Investigation of the acid-mediated cyclisation of amide acetal for the synthesis of benzazepinone

Article abstract of DOI:10.3987/COM-08-11366

On treatment with acids, the open-chained amide acetals (1) gave the 7,7 membered ring fused benzazepinones (4) via the bis-cyclisation.

Full text of DOI:10.3987/COM-08-11366

HETEROCYCLES, Vol. 75, No. 8, 2008, pp. 1963 - 1970. © The Japan Institute of Heterocyclic Chemistry
Received, 12th February, 2008, Accepted, 19th March, 2008, Published online, 21st March, 2008. COM-08-11366
INVESTIGATION OF THE ACID-MEDIATED CYCLISATION OF
AMIDE ACETAL FOR THE SYNTHESIS OF BENZAZEPINONE
Wong Phakhodee,a,c Poolsak Sahakitpichan,a,b Songpon Deechongkit,a,b and
Somsak Ruchirawat*a,b,d
aLaboratory of Medicinal Chemistry, Chulabhorn Research Institute and
bChulabhorn Graduate Institute, Vipavadee-Rangsit Road, Laksi, Bangkok 10210,
cDepartment of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road,
d
Bangkok 10400, Programme on Research and Development of Synthetic Drugs,
Institute of Science and Technology for Research and Development, Mahidol
University, Salaya Campus, Nakhonpathom 73170, Thailand
E-mail: somsak@cri.or.th
Abstract – On treatment with acids, the open-chained amide acetals (1) gave the
7,7 membered ring fused benzazepinones (4) via the bis-cyclisation.
INTRODUCTION
New approaches for the synthesis of heterocyclic and carbocyclic molecules by cyclisation through
various means of carbon-carbon bond formation are important in organic chemistry because they can be
potentially applied for the syntheses of many biologically active compounds having heterocyclic and
carbocylic structures.1 An attractive strategy to form heterocyclic molecules is the acid-mediated ring
closure of acetal,2 some examples are known for intramolecular acid-mediated cyclisation of acetals with
direct participation of aromatic moiety as a nucleophile. Takayama et al.3 reported the acid-mediated
cyclisation of dibenzylaminoacetaldehyde dimethyl acetal using hydrochloric acid in acetone for the
synthesis of (-)-cherylline. Recently, Chrzanowska et al.4 reported acid-mediated cyclisation of
isoquinolone acetal, derived from the condensation of acyclic imine acetal with (S)-(-)-O-toluamide under
basic condition, to furnish a protoberberine alkaloid (S)-(-)-O-methylbharatamine. Alternatively,
Domínguez et al.5 reported acid-mediated cyclisation of sulfonamide acetal to form chromeno-3-
benzazepine. To the best of our knowledge, the acid-mediated bis-cyclisation reactions of amide acetal
have not been studied previously. This paper explores the acid-mediated cyclisation of amide acetals 1
using mixtures of HCl/AcOH, H2SO4/AcOH, and refluxing HCO2H aiming at a possible synthesis of the
tetracyclic benzazepinone. Theoretically, three possible cyclisation products 2-4 could be formed by the

treatment of the open chain amide acetals with acids as shown in Scheme 1. The results of the
investigation are reported herein.
O
R
R
R
R
O
N
N
R
R
O
N
2
3
OMe
MeO
MeO
OMe
OMe
MeO
1
OMe
O
R
R
OMe
N
R = OMe or OCH2O
4
MeO
OMe
Scheme 1
RESULTS AND DISCUSSION
The amide acetals 1a and 1b could be prepared by the reaction of amine acetal 8 with either
homoveratroyl chloride 6a or 3, 4-methylenedioxyphenylacetyl chloride 6b respectively. The required
amide acetals 1a and 1b were obtained in 77 and 73% yield respectively. The amine acetal 8 was
conveniently prepared by reduction of the corresponding amide 7 using lithium aluminium hydride. The
acid chlorides 6 were obtained by reaction of the corresponding acids 5a and 5b with oxalyl chloride as
1
shown in Scheme 2. Interestingly, H NMR analyses of both amide acetals (1a and 1b) were
complicated by the presence of amide rotamers.6
R
R
O
R
O
oxalyl chloride (1.5 equiv),
benzene, DMF(3 drops)
Cl
9
OH
O
10
13
R
R
R
O
R
R
6
acid conditions
See Table 1
7
5a; R = OMe
5b; R = OCH2O
N
6
Na2CO3 (2 equiv),
H2O/CH2Cl2, 2h
N
15
14
1
5
4
MeO
OMe
OMe
LiAlH4 (3 equiv),
O
MeO
MeO
MeO
MeO
OMe
THF, reflux, 6h
1a : R = OMe, 77%
1b : R = OCH2O, 73%
MeO
4a : R = OMe
4b : R = OCH2O
OMe
NH
NH
OMe
MeO
OMe
MeO
8
7
Scheme 2
When cyclisation of amide acetal 1a was performed under acidic conditions including HCl/AcOH,
H2SO4/AcOH, and refluxing HCO2H, one main product was obtained from the reaction as shown in Table
1. High resolution MS showed MW of 384.1811 (M++1) for the reaction product of 1a, corresponding to

our expected products (2a-4a). Note that compounds 2a-4a are isomers of one another (formula:
C22H25NO5 expected MW 384.1805 (M++1)). The NMR spectrum of the product clearly indicated the
presence of only four aromatic protons, suggesting that the double cyclisation indeed occurred.
Comparing the structures of compounds 2a-4a, there are two different types of connectivity of the fused
rings, namely Ar2CH2CHNCO (2a or 3a) or Ar2CHCH2NCO (4a). The benzazepinone core structure
was confirmed using HMQC spectral data. It was shown that one of the methylene protons at C-15
showed a doublet at 3.59 ppm (1H, J = 15.2 Hz) and another proton appeared as a doublet of doublet at
4.61 (1H, J = 15.2, 3.4 Hz). It was also shown that one benzylic proton at C-14 displayed a doublet at
4.04 (1H, J = 3.4 Hz). The relatively downfield chemical shift of one proton attached to C-15 at 4.61
ppm was apparently due to the anisotropic effect of the carbonyl group at C-8. The NMR data are
sufficient to confirm the structure of 2,3,11,12-tetramethoxy-5,8,9,14-tetrahydro-6H-7,14-
methanodibenz[d,g]azecin-8-one (4a). By the same structural and NMR data analysis for compound 4a,
it can be concluded that product of the acid-mediated reaction of 1b is 4b.
The acid-mediated cyclisation converting the amide acetals 1 to tetracyclic benzazepinones 4 was studied.
Table 1 summerizes the results of our investigation when different acidic conditions were employed.
The results show that changing HCl to H2SO4 in the mixed acid system with AcOH resulted in 31- 41%
yields of the products. However, formic acid, a weaker acid, could be employed in the above
transformation furnishing the desired benzazepinones 4a and 4b in 36-38% yields. The maximum yield
was obtained within 4 h. Longer reaction time did not result in improved yield, as the starting amide
acetals were depleted by that time.
Table 1. Tetracyclic products 4 from acid-mediated cyclisation
Amide
Acetals
1a
Acids
Conditions Time (h)
Yieldc (4a)
Yieldc (4b)
HCl/AcOHa
H2SO4/AcOHb
HCO2H
0 oC to rt
0 oC to rt
reflux
17
17
4
35
41
36
1a
1a
1b
HCl/AcOHa
H2SO4/AcOHb
HCO2H
0 oC to rt
0 oC to rt
reflux
20
20
4
31
33
38
1b
1b
b
c
aAll reactions were performed with HCl/AcOH (1:1). All reactions were performed with H2SO4/AcOH (1:5). Isolated yield
after chromatography on silica gel
A possible mechanism of acid-mediated cyclisation of amide acetals 1 to the benzazepinone products 4 is
shown in Scheme 3.

R1O
R2O
O
H
N
O
MeO
OMe
Me
OMe
1
-MeOH
O
R1O
R2O
O
O
N
R1O
R2O
R1O
R2O
N
path B
N
path A
MeO
H
MeO
H
MeO
OMe
12
10
9
OMe
MeO
MeO
-MeOH
MeO
OMe
O
-MeOH
O
O
N
R1O
R2O
R1O
R2O
R1O
R2O
N
N
OMe
MeO
OMe
MeO
MeO
OMe
11
4
13
Scheme 3
The reaction was initiated by the formation of oxonium ion 9 from acetal 1 which was followed by further
intramolecular cyclisation reaction of one of the aromatic rings with oxonium ion to form the
seven-membered rings of compounds 10 or 12 (path A or path B). Loss of methanol from 10 and 12
facilitated by the lone pair electrons on oxygen could lead to the reactive quinone methide intermediates
11 and 13 which could be trapped by the electron rich aromatic ring to give the bis-cyclisation product 4.
In conclusion the new benzazepinone core structures,7 2,3,11,12-tetramethoxy-5,8,9,14-tetrahydro-6H-
7,14-methanodibenz[d,g]azecin-8-one 4a and 2,3-dimethoxy-11,12-methylenedioxy-5,8,9,14-tetrahydro-
6H-7,14-methanodibenz[d,g]azecin-8-one 4b, could be conveniently obtained from acid-mediated
cyclisation of amide acetals using different acidic conditions.
EXPERIMENTAL
All common reagents were purchased from Merck, Fluka, and Labscan Asia Co., Ltd., Bangkok Thailand.
13
Melting points were determined on a Electrothermal 9100 apparatus and are uncorrected. 1H and C
NMR spectra were recorded on a Bruker DPX-300 using deuterated chloroform (CDCl3) as the solvent.
IR spectra were collected on a Perkin Elmer system 2000 FT-IR and JASCO A-302 spectromerters.
Mass spectra were recorded on a Finnigan INCOS.50 and Bruker Daltonics (microTOF). Elemental
analyses were performed on a Perkin Elmer Elemental Analyzer 2400 CHN.

General procedure for the preparation of N-(3,4-dimethoxyphenethyl)-2,2-dimethoxyethanamine
(8) Amide 7 (2 mmol) dissolved in dry THF was slowly added into a stirred solution of lithium
aluminium hydride (6 mmol) in dry THF at 0 oC. The reaction was allowed to stir at rt and then refluxed
for 6 h. Excess lithium aluminium hydride was decomposed with saturated aqueous sodium sulfate. The
inorganic material was removed by filtration and washed with EtOAc. The aqueous layer was extracted
twice with EtOAc. The combined organic layer was washed with water, dried (anh. Na2SO4), filtered,
and evaporated in vacuo to give the crude N-(3,4-dimethoxyphenethyl)-2,2-dimethoxyethanamine 8
which was used in the next step without further purification.
General procedure for the synthesis of amide acetals (1) Oxalyl chloride (1.5 mmol) was added
slowly to the stirred solution either of homoveratric acid 5a (1 mmol) or 3,4-methylenedioxyphenylacetic
acid 5b and DMF (2 drops) in benzene (5 mL). The reaction was stirred at rt for 1 h, then concentrated
under reduced pressure to give the crude acid chloride 6. To the mixture of amine 8 (2 mmol), prepared
in situ from LiAlH4 reduction, in CH2Cl2 and sodium carbonate (4 mmol) in water, was added the
solution of acid chloride in CH2Cl2. The reaction was stirred at rt for 2 h. Water was added
and the two phases were separated. The aqueous layer was extracted with CH2Cl2 (3 x 10 mL). The
combined organic extracts were washed with water, dried (anh. Na2SO4), filtered, and evaporated to give
the crude amide acetals which were purified by PTLC using 50% EtOAc in hexane to give amide acetals
1 as a mixture of rotational isomers.
N-(2,2-Dimethoxyethyl)-N-(3,4-dimethoxyphenethyl)-2-(3,4-dimethoxyphenyl)acetamide
(1a)
From 0.1962 g of homoveratric acid 5a, (1 mmol), N-(2,2-dimethoxyethyl)-N-(3,4-dimethoxyphenethyl)-
2-(3,4-dimethoxyphenyl)acetamide 1a was obtained in 77% yield (0.3465 g) using the general procedure
for the synthesis of amide acetal as described above. FTIR (UATR): νmax 1636, 1513, 1451, 1417, 1025
1
cm-1; H NMR (CDCl3, 300 MHz) δ (ppm) major isomer 2.72 (t, J = 7.3 Hz, 2H), 3.40 (s, OCH3, 6H),
3.42 (s, 2H), 3.45 (d, J = 5.3 Hz, 2H), 3.59 (t, J = 7.3 Hz, 2H), 3.85, 3.86, 3.87 (3s, Ar-OCH3, 12H), 4.57
(t, J = 5.3 Hz, 1H), 6.61-6.84 (m, ArH, 6H); minor isomer 2.79 (t, J = 7.3 Hz, 2H), 3.28 (d, J = 5.3 Hz,
2H), 3.37 (s, OCH3, 6H), 3.59 (t, J = 7.3 Hz, 2H), 3.74 (s, 2H), 3.85, 3.86, 3.87, 3.88 (4s, Ar-OCH3, 12H),
4.25 (t, J = 5.3 Hz, 1H), 6.61-6.84 (m, ArH, 6H); 13C NMR (CDCl3, 75 MHz): δ (ppm) major isomer 34.5,
40.0, 48.4, 51.2, 55.3, 55.84, 55.86, 103.4, 111.1, 111.4, 111.92, 111.95, 120.7, 120.8, 127.5, 130.7,
147.79, 147.83, 148.97, 149.01, 171.7; minor isomer 33.3, 40.4, 51.0, 48.4, 55.1, 55.76, 55.81, 103.9,
111.1, 111.2, 111.9, 111.95, 120.6, 120.9, 127.5, 130.7, 147.4, 147.8, 148.8, 149.0, 171.8; LRMS (EI)
448 (M++1, 4), 447 (M+, 11), 283 (28), 264 (35), 251 (46), 166 (41), 165 (100), 151 (35); HRMS (TOF).
Calcd. for C24H34NO7[M+H]+: 448.2330. Found: 448.2336.
N-(2,2-Dimethoxyethyl)-N-(3,4-dimethoxyphenethyl)-2-(3,4-methylenedioxyphenyl)acetamide (1b)
From 0.1802 g of 3,4-methylenedioxyphenylacetic acid 5b (1 mmol), N-(2,2-dimethoxyethyl)-

N-(3,4-dimethoxyphenethyl)-2-(3,4dimethoxyphenyl)acetamide 1b was obtained in 73% yield (0.3158 g)
using the general procedure for the synthesis of amide acetal as described above. FTIR (UATR): νmax
1
1639, 1514, 1442, 1237, 1020 cm-1; H NMR (CDCl3, 300 MHz) δ (ppm) major isomer 2.72 (t, J = 7.2
Hz, 2H), 3.41 (s, 6H, OCH3), 3.43 (s, 2H), 3.44 (d, J = 5.3 Hz, 2H), 3.58 (t, J = 7.2 Hz, 2H), 3.87 (s,
Ar-OCH3, 6H), 4.57 (t, J = 5.3 Hz, 1H), 5.92 (s, 2H), 6.56-6.83 (m, 6H, ArH); minor isomer 2.79 (t, J =
7.3 Hz, 2H), 3.29 (d, J = 5.2 Hz, 2H), 3.38 (s, OCH3, 6H), 3.58 (t, J = 7.3 Hz, 2H), 3.69 (s, 2H), 3.82,
3.85, (2s, Ar-OCH3, 6H), 4.29 (t, J = 5.2 Hz, 1H), 5.93 (s, 2H), 6.56-6.83 (m, ArH, 6H); 13C NMR
(CDCl3, 75 MHz): δ (ppm) major isomer 34.5, 40.0, 48.5, 51.2, 55.4, 55.87, 100.9, 103.4, 1082, 109.2,
111.4, 111.9, 120.8, 121.7 128.7, 130.7, 146.4, 147.77, 147.82, 149.0, 171.5; minor isomer 33.3, 40.3,
48.4, 50.9, 55.2, 55.75, 55.81, 100.9, 103.9, 108.3, 109.4, 111.1, 112.0, 120.7, 121.9, 128.8, 131.6, 146.4,
147.4, 147.8, 148.8, 171.6; LRMS (EI) 432 (M++1, 2), 431 (M+, 6), 248 (13), 165 (20), 164 (100); HRMS
(TOF). Calcd. for C23H30NO7[M+H]+: 432.2017. Found: 432.2019.
Synthesis of 2,3,11,12-tetramethoxy-5,8,9,14-tetrahydro-6H-7,14-methanodibenz[d,g]azecin-8-one
(4a) Reaction with HCl/AcOH; Amide acetal 1a (0.10 g, 0.22 mmol) was dissolved in AcOH (2.50 mL).
Then, concentrated HCl (2.50 mL) was slowly added drop-wise at 0 oC and subsequently stirred at rt for
17 h. The reaction was then quenched with water, neutralized with 25% NH4OH, and extracted
with CH2Cl2 (3 x 20 mL). The combined organic layers were washed with water and brine, dried (anh.
Na2SO4), filtered, and evaporated under reduced pressure to give crude product which was purified by
PTLC using 60% EtOAc in hexane as a mobile phase to give the required benzazepinone 4a in 35% yield
1
(0.0302 g). Mp 243-244 °C; FTIR (KBr): νmax 1656, 1633, 1607, 1516, 1464 cm-1; H NMR (CDCl3,
300 MHz) δ (ppm) 2.46 (dd, J = 13.6, 4.1 Hz, 1H), 2.61 (t, J = 13.6, Hz, 1H), 2.81 (t, J = 13.6 Hz, 1H),
3.35 (d, J = 14.2 Hz, 1H), 3.59 (d, J = 15.2 Hz, 1H), 3.60, 3.86, 3.87, 3.95 (4s, 12H), 4.04 (d, J = 3.4 Hz,
1H), 4.51 (d, J = 14.2 Hz, 1H), 4.61 (dd, J = 15.2, 3.4 Hz, 1H), 4.63 (dd, J = 13.6, 4.1 Hz, 1H), 6.51 (s,
13
ArH, 1H), 6.66 (s, ArH, 1H), 6.67 (s, ArH, 1H), 6.85 (s, ArH, 1H); C NMR (75 MHz, CDCl3) δ 36.6,
43.2, 47.6, 49.0, 52.1, 55.6, 55.78, 55.82, 56.3, 113.9, 114.0, 115.4, 115.5, 123.6, 129.1, 132.1, 136.6,
146.4, 147.2, 147.5, 147.9, 172.8; LRMS (EI) 384 (M++1, 22), 383 (M+, 100), 382 (25), 355 (15), 354 (59),
340 (21), 311 (20), 296 (15), 295 (38), 165 (15); HRMS (TOF). Calcd. for C22H26NO5[M+H]+: 384.1805.
Found: 384.1811; Anal. calcd for C22H25NO5: C, 68.91; H, 6.57; N, 3.65. Found: C, 68.84; H, 6.74; N,
3.79.
Reaction with H2SO4/AcOH; Amide acetal 1a (0.10 g, 0.22 mmol) was dissolved in AcOH (5.00 mL).
Then conc. H2SO4 (1.00 mL) was slowly added drop-wise at 0 oC and subsequently stirred at rt for 17 h.
Similar work-up and purification gave the required benzazepinone 4a in 41% yield (0.0351 g).
Reaction with refluxing HCO2H; Amide acetal 1a (0.10 g, 0.22 mmol) was refluxed in HCO2H (10 mL)
for 4 h. Similar work-up and purification gave the required benzazepinone 4a in 36% yield (0.0310 g).

Synthesis of 2,3-dimethoxy-11,12-methylendioxy-5,8,9,14-tetrahydro-6H-7,14-methanodibenz[d,g]-
azecin-8-one (4b) Reaction with HCl/AcOH; Amide acetal 1b (0.10 g, 0.23 mmol) was dissolved in
o
AcOH (2.50 mL). Then, conc. HCl (2.50 mL) was slowly added dropwise at 0 C and subsequently
stirred at rt for 20h. Similar work-up and purification gave benzazepinone 4b in 31% yield (0.0267 g).
1
Mp 257.0-257.5 °C; FTIR (KBr) νmax 1659, 1606, 1517, 1482, 1459, 1445 cm-1; H NMR (300 MHz,
CDCl3) δ (ppm) 2.46 (dd, J = 13.9, 4.3 Hz, 1H), 2.62 (t, J = 13.9 Hz, 1H), 2.81 (t, J = 13.9 Hz, 1H), 3.31
(d, J = 14.2 Hz, 1H), 3.57 (d, J = 15.5 Hz, 1H), 3.87, 3.95 (2s, 6H), 4.01 (d, J = 4.0 Hz, 1H), 4.49 (d, J =
14.2 Hz, 1H), 4.60 (dd, J = 15.5, 4.0 Hz, 1H), 4.64 (dd, J = 13.9, 4.3 Hz, 1H), 5.84 (d, J = 1.3 Hz, 1H),
5.89 (d, J = 1.3 Hz, 1H), 6.50 (s, ArH, 1H), 6.65 (s, ArH, 1H), 6.68 (s, ArH, 1H), 6.81 (s, ArH, 1H); 13C
NMR (75 MHz, CDCl3) δ 36.5, 43.3, 47.5, 49.4, 52.0, 55.9, 56.2, 101.1, 110.6, 111.0, 115.4, 115.5, 124.5,
130.6, 131.8, 136.7, 146.3, 146.4, 146.9, 147.2, 172.6; LRMS (EI) 368 (M++1, 22), 367 (M+, 100), 338
(51), 279 (39), 178 (47); HRMS (TOF). Calcd. for C21H22NO5[M+H]+: 368.1492 Found: 368.1500.
Anal. calcd. for C21H21NO5: C, 68.65; H, 5.76; N, 3.81. Found: C, 68.50; H, 5.54; N, 3.55.
Reaction with H2SO4/AcOH; Amide acetal 1b (0.10 g, 0.23 mmol) was dissolved in AcOH (5.00 mL)
o
then conc. H2SO4 (1.00 mL) was slowly added drop-wise at 0 C and subsequently stirred at rt for 20 h.
Similar work-up and purification gave benzazepinone 4b in 33% yield (0.0283 g).
Reaction with refluxing HCO2H; Amide acetal 1b (0.10 g, 0.23 mmol) was refluxed in HCO2H (10 mL)
for 4 h. Similar work-up and purification gave benzazepinone 4b in 38% yield (0.0325 g).
ACKNOWLEDGEMENTS
We acknowledge the financial support from the Center for Toxicology, Environmental Health and
Management of Toxic Chemicals (CTEM). One of us (W.P.) acknowledges partial financial support
from Center for Innovation in Chemistry: Postgraduate Education and Research in Chemistry
(CIC-PERCH).
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