Subscriber access provided by University of Sunderland
Note
Semi-Synthesis of Chondroitin Sulfate E Tetrasaccharide From Hyaluronic Acid Wang Yao, Yong Zhu, Xiao Zhang, Meng Sha, Xiangbao Meng, and Zhongjun Li J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Organic Chemistry
Semi‐Synthesis of Chondroitin Sulfate E Tetrasaccharide From Hyalu ronic Acid Wang Yao, Yong Zhu, Xiao Zhang, Meng Sha, Xiangbao Meng, Zhongjun Li* State Key Laboratory of Natural and Biomimetic Drugs; Department of Chemical Biology, School of Pharmaceutical Sciences, Peking University, Beijing, 100191, P. R. China. ABSTRACT: Chondroitin sulfate is crucial glycosaminoglycan that regulates key functions of nervous system. CS-E is one of the key CS subtypes that modulates the biological function of CS. Herein, N-protecting-group free semi-synthesis of CS-E tetrasaccharide is reported using hyaluronic acid as a readily available starting material. The synthetic process utilizes the enzymatic degradation and selective C4 hydroxyl group conversion as key approaches for direct construction of the CS tetrasaccharide precursor, which furnish the neuroactive CS-E tetrasaccharide in 15 steps.
tetrasaccharide backbone with disaccharide building block 7. Chondroitin sulfate (CS) is one of the major components in Although these approaches allowed for the further biochemextracellular matrix that regulates the functions of nervous ical studies, all the cases required multisteps synthesis of system involved in neural plasticity, migration, neuronal glycosyl donor and acceptor. pathfinding and neural stem cell differentiation.1-6 CS is composed of sulfated →4GlcAβ1→3GalNAcβ1 disaccharide repeating unit. In human body, there are 4 subtypes of CS (Figure 1) among which CS-E has been found as key neuroactive species that directed important neurological events such as regulating the neurite outgrowth. Hsieh-Wilson and co-workers found CS-E tetrasaccharide could stimulate the embryonic hippocampal neurite extension in vitro7 and they reported CS-E tetrasaccharide is the minimum length that presented such activity and CS-E could bind firmly to midkine and BDNF7-8. Further, CS-E disaccharide Figure 1. Structure of chondroitin sulfate in human body. containing glycoclusters which mimicked CS-E polysaccharide was applied to trigger the downstream signaling of NGF/TrkA pathway.9 To date, CS-E has been found to bind Hyaluronic acid (HA), composed of →4GlcAβ1→3Glcto Nogo receptors,3 trans-membrane protein tyrosine phosNAcβ1 disaccharide repeating unit is a widely used GAG phatases (PTPσ),10 Wnt-3a,11 semaphorin 3A,12 contactinmaterial. Except for the configuration of the C4-OH on hex1,13 pleiotrophin14 and fibroblast factors.14 Chemical syntheosamine moiety, HA and CS possessed backbone with same sis is a major way to obtain structural defined CS-E oligotype of glycosidic bond. Compared to CS, HA is non-sulsaccharides. Though there have been many synthetic apfated, non-protein linked GAGs. Many hyaluronidases proaches,7-9, 15-23 the processes are still challenging.24 As repwhich could selectively hydrolyze HA polysaccharide into resentative samples, Hsieh-Wilson and co-workers8 acoligosaccharide fragments have been discovered.25 We hycessed CS-E tetrasaccharide 1 with an allyl group at reducpothesized that defined CS oligosaccharides could be direcly ing end in 33 collective steps from galactosamine hydrochlosynthesized from HA oligosaccharides (Scheme 1). Herein, ride 3 and tetra-acetyl glucopyranosyl bromide 2. The glytaking advantage of enzymatic degradation techniques, we cosylation was accomplished with trichloroacetimidates 4. developed an easy approach toward CS-E tetrasaccharide 8 Jacquinet and co-workers19c reported a 22-step synthesis of without the need for glycosylation coupling. Naphthylated CS-E tetrasaccharide 5 from CS polysaccharide. They degradated the CS polymer into de-N-acetyl disaccharide 6 under acidic condition, then constructed the ACS Paragon Plus Environment
The Journal of Organic Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Scheme 1. Previous syntheses of CS-E tetrasaccharide and retrosynthetic analysis, TCA = trichloroacetimidate, NAP = 2naphthylmethyl, pMP = p-methoxyphenyl
Our synthesis began with degradation of HA. As shown in Scheme 2, HA (M.W. = 500 kDa, $328/kg) was digested with 2.4% w/w bovine testis (or testicular) hyaluronidase (BTH) ($145/g, 1.2 g for 50 g HA) in acetate saline (pH = 5.0) for 14 days. Previously, we have developed a continuous esterification method by sequential methylating then acetylating the degradation mixture 9.26 This protocol enabled us to afford the protected tetrasaccharide 10 (α/β=1.7/1) at a quantity of 25 g from 9 in 42% overall yield. 10 was then subjected to coupling with azido-hexanol 11 to yield tetrasaccharide 13 which possesses an azido group at reducing end. An initial attempt was made to convert 10 to 13 using SnCl4 to activate the anomeric -OAc group. Although 13 was generated under this condition, the yield was only 15% which accompanied with oxazoline 12 (26%) and a large amount (>50%) of unreacted donor 10. The sluggishness of the acetate donor urged us to transform 10 firstly into oxazoline 12 through an easier intramolecular pathway. We attempted to use a cocktail of TMSBr, collidine and BF3ꞏOEt2, which has previously been used to transform a high mannoside per-acetate oligosaccharide into the corresponding oxazoline.27 Conducting this reaction in 1,2-dichloroethane smoothly gave 12 in 79% at 15 g scale. Oxazoline 12 was then converted into azido 13 by using CuCl2 to promote the
Page 2 of 11
opening of the oxazoline ring.28 After the reaction was finished, CuCl2 was precipitated with NaHCO3 aqueous solution and 13 was successfully crystallized in Petroleum ether/Et2O in 95% yield at 11 g scale. With adequate quantity of 13 in hand, a series of functional group transformations were carried out. In methanolic HCl, 13 was successfully de-acetylated to yield alcohol 14. Jacquinet and coworkers had reported the de-acetylation process on CS-type building blocks under classical Zemplén condition,19c notably, under this condition, 14 was obtained in a much lower yield (ca. 50%). Treating 14 with PhCH(OMe)2 in DMF gave pentaol 15 in 84% over two steps from 13. To our delight, 15 has poor solubility in MeOH (
