Letter Cite This: Org. Lett. 2017, 19, 6482−6485
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.
D
Scheme 1. Retrosynthetic Analysis of Digalactosyl Diacylglycerols 1a−e
igalactosyl diacylglycerols (DGDGs) are found widely in natural products made from higher plants (including various edible plants), marine algae, and bacteria (Figure 1).1 DGDGs have been reported to possess a variety of biological activities, such as antitumor,2 antiparasitic,3 anti-inflammatory,4 and antihyperlipidemic5 activities. In addition to DGDGs, related glycoglycerolipids such as monogalactosyldiacylglycerol (MGDG) or sulfoquinovosyldiacylglycerol (SQDG) also exhibit broad biological activities.1b 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 their biological activities.5a 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 synthesis of DGDGs,5a including saturated and unsaturated Received: September 28, 2017 Published: November 28, 2017
Figure 1. Structure of digalactosyl diacylglycerols 1a−e. © 2017 American Chemical Society
6482
DOI: 10.1021/acs.orglett.7b03043 Org. Lett. 2017, 19, 6482−6485
Letter
Organic Letters
saturated fatty acids through glycosylation using iodosuccimide (NIS) and trifluoromethanesulfonic acid (TfOH).7 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 unsaturated 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 acids to the diacylglycerol moiety.5a 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 removed under neutral conditions using a Ru catalyst,8 an application of which was also recently reported by our group.9 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., αselectivity 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, βselectivity is needed in the absence of neighboring group participation at C-2.10 In fact, in most previous syntheses of MGDGs and DGDGs, the construction of galactose−glycerol β-linkages relies on neighboring assisting groups such as the Ac group at C-2.5a,6,11 Our preparation of the glycosyl donor 5 is shown in Scheme 2. The diol 8 was easily prepared from D-galactose in a 37% yield in four steps by following a published procedure.12 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 trifluoroacetimidate.13 The synthesis of the glycosyl acceptor 6 is shown in Scheme 3. The glycosyl acceptor 6 was easily
Scheme 2. Synthesis of Glycosyl Donor 5
Scheme 3. Synthesis of Glycosyl Acceptor 6
Table 1. Glycosylation of Acceptor 6 with Donor 5
entrya
solvent
equiv of 5
concn of 6 (M)
yield (%)b
α:βc
1 2 3 4 5
CH2Cl2 CH2Cl2 CH2Cl2 Et2O CH2Cl2
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 90:10 93:7 82:18 92:8
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. bYields of the isolated products. cDetermined by 1H NMR analysis.
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 fatty acids and used them as substrates for GPLRP2 galactolipase.6 Nishida et al. developed a synthetic route to DGDGs with 6483
DOI: 10.1021/acs.orglett.7b03043 Org. Lett. 2017, 19, 6482−6485
Letter
Organic Letters Scheme 4. Synthesis of the DGDG Derivative 16
Scheme 5. Total Synthesis of DGDGs 1a−e
prepared from the common intermediate 7.14 The introduction of the Lev14b 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 trifluoromethanesulfonate (TMSOTf) in CH2Cl2 gave the desired product 4 in 54% 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 Et2O did not improve the yield and selectivity (entry 4). In contrast, when the reaction was performed at 0.2 M in CH2Cl2, it gave the desired product 4 in 93% yield and α:β ratio = 92:8 (entry 5), 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).15 With the digalactosyl derivative 4 in hand, we next attempted introduction of the glycerol moiety 1416 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 SiO2 column chromatography. The
Gal(α1−6)Gal derivative 12α was converted to the corresponding 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 tend to favor 1,2-trans-glycoside formation,17 which were consistent with our results. However, further investigation would be necessary for understanding of the detailed mechanism for β-selectivity. 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 catalyst 24 (prepared from [CpRu(CH3CN)3]PF6)8 to afford 6484
DOI: 10.1021/acs.orglett.7b03043 Org. Lett. 2017, 19, 6482−6485
Letter
Organic Letters
(5) (a) Tanaka, R.; Sakano, Y.; Nagatsu, A.; Shibuya, M.; Ebizuka, Y.; Goda, Y. Bioorg. Med. Chem. Lett. 2005, 15, 159. (b) Sakano, Y.; Mutsuga, M.; Tanaka, R.; Suganuma, H.; Inakuma, T.; Toyoda, M.; Goda, Y.; Shibuya, M.; Ebizuka, Y. Biol. Pharm. Bull. 2005, 28, 299. (6) Lafont, D.; Carriere, F.; Ferrato, F.; Boullanger, P. Carbohydr. Res. 2006, 341, 695. (7) Miyachi, A.; Miyazaki, A.; Shingu, Y.; Matsuda, K.; Dohi, H.; Nishida, Y. Carbohydr. Res. 2009, 344, 36. (8) Tanaka, S.; Saburi, H.; Ishibashi, Y.; Kitamura, M. Org. Lett. 2004, 6, 1873. (9) Choy, S. L.; Bernin, H.; Aiba, T.; Bifeld, E.; Lender, S. C.; Mühlenpfordt, M.; Noll, J.; Eick, J.; Marggraff, C.; Niss, H.; Roldán, N. G.; Tanaka, S.; Kitamura, M.; Fukase, K.; Clos, J.; Tannich, E.; Fujimoto, Y.; Lotter, H. Sci. Rep. 2017, 7, 9472. (10) Demchenko, A. V. General Aspects of the Glycosidic Bond Formation. In Handbook of Chemical Glycosylation; Demchenko, A. V., Ed.; Wiley-VCH: Weinheim, 2008; pp 1−27. (11) Mannock, D. A.; Lewis, R. N. A. H.; Mcelhaney, R. N. Chem. Phys. Lipids 1987, 43, 113. (12) Ohlsson, J.; Magnusson, G. Carbohydr. Res. 2000, 329, 49. (13) Yu, B.; Tao, H. Tetrahedron Lett. 2001, 42, 2405. (14) (a) Jacquinet, J. C. Carbohydr. Res. 2006, 341, 1630. (b) Gu, G.; Adabala, P. J.; Szczepina, M. G.; Borrelli, S.; Pinto, B. M. J. Org. Chem. 2013, 78, 8004. (15) (a) Demchenko, A.; Stauch, T.; Boons, G.-J. Synlett 1997, 818. (b) Takatani, M.; Nakano, J.; Arai, M. A.; Ishiwata, A.; Ohta, H.; Ito, Y. 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. (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. 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. (18) (a) Murakami, N.; Morimoto, T.; Imamura, H.; Ueda, T.; Nagai, S.; Sakakibara, J.; Yamada, N. Chem. Pharm. Bull. 1991, 39, 2277. (b) Jung, J. H.; Lee, H.; Kang, S. S. Phytochemistry 1996, 42, 447.
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 DGDGs.5b,18 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 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 αselectivity. 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.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03043. Experimental procedures and characterization of the products (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yukari Fujimoto: 0000-0001-5320-3192 Notes
The authors declare no competing financial interest.
■
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.
■
REFERENCES
(1) (a) Sugawara, T.; Miyazawa, T. Lipids 1999, 34, 1231. (b) Zhang, J.; Li, C.; Yu, G.; Guan, H. Mar. Drugs 2014, 12, 3634. (c) Diercks, H.; Semeniuk, A.; Gisch, N.; Moll, H.; Duda, K. A.; Holzl, G. J. Bacteriol. 2015, 197, 497. (2) (a) Morimoto, T.; Nagatsu, A.; Murakami, N.; Sakakibara, J.; Tokuda, H.; Nishino, H.; Iwashima, A. Phytochemistry 1995, 40, 1433. (b) Maeda, N.; Kokai, Y.; Ohtani, S.; Hada, T.; Yoshida, H.; Mizushina, Y. Food Chem. 2009, 112, 205. (c) Zhang, H.; Oh, J.; Jang, T.-S.; Min, B. S.; Na, M. Food Chem. 2012, 131, 1097. (3) Sashidhara, K. V.; Singh, S. P.; Misra, S.; Gupta, J.; MisraBhattacharya, S. Eur. J. Med. Chem. 2012, 50, 230. (4) (a) Manez, S.; del Carmen Recio, M.; Gil, I.; Gomez, C.; Giner, R.-M.; Waterman, P. G.; Rios, J.-L. J. Nat. Prod. 1999, 62, 601. (b) Bruno, A.; Rossi, C.; Marcolongo, G.; Di Lena, A.; Venzo, A.; Berrie, C. P.; Corda, D. Eur. J. Pharmacol. 2005, 524, 159. 6485
DOI: 10.1021/acs.orglett.7b03043 Org. Lett. 2017, 19, 6482−6485
