1H), 7.30-7.50 (m, 3H), 7.70-
7.90 (m, 2H), 11.81 (br s, 1H).
13C NMR: d = 7.9, 11.9, 24.6, 25.4, 33.9, 51.5, 85.4, 126.7, 128.0,
130.1, 140.9, 168.5, 187.4.
a Isolated yields.
tached to carbonyl causes some side reactions, the yields
are good to excellent (average 77% for thirteen exam-
ples).
Anal. Calcd for C18H23NO (269.39): C, 80.25; H, 8.61; N, 5.20.
Found: C, 79.89; H, 8.93; N, 5.21.
(Z)-5-(Butylamino)-2,2,6-trimethylhept-4-en-3-one (3d)
Yield: 73%; oil.
1H NMR: d = 0.94 (t, J = 7.2 Hz, 3H), 1.00-1.30 (m, 15H), 1.30-
1.50 (m, 2H), 1.52-1.70 (m, 2H), 2.62-2.80 (m, 1H), 3.20-3.30
(m, 2H), 5.19 (s, 1H), 11.24 (br s, 1H).
13C NMR: d = 13.8, 20.2, 21.3, 28.1, 28.7, 32.3, 41.5, 41.9, 84.6,
173.4, 203.9.
Figure
Anal. Calcd for C14H27NO (225.38): N, 6.21. Found: N, 6.16.
Mps were determined on a hot stage apparatus and are uncorrected.
1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded in
CDCl3 with TMS or CDCl3 as internal reference. Elemental analy-
ses were performed on a Carlo Erba-1106 instrument. High resolu-
tion mass spectra were measured on a Kratos/AE1-MS 30 mass
spectrometer. THF and Et2O were distilled from sodium/benzophe-
none under N2 immediately prior to use. All reactions with air-sen-
sitive compounds were carried out under an Ar atm.
(Z)-1-(tert-Butylamino)-4,4-dimethyl-1-phenylpent-1-en-3-one
(3e)
Yield: 89%; mp: 123-124 ∞C.
1H NMR: d = 1.15 (s, 9H), 1.16 (s, 9H), 5.02 (s, 1H), 7.26-7.39 (m,
5H), 11.23 (br s, 1H).
13C NMR: d = 27.9, 31.7, 41.6, 53.5, 93.8, 127.6, 128.3, 128.6,
137.8, 166.0, 203.9.
Enaminones 3a-p; General Procedure
Anal. Calcd for C17H25NO (259.39): N, 5.40. Found: N, 5.20.
To a stirred solution of LDA (5 mmol) in THF (10 mL) at -10 ∞C
was added dropwise the solution of ketimine 2a-c (2.5 mmol) in
THF (15 mL), and the resulting solution was stirred at 0 ∞C for 30
min. After cooling to -78 ∞C, a solution of benzotriazole derivative
1a-f (2.5 mmol) in THF (10 mL) was added dropwise. The reaction
was allowed to warm up to r.t. while stirring overnight, quenched
(Z)-1-Cyclohexylamino-1-cyclopropyl-4,4-dimethylpent-1-en-
3-one (3f)
Yield: 90%; oil.
Synthesis 2000, No. 14, 2029–2032 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Imine Acylation via Benzotriazole Derivatives: The Preparation of Enaminones
2031
1H NMR: d = 0.70-0.80 (m, 2H), 0.90-1.00 (m, 2H), 1.12 (s, 9H),
1.20-1.50 (m, 5H), 1.50-1.70 (m, 2H), 1.70-1.90 (m, 2H), 1.90-
2.10 (m, 2H), 3.60-3.80 (m, 1H), 4.84 (s, 1H), 11.28 (br s, 1H).
13C NMR: d = 7.5, 11.8, 24.8, 25.4, 28.1, 34.0, 41.4, 51.3, 83.7,
167.2, 203.4.
(Z)-4-(Butylamino)-1-chloro-5-methylhex-3-en-2-one (3l)
Yield: 35%; oil.
1H NMR: d = 0.95 (t, J = 7.2 Hz, 3H), 1.17 (d, J = 6.8 Hz, 6H),
1.36-1.56 (m, 2H), 1.56-1.70 (m, 2H), 2.70-2.84 (m, 1H), 3.20-
3.40 (m, 2H), 3.98 (s, 2H), 5.50 (s, 1H), 11.30 (br s, 1H).
Anal. Calcd for C16H27NO (249.40): N, 5.62. Found: N, 5.89.
13C NMR: d = 13.6, 19.9, 20.9, 28.6, 31.9, 47.2, 86.3, 175.4, 187.8.
HRMS (CI): m/z calcd for C11H21ClNO (M+1): 218.1312. Found:
218.1319.
(Z)-3-(Butylamino)-1-(4-methoxyphenyl)-4-methylpent-2-en-1-
one (3g)
Yield: 70%; mp: 68-70 ∞C.
(Z)-4-(tert-Butylamino)-1-chloro-4-phenylbut-3-en-2-one (3m)
Yield: 33%; mp: 130-131 ∞C.
1H NMR: d = 1.18 (s, 9H), 4.00 (s, 2H), 5.14 (s, 1H), 7.22-7.43 (m,
5H), 11.40 (br s, 1H).
13C NMR: d = 31.6, 47.2, 54.4, 94.7, 127.8, 129.1, 136.5, 167.8,
187.6.
1H NMR: d = 0.96 (t, J = 5.0 Hz, 3H), 1.21 (d, J = 2.2 Hz, 6H),
1.40-1.60 (m, 2H), 1.60-1.78 (m, 2H), 2.75-2.90 (m, 1H), 3.30-
3.40 (m, 2H), 3.83 (s, 3H), 5.67 (s, 1H), 6.88 (d, J = 8.8 Hz, 2H),
7.83 (d, J = 8.8 Hz, 2H), 11.68 (br s, 1H).
13C NMR: d = 13.7, 20.0, 21.2, 28.7, 32.2, 41.9, 55.2, 85.9, 113.2,
128.4, 133.5, 161.2, 173.9, 187.1.
HRMS (CI): m/z calcd for C14H19ClNO (M+1): 252.1155. Found:
252.1154.
Anal. Calcd for C17H25NO2 (275.39): N, 5.09. Found: N, 5.41.
(Z)-3-(tert-Butylamino)-1-(4-methoxyphenyl)-3-phenylprop-2-
en-1-one (3h)
Yield: 71%; mp: 98-100 ∞C.
1H NMR: d = 1.21 (s, 9H), 3.82 (s, 3H), 5.56 (s, 1H), 6.85 (d, J = 8.8
Hz, 2H), 7.38-7.50 (m, 5H), 7.80 (d, J = 8.8 Hz, 2H), 11.72 (br s,
1H).
(Z)-1-Chloro-4-(cyclohexylamino)-4-cyclopropylbut-3-en-2-
one (3n)
Yield: 29%; mp: 61-63 ∞C.
1H NMR: d = 0.75-0.87 (m, 8H), 0.96-1.12 (m, 3H), 1.16-1.32
(m, 1H), 1.32-1.72 (m, 1H), 1.80-2.05 (m, 3H), 3.58 (s, 2H), 5.89
(s, 1H).
13C NMR: d = 6.6, 8.3, 8.8, 11.3, 19.8, 24.3, 33.5, 40.0, 105.4,
138.1, 157.9.
13C NMR: d = 21.3, 31.8, 55.2, 94.6, 113.2, 127.7, 128.2, 128.5,
128.7, 132.9, 137.8, 161.6, 166.4, 186.7.
Anal. Calcd for C20H23NO2 (309.41): N, 4.53. Found: N, 4.66.
HRMS (CI): m/z calcd for C13H21ClNO (M+1): 242.1312. Found:
242.1341.
(Z)-3-(Cyclohexylamino)-3-cyclopropyl-1-(4-methoxyphenyl)-
prop-2-en-1-one (3i)
Yield: 85%; oil.
1H NMR: d = 0.80-0.95 (m, 2H), 0.95-1.00 (m, 2H), 1.20-1.56
(m, 6H), 1.58-1.76 (m, 2H), 1.78-1.88 (m, 2H), 1.92-2.05 (m,
2H), 3.81 (s, 3H), 5.33 (s, 1H), 6.87 (d, J = 9.0 Hz, 2H), 7.81 (d,
J = 9.0 Hz, 2H), 11.65 (br s, 1H).
(1E,4Z)-5-(Butylamino)-6-methyl-1-phenylhepta-1,4-dien-3-
one (3o)
Yield: 61%; oil.
1H NMR: d = 0.96 (t, J = 7.2 Hz, 3H), 1.18 (d, J = 5.6 Hz, 6H),
1.40-1.60 (m, 2H), 1.60-1.75 (m, 2H), 2.70-2.84 (m, 1H), 3.20-
3.40 (m, 2H), 5.19 (s, 1H), 6.60 (d, J = 15.6 Hz, 1H), 7.20-7.40 (m,
3H), 7.40-7.60 (m, 3H), 11.85 (br s, 1H).
13C NMR: d = 13.7, 20.0, 21.1, 28.5, 32.1, 42.1, 91.7, 127.6, 128.6,
128.7, 129.1, 136.1, 136.5, 174.7, 185.3.
13C NMR: d = 7.6, 11.8, 20.6, 24.4, 25.3, 29.6, 33.8, 51.2, 55.1,
84.8, 113.1, 128.3, 133.4, 161.2, 167.9, 186.6.
Anal. Calcd for C19H25NO2 (299.42): N, 4.68. Found: N, 4.97.
Anal. Calcd for C18H25NO (271.41): N, 5.16. Found: N, 5.32.
(Z)-3-(Butylamino)-4-methyl-1-(4-methylphenyl)pent-2-en-1-
one (3j)
Yield: 78%; oil.
(1Z,4E)-1-(Cyclohexylamino)-1-cyclopropyl-5-phenylpenta-
1,4-dien-3-one (3p)
Yield: 75%; mp: 75-77 ∞C.
1H NMR: d = 0.80-0.90 (m, 2H), 0.90-1.00 (m, 2H), 1.20-1.60
(m, 5H), 1.60-1.76 (m, 2H), 1.76-1.90 (m, 2H), 1.90-2.04 (m,
2H), 3.72-3.90 (m, 1H), 4.80 (s, 1H), 6.64 (d, J = 15.7 Hz, 1H),
7.24-7.40 (m, 4H), 7.40-7.60 (m, 2H), 12.00 (br s, 1H).
1H NMR: d = 0.96 (t, J = 7.5 Hz, 3H), 1.22 (d, J = 6.8 Hz, 6H),
1.40-1.60 (m, 2H), 1.60-1.76 (m, 2H), 2.37 (s, 3H), 2.72-2.90 (m,
1H), 3.30-3.40 (m, 2H), 5.70 (s, 1H), 7.18 (d, J = 8.0 Hz, 2H), 7.76
(d, J = 8.0 Hz, 2H), 11.70 (br s, 1H).
13C NMR: d = 13.7, 20.0, 21.2, 21.3, 28.7, 32.2, 42.0, 86.3, 126.8,
128.7, 138.3, 140.3, 174.2, 187.9.
13C NMR: d = 8.08, 11.4, 24.4, 25.4, 33.7, 51.4, 90.5, 127.6, 128.6,
129.0, 136.1, 136.4, 169.0, 184.6.
Anal. Calcd for C17H25NO (259.39): N, 5.40. Found: N, 5.28.
Anal. Calcd for C20H25NO (295.43): N, 4.74. Found: N, 4.69.
(Z)-3-(tert-Butylamino)-1-(4-methylphenyl)-3-phenylprop-2-
en-1-one (3k)
Yield: 66%; mp: 100-102 ∞C.
References
1H NMR: d = 1.22 (s, 9H), 2.36 (s, 3H), 5.57 (s, 1H), 7.17 (d, J = 7.9
Hz, 2H), 7.30-7.55 (m, 5H), 7.77 (d, J = 8.2 Hz, 2H), 11.90 (br s,
1H).
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C NMR: d = 21.4, 31.8, 53.9, 94.9, 127.0, 127.8, 128.2, 128.8,
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Anal. Calcd for C20H23NO (293.41): N, 4.77. Found: N, 4.57.
Synthesis 2000, No. 14, 2029–2032 ISSN 0039-7881 © Thieme Stuttgart · New York

2032
A. R. Katritzky et al.
PAPER
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Article Identifier:
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Synthesis 2000, No. 14, 2029–2032 ISSN 0039-7881 © Thieme Stuttgart · New York