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Multigram Chromatography-Free Synthesis of Octa (Ethylene Glycol) P ORGANIC CHEMISTRY FRONTIERS View Article Online METHOD View Journal | View Issue Multigram chromatography-free synthesis of octa(ethylene glycol) p-toluenesulfonate† Cite this: Org. Chem. Front., 2016, 3, 1524 Adam M. Wawro,a Takahiro Muraoka,b,c Maho Katob and Kazushi Kinbara*b Here we report a detailed synthetic procedure for monodisperse octa(ethylene glycol) p-toluenesulfonate, Received 27th July 2016, a member of the heterobifunctional oligo(ethylene glycol) derivatives family. The method offers Accepted 3rd September 2016 completely chromatography-free workup and purification, resulting in higher yields and a lower cost than DOI: 10.1039/c6qo00398b any of the alternative strategies. We also introduce several upgrades, simplifying the method even more, rsc.li/frontiers-organic and discuss the chromatography-free design limitations. Introduction building blocks of functional compounds. Although tradition- ally PEGs have been produced by polymerization, yielding stat- Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Poly(ethylene glycol)s (PEGs) are frequently used in chemistry, istically distributed mixtures of individual polymer molecules, most notably for protein cross-linking and modification, the recent demand for more uniform PEGs has stimulated surface functionalization, and as flexible and hydrophilic studies on well-defined, single-molecule polymers. These PEGs, known as ‘discrete’, ‘unimolecular’ or ‘monodisperse’, consist of a single chemical species with a specific molecular a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, weight. Monodisperse PEGs are obtained by classical step- 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan bSchool of Life Science and Technology, Tokyo Institute of Technology, by-step chemical synthesis, usually from inexpensive short 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan. (n ≤ 4) PEG oligomers, rather than by polymerization. Due This article is licensed under a E-mail: [email protected]; http://www.kinbara.bio.titech.ac.jp to their discrete structure, monodisperse PEGs require the cPRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, same purification and analytical regime as small-molecule Saitama 332-0012, Japan compounds. †Electronic supplementary information (ESI) available: 1H and 13C NMR spectra; qNMR experiment; HPLC oligomer purity analysis; report from the experiment Among PEG derivatives, the highest versatility can be attri- Open Access Article. Published on 06 September 2016. Downloaded 7/8/2021 12:15:59 PM. reproduced by M. K.; extraction process details. See DOI: 10.1039/c6qo00398b buted to heterobifunctional PEGs. Due to their asymmetric Adam M. Wawro, born in 1988, Dr Takahiro Muraoka is an received his MSc degree in assistant professor at the Tokyo Chemical Technology from the Institute of Technology since Warsaw University of Technology 2015. He obtained his Ph.D. in in Poland. Currently he is pursu- 2007 from the University of ing his PhD as a MEXT Scholar Tokyo under the supervision of at Tohoku University in Japan Professor Takuzo Aida. After under the supervision of working as a JSPS postdoctoral Professor Kazushi Kinbara. researcher in Professor Samuel I. Stupp’s group at Northwestern University, he was promoted to assistant professor at Tohoku Adam M. Wawro Takahiro Muraoka University in 2008. Since 2013, he has been concurrently serving as a PRESTO researcher at the Japan Science and Technology Agency. His present research focuses on functional synthetic mole- cules inspired by proteins on the basis of monodisperse PEG and amphiphilic molecular design. 1524 | Org. Chem. Front.,2016,3, 1524–1534 This journal is © the Partner Organisations 2016 View Article Online Organic Chemistry Frontiers Method structure, the two chain termini can be functionalized to show Li et al.3 designed a benzyl-type fluorinated tag attached to orthogonal reactivity. Such asymmetric oligomeric PEGs are the grown PEG chain. This feature allowed the use of chrom- used for the surface modification, protein cross-linking and atography based on fluorine interactions, simplifying the PEGylation of small drug molecules. Unfortunately, heterobi- workup and purification process. However, purity of the pro- functional PEGs are the most challenging derivatives from the ducts was not discussed. synthetic point of view. Most of the known methods give low Székely et al.4,5 employed a trifunctional benzyl-type core on overall yields and are either hampered by numerous chromato- which three PEG chains are grown simultaneously. PEG arms graphic separations or require the use of expensive reagents. of the resulting star-shaped molecule are elongated and even- On top of that, commercially available asymmetrically substi- tually the core is cleaved. This approach requires reverse-phase tuted PEGs usually cost above US $500 per gram. column chromatography with a relatively small amount of the stationary phase. Previous methods Finally, Zhang et al.6,7 proposed a new type of building Methods of asymmetric PEG derivatives’ synthesis can be block. Instead of using asymmetric PEG tosylates, they syn- divided into two types. The first group involves firstly the syn- thesized macrocyclic PEG sulfates. These sulfates are proasym- thesis of unsubstituted PEGs of the desired length, followed by metric, i.e. upon ring-opening reaction an asymmetrically non-selective monofunctionalization. Although the initial part substituted derivative is obtained. This method requires fewer is relatively simple, the final step gives moderate yields, since a chromatographic separations, but employs toxic CCl4, tran- symmetric bis-substituted product is obtained concurrently. sition metal catalyst and a large volume of the solvent for the Excess of PEG can shift the product ratio towards the mono- cyclization process. substituted derivative, but in this case recovery and purifi- We have recently reported a new synthetic strategy for the cation of the unreacted, precious starting material is highly synthesis of monodisperse oligo(ethylene glycol) p-toluene- desired. The other group of methods focuses on the direct syn- sulfonates.8 The toluenesulfonates are heterobifunctional and Creative Commons Attribution-NonCommercial 3.0 Unported Licence. thesis of heterobifunctional PEGs. The asymmetric molecular easily derivatizable to give a wide range of orthogonally reac- structure is introduced at one of the early steps and then the tive functional PEGs. Our method has significant advantages PEG chain is extended until the desired length is obtained. over the previously known ones: requires no chromatography, This approach is generally more convenient and most of the uses only simple and inexpensive reagents and offers the recent reports utilize it. highest overall yields, with respect to the corresponding asym- There are four main strategies belonging to the latter metric PEGs obtained by other methods. These advantages group. Loiseau et al.1 and later French et al.2 proposed the use have been achieved by focusing on three particular aspects: of two orthogonally reactive protective groups. By selective rational arrangement of the individual processes of a multi- manipulation of the terminal functional groups, shorter PEG step procedure, carefully developed workup conditions and This article is licensed under a fragments are assembled into longer chains. The protective taking advantage of unique PEG physical properties, namely groups can be removed at any stage and the product can be substituent-dependent partitioning between the organic further functionalized. This approach generates a small and aqueous layers. In this paper we report an improved amount of by-products, but requires chromatographic purifi- Open Access Article. Published on 06 September 2016. Downloaded 7/8/2021 12:15:59 PM. procedure, featuring a shorter reaction time and no use of cation after nearly every step. hydrogen gas. Maho Kato was born in 1993 in Dr Kazushi Kinbara obtained a Chiba, Japan. She completed her Ph.D. in Organic Chemistry in undergraduate course from the 1996 under the direction of Tokyo Institute of Technology. Professor Kazuhiko Saigo from Now she continues her research the University of Tokyo and then as an MSc graduate student in began an academic career there. the Kinbara Group. In 2001, he moved to Professor Takuzo Aida’s group at the School of Engineering, University of Tokyo, as a lecturer and associate professor. In 2008, he was promoted to professor at the Maho Kato Kazushi Kinbara Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. In 2015, he moved to the Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology. This journal is © the Partner Organisations 2016 Org. Chem. Front.,2016,3,1524–1534 | 1525 View Article Online Method Organic Chemistry Frontiers Method description quality of compound 5. In fact, chromatographic purifications on silica would lower the total yield, increase significantly the reac- Design tion costs and required time, as well as potentially lead to the Our method consists of five main stages (Fig. 1). Initially, an acid-triggered undesired cleavage of trityl groups. In the excess of PEG4 is protected with the trityl group, leading to Procedure and characterization section we indicated that the PEG4 trityl ether (1) contaminated with a small amount of sym- intermediates were used without additional purification, but we metric bis-trityl ether. The by-product is not removed
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