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Synthesis of Isobemisiose, Neosartose, and Fischerose: Three Α-1,6-Linked

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Synthesis of Isobemisiose, Neosartose, and Fischerose: Three Α-1,6-Linked RSC Advances View Article Online PAPER View Journal | View Issue Synthesis of isobemisiose, neosartose, and fischerose: three a-1,6-linked trehalose-based Cite this: RSC Adv., 2020, 10, 22726 oligosaccharides identified from Neosartorya fischeri† E. J. Kuenstner, ‡§ E. A. Palumbo, J. Levine and N. L. Snyder * Three complex a-1,6-linked trehalose-based oligosaccharides with unique preservation properties, isobemisiose, neosartose, and fischerose, were recently identified from the extreme stress-tolerant ascospores of Neosartorya fischeri. Herein, we report the first concise, scalable, and iterative chemical synthesis of these oligosaccharides from a differentially protected thioglycoside donor and a selectively Received 8th May 2020 protected, asymmetric trehalose acceptor. This work constitutes an improved synthesis of isobemisiose, Accepted 4th June 2020 and is also the first reported synthesis of neosartose, a tetrasaccharide, and fischerose, DOI: 10.1039/d0ra04137h a pentasaccharide, in good yield. Importantly, in-depth studies of biological function are enabled by this Creative Commons Attribution 3.0 Unported Licence. rsc.li/rsc-advances synthetic platform. Introduction and scherose 6. In aggregate, the data suggested that the trehalose-containing oligosaccharides form a high-density glass Trehalose 1, a disaccharide comprised of two glucose mono- that is postulated to serve a protective role for cytosolic mers linked in a a,a-1,1 fashion, is found in over 80 different biomolecules in vivo. Moreover, recent studies have indicated species of plants, insects, algae, fungi, and bacteria (Fig. 1).1 The that sol gels embedded with bovine LDH have increased oligosaccharides and mycolic acids that contain trehalose (e.g. 2 stability in the presence of neosartose 5 and scherose 6 when This article is licensed under a 13 & 3) are important biomolecules involved in a variety of func- compared to trehalose. The complexity of these compounds tions ranging from pathogenicity and autophagy induction to and their unique preservation properties ultimately led us to membrane stability.2–4 Notably, the atypical a,a-1,1 glycosidic engage in a synthetic effort to prepare these compounds to Open Access Article. Published on 12 June 2020. Downloaded 9/25/2021 7:04:20 PM. linkage of trehalose renders the disaccharide non-reducing, supplement our ongoing studies to broaden our understanding which is believed to impart signicant resistance to degrada- of the role of these unique compounds in protein preservation. tion by chemical, thermal, and desiccating means.5,6 The Our results are reported below. precise biophysical mechanism by which trehalose derivatives stabilize lipid membranes is the subject of ongoing investiga- tion, and has previously been explained by a number of different hypotheses.7–9 Results and discussion In 2015, Wyatt et al. identied and characterized a series of We envisioned a linear assembly of the oligosaccharides 4, 5, trehalose-containing oligosaccharides responsible for the and 6 via a series of iterative glycosylations. The crux of our unique preservation properties in the extreme stress-resistant strategy was the unication of a bifunctional glycosyl donor ascospores of Neosartorya scheri (Aspergillus scheri) 10–12 with an asymmetric trehalose acceptor. Whereas differential (Fig. 2). FTIR spectroscopy was used to ascertain Tg and removal of the glucosyl C6 protecting group would allow for WTC values for combinations of isobemisiose 4, neosartose 5 additional glycosylations, global deprotection would afford the requisite targets. These considerations led to the identication Department of Chemistry, Davidson College, Box 7120, Davidson, NC 28036, USA. of readily accessible thioglycoside 7 (ref. 14) and trehalose E-mail: [email protected] derivative 8 (ref. 15) as suitable building blocks (Fig. 3). Thio- † Electronic supplementary information (ESI) available: Experimental procedures, glycoside 7 was particularly attractive because of the precedence 13 1 4 5 6 9 10 11 b in addition to C& H NMR spectra for , , , , , , and select anomers, is for using this donor to generate a-glucosidic linkages with high available online. See DOI: 10.1039/d0ra04137h ff ‡ diasteroselectivity, albeit with di erent nucleophiles, and for its Present address: Entasis Therapeutics, 35 Gatehouse Drive, Waltham, MA 14 02451, USA. straightforward synthesis on multigram scales. Both thio- 7 8 § Previous address: Department of Chemistry, Hamilton College, 198 College Hill glycoside and trehalose analog were readily synthesized Road, Clinton, NY 13323, USA. from commercially available building blocks. 22726 | RSC Adv.,2020,10, 22726–22729 This journal is © The Royal Society of Chemistry 2020 View Article Online Paper RSC Advances Fig. 1 The structures of trehalose and representative derivatives of biological importance, with the trehalose core highlighted in red. Our synthesis began with a moderately stereoselective had been presumed to be of the a conguration, fully matched glycosylation of donor 7 with acceptor 8 via the use of NIS and the previous report, conrming our previous assignments of Creative Commons Attribution 3.0 Unported Licence. catalytic triic acid to form trisaccharide 9 (Scheme 1). Despite relative stereochemistry.10 the use of a non-participating benzyl group at C2 in conjunction The stereoselectivity of our glycosylation, while modest, with a binary solvent mixture, we were unable to achieve greater merits additional discussion. Indeed, the reactivity of carbo- than 3 : 1 d.r. to favor the desired a anomer.16,17 Purication of hydrate building blocks is critical for forming regio- and dia- these diastereomers was best achieved following saponication stereoselective glycosidic linkages.19–22 First, our results with to alcohol 9 and the corresponding b-linked epimer (not thioglycoside donor 7 differs from an earlier report by Koto and shown). Tentative assignment of the relative stereochemistry of co-workers18 that remarkably predates the isolation of iso- the newly-formed glycosidic bond was rst accomplished by bemisiose from a natural source. Their report, which involves 13 This article is licensed under a examination of anomeric resonances in C NMR, and later glycosylation of acceptor 8 with the lactol (C1 hemiacetal) conrmed via comparison to a previously-synthesized sample of analog of 7 in the presence of excess pyridine and TMS triate, 9.18 Iterative glycosylations under identical conditions gave 10 gave ca 12 : 1 d.r. to favor the a-anomer albeit in lower yields 11 Open Access Article. Published on 12 June 2020. Downloaded 9/25/2021 7:04:20 PM. and , which proceeded with comparable diastereoselectivity. and on smaller scales. While the original authors did not Removal of the remaining benzyl protecting groups via hydro- comment on the origin of this high diastereoselectivity, other genolysis required the addition of a few drops of water as a co- reports have speculated that it may arise via remote participa- solvent to effect full dissolution of the partially deprotected tion of the C6 acetyl group.23 Indeed, similar mechanisms of oligosaccharides, but proceeded in near-quantitative yields. All selectivity have been noted in the synthesis of an epimeric 13C and 1H NMR spectra of the deprotected compounds, which trehalose-containing galactoside.24 However, the presence of Fig. 2 Trehalose-containing oligosaccharides with preservation capabilities identified in Neosartorya fischeri (Aspergillus fischeri). This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10, 22726–22729 | 22727 View Article Online RSC Advances Paper Fig. 3 Iterative oligosaccharide synthesis by incorporation of a bifunctional glycosyl linchpin. a C6 acetate in 7 suggests a more complex interplay between generally display a-selectivity.22 It might be expected, then, that remote participation and various ion-pair species. Recent replacement of the benzyl protecting groups in 8 for benzoyl reports by Zhu25 and Kowalska,26 which were published while groups could improve the diastereoselectivity of our glycosyla- this manuscript was in preparation, suggest that the a-selec- tion. These speculations aside, the present paper emphasizes tivity of this reaction might be improved by using tri- the impact of seemingly subtle changes in stereoelectronic uoroacetimidates instead of thioglycoside donors. In contrast, environments during the formation of reactive intermediates in the role of glycosylation acceptors in promoting stereochemical glycosylations. outcomes is less established than that of glycosylation donors, In conclusion, we have described a concise synthesis of three but research in this area may still provide relevant insight to our complex oligosaccharides: isobemisiose (4), neosartose (5), and results. Numerous studies across various reaction manifolds scherose (6). Our sequence to 6, which is the largest synthe- have correlated reactive acceptors (e.g. primary alcohols) with b- sized a-linked trehalose-derived oligosaccharide described to selectivity.27,28 Conversely, less reactive counterparts (e.g. date, is unprecedented. Sequences to the former species Creative Commons Attribution 3.0 Unported Licence. secondary alcohols with adjacent electron-withdrawing groups) compare favorably to reports of enzymatic incubations and de This article is licensed under a Open Access Article. Published on 12 June 2020. Downloaded 9/25/2021 7:04:20 PM. Scheme 1 Synthesis of a-1,6-linked oligosaccharides. 22728 | RSC Adv., 2020, 10, 22726–22729 This journal is ©
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