Synthesis of pyrrolin-4-ones by Pt-catalyzed cycloisomerization in PEG under microwaves

Article abstract of DOI:10.1021/jo302474r

The unprecedented eco-friendly Pt-catalyzed 5-endo-dig cycloisomerization of readily available α-amino ynones is reported under microwave irradiation in PEG-3400 as reaction matrix. The corresponding pure pyrrolin-4-ones were obtained in excellent yields (80-98%) directly after a straightforward precipitation-filtration workup, thus avoiding any chromatographic purification. The catalytic system was recycled and the chiral purity of all the products was also investigated.

Full text of DOI:10.1021/jo302474r

Note
pubs.acs.org/joc
Synthesis of Pyrrolin-4-ones by Pt-Catalyzed Cycloisomerization in
PEG under Microwaves
Rosella Spina,†,‡ Evelina Colacino,† Bartolo Gabriele,‡ Giuseppe Salerno,§ Jean Martinez,†
and Frederic Lamaty*,†
́
́
†Institut des Biomolec
Bataillon, 34095 Montpellier Cedex 5, France
‡Dipartimento di Scienze Farmaceutiche and §Dipartimento di Chimica, Universita
Italy
́
ules Max Mousseron (IBMM), UMR 5247 CNRS - UM I-UM II, Universite
́
de Montpellier II, Place E.
̀
della Calabria, 87036 Arcavacata di Rende (CS),
S
* Supporting Information
ABSTRACT: The unprecedented eco-friendly Pt-catalyzed 5-
endo-dig cycloisomerization of readily available α-amino ynones is
reported under microwave irradiation in PEG-3400 as reaction
matrix. The corresponding pure pyrrolin-4-ones were obtained in
excellent yields (80−98%) directly after a straightforward
precipitation−filtration workup, thus avoiding any chromato-
graphic purification. The catalytic system was recycled and the
chiral purity of all the products was also investigated.
he field of environmentally friendly (or green) chemistry
where ynones 1 were used as substrates in toxic mercury(II)- or
gold(III)-catalyzed heterocyclization in volatile organic solvents
under reflux.9−11 In most of the cases, the yields were good, but
the enantiomeric excess was not always satisfying and no
recycling experiments of the catalyst were performed. In order
to alleviate the potential loss of chiral integrity, we initiated a
study with metals other than mercury or gold. α-Amino ynone
1a3g served as a model to study the cycloisomerization reaction
using various PEG-3400/metal catalyst associations under
microwaves in order to compare conversions, yields, and chiral
integrity (Table 1). Microwave activation (monomode waves)
is particularly adapted in the reactions using solid PEG since
the focused heating generated by the microwaves together with
the good absorption of the PEG quickly provides a liquid-phase
environment once PEG is melted (above 50 °C).2b The
viscosity of the reaction medium remains high, but microwaves
provide a homogeneous heating.
T
is currently attracting increasing attention.1 One im-
portant area in current chemical science is to develop mild and
sustainable processes to prepare organic molecules. Previous
work developed in our laboratory made use of poly(ethylene
glycol) (PEG)2 as an alternative and eco-compatible solvent
under microwave irradiation.3 In the past decades, the use of
transition-metal catalysis to provide new reactivities of organic
substrates has emerged.4 The metal-catalyzed cycloisomeriza-
tion is now commonly used for the synthesis of numerous
families of heterocycles,5 and several strategies have been also
developed in our laboratories.3f,g,6
Starting from α-amino ynones 1, easily obtained from the
addition to organolithium reagents to urethane-protected N-
carboxyanhydrides3g (UNCAs), pyrrolin-4-ones 2 were synthe-
tized by an unprecedented platinum-catalyzed cycloisomeriza-
tion reaction under mild conditions (Scheme 1).
From the various metals which were studied (Table 1), the
complete conversion of substrate 1a was possible only when
PtCl2 was the catalyst, leading to pyrrolinone 2a with an
excellent yield after 30 min. The amount of catalyst was
reduced from 5 to 1 mol % (Table 1, entry 5) resulting in an
increase of the yield of the reaction. Unfortunately, it was
difficult to reproduce the results with such a small amount of
catalyst, and the study was carried out on a 5 mol % basis. In
both cases, however, the enantiomeric ratio was affected,
dropping from 90/10 of the starting material to 68/32 (Table
1, entries 5 and 6). The epimerization could be due to the
keto−enol equilibrium favored by the presence of HCl released
during the reaction.11,12 In order to neutralize the acid formed
Scheme 1. Platinum-Catalyzed Cycloisomerization Reaction
of 1 Using PEG-3400 as a Green Solvent under Microwaves
A pyrrolinone scaffold similar to 2 is present in diverse
pharmaceutically active compounds,7 and various families of
nitrogen-containing heterocycles8 can be easily obtained by
simple transformation of α-amino ynones 1. However,
straightforward procedures to pyrrolinones were poorly
investigated in the past, and only rare examples are reported
Received: November 11, 2012
Published: February 1, 2013
© 2013 American Chemical Society
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The Journal of Organic Chemistry
Note
optimized conditions to afford the corresponding pyrrolinones
2a−h always in excellent isolated yields (90−98%) and with
good to excellent preservation of the enantiomeric ratio (Table
3). As described previously in the literature,11 the epimerization
was very limited for the hindered substrates (Table 3, entries
1−3). In the case of less hindered substrates, epimerization
occurred but was restrained compared to the one observed with
a gold catalyst.11
Table 1. Screening of Different Catalysts for the
Cycloisomerization Reaction of 1a
a
b
c
d
entry
catalyst (mol %)
conversion (%)
yield (%)
er (%)
The recyclability of the catalytic system PEG-3400/PtCl2/
K2CO3 was also investigated for the first time in this type of
transformation. Two sets of experiments were carried out on
two different α-amino ynones 1b and 1c. After a first run
(performed as described in Table 2) on each of the starting
materials (Table 4, entries 1 and 3), the valine-based
pyrrolinones 2b and 2c were obtained in very good yield and
retention of stereochemistry after precipitation. The precipitate
recovered after the precipitation−filtration workup for each
experiment was reused3 in the next runs (Table 4, entries 2 and
4) by charging the reaction vessel with 1b or 1c, respectively, to
provide again 2b or 2c, respectively, with high yields and
preservation of the starting optical purity. One more run could
be efficiently performed on 1c (Table 4, entry 5) but not on 1b,
which remained unreacted in the next cycle.
In conclusion, we disclose herein a new route to the synthesis
of pyrrolin-4-ones using unprecedented Pt-catalyzed 5-endo-dig
cycloisomerization of readily available α-amino ynones 13g in
PEG-3400 as an alternative and eco-friendly solvent under
microwave irradiation. The final products were always
recovered in excellent yields and purity without any time- or
solvent-consuming column chromatography after a simple
precipitation−filtration workup that allowed catalytic system
recovering and recycling.
e
1
−
0
0
0
0
−
−
57/43
2
3
4
5
6
CuO (5)
CuCl2 (5)
PdI2(2)
PtCl2(1)
PtCl2 (5)
90
75
30
99
21
nd
100
100
68/32
68/32
95
a
b
Enantiomeric ratio (er) of 1a is 90/10. Conversions were
c
determined by HPLC. Yields were calculated by 1H NMR using
d
CH2Br2 as an internal standard. Enantiomeric ratio (er) was
determined by chiral HPLC analyses. Reaction was carried out for
60 min.
e
in situ and to slow down the epimerization process, different
combinations of platinum(II) dichloride and various bases were
tested, and a selection of results is summarized in Table 2.
Table 2. PtCl2-Catalyzed Cycloisomerization Reaction of α-
Amino Ynone 1a in Basic Medium
a
a
b
T
(°C)
time
(min)
er 1a
(%)
er 2a
(%)
yield
entry
base
(%)
EXPERIMENTAL SECTION
■
1
2
3
4
Cs2CO3
NaHCO3
Et3N
70
70
70
70
80
80
70
60
30
60
60
60
30
30
90/10
90/10
90/10
90/10
90/10
90/10
90/10
74/26
80/20
79/21
59/41
−
81
87
83
54
0
Typical Procedure for the PtCl2-Catalyzed Heterocyclization
of α-Amino Ynones 1a−h. A mixture of PtCl2 (1.3 mg, 0.005
mmol), K2CO3 (1.4 mg, 0.01 mmol), PEG-3400 (400 mg), and α-
amino ynones 13g (0.1 mmol) placed in a sealed vessel was heated at
70 °C under microwave irradiation (initial power 400 W) for 30 min.
The reaction mixture was solubilized in a small amount of CH2Cl2 (2.0
mL) and precipitated in Et2O (250 mL). After 3 h at −18 °C, filtration
of the catalytic system (PEG-3400/Pt/base) and evaporation of
diethyl ether afforded pure product 2. When necessary, an aqueous
washing to remove traces of PEG was performed.
Typical Procedure to Recycle the Catalytic System. α-Amino
ynone 1 (0.1 mmol) was added to the precipitate PEG-3400/Pt/base
obtained after the precipitation−filtration workup from a previous
experiment (Table 4) and placed in a sealed vessel then heated up
using microwave irradiation (initial power 400W) at 70 °C (the
instrument adjusted the heating power to keep this temperature
constant) for the indicated time (Table 4), to afford pure product 2
after precipitation/filtration workup as previously described.3
2-Isopropyl-3-oxo-5-phenyl-2,3-dihydropyrrole-1-carbox-
ylic Acid tert-Butyl Ester (2a).
t-BuOK
K2CO3
K2CO3
K2CO3
c
5
6
7
86/14
70
90
89/11
a
Enantiomeric ratio (er) was determined by chiral HPLC analyses.
b
Yields were calculated by 1H NMR using CH2Br2 as an internal
c
standard. The reaction was carried out in the absence of catalyst.
When Cs2CO3, Et3N, or t-BuOK (Table 2, entries 1, 3, and
4) was used, the reaction time was longer and full conversion of
substrate 1a was complete after 60 min. Furthermore, the
enantiomeric excess of product 2a was decreased (18−60%),
while in the absence of catalyst the cyclization did not take
place (Table 2, entry 5). Under the optimized reaction
conditions (Table 2, entry 7), the starting enantiomeric ratio
in 1a was preserved in the final product 2a, which could be
easily recovered as a pure compound after a practical and
simple precipitation/filtration workup. The crude was dissolved
in a small amount of dichloromethane and then precipitated in
Et2O. After filtration to separate PEG-based solids, pure
product 2a was recovered in 94% yield after evaporation of
the filtrate. In some cases, an aqueous washing to remove traces
of PEG was necessary. Compared to other procedures,11 no
column chromatography was required decreasing the use of
organic solvent (limited to the precipitation procedure).
Various α-amino ynones 1a−h were reacted under the
Starting from 1a (24.1 mg, 0.08 mmol), 22.6 mg was obtained (94%
isolated yield): CAS Registry No. 1169845-39-9; yellow solid; mp 65−
68 °C (lit.11 70−72 °C; ee 90%; H NMR (CDCl3, 400 MHz) δ
1
(ppm) 7.36−7.34 (m, 5H), 5.53 (s, 1H), 4.11 (d, 1H, J = 3.5 Hz), 2.54
(septuplet, 1H, J = 3.5 Hz), 1.16 (s, 9H), 1.10 (d, 3H, J = 7.0 Hz), 0.95
(d, 3H, J = 7.0 Hz); 13C NMR (CDCl3, 100 MHz) δ (ppm) 201.1,
172.9, 150.3, 133.1, 130.2, 128.0, 127.0, 113.6, 82.6, 71.5, 32.1, 27.6,
17.2, 17.1; ESIMS m/z 302.1 (M + H)+, 246.1 (M + H − t-Bu)+, 603.3
2699
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Note
Table 3. Pt-Catalyzed Cycloisomerization of α-Amino Ynone Derivatives under Microwave Irradiation in PEG-3400
a
b
Enantiomeric ratio (er) was determined by chiral HPLC analysis. The reaction was carried out in the absence of base.
(2M + H)+; HRMS (ESI) calcd for C18H24NO3 (M + H)+ 302.1756,
found 302.1748; HPLC Chiralpak AD-H, 2-propanol/hexane = 2/98,
flow rate 1.0 mL/min, λ = 214 nm, tmajor = 9.333 min, tminor = 8.417
min.
5-Butyl-2-isopropyl-3-oxo-2,3-dihydropyrrole-1-carboxylic
Acid tert-Butyl Ester (2b).
H)+, 304.2 (M + Na)+, 226.2 (M + H − t-Bu)+, 563.3 (2M + Na)+;
HRMS (ESI) calcd for C16H28NO3 (M + H)+ 282.2069, found
282.2063; HPLC Chiralpak AD-H, 2-propanol/hexane = 2/98, flow
rate 1.0 mL/min, λ 214 nm, tmajor = 6.833 min, tminor = 4.817 min.
5-Cyclopropyl-2-isopropyl-3-oxo-2,3-dihydropyrrole-1-car-
boxylic Acid tert-Butyl Ester (2c).
Starting from 1b (22.5 mg, 0.08 mmol), 21.1 mg was obtained (95%
isolated yield): yellow oil, ee 97%; H NMR (CDCl3, 400 MHz) δ
Starting from 1c (26.5 mg, 0.1 mmol), 26.0 mg was obtained (98%
isolated yield): yellow solid; mp 70−72 °C, ee 95%; 1H NMR (CDCl3,
400 MHz) δ (ppm) 5.01 (s, 1H), 3.98 (d, 1H, J = 3.1 Hz), 2.74−2.65
(m, 1H), 2.52−2.42 (m, 1H), 1.53 (s, 9H), 1.17−1.13 (m, 2H), 1.14
(d, 3H, J = 7.0 Hz), 0.84−0.75 (m, 2H), 0.81 (d, 3H, J = 7.0 Hz); 13C
NMR (CDCl3, 100 MHz) δ (ppm) 200.3, 179.7, 149.9, 105.0, 82.7,
70.8, 30.9, 28.2, 17.5, 16.1, 11.4, 11.3, 11.2; ESIMS m/z 266.2 (M +
1
(ppm) 5.34 (s, 1H), 3.96 (d, 1H, J = 3.4 Hz), 2.97−2.76 (m, 2H),
2.50−2.45 (m, 1H), 1.60−1.40 (m, 4H), 1.47 (s, 9H), 1.15 (d, 3H, J =
7.1 Hz), 0.95 (t, 3H, J = 7.0 Hz), 0.83 (d, 3H, J = 7.0 Hz); 13C NMR
(CDCl3, 100 MHz) δ (ppm) 200.5, 177.0, 149.4, 110.1, 82.6, 70.2,
31.0, 30.6, 29.7, 28.0, 22.4, 17.3, 15.9, 13.7; ESIMS m/z 282.2 (M +
2700
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Note
Table 4. Recycling of the Catalytic System
a
b
c
Enantiomeric ratio (er) was determined by chiral HPLC analyses. Conversion was determined by HPLC. Isolated yields.
H)+, 210.2 (M + H − t-Bu)+, 531.4 (2M + H)+; HRMS (ESI) calcd for
C15H24NO3 (M + H)+ 266.1756, found 266.1761; HPLC Chiralpak
AD-H, 2-propanol/hexane = 2/98, flow rate 1.0 mL/min, λ = 214 nm,
tmajor = 12.6 min, tminor = 9.083 min.
2-Methyl-3-oxo-5-phenyl-2,3-dihydropyrrole-1-carboxylic
Acid tert-Butyl Ester (2d).
(s, 9 Hz), 1.45 (d, 3H, J = 7.0 Hz), 1.21−1.16 (m, 2H), 0.87−0.82 (m,
2H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 200.6, 179.4, 149.6,
102.4, 82.8, 63.2, 28.2, 17.3, 11.9, 11.5, 11.1; ESIMS m/z 238.1 (M +
H)+, 260.2 (M + Na)+, 182.1 (M + H − t-Bu)+; HRMS (ESI) calcd for
C13H20NO3 (M + H)+: 238.1443, found 238.1442; HPLC Chiralcel
OD-H, 2-propanol/hexane = 2/98, flow rate 1.0 mL/min, λ = 214 nm,
tmajor = 10.683 min, tminor = 9.317 min.
2-Benzyl-5-cyclopropyl-3-oxo-2,3-dihydropyrrole-1-carbox-
ylic Acid tert-Butyl Ester (2g).
Starting from 1d (27.4 mg, 0.1 mmol), 24.7 mg was obtained (90%
isolated yield): CAS Registry No. 1169845-42-4; yellow solid; mp 80−
85 °C (lit.11 90−92 °C); ee 48%; H NMR (CDCl3, 400 MHz) δ
1
(ppm) 7.46−7.41 (m, 5H), 5.58 (s, 1H), 4.24 (q, 1H, J = 7.1 Hz), 1.61
(d, 3H, J = 7.1 Hz), 1.29 (s, 9H); 13C NMR (CDCl3, 100 MHz) δ
(ppm) 201.0, 172.1, 149.7, 132.8, 130.2, 127.9, 127.4, 111.2, 82.8, 63.6,
27.8, 17.6; ESIMS m/z 274.2 (M + H)+, 296.1 (M + Na)+, 218 (M +
H − t-Bu)+, 174.2 (M + H − Boc)+; HRMS (ESI) calcd for
C16H20NO3 (M + H)+ 274.1443, found 274.1431; HPLC Chiralpak
AD-H, 2-propanol/hexane = 2/98, flow rate 1.0 mL/min, λ = 214 nm,
tmajor = 11.483 min, tminor = 10.917 min.
Starting from 1g (25.0 mg, 0.08 mmol), 22.5 mg was obtained (90%
isolated yield): yellow oil, ee 12%; H NMR (CDCl3, 400 MHz) δ
1
(ppm) 7.20−7.17 (m, 3H), 7.07−7.04 (m, 2H), 4.84 (s, 1H), 4.3 (dd,
1H, J = 2.6, 6.1 Hz), 3.44 (dd, 1H, J = 6.1, 13.4 Hz), 3.24 (dd, 1H, J =
2.6, 13.4 Hz), 2.48−2.42 (m, 1H), 1.61 (s, 9H), 1.02−0.93 (m, 1H),
0.96−0.84 (m, 1H), 0.62−0.59 (m, 1H), 0.15−0.07 (m, 1H); 13C
NMR (CDCl3, 100 MHz) δ (ppm) 199.7, 180.0, 149.6, 134.5, 129.7,
127.9, 126.8, 105.3, 83.0, 67.1, 36.8, 28.3, 11.3, 10.9, 9.9; ESIMS m/z
314.1 (M + H)+, 336.0 (M + Na)+, 258.1 (M + H − t-Bu)+, 627.2 (2M
+ H)+, 649.2 (2M + Na)+; HRMS (ESI) calcd for C19H24NO3 (M +
H)+ 314.1756, found 314.1762; HPLC Chiralcel AD-H, 2-propanol/
hexane = 2/98, flow rate 1.0 mL/min, λ = 214 nm, tmajor = 16.443 min,
tminor = 12.917 min.
5-Butyl-2-methyl-3-oxo-2,3-dihydropyrrole-1-carboxylic
Acid tert-Butyl Ester (2e).
2-Benzyl-5-(4-dimethylaminophenyl)-3-oxo-2,3-dihydropyr-
role-1-carboxylic Acid tert-Butyl Ester (2h).
Starting from 1e (30.1 mg, 0.1 mmol), 27.0 mg was obtained (90%
isolated yield): yellow oil, ee 26%; H NMR (CDCl3, 400 MHz) δ
1
(ppm) 5.37 (s, 1H), 4.01 (q, 1H, J = 7.0 Hz), 2.92 (m, 2H), 1.60−1.39
(m, 4H), 1.49 (s, 9H), 1.40 (d, 3H, J = 7.0 Hz), 0.95 (t, 3H, J = 7.0
Hz); 13C NMR (CDCl3, 100 MHz) δ (ppm) 201.1, 176.7, 149.2,
107.9, 82.7, 62.7, 30.9, 29.5, 28.2, 22.4, 17.3, 13.8; ESIMS m/z 254.1
(M + H)+, 276.1 (M + Na)+, 198.1 (M + H − t-Bu)+, 507.2 (2M +
H)+; HRMS (ESI) calcd for C14H24NO3 (M + H)+: 254.1756, found
254.1759. HPLC Chiralcel OD-H, 2-propanol/hexane = 2/98, flow
rate 1.0 mL/min, λ = 214 nm, tmajor = 7.7 min, tminor = 6.983 min.
5-Cyclopropyl-2-methyl-3-oxo-2,3-dihydropyrrole-1-car-
boxylic Acid tert-Butyl Ester (2f).
Starting from 1h (21.0 mg, 0.5 mmol), 17.7 mg was obtained (90%
isolated yield): orange oil; ee 57%; H NMR (CDCl3, 300 MHz) δ
1
(ppm) 7.20−7.16 (m, 5H), 7.02 (d, 2H, J = 8.9 Hz), 6.58 (d, 2H, J =
8.9 Hz), 5.35 (s, 1H), 4.42 (dd, 1H, J = 2.8, 6.9 Hz), 3.54 (dd, 1H, J =
6.3, 13.3 Hz), 3.35 (dd, 1H, J = 2.8, 13.3 Hz), 2.98 (s, 6H), 1.43 (s,
9H); 13C NMR (CDCl3, 75 MHz) δ (ppm) 199.2, 174.0, 151.9, 150.2,
135.1, 129.9, 128.0, 126.8, 118.9, 111.1, 110.5, 82.5, 67.6, 40.1, 37.5,
28.0; ESIMS m/z 393.2 (M + H)+, 337.1 (M + H − t-Bu)+; HRMS
(ESI) calcd for C24H29N2O3 (M + H)+ 393.2178, found 393.2168;
HPLC Chiralpak AD-H, 2-propanol/hexane = 10/90, flow rate 1.0
mL/min, λ = 214 nm, tmajor = 9.667 min, tminor = 8.45 min.
Starting from 1f (23.7 mg, 0.1 mmol), 23.4 mg was obtained (98%
1
isolated yield): yellow oil, ee 72%; H NMR (CDCl3, 400 MHz) δ
(ppm) 5.02 (s, 1H), 4.02 (q, 1H, J = 7.0 Hz), 2.83−2.74 (m, 1H), 1.54
2701
dx.doi.org/10.1021/jo302474r | J. Org. Chem. 2013, 78, 2698−2702

The Journal of Organic Chemistry
Note
Rhodes, A. J.; Sarpong, R. Tetrahedron 2008, 64, 7008−7014.
(d) Palacios, F.; Alonso, C.; Rubiales, G.; Villegas, M. Tetrahedron
2009, 65, 1119−1124.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra and chiral HPLC chromatograms for
compounds 2a−h. This material is available free of charge via
the Internet at http://pubs.acs.org.
(11) Gouault, N.; Le Roch, M.; Cornee, C.; David, M.; Uriac, P. J.
Org. Chem. 2009, 74, 5614−5617.
(12) Capon, B.; Kwok, F. C. J. Am. Chem. Soc. 1989, 111, 5346−
5356.
AUTHOR INFORMATION
Corresponding Author
*Tel: +33 (0)4 67 14 38 77. Fax: +33 (0)4 67 14 48 66. E-mail:
frederic.lamaty@univ-montp2.fr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the CNRS and the MESR for financial support. R.S.
■
is grateful to Universita
fellowship. We thank ISOCHEM (Vert le Petit, France) for a
gift of UNCAs.
̀
della Calabria (Italy) for a Ph.D.
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