Microwave assisted one pot synthesis of some novel 2,5-disubstituted 1,3,4-oxadiazoles as antifungal agents

Article abstract of DOI:10.1016/j.bmcl.2010.10.120

Sodium bisulfite has been reported first time for the synthesis of 2,5-disubstituted 1,3,4-oxadiazole using microwave and conventional method in ethanol-water. The yields obtained are in the range of 90-95% using microwave and 87-91% using conventional method. All the synthesized compounds (8a-8s) are novel and were evaluated for their in vitro antifungal activity. SAR for the series has been developed by comparing their MIC values with miconazole and fluconazole. Some of the compounds from the series like 8k was equipotent with miconazole against Candida albicans and Fusarium oxysporum. Also compound 8n was equipotent with miconazole against F. oxysporum.

Full text of DOI:10.1016/j.bmcl.2010.10.120

Bioorganic & Medicinal Chemistry Letters 21 (2011) 444–448
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Microwave assisted one pot synthesis of some novel 2,5-disubstituted
1,3,4-oxadiazoles as antifungal agents
Jaiprakash N. Sangshetti a, Aniruddha R. Chabukswar b, Devanand B. Shinde a,
⇑
a Department of Chemical Technology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431 004 (MS), India
b Maharashtra Institute of Pharmacy, Kothrud, Pune (MS), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2010
Revised 5 October 2010
Accepted 25 October 2010
Available online 30 October 2010
Sodium bisulfite has been reported first time for the synthesis of 2,5-disubstituted 1,3,4-oxadiazole using
microwave and conventional method in ethanol–water. The yields obtained are in the range of 90–95%
using microwave and 87–91% using conventional method. All the synthesized compounds (8a–8s) are
novel and were evaluated for their in vitro antifungal activity. SAR for the series has been developed
by comparing their MIC values with miconazole and fluconazole. Some of the compounds from the series
like 8k was equipotent with miconazole against Candida albicans and Fusarium oxysporum. Also
compound 8n was equipotent with miconazole against F. oxysporum.
Keywords:
Microwave assisted one pot synthesis
Sodium bisulfite catalyst
Hydrazides
Ó 2010 Elsevier Ltd. All rights reserved.
1,3,4-Oxadiazole
1,2,3-Triazole
In vitro antifungal activity
MIC values
1,3,4-Oxadiazoles are
a
class of heterocycles, which have
Recently CAN has been reported for one pot synthesis of 2,5-disub-
stituted 1,3,4-oxadiazoles from hydrazides and substituted aro-
matic aldehydes.20
However, many of these methods suffer from drawbacks such
as long reaction times,21 unsatisfactory yields, special care in han-
dling and storing the reagents, undesired side products in reaction
with harsh reagents,22 using heavy metal oxidants,23 cumbersome
product isolation procedure and environmental pollution. There-
fore, a need still exists for further development of versatile and
milder reaction conditions.
Sodium bisulfite (NaHSO3) is important reagent in organic
transformation. We have reported NaHSO3 in C–C bond forming
reaction.24 In the present work utility of sodium bisulfite in synthe-
sis of some novel 2,5-disubstituted 1,3,4-oxadiazoles using micro-
wave (Microsynth microwave lab station-Ethosi Milestone) and
conventional method has been demonstrated.
1,2,3-Triazole and its derivatives are important heterocycles
with different activities like potent antineoplastic,25 antimicro-
bial,26–28 analgesic,29 antiinflammatory, local anesthetic,30 anticon-
vulsant,31 antimalarial,32 anti HIV agents.33 Some 1,2,3-triazole
derivatives were used as DNA cleaving agents34 and potassium
channel activators,35 cannabinoid CB1 receptor antagonists36 and
antitubercular agents.37
attracted significant interest in medicinal chemistry.1 Among the
1,3,4-oxadiazoles, 2,5-unsymmetrical disubstituted derivatives
have attracted considerable attention because of their biological
importance.2 The 1,3,4-oxadiazole-ring system has been identified
as the main core of many bioactive molecules. Compounds con-
taining these aromatic five-membered heterocycles have been
shown to exert antiinflammatory,3 antimicrobial,4 anticonvulsant,
and hypoglycemic activities.5
Several synthetic methods have been reported for the prepara-
tion of 1,3,4-oxadiazoles. One of the popular methods involves
cyclization of diacylhydrazines prepared from the reaction of acyl
chlorides and hydrazine. Several cyclodehydrating agents such as
BF3–OEt2,6 1,1,1,3,3,3-hexamethydisilazane,2 triflic anhydride,7
phosphorus pentoxide,8 polyphosphoric acid,9 thionyl chloride,10
phosphorus oxychloride,11 and sulfuric acid12 have been used.
One-pot syntheses of 1,3,4-oxadiazoles from hydrazine with
carboxylic acids have also been reported.13 Another synthetic route
for the preparation of these compounds is via acylation of
tetrazoles.14 1,3,4-Oxadiazoles have also been prepared by oxida-
tion of acyl hydrazones with different oxidizing agents.15–17
Reaction of acyl hydrazides with orthoesters in the presence of
an acidic catalyst,18 and solid phase synthesis of oxadiazoles19
are other approaches for the synthesis of this group of compounds.
Considering the biological significance of 1,3,4-oxadiazole
and 1,2,3-triazole and in continuation of our work on synthesis
of pharmacologically significant heterocycles,38 a novel series
of 1,3,4-oxadiazole has been synthesized by one pot reaction of
⇑
Corresponding author. Tel.: +91 240 2403307; fax: +91 240 2400413.
E-mail address: dbshinde.2007@rediffmail.com (D.B. Shinde).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.10.120

J. N. Sangshetti et al. / Bioorg. Med. Chem. Lett. 21 (2011) 444–448
445
hydrazide, aromatic aldehyde in ethanol: water using sodium
O
O
bisulfite as a catalyst. The synthesized compounds are novel and
evaluated for in vitro antifungal activity.
NHNH2
O
N
N
The ester compound 5 was prepared from commercially avail-
able N-boc piperidone as described in our previously reported
method (Scheme 1).38d,e Starting hydrazide compound has been
prepared from ester compound 5 using hydrazine hydrate in
methanol and further deprotection using trifluoroacetic acid.
Deprotected hydrazide compound on alkylation or acylation in
presence of triethylamine in tetrahydrofuran gave compounds
6a–6s (Scheme 2).Corresponding hydrazide compounds and aro-
matic aldehydes were heated at 100 °C in ethanol–water (1:2)
using 20 mol % NaHSO3 to get the target compounds 8a–8s
(Scheme 3).
N
N
N
N
N
1) a
2)
3)
b
c
N
R
Boc
5
6a-6s
NaHSO3 promotes the reaction of hydrazide with aromatic alde-
hyde with elimination of water molecule followed by cyclization to
form substituted 1,3,4-oxadiazole.
Scheme 2. Synthesis of hydrazide compounds (6a–6s) from compound 5. (a)
Hydrazine hydrate, methanol, reflux, 8 h; (b) TFA, dichloromethane, rt, 16 h; (c)
triethylamine, R-X or RCOX, tetrahydrofuran, 0–5 °C to rt, 2.5 h.
Catalytic property of NaHSO3 has been studied considering
synthesis of (8a). Effect of various solvents like THF, acetonitrile,
ethanol have also been studied. Among the results obtained, use
of 20-mol % NaHSO3 in ethanol–water gave the better yield (95%
and 91% using microwave and conventional method, respectively)
for the synthesis of 8a (Table 1). The use of environmental benign
solvent such as water has got very much importance in ‘Green
Chemistry’. To study this aspect, the reaction was carried out for
synthesis of 8a using 20-mol % NaHSO3 and corresponding sub-
strates in water. The reaction was found to be sluggish and it
may be due to the less solubility of substrates. To avoid this prob-
lem, the ethanol–water (1:2; v/v) solvent was used and found to be
effective for synthesis of 8a. The synthetic procedure was extended
for synthesis of all the compounds 8a–8s using different hydra-
zides, and aromatic aldehydes. Results are summarized in Table
2. The yields were obtained in the range of 87–91%. All synthesized
derivatives were characterized using mass and 1H NMR.
All the synthesized compounds were screened for in vitro anti-
fungal activity. The antifungal activity was evaluated against differ-
ent fungal strains such as Candida albicans, Fusarium oxysporum,
Aspergillus flavus, Aspergillus niger, and Cryptococcus neoformans.
Minimum inhibitory concentration (MIC) values were determined
using standard agar method.39 Miconazole and Fluconazole were
used as a standard for the comparison of antifungal activity.
Dimethyl sulfoxide was used as solvent control. MIC values of the
tested compounds are presented in Table 3.
Many of newly synthesized compounds were found to show good
antifungal activity. From the antifungal activity data (Table 3), it is
observed that compound 8k and 8n are the most active among all
tested compounds against most of the tested organisms. N-Pro-
tected compound with phenyl substituents at 5 position (8a) shows
very less antifungal activity comparable to miconazole and fluco-
nazole. Deprotected compound 8b shows significant rise in activity
OSO2CH3
N3
OH
O
c
b
a
N
Boc
N
N
Boc
N
Boc
Boc
2
3
4
1
d
O
O
N
N
N
N
Boc
5
Scheme 1. Synthesis of compound 5 from N-Boc piperidone. (a) NaBH4, ethanol, rt, 2 h; (b) methanesulfonyl chloride, triethylamine, dichloromethane; (c) NaN3, DMF, 80 °C,
8 h; (d) ethyl propiolate, CuI, acetonitrile, rt, 12 h; (e) TFA, dichloromethane, rt, 14 h; (f) triethylamine, R-X or RCOX, tetrahydrofuran, 0–5 °C to rt, 2 h.

446
J. N. Sangshetti et al. / Bioorg. Med. Chem. Lett. 21 (2011) 444–448
N
O
R1
N
NHNH2
O
N
N
N
N
CHO
N
N
a
+
N
R
R1
N
R
6a-6s
8a-8s
7
Scheme 3. Synthesis of 2-substituted phenyl-5-(1-(substituted piperidin-4-yl)-1H-1,2,3-triazol-4-yl)-1,3,4-oxadiazole. (a) Sodium bisulfite (20 mol %), ethanol–water (1:2),
100 °C, 9–10.5 h using conventional method and 10–15 min using microwave.
compared to 8a. Substitution of methyl group (8c) on piperidine
nitrogen increases the antifungal activity compared with unsubsti-
tuted nitrogen (8b). Substitution of ethyl group (8d) on nitrogen fur-
Table 1
Optimization of reaction conditions and the quantity of NaHSO3 for the synthesis tert-
ther enhance activity against C. albicans and F. oxysporum whereas
there is no effect on activity against A. niger and A. flavus and as evi-
denced by same MIC. Introduction of mesyl group on nitrogen (8e)
increases the antifungal activity by 2–3-fold compared with unsub-
stituted piperidine against all tested organisms. Introduction of ben-
zoyl group on nitrogen (3f) shows significant loss of activity
compared with unsubstituted nitrogen. No activity was reported
up to 200 lg/mL against C. albicans. Introduction of Cl group at p
position of Phenyl of benzoyl (8g) shows slight increase in the anti-
fungal activity compared with unsubstituted benzoyl.
butyl
boxylate (8a)
4-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)-1H-1,2,3-triazol-1-yl)piperidine-1-car-
Solvent
Mol of
NaHSO3
Microwave
Conventional
Reaction
Yielda
Reaction
Yielda
(%)
time (min)
time (h)
(%)
THF
Acetonitrile
20
20
20
20
20
10
5
30
30
40
10
10
20
30
72
75
70
95
95
88
76
20
18
14
09
09
12
15
68
74
70
91
91
79
72
Dichloromethane
Ethanol
Ethanol–water (1:2)
Ethanol–water (1:2)
Ethanol–water (1:2)
Introduction of –Cl group on 5 phenyl (8h–8k) shows increase
in activity against all tested organisms except C. neoformans
compared to compounds with unsubstituted 5 phenyl group.
a
Yields refer to the isolated pure products.
Table 2
Experimental data of the synthesized compounds 8a–8s
Entry
R
R1
Microwave
Time (min)
Conventional
Time (h)
Molecular formula/molecular weight
Yielda
95
Yielda
91
8a
8b
8c
8d
8e
8f
–Boc
–H
10
10
12
15
12
12
12
10
15
10
10
15
15
15
09
09
9.5
10
09
9.5
9.5
09
10
9.5
9
C20H24N6O3
396
C15H16N6O
296
C16H18N6O
310
C17H20N6O
324
C16H18N6O8s
374
C22H20N6O2
400
C22H19 Cl N6O2
434
C16H17 Cl N6O
344
C17H19 Cl N6O
358
C17H17 Cl N6O2
372
C16H17 Cl N6O8s
408
C16H18 N6O2
327
C17H20N6O2
341
–H
–H
91
94
91
95
90
94
94
92
92
94
95
92
95
88
91
87
91
88
90
89
87
88
90
89
87
90
–CH3
–H
–CH2CH3
–SO2CH3
–COC6H5
–COC6H5, 4 Cl
–CH3
–H
–H
–H
8g
8h
8i
–H
–Cl
–Cl
–Cl
–Cl
–OH
–OH
–OH
–CH2CH3
–COCH3
–SO2CH3
–CH3
8j
8k
8l
10
10
10
8m
8n
–CH2CH3
–SO2CH3
C16H18N6O4S

J. N. Sangshetti et al. / Bioorg. Med. Chem. Lett. 21 (2011) 444–448
447
Table 2 (continued)
Entry
R
R1
Microwave
Time (min)
Conventional
Time (h)
Molecular formula/molecular weight
Yielda
Yielda
390
8o
8p
8q
8r
8s
–CH3
–OCH3
–OCH3
–NO2
–CH3
15
15
15
15
15
92
90
95
92
92
10
89
88
90
89
87
C17H20 N6O2
340
C18H19 Cl N6O2
354
C16H17N7O5S
419
C18H20 N6O2
352
–CH2CH3
–SO2CH3
–COCH3
–COC6H5
10.5
10
10
–CH3
10
C23H22 N6O2
414
a
Yields refer to the isolated pure products.
Table 3
Antifungal activity of the synthesized compouunds
Compound
MIC values (l
g/mL)a,b
C. albicans
F. oxysporum
A. flavus
A. niger
C. neoformans
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
8m
8n
8o
8p
100 2.88
80 1.443
70 2.886
55 2.886
35 1.443
a
150 2.886
55 1.443
40 1.154
35 1.443
25 1.154
40 1.443
35 1.443
30 2.886
60 2.886
50 1.154
40 1.154
45 1.154
a
125 0.577
85 0.2.886
70 1.443
50 1.443
40 0.2.886
100 2.500
90 2.500
50 1.154
35 2.500
30 2.500
25 1.443
35 2.500
35 1.154
25 2.500
65 1.443
60 1.443
55 1.154
40 1.154
100 2.500
25 1.443
100 2.886
90 0.577
60 0.577
60 1.443
45 0.577
150 1.154
90 1.443
40 1.154
25 2.886
20 0.577
15 1.443
25 1.443
20 1.154
15 2.886
50 2.886
50 1.443
50 2.886
35 2.500
150 1.154
12.5 1.443
150 2.886
125 2.886
80 1.443
80 2.500
40 2.886
175 1.443
100 1.154
45 2.500
50 1.154
25 1.443
20 2.500
35 2.886
20 2.500
15 2.886
70 2.500
75 2.886
70 2.500
40 2.886
a
150 2.886
90 2.886
80 2.500
55 2.886
a
a
150 2.886
50 1.154
a
50 2.500
a
60 2.886
100 1.443
80 2.500
100 2.886
90 1.443
a
8q
8r
8s
a
a
Miconazole
Fluconazole
25 2.886
12.5 1.443
10 2.500
25 2.886
5
2.500
5
1.154
5
1.154
5
2.500
a
No activity was observed up to 200
Values are the average of three readings standard deviation.
l
g/mL.
b
Compound 8k with methyl sulfone group on piperidine nitrogen
and –Cl group on 5 phenyl substituents shows better activity. Com-
pound 8k was equipotent with miconazole against F. oxysporum
and C. albicans.
From activity it is observed that introduction of mesyl on piper-
idine nitrogen along with introduction of chloro and hydroxyl
group on phenyl serves as an important arrangement to give the
most active compounds from series.
Introduction of –OH group on 5 phenyl (8l–8n) shows increase in
activity against all tested organisms except C. neoformans compared
to compounds with unsubstituted 5 phenyl group. Compound 8n
with methyl sulfone group on piperidine nitrogen and –OH group
on 5 phenyl substituents shows better activity. Compound 8n was
equipotent with miconazole against F. oxysporum Activity of 8n was
comparable with miconazole againstC. albicans, A. flavus, and A. niger.
Introduction of –OCH3 group on 5 phenyl (8o–8p) shows in-
crease in activity against all tested organisms except C. neofor-
mans compared to compounds with unsubstituted 5 phenyl group.
Introduction of –NO2 group on 5 phenyl (8q) shows decrease in
activity against all tested organisms except A. niger compared to
compounds with unsubstituted 5 phenyl group.
In conclusion, we have first time demonstrated use of sodium
bisulfite for the synthesis of 2,5-disubstituted 1,3,4-oxadiazole
from hydrazides and aromatic aldehydes using conventional as
well as microwave synthesis in good yield. Also all the synthesized
compounds are novel with 1,2,3-triazolo piperidine scaffold. All
the synthesized compounds were tested for in vitro antifungal
activity. Based on the activity data, SAR for the series has been
developed. From the series 8k and 8n can serve as an important
pharmacophore for the design and development of new lead as
antifungal agent.
Acknowledgments
Introduction of CH3 group on 5 phenyl (8r) shows decrease in
activity against C. albicans, A. flavus and A. niger compared to com-
pound with Cl substituted 5 phenyl group (8j). Compound from
same series with benzoyl group on piperidine nitrogen (8s) shows
very less antifungal activity.
The authors are thankful to Council for Scientific and Industrial
Research (CSIR), New Delhi for financial assistance. The authors are
also thankful to the Head, Department of Chemical Technology,
Dr. Babasaheb Ambedkar Marathwada University, Aurangabad
431 004 (MS), India for providing the laboratory facility.

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22. Katritzky, A. R.; Mohapatra, P. P.; Huang, L. ARKIVOC 2008, 62.
Supplementary data
23. Hwu, J. R.; Chen, C. N.; Shaio, S. S. J. Org. Chem. 1995, 60, 826.
24. Sangshetti, J. N.; Kokare, N. D.; Kotharkar, S. A.; Shinde, D. B. Monatsh. Chem.
2008, 139, 125.
25. Passannanti, A.; Diana, P.; Barraja, P.; Mingoia, F.; Lauria, A.; Cirrincione, G.
Heterocycles 1998, 48, 1229.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.10.120.
26. Chen, M. D.; Lu, S. J.; Yuag, G. P.; Yang, S. Y.; Du, X. L. Heterocycl. Commun. 2000,
6, 421.
References and notes
27. Sherement, E. A.; Tomanov, R. I.; Trukhin, E. V.; Berestovitskaya, V. M. Russ. J.
Org. Chem. 2004, 40, 594.
1. Tully, W. R.; Cardner, C. R.; Gillespie, R. J.; Westwood, R. J. Med. Chem. 1991, 34,
2060.
28. Holla, B. S.; Mahalinga, M.; Karthikeyan, M. S.; Poojary, B.; Mohammed
Akberali, P.; Kumari, N. S. Eur. J. Med. Chem. 2005, 40, 1173.
29. André, A.; Jean, M.; (Roussel–UCLAF) Ger. Offen, 1815467, 24 July 1969.
30. Banu, K. M.; Dinakar, A.; Ananthanarayanan, C. Indian J. Pharm. Sci. 1999, 4, 202.
31. Meier, R. EP,62 (1992); eidem, U.S. 4789680, 1986.
32. Jilino, M.; Stevens, F. G. J. Chem. Soc., Perkin Trans. 1 1998, 1677.
33. Velarquez, S.; Alvarez, R.; Perez, C.; Gago, F.; De, C.; Balzarini, J.; Camarasa, M. J.
Antivir. Chem. Chemother. 1998, 9, 481.
34. Danoun, S.; Baziard Mouysset, G.; Stigliani, J.; Payard, M.; Selkti, M.; Tomas, V.
A. Heterocycl. Commun. 1998, 4, 45.
35. Biagi, G.; Calderone, V.; Giorgi, I.; Livi, O.; Martinotti, E.; Mtartelli, A.; Nardi, A.
Farmaco 2004, 59, 397.
36. Hou, D. R.; Alam, S.; Kuan, T. C.; Ramanathan, M.; Lin, T. P.; Hung, M. S. Bioorg.
Med. Chem. Lett. 2009, 19, 1022.
37. Gill, C.; Jadhav, G.; Shaikh, M.; Kale, R.; Ghawalkar, A.; Nagargoje, D.; Shiradkar,
M. Bioorg. Med. Chem. Lett. 2008, 18, 6244.
38. (a) Nagawade, R. R.; Khanna, V. V.; Bhagwat, S. S.; Shinde, D. B. Eur. J. Med.
Chem. 2005, 40, 1325; (b) Kokare, N. D.; Sangshetti, J. N.; Shinde, D. B. Synthesis
2007, 18, 2829; (c) Sangshetti, J. N.; Kokare, N. D.; Shinde, D. B. J. Heterocycl.
Chem. 2008, 45, 1191; (d) Sangshetti, J. N.; Nagawade, R. R.; Shinde, D. B. Bioorg.
Med. Chem. Lett. 2009, 19, 3564; (e) Sangshetti, J. N.; Shinde, D. B. Bioorg. Med.
Chem. Lett. 2009, 20, 742–745.
2. Diana, G. D.; Volkots, D. L.; Nitz, T. J.; Biailly, T. R.; Long, M. A.; Vesico, N.;
Aldous, A.; Pevear, D. C.; Dukto, F. J. J. Med. Chem. 1994, 37, 2421.
3. Amir, M.; Shawano, S. Indian J. Heterocycl. Chem. 1998, 8, 107.
4. Ashour, F. A.; Al Mazoroa, S. A. J. Pharm. Sci. 1990, 4, 29.
5. Nassar, O. M. Indian J. Heterocycl. Chem. 1997, 7, 105.
6. Mohamed, N. A. Polym. Degrad. 1994, 44, 33.
7. Tandon, V. K.; Chhor, R. B. Synth. Commun. 2001, 31, 1727.
8. Liras, S.; Allen, M. P.; Segelstein, B. E. Synth. Commun. 2000, 30, 437.
9. Carlsen, H. J.; Jorgensen, K. B. J. Heterocycl. Chem. 1994, 31, 805.
10. Al-Talib, M.; Tashtoush, H.; Odeh, N. Synth. Commun. 1990, 20, 1811.
11. Theocharis, A. B.; Alexandrou, N. E. J. Heterocycl. Chem. 1990, 27, 1685.
12. Short, F. W.; Long, L. M. J. Heterocycl. Chem. 1969, 6, 707.
13. Bentiss, F.; Lagrenee, M. J. Heterocycl. Chem. 1999, 36, 1029.
14. Osipova, T. F.; Koldobskii, G. I.; Ostrovskii, V. A. Zh.Org. Khim. 1984, 20, 1119.
15. Balachandran, K. S.; George, M. V. Tetrahedron 1973, 29, 2119.
16. (a) Chiba, T.; Okimoto, M. J. Org. Chem. 1992, 57, 1375; (b) Yang, R.; Dai, L. J. Org.
Chem. 1993, 58, 3381; (c) Rostamizadeh, S.; Housanini, G. Tetrahedron Lett.
2004, 45, 8753.
17. Reddy, P. S. N.; Reddy, P. R. Indian J. Chem. 1987, 26B, 890.
18. Shafiee, A.; Naimi, E.; Mansobi, P.; Foroumadi, A.; Shekari, M. J. Heterocycl.
Chem. 1995, 32, 1235.
39. (a) Greenwood, D.; Slack, R. C. B.; Peutherer, J. F., 14th ed. In Medical
Microbiology; ELBS: London, 1992; Vol. 1, (b) Duraiswamy, B.; Mishra, S. K.;
Subhashini, V.; Dhanraj, S. A.; Suresh, B. Indian J. Pharm. Sci. 2006, 68, 389; (c)
Saundane, A. R.; Rudresh, K.; Satynarayan, N. D.; Hiremath, S. P. Indian J. Pharm.
Sci. 1989, 60, 379; (d) Therese, K. L.; Bhagylaxmi, R.; Madhavan, H. N.; Deepa, P.
Indian J. Med. Microbiol. 2006, 24, 273–279.
19. Coppo, F. T.; Evans, K. A.; Graybill, T. L.; Burton, G. Tetrahedron Lett. 2004, 45,
3257.
20. Dabiri, M.; Salehi, P.; Baghbanzadeha, M.; Bahramnejada, M. Tetrahedron Lett.
2006, 47, 6983.
21. Cesarini, S.; Colombo, N.; Pulici, M.; Felder, E. R.; Brill, W. K. D. Tetrahedron
2006, 62, 10223.