Synthesis and stereochemistry of some novel dihydropyrrolo[3,4-c]pyrazoles

Article abstract of DOI:10.1007/s00706-011-0618-z

Eleven novel dihydropyrrolo[3,4-c]pyrazole derivatives were obtained by the reaction of chiral (1R)-N-(1-phenylethyl)maleimide and C,N-aryl-substituted nitril-imines. The reaction afforded the cycloadducts as a regioisomeric mixture which can be separable in some cases. The structure and stereochemistry of cycloadducts were assigned on the basis of infrared (IR), ¹H and ¹³C nuclear magnetic resonance (NMR), mass and X-ray spectra, optical rotation measurements, and CHN analyses.

Full text of DOI:10.1007/s00706-011-0618-z

Monatsh Chem (2012) 143:127–138
DOI 10.1007/s00706-011-0618-z
ORIGINAL PAPER
Synthesis and stereochemistry of some novel
dihydropyrrolo[3,4-c]pyrazoles
•
•
Yas¸ar Du¨ru¨st Muhammet Yıldırım
•
Chris F. Fronczek Frank R. Fronczek
Received: 16 October 2010 / Accepted: 5 August 2011 / Published online: 16 September 2011
Ó Springer-Verlag 2011
Abstract Eleven novel dihydropyrrolo[3,4-c]pyrazole
derivatives were obtained by the reaction of chiral (1R)-N-
(1-phenylethyl)maleimide and C,N-aryl-substituted nitril-
imines. The reaction afforded the cycloadducts as a
regioisomeric mixture which can be separable in some
cases. The structure and stereochemistry of cycloadducts
very electron-deficient alkenes give significant amounts of
the 4-substituted 2-pyrazolines [27]. The rationalization of
such regiochemical results became evident in terms of the
frontier molecular orbital (FMO) theory of Fukui [28–37].
On the other hand, maleimides are an important class
of substrates for biological and chemical applications
[38–43]. Their reactivity as a dipolarophile in 1,3-dipolar
cycloadditions has been utilized in many examples of
cycloaddition chemistry [44].
1
were assigned on the basis of infrared (IR), H and 13C
nuclear magnetic resonance (NMR), mass and X-ray
spectra, optical rotation measurements, and CHN analyses.
As part of our continued interest in nitrilimine cycloaddi-
tion research [45], due to limited numbers of cycloaddition
reactions of chiral maleimides with dipolar compounds such
as nitrones, nitriloxides, and anthrones reported in the litera-
ture [46–54]tothebestofourknowledge, wewereencouraged
to carry out 1,3-dipolar cycloadditions of C,N-substituted
nitrilimines to chiral (1R)-N-(1-phenylethyl)maleimide to
assemble pyrrolo[3,4-c]pyrazoles bearing chiral centers.
Keywords Nitrilimine Á Chiral maleimide Á
1,3-Dipolar cycloaddition Á Ylide
Introduction
Nitrilimines, which were discovered by Huisgen and
coworkers in 1959, are important precursors of 1,3-dipolar
structure which can be used to assemble a wide variety of
five-membered nitrogen-containing heterocyclic com-
pounds with 1,3-dipolar cycloaddition reactions to various
dipolarophiles leading to five- or six-membered, or larger
heterocycles [1–22]. Experimental observations showed
that the orientation observed for alkenes substituted with
electron-donor, conjugating, and moderate electron-
acceptor substituents is the 5-substituted regioisomer
[23–26]. Sometimes the cycloadditions of nitrilimines to
Results and discussion
Hydrazones 3 were prepared from the appropriate para-
substituted aromatic aldehydes and hydrazines according to
literature procedures [53, 54], and hydrazonyl chlorides 4
were obtained from the above hydrazones by using the
method reported by Kim and Corey (N-chlorosuccinimide-
dimethyl sulfide) [53–55] (Scheme 1). Chiral maleimide 6
was prepared by reacting chiral amine 5 with maleic
anhydride followed by acetic anhydride (Scheme 2).
Cycloaddition reactions of the nitrilimines which were
generated in situ by the effect of triethylamine on hyd-
razonyl chlorides to chiral maleimide 6 were carried out in
dry acetonitrile at room temperature. Thus, 11 novel pyr-
rolo[3,4-c]pyrazoles 7a–8k were obtained in good yields
(Scheme 3, Table 1).
Y. Du¨ru¨st (&) Á M. Yıldırım
˙
Department of Chemistry, Abant Izzet Baysal University,
14280 Bolu, Turkey
e-mail: yasardurust@ibu.edu.tr
C. F. Fronczek Á F. R. Fronczek
Department of Chemistry, Louisiana State University,
Baton Rouge, LA 70803-1804, USA
123

128
Y. Du¨ru¨st et al.
R1
R2
R1
R2
NCS-DMS
EtOH, H2O
RT, 1h
O
H
R1
R2
N
+
NHNH2
N
0 to -40 °C,
2-4 h, CH2Cl2
N
N
H
Cl
H
H
1
2
3a-3k
4a-4k
R1
H
Cl
Br
F
CN
CF3
R2
H
H
H
H
H
H
,
R1
OCH3
SCH3
OCOCH3
CH3
NO2
R2
H
H
H
CN
Cl
3 4
a
3 4
,
g
h
i
j
k
b
c
d
e
f
Scheme 1
clearly seen that 3a–H and 6a–H protons make couplings
with the ortho hydrogens of the phenyl groups as depicted
in the structure of 8f. Also there is a quintet splitting pat-
tern arising from the couplings of methyl protons and
phenyl ortho hydrogen with the C–H proton. In the case of
dimethyl sulfoxide (DMSO)-d6 as NMR solvent it
appeared as triplet (Figs. 1, 2). Methyl protons of the chiral
maleimide portion were also coupled with both C–H and
phenyl ortho hydrogens, causing a triplet splitting (Fig. 3).
Stereochemistry of the cycloadducts was determined
based on the X-ray diffraction data and NMR spectra.
Theoretically, the unsymmetrical nitrilimine 1,3-dipole
may approach chiral maleimide in two different orienta-
tions, leading to the formation of cycloadducts 7 and 8
regioselectively (Scheme 4).
1. Maleic anhydride,
acetone, 4 h, RT
(R) CH3
(R)
CH3
H
2. NaOAc, acetic anhydride,
95-100 °C, 30 min
N
O
H
O
NH2
6
5
Scheme 2
Structure determination of the new compounds was
1
performed on the basis of IR, H NMR, 13C NMR, mass
spectrometry (MS), X-ray diffraction spectra, and ele-
mental analyses. All cycloadducts showed strong
absorption around 1,705–1,712 cm-1 due to two carbonyl
groups and absorptions relating to the C=N around
1,600 cm-1 in the IR spectra. Crude H NMR spectra of
1
The structure elucidation of the chiral cycloadducts 7/8
was established by means of spectroscopic and physical
data including IR, NMR, gas chromatography (GC)–MS,
CHN analyses, and most importantly by X-ray diffraction
data. After recrystallization of the cycloadducts from
appropriate solvents, fine crystals were obtained, which
allowed us to determine the stereochemistry of the chiral
the cycloadducts obtained after the usual work-up showed
only one single diastereomer in each case, and after puri-
fication doublet of doublet patterns related to the bridge
protons (3a–H and 6a–H) and a quintet pattern related to
the C–H (phenethyl on maleimide nitrogen) were observed.
At first sight, when we examined the doublets, it could be
Scheme 3
O
R2
R1
Et3N
(R)
R1
N N
R2
N
+
N
N
H
CH3
H
Cl
O
6
4a-4k
CH3CN,
2-4 h, RT
R1
H
R2
H
7a/8a
7b/8b
7c/8c
7d/8d
7e/8e
7f/8f
7g/8g
7h/8h
7i/8i
Cl
Br
F
CN
CF3
OCH3
SCH3
OCOCH3
CH3
NO2
H
H
H
H
H
H
H
H
(R)
(R)
CH3
CH3
H
H
+
N
N
O
H
O
H
O
H
O
H
(R)
(R)
(S)
N(S)
N
N
N
R1
R2
R2
R1
7
8
7j/8j
7k/8k
CN
Cl
(3aS,6aS)
(3aR,6aR)
123

Synthesis and stereochemistry
129
Fig. 3 1H NMR splitting of methyl protons on chiral carbon
Fig. 1 Splitting patterns of the protons in 8f
starting materials, were determined from resonant scat-
tering of light using Cu Ka radiation, using the Flack
X and Hooft Y parameters [57, 58]: for 7a/8a, X = 0.2(2)
and Y = 0.11(9); for 7d/8d, X = 0.04(14) and
Y = 0.05(6). This corresponds to a probability of 1.000
that the depicted and expected configuration is correct in
both cases.
O
H
H3C
H
H
N
N
3a
H
H
6a
N
C
O
H
H
The formation of a single regioisomer of the compounds
8f and 7i was demonstrated by their X-ray diffraction data
(Fig. 6). According to these results, the cycloaddition
reactions yielded the cycloadducts 8f and 7i as major
product in fully regiocontrolled manner by the orientation
of the unsymmetrical dipole [59]. Splitting patterns as a
representative example of the protons attached to the chiral
carbons in compound 8f are shown in Fig. 7.
The 1H NMR spectra of all new compounds showed two
doublets at around 4.7 and 5.0 ppm, which were assigned
to the protons at C–6a and C–3a, respectively. Chemical
shifts and coupling constants are shown in Table 2. Elec-
tron-withdrawing or electron-releasing substituents on
para-position of the phenyl ring attached to azomethine
carbon caused slight shielding or deshielding effect on Hb
bridge protons. However, the vicinal coupling constants
have been shown to be diagnostic (Jtrans \ 6 Hz,
Jcis & 9–12 Hz), so observed coupling constants for Ha
and Hb protons are compatible with cis-configuration, as
supported by the X-ray ORTEP views and previously
reported literature values for similar stereochemistry
F3C
Fig. 2 1H NMR couplings in 8f
centers by X-ray crystallography and regioisomeric ratios
of some cycloadducts. The results of X-ray single-crystal
diffraction measurements indicated that the compounds
7a/8a and the compounds 7d/8d were as a regioisomeric
mixture in approximately 1:1 ratio (Figs. 4, 5). This result
also revealed that these cycloaddition reactions are ste-
reospecific due to formation of two regioisomers
depending on the starting chiral maleimide with R con-
figuration. In both structures depicted in Figs. 4 and 5, the
C5 and N2 sites are disordered, with swapped identities,
due to the mixture of inseparable diastereomers present in
the crystals. Refined population parameters yielded
0.486(9) 7a:0.514(9) 8a and 0.428(2) 7d:0.572(2) 8d. The
absolute configurations, which agreed with that of the
123

130
Y. Du¨ru¨st et al.
R1
Scheme 4
O
H
O
H
R1
H
H3C
H
H3C
H
N
O
(S)
N
N
(S)
(R)
δ
N
H
δ
O
N
N
R2
7
R2
R2
H
O
H
H3C
H
R2
O
H
N
H3C
H
N
δ
N
(R)
O
N
N
(R)
(R)
N
H
δ
O
R1
8
R1
Fig. 4 Structures and X-ray
ORTEP views of the
regioisomers 7a and 8a
O
H
H3C
(S)
N
N
H
(S)
(R)
N
H
O
7a
O
H
H3C
N
(R)
N
N
H
(R)
(R)
H
O
8a
123

Synthesis and stereochemistry
131
Fig. 5 Structures and X-ray
ORTEP views of the
regioisomers 7d and 8d
F
O
H
H C
3
(S)
N
N
H
(S)
(R)
N
H
O
7d
O
H
H C
3
N
(R)
N
N
H
(R)
(R)
H
O
8d
F
Table 1 Cycloaddition products 7a–8k and reaction conditions
Comp.
R1
R2
Reaction conditions
Yield/%
[a]2D1 a
Regioisomeric ratio (7:8)
Solvent
Time/h
7a
7b
7c
7d
7e
7f
H
H
Toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
12
20
48
20
12
8
51
84
72
85
92
70
97
91
71
96
88
?80.0
?70.0
?60.0
?60.0
?68.0
?12.0
?40.0
?44.0
?79.0
?80.0
?53.0
50:50
Cl
H
Inseparable mixture
Inseparable mixture
57:43
Br
H
F
H
CN
H
Inseparable mixture
0:100
CF3
OCH3
SCH3
OCOCH3
CH3
NO2
H
7g
7h
7i
H
20
20
3
Inseparable mixture
Inseparable mixture
100:0
H
H
7j
CN
Cl
12
12
Inseparable mixture
Inseparable mixture
7k
All reactions carried out at r.t.
a
c = 0.01 g cm-3, acetone
[60, 61]. The IR spectra of compounds 7a–8k exhibited
amide carbonyl absorption bands at around
1,705–1,710 cm-1. The 13C NMR spectra of all chiral
products showed the signals corresponding to amide car-
bonyl carbons at around 170–174 ppm and the signals
corresponding to C=N carbon at around 143–147 ppm.
In the electrospray ionization (EI) mass spectra of cyc-
loadducts 7a–8k, almost all cycloadducts displayed
molecular ions as base peak. Only compounds 7g and 7k
showed different fragmentation pattern and ionic species:
M?–CH3OC6H5C=NNC6H5, which can be accounted for
by retrocycloaddition of cycloadduct 7g, and M?–NO2Cl,
123

132
Y. Du¨ru¨st et al.
Fig. 6 Structures and X-ray
ORTEP views of the
regioisomers 8f and 7i
O
H
H3C
N
(R)
N
N
H
(R)
(R)
H
O
8f
CF3
OCOCH3
O
H
H3C
(S)
N
N
H
(S)
(R)
N
H
O
7i
Fig. 7 Representative
1
expanded H NMR spectrum of
compound 8f, indicating
splitting patterns of the aliphatic
protons
123

Synthesis and stereochemistry
133
Table 2 1H NMR chemical shifts and coupling constants for aliphatic protons of compounds 7a–8k in CDCl3
Comp.
Hc/ppm
J/Hz
Hb/ppm
J/Hz
Ha/ppm
J/Hz
Hd/ppm
J/Hz
7a
7b
7c
7d
7e
7f
5.30, q
7.2
7.0
7.0
7.4
6.7
7.4
7.3
7.1
7.0
9.7
5.8
5.42, dd
5.01, dd
5.01, dd
5.01, dd
5.15, dd
4.95, dd
4.98, dd
4.95, dd
5.08, dd
5.28–5.34, td
5.10, dd
10.8, 3.3
5.24, dd
10.8, 1.7
22.1, 10.9
22.9, 10.9
20.9, 10.9
20.1, 11.1
24.1, 10.7
24.4, 10.9
21.5, 11.0
23.2, 11.0
–
1.70, t
1.80, t
1.81, t
1.83, t
1.81, t
1.81, t
1.80, t
1.80, t
1.82, t
1.69, t
1.80, t
4.2
7.2
7.3
7.1
7.0
7.1
7.4
6.8
7.3
6.5
5.7
5.44, t
35.7, 10.9
34.8, 11.0
34.6, 10.9
32.2, 11.1
33.4, 10.7
32.7, 10.9
51.1, 10.9
29.8, 11.0
10.8, 3.3
4.70, dd
5.43, t
4.70, dd
5.44, quint
5.44, quint
5.37, quint
5.43, quint
5.44, t
4.70, dd
4.76, dd
4.75, dd
7g
7h
7i
4.75, dd
4.70, dd
5.44, quint
5.57, t
4.76, dd
7j
Overlap with Hb signal
4.86, dd
7k
5.42, quint
29.7, 10.7
23.2, 10.7
Scheme 5
OCH3
O
O
CH3
H
H
CH3
N
H
COC6H4C=NNPh
+
N
N
N
m/z = 224 (100%)
O
H
O
m/z = 201 (100%)
7g
NO2
O
O
H
H
H
H
CH3
CH3
-NO2Cl
H
H
N
N
N
N
N
N
O
O
7k
m/z = 395 (100%)
Cl
123

134
Y. Du¨ru¨st et al.
Fig. 8 Gas chromatogram of 7c
Fig. 9 Gas chromatogram of 7i
by producing more stable unsubstituted pyrrolo[3,4-c]pyr-
azole-4,6-dione from cycloadduct 7k (Scheme 5).
two adjacent peaks which are almost fused, related to the
diastereomers (regioisomers). A typical gas chromatogram
of 7c/8c is shown in Fig. 8. Mass spectra of the two iso-
meric compounds are the same.
In the 13C NMR spectra of the cycloadducts, among the
aliphatic carbons the most deshielded one is 3a–C due to its
proximity to two nitrogens, and it resonated at around
64–66 ppm for 7a–8k. The signals of the carbonyl carbons
were found at around 171–173 ppm and the azomethine
C=N carbons at around 143–151 ppm.
In some cases, besides the GC peaks related to the ste-
reoisomeric mixture, there was a third peak with two
hydrogen atoms fewer, which can easily be attributed to the
aromatization product on the GC column. We observed this
peak in the gas chromatograms of 7i and 7j with relatively
high abundance. A typical gas chromatogram of 7i
Upon examination of the gas chromatograms of the pure
samples of cycloadducts 7a–8k, in some cases we observed
123

Synthesis and stereochemistry
135
three-necked flask. The yellow reaction mixture was heated
to 95–100 °C for 3 h, and after cooling to room temper-
ature, the brown reaction mixture was poured into 40 cm3
ice-water mixture with stirring for 1 h. The reaction
mixture was extracted with ethyl acetate (3 9 25 cm3),
and the combined ethyl acetate layers were dried over
Na2SO4. Finally, the solvent was evaporated under reduced
pressure, and the remaining orange oil was dried in vacuo
and purified by column chromatography (ethyl acetate/
n-hexane 1:3). (R)-(?)-N-(1-Phenylethyl)maleimide (6) was
obtained as light-yellow oil (1.6 g, 54%); [a]2D1 = ?88.7°
(c = 0.01 g cm-3, CHCl3). Physical data were found
to agree with those published for the (S)-enantiomer in
Ref. [56].
indicating the above observation is shown in Fig. 9. The
retention time peak at 38.16 min is possibly of aromatized
product.
Conclusions
1,3-Dipolar cycloaddition of nitrilimines to chiral (1R)-N-
(1-phenylethyl)maleimide gives rise to straightforward
formation of cycloadducts as a regioisomeric mixture in
good to excellent yields. In some cases, regioselectivity is
very high, as much as 100% (e.g., 8f and 7i). Gas chro-
matograms of the cycloadducts clearly show the
regioisomeric mixture and fully aromatized triazoles in
some samples. Exact structures of the stereoisomers were
determined by X-ray diffraction and NMR data.
General procedure for the synthesis of chiral
dihydropyrrolo[3,4-c]pyrazolediones 7a–8k
The corresponding hydrazonyl chloride 4 (0.5 mmol) and
maleimide 6 (0.5 mmol) were dissolved in 20 cm3 dry ace-
tonitrile. Et3N (0.202 g, 2 mmol) was added dropwise into
the mixture with stirring, and the reaction mixture was stirred
at room temperature for 2–4 h. The progress of reaction was
monitored by TLC. After the reaction was complete, all
CH3CN was evaporated, and the reaction mixture was mixed
with 50 cm3 water. The precipitated crude cycloadduct 7/8
was collected by suction filtration, washed with 50 cm3
water and 25 cm3 hexane, and dried in vacuo for 1–2 h.
Finally, chiral cycloadducts 7/8 were purified with a Chro-
matotron (centrifugal thin-layer chromatograph) using
n-hexane/ethyl acetate as eluent and recrystallized from
appropriate solvent mixtures as indicated.
Experimental
1H and 13C NMR spectra were recorded on BRUKER and
VARIAN spectrometers (200, 300, and 400 MHz for pro-
ton, and 75 and 100 MHz for carbon). IR spectra were
recorded on a SHIMADZU FTIR-8400S instrument from
KBr pellets. GC–MS spectra were run on an Agilent 6890
GC System 5973 MSD instrument. Optical rotations were
measured by a Rudolph Research Analytical Autopol I
automatic polarimeter using path length of 10 cm. Ele-
mental analyses were performed on EuroVector EA 3000
and Elementar instruments and results agreed satisfactorily
( 0.2%) with calculated values. A ChromatotronTM cen-
trifugal thin-layer chromatograph system was used for
purification and isolation of cycloadducts. Melting points
were determined on a MELTEMP apparatus. Thin-layer
chromatography (TLC) was done using precoated plates
with fluorescent indicator (Merck 5735). Stain solutions of
permanganate, p-anisaldehyde, and iodine were used for
visualization of TLC spots.
(3aS,6aS)- and (3aR,6aR)-5,6a-Dihydro-1,3-diphenyl-5-
[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-4,6(1H,3aH)-
dione (7a/8a, C25H21N3O2)
Inseparable stereoisomers, detected by X-ray diffraction.
Colorless plate crystals; yield 100 mg (51%); m.p.:
150–152 °C (acetone); [a]2D1 = ?80.0° cm3 g-1 dm-1
(c = 0.01 g cm-3, acetone); Rf = 0.75 (ethyl acetate/n-
hexane 1:2); IR (KBr): vꢀ = 3,483, 3,059, 2,926 (C–H),
1,710 (C=O), 1,597 (C=N), 1,494, 1,386, 1,346, 1,222,
(R)-N-(1-Phenylethyl)maleimide (6)
(R)-1-Phenylethylamine (5, 2.42 g, 20.0 mmol) was dis-
solved and stirred in 25 cm3 acetone. Solution of 4.10 g
maleic anhydride (42.0 mmol) in 25 cm3 acetone was
added dropwise over 15 min, and the resulting yellow
solution was stirred at room temperature for 4 h. The
reaction mixture was poured into 50 cm3 ice-water to
precipitate the white product. The precipitate was isolated
by filtration and dried under vacuum to yield 73% (R,E)-4-
oxo-4-(1-phenylethylamino)but-2-enoic acid, which was
used in the next step without purification.
1
1,192, 754, 696 cm-1; H NMR (400 MHz, DMSO-d6):
d = 7.98 (t, J = 6.9 Hz, 2H), 7.43–7.50 (m, 5H),
7.24–7.36 (m, 7H), 6.95 (q, J = 7.3 Hz, 1H), 5.42 (dd,
J = 10.8, 3.3 Hz, 1H), 5.30 (q, J = 7.2 Hz, 1H), 5.24 (dd,
J = 10.8, 1.7 Hz, 1H), 1.70 (t, J = 4.2 Hz, 3H) ppm; 13C
NMR (100 MHz, DMSO-d6): d = 173.8 (C=O), 172.8
(C=O), 145.0 (C=N), 144.5, 139.8, 131.0, 129.5, 128.9,
128.9, 128.8, 127.9, 127.5, 127.0, 121.0, 114.4, 66.0
(–CH), 54.0 (–CH), 50.3 (–CH), 16.8 (–CH3) ppm; MS
(70 eV): m/z (%) = 395 (100) [M]?, 291 (65), 247 (40), 219
(25), 105 (20), 77 (20).
(R,E)-4-Oxo-4-(1-phenylethylamino)but-2-enoic acid
(14.6 mmol), 1.43 g anhydrous sodium acetate (17.5 mmol),
and 45 g (438 mmol) acetic anhydride were added into a
123

136
Y. Du¨ru¨st et al.
(3aS,6aS)-3-(4-Chlorophenyl)-5,6a-dihydro-1-phenyl-5-
[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-4,6(1H,3aH)-
dione (7b, C25H20ClN3O2)
4.70 (dd, J = 20.9, 10.9 Hz, 1H), 1.83 (t, J = 7.1 Hz, 3H)
ppm; 13C NMR (100 MHz,CDCl3): d = 172.5 (C=O), 171.5
(C=O), 144.6 (C=N), 142.0, 138.7, 129.2, 129.1, 129.0,
128.6, 128.2, 127.6, 121.5, 115.8, 115.6, 114.4, 65.4 (–CH),
53.3 (–CH), 51.5 (–CH), 16.3 (–CH3) ppm; MS (70 eV): m/z
(%) = 413 (100) [M]?, 309 (78), 265 (35),105 (29), 70 (21).
Light-yellow needle crystals; yield 181 mg (84%); m.p.:
189–191 °C (dichloromethane/n-hexane); [a]2D1 = ?70.0°
cm3 g-1 dm-1 (c = 0.01 g cm-3, acetone); Rf = 0.76
(ethyl acetate/n-hexane 1:2); IR (KBr): vꢀ = 3,470, 3,090,
2,939 (C–H), 1,705 (C=O), 1,597 (C=N), 1,492, 1,379,
4-[(3aS,6aS)-1,3a,4,5,6,6a-Hexahydro-4,6-dioxo-1-phenyl-
5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazol-3-yl]benzoni-
trile (7e, C26H20N4O2)
1,356, 1,220, 1,193, 831, 750, 696 cm-1
;
1H NMR
(400 MHz, DMSO-d6): d = 8.01 (d, J = 8.0 Hz, 2H),
7.58 (d, J = 6.7 Hz, 2H), 7.19–7.48 (m, 9H), 7.02 (t,
J = 6.5 Hz, 1H), 5.44 (t, J = 7.0 Hz, 1H), 5.01 (dd,
J = 35.7, 10.9 Hz, 1H), 4.70 (dd, J = 22.1, 10.9 Hz, 1H),
1.80 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (100 MHz,
DMSO-d6): d = 171.5 (C=O), 170.5 (C=O), 144.5 (C=N),
142.3, 142.1, 139.0, 135.4, 129.3, 128.9, 128.8, 128.6,
128.3, 127.6, 121.7, 114.4, 65.3 (–CH), 53.1 (–CH), 51.5
(–CH), 16.3 (–CH3) ppm; MS (70 eV): m/z (%) = 429
(100) [M]?, 325 (82), 281 (35), 253 (29), 105 (35), 77 (29).
Yellow powder; yield 194 mg (92%); m.p.: 207–209 °C;
[a]2D1 = ?68.0° cm3 g-1 dm-1 (c = 0.01 g cm-3, acetone);
Rf = 0.7 (ethyl acetate/n-hexane 1:2); IR (KBr): vꢀ = 3,398,
3,059, 2,941 (C–H), 2,225 (C:N), 1,712 (C=O), 1,597
(C=N), 1,496, 1,384, 1,356, 1,220, 1,193, 750, 698 cm-1; 1H
NMR (400 MHz, CDCl3): d = 8.15 (d, J = 7.1 Hz, 2H),
7.70 (d, J = 8.1 Hz, 2H), 7.60 (d, J = 5.7 Hz, 2H), 7.45 (t,
J = 6.5 Hz, 2H), 7.30–7.41 (m, 5H), 7.06 (t, J = 7.2 Hz,
1H), 5.44 (quintet, J = 6.7 Hz, 1H), 5.15 (dd, J = 32.2,
11.1 Hz, 1H), 4.76 (dd, J = 20.1, 11.1 Hz, 1H), 1.81 (t,
J = 7.0 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3):
d = 171.9 (C=O), 171.3 (C=O), 143.5 (C=N), 140.7, 138.5,
134.7, 132.3, 129.3, 128.7, 128.3, 127.6, 127.2, 122.3, 118.8,
114.6, 112.1, 65.4 (–CH), 52.7 (–CH), 51.6 (–CH), 16.4
(–CH3) ppm; MS (70 eV): m/z (%) = 420 (100) [M]?, 316
(78), 272 (25), 244 (16), 105 (33), 70 (38).
(3aS,6aS)-3-(4-Bromophenyl)-5,6a-dihydro-1-phenyl-5-
[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-4,6(1H,3aH)-
dione (7c, C25H20BrN3O2)
Light-yellow crystals; yield 170 mg (72%); m.p.:
146–148 °C (acetone); [a]2D1 = ?60.0° cm3 g-1 dm-1
(c = 0.01 g cm-3, acetone); Rf = 0.76 (ethyl acetate/n-
hexane 1:2); IR (KBr): vꢀ = 3,471, 3,032, 2,939 (C–H),
1,708 (C=O), 1,599 (C=N), 1,573, 1,491, 1,379, 1,356, 1,193,
(3aR,6aR)-5,6a-Dihydro-1-phenyl-5-[(1R)-1-phenylethyl]-
3-[4-(trifluoromethyl)phenyl]pyrrolo[3,4-c]pyrazole-
4,6(1H,3aH)-dione (8f, C26H20F3N3O2)
1
750, 698 cm-1; H NMR (400 MHz, CDCl3): d = 7.93 (d,
J = 8.5 Hz, 2H), 7.57 (q, J = 4.0 Hz, 4H), 7.46 (t,
J = 6.4 Hz, 2H), 7.28–7.39 (m, 5H), 7.02 (t, J = 7.2 Hz,
1H), 5.43 (t, J = 7.0 Hz, 1H), 5.01 (dd, J = 34.8, 11.0 Hz,
1H), 4.70 (dd, J = 22.9, 10.9 Hz, 1H), 1.81 (t, J = 7.3 Hz,
3H) ppm; 13C NMR (100 MHz,CDCl3): d = 172.3 (C=O),
171.5 (C=O), 144.3 (C=N), 141.8, 138.7, 131.7, 129.4, 129.2,
128.6, 128.5, 128.2, 127.6, 123.7, 121.7, 114.4, 65.3 (–CH),
53.1 (–CH), 51.5 (–CH), 16.3 (–CH3) ppm; MS (70 eV): m/z
(%) = 473 (100) [M]?, 369 (74), 327 (32), 299 (29), 247 (15),
105 (53), 77 (47).
Light-green crystals; yield 161 mg (70%); m.p.: 174–
176 °C (acetic acid); [a]2D1 = ?12.0° cm3 g-1 dm-1
(c = 0.01 g cm-3, acetone); Rf = 0.68 (ethyl acetate/n-
hexane 1:2); IR (KBr): vꢀ = 3,452, 3,064, 2,941 (C–H),
1,708 (C=O), 1,599 (C=N), 1,500, 1,327, 1,193, 1,166,
1
1,068, 844, 750, 696 cm-1; H NMR (400 MHz, CDCl3):
d = 8.17 (q, J = 3.8 Hz, 2H), 7.70 (t, J = 4.7 Hz, 2H),
7.61 (t, J = 7.6 Hz, 2H), 7.47 (t, J = 6.5 Hz, 2H), 7.28–
7.41 (m, 5H), 7.05 (t, J = 7.0 Hz, 1H), 5.44 (quintet,
J = 7.0 Hz, 1H), 5.08 (dd, J = 29.8, 11.0 Hz, 1H), 4.76
(dd, J = 23.2, 11.0 Hz, 1H), 1.82 (t, J = 7.3 Hz, 3H) ppm;
13C NMR (100 MHz, CDCl3): d = 172.1 (C=O), 171.4
(C=O), 143.9 (C=N), 141.2, 138.6, 133.8, 130.9, 129.3,
128.6, 128.3, 127.6, 127.1, 125.5 (–CF3), 122.6, 121.9,
114.5 (CF3), 65.3 (–CH, 3aC), 52.9 (–CH3CH), 51.6 (–CH,
6aC), 16.4 (–CH3) ppm; MS (70 eV): m/z (%) = 463 (100)
[M]?, 359 (80), 315 (30), 269 (10), 105 (40), 70 (43).
(3aS,6aS)- and (3aR,6aR)-3-(4-Fluorophenyl)-5,6a-dihydro-
1-phenyl-5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-4,
6(1H,3aH)-dione (7d/8d, C25H20FN3O2)
Inseparable stereoisomers, detected by X-ray diffraction.
White crystals; yield 175 mg (85%); m.p.: 205–206 °C
(dichloromethane/n-pentane);
[a]2D1 = ?60.0° cm3 g-1
dm-1 (c = 0.01 g cm-3, acetone); Rf = 0.75 (ethyl ace-
tate/n-hexane 1:2); IR (KBr): vꢀ = 3,481, 3,061, 2,980
(C–H), 2,941, 1,708 (C=O), 1,597 (C=N), 1,498, 1,379,
(3aS,6aS)-5,6a-Dihydro-3-(4-methoxyphenyl)-1-phenyl-5-
[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-4,6(1H,3aH)-
dione (7g, C26H23N3O3)
1,356, 1,224, 1,193, 1,128, 750, 698 cm-1 1H NMR
;
(400 MHz, CDCl3): d = 8.07 (t, J = 3.1 Hz, 2H), 7.58 (d,
J = 7.2 Hz, 2H), 7.47 (t, J = 6.8 Hz, 2H), 7.28–7.37 (m,
5H), 7.13–7.22 (m, 2H), 7.02 (t, J = 7.0 Hz, 1H), 5.44
(quintet, J = 7.4 Hz, 1H), 5.01 (dd, J = 34.6, 10.9 Hz, 1H),
Light-yellowneedles;yield209 mg(97%);m.p.:135–137 °C
(dichloromethane/n-hexane); [a]2D1 = ?40.0° cm3 g-1 dm-1
, acetone); Rf = 0.75 (ethyl acetate/
(c = 0.01 g cm-3
123

Synthesis and stereochemistry
137
n-hexane 1:2); IR (KBr): vꢀ = 3,487, 3,056, 2,935 (C–H),
2,837, 1,708 (C=O), 1,597 (C=N), 1,496, 1,379, 1,356, 1,253,
51.4 (–CH), 21.1(–CH3), 16.4 (–CH3) ppm; MS (70 eV):
m/z (%) = 453 (100) [M]?, 411 (65), 307 (100), 236 (50),
207 (10), 105 (33), 70 (10).
1
1,176, 835, 750, 696 cm-1; H NMR (400 MHz, CDCl3):
d = 8.01 (t, J = 6.3 Hz, 1H), 7.59 (d, J = 7.3 Hz, 2H), 7.46
(q, J = 7.6 Hz, 2H), 7.20–7.35 (m, 5H), 6.98 (d, J = 8.0 Hz,
3H), 6.64 (s, 1H), 5.37 (quintet, J = 7.4 Hz, 1H), 4.95 (dd,
J = 33.4, 10.7 Hz, 1H), 4.75 (dd, J = 24.1, 10.7 Hz, 1H),
3.80 (s, 3H), 1.81 (t, J = 7.1 Hz, 3H) ppm; 13C NMR
(100 MHz,CDCl3): d = 172.7 (C=O), 171.7 (C=O), 160.8,
145.0 (C=N), 142.9, 134.0, 129.2, 128.8, 128.5, 128.1, 127.7,
127.2, 123.1, 121.2, 114.3, 65.3 (–CH, 3aC), 55.4 (OCH3),
53.4 (–CH), 51.4 (–CH), 16.4 (–CH3) ppm; MS (70 eV): m/z
(%) = 201(100) [M]?–C16H5N–N=C–C6H4OCH3, 158 (25),
104 (40), 77 (40), 51 (15).
4-[(3aS,6aS)-4,5,6,6a-Tetrahydro-3-(4-methylphenyl)-4,6-
dioxo-5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazol-
1(3aH)-yl]benzonitrile (7j, C27H22N4O2)
White needle crystals; yield 213 mg (96%); m.p.:
234–236 °C (ethanol/n-hexane/acetone/dichloromethane);
[a]2D1 = ?80.0° cm3 g-1 dm-1 (c = 0.01 g cm-3
,
ace-
tone); Rf = 0.58 (ethyl acetate/n-hexane 1:2); IR (KBr):
vꢀ = 3,470, 3,032, 2,980, 2,941 (C–H), 2,218 (C:N),
1,712 (C=O), 1,602 (C=N), 1,508, 1,390, 1,186, 829,
;
696 cm-1 1H NMR (400 MHz, CDCl3): d = 7.96 (dd,
J = 3.5, 4.6 Hz, 2H), 7.60 (s, 4H), 7.44 (t, J = 6.9 Hz,
2H), 7.26–7.36 (m, 5H), 5.42 (quintet, J = 5.8 Hz, 1H),
5.10 (dd, J = 29.7, 10.7 Hz, 1H), 4.86 (dd, J = 23.2,
10.7 Hz, 1H), 2.41 (s, 3H), 1.80 (t, J = 5.7 Hz, 3H) ppm;
13C NMR (100 MHz, CDCl3): d = 172.5 (C=O), 171.5
(C=O), 147.5 (C=N), 146.0, 142.0, 139.0, 133.5, 129.5,
128.7, 127.6, 127.5, 127.4, 120.0, 114.2, 103.0, 64.5
(–CH), 54.0 (–CH), 51.5 (–CH), 22.0 (–CH3), 16.0 (–CH3)
ppm; MS (70 eV): m/z (%) = 434 (100) [M]?, 330 (78),
286 (53), 105 (46), 70 (23).
(3aS,6aS)-5,6a-Dihydro-3-[4-(methylthio)phenyl]-1-
phenyl-5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-
4,6(1H,3aH)-dione (7h, C26H23N3O2S)
Light-yellow crystals; yield 200 mg (91%); m.p.: 82–84 °C
(dichloromethane/n-pentane); [a]2D1 = ?44.0° cm3 g-1
dm-1 (c = 0.01 g cm-3, acetone); Rf = 0.80 (ethyl
acetate/n-hexane 1:2); IR (KBr): vꢀ = 3,483, 3,056, 2,980,
2,920 (C–H), 1,708 (C=O), 1,597 (C=N), 1,494, 1,388,
1
1,356, 1,226, 1,093, 750, 696 cm-1; H NMR (400 MHz,
DMSO-d6): d = 7.95–7.99 (m, 2H), 7.58 (q, J = 4.0 Hz,
2H), 7.46 (t, J = 6.5 Hz, 2H), 7.28–7.38 (m, 7H), 7.01 (t,
J = 7.2 Hz, 1H), 5.43 (quintet, J = 7.3 Hz, 1H), 4.98 (dd,
J = 32.7, 10.9 Hz, 1H), 4.75 (dd, J = 24.4, 10.9 Hz, 1H),
2.53 (s, 3H), 1.80 (t, J = 7.4 Hz, 3H) ppm; 13C NMR
(100 MHz, CDCl3): d = 172.5 (C=O), 171.6 (C=O), 144.6
(C=N), 142.6, 140.7, 134.0, 129.2, 128.6, 128.2, 127.6,
127.4, 127.2, 125.8, 121.4, 114.3, 65.2 (–CH, 3aC), 53.2
(–CH), 51.4 (–CH), 16.4 (–CH3), 15.3 (–SCH3) ppm; MS
(70 eV): m/z (%) = 441 (100) [M]?, 337 (65), 295 (22),
266 (45), 250 (33), 105 (22), 70 (15).
(3aS,6aS)-1-(4-Chlorophenyl)-5,6a-dihydro-3-(4-nitro-
phenyl)-5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-
4,6(1H,3aH)-dione (7k, C25H19ClN4O4)
Orange powder; yield 210 mg (88%); m.p.: 242–244 °C;
[a]2D1 = ?53.0° cm3 g-1 dm-1 (c = 0.01 g cm-3
tone); Rf = 0.62 (ethyl acetate/n-hexane 1:2); IR (KBr):
,
ace-
vꢀ = 3,475, 3,057, 2,935 (C–H), 1,708 (C=O), 1,593 (C=N),
1,550, 1,492, 1,392, 1,334, 1,222, 1,195, 852, 690 cm-1
;
1H NMR (400 MHz, DMSO-d6): d = 8.32 (q, J = 4.5 Hz,
2H), 8.20 (t, J = 7.6 Hz, 2H), 7.53 (t, J = 8.8 Hz, 2H),
7.42 (q, J = 4.5 Hz, 2H), 7.24–7.29 (m, 5H), 5.57 (t,
J = 9.7 Hz, 1H), 5.28–5.34 (td, J = 10.8, 3.3 Hz, 2H),
1.69 (t, J = 6.5 Hz, 3H) ppm; 13C NMR (100 MHz,
DMSO-d6): d = 173.5 (C=O), 172.5 (C=O), 147.5 (C=N),
143.0, 142.5, 139.7, 137.0, 129.4, 128.9, 128.2, 128.0,
127.1, 125.6, 124.2, 116.3, 65.9 (–CH), 53.8 (–CH), 50.6
(–CH), 16.8 (–CH3) ppm; MS (70 eV): m/z (%) = 395
(100) [M]?–NO2Cl, 291 (70), 247 (40), 218 (25), 105 (20),
70 (10).
(3aS,6aS)-3-[4-(Acetyloxy)phenyl]-5,6a-dihydro-1-phenyl-
5-[(1R)-1-phenylethyl]pyrrolo[3,4-c]pyrazole-
4,6(1H,3aH)-dione (7i, C27H23N3O4)
Yellow needle crystals; yield 160 mg (71%); m.p.: 86–
88 °C
(dichloromethane/n-hexane/acetone);
[a]2D1
=
?79.0° cm3 g-1 dm-1 (c = 0.01 g cm-3, acetone); Rf =
0.60 (ethyl acetate/n-hexane 1:2); IR (KBr): vꢀ = 3,412,
3,063, 2,937 (C–H), 1,757 (C=O), 1,710 (C=O), 1,599
(C=N), 1,498, 1,452, 1,357, 1,197, 750 cm-1; H NMR
1
(400 MHz, CDCl3): d = 8.10 (q, J = 3.8 Hz, 2H), 7.58 (t,
J = 7.6 Hz, 2H), 7.47 (t, J = 7.2 Hz, 2H), 7.24–7.36 (m,
5H), 7.18 (q, J = 4.6 Hz, 2H), 7.01 (t, J = 7.3 Hz, 1H),
5.44 (t, J = 7.1 Hz, 1H), 4.95 (dd, J = 51.1, 10.9 Hz, 1H),
4.70 (dd, J = 21.5,11.0 Hz, 1H), 2.35 (s, 3H) 1.80 (t,
J = 6.8 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3):
d = 172.4 (C=O), 171.5 (C=O), 169.4 (C=O), 151.5
(C=N), 144.5, 142.0, 138.7, 129.2, 128.9, 128.6, 128.3,
128.2, 127.6, 121.8, 121.5, 114.4, 65.4 (–CH), 53.3 (–CH),
X-ray crystallographic data
X-ray diffraction crystallographic data for the structural
analysis of compounds 7a/8a (CCDC 789119) and 7d/8d
(CCDC 789120) have been deposited with the Cambridge
Crystallographic Data Center. Copies of this information
can be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: ?44-
123

138
Y. Du¨ru¨st et al.
29. Sustmann R (1974) Pure Appl Chem 40:569
30. Houk KN, Sims J, Duke RE, Strozier RW, George JK (1973) J
Am Chem Soc 95:7287
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or http://
ccdc.cam.ac.uk).
31. Houk KN, Sims J, Watts CR, Lukus LJ (1973) J Am Chem Soc
95:7301
32. Bastide J, El-Ghandour N, Henrirou O (1973) Bull Chim Fr
7–8:2290
33. Henrirou O (1973) Bull Chim Fr 7–8:2294
34. Ess DH, Houk KN (2008) J Am Chem Soc 130:10187
35. Houk KN, Yamaguchi K (1984) In: Padwa A (ed) 1,3-Dipolar
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p 439
˙
Acknowledgments Abant Izzet Baysal University, Directorate of
Scientific Research Projects Unit (BAP grant 2007.03.03.260) and
¨
˙
TUBITAK (The Scientific and Technological Research Council of
Turkey, grant 106T645) are gratefully acknowledged for financial
support.
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