Efficient and stereoselective synthesis of the disaccharide fragment of incednine

Article abstract of DOI:10.1021/ol202639v

Efficient and stereoselective synthesis of a disaccharide fragment, 2-deoxy-4-O-(N′-monodemethyl-d-forosaminyl)-2-methylamino-β-d- xylopyranoside, of a novel antibiotic, incednine (1), is described. The key β-stereoselective formation of a 2,3,4,6-tetradeoxy-4-methylamino glycoside bond was achieved by remote participation-assisted glycosylation.

Full text of DOI:10.1021/ol202639v

ORGANIC
LETTERS
2011
Vol. 13, No. 22
6126–6129
Efficient and Stereoselective Synthesis of
the Disaccharide Fragment of Incednine
Takashi Ohtani, Shohei Sakai, Akira Takada, Daisuke Takahashi, and
Kazunobu Toshima*
Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
toshima@applc.keio.ac.jp
Received September 30, 2011
ABSTRACT
Efficient and stereoselective synthesis of a disaccharide fragment, 2-deoxy-4-O-(N0-monodemethyl-D-forosaminyl)-2-methylamino-β-D-
xylopyranoside, of a novel antibiotic, incednine (1), is described. The key β-stereoselective formation of a 2,3,4,6-tetradeoxy-4-methylamino
glycoside bond was achieved by remote participation-assisted glycosylation.
Incednine (1), a novel macrolactam glycoside antibiotic
that was isolated from Streptomyces sp. by Imoto and co-
workers,1 exhibits significant inhibitory activityagainstthe
antiapoptotic oncoproteins Bcl-2 and Bcl-xL, with a mode
of action different from those of other inhibitors. It is
known that these proteins are overexpressed in many
cancer cells, resulting in the expansion of transformed
populations and advancement of the multi-drug-resistant
stage.2ꢀ4 Therefore, 1 isexpected tobe ofimportanceinthe
development of novel antitumor drugs. Furthermore, 1 is
likely to be a useful tool for further study of the functions
ofBcl-2 and Bcl-xL. Structurally, 1 contains several unique
features: an R-methoxy-R,β-unsaturated amide moiety,
two independent conjugated polyene systems embedded
in the 24-membered macrolactam ring, and a disaccharide
fragment comprising two unusual deoxyamino sugars,
N-monodemethyl-D-forosamine and 2-deoxy-2-methyl-
amino-D-xylose, attached by β-glycoside bonds.
chemical synthesis. We recently reported the total synth-
esis of incednam, the aglycon of 1, employing a Stille
coupling reaction and macrolactamization as key steps,5
with efficient and stereoselective construction of the
disaccharide domain as the remaining task in the synth-
esis of 1.
The highly deoxygenated 2,3,4,6-tetradeoxy-4-methyl-
amino-β-glycoside residue N-monodemethyl-β-D-forosa-
minide is known as one of the most acid-labile glycoside
linkages.6 In addition, stereoselective and direct con-
struction of the 2,3,6-trideoxy-β-glycoside linkage by
glycosylation is a difficult and challenging task for syn-
thetic chemists due to the missing control element at the
C-2 position.7 Herein we report the stereoselective synth-
esis of the disaccharide fragment (2) of incednine (1)
employing a highly β-selective and direct glycosylation
assisted by remote and neighboring group participation
as key steps.
Because of its important biological activity and novel
molecular architecture, 1 has been a prime target for
The retrosynthetic analysis of 2 is depicted in Figure 1.
We envisaged that the desired disaccharide 2 could be
derived by β-selective glycosylation of the disaccharide
(1) Futamura, Y; Sawa, R; Umezawa, Y; Igarashi, M; Nakamura, H;
Hasagawa, K; Yamasaki, M; Tashiro, E; Takahashi, Y; Akamatsu, Y;
Imoto, M. J. Am. Chem. Soc. 2008, 130, 1822.
(2) Tsujimoto, Y.; Finger, L. R.; Yunis, J.; Nowell, P. C.; Croce,
C. M. Science 1984, 226, 1097.
(3) Reed, J. C.; Cuddy, M.; Slabiak, T.; Croce, C. M.; Nowell, P. C.
Nature 1988, 336, 259.
(4) Gross, A.; McDonnell, J. M.; Korsmeyer, S. J. Genes Dev. 1999,
13, 1899.
(5) (a) Ohtani, T.; Kanda, H.; Misawa, K.; Urakawa, Y.; Toshima,
K. Tetrahedron Lett. 2009, 50, 2270. (b) Ohtani, T.; Tsukamoto, S.;
Kanda, H.; Misawa, K.; Urakawa, Y.; Fujimaki, T.; Imoto, M.;
Takahashi, Y.; Takahashi, D.; Toshima, K. Org. Lett. 2010, 12, 5068.
(6) Evans, D. A.; Black, W. C. J. Am. Chem. Soc. 1993, 115, 4497.
(7) Tanaka, H.; Yoshizawa, A.; Takahashi, T. Angew. Chem., Int. Ed.
2007, 46, 2505.
r
10.1021/ol202639v
Published on Web 10/25/2011
2011 American Chemical Society

activator, a 3-O-TBS-4-O-Ac or 3-O-Bn-4-O-Bz donor
was found to show better β-selectivity than 3,4-di-O-Bn
donors, probably due to the participation effect of the C-4
acyl protective groups.10 In addition, other reports have
noted that remote participation of the C-4 acyl group
induced β-selectivity in mannosylations and glucosyla-
tions.11 On the basis of these previous findings, we ex-
pected that the glycosyl donor 5, with high conformational
flexibility, would facilitate the remote participation of the
N-Troc group at the C-4, and glycosylation withacceptor 4
would proceed effectively to give the corresponding dis-
accharide with high β-selectivity.
Scheme 1. Synthesis of Glycosyl Acceptor 4
Figure 1. Retrosynthetic analysis of the disaccharide fragment 2.
Figure 2. Presumed mechanism for remote participation-as-
sisted β-selective glycosylation.
imidate 3 using a mild Lewis acid activator, such as
Yb(OTf)3,8 assisted by participation of the neighboring
2-N-carbamoylgroup,8 withoutanomerization orcleavage
of the preinstalled 2,3,4,6-tetradeoxy-4-methylamino-β-
glycoside bond. For the N-carbamoyl group at the C-2
position, we chose a N-trichloroethoxycarbamoyl (Troc)
group because it is widely used in neighboring group-
assisted glycosylations9 and easy to remove under mild
conditions. For the stereoselective formation of a N-
monodemethyl-β-D-forosaminide bond in the glycosyl do-
nor 3, we focused on remote participation-assisted glyco-
sylation using 5, which possesses a Troc group at the C-4
position (Figure 2). In our previous study on the glycosyla-
tion of D-olivoses and D-olivosyl phosphates, which pos-
sess different protecting groups at the C-3 and C-4
positions, in the presence of montmorillonite K-10 as an
The synthesis of the 2-deoxy-2-methylamino-D-xylopyr-
anoside derivative 4 is summarized in Scheme 1. Prepara-
tion of the known sugar 7 from D-xylose (6) was achieved
based on the Hashimoto’s procedure,12 and 7 led to the
corresponding thioglycoside as a mixture of anomeric
isomers (R/β = 9/1) by treatment with TMSOTf and
TMSSPh. Deprotection of the acetyl groups provided 8,
and then the separated R-anomer was protected as a
diethylisopropylsilyl (DEIPS) ether.13 The azide group of
(10) (a) Jyojima, T.; Miyamoto, N.; Ogawa, Y.; Matsumura, S.;
Toshima, K. Tetrahedron Lett. 1999, 40, 5023. (b) Nagai, H.; Matsumura,
S.; Toshima, K. Chem. Lett. 2002, 31, 1100. (c) Nagai, H.; Sasaki, K.;
Matsumura, S.; Toshima, K. Carbohydr. Res. 2005, 340, 337.
(11) (a) Beak, J. Y.; Lee, B.-Y.; Jo, M. G.; Kim, K. S. J. Am. Chem.
Soc. 2009, 131, 17705. (b) De Mao, C.; Kamat, M. N.; Demchemko,
A. V. Eur. J. Org. Chem. 2005, 706. (c) Crich, D.; Hu, T.; Cai, F. J. Org.
Chem. 2008, 73, 8942. (d) Ma, Y.; Lian, G.; Li, Y.; Yu, B. Chem.
Commun. 2011, 47, 7515.
(8) Adinolfi, M.; Iadonisi, A.; Schiattarella, M. Tetrahedron Lett.
2003, 44, 6479.
(12) Hashimoto, H.; Araki, K.; Saito, Y.; Kawa, M.; Yoshimura, J.
Bull. Chem. Soc. Jpn. 1986, 59, 3131.
(9) Bongat, A. F. G.; Demchenko, A. V. Carbohydr. Res. 2007, 342,
374.
(13) Toshima, K.; Mukaiyama, S.; Kinoshita, M.; Tatsuta, K. Tetra-
hedron Lett. 1989, 30, 6413.
Org. Lett., Vol. 13, No. 22, 2011
6127

9 was converted to the corresponding N-monomethyla-
mine 10 by a convenient one-pot procedure reported by
Suzuki and co-workers.14 At this stage, conformational
change, in which the typical 4C1 conformation flipped to
Table 1. Stereo- and Regioselective Glycosylation of 4 with 5
1
the less common C4 conformation, was confirmed by
NOESY correlation between H-1 and H-5. This phenom-
enon was previously reported by Matsuda and Shuto when
bulky and robust tert-butyldimethylsilyl protecting groups
were installed at the C-3 and C-4 positions of xylopyrano-
side derivatives.15 The undesired amino sugar 11, which
was produced along with 10, could be converted to the N-
monomethylamino sugar 10 via reductive amination in
75% yield. The N-methylamino group was protected as a
trichloroethoxycarbamate, and subsequent deprotection
of the DEIPS groups gave the glycosyl acceptor 4. Mean-
while, glycosyl donor 5 was prepared from methyl R-D-
glucopyranoside (12), as shown in Scheme 2. The known
methylglycoside13was synthesizedin six steps, referring to
Baer’s methodology.16 The N-methylamino group of 13
was protected by a Troc group, and subsequent hydrolysis
and acetylation gave the glycosyl donor 5.
yield (%)
entry
activator
solvent temp (°C) 15β 15r 16β
1
2
3
4
5
6
7
8
TMSOTf (0.3 equiv)
TMSOTf (0.3 equiv)
TMSOTf (0.3 equiv)
TMSOTf (0.3 equiv)
TMSOTf (0.3 equiv)
TMSOTf (0.3 equiv)
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
Et2O
ꢀ60
ꢀ40
ꢀ20
ꢀ40
ꢀ40
ꢀ40
ꢀ40
ꢀ20
31 58
6
20
17
69
66
9
7
29 54 12
10 70
Scheme 2. Synthesis of Glycosyl Donor 5
toluene
67 13
42 35
9
9
BF3 OEt2 (1.5 equiv) CH2Cl2
3
3
BF3 OEt2, (1.5 equiv) CH2Cl2
68 13 17
hindranceatthe C-4position. However, the pyranosechair
flip, which induces remote participation, did not take place
effectively at ꢀ60 °C; therefore, the reaction was also
examined at ꢀ40 and ꢀ20 °C. The β-selectivity of dis-
accharide 15 was dramatically improved, as expected, and
the desired disaccharide 15β was obtained in 69 and 66%
yields, respectively(entries 2 and 3). Next, the solvent effect
on the glycosylation reaction was investigated using
MeCN, Et2O, and toluene at ꢀ40 °C.
With both glycosyl acceptor 4 and donor 5 in hand, we
examined the glycosylation reaction under various condi-
tions to investigate how the N-Troc group at the C-4
position of 5 affected the stereoselectivity of the reaction.
The results are summarized in Table 1. It was found that
glycosylation of 4 (1.5 equiv) with 5 (1.0 equiv) in the
Interestingly, when a polar solvent, MeCN or Et2O, was
used, the stereoselectivity of 15was found to changefrom β
to R, and 15r was obtained as the major product (entries 4
and 5); in contrast, when an apolar solvent, CH2Cl2 or
toluene, was used, the desired 15β was obtained as the
major product (entries 2 and 6). These results suggested
that polar solvents that can coordinate to an oxocarbe-
nium intermediate prevented conformational change
of the pyranose ring and remote participation of the
N-Troc group at the C-4 position, leading to decreased
β-selectivity.
˚
presence of TMSOTf (0.3 equiv) and 5 A molecular sieves
(100 wt % with respect to 5) in CH2Cl2 at ꢀ60 °C
proceeded smoothly to provide the desired β-(1,4)-linked
deoxyglycoside 15β, the R-(1,4)-linked deoxyglycoside
15r, and the β-(1,3)-linked deoxyglycoside 16β in yields
of 31, 58, and 6%, respectively, with excellent regioselec-
tivity and poor stereoselectivity (entry 1). The configura-
tion of the anomeric positions of 15 and 16 was clearly
confirmed by the corresponding amino products which
were obtained after deprotection of the Troc groups. The
high regioselectivity was, presumably, due to low steric
In order to determine whether the major factor control-
ling β-selectivity was the remote participation effect of the
N-Troc group at the C-4 or the formation of a glycosyl R-
triflate intermediate,17 we performed the glycosylation
reaction using BF3 OEt2 (1.5 equiv) instead of TMSOTf
3
as an activator in CH2Cl2 at ꢀ40 and ꢀ20 °C (entries 7 and
8). Although the glycosylation reaction at ꢀ40 °C afforded
the desired disaccharide 15 in high yield with low β-
selectivity, the β-selectivity was found to be improved by
(14) Kato, H.; Ohmori, K.; Suzuki, K. Synlett 2001, 1003.
(15) (a) Abe, H.; Shuto, S.; Matsuda, A. J. Am. Chem. Soc. 2001, 123,
11870. (b) Yamada, H.; Nakatani, M.; Ikeda, T.; Marumoto, Y.
Tetrahedron Lett. 1999, 40, 5573. (c) Hosoya, T.; Ohashi, Y.; Matsumoto,
T.; Suzuki, K. Tetrahedron Lett. 1996, 37, 663.
(16) Baer, H. H.; Hanna, Z. S. Carbohydr. Res. 1981, 94, 43.
(17) Crich, D.; Sun, S. J. Org. Chem. 1996, 61, 4506.
Org. Lett., Vol. 13, No. 22, 2011
6128

increasing the reaction temperature to ꢀ20 °C. These
results clearly suggested that the remote participation
effect of the N-Troc group at the C-4 position was suffi-
cient for highly stereoselective construction of the N-
monodemethyl-β-D-forosaminide bond.
Hydrolysis of the thiophenyl group using Sc(OTf)3 and
NIS afforded the corresponding lactol without cleavage
of the N-monodemethyl-β-D-forosaminide bond, and
subsequent trichloroacetimidate formation provided
the key disaccharide donor 3 by treatment with a cata-
lytic amount of DBU and trichloroacetonitril. Next,
glycosylation of cyclohexanol (1.0 equiv) as a model
aglycon with the resulting glycosyl donor 3 (1.5 equiv)
was conducted using Yb(OTf)3 as a mild Lewis acid
promoter in CH2Cl2 at ꢀ20 °C. In this way, the desired
β-glycoside 18 was obtained in 92% yield as the sole
product without anomerization of the preinstalled 2,3,
4,6-tetradeoxy-4-methylamino-β-glycoside bond. Finally,
cleavage of the chloroacetyl group and subsequent
deprotection of the N-Troc groups using Zn and
NH4OAc in MeOH accomplished the synthesis of the
target disaccharide fragment 2.
Scheme 3. Completion of the Synthesis of 2
In conclusion, the disaccharide fragment 2 of incednine
(1) was efficiently and stereoselectively synthesized starting
from D-xylose and methyl R-D-glucopyranoside (12).
Stereoselective construction of the N-monodemethyl-β-D-
forosaminide bond was achieved via remote participation-
assisted glycosylation using an N-Troc group at the C-4
position. Additional studies on the total synthesis of
incednine (1) using the disaccharide glycosyl donor 3 are
currently underway in our laboratory.
Acknowledgment. This research was supported in part
by the 21st Century COE Program “Keio Life-Conjugated
Chemistry”, High-Tech Research Center Project for Pri-
vate Universities: Matching Fund Subsidy, 2006-2010,
and JSPS Fellow 22 5820 from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan
(MEXT).
3
Supporting Information Available. Experimental pro-
cedures and compound characterizations. This material is
available free of charge via the Internet at http://pubs.acs.
org.
The completion of the synthesis of 2 is summarized in
Scheme 3. Protection of the hydroxy group at the C-3
position of 15β as a chloroacetate gave thioglycoside 17.
Org. Lett., Vol. 13, No. 22, 2011
6129