Synthesis of orthogonally protected d-olivoside, 1,3-di-O-acetyl-4-O-benzyl-2,6-dideoxy-d-arabinopyranose, as a C-glycosyl donor

Article abstract of DOI:10.1016/j.tet.2009.03.092

1,3-Di-O-acetyl-4-O-benzyl-2,6-dideoxy-d-arabinopyranose (11) was synthesised from thiophenyl α-d-mannopyranoside (21) in an eight-step sequence. Tosylation of 21 and subsequent reaction with 2,2-dimethoxypropane gave tosylate 22, which upon treatment with lithium aluminium hydride furnished 6-deoxy glycoside 24 and by-product thiophenyl 6-deoxy-2-O-isopropyl-α-d-arabinopyranoside. The X-ray crystal structure of the latter was determined. Benzylation of the 4-hydroxyl group of 24 and subsequent protecting group manipulation gave d-rhamnosyl bromide 29, which on treatment with zinc-copper couple gave the orthogonally protected d-rhamnal 30. Triphenylphosphine hydrogen bromide catalysed addition of acetic acid to 30 furnished the target molecule 11. The scandium(III) triflate promoted reaction of 11 and 2-naphthol gave the corresponding C-glycoside 36 in 86% yield. Crown Copyright

Full text of DOI:10.1016/j.tet.2009.03.092

Tetrahedron 65 (2009) 4092–4098
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of orthogonally protected
D
-olivoside, 1,3-di-O-acetyl-4-O-benzyl-2,6-
dideoxy-D-arabinopyranose, as a C-glycosyl donor
,
b
b
Hasnah Osman a , David S. Larsen , Jim Simpson
*
a Universiti Sains Malaysia, Gelugor 11800, Penang, Malaysia
b Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Di-O-acetyl-4-O-benzyl-2,6-dideoxy-D-arabinopyranose (11) was synthesised from thiophenyl a-D-
Received 6 December 2008
Received in revised form 7 March 2009
Accepted 26 March 2009
Available online 1 April 2009
mannopyranoside (21) in an eight-step sequence. Tosylation of 21 and subsequent reaction with 2,2-
dimethoxypropane gave tosylate 22, which upon treatment with lithium aluminium hydride furnished
6-deoxy glycoside 24 and by-product thiophenyl 6-deoxy-2-O-isopropyl-
X-ray crystal structure of the latter was determined. Benzylation of the 4-hydroxyl group of 24 and
subsequent protecting group manipulation gave -rhamnosyl bromide 29, which on treatment with
zinc–copper couple gave the orthogonally protected -rhamnal 30. Triphenylphosphine hydrogen bro-
a-D-arabinopyranoside. The
D
D
mide catalysed addition of acetic acid to 30 furnished the target molecule 11. The scandium(III) triflate
promoted reaction of 11 and 2-naphthol gave the corresponding C-glycoside 36 in 86% yield.
Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
Me
O
1. Introduction
Me
O
O
O
OH
HO
The angucycline family of antibiotics is secondary metabolites
produced by microorganisms1,2 of which the first member was
discovered in 1965.3,4 These antibiotics have attracted much
attention due to the wide range of biological activities that they
exhibit, which include antibacterial,5–7 antiviral,8 antitumour9–11
and enzyme inhibition properties.12 2,6-Dideoxy- and 2,3,6-tri-
HO
Me
OH
O
RO
1
HO
OH
OH
O
O
Me
Me
OH
Me
3 R =
O
HO
O
HO
deoxy-sugars such as
D-olivose (1) and L-rhodinose (2) are
2
present in many of these antibiotics and may be attached at
different positions of the angucycline framework either by C- or
O-glycosidic linkages.1 Urdamycin B (3), a secondary metabolite
of Streptomyces fradiae, shows glycosylation typical of the C-
glycosyl angucycline subgroup.13 The oligosaccharide moiety of 3
4 R = H
ring system. Further modifications to the tetracyclic core has
resulted in the syntheses of (ꢀ)-rubiginone B114 and B2,15
emycin A,14,16 (þ)-ochromycinone16 and (ꢁ)-tetrangomycin.17–19
Furthermore, the reaction of C-glycosyl-napthoquinone 5 and
diene (ꢀ)-6 provided, after further modification, C-glycosidic
angucycline 7.20 Other groups have utilised C-glycosidic naphtho-
quinones as dienophiles for the synthesis of angucycline an-
tibiotics. Sulikowski et al.21,22 used C-glycosyl dienophile 8 in
their elegant synthesis of urdamycinone B (4), while Matsuo
et al.23,24 used the unprotected C-glycoside 9 as a dienophile in
their approach to 4.
consists of a trisaccharide containing two
D-olivose (1) and one
L
-rhodinose (2) residues. A -olivose residue is attached to C-9
b-D
of the benzo[a]anthraquinone ring system by a C-glycosidic
linkage. The oligosaccharide chain then extends from O-3 of this
sugar.1
In an ongoing synthetic programme we have developed
a Diels–Alder strategy to access certain members of the angu-
cycline antibiotics. The key part of the strategy involves the
reaction of substituted naphthoquinones with dienes resulting
in the formation of a functionalised benzo[a]anthraquinone
Given the success of these C-glycosides as dienophiles for the
synthesis of 4, we reasoned that orthogonally protected C-glycosyl-
naphthoquinone 10 would expedite the synthesis of more highly
glycosylated angucyclines such as urdamycin B (3). Given this, we
focussed on the preparation of the corresponding C-glycosyl donor,
* Corresponding author. Tel.: þ604 653 3262; fax: þ604 657 4854.
E-mail address: ohasnah@usm.my (H. Osman).
0040-4020/$ – see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.03.092

H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
4093
1,3-di-O-acetyl-4-O-benzyl-2,6-dideoxy-
D
-arabinopyranose 11. Not
reflux and monitored by TLC. After 72 h the intermediary methyl
glycoside had been consumed. Unfortunately, analysis of the crude
reaction product showed that it was a complex mixture of products.
Given the problem associated with the hydrolysis of 20, an
only would this compound allow access to C-glycosyl-naph-
thoquinones, it would also serve as a precursor to 2,6-dideoxy-
glycosyl trichloroacetimidates.25 Tanaka et al. have shown that this
latter class of glycosyl donor can be used for the direct synthesis of
alternative route to 4-O-benzyl-D-rhamnal derivatives was de-
b
-2,6-dideoxyoligosaccharides.
veloped (Scheme 2).
O
AcO
Me
O
O
O
O
Me
Me
O
O
HO
RO
HO
RO
OH
OH
OMe
( )-6
5 R = Ac
9 R = H
7
O
O
O
Me
Br
Me
Me
O
BnO
AcO
O
O
AcO
BnO
OAc
AcO
AcO
OAc O
OH
10
8
11
2. Result and discussion
Our initial approach to 11 was based upon modification of
Tosylation and subsequent treatment of thiophenyl glycoside
2129 with dimethoxypropane under acidic conditions gave aceto-
nide 22 in 56% yield. A by-product, thiophenyl 3,6-di-O-tosyl-a-D-
6-deoxy-
D
-glucal 12, which can be prepared from 6-O-tosyl-
D
-
mannopyranoside (23), was also isolated in 7% yield from the
reaction. Reduction of 22 was achieved with lithium aluminium
hydride in dry ether to give 24 in 63% yield. The spectroscopic data
were in good agreement with those reported for ent-24, which has
glucal 13 by treatment with lithium aluminium hydride.26,27
OTs
O
O
Me
O
O
been prepared previously from L
-rhamnose by Crich and Picione30
HO
HO
HO
HO
and also by Zegelaar-Jaarsveld et al.31 An unexpected by-product,
isopropyl ether 25 was also isolated from the reduction. The
analytical and spectroscopic data were consistent with the struc-
ture of 25. A strong HMBC correlation between the isopropyl
methine-proton and C-2 indicated that the isopropyl group was
attached to O-2 of the sugar. Furthermore, the molecular structure
of 25 was confirmed from a single X-ray crystal diffraction study
and is shown in Figure 1.32
Benzylation of 24 using benzyl bromide and sodium hydride in
DMF afforded 26 in 93% yield. The spectroscopic data of 26 were
consistent with those of its enantiomer.30,31 Hydrolysis of 26 was
effected using trifluoroacetic acid in methanol to give 27, which on
subsequent acetylation furnished 28 in 83% yield for the two steps.
Rhamnoside 28 has been previously reported by Yu and Wang,33
however, no details of the synthesis or data were given. Reaction of
28 with iodine monobromide gave rhamnosyl bromide 29, which
proved extremely labile. To circumvent this, the crude reaction
product was immediately treated with zinc–copper couple using the
OH
14
12
13
We found this reaction to be very variable, especially on larger
scale where a significant quantity of the anhydro-sugar 14 was
formed. It was envisaged that 11 could be synthesised from methyl
4-O-benzyl-
starting from methyl
Methyl -mannopyranoside (16) was converted, via tosylate
17 and acetonide 18, into -rhamnoside 19 in 41% overall yield using
D-rhamnose 15. A strategy was devised to access 15
a-D
-mannopyrannoside 16 (Scheme 1).
a-D
D
the chemistry of Nishio et al.28 Benzylation of 19 under standard
conditions gave 20 in good yield (65%). The next step in the syn-
thetic sequence was the hydrolysis of the isopropylidene group and
methyl glycoside of 20 using aqueous acetic acid and ion exchange
resin to give 4-O-benzyl-D-rhamnopyranose (15). The reaction was
monitored by TLC and after a few hours a new compound was
formed. A preliminary examination of the reaction mixture by 1H
NMR spectroscopy showed that the isopropylidene group had been
hydrolysed but not the methyl glycoside. The mixture was heated at
OMe
OMe
OMe
OH
OH
OH
OH
O
O
O
O
O
py, p-TsCl
DMP, p-TSA
HO
TsO
TsO
CH2Cl2
acetone
OH
16
OH
17
OH
18
LiAlH4, Et2O
OH
OMe
OMe
OH
OH
O
O
O
O
Amberlite IR-120 (H+)
O
O
BnBr, NaH
DMF
O
Me
Me
Me
OBn
15
OBn
20
OH
19
Scheme 1.

4094
H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
SPh
SPh
OH
SPh
OH
SPh
OiPr
OH
OH
OH
O
O
O
O
O
O
O
1) p-TsCl, Py
LiAlH4
Et2O
+
Me
HO
TsO
2) DMP, p-TSA
Me
O
OH
OH
56%
25 (14%)
22
24 R = H (63%)
21
BnBr, NaH, DMF
(93%)
Br
O
SPh
SPh
OBn
1) TFA, MeOH
2) Ac2O, py
OAc
OAc
O
O
OR
OR
O
IBr
CH2Cl2
O
Me
Me
Me
OBn
29
OBn
27 R = H
26
28 R = Ac (83%, 2 steps)
Zn/Cu, THF
50% (2 steps)
OR
O
AcOH, TPPHBr
O
CH2Cl2
Me
OAc
Me
OAc
OBn
30
OBn
11 R = Ac, 42%
32 R = H, 30%
SPh
OH
OH
OAc
OAc
O
O
TsO
OTs
Me
OH
23
OBn
31
Scheme 2.
method of Bredenkamp et al.34 to give the target rhamnal 30 in 50%
yield for the two steps along with hydrolysis product 31 in 36% yield.
The stabilityof bromide 29 is in contrast tothatof 34 (Scheme 3). The
reaction of thiophenyl glycoside 33 with iodine monobromide gave
34, which upon subsequent treatment with zinc–copper couple
furnished tri-O-acetyl-D-rhamnal 35 in 72% yield for the two steps.
The last step in the sequence to 11 (Scheme 2) was the triphe-
nylphosphine hydrogen bromide catalysed addition of acetic acid to
glycal 30. The expected 2-deoxy glycosyl acetate 11 was obtained in
42% yield as a 4:1 mixture of - and -anomers, along with a 30%
yield of a 4.5:1 mixture of - and -anomers the corresponding
pyranose 32 and starting material (17%) was also recovered from
the reaction. Olivose 32 presumably arises from addition of ad-
ventitious water to 30 or by hydrolysis of 11. Acetylation of 32
under standard conditions gave the target compound 11 in quan-
titative yield. Olivoside 11 was then tested as a glycosyl donor
a
b
b
a
Figure 1. The asymmetric unit of 25 showing the atom numbering scheme for the two unique molecules with ellipsoids drawn at the 50% probability level. For clarity only the
major disorder components of the phenyl rings are shown.

H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
4095
SPh
Br
OAc
OAc
OAc
OAc
1) TFA, MeOH
2) Ac2O, py
O
IBr
O
O
Zn/Cu, THF
24
CH2Cl2
72%
Me
OAc
Me
Me
(2 steps)
95%
OAc
OAc
OAc
33
34
35
Scheme 3.
dichloromethane (75 mL) and pyridine (75 mL) at 0 ꢂC. The mixture
was slowly warmed to room temperature and stirred overnight.
The solvent was removed in vacuo and the residue was dissolved in
dichloromethane. The solution was washed with 1.0 M HCl
(150 mL), saturated aqueous sodium hydrogen carbonate solution
(2ꢃ100 mL), water (2ꢃ100 mL) and brine (50 mL). The organic
layer was dried over anhydrous magnesium sulfate and concen-
trated in vacuo. The residue was dissolved in dichloromethane
(50 mL), p-toluenesulfonic acid (1.00 g, 5.26 mmol) and 2,2-dime-
thoxypropane (50 mL) were added. The pH of the mixture was
tested with indicator paper to ensure that the solution was acidic.
The mixture was stirred for 17 h at room temperature then diluted
with dichloromethane, washed with saturated aqueous sodium
hydrogen carbonate solution (2ꢃ100 mL), water (2ꢃ100 mL) and
brine (50 mL). The organic layer was separated and dried over an-
hydrous magnesium sulfate and the solvent removed. Purification
of the residue by column chromatography [hexane/diethyl ether
1:1 to 1:3 as eluant] gave two fractions. The higher Rf fraction gave
the title compound 22 (9.50 g, 56%) as a white foam. nmax (KBr):
Me
Me
O
HO
O
BnO
AcO
BnO
AcO
Sc(OTf)3
OAc
ClCH2CH2Cl
(86%)
OH
36
11
Scheme 4.
(Scheme 4). Gratifyingly, reaction of 11 and 2-naphthol in 1,2-di-
chloroethane catalysed by scandium(III) triflate35,36 proceeded
smoothly and gave b-C-glycoside 36 in 86% yield.
3. Conclusions
In conclusion, we have developed a reliable eight-step synthesis of
the orthogonally protected C-glycosyl donor, 1,3-di-O-acetyl-4-O-
benzyl-2,6-dideoxy-
mannopyranoside. Thisstrategyalsoprovides analternative synthesis
of 3,4-di-O-acetyl- -rhamnal(35) a key intermediate for the synthesis
of -olivosides. Olivoside 11 proved to be an effective C-glycosyl donor
D-arabinopyranose (11) from thiophenyl a-D-
3693, 3599, 1594, 1386, 1370, 1063 cmꢁ1; [
a
]
þ114.7 (c 0.2,
D
D
D
CH2Cl2); dH (300 MHz, CDCl3): 1.36 (3H, s, CH3), 1.58 (3H, s, CH3),
2.43 (3H, s, Ts-CH3), 3.73 (1H, m, H-4), 4.11–4.21 (3H, m, H-2, H-3,
H-5), 4.32 (2H, ddd, J¼9.0, 6.9, 4.5 Hz, H-6), 5.70 (1H, s, H-1), 7.27–
7.43 (7H, m, Ph-H), 7.71 (2H, d, J¼9.0 Hz, Ph-H) ppm; dC (125 MHz,
CDCl3): 21.75, 26.39, 28.16, 68.54, 69.06, 69.25, 76.20, 78.08, 84.14,
110.10, 127.97, 128.07, 129.21, 129.88, 132.18, 132.72, 144.99 ppm.
Found: C, 56.28; H, 5.61; S, 13.48%. C22H26O7S2 requires: C, 56.63; H,
5.62; S, 13.75%. The lower Rf fraction gave thiophenyl 3,6-di-O-p-
in its scandium(III) triflate promoted glycosylation of 2-naphthol.
4. Experimental
4.1. General
Melting points were recorded on a Gallenkamp capillary melt-
ing point apparatus or a Mettler Toledo FP62 automatic melting
point apparatus and are uncorrected. 1H and 13C NMR assignments
were made on the basis of chemical shift and the coupling
information obtained from one or two-dimensional experiments
(e.g., COSY, HSQC, HMBC, DEPT and NOESY). Infrared (IR) spectra
were recorded on a Perkin Elmer 1600 series FTIR spectropho-
tometer. Low resolution mass spectra were run on a Shimadzu
QP8000 alpha mass detector with APCI or ESI probes using a man-
ual Rheodyne injector and a Shimadzu LC10AD HPLC pump to
provide direct sample injection. High resolution mass spectra were
recorded by Bruce Clark using a Kratos MSORF mass spectrometer.
Elemental analyses were carried out using a Carlo Erba 1108 CHNS
combustion analyser. Polarimetry was performed using a Jasco
DIP1000 Digital polarimeter with a cell of 10 cm in length and
3.5 mm internal diameter and the observed rotation was measured
at 589 nm (sodium D line). Thin-layer chromatography (TLC) was
toluenesulfonyl-
a
-
D
-mannopyranoside (23) (1.56 g, 7%) as a white
þ109.1 (c 0.2,
foam. nmax (KBr): 3594, 1594, 1371, 1077 cmꢁ1; [
a]
D
CH2Cl2); dH (300 MHz, CDCl3): 2.43 (3H, s, Ts-CH3), 2.48 (3H, s, Ts-
CH3), 4.01–4.04 (1H, m, H-5), 4.23–4.31 (2H, m, H-6), 4.21–4.28 (3H,
m, H-4, H-6), 4.33 (1H, d, J¼4.8 Hz, H-2), 4.62 (1H, dd, J¼9.6, 3.0 Hz,
H-3), 5.39 (1H, s, H-1), 7.27–7.41 (9H, m, Ph-H), 7.73 (2H, d,
J¼8.1 Hz, Ph-H), 7.86 (2H, d, J¼8.1 Hz, Ph-H) ppm; dC (125 MHz,
CDCl3): 21.72, 21.83, 64.52, 68.41, 70.93, 71.34, 81.93, 87.52, 127.95,
128.13, 128.17, 129.25, 129.87, 130.19, 131.74, 132.62, 132.71, 132.77,
145.05, 145.70 ppm. Found: C, 53.93; H, 5.14; S, 16.26%. C26H28O9S3
requires: C, 53.78; H, 4.86; S, 16.57%.
4.1.2. Thiophenyl 2,3-O-isopropylidene-6-deoxy-a-D-
mannopyranoside (24)
LiAlH4 (0.590 g, 15.6 mmol) was added to a solution of tosylate
22 (2.00 g, 4.30 mmol) in diethyl ether (100 mL) cooled in an ice-
salt bath. The ice bath was removed and the mixture was stirred
under nitrogen for 12 h. The reaction mixture was cooled in an ice
bath and the reaction quenched by the addition of 1 M sodium
hydroxide (5 mL). The mixture was diluted with diethyl ether,
washed with brine (50 mL) and water (2ꢃ100 mL). The organic
layer was separated and dried over anhydrous magnesium sulfate.
The solvent was removed in vacuo. Purification of the residue by
silica gel column chromatography [hexane/diethyl ether 1:1 to 2:1
as eluant] afforded two fractions. The higher Rf fraction gave the
title compound 24 (0.800 g, 63%) as a white crystalline solid. Mp
76 ꢂC (diethyl ether/hexane); nmax (KBr): 3597, 2938, 2923, 1603,
performed on Merck silica gel (DC Alurolle Kieselgel 60 F254
,
0.2 mm layer) plates in the solvent system indicated. Organic re-
agents for moisture sensitive reactions were distilled from the
following drying agents: tetrahydrofuran and ether (sodium-benzo-
phenone ketyl), dichloromethane and triethylamine (calcium hy-
dride). All moisture sensitive reactions were performed under
a nitrogen atmosphere.
4.1.1. Thiophenyl 2,3-O-isopropylidene-6-O-p-toluenesulfonyl-a-D-
mannopyranoside (22)
p-Toluenesulfonyl chloride (8.66 g, 45.4 mmol) was added to
mixture of thioglycoside (21) (9.90 g, 36.0 mmol) in
a
1382, 1214, 1062 cmꢁ1; [
a]
þ199.3 (c 0.6, CH2Cl2); dH (300 MHz,
D

4096
H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
CDCl3): 1.26 (3H, d, J¼6.0 Hz, H-6), 1.39 (3H, s, CH3), 1.56 (3H, s,
CH3), 2.2 (1H, OH), 3.50 (1H, m, H-5), 4.10 (1H, d, J¼6.4 Hz, H-2),
4.17 (1H, t, J¼7.1 Hz, H-4), 4.37 (1H, d, J¼0.9 Hz, H-3), 5.76 (1H, s, H-
1), 7.32–7.50 (5H, m, Ph-H) ppm; dC (125 MHz, CDCl3): 17.13, 26.47,
28.22, 67.02, 75.32, 76.66, 77.50, 78.41, 88.31, 109.84, 127.67, 129.11,
131.93, 133.47 ppm. Found: C, 60.57; H, 6.80; S, 10.65%. C15H20O4S
requires: C, 60.79; H, 6.80; S, 10.82%.
and acetic anhydride (10 mL) was added. The mixture was allowed
to warm to room temperature and stirred overnight. The solvent
was removed in vacuo and the residue was dissolved in dichloro-
methane. The mixture was washed with 1 M HCl (50 mL) and
extracted with dichloromethane (3ꢃ50 mL). The extract was
washed with saturated aqueous sodium hydrogen carbonate solu-
tion (40 mL) and water (100 mL). The phases were separated and
the organic phase dried over anhydrous magnesium sulfate. Puri-
fication by silica gel column chromatography [hexane/diethyl ether
(1:1) as an eluant] afforded the title compound 28 (2.54 g, 83%) as
A lower Rf fraction gave thiophenyl 2-O-isopropyl-6-deoxy-a-D-
mannopyranoside (25) (0.200 g, 14%) as white crystals. Mp 79.2 ꢂC
(diethyl ether/hexane); nmax (KBr): 3584, 3545, 2988, 2933, 1603,
1476 cmꢁ1; [
a
]
þ153.0 (c 1.2, CH2Cl2); dH (500 MHz, CDCl3): 1.18
a clear syrup. nmax (KBr): 1748, 1374, 1236, 1099, 1079 cmꢁ1; [
a]
D
D
(3H, d, J¼6.0 Hz, CH3), 1.20 (3H, d, J¼6.0 Hz, CH3), 1.32 (3H, d,
J¼6.0 Hz, H-6), 2.38 (1H, br d, J¼10.0 Hz, OH), 2.55 (1H, br s, OH),
3.45 (1H, t, J¼9.5 Hz, H-4), 3.68–3.77 (2H, m overlapping a sept,
J¼6.0 Hz, H-3 and isopropyl CH), 3.91 (1H, dd, J¼4.0, 1.0 Hz, H-2),
4.06–4.15 (1H, m, H-5), 5.50 (1H, s, H-1), 7.24–7.46 (5H, m, Ph-
H) ppm; dC (125 MHz, CDCl3): 17.51, 21.83, 23.31, 68.88, 71.66, 71.78,
74.33, 77.50, 86.16, 127.44, 129.50, 131.52, 134.45 ppm. Found: C,
60.20; H, 7.27; S, 10.56%. C15H22O4S requires: C, 60.38; H, 7.43; S,
10.75%.
þ58.9 (c 0.4, CH2Cl2); dH (300 MHz, CDCl3): 1.36 (3H, d, J¼6.0 Hz, H-
6), 1.99, 2.13 (2ꢃ3H, 2ꢃOAc), 3.52 (1H, t, J¼9.6 Hz, H-4), 4.25 (1H,
dq, J¼11.2, 5.6 Hz, H-5), 4.66 (1H, d, J¼11.2 Hz, CH2Ph), 4.72 (1H, d,
J¼11.2 Hz, CH2Ph), 5.31 (1H, dd, J¼9.5, 3.2 Hz, H-3), 5.37 (1H, d,
J¼1.7 Hz, H-1), 5.50 (1H, dd, J¼3.4, 1.9 Hz, H-2), 7.26–7.47 (10H, m,
Ph-H) ppm; dC (125 MHz, CDCl3): 17.89, 20.91, 20.98, 69.14, 71.88,
71.91, 75.15, 78.98, 85.72, 127.69, 127.76, 127.92, 128.52, 129.14,
131.96, 133.66, 137.99, 169.83, 169.99 ppm; HRMS-ESI (þ): Found m/z
469.1087 (MKþ,100%). C23H26O6SK requires m/z 469.1087.
4.1.3. Thiophenyl 6-deoxy-2,3-O-isopropylidene-4-O-benzyl-
4.1.5. 1,5-Anhydro-3-O-acetyl-4-O-benzyl-2,6-dideoxy-D-arabino-
a-D
-mannopyranoside (26)
hex-1-enitol (30)
Benzyl bromide (2.0 mL, 16.1 mmol) was added to a mixture of
A 1 M solution of IBr in dichloromethane (1.72 mL, 1.74 mmol)
was added to an ice cold solution of the thioglycoside (28) (0.500 g,
1.16 mmol) in dry dichloromethane (10 mL). The mixture was stir-
red for 8 min and quenched by the addition of aqueous sodium
thiosulfate (2 mL). The mixture was diluted with dichloromethane,
washed with saturated aqueous sodium thiosulfate solution
(50 mL) and water (100 mL). The organic layer was separated, dried
over anhydrous magnesium sulfate and the solvent was removed to
thioglycoside 24 (2.40 g, 8.10 mmol) and sodium hydride (0.810 g,
20.0 mmol) in DMF (20 mL) at 0 ꢂC. The mixture was allowed to
warm to room temperature and stirred overnight. The mixture was
quenched by the addition of water (2 mL) and then diluted with
diethyl ether (150 mL). The mixture was washed with water
(2ꢃ100 mL), brine (50 mL), dried over anhydrous magnesium sul-
fate and concentrated in vacuo. Purification of the residue by silica
gel column chromatography [hexane/diethyl ether (4:1) as eluant]
gave the title compound 26 (2.90 g, 93%) as a white crystalline solid.
Mp 83 ꢂC (hexane); nmax (KBr): 2936, 2920, 1612, 1379, 1217,
give thiophenyl 6-deoxy-2,3-di-O-acetyl-4-O-benzyl-
pyranosyl bromide (29), which was used without further purifica-
tion. nmax (KBr): 1757, 1375, 1259, 1236 cmꢁ1
dH (300 MHz, CDCl3):
a-D-manno-
;
1115 cmꢁ1; [
a
]
þ220.9 (c 0.3, CH2Cl2); dH (300 MHz, CDCl3): 1.23
inter alia 1.38 (3H, d, J¼6.6 Hz, H-6), 1.95, 2.11 (2ꢃ3H, s, 2ꢃOAc),
3.62 (1H, t, J¼9.9 Hz, H-4), 4.06–4.14 (1H, m, H-5), 4.67 (1H, d,
J¼11.2 Hz, CH2Ph), 4.74 (1H, d, J¼11.2 Hz, CH2Ph), 5.49 (1H, dd,
J¼3.3, 1.5 Hz, H-2), 5.70 (1H, dd, J¼9.9, 3.3 Hz, H-3), 6.26 (1H, d,
J¼1.5 Hz, H-1), 7.28–7.36 (5H, m, Ph-H) ppm. Zinc–copper couple
(2.00 g) was added to a cooled (0 ꢂC) solution of bromide (29),
D
(3H, d, J¼6.1 Hz, H-6), 1.39 (3H, s, CH3), 1.52 (3H, s, CH3), 3.31 (1H,
dd, J¼9.8, 6.6 Hz, H-4), 4.14 (1H, m, H-5), 4.30–4.38 (2H, m, H-2, H-
3), 4.64 (1H, d, J¼11.7 Hz, CH2Ph), 4.92 (1H, d, J¼11.7 Hz, CH2Ph),
5.74 (1H, s, H-1), 7.27–7.46 (10H, m, Ph-H) ppm; dC (125 MHz,
CDCl3): 17.79, 26.56, 28.10, 66.30, 73.20, 76.79, 78.49, 81.53, 83.90,
109.53, 127.57, 127.56, 127.85, 128.07, 128.37, 129.06, 131.86, 133.63,
138.29 ppm. Found: C, 68.24; H, 6.91; S, 8.20%. C22H26O4S requires:
C, 68.37; H, 6.78; S, 8.30%.
sodium acetate (116 mg, 1.41 mmol) and acetic acid (127.0 mL,
2.22 mmol) in tetrahydrofuran (30 mL). The mixture was stirred
overnight. Sodium carbonate (103 mg, 0.97 mmol) was then added
and the mixture was stirred 30 min at room temperature. The
solvent was removed in vacuo. Dichloromethane (50 mL) was
added and the organic phase was washed with water (3ꢃ20 mL)
and saturated aqueous sodium hydrogen carbonate solution
(30 mL). The organic phase was dried over anhydrous magnesium
sulfate and solvent was removed in vacuo. Purification of the res-
idue by silica gel column chromatography [hexane/diethyl ether
(1:1)] afforded two fractions. The higher Rf fraction gave the title
compound (30) (152 mg, 50%) as a colourless syrup. HRMS-ESI (þ):
Found m/z 285.1103 (MNaþ, 100%). C15H18O4Na requires m/z
4.1.4. Thiophenyl 6-deoxy-4-O-benzyl-a-D-mannopyranoside (27)
and thiophenyl 6-deoxy-1,2-di-O-acetyl-4-O-benzyl-a-D-
mannopyranoside (28)
A mixture of the acetal 26 (2.75 g, 7.12 mmol) and trifluoroacetic
acid (5.00 mL, 35.6 mmol) in methanol (50 mL) was stirred at room
temperature for 40 h. The solvent was removed and the residue was
diluted with dichloromethane (100 mL). The mixture was washed
with water (2ꢃ50 mL) and brine (50 mL). The organic layer was
separated, dried over anhydrous magnesium sulfate and concen-
trated in vacuo. A small portion of the solid residue was crystallised
from hexane/diethyl ether and gave the title compound 27 for
characterisation. Mp 106.2 ꢂC (hexane/diethyl ether); nmax (KBr):
285.1103; nmax (KBr): 1730, 1656, 1374, 1234, 1116 cmꢁ1; [
a
]D ꢁ59.3
(c 0.95, CHCl3); dH (300 MHz, CDCl3): 1.38 (3H, d, J¼6.6 Hz, H-6),
2.03 (3H, s, OAc), 3.50–3.55 (1H, dd, J¼8.4, 6.0 Hz, H-4), 3.98–4.06
(1H, m, H-5), 4.71 (1H, d, J¼11.7 Hz, CH2Ph), 4.73 (1H, d, J¼11.7 Hz,
CH2Ph), 4.76 (1H, dd, J¼6.0, 2.7 Hz, H-2), 5.41 (1H, dddd, J¼5.7, 3.0,
1.5, 0.6 Hz, H-3), 6.38 (1H, dd, J¼6.0, 1.5 Hz, H-1), 7.28–7.36 (5H, m,
Ph-H) ppm; dC (125 MHz, CDCl3): 17.29, 21.31, 71.05, 73.67, 74.01,
78.24, 99.22, 127.94 (2C), 128.51, 137.98, 145.87, 170.71 ppm; The
3685, 3576, 3089, 2923, 1597, 1382, 1269, 1081 cmꢁ1; [
a
]D þ232.9 (c
0.4, CH2Cl2); dH (300 MHz, CDCl3): 1.36 (3H, d, J¼6.3 Hz, H-6), 2.36–
2.42 (1H, br s, OH), 2.28–2.60 (1H, br s, OH), 3.43 (1H, t, J¼9.3 Hz, H-
4), 3.93 (1H, dd, J¼9.3, 3.2 Hz, H-3), 4.18–4.26 (2H, m, H-2, H-5),
4.72 (1H, d, J¼11.5 Hz, CH2Ph), 4.76 (1H, d, J¼11.5 Hz, CH2Ph), 5.48
(1H, d, J¼1.5 Hz, H-1), 7.25–7.49 (10H, m, Ph-H) ppm; dC (125 MHz,
CDCl3): 17.98, 68.69, 71.91, 72.63, 75.15, 81.89, 87.45, 127.47, 128.05,
128.18,128.77, 129.12, 131.51, 134.17, 138.17 ppm. Found: C, 65.58; H,
6.60; S, 9.35%. C19H22O4S requires: C, 65.87; H, 6.40; S, 9.26%. The
remainder of the residue was dissolved in pyridine (10 mL) at 0 ꢂC
lower Rf fraction gave 6-deoxy-2,3-di-O-acetyl-4-O-benzyl-a-D-
mannopyranose (31) (118 mg, 30%) (a/b; 1.4:1) as a colourless syrup.
HRMS-ESI (þ): Found m/z 361.1258 (MNaþ, 100%). C17H22O7Na re-
quires m/z 361.1263; nmax (KBr): 3589, 2935, 1749, 1369, 1247,
1068 cmꢁ1; Data for
a-anomer: dH (300 MHz, CDCl3): inter alia 1.33

H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
4097
(3H, d, J¼6.3 Hz, H-6), 1.98, 2.14 (2ꢃ3H, 2ꢃOAc), 3.08 (1H, OH), 3.48
(1H, t, J¼9.9 Hz, H-4), 4.04–4.14 (1H, m, H-5), 4.63 (1H, d, J¼11.4 Hz,
CH2Ph), 4.70 (1H, d, J¼11.4 Hz, CH2Ph), 5.11 (1H, d, J¼3.2 Hz, H-1),
5.28 (1H, d, J¼1.7 Hz, H-2), 5.37 (1H, dd, J¼3.4, 1.9 Hz, H-3), 7.32–
7.36 (5H, m, Ph-H) ppm; dC (125 MHz, CDCl3): inter alia 17.93, 20.95,
20.98, 67.79, 70.80, 71.34, 75.00, 78.81, 92.20, 127.67, 127.85, 127.96,
0.024 mmol), 2-naphthol (8 mg, 0.055 mmol) and Drierite
(200 mg) in dichloroethane (1.0 mL) and a reaction time of 5 h
gave, after purification by silica gel column chromatography
[hexane/ethyl acetate (2:1) as eluant], the title compound (36)
(15 mg, 86%) as a white solid.
HRMS-ESI (þ): Found m/z 429.1678 (MNaþ, 100%). C25H26O5Na
128.47, 129.49, 138.07, 170.04, 170.24 ppm; Data for b-anomer: dH
requires m/z 429.1678. nmax (neat): 3702, 2961, 1748, 1602, 1468,
(300 MHz, CDCl3): inter alia 1.39 (3H, d, J¼6.3 Hz, H-6) ppm; dC
(125 MHz, CDCl3): inter alia 17.93, 20.95, 70.98, 71.89, 73.69, 75.22,
77.99, 92.21, 127.85, 127.86, 128.49, 137.86, 170.04, 170.24 ppm.
1379, 1233, 1099 cmꢁ1; [
a]
þ53 (c 0.4, CH2Cl2); dH (500 MHz,
D
CDCl3): inter alia 1.29 (3H, d, J¼6.5 Hz, H-60), 1.98 (3H, s, 1ꢃOAc),
1.95–2.04 (1H, m, H-20ax), 2.51 (1H, ddd, J¼13.5, 5.0, 2.0 Hz, H-
20eq), 3.41 (1H, t, J¼9.5 Hz, H-40), 3.75 (1H, m, H-50), 4.74 (1H, d,
J¼11.5 Hz, CH2Ph), 4.77 (1H, d, J¼11.5 Hz, CH2Ph), 5.26 (1H, m, H-
30), 5.52 (1H, dd, J¼12.0, 2.0 Hz, H-10), 7.08–7.79 (11H, m, Ph-H, Ar-
4.1.6. 1,3-Di-O-acetyl-4-O-benzyl-2,6-dideoxy-D-arabino-
pyranose (11)
0
Glacial acetic acid (45
mL, 0.79 mmol) was added to a stirred
H), 8.85 (1H, s, OH) ppm; dC (125 MHz, CDCl3): 18.69 (C6 ), 21.18
0
0
0
0
solution of 1,5-anhydro-3-O-acetyl-4-O-benzyl-2,6-dideoxy-
D
-ara-
(OAc), 36.47 (C2 ), 74.35 (C3 ), 75.27 (CH2Ph), 75.95 (C1 ), 76.90 (C5 ),
0
bino-hex-1-enitol (30) (118 mg, 0.45 mmol) and triphenylphos-
phine hydrogen bromide (10 mg, 0.03 mmol) in anhydrous
dichloromethane (5 mL). The mixture was stirred overnight at room
temperature. Removal of the solvent under reduced pressure and
purification by silica gel column chromatography [diethyl ether/
hexane (1:2)] as eluant gave two fractions. The higher Rf fraction
gave recovered starting material 30 (20 mg, 17%). The next fraction
82.10 (C4 ), 109.54, 114.47, 117.80, 119.78, 120.78, 123.07, 127.83,
127.92, 128.05, 128.61, 128.76, 128.91, 129.91 (C1), 134.67 (2 ipso Ph),
137.98 (2 ipso Ph), 153.61 (C2), 170.31 (OAc) ppm.
Acknowledgements
The authors thank the Universiti Sains Malaysia for financial
support.
gave the title compound 11 (51 mg, 42%) (a/b; 4.5:1) as a colourless
syrup. HRMS-ESI (þ): Found m/z 345.1314 (MNaþ, 100%).
C17H22O6Na requires m/z 345.1314; nmax (KBr): 1747, 1367, 1237,
References and notes
1103 cmꢁ1; Data for
a-anomer: dH (500 MHz, CDCl3): inter alia 1.30
(3H, d, J¼6.3 Hz, H-6), 1.81 (1H, ddd, J¼13.5, 11.4, 3.9 Hz, H-2ax),
2.01, 2.09 (2ꢃ3H, s, 2ꢃOAc), 2.36 (1H, ddd, J¼13.5, 11.7, 5.1 Hz, H-
2eq), 3.23 (1H, t, J¼9.6 Hz, H-4), 3.89 (1H, m, H-5), 4.67 (1H, d,
J¼11.3 Hz, CH2Ph), 4.73 (1H, d, J¼11.3 Hz, CH2Ph), 5.27 (1H, ddd,
J¼11.4, 9.0, 5.2 Hz, H-3), 6.14 (1H, dd, J¼3.7, 1.8 Hz, H-1), 7.28–7.37
(5H, m, Ph-H) ppm; dC (125 MHz, CDCl3): inter alia 18.28, 21.19,
21.26, 34.36, 69.69, 71.22, 75.12, 82.09, 91.05, 127.85, 127.98, 128.55,
1. Rohr, J.; Thiericke, R. Nat. Prod. Rep. 1992, 9, 103–137.
2. Carreno, M. C.; Urbano, A. Synlett 2005, 1–25.
3. Dann, M.; Lefemine, D. V.; Barbatschi, F.; Shu, P.; Kunstmann, M. P.; Mitscher, L.
A.; Bohonos, N. Antimicrob. Agents Chemother. 1965, 832–835.
4. Kuntsmann, M. P.; Mitscher, L. A. J. Org. Chem. 1966, 31, 2920–2925.
5. Sawa, R.; Matsuda, N.; Uchida, T.; Ikeda, T.; Sawa, T.; Naganawa, H.; Hamada, M.;
Takeuchi, T. J. Antibiot. 1991, 44, 396–402.
6. Hayakawa, Y.; Adachi, K.; Iwakiri, T.; Imamura, K.; Furihata, K.; Seto, H.; Otake,
N. Agric. Biol. Chem. 1987, 51, 1397–1405.
138.01, 169.57, 170.22 ppm; Data for
b
-anomer: dH (500 MHz,
7. Omura, S.; Tanaka, H.; Oiwa, R.; Awaya, J.; Masuma, R.; Tanaka, K. J. Antibiot.
1977, 30, 908–916.
8. Kondo, S.; Gomi, S.; Ikeda, D.; Hamada, M.; Takeuchi, T.; Iwai, H.; Seki, J.;
Hoshino, H. J. Antibiot. 1991, 44, 1228–1236.
9. Rasmussen, R. R.; Nuss, M. E.; Scherr, M. H.; Mueller, S. L.; McAlpine, J. B.;
Mitscher, L. A. J. Antibiot. 1986, 39, 1515–1526.
10. Hayakawa, Y.; Ha, S. C.; Kim, Y. J.; Furihata, K.; Seto, H. J. Antibiot. 1991, 44,
1179–1186.
11. Drautz, H.; Zaehner, H.; Rohr, J.; Zeeck, A. J. Antibiot. 1986, 39, 1657–1669.
12. Omura, S.; Nakagawa, A.; Fukamachi, N.; Miura, S.; Takahashi, Y.; Komiyama, K.;
Kobayashi, B. J. Antibiot. 1988, 41, 812–813.
13. Yamaguchi, M.; Okuma, T.; Horiguchi, A.; Ikeura, C.; Minami, T. J. Org. Chem.
1992, 57, 1647–1649.
14. Larsen, D. S.; O’Shea, M. D. Tetrahedron Lett. 1993, 34, 3769–3772.
15. Larsen, D. S.; O’Shea, M. D. J. Chem. Soc., Perkin Trans. 1 1995, 1019–1028.
16. Larsen, D. S.; O’Shea, M. D.; Brooker, S. Chem. Commun. 1996, 203–204.
17. Landells, J. S.; Larsen, D. S.; Simpson, J. Tetrahedron Lett. 2003, 44, 5193–5196.
18. Krohn, K.; Khanbabaee, K. Liebigs Ann. Chem. 1994, 1109–1112.
19. Krohn, K.; Boeker, N.; Floerke, U.; Freund, C. J. Org. Chem. 1997, 62, 2350–2356.
20. Andrews, F. L.; Larsen, D. S.; Larsen, L. Aust. J. Chem. 2000, 53, 15–24.
21. Boyd, V. A.; Sulikowski, G. A. J. Am. Chem. Soc. 1995, 117, 8472–8473.
22. Kim, K.; Boyd, V. A.; Sobti, A.; Sulikowski, G. A. Isr. J. Chem. 1997, 37, 3–22.
23. Matsuo, G.; Miki, Y.; Nakata, M.; Matsumura, S.; Toshima, K. Chem. Commun.
1996, 225–226.
CDCl3): inter alia 1.32 (3H, d, J¼6.3 Hz, H-6), 1.64–1.73 (1H, m, H-
2ax), 2.00, 2.09 (2ꢃ3H, s, 2ꢃOAc), 2.36 (1H, ddd, J¼12.0, 9.9, 5.4 Hz,
H-2eq), 3.21 (1H, t, J¼9.6 Hz, H-4), 3.56 (1H, m, H-5), 4.65 (1H, d,
J¼11.3 Hz, CH2Ph), 4.67 (1H, d, J¼11.3 Hz, CH2Ph), 5.01 (1H, ddd,
J¼11.6, 9.2, 5.2 Hz, H-3), 5.75 (1H, dd, J¼9.9, 2.1 Hz, H-1), 7.28–7.37
(5H, m, Ph-H) ppm; dC (125 MHz, CDCl3): inter alia 35.65, 67.32,
72.36, 72.98, 97.10,138.01,169.61 ppm; The lower Rf fraction gave 3-
O-acetyl-4-O-benzyl-2,6-dideoxy-
D-mannopyranose (32), which
crystallised from diethyl ether/hexane (38 mg, 30%) (a/b
; 2:1). Mp
139.2 ꢂC (diethyl ether/hexane); HRMS-ESI (þ): Found m/z 303.1207
(MNaþ, 100%). C15H20O5Na requires m/z 303.1208; nmax (KBr): 3680,
2986, 1741, 1604, 1369, 1239, 1099 cmꢁ1; Data for
a-anomer: dH
(300 MHz, CDCl3): inter alia 1.29 (3H, d, J¼6.3 Hz, H-6), 1.70 (1H, m,
H-2ax), 2.00 (3H, s, OAc), 2.28 (1H, ddd, J¼13.1, 5.4, 1.8 Hz, H-2eq),
2.49 (1H, dd, J¼3.3, 2.1 Hz, OH), 3.20 (1H, t, J¼9.3 Hz, H-4), 4.15 (1H,
m, H-5), 4.65 (1H, d, J¼11.1 Hz, CH2Ph), 4.71 (1H, d, J¼11.1 Hz,
CH2Ph), 5.29–5.35 (2H, m, H-1, H-3), 7.28–7.36 (5H, m, Ph-H) ppm;
dC (125 MHz, CDCl3): inter alia 17.83, 17.86, 35.20, 66.84, 70.94,
74.32, 81.45, 91.18, 127.39, 127.49, 128.05, 128.09, 137.85,
24. Matsuo, G.; Miki, Y.; Nakata, M.; Matsumura, S.; Toshima, K. J. Org. Chem. 1999,
64, 7101–7106.
25. Tanaka, H.; Yoshizawa, A.; Takahashi, T. Angew. Chem., Int. Ed. 2007, 46,
2505–2507.
170.22 ppm; Data for b-anomer: dH (300 MHz, CDCl3): inter alia 1.34
(3H, d, J¼6.3 Hz, H-6), 1.65 (1H, q, J¼12.0, 12.0, 9.5 Hz, H-2ax), 2.01
(3H, s, OAc), 2.40 (1H, ddd, J¼12.0, 5.2, 2.1 Hz, H-2eq), 2.97 (1H, d,
J¼6.0 Hz, OH), 3.19 (1H, t, J¼9.3 Hz, H-4), 3.44 (1H, m, H-5), 4.64 (1H,
d, J¼11.1 Hz, CH2Ph), 4.65 (1H, d, J¼11.1 Hz, CH2Ph), 4.85 (1H, ddd,
J¼9.3, 6.0, 2.0 Hz, H-1), 4.89 (1H, ddd, J¼11.7, 9.0, 5.0 Hz, H-3), 7.28–
7.36 (5H, m, Ph-H) ppm; dC (125 MHz, CDCl3): inter alia 20.87, 37.74,
72.86, 74.62, 81.45, 93.04, 127.75, 137.85, 170.32 ppm.
26. Fraser-Reid, B.; Kelly, D. R.; Tulshian, D. B.; Ravi, P. S. J. Carbohydr. Chem. 1983, 2,
105–114.
27. Pathak, V. P. Synth. Commun. 1993, 23, 83–85.
28. Nishio, T.; Miyake, Y.; Kubota, K.; Yamai, M.; Miki, S.; Ito, T.; Oku, T. Carbohydr.
Res. 1996, 280, 357–363.
29. Maity, S. K.; Dutta, S. K.; Banerjee, A. K.; Achari, B.; Singh, M. Tetrahedron 1994,
50, 6965–6974.
30. Crich, D.; Picione, J. Org. Lett. 2003, 5, 781–784.
31. Zegelaar-Jaarsveld, K.; Van der Marel, G. A.; Van Boom, J. H. Tetrahedron 1992,
48, 10133–10148.
4.1.7. 2-Hydroxy-1-[30-O-acetyl-40-O-benzyl-2,6-dideoxy-
b-D-
manno-hexopyranosyl]-naphthalene (36)
32. Yu, B.; Wang, P. Org. Lett. 2002, 4, 1919–1922.
33. Crystallographic data for 25: C15H22O4S, Mw¼298.39, monoclinic, C centred,
a¼18.8001(8), b¼5.0848(2), c¼33.1695(13) Å,
b
¼106.036(2)ꢂ, V¼3047.4(2) Å3,
space group C2 (# 5), Z¼8, Dc¼1.301 g cmꢁ3, Bruker APEXII diffractometer,
3-O-Acetyl-4-O-benzyl-2,6-dideoxy-
D-mannopyranose (11)
(14 mg 0.043 mmol) in dichloroethane (1.0 mL), Sc(OTf)3 (12 mg,
Radiation Mo K
a
(
l
¼0.71073, T¼293(2)K), R¼0.0518, Rw¼0.1387, R1¼0.0416,

4098
H. Osman et al. / Tetrahedron 65 (2009) 4092–4098
s(I) out of 9533 reflections col- Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
GOF¼1.000 for 8613 reflections with I>2
lected, Flack parameter 0.03(6) with 4096 Friedel pairs. CCDC 704190 con-
tains the supplementary crystallographic data for 25. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
þ44 1223 336033.
34. Bredenkamp, M. W.; Holzapfel, C. W.; Toerien, F. Synth. Commun.1992,22, 2459–2477.
35. Matsumoto, T.; Hosoya, T.; Suzuki, K. Tetrahedron Lett. 1990, 31, 4629–4632.
36. Ben, A.; Yamauchi, T.; Matsumoto, T.; Suzuki, K. Synlett 2004, 225–230.