Convergent Synthesis of Digalactosyl Diacylglycerols

Article abstract of DOI:10.1021/acs.orglett.7b03043

Efficient convergent chemical syntheses of digalactosyl diacylglycerols (DGDGs), which have both a galactose-galactose α(1→6)-linkage and a galactose-glycerol β-linkage along with a diacylglycerol containing various kinds of fatty acids, have been accomplished. In order to achieve a concise synthesis, we chose to use allylic protective groups as permanent protective groups. We have also achieved α- and β-selective glycosylations for the respective linkages with high yields as the key steps.

Full text of DOI:10.1021/acs.orglett.7b03043

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Convergent Synthesis of Digalactosyl Diacylglycerols
Shinsuke Inuki,†,‡ Junichiro Kishi, Emi Kashiwabara, Toshihiko Aiba, and Yukari Fujimoto*,†
†
†
†,§
†
Graduate School of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
‡
Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
§
Graduate School of Science, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
*
S Supporting Information
ABSTRACT: Efficient convergent chemical syntheses of digalactosyl diacylglycerols (DGDGs), which have both a galactose−
galactose α(1→6)-linkage and a galactose−glycerol β-linkage along with a diacylglycerol containing various kinds of fatty acids,
have been accomplished. In order to achieve a concise synthesis, we chose to use allylic protective groups as permanent
protective groups. We have also achieved α- and β-selective glycosylations for the respective linkages with high yields as the key
steps.
igalactosyl diacylglycerols (DGDGs) are found widely in
Scheme 1. Retrosynthetic Analysis of Digalactosyl
D
natural products made from higher plants (including
Diacylglycerols 1a−e
1
various edible plants), marine algae, and bacteria (Figure 1).
DGDGs have been reported to possess a variety of biological
2
3
4
activities, such as antitumor, antiparasitic, anti-inflammatory,
and antihyperlipidemic activities. In addition to DGDGs,
related glycoglycerolipids such as monogalactosyldiacylglycerol
5
(
MGDG) or sulfoquinovosyldiacylglycerol (SQDG) also
1
b
exhibit broad biological activities. The representative
structure of DGDGs consists of a galactose−galactose α(1→
6)-linkage and diacylglycerol moieties. The diacylglycerol
moieties contain various kinds of acyl chains, including
saturated and unsaturated fatty acids. The fatty acid
composition of diacylglycerol moieties can have effects on
5a
their biological activities. However, the separation of DGDGs
with different acyl groups from natural product extracts is
difficult, because of their similar physical and chemical
properties. Therefore, the chemical synthesis of pure DGDGs
is a worthwhile endeavor to expedite detailed studies of their
biological functions and activities. Goda et al. reported the total
5a
synthesis of DGDGs, including saturated and unsaturated
Received: September 28, 2017
Figure 1. Structure of digalactosyl diacylglycerols 1a−e.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03043
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Synthesis of Glycosyl Donor 5
saturated fatty acids through glycosylation using iodosuccimide
7
(
NIS) and trifluoromethanesulfonic acid (TfOH). Here, we
planned to develop a concise and convergent synthesis of
DGDGs for structure−activity relationship (SAR) studies.
A challenge to accomplishing the total synthesis of DGDG is
the choice of a protecting group which must be capable of
being removed under mild conditions at the final stages because
diacylglycerol moieties in DGDGs contain unstable ester
linkages that cannot withstand basic conditions. The unsatu-
rated fatty acids are also sensitive to reduction. For example, in
Goda’s synthesis, the p-methoxybenzyl (PMB) group was used
as protecting groups, and Ac groups on the sugar moiety were
converted to PMB groups before introducing unsaturated fatty
5a
acids to the diacylglycerol moiety. The construction of the
galactose−galactose α(1→6)-linkages and galactose−glycerol
β-linkages is another major issue of concern in any DGDG
synthesis. In general, the properties of protecting groups can
affect the reactivity and selectivity of sugar moieties during
glycosylation reactions, which may cause the number of total
steps for the synthesis to increase due to the use of various
types of protecting groups in a synthetic sequence. Thus, the
selection of appropriate protecting groups is an important
factor for minimizing the total number of synthesis steps. In an
effort to achieve a concise and convergent synthesis of DGDGs,
we selected allyl/alloc protecting groups as permanent
protective groups. These protecting groups can be easily
Scheme 3. Synthesis of Glycosyl Acceptor 6
8
removed under neutral conditions using a Ru catalyst, an
9
application of which was also recently reported by our group.
Additionally, we planned to use a common monosaccharide
intermediate containing allyl/alloc protecting groups for the
concise preparation of the disaccharide.
Our retrosynthetic analysis of DGDGs is outlined in Scheme
1
. We envisioned that the allyl/alloc protected DGDG 2 could
be constructed by β-selective glycosylation of the glycerol
moiety 3 with the digalactosyl derivative 4, followed by the
subsequent introduction of the various fatty acids upon the
glycerol moiety. The digalactosyl derivative 4 could be accessed
through α-selective glycosylation of the glycosyl acceptor 6 with
the glycosyl donor 5. Both the allyl/alloc protected galactose
derivatives 5 and 6 could be readily obtained from a common
monosaccharide intermediate 7. Apparently, the key to the
success of this synthetic route depends on the selectivity of two
glycosylation reactions using substrates containing the 2-O-allyl
group, derived from the common intermediate 7; i.e., α-
selectivity is required in the first glycosylation to construct the
Gal(α1−6)Gal linkage. In contrast, during the second
glycosylation, to generate the galactose−glycerol β-linkage, β-
selectivity is needed in the absence of neighboring group
Table 1. Glycosylation of Acceptor 6 with Donor 5
a
b
c
entry
solvent
equiv of 5 concn of 6 (M) yield (%)
α:β
1
2
3
4
5
CH Cl2
1.5
2.0
3.3
2.0
1.5
0.1
0.1
0.1
0.1
0.2
54
46
63
57
93
91:9
2
CH Cl2
90:10
93:7
10
2
participation at C-2. In fact, in most previous syntheses of
CH Cl2
2
MGDGs and DGDGs, the construction of galactose−glycerol
Et O
2
82:18
92:8
β-linkages relies on neighboring assisting groups such as the Ac
2
CH Cl2
5a,6,11
group at C-2.
a
Reactions were carried out with acceptor (1.0 equiv), donor
indicated, TMSOTf (0.2 equiv), and MS 4 Å in solvent indicated at
20 °C. Yields of the isolated products. Determined by H NMR
Our preparation of the glycosyl donor 5 is shown in Scheme
b
c
1
2. The diol 8 was easily prepared from D-galactose in a 37%
−
12
yield in four steps by following a published procedure.
analysis.
Treatment of 8 with allylBr and NaH gave the compound 9,
and the benzylidene acetal group was removed with TFA to
generate the common intermediate diol 7. The introduction of
Alloc groups to the 4,6-hydroxyl groups followed by the
removal of the PMP group led to 10, which was subsequently
converted to the glycosyl donor 5 containing the N-phenyl
fatty acids, and their structure-specific inhibitory activity on
human lanosterol synthase revealed that a dimyristoyl derivative
indicated strong activity among DGDGs with various acyl
groups. Lafont et al. synthesized DGDGs with saturated fatty6
acids and used them as substrates for GPLRP2 galactolipase.
Nishida et al. developed a synthetic route to DGDGs with
13
trifluoroacetimidate. The synthesis of the glycosyl acceptor 6
is shown in Scheme 3. The glycosyl acceptor 6 was easily
B
DOI: 10.1021/acs.orglett.7b03043
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Scheme 4. Synthesis of the DGDG Derivative 16
Scheme 5. Total Synthesis of DGDGs 1a−e
1
4
prepared from the common intermediate 7. The introduction
Gal(α1−6)Gal derivative 12α was converted to the corre-
sponding imidate 13 (anomer ratio at the reducing end of the
disaccharide, α:β = 70:30). We next investigated the β-selective
glycosylation of glycerol 14 with the imidate 13. Reaction of
glycosyl donor 13 (α:β = 70:30) with acceptor 14 in the
presence of TMSOTf at −20 °C provided a trace amount of the
desired product 16. Instead, an undesired side product 15 was
obtained in 82% yield and low selectivity (α:β = ca. 50:50).
Intriguingly, the corresponding glycosylation at −78 °C did
improve the yield and selectivity of the desired protected
DGDG 16 (91% yield, α:β = 21:79). Several groups reported
that kinetically controlled glycosylations at low temperatures
1
4b
of the Lev and Alloc groups (70% yield, α:β = 38:62) into
compound 7, followed by the separation, gave the β-anomer 11.
The removal of the Lev group at the 6-hydroxyl group afforded
the desired glycosyl acceptor 6.
We proceeded to investigate the glycosylation of the glycosyl
donor 5 and acceptor 6, as shown in Table 1. Reaction of 6
with the donor 5 mediated by trimethylsilyl trifluoromethane-
sulfonate (TMSOTf) in CH Cl gave the desired product 4 in
4% yield with high α-selectivity (entry 1, α:β = 91:9).
Increasing the equivalent amount of glycosyl donor 5 (from 1.5
to 3.3 equiv) did not affect the yield of the desired product
significantly (entries 2 and 3). Changing the solvent to Et O
did not improve the yield and selectivity (entry 4). In contrast,
when the reaction was performed at 0.2 M in CH Cl , it gave
the desired product 4 in 93% yield and α:β ratio = 92:8 (entry
), indicating that the concentration of donor 5 had noticeable
effects on the reaction course compared with the equivalents of
glycosyl donor (entries 2, 3, vs 5).
With the digalactosyl derivative 4 in hand, we next attempted
introduction of the glycerol moiety 14 into the disaccharide
Scheme 4). Removing the PMP group with CAN gave the
Gal(α1−6)Gal and Gal(β1−6)Gal mixture (12α and 12β),
which was separated by SiO column chromatography. The
2
2
5
17
tend to favor 1,2-trans-glycoside formation, which were
consistent with our results. However, further investigation
would be necessary for understanding of the detailed
mechanism for β-selectivity.
2
2
2
5
We sought to convert 16 to the DGDGs 1a−e (Scheme 5).
The removal of the TBDPS group using TBAF followed by
acylation with various fatty acids provided the desired products
18−20. Next, the PMB group was removed, and the
introduction of a second fatty acid at the sn-2 position provided
the protected glycolipid derivatives 2a−e. Finally, all protective
groups were removed from 2a−e independently with Ru
15
16
(
8
catalyst 24 (prepared from [CpRu(CH CN) ]PF ) to afford
2
3
3
6
C
DOI: 10.1021/acs.orglett.7b03043
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the DGDGs 1a−e in moderate to high yields without
isomerization of olefins. The structures of DGDGs 1a−e
were determined by 1D and 2D NMR spectroscopy and mass
spectrometry, which were in agreement with those of isolated
(5) (a) Tanaka, R.; Sakano, Y.; Nagatsu, A.; Shibuya, M.; Ebizuka, Y.;
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5
b,18
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In conclusion, we have achieved concise chemical syntheses
of the DGDGs 1a−e by using allylic protective groups as
permanent protective groups. The galactose−galactose α(1→
(
6)-linkage and galactose−glycerol β-linkage were constructed
6, 1873.
by α/β selectively under the optimized reaction conditions. In
the first glycosylation, controlling the concentration of donor 5
provided the desired product in high yield and with high α-
selectivity. On the other hand, the second β-selective
glycosylation was accomplished at low temperature to give
the desired product 16 in high yield. These newly developed
methods will contribute to the comprehensive synthesis of
various DGDGs and also to the analysis of their biological
functions. Further research including biological assays and SAR
studies is currently underway.
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(
ASSOCIATED CONTENT
Supporting Information
■
2
(
(
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03043.
Tetrahedron Lett. 2004, 45, 3929. (c) Kononov, L. O.; Malysheva, N.
N.; Orlova, A. V.; Zinin, A. I.; Laptinskaya, T. V.; Kononova, E. G.;
Kolotyrkina, N. G. Eur. J. Org. Chem. 2012, 2012, 1926. (d) Yang, F.;
Zhu, Y.; Yu, B. Chem. Commun. 2012, 48, 7097. (e) Kusumi, S.;
Tomono, S.; Okuzawa, S.; Kaneko, E.; Ueda, T.; Sasaki, K.; Takahashi,
D.; Toshima, K. J. Am. Chem. Soc. 2013, 135, 15909.
Experimental procedures and characterization of the
products (PDF)
AUTHOR INFORMATION
Corresponding Author
(
16) Aiba, T.; Sato, M.; Umegaki, D.; Iwasaki, T.; Kambe, N.; Fukase,
K.; Fujimoto, Y. Org. Biomol. Chem. 2016, 14, 6672.
17) (a) Andersson, F.; Fugedi, P.; Garegg, P. J.; Nashed, M.
■
(
*E-mail: fujimotoy@chem.keio.ac.jp.
Tetrahedron Lett. 1986, 27, 3919. (b) Nishizawa, M.; Shimomoto, W.;
Momii, F.; Yamada, H. Tetrahedron Lett. 1992, 33, 1907. (c) Shimizu,
H.; Ito, Y.; Ogawa, T. Synlett 1994, 1994, 535. (d) Chenault, H. K.;
Castro, A.; Chafin, L. F.; Yang, J. J. Org. Chem. 1996, 61, 5024.
ORCID
Yukari Fujimoto: 0000-0001-5320-3192
Notes
(18) (a) Murakami, N.; Morimoto, T.; Imamura, H.; Ueda, T.; Nagai,
The authors declare no competing financial interest.
S.; Sakakibara, J.; Yamada, N. Chem. Pharm. Bull. 1991, 39, 2277.
(b) Jung, J. H.; Lee, H.; Kang, S. S. Phytochemistry 1996, 42, 447.
ACKNOWLEDGMENTS
■
We thank Prof. K. Suenaga for the assistance in measuring
physical data of a compound. This research was supported by
grants from the Grant-in-Aid for Scientific Research (Nos.
JP26282211, JP26102732, JP26882036, JP17H02207,
JP17H05800, JP16H01162, and JP16K16638), the Mizutani
Foundation for Glycoscience, Mishima Kaiun Memorial
Foundation, Takeda Science Foundation, and Protein Research
Foundation.
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DOI: 10.1021/acs.orglett.7b03043
Org. Lett. XXXX, XXX, XXX−XXX