Design and synthesis of 3′-deoxy-3′-fluoro-2′-O-3′- C-vinylene-linked bicyclic purine nucleoside

Article abstract of DOI:10.1055/s-0028-1087227

Starting from D-xylose via a 3-deoxy-3α-C-ethynyl-3-fluoro sugar as a key intermediate, a novel fluorinated bicyclic purine nucleoside has been synthesized. A neighboring-group participation pathway and a mechanism involving an intramolecular cycloaddition induced by fluorine are proposed for the formation of the key intermediate and the bicyclic nucleoside, respectively. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087227

LETTER
2993
Design and Synthesis of 3¢-Deoxy-3¢-Fluoro-2¢-O,3¢-C-Vinylene-Linked
Bicyclic Purine Nucleoside
D
esign and Synth
a
e
sis of Flu
o
orinated Bicy
j
c
lic Pu
u
rine
N
ucleos
a
ide n Zhao,a Junbiao Chang,*a,b Qiang Wang,b Yunfei Du,a Kang Zhaoa
a
School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, P. R. of China
Department of Chemistry, Zhengzhou University, Zhengzhou 450001, P. R. of China
Fax +86(371)67783017; E-mail: changjunbiao@zzu.edu.cn
b
Received 27 June 2008
ate 4 was observed to furnish the 3-fluoro-3-deoxy-3a-C-
ethynyl sugar 6 and not the expected fluorination product
5. In a subsequent transformation, we have observed a
fluorine-induced intramolecular cyclization to produce
Abstract: Starting from D-xylose via a 3-deoxy-3a-C-ethynyl-3-
fluoro sugar as a key intermediate, a novel fluorinated bicyclic pu-
rine nucleoside has been synthesized. A neighboring-group partici-
pation pathway and a mechanism involving an intramolecular
cycloaddition induced by fluorine are proposed for the formation of the novel fluorinated bicyclic purine nucleoside
the key intermediate and the bicyclic nucleoside, respectively.
{(3aS,4R,6S,6aS)-3a-fluoro-6-(6-methoxy-7H-purin-7-yl)-
3a,4,6,6a-tetrahydrofuro[3,2-c]-furan-4-yl} methanol (10).
Key words: ethynyl group, fluorine, bicyclic nucleoside, neighbor-
ing-group participation, intramolecular cyclization
The starting material for our synthetic route (Scheme 1)
was D-xylose (1), which was converted to the 3b-C-ethy-
nyl sugar 2 in five steps by a known procedure.12
Nucleoside analogues, some of which remain in the fore-
front of anti-HIV and anti-HBV chemotherapeutic regi-
mens,1,2 play an important role in the control of viral
diseases and cancer. However, side effects and drug-resis-
tant mutants continue to be problematic with these agents.
As a result, the development of structurally new, more po-
tent, and less toxic nucleoside derivatives is in demand.2
Modification of the carbohydrate moiety of nucleosides
with the electron-withdrawing fluorine atom has been ex-
tensively applied in the design of such analogues, as this
can greatly affect the electronic structure of a nucleoside
often leading to dramatically improved activities3 without
significantly altering its shape and size.4 In addition, the
ethynyl group, which has characteristic electronic and
structural features due to its sp-hybridization, is present in
many biologically active compounds,5 and its introduc-
tion into the natural nucleoside structure sometimes pro-
duces potent antimetabolites.6–9,2b The 1-(3-C-ethynyl-b-
D-ribopentofuranosyl) cytosine (Ecyd), for example, ex-
hibits good activity against various human solid tumors
and is undergoing human clinical trials.10 These encourag-
ing results have prompted us to synthesize nucleosides
modified with both fluoro and ethynyl groups, such as 2¢-
fluoro-2¢-deoxy-3¢b-C-ethynyl nucleosides or 3¢-fluoro-
3¢-deoxy-3¢a-C-ethynyl nucleosides, in order to discover
novel drugs. The related compounds, 3¢-fluoro-2¢,3¢-
dideoxy-2¢,3¢-didehydro-4¢-ethynyl nucleosides2a and 2¢-
fluoro-2¢-deoxy-3¢a-C-ethynyl nucleosides11 have been
previously prepared among which the 3¢-fluoro-2¢,3¢-
dideoxy-2¢,3¢-didehydro-4¢-ethynyl-D-adenine showed
moderate anti-HIV activity without cytotoxicity. In the
course of preparing a 2¢-fluoro-2¢-deoxy-3¢b-C-ethynyl
derivative, an unusual fluorination of hydroxyl intermedi-
Benzoylation of 2 in a mixture of BzCl, DMAP, and Et3N
followed by methanolysis with concentrated HCl in aque-
ous MeOH at 55 °C for 4 hours gave the intermediate
4
22,23 as the sole isomer in 75% yield. 1H NMR spectrum
of 4 showed that the proton signal for H-1 (d = 5.56 ppm)
appeared as a doublet (J = 2.5 Hz), which suggested that
the a-isomer was formed.13a When 4 was treated with di-
ethylaminosulfur trifluoride (DAST, 4.5 equiv) and pyri-
dine in CH2Cl2 from –78 °C to room temperature, sugar 6
was obtained in 42% yield as the major product with no 2-
fluoro derivative 5 observed. The assignment of com-
pound 6 as the 3-fluoro-3-deoxy-3a-C-ethynyl sugar is
1
consistent with its H NMR spectrum. The J2,F coupling
constants have a diagnostic value in differentiating be-
tween compounds analogous to the 2-fluoro-2-deoxy-3b-
C-ethynyl sugar 5 (J2,F = ca. 50.0 Hz, a larger geminal
coupling constant)13 and 3-fluoro-3-deoxy-3a-C-ethynyl
sugar 6 (J2,F = 12.5 Hz, a smaller vicinal coupling con-
stant). The signal of the ethynyl proton is a doublet
(J = 5.5 Hz) suggesting a close spacial interaction be-
tween the ethynyl proton and the fluorine. In addition, the
singlet peak corresponding to H-1 indicates that the b-
anomer was formed.13a Further substantiation for the
structure 6 comes from an X-ray structure of the final nu-
cleoside product 10. The intermediate 6 was then subject-
ed to acetolysis by a known method12b,13c,14 to give only
the b-anomer of 1-acetate 7 in 64% yield as a syrup.
The formation of 6 by the DAST reaction can be rational-
ized by a neighboring-group participation of the 3-benz-
oate ester and the generation of the benzoxonium ion
B13b,15 which would react with fluoride anion present in
the reaction system (Scheme 2). Because the allylic stabi-
lization of the ethynyl group makes the C-3 more electro-
philic, the attack of the fluoride anion at C-3 is preferred
to give the fluorinated product 6.24
SYNLETT 2008, No. 19, pp 2993–2996
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1087227; Art ID: W10308ST
© Georg Thieme Verlag Stuttgart · New York

2994
B. Zhao et al.
LETTER
BzCl, DMAP
Et3N, CH2Cl2
0 °C to r.t., 3 h
OH
O
RO
RO
ref. 12
5 steps
HO
O
O
67%
OH
OH
O
O
OBz O
OH
O
R = 4-ClBz
D-xylose (1)
R = 4-ClBz
3
2
OMe
F
MeOH–H2O
concd HCl
55 °C, 4 h
RO
RO
DAST, pyridine
CH2Cl2, –78 °C to r.t.
24 h
O
O
OMe
OBz OH
OBz
75%
42%
R = 4-ClBz
R = 4-ClBz
4
6
concd H2SO4
Ac2O, AcOH
5 °C, 8 h
OAc
F
RO
DAST
pyridine
CH2Cl2
F
RO
O
O
OMe
64%
OBz
OBz
R = 4-ClBz
R = 4-ClBz
5
7
Scheme 1 Synthesis of the glycosyl donor 7
RO
RO
O
O
DAST
OMe
OH
OMe
OBz
O
O
S
R = 4-ClBz
F
F
O
4
NEt2
A
OMe
RO
RO
F
F–
O
O
Figure 1 X-ray crystallographic structure of compound 10
OMe
O
OBz
O
dition reaction. The structure and the absolute configura-
tion of 10 were further confirmed by X-ray
crystallography (Figure 1).17
R = 4-ClBz
6
In Scheme 4 we suggest a plausible mechanism involving
an intramolecular cycloaddition induced by fluorine
whereby the protected nucleoside 9 was treated with
K2CO3 and MeOH to yield the novel fluorinated bicyclic
nucleoside 10. Once deprotected, the 2¢-OH could attack
the ethynyl group and create a vinyl carbanion intermedi-
ate E. Both the electron-withdrawing effect of the fluorine
at C-3a stabilizing the intermediate E and the energetics
of the dihydrofuran cyclization could promote the reac-
tion.18
B
Scheme 2 The proposed mechanistic pathway for the synthesis of
compound 6
Next, condensation of the acetate 725 with persilylated 6-
chloropurine proceeded under Vorbrüggen conditions us-
ing SnCl4 as a catalyst.
The 7-isomer 926 was the major product obtained under
the kinetic conditions (SnCl4, room temperature), as men-
tioned previously.16 When the protected nucleoside 9 was
treated with K2CO3 in MeOH at 65 °C for 2 hours, the
fluorinated bicyclic nucleoside 1027 was isolated in 55%
yield as the major product (Scheme 3). The structure of 10
was unequivocally identified by 1H NMR, 13C NMR, and
The conformationally restricted 2¢-O,3¢-C-vinylene-
linked bicyclic nucleoside analogue 10, which fuses an
unsaturated vinyl group and a fluorine atom in the carbo-
hydrate moiety, is important to our antiviral and antican-
cer nucleoside research program because of its structural
novelty within the field as suggested by Wengel and co-
workers.21b
1
19F NMR spectroscopy and HRMS. The H NMR spec-
trum of 10 (DMSO-d6) showed the disappearance of the
ethynyl hydrogen doublet of 9 at d = 2.94 ppm (d, 1 H,
J = 5.5 Hz) and the concomitant appearance of the vinyl
protons at d = 7.03 ppm (t, 1 H, J = 2.5 Hz, H-3) and d =
5.62 ppm (d, 1 H, J = 2.5 Hz, H-2), verifying the cycload-
Other naturally occuring bicyclic nucleosides show im-
portant biological activities,19 and a plethora of conforma-
tionally restricted bicyclic nucleosides have been
synthesized for potential antiviral activities.20 Finally, the
Synlett 2008, No. 19, 2993–2996 © Thieme Stuttgart · New York

LETTER
Design and Synthesis of Fluorinated Bicyclic Purine Nucleoside
2995
Cl
N
N
N
N
N
N
N
N
F
F
HO
RO
O
O
Cl
OH
OBz
R = 4-ClBz
11
8
persilylated
6-chloropurine,
DCE, SnCl4
K2CO3, MeOH
N
N
N
N
N
N
N
N
F
F
OAc
O
F
RO
RO
persilylated
6-chloropurine
DCE, SnCl4
HO
O
O
K2CO3
MeOH
reflux, 2 h
Cl
OMe
0 °C to r.t., 3 h
OBz
OBz
O
65%
55%
R = 4-ClBz
R = 4-ClBz
9
7
10
Scheme 3 Synthesis of the nucleoside 10
N
N
sponsible for the production of fluorinated bicyclic purine
nucleoside 10. Application of this experiment to the syn-
thesis of other related bicyclic nucleosides is of interest to
our antiviral and anticancer studies.
N
N
N
N
N
N
F
RO
F
RO
O
O
Cl
OMe
K2CO3
MeOH
reflux, 2 h
OBz
OH
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
R = 4-ClBz
H
9
C
N
N
Acknowledgment
N
N
N
J. C. thanks the National Natural Science Foundation of China
(#20672030, #30825043) and the Program of New Century Excel-
lent Talent, The Ministry of Education of China (NCET #050613)
for financial support.
N
HO
F
N
N
O
HO
O
OMe
F
OMe
O
–
+ OH2
–
BH
BH
+
+
BH
D
References and Notes
E
(1) (a) Patani, G. A.; Lavoie, E. J. Chem. Rev. 1996, 96, 3147.
(b) De Clereq, E. Antiviral Res. 2005, 67, 56. (c) Nair, V.;
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Chem. 2004, 69, 6034. (b) Ohrui, H.; Kohgo, S.; Kitano, K.;
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(c) Yoshimura, Y.; Asami, K.; Matsui, H.; Tanaka, H.;
Takahata, H. Org. Lett. 2006, 8, 6015.
BH
N
N
N
N
F
HO
O
OMe
O
10
(3) (a) Watanabe, K. A.; Reichman, U.; Hirota, K.; Lopez, C.;
Fox, J. J. J. Med. Chem. 1979, 22, 21. (b) Hertel, L. W.;
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Chu, C. K.; Cheng, Y.-C. Antimicrob. Agents Chemother.
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Scheme 4 The proposed mechanism for the formation of compound 10
monomeric nucleotides containing bicyclic furanose moi-
eties could effectively improve the binding affinity of
oligonucleotide analogues composed thereof towards
complementary DNA and RNA.21 A similar role for mod-
ified nucleoside 10 is under investigation in our laborato-
ry.
In conclusion, a mechanism involving an intramolecular
cycloaddition induced by fluorine is proposed to be re-
Synlett 2008, No. 19, 2993–2996 © Thieme Stuttgart · New York

2996
B. Zhao et al.
LETTER
(5) (a) Kando, J. C.; Kalunian, D. A. Hosp. Formul. 1990, 25,
849. (b) Harada, S.; Takayama, S. Yakuri to Chiryo 1991,
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Ford, H. Jr; Feldman, R. J.; Mitsuya, H.; Geoge, C.; Barchi,
J. J. Jr. J. Am. Chem. Soc. 1998, 120, 2780. (b) Lescop, C.;
Huet, F. Tetrahedron 2000, 56, 2995. (c) Marquez, V. E.;
Siddiqui, M. A.; Ezzitouni, A.; Russ, P.; Sidiqui, M. A.;
Wang, J.; Wagner, R. W.; Matteucci, M. D. J. Med. Chem.
1996, 39, 3739. (d) Olsen, A. G.; Rajwanshi, V. K.; Nielsen,
C.; Wengel, J. J. Chem. Soc., Perkin Trans. 1 2000, 3610.
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M.; Wengel, J. J. Chem. Soc., Perkin Trans. 1 2000, 3539.
(c) Leumann, C. J. Bioorg. Med. Chem. 2002, 10, 841.
(22) Compound 3: white solid, mp 168 °C. 1H NMR (500 MHz,
CDCl3): d = 8.04–8.00 (m, 4 H, Ar), 7.60–7.57 (m, 1 H, Ar),
7.43–7.36 (m, 4 H, Ar), 6.02 (d, 1 H, J = 3.5 Hz, H-1), 5.31
(d, 1 H, J = 3.5 Hz, H-2), 4.82–4.79 (m, 1 H, H-5), 4.75–4.71
(m, 1 H, H-4), 4.66–4.64 (m, 1 H, H-5), 2.74 (s, 1 H, H-7),
1.51 (s, 3 H, H-9), 1.35 (s, 3 H, H-10). 13C NMR (75 MHz,
CDCl3): d = 165.4, 164.1, 139.7, 133.5, 131.3, 129.9, 129.1,
128.7, 128.4, 128.2, 113.6, 104.7, 82.6, 78.1, 77.9, 77.8,
77.2, 63.8, 26.8, 26.6.
(6) (a) Minakawa, N.; Takeda, T.; Sasaki, T.; Matasuda, A.;
Ueda, T. J. Med. Chem. 1991, 34, 778. (b) De Clercq, E.;
Cools, M.; Balzarini, J.; Snoeck, R.; Andrei, G.; Hosoya, M.;
Shigeta, S.; Ueda, T.; Minakawa, N.; Matasuda, A.
Antimicrob. Agents Chemther. 1991, 679. (c) Minakawa,
N.; Matasuda, A. Curr. Med. Chem. 1999, 6, 615.
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Med. Chem. Lett. 1996, 6, 1887. (b) Hattori, H.; Tanaka,
M.; Fukushima, M.; Sasaki, T.; Matasuda, A. J. Med. Chem.
1996, 39, 5005. (c) Hattori, H.; Nozawa, E.; Lino, T.;
Yoshimura, Y.; Shuto, S.; Shimamoto, M.; Nomura, Y.;
Fukushima, M.; Tanaka, M.; Sasaki, T.; Matsuda, A. J. Med.
Chem. 1998, 41, 2892.
(8) (a) Sugimoto, I.; Shuto, S.; Mori, S.; Shigeta, S.; Matasuda,
A. Bioorg. Med. Chem. Lett. 1999, 9, 385. (b) Nomura, M.;
Shuto, S.; Tanaka, M.; Sasaki, T.; Mori, S.; Shigeta, S.;
Matasuda, A. J. Med. Chem. 1999, 42, 2901.
(9) Siddiqui, M. A.; Hughes, S. H.; Boyer, P. L.; Mitsuya, H.;
Van Q, N.; George, C.; Sarafinanos, S. G.; Marquez, V. E.
J. Med. Chem. 2004, 47, 5041.
(23) Compound 4: light yellow solid, mp 106–107 °C. 1H NMR
(500 MHz, CDCl3): d = 8.09–8.07 (m, 4 H, Ar), 7.62 (m,
1 H, Ar), 7.43–7.50 (m, 4 H, Ar), 5.61 (d, 1 H, J = 2.0 Hz,
H-1), 5.17 (d, 1 H, J = 2.0 Hz, H-2), 4.74–4.71 (m, 1 H, H-
5), 4.64–4.60 (m, 1 H, H-4), 4.47–4.44 (m, 1 H, H-5), 3.47
(10) Hrdlicka, P. J.; Jepsen, J. S.; Nielsen, C.; Wengel, J. Bioorg.
Med. Chem. 2005, 13, 1249.
(s, 3 H, H-8), 2.76 (br s, 1 H, 2-OH), 2.74 (s, 1 H, H-7). 13
NMR (75 MHz, CDCl3): d = 165.7, 165.5, 139.6, 133.8,
131.2, 130.0, 128.8, 128.6, 128.4, 107.1, 83.1, 81.4, 80.0,
77.2, 73.7, 65.2, 56.1.
C
(11) Moukhachafiq, O.; Tiwari, K. N.; Secrist, J. A. III
Nucleosides, Nucleotides Nucleic Acids 2005, 24, 713.
(12) (a) Poopeiko, N. E.; Kvasyuk, E. I.; Mikhailopuio, I. A.;
Lidaks, M. J. Synthesis 1985, 605. (b) Momura, M.; Sato,
T.; Washinosu, M.; Tanaka, M.; Asao, T.; Shuto, S.;
Matsuda, A. Tetrahedron 2002, 58, 1279.
(13) (a) Binkley, R. W. Modern Carbohydrate Chemistry;
Marcel Dekker Inc.: New York, 1988, 53–60. (b) Watts,
J. K.; Sadalapure, K.; Choubdar, N.; Pinto, B. M.; Damha,
M. J. J. Org. Chem. 2006, 71, 921. (c) Ma, T. W.; Lin, J.;
Newton, M. G.; Cheng, Y. C.; Chu, C. K. J. Med. Chem.
1997, 40, 2750. (d) Ma, T.; Pai, B.; Zhu, Y. L.; Lin, J. S.;
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7469.
(24) Compound 6: light yellow oil. 1H NMR (500 MHz, CDCl3):
d = 8.08–8.04 (m, 4 H, Ar), 7.64–7.61 (m, 1 H, Ar), 7.50–
7.42 (m, 4 H, Ar), 5.59 (d, 1 H, J = 12.5 Hz, H-2), 5.13 (s,
1 H, H-1), 4.78–4.64 (3 H, m, H-4 and H-5), 3.50 (s, 3 H,
H-8), 2.79 (d, 1 H, J = 5.5 Hz, H-7). ESI-HRMS: m/z calcd
for C22H18ClFO6 [M + Na+]: 455.0674; found: 455.0672.
(25) Compound 7: light yellow oil. 1H NMR (400 MHz, CDCl3):
d = 8.07–8.02 (m, 4 H, Ar), 7.65–7.61 (m, 1 H, Ar), 7.50–
7.41 (m, 4 H, Ar), 6.34 (s, 1 H, H-1), 5.72 (d, 1 H, J = 11.6
Hz, H-2), 4.85–4.62 (m, 3 H, H-4 and H-5), 2.81 (d, 1 H,
J = 5.2 Hz, H-7), 2.15 (s, 3 H, H-9). 13C NMR (100 MHz,
CDCl3): d = 169.4, 165.4, 164.4, 140.0, 134.2, 131.4, 130.2,
129.0, 128.8, 128.7, 128.2, 99.3, 93.2, 84.8, 81.1, 80.0,
74.46, 62.1, 21.1. 19F NMR (CDCl3): d = –185.7.
(14) Yoshimura, Y.; Endo, M.; Miuura, S.; Sakata, S. J. Org.
Chem. 1999, 64, 7912.
(26) Compound 9: white solid, mp: 211–212 °C. 1H NMR (500
MHz, CDCl3): d = 8.94 (s, 1 H, H-2), 8.67 (s, 1 H, H-8),
8.11–8.09 (m, 2 H, Ar), 8.05–8.03 (m, 2 H, Ar), 7.70–7.67
(m, 1 H, Ar), 7.54–7.51 (m, 2 H, Ar), 7.46–7.44 (m, 2 H, Ar),
6.88 (s, 1 H, H-1¢), 5.92 (dd, 1 H, J = 13.5, 1.0 Hz, H-2¢),
4.98–4.93 (m, 1 H, H-4¢), 4.90–4.82 (m, 2 H, H-5¢), 2.94 (d,
1 H, J = 6.0 Hz, H-7¢). 13C NMR (125 MHz, CDCl3): d =
165.4, 164.2, 162.6, 153.2, 146.7, 142.8, 140.4, 134.7,
131.4, 130.3, 129.1, 128.0, 127.8, 122.0, 93.8, 90.3, 84.0,
83.0, 80.9, 72.7, 60.9.
(15) (a) Wang, Y.; Inguaggiato, G.; Jasamai, M.; Shah, M.;
Hughes, D.; Slater, M.; Simons, C. Bioorg. Med. Chem.
1999, 7, 481. (b) Hudlicky, M. Org. React. 1988, 35, 513.
(16) (a) Robins, M. J.; Zou, R.; Guo, Z.; Wnuk, S. F. J. Org.
Chem. 1996, 61, 9207. (b) Garner, P.; Ramakanth, S. J. Org.
Chem. 1988, 53, 1294. (c) Wright, G. E.; Dudycz, L. W.
J. Med. Chem. 1984, 27, 175.
(17) Compound 10: space group P212121 with cell parameters:
a = 7.4156(15) nm, b = 20.417(4) nm, c = 23.585(5) nm;
CCDC 701133.
(27) Compound 10: white solid, mp 112–115 °C. 1H NMR (500
Hz, DMSO-d6): d = 8.86 (s, 1 H), 8.62 (s, 1 H), 7.03 (t,
J = 2.5 Hz, 1 H), 5.95 (d, J = 6.0 Hz, 1 H), 5.68 (dd, J = 6.0,
26.5 Hz), 5.62 (d, J = 2.5 Hz, 1 H), 5.01 (t, J = 5.0 Hz, 1 H),
4.25–4.20 (m, 1 H), 4.06 (s, 3 H), 3.82–3.77 (m, 1 H), 3.71–
3.66 (m, 1 H). 13C NMR (75 Hz, DMSO-d6): d = 162.86,
157.15, 153.48 (d, J = 9.0 Hz, C-2), 152.88, 147.27, 112.07,
111.17 (d, 2JC–F = 202.1 Hz, C-3a), 103.91 (d, 3JC–F = 24.5
Hz, C-3), 91.16 (d, 3JC–F = 25.3 Hz, C-4), 90.63 (d, J = 13.0
Hz, C-6), 86.39 (d, J = 31.3 Hz, C-6a), 60.07 (d, J = 9.0 Hz,
CH2OH), 55.60 (OCH3). 19F NMR (400 Hz, DMSO-d6): d =
–167.61. ESI-HRMS: m/z calcd for C13H13FN4O4 [M + Na+]:
331.0819; found: 331.0811.
(18) Shapiro, S. L.; Bandurco, V.; Freedman, L. J. Org. Chem.
1961, 26, 3710.
(19) (a) Nakagawa, F.; Okazaki, T.; Naito, A.; Iijima, Y.;
Yamazaki, M. J. Antibiot. 1985, 38, 823. (b) Arai, M.;
Haneishi, T.; Kitahara, N.; Enokita, R.; Kawakubo, K.;
Kondo, Y. J. Antibiot. 1976, 29, 863. (c) Terahara, A.;
Haneishi, T.; Arai, M.; Hata, T.; Kuwano, H.; Tamura, C.
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Synlett 2008, No. 19, 2993–2996 © Thieme Stuttgart · New York