The synthesis of the new C-nucleoside 6-deazaformycin B

Article abstract of DOI:10.1055/s-2007-1000870

The synthesis of the 6-deaza analogue of formycin B is described, through the condensation of lithiated 4-methoxy-2-methyl-3-trifluoroacetamidopyridine with a suitably protected ribonolactone, dehydration of the resulting hemiacetal, reduction and subsequent ring closure followed by protecting group manipulation. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-1000870

LETTER
181
The Synthesis of the New C-Nucleoside 6-Deazaformycin B
Synthesis of
6
t
eformycinBlla Korouli, Nikolaos Lougiakis, Panagiotis Marakos, Nicole Pouli*
Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Athens,
Panepistimiopolis Zografou, 15771 Athens, Greece
Fax +30(210)7274747; E-mail: pouli@pharm.uoa.gr
Received 13 June 2007
NH2
O
N
Abstract: The synthesis of the 6-deaza analogue of formycin B is
H
N
H
N
described, through the condensation of lithiated 4-methoxy-2-meth-
yl-3-trifluoroacetamidopyridine with a suitably protected ribono-
lactone, dehydration of the resulting hemiacetal, reduction and
subsequent ring closure followed by protecting group manipulation.
N
HN
N
O
O
HO
HO
H
H
Key words: C-nucleosides, heterocycles, pyrazolo[4,3-b]pyridine,
lithiation, ring closure
HO
OH
HO
OH
9-deazaadenosine
9-deazainosine
NH2
H
O
H
Nucleosides and nucleotides are involved, either by them-
selves or in combination with other molecules, in almost
all cell activities, including catalysis, transfer of energy
and mediation of hormone signals. Structural modifica-
tions of naturally occurring nucleosides have been used by
medicinal chemists to provide new compounds, which can
serve as valuable tools in structure–activity studies.1 A
great number of the synthesized molecules display antibi-
otic,2 antifungal3 and antineoplastic properties4 and, most
importantly, the majority of antiviral drugs currently in
clinical use are nucleoside analogues.5 A very simple al-
teration of the purine or pyrimidine base of nucleosides is
the replacement of the nitrogen, linking the heterocyclic
part to the sugar, with a methine unit. The resulting C-nu-
cleosides show interesting biological activity and they are
also endowed with stability towards chemical or enzymat-
ic cleavage.6
C-Nucleoside analogues of cytidine (1-deazacytidine),7
guanosine (9-deazaguanosine),8 adenosine (9-deazaade-
nosine, Figure 1)9 and inosine (9-deazainosine,
Figure 1)10 have been synthesized and studied. In fact, 9-
deazainosine is active against Trypanosoma brucei subsp.
experimental infections and against Pneumonocystis car-
inii pneumonias in rats,11 while 9-deazaadenosine dis-
N
N
N
HN
N
N
N
O
N
O
HO
HO
H
H
HO
OH
HO
OH
formycin B
formycin A
Figure 1 Structures of C-nucleosides
plays potent antineoplastic activity against a variety of
human tumor cell lines.12
We are involved in the synthesis of purine-like C-nucleo-
sides bearing structural similarity with the natural antibi-
otics formycin A and formycin B (Figure 1), which mimic
the isosteric adenosine and inosine molecules, respective-
ly, and substitute for them in many enzymatic reactions.13
In view of our interest towards the importance of the 4- or
the 6-nitrogen of these molecules, concerning their ability
to behave as antimetabolites, we have previously accom-
plished the synthesis of 4-deazaformycin A14 and 4-dea-
zaformycin B.15 As a continuation of this ongoing
research effort we present here the synthesis of 1,4-dihy-
dro-3-(b-D-ribofuranosyl)-7H-pyrazolo[4,3-b]pyridin-7-
one (6-deazaformycin B).
R
OMe
OMe
OMe
OMe
e
NO2 O2N
NH2
Me
(from 5)
c
d
+
+
N
Me
N
Me
N
Me
N
Me
N
O–
1 R = H
2 R = NO2
3 R = OMe
7 R = NH2
8 R = NHCOMe
9 R = NHCOCF3
4
5
6
a
f
g
b
Scheme 1 Reagents and conditions: (a) H2SO4, HNO3 (65%), 160 °C, 3 h; (b) MeONa, MeOH, 80 °C, 2 h; (c) Fe, AcOH, 130 °C, 2 h; (d)
H2SO4, HNO3 (65%), 65 °C, 12 h; (e) H2, 10% Pd/C, EtOH, r.t., 4 h.; (f) Ac2O, CH2Cl2, r.t., 10 h; (g) (CF3CO)2O, CH2Cl2, r.t., 1 h.
SYNLETT 2008, No. 2, pp 0181–0184
x
x.
x
x.
2
0
0
7
Advanced online publication: 21.12.2007
DOI: 10.1055/s-2007-1000870; Art ID: D18207ST
© Georg Thieme Verlag Stuttgart · New York

182
S. Korouli et al.
LETTER
OMe
N
For the synthesis of the target derivative we used 2-pi-
coline N-oxide (1), which was easily converted into 4-
methoxy-2-picoline (4, Scheme 1).16
OMe
NHCOMe
OH
NHCOMe
a
Compound 4 was nitrated and upon separation of the mix-
ture of the resulting nitro compounds, the desired isomer
5 was reduced to provide the aminopicoline 7. This com-
pound has been previously reported in the literature,17 as
a side product of the ammonolysis of 3-methoxy-2-acyl-
furan, although its structure was not unambiguously con-
firmed. The aminopicoline 7 was then converted into the
corresponding acetamide 8, which was lithiated using n-
butyllithium in anhydrous THF and the resulting anion
was allowed to attack the carbonyl of the easily accessible
2,3,5-tri-O-benzyl-D-ribonolactone.18 However, this reac-
tion provided the expected anomeric mixture of hemiace-
tals 10 (Scheme 2) in only 13% yield, while the mixture
of compounds 11 was obtained as the major product (35%
yield); this is indicative of anion formation on the acet-
amide’s methyl, which then attacks the ribonolactone.
N
Me
O
BnO
BnO
OBn
8
10
+
OMe
N
NH(O)CH2C
O
BnO
OH
Me
OBn
BnO
11
Scheme 2 Reagents and conditions: (a) (i) n-BuLi (2 equiv), THF,
–78 °C → –55 °C, 1 h; (ii) 2,3,5-tri-O-benzyl-D-ribonolactone, THF,
–78 °C → –20 °C, 3 h.
OMe
OMe
OMe
OMe
F3COCHN
NHCOCF3
NHCOCF3
NHCOCF3
a
b
c
O
BnO
N
N
N
O
N
Me
O
H
BnO
BnO
9
H
OBn
OH
OBn
BnO
OBn
BnO
BnO
d
12
13
14
OMe
N
OMe
N
F3COCHN
NHCOCF3
e
O
HO
O
HO
H
H
HO
OH
HO
OH
16
15
OMe
N
f
NHCOCF3
OMe
F3C(O)CHN
H
O
O
AcO
+
AcO
H
OAc
N
OAc
AcO
AcO
17
18
g
O
OMe
OMe
Ac
N
N
H
N
H
N
h
i
N
H
N
N
O
N
N
O
H
AcO
HO
O
HO
H
OAc
OAc
HO
OH
AcO
HO
OH
20
19
21
Scheme 3 Reagents and conditions: (a) (i) n-BuLi (2 equiv), THF, –78 °C → –45 °C, 1 h; (ii) 2,3,5-tri-O-benzyl-D-ribonolactone, THF,
–78 °C → –20 °C, 3 h; (b) BF3·Et2O, CH2Cl2, 0 °C, 2 h; (c) H2, 10% Pd/C, EtOH, r.t., 24 h; (d) BCl3, CH2Cl2, –78 °C, 1 h; (e) H2, 10% Pd/
C, EtOH, r.t., 5 h; (f) Ac2O, Et3N, CH2Cl2, r.t., 12 h; (g) AcOK, Ac2O, isoamyl nitrite, C6H6, reflux, 12 h; (h) NH3–MeOH, r.t., 10 h; (i) NaI,
(Me)3SiCl, MeCN, 80 °C, 4 h.
Synlett 2008, No. 2, 181–184 © Thieme Stuttgart · New York

LETTER
Synthesis of 6-Deazaformycin B
183
Consequently, we decided to use the trifluoroacetamide
9,19 which was easily prepared by treatment of 7 with tri-
fluoroacetic anhydride. The lithium-mediated anion of 9
was reasonably stable at –45 °C and upon reaction with
2,3,5-tri-O-benzyl-D-ribonolactone provided the mixture
of hemiacetals 12 in 62% yield (Scheme 3). This mixture
was treated with BF3·OEt2 in CH2Cl2 solution and provid-
ed almost quantitatively the olefin 13.20 Unfortunately,
Acknowledgment
This study was supported by a Research Grant provided by the
Greek Ministry of Education (Business Program of Education and
Initial Professional Preparing-EPEAEK).
References and Notes
(1) (a) Ichikawa, E.; Kato, K. Curr. Med. Chem. 2001, 8, 385.
(b) Yokoyama, M.; Momotake, A. Synthesis 1999, 1541.
(c) Merino, P. Curr. Med. Chem. 2006, 13, 539.
(2) Somu, R. V.; Boshoff, H.; Qiao, C. H.; Bennett, E. M.;
Barry, C. E.; Aldrich, C. C. J. Med. Chem. 2006, 49, 31.
(3) Barrett, A. G. M.; Doubleday, W. W.; Hamprecht, D.;
Kasdorf, K.; Tustin, G. J. Pure Appl. Chem. 1997, 69, 383.
(4) Jordheim, L.; Calmarini, C. M.; Dumontet, C. Lancet Oncol.
2002, 3, 415.
(5) (a) Vittori, S.; Dal Ben, D.; Lambertucci, C.; Marucci, G.;
Volpini, R.; Cristalli, G. Curr. Med. Chem. 2006, 13, 3529.
(b) De Clercq, E. J. Clin. Virol. 2004, 30, 115.
(6) (a) Wu, O.; Simons, C. Synthesis 2004, 1533.
(b) Watanabe, K. A. In Chemistry of Nucleosides and
Nucleotides, Vol. 3; Townsend, L. B., Ed.; Plenum Press:
New York, 1994, 421–535.
(7) Sollogoub, M.; Fox, K. R.; Powers, V. E. C.; Brown, T.
Tetrahedron Lett. 2002, 43, 3121.
(8) Lim, M.-I.; Ren, W.-Y.; Otter, B. A.; Klein, R. S. J. Org.
Chem. 1983, 48, 780.
(9) Lim, M.-I.; Klein, R. S. Tetrahedron Lett. 1981, 22, 25.
(10) Lim, M.-I.; Klein, R. S.; Fox, J. J. Tetrahedron Lett. 1980,
21, 1013.
(11) (a) Bacchi, C. J.; Berens, R. L.; Nathan, H. C.; Klein, R. S.;
Elegbe, I. A.; Rao, K. V. B.; McCann, P. P.; Marr, J. J.
Antimicrob. Agents Chemother. 1987, 31, 1406. (b) Smith,
J. W.; Bartlett, M. S.; Queener, S. F. Diagn. Microbiol.
Infect. Dis. 1987, 7, 113.
1
even if the H NMR spectrum of the isolated compound
corresponded to only one isomer, the NOE spectral data
did not provide clear evidence concerning the exact stereo-
chemistry of the olefin 13. The olefin was submitted to
catalytic hydrogenation over Pd/C, to result in an anomer-
ic mixture of compounds 14, while no reductive debenzy-
lation took place. Only the major and more polar
component of the abovementioned mixture 14 could be
isolated pure by column chromatography and according to
NOE experiments, proved to be the a-anomer, since we
observed correlation peaks between H-1¢ and H-3¢, as well
as between H-1¢ and H-5¢. Due to the extremely low yield
of the desired b-anomer, we decided to cleave the benzyl
groups of the olefin 13 and then reduce the resulting com-
pound. For this purpose, a CH2Cl2 solution of 13 was
treated with boron trichloride at –78 °C to provide exclu-
sively the Z-isomer of the olefin 15.21 The stereochemistry
was unambiguously determined on the basis of NOE data,
where a strong correlation peak between the olefinic pro-
ton and H-2¢ was obvious.
Catalytic hydrogenation of compound 15 over Pd/C pro-
vided the mixture of anomers 16 in a 1:1.5 ratio, as esti-
1
mated by H NMR. The chromatographic separation of
the mixture proved to be very difficult at this point, there-
fore both anomers were acetylated and the resulting ace-
tates were separated by column chromatography and
identified. The configuration at C-1¢ was assigned on the
basis of NOE experiments, where we observed clear cor-
relation peaks between H-1¢ and H-3¢ for 17 (a-anomer)22
and between H-4¢ and H-1¢ for compound 18 (b-ano-
mer).23 The anomer 18 was then treated with isoamyl ni-
trite in benzene at reflux in the presence of acetic
anhydride and potassium acetate,24 to result upon rear-
rangement of the intermediate N-nitroso compound in the
1-acetylpyrazolopyridine 19. The acetyl groups of 19
were easily cleaved upon treatment with methanolic am-
monia and the resulting C-nucleoside 20 was converted
into the target 6-deazaformycin B (21)25 by reaction with
trimethylsilyl chloride in the presence of sodium iodide.26
(12) Chu, M. Y.; Zuckerman, L. B.; Sato, S.; Crabtree, G. W.;
Bogden, A. E.; Lim, M.-I.; Klein, R. S. Biochem.
Pharmacol. 1984, 33, 1229.
(13) (a) Robins, R. K.; Townsend, L. B.; Cassidy, F.; Gerster, J.
F.; Lewis, A. F.; Miller, R. L. J. Heterocycl. Chem. 1966, 3,
110. (b) Buchanan, J. G. Fortschr. Chem. Org. Naturst.
1983, 44, 243.
(14) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Townsend, L. B.
J. Org. Chem. 2003, 68, 6466.
(15) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Townsend, L. B.
Synlett 2002, 1479.
(16) Olah, G. A.; Arvanaghi, M.; Vankar, Y. D. Synthesis 1980,
660.
(17) Fischer, B. E.; Hodge, J. E. J. Org. Chem. 1964, 29, 776.
(18) (a) Cousineau, T. J.; Secrist, J. A. III. J. Org. Chem. 1979,
44, 4351. (b) Timpe, W.; Dax, K.; Wolf, N.; Weidmann, H.
Carbohydr. Res. 1975, 69, 51.
(19) Data for N-(4-Methoxy-2-methylpyridin-3-yl)trifluoro-
acetamide (9). Yield: 96%; mp 164–165 °C (EtOH). 1H
NMR (DMSO-d6): d = 2.31 (s, 3 H, CH3), 3.86 (s, 3 H,
CH3O), 7.05 (d, J5,6 = 5.80 Hz, 1 H, H-5), 8.35 (d, J6,5 = 5.80
Hz, 1 H, H-6), 10.93 (br s, D2O exch., 1 H, NH). 13C NMR
(DMSO-d6): d = 20.30 (CH3), 56.18 (CH3O), 106.00 (C-5),
107.50, 113.22, 118.96, 124.69 (CF3CO), 118.23 (C-3),
149.93 (C-6), 154.15, 154.88, 155.60, 156.31 (CF3CO),
156.09 (C-2), 160.89 (C-4). Anal. Calcd for C9H9F3N2O2: C,
46.16; H, 3.87; N, 11.96. Found: C, 45.87; H, 3.76; N, 12.13.
(20) Data for N-[4-Methoxy-2-(2,3,5-tri-O-benzyl-D-ribo-
furanosylidene)methylpyridin-3-yl]trifluoroacetamide (13).
Yield: 71%; mp 146–151 °C (EtOH). 1H NMR (CDCl3): d =
3.56 (m, 1 H, H-5¢), 3.74 (m, 1 H, H-5¢), 3.89 (s, 3 H, CH3O),
In conclusion, we have prepared 6-deazaformycin B using
an efficient method, through the attachment of a protected
ribonolactone to a suitably substituted picoline, cycliza-
tion of the intermediate to elaborate the pyrazolo[4,3-
b]pyridine ring system, followed by cleavage of the pro-
tective groups.
Synlett 2008, No. 2, 181–184 © Thieme Stuttgart · New York

184
S. Korouli et al.
LETTER
3.97 (m, 1 H, H-3¢), 4.40 (m, 2 H, CH2), 4.43 (m, 1 H, H-2¢),
4.47 (m, 2 H, CH2), 4.56 (m, 2 H, CH2), 4.70 (m, 1 H, H-4¢),
5.85 (s, 1 H, =CH), 6.79 (d, J5,6 = 5.62 Hz, 1 H, H-5), 7.18–
7.42 (m, 15 H, PhH), 8.45 (d, J6,5 = 5.62 Hz, 1 H, H-6), 8.79
(br s, D2O exch., 1 H, NH). 13C NMR (CDCl3): d = 56.10
(CH3O), 68.52 (C-5¢), 71.28 (CH2), 72.27 (CH2), 72.52
(=CH), 72.99 (C-2¢), 73.20 (CH2), 75.80 (C-3¢), 76.89 (C-1¢),
85.18 (C-4¢), 105.81 (C-5), 107.52, 113.25, 118.99, 124.73
(CF3CO), 118.15 (C-3), 127.61, 127.81, 128.10, 128.16,
128.22, 128.31, 128.41, 128.51, 128.56, 137.04, 137.47
(CPh), 147.07 (C-6), 150.04 (C-2), 153.80, 154.52, 155.26,
155.99 (CF3CO), 163.17 (C-4). Anal. Calcd for
(CF3CO), 119.35 (C-3), 150.32 (C-6), 154.98 (C-2), 154.18,
154.91, 155.63, 156.35 (CF3CO), 160.92 (C-4), 168.98
(CH3CO), 169.82 (CH3CO), 170.69 (CH3CO). Anal. Calcd
for C20H23F3N2O9: C, 48.78; H, 4.71; N, 5.69. Found: C,
48.63; H, 4.67; N, 5.52.
(23) Data for N-[4-Methoxy-2-(2,3,5-tri-O-acetyl-b-D-ribo-
furanosyl)methylpyridin-3-yl]trifluoroacetamide (18):
yield: 34%; light yellow oil; [a]D –30.2° (c = 0.227, CH2Cl2).
1H NMR (CDCl3): d = 1.98 (s, 3 H, CH3CO), 2.03 (s, 3 H,
CH3CO), 2.07 (s, 3 H, CH3CO), 3.03 (m, 1 H, CH2), 3.22 (m,
1 H, CH2), 3.89 (s, 3 H, CH3O), 4.00 (m, 1 H, H-5¢), 4.10 (m,
1 H, H-5¢), 4.33 (m, 1 H, H-4¢), 4.37 (m, 1 H, H-1¢), 4.89 (m,
1 H, H-2¢), 4.96 (m, 1 H, H-3¢), 6.82 (d, J5,6 = 5.86 Hz, 1 H,
H-5), 8.40 (d, J6,5 = 5.86 Hz, 1 H, H-6), 8.71 (br s, D2O exch.,
1 H, NH). 13C NMR (CDCl3): d = 20.49 (CH3CO), 20.58
(CH3CO), 20.64 (CH3CO), 36.77 (CH2), 56.21 (CH3O),
63.56 (C-5¢), 71.32 (C-2¢), 72.47 (C-3¢), 80.79 (C-1¢), 80.98
(C-4¢), 106.46 (C-5), 106.60, 112.33, 118.07, 123.82
(CF3CO), 119.98 (C-3), 150.35 (C-6), 154.40 (C-2), 154.51,
155.22, 155.98, 156.64 (CF3CO), 161.11 (C-4), 169.87 (2 ×
CH3CO), 170.62 (CH3CO). Anal. Calcd for C20H23F3N2O9:
C, 48.78; H, 4.71; N, 5.69. Found: C, 48.92; H, 4.64; N, 5.76.
(24) Marakos, P.; Pouli, N.; Wise, S. D.; Townsend, L. B. Synlett
1997, 561.
C35H33F3N2O6: C, 66.24; H, 5.24; N, 4.41. Found: C, 66.02;
H, 5.31; N, 4.29.
(21) Data for (Z)-N-[4-Methoxy-2-(D-ribofuranosyl-
idene)methylpyridin-3-yl]trifluoroacetamide (15): yield:
86%; mp >250 °C (dec.; EtOH). 1H NMR (DMSO-d6): d =
3.63 (m, 2 H, H-5¢), 4.05 (s, 3 H, CH3O), 4.07 (m, 1 H, H-
3¢), 4.53 (m, 1 H, H-4¢), 4.81 (m, 1 H, H-2¢), 5.28–5.46 (br s,
D2O exch., 2 H, 2 × OH), 5.51 (s, 1 H, =CH), 5.77–5.84 (br
s, D2O exch., 1 H, OH), 7.45 (d, J5,6 = 7.09 Hz, 1 H, H-5),
8.17 (br s, D2O exch., 1 H, NH), 8.64 (d, J6,5 = 7.09 Hz, 1 H,
H-6). 13C NMR (DMSO-d6): d = 57.52 (CH3O), 60.33 (C-5¢),
69.71 (C-3¢), 73.46 (C-2¢), 74.07 (C-1¢), 84.71 (=CH), 93.15
(C-4¢), 104.41 (C-5), 107.45, 113.18, 118.91, 124.65
(CF3CO), 116.28 (C-3), 143.80 (C-6), 148.17 (C-2), 153.94,
154.65, 155.40, 156.12 (CF3CO), 166.92 (C-4). Anal. Calcd
for C14H15F3N2O6: C, 46.16; H, 4.15; N, 7.69. Found: C,
46.22; H, 3.96; N, 7.51.
(22) Data for N-[4-Methoxy-2-(2,3,5-tri-O-acetyl-a-D-
ribofuranosyl)methylpyridin-3-yl]trifluoroacetamide (17).
Yield: 51%; light yellow oil; [a]D +33.1° (c = 0.124,
CH2Cl2). 1H NMR (CDCl3): d = 2.03 (s, 3 H, CH3CO), 2.05
(s, 3 H, CH3CO), 2.12 (s, 3 H, CH3CO), 2.88 (m, 1 H, CH2),
3.17 (m, 1 H, CH2), 3.90 (s, 3 H, CH3O), 4.03 (m, 1 H, H-5¢),
4.24 (m, 1 H, H-5¢), 4.30 (m, 1 H, H-4¢), 4.54 (m, 1 H, H-1¢),
5.26 (m, 1 H, H-3¢), 5.55 (m, 1 H, H-2¢), 6.81 (d, J5,6 = 5.86
Hz, 1 H, H-5), 8.38 (d, J6,5 = 5.86 Hz, 1 H, H-6), 8.84 (br s,
D2O exch., 1 H, NH). 13C NMR (CDCl3): d = 20.57 (2 ×
CH3CO), 20.77 (CH3CO), 35.49 (CH2), 56.22 (CH3O),
63.77 (C-5¢), 72.17 (C-3¢), 72.69 (C-2¢), 77.57 (C-4¢), 80.48
(C-1¢), 106.35 (C-5), 106.51, 112.24, 117.97, 123.71
(25) Data for 1,4-Dihydro-3-(b-D-ribofuranosyl)-7H-pyrazo-
lo[4,3-b]pyridin-7-one(21): yield: 78.9%; mp 200 °C (dec.;
EtOH); [a]D –43.0° (c = 0.330, MeOH). 1H NMR (DMSO-
d6): d = 3.61 (m, 2 H, H-5¢), 3.86 (m, 1 H, H-4¢), 3.94 (m, 1
H, H-3¢), 4.11 (m, 1 H, H-2¢), 4.87 (m, 1 H, H-1¢), 4.93 (br s,
D2O exch., 1 H, OH-3¢), 5.11 (br s, D2O exch., 1 H, OH-2¢),
5.76 (br s, D2O exch., 1 H, OH-5¢) 5.88 (d, J6,5 = 7.14 Hz, 1
H, H-6), 7.59 (d, J5,6 = 7.14 Hz, 1 H, H-5), 11.66 (br s, D2O
exch., 1 H, NH-4), 13.46 (br s, D2O exch., 1 H, NH-1). 13
C
NMR (DMSO-d6): d = 61.27 (C-5¢), 71.58 (C-3¢), 75.33 (C-
2¢), 79.09 (C-1¢), 84.71 (C-4¢), 109.10 (C-6), 126.00 (C-3a),
133.50 (C-7a), 137.23 (C-5), 138.20 (C-3), 168.20 (C-7).
Anal. Calcd for C11H13N3O5: C, 49.44; H, 4.90; N, 15.72.
Found: C, 49.27; H, 4.73; N, 15.59.
(26) (a) Olah, G. A.; Narang, S. C.; Gupta, B. G. B.; Malhotra, R.
J. Org. Chem. 1979, 44, 1247. (b) Ramasany, K.; Imamura,
N.; Robins, R. K.; Revankar, G. R. J. Heterocycl. Chem.
1988, 25, 1893.
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