Synthesis of cyclopentene-fused pyrroloisoquinolinone derivative via N-acyliminium ion cyclization

Article abstract of DOI:10.1081/SCC-120003411

In this research, a new class of pyrrolo[2,1-a]isoquinolinone derivative 7 was prepared. C-4 Hydroxylated 5-ethoxylactam 1b gave 4,5-epoxylactam 2 in high yield instead of isoquinoline derivative 3b under the N-acyliminium ion cyclization condition. The 4

Full text of DOI:10.1081/SCC-120003411

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SYNTHESIS OF CYCLOPENTENE-FUSED
PYRROLOISOQUINOLINONE DERIVATIVE VIA N-
ACYLIMINIUM ION CYCLIZATION
Dong Jin Hwang a , Soo Sung Kang a , Jae Yeol Lee a , Jung Hoon Choi b , Hokoon Park a &
Yong Sup Lee c
a Division of Life Sciences , Korea Institute of Science and Technology , P.O. Box 131,
Cheongryang, Seoul, 130-650, Korea
b Department of Chemistry , Hanyang University , Seoul, 133-791, Korea
c Division of Life Sciences , Korea Institute of Science and Technology , P.O. Box 131,
Cheongryang, Seoul, 130-650, Korea
Published online: 16 Aug 2006.
To cite this article: Dong Jin Hwang , Soo Sung Kang , Jae Yeol Lee , Jung Hoon Choi , Hokoon Park & Yong Sup Lee (2002)
SYNTHESIS OF CYCLOPENTENE-FUSED PYRROLOISOQUINOLINONE DERIVATIVE VIA N-ACYLIMINIUM ION CYCLIZATION, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:16, 2499-2505, DOI:
10.1081/SCC-120003411
To link to this article: http://dx.doi.org/10.1081/SCC-120003411
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 16, pp. 2499–2505, 2002
SYNTHESIS OF CYCLOPENTENE-FUSED
PYRROLOISOQUINOLINONE
DERIVATIVE VIA N-ACYLIMINIUM
ION CYCLIZATION
Dong Jin Hwang,1,2 Soo Sung Kang,1,2 Jae Yeol Lee,1
Jung Hoon Choi,2 Hokoon Park,1 and Yong Sup Lee1,
*
1Division of Life Sciences, Korea Institute of Science
and Technology, P.O. Box 131, Cheongryang,
Seoul 130-650, Korea
2Department of Chemistry, Hanyang University,
Seoul 133-791, Korea
ABSTRACT
In this research, a new class of pyrrolo[2,1-a]isoquinolinone
derivative 7 was prepared. C-4 Hydroxylated 5-ethoxylactam
1b gave 4,5-epoxylactam 2 in high yield instead of isoquino-
line derivative 3b under the N-acyliminium ion cyclization
condition. The 4,5-epoxylactam 2 was converted to another
N-acyliminium ion precursor 6 through base-promoted
epoxide ring opening, silylation, and thermal Diels–Alder
reaction with cyclopentadiene in high yield. Finally,
compound 6 was cyclized in the presence of TiCl4 to provide
cyclopentene-fused pyrroloisoquinoline derivative 7.
*Corresponding author. E-mail: yslee@kist.re.kr
2499
DOI: 10.1081/SCC-120003411
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2500
HWANG ET AL.
INTRODUCTION
For many researchers, the N-acyliminium ion has been used as a
reactive intermediate for efficient synthetic method, in particular for
carbon–carbon bond formation.[1] We also recently reported synthetic
routes to several heterocyclic systems such as azepinoindole, quinolizi-
dine and indolizidine rings through the N-acyliminium ion cyclization
strategy.[2] In our previous synthesis of pyrroloisoquinoline derivatives via
N-acyliminium ion cyclization of 5-ethoxy or 5-hydroxylactams (for ex-
ample, 1a) derived from L-malic acid and L-tartaric acid, the protection of
C-4 hydroxyl group was important in getting high yields of cyclized prod-
ucts (Scheme 1).[3]
In case of refluxing 4-acetoxy-5-hydroxylactam 1a in formic acid, the
cyclized product 3a was obtained in high yield (74%) whereas the cyclized
product 3b was obtained only in less than 20% yield when 4-hydroxy-5-
ethoxylactam 1b was used in the same reaction condition. We presumed
that the low yield of the desired cyclized product was due to the formation
of less reactive 4,5-epoxylactam 2 as an intermediate. Herein we wish
to report the synthesis and potential utility of 2 by the synthesis of cyclo-
pentene-fused pyrroloisoquinoline derivative 7.
Scheme 1.

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PYRROLOISOQUINOLINONE DERIVATIVE
RESULTS AND DISCUSSION
2501
During the investigation of cyclization of 1b, we found that 4,5-epoxy-
lactam 2 could be obtained in high yield by the treatment of 1b with
BF3 ꢀ Et2O at room temperature (Scheme 1). The presence of epoxide ring
in 2 was proved by the ready conversion to the starting 5-ethoxylactam 1b
upon treatment of 1 N HCl in ethanol. The 4,5-epoxylactam 2 can also
be transformed to the cyclized product 3b in 33% yield when heated at
reflux in formic acid. To examine the synthetic utility of 2, the openings
of epoxide ring by several nucelophiles were tried. However, the attempts
to open the ring by using ethylmagnesium bromide, allyltrimethylsilane,
or trimethylaluminum were unsuccessful. The less reactivity of epoxide
ring was unexpected since the epoxide ring-opening reaction of 1-phenoxy-
carbonyl-2,3-epoxy-pyrrolidine with allyltrimethylsilane has been known in
the literature.[4]
On the other hand, when 2 was treated with LiHDMS, unsaturated
5-hydroxy-enlactam 4 was obtained in 29% yield. The yield of 4 was
improved to 88% yield by using LDA as a base (Scheme 2). Since the
1
enantiomeric purity could not be checked by chiral HPLC or H NMR
using chiral shifting agent at this stage, compound 4 was converted to
5-silyloxy-enlactam 5 in 81% yield. The enantiomeric excess of 5 was deter-
mined to be 77% by 1H NMR analysis using the chiral shifting agent
Eu(hfc)3.[5] The configuration of C-5 was assigned to S by comparison of
the optical rotation of 5 with that of (5S )-1-benzyl-5-(tert-butyldimethylsi-
lyloxy)-1,5-dihydro-pyrrol-2-one, which was prepared by Yoda group.[6]
The moderate enantiomeric purity of 5 seems to be due to the slight
equilibrium of 5-hydroxy-enlactam 4 to its ring-opened aldehydo–enamide
structure during the formation of 4 or the silylation step.
Scheme 2.

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2502
HWANG ET AL.
The 5-silyloxy-enlactam 5 was used as a dienophile in the thermal
Diels–Alder reaction with cyclopentadiene[7a] to gain a tricyclic lactam 6
in 90% yield. The thermal Diels–Alder reaction of 5 is expected to proceed
with a high endoselectivity and diastereoselectivity as was previously
reported.[7] The tricyclic lactam 6 was subjected to the N-acyliminium
ion cyclization condition to form pentacyclic pyrrolo[2,1-a]isoquinolinone
ring 7. Interestingly, the tricyclic lactam 6 did not cyclize efficiency under
the influence of BF3 ꢀ Et2O to give the cyclized product 7 only in 10% yield.
On the other hand, the stronger Lewis acid, TiCl4 was an efficient activator[8]
for the cyclization to provide 7 in high yield (77%) and as a single diaste-
reomer resulting from a diastereoselective attack of benzene ring anti to
the cyclopentene ring during the cyclization. The chemical shifts of each
protons and the stereochemistry of angular position in 7 could be fully
1
1
assigned by using H NMR, in particular H-1H COSY techniques.
In conclusion, we have found that C-4 hydroxylated 5-ethoxylactam
1b gave 4,5-epoxylactam 2 in high yield instead of isoquinoline ring under
the N-acyliminium ion cyclization condition. The 4,5-epoxylactam 2
was converted to another N-acyliminium ion precursor 6 through base-
promoted epoxide ring opening, silylation, and thermal Diels–Alder reac-
tion with cyclopentadiene in high yield. Finally, compound 6 was cyclized in
the presence of TiCl4 to provide cyclopentene-fused pyrroloisoquinoline
derivative 7. Although, we have shown one example on the utility of
4,5-epoxylactam by synthesizing cyclopentene-fused pyrroloisoquinoline
derivative and the enantiomeric excess of 5-silyloxy-enlactam 5 was not
high enough, it seems that 4,5-epoxylactam can be useful intermediate in
the synthesis of several chiral or racemic form of isoquinoline or pyrrolidine
derivatives by chemical transformation. We are now investigating to
improve the enatiomeric excess of 5-silyloxy-enlactam 5.
EXPERIMENTAL
Melting points were determined with a Thomas-Hoover capillary
apparatus and uncorrected. IR spectra were obtained on a Perkin Elmer
16F PC FT-IR spectrometer. The NMR spectra were recorded on Varian
Gemini 300 FT or Brucker Advance 300 spectrometers at 300 MHz
(for 1H NMR) and 75 MHz (for 13C NMR). The chemical shifts are reported
in ppm downfield relative to tetramethylsilane. Mass spectra were
determined with a HP 590 GC/MS 5972 MSD spectrometer and high
resolution mass spectra were recorded on a Jeol JMS-AX 505WA mass
spectrometer by chemical ionization method (CI) using methane as a carrier
gas. Optical rotations were measured on an AUTOPOL III automatic

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PYRROLOISOQUINOLINONE DERIVATIVE
2503
polarimeter and all concentrations are given in g/mL. All starting materials
were obtained from commercial supplies, and used without further purifica-
tion. Analytical thin layer chromatography was carried out on pre-coated
silica gel (Merck Kiesegel 60 F254 layer thickness 0.25 mm). Flash column
chromatography was carried out using Kiesegel 60 (230 ꢁ 400 mesh, Merck).
(1S,5S )-2-Phenethyl-6-oxa-2-aza-bicyclo[3,1,0]hexan-3-one (2): To a
solution of 1b[3b] (1.65 g, 6.62 mmol) in CH2Cl2 was added dropwise
BF3ꢀEt2O at ꢂ78ꢃC for 10 min under argon atmosphere. After 2 h, the
mixture was allowed to warm to r.t. and further stirred for 30 min. The
mixture was poured into ice-water containing Na2CO3 and extracted
with CH2Cl2. The organic layer was dried over MgSO4, concentrated, and
purified by flash column chromatography (EtOAc/hexane¼2 : 1) to afford
3 (1.20 g, 89%) as a white solid. Rf ¼ 0.34 (EtOAc/hexane ¼ 2 : 1) M.p.
112–114ꢃC; MS m/z: 203 (M ); HRMS (Cl) Calcd. for C12H14NO2 (MþH)þ:
þ
m/z 204.1025. Found: 204.1020; ½ꢀŠ2D0+52:6ꢃ (c 1.48, CHCl3): IR (KBr,
1
cmꢂ1) 2930, 1704, 1448, 1382, 1284, 1242, 1072, 700; H NMR (CDCl3)
ꢁ 7.28–7.18 (5H, m, phenyl), 4.48 (1H, d, J ¼ 5.3Hz, H-5), 4.10–3.98 (1H,
m, H-4), 3.71 (1H, m, PhCH2CH2–), 3.42 (1H, m, PhCH2CH2–), 2.96–2.78
(2H, m, PhCH2CH2–), 2.71 (1H, dd, J ¼ 17.6, 8.3Hz, H-3), 2.55 (1H, dd,
J ¼ 17.6, 4.9 Hz, H-30); 13C NMR (CDCl3) ꢁ 171.8, 138.6, 128.7, 128.6,
126.6, 84.3, 66.4, 42.5, 36.0, 34.1.
(5S)-5-Hydroxy-1-phenethyl-1,5-dihydropyrrol-2-one (4): To a stirred
solution of freshly distilled diisopropylamine (0.26 mL, 1.86 mmol), in
THF (10 mL) was added a solution of n-butyllithium (0.69 mL, 2.05 mmol,
2.5 M solution in hexane) ꢂ78ꢃC under argon atmosphere. The solution of
LDA was slowly warmed to 0ꢃC and stirred for 10 min and cooled again to
ꢂ78ꢃC. To the LDA solution was added dropwise a solution of 2 (315 mg,
1.55 mmol) in THF (10 mL) for 20 min. The reaction mixture was stirred at
the same temperature for 30 min and quenched by addition of sat. NH4Cl.
The solution was dispersed into excess CH2Cl2 and dried over MgSO4. The
solution was concentrated and the resulting brownish solid was recrystal-
lized from EtOAc/hexane to give 4 (229 mg) as a white solid. The mother
liquor was concentrated and purified by flash column chromatography
(EtOAc/hexane ¼ 2 : 1) to give 49 mg of 4 additionally. Yield ¼ 88%.
Rf ¼ 0.33 (EtOAc/hexane¼2/1). M.p. 102–104ꢃC; MS m/z: 203(M þ); HRMS
(CI) Calcd. for C12H14NO2 (MþH)þ: m/z 204.1025. Found: 204.1030;
½ꢀŠ2D0+37:8ꢃ (c 1.08, CHCl3); IR (KBr, cmꢂ1) 3158, 2832, 1662, 1590,
1428, 1312, 1120, 1090, 694; 1H NMR (CDCl3) ꢁ 7.30–7.17 (5H, m, phenyl),
6.85 (1H, d, J ¼ 6.0 Hz, H-4), 6.05 (1H, d, J ¼ 6.0 Hz, H-3), 5.12 (1H, br d,
J ¼ 5.7 Hz, H-5), 3.75 (1H, m, PhCH2CH2–), 3.51 (1H, m, PhCH2CH2–),
2.90 (2H, t, J ¼ 7.1 Hz, PhCH2CH–); 13C NMR (CDCl3) ꢁ 170.1, 146.4,
139.2, 129.1, 129.0, 128.6, 126.3, 84.2, 41.1, 31.0.

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2504
HWANG ET AL.
(5S)-5-(tert-Butyldimethylsilyloxy)-1-phenethyl-1,5-dihydropyrrol-2-one
(5): To a stirred solution of 4 (915 mg, 4.5 mmol) in CH2Cl2 (20 mL) was
added portionwise TBDMSCl (882 mg, 5.9 mmol) and imidazole (460 mg,
6.8 mmol) at r.t. and stirred for 1 h. The mixture was diluted with CH2Cl2
and washed with water. The organic layer was dried over MgSO4, concen-
trated, and purified by flash column chromatography (EtOAc/hexane ¼
1 : 5) to give 5 (1.15 g, 81%) as a white solid. Rf ¼ 0.23(EtOAc/hexane ¼
1/5). M.p. 96–98ꢃC; MS m/z: 317(Mþ); HRMS (CI) Calcd. for
C18H28NO2Si (MþH)þ: m/z 318.1889. Found: 318.1881; ½ꢀŠ2D0+41:9ꢃ (c 1.40,
CHCl3); IR (KBr, cmꢂ1) 2932, 1690, 1444, 1422, 1362, 1252, 1082, 862;
1H NMR (CDCl3) ꢁ 7.24–7.11 (5H, m, phenyl), 6.69 (1H, dd, J ¼ 6.0,
1.2 Hz, H-4), 6.06 (1H, d, J ¼ 6.0 Hz, H-3), 5.17 (1H, s, H-5), 3.75 (1H, m,
PhCH2CH2–), 3.26 (1H, m, PhCH2CH2–), 2.79 (2H, m, PhCH2CH2–), 0.83
(9H, s, –Si(C(CH3)3), 0.04 (3H, s, SiCH3), 0.00 (3H, s, SiCH3); 13C NMR
(CDCl3) ꢁ 169.5, 145.8, 139.4, 129.2, 129.0, 128.9, 126.9, 84.2, 41.1, 35.2,
26.0, 8.4, ꢂ3.8, ꢂ3.9.
(1S,2S,5S,6R,7R)-5-(tert-Butyldimethylsilyloxy)-4-phenethyl-4-aza-tri-
cyclo [5,2,1,02,6]dec-8-en-3-one (6): To a dried 10 mL, round-bottom-flask
was added 5 (779 mg, 2.45 mmol) and fresh distillated cyclopentadiene
(0.6 mL, 7.36 mmol) in dry toluene (20 mL). The reaction solution was
heated at reflux for 18 h. After cooling to r.t., the mixture was concentrated
under reduced pressure and purified by flash column chromatography
(EtOAc/hexane ¼ 1 : 5) to give 6 (842 mg, 90%) as a white solid. Rf ¼ 0.26
(EtOAc/hexane ¼ 1 : 5) M.p. 148–150ꢃC; MS m/z: 383 (Mþ); HRMS (CI)
Calcd. for C23H34NO2Si (MþH)þ: m/z 384.2359. Found: 384.2368;
½ꢀŠ ꢂ 3:2ꢃ (c 0.43, CHCl3); IR (KBr, cmꢂ1) 2956, 1674, 1454, 1388, 1252,
20
D
1
1054, 840; H NMR (CDCl3) ꢁ 7.16–7.02 (5H, m, phenyl), 5.86 (1H, dd,
J ¼ 5.6, 2.8 Hz, H-7 or H-8), 5.69 (1H, dd, J ¼ 5.6, 2.8 Hz, H-7 or H-8), 4.38
(1H, s, H-5), 3.49 (1H, m, PhCH2CH–), 3.47–2.89 (5H, m, PhCH2CH–, H-2,
H-6 and H-9), 2.56 (2H, m, PhCH2CH2–), 2.42 (1H, m, H-1), 1.39 (1H, d,
J ¼ 8.4 Hz, H-10), 2.21 (1H, d, J ¼ 8.4 Hz, H-100), 0.78 (9H, s, ꢂSi(C(CH3)3),
0.00 (3H, s, SiCH3), ꢂ 0.01 (3H, s, SiCH3); 13C NMR (CDCl3) ꢁ 175.2,
139.2, 136.9, 133.5, 129.1, 128.8, 126.7, 85.6, 51.7, 49.8, 48.3, 45.2, 44.8,
41.2, 34.2, 26.1, 18.3, ꢂ 3.8, ꢂ 4.3.
(8aS,9S,12R,12aR,12bR)-5,8a,9,12,12a,12b-Hexahydro-6H-9,12-methano-
indeno[1,2-a]isoquinolin-8-one (7): To a solution of 6 (295 mg, 0.77 mmol) in
CH2Cl2 (10 mL) was added dropwise a solution of TiCl4 (2.31 mL,
2.31 mmol, 1 M solution in CH2Cl2) at ꢂ78ꢃC for 10 min under argon atmos-
phere. After stirring for 2 h at the same temperature, the mixture was warmed
to r.t. After 30 min, the mixture was poured into ice-water containing Na2CO3
and extracted with CH2Cl2. The organic layer was dried over MgSO4, concen-
trated, and purified by flash column chromatography (EtOAc/hexane ¼ 1:5)

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PYRROLOISOQUINOLINONE DERIVATIVE
2505
to afford 7 (149 mg, 77%) as a white solid. Rf ¼ 0.21 (EtOAc/hexane ¼ 1: 5).
M.p. 96–98ꢃC; MS m/z: 251 (Mþ); HRMS (CI) Calcd for C17H18NO (MþH)þ:
m/z 252.1388. Found: 252.1394; ½ꢀŠ2D0+102:4ꢃ (c 0.81, CHCl3); IR (KBr, cmꢂ1
)
1
3454, 2926, 1680, 1458, 1300, 1250, 1044, 758; H NMR (CDCl3) ꢁ 7.29–7.06
(4H, m, phenyl), 6.3(1H, m, H-11), 6.26 (1H, m, H-10), 4.23(1H, m,
PhCH2CH–), 4.11 (1H, d, J¼ 2.9 Hz, H-12b), 3.32–3.29 (2H, m, H-9 and
H-12), 3.12 (1H, m, PhCH2CH2–), 2.98 (1H, m, H-8a), 2.66 (2H, m,
PhCH2CH2–), 2.64 (1H, m, PhCH2CH2–), 1.69 (1H, d, J¼ 8.5 Hz, H-13),
1.47 (1H, d, J¼ 8.5 Hz, H-130); 13C NMR (CDCl3) ꢁ 173.5, 138.9, 137.3, 134.7,
134.0, 129.5, 127.1, 127.1, 125.2, 60.1, 51.8, 51.6, 46.9, 46.2, 44.9, 37.6, 28.6.
REFERENCES
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5. Tris[3-[(heptafluoropropyl)hydroxymethylene]-d-camphorato]europium(III)
(Eu(hfc)3) was purchased from Aldrich company. For comparison, race-
mic form of 5 was prepared from (ꢄ)-malic acid by the same procedure
for the synthesis of (S )-5. Enantiomeric excess was determined by
integration of H-3peaks at 6.87 and 6.76 ppm from 1H NMR spectrum
of (S )-5 in the presence of Eu(hfc)3.
6. Yoda, H.; Kitayama, H.; Katagiri, T.; Takable, K. Tetrahedron 1992,
48, 3313.
7. (a) Speckamp, W.N.; Hiemstra, H.; Koot, W.-J. J. Org. Chem. 1992, 57,
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Received in the Japan June 8, 2001

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