Electrophilic substitution of indole on the benzene moiety: A synthesis of 5-acyl- and 5-aroylindoles

Article abstract of DOI:10.1055/s-1998-2167

The Vilsmeier-Haack intermediate of indole is acylated, in the presence of a certain excess of AlCl₃, exclusively at the benzene ring and especially at position-5. The formed acylindolecarboxaldehydes are readily decarbonylated on heating with Pd/C in mesitylene. Thus, 5-acylindoles and 5- aroylindoles are synthesized in a two step sequence from indole with overall yield of 30-46%.

Full text of DOI:10.1055/s-1998-2167

October 1998
SYNTHESIS
1519
Electrophilic Substitution of Indole on the Benzene Moiety: A Synthesis of 5-Acyl- and
-Aroylindoles
5
Vassilis J. Demopoulos,* Ioannis Nicolaou
Department of Pharmaceutical Chemistry, School of Pharmacy, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece
Fax +30(31)997622, E-mail: vdem@pharm.auth.gr
Received 11 September 1997; revised 2 March 1998
Abstract: The Vilsmeier-Haack intermediate of indole is acy-
AlCl was the catalyst of choice in this type of reaction.
3
lated, in the presence of a certain excess of AlCl , exclusively at
3
The use of FeCl resulted in extensive decomposition and
3
the benzene ring and especially at position-5. The formed acylin-
dolecarboxaldehydes are readily decarbonylated on heating with
Pd/C in mesitylene. Thus, 5-acylindoles and 5-aroylindoles are
synthesized in a two step sequence from indole with overall yield
of 30–46%.
isolation of only low yields of acylated product. It is worth
noting that the selectivity in this case is reversed, giving
rise to a predominant substitution at the 6-position (entry
5). SnCl , TiCl and BCl were all ineffective catalysts, re-
4
4
3
Key words: indole Vilsmeier-Haack intermediate, Friedel-Crafts
acylation, Pd/C decarbonylation, 5-acylindoles, 5-aroylindoles
sulting in no acetylation products. We noted that with the
addition of the excess AlCl (entries 2–4), and after ap-
3
proximately 10 minutes, the Vilsmeier–Haack intermedi-
ate of indole insoluble in 1,2-dichloroethane had dis-
solved. This did not happen in the cases of the unsuccess-
ful reactions (entries 1 and 5–8). Increasing the length of
the side chain or introducing a branching in the acylating
agent (e.g., propionyl and isobutyryl chlorides, entries 9
and 10) did not substantially affect the overall yield or the
ratio of the formed products. On the other hand, attempts
to acetylate directly indole-3-carboxaldehyde resulted in a
low yield (20%) of almost equimolar quantities of 4a and
The addition of electrophiles to the C-3 position of the in-
dole ring is perhaps the most characteristic reaction of this
class of heterocycles. Acylation of indoles has been
1
widely used and Friedel–Crafts acylation is one of the best
methods for introducing an acyl group at the 3-position of
2
,3
1
-phenylsulfonylindole. 1-Acylindoles are also acylat-
ed at the 3-position. However, their reaction with excess
of halogenacyl chlorides in the presence of excess alumi-
num chloride gives the corresponding 1,6-diacylindoles.4
The acylation of indoles bearing electron withdrawing
groups (ethoxycarbonyl or acetyl) at positions 2 or 3, un-
der certain conditions, leads to substitution of the benzene
part of the heterocycle, especially at the C-5 position.5–7
Analogous electrophilic reactions have also been studied
with pyrroles deactivated with electron withdrawing func-
tionalities, and it has been shown that the Vilsmeier–
Haack intermediate of pyrrole readily reacts, under
Friedel–Crafts acylation conditions, leading to a one-pot
5
a along with 4% of 6a and a 62% recovery of the starting
aldehyde.
8
synthesis of 4-acylpyrrole-2-carboxaldehydes. The latter
could be decarbonylated with the action of Raney Ni or on
heating with Pd/C.
In the present study, we investigated the Friedel–Crafts
acylation of the Vilsmeier–Haack intermediate of indole
(1) (Scheme 1) and the subsequent decarbonylation of the
formed acylindole-3-carboxaldehydes 4–5 (Scheme 2).
The target 5-substituted indoles 7 are of interest as, for
example, in the synthesis of serotonin receptor agonists or
9
,10
antagonists.
Scheme 1
The results of these Friedel–Crafts reactions are presented
in the Table. We found that the reaction of the Vilsmeier–
Haack intermediate of indole (1) with acetyl chloride and
a certain excess of AlCl (entry 2) leads to the formation The use of an aroyl halide (eg. benzoyl or p-toluoyl chlo-
3
of the isomeric acylindoles 4a, 5a and 6a. The isomer 4a ride, entries 11 and 12) as the acylating agent, resulted in
substituted at the 5-position was the major product and the formation of the corresponding aroylindole isomers,
could be isolated in 42% yield from indole in this one-pot 4d–5d and 4e–5e. In these reactions, for reproducible re-
reaction. Smaller amounts of the Lewis acid catalyst (en- sults, we found it necessary to use a greater excess of the
try 1) resulted in insignificant acetylation. Larger amounts Lewis acid catalyst. Again, the 5-substituted isomers (4d
of AlCl (entry 4) or the use of nitromethane (entry 3) did and 4e) were the predominant products and could be iso-
3
not change the preferred position of electrophilic attack. lated in 59% and 46% yield, respectively.

1
520
Papers
Table. Friedel-Crafts Reactions of the Vilsmeier-Haack Intermediate of Indole
SYNTHESIS
Entry
Acylating Chloride
Lewis Acid
Ratio
Yield (%)
1
: 2 : Lewis Acid
3
4 and 5
(ratio 4 : 5)
6
%
1
2
3
4
5
6
7
8
9
2a (Me)
2a (Me)
2a (Me)
2a (Me)
2a (Me)
2a (Me)
2a (Me)
2a (Me)
2b (Et)
AlCl3
AlCl3
AlCl3
AlCl3
FeCl3
SnCl4
TiCl4
BCl3
AlCl3
AlCl3
AlCl3
AlCl3
1 : 1.12 : 1.22
1 : 1.12 : 2.44
1 : 1.12 : 2.44b
1 : 1.12 : 3.66
1 : 1.12 : 2.44
1 : 1.12 : 2.44
1 : 1.12 : 2.44
1 : 1.12 : 2.44
1 : 1.12 : 2.44
1 : 1.12 : 2.44
1 : 1.12 : 3.66
1 : 1.12 : 3.66
79
0
0
0
7
–
–
–
0
0
0
0
2
0
12
13
9
6
0
0
0
11
7
0
a
a
a
a
80 (75 : 25)
64 (75 : 25)
77 (70 : 30)
14 (20 : 80)
0
c
c
0
0
c
a
a
d
70 (80 : 20)
76 (75 : 25)
79 (75 : 25)
1
1
1
0
1
2
2c (i-Pr)
2d (Ph)
2e (4-MeC H )
63 (73 : 27)d
0
6
4
a
1
The ratio is based on the H NMR spectrum of the mixture. The peak at δ = 10 (which corresponds to the CHO of both 4a–c and 5a–c) was
compared with the peak at δ = 8.79 (which corresponds to the H-4 of 4a–c).
b
c
d
Nitromethane (equimolar to AlCl ) was added.
Sole product based on TLC.
The ratio is based on the isolated isomers.
3
Deformylation or deacylation of the C-3 substituent of
certain indoles has been reported by the action of ethylene
glycol and p-toluenesulfonic acid in refluxing ben-
1
1,12
zene
or on heating in strongly basic solutions for ex-
1
3
tended periods. However, under these conditions only
small amounts of the desired product from 4a were
formed. It has been shown that ethyl 5-formylpyrrole-3-
glyoxylate is decarbonylated by the action of Pd/C in re-
1
4
fluxing mesitylene. We found that 4a is readily decar-
bonylated under similar conditions. Also, it could be de-
carbonylated in refluxing xylene, but this reaction re-
quires extended periods of time for completion. For
preparative purposes, it is not necessary to separate 4a
from 5a. Deformylation of this mixture of isomers result-
ed in the isolation of 5-acetylindole (7a) in 46% overall
yield from indole in a two step sequence. In a similar man-
ner 7b and 7c were formed in 37 and 41% overall yield
from indole, respectively. This represents a facile method-
ology and compares favorably with previously reported
procedures which either involve substitution of 1-potas-
Scheme 2
isolated compounds (4a, 4d,e, 5d,e, 6a–c, 7a–e and 8a) satisfactory
microanalyses were obtained: C±0.34, H±0.33, N±0.40.
Formation and Friedel–Crafts Reaction of the Vilsmeier–Haack
Intermediate of Indole (1) (Entries 2 and 9–12, Table); General
Procedure:
Oxalyl chloride (0.7 g, 5.5 mmol) in 1,2-dichloroethane (10 mL) was
added dropwise over a period of 5 min into a stirred, ice cold solution
of DMF (0.41 g, 5.6 mmol) in 1,2-dichloroethane (10 mL) under a N2
atmosphere, and then the mixture was allowed to warm to r.t. for
15 min. The mixture was cooled (ice bath) and a solution of indole
9
sio-5-lithioindole or are roundabout routes via N-acetyl-
1
5,16
indoline.
5-Benzoylindole-3-carbaldehyde (4d) was
(
0.59 g, 5 mmol) in 1,2-dichloroethane (10 mL) was rapidly added.
The resulting mixture was warmed to r.t. for 15 min and then cooled
ice bath). AlCl (12.2 or 18.3 mmol) was rapidly added and the mix-
also readily decarbonylated to 7d in 62% yield. Thus, the
overall yield of formation of 5-benzoylindole is 37% from
indole. To the best of our knowledge, 7d has been previ-
(
3
ture was allowed to warm to r.t. for 15 min. The mixture was cooled
ice bath), a solution of the appropriate acyl or aroyl chloride (5.6
1
7
ously reported only as a minor product in the photo-
Fries rearrangement of 1-benzoylindole; no physicochem-
ical properties were presented. Finally, decarbonylation
of 4e afforded 7e in 65% yield (30% from indole).
(
mmol) in 1,2-dichloroethane (5 mL) was rapidly added and the reac-
tion was allowed to proceed overnight at r.t.
1. Acylation Reactions: The reaction mixture was poured into ice/
water, basified with solid NaOH, stirred vigorously for 2 h, cooled
(ice bath) and acidified with concd HCl. It was filtered (if necessary),
the two phases were separated and the aqueous phase was extracted
with CH Cl . The combined organic extracts were washed with brine,
Melting points are uncorrected and were determined in open glass
capillaries using a Mel-Temp II apparatus. IR spectra were recorded
1
on a Perkin-Elmer 597 spectrophotometer and H NMR spectra on a
2
2
Bruker AW-80 spectrometer with internal TMS as reference. Elemen-
tal analyses were performed using a Perkin-Elmer 2400 CHN analyz-
er. Flash chromatography was carried out using Merck 9385 silica
filtered through a pad of anhyd MgSO and evaporated under reduced
4
pressure. The residue, combined with the precipitate, was flash chro-
matographed on silica gel with petroleum ether/EtOAc (2:1 to 0:1) as
eluent to afford the products as described below.
o
gel. Petroleum ether refers to the fraction with bp 40–60 C. For all

October 1998
SYNTHESIS
1521
7
-Acetyl-1H-indole-3-carboxaldehyde (6a); yield: 0.112 g (12%); mp
5-(4-Methylbenzoyl)-1H-indole-3-carboxaldehyde (4e); yield: 0.6 g
1
62–164°C (CH Cl /petroleum ether).
(46%); mp 206–207°C (toluene).
2
2
–
1
–1
IR (nujol): ν = 3320, 1640 cm .
IR (nujol): ν = 3170, 1630 cm .
1
1
H NMR (CDCl ): δ = 2.69 (s, 3H, CH ), 7.34 (t, 1H, J = 8 Hz, H-5),
H NMR (1:1 CDCl /DMSO-d ): d = 2.41 (s, 3H, CH ), 7.27 (d, 2H,
3
3
3
6
3
7
.81–7.93 (m, 2H, H-2,6), 8.56 (d, 1H, J = 8 Hz, H-4), 10.08 (s, 1H,
J = 7 Hz, phenyl H), 7.52–8.07 (m, 5H, H-2,6,7 and phenyl H), 8.6 (s,
CHO), 11 (br s, 1H, NH).
1H, H-4), 9.98 (s, 1H, CHO).
A mixture (75:25) of 5-acetyl-1H-indole-3-carboxaldehyde (4a) and
6
-acetyl-1H-indole-3-carboxaldehyde (5a); yield: 0.745 g (80%).
This mixture was twice recrystallized from absolute EtOH to afford
-acetyl-1H-indole-3-carboxaldehyde (4a) which was homogeneous
by TLC (silica gel) (CH Cl /Et O,1:1); yield: 0.39 g (42%); mp 238–
5
-Acylindoles 7a–c; General Procedure:
To a stirred suspension of the mixtures of isomers 4a–5a, 4b–5b and
c–5c (1.6 mmol) in mesitylene (10 mL) under a nitrogen atmosphere
5
4
2
2
2
was added Pd/C (10%, 0.06 g) and the mixture was refluxed for 4–7 h.
It was cooled to r.t., diluted with CH Cl , filtered through celite and
concentrated under reduced pressure.
2
40°C .
–
1
2
2
IR (nujol): ν = 3150, 1635 cm .
1
H NMR (1:1 CDCl /DMSO-d ): δ = 2.62 (s, 3H, CH ), 7.51 (dAB,
3
6
3
1
2
H, J = 8.6 Hz, H-7), 7.87 (dAB, 1H, J = 8.6 Hz, H-6), 8.08 (s, 1H, H-
), 8.79 (s, 1H, H-4), 10 (s, 1H, CHO).
1
-(1H-Indol-5-yl)ethanone (7a):
The residue was flash chromatographed on silica gel with petroleum
ether/EtOAc (1:0 to 5:1) as the eluent to afford 7a; yield: 0.128 g
7
-Propionyl-1H-indole-3-carboxaldehyde (6b); yield: 0.11 g (11%);
9
(
46% from indole); mp 72–74°C (CH Cl /petroleum ether) (Lit. 73–
mp 132–133°C (CH Cl /petroleum ether).
2
2
2
2
–
1
75°C).
IR (nujol): ν = 3320, 1640 cm .
–1
1
IR (nujol): ν = 3230, 1645 cm .
H NMR (CDCl ): δ = 1.25 (t, 3H, J = 7.3 Hz, CH ), 3.13 (q, 2H, J =
3
3
1
H NMR (CDCl ): δ = 2.65 (s, 3H, CH ), 6.63 (m, 1H, H-3, after addi-
7
.3 Hz, CH ), 7.32 (t, 1H, J =7.6 Hz, H-5), 7.8–7.98 (m, 2H, H-2,6),
3
3
2
tion of D O changes to d, J = 3 Hz), 7.26 (m, 1H, H-2, after addition of
8.54 (d, 1H, J = 7.6 Hz, H-4), 10.08 (s, 1H, CHO), 11.2 (br s, 1H, NH).
2
D O changes to d, J = 3 Hz), 7.36 (d, 1H, J = 9 Hz, H-7), 7.84 (dd, 1H,
A mixture (80:20) of 5-propionyl-1H-indole-3-carboxaldehyde (4b)
and 6-propionyl-1H-indole-3-carboxaldehyde (5b); yield: 0.704 g
2
J = 1.6, 9 Hz, H-6), 8.32 (s, 1H, H-4), 9.00 (br s, 1H, NH).
(70%).
1
-(1H-Indol-6-yl)ethanone (8a); yield: 0.01 g (4% from indole); mp
18
7
-Isobutyroyl-1H-indole-3-carboxaldehyde (6c); yield: 0.07 g (7%);
120–121°C (CH Cl /petroleum ether) (Lit. mp 122–122.5°C).
2 2
–1
mp 134–135°C (CH Cl /petroleum ether).
IR (nujol): ν = 3350, 1655 cm .
2
2
–
1
1
IR (nujol): ν = 3340, 1660, 1640 cm .
H NMR (CDCl ): δ = 1.29 (d, 6H, J = 6.6 Hz, CH ), 3.85 (q, 1H, J =
H NMR (CDCl ): δ = 2.6 (s, 3H, CH ), 6.6 (m, 1H, H-3, after addi-
3
3
1
tion of D O changes to d, J = 3 Hz), 7.38 (m, 1H, H-2, after addition
3
3
2
6
8
.6 Hz, CH), 7.36 (t, 1H, J = 7.4 Hz, H-5), 7.85–8.00 (m, 2H, H-2,6),
.58 (d, 1H, J = 7.4 Hz, H-4), 10.1 (s, 1H, CHO), 11.1 (br s, 1H, NH).
of D O changes to d, J = 3 Hz), 7.55–7.84 (m, 2H, H-4,5), 8.1 (s, 1H,
H-7), 8.8 (br s, 1H, NH).
2
A mixture (75:25) of 5-isobutyroyl-1H-indole-3-carboxaldehyde (4c)
and 6-isobutyroyl-1H-indole-3-carboxaldehyde (5c); yield: 0.813 g
1-(1H-Indol-5-yl)propan-1-one (7b):
(
76%).
The residue was flash chromatographed on silica gel with petroleum
ether/CH Cl (1:0 to 1:2) to afford 7b; yield: 0.146 g (37% from in-
2
2
Aroylation Reactions: The reaction mixture was poured into ice/wa-
ter, stirred vigorously for 4 h, the two phases were separated and the
aqueous phase was extracted with CH Cl . The aqueous phase was
basified with solid NaOH, stirred briefly (30 min), acidified with con-
cd HCl and extracted with CH Cl . The combined organic extracts
dole); mp 111–112°C (CH Cl /petroleum ether).
2
2
–
1
IR (nujol): ν = 3280, 1650 cm .
H NMR (CDCl ): δ = 1.23 (t, 3H, J = 6.4 Hz, CH ), 3.06 (q, 2H, J =
2
2
1
3
3
6
.4 Hz, CH ), 6.62 (m, 1H, H-3, after addition of D O changes to d, J
2
2
2
2
=
3.3 Hz), 7.25 (m, 1H, H-2, after addition of D O changes to d, J =
2
were washed with brine, dried (MgSO ) and evaporated under re-
4
3.3 Hz), 7.4 (d, 1H, J = 8.4 Hz, H-7), 7.87 (d, 1H, J = 8.4 Hz, H-6),
duced pressure. The residue was flash chromatographed on silica gel
with petroleum ether/EtOAc (2:1 to 0:1) as the eluent to afford the
products as given below.
8.33 (s, 1H, H-4), 8.87 (br s, 1H, NH).
No attempt was made to isolate and characterize the corresponding C-
6
isomer.
6
-Benzoyl-1H-indole-3-carboxaldehyde (5d); yield: 0.25 g (20%);
1
-(1H-Indol-5-yl)2-methylpropan-1-one (7c):
mp 157–159°C (CH Cl /petroleum ether).
2
2
–
1
The residue was flash chromatographed on silica gel with petroleum
ether/CH2Cl2 (1:0 to 1:2) to afford 7c; yield: 0.162 g (41% from in-
dole); mp 70°C (CH Cl /petroleum ether).
IR (nujol): ν = 3160, 1635, 1615 cm .
1
H NMR (CDCl ): δ = 7.4–7.66 (m, 3H, phenyl H), 7.7–7.87 (m, 3H,
3
2
2
H-2 and phenyl H), 7.93–8.08 (m, 2H, H-5,7), 8.32 (d, 1H, J = 8 Hz,
H-4), 10.04 (s, 1H, CHO), 10.2 (br s, 1H, NH).
–1
IR (nujol): ν = 3280, 1655 cm .
1
H NMR (CDCl ): δ = 1.23 (d, 6H, J = 6.6 Hz, CH ), 3.67 (q, 1H, J =
3
3
6
=
.6 Hz, CH), 6.63 (m, 1H, H-3, after addition of D O changes to d, J
3 Hz), 7.4 (d, 1H, J = 8.4 Hz, H-7), 7.89 (d, 1H, J = 8.4 Hz, H-6),
2
5
-Benzoyl-1H-indole-3-carboxaldehyde (4d); yield: 0.739 g (59%);
mp 202–204°C (EtOAc/petroleum ether).
–
1
8.33 (s, 1H, H-4), 8.8 (br s, 1H, NH).
No attempt was made to isolate and characterize the corresponding C-
6
IR (nujol): ν = 3100, 1640, 1605 cm .
1
H NMR (1:1 CDCl /DMSO-d ): δ = 7.48–7.59 (m, 4H, H-7 and
3
6
isomer.
phenyl H), 7.75–7.97 (m, 4H, H-2,6 and phenyl H), 8.66 (s, 1H, H-4),
1
0 (s, 1H, CHO).
5-Aroylindoles 7d and 7e; General Procedure:
6-(4-Methylbenzoyl)-1H-indole-3-carboxaldehyde (5e); yield: 0.224 g
A mixture of 4d or 4e (1.6 mmol) and Pd/C (10%, 0.06 g) in mesity-
(
17%); mp 223–224°C (CH Cl /petroleum ether).
lene (10 mL) was refluxed under a N atmosphere for 2 h. Workup as
above and flash chromatography on silica gel with petroleum ether/
EtOAc (1:0 to 4:1) afforded 7d and 7e.
2
2
2
–
1
IR (nujol): ν = 3160, 1660, 1625 cm .
1
H NMR (1:1 CDCl /DMSO-d ): δ = 2.42 (s, 3H, CH ), 7.28 (d, 2H,
3
6
3
J = 7.6 Hz, phenyl H), 7.57–8.09 (m, 5H, H-2,5,7 and phenyl H),
.26 (d, 1H, J = 8.4 Hz, H-4), 10.02 (s, 1H, CHO), 12.05 (br s, 1H,
NH).
(1H-Indol-5-yl)phenylmethanone (7d); yield: 0.22 (62% or 37% from
indole); mp 151–152°C (CH Cl /petroleum ether).
IR (nujol): ν = 3275, 1605 cm .
8
2
2
–1

1
522
Papers
SYNTHESIS
1
H NMR (CDCl ): δ = 6.6 (m, 1H, H-3, after addition of D O changes
(7) Tani, M.; Aoki, T.; Ito, S.; Matsumoto, S.; Hideshima, M.; Fu-
kushima, K.; Nozawa, R.; Maeda, T.; Tashiro, M.; Yokoyama,
Y.; Murakami, Y. Chem. Pharm. Bull. 1990, 38, 3261.
3
2
to d, J = 3.2 Hz), 7.2 (m, 1H, H-2, after addition of D O changes to d,
J = 3.2 Hz), 7.3–7.56 (m, 4H, H-7 and phenyl H), 7.67–7.88 (m, 3H,
H-6 and phenyl H), 8.12 (s, 1H, H-4), 8.95 (br s, 1H, NH).
2
(8) Anderson, H.J.; Loader, C.E.; Foster, A. Can. J. Chem. 1980,
58, 2527.
(
1H-Indol-5-yl)-p-tolylmethanone (7e); yield: 0.243 g (65 or 30%
(9) Yang, Y.; Martin, A.R.; Nelson, D.L.; Regan, J. Heterocycles
1992, 34, 1169.
from indole); mp 165–166°C (CH Cl /petroleum ether).
2
2
–
1
IR (nujol): ν = 3220, 1620 cm .
(10) Agarwal, A.; Pearson, P.P.; Taylor, E.W.; Li, H.B.; Dahlgren,
T.; Herslöf, M.; Yang, Y.; Lambert, G.; Nelson, D.L.; Regan,
J.W.; Martin, A.R. J. Med. Chem. 1993, 36, 4006.
(11) Kametani, T.; Kigawa, Y.; Takahashi, K.; Nemoto, H.; Fuku-
moto, K. Chem. Pharm. Bull. 1978, 26, 1918.
1
H NMR (1:1 CDCl /DMSO-d ): δ = 2.41 (s, 3H, CH ), 6.56 (m, 1H,
3
6
3
H-3, after addition of D O changes to d, J = 3.3 Hz), 7.15–7.82 (m,
7
2
H, H-2,6,7 and phenyl H), 8.06 (s, 1H, H-4), 10.6 (br s, 1H, NH).
(12) Nagarathnam, D. J. Heterocyclic Chem. 1992, 29, 1371.
(
(
(
1) Sundberg, R.J. Indoles; Academic Press: London, 1996; p 105.
2) Ketcha, D.M.; Gribble, G.W. J. Org. Chem. 1985, 50, 5451.
3) Wenkert, E.; Moeller, P.D.R.; Piettre, S.R.; McPhail, A.T.
J. Org. Chem. 1988, 53, 3170.
(13) Powers, J. C. Tetrahedron Lett. 1965, 655.
(14) Demopoulos, B.J.; Anderson, H.J.; Loader, C.E.; Faber, K. Can.
J. Chem. 1983, 61, 2415.
(15) Akagi, M.; Ozaki, K. Heterocycles 1987, 26, 61.
(16) Terent'ev, A.P.; Preobrazhenskaya, M.N.; Sorokina, G.M. Zhur.
Obshchei Khim. 1959, 29, 2875; Chem. Abstr. 1960, 54, 12098f.
(17) Oldroyd, D.L.; Weedon, A.C. J. Photochem. Photobiol., A
1991, 57, 207.
(
(
(
4) Nakatsuka, S.; Teranishi, K.; Goto, T. Tetrahedron Lett. 1994,
3
5, 2699.
5) Hino, T.; Torisawa, Y.; Nakagawa, M. Chem. Pharm. Bull.
982, 30, 2349.
6) Murakami, Y.; Tani, M.; Tanaka, K.; Yokoyama, Y. Chem.
Pharm. Bull. 1988, 36, 2023.
1
(18) Somei, M.; Natsume, M. Tetrahedron Lett. 1973, 27, 2451.