A novel synthesis of 4,5-diaryl-6-arylamino-2,3-benzo-1,3a,6a-triazapentalenes

Article abstract of DOI:10.1016/j.tetlet.2003.08.012

Treatment of N-alkyl- and N-aryl-imines of 2,3-diaryl- and 2-alkyl-3-aryl-3-(benzotriazol-1-yl)propenals with trifluoroacetic anhydride in THF at room temperature gave 5-alkyl-4-aryl-6-[N-alkyl (and aryl)-N-trifluoroacetyl]amino-2,3-benzo-1,3a,6a-triazape

Full text of DOI:10.1016/j.tetlet.2003.08.012

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7507–7511
A novel synthesis of
4,5-diaryl-6-arylamino-2,3-benzo-1,3a,6a-triazapentalenesꢀ
Yu-Ah Choi, Kyongtae Kim* and Young Ja Park
School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-742, South Korea
Received 5 June 2003; revised 5 August 2003; accepted 6 August 2003
Abstract—Treatment of N-alkyl- and N-aryl-imines of 2,3-diaryl- and 2-alkyl-3-aryl-3-(benzotriazol-1-yl)propenals with tri-
fluoroacetic anhydride in THF at room temperature gave 5-alkyl-4-aryl-6-[N-alkyl (and aryl)-N-trifluoroacetyl]amino-2,3-benzo-
1,3a,6a-triazapentalenes in moderate to good yields. On heating triazapentalenes having R2=aryl in MeOH at reflux, detrifl-
uoroacetylation of triazapentalene occurred to give title compounds in good yields. However, the same treatment of
triazapentalenes having R2=alkyl did not give the corresponding detrifluoroacetylation product. The title compounds and
5-alkyl-4-aryl-6-(N-alkyl-N-trifluoroacetyl)amino-2,3-benzo-1,3a,6a-triazapentalenes were found to be good precursors for the
synthesis of 1-(o-aminophenyl)-3-arylamino-4-alkyl (and aryl)-5-arylpyrazoles and 1-(o-aminophenyl)-3-(N-alkyl-N-tri-
fluoroacetyl)amino-4-alkyl (and aryl)-5-arylpyrazoles, respectively.
© 2003 Elsevier Ltd. All rights reserved.
Heteropentalene mesomeric betaines of type B have
been the subject of extensive investigation, mainly
because of their electronic structure and potential syn-
thetic utility.1 Among representatives of four of these
types, i.e. pyrazolo[1,2-a]pyrazoles, pyrazolo[1,2-a]-
1,2,3-triazoles, 1,2,3-triazolo[1,2-b]-1,2,3-triazoles, and
different methods involving the reactions of 1-(o-
nitroaryl)pyrazoles 1a2 or indazoles 1b3 with tri-
ethylphosphite (Scheme 1) and photolysis of
1-(o-azidoaryl)indazoles 1c.3a In these reactions, free
nitrenes or nitrenoid-like species may be involved as
intermediates. Conversion of 2-(o-benzoylphenyl)-2H-
benzotriazole 3 into triazapentalene 2c (R1=R2=H,
R3=PhCO) on heating in triethylphosphite at 180–
200°C4 may be an analogous method to the reaction
depicted in Scheme 1 but a carbene is involved as an
intermediate.
1,2,3-triazolo[1,2-a]-1,2,3-triazoles,
pyrazolo[1,2-a]-
1,2,3-triazoles 2 have been basically synthesized by two
Very recently, we reported the synthesis of 4,5-diaryl-6-
phenylsulfanyl-2,3-benzo-1,3a,6a-triazapentalenes
6
through Pummerer-type reactions of g-(benzotriazol-1-
yl)allylic sulfoxides 5 with trifluoroacetic anhydride
(TFAA) in THF5 (Scheme 2). This was the first exam-
ple of the involvement of N-2 of benzotriazole for the
intramolecular cyclization leading to 6. In continuing to
explore new benzo-1,3a,6a-triazapentalenes analogous
to 6, we intended to introduce various substituents
instead of arylsulfanyl groups at position 6 by employ-
Scheme 1.
Keywords: betaines; pentalenes; pyrazoles.
ꢀ Supplementary data associated with this article can be found at
doi:10.1016/j.tetlet.2003.08.012
* Corresponding author. Tel.: 82-2-880-6636; fax: 82-2-874-8858;
e-mail: kkim@plaza.snu.ac.kr
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.08.012

7508
Y.-A. Choi et al. / Tetrahedron Letters 44 (2003) 7507–7511
Scheme 2. Reagents and conditions: (i) (a) n-BuLi, THF,
−78°C, (b) 1-aryl-2-chloroethanones; (ii) (a) n-BuLi, THF,
−78°C, (b) H3O+; (iii) SOCl2, THF, rt; (iv) R2SH, NaOEt,
THF, rt; (v) m-CPBA (1 equiv.); (vi) TFAA (5 equiv.), THF,
rt.
Scheme 3. Reagents and conditions: (i) (a) DMSO, (COCl)2,
CH2Cl2, −78°C, N2, 15 min, (b) TEA, −78°C, N2, 2 h; (ii)
R2NH2, absolute EtOH, 12 h, rt; (iii) TFAA (1.2 equiv.),
THF, rt; (iv) MeOH, reflux.
ing our methodology. The key point of the methodol-
ogy leading to 6 is to generate an electron-deficient
center at the carbon atom a to the double bond of 5.
10 were formed along with a small amount of the
corresponding N-aryltrifluoroacetamides.10 It is envis-
aged that the latter compounds may be formed by the
reaction of TFAA with aniline derivatives formed by
hydrolysis of 8 during the course of the reactions. On
the other hand, N-alkylimines were sticky liquids,
which underwent rapidly hydrolysis to give 7. Conse-
quently, compounds 10j,k and 10p were prepared by
treatment of 7h and 7o with the corresponding alkyl-
amines in dried Et2O (10 mL) for 24 h at room temper-
ature, followed by immediate addition of TFAA with a
stirring for 1 h.
Since an allylic cation 9 having an amino group, which
is a resonance form of protonated imine 8 was envis-
aged to be a precursor of triazapentalenes 11 bearing an
alkyl- and arylamino group instead of an R2S group at
position 6, the imine 8 was prepared from compound 4
via an aldehyde 7 (Scheme 3).
The structures of 10 were determined based on the
spectroscopic (1H, 13C, and 19F NMR, IR) and analyti-
cal data.10 The X-ray single-crystal structure of 10p11
clearly shows that a trifluoroacetyl group is bonded to
the amino nitrogen instead of the benzotriazol moiety
(Fig. 1). Compounds 10 were stable under dry condi-
tions, but decomposed slowly in the air. On heating in
methanol at reflux, compounds 10a–c, 10e, 10g–i, and
10l–o underwent deacylation to give the desired com-
pounds 11,11 which slowly decomposed in the air.
The stereochemistry of compound 4 was found to be
maintained through Swern oxidation reaction6 in view
1
of the H NMR spectroscopic data.7
Treatment of 7 with primary arylamines in absolute
EtOH at room temperature gave a,b-unsaturated
imines 8 in good yields.8 Since the imines 8a–i and 8l–o
undergo hydrolysis during the chromatographic purifi-
cation and recrystallization from EtOH, the solid
imines obtained after stripping off the solvent from the
reaction mixture were used for the subsequent reaction
and recording of spectroscopic data. The stereochem-
istry around the CꢀC double bond of 8 was retained.
The NOE study9 on the compound 8m revealed that the
CꢀN double bond had an (E)-configuration.
In the meantime, the reactions of 7e (Ar=4-ClC6H4,
R1=4-BrC6H4) with diethylamine in CH2Cl2 at room
and reflux temperatures did not occur. Only reactant 7e
was quantitatively recovered.
Treatment of a,b-unsaturated aldimines 8a–i and 8l–o
with Lewis acids such as CF3CO2H, CF3SO3H, p-
TsOH, and BF3·OEt in dried CH2Cl2 at room tempera-
ture quantitatively led to the corresponding compounds
7. However, when compounds 8 were treated with
TFAA in THF at room temperature, triazapentalenes
Figure 1.

Y.-A. Choi et al. / Tetrahedron Letters 44 (2003) 7507–7511
7509
Table 1. Yields of compounds 7–11
Product
Ar
R1
R2
7
Yielda (%)
8
10
11
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
4-MeOC6H4
4-MeOC6H4
3-MeC6H4
3-MeC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-FC6H4
4-FC6H4
4-FC6H4
4-FC6H4
Ph
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
Me
4-ClC6H4
4-MeC6H4
Ph
C6H4Ph
4-MeOC6H4
2-Me-4-MeOC6H4
4-MeC6H4
Ph
4-ClC6H4
n-Pr
Allyl
Ph
Ph
69b
90c
88e
79
66
89
89
85
55
0
89
79
76
35
38
67
88
84
75
42
87
78
87
0
82
0
75
80
83
83
85d
78
90
76
88f
85
91g
84h
80i
90j
83
85
80
81
4-FC6H4
4-FC6H4
Ph
tert-Bu
tert-Bu
tert-Bu
tert-Bu
52k
77l
4-MeC6H4
Ph
n-Pr
72m
73o
64n
Ph
a Isolated yields.
b (E)/(Z)=4.85:1.
c (E)/(Z)=2.03:1.
d (E)/(Z)=1.89:1.
e (E)/(Z)=1:1.02.
f (E)-Isomer only.
g (E)/(Z)=2.46:1.
h (E)/(Z)=1.37:1.
i (E)/(Z)=3.14:1.
j (E)/(Z)=2.04:1.
k (E)-Isomer only.
l (E)-Isomer only.
m (E)-Isomer only.
n (E)-Isomer only.
o (E)-Isomer only. d, f, h, j, l, m, oThe compounds 8 with the specified (E)/(Z)-ratios were used to prepare 10.
However, compounds 10j,k and 10p did not give de-
acylation products 11 under the same foregoing condi-
tions. Presumably this may be due to lack of the
stabilization of the amide ions to be formed.
Raney Ni under the same conditions gave pyrazoles 13
having a [N-(n-propyl)-N-trifluoroacetyl]amino group
at C-3. Yields of 12 are summarized in Table 2.
Preparation of single crystals of both 10 and 11 for
X-ray structural determinations was unsuccessful.
Yields of compounds 7–11 are summarized in Table 1.
Interestingly it has been found that compounds 11 can
be utilized for the synthesis of 1-(o-aminophenyl)-5-
aryl-4-aryl
(or
alkyl)-3-arylaminopyrazoles
1212
(Scheme 4). Similar treatment of 10j and 10p with
Scheme 4.
Table 2. Yields of pyrazole derivatives 12
Compound
Ar
R1
R2
Product
Yielda (%)
11b
11c
11e
11h
11l
4-MeOC6H4
3-MeC6H4
4-ClC6H4
4-FC6H4
Ph
4-MeC6H4
4-MeC6H4
4-BrC6H4
4-BrC6H4
Me
4-MeC6H4
Ph
4-MeOC6H4
Ph
Ph
Ph
12b
12c
12e
12h
12l
88
95
92
80
86
76
11o
Ph
tert-Bu
12o
a Isolated yields.

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Y.-A. Choi et al. / Tetrahedron Letters 44 (2003) 7507–7511
(1150 mg, 11.36 mmol). After stirring for 2 h at −78°C
under a nitrogen atmosphere, water (100 mL) was added
and the reaction mixture was extracted with
dichloromethane (50 mL×4). The combined extract was
dried over MgSO4. After removal of the solvent under
vacuo, the residue was chromatographed on a silica gel
(70–230 mesh, 3×18 cm) using a mixture of n-hexane and
EtOAc (7:1) to give a unknown compound (31 mg), and
3-(benzotriazol-1-yl)-3-(4-methoxyphenyl)-2-(p-
In summary, we have explored a novel synthesis of
2,3-benzo-1,3a,6a-triazapentalenes bearing an aryl-
amino group at position 6 starting from 2,3-diaryl- and
2-alkyl-3-aryl-2-(benzotriazol-1-yl)propenals in three
steps. The key step leading to the desired products is
the generation of iminium ions from a,b-unsaturated
imines using trifluoroacetic anhydride. Title compounds
are found to be good precursors for the synthesis of
1-(o-aminophenyl)-3-arylamino-4-alkyl (and aryl)-5-
arylpyrazoles, which have been biologically impor-
tant.13 The scope of the reaction is in progress.
tolyl)propenal 7a (413 mg, 69%): IR (neat) 2928, 1721,
1667, 1600, 1507, 1443, 1382, 1251, 1174, 1024, 905, 825,
1
745, 668, 518 cm−1; H NMR (300 MHz, CDCl3) l 2.16
(s, 3H, CH3), 3.87 (s, 3H, OCH3), 6.71–7.29 (m, 1H,
ArH), 8.00 (d, J=4.1 Hz, 1H, ArH), 9.92 (d, J=7.6 Hz,
1H, CHO). 2.33 (s, 3H, CH3), 3.75 (s, 3H, OCH3),
6.71–7.29 (m, 11H, ArH), 8.21 (d, J=4.1 Hz, 1H, ArH),
9.35 (d, J=7.6 Hz, 1H, CHO); 13C NMR (75 MHz,
CDCl3) l 21, 55, 110, 111, 114, 120, 124, 125, 128, 129,
130, 131, 132, 133, 135, 138, 145, 149, 161, 162, 192.
Anal. calcd for C23H19N3O2: C, 74.78; H, 5.18; N, 11.37.
Found: C, 74.72; H, 5.16; N, 11.34.
7. The (E)/(Z)-ratios of 7 were determined on the basis of
1
Acknowledgements
the H NMR intensities of the CHO group.
8. Typical procedure: To a solution of 3-(benzotriazol-1-yl)-
2-methyl-3-phenylpropenal 7l (85 mg, 0.32 mmol) in
absolute EtOH (10 mL) was added aniline (30 mg, 0.32
mmol). The mixture was stirred for 12 h at room temper-
ature, followed by addition of water (50 mL). The
aqueous solution was extracted with CH2Cl2 (30 mL×3).
The combined extract was dried over MgSO4 and
removal of the solvent in vacuo gave a viscous residue (98
mg, 90%) which was a mixture of N-phenylimine of (E)-
and (Z)-7l (2.84:1): IR (neat) 3056, 1590, 1478, 1440,
1372, 1273, 1222, 1152, 1056, 1020, 905, 841, 748, 694,
and 528 cm−1; 1H NMR (300 MHz, CDCl3) l 2.15 (s, 3H,
CH3, E), 2.46 (s, 3H, CH3, Z), 7.01–7.40 (m, 26H, ArH,
E and Z), 7.84 (s, 1H, CH, Z), 8.10–8.12 (m, 2H, ArH,
E and Z), 8.45 (s, 1H, CH, E); 13C NMR (75 MHz,
CDCl3) l 15.0, 15.5, 110.6, 115.0, 120.1, 120.8, 120.9,
124.3, 126.3, 128.2, 128.3, 128.7, 128.8, 129.0, 129.1,
129.2, 130.1, 130.2, 133.2, 134.0, 134.1, 134.6, 141.7,
145.7, 151.6, 159.9, 160.0. Anal. calcd for C22H18N4: C,
78.08; H, 5.36; N, 16.56. Found: C, 78.01; H, 5.32; N,
16.49.
This work was supported by the S.N.U. foundation of
Overhead Research Fund.
References
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6928–6934; (e) Albini, A.; Bettinetti, G.; Minoli, G. J.
Am. Chem. Soc. 1997, 119, 7308–7315.
9. Compound (E)-8m (Ar=4-FC6H4, R1=tert-Bu, R2=
Ph), with a singlet at l=8.25 (600 MHz, CDCl3) assigned
to the imine proton, displayed NOE effects between one
ortho proton (l=6.88–6.89) of the 4-FC6H4 group and
one ortho proton (l=7.23–7.26) of the Ph group.
10. Typical procedure: To a solution of 8b (68 mg, 0.15
mmol) in THF (5 mL) was added trifluoroacetic anhy-
dride (62 mg, 0.30 mmol). The mixture was stirred for 1
h at room temperature, followed by the addition of water
(50 mL). The aqueous solution was extracted with
CH2Cl2 (30 mL×3). The combined extract was dried over
MgSO4. After removal of the solvent in vacuo, the
residue was chromatographed on a silica gel (70–230
mesh, 3×10 cm) using a mixture of n-hexane and EtOAc
(10:1) to give 4-(4-methoxyphenyl)-5-(p-tolyl)-6-[N-(p-
tolyl)-N-trifluoroacetyl]amino-2,3-benzo-1,3a,6a-triaza-
pentalene 10b (73 mg, 89%): mp 198–200°C (n-hexane);
IR (neat) 1715, 1600, 1507, 1456, 1379, 1209, 1116, 1030,
4. Lee, J. H.; Matsumoto, A.; Yoshida, M.; Simamura, O.
Chem. Lett. 1974, 951–954.
5. Kim, T.; Kim, K.; Park, Y. J. Eur. J. Org. Chem. 2002,
493–502.
6. Typical procedure: A mixture of DMSO (634 mg, 3.12
mmol) and oxalyl chloride (618 mg, 4.87 mmol) in
CH2Cl2 (30 mL) was stirred for 15 min at −78°C under a
nitrogen atmosphere, followed by addition of 3-(benzotri-
azol-1-yl)-3-(4-methoxyphenyl)-2-(p-tolyl)propenol
4a
(603 mg, 1.62 mmol). The mixture was stirred for an
additional 2 h, followed by the addition of triethylamine
908, 825, 732, and 534 cm−1 1H NMR (300 MHz,
;

Y.-A. Choi et al. / Tetrahedron Letters 44 (2003) 7507–7511
7511
CDCl3) l 2.27 (s, 3H, CH3), 2.39 (s, 3H, CH3), 3.84 (s,
3H, OCH3), 6.82–7.38 (m, 15H, ArH), 7.60 (d, J=8.2 Hz,
1H, ArH); 13C NMR (75 MHz, CDCl3) l 21.1, 21.3, 55,
111, 113, 114, 116, 119, 120, 126.4, 126.8, 127, 129.5,
129.7, 129.8, 131, 138, 159; 19F NMR (l=−72.6). Anal.
calcd for C32H25F3N4O2: C, 69.31; H, 4.54; N, 10.10.
Found: C, 69.12; H, 4.58; N, 10.09.
of EtOAc and n-hexane (1:3) to give [1-(2-aminophenyl)-
4-tert-butyl-5-phenyl-1H-pyrazol-3-yl]phenylamine 12o
(46 mg, 76%): mp 120–121°C (CH2Cl2-n-hexane); IR
(neat) 3456, 3360, 2960, 1603, 1500, 1440, 1350, 1302,
1
1241, 1024, 963, 905, 742, and 544 cm−1; H NMR (300
MHz, CDCl3) l 1.23 (s, 9H, C(CH3)3), 4.01 (s, 1H, NH2),
5.64 (s, 1H, NH), 5.70 (s, 1H, NH), 6.55–6.75 (m, 3H,
ArH), 6.96–7.47 (m, 13H, ArH); 13C NMR (75 MHz,
CDCl3) l 30.9, 31.8, 115.3, 116.2, 117.4, 119.0, 119.1,
125.8, 127.5, 128.3, 128.4, 128.7, 129.0, 131.0, 132.7,
141.4, 143.6, 144.8, 148.9. Anal. calcd for C25H26N4: C,
78.50; H, 6.85; N, 14.65. Found: C, 78.59; H, 7.00; N,
14.85.
11. Typical procedure: A solution of 10b (70 mg, 0.13 mmol)
in MeOH (10 mL) was heated for 1 h at reflux and then
cooled to room temperature. Water (50 mL) was added
to the reaction mixture and then the aqueous solution
was extracted with CH2Cl2 (30 mL×3). The combined
extract was dried over MgSO4. After removal of the
solvent, the residue was chromatographed on a silica gel
(70–230 mesh, 3×10 cm) using a mixture of EtOAc and
n-hexane (1:5) to give 4-(4-methoxyphenyl)-6-(p-tolu-
idino)-5-(p-tolyl)-2,3-benzo-1,3a,6a-triazapentalene 11b
(45 mg, 78%): mp 150–152°C (CH2Cl2–n-hexane); IR
(neat) 3264, 3024, 1603, 1536, 1507, 1456, 1350, 1286,
13. (a) Elguero, J. In Comprehensive Heterocyclic Chemistry;
Katritzky, A. R.; Rees, C. W.; Scriven, E. F. V., Eds.;
Pergamon Press: Oxford, 1996; Vol. 3, pp. 1–75; (b)
Almirante, N.; Cerri, A.; Fedrizzi, G.; Marazzi, G.; San-
tagostino, M. Tetrahedron Lett. 1998, 39, 3287–3290; (c)
Grosche, P.; Holtzel, A.; Walk, T. B.; Trautwein, A. W.;
Jung, G. Synthesis 1999, 1961–1970; (d) Genin, M. J.;
Biles, C.; Keiser, B. J.; Poppe, S. M.; Swaney, S. M.;
Tarpley, W. G.; Yagi, Y.; Romero, D. L. J. Med. Chem.
2000, 43, 1034–1040; (e) Stauffer, S. R.; Coletta, C. J.;
Tedesco, R.; Nishiguchi, G.; Carlson, K.; Sun, J.;
Katzenellenbogen, B. S.; Katzenellenbogen, J. A. J. Med.
Chem. 2000, 43, 4934–4947; (f) Lyga, J. W.; Patera, R.
M.; Plummer, M. J.; Halling, B. P. Pestic. Sci. 1994, 42,
29; (g) Habeeb, A. G.; Rao, P. N. P.; Knaus, E. E. J.
Med. Chem. 2001, 44, 3039–3042; (h) Hashimoto, H.;
Imamura, K.; Haruta, J.-i.; Wakitani, K. J. Med. Chem.
2002, 45, 1511–1517; (i) Palomer, A.; Cabre, F.; Pascual,
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2002, 45, 1402–1411.
1248, 1177, 1107, 1027, 905, 816, 726, 643, and 518 cm−1
;
1H NMR (300 MHz, CDCl3) l 2.23 (s, 3H, CH3), 2.27 (s,
3H, CH3), 3.87 (s, 3H, OCH3), 5.70 (s, 1H, NH), 6.55–
6.75 (m, 3H, ArH), 6.96–7.47 (m, 13H, ArH); 13C NMR
(75 MHz, CDCl3) l 20.5, 21.1, 55.2, 110.9, 112.9, 114.2,
114.3, 115.1, 115.4, 117.2, 118.8, 120.2, 120.4, 125.9,
127.6, 128.9, 129.2, 129.5, 129.8, 131.2, 137.1, 142.0,
147.2, 159.8. Anal. calcd for C30H26N4O: C, 78.58; H,
5.72; N, 12.22. Found: C, 78.46; H, 5.68; N, 12.20.
12. Typical procedure: A solution of 11o (60 mg, 0.16 mmol)
and Raney-Ni (50 mg) in MeOH (10 mL) was heated for
1.5 h at reflux under hydrogen. Removal of the insoluble
materials by filtration followed by evaporation of the
solvent, gave a residue, which was chromatographed on a
silica gel column (70–230 mesh, 2×10 cm) using a mixture