Reactions of ninhydrin with benzo[b]thiophenes

Article abstract of DOI:10.3987/COM-09-11744

The reaction of ninhydrin with benzo[b]thiophene in acetic acid in the presence of a small amount of sulfuric acid afforded a novel fluorenone compound fused to benzo[b]thiophene rings. In the reaction using 2,2′-bibenzo[b]thiophene, phthalide conjoined i

Full text of DOI:10.3987/COM-09-11744

HETEROCYCLES, Vol. 78, No. 10, 2009
2467
HETEROCYCLES, Vol. 78, No. 10, 2009, pp. 2467 - 2475. © The Japan Institute of Heterocyclic Chemistry
Received, 5th May, 2009, Accepted, 2nd June, 2009, Published online, 3rd June, 2009
DOI: 10.3987/COM-09-11744
REACTIONS OF NINHYDRIN WITH BENZO[b]THIOPHENES
Nono Suzue, Rie Ishii, Ryo Kamiya, Hidetsugu Wakabayashi, and Keiji
Kobayashi*
Department of Chemistry, Graduate School of Material Science, Josai University,
Sakado, Saitama 350-0295, Japan; E-mail: kobayak@josai.ac.jp
Abstract – The reaction of ninhydrin with benzo[b]thiophene in acetic acid in the
presence of a small amount of sulfuric acid afforded a novel fluorenone
compound fused to benzo[b]thiophene rings.
In the reaction using
2,2’-bibenzo[b]thiophene, phthalide conjoined in a spiro framework was
unexpectedly isolated along with an isocoumarin derivative which is fused to
benzo[b]thiophene and benzene rings. The product related to the latter was
obtained in the reaction of 3,3’-bithiophene with ninhydrin. The structures of
these novel heterocycles were supported by spectroscopic studies and X-ray
crystallography.
INTRODUCTION
Ninhydrin is known as a highly electrophilic compound because of its activated ketone functional group
at the 2-position.1 It reacts with a variety of nucleophiles, such as amines,2 alkenes,3 and methylene
compounds activated by neighboring carbonyl groups.4 The reactions are in most cases acid-catalyzed.
Arenes also enter into a Friedel-Crafts-type reaction with ninhydrin, affording 2-monoaryl- and/or
2,2-diaryl-1,3-indandiones.5
Phenols
undergo
ortho-selective
substitution
to
give
2-(2-hydroxyphenyl)-2-hydroxy-1,3-indandiones, which actually equilibrate to intramolecular hemiacetal
structures.6 Despite the versatility of the reactions of ninhydrin investigated thus far, studies of reactions
with heteroaromatic compounds have been extremely limited. Musgrave et al. have reported the
reaction of ninhydrin with thiophene in aqueous 75% v/v sulfuric acid.7 Other than this paper, to the
best of our knowledge, none has reported the reactions of ninhydrin with thiophene derivatives. Herein,
we report the results of our investigation on the reactions of benzo[b]thiophene derivatives with ninhydrin,
demonstrating the formation of unexpected compounds.

2468
HETEROCYCLES, Vol. 78, No. 10, 2009
RESULTS AND DISCUSSION
The thiophene compounds as illustrated below were used in the reactions with ninhydrin.
S
S
S
S
S
The reaction of benzo[b]thiophene was carried out in acetic acid under reflux by adding a small amount
of sulfuric acid as a catalyst. At the outset, the substitution of one and/or two thiophene rings at the
2-position of ninhydrin was anticipated because of the high reactivity to electrophiles of the carbon at this
position. In addition to such substituted products, 1 and 2, a novel product 3 was obtained.
S
S
S
O
O
O
O
S
OH
S
O
1
3
2
1
Compound 3 was isolated in 68% yield as an orange solid. Its H NMR spectrum is composed of six
doublets and six triplets in aromatic regions, indicating the existence of three sets of ortho-disubstituted
benzene rings. In the 13C NMR spectrum, twenty five signals were observed, among which twenty four
are assignable to aromatic carbons and one to a carbonyl carbon. On the basis of these data together
with the mass spectrum (m/e M+ 392) and IR spectrum (1703 cm-1), four isomeric structures 3A-3D are
conceivable.
S
S
S
S
S
S
S
S
O
O
O
O
3B
3A (3)
3C
3D
1
The most characteristic feature in the H NMR spectrum is the occurrence of one doublet at a low
magnetic field of 9.95 ppm. This proton should be located in close proximity to the carbonyl
functionality and be subject to the anisotropic effect. This suggestion was supported by the NMR
measurements in the presence of the shift reagent, Eu(fod)4. With increasing sift reagent concentration,
the signal at 9.95 ppm significantly moves downfield, whereas the other signals are not affected except
for the peri-proton at 7.75 ppm, which shows a slight downfield shift. On the basis of these
observations, 3A and 3B are the most plausible structures among the four isomers. Then, 3B is excluded,
because the compound in question showed no NOE effects between any protons, indicating that there are

HETEROCYCLES, Vol. 78, No. 10, 2009
2469
no protons close to each other through space. The protons separated spatially by a bay region, as shown
in compound 4, have been reported to exhibit NOE.8 The chemical shifts of 9.95 ppm and others are
consistent with those observed for protons in similar environments structurally, for example, in
compound 5.9 Thus, the structure of the orange product was identified as 3.
S
H
NOE
H
O
H
9.90 ppm
O
5
4
The formation of 3 is formally a result of dehydration from two benzo[b]thiophenes and one ninhydrin
molecule. A possible mechanism for the formation of 3 is depicted in Scheme 1. Benzo[b]thiophene is
known to undergo electrophilic attack at the -carbon atom. Thus, it is reasonable that the reaction of
benzo[b]thiophene with ninhydrin affords 1 and 2. The cationic intermediate yielding these substituted
products is capable of entering into a reaction with a second molecule of benzo[b]thiophene, followed by
intramolecular condensation to connect the carbonyl carbon and the -carbon. An analogous
thiophene-condensed fluorenone product has been isolated in the reaction of ninhydrin with thiophene,7
whereas the condensation pattern of the thiophene rings is different from the case in question. This is
not unusual, because benzo[b]thiophene undergoes electrophilic attack at the -position, whereas
thiophene undergoes it at the -position.
S
S
O
O
S
O
+
+
H+
OH
OH
H
H
H
S
OH
O
O
O
H
-H2O
-H2O
S
O
H
OH
O
S
S
H+
H
S
H
H
HO
+
O
O
OH
3
H
S
H
-H+
S
-H+
-H2O
S
+
O
O
Scheme 1
The results obtained for benzo[b]thiophene prompted us to examine the reaction of ninhydrin with
2,2’-bibenzo[b]thiophene, since this might form 3B. Unexpectedly, actual products were lactone
compounds 6 (70%) and 7 (5%). Products due to substitution at the 2-carbon of ninhydrin were not
obtained.

2470
HETEROCYCLES, Vol. 78, No. 10, 2009
O
O
O
O
O
S
S
S
S
7
6
Compound 6, a colorless solid, is formulated as C25H12O2S2 on the basis of the mass spectrum and
combustion data. The IR spectrum of 6 displays carbonyl absorption at 1743 cm-1, and the 1H-NMR
spectrum has three sets of doublet-triplet-triplet-doublet patterns. The structure of 6 was eventually
established by X-ray crystallographic analysis for a single crystal obtained by recrystallization from
chloroform (Figure 1). The molecule is not planar but significantly twisted. The isocoumarin
substructure deviates more significantly from planarity than the benzobis(benzothiophene) ring.
Because of this chiral structure, the crystal of 6 includes a pair of dl molecules and, that is a racemic
crystal of space group Pca21. Compound 7 is a yellow colored solid, showing a UV/Vis absorption
maximum at 418 nm. The structure of spirolactone 7 was also elucidated by X-ray crystallographic
analysis for a single crystal obtained by recrystallization from chlorobenzene (Figure 1).
(a)
(b)
Figure 1
(a) ORTEP drawing of 6. (b) ORTEP drawing of 7.
The formation of 6 is interpreted as a result of dehydration accompanied by ring expansion. We assume
a possible mechanism of the formation of 6 as shown in Scheme 2, with reference to the analogous
formation of indole-condensed isocoumarin using ninhydrin as the starting compound, as reported by
Bullington.10 The acid-promoted addition of benzo[b]thiophene to ninhydrin would lead to the
formation of an , -dihydroxyketone and subsequently to a hydroxyl epoxide intermediate. The latter
could undergo dehydration followed by ring expansion through the opening of the epoxide ring to

HETEROCYCLES, Vol. 78, No. 10, 2009
2471
neutralize the positive charge. The literature indicates the formation of isocoumarin compounds from
hemiacetals derived from ninhydrin and phenols. 11
O
OH
HO
O
O
HO
HO
OH
H+
O
OH
OH
HO
OH
H+
S
S
H2O
S
S
S
S
S
S
HO
+
O
6
H+
S
S
HO
O
H+
+
H2O
HO
HO
H O
S
S
O
2
O
HO
+
7
H+
H+
H
oxidation
S
S
S
S
Scheme 2
The rationale for the formation of spirolactone 7 is more difficult because it must involve oxidation. We
assume that oxygen in the air participates in the oxidation process, since the reaction under nitrogen
atmosphere extremely lowered the yield of 7. At this stage, however, the detailed mechanism for the
formation of 7 is not confirmative.
Likewise in the formation of 6 and/or 7 from 2,2’-bibenzo[b]thiophene, 3,3’-bithiophene is considered to
form intramolecular cyclization products because the reactive -positions of two thiophene units are open
for electrophilic attack. As anticipated, compound 8 was isolated as a colorless solid in 64% yield.
The spirolactone product corresponding to 7 was not detected this time. The formation of 8 may be
consistently explained by the intervention of an epoxide as an intermediate. It is, therefore, reasonable
that in this reaction, the two thiophene rings are condensed with the isocoumarin ring in different
orientation from that in the case of 6.
O
O
S
S
8

2472
HETEROCYCLES, Vol. 78, No. 10, 2009
In conclusion, we have demonstrated that ninhydrin shows the high electrophilic reactivity to
benzo[b]thiophenes. Principally, three types of reaction were observed: substitution at the 2-carbon by
the thiophenes, condensation by dehydration to give benzo[b]thiophene-fused fluorene, and the
rearrangements of the cationic intermediate to give novel isocoumarin and spirolactone compounds, 6 and
7.
EXPERIMENTAL
All melting points were determined using a Yanaco MS-500V apparatus and uncorrected. The IR
spectra were obtained using a Shimadzu FT-IR 8200PC spectrometer. The UV spectra were recorded by
1
13
Shimadzu FTUV-2200PC. The H-NMR (500 MHz) and C-NMR (125 MHz) spectra were recorded
using a JEOL -500 spectrometer. Chemical shifts are given in  values (ppm) using TMS as the
internal standard. The mass spectra were taken on a Shimadzu GCMS-QP5050A mass spectrometer.
Elementary combustion analyses were performed using a Yanaco CHN CORDER MT-6 analyzer. All
reactions were monitored by TCL employing a 0.25 mm silica gel plate (Merck 60F 254). Column
chromatography was carried out on silica gel (Merck 60N spherical). The yields of products are based
on the initial weights of ninhydrin used.
(a) Reaction with benzo[b]thiophene. A solution of ninhydrin (0.89 g, 5 mmol) and benzo[b]thiophene
(1.32 g, 10 mmol) in acetic acid (60 mL) was refluxed with conc H2SO4 (0.2 mL) for 6 h. The usual
workup and chromatography of the reaction mixture gave the fraction of 2 first, followed by the fraction
of 1 and lastly the fraction of 3. Compound 3 was recrystallized from hot toluene or EtOAc.
2-Hydroxy-2-benzo[b]thien-3-yl-1,3-indandione (1): 0.29 g, 20%. yellow crystals. mp 212-214 ºC. UV
1
max (CHCl3) 358 nm (log =3.08); H-NMR (CDCl3)  3.44 (1H, s, OH), 7.19 (1H, s), 7.38 (1H, ddd,
J=8.2, 7.0, 0.9 Hz), 7.46 (1H, ddd, J=8.2, 7.3, 1.2 Hz), 7.80 (1H, dd, J=7.9, 0.9 Hz), 7.93 (2H, m), 8.06
13
(2H, m), 8.57 (1H, dd, J=7.9, 0.9 Hz); C-NMR (CDCl3)  122.54, 124.34, 124.68, 125.05, 125.30,
127.18, 130.80, 136.71, 136.80, 140.11, 140.49, 196.75; MS m/Z=294 (100%, M), 266 (19%, M-CO); IR
(KBr) 3360, 1705 cm-1; Anal. Calcd for C17H10O3S: C, 69.37; H, 3.42; S, 10.89%. Found: C, 69.42; H,
3.50, S, 10.71%.
2,2-Bisbenzo[b]thien-3-yl-1,3-indandione (2): 0.33 g, 16%. yellow crystals. mp 195-198 ºC. UV max
1
(CH3Cl) 370 nm (log =3.31); H-NMR (CDCl3)  7.17 (1H, t, J=8.2 Hz), 7.26 (2H, s), 7.28 (2H, t,
J=8.2 Hz), 7.64 (2H, d, J=8.2 Hz), 7.81 (2H, d, J=8.2 Hz), 7.95 (2H, m), 8.12 (2H, m); 13C-NMR
(CDCl3)  122.68, 124.20, 124.35, 124.40, 124.43, 124.54, 127.40, 129.82, 136.52, 137.12, 140.44,
140.59, 197.35, MS m/Z=410 (100%, M), 365 (24%, M-CO-OH), 353 (23%, M-CO-CHO); IR (KBr)
1708 cm-1; Anal. Calcd for C25H14O2S2: C, 73.17; H, 3.41; S, 15.61%. Found: C, 73.42; H,3.53, S,

HETEROCYCLES, Vol. 78, No. 10, 2009
2473
15.71%.
Indeno[2,3-i]benzo[1,2-b:3,4-b’]bis[1]benzothiophen-6-one (3): 1.32 g, 68%. orange solid. mp 414-416
ºC. UV max (CHCl3) 478 nm (log =2.82); 1H-NMR (CDCl3)  7.37 (1H, t, J=6.7 Hz), 7.57 (1H, t), 7.59
(1H, t), 7.61 (1H, t), 7. 63 (1H, t), 7.69 (1H, t, J=7.3 Hz), 7.75 (1H, d, J=7.3 Hz), 7.80 (1H, d, J=7.3 Hz),
7. (1H, d, J=7.9 Hz), 8.05 (1H, d, J= 7.9 Hz), 8.60 (1H, d, J=7.9 Hz), 9.95 (1H, d, J=8.6 Hz); 13C-NMR
(CDCl3)  121.84, 122.18, 123.10, 124.17, 125.10, 125.29, 125.61, 127.43, 127.79, 127.93, 129.01,
130.94, 132.59, 133.63, 133.65, 134.19, 134.69, 134.85, 135.30, 135.46, 136.48, 138.82, 141.48, 142.85,
195.00; MS m/Z=392 (100%, M), 364 (19%, M-CO), 362 (20%, M-H2CO); IR (KBr) 1703 cm-1; Anal.
Calcd for C25H12OS2: C, 76.50; H, 3.08; S, 16.34%. Found: C, 76.20; H, 3.16, S, 15.75%.
(b) Reaction with 2,2’-bibenzo[b]thiophene.
A solution of ninhydrin (0.89 g, 5 mmol),
2,2’-bibenzo[b]thiophene (2.66 g, 10 mmol) in acetic acid (70 mL) was refluxed with conc H2SO4 (0.2
mL) was refluxed for 5 h and treated in a manner similar to that described above. The fraction of 6 was
eluted faster than that of 7. Single crystals of 6 and 7 suited for X-ray analyses were obtained by
recrystallization from hot toluene and chlorobenzene, respectively.
7H-Bis[1]benzothieno[3,2-a:2’,3’-c]dibenzo[b,d]pyran-7-one (6): 1.43 g, 70%. colorless solid. mp
344.2-346 ºC. 1H-NMR (CDCl3)  7.39 (1H, t, J=7.6 Hz), 7.50 (1H, t, J=7.6 Hz), 7.56 (1H, t, J=7.3 Hz),
7.63 (1H, t, J=8.0 Hz), 7.66 (1H, t, J=7.3 Hz), 7.79 (1H, t, J=7.3 Hz), 7.95 (1H, d, J=8.0 Hz), 7.98 (1H, d,
J=7.6 Hz), 8.52 (1H, d, J=8.0 Hz), 8.51 (1H, d, J=8.0 Hz), 8.65 (1H, d, J=8.2 Hz), 9.10 (1H, d, J=7.3
Hz); 13C-NMR (CDCl3)  121.48, 122.61, 122.77, 123.52, 123.71, 124.94, 124.82, 125.44, 125.84,
126.64, 126.96, 126.99, 127.09, 128.72, 129.27, 130.19, 133.25, 134.41, 134.65, 135.81, 135.93, 138.95,
140.45, 147.72, 177.81, MS m/Z=408 (100%, M), 380 (10%, M-CO), 351 (26%, M-CO-CHO); IR (KBr)
1743 cm-1; Anal. Calcd for C25H12O2S2: C, 73.53; H, 2.94; S, 15.68%. Found: C, 73.42; H, 3.21, S,
16.10%.
Spiro[6,7-dihydro-6-oxobenzo[2,1-b:3,4-b’]bis[1]benzothiophene-7,1’-3’-oxoisobenzofuran] (7): 0.11
1
g, 5%. yellow crystals. mp 294.5-296 ºC. UV max(CHCl3) 418 nm (log ); H-NMR (CDCl3) 
7.10 (1H, d, J=7.6 Hz), 7.22 (1H, d, J=7.0 Hz), 7.23 (1H, t, J=8.2 Hz), 7.35 (1H, t, J=8.2 Hz), 7.46 (1H, t),
7.48 (1H, t), 7.50 (1H, t), 7.58 (1H, t, J=7.3 Hz), 7.85 (1H, d, J=7.9 Hz), 7.90 (1H, d, J=7.3 Hz), 8.13 (1H,
d, J=7.6 Hz), 8.50 (1H, d, J=6.7 Hz); MS m/Z=424 (100%, M), 408 (81%, M-O), 380 (78%, M-CO2), 351
(58%, M-CO2-CHO); Anal. Calcd for C25H12O3S2: C, 70.75; H, 2.83; S, 15.09%. Found: C, 70.02; H,
3.01, S, 14.81%.
(c) Reaction with 3,3’-bithiophene. A solution of ninhydrin (0.89 g, 5 mmol) and 3,3’-bithiophene (2.66

2474
HETEROCYCLES, Vol. 78, No. 10, 2009
g, 10 mmol) was refluxed in acetic acid (70 mL) with conc H2SO4 (0.2 mL) for 3 h. The usual workup
and chromatography of the reaction mixture gave 8.
8H-Bisthieno[2,3-a:3’,2’-c]dibenzo[b,d]pyran-8-one (8): 0.98 g, 64 %. colorless solid. mp 244 ºC.
1H-NMR (CDCl3)  7.65 (1H, t, J=7.0 Hz), 7.67 (1H, d, J=5.5 Hz), 7.73 (1H, d, J=9.2 Hz), 7.74 (1H, d,
J=9.2 Hz), 7.81（1H, d, J=5.5 Hz), 7.98 (1H, t, J=7.8 Hz), 8.57 (1H, d, J=7.9 Hz), 8.68 (1H, d, J=8.2 Hz);
13C-NMR (CDCl3)  108.89, 121.07, 121.77, 122.29, 124.79, 125.64, 126.76, 128.15, 130.01, 131.22,
131.66, 132.45, 134.83, 135.20, 136.50, 145.49, 160.42; IR (KBr) 1740 cm-1; MS m/Z=308 (100%, M),
280 (31%, M-CO), 251 (22%, M-CO-CHO); Anal. Calcd for C17H8O2S2: C, 66.21; H, 2.61; S, 20.80%.
Found: C,66.49; H,2.99, S, 20.76%.
X-Ray crystal data of 6. C25H12O2S2, M=408.49. orthorhombic, Pca21 (#29), a=20.2280(7),
b=11.4336(3), c=7.7505(2) Å, U=1792.53(9) Å3, Z=4, Dcalc=1.514 g/cm3,  (Mo-K)=3.176 cm-1, 4828
reflections (2θ<59.1o), Rint 0.028, R1 (I > 2.00(I)) 0.0274, wR2 (I > 2.00(I)) 0.0479. CCDC reference
number 704947.
X-Ray crystal data of 7. C25H12O3S2, M=424.49. monoclinic, C2/c (#15), a=24.7951(11), b=10.2854(3),
c=15.0316(5) Å,  = 90.9718(15), U=3832.9(2) Å3, Z=8, Dcalc=1.471 g/cm3, -=3.038 cm-1,
o
4384 reflections (2θ<54.9 ), R1 (I > 2.00(I)) 0.0312, wR2 (I > 2.00(I)) 0.0526. CCDC reference
number 704948.
REFERENCES
1. M. B. Rubin and R. Gleiter, Chem. Rev., 2000, 100, 1121.
2. (a) J. Jayashankaraèn, R. Durga, R. S. Manian, M. Sivaguru, and R. Raghunathan, Tetrahedron Lett.,
2006, 47, 5535; (b) J. Jayashankaran, R. D. R. S. Manian, and R. Raghunathan, Synthesis, 2006,
1028; (c) M. S. Leonard, D. B. Hauze, P. J. Carroll, and M. M. Joullié, Tetrahedron, 2003, 59, 6933;
(d) M. F. Aly, H. Ardill, R. Grigg, S. Leong-Ling, S. Rajviroongit, and S. Surendrakumar,
Tetrahedron Lett., 1987, 28, 6077; (e) H. Ardill, R. Grigg, V. Sridharan, S. Surendrakumar, S.
Thianpetanagul, and S. Kanajun, J. Chem. Soc., Chem. Commun., 1986, 602; (f) R. Grigg, J. F.
Malone, T. Mongkolaussavaratana, and S. Thianpatanagul, J. Chem. Soc., Chem. Commun., 1986,
421; (g) P. A. Crooks, Chem. Ind., 1980, 467; (h) A. Schonberg, E. Singer, B. Eschenhof, and G-A.
Hoyer, Chem. Berichte, 1978, 3058; (i) M. Israel, L. C. Jones, and E. J. Modest, J. Heterocyclic.
Chem., 1972, 255; (j) R. H. F. Manske and Q. A. Ahmed, Canad. J. Chem., 1970, 48, 1280; (k) R.
Shapiro and N. Chatterjie, J. Org. Chem., 1970, 35, 447.
3. (a) N. P. Peet, E. W. Huber, and J. C. Huffman, J. Heterocycl. Chem., 1995, 32, 33; (b) G. B. Gill, M.
S. H. Idris, and K. S. Kirollos, J. Chem. Soc., Perkin Trans. 1, 1992, 2355.

HETEROCYCLES, Vol. 78, No. 10, 2009
2475
4. (a) S. Das, A. Pramanik, R. Frölich, and A. Patra, Tetrahedron, 2004, 60, 10197; (b) G. Kaupp, M. R.
Naimi-Jamal, and J. Schmeyers, Chem. Eur. J., 2002, 8, 594; (c) D. G. Schmidt, P. O. Seemuth, and
H. Zimmer, J. Org. Chem., 1983, 48, 1914; (d) A. Schönberg and E. Singer, Tetrahedron Lett., 1969,
4571.
5. (a) H. N. Song, H. J. Lee, H. R. Kim, E. K. Ryu, and J. N. Kim, Synth. Commun., 1999, 29, 3303; (b)
H. N. Song, M. R. Seong, H. J. Lee, and J. N. Kim, Synth. Commun., 1999, 29, 2759; (c) S. K. Kundu,
S. Das and A. Pramanik, J. Chem. Res., 2004, 781; (d) H. N. Song, M. R. Seong, J. S. Son, and J. N.
Kim, Synth. Commun., 1998, 28, 1865; (e) D. A. Klumpp, F. Shyla, S. Lau, K. K. Jin, R. Bau, G. K.
S. Prakash, and G. A. Olah, J. Org. Chem., 1999, 64, 5152.
6. (a) S. Das, A. Pramanik, R. Fröhlich, and A. A. Pramanik, Org. Lett., 2006, 8, 4263; (b) G. Schmitt,
N. D. An, J-P. Poupelin, J. Vebrel, and B. Laude, Synthesis, 1984, 758; (c) J. E. Na, K. Y. Lee, J. S.
Seo, and J. N. Kim, Tetrahedron Lett., 2005, 46, 4505; (d) J.-P. Poupelin, G. S.-Ruf, J.-C. Perche,
J.-C. Roussey, B. Laude, G. Narcisse, and F. B.-Logeaisubeut, Eur. J. Med. Chem., 1980, 15, 253; (e)
J.-P. Poupelin, G. S.-Ruf, J.-C. Perche, R. Lacroix, G. U.-Ernouf, G. Narcisse, and F. Hubert, Eur. J.
Med. Chem., 1979, 14, 171.
7. W. T. A. Harrison, B. J. Morrison, and O. C. Musgrave, Tetrahedron, 2004, 60, 9255.
8. A. Marrocchi, L. Minuti, A.Taticchi, and H. W. Scheeren, Tetrahedron, 2001, 57, 4959.
9. L. Minuti, A. Taticchi, E. G.-Baitz, and A. Marrochi, Tetrahedron, 1995, 51, 8953.
10. J. L. Bullington and J. H. Dodd, J. Org. Chem., 1993, 58, 4833.
11. S. Das, R. Fröhlich, and A. Pramanik, Synlett, 2006, 207.