A reinvestigation of the reactions of 3-substituted chromones with hydroxylamine. Unexpected synthesis of 3-amino-4H-chromeno[3,4-d]isoxazol-4-one and 3-(diaminomethylene)chroman-2,4-dione

Article abstract of DOI:10.1016/j.tetlet.2008.09.091

Reactions of 3-substituted chromones (3-formylchromone, 3-formylchromone-3-oxime, and 3-cyanochromone) with hydroxylamine in alkaline medium were reinvestigated, and a proof of structures and a probable reaction pathway are presented. Syntheses of 2-aminochromone-3-carboxamide, 3-amino-4H-chromeno[3,4-d]isoxazol-4-one, and 3-(diaminomethylene)chroman-2,4-dione were developed.

Full text of DOI:10.1016/j.tetlet.2008.09.091

Tetrahedron Letters 49 (2008) 6856–6859
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A reinvestigation of the reactions of 3-substituted chromones with
hydroxylamine. Unexpected synthesis of 3-amino-4H-chromeno[3,4-d]-
isoxazol-4-one and 3-(diaminomethylene)chroman-2,4-dione
a
b
Vyacheslav Ya. Sosnovskikh a, , Vladimir S. Moshkin , Mikhail I. Kodess
*
a Department of Chemistry, Ural State University, pr. Lenina 51, 620083 Ekaterinburg, Russia
b Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 620041 Ekaterinburg, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2008
Revised 5 September 2008
Accepted 15 September 2008
Available online 19 September 2008
Reactions of 3-substituted chromones (3-formylchromone, 3-formylchromone-3-oxime, and 3-cyanochr-
omone) with hydroxylamine in alkaline medium were reinvestigated, and a proof of structures and a
probable reaction pathway are presented. Syntheses of 2-aminochromone-3-carboxamide, 3-amino-
4H-chromeno[3,4-d]isoxazol-4-one, and 3-(diaminomethylene)chroman-2,4-dione were developed.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
3-Substituted chromones
Hydroxylamine
2-Aminochromone-3-carboxamide
3-Amino-4H-chromeno[3,4-d]isoxazol-4-
one
3-(Diaminomethylene)chroman-2,4-dione
Derivatives of 4H-1-benzopyran-4-one, also known as 4H-chro-
men-4-ones or chromones, are important natural products pos-
sessing a wide range of valuable physiological activities.1 In
addition, they represent useful synthetic building blocks in organic
and medicinal chemistry.2 The introduction of an electron-with-
drawing group at the 3-position of the chromone system changes
crucially the reactivity of the pyrone ring with respect to nucleo-
philes, and provides a broad synthetic potential of 3-substituted
chromones. The diversity of properties of these compounds is
due to the fact that, being highly reactive geminally activated
push–pull alkenes with a good leaving group at the b-carbon atom,
whose role is played by the phenolate anion (a fragment of the enol
ether), they acquire the ability to undergo additional transforma-
from 2-hydroxyacetophenones and hydroxylamine under the Vils-
meier–Haack conditions.7 Among the diverse transformations of 3,
one of its more interesting reactions is its ability to interact with
water at the C-2 atom followed by pyrone ring opening and cycli-
zation at the CN group, leading to the formation of 2-amino-3-for-
mylchromone 4,6,8 which in turn can be converted into 2-amino-3-
formylchromone-3-oxime 5 and 2-amino-3-cyanochromone 6.6
Thus, the nitrile 3 is ‘chemically equivalent’ to the aminoaldehyde
4 under certain reaction conditions9 (Scheme 1).
Continuing our studies on the synthetic potential of the chro-
mone system,10 we were interested in the course of oximation
reactions of 3-substituted chromones 1–3. This, at first glance sim-
ple reaction, has already been the subject of investigations. Ghosh
et al. in 197911 studied the reaction of hydroxylamine hydrochlo-
ride with 3 in the presence of sodium acetate, and obtained a com-
pound with mp 263 °C. Its composition corresponded to a 1:1
adduct, C10H8N2O3, which was assigned the structure 7 on the ba-
sis of 1H NMR, IR, and mass spectra. The authors believed that the
reaction occurred via 1,2-addition of hydroxylamine to the cyano
group of 3, and the stability of 7 was explained by two intramolec-
ular hydrogen bonds. In this case, the structure of 2-amino-3-for-
tions related to
c-pyrone ring opening and heterocyclizations at
the C-4 atom and/or at the substituent at the 3-position.3
The chemistry of 3-substituted chromones, mainly 3-formyl-
and 3-cyanochromones, has been developed since the mid-1970s
after the simple and convenient Vilsmeier–Haack method was pro-
posed for the synthesis of 3-formylchromones from 2-hydroxyace-
tophenones, DMF, and POCl3.4 Treatment of 3-formylchromones 1
with hydroxylamine via 3-formylchromone-3-oximes 2 leads to 3-
cyanochromones 3.5,6 In addition, 3 can be synthesized directly
mylchromone-3-oxime
5 was not considered a possibility,
because this oxime had been prepared earlier from 4 and its spec-
tral characteristics differed from those of 7.6a A compound of the
same composition and spectral properties has been synthesized
* Corresponding author. Fax: +7 343 261 59 78.
E-mail address: vyacheslav.sosnovskikh@usu.ru (V. Ya. Sosnovskikh).
12
´
by Basinski and Jerzmanowska in reactions of chromones 1–3
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.091

V. Ya. Sosnovskikh et al. / Tetrahedron Letters 49 (2008) 6856–6859
6857
Ha
O
O
O
Hb
H
N
O
O
N
N
X
X
N2H4,
NaOH
51%
NH2
O
Hc
O
O
NH2
N
O
O
1-3
4-6
Hd
15
11
for 3
for 1-3
ref.12
NH2OH
ref.11
NH2OH
64%
H
O
O
O
N
H
OH
OH
a
H
H
H
b
H
O
N
O
O
N
O
N
H
NH
c
H
H
N
N
N
H
Hd
8
7
9
O
O
14
14'
O
O
O
O
O
NH2OH
52%
H2O
ref.12
NH
NH
N
N
O
N
H
N
H
OH
O
O
H
O
NH2
NH2
10
HX
53-82%
COX
O
O
X = CHO (1,4), CH=NOH (2,5), CN (3,6)
12
13a-c
X = OH (a), MeO (b), NH2 (c)
Scheme 1.
Scheme 2.
with hydroxylamine under different conditions affording contrast-
ing yields. Based on a very doubtful mechanism, they proposed
pyrazolone 8 as their product. In the same work,12 the authors per-
formed an important transformation, which, however, was not
fully appreciated by them. When chromone 2 was treated with
hydroxylamine hydrochloride (2 equiv) in the presence of NaOH
(4 equiv) in aqueous ethanol, a compound of molecular formula
C10H6N2O3 was obtained, which easily added water to form a prod-
uct of molecular formula C10H8N2O4. Structures 9 and 10, respec-
tively, were assigned to these products, and chromones 3, 5, and
6 were indicated as non-isolable intermediates of the reaction;12b
their claim has since been reviewed favorably9 (Scheme 1).
the validity of the new structures using 1H, 13C, 15N NMR, and IR
spectroscopies. In addition, all the signals in the 1H and 13C NMR
spectra of compound 12 were assigned on the basis of 2D 1H–13
C
HSQC and HMBC experiments.15 Thus, the claimed compounds 9
and 10 are implausible, and are instead coumarino[3,4-d]isoxazole
12 and its acid 13a.
It was also found that the coumarin ring can easily be opened by
the action not only of water,12 but also of methanol and ammonia,
which is a new route to prepare densely functionalized isoxazoles
such as 13a–c (Scheme 2). As mentioned above, compound 13a
had previously been incorrectly formulated as 10,12 which is con-
tradicted by the spectral properties of this compound. For example,
in the NMR spectrum of 13a in DMSO-d6, all the protons of the ben-
zene ring are shifted to a higher field in comparison with those of
We studied repeatedly the reactions of chromones 1–3 with
hydroxylamine under the conditions described in Refs. 11 and 12
and confirmed the identity of compounds 7 and 8. On the basis
of 1H, 13C, and 15N NMR spectroscopy and using 2D HSQC and
HMBC experiments we found that the product was, in fact, 2-ami-
no-3-carbamoylchromone 11 (Scheme 2). The structural assign-
ment for this compound as the aminoamide was based on four
CD3CO2D exchangeable NH protons in the 1H NMR spectrum
[dH = 7.44 (Hb), 9.11 (Hd), 9.66 (Ha), and 10.50 (Hc)]. A literature
search proved that this compound had been obtained before the
12 and, hence, opening of the a-pyrone ring took place under the
action of water. The amino group and the phenolic hydroxyl ap-
peared as two singlets at d 5.90 (2H) and 10.03 (1H) ppm; the
broad signal at d 11.6–13.6 ppm is due to the resonance of the
CO2H proton. The IR spectra are also of diagnostic value in this
isoxazole series (m
CO = 1683–1712 cmꢀ1).
reports of Ghosh11 and Basinski12 via acidic hydrolysis of 6, which
Following the same procedure,12b we also found that coumari-
no[3,4-d]isoxazole 12, when treated with hydroxylamine under
basic conditions, underwent transformation into a product of
molecular formula C10H8N2O3, which was shown by a combination
´
in turn was synthesized from 2-acetoxybenzoyl chloride and mal-
ononitrile in the presence of NaOH.13 Although in 1978, the iden-
tity of the products prepared by hydrolysis of 6 and oximation of
3 was described in a Japanese patent,14 the structures 7 and 8
are still valid in the chemical literature.9
of 1H, 13C, and 15N NMR spectroscopy, including 2D 1H–13C, 1H–15
N
HSQC, and HMBC experiments, to be 3-(diaminomethylene)chro-
man-2,4-dione 14 (yield 52%). Thus, it turned out that the O–N
bond in 12 is reduced under the action of hydroxylamine affording
previously unknown compound 14. Although the precise mecha-
nism for this unusual reaction is unclear, the driving force for fis-
sion of the O–N bond is presumably the formation of a stable
highly delocalized structure with two intramolecular hydrogen
bonds. It is notable that the 13C spectroscopic data for compound
14 show significant polarization of the C@C double bond
[dC = 86.3 (C3) and 164.0 ppm (C30)] and that the two nitrogen
atoms are apparently equivalent (dN = 94.2 ppm). This may indi-
cate a significant contribution from a fully charge-separated form
Moreover, we repeated the treatment of chromone 2 with
12b
´
hydroxylamine according to Basinski and Jerzmanowska,
and a
perfect reproducibility of the reaction, as well as agreement of
the spectral and chemical properties of the product with their
description has been established. The same compound was also ob-
tained as a result of the oximation of chromone 4 (yields 40–45%
from 2 and 4). However, we found that the reaction of both 2
and 4 with hydroxylamine in strongly basic solution leads not to
9, but instead to 3-amino-4H-chromeno[3,4-d]isoxazol-4-one 12,
which was converted into acid 13a when heated to boiling with
a 10% solution of sodium hydroxide (Scheme 2). We confirmed

6858
V. Ya. Sosnovskikh et al. / Tetrahedron Letters 49 (2008) 6856–6859
140 in which there is a single bond but no free rotation of the
(NH2)2C+ grouping due to two strong intramolecular hydrogen
bonds (Scheme 2). Besides the signals expected for the aromatic
protons, the 1H NMR spectrum of 14 showed two singlets (2H) at
d 9.82 (Ha, Hd) and 7.69 (Hb, Hc) ppm.16 The simple and efficient
syntheses of compounds 12–14 are of interest, because this class
of organic compounds has remained inaccessible and unstudied
until now.
Interestingly, when aminoamide 11 was treated with hydroxyl-
amine under the same conditions, chroman-2,4-dione 14 was ob-
tained in 64% yield. This unprecedented isomerization evidently
involved firstly the formation of the coumarino[3,4-d]isoxazole
12, followed by cleavage of the isoxazole ring. In accordance with
this, the reaction of 11 with hydrazine hydrate in the presence of
NaOH gave 3-aminochromeno[4,3-c]pyrazol-4-one 15 in 55% yield
(Scheme 3). Previously, this compound was obtained by the reac-
tion of 4-chloro-3-cyanocoumarin with hydrazine hydrate.17 Of
note is the fact that the 1H NMR spectrum of 15 showed two sets
of signals due to the possibility of annular prototropy of the pyr-
azole ring. Although annular tautomerism in NH azoles is a very
fast process on the NMR time scale,18 in our case, two singlets
(NH2) exhibiting broadening at lower frequency and two singlets
(NH) at higher frequency were observed, indicating that coumari-
no[4,3-c]pyrazole 15 exists in DMSO-d6 as a 77:23 mixture of
1H- and 2H-tautomers, respectively. The structural assignment
was based on the signals due to the NH2 groups at d 6.60 ppm
(1H-tautomer) and d 5.62 ppm (2H-tautomer), the former of which
is very similar in structure to that of 12.
A possible route for the multi-step reaction of 3-formylchro-
mone 1 with hydroxylamine in alkaline medium is outlined in
Scheme 3. It includes several sequential nucleophilic 1,4- and
1,2-attacks of hydroxylamine and water molecules at the activated
C-2 atom or at the 3-substituent of the chromone system. For the
transformation 5?11 we favor a pathway involving the intermedi-
acy of the 5-aminoisoxazole B, since its formation via intramolec-
ular recyclization is especially facile under basic conditions.
Subsequent deprotonation of B with concomitant isoxazole ring
opening leads to the open chain intermediate C, which is cyclized
involving the phenolic hydroxyl and cyano group to form amino-
amide 11. We suggest, therefore, that formation of 11 from
5 corresponds to the operation of a ring-to-ring interconver-
sion.2e,10b In this context, it is important that when oxime 5 was
treated with alkali without hydroxylamine, a 1:1 mixture of 11
and salicylic acid was isolated. An alternative mechanism, involv-
ing initial dehydration of 5 to form 2-amino-3-cyanochromone 6
followed by addition of water to form 11 was discarded owing to
the failure of 6 to react with water to produce 11 under the condi-
tions employed. In addition, attempts to detect 6 among the reac-
tion products by 1H NMR spectroscopy were unsuccessful,
although it can be readily prepared from 4 and 5 in acidic
solutions.6
O
O
CHO
CN
OH
NH2OH
-H2O
N
O
O
O
O
1
3
2
-H2O
CN
H2X
O
OH
X
A
X = O
X = NOH
O
O
O
O
CHO
NH2
OH
NH2OH
-H2O
N
NH2
4
5
O
OH
NH2
O
OH
O
H
NH2
O
CN
N
C
B
O
O
NH2
O
OH
O
CONH2
NOH
NH2OH
NH2
H2N
11
D
-NH3
OH
O
O
NOH
NH2
O
O
O
NH2
X
N
O
12'
14
E
-H2O
N
O
O
NH2
O
NH2
O
NH2OH
NH2
O
O
12
Scheme 3.
mone-3-carboxylic acids with phenylhydrazine leads to 1-
phenylchromeno[4,3-c]pyrazol-4(1H)-ones.20
We believe that hydroxylamine in alkaline medium first converts
chromones 1–5 into the key intermediate 11, which then undergoes
further recyclization and reduction to give compound 14 via 12.
These products can result from the initial nucleophilic 1,2- or 1,4-
additions of hydroxylamine to 11 due to the chemical equivalency
of the 3-CONH2 group and the C-2 atom (intermediate D). However,
it seems more logical to us that the more nucleophilic nitrogen atom
of the hydroxylamine would attack at the C-2 position of the chro-
mone rather than at the 3-CONH2 group. This behavior of 11 closely
resembles (with the exception of a reduction stage) to that already
reported for ethyl 2-alkyl- and 2-phenylchromone-3-carboxylates,
which on treatment with hydroxylamine hydrochloride in refluxing
acetic acid give 3-alkyl- and 3-phenyl-4H-chromeno[3,4-d]iso-
xazol-4-ones;19 condensation of chromone- and 2-methylchro-
It should be noted that the alternative cyclization of E involving
the amidoxime hydroxyl and the lactone carbonyl to give 3-amino-
4H-chromeno[3,2-d]isoxazol-4-one 120 does not occur. The struc-
ture of the 3-amino-4H-chromeno[3,4-d]isoxazol-4-one 12 is not
in doubt. That the compound is a coumarin and not a chromone
is shown by the IR band at 1757 cmꢀ1, typical of the carbonyl
group in coumarins19 (the chromone carbonyl band is usually near
1660 cmꢀ1).
In support of the proposed pathway (Scheme 3), the reactions of
chromones 1, 3, and 5 with hydroxylamine were studied under the
conditions used for the synthesis of isoxazole 12 from 2 and 4 and
chroman-2,4-dione 14 from 11 and 12. In these cases, different

V. Ya. Sosnovskikh et al. / Tetrahedron Letters 49 (2008) 6856–6859
6859
5. (a) Nohara, A. Tetrahedron Lett. 1974, 1187–1190; (b) Klutchko, S.; Cohen, M. P.;
Shavel, J.; von Strandtmann, M. J. Heterocycl. Chem. 1974, 11, 183–188.
6. (a) Petersen, U.; Heitzer, H. Liebigs Ann. Chem. 1976, 1659–1662; (b) Hagen, H.;
Nilz, G.; Walter, H.; Landes, A.; Freund, W. DE Patent 4039281, 1992; Chem.
Abstr. 1992, 117, 111473k.
compositions of the mixtures of 11, 12, and 14 were obtained
(11:12:14 = 8:30:62 from 1 and 33:17:50 from 5) in good com-
bined yields (50–60%), whereas the same reaction with 3 yielded
a mixture of compounds 12 and 14 in about equal amounts and
11 was not detected at all (the ratio of the products was deter-
mined by integration of the signals in the 1H NMR spectra). Thus,
the origin of these materials is explained and the overall reaction
can be represented as a coherent process, the final product of
which is chroman-2,4-dione 14. Compounds 11 and 12 are isolable
intermediates in this process, and the product structure depends
on the nature of the starting materials and the reaction conditions.
Since coumarins with 3,4-heterocyclic fused ring systems serve as
useful synthetic intermediates and have attracted attention as key
compounds for drug design,21 it is expected that these reactions
will have significant synthetic application.
In conclusion, 3-formylchromone represents a very reactive
system and its reactions with hydroxylamine give a variety of
products. Since the identity of some of these products was in
doubt, we have reinvestigated the reactions and found that by
varying the conditions, 2-aminochromone-3-carboxamide, 3-
amino-4H-chromeno[3,4-d]isoxazol-4-one, and 3-(diaminometh-
ylene)chroman-2,4-dione could be prepared in moderate to good
yields. The resulting products are of considerable interest as useful
precursors in the synthesis of other biologically and medicinally
important organic materials. The generality of this simple and
unrecognized method is being investigated further.
7. Reddy, G. J.; Latha, D.; Thirupathaiah, C.; Rao, K. S. Tetrahedron Lett. 2004, 45,
847–848.
8. Nohara, A.; Ishiguro, T.; Ukawa, K.; Sugihara, H.; Maki, Y.; Sanno, Y. J. Med.
Chem. 1985, 28, 559–568.
9. Ghosh, C. K.; Karak, S. K. J. Heterocycl. Chem. 2005, 42, 1035–1042.
10. (a) Sosnovskikh, V. Ya. Uspekhi Khim. 2003, 72, 550–578. Russ. Chem. Rev. 2003,
72, 489–516; (b) Sosnovskikh, V. Ya.; Moshkin, V. S.; Kodess, M. I. Tetrahedron
2008, 64, 7877–7889.
11. Ghosh, C. K.; SinhaRoy, D. K.; Mukhopadhyay, K. K. J. Chem. Soc., Perkin Trans. 1
1979, 1964–1968.
12. (a) Jerzmanowska, Z.; Basin´ ski, W.; Zielin´ ska, L. Pol. J. Chem. 1980, 54, 383–386;
(b) Basin´ ski, W.; Jerzmanowska, Z. Pol. J. Chem. 1983, 57, 471–481.
13. (a) Brown, R. E.; Lustgarten, D. M. U.S. Patent 3,932,466, 1976; Chem. Abstr.
1976, 84, 135471j.; (b) Brown, R. E.; Lustgarten, D. M. U.S. Patent 4,024,160,
1977; Chem. Abstr. 1977, 87, 84821a.; (c) Bevan, P. S.; Ellis, G. P.; Hudson, H. V.;
Romney-Alexander, T. M. J. Chem. Soc., Perkin Trans. 1 1986, 1643–1649.
14. Nohara, A.; Sugihara, H.; Ukawa, K. Japan Patent 53111071, 1978; Chem. Abstr.
1979, 90, 54827.
15. 3-Amino-4H-chromeno[3,4-d]isoxazol-4-one 12. This compound was prepared
from chromones 2 and 4 according to the procedure described previously for
9.12b Yields 40–45%, mp 229–230 °C (lit.12b mp 228–229 °C); IR (KBr) 3457,
3364, 3300, 3195, 1757, 1646, 1616, 1597, 1566, 1528 cm–1
;
1H NMR
(400 MHz, DMSO-d6) d 6.45 (s, 2H, NH2), 7.49 (ddd, 1H, H-8, J = 7.8, 7.4,
1.0 Hz), 7.58 (dd, 1H, H-6, J = 8.5, 1.0 Hz), 7.78 (ddd, 1H, H-7, J = 8.5, 7.4,
1.6 Hz), 7.99 (dd, 1H, H-9, J = 7.8, 1.6 Hz); 13C NMR (100 MHz, DMSO-d6) d
97.23 (C3a), 110.58 (C9a), 117.31 (C6), 122.47 (C9), 125.08 (C8), 133.74 (C7),
153.63 (C5a), 156.40 (C3), 160.51 (C4), 168.74 (C9b); 15N NMR (40 MHz,
DMSO-d6) d 50.0 (NH2), 342.3 (@N); MS (EI): m/z (%) 202 [M]+ (100), 173 (11),
130 (10), 121 [HOC6H4CO]+ (84), 120 [OC6H4CO]+ (27), 104 [C6H4CO]+ (18), 92
[C6H4O]+ (33), 76 (27), 63 (23), 53 (10), 50 (16). Anal. Calcd for C10H6N2O3: C,
59.41; H, 2.99; N, 13.86. Found: C, 59.25; H, 2.90; N, 13.74.
Acknowledgments
16. 3-(Diaminomethylene)chroman-2,4-dione 14: Yield 64% (from 11), 52% (from
12), mp 259–261 °C; IR (KBr) 3429, 3290, 3157, 1684, 1633, 1610, 1568, 1487,
1473 cmꢀ1 1H NMR (400 MHz, DMSO-d6) d 7.28 (dd, 1H, H-8, J = 8.2, 1.2 Hz),
;
This work was financially supported by the RFBR (Grant 06-03-
32388) and CRDF (Grant BP2M05).
7.30 (ddd, 1H, H-6, J = 7.8, 7.3, 1.2 Hz), 7.63 (ddd, 1H, H-7, J = 8.2, 7.3, 1.7 Hz),
7.69 (br s, 2H, NH2), 7.94 (dd, 1H, H-5, J = 7.8, 1.7 Hz), 9.82 (br s, 2H, NH2); 13
C
NMR (100 MHz, DMSO-d6) d 86.26 (C3), 116.22 (C8), 120.71 (4a), 123.60 (C6),
125.44 (C5), 133.36 (C7), 152.46 (C8a), 163.93 (C2/C30), 164.12 (C30/2), 177.96
(C4); 15N NMR (40 MHz, DMSO-d6) d 94.2 (NH2); MS (EI): m/z (%) 204 [M]+
(100), 187 (19), 159 (13), 121 [HOC6H4CO]+ (40), 120 [OC6H4CO]+ (39), 109
(17), 92 [C6H4O]+ (24), 84 (20), 68 (20), 65 (13), 64 (13), 63 (12). Anal. Calcd for
C10H8N2O3ꢁ0.25H2O: C, 57.56; H, 4.11; N, 13.42. Found: C, 57.39; H, 3.80; N,
13.04.
References and notes
1. Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem. Rev. 2003, 103, 893–930.
2. (a) Nohara, A.; Kuriki, H.; Saijo, T.; Sugihara, H.; Kanno, M.; Sanno, Y. J. Med.
Chem. 1977, 20, 141–145; (b) Nohara, A.; Umetani, T.; Ukawa, K.; Sanno, Y.
Chem. Pharm. Bull. 1974, 22, 2959–2965; (c) Ukawa, K.; Ishiguro, T.; Wada, Y.;
Nohara, A. Heterocycles 1986, 24, 1931–1941; (d) Hsung, R. P. J. Org. Chem. 1997,
62, 7904–7905; (e) Hsung, R. P.; Zificsak, C. A.; Wei, L.-L.; Zehnder, L. R.; Park, F.;
Kim, M.; Tran, T.-T. T. J. Org. Chem. 1999, 64, 8736–8740; (f) Sosnovskikh, V. Ya.;
Irgashev, R. A.; Levchenko, A. A. Tetrahedron 2008, 64, 6607–6614.
3. (a) Ghosh, C. K. J. Heterocycl. Chem. 1983, 20, 1437–1445; (b) Sabitha, G.
Aldrichim. Acta 1996, 29, 15–25; (c) Ghosh, C. K. Heterocycles 2004, 63, 2875–
2898; (d) Gašparová, R.; Lácová, M. Molecules 2005, 10, 937–960.
4. (a) Harnisch, H. Liebigs Ann. Chem. 1972, 765, 8–14; (b) Nohara, A.; Umetani, T.;
Sanno, Y. Tetrahedron Lett. 1973, 1995–1998; (c) Nohara, A.; Umetani, T.; Sanno,
Y. Tetrahedron 1974, 30, 3553–3561.
17. (a) Checchi, S.; Pecori Vettori, L.; Vincieri, F. Gazz. Chim. Ital. 1968, 98, 1488–
1502; (b) Kaljaj, V.; Trkovnik, M.; Stefanovic-Kaljaj, L. J. Serb. Chem. Soc. 1987,
52, 183–185. Chem. Abstr. 1988, 108, 150410w.
18. Elguero, J.. In Comprehensive Heterocyclic Chemistry II; Katritzky, A. R., Rees, C.
W., Scriven, E. F. V., Eds.; Pergamon: Oxford, 1996; Vol. 3, p 18.
19. Chantegrel, B.; Nadi, A. I.; Gelin, S. J. Org. Chem. 1984, 49, 4419–4424.
20. Chantegrel, B.; Nadi, A. I.; Gelin, S. Tetrahedron Lett. 1983, 24, 381–384.
21. Darbarwar, M.; Sundaramurthy, V. Synthesis 1982, 337–388.