Oxidative N-heterocyclic carbene (NHC) catalysis for the rapid access to functionalized pyrrolo-oxazinones

Article abstract of DOI:10.1016/j.tet.2021.132330

The N-heterocyclic carbene (NHC)-catalyzed intramolecular cyclization of N-substituted pyrrole 2-carboxaldehydes under oxidative conditions allowing the facile synthesis of pyrrolo-oxazinone derivatives is reported. The keys to the success of this strateg

Full text of DOI:10.1016/j.tet.2021.132330

Tetrahedron 94 (2021) 132330
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Oxidative N-heterocyclic carbene (NHC) catalysis for the rapid access
to functionalized pyrrolo-oxazinones
Arghya Ghosh, Shilpa Barik, Soumen Barik, Sayan Shee, Akkattu T. Biju*
Department of Organic Chemistry, Indian Institute of Science, Bangalore, 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The N-heterocyclic carbene (NHC)-catalyzed intramolecular cyclization of N-substituted pyrrole 2-
carboxaldehydes under oxidative conditions allowing the facile synthesis of pyrrolo-oxazinone de-
rivatives is reported. The keys to the success of this strategy are the generation of acylazoliums using
NHC and the base-mediated formation of the enolate. Subsequent intramolecular acylation of the enolate
with the acylazoliums afforded the desired product. Mild reaction conditions, good functional group
compatibility and high yields of products are the advantages of the present annulation.
© 2021 Elsevier Ltd. All rights reserved.
Received 9 June 2021
Received in revised form
28 June 2021
Accepted 29 June 2021
Available online 12 July 2021
Keywords:
Acylazolium
Organocatalysis
Pyrrolo-oxazinones
N-heterocyclic carbenes
Intramolecular cyclization
1. Introduction
can be generated by the stoichiometric oxidation of the initially
formed Breslow intermediate [8]. In 2010, Studer and co-workers
Functionalized pyrroles are endowed with remarkable biolog-
ical properties as this heterocyclic scaffold is frequently found in a
variety of natural products and drugs [1]. Among the various pyr-
role natural products, pyrrolo-oxazinones are important as this
core is found in marine natural products including Lukianol A and B
having cytotoxic and antitumor activity (Fig. 1) [2]. Moreover,
peramine is a naturally occurring pyrrolo-pyrazine alkaloid found
application as insect feeding deterrent [3]. The intramolecular
cyclization of the N-alkyne-functionalized pyrrole derivative has
been a convenient method for the direct access to pyrrolo-
oxazinones [4]. Herein, we demonstrate the potential of N-het-
erocyclic carbene (NHC) organocatalysis for the one-step synthesis
reported an elegant NHC-catalyzed chemoselective acylation of
alcohols leading to the formation of esters using the bisquinone
oxidant (Scheme 1, eq 1) [9,10]. Since then, this oxidative strategy
has been conveniently employed for the generation of activated
carboxylates from aldehydes [11,12]. Recently, Lang and Wang uti-
lized the oxidative NHC catalysis for the efficient synthesis of
benzoxazinones from N-(2-formylphenyl)benzamide derivatives
(eq 2) [13]. The reaction proceeded via a tandem isomerization-
cyclization strategy. Inspired by this work, and given the signifi-
cance of pyrrolo-oxazinones [2,3], we envisioned the NHC-
catalyzed intramolecular cyclization of N-substituted pyrrole 2-
carboxaldehydes leading to the synthesis of pyrrolo-oxazinones,
where the reaction proceeds via the acylazolium intermediate (eq
3).
of
pyrrolo-oxazinones
from
N-substituted
pyrrole
2-
carboxaldehydes under oxidative conditions [5].
In the last three decades, NHC catalysis has been widely applied
for the synthesis of various heterocycles [6]. In addition to the
traditional reactivity of polarity reversal using NHCs, these cata-
lytically active species are also useful for the non-umpolung
transformations [7]. The generation of acylazolium intermediates
is one of the important modes in this domain and this intermediate
2. Results and discussion
The present study was initiated by the treatment of pyrrole-2-
carbaldehyde 1a with the carbene engendered from the N-mesi-
tyl triazolium salt A using Cs2CO3, and oxidative conditions using
the bisquinone 3 in THF. Interestingly, under these conditions, the
desired pyrrolo-oxazinone derivative 2a was formed in 67% yield
(based on 1H NMR yield, Table 1, entry 1). Screening various NHC
pre-catalysts revealed that the carbene generated from the
* Corresponding author.
E-mail address: atbiju@iisc.ac.in (A.T. Biju).
https://doi.org/10.1016/j.tet.2021.132330
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

A. Ghosh, S. Barik, S. Barik et al.
Tetrahedron 94 (2021) 132330
triazolium salt B afforded 2a in an enhanced yield of 70% (entry 2)
whereas other NHC pre-catalysts provided reduced yield of 2a
(entries 3e5). Further experiments were performed using NHC
derived from B. The reactions performed in various solvents indi-
cated that toluene is the so far best solvent furnishing 2a in 72%
yield, and the reactions performed in other solvents such as CHCl3,
DME and CH3CN returned moderate yield of 2a (entries 6e9).
Cs2CO3 was found to be the optimal base for this reaction, and the
other bases such as DBU, Na2CO3 and K2CO3 are less effective (en-
tries 10e12). Notably, reducing the catalyst loading of B to 10 mol %
reduced the formation of 2a to 57% (entry 13). Control experiments
indicated that there is no product formation in the absence of the
NHC pre-catalyst B (entry 14). So far the best condition for this
transformation is use of NHC pre-catalyst B (20 mol %) in the
presence of Cs2CO3 (1.5 equiv), and bisquinone 3 (1.5 equiv) in
toluene (1.0 mL) at 30 ꢀC for 12 h (entry 8).
Fig. 1. Naturally occurring pyrrolo-oxazinones and an analogue.
Having established the adequate reaction conditions, the scope
and limitations of this intramolecular cyclization has been evalu-
ated (see Scheme 2). The unsubstituted benzoyl methyl substituted
pyrrole substrate worked well and various electronically dissimilar
substituents at the 4-position of the benzoyl ring worked well and
the corresponding substituted pyrrolo-oxazinones were formed in
good yields (2a-2e). Moreover, substitution at the 3-position as
well as 2-position of the arene did not affect the outcome of the
reaction and the desired annulated products are formed in mod-
erate to good yields (2f-2i). In addition, versatile pyrrolo-
oxazinones bearing the disubstituted aryl moiety at the 3-
position was smoothly synthesized starting from the correspond-
ing pyrrole aldehydes (2j-2m). Interestingly, 3-heteroaryl pyrrolo-
Scheme 1. NHC catalysis under oxidative conditions.
Table 1
Optimization studiesa.
entrya
NHC.HX
base
solvent
yield of 2a (%)b
1
2
3
4
5
6
7
8
A
B
C
D
E
B
B
B
B
B
B
B
B
-
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DBU
THF
THF
THF
THF
67
70
13
32
<5
69
39
73 (72)
59
34
THF
CHCl3
CH3CN
toluene
DME
toluene
toluene
toluene
toluene
toluene
9
10
11
12
13c
14
K2CO3
Na2CO3
Cs2CO3
Cs2CO3
11
<5
57
<5
a
Standard conditions: 1a (0.1 mmol), 2a (0.15 mmol), 3 (0.15 mmol) base
(0.15 mmol), NHC.HX (20 mol %), solvent (1.0 mL), 30 ꢀC, 12 h.
b
The yields were determined by 1H NMR analysis of crude product using CH2Br2
as the internal standard. Isolated yield is provided in parenthesis.
c
Reaction carried out using 10 mol % of B.
Scheme 2. Substrate Scope of the Reactiona.
2

A. Ghosh, S. Barik, S. Barik et al.
Tetrahedron 94 (2021) 132330
oxazinones could be synthesized in moderate yields using this
NHC-catalyzed strategy (2n, 2o). Furthermore, reaction performed
with substrates bearing iodo, bromo and phenyl group on the
pyrrole ring underwent smooth annulation reaction and the
desired products are formed in moderate to good yields (2p-2r).
Notably, the reaction of a-methyl substituted protected pyrrole 2-
aldehyde substrate furnished the target annulated product 2s in
53% yield further demonstrating the versatility of this annulation.
This carbene-catalyzed intramolecular cyclization is not limited
to aryl ketones but instead alkyl ketones can also be utilized in this
annulation. Thus, the reaction of the methyl ketone 1t under the
optimized reaction conditions afforded the annulated product 2t in
54% yield expanding the scope of this reaction (Scheme 3, eq 4).
Moreover, when the reaction of 1a was conducted in a 1.0 mmol
scale, the product 2a was obtained in 70% yield demonstrating that
the present NHC-catalyzed process is easily scalable (eq 5).
To shed light on the mechanism of the reaction, we have per-
formed few mechanistic experiments (Scheme 4). When the NHC-
catalyzed reaction was performed in open air, exploring the role of
air as oxidant, the desired product 2a was formed only in traces (eq
6). This indicates the role of the Kharasch oxidant 3 [10] in oxidizing
the initially generated Breslow intermediate [8]. Moreover, when
the reaction was carried out under optimized conditions in the
presence of cinnamaldehyde, the annulated product 2a was not
formed. The formation of cinnamic acid under these conditions
indicates the preferential addition of NHC to enal and the formation
Scheme 4. Mechanistic Experiments.
of a,b-unsaturated acylazolium [7] over the carbene addition to 1a
leading to the acylazoliums. Interestingly, with excess of oxidant 3
(2.0 equiv), 36% of 2a was formed (eq 7). In addition, treatment of
1a with benzyl alcohol under the optimized conditions produced
the benzyl ester 4a in 69% yield with only traces of 2a (eq 8). This
experiment is likely an indication that the direct acylation of the
acylazolium using benzyl alcohol is faster than the enolate gener-
ation/isomerization to enol followed by the intramolecular
cyclization.
Mechanistically, the reaction proceeds via the initial formation
of the free carbene from the triazolium salt B using the base, and a
subsequent nucleophilic attack on the pyrrole-2-carbaldehyde 1a
followed by a proton transfer results in the formation of the
nucleophilic Breslow intermediate I (Scheme 5). The two-electron
oxidation of the intermediate I using the bisquinone generates
the key acylazolium intermediate II. Under basic conditions, the
intermediate II is converted into the enolate intermediate III, which
undergoes an intramolecular acylation leading to the formation of
the pyrrolo-oxazinones 2 with the regeneration of the NHC catalyst.
Alternatively, the isomerization of the acylazolium II to the corre-
sponding enol form followed by the nucleophilic acylation can also
result in the formation of 2.
Scheme 5. Proposed Mechanism of the Reaction.
(Scheme 6). Treatment of 2a with 40% aqueous methyl amine so-
lution resulted in the formation of the aminal, which was subse-
quently dehydrated using catalytic amount of PTSA resulting in the
We also performed the functionalization of pyrrolo-oxazinones
Scheme 3. Synthetic Utility of the Reaction.
Scheme 6. Functionalization of Pyrrolo-oxazinones.
3

A. Ghosh, S. Barik, S. Barik et al.
Tetrahedron 94 (2021) 132330
formation of the pyrrolo-pyrazine derivative 5a in 67% yield (for
two steps). Moreover, ring-opening of 2a using benzyl amine
afforded the amide derivative 6a in 63% yield. In addition, ring-
opening using methanol furnished the methyl ester 7a in 71%
yield. The conversion to 6a and 7a can be considered as the two-
step transformation of the aldehyde 1a to the amide and ester
respectively via the NHC-catalyzed annulation/ring-opening
strategy.
4.2.1. 3-Phenyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2a) [4a]
White solid (39 mg on 0.25 mmol scale, 72% yield). Rf (Pet. ether/
EtOAc ¼ 85/15): 0.52; 1H NMR (400 MHz, CDCl3)
d 7.71e7.69 (m,
2H), 7.46e7.36 (m, 4H), 7.27e7.26 (m, 1H), 7.19e7.18 (m, 1H),
6.58e6.57 (m, 1H). 13C NMR (100 MHz, CDCl3)
d 154.9, 142.3, 130.6,
129.52, 129.0, 124.5, 121.6, 116.9, 115.8, 113.7, 104.5. HRMS (ESI) m/z:
[MþH]þ calcd for C13H10NO2 212.0706; found 212.0711. FTIR (cmꢁ1
)
3122, 2923, 1714, 1535, 1477, 1360, 1255, 1191, 1054, 913.
3-(4-Methoxyphenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2b)
[4a]: White solid (49 mg, on 0.25 mmol scale, 81% yield). Rf (Pet.
3. Conclusion
ether/EtOAc ¼ 80/20): 0.55; 1H NMR (400 MHz, CDCl3)
d 7.60 (d,
J ¼ 8.7 Hz, 2H), 7.33 (s, 1H), 7.23 (d, J ¼ 3.4 Hz, 1H), 7.15 (m, 1H), 6.92
In summary, we have developed a mild, scalable and one-pot
strategy for the synthesis of pyrrolo-oxazinone derivatives. The
NHC-catalyzed intramolecular cyclization of N-substituted pyrrole
2-carboxaldehydes under oxidative conditions afforded the prod-
ucts in moderate to good yields with good functional group toler-
ance. The key step in this annulation reaction is the generation of
acylazolium intermediates under oxidative conditions. The appli-
cation of the present strategy to the synthesis of marine natural
products Lukianol A and B starting from the phenol derivatives
appears interesting.
(d, J ¼ 8.7 Hz, 2H), 6.56e6.54 (m, 1H), 3.82 (s, 3H). 13C NMR
(100 MHz, CDCl3)
d 159.9, 154.4, 141.6, 125.2, 122.3, 120.8, 116.0,
114.8, 113.7, 112.8, 102.5, 54.8. HRMS (ESI) m/z: [MþH]þ calcd for
C
14H12NO3 242.0812; found 242.0816. FTIR (cmꢁ1) 3123, 2921, 1710,
1536, 1469, 1365, 1254, 1194, 1054, 905.
3-(p-Tolyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2c): White
solid (41 mg, on 0.25 mmol scale, 72% yield). Rf (Pet. ether/
EtOAc ¼ 85/15): 0.53; 1H NMR (400 MHz, CDCl3)
d 7.56 (d,
J ¼ 8.2 Hz, 2H), 7.40 (s, 1H), 7.24e7.16 (m, 4H), 6.56e6.55 (m, 1H),
2.36 (s, 3H). 13C NMR (100 MHz, CDCl3)
d 155.0, 142.3, 139.6, 129.6,
127.7, 124.3, 121.6, 116.8, 115.6, 113.6, 103.9, 21.4. HRMS (ESI) m/z:
[MþH]þ calcd for C14H12NO2 226.0863; found 226.0869. FTIR
(cmꢁ1) 3121, 2923, 1714, 1539, 1470, 1358, 1255, 1192, 1058, 905.
3-(4-Chlorophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2d):
White solid (51 mg, on 0.25 mmol scale, 83% yield). Rf (Pet. ether/
4. Experimental section
4.1. General information
EtOAc ¼ 90/10): 0.46; 1H NMR (400 MHz, CDCl3)
d 7.65e7.62 (m,
Unless otherwise specified, all reactions were performed under
argon atmosphere using oven-dried reaction vessels with Teflon
screw caps. 30 ꢀC corresponds to the room temperature (rt) of the
lab when the experiments were carried out. Dry toluene was pur-
chased from commercial sources and stored under argon over so-
dium wire. The pyrrole-2-carboxaldehyde was purchased from
commercial sources. All the 1-(2-oxo-2-phenylethyl)-1H-pyrrole-
2-carbaldehyde derivatives were prepared following the literature
procedure [14]. The triazolium salt B was synthesized following the
literature procedure [15]. Cs2CO3 was purchased from commercial
sources and was used without further purification. Analytical thin
layer chromatography was performed on TLC Silica gel 60 F254.
Visualization was accomplished with short wave UV light or KMnO4
staining solutions followed by heating. Flash chromatography was
performed on silica gel (230e400 mesh) by standard techniques
eluting with Pet. Ether-EtOAc solvent system. All compounds were
fully characterized. 1H and 13C NMR spectra were recorded on
Bruker AV 400 or Bruker Ultra shield spectrometer in solvents as
2H), 7.46 (s,1H), 7.41e7.39 (m, 2H), 7.28e7.27 (m,1H), 7.20e7.19 (m,
1H), 6.61e6.59 (m, 1H). 13C NMR (100 MHz, CDCl3)
d 154.6, 141.3,
135.4,129.3,129.1,125.7,121.8,116.8,116.15,113.9,104.7. HRMS (ESI)
m/z: [MþH]þ calcd for C13H9ClNO2 246.0322; found 246.0320. FTIR
(cmꢁ1) 3123, 2918, 1722, 1710, 1476, 1469, 1393, 1362, 1187.
3-(4-Fluorophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2e):
White solid (45 mg, on 0.25 mmol scale, 78% yield). Rf (Pet. ether/
EtOAc ¼ 85/15): 0.51; 1H NMR (400 MHz, CDCl3)
d 7.71e7.68 (m,
2H), 7.43 (s, 1H), 7.28 (s, 1H), 7.21 (s, 1H), 7.15e7.12 (m, 2H), 6.60 (s,
1H). 13C NMR (100 MHz, CDCl3)
d
163.4 (d, J ¼ 250 Hz), 154.7, 141.5,
126.8 (d, J ¼ 3.4 Hz), 126.5 (d, J ¼ 8.7 Hz), 121.7, 116.2 (d, J ¼ 20 Hz),
116.1, 115.9, 113.8, 104.3. HRMS (ESI) m/z: [MþH]þ calcd for
C
13H9FNO2 230.0612; found 230.0615. FTIR (cmꢁ1) 3111, 2966,
2923, 2841, 1721, 1607, 1473, 1358, 1254, 1187, 1037, 951.
3-(3-Methoxyphenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2f):
White solid (52 mg, on 0.25 mmol scale, 86% yield). Rf (Pet. ether/
EtOAc ¼ 90/10): 0.34; 1H NMR (400 MHz, CDCl3)
d 7.45 (s, 1H),
7.33e7.19 (m, 5H), 6.91 (d, J ¼ 8.2, 1H), 6.58 (s, 1H), 3.84 (s, 3H). 13
C
indicated. Chemical shifts (d) are given in ppm. The residual solvent
signals were used as references and the chemical shifts converted
NMR (100 MHz, CDCl3) d 160.1, 154.8, 141.9, 131.9, 129.9, 121.7, 116.9,
116.6, 115.8, 115.3, 113.7, 109.8, 104.7, 55.5. HRMS (ESI) m/z: [MþH]þ
calcd for C14H12NO3 242.0812; found 242.0817. FTIR (cmꢁ1) 3100,
2965, 2921, 2840, 1730, 1604, 1477, 1363, 1256, 1180, 1037, 951.
3-(3-Bromophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2g):
White solid (49 mg, on 0.25 mmol scale, 67% yield). Rf (Pet. ether/
to the TMS scale (CDCl3: dH ¼ 7.26 ppm, dC ¼ 77.16 ppm). Infrared
(FT-IR) spectra were recorded on a Bruker Alfa FT-IR, n .
-max in cmꢁ1
HRMS (ESI) data were recorded on a Waters Xevo G2-XS Q-TOF
instrument.
4.2. General procedure for the synthesis of pyrrolo-oxazinones
EtOAc ¼ 90/10): 0.47; 1H NMR (400 MHz, CDCl3)
d 7.90 (m, 1H), 7.66
(d, J ¼ 7.8 Hz, 1H), 7.54e7.50 (m, 2H), 7.35e7.29 (m, 2H), 7.23 (m,
To an oven-dried Schlenk reaction vessel equipped with a
magnetic stir bar was taken the 1-(2-oxo-2-phenylethyl)-1H-pyr-
role-2-carbaldehydes 1 (0.25 mmol, 1.0 equiv), triazolium salt B
(11.3 mg, 0.05 mmol), oxidant 3 (153.2 mg, 0.375 mmol). The
mixture was kept under argon atmosphere. To this mixture was
added toluene (2.5 mL), followed by Cs2CO3 (122.2 mg,
0.375 mmol) under a positive pressure of argon and the reaction
mixture was stirred at 30 ꢀC for 12 h. Then, the solvent was evap-
orated and the crude mixture was purified using silica gel flash
column chromatography (eluent: Pet. ether/EtOAc) to afford the
pyrrolo-oxazinone derivatives 2.
1H), 6.64e6.63 (m, 1H). 13C NMR (100 MHz, CDCl3)
d 154.5, 140.8,
132.7, 132.4, 130.5, 127.4, 123.3, 122.9, 121.9, 116.9, 116.3, 114.1, 105.2.
HRMS (ESI) m/z: [MþH]þ calcd for C13H9BrNO2 289.9817; found
289.9819. FTIR (cmꢁ1) 2926,1771,1740,1646,1515,1454,1365,1287.
3-(3-Chlorophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2h):
White solid (52.2 mg, on 0.25 mmol scale, 85% yield). Rf (Pet. ether/
EtOAc ¼ 90/10): 0.38; 1H NMR (400 MHz, CDCl3)
d 7.71 (s, 1H),
7.59e7.57 (m, 1H), 7.48 (s, 1H), 7.38e7.35 (m, 2H), 7.29e7.26 (m,
1H), 7.20 (bs, 1H), 6.61e6.60 (m, 1H). 13C NMR (100 MHz, CDCl3)
d
154.5, 140.9, 135.2, 132.4, 130.3, 129.5, 124.5, 122.4, 121.9, 116.9,
116.3, 114.1, 105.2. HRMS (ESI) m/z: [MþH]þ calcd for C13H9ClNO2
4

A. Ghosh, S. Barik, S. Barik et al.
Tetrahedron 94 (2021) 132330
246.0316; found 246.0322. FTIR (cmꢁ1) 3110, 2966, 2923, 2841,
1731, 1607, 1475, 1361, 1254, 1183, 1037, 949.
(100 MHz, CDCl3)
d
154.4, 138.5, 133.5, 128.1, 126.2, 125.3, 121.7,
116.8, 116.3, 113.8, 103.6. HRMS (ESI) m/z: [MþH]þ calcd for
3-(2-Methoxyphenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2i):
White solid (32 mg, on 0.25 mmol scale, 53% yield). Rf (Pet. ether/
C
11H8NO2S 218.0270; found 218.0274. FTIR (cmꢁ1) 3610, 2958,
2923, 2849, 2364, 1731, 1529, 1517, 1465, 1238, 1080, 852.
EtOAc ¼ 90/10): 0.35; 1H NMR (400 MHz, CDCl3)
d
8.03 (s, 1H), 7.94
7-Iodo-3-phenyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2p):
(d, J ¼ 7.6 Hz, 1H), 7.33 (t, J ¼ 7.7 Hz, 1H), 7.26 (d, J ¼ 3.7 Hz, 1H), 7.18
White solid (44.2 mg, on 0.25 mmol scale, 52% yield). Rf (Pet. ether/
(s, 1H), 7.06 (t, J ¼ 7.5 Hz, 1H), 6.98 (d, J ¼ 8.4 Hz, 1H), 6.57 (m, 1H),
EtOAc ¼ 90/10): 0.48; 1H NMR (400 MHz, CDCl3)
d 7.68 (d,
3.96 (s, 3H). 13C NMR (100 MHz, CDCl3)
d
156.2, 155.2, 138.4, 130.0,
J ¼ 6.9 Hz, 2H), 7.45e7.38 (m, 4H), 7.34 (s, 1H), 7.23 (s, 1H). 13C NMR
127.8, 121.8, 121.1, 119.0, 117.1, 115.4, 113.6, 111.2, 109.2, 55.7. HRMS
(ESI) m/z: [MþH]þ calcd for C14H12NO3 242.0812; found 242.0816.
FTIR (cmꢁ1) 3145, 2961, 2922, 2850, 1734, 1598, 1452, 1388, 1349,
1252, 1183, 1052.
(100 MHz, CDCl3) d 153.2, 142.8, 130.1, 129.9, 129.1, 125.7, 124.6,
122.3, 118.8, 103.5, 66.15. HRMS (ESI) m/z: [MþH]þ calcd for
C
13H9INO2 337.9672; found 337.9674. FTIR (cmꢁ1) 3125, 2956, 2918,
2844, 1711, 1448, 1379, 1341, 1222, 1051, 1024.
3-(3,4-Dimethoxyphenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2j): White solid (56.3 mg, on 0.25 mmol scale, 83% yield). Rf (Pet.
7-Bromo-3-phenyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2q):
White solid (52.1 mg, on 0.25 mmol scale, 72% yield). Rf (Pet. ether/
ether/EtOAc ¼ 90/10): 0.35; 1H NMR (400 MHz, CDCl3)
d
7.38 (s,
EtOAc ¼ 90/10): 0.47; 1H NMR (400 MHz, CDCl3)
7.69 (d, J ¼ 6.1 Hz,
d
1H), 7.25e7.22 (m, 2H), 7.19e7.17 (m, 2H), 6.88 (d, J ¼ 8.3, 1H),
2H), 7.44e7.39 (m, 4H), 7.26e7.25 (m, 1H), 7.18 (s, 1H). 13C NMR
6.57e6.55 (m, 1H), 3.93e3.90 (m, 6H). 13C NMR (100 MHz, CDCl3)
(100 MHz, CDCl3)
d 153.5, 143.0, 130.1, 129.9, 129.1, 124.6, 121.1,
d
154.9, 150.1, 149.2, 142.1, 123.3, 121.5, 117.2, 116.6, 115.5, 113.5,
117.5, 117.2, 103.7, 101.9. HRMS (ESI) m/z: [MþH]þ calcd for
111.2, 107.5, 103.5, 56.1, 55.9. HRMS (ESI) m/z: [MþH]þ calcd for
C
13H9BrNO2 289.9811; found 289.9818. FTIR (cmꢁ1) 3127, 2957,
C
15H14NO4 272.0917; found 272.0923. FTIR (cmꢁ1) 3011, 2965,
2918, 2845, 1710, 1447, 1387, 1256, 1221, 1166, 1049.
2935, 2837, 1731, 1591, 1416, 1331, 1258, 1188, 1021, 952.
3-(3-Bromo-4-fluorophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-
one (2k): White solid (37 mg, on 0.25 mmol scale, 48% yield). Rf
3,7-Diphenyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2r): White
solid (55 mg, on 0.25 mmol scale, 76% yield). Rf (Pet. ether/
EtOAc ¼ 90/10): 0.47; 1H NMR (400 MHz, CDCl3)
7.72 (d, J ¼ 7.2 Hz,
d
(Pet. ether/EtOAc ¼ 85/15): 0.51; 1H NMR (400 MHz, CDCl3)
d
7.90
2H), 7.58e7.54 (m, 3H), 7.47e7.40 (m, 7H), 7.30 (t, J ¼ 7.2 Hz,1H). 13
C
(m, 1H), 7.66 (d, J ¼ 7.8 Hz, 1H), 7.54e7.50 (m, 2H), 7.35e7.29 (m,
NMR (100 MHz, CDCl3) d 154.8, 142.6, 133.2, 130.5, 129.8, 129.6,
2H), 7.23 (m, 1H), 6.64e6.63 (m, 1H). 13C NMR (100 MHz, CDCl3)
129.2, 129.1, 127.5, 125.9, 124.5, 117.9, 117.7, 112.9, 104.3. HRMS (ESI)
m/z: [MþH]þ calcd for C19H14NO2 288.1019; found 288.1021. FTIR
(cmꢁ1) 3108, 2959, 2924, 2851, 1709, 1406, 1222, 1028, 994.
d
159.7 (d, J ¼ 253 Hz), 154.4, 140.2, 129.8, 128.3 (d, J ¼ 4.2 Hz), 125.1
(d, J ¼ 7.5 Hz), 121.9, 117.2 (d, J ¼ 23.6 Hz), 116.8, 116.4, 114.1, 110.1 (d,
J ¼ 23.6 Hz), 104.9. HRMS (ESI) m/z: [MþH]þ calcd for C13H8FBrNO2
307.9722; found 307.9720. FTIR (cmꢁ1) 2920, 1725, 1646, 1513, 1478,
1395, 1362, 1045.
4-Methyl-3-phenyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2s):
White solid (30.0 mg, on 0.25 mmol scale, 53% yield). Rf (Pet. ether/
EtOAc ¼ 90/10): 0.37; 1H NMR (400 MHz, CDCl3)
d 7.55e7.53 (m,
3-(3,5-Difluorophenyl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2l):
White solid (44.0 mg, on 0.25 mmol scale, 71% yield). Rf (Pet. ether/
2H), 7.48e7.42 (m, 3H), 7.33e7.32 (m, 1H), 7.23e7.22 (m, 1H), 6.64
(dd, J1 ¼ 3.9 Hz, J2 ¼ 2.6 Hz, 1H), 2.42 (s, 3H). 13C NMR (100 MHz,
EtOAc ¼ 90/10): 0.17; 1H NMR (400 MHz, CDCl3)
d
7.49 (s, 1H), 7.31
CDCl3) d 155.4, 139.2, 131.8, 129.4, 128.5, 119.4, 117.7, 115.9, 113.6,
(d, J ¼ 3.7 Hz, 1H), 7.26e7.21 (m, 3H), 6.86e6.81 (m, 1H), 6.64e6.62
113.3, 13.6. HRMS (ESI) m/z: [MþH]þ calcd for C14H12NO2 226.0863;
found 226.0871. FTIR (cmꢁ1) 3133, 2959, 2922, 2850, 1721, 1469,
1352, 1304, 1195, 1130, 1038, 739.
(m, 1H). 13C NMR (100 MHz, CDCl3)
d
163.5 (dd, J1 ¼ 249.5 Hz,
J2 ¼ 13.5 Hz), 154.1, 140.0, 131.9, 122.0, 116.9, 116.7, 114.3, 107.4 (dd,
J1 ¼19.6 Hz, J2 ¼ 8.1 Hz),105.1,104.8 (t, J ¼ 25.4 Hz). HRMS (ESI) m/z:
[MþH]þ calcd for C13H8F2NO2 248.0518; found 248.0526. FTIR
(cmꢁ1) 2956, 2917, 2869, 2842, 1725, 1706, 1590, 1448, 1361, 1118,
99, 860, 732.
3-Methyl-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2t): White solid
(20 mg, on 0.25 mmol scale, 54% yield). Rf (Pet. ether/EtOAc ¼ 90/
10): 0.42; 1H NMR (400 MHz, CDCl3)
d
7.20 (d, J ¼ 4.0 Hz, 1H),
7.03e7.02 (m, 1H), 6.79 (s, 1H), 6.51e6.49 (m, 1H), 2.15 (s, 3H). 13
C
3-(Naphthalen-1-yl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2m):
White solid (54.1 mg, on 0.25 mmol scale, 83% yield). Rf (Pet. ether/
NMR (100 MHz, CDCl3) d 155.6,141.2,120.8,116.6,115.3,112.9,105.0,
16.9. HRMS (ESI) m/z: [MþH]þ calcd for C8H8NO2 150.0555; found
150.0558. FTIR (cmꢁ1) 2958, 1726, 1687, 1533, 1481, 1353, 1219.
2-Methyl-3-phenylpyrrolo[1,2-a]pyrazin-1(2H)-one (5a) [16]:
EtOAc ¼ 90/10): 0.40; 1H NMR (400 MHz, CDCl3)
d 8.18e8.16 (m,
1H), 7.94e7.88 (m, 2H), 7.62 (d, J ¼ 7.0 Hz, 1H), 7.56e7.46 (m, 3H),
7.32 (d, J ¼ 3.7 Hz, 1H), 7.26 (s, 1H), 7.20 (s, 1H), 6.63e6.61 (m, 1H).
To
a
dry Schlenk tube containing compound 2a (26.4 mg,
L, 0.5 mmol) was added. The
13C NMR (100 MHz, CDCl3)
d
155.2, 142.6, 133.8, 131.2, 130.7, 128.7,
0.125 mmol) 40% aqueous MeNH2 (55 m
128.5, 127.8, 127.2, 126.5, 125.1, 121.6, 116.9, 115.8, 113.6, 108.3.
HRMS (ESI) m/z: [MþH]þ calcd for C17H12NO2 262.0863; found
262.0865. FTIR (cmꢁ1) 3050, 2953, 2920, 2851, 1730, 1601, 1430,
1390, 1280, 1207, 1180, 952.
3-(Furan-2-yl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one (2n) [4a]:
White solid (25.2 mg, on 0.25 mmol scale, 50% yield). Rf (Pet. ether/
resulting mixture was allowed to stir overnight at 30 ꢀC. Then the
solution was concentrated under reduced pressure and the residue
was purified by flash column chromatography using silica gel and
petroleum ether/EtOAc (70:30) as eluent. The aminal thus formed
was treated with catalytic PTSA (1.0 mg) in benzene (1.0 mL) and
the mixture was heated under reflux for 1 h. The reaction mixture
was cooled, and the solvent was evaporated and the residue was
purified by flash column chromatography on silica gel using pe-
troleum ether/EtOAc (80:20) to afford the 2-methyl-3-
EtOAc ¼ 90/10): 0.53; 1H NMR (400 MHz, CDCl3)
d 7.43 (s, 2H), 7.28
(d, J ¼ 3.9 Hz, 1H), 7.17 (s, 1H), 6.80 (d, J ¼ 3.3 Hz, 1H), 6.58 (t,
J ¼ 3.2 Hz, 1H), 6.50 (m, 1H). 13C NMR (100 MHz, CDCl3)
d 154.3,
145.4,143.2,135.6, 121.9, 116.8, 116.5, 113.8,112.1, 109.1, 103.3. HRMS
(ESI) m/z: [MþH]þ calcd for C11H8NO3 202.0499; found 202.0504.
FTIR (cmꢁ1) 3610, 3104, 2957, 2921, 2365, 1727, 1571, 1495, 1348,
1194, 1107, 873.
phenylpyrrolo[1,2-a]pyrazin-1(2H)-one 5a as
a white solid
(19 mg, 67% yield). Rf (Pet. ether/EtOAc ¼ 70/30): 0.48; 1H NMR
(400 MHz, CDCl3) d 7.46e7.43 (m, 3H), 7.39e7.36 (m, 2H), 7.11e7.10
(m, 1H), 7.07e7.06 (m, 1H), 6.89 (m, 1H), 6.57e6.55 (m, 1H), 3.24 (s,
3-(Thiophen-2-yl)-1H-pyrrolo[2,1-c][1,4]oxazin-1-one
(2o):
3H). 13C NMR (100 MHz, CDCl3)
d 157.1, 133.2, 130.4, 129.5, 129.2,
White solid (24.6 mg, on 0.25 mmol scale, 45% yield). Rf (Pet. ether/
128.8, 123.5, 118.1, 112.7, 109.9, 107.9, 32.1. HRMS (ESI) m/z: [MþH]þ
calcd for C14H13N2O4 225.1028; found 225.1026. FTIR (cmꢁ1) 2920,
2367, 1668, 1627, 1473, 1424, 1366, 1292.
EtOAc ¼ 90/10): 0.40; 1H NMR (400 MHz, CDCl3)
d 7.45 (d,
J ¼ 3.5 Hz,1H), 7.36 (s,1H), 7.33 (d, J ¼ 5.0 Hz,1H), 7.28 (d, J ¼ 3.8 Hz,
1H), 7.16 (s, 1H), 7.09e7.07 (m, 1H), 6.59e6.57 (m, 1H). 13C NMR
N-Benzyl-1-(2-oxo-2-phenylethyl)-1H-pyrrole-2-carboxamide
5

A. Ghosh, S. Barik, S. Barik et al.
Tetrahedron 94 (2021) 132330
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(6a): To a dry Schlenk tube containing compound 2a (26.4 mg,
0.125 mmol) was added BnNH2 (68.3 mg, 0.625 mmol). The
resulting mixture was allowed to stir for 24 h at 60 ꢀC. Then the
residue was purified by flash column chromatography on silica gel
using petroleum ether/EtOAc (75:25) to afford the N-benzyl-1-(2-
oxo-2-phenylethyl)-1H-pyrrole-2-carboxamide 6a as a white solid
(25 mg, 63% yield). Rf (Pet. ether/EtOAc ¼ 70/30): 0.41; 1H NMR
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(400 MHz, CDCl3)
d
8.04 (d, J ¼ 7.5 Hz, 2H), 7.64e7.60 (m, 1H),
7.53e7.49 (m, 2H), 7.37e7. 26 (m, 5H), 6.80 (m, 1H), 6.68e6.66 (m,
1H), 6.29e6.23 (m, 2H), 5.90 (s, 2H), 4.49 (d, J ¼ 5.7 Hz, 2H). 13C
NMR (100 MHz, CDCl3)
d 194.0, 161.8, 138.5, 135.1, 133.8, 128.9,
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128.8, 128.7, 128.4, 128.2, 127.8, 127.5, 126.7, 125.4, 112.2, 108.2, 55.3,
43.3. HRMS (ESI) m/z: [MþH]þ calcd for C20H19N2O2 319.1447;
found 319.1443. FTIR (cmꢁ1) 2923, 2370, 1696, 1644, 1628, 1545,
1513, 1461, 1225, 1115.
Methyl 1-(2-oxo-2-phenylethyl)-1H-pyrrole-2-carboxylate (7a)
[17] To a dry Schlenk tube containing compound 2a (26.4 mg,
0.125 mmol), MeOH (3.0 mL) and DMAP (15.3 mg, 0.125 mmol)
were added. The resulting mixture was allowed to stir overnight at
60 ꢀC. Then the solution was concentrated under reduced pressure
and the residue was purified by flash column chromatography on
silica gel using petroleum ether/EtOAc (90:10) to afford the methyl
1-(2-oxo-2-phenylethyl)-1H-pyrrole-2-carboxylate 7a as a white
solid (21.6 mg, 71% yield). Rf (Pet. ether/EtOAc ¼ 80/20): 0.48; 1H
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NMR (400 MHz, CDCl3)
7.53e7.49 (m, 2H), 7.06e7.05 (m, 1H), 6.85e6.84 (m, 1H), 6.27e6.25
(m, 1H), 5.76 (s, 2H), 3.72 (s, 3H). 13C NMR (100 MHz, CDCl3)
193.5,
d
8.01 (d, J ¼ 7.5 Hz, 2H), 7.64e7.60 (m, 1H),
d
161.9, 134.9, 133.8, 129.9, 128.9, 128.1, 122.3, 118.4, 108.8, 55.2, 51.2.
HRMS (ESI) m/z: [MþNa]þ calcd for C14H13NO3Na 266.0793; found
266.0790. FTIR (cmꢁ1) 2921, 2346, 1701, 1574, 1532, 1438, 1406,
1330, 1224.
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A.T. Biju, Angew. Chem. Int. Ed. 60 (2021) 13712;
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
(b) S. Mondal, S.R. Yetra, S. Mukherjee, A.T. Biju, Acc. Chem. Res. 52 (2019)
425;
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Acknowledgments
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Generous financial support by the Science and Engineering
Research Board (SERB), Government of India (File Number: EMR/
2016/007021), is gratefully acknowledged. A. G. thanks CSIR (for
SRF), Sh. B., So. B. and S. S. thank IISc for the fellowships.
(b) C.-L. Zhang, Y.-Y. Gao, H.-Y. Wang, B.-A. Zhou, S. Ye, Angew. Chem., Int. Ed.
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(c) J. Jin, X. Huang, J. Xu, T. Li, X. Peng, X. Zhu, J. Zhang, Z. Jin, Y.R. Chi, Org. Lett.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.132330.
(d) Z. Chen, X. Kong, S. Niu, S. Yang, J. Liu, B. Chen, B. Luo, C. Zhou, C. Ding,
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