Double bond formation based on nitroaldol reaction and radical elimination: A prototype segment connection method for the total synthesis of nigricanoside A dimethyl ester

Article abstract of DOI:10.1016/j.tetlet.2018.03.091

During the course of our studies toward the total synthesis of nigricanoside A dimethyl ester, a prototype method for the connection of the left- and right-half segments at the C9′–C10′ double bond was developed using a model system. The method was based

Full text of DOI:10.1016/j.tetlet.2018.03.091

Accepted Manuscript
Double bond formation based on nitroaldol reaction and radical elimination: a
prototype segment connection method for the total synthesis of nigricanoside A
dimethyl ester
Takayuki Tsunoda, Kenshu Fujiwara, Satoshi Okamoto, Yoshihiko Kondo,
Uichi Akiba, Yusuke Ishigaki, Ryo Katoono, Takanori Suzuki
PII:
S0040-4039(18)30428-3
https://doi.org/10.1016/j.tetlet.2018.03.091
TETL 49860
DOI:
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
1 March 2018
24 March 2018
30 March 2018
Please cite this article as: Tsunoda, T., Fujiwara, K., Okamoto, S., Kondo, Y., Akiba, U., Ishigaki, Y., Katoono, R.,
Suzuki, T., Double bond formation based on nitroaldol reaction and radical elimination: a prototype segment
connection method for the total synthesis of nigricanoside A dimethyl ester, Tetrahedron Letters (2018), doi: https://
doi.org/10.1016/j.tetlet.2018.03.091
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Double bond formation based on nitroaldol
reaction and radical elimination: a prototype
segment connection method for the total
synthesis of nigricanoside A dimethyl ester
Leave this area blank for abstract info.
Takayuki Tsunoda, Kenshu Fujiwara,* Satoshi Okamoto, Yoshihiko Kondo, Uichi Akiba, Yusuke Ishigaki, Ryo
Katoono, Takanori Suzuki

1
Tetrahedron Letters
Double bond formation based on nitroaldol reaction and radical elimination: a
prototype segment connection method for the total synthesis of nigricanoside A
dimethyl ester
Takayuki Tsunoda a, Kenshu Fujiwara b, , Satoshi Okamoto a, Yoshihiko Kondo b, Uichi Akiba b, Yusuke
Ishigakia, Ryo Katoono a and Takanori Suzuki a
aDepartment of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
bDepartment of Life Science, Graduate School of Engineering Science, Akita University, Akita 010-8502, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
During the course of our studies toward the total synthesis of nigricanoside A dimethyl ester, a
prototype method for the connection of the left- and right-half segments at the C9'-C10' double
bond was developed using a model system. The method was based on a simple three-step
process including: (i) a nitroaldol reaction, (ii) chlorination or thionocarbonylation, and (iii)
radical elimination.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Natural product synthesis
Nitroaldol reaction
Radical elimination
Double bond formation
β-chloronitroalkane
Nigricanoside A dimethyl ester (1) (Scheme 1), isolated from
the green alga Avrainvillea nigricans by Andersen in 2007, is a
unique monogalactosyl diacyl glycerol skeleton featuring ether
bonds that connect two oxygenated fatty acids and a galactose
moiety.1 Although the stereochemistry of 1 was not completely
determined in the first isolation report, its novel structure and a
possible strong cytotoxicity against cancer cells, suggested in the
report, attracted significant attention from synthetic chemists.
Therefore, several research groups,2,3 including ours,4 have
studied the total synthesis of 1 aimed at the determination of the
absolute configuration and demonstration of the cytotoxicity. In
our study, a stereoselective method for the construction of the
C8'-O-C6'' ether of 1 was developed based on the chirality
transferring Ireland-Claisen rearrangement.4 In 2015, MacMillan
and Ready achieved the stereochemical assignment of the natural
product by stereoselective total synthesis and demonstrated the
absence of cytotoxicity.2 Since then, our interest in the synthesis
of 1 shifted toward the development of a new efficient
double bond of 1 based on a nitroaldol reaction and radical
elimination.
construction process for the novel structure. Thus, we have
explored a convergent strategy to construct 1 from the left-half
segment 2 and the right-half segment 3 by the union at the C9'-
C10' double bond, which has ether groups at both allylic
positions (C8' and C11'), a rare structure in acyclic natural
products (Scheme 1). Here, we describe the development of a
prototype method for the segment connection at the C9'-C10'
Scheme 1. Plan for the convergent synthesis of nigricanoside A
dimethyl ester (1).
The segment connection at the C9'-C10' double bond of 1
presents some difficult challenges due to the presence of ether
groups at C8' and C11' in addition to other internal double bonds.
———
 Corresponding author. e-mail: fjwkn@gipc.akita-u.ac.jp

2
Tetrahedron
Cross-metathesis is a well-known method for the formation of
and tetramethylguanidine also reacted with enone 18 without
simple ether-group-adjacent double bonds.5 However, the C9'-
C10' double bond is predicted to react with other internal double
bonds at C14'-C15', C7-C8, and C12-C13 by ring-closing olefin
metathesis as a side reaction under the cross-metathesis
conditions.6 Therefore, cross-metathesis is inappropriate for the
segment connection. Olefination, such as the Julia-Kocienski
reaction,7 includes an addition reaction of a carbanion stabilized
by an electron withdrawing group to an aldehyde in the first step.
In the application of olefination to the formation of the C9'-C10'
double bond, the stabilized carbanion possesses a bulky alkoxy
group (C1-C16 chain or the galactose moiety) at the β-position.
Since the alkoxy group is a potential leaving group, unfavorable
β-elimination is predicted when the carbanion is less stable than
the eliminating alkoxide ion. In fact, when a β-alkoxyalkyl 1-
phenyl-1H-tetrazol-5-yl sulfone was used in a Julia-Kocienski
olefination in our preliminary study, the β-elimination of the
alkoxide group predominated to produce a vinyl sulfone as a
byproduct.8 Accordingly, careful selection of the electron
withdrawing group to appropriately stabilize the adjacent
carbanion is important for the successful segment connection by
olefination.
elimination of the β-silyloxy group.14
There are only a few examples of the eliminative alkenylation
of nitroaldols under radical conditions (Scheme 4). However,
Kobertz13 reported a successful application of Robins'
procedure15 to nitroaldol 17, which was subjected to
thionocarbonylation under basic conditions followed by radical
elimination using AIBN and Bu3SnH, to produce alkene 22
having a 1,4-dialkoxybut-2-ene unit closely related to the C8'-
C11' part of 1. For an alternative method, a three-step sequence
including dehydration of a nitroaldol to a nitroalkene, conjugate
addition of a trithiocarbonic acid or a xanthate to the nitroalkene,
and radical elimination to form an alkene was reported by
Barton16 and Zard.17 Thus, encouraged by these successful
reports, we set out to find suitable conditions for the segment
connection process using model compounds 8 and 9a-c.
Scheme 2. A plan for segment connection at the double bond
adjacent to an ether group.
From the above consideration, we initially planned to connect
segments 4 (corresponding to 2) and 5 (corresponding to 3) by a
process including Henry (nitroaldol) reaction9 to give a nitroaldol
(6), followed by elimination of the hydroxy and nitro groups to
afford an allyl ether (7) (Scheme 2). Since nitroalkanes generally
have smaller pKa values than alcohols, -alkoxynitroalkanes are
expected to generate stable -alkoxy--nitrocarbanions (5),
which would react with aldehydes (4) without elimination of the
-alkoxy group to give nitroaldols (6). The eliminative
Scheme 3. Previous examples for nitroaldol and conjugate addition
reactions of β-alkoxynitro compounds. THP: tetrahydropyran-1-yl;
TBS: tert-butyldimethylsilyl; THF: tetrahydrofuran.
alkenylation of nitroaldols (6) would be achievable based on the
high radical reactivity of the nitro group. In this study, simple
model compound 8 and advanced model compounds 9a-c (Figure
1), in which the hept-4-yloxy group at the β-position of the nitro
group was modeled on the C1-C16 chain of 1, were employed to
demonstrate the segment connection process.10
Figure 1. Model compounds 8 and 9a-c. PBB: p-bromobenzyl;
MOM: methoxymethyl.
Scheme 4. Previous examples for double bond formation from
nitroaldol compounds. AIBN: 2,2'-azodiisobutyronitrile.
So far, several successful examples have been reported for
nitroaldol reactions of β-alkoxynitroalkanes (Scheme 3). Seebach
connected dianion 11, generated from β-alkoxynitro compound
10, with benzaldehyde to give nitropolyol 12 in high yield after
hydrolysis.11 Fluoride ion-promoted nitroaldol reactions using β-
alkoxynitro compounds 13 and 16, reported by Goméz-Sanchéz12
and Kobertz,13 produced nitroaldols 14 and 17, respectively, in
good yields. The anion generated by β-silyloxynitropropane 19
First, we examined the nitroaldol reaction of the model
compounds 8 and 9a-c with 3-phenylpropanal (27). Selected
results are shown in Table 1. When 8 was reacted with 27 in the
presence of KF and 18-crown-6 in MeCN according to the
Kobertz procedure,13 a complex mixture of unknown products
was obtained (entry 1). Under the Goméz-Sanchéz conditions,12
model 8 was treated with Bu4NF·3H2O, 27, Et3N, and TBSCl in

3
that order in THF to produce nitroaldol 28 (a mixture of four
diastereomers) in 55% yield (entry 2). However, the Goméz-
Sanchéz conditions were ineffective in the reaction of advanced
model 9a (entry 3). The reaction of 8 with 27 in the presence of
1,5-diazabicyclo[4.3.0]non-5-ene (DBN)18 in DMF afforded 28
in 53% yield (entry 4). In the reaction, unreacted 8 and 27 were
almost completely recovered, and, therefore, the reaction marked
a high yield (81%) based on recovered starting material (BRSM).
DBN also mediated the nitroaldol reaction of advanced model 9a
with 27 to give 29a in 59% yield and 88% BRSM yield (entry 5).
Advanced models 9b,c also produced the corresponding products
29b,c with high BRSM yields (93% and 86%, respectively) and
good reproducibility under the same conditions (entries 6 and 7).
Besides the conditions in Table 1, we also examined the reaction
of the dianion from 8 according to the Seebach procedure;11
however, it gave a complex mixture of unknown products. Thus,
these model studies suggested the suitability of a DBN-mediated
nitroaldol reaction between 2 and 3.
form under standard conditions without elimination. Although
elimination of the mesylate with Et3N at lower temperature was
found to show a decreased ratio of double bond migration,
migrated alkene 3320 was still a major product (D).21 Thus, no
thionocarbonate derivatives 30, 31, and 34 were obtained due to
instability of the nitroaldol 28 and nitroalkene 32 under basic
conditions, which induced the retro-nitroaldol reaction of 28 and
migration of the double bond of 32.
Table 1. Nitroaldol reaction of model compounds 8 and 9a-c.
DBN:
1,5-diazabicyclo[4.3.0]non-5-ene;
DMF:
N,N-
dimethylformamide.
Scheme 5. Attempts of the formation of thionocarbonate
derivatives 30, 31, and 34 from nitroaldol 28.
Therefore, we examined the formation of a thionocarbonate
derivative from 28 under neutral conditions. After several
attempts, it was found that the treatment of 28 with
thiocarbonyldiimidazole in refluxing toluene gave
Product
imidazolylthionocarbonate 35 (Scheme 6). Due to its lability, the
resulting 35 was immediately treated with Bu3SnH (10 eq) and
AIBN (2 eq) in refluxing toluene to produce E-alkene 36 in 25%
yield. Although we also attempted a tandem one-pot operation
for the thionocarbonylation/radical elimination reactions, no
alkene 36 was obtained. For the successful formation of alkene
36, the removal of thiocarbonyldiimidazole and imidazole by
extraction was required in the thionocarbonylation step. Thus,
although the reaction proceeded in low yield, an easy access of
alkene 36 from nitroaldol 28 was realized by a simple two-step
reaction method.
Entry Conditions
Yield
1
2
8 (1 eq), 27 (1 eq), KF (cat.), 18-crown-6 (cat.),
MeCN, 21 °C, 26 h.
complex
mixture
8 (1 eq), Bu4NF·3H2O (0.25 eq), 27 (0.67 eq),
Et3N (0.67 eq), TBSCl (1 eq), THF, 0 → 23 °C,
0.5 h.
28: 55%
(from 27)
3
9a (1 eq), Bu4NF·3H2O (0.25 eq), 27 (0.67 eq),
Et3N (0.67 eq), TBSCl (1 eq), THF, 0 → 23 °C,
0.5 h.
no reaction
4
5
6
7
8 (1 eq), 27 (1 eq), DBN (3 eq), DMF, 22 °C, 1 h.
28: 53%
(brsm 81%)
9a (1 eq), 27 (1 eq), DBN (3 eq), DMF, 22 °C, 2 h. 29a: 59%
(brsm 88%)
9b (1 eq), 27 (1 eq), DBN (3 eq), DMF, 22 °C, 2 h. 29b: 52%
(brsm 93%)
9c (1 eq), 27 (1 eq), DBN (3 eq), DMF, 22 °C, 2 h. 29c: 54%
(brsm 86%)
Next, conversion of γ-alkoxy-β-nitroalkanol 28 to xanthate 30,
thionocarbonate 31, and xanthate 34, which would be
transformed to an alkene under radical elimination (Barton-
McCombie) conditions,19 was attempted (Scheme 5). Although
several bases were used, standard conditions for xanthate
formation resulted in decomposition of 28 due to the retro-
nitroaldol reaction under strong basic conditions (A). When
nitroaldol 28 was treated with phenyl chlorothionoformate in the
presence of DMAP according to the Robins procedure,15 no
thionocarbonate 31 was produced (B). Next, formation of
nitroalkene 32, which would be an ideal precursor for xanthate
34, was attempted.16,17 While treatment of 28 with TFAA in the
presence of 2,6-lutidine and DMAP at –78 °C promoted
dehydration, undesired migration of the double bond took place
under the reaction conditions to give 3320 as a major product (C).
After several attempts, the mesylate ester of 28 was found to
Scheme 6. Formation of alkene 36 from 28 by
thionocarbonylation/radical elimination sequence.
a
During exploration of the reaction conditions for the
conversion of 28 to 32, we also found that the treatment of 28 (a
mixture of diasteromers) with thionyl chloride in DMF at 60 °C
for 5 min produced β-chloronitroalkane 37 in good yield (62%
after purification) without formation of 32 (Scheme 7). While
reaction conditions are known for the conversion of simple
nitroaldols to β-chloronitroalkanes,22 the above reaction is a rare
preparation of a β-chloro-β'-alkoxynitroalkane from a γ-alkoxy-
β-nitroalkanol. Next, the transformation of 37 to 36 was
examined under radical elimination conditions. The treatment of
37 with Bu3SnH and AIBN in refluxing toluene for 2.5 h

4
Tetrahedron
produced E-alkene 36 stereoselectively along with unreacted
thionyl chloride in DMF gave the corresponding chloride in
relatively good yield, which was confirmed by NMR analysis on
the crude mixture. Although the reduction of the PBB group to a
Bn group was anticipated at the stage of radical elimination, the
PBB group of the chloride intermediate was, in fact,
37. The mixture of 36 and 37 was subjected again to the same
reaction conditions to complete the transformation to 36 (61%).
unexpectedly damaged and removed under the radical conditions
to give a complex mixture of byproducts, which had no PBB or
Bn groups. Nitroaldols 29b and 29c were decomposed in the
chlorination step, in which the MOM ether of 29b and even the
methyl ether of 29c were removed. The addition of pyridine
slightly inhibited removal of the MOM group to afford 38b as a
minor component (Scheme 9). Chloride 38b was reacted with
Bu3SnH and AIBN to produce alkene 39b23 in 7.9% total yield.
The small production of 39b was attributable to the low yield of
the chlorination step. The radical elimination of 38b itself
proceeded smoothly. Therefore, for the efficient conversion of a
γ,δ-dialkoxy-β-nitroalkanol (29) to a 3,4-dialkoxy-1-alkene (39)
via the chlorination/radical elimination sequence, the suppression
of the removal of the protecting group of the oxygen atom at the
δ-position of the nitroalkanol is required in the chlorination step.
Scheme 7. Double bond formation from nitroaldol 28 via chloride
37.
Diastereomers of 37 exhibited different stabilities. During
silica gel chromatography, one of them decomposed, but the rest
were fairly stable. Therefore, the β-chloro-β'-alkoxynitroalkanes
were used without purification. When chlorination of 28 and
radical elimination were performed sequentially without isolation
of 37, the two-step yield of E-alkene 36 was markedly improved
(83%) as expected (Scheme 8). Thus, a new method for the
conversion of a γ-alkoxy-β-nitroalkanol to a 3-alkoxy-1-alkene
has been developed by chlorination followed by radical
elimination.
Scheme 8. Sequential two-step double bond formation from
nitroaldol 28.
Scheme 10. An alternative plan for the total synthesis of 1.
Double bond formation from the advanced models was
examined next. The stepwise treatment of nitroaldol 29b with
thiocarbonyldiimidazole and subsequently with Bu3SnH and
AIBN gave a complex mixture of byproducts and no desired
alkene 39b was detected (Scheme 9). This disappointing result
might be attributable to the steric effect of the protected hydroxy
group at the δ-position of the thionocarbonate intermediate, by
which the disturbance of radical elimination might enhance
undesired radical side reactions or the reverse reaction from the
labile thionocarbonate intermediate to afford degradation
products.
Scheme 11. Double bond formation from nitroaldol 45.
Scheme 9. Double bond formation from nitroaldol 29b.
Finally, we designed an alternative plan for the union at the
C9'-C10' double bond of 1 (Scheme 10). This plan involved the
installation of a nitro group to the left-half segment (40) and an
The alkene formation by chlorination/radical elimination
sequence was then applied to 29a-c. The reaction of 29a with

5
7.
Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A.
Synlett 1998, 9, 26.
Kinashi, N.; Fujiwara, K.; Suzuki, T. unpublished result.
(a) Henry, L. Compt. Rend. 1895, 120, 1265. (b) Henry, L. Bull.
Soc. Chim. France 1895, 13, 999.
aldehyde to the right-half segment (41), thereby avoiding the
presence of a problematic alkoxy group at the -position in the β-
nitro alcohol intermediate (42). The plan was demonstrated by
use of model aldehyde 44 corresponding to 41 and nitropropane
corresponding to 40 (Scheme 11). Aldehyde 44 was prepared
from alcohol 43 by Swern oxidation. The reaction of 44 with
nitropropane in the presence of DBN in DMF produced
nitroaldol 45 as a mixture of diastereomers in 84% yield from 43.
Upon treatment with thionyl chloride in DMF, nitroaldol 45 was
only dehydrated to give nitroalkene 46,24 while the methyl ether
remained intact. On the other hand, reaction of 45 with
thiocarbonyldiimidazole in 1,2-dichloroethane at 40 °C followed
by treatment with Bu3SnH and AIBN in refluxing toluene
furnished alkene 4823 in 25% yield. Thus, although further
optimization is required, the three-step process including a
nitroaldol reaction, thionocarbonylation, and radical elimination
shows promise for the union at the C9'-C10' double bond of 1.
8.
9.
10. Model 8 was prepared as a racemate. Each advanced model was a
single (2R,3S)-form. The synthesis of compounds 8, 9a, 9b, and
9c is outlined in Supplementary data.
11. Eyer, M.; Seebach, D. J. Am. Chem. Soc. 1985, 107, 3601.
12. Fernández, R.; Gasch, C.; Gómez-Sánchez, A.; Vílchez, J. E.
Tetrahedron Lett. 1991, 32, 3225.
13. Kobertz, W. R.; Bertozzi, C. R.; Bednarski, M. D. J. Org. Chem.
1996, 61, 1894.
14. Kitayama, T. Tetrahedron 1996, 52, 6139.
15. Robins, M. J.; Wilson, J. S. J. Am. Chem. Soc. 1981, 103, 932.
16. Barton, D. H. R.; Dorchak, J.; Jaszberenyi, J. C. Tetrahedron Lett.
1993, 34, 8051.
17. Ouvry, G.; Quiclet-Sire, B.; Zard, S. Z. Org. Lett. 2003, 5, 2907.
18. (a) Ono, N.; Katayama, H.; Nisyiyama, S.; Ogawa, T. J.
Heterocycl. Chem. 1994, 31, 707. (b) Kutovaya, I. V.; Shmatova,
O. I.; Tkachuk, V. M.; Melnichenko, N. V.; Vovk M. V.;
Nenajdenko, V. G. Eur. J. Org. Chem., 2015, 30, 6749.
19. Barton, D. H. R.; McCombie S. W. J. Chem. Soc., Perkin Trans. 1
1975, 1574.
In conclusion, during the course of our studies toward the total
synthesis of nigricanoside A dimethyl ester (1), a prototype
method for the connection of the left- and right-half segments (2
and 3 / 40 and 41) at the C9'-C10' double bond of 1 was
developed using a model system. The method was based on a
simple three-step process including nitroaldol reaction,
chlorination or thionocarbonylation, and radical elimination.
Further studies toward the total synthesis of 1 are currently
underway in our laboratory.
20. Compound 33 was obtained as a 1:1 mixture of diastereomers,
both of which had an E-double bond.
21. Compound 32 was produced as a single isomer. The
stereochemistry of the trisubstituted double bond of 32 was not
determined.
22. (a) Fourneau, J. P. Bull. Soc. Chim. France 1940, 7, 603. (b)
Novikov, S. S.; Belikov, V. M.; Epishina, L. V. Bull. Acad. Sci.
USSR, Div. Chem. Sci. (Engl. Transl.) 1962, 11, 1042. (c)
Fukunaga, K.; Okamoto, A.; Kimura, M. Nippon Kagaku Kaishi
1983, 542.
23. The stereochemistry of the newly formed double bond was E.
24. Compound 46 was observed as a single isomer. The
stereochemistry of the nitro-substituted double bond was not
determined.
Acknowledgments
We thank Dr. Eri Fukushi and Mr. Yusuke Takata (GC-MS
and NMR Laboratory, Faculty of Agriculture, Hokkaido
University) for the measurements of mass spectra. We are also
grateful to Prof. Mitsutoshi Jikei and Prof. Kazuya Matsumoto
(Department of Materials Science, Graduate School of
Engineering Science, Akita University) for the measurements of
NMR spectra and to Prof. Tetsuo Tokiwano (Faculty of
Bioresource Sciences, Akita Prefectural University) for the
measurements of optical rotation. This work was supported by
JSPS KAKENHI Grant Number JP15K01794.
Highlights
A prototype segment connection method for the
synthesis of nigricanoside A dimethyl ester
Olefination to give a double bond, which has an
ether group at an allylic position
Olefination by nitroaldol reaction, chlorination or
thionocarbonylation, and radical elimination.
Supplementary data
Outline of the synthesis of compounds 8 and 9a-c, selected
spectral data of compounds 8, 9a-c, 28, 29a-c, 36, 39b, 45, and
48, and synthetic procedures for 36 and 48. Supplementary data
associated with this article can be found, in the online version, at
https://doi.org/##.####/j.tetlet.####.##.###
References and notes
1.
2.
Williams, D. E.; Sturgeon, C. M.; Roberge, M.; Andersen, R. J. J.
Am. Chem. Soc. 2007, 129, 5822.
For total synthesis, see: Chen, J.; Koswatta, P.; DeBergh, J. R.; Fu,
P.; Pan, E.; MacMillan, J. B.; Ready, J. M. Chem. Sci. 2015, 6,
2932.
3.
For other synthetic studies on nigricanosides, see: (a) Kurashina,
Y.; Kuwahara, S. Biosci. Biotechnol. Biochem. 2012, 76, 605. See,
also: (b) Espindola, A. P. D. M.; Crouch, R.; DeBergh, J. R.;
Ready, J. M.; MacMillan, J. B. J. Am. Chem. Soc. 2009, 131,
15994. (c) Tortosa, M. Angew. Chem. Int. Ed. 2011, 50, 3950.
Kinashi, N., Fujiwara, K.; Tsunoda, T.; Katoono, R.; Kawai, H.;
Suzuki, T. Tetrahedron Lett. 2013, 54, 4564.
For a review, see: Chatterjee, A. K. In Handbook of Metathesis;
Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, 2003; Vol. 2, pp
246–295.
4.
5.
6.
For an example of the coincidence of cross, ring-opening, and
ring-closing olefin metathesis reactions, see: Oguri, H.; Sasaki, S.;
Oishi, T.; Hirama, M. Tetrahedron Lett. 1999, 40, 5405.