Macrocyclic Sulfates as Versatile Building Blocks in the Synthesis of

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Macrocyclic Sulfates as Versatile Building Blocks in the Synthesis of Monodisperse Poly(ethylene glycol)s and Monofunctionalized Derivatives Yu Li,† Xiaolong Qiu,‡ and Zhong-Xing Jiang*,† †

Key Laboratory of Combinatorial Biosynthesis and Drug Discovery of the Ministry of Education, School of Pharmaceutical Sciences, Wuhan University, Wuhan 430071, China ‡ Wisdom Pharmaceutical Co., Ltd, 18 Qinghua Road, Haimen, Jiangsu 226123, China ABSTRACT: Even after decades of effort, the efficient synthesis of the structurally simple but highly valuable molecules monodisperse poly(ethylene glycol)s (M-PEGs) remains a long-standing challenge. In this contribution, we give a brief review of the macrocyclic-sulfate-based strategy developed in our lab for the synthesis of M-PEGs and their monofunctionalized derivatives with a focus on the synthetic efficacy and versatility.

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INTRODUCTION Poly(ethylene glycol)s (PEGs) are nontoxic, water-soluble, and biocompatible polymers with diverse applications. As the mostused polymer in biopharmaceuticals, PEGs are routinely employed to improve solubility and stability, reduce immunogenicity and dosing frequency, diminish reticuloendothelial system uptake, prolong blood circulation, and induce the enhanced permeability and retention (EPR) effect.1−4 As of 2012, 12 PEGylated drugs had been approved by the U.S. Food and Drug Administration. PEGylation has already become one of the most successful strategies in biopharmaceutical research and development. Regular PEGs are produced on industrial scales by anionic polymerization of ethylene oxide, and therefore, they are always complex mixtures of different-length oligomers. The heterogeneity of PEGs causes a range of problems in their biomedical applications, such as purification and characterization of PEG− drug conjugates, drug quality control and efficacy variation due to batch differences, and drug regulatory approval.5,6 To this end, monodisperse PEGs (M-PEGs) can completely avoid these issues associated with regular PEGs. Although M-PEGs are structurally simple molecules, their synthesis has remained a long-standing challenge for two reasons.7−17 First, long synthesis and tedious purification severely deteriorate the synthetic efficacy. To form an ether bond in M-PEGs, three synthetic steps are usually required: hydroxyl group protection/deprotection, activation, and Williamson ether synthesis. In each synthetic step, chromatographic purification is usually required. Second, monofunctionalization (or desymmetrization) M-PEGs is a difficult task because it requires large excess amounts of M-PEGs, expensive reagents, tedious manipulation of protecting and activating groups, and difficult separation of the mono-, di-, and nonfunctionalized MPEG mixture.7−19 For these reasons, no M-PEGs above 3000 Da have ever been synthesized, and the price of M-PEGs is unbelievably high, severely limiting the applications of M-PEGs in biopharmaceuticals. Therefore, it is of great importance to develop efficient synthetic strategies for M-PEGs. © XXXX American Chemical Society

M-PEGs have been extensively employed as solubility enhancers and hydrophilic linkers in our biomaterials research.20−23 However, the challenging synthesis of M-PEGs seriously slowed down our research, prompting us to develop efficient, versatile, and operationally simple strategies for the synthesis of M-PEGs on large scales. To avoid the tedious chromatographic purification of synthetic intermediates, a fluorous synthesis of M-PEGs was recently developed in our lab in which all of the intermediates were conveniently purified by a combination of fluorous and regular silica gel-based solidphase extraction (Scheme 1).15 Small-sized cyclic sulfates were used in our previous syntheses of fluorinated amino acids, where the sulfate group acted as both a protecting group and an activating group during the monofunctionalization of diols.24,25 High efficacy and selectivity were achieved in these syntheses. As over half of the synthetic steps in existing M-PEG syntheses were consumed by hydroxyl group protection, deprotection, and activation, we envisioned that these tedious manipulations could be avoided through macrocyclization of oligo(ethylene glycol)s (OEGs) into macrocyclic sulfates, which would dramatically improve the synthetic efficacy of M-PEGs.26

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MACROCYCLIZATION OF OLIGO(ETHYLENE GLYCOL)S Macrocyclization of commercially available tetra(ethylene glycol) (1c) through transesterification with diisopropyl sulfite27 was unsuccessful because of the slow reaction kinetics and low stability of the resulting macrocyclic sulfite 2c at elevated temperature. Then the reaction was carried out with more reactive SOCl2 in the presence of Et3N at 0 °C to give 2c in 51% yield. After systematic optimization, it was found that slow addition of SOCl2 (2.0 equiv) to a dilute solution of 1c (0.04 mol/L), DIPEA (4.8 equiv), and DMAP (0.05 equiv) in CH2Cl2 at 0 °C gave the best result. Monitoring the reaction with 1H NMR spectroscopy indicated that the macrocyclization Received: May 5, 2015

A

DOI: 10.1021/acs.oprd.5b00142 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Review

Scheme 1. Fluorous synthesis of M-PEGs

in decent yields. However, under the same reaction conditions, non-OEG diols gave much lower yields than equally sized OEGs (Table 2). The preferred conformation resulting from heteroatoms and the enhanced solubility in CH2Cl2 may account for the high yields of OEG macrocyclic sulfites. The X-ray crystal structure of 26-membered macrocyclic sulfate 3g shows that oxygen atoms may contribute to the twisted conformation of 3g resembling a partially folded Arabic number 8. OEGs are actually ideal substrates for this macrocyclization. Even eicos(ethylene glycol) was macrocyclized to give 62-membered macrocyclic sulfite 2l in 49% yield. In terms of synthetic chemistry, it is very challenging to form such a huge ring without a template.

proceeded rapidly, and the slow addition of SOCl2 dramatically improved the yield of 2c. Under the optimized conditions, 2-mer to 20-mer OEGs were conveniently macrocyclized to the corresponding macrocyclic sulfites (Table 1). It was found that as the size of the OEG increased, the macrocyclization reaction gradually slowed. Therefore, extended reaction times and elevated reaction temperatures for OEGs above the 7-mer were necessary to drive the reaction to completion. Further dilution of OEGs above 7-mer gave much better yields of the macrocyclic sulfites because the formation of polymers and catenane-like side products was effectively avoided. Oxidation of macrocyclic sulfites 2 with in situ-generated RuO4 afforded the corresponding macrocyclic sulfates 3 in good to excellent yields (Table 1). Macrocyclic sulfites 2 slowly decomposed, especially at elevated temperature, and their stabilities decreased dramatically with increasing macrocyclic sulfite ring size. Therefore, direct oxidation of crude 2 without chromatographic purification was also carried out, and macrocyclic sulfates 3 were obtained with comparable yields. It was found that macrocyclic sulfates 3 are much more stable than the corresponding macrocyclic sulfites 2. Macrocyclic sulfate 3c showed outstanding stability toward air and moisture, as no change was detected by 1H NMR spectroscopy after it had been placed in open air for 2 months. On the basis of its outstanding stability and easily available starting material 1c ($45/kg), macrocyclic sulfate 3c is a useful building block for M-PEG synthesis. Then 3c was prepared on multihundred-gram scales without purification of macrocyclic sulfite 2c (Scheme 2). During the large-scale preparation, 3c was conveniently purified by recrystallization instead of flash chromatography, and the reaction solvent was conveniently recovered and reused. The macrocyclization reaction was also expanded to non-OEG diols, and the corresponding macrocyclic sulfates were obtained

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MONOFUNCTIONALIZATION OF OLIGO(ETHYLENE GLYCOL)S The one-pot nucleophilic ring-opening reaction of macrocyclic sulfates 3 provides a straightforward method for monofunctionalization of OEGs. The difunctionalized side products can be completely avoided, which dramatically improves the product yield and simplifies the purification. Monofunctionalization of OEGs through macrocyclic sulfates omits the protecting/ activating group manipulations and requires only a single step, which considerably improves the synthetic efficacy. Through the one-pot nucleophilic ring-opening reaction of macrocyclic sulfates with a range of O-, S-, C-, N- and Fcontaining nucleophiles, a variety of monofunctionalized OEGs were conveniently prepared (Table 3). From readily available 3c and O-containing nucleophiles, many valuable monofunctionalized tetra(ethylene glycols) were prepared with high efficacy, including higher-order OEGs 4a−c, widely used synthetic intermediates 4d−I, phenol ethers 4j and 4k, nonionic surfactants 4l and 4m, the fluorescence imaging agent 4n, the 19 F MRI agent 4o, and clickable OEG 4p. It is noteworthy that B



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Table 1. Macrocyclization of oligo(ethylene glycol)s

Scheme 2. Large-scale preparation of macrocyclic sulfate 3c

Table 2. Macrocyclization of non-OEG diols

even KOtBu, which is a preferred base in M-PEGs synthesis, is able to perform nucleophilic attack on 3c as a result of the high reactivity of the macrocyclic sulfate. Mono-tert-butyl ether 4g is an important building block that is very difficult to prepare through other means.13,16 Sulfur can be efficiently introduced into OEGs through the use of S-containing nucleophiles. The reaction between 3c and AcSK was also carried out in water, and 3c was consumed in several minutes, illustrating the potential application of macrocyclic sulfates in aqueous PEGylation of cysteine-containing peptides or proteins. With the C-containing nucleophile diethyl malonate, a carbon−carbon single bond was constructed in 4s. Nitrogen can also be conveniently incorporated into OEGs through the use of either amines or azide. When a primary amine, BnNH2, was employed, a one-pot dual nucleophilic ring-opening reaction afforded tertiary amine

4t in high yield. The F-containing nucleophile NaF successfully attacked 3c to give monofluorinated OEG 4v in high yield. The ring-opening reaction was completed in just a few minutes at 120 °C, illustrating another potential application of macrocyclic sulfates in the radiochemical synthesis of 18F-labeled PEGs. These results indicate that macrocyclic sulfates of OEGs are versatile building blocks with high reactivity. The monofunctionalization of macrocyclic-sulfate-based OEGs, which successfully avoids the formation of difunctionalized side products and the need for manipulation of protecting/activating group(s), C



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Table 3. Monofunctionalization of OEGs through macrocyclic sulfates

a

After the ring-opening reaction, the residue was treated with TosH in MeOH. bDMF was used as the solvent. cThe reaction was run at 120 °C.

provides a highly efficient and alternative strategy for many important OEG derivatives.

Scheme 4. Synthesis of monomethoxy-functionalized 64-mer M-PEGsa

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SYNTHESIS OF MONODISPERSE POLY(ETHYLENE GLYCOL)S The macrocyclic-sulfate-based strategy showed good generality and high efficacy in the synthesis of M-PEGs and derivatives. Scheme 3. Synthesis of 36-mer M-PEGsa

a

Reaction conditions: (a) NaH, 3g, DMF, rt, then H2O, H2SO4, THF, rt; (b) NaH, 3g, THF, rt, then H2O, H2SO4, rt.

Scheme 5. Synthesis of heterofunctionalized 12-mer MPEGsa

a

Reaction conditions: (a) NaH, 3c, DMF, rt, then H2O, H2SO4, THF, rt.

First, convenient monofunctionalization of OEGs through macrocyclic sulfates provides an easy access to various non-, mono-, di-, and heterofunctionalized M-PEGs, as illustrated in Table 3. Second, the macrocyclic-sulfate-based strategy eliminates more than half of the synthetic steps by omitting the hydroxyl group protection and activation steps. In addition, highly flexible choice of 2- to 20-mer macrocyclic sulfates of OEGs enables the synthesis of M-PEGs with the minimal number of synthetic steps. Finally, the polarity switches during the reaction of macrocyclic sulfates [nonionic (starting materials) → ionic (stable intermediates) → nonionic (products)] may dramatically simplify the purification process.

a Reaction conditions: (a) NaH, 3c, THF, rt, then H2O, H2SO4, reflux; (b) TosCl, NaOH(aq), THF, 45 °C; (c) NaN3, DMF, 60 °C; (d) Ph3P, H2O, THF, 45 °C.

To illustrate the generality and efficacy of the macrocyclicsulfate-based strategy, 36-mer, monomethoxy-functionalized 64D



Review

Figure 1. MALDI-TOF-MS spectra of (a) M-PEG 8, (b) M-PEG 16, and (c) regular PEG1500.

M-PEGs, such as N3-, F-, TrtS-, Bn-, and tBu-substituted MPEGs. Finally, from monomethylated tetra(ethylene glycol) 4f, heterofunctionalized 12-mer M-PEGs were conveniently prepared in five steps in an overall yield of 73% (Scheme 5).28 Amine 21 was employed as a solubility enhancer and an 19F MRI and fluorescence dual imaging agent.28 M-PEGs synthesized through this macrocyclic sulfate strategy exhibit high monodispersity. Negligible impurities, derived from starting materials and depolymerization, can be detected from the MALDI-TOF-MS spectra of 12−19, which is a superior method for assessing the monodispersity of M-PEGs12−14,16 (Figure 1). On the basis of the MALDI-TOF-MS data, the polydispersity indexes (PDIs) of these M-PEGs were calculated as 1.00007 (36-mer 8) and 1.00003 (64-mer 16), which are far beyond the range of PDIs for regular PEGs.

mer, and heterofunctionalized 12-mer M-PEGs were chosen as the target molecules. First, from tetra(ethylene glycol) 1c and its macrocyclic sulfate 3c, a series of M-PEGs were sequentially prepared by an iterative one-pot dual nucleophilic ring-opening reaction of 3c (Scheme 3). Eventually, 36-mer 8 was prepared from 4-mer 1c through four cycles of the one-pot dual chain extension reaction in an overall yield of 51% on a multigram scale. During the one-pot dual chain extension reaction, two OEG fragments were introduced at a time, which considerably improved the efficacy of the M-PEG synthesis. The bis(sodium sulfate) intermediates were insoluble in THF, and therefore, DMF was used instead to promote the nucleophilic ring-opening reaction. Selective extraction of the bis(sodium sulfate) intermediates into the aqueous phase while leaving most of the impurities in the organic phase dramatically simplified the purification. The bis(sodium sulfate) intermediates hydrolyzed with H2SO4 and water in refluxing THF because of the low solubility of these intermediates in THF at room temperature. Second, from MeONa and macrocyclic sulfate 3g, a series of monomethoxy-functionalized M-PEGs were conveniently prepared. The 64-mer 16, which is the longest M-PEG reported to date, was prepared in eight steps in an overall yield of 15% (Scheme 4). It is noteworthy that monomethoxy-functionalized M-PEGs are the most used forms in PEGylation, and this strategy provides them with high efficacy. Besides monomethoxyfunctionalized M-PEGs, this macrocyclic-sulfate-based strategy can be easily expanded to various valuable monofunctionalized

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CONCLUSION M-PEGs are next-generation pharmaceutical ingredients that can effectively avoid the problems associated with regular PEGs. However, their tedious synthesis and high price dramatically limit the applications of M-PEGs. Substantial progress in the synthesis of M-PEGs has been made in recent years, but this field is far from mature. It is noteworthy that all high-molecular-weight M-PEGs of great biopharmaceutical importance are still beyond the reach of existing strategies. Strategies to simplify M-PEGs synthesis, such as efficient monofunctionalization of OEGs, E



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omission of protecting/activating group manipulations, convenient purification of the intermediates and products, etc., are particularly valuable in this field. Macrocyclic sulfates are versatile building blocks that considerably simplify the synthesis of MPEGs. The synthetic simplicity coupled with the versatility of this macrocyclic-sulfate-based strategy may pave the way for broader applications of M-PEGs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21372181). REFERENCES

(1) Harris, J. M.; Chess, R. B. Nat. Rev. Drug Discovery 2003, 2, 214. (2) Veronese, F. M.; Pasut, G. Drug Discovery Today 2005, 10, 1451. (3) Knop, K.; Hoogenboom, R.; Fischer, D.; Schubert, U. S. Angew. Chem., Int. Ed. 2010, 49, 6288. (4) Pelegri-O’Day, E. M.; Lin, E.-W.; Maynard, H. D. J. Am. Chem. Soc. 2014, 136, 14323. (5) Vicent, M. J.; Dieudonné, L.; Carbajo, R. J.; Pineda-Lucena, A. Expert Opin. Drug Delivery 2008, 5, 593. (6) Gaberc-Porekar, V.; Zore, I.; Podobnik, B.; Menart, V. Curr. Opin. Drug Discovery Dev. 2008, 11, 242. (7) Fordyce, R.; Lovell, E. L.; Hibbert, H. J. Am. Chem. Soc. 1939, 61, 1905. (8) Keegstra, E. M. D.; Zwikker, J. W.; Roest, M. R.; Jenneskens, L. W. J. Org. Chem. 1992, 57, 6678. (9) Harada, A.; Li, J.; Kamachi, M. J. Am. Chem. Soc. 1994, 116, 3192. (10) Loiseau, F. A.; Hii, K. K.; Hill, A. M. J. Org. Chem. 2004, 69, 639. (11) Ahmed, S. A.; Tanaka, M. J. Org. Chem. 2006, 71, 9884. (12) Niculesco-Duvaz, D.; Getaz, J.; Springer, C. J. Bioconjugate Chem. 2008, 19, 973. (13) French, A. C.; Thompson, A. L.; Davis, B. G. Angew. Chem., Int. Ed. 2009, 48, 1248. (14) Székely, G.; Schaepertoens, M.; Gaffney, P. R. J.; Livingston, A. G. Polym. Chem. 2014, 5, 694. (15) Li, Y.; Guo, Q.; Li, X.; Zhang, H.; Yu, F.; Yu, W.; Xia, G.; Fu, M.; Yang, Z.; Jiang, Z.-X. Tetrahedron Lett. 2014, 55, 2110. (16) Maranski, K.; Andreev, Y. G.; Bruce, P. G. Angew. Chem., Int. Ed. 2014, 53, 6411. (17) Székely, G.; Schaepertoens, M.; Gaffney, P. R. J.; Livingston, A. G. Chem.Eur. J. 2014, 20, 10038. (18) Gibson, T. J. Org. Chem. 1980, 45, 1095. (19) Reed, N. N.; Janda, K. D. J. Org. Chem. 2000, 65, 5843. (20) Jiang, Z.-X.; Liu, X.; Jeong, E.-K.; Yu, Y. B. Angew. Chem., Int. Ed. 2009, 48, 4755. (21) Jiang, Z.-X.; Feng, Y.; Yu, Y. B. Chem. Commun. 2011, 47, 7233. (22) Li, Y.; Thapa, B.; Zhang, H.; Li, X.; Yu, F.; Jeong, E.-K.; Yang, Z.; Jiang, Z.-X. Tetrahedron 2013, 69, 9586. (23) Yu, W.; Yang, Y.; Bo, S.; Li, Y.; Chen, S.; Yang, Z.; Zheng, X.; Jiang, Z.-X.; Zhou, X. J. Org. Chem. 2015, 80, 4443. (24) Jiang, Z.-X.; Qin, Y.-Y.; Qing, F.-L. J. Org. Chem. 2003, 68, 7544. (25) Jiang, Z.-X.; Yu, Y. B. J. Org. Chem. 2007, 72, 1464. (26) Zhang, H.; Li, X.; Shi, Q.; Li, Y.; Xia, G.; Chen, L.; Yang, Z.; Jiang, Z.-X. Angew. Chem., Int. Ed. 2015, 54, 3763. (27) King, S. A.; Pipik, B.; Conlon, D. A.; Bhupathy, M. Synth. Commun. 1997, 27, 701. (28) Bo, S.; Song, C.; Li, Y.; Yu, W.; Chen, S.; Zhou, X.; Yang, Z.; Zheng, X.; Jiang, Z.-X. J. Org. Chem. 2015, 80, 6360.

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Macrocyclic Sulfates as Versatile Building Blocks in the Synthesis of

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