A new series of non-classical type C heteropentalenes: 2H-pyrrolo[2,1-c][1, 2,4]triazoles

Article abstract of DOI:10.1002/jhet.5570440218

(Chemical Equation Presented) 7-Functionalized title compounds 5 are obtained by cyclization of 3-acetonyl- or 3-[(alkoxycarbonyl)-methyl]-4- phenacyl-1,2,4-triazolium salts 2 having methyl at C(5); the process can be effected in an acetate buffer or by b

Full text of DOI:10.1002/jhet.5570440218

Mar-Apr 2007
393
A New Series of Non-classical Type C Heteropentalenes:
2H-Pyrrolo[2,1-c][1,2,4]triazoles
Dietrich Moderhack * and Jan-Christoph Schneider
Institute of Pharmaceutical Chemistry, Technical University
D-38106 Braunschweig, Beethovenstrasse 55, Germany
*E-mail: d.moderhack@tu-bs.de
Received July 10, 2006
COR'
R
+
R
R'
Me
+
Ph
+
Me
(i)
(ii)
N
N
N
_
_
N
O
_
N
N
N
N
Br
Ph
Ph
Ph
N
EWG
EWG
EWG
R = H, Me; EWG = COMe
R = Me; EWG = COOEt
R = H, Me; R' = Ph, OEt;
EWG = CO(O)R''
R' = Ph, OEt; EWG = COMe
R' = Ph; EWG = COOEt
(i) Acetate buffer or NaHCO3; (ii) Ac2O / Et3N
7-Functionalized title compounds 5 are obtained by cyclization of 3-acetonyl- or 3-[(alkoxycarbonyl)-
methyl]-4-phenacyl-1,2,4-triazolium salts 2 having methyl at C(5); the process can be effected in an
acetate buffer or by base, irrespective of the function at C(3). 5-Unsubstituted salts 2 do not react unless
the side chain at C(3) is an acetonyl group. Cyclization of 2 with acetic anhydride−base gives rise to 5,7-
difunctionalized compounds 8; again methyl at C(5) of 2 is compulsory, but here the reaction can be
extended to salts having an (alkoxycarbonyl)methyl group at C(4). Regarding defunctionalization, acetyl
groups can be split from C(5) only, whereas ester functions are removable also from C(7). Title com-
pounds devoid of acceptor groups (XIII) are unstable but can be trapped by electrophilic reagents (DMAD,
acetic anhydride, and phenyl isocyanate) to give the derivatives 10 and 12. The 7-functionalized products
5 are likewise susceptible to SE-reactions. By comparison, all title compounds appear to be more reactive
toward this kind of reagents than the isomeric 1H-pyrrolotriazoles (13) including 2H-pyrrolotetrazoles
(III). This is consistent with B3LYP-DFT calculations using appropriate models.
J. Heterocyclic Chem., 44, 393 (2007).
INTRODUCTION
Scheme 1
Mesoionic (′non-classical′) heteropentalenes of type C
[1] bearing a pyrrolic half-ring were first encountered in
1980; thence studies have developed gradually to include
pyrrolo[1,2-c]thiazoles (I) [2], 2H-pyrrolo[1,2-c]imidaz-
oles (II) [3], and 2H-pyrrolotetrazoles (III) [4a], i.e. the
isomers of the ′classical′ systems V [5], VI [5], and VII
[4a,6]. Herein we report on the new title series IV [7].
Considering the synthetic approach by base-mediated
cyclization of N-(acylmethyl)-α-methylazolium salts
(Tschitschibabin reaction) [5], earlier work has shown that
triazolium salts such as IX (Y = H) are unsuitable
reactants because the 3-methyl group is inactive. As a
consequence, salts IX having R′ = Me and Y = H led to
the isomeric type VIII rather than the desired product
IV′, and representatives IX with R′ = H failed to cyclize
at all [8a]. Thus we focused on educts IX having for Y an
activating group (COMe, COOR) to be removed after
ring closure. This concept has recently proved fruitful in
preparing 2H-pyrrolotetrazoles (III) [4a]. Nevertheless,
starting from IX with Y = EWG and R′ = H, one should
be aware of possible shortcomings: 5-Unsubstituted 1,2,4-
triazolium salts bear an acidic hydrogen at this position
[9]; if the electron-withdrawing influence of the acceptor
I, V: a = S, b = c = CH
II, VI: a = NR, b = c = CH
III, VII: a = NR, b = c = N
IV, VIII: a = NR, b = N, c = CH
c
N
c N
+
_
b
a
b
a
I_ IV
V_ VIII
'
R
+
'
R
R''
+
R''
N
N
//
O
_
N
_
N
Z
Y = H [8a]
R
R
N
N
Y
IV'
IX
Y = EWG
'
R
+
R''
N
_
N
N
R
EWG
IV''
group is too weak, ring deprotonation will be favored over
proton loss at the side chain and thus vitiate pyrrole ring
closure.
RESULTS AND DISCUSSION
As candidates for cyclization we chose the triazolium
salts 2a-f. These compounds were obtained by quaterniz-

394
D. Moderhack and J.-C. Schneider
Vol 44
ation of the triazoles 1a-e with the corresponding alkyl
bromide in nitromethane solution [10] (Scheme 2).
Access to 1a-e was effected as follows. Compound 1b
resulted from esterification of the known carboxylic acid
[11], whereas the congeners 1a and 1c were provided by
ring closure of the amidrazones 3a and 3b [12] with
triethyl orthoformate and acetic anhydride, respectively.
Regarding the acetonyltriazoles 1d,e, the 5-unsubstituted
Scheme 3
Me
+
Ph
Ph
N
N
+
_
N
_
N
N
N
R2
COOEt/Me
COR3
Ph
5a: R2 = Me
b: R2 = Ph
5c: R3 = OEt
d: R3 = Me
(i) or (ii)
(i) or (ii)
(2a,b)
(2c,d)
//
R1
N
Ph
+
N
_
O
Scheme 2
Br
R2
N
COR3
R1
R4
R1
2a-e
+
N
N
(i)
(ii)
(i)
+
_
O
(2e)
(2e)
N
N
Br
R2
N
R2
N
COMe
COR3
COR3
Ph
Ph
(iii)
1a-e
2a-f
N
N
+
_
N
_
N
N
N
a: R1 = H, R2 = Me, R3 = OEt, R4 = Ph
b: R1 = H, R2 = Ph, R3 = OMe, R4 = Ph
c: R1 = Me, R2 = Ph, R3 = OEt, R4 = Ph
d: R1 = Me, R2 = Ph, R3 = Me, R4 = Ph
e: R1 = H, R2 = Ph, R3 = Me, R4 = Ph
f : R1 = Me, R2 = Ph, R3 = Me, R4 = OEt
Ph
Ph
(iv)
COMe
COMe
5e
6a
(?)
OH
Me
MeCOO
Ph
+
O
NH2
N
(ii)
(iii)
N
N
Ph
R2NHNH
Ph
N
Ph
COMe
1a
1c
1e
N
N
(3a)
(3b)
_
COOEt
Cl
COMe
3a: R2 = Me
X
XI
b: R2 = Ph
R1
Z
Me
+
Ph
(iv)
N
N
N
5c
(iv)
(v)
_
N
Cl
1d
Ph
N
(4a)
(4b)
Me
O
7
4a: R1 = Me, Z = OEt
b: R1 = H, Z = Het
Reagents and conditions: (i) NaOAc / AcOH, 100-110 °C, 1 h;
(ii) NaHCO3, 100 °C, 1 h; (iii) AcOH, 100 °C, 20 h; (iv) 12 N
HCl, 100 °C, 2 h
Het = (5-methylisoxazol-3-yl)amino
Reagents and conditions: (i) BrCH2COPh, 70 °C, 2 d; or BrCH2COOEt,
70 °C, 24 h (for 2f); (ii) CH(OEt)3, 80 °C, 1 h; (iii) Ac2O, 140 °C, 2-3 h;
(iv) PhNH2, 60 °C, 8 h; (v) PhNH2, then NaOEt [13]
exchange experiments (uncatalyzed): Whereas 2b
exhibits the most active hydrogen at C(5), in the case
of 2e the acetonyl group is preferentially affected.
Hence, we repeated cyclization of the latter salt with
sodium hydrogencarbonate (conditions of the Tschi-
tschibabin reaction) and thereby could secure the
missing compound 5e. As regards the unexpected
formation of 6a, we tentatively assume that acetic acid
protonates C(5) of 5e giving a cation akin to 7 which,
according to the sensitivity of 5-unsubstituted 1,2,4-
triazolium ions towards nucleophiles [14], takes up
acetate ion to yield the intermediate X. This species
may cyclize to XI. Ensuing dehydration would lead to
the tricyclic valence isomer of 6a [15] which in turn
ring-opens. Simply heating of 5e in acetic acid also
gave 6a [16]. Finally, concerning cyclization of the
remainder of 2 with sodium hydrogencarbonate, the
salts 2c,d are reactive too (→ 5c,d), whereas the 5-
unsubstituted representatives 2a and 2b under these
conditions again turned out to be inert candidates.
derivative 1e was made from the symmetrical N,N′-
bis(isoxazolyl)amidine 4b by successive treatment with
aniline and sodium ethoxide as described earlier [13].
Preparation of 1d, however, required a modification:
Since the homologue of 4b (Me in place of H) failed to
react with aniline, we used the imidate 4a which on
heating with the amine directly produced 1d.
Attempts to cyclize the salts 2 were started by heating
2a-e with sodium acetate in acetic acid (cf. [4a]). We
found that the derivatives 2c,d readily gave the anticipat-
ed products 5c,d (Scheme 3). However, educts having a
free 5-position (2a,b,e) either failed to undergo ring
closure such as 2a,b or led to a bicycle that bears an
acetyl function also at C(5) (→ 6a). The first
observation shows that the electron-withdrawing force
of the ester group is not sufficient to set off ring
deprotonation. This can be demonstrated by H/D

Mar-Apr 2007
A New Series of Non-classical Type C Heteropentalenes
395
Scheme 4
Also no bicycle of type 5 was obtained from the salt 2f:
Instead of forming a derivative 5 having an oxygen
function at C(6) (cf. [17]), 2f reverted to the triazole 1e
by requaternization [18].
Ph
O
O
N
Ph
Me
+
_
N
COMe
+
_
N
Defunctionalization of the pyrrolotriazoles 5 and 6,
performed with hot mineral acid according to the general
practice (cf. [4a,8]), proceeded easily as regards the
acceptor groups at C(5). However, removing them from
C(7), there are limitations: Treatment of the derivatives
5d,e did no affect the acetyl group and the materials were
recovered unchanged; hence, compound 5e was the sole
product from defunctionalization of 6a. To obtain an ac-
ceptor-free bicycle, we resorted to the ester 5c. In this
case, the hydrochloride salt 7 [19] separated directly from
the reaction mixture; attempts to isolate the free base XIII
met with failure and the species had to be trapped by
reagents (see later; Scheme 5). This elusive character
markedly contrasts with the stability of acceptor-free 2H-
pyrrolotetrazoles (III) [4a] and recalls properties of the
2H-pyrrolo[1,2-c]imidazole system (II) [3a].
In a complementary range of cyclization experiments
we sought access to the title class by treating 2 with acetic
anhydride in the presence of base, i.e. by applying the
synthetic principle known from other pyrroloazoles
[4,8a,20]. By this procedure the bicycles 8b-d were
obtained from the salts 2c,d, and f, respectively (Scheme
4). The reaction proceeds via intermediates such as XII;
this is inferred from related processes [4a,20b,21,22].
However, for direct observation the species XII cyclized
too rapidly [23]. The extent of their formation depends on
how strongly activation of the C(3)-linked methylene
N
N
Ph
Ph
N
COMe
8a
Me
OCOMe
(i)
(i)
9
//
(2e)
(2e)
R1
R4
+
N
_
O
N
Br
Ph
N
COR3
2c-f
(i) or (ii)
(2c,d,f)
O
N
_
N
R4
O
R4
Me
Me
Me
+
N
Me
+
N
_
N
COR3
Ph
O
COR3
Ph
N
8b: R3 = OEt, R4 = Ph
c: R3 = Me, R4 = Ph
d: R3 = Me, R4 = OEt
XII
Reagents and conditions: (i) Ac2O / Et3N, 25 °C, 2 d (for 8b,c) or 2.5 h
(for 9); (ii) Ac2O / Et3N, 130 °C, 1 h (for 8d)
Table 1. Selected 13C NMR Data of Pyrrolotriazoles [a]
N°
C(5)
C(6)
C(7)
C(7a)
5c
5e
6a
6b
6c
8b
8c
8d
10a
94.6
140.5
138.5
145.4
148.3
146.7
147.1
146.6
146.8
148.6
82.7
93.1
152.7
154.4
152.6
152.5
152.9
153.2
154.1
154.0
153.5
100.9
115.5
116.4
116.8
115.5
116.0
106.0
114.0
103.4
91.2
101.9
91.0
100.5
99.1
88.2
group in
2 exceeds that of the N(4)-linked one.
Comparing the educts 2c and 2f, the latter is clearly better
prepared for attack, since here the relevant side chain at
C(3) bears the more efficient activator. Hence the yield of
8d is nearly twice that of 8b whereas, conceivably too,
that of 8c holds the mid between them. As a result of the
less favored acetylation of the C(3)-linked methylene
group of 2c, the 5-acetyl derivative 6b arose as a side
product of 8b: Here acetic anhydride simply worked as a
dehydrating agent [4a,22b] to form intermediary 5c, which
in turn underwent electrophilic substitution (cf. Scheme
5). The salt 2e having an unsubstituted 5-position could
not be cyclized to the derivative 8a but instead gave the
open-chain ylide 9 [24]. Owing to the absence of the
electron-releasing methyl group at C(5), activation of the
N(4)-linked methylene group of 2e is obviously too high
with respect to the C(3)-linked one. Heating of 9 in
methanol slowly produced compound 5e − a reaction that
is comparable to the recently described conversion of
similarly functionalized tetrahydroquinolizinium ylides
into indolizines [25]. Attempts to transform 9 into the
bicycle 8a by heating the compound with base or by ther-
molysis remained largely unrewarded [16].
[a] CDCl3 or DMSO-d6 (5e), δ ppm; all signals are singlets except
for C(5) of 5c,e and C(7) of 10a.
All of the above cyclization products 5, 6, and 8 are
reliably identifiable by 13C NMR. Diagnostic are the
resonances of the pyrrolic carbon atoms since they
appear outside the areas typical of the substitutents and
of C(3) of the triazole unit. Of particular value is the
singlet of the ring junction carbon [C(7a)] which occurs
within the narrow range of δ 152–154 ppm (Table 1).
As anticipated, this carbon atom absorbs at lower field
compared to C(7a) of the isomeric system VIII [26].
The same deshielding is observed with 2H-pyrrolote-
trazoles (III) [4a] and other ′non-classical′ type C
heteropentalenes [28, 29].
As expected from our experience with the 2H-pyrrolo-
tetrazole series (III) [4b] and the ample material avail-
able on systems V−VII [30], the new heteropentalenes 5

396
D. Moderhack and J.-C. Schneider
Vol 44
Scheme 6
and XIII should exhibit appreciable reactivity towards
electrophiles. Site of attack should be the pyrrolic half-
ring and here, in the case of free 5- and 7-positions, pre-
ferentially C(5). This view is consistent with calcu-
lations of the π-electron densities (Table 2). From an
inspection of these figures it is likewise apparent that the
reactivity of the title system (IV) should exceed not only
that of the 1H-isomer (VIII) but also that of III. These
presuppositions are borne out by the following. (i)
Treatment of XIII with acetic anhydride or phenyl
isocyanate led to the 5-functionalized derivatives 10a and
10b, respectively (Scheme 5). (ii) Prolonged heating of
10a with the anhydride introduced a second acetyl group
at C(7) to give 6c. This additional substitution failed not
only with the 1H-isomer 14 (easily made from 13;
Scheme 6) but also with the 2H-pyrrolotetrazole
analogue of 10a (N in place of CMe) [31]. Moreover,
while the compounds 5e and 5d are readily converted
into the diacetyl derivatives 6a and 6c, respectively, the
tetrazole congener of 5d,e (N in place of CR1) could not
be induced to undergo further acetylation [4b].
Me
Ph
N
N
N
Ph
13 [8a]
(i)
(iii)
Me
H
N
COMe
E
E
Me
Ph
N
Ph
N
N
N
R
N
Ph
14: R = H
15: R = COMe
Ph
16: E = CO2Me
(ii)
//
Reagents and conditions: (i) Ac2O, 65 °C, 1 h; (ii) Ac2O, 100 °C, 2 h;
(iii) DMAD, 65 °C, 1 h
To illustrate the reactivity towards activated multiple
bonds, we allowed dimethyl acetylenedicarboxylate
(DMAD) to react with the deprotonated salt 7. From this
procedure a 1:2 product was obtained. Considering the
behavior of a 2H-pyrrolo[1,2-c]imidazole derivative (II;
R = Me) which by virtue of the inherent azomethine ylide
moiety yields a 1:2 cycloadduct [3a], the above product
might have an analogous constitution (11). However, the
13C NMR spectrum revealed that a twofold Michael-type
addition had occurred, leading to the 5,7-disubstituted
derivative 12a with E and Z configurated side chains. The
stereochemical assignment was made on the basis of the
Scheme 5
COMe
R1
+
R1
+
Ph
Ph
(i)
N
N
_
N
_
N
N
N
COR3
COR3
Ph
Ph
5c-e
6a-c
5e; 6a: R1 = H, R3 = Me
c; b: R1 = Me, R3 = OEt
d; c: R1 = R3 = Me
3JC(5),H(vinyl) and JC(7),H(vinyl) coupling constants (9.6 and 6.4
3
(iv)
(10a)
Hz, respectively), in conjunction with the typically dis-
cerned resonances of the β carbon atom of the vinyl
groups [32] which appear at δ 123.2 (E) and 106.9 (Z),
COR
Me
+
Me
Ph
Ph
_
N
N
_
N
respectively. A second stereoisomer (12b) could also be
+
N
N
Cl
Ph
Ph
3
detected, albeit in traces (3JC(5),H(vinyl): 6.2 Hz; JC(7),H(vinyl)
:
N
7
(ii)
(iii)
10a: R = Me
b: R = NHPh
6.5 Hz). To compare the above behavior with that of the
1H-isomeric class VIII, we reacted the derivative 13
under the same conditions. Here we only found the 1:1
product 16, in parallel to the findings with the pyrrolo-
tetrazole series III and VII [4b]. This again shows the re-
markably high reactivity of the title class IV.
Me
+
Ph
N
_
N
N
Ph
XIII
(v)
(v)
\\
EXPERIMENTAL
X
N
E
N
E
E
Y
_
E
E
Melting points were determined on a Kofler microscope.
Elemental analyses were obtained on a Carlo-Erba C-H-N-O
Elemental Analyser 1106 or Thermo Quest 1112 Elemental
Analyzer (with three compounds optimum values obtained for
carbon were within 0.6−0.8 only, although the materials were pure
by NMR). The IR spectra were recorded on a Philips PU-9800
FTIR or Thermo Nicolet FT-IR 200 instrument. The NMR spec-
tra were run on a Bruker DRX-400 spectrometer. The UV/VIS
spectra were determined on a Philips PU-8730 spectrometer
or Jena Specord 200 UV/VIS-Spektrometer. The mass spectra
Me
Ph
H
Me
Ph
H
N
+
N
N
Ph
Ph
N
E
E
11
12a: X = H, Y = E (major)
b: X = E, Y = H
E = CO2Me
Reagents and conditions: (i) Ac2O, 65 °C, 1 h; (ii) DMF / Et3N;
(iii) Ac2O, 80 °C, 1 h; or PhNCO, 20 °C, 1 h; (iv) Ac2O, 100 °C,
2 h (for 6c); (v) DMAD, 65 °C, 1 h

Mar-Apr 2007
A New Series of Non-classical Type C Heteropentalenes
397
Table 2. Energies and π-Electron Densities of Pyrrolotetrazole and -triazole Models [a]
5
a
N
a
N
a
N
a N
+
+
_
N
1
_
N
N
N
N
N
Me
O
Me
Me
N
N
O
Me
Me
Me
VIIb, VIIIb
IIIa, IVa
-E [b]
VIIa, VIIIa
IIIb, IVb
No
a
Erel
N(1)
N(2)
C/N(3)
1.288
N(4)
C(5)
C(6)
C(7)
C(7a)
1.104
IIIa
N
413.16130
4.07
1.208
1.430
1.401
1.189
1.018
1.146
VIIa
IVa
VIIIa
IIIb
VIIb
IVb
VIIIb
N
CH
CH
N
N
CH
CH
413.16778
397.14195
397.15732
565.86054
565.87077
549.84434
549.86166
0.00
9.64
0.00
6.42
0.00
10.87
0.00
1.515
1.279
1.549
1.203
1.477
1.265
1.502
1.158
1.447
1.203
1.419
1.149
1.443
1.195
1.187
1.136
1.064
1.258
1.174
1.098
1.048
1.441
1.405
1.440
1.391
1.423
1.401
1.430
1.161
1.226
1.185
1.172
1.147
1.202
1.166
1.034
1.012
1.031
0.973
1.038
0.966
1.037
1.162
1.172
1.177
1.077
1.100
1.110
1.108
1.114
1.082
1.095
1.131
1.097
1.106
1.081
[a] Calculated at the B3LYP/6-311+G(d,p) level of theory; all molecules are fully planar; acetyl derivatives shown are lowest energy
conformers; [b] E in a.u., Erel in kcal/mol.
were taken on a Finnigan MAT 8430 or MAT 90 machine.
Chromatography was carried out on silca gel Woelm 63−200 µm.
3-[(Ethoxycarbonyl)methyl]-1-methyl-4-phenacyl-1,2,4-
triazolium bromide (2a). Adopting the procedure described in
[33], methylhydrazine (4.60 g, 100 mmol) was added with stir-
ring and ice-cooling to a suspension of ethyl 2-(ethoxy-
carbonyl)acetimidate hydrochloride (19.5 g, 100 mmol) [34] in
dry pyridine (50 mL). After 2 hours diethyl ether was added and
the mixture was kept overnight at 0−5 °C to allow crystallization
of 10.7 g (55%) of 2-(ethoxycarbonyl)-N2 -methylacetamidra-
zone hydrochloride (3a) [1H NMR (CDCl3; 90 MHz): δ 1.23 (t,
J = 7 Hz, 3H), 2.52 (s, 3H), 3.84 (s, 2H), 4.12 (q, J = 7 Hz, 2H),
11.53 (br s, 1H)]. This material (1.00 g, 6.3 mmol) was
dissolved in dry ethanol (25 mL) and, after adding triethyl
orthoformate (0.94 g, 6.3 mmol), heated at reflux for 1 hour.
The mixture was made alkaline with aqueous sodium carbonate
(10%) and extracted with dichloromethane to give 0.88 g (83%)
of oily ethyl 1-methyl-1,2,4-triazole-3-carboxylate (1a) [1H
NMR (CDCl3): δ 1.27 (t, J = 7.1 Hz, 3H), 3.78 (s, 2H), 3.88 (s,
3H), 4.19 (q, J = 7.1 Hz, 2H), 7.98 (s, 1H); 13C NMR (CDCl3):
δ 13.9 (q), 34.4 (t), 35.9 (q), 60.9 (t), 144.1 (d), 158.1 (s), 169.3
(s)]. This material (1.00 g, 5.9 mmol) and 2-bromo-1-phenyl-
ethanone (1.20 g, 6 mmol) were dissolved in nitromethane (20
mL) and heated at 70 °C for 48 hours. Concentration in vacuo
left an oil that on treatment with ethyl acetate−diethyl ether
crystallized to afford 1.37 g (63%) of 2a; mp 199−201 °C (de-
comp.; ethanol); IR (KBr): ν˜ 1737, 1711 cm−1; 1H NMR
(DMSO-d6): δ 1.09 (t, J = 7.1 Hz, 3H), 4.05 (q, J = 7.1 Hz, 2H),
4.20 (s, 3H), 4.38 (s, 2H), 6.25 (s, 2H), 7.62−7.79 (m, 3H),
8.07−8.12 (m, 2H), 10.18 (s, 1H); 13C NMR (DMSO-d6): δ 13.7
(q), 30.2 (t), 39.0 (q), 53.1 (t), 61.6 (t), 128.4 (d, 2C), 129.0 (d,
2C), 133.4 (s), 134.6 (d), 145.0 (d), 150.7 (s), 166.3 (s), 190.1
(s). Anal. Calcd. for [C15H18N3O3]Br: C, 48.93; H, 4.93; N,
11.41. Found: C, 48.89; H, 4.95; N, 11.20.
give 0.60 g (59%) of methyl (1-phenyl-1,2,4-triazol-3-yl)acetate
(1b) [mp 51−54 °C (diethyl ether−light petroleum); IR (KBr):
1
ν˜ 1750 cm−1; H NMR (CDCl3): δ 3.76 (s, 3H), 3.92 (s, 2H),
7.36−7.39 (m, 1H), 7.46−7.50 (m, 2H), 7.64−7.66 (m, 2H), 8.53
(s, 1H); 13C NMR (CDCl3): δ 34.3 (t), 52.3 (q), 119.8 (d, 2C),
128.0 (d), 129.6 (d, 2C), 136.7 (s), 141.1 (d), 158.7 (s), 169.6
(s)]. This material (1.50 g, 6.9 mmol) and 2-bromo-1-phenyl-
ethanone (1.65 g, 8.3 mmol) were dissolved in nitromethane (20
mL) and heated at 70 °C for 48 hours. Work-up as described
above gave 1.90 g (66%) of 2b; mp 165 °C (ethyl acetate−acetone);
1
IR (KBr): ν˜ 1738, 1700 cm−1; H NMR (DMSO-d6): δ 3.63 (s,
3H), 4.54 (s, 2H), 6.34 (s, 2H), 7.66−7.83 (m, 6H), 7.97−7.99
(m, 2H), 8.14−8.16 (m, 2H), 11.13 (s, 1H); 13C NMR (DMSO-
d6): δ 30.3 (t), 52.8 (q), 53.5 (t), 120.6 (d, 2C), 128.7 (d, 2C),
129.1 (d, 2C), 130.4 (d, 2C), 130.8 (d), 133.3 (s), 134.6 (s),
134.8 (d), 143.6 (d), 151.5 (s), 166.7 (s), 189.7 (s). Anal. Calcd.
for [C19H18N3O3]Br: C, 54.82; H, 4.36; N, 10.09. Found: C,
54.56; H, 4.32; N, 10.00.
3-[(Ethoxycarbonyl)methyl]-5-methyl-4-phenacyl-1-phen-
yl-1,2,4-triazolium bromide (2c). A mixture of 3b (1.00 g, 4
mmol) [12] and acetic anhydride (5.00 g, 50 mmol) was heated
at reflux until a clear solution had formed (2−3 hours). After
addition of water (to allow hydrolysis of unconsumed reagent) it
was made alkaline with aqueous sodium carbonate (10%) and
extracted several times with dichloromethane. The combined
organic layers were dried (Na2SO4) and concentrated in vacuo to
afford 0.76 g (78%) of oily ethyl (5-methyl-1-phenyl-1,2,4-
triazol-3-yl)acetate (1c) [1H NMR (CDCl3): δ 1.21 (t, J = 7.1 Hz,
3H), 2.43 (s, 3H), 3.73 (s, 2H), 4.14 (q, J = 7.1 Hz, 2H),
7.33−7.44 (m, 5H); 13C NMR (CDCl3): δ 13.0 (q), 14.0 (q), 34.3
(t), 61.1 (t), 124.4 (d, 2C), 128.6 (d), 129.2 (d, 2C), 137.1 (s),
152.7 (s), 157.0 (s), 169.4 (s)]. This material (4.90 g, 20 mmol)
and 2-bromo-1-phenylethanone (4.20 g, 21 mmol) were dis-
solved in nitromethane (20 mL) and heated at 70 °C for 48
hours. Work-up as above gave 6.50 g (73%) of 2c; mp 153 °C
(ethanol); IR (KBr): ν˜ 1718, 1689 cm−1; 1H NMR (DMSO-d6): δ
1.08 (t, J = 7.1 Hz, 3H), 2.78 (s, 3H), 4.01 (q, J = 7.1 Hz, 2H), 4.47
(s, 2H), 6.33 (s, 2H), 7.66−7.75 (m, 5H), 7.79−7.83 (m, 3H),
8.13−8.15 (m, 2H); 13C NMR (DMSO-d6): δ 11.2 (q), 13.7 (q), 30.6
(t), 52.3 (t), 61.7 (t), 125.3 (d, 2C), 128.7 (d, 2C), 128.9 (d, 2C),
130.1 (d, 2C), 131.3 (d), 133.8 (s), 134.5 (s), 134.7 (d), 150.2 (s),
3-[(Methoxycarbonyl)methyl]-4-phenacyl-1-phenyl-1,2,4-
triazolium bromide (2b). To (1-phenyl-1,2,4-triazol-3-yl)acetic
acid (1.00 g, 5 mmol) [11] was added methanol (10 mL) with a
drop of concentrated sulfuric acid. The mixture was heated at
reflux for 2 hours, then diluted with water and repeatedly
extracted with dichloromethane. The combined organic layers
were dried (Na2SO4) and the solvent was evaporated in vacuo to

398
D. Moderhack and J.-C. Schneider
Vol 44
154.4 (s), 166.3 (s), 189.7 (s). Anal. Calcd. for [C21H22N3O3]Br: C,
56.77; H, 4.99; N, 9.46. Found: C, 56.06; H, 4.89; N, 9.27 [35].
3-Acetonyl-5-methyl-4-phenacyl-1-phenyl-1,2,4-triazolium
bromide (2d). A mixture of 5-methylisoxazol-3-amine (3.00 g,
30 mmol) and triethyl orthoacetate (4.90 g, 30 mmol) was
heated at reflux for 1.5 hours. In vacuo concentration gave 4.90
g (96%) of oily ethyl N-(5-methylisoxazol-3-yl)acetimidate (4a)
[1H NMR (CDCl3): δ 1.32 (t, J = 7.1 Hz, 3H), 2.01 (s, 3H), 2.37
(d, J = 0.8 Hz, 3H), 4.28 (q, J = 7.1 Hz, 2H), 5.68 (d, J = 0.8 Hz,
1H); 13C NMR (CDCl3): δ 12.5 (q), 14.0 (q), 17.9 (q), 62.4 (t),
98.2 (d), 165.7 (s), 166.7 (s), 169.7 (s)]. This material (4.00 g,
24 mmol) was heated with aniline (2.24 g, 24 mmol) at 60 °C for
8 hours. Removal of formed ethanol under reduced pressure left
4.90 g (95%) of oily (5-methyl-1-phenyl-1,2,4-triazol-3-yl)-
acetone (1d) [1H NMR (CDCl3): δ 2.28 (s, 3H), 2.52 (s, 3H),
3.87 (s, 2H), 7.43−7.46 (m, 3H), 7.49−7.51 (m, 2H); 13C NMR
(CDCl3): δ 13.1 (q), 29.6 (q), 43.3 (t), 124.4 (d, 2C), 128.7 (d),
129.3 (d, 2C), 137.1 (s), 152.8 (s), 157.3 (s), 203.8 (s)]. This
material (4.30 g, 20 mmol) and 2-bromo-1-phenylethanone (4.80
g, 24 mmol) were dissolved in nitromethane (20 mL) and heated
at 70 °C for 48 hours. Work-up as above gave 7.10 g (86%) of
2d; mp 188 °C (decomp.; ethanol); IR (KBr): ν˜ 1733, 1693 cm−1;
1H NMR (DMSO-d6): δ 2.26 (s, 3H), 2.78 (s, 3H), 4.67 (s, 2H),
6.27 (s, 2H), 7.64−7.68 (m, 2H), 7.72−7.74 (m, 3H), 7.77−7.83
(m, 3H), 8.15−8.17 (m, 2H); 13C NMR (DMSO-d6): δ 11.3 (q),
29.7 (q), 39.1 (t), 52.5 (t), 125.2 (d, 2C), 128.8 (d, 2C), 128.9 (d,
2C), 130.1 (d, 2C), 131.2 (d), 133.5 (s), 134.5 (s), 134.6 (d),
151.0 (s), 154.1 (s), 190.0 (s), 201.6 (s). Anal. Calcd. for
[C20H20N3O2]Br: C, 57.98; H, 4.87; N, 10.14. Found: C, 58.19;
H, 4.56; N, 10.15.
extracted with dichloromethane. The combined organic layers
were dried (Na2SO4) and concentrated in vacuo to give a residue
that was chromatographed on silica gel using dichloromethane−
ethyl acetate (4/1) as eluent.
Procedure (ii): To a suspension of 2c (0.89 g, 2 mmol) in
water (20 mL) was added sodium hydrogencarbonate (0.17 g, 2
mmol) and the mixture was heated for 1 hour at 100 °C. After
cooling the precipitate was collected by filtration.
5c: Yield from (i) and (ii): 0.12 g (35%) and 0.58 g (47%),
1
respectively; mp 158 °C (ethanol); IR (KBr): ν˜ 1658 cm−1; H
NMR (CDCl3): δ 1.26 (t, J = 7.1 Hz, 3H), 2.66 (s, 3H), 4.26 (q,
J = 7.1 Hz, 2H), 6.50 (s, 1H), 7.29−7.36 (m, 3H), 7.47−7.58 (m,
7H); 13C NMR (CDCl3): δ 10.1 (q), 14.7 (q), 58.3 (t), 82.7 (s),
96.4 (d), 125.2 (d, 2C), 127.1 (d), 127.3 (d, 2C), 129.3 (d, 2C),
129.5 (d, 2C), 129.7 (d), 129.8 (s), 135.0 (s), 136.6 (s), 140.5 (s),
152.7 (s), 164.0 (s); UV (MeOH): λ (log ε) 221 (4.31), 253
(4.43), 274 (4.26), 348 (3.84) nm; MS (m/z, %): 345 (M+, 48),
118 (100). Anal. Calcd. for C21H19N3O2: C, 73.03; H, 5.54; N,
12.17. Found: C, 72.62; H, 5.43; N, 11.90.
7-Acetyl-3-methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]-
triazole (5d). Procedure (i): A solution of 2d (1.00 g, 2.4 mmol)
in acetic acid (10.0 g, ca. 170 mmol) was heated with anhydrous
sodium acetate (1.10 g, 13.4 mmol) as above and worked up
accordingly.
Procedure (ii): To a suspension of 2d (4.00 g, 9.7 mmol) in
water (20 mL) was added sodium hydrogencarbonate (0.90 g,
10.7 mmol) and the mixture was stirred at 25 °C for 3 days,
whereupon the solid was filtred off.
5d: Yield from (i) and (ii): 0.24 g (32%) and 2.90 g (95%),
respectively; mp 231 °C (decomp.; ethanol); IR (KBr): ν˜ 3122,
1588 cm−1; 1H NMR (CDCl3): δ 2.48 (s, 3H), 2.73 (s, 3H), 6.58
(s, 1H), 7.30−7.36 (m, 4H), 7.56−7.59 (m, 6H); 13C NMR:
spectrum not available because of poor solubility; UV (MeOH):
λ (log ε) 222 (4.40), 257 (4.28), 295 (4.10), 351 (4.15) nm; MS
(m/z, %): 315 (M+, 65), 300 (90), 77 (100). Anal. Calcd. for
C20H17N3O: C, 76.19; H, 5.43; N, 13.32. Found: C, 75.72; H,
5.48; N, 13.15.
7-Acetyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]triazole (5e).
To a suspension of 2e (0.80 g, 2 mmol) in water (20 mL) was
added sodium hydrogencarbonate (0.17 g, 2 mmol) and the
mixture was heated at 100 °C for 1 hour; the solid formed was
collected by filtration.
5e: Yield: 0.57 g (89%); mp 217 °C (decomp.; ethanol); IR
(KBr): ν˜ 3118, 1589 cm−1; 1H NMR (DMSO-d6): δ 2.47 (s, 3H),
7.13 (s, 1H), 7.30−7.37 (m, 3H), 7.53−7.57 (m, 3H), 7.64−7.68
(m, 2H), 8.00−8.03 (m, 2H); 13C NMR (DMSO-d6): δ 29.4 (q),
93.1 (s), 100.9 (d), 120.5 (d, 2C), 123.3 (d), 126.9 (d), 127.3 (d,
2C), 129.1 (d), 129.3 (d, 2C), 129.8 (d, 2C), 135.1 (s), 137.0 (s),
138.5 (s), 154.4 (s), 185.4 (s); UV (MeOH): λ (log ε) 226 (4.34),
263 (4.28), 295 (4.38), 360 (3.93) nm; MS (m/z, %): 301 (M+,
85), 286 (100). Anal. Calcd. for C19H15N3O ⋅H2O: C, 71.46; H,
5.37; N, 13.16. Found: C, 71.23; H, 5.44; N, 13.03.
5,7-Diacetyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]triazole
(6a). Procedure (i): A solution of 2e (0.40 g, 1 mmol) in acetic
acid (10.0 g, ca. 83 mmol) was heated with anhydrous sodium
acetate (0.55 g, 6.7 mmol) as described above for 5c [procedure
(i)] and worked up accordingly.
Procedure (ii): A mixture of 5e (0.30 g, 1 mmol) and acetic
anhydride (5.00 g, 50 mmol) was stirred at 65 °C for 1 hour.
The cooled solution was diluted with water (10 mL) to allow
hydrolysis of unconsumed reagent and neutralized with aqueous
sodium carbonate (20%); then the solid was filtered off.
3-Acetonyl-4-phenacyl-1-phenyl-1,2,4-triazolium bromide
(2e). A mixture of 1e (4.02 g, 20 mmol) [13] and 2-bromo-1-
phenylethanone (4.80 g, 24 mmol) was heated in nitromethane (20
mL) at 70 °C for 48 hours. Work-up as described above gave 5.20 g
(65%) of 2c; mp 205 °C (decomp.; ethanol); IR (KBr): ν˜ 1731,
1
1700 cm−1; H NMR (DMSO-d6): δ 2.29 (s, 3H), 4.64 (s, 2H),
6.16 (s, 2H), 7.65−7.83 (m, 6H), 7.95−7.98 (m, 2H), 8.12−8.15
(m, 2H), 10.98 (d, J = 1.5 Hz, 1H); 13C NMR (DMSO-d6):
δ 29.8 (q), 38.8 (t), 53.2 (t), 120.6 (d, 2C), 128.6 (d, 2C), 129.1
(d, 2C), 130.4 (d, 2C), 130.8 (d), 133.3 (s), 134.7 (s), 134.8 (d),
143.5 (d), 152.3 (s), 189.7 (s), 201.5 (s). Anal. Calcd. for
[C19H18N3O2]Br: C, 57.01; H, 4.53; N, 10.50. Found: C, 56.57;
H, 4.62; N, 10.22.
3-Acetonyl-4-[(ethoxycarbonyl)methyl]-5-methyl-1-phenyl-
1,2,4-triazolium bromide (2f). A mixture of 1c (4.30 g, 20
mmol; material as above) and ethyl bromoacetate (3.30 g, 20
mmol) was heated in nitromethane (20 mL) at 70 °C for 24
hours. Work-up as described above gave 1.00 g (13%) of 2f; mp
1
179−181 °C (ethanol); IR (KBr): ν˜ 1740, 1722 cm−1; H NMR
(DMSO-d6): δ 1.28 (t, J = 7.1 Hz, 3H), 2.32 (s, 3H), 2.81 (s,
3H), 4.26 (q, J = 7.1 Hz, 2H), 4.64 (s, 2H), 5.43 (s, 2H),
7.71−7.73 (m, 3H), 7.76−7.78 (m, 2H); 13C NMR (DMSO-d6):
δ 11.3 (q), 13.9 (q), 29.7 (q), 38.9 (t), 46.8 (t), 62.2 (t), 125.2 (d,
2C), 130.1 (d, 2C), 131.3 (d), 134.4 (s), 150.7 (s), 154.0 (s),
165.3 (s), 201.6 (s). Anal. Calcd. for [C16H20N3O3]Br: C, 50.27;
H, 5.27; N, 10.99. Found: C, 50.14; H, 5.28; N, 10.84.
Ethyl 3-methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]tri-
azole-7-carboxylate (5c). Procedure (i): A solution of 2c (0.44
g, 1 mmol) in acetic acid (5.00 g, ca. 83 mmol) was heated with
anhydrous sodium acetate (0.55 g, 6.7 mmol) for 1 hour at
100−110 °C. After cooling the mixture was made weakly
alkaline with aqueous sodium carbonate (10%) and repeatedly

Mar-Apr 2007
A New Series of Non-classical Type C Heteropentalenes
399
6a: Yield from (i) and (ii): 0.12 g (35%) and 0.19 g (55%),
respectively; mp 209 °C (decomp.; ethanol); IR (KBr): ν˜ 1646,
1610 cm−1; 1H NMR (CDCl3): δ 1.80 (s, 3H), 2.24 (s, 3H),
7.41−7.62 (m, 8H), 7.91−7.94 (m, 2H), 10.14 (s, 1H); 13C NMR
(CDCl3): δ 26.7 (q), 29.6 (q), 103.4 (s), 115.5 (s), 121.0 (d, 2C),
124.6 (d), 128.4 (d, 2C), 128.5 (d), 129.0 (d, 2C), 129.8 (d),
130.0 (d, 2C), 134.8 (s), 136.9 (s), 145.4 (s), 152.6 (s), 186.0 (s),
190.8 (s); UV (MeOH): λ (log ε) 265 (4.53), 368 (4.43) nm; MS
(m/z, %): 343 (M+, 60), 328 (100). Anal. Calcd. for C21H17N3O2:
C, 73.45; H, 4.99; N, 12.24. Found: C, 73.20; H, 4.91; N, 11.91.
Ethyl 5-acetyl-3-methyl-2,6-diphenyl-2H-pyrrolo[2,1-c]-
[1,2,4]triazole-7-carboxylate (6b). Following the above
procedure (ii), 5c (0.35 g, 1 mmol) was heated with acetic
anhydride (5.00 g, 50 mmol) and the mixture was worked up
accordingly.
6b: Yield: 0.26 g (67%); mp 192−194 °C (ethanol); IR (KBr):
ν˜ 1685 cm−1; 1H NMR (CDCl3): δ 1.05 (t, J = 7.1 Hz, 3H), 1.74
(s, 3H), 3.15 (s, 3H), 4.12 (q, J = 7.1 Hz, 2H), 7.36−7.38 (m,
2H), 7.42−7.45 (m, 3H), 7.58−7.59 (m, 5H); 13C NMR (CDCl3):
δ 14.1 (q), 14.5 (q), 28.2 (q), 59.2 (t), 91.2 (s), 116.4 (s), 126.2
(d, 2C), 127.8 (d, 2C), 127.9 (d), 129.0 (d, 2C), 129.5 (d, 2C),
130.3 (d), 135.7 (s), 136.7 (s), 137.8 (s), 148.3 (s), 152.5 (s),
163.1 (s), 185.0 (s). Anal. Calcd. for C23H21N3O3: C, 71.30; H,
5.46; N, 10.85. Found: C, 71.03; H, 5.53; N, 10.62.
5,7-Diacetyl-3-methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]-
triazole (6c). Procedure (i): Compound 5d (0.31 g, 1 mmol) was
heated with acetic anhydride (5.00 g, 50 mmol) according to the
above procedure (ii) for 6a. The cooled mixture was diluted with
water (10 mL) and, after hydrolysis of unconsumed reagent,
neutralized with aqueous sodium carbonate (20%) to be followed
by repeated extractions with dichloromethane. The combined
organic layers were dried (Na2SO4) and concentrated in vacuo and
the residue was chromatographed on silica gel using dichloro-
methane−ethyl acetate (4/1) as eluent.
Ethyl 5-benzoyl-3,6-dimethyl-2-phenyl-2H-pyrrolo[2,1-c]-
[1,2,4]triazole-7-carboxylate (8b). To a solution of 2c (0.74 g,
1.7 mmol) in acetic anhydride (15 mL) was added triethylamine
(0.8 mL, 5.6 mmol) and stirred for 2 days at room temperature.
Then the mixture was diluted with water (20 mL) to allow hy-
drolysis of unconsumed reagent and repeatedly extracted with
dichloromethane. The combined organic layers were dried
(Na2SO4) and concentrated in vacuo to afford a solid that,
1
according to H NMR, consisted of a 3:1 mixture of 8b and 6b.
On treatment with methanol (20 mL) the latter component (data
as above) was separated from 8b.
8b: Yield: 0.16 g (25%); mp 198 °C (methanol); IR
1
(KBr): ν˜ 1671 cm−1; H NMR (CDCl3): δ 1.36 (t, J = 7.1 Hz,
3H), 2.24 (s, 3H), 3.03 (s, 3H), 4.36 (q, J = 7.1 Hz, 2H),
7.45−7.49 (m, 2H), 7.52−7.60 (m, 6H), 7.72−7.74 (m, 2H); 13C
NMR (CDCl3): 13.6 (q), 14.7 (q), 15.1 (q), 59.5 (t), 91.0 (s),
115.5 (s), 126.1 (d, 2C), 128.4 (d, 2C), 128.9 (d, 2C), 129.6 (d,
2C), 130.3 (d), 131.4 (d), 136.7 (s), 136.9 (s), 141.0 (s), 147.1
(s), 153.2 (s), 164.1 (s), 183.1 (s); UV (MeOH): λ (log ε) 202
(4.51), 237 (4.43), 368 (4.33) nm. Anal. Calcd. for C23H21N3O3:
C, 71.30; H, 5.46; N, 10.85. Found: C, 71.14; H, 5.46; N, 10.78.
7-Acetyl-5-benzoyl-3,6-dimethyl-2-phenyl-2H-pyrrolo[2,1-
c][1,2,4]triazole (8c). A mixture of 2d (0.83 g, 2 mmol), acetic
anhydride (15 mL), and triethylamine (0.8 mL, 5.6 mmol) was
treated and worked up accordingly to give a residue that was
chromatographed on silica gel using dichloromethane−ethyl
acetate (4/1) as eluent.
8c: Yield: 0.21 g (29%); mp 238−239 °C (decomp.; ethanol);
IR (KBr): ν˜ 1619, 1594 cm−1; 1H NMR (CDCl3): δ 2.28 (s, 3H),
2.63 (s, 3H), 2.99 (s, 3H), 7.45−7.49 (m, 2H), 7.53−7.62 (m,
6H), 7.73−7.75 (m, 2H); 13C NMR (CDCl3): 13.5 (q), 15.6 (q),
30.6 (q), 100.5 (s), 116.0 (s), 125.8 (d, 2C), 128.4 (d, 2C), 129.0
(d, 2C), 129.8 (d, 2C), 130.4 (d), 131.7 (d), 136.5 (s), 137.0 (s),
140.6 (s), 146.6 (s), 154.1 (s), 183.9 (s), 191.5 (s); UV (MeOH):
λ (log ε) 239 (4.36), 379 (4.35) nm; MS (m/z, %): 357 (M+,
100), 342 (81). Anal. Calcd. for C22H19N3O2 ⋅½ H2O: C, 72.12;
H, 5.50; N, 11.47. Found: C, 72.39; H, 5.20; N, 11.41.
Procedure (ii): A stirred mixture of 10a (0.10 g, 0.3 mmol; for
preparation, see below) and acetic anhydride (3.00 g, 30 mmol)
was heated at 100 °C for 2 hours. Work-up was performed as
with (i).
Ethyl 7-acetyl-3,6-dimethyl-2-phenyl-2H-pyrrolo[2,1-c]-
[1,2,4]triazole-5-carboxylate (8d). To a solution of 2f (0.38 g, 1
mmol) in acetic anhydride (15 mL) was added at 120 °C triethyl-
amine (0.2 mL, 1.4 mmol) and the mixture was heated at 130 °C
for 1 hour. On addition of water (20 mL) to the cooled solution the
product began to precipitate and was collected by filtration.
6c: Yield from (i) and (ii): 0.17 g (48%) and 0.09 g (80%),
respectively; mp 236−238 °C (ethanol); IR (KBr): ν˜ 1640, 1617
1
cm−1; H NMR (CDCl3): δ 1.73 (s, 3H), 2.12 (s, 3H), 3.14 (s,
3H), 7.38−7.41 (m, 2H), 7.46−7.49 (m, 3H), 7.58−7.60 (m, 5H);
13C NMR (CDCl3): δ 14.5 (q), 28.3 (q), 29.5 (q), 101.9 (s),
116.8 (s), 126.1 (d, 2C), 128.34 (d), 128.35 (d, 2C), 128.9 (d,
2C), 129.6 (d, 2C), 130.4 (d), 135.7 (s), 136.6 (s), 137.9 (s),
146.7 (s), 152.9 (s), 185.7 (s), 190.4 (s); UV (MeOH): λ (log ε)
203 (4.48), 366 (4.49) nm; MS (m/z, %): 357 (M+, 50), 342
(100). Anal. Calcd. for C22H19N3O2: C, 73.93; H, 5.36; N, 11.76.
Found: C, 73.15; H, 5.50; N, 11.41 [35].
3-Methyl-2,6-diphenyl-5H-pyrrolo[2,1-c][1,2,4]triazolium
chloride (7). To 5c (0.35 g, 1 mmol) was added 12 N HCl (10
mL) and the mixture was heated at 100 °C for 2 hours. On
neutralization with aqueous sodium carbonate (10%) the product
precipitated and was collected by filtration.
8d: Yield: 0.13 g (40%); mp 213 °C (ethanol); IR
1
(KBr): ν˜ 1685 cm−1; H NMR (CDCl3): δ 1.40 (t, J = 7.1 Hz,
3H), 2.59 (s, 3H), 2.87 (s, 3H), 3.05 (s, 3H), 4.34 (q, J = 7.1 Hz,
2H), 7.55−7.61 (m, 5H); 13C NMR (CDCl3): 13.6 (q), 14.0 (q),
14.5 (q), 30.6 (q), 60.0 (t), 99.1 (s), 106.0 (s), 126.0 (d, 2C),
129.7 (d, 2C), 130.4 (d), 136.0 (s), 136.7 (s), 146.8 (s), 154.0 (s),
161.7 (s), 191.2 (s). Anal. Calcd. for C18H19N3O3: C, 66.45; H,
5.89; N, 12.91. Found: C, 66.23; H, 5.88; N, 12.94.
5-Acetyl-3-methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]-
triazole (10a). In analogy to the method of ref. [36],
dimethylformamide (10 mL) was added to 7 (0.31 g, 1 mmol),
followed by acetic anhydride (0.11 g, 1.1 mmol) and triethyl-
amine (0.10 g, 1 mmol). The mixture was stirred at 80 °C for 1
hour and poured into water (30 mL). The solid formed was
filtered off.
7: Yield: 0.09 g (29%); mp 184 °C (ethanol); IR (KBr): ν˜ 3045,
1
1611 cm−1; H NMR (DMSO-d6): δ 2.85 (s, 3H), 5.73 (s, 2H),
7.56−7.59 (m, 3H), 7.70−7.78 (m, 6H), 7.87−7.88 (m, 2H); 13C
NMR (DMSO-d6): δ 11.2 (q), 52.3 (t), 108.9 (d), 125.1 (d, 2C),
126.9 (d, 2C), 129.2 (d, 2C), 130.0 (d, 2C), 130.7 (s), 130.9 (d),
131.2 (d), 135.2 (s), 149.5 (s), 157.5 (s), 160.9 (s). Anal. Calcd.
for [C18H16N3]Cl ⋅H2O: C, 65.95; H, 5.53; N, 12.82. Found: C,
65.51; H, 5.52; N, 12.53.
10a: Yield: 0.20 g (63%); mp 149−153 °C (acetone− water);
1
IR (KBr): ν˜ 1604 cm−1; H NMR (CDCl3): δ 1.85 (s, 3H), 3.23
(s, 3H), 5.82 (s, 1H), 7.39−7.46 (m, 5H), 7.53−7.59 (m, 5H); 13
C
NMR (CDCl3): 14.9 (q), 27.9 (q), 88.2 (d), 114.0 (s), 126.1 (d,

400
D. Moderhack and J.-C. Schneider
Vol 44
2C), 128.2 (d), 128.3 (d, 2C), 129.7 (d, 2C), 129.9 (d, 2C), 130.4
(d), 137.4 (s), 137.5 (s), 137.7 (s), 148.6 (s), 153.5 (s), 181.6 (s).
Anal. Calcd. for C20H17N3O: C, 76.17; H, 5.43; N, 13.32.
Found: C, 75.44; H, 5.37; N, 13.07 [35].
added, successively, methanol (20 mL) and dimethyl
acetylenedicarboxylate (DMAD; 0.50 g, 3.5 mmol), and the
mixture was stirred at 80 °C (bath) for 1 hour. The solvent was
removed in vacuo and the residue was recrystallized.
3-Methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]triazole-5-
carbanilide (10b). In analogy to the method of ref. [36],
dimethylformamide (10 mL) was added to 7 (0.31 g, 1 mmol),
followed by phenyl isocyanate (0.12 g, 1 mmol) and triethyl-
amine (0.10 g, 1 mmol). The mixture was stirred at room temp-
erature for 1 hour and poured into water (30 mL). The solid
formed was filtered off.
16: Yield: 0.15 g (36%); mp 82−85 °C (ethanol−water); IR
1
(KBr): ν˜ 1729, 1712 cm−1; H NMR (CDCl3): δ 2.37 (s, 3H),
3.53 (s, 3H), 3.54 (s, 3H), 6.14 (s, 1H), 6.93 (s, 1H), 7.14−7.18
(m, 1H), 7.24−7.28 (m, 1H), 7.32−7.39 (m, 4H), 7.43−7.48 (m,
2H), 7.66−7.69 (m, 2H); 13C NMR (CDCl3): 11.9 (q), 51.8 (q),
52.9 (q), 82.2 (d), 104.8 (s), 116.1 (d, 2C), 123.9 (d), 126.9 (d),
127.1 (d), 128.3 (d, 2C), 128.4 (d, 2C), 129.4 (d, 2C), 134.3 (s),
136.0 (s), 136.9 (s), 138.1 (s), 138.7 (s), 139.3 (s), 165.2 (s),
167.3 (s). Anal. Calcd. for C24H21N3O4: C, 69.39; H, 5.10; N,
10.11. Found: C, 69.39; H, 5.11; N, 9.86.
10b: Yield: 0.27 g (67%); mp 179−181 °C (acetone−water);
IR (KBr): ν˜ 3402, 1647 cm−1; 1H NMR (CDCl3): δ 3.21 (s, 3H),
5.80 (s, 1H), 6.93−6.99 (m, 2H), 7.07−7.09 (m, 2H), 7.16−7.20
(m, 2H), 7.50−7.60 (m, 10H); 13C NMR (CDCl3): 14.1 (q), 83.7
(d), 103.4 (s), 118.8 (d, 2C), 122.5 (d), 125.8 (d, 2C), 128.5 (d),
128.8 (d, 2C), 128.9 (d, 2C), 129.6 (d, 2C), 129.7 (d, 2C), 130.0
(d), 134.5 (s), 136.4 (s), 137.3 (s), 139.0 (s), 143.1 (s), 152.5 (s),
158.7 (s). Anal. Calcd. for C25H20N4O ⋅ ½ H2O: C, 74.80; H,
5.27; N, 13.96. Found: C, 75.10; H, 5.10; N, 13.69.
REFERENCES AND NOTES
[1] For classification, see: C. A. Ramsden, Tetrahedron, 33, 3203
(1977); C. A. Ramsden, in Comprehensive Heterocyclic Chemistry, Vol.
6, K. T. Potts, ed. (series eds.: A. R. Katritzky and C. W. Rees), Pergamon
Press, Oxford, 1984, pp.1027−1048.
[2a] J. M. Kane, J. Org. Chem., 45, 5396 (1980); [b] O. B.
Sutcliffe, R. C. Storr, T. L. Gilchrist, and P. Rafferty, Tetrahedron, 56,
10011 (2000).
Reaction of 3-methyl-2,6-diphenyl-5H-pyrrolo[2,1-c][1,2,4]-
triazolium chloride (7) with dimethyl acetylenedicarboxylate
(DMAD). To 7 (0.31 g, 1 mmol) was added, successively, me-
thanol (20 mL), triethylamine (0.10 g, 1 mmol), and DMAD
(0.50 g, 3.5 mmol), and the mixture was stirred at 80 °C (bath)
for 1 hour. The solvent was removed in vacuo and the residue
was chromatographed on silica gel using dichloromethane−ethyl
acetate (1/2) as eluent to give 0.16 g of a dark red, viscous oil
containing dimethyl 5-[(E)-1,2-bis(methoxycarbonyl)vinyl]-3-
methyl-2,6-diphenyl-2H-pyrrolo[2,1-c][1,2,4]triazole-7-maleate
(12a) and a trace of tetramethyl 3-methyl-2,6-diphenyl-2H-pyr-
rolo[2,1-c][1,2,4]triazole-5,7-dimaleate (12b); attempts to
separate the components failed.
[3a] W. D. Ollis, S. P. Stanforth, and C. A. Ramsden, J. Chem. Soc.,
Perkin Trans. 1, 957 (1989); [b] B. Musicky, J. Org. Chem., 55, 910
(1990); B.Musicky, J.Org.Chem., 56, 110 (1991).
[4a] D. Moderhack, D. Decker, and B. Holtmann, J. Chem. Soc.,
Perkin Trans. 1, 720 (2001); [b] D. Moderhack, D. Decker, and B.
Holtmann, J.Chem.Soc., Perkin Trans.1, 729 (2001).
[5] J. Elguero, R. M. Claramunt, and A. J. H. Summers, Adv.
Heterocycl.Chem., 22, 183 (1978).
[6a] D. Moderhack and D.-O. Bode, J. Chem. Soc., Perkin Trans.
1, 1483 (1992); [b] D. Moderhack and D. Decker, Heterocycles, 37, 683
(1994).
[7] For naming IV (and also II and III) as ′2H-pyrrolo...azole′
instead of using the IUPAC name ′1H-pyrrolo...azol-2-ium-1-ide′ [Pure
Appl.Chem., 65, 1357 (1993)], cf. the established literature practice as
detailed in [4a].
[8a] F. S. Babichev, V. A. Kovtunenko, and L. N. Didenko, Ukr.
Khim.Zh. (Russ. Ed.), 40, 245 (1974); Chem.Abstr., 80, 133356u (1974);
[b] V. A. Kovtunenko, L. N. Didenko, and F. S. Babichev, Ukr. Khim. Zh.
(Russ.Ed.), 40, 402 (1974); Chem. Abstr., 81, 13441s (1974).
[9] H. G. O. Becker, D. Nagel, and H.-J. Timpe, J. Prakt. Chem.,
315, 97 (1973); H. G. O. Becker, G. Hoffmann, K. M. Gwan, and L.
Knüpfer, J.Prakt.Chem., 330, 325 (1988).
[10] Brief heating of the components without solvent, as recom-
mended for the preparation of IX (a=CH/CMe, Y=H) [8b], proved to be
unsuitable. – Reactons of 1d,e with acetonyl bromide (25°C, 2.5 d,
nitromethane) gave quaternary salts 2 that could not be purified; for
details including cyclization experiments, see [16].
[11] G.E. Wright, J.Heterocyclic Chem., 14, 701 (1977).
[12] A. Weissberger, H. D. Porter, and W. D. Gregory, J. Am.
Chem.Soc., 66, 1851 (1944).
[13] H.Kano and E.Yamazaki, Tetrahedron, 20, 159 (1964).
[14] E.g., H. G. O. Becker, L. Krahnert, G. Rasch, W. Riediger, and
J.Witthauer, J.Prakt. Chem., 311, 477 (1969).
1
12a: IR (neat): ν˜ 1740 cm−1; H NMR (CDCl3): δ 2.46 (s,
3H), 3.23 (s, 3H), 3.41 (s, 3H), 3.645 (s, 3H), 3.649 (s, 3H), 6.44
(s, 1H), 6.60 (s, 1H), 7.25−7.35 (m, 5H), 7.57−7.60 (m, 5H); 13
C
NMR (CDCl3): 11.3 (q), 51.0 (q), 51.6 (q), 51.9 (q), 52.8 (q),
91.6 (s, 3JC,H = 6.4 Hz), 106.7 (s, 3JC,H = 9.5 Hz), 106.9 (d), 123.2
(d), 125.6 (d, 2C), 127.4 (d, 2C), 127.9 (d), 129.7 (d, 2C), 130.3
(d), 131.1 (d, 2C), 132.0 (s), 132.8 (s), 134.1 (s), 136.5 (s), 140.4
(s), 143.3 (s), 151.2 (s), 165.4 (s), 167.2 (s), 167.4 (s), 168.0 (s).
12b: 1H NMR (CDCl3): δ 5.64 (s, 1H), 6.40 (s, 1H); 13C NMR
(CDCl3): 93.4 (s, 3JC,H = 6.5 Hz), 108.0 (s, 3JC,H = 6.2 Hz).
5-Acetyl-3-methyl-1,6-diphenyl-1H-pyrrolo[2,1-c][1,2,4]-
triazole (14). A mixture of 13 (0.27 g, 1 mmol) [8a] and acetic
anhydride (5.00 g, 50 mmol) was stirred at 65 °C for 1 hour.
The cooled solution was diluted with water (10 mL) to allow
hydrolysis of unconsumed reagent and neutralized with aqueous
sodium carbonate (20%). Repeated extraction with dichloro-
methane gave a material that was chromatographed on silica gel
using dichloromethane−ethyl acetate (1/1) as eluent.
14: Yield: 0.08 g (25%); mp 118 °C (ethanol); IR (KBr):
1
ν˜ 1626, 1600 cm−1; H NMR (CDCl3): δ 1.92 (s, 3H), 2.97 (s,
3H), 5.97 (s, 1H), 7.18−7.23 (m, 1H), 7.41−7.46 (m, 7H),
7.64−7.67 (m, 2H); 13C NMR (CDCl3): 15.2 (q), 28.6 (q), 86.6
(d), 116.6 (s), 117.1 (d, 2C), 125.1 (d), 128.1 (d), 128.2 (d, 2C),
129.45 (d, 2C), 129.46 (d, 2C), 136.7 (s), 138.0 (s), 141.7 (s),
142.4 (s), 144.5 (s), 185.3 (s). Anal. Calcd. for C20H17N3O: C,
76.17; H, 5.43; N, 13.32. Found: C, 76.16; H, 5.43; N, 13.23.
Dimethyl 3-methyl-1,6-diphenyl-1H-pyrrolo[2,1-c][1,2,4]-
triazole-5-fumarate (16). To 13 (0.27 g, 1 mmol) [8a] was
[15] For a comparable structure, see H. Shimoharada, S. Ikeda, S.
Kajigaeshi, and S.Kanemasa, Chem.Lett., 1237 (1977).
[16] J.-C. Schneider, Dissertation, Technical University, Braun-
schweig, 2006.
[17] S. Ohta, Y. Narita, T. Yuasa, S. Hatakeyama, M. Kobayashi,
K. Kaibe, I. Kawasaki, and M. Yamashita, Chem. Pharm. Bull., 39, 2787
(1991).

Mar-Apr 2007
A New Series of Non-classical Type C Heteropentalenes
401
[18] For requaternization occurring instead of cyclization, cf.: D.
Moderhack and A. Lembcke, Chem.-Ztg., 109, 432 (1985), and
references cited therein.
[19] For site of protonation [C(5)], cf. [4b] and references therein.
− Calculations performed at the B3LYP/6-311+G(d,p) level of theory
show that the model ion 2-methyl-5H-pyrrolo[2,1-c][1,2,4]triazolium is
more stable than the 7H-tautomer (by 2.65 kcal/mol; absolute minima:
-397.54126 and -397.53704 a.u.).
[20a] V. A. Kovtunenko and F. S. Babichev, Ukr. Khim. Zh. (Russ.
Ed.) 38, 1244 (1972); Chem. Abstr., 78, 72007 r (1973); [b] T. A.
Kuz´menko, V. V. Kuz´menko, and V. A. Anisimova, Zh. Org. Khim., 40,
249 (2004); Russ. J.Org.Chem., 40, 221 (2004).
[21] F. W. Kröck and F. Kröhnke, Chem. Ber., 102, 659 (1969); F.
W.Kröck and F.Kröhnke, Chem.Ber., 102, 669 (1969).
[22a] A. Knoll and J. Liebscher, Z. Chem., 28, 214 (1988); [b] A.
Čopar, R. Stanovnik, and M. Tišler, J. Heterocyclic Chem., 30, 1577
(1993).
[23] The tetrazole congener of XII (N in place of CMe; R3 = Me,
R4 = Ph) could be detected by 1H NMR [16].
[24] 5-Unsubstituted 1H-1,2,4-triazolium-4-methanides bearing
different acyl groups at the ylide carbon do not seem to have been
described until now.
[25] E. I. Kostik, A. Abico and A. Oku, J. Org. Chem., 66, 2618
(2001).
[26] The carbon atom C(7a) of class VIII absorbs near δ 140 [16, 27].
[27] D.-T. Decker, Dissertation, Technical University, Braun-
schweig, 1996.
[28] D. Moderhack, in Comprehensive Heterocyclic Chemistry II,
Vol. 8, G. Jones, ed. (series eds.: A. R. Katritzky, C. W. Rees, and E. F. V.
Scriven), Pergamon, Oxford, 1996, pp.227−236.
[29] D. Moderhack and B. Holtmann, J. Prakt. Chem., 342, 591
(2000).
[30] See references collected in [4b].
[31] B. Holtmann, Dissertation, Technical University, Braun-
schweig, 1998.
[32] Cf. R. A. Jones and J. Sepulveda Arques, Tetrahedron, 37,
1597 (1981).
[33] M.R.Atkinson and J.B. Polya, J.Chem.Soc., 3319 (1954).
[34] A. Pinner and C. Oppenheimer, Ber. Dtsch. Chem. Ges., 28,
473 (1895); see p.478.
[35] For approximate analytical figure of carbon, cf.
instrumentation.
[36] Z. V. Voitenko, T. V. Yegorova, A. I. Kysil′, C. André, and J.
G.Wolf, Tetrahedron, 60, 195 (2004).