Tert -Amino Effect in peri -Substituted Naphthalenes: Syntheses of Naphthazepine and Naphthazonine Ring Systems

Article abstract of DOI:10.1055/s-0030-1258536

Novel straightforward syntheses of naphtho-fused azepines and benzazonine via tert-amino effect are described. Starting from 1-naphthylamine, 8-N,N-dialkylaminonaphthalene-1-carbaldehydes could be obtained in two steps. The aldehyde was prepared by a Suzuki reaction of 8-bromonaphthalene-1- carbaldehyde with ortho-pyrrolidinophenylboronic acid. Treatment of aldehydes with active methylene compounds afforded naphthazepines and novel benzazonine ring system, respectively, through rearrangement of isolable vinyl intermediates or benzo[de]quinolinium derivatives or without isolation of any intermediates. A mechanistic investigation supports an intramolecular hydride transfer for the ring closure to azepine or azonine. Our results indicate that the tert-amino effect may provide a valuable approach to the synthesis of ortho- and peri-fused aza-ring systems.

Full text of DOI:10.1055/s-0030-1258536

LETTER
2109
tert-Amino Effect in peri-Substituted Naphthalenes: Syntheses of
Naphthazepine and Naphthazonine Ring Systems
S
ynthese
s
of Naph
g
thazepine an
o
d
N
aphthazo
t
nine
R
a
ingSystems Anna Földi,a Krisztina Ludányi,b Attila Csaba Bényei,c Péter Mátyus*a
a
Department of Organic Chemistry, Semmelweis University, Hőgyes Endre utca 7., 1092 Budapest, Hungary
Fax +36(1)2170851; E-mail: peter.matyus@szerves.sote.hu
b
c
Department of Pharmacy, Semmelweis University, Hőgyes Endre utca 7., 1092 Budapest, Hungary
Department of Chemistry, Laboratory for X-ray Diffraction, University of Debrecen, Egyetem tér 1., 4010 Debrecen, Hungary
Received 20 May 2010
Dedicated to Professor Tamás Roska on the occasion of his 70th birthday
A
A
A
A
H
Abstract: Novel straightforward syntheses of naphtho-fused
azepines and benzazonine via tert-amino effect are described. Start-
ing from 1-naphthylamine, 8-N,N-dialkylaminonaphthalene-1-carb-
aldehydes could be obtained in two steps. The aldehyde was
prepared by a Suzuki reaction of 8-bromonaphthalene-1-carbalde-
hyde with ortho-pyrrolidinophenylboronic acid. Treatment of alde-
hydes with active methylene compounds afforded naphthazepines
and novel benzazonine ring system, respectively, through rear-
rangement of isolable vinyl intermediates or benzo[de]quinolinium
derivatives or without isolation of any intermediates. A mechanistic
investigation supports an intramolecular hydride transfer for the
ring closure to azepine or azonine. Our results indicate that the tert-
amino effect may provide a valuable approach to the synthesis of
ortho- and peri-fused aza-ring systems.
a
Ha
R1
H
H
∆
Hb
R1
Hb
••
N
N
R2
R2
A
A
Ha
H
A
H
Ha
[1,5]-H
A
Hb
R1
Hb
R1
••
N
N
R2
R2
Key words: tert-amino effect, naphthazepine, naphthazonine, ben-
zo[d,e]quinolinium, peri-interaction
A: electron-withdrawing group
Figure 1 Isomerization via type 2 tert-amino effect
The term ‘tert-amino effect’ was introduced by Meth-
Cohn and Suschitzky nearly forty years ago, to describe
thermal rearrangement reactions of ortho-substituted tert-
anilines via cyclization to benzofused aza-ring systems.1
Seven types of the tert-amino effect have been distin-
guished so far, according to the size of the ring formed and
its mode of formation.2 One version of type 2 reactions,
the isomerization reaction of tert-anilines with an ortho-
vinyl group and their heterocyclic analogues has received
much attention, due to its high synthetic value to obtain
biologically useful tetrahydroquinolines and related fused
ring systems with predictable regio- and stereochemis-
try.3a–3f This type of tert-amino effect can occur in two
steps: the rate-limiting first step involves a hydrogen mi-
gration from the N-methylene carbon to the a-vinyl car-
bon affording a 1,5-dipolar intermediate that cyclizes in
the second step to a tetrahydropyrido-fused system with
the formation of a new carbon–carbon s-bond (Figure 1).
An additional feature of such reactions is the requirement
of strongly electron-withdrawing substituents in the vinyl
group to stabilize the negative end of the dipolar interme-
diate. In context of the hydrogen migration, a sigmatropic
[1,5]-hydrogen shift and an ionic mechanism with hydride
transfer have been proposed.
In our studies, kinetics, thermodynamics, steric features,
the synthetic scope, and methodological limitations of this
type of tert-amino effect have been investigated and vari-
ous types of pyrido-fused diazines have been prepared.4a–f
An impressive number and diversity of very recent exam-
ples,5a–e illustrate further the synthetic potential of type 2
tert-amino effect.
Interestingly, only very few isomerization reactions have
been reported, which led to the formation of a seven-
membered or larger ring.6 In our approaches, vinyl-substi-
tuted bi- or triaryl-tert-amines A and B could be cyclized
to diarene-fused azocines7a and triarene-fused azecines,7b
respectively, via type 2 tert-amino effect (Figure 2). Fol-
lowing this line, we also decided to study ortho-fused
aromatic ring systems, the prototype of which is naphtha-
lene, possessing key functionalities in peri-positions.
Herein we report on novel extensions of the tert-amino ef-
fect to 1-dialkylamino- and 1-(2-dialkylaminophenyl)-8-
vinylnaphthalenes, C and D, respectively, to open new
routes to ortho- and peri-fused naphthazepine and napht-
hazonine ring systems (Figure 2); these compounds show,
in turn, some structural resemblances to naphthalenes I
and II (Figure 3) and related compounds exhibiting anti-
HIV activity.8
SYNLETT 2010, No. 14, pp 2109–2113
x
x
.x
x
.2
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1
0
Advanced online publication: 30.07.2010
DOI: 10.1055/s-0030-1258536; Art ID: G13310ST
© Georg Thieme Verlag Stuttgart · New York

2110
Á. A. Földi et al.
LETTER
A
A
A
A
R2
R1
A
A
Ar
A
A
R2
N
N
R2
R2
R1
N
N
Ar
R1
A
R1
A
A
A
N
A
A
A
A
R2
R2
N
Ar
Ar
R1
R1
R1
N
N
R2
R2
R1
A
B
C
D
Figure 2 Some recent and new extensions of type 2 tert-amino effect: syntheses of diarene-fused azocines, triarene-fused azecines, and naph-
thalene-fused azepine or azonine ring systems
We first carried out Knoevenagel condensation of 8-di-
methylaminonaphthalene-1-carbaldehyde (3a) with ac-
OH
S
S
S
S
tive methylene compounds at room temperature in
ethanol. With acyclic malononitrile (MN), the expected
vinyl compound 4 could be smoothly obtained. However,
treatment of 3a with cyclic active methylene compounds,
such as 1,3-dimethylbarbituric acid (DMBA) or 1H-in-
dene-1,3(2H)-dione (ID) led, not fully unexpectedly,7a to
zwitterionic benzo[d,e]quinolinium derivatives 5 and 6 as
only isolable products in excellent yields (Scheme 2).
Compounds 5 and 6 could be formed from intermediate
vinyl compounds by cyclization via a tert-amino effect.
The cyclic vinyl substituent might facilitate the ring for-
mation by efficient delocalization of the developing neg-
ative charge and forcing the nucleophilic and electrophilic
centers closer to each other by steric buttressing.
O
O
OH
Ar
Ar
I
II
Figure 3 Synthetic lignan analogues I and II evaluated as anti-HIV
agents
In our study, acyclic and cyclic amino and vinyl substi-
tutents were employed. Synthesis of vinyl derivatives was
accomplished in three steps, starting from commercially
available 1-naphthylamine (1). Its dimethylation accord-
ing to the published protocol led to dimethylaminonaph-
thalene (2a).9a Pyrrolidino derivative 2b9b was obtained
by alkylation with 1,4-dibromobutane. The peri-selective
formylation afforded aldehydes 3a10 and 3b (Scheme 1).
Compounds 5 and 6 could be easily distinguished from
the vinyl compounds, based on upfield NMR shifts of the
a-CH group (Figure 4). The X-ray analysis of 5 unambig-
uously revealed that C1–N bond formation (bond distance
of 1.630 Å, see Figure 5) did occur. The Wallis group, in
their excellent comprehensive study, provided X-ray evi-
dences for through-space attractive peri-interactions be-
tween nucleophilic donor and electron-deficient acceptor
atoms in naphthalenes.11 Moreover, they also reported on
the formation of a naphthazepine from b-benzoyl-b-nitro-
vinyl analogue of 4 by heating a sample in an NMR tube,
although the product was not isolated;11a interestingly, the
synthetic value of the isomerization has not been ex-
plored.
R1
R2
R1
R2
O
N
N
NH2
b
a
2a9a
3a10
3a-d6 (45%)
3b (55%)
1
2a-d6 (65%)
2b9b
2a, 3a R1, R2 = Me
2a-d6, 3a-d6 R1, R2 = CD3
2b, 3b R1 + R2 = (CH2)4
Scheme 1 Reagents and conditions: for 2a (a): Me2SO4, H2O,
NaOH, r.t., 4 h; for 2a-d6 (a): (CD3)2SO4, H2O, NaOH, r.t., 4 h; for 2b
(a): Br(CH2)4Br, (i-Pr)2EtN, toluene, 110 °C, 15 h; for 3a,b (b): i) n-
BuLi, Et2O, r.t., 48 h; ii) DMF, –60 °C, 1 h; iii) MeOH, –20 °C, 3 h.
We particularly focused on the possible transformations
of compounds 4–6 to obtain novel series of 1,2-dihy-
dronaphtho[1,8-b,c]azepines. Reactions were carried out
in DMSO, neat, at different temperatures with traditional
and microwave heating. From vinyl derivative 4, azepine
Synlett 2010, No. 14, 2109–2113 © Thieme Stuttgart · New York

LETTER
Syntheses of Naphthazepine and Naphthazonine Ring Systems
2111
NC
NC
A
H
C
H
H
C
N
NMe2
a
A
A
R2
H
A
A
A
C
A
R2
c
N
R1
A
O
R1
Me
NMe2
N
4 (91%)
4 8.81 / 166.3
13 7.64 / 165.1
5 7.08 / 93.5
6 6.81 / 91.5
7 3.87 / 42.4
8 3.73 / 44.7
9 3.50 / 40.5
A
10 3.38, 4.18 / 43.0
11 2.87, 4.36 / 42.6
12 3.16, 4.43 / 41.3
14 3.16, 4.43 / 41.3
Me
Me
c
N
3a
A
7–9
b
Figure 4 Some characteristic NMR data [1H NMR, 13C NMR in
CDCl3, d (ppm)] of compounds 4–14
5 (94%)
6 (95%)
Me
O
O
Me
O
N
O
3
Me
CN
N
N
Me
A
A
N
3 Me
2
O
2
1
NC
N
N
Me
1
Me
O
O
O
Me
6
5
O
Me
O
N
N
4
dist.
2.430
3.125
3.361
5
dist.
CN
CN
A
A
C1–N
C1–N
1.630
2.563
2.938
=
O
C1–C3
C2–C3
C1–C3H 3.064
C1–C3
C2–C3
C1–C3H 2.719
O
7
O
Me
8
9
CN
Scheme 2 Reagents and conditions: for 4: MN, EtOH, piperidine,
r.t., 7 h; for 5: DMBA, EtOH, piperidine, r.t., 5 h; for 6: ID, EtOH,
piperidine, r.t., 6 h.
2
3
1
NC
N
Table 1 Effect of Different Reaction Conditions on Ring Closurea
15 dist.
Reaction
4 → 7
4 → 7
4 → 7
4 → 7
4 → 7
5 → 8
5 → 8
5 → 8
5 → 8
5 → 8
6 → 9
Solvent
DMSO
DMSO
DMSO
neat
Yield (%)
Time (h) Temp (°C)
C1–N
3.368
4.234
3.901
C1–C3
C2–C3
85
81
81
61
80
0
23.5
2.5
0.2
3
60
100
100b
160
162b
60
C1–C3H 3.871
Figure 5 Characteristic atomic distances (Å) measured by X-ray
diffraction
isomerization reactions of 4 and 4-d6 (Scheme 3) were
monitored by 1H NMR spectroscopy at 100 °C in DMSO-
d6 and the rate constants of reactions were calculated. The
value of kinetic isotope effect (kH/kD = 2.5420/
0.8920 = 2.85) supports that a hydrogen (or deuterium, re-
spectively) located in the dimethyl group migrates in the
rate limiting step. NMR and MS analyses of the product
obtained from 4-d6 indicated that no deuterium loss oc-
curred in the reaction (accordingly, it could be assigned to
7-d6), moreover, there could be detected no incorporation
of deuterium when rearrangement of 4 to 7 was carried out
in D2O. On the basis of these findings, we propose that the
isomerization of 4 affording azepine 7 proceeds in an in-
tramolecular pathway. It could also be suggested that
isomerization takes place in two steps, starting with a hy-
dride transfer from the N-methyl group to the electron-de-
ficient vinyl carbon affording a dipolar intermediate,
following cyclization to azepine with the formation of a
new C–C bond between the oppositely polarized carbon
atoms in the second step. The short distances between at-
neat
0.3
24
5
DMSO
DMSO
DMSO
neat
81
80
67
60
85
100
100b
180
180b
80
0.7
2
neat
0.7
15
DMSO
a Conditions: 2 mmol/10 mL DMSO.
b Microwave assisted reactions at 105 W max. power.
7 could only be isolated in a fairly good yield (Table 1).
To investigate whether the rearrangement reaction takes
place intramolecularly, compound 4-d6, the hexadeuterat-
ed dimethyl analogue of 4 was prepared from amine 1 fol-
lowing the methods described above, except using
(CD3)2SO4 for dimethylation of the amino group. The
Synlett 2010, No. 14, 2109–2113 © Thieme Stuttgart · New York

2112
Á. A. Földi et al.
LETTER
oms involved in the hydride transfer also support this The scope of rearrangements was further investigated in
mechanism (Figure 5).
system D (Figure 2) possessing a phenylnaphthyl skeleton
with key groups positioned at the phenyl and naphthalene
rings. Preparation of 8-(2-pyrrolidinophenyl)-1-vinyl-
naphthalene (15) was easily accomplished from 8-bromo-
naphthalene-1-carbaldehyde (13)12 in three steps
(Scheme 5). Suzuki cross-coupling reaction of 13 with
ortho-pyrrolidinophenylboronic acid afforded 14. The
Knoevenagel condensation of 14 with MN led to vinyl de-
rivative 15 in excellent yield. Interestingly, but not sur-
prisingly, the X-ray analysis13–16 of vinyl compound 15
showed that the distance between donor nitrogen and ac-
ceptor vinyl carbon was significantly longer than that in 4,
as a consequence of unfavorable steric effects (Figure 5).
Upon heating of 15 in DMSO even at higher temperatures
(microwave irradiation, 190 °C for 5 h), no cyclization
was detected.
CN
D
CN
CD2
NC
D
CD2
N
NC
N
CD3
CD3
4-d6
CN
D
N
NC
D
D
CD3
7-d6 (98%)
Scheme 3 Reagents and conditions: for 7-d6: DMSO-d6, 100 °C, 4 h.
O
Transformation of zwitterionic compounds 5 and 6 to
azepines 8 and 9, respectively, could also be rationalized.
It could occur through intermediate vinyl compounds
formed with opening of the five-membered ring. When
we monitored the rearrangement of 5 with 1H NMR spec-
troscopy at temperatures 50–80 °C in DMSO-d6, a broad
signal was indeed identified that was assigned to a vinyl
proton; upon elevating the temperature to 80 °C, there
also appeared the signal set of naphthazepine 8.
O
Br
N
a
b
13
14 (60%)
NC
NC
CN
N
NC
N
c
In the second series of reactions, 8-pyrrolidinonaphtha-
lene-1-carbaldehyde (3b) was treated with active methyl-
ene compounds MN, DMBA, ID. Under mild conditions
at room temperature, naphthazepines 10–12 could be iso-
lated, that is, cyclization via type 2 tert-amino effect oc-
curred. To achieve higher rates for formation of azepines
10–12, reactions were carried out at 80 °C (Scheme 4).
The increased reactivity of pyrrolidino vs. dimethylamino
derivatives might be explained by more efficient overlap
of nonbonding electron pair of pyrrolidine nitrogen with
the aromatic p-system and a more favorable geometric po-
sition of the hydrogen for migration.
15 (91%)
16 (65%)
Scheme 5 Reagents and conditions: for 14: 2-(pyrrolidin-1-yl)phe-
nylboronic acid, Pd[PPh3]4, 0.2 M Na2CO3, DME, reflux, 1 h; for 15
malononitrile, piperidine, EtOH, r.t., 4 h; for 16: [bmim]BF4, 190 °C,
3 h.
Next, we thought to apply an ionic liquid as a particularly
suitable solvent for high temperature polar reactions. The
solution of 15 was heated in [bmim]BF4 at 190 °C. Under
these conditions the isomerization could indeed be
achieved and pyrrolonaphtho[1,8-e,f]azonine 16, repre-
senting a novel polycyclic ring system, was isolated in
good yield (Scheme 5).17
A
A
O
N
N
In conclusion, novel 1,2-dihydrobenzo[c,d]indolium,
naphtho[1,8-b,c]azepine and naphtho[1,8-e,f]azonine ring
systems could be prepared from easily available peri-sub-
stituted naphthylamines by new extensions of tert-amino
effect. Our results further demonstrate the high synthetic
potential of tert-amino effect in the syntheses of otherwise
hardly accessible medium-sized or larger fused aza-ring
systems.
3b
10 (94%)
11 (87%)
12 (87%)
Me
O
O
O
N
N
A
CN
CN
O
12
10
11
=
A
O
Me
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Scheme 4 Reagents and conditions: for 10: malononitrile, piperi-
dine, EtOH, 80 °C, 7 h; for 11: 1,3-dimethylbarbituric acid, piperidi-
ne, EtOH, 80 °C, 4.5 h; for 12: 1H-indene-1,3(2H)-dione, piperidine,
EtOH, 80 °C, 3 h.
Synlett 2010, No. 14, 2109–2113 © Thieme Stuttgart · New York

LETTER
Syntheses of Naphthazepine and Naphthazonine Ring Systems
2113
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Acknowledgment
The authors thank the Hungarian Scientific Research Fund
(K73389), Ministry of Health (ETT 099-03/2009) for financial sup-
ports, Dr. Tamás Gáti for his contribution to the assignments of
NMR spectra, Dr. Péter Tétényi for recording IR spectra, and Dr.
Petra Dunkel for helpful discussion. The X-ray analyses were sup-
ported by the EU and co-financed by the European Social Fund
through the Social Renewal Operational Programme under the pro-
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Synlett 2010, No. 14, 2109–2113 © Thieme Stuttgart · New York