Electrooxidative cyclization of hydroquinolyl alcohols, hydroquinolylamines, and dimethyl aminomalonates

Article abstract of DOI:10.1071/CH07010

Several hydroquinolyl alcohols and amines were electrochemically oxidized in methanol in the presence of sodium methoxide and potassium iodide to afford the corresponding intramolecular cyclization products. Furthermore, several amino malonates were electrochemically oxidized to yield the corresponding heterocyclic compounds through an intramolecular carbon-carbon bond formation in the presence of sodium cyanide in methanol. CSIRO 2007.

Full text of DOI:10.1071/CH07010

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2007, 60, 236–242
www.publish.csiro.au/journals/ajc
Electrooxidative Cyclization of Hydroquinolyl Alcohols,
Hydroquinolylamines, and Dimethyl Aminomalonates
Mitsuhiro Okimoto,A,B TakashiYoshida,A Masayuki Hoshi,A Kazuyuki Hattori,A
Masashi Komata,A Kaori Numata,A and Kenta TomozawaA
ADepartment of Applied and Environmental Chemistry, Kitami Institute of Technology,
Koen-cho 165, Kitami, Hokkaido 090-8507, Japan.
BCorresponding author. Email: oki@chem.kitami-it.ac.jp
Several hydroquinolyl alcohols and amines were electrochemically oxidized in methanol in the presence of sodium methox-
ide and potassium iodide to afford the corresponding intramolecular cyclization products. Furthermore, several amino
malonates were electrochemically oxidized to yield the corresponding heterocyclic compounds through an intramolecular
carbon–carbon bond formation in the presence of sodium cyanide in methanol.
Manuscript received: 15 January 2007.
Final version: 13 March 2007.
CN
Introduction
R1
R1
CN
R2
R1
R1
R1
R1
Oxidation
N
ꢀ
N
An important and unique technique in organic synthesis is the
electrochemical process, which can be carried out under very
mild reaction conditions without the use of harsh oxidizing or
reducing reagents.[1] Our laboratories have employed such elec-
trochemical techniques in the oxidation of organic compounds,
in particular, nitrogenous compounds such as amines, enamines,
and hydrazones.[2] For tertiary aliphatic amines, it is well known
that the α-carbon of the nitrogen atom can be activated under
appropriate anodic conditions.[3] Although numerous reports
describethecharacteristicbehaviourofaminesinthepreparation
of organic compounds, studies that examine the electrooxida-
tive intramolecular cyclization of amines are limited.[4] Chiba
and coworkers have reported on the electrooxidative cyanation
of tertiary aliphatic amineA, as illustrated in Scheme 1, in which
the α-carbon of amine A is attacked by a cyano anion (formed
from sodium cyanide, which is employed as the electrolyte) to
yield a mixture of the corresponding nitriles B and C.[5]
N
R2
NaCN in MeOH
R2
A
B
C
Scheme 1. Electrooxidative cyanation of tertiary aliphatic amines.
O
Oxidation
(CH2)n
(CH2)n
OH
N
N
N
N
n ꢁ 2 or 3
D
E
NH
Oxidation
(CH2)n
NH2
(CH2)n
n ꢁ 2 or 3
F
G
From their report, we reasoned that a similar nucleophilic
attack on the α-carbon of a nitrogen atom could be achieved
using the oxygen atom of a hydroxy group or the nitrogen atom
of an amino group. Specifically, amino alcohol D and diamine
F would produce the corresponding intramolecular heterocyclic
compounds E and G, respectively, as illustrated in Scheme 2.
Scheme 2. Electrooxidative cyclization of amino alcohols and diamines.
compounds (2 and 4, respectively). In contrast, diamines 7 and
10 afforded heterocyclic imino amines 8 and 11, respectively,
instead of the expected type-G diamines.
Investigations on the electrooxidation of hydroquinolyl and
hydroisoquinolyl alcohols (Scheme 3, compounds 1 and 3,
respectively) and amines (Scheme 3, compounds 7 and 10,
respectively) began with the optimization of the reaction con-
ditions. Using 2-(3,4-dihydroquinolin-1(2H)-yl)ethanol 1a as
a model substrate, electrooxidation reactions were performed
in the presence of various supporting electrolytes, as listed
in Table 1, to yield the corresponding fused-ring heterocyclic
compound 2,3a,4,5-tetrahydro-1H-oxazolo[3,2-a]quinoline 2a.
In the presence of basic electrolytes, such as NaOAc, NaCN,
NaOH, and NaOMe (runs 1–4), the yields of 2a varied from
25 to 80%. Increased basicity of the electrolyte resulted in high
Results and Discussion
Initially, because of the simple synthetic procedures, numer-
ous amino alcohols D and diamines F were examined. In these
cases, the expected corresponding cyclized products E and G are
presumably stable against methanolysis and hydrolysis. Based
on the preliminary results, hydroquinolyl and hydroisoquinolyl
alcohols (Scheme 3, compounds 1 and 3, respectively) and
amines (Scheme 3, compounds 7 and 10, respectively) were
chosen as the starting substrates. As shown in Scheme 3, amino
alcohols 1 and 3 resulted in the formation of an intramoleclar
carbon–oxygen bond to afford the expected heterocyclic type-E
© CSIRO 2007
10.1071/CH07010
0004-9425/07/040236

Electrooxidation of Hydroquinolyl Alcohols
237
NH2
O
N
OH
N
N
N
N
(CH2)n
(CH2)n
(CH2)n
(CH2)n
1
2
4
7
8
OH
N
N
NH2
N
N
N
(CH2)n
(CH2)n
O
n ꢁ 2 or 3
n ꢁ 2 or 3
(CH2)n
(CH2)n
3
10
11
Scheme 3. Electrooxidative cyclization of hydroquinolyl alcohols and diamines.
Table 1. Influence of supporting electrolytes for the cyclization of
amino alcohol 1a
Reaction conditions: 1a (5 mmol), MeOH (40 mL), current passed
(2.0 F mol−1), constant current (0.3A), ∼15◦C
Oxidation
a: n ꢁ 2
b: n ꢁ 3
O
N
N
OH
(CH2)n
(CH2)n
2a ꢁ 87% (2.3 F molꢂ1), 2b ꢁ 88% (3.0 F molꢂ1
)
1a, 1b
Oxidation
O
in MeOH
N
N
OH
Oxidation
N
1a
2a
OH
(CH2)n
N
O
(CH2)n
Run Supporting
electrolyte
Amount used RecoveryA of 1a YieldA of 2a
4a ꢁ 62% (6.2 F molꢂ1), 4b ꢁ 73% (7.8 F molꢂ1
)
)
3a, 3b
[mmol]
[%]
[%]
1
2
3
4
5
6
7
8
9
NaOAc
NaCN
NaOH
NaOMe
LiClO4
p-TsONEt4
KBr
10
10
10
10
10
8
10
10
5 + 5
37
14
4
17
59
15
23
22
4
25
50
70
80
1
2
7
23
93
Oxidation
OH
(CH2)n
OH
N
(CH2)n
N
6a ꢁ 65% (2.5 F molꢂ1), 6b ꢁ 63% (2.7 F molꢂ1
5a, 5b
Scheme 4. Electrooxidation of the tetrahydroquinolyl alcohols, tetra-
hydroisoquinolyl alcohols, and indolinyl alcohols. Reaction conditions: sub-
strate (5 mmol), NaOMe (5 mmol), KI (5 mmol), MeOH (40 mL), constant
current (0.3A), ∼15◦C. Isolated yields are shown.
KI
KI + NaOMe
ARecovery of 1a and yields of 2a were determined by GC analysis.
ꢂe
OH
ꢂe, ꢂHꢀ
ꢀ
N
yields of 2a. In contrast, neutral electrolytes, such as LiClO4,
tetraethylammonium p-toluenesulfonate, KBr, and KI (runs 5–
8), gave very poor yields of 2a. In such cases, the purification
procedures resulted in the recovery of unreacted 1a and/or the
depositofconsiderableamountsofdarkbrownishmaterialonthe
silica gel column. It is noteworthy that, although the presence
of NaOMe or KI afforded 2a with yields of 80% (run 4) and
23% (run 8), respectively, the concurrent use of both electrolytes
resulted in an improved yield of 93% (run 9). Our studies showed
that varying the amount of KI, over a range of 2 to 10 mmol, did
not affect the yield of 2a (90 and 91%, respectively), whereas
using an excessive amount of NaOMe (20 vs 5 mmol) decreased
the yield of 2a (73 vs 93%, respectively).
N
Base
OH
(CH2)n
(CH2)n
H
H
H
H
H
3
ꢂHꢀ
n ꢁ 2 or 3
(CH2)n
ꢀ
N
N
Base
(CH2)n
H
O
O
H
I
4
Scheme 5. Proposed scheme of the oxidative cyclization of quinolyl
alcohols.
Based on the yields listed in Table 1, the corresponding opti-
mal reaction conditions were employed for the electrooxidation
of other hydroquinolyl alcohols 1b, 3a, and 3b, as shown in
Scheme 4. During the course of the electrooxidation reactions,
the composition of the reaction mixture was monitored using gas
chromatography(GC) analysis todetermine whether the reaction
had reached completion.
Significantly higher amounts of electrical current were
required for the electrooxidation of aliphatic amines 3 than
that for the aromatic amines 1, for example, substrates 1a
and 1b required electric currents of 2.3 F mol−1 (62 min) and
3.0 F mol−1 (81 min), respectively, to form the corresponding
heterocyclic products in good yields (87 and 88%, for 2a and 2b,
respectively), whereas substrates 3a and 3b required electric
currents of 6.2 F mol−1 (167 min) and 7.8 F mol−1 (210 min),
respectively, to form the products in moderate yields (62 and
73% for 4a and 4b, respectively). As a note, in the cases of 3a
and 3b, the oxygen atom on the hydroxy group carried out the
nucleophilic attack selectively on the benzylic carbon, which
is presumably because of the stable molecular structure of the
conjugated intermediate (Scheme 5).

238
M. Okimoto et al.
As shown in Scheme 5, the most reasonable pathway for
In the case of 10a and 10b, a selective nucleophilic attack
on the carbon at the benzilic position, similar to that of amino
alcohol 3, resulted in the formation of imino amines 11a (52%)
and 11b (28%), respectively. Except for 8a, imino amines 8b,
11a, and 11b resisted further oxidation, in spite of excessive
amounts of electric current passed, to form the oxidized prod-
ucts such as 9. In comparison with the electrooxidation process,
Finch and coworkers produced an L-type product (Scheme 8)
in a yield of 27% from 7b using mercuric acetate as the
oxidant.[10] Moreover, Wendelin and coworkers reported on
the three-step synthesis of the picrates of 8a and 8b using
3,4-dihydroquinoline-2(1H)-thioine as the starting substrate.[11]
As a note, the electrooxidation of indolinylamines 12a and 12b
resulted in only very poor yields of dehydrogenation products
13a (20%) and 13b (5%) along with a large amount of tar-like
material. The proposed mechanism that describes the aromati-
zation of tetrahydroquinolylamine 7a to 9 through the partially
oxidized product 8a is shown in Scheme 8.
the cyclization of tetrahydroisoquinolyl alcohol 3 to 4 can be
described as three reaction steps: (i) loss of one electron from
the lone electron pair of the nitrogen atom of substrate 3 to the
anode to form cationic radical H; (ii) immediate one-electron
oxidation and deprotonation to form iminium ion I; and then
(iii) subsequent nucleophilic attack by the oxygen atom of the
hydroxy group. Essentially, substrate 3 loses two protons and
two electrons (two-electron oxidation) during the course of the
reaction. During these steps, NaOMe would serve as the base in
facilitating the deprotonation of both cation radical H and the
hydroxy group of I.
Interestingly, electrooxidations of indolinyl alcohols 5a and
5bundersimilarreactionconditionsresultedinthedehydrogena-
tion products 6a (65%) and 6b (63%), respectively (Scheme 4),
instead of the expected heterocyclic compounds.Apparently, the
electrooxidation of 5 favours aromatization over cyclization to
afford the corresponding indolyl alcohol 6,[6] presumably by a
reaction pathway as shown in Scheme 6.
In order to produce 8a, substrate 7a would lose four protons
and four electrons. In the formation of 9, however, substrate
7a must lose a total of eight protons and eight electrons
(eight-electron oxidation). Although expected product 8a could
Although the pathway is initially identical to that of 1 and 3,
the formation of iminium ion K allows the deprotonation of the
benzilic carbon to afford indole derivatives 6.
In reference to the cyclized product obtained from 1 and
3 by electrooxidation, Schneider and coworkers first reported
the formation of 4a using 3,4-dihydroquinoline as the starting
substrate.[7] Consequently, the conversion of 4a into several use-
ful derivatives were investigated.[8] Lohray and coworkers were
able to obtain 2a in a yield of 56% by reduction of the amide
esters.[9]
Oxidation
N
N
N
N
Oxidation
NH2
8a ꢁ 45% (6.0 F molꢂ1), 9 ꢁ 41% (14.4 F molꢂ1
)
N
(CH2)n
The electrochemical oxidations of hydroquinolyl- and
hydroisoquinolyl-amines (7 and 10, respectively) were carried
out under optimal reaction conditions (NaOMe–KI in MeOH
system) that were independently determined using 7b as a model
substrate. Not only were the yields low, but the main products
were the unexpected cyclized imino amines 8b, 9, and 11, which
can be attributed to the further oxidized products of the first
expected compound type-G (Scheme 2). In the case of 7a, after
the passage of 6.0 F mol−1 of electric current, the corresponding
cyclized imino amine 8a was formed in a yield of 45% (deter-
mined by GC analysis). Interestingly, however, after the passage
of 14.4 F mol−1 of electricity, fused aromatic compound 9 was
obtained as the final product, as shown in Scheme 7.While moni-
toring the conversion of 8a into 9 using GC analysis, a peak that
would correspond to either intermediate M or N (Scheme 8)
was observed. Unfortunately, isolation of the compound was
unsuccessful.
7a, 7b
N
N
a: n ꢁ 2
b: n ꢁ 3
8b ꢁ 42% (6.1 F molꢂ1
)
Oxidation
NH2
(CH2)n
N
N
N
(CH2)n
10a, 10b
11a ꢁ 52% (4.9 F molꢂ1), 11b ꢁ 28% (4.6 F molꢂ1
)
Oxidation
NH2
NH2
(CH2)n
N
N
(CH2)n
13a ꢁ 20% (4.5 F molꢂ1), 13b ꢁ 5% (4.7 F molꢂ1
)
12a, 12b
Scheme 7. Electrooxidation of the tetrahydroquinolylamines, tetra-
hydroisoquinolylamines, and indolinylamines. Reaction conditions: sub-
strate (5 mmol), NaOMe (5 mmol), KI (5 mmol), MeOH (40 mL), constant
current (0.3A), ∼15◦C. Isolated yields are shown, except for 8a.
H
H
H
H
ꢂ2e, ꢂ2Hꢀ
ꢂ2e, ꢂ2Hꢀ
H
H
ꢂe, ꢂHꢀ
ꢂe
Base
NH2
Base
NH
N
ꢀ
N
N
N
N
Base
H
H
N
(CH2)nꢂOH
(CH2)nꢂOH
7a
L
8a
5
J
6
H
H
H
H
ꢂHꢀ
N
N
N
M
or
ꢂ2e, ꢂ2Hꢀ
ꢂ2e, ꢂ2Hꢀ
n ꢁ 2 or 3
Base
ꢀ
N
Base
N
N
Base
N
H
N
(CH2)nꢂOH
(CH2)nꢂOH
N
9
K
Scheme 6. Proposed scheme of the oxidative aromatization of indolinyl
alcohols.
Scheme 8. Proposed scheme of the oxidative aromatization of quinolyl-
amine 7a.

Electrooxidation of Hydroquinolyl Alcohols
239
not be isolated, the reaction mechanism for formation of the
L-type compound from substrate 7a should resemble that of the
amino alcohols. In every step, NaOMe would serve as the base
in facilitating the deprotonation.
Furthermore, we have also investigated the electrooxidation
of dimethyl aminomalonates O. As shown in Scheme 9, the
electrooxidation resulted in a carbon–carbon bond formation
between the carbon at the α-position of the nitrogen atom and
the active methine carbon of the dimethyl malonate moiety to
give the intramolecular cyclization product P.[12]
To optimize the reaction conditions of the cyclization of
O to P, electrooxidations were performed using dimethyl
2-(piperidin-1-yl)ethylmalonate 14c as the model substrate
to yield the corresponding fused-ring heterocyclic dimethyl
hexahydroindolizine-1,1-dicarboxylate 15c. At first, the elec-
trooxidations were examined in the presence of various support-
ing electrolytes, as listed inTable 2. In the presence of NaOH (run
1) or neutral salts (runs 2–5), the yields were less than 10%. The
yield improved slightly using an equimolar amount of NaOMe
(run 8). Unfortunately, the concurrent use of NaOMe and KI,
which gave favorable results earlier, did not function as a suit-
able electrolyte system for the cyclization of compound 14c. In
the cases of low yields of 15c (runs 1–8), approximately 50% of
14c was recovered unchanged and/or a considerable amount of
tar-like material remained after evaporation of the reaction mix-
ture. Surprisingly, the use of NaCN as the supporting electrolyte
resulted in the highest yield (63%, run 9). Because the amount
of NaCN, over the wide range of 5 to 40 mmol, did not affect
the yield of 15c (63 and 61%, respectively), it can be assumed
that NaCN does not directly relate to this cyclization mecha-
nism. Moreover, during the first half of the electrooxidation, the
yield of 15c increased in proportion to the current passed (18%,
0.5 F mol−1; 36%, 1.0 F mol−1; 52%, 1.5 F mol−1); during the
final stages of the electrooxidation, however, the rate of increase
gradually declined (58%, 2.0 F mol−1).The highest yield of 63%
was obtained after passage of ∼2.6 F mol−1 (70 min), at which
point almost all of 14c was oxidized. Cyanation adducts (type-B
and/or C compounds in Scheme 1) were not detected during the
electrooxidation of 14c.[5]
CO2Me
CO2Me
CO2Me
CO2Me
Oxidation
(CH2)n
N
(CH2)n
N
n ꢁ 2 or 3
O
P
Scheme 9. Electrooxidative cyclization of aminomalonates.
Table 2. Influence of supporting electrolytes for the cyclization of
aminomalonate 14c
Reactionconditions: 14c (5 mmol), MeOH(40 mL), constantcurrent(0.3A),
current passed (2.6 F mol−1), ∼15◦C
CO2Me
MeO2C
CO2Me
CO2Me
Oxidation
in MeOH
N
N
14c
15c
The results of electrooxidations of aminomalonates 14a–g,
using the optimal reaction conditions as described above, are
shown in Table 3. All of the substrates were consumed after
passing 2.5 to 3.0 F mol−1 of electric current.As listed inTable 3,
the corresponding cyclization product can be obtained, not only
from five- and six-membered cyclic aminomalonates 14a–e, but
also from non-cyclic aminomalonates 14f and 14g.The products
were isolated as a slightly yellow viscous oil with yields of 45%
(15a) to 71% (15b).
Although most of the products obtained in this reaction
are new compounds, similar skeletons are widely found in
plants,[13] and hence have attracted widespread attention as
targets of organic synthesis.[14] Among such syntheses, two
Run
Supporting
electrolyte
Amount used
[mmol]
YieldA of 15c
[%]
1
2
3
4
5
6
7
8
9
NaOH
p-TsONEt4
KBr
20
10
20
20
20
20
20
5
4
4
5
5
6
10
10
25
63
KI
LiClO4
NaOAc
NaOMe
NaOMe
NaCN
20
AYields of 15c were determined by GC analysis.
Table 3. Electrooxidation of the dimethyl aminomalonates
Reaction conditions: 14 (5 mmol), NaCN (10 mmol), MeOH (40 mL), constant current (0.3A), ∼15◦C
CO2Me
R1
R1
CO2Me
CO2Me
Oxidation
(CH2)n
N
(CH2)n
N
CO2Me
NaCN/MeOH
R2
R2
14
15
Substrate 14
R1
R2
n
Current passed
YieldA of 15
[F mol−1
]
[%]
14a
14b
14c
14d
14e
14f
–(CH2)3–
–(CH2)3–
–(CH2)4–
–(CH2)4–
2
3
2
3
2
2
2
2.6
2.5
2.6
2.5
2.8
2.7
3.0
45 (53)
71 (73)
59 (63)
53 (62)
68 (72)
61 (71)
60 (66)
–CH2CH(CH3)CH2CH2–
Et
Prn
Prn
Bun
14g
AIsolated yields based on 14. Values in parentheses were determined by GC analysis.

240
M. Okimoto et al.
notable examples include the iminium salt from α-amino acid
decarboxylation[15] or the anodically prepared carbamate;[16]
unfortunately, their syntheses require multi-step reactions. A
reaction pathway of this cyclization is proposed in Scheme 10.
Up to the formation of iminium ion R, the pathway for both
amino alcohol and diamine should be identical. A nucleophilic
attack by an active methine carbon of a malonate moiety in the
activated R would produce cyclization product 15 under basic
conditions.
Although the role of NaCN remains unclear, in this reaction,
NaCN may serve as a suitable base and facilitate the deproto-
nation of both cation radical Q and the methine carbon of R.
Essentially, substrate 14 loses two protons and two electrons
(two-electron oxidation) during the course of the reaction.
was electrooxidized in MeOH (40 mL) that contained NaCN
(0.49 g, 10 mmol). In all cases, the electrooxidation was car-
ried out using a constant current of 0.3A. During the course of
the electrooxidation, the anolyte was magnetically stirred and
the temperature of the cell was maintained at ∼15◦C. Upon
completion of the electrooxidation, the reaction mixture was
treated using a three-fold increase in the amount of anolyte. The
combined reaction mixture was concentrated under vacuum at
∼45◦C to remove most of the methanol, and then treated with
water (∼20 mL). The resulting oily layer was extracted using
ether (3 × 60 mL), the ether layers were combined, washed with
aqueous sodium thiosulfate solution (in the case of 15, brine),
and dried over magnesium sulfate. Following removal of the
solvent, the residue was purified by silica gel column chro-
matography (diameter 25 mm, length 500 mm, diethyl ether as
the elution solvent; in the case of 15a, 7:3 ether/MeOH was
used) or by distillation under reduced pressure.
R1
H
R1
H
ꢀ
N
H
ꢂe
CO2Me
CO2Me
N
(CH2)n
14
(CH2)n
CO2Me
H
H
H
R2
2,3a,4,5-Tetrahydro-1H-oxazolo[3,2-a]quinoline 2a
R2
CO2Me
Bp 108–110◦C (2 mmHg). νmax (neat)/cm−1 2939, 2864, 1605,
1504, 1460, 1346, 1312, 1061, 1053, 1001, 746. δH (CDCl3) 1.4–
1.9 (m, 1H, CH2), 2.1–2.4 (m, 1H, CH2), 2.6–2.9 (m, 2H, CH2),
3.2–3.6 (m, 2H, CH2), 3.8–4.3 (m, 2H, CH2), 4.7–4.9 (m, 1H,
CH), 6.4–6.8 (m, 2H, ArH), 6.9–7.3 (m, 2H, ArH). δC (CDCl3)
24.76 (CH2), 26.18 (CH2), 47.36 (CH2), 65.68 (CH2), 87.96
(CH), 111.90 (CH), 116.99 (CH), 122.33 (C), 127.37 (CH),
128.07 (CH), 143.87 (C). m/z 175 (M+, 87%), 174 (100), 145
(26), 144 (25), 132 (21), 130 (17), 118 (14), 117 (27), 91 (18), 77
(13). Found: 175.0993 [M+]. Calc. for C11H13NO: 175.0997.
Q
CO2Me
CO2Me
H
R1
H
H
R1
ꢂHꢀ
ꢂe, ꢂHꢀ
CO2Me
CO2Me
(CH2)n
N
ꢀ
N
R2
Base
Base
(CH2)n
R2
R
15
Scheme 10. Proposed scheme of the oxidative cyclization of
aminomalonates.
1,2,3,4a,5,6-Hexahydro[1,3]oxazino[3,2-a]quinoline 2b
Bp 110–112◦C (1.5 mmHg). Mp 39–41◦C (recrystallized from
EtOH). νmax (neat)/cm−1 2953, 2845, 1603, 1495, 1458, 1315,
1263, 1171, 1076, 1061, 748. δH (CDCl3) 1.2–1.5 (m, 1H, CH2),
1.7–2.1 (m, 3H, CH2), 2.4–3.4 (m, 3H, CH2), 3.7–4.3 (m, 3H,
CH2), 4.6–4.8 (m, 1H, CH), 6.4–6.8 (m, 2H, ArH), 6.9–7.3 (m,
2H, ArH). δC (CDCl3) 23.67 (CH2), 24.43 (CH2), 28.38 (CH2),
46.91 (CH2), 67.84 (CH2), 85.96 (CH), 112.84 (CH), 118.13
(CH), 125.62 (C), 127.09 (CH), 128.68 (CH), 143.91 (C). m/z
189 (M+, 100%), 188 (50), 170 (13), 158 (26), 146 (29), 132
(36), 131 (72), 130 (52), 91 (13), 77 (11). Found: 189.1179 [M+].
Calc. for C12H15NO: 189.1154.
Conclusions
In conclusion, our studies have demonstrated the electrooxida-
tion of hydroquinolyl alcohols, hydroquinolylamines, and amino
malonates toward the synthesis of several heterocyclic com-
pounds. Although the actual yields depend upon the structure
of the substrates, our methodology is advantageous as follows:
(i) the absence of oxidants and/or special reagents; (ii) very
mild reaction conditions; (iii) availability of the substrates; and
(iv) simple, one-pot procedure.
Experimental
All electrooxidation products were identified from their physical
and/or spectroscopic data. Hydroquinolyl alcohols and amines
were synthesized by typical N-alkylation of hydroquinoline with
the corresponding chloro alcohol or chloro amine, respectively.
The aminomalonates were prepared by a three-step reaction
from the corresponding secondary amine: (i) N-alkylation of
the amine by chloro alcohol to yield amino alcohol; (ii) bromi-
nation of the amino alcohol by concentrated hydrobromic acid in
toluene to yield the hydrobromide salt of the bromo amine; and
(iii) reaction between the salt and dimethyl malonate in MeOH in
the presence of NaOMe to give the dimethyl aminomalonate.[17]
Preparative-scale electrooxidations were carried out in a tall
50 mL beaker equipped with a fine frit cup (porosity ∼100 µm)
as the cathode compartment with a nickel coil (diameter 0.8 mm,
length 220 mm) as the cathode, and an insert cylindrical plati-
num net (diameter 32 mm, height 35 mm, 55 mesh) as the
anode. Amino alcohols 1, 3, 5 (5 mmol) and diamines 7, 10,
12 (5 mmol) were electrooxidized in MeOH (40 mL) in the
presence of KI (0.83 g, 5 mmol) and NaOMe (5 mmol). In the
case of the dimethyl aminomalonates 14, the substrate (5 mmol)
3,5,6,10b-Tetrahydro-2H-oxazolo[2,3-a]isoquinoline 4a
Mp 50–52◦C (recrystallized from EtOH). νmax (neat)/cm−1
1464, 1394, 1290, 1078, 1057, 1016, 991, 889, 777, 746. δH
(CDCl3)2.5–4.0(m, 8H, CH2), 5.19(s, 1H, CH), 6.8–7.5(m, 4H,
ArH). δC (CDCl3) 29.73 (CH2), 45.65 (CH2), 54.73 (CH2), 62.18
(CH2), 90.40 (CH), 126.28 (CH), 128.07 (CH), 128.23 (CH),
128.84 (CH), 132.14 (C), 134.95 (C). m/z 175 (M+, 69%), 174
(100), 145 (53), 130 (25), 117 (53), 115 (26), 103 (15), 91 (21),
77 (17), 56 (16). Found: 175.0973 [M+]. Calc. for C11H13NO:
175.0997.
2,3,4,6,7,11b-Hexahydro[1,3]oxazino-
[2,3-a]isoquinoline 4b
Bp114–116◦C(1.5 mmHg). νmax (neat)/cm−1 2947, 2839, 1464,
1393, 1371, 1294, 1142, 1086, 964, 885, 745. δH (CDCl3) 1.2–
1.5 (m, 1H, CH2), 1.8–3.4 (m, 7H, CH2), 3.7–4.3 (m, 2H, CH2),
4.87(s, 1H, CH), 7.0–7.4(m, 4H,ArH). δC (CDCl3)22.97(CH2),
29.08 (CH2), 46.22 (CH2), 53.22 (CH2), 68.33 (CH2), 89.95
(CH), 125.95 (CH), 127.29 (CH), 127.78 (CH), 128.31 (CH),

Electrooxidation of Hydroquinolyl Alcohols
241
132.20 (C), 134.83 (C). m/z 189 (M+, 38%), 188 (77), 158 (37),
146 (100), 145 (31), 133 (46), 132 (66), 131 (42), 130 (35), 104
(79). Found: 189.1174 [M+]. Calc. for C12H15NO: 189.1154.
(CH2), 126.19 (CH), 126.52 (CH), 127.21 (CH), 129.41 (CH),
131.28 (C), 135.97 (C), 151.89 (C). m/z 186 (M+, 95%), 185
(100), 171 (12), 158 (11), 157 (21), 131 (23), 130 (24), 116
(8), 103 (10), 77 (8). Calc. for C12H14N2: 186.1157. Found:
186.1155 (M+).
2-(1H-Indol-1-yl)ethanol 6a
Bp141–142◦C(2.5 mmHg). νmax (neat)/cm−1 3380, 1510, 1481,
1464, 1362, 1337, 1315, 1063, 764, 743. δH (CDCl3) 1.94 (br s,
1H, OH), 3.69 (t, J 7, 2H, CH2), 4.07 (t, J 7, 2H, CH2), 6.4–6.5
(m, 1H, ArH), 6.9–7.3 (m, 4H, ArH), 7.5–7.7 (m, 1H, ArH). δC
(CDCl3) 48.54 (CH2), 61.61 (CH2), 101.31 (CH), 109.34 (CH),
119.51 (CH), 120.98 (CH), 121.59 (CH), 128.35 (CH), 128.68
(C), 136.09(C). m/z161(M+, 38%), 131(10), 130(100), 103(7),
77 (8). Found: 161.0841 [M+]. Calc. for C10H11NO: 161.0841.
2-(1H-Indol-1-yl)ethanamine 13a
νmax (neat)/cm−1 2936, 1670, 1609, 1510, 1464, 1398, 1313,
1084, 1013, 741. δH (CDCl3) 1.28 (br s, 2H, NH2), 2.93 (t, J 7,
2H, CH2), 4.01 (t, J 7, 2H, CH2), 6.3–6.6 (m, 1H, ArH), 6.8–7.4
(m, 4H, ArH), 7.5–7.8 (m, 1H, ArH). δC (CDCl3) 41.94 (CH2),
49.39 (CH2), 101.23 (CH), 109.29 (CH), 119.35 (CH), 120.94
(CH), 121.47 (CH), 127.99 (CH), 128.68 (C), 136.05 (C). m/z
160 (M+, 43%), 132 (9), 131 (63), 130 (100), 117 (5), 103 (10),
77 (12), 63 (3), 51 (3), 30 (13). Calc. for C10H12N2: 160.1000.
Found: 160.0974 [M+].
3-(1H-Indol-1-yl)propan-1-ol 6b
Bp 148–150◦C (2 mmHg). νmax (neat)/cm−1 3380, 2941, 1510,
1485, 1464, 1337, 1315, 1065, 1015, 743. δH (CDCl3) 1.87
(quin., J 7, 2H, CH2), 2.7 (br s, 1H, OH), 3.38 (t, J 7, 2H, CH2),
4.09 (t, J 7, 2H), 6.6–6.7 (m, 1H, ArH), 7.1–7.6 (m, 4H, ArH),
7.8–7.9 (m, 1H, ArH). δC (CDCl3) 32.58 (CH2), 42.51 (CH2),
59.29 (CH2), 101.07 (CH), 109.34 (CH), 119.23 (CH), 120.90
(CH), 121.39 (CH), 127.90 (CH), 128.56 (C), 135.97 (C). m/z
175 (M+, 45%), 131 (36), 130 (100), 117 (7), 103 (7), 89 (7),
77 (8). Found: 175.0993 [M+]. Calc. for C11H13NO: 175.0997.
3-(1H-Indol-1-yl)propan-1-amine 13b
νmax (neat)/cm−1 2934, 2870, 1670, 1609, 1508, 1464, 1313,
1244, 1186, 743. δH (CDCl3) 1.55 (br s, 2H, NH2), 1.91 (quin.,
J 7, 2H, CH2), 2.65 (t, J 7, 2H, CH2), 4.16 (t, J 7, 2H, CH2), 6.4–
6.6 (m, 1H, ArH), 6.8–7.4 (m, 4H, ArH), 7.5–7.8 (m, 1H, ArH).
δC (CDCl3) 33.68 (CH2), 39.34 (CH2), 43.78 (CH2), 101.12
(CH), 109.29 (CH), 119.23 (CH), 120.94 (CH), 121.39 (CH),
127.70 (CH), 128.64 (C), 136.01 (C). m/z 174 (M+, 57%), 156
(20), 144 (15), 131 (100), 130 (65), 117 (26), 89 (11), 77 (12),
63 (5), 30 (20). Found: 174.1162 [M+]. Calc. for C11H14N2:
174.1157.
2,3,5,6-Tetrahydro-1H-pyrimido[1,2-a]quinoline 8b
Bp136–138◦C(1.5 mmHg). νmax (neat)/cm−1 2941, 2854, 1655,
1649, 1637, 1456, 1392, 1313, 1265, 752. δH (CDCl3) 2.02
(quin., J 6, 2H, CH2), 2.4–3.0 (m, 4H, CH2), 3.2–3.7 (m, 4H,
CH2), 6.6–7.4 (m, 4H, ArH). δC (CDCl3) 21.91 (CH2), 26.18
(CH2), 32.33 (CH2), 43.49 (CH2), 43.86 (CH2), 112.23 (CH),
120.94 (CH), 126.48 (C), 127.29 (CH), 127.70 (CH), 141.55 (C),
153.15 (C). m/z 186 (M+, 93%), 185 (100), 158 (17), 130 (41),
118 (8), 103 (8), 91 (8), 77 (10), 65 (5), 51 (4). Found: 186.1191
[M+]. Calc. for C12H14N2: 186.1157.
Dimethyl Tetrahydropyrrolizine-1,1-dicarboxylate 15a
νmax (neat)/cm−1 2955, 2874, 1734, 1456, 1437, 1265, 1225,
1198, 1134, 1076. δH (CDCl3) 1.1–2.7 (m, 9H, CH, CH2 × 4),
2.8–3.2 (m, 2H, CH2), 3.73 (s, 3H, CH3), 3.76 (s, 3H, CH3). δC
(CDCl3) 23.62 (CH2), 26.35 (CH2), 28.71 (CH2), 32.70 (CH2),
52.37 (CH3), 52.82 (CH3), 55.50 (CH2), 63.65 (C), 68.70 (CH),
170.66 (C=O), 171.64 (C=O). m/z 227 (M+, 41%), 196 (44),
168 (M+ − CO2Me, 23), 140 (45), 127 (22), 108 (19), 84 (40),
83 (100), 55 (47), 42 (23). Found: 227.1140 [M+]. Calc. for
C11H17NO4: 227.1158.
Imidazo[1,2-a]quinoline 9
Bp 156–158◦C (2 mmHg). νmax (neat)/cm−1 1612, 1535, 1447,
1420, 1317, 806, 752. δH (CDCl3) 6.8–7.5 (m, 4H,ArH), 7.6–8.0
(m, 4H, ArH). δC (CDCl3) 110.96 (CH), 115.00 (CH), 115.53
(CH), 117.23 (CH), 123.26 (C), 124.57 (CH), 125.83 (C), 128.60
(CH), 128.96 (CH), 132.55 (CH), 143.91 (C). m/z 168 (M+,
100%), 140 (7), 128 (14), 114 (11), 101 (4), 84 (9), 75 (3), 63
(3). Found: 168.0666 [M+]. Calc. for C11H8N2: 168.0687.
Dimethyl Hexahydroindolizine-8,8-dicarboxylate 15b
νmax (neat)/cm−1 2953, 2789, 1732, 1435, 1304, 1261, 1217,
1115, 1080, 1000. δH (CDCl3) 1.4–2.8 (m, 11H, CH, CH2 × 5),
2.9–3.3 (m, 2H, CH2), 3.70 (s, 3H, CH3), 3.75 (s, 3H, CH3). δC
(CDCl3) 21.18 (CH2), 22.60 (CH2), 26.18 (CH2), 32.45 (CH2),
51.88 (CH3), 52.29 (CH3), 53.26 (CH2), 55.10 (CH2), 57.01 (C),
67.76 (CH), 169.97 (C=O), 171.56 (C=O). m/z 241 (M+, 42%),
210 (39), 182 (M+ − CO2Me, 55), 122 (28), 97 (100), 96 (75),
83 (40), 82 (36), 69 (58), 41 (25). Found: 241.1313 [M+]. Calc.
for C12H19NO4: 241.1314.
2,3,5,6-Tetrahydroimidazo[2,1-a]isoquinoline 11a
Bp 127–129◦C (2 mmHg). νmax (neat)/cm−1 2945, 2845, 1616,
1461, 1410, 1339, 1283, 1258, 1016, 739. δH (CDCl3) 2.6–3.5
(m, 6H, CH2), 3.7–4.0 (m, 2H, CH2), 7.0–8.2 (m, 4H, ArH). δC
(CDCl3) 28.95 (CH2), 45.49 (CH2), 52.65 (CH2), 53.14 (CH2),
126.19 (C), 126.56 (CH), 126.68 (CH), 127.86 (CH), 130.67
(CH), 136.98 (C), 162.39 (C). m/z 172 (M+, 59%), 171 (100),
169 (7), 156 (5), 144 (10), 130 (4), 115 (8), 89 (3), 77 (4), 56
(4). Found: 172.1003 [M+]. Calc. for C11H12N2: 172.1000.
Dimethyl Hexahydroindolizine-1,1-dicarboxylate 15c
Mp 56–58◦C. νmax (KBr)/cm−1 2924, 2793, 1730, 1441, 1271,
1248, 1150, 1117, 1088, 1003. δH (CDCl3) 0.9–2.8 (m, 11H, CH,
CH2 × 5), 3.0–3.3 (m, 2H, CH2), 3.73 (s, 3H, CH3), 3.74 (s, 3H,
CH3). δC (CDCl3)24.31(CH2), 24.84(CH2), 28.02(CH2), 31.19
(CH2), 52.33 (CH3), 52.45 (CH3), 53.39 (CH2), 53.83 (CH2),
61.94 (C), 69.31 (CH), 170.95 (C=O), 172.04 (C=O). m/z 241
(M+, 47%), 210 (55), 182 (M+ − CO2Me, 93), 150 (24), 122
(27), 98(41), 97(100), 69(38), 59(28), 41(23). Found:241.1305
[M+]. Calc. for C12H19NO4: 241.1314.
3,4,6,7-Tetrahydro-2H-pyrimido[2,1-a]isoquinoline 11b
Bp 140–142◦C (2 mmHg). νmax (neat)/cm−1 2934, 2843, 1622,
1601, 1576, 1456, 1418, 1366, 1308, 741. δH (CDCl3) 1.93
(quin., J 6, 2H, CH2), 2.7–3.0 (m, 2H, CH2), 3.1 –3.4 (m, 4H,
CH2), 3.5–3.8 (m, 2H, CH2), 7.0–8.2 (m, 4H, ArH). δC (CDCl3)
22.07 (CH2), 28.95 (CH2), 44.79 (CH2), 48.13 (CH2), 48.62

242
M. Okimoto et al.
Dimethyl Hexahydroquinolizine-1,1-dicarboxylate 15d
[2] (a) T. Chiba, M. Okimoto, H. Nagai, Y. Takata, J. Org. Chem. 1983,
48, 2968. doi:10.1021/JO00166A006
νmax (neat)/cm−1 2937, 2858, 1732, 1445, 1304, 1257, 1150,
1108, 1036, 997. δH (CDCl3) 1.1–2.6 (m, 13H, CH, CH2 × 6),
2.8–3.1 (m, 2H, CH2), 3.71 (s, 3H, CH3), 3.74 (s, 3H, CH3). δC
(CDCl3) 22.32 (CH2), 24.76 (CH2), 25.13 (CH2), 26.92 (CH2),
32.21 (CH2), 51.96 (CH3), 52.57 (CH3), 55.58 (CH2), 57.70
(CH2), 58.35 (C), 66.09 (CH), 170.29 (C=O), 171.80 (C=O).
m/z 255 (M+, 30%), 224 (29), 196 (M+ − CO2Me, 32), 124
(26), 111 (79), 97 (38), 96 (25), 83 (100), 55 (21), 41 (14).
Found: 255.1505 [M+]. Calc. for C13H21NO4: 255.1471.
(b) M. Okimoto, T. Chiba, J. Org. Chem. 1990, 55, 1070.
doi:10.1021/JO00290A048
(c) T. Chiba, M. Okimoto, J. Org. Chem. 1992, 57, 1375.
doi:10.1021/JO00031A014
(d) M. Okimoto, T. Itoh, T. Chiba, J. Org. Chem. 1996, 61, 4835.
doi:10.1021/JO960282E
(e) M. Okimoto, Y. Takahashi, Bull. Chem. Soc. Jpn. 2002, 75, 2059.
doi:10.1246/BCSJ.75.2059
(f) M. Okimoto, Y. Takahashi, T. Kakuchi, Synthesis (Mass.) 2003,
2057. doi:10.1055/S-2003-41048
(g) M. Okimoto, K. Numata, K. Tomozawa, T. Shigemoto, M. Hoshi,
Y. Takahashi, Aust. J. Chem. 2005, 58, 560. doi:10.1071/CH05090
[3] (a) K. Nyberg, R. Servin, Acta Chem. Scand. B 1976, 30, 640.
(b) T. Shono, H. Hamaguchi, Y. Matsumura, J. Am. Chem. Soc. 1975,
97, 4264. doi:10.1021/JA00848A020
Dimethyl 7-Methylhexahydroindolizine-
1,1-dicarboxylate 15e
νmax (neat)/cm−1 2953, 2795, 1734, 1456, 1437, 1273, 1233,
1200, 1150, 1084. δH (CDCl3) 0.95 (d, J 7, 3H, CH3), 1.0–
2.8 (m, 10H, CH × 2, CH2 × 4), 3.0–3.3 (m, 2H, CH2), 3.73 (s,
3H, CH3), 3.74 (s, 3H, CH3). δC (CDCl3) 21.99 (CH3), 31.23
(CH), 31.60 (CH2), 33.51 (CH2), 36.32 (CH2), 52.33 (CH3),
52.45 (CH3), 52.98 (CH2), 53.10 (CH2), 61.77 (C), 69.18 (CH),
170.91 (C=O), 172.00 (C=O). m/z 255 (M+, 26%), 224 (31),
196 (M+ − CO2Me, 60), 164 (15), 112 (81), 111 (100), 96 (23),
69 (25), 59 (21), 41 (19). Found: 255.1441 [M+]. Calc. for
C13H21NO4: 255.1471.
(c) S. D. Ross, M. Finkelstein, E. J. Rudd, J. Org. Chem. 1972, 37,
2387. doi:10.1021/JO00980A006
(d) J. E. Barry, M. Finkelstein, E.A. Mayeda, S. D. Ross, J. Org. Chem.
1974, 39, 2695. doi:10.1021/JO00932A006
[4] (a) I.Tabakovic, M.Trikovnik, D. Galijas, J. Electroanal. Chem. 1978,
86, 241. doi:10.1016/S0022-0728(78)80373-8
(b) R. Hazard, A. Tallec, Bull. Soc. Chim. Fr. 1975, 679.
[5] T. Chiba, Y. Takata, J. Org. Chem. 1977, 42, 2973. doi:10.1021/
JO00438A005
[6] S. Torii, T. Yamanaka, H. Tanaka, J. Org. Chem. 1978, 43, 2882.
doi:10.1021/JO00408A029
Dimethyl 2-Ethyl-1-propylpyrrolidine-
3,3-dicarboxylate 15f
[7] W. Schneider, B. Müller, Arch. Pharm. (Weinheim) 1961, 294, 360.
doi:10.1002/ARDP.19612940607
[8] (a) W. Schneider, B. Müller,Arch. Pharm. (Weinheim) 1962, 295, 571.
doi:10.1002/ARDP.19622950803
(b) H. Möhrle, E. Tot, S. Steiner, J. Prakt. Chem. 1996, 338, 711.
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(c) U. Azzena, L. Pisano, M. Pittalis, Heterocycles 2004, 63, 401.
[9] B. B. Lohray, V. Bhushan, A. S. Reddy, V. V. Rao, Indian J. Chem.
2000, 39B, 297.
Bp 108–110 (1.5 mmHg). νmax (neat)/cm−1 2959, 2876, 1736,
1456, 1435, 1267, 1232, 1194, 1157, 1072. δH (CDCl3) 0.89
(t, J 7, 3H, CH3), 0.92 (t, J 7, 3H, CH3), 1.2–1.7 (m, 3H, CH,
CH2), 2.0–2.8 (m, 6H, CH2), 3.0–3.4 (m, 2H, CH2), 3.71 (s,
3H, CH3), 3.74 (s, 3H, CH3). δC (CDCl3) 10.91 (CH3), 11.85
(CH3), 22.15 (CH2), 25.37 (CH2), 32.62 (CH2), 51.19 (CH2),
52.25 (CH3), 52.65 (CH3), 58.23 (CH2), 63.57 (C), 69.43 (CH),
170.38 (C=O), 172.33 (C=O). m/z 257 (M+, 8%), 229 (15), 228
(M+ − Et, 100), 198 (M+ − CO2Me, 28), 159 (5), 140 (12), 114
(9), 84 (8), 59 (7). Found: 257.1556 [M+]. Calc. for C13H23NO4:
257.1627.
[10] N. Finch, C. W. Gemenden, J. Org. Chem. 1973, 38, 437.
doi:10.1021/JO00943A006
[11] W. Wendeline, H. Keimelmayr, M. Huber, Sci. Pharm. 1989, 57, 391.
[12] M. Okimoto,T.Yoshida, M. Hoshi, K. Hattori, M. Komata, K. Numata,
K. Tomozawa, Synlett 2006, 1753. doi:10.1055/S-2006-944205
[13] (a) N. K. Hart, S. R. Johns, J. A. Lamberton, Aust. J. Chem. 1972,
25, 817.
Dimethyl 1-Butyl-2-propylpyrrolidine-
3,3-dicarboxylate 15g
(b) N. J. Leonard, The Alkaloids (Ed. R. H. F. Manske) 1960, Vol. 6,
p. 35 (Academic Press: New York, NY).
(c) J. S. Glasby, Encyclopedia of the Alkaloids 1975 (Plenum Press:
New York, NY, London).
Bp 124–126 (1.5 mmHg). νmax (neat)/cm−1 2957, 2934, 2874,
2808, 1736, 1456, 1435, 1269, 1223. δH (CDCl3) 0.90 (t, J 7,
3H, CH3), 0.91 (t, J 7, 3H, CH3), 1.1–1.7 (m, 9H, CH2 × 4, CH),
1.9–2.8 (m, 4H, CH2), 3.0–3.5 (m, 2H, CH2), 3.71 (s, 3H, CH3),
3.73 (s, 3H, CH3). δC (CDCl3) 14.09 (CH3), 14.42 (CH3), 19.79
(CH2), 20.61 (CH2), 31.19 (CH2), 32.62 (CH2), 34.90 (CH2),
51.23 (CH2), 52.25 (CH3), 52.69 (CH3), 55.38 (CH2), 63.67 (C),
68.04 (CH), 170.46 (C=O), 172.37 (C=O). m/z 285 (M+, 10%),
254 (15), 243 (27), 242 (M+ − Pr, 100), 226 (M+ − CO2Me,
3), 200 (10), 140 (23), 114 (9), 98 (14), 59 (8), 42 (9). Found:
285.1926 [M+]. Calc. for C15H27NO4: 285.1940.
[14] (a) N. A. Khatri, H. F. Schmitthenner, J. Shringarpure, S. M. Weinred,
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1980, 21, 1373. doi:10.1016/S0040-4039(00)74580-1
(c) T. L. Macdonald, B. Narayanan, J. Org. Chem. 1983, 48, 1129.
doi:10.1021/JO00155A049
(d) R. V. Stevens, The Total Synthesis of Natural Products (Ed.
J.ApSimon) 1977,Vol. 3, p. 439 (Wiley–Interscience: NewYork, NY).
(e) D. J. Robins, Advances in Heterocyclic Chemistry (Eds
A. R. Katritzky, A. J. Boulton) 1979, Vol. 24, p. 247 (Academic Press:
New York, NY).
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