Improved Efficacy as a Photosensitizer

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tosylation and sulfonylation reactions have been probably the most extensively studied in that regard. Selective alkylations or silylations have also been reported ...
DOI: 10.1002/chem.201700782

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& Aggregation

Secondary-Rim g-Cyclodextrin Functionalization to Conjugate with C60 : Improved Efficacy as a Photosensitizer Xiaolei Zhu,[a] Annamaria Quaranta,[c] Ren8 V. Bensasson,[d] Matthieu Sollogoub,*[a] and Yongmin Zhang*[a, b] Abstract: DIBAL-H-mediated demethylation provides a novel method to access secondary-rim functionalized gcyclodextrin. 2A,3B-Dihydroxyl-per-O-methylated-g-cyclodextrin has been obtained, whose conjugation with C60 allows access to the most water-soluble C60 conjugate described so far. The water solubility of 0.12 m (550 mg mL@1) is 150 times higher than that of the native g-CD/C60 complex. Its singlet oxygen (1O2) quantum yield is 0.39, an increase of one to two orders of magnitude compared to that of a(b)CD–C60 conjugates.

Water-soluble C60 derivatives serve as radical scavengers, reactive oxygen species (ROS) producers, O2 uptake inhibitors, HIV inhibitors, and DNA or drug vectors.[1–5] However, inherent aggregation of such C60 conjugates in polar solvents is a bottleneck in their therapeutic developments, as it prevents C60 from interacting molecularly with the target and decreases the photoproduction efficacy of ROS.[6, 7a] To enhance the photosensitivity of C60, It is necessary to prevent C60 aggregation and enhance its water solubility. Ikeda et al. adopted liposome to solubilize and locate C60 derivatives in the surface of liposome to improve the photodynamic activity.[7b] Cationic porphyrin not only enhanced the solubility of C60, but also strengthened the affinity to DNA.[7c] s-Triazine scaffolds were attached to ameliorate the hydrophilicity of C60 and bring C60 to the DNA

[a] X. Zhu, Prof. M. Sollogoub, Dr. Y. Zhang Sorbonne Universit8s, UPMC Univ Paris 06, CNRS Insititut Parisien de Chimie Mol8culaire (UMR 8232) 4 Place Jussieu, 75005 Paris (France) E-mail: [email protected] [email protected] [b] Dr. Y. Zhang Jianghan University, Institute for Interdisciplinary Research 430056 Wuhan (P. R. China) [c] Dr. A. Quaranta CEA Saclay Service de Bio8nerg8tique Biologie Structurale et M8canismes 91191 Gif-sur-Yvette (France) [d] Prof. R. V. Bensasson Mus8um National d’Histoire Naturelle CNRS, Mol8cules de Communication et Adaptation des Microorganismes (UMR 7245) 63 Rue Buffon, 75005 Paris (France) Supporting information for this article can be found under: https://doi.org/10.1002/chem.201700782. Chem. Eur. J. 2017, 23, 9462 – 9466

minor groove.[7d] Furthermore, sugar-, protein-, and polymerbased C60 derivatives were applied as well.[ 7e–h, k] Torus-shaped cyclodextrins (CDs) are water-soluble concave molecules possessing a hydrophobic cavity. They are able to form inclusion complexes with small hydrophobic molecules in water. They have been used as drug-carriers, precisely because they allow drug solubilization. CDs thus are able to solubilize C60 and improve the photosensitivity.[7i–k] Some of us have synthesized a family of CD-C60 conjugates 1–9 (see Figure 1),[8–12] but in all cases aggregation was observed. From these studies, it appears that b-CD is a better solubilizing agent than a-CD (4 vs. 8), that two CDs are better than one (4 vs. 7), and that secondary rim attachment is better than primary rim connection (4 vs. 1). Only one g-CD conjugate has been synthesized with a primary rim attachment to C60. It displays better water solubilizing effect than comparable b-CD (9 vs. 2), but slightly inferior to the b-CD derivative attached to C60 by its secondary rim (9 vs. 3). This is linked to precedents in the literature showing that g-CD forms a more stable inclusion complex with C60 compared to a- and b-CDs.[13] This is related to the difference of internal diameters of these concave molecules: approximately 4.2 and 8.8 a for the primary and secondary rim of the a-CD, 5.6 and 10.8 a for b-CD, 6.8 and 12.0 a for g-CD, the cavity depth being 7.8 a for the three CDs, whereas the diameter of the spherical C60 is 10 a.[14] It was therefore obvious to envisage the synthesis of a g-CD-C60 conjugate with secondary rim attachment. However, the main challenge in the synthesis of such a species is the access to regioselectively functionalized g-CD on its secondary rim. Regioselective functionalization of CDs is a well-known challenge in this area of chemistry to widen the scope of applications of these molecules. Functionalization of the primary rim has been the most studied. Because the primary hydroxyls are the most nucleophilic, they are easy to distinguish from the secondary ones. Many strategies have been implemented to mono-functionalize the primary rim. Among various strategies, tosylation and sulfonylation reactions have been probably the most extensively studied in that regard. Selective alkylations or silylations have also been reported.[15, 16] We have delineated a blueprint strategy consisting of deprotecting per-O-benzylated CDs using diisobutylaluminium hydride (DIBAL-H), which proves to be very efficient because it allows access to CDs with one, two, and up to six different functions on their primary rim.[17–19] These strategies have been applied to all three a-, b-, and g-CDs, but the regioselectivities for g-CDs can be low because of their larger size.[19] In comparison with 6-posi-

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Figure 1. Water solubilities of CD-C60 conjugates.

tion, the regioselective functionalization on the secondary rim is more difficult. However, the strategies stay the same and the regioselective sulfonylation remains the main method,[20–22, 30] especially for g-CD.[23, 24] Nevertheless, because it is secondary hydroxyl groups that are sulfonylated, a major difference with the sulfonylation of primary hydroxyls lies in the stereochemical outcome of the following substitution reaction. Indeed, the sulfonyl group is displaced by the neighbouring 3-OH to form a manno-2,3-epoxy, whose opening under the action of a nucleophile results in the epimerization at both C2 and C3 of the g-CD, hence changing both structure and complexation properties. In fact, only one report of alkylation of the secondary rim of g-CD can be found in the literature. In this paper, Jindrˇich et al. reported that g-CD was alkylated by allyl or propargyl bromide to afford the mixture of 2I-O-allyl (propargyl)-gCD and 3I-O-allyl (propargyl)-g-CD. After peracetylatyion, it is possible to separate the mixture of regioisomers to give an isolated yield of 19 %.[25] There is a need for regioselective and efficient secondary rim functionalization of g-CD. Some of us have reported the DIBAL-H deprotection strategy on per-O-methylated a- and b-CDs. The regioselective deprotection occurred on the secondary rim due to the less hindering character of methyl groups.[10, 21, 22] Therefore, we embarked in the implementation of this methodology on per-O-methylated g-CDs. We previously showed that reaction conditions had to be carefully optimized. For instance, 9 equiv of a 0.2 m DIBAL-H solution in toluene at 0 8C for 18 h removed two methyl groups of per-O-methylated b-CD to afford 2A,3B-b-CD-diol in 56 % yield and 6A-b-CD-monol was barely produced.[10, 22] Nevertheless, when these reaction conditions were applied to perO-methylated g-CD, they only led to the formation of 6A-g-CDmonol 12. The reaction conditions, including DIBAL-H concentration, temperature, and reaction time, were then optimized. Chem. Eur. J. 2017, 23, 9462 – 9466

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Hence, per-O-methylated g-CD was treated with 79 equiv of a 1.2 m DIBAL-H solution in toluene at 25 8C for 1 h to afford 2A,3B-g-CD-diol 10 as the major product in 32 % yield. Formation of triols and tetrols increased significantly if the reaction time was prolonged. Treatment of per-O-methylated g-CD with the same concentration of DIBAL-H for 4 h at @10 8C gave 6Ag-CD-monol 12 as the major product in 31 % yield. We have therefore delineated a new strategy for the regioselective functionalization of g-CD either on its primary or secondary rim using DIBAL-H (see Scheme 1). The detailed structures of 2A,3B-g-CD-diol 10 and 6A-g-CDmonol 12 were further elucidated by NMR analysis. The 1 H NMR spectrum of 2A,3B-g-CD-diol 10 displayed a strongly deshielded triplet at d 4.01 ppm, which was assigned to H3B. In addition, H2A was found to be at d 3.58 ppm, which was more deshielded than other H2 around d 3.20 ppm. The 13C NMR spectrum displayed two signals at d 72.01 and 74.06 ppm, which were C3B and C2A. Meanwhile, both H6a and H6b (d 3.77 and 3.90 ppm) were deshielded in the 1H NMR spectrum of 6Ag-CD-monol 12. It was then possible to synthesize both 2A-g-CD-C60 conjugate 11 and 6A-g-CD-C60 conjugate 13 using the same strategy. Selective deprotonation of the more acidic OH at position 2 of diol 10 with NaH allowed its preferential alkylation with 2-(2azidoethoxy)ethyl 4-methylbenzenesulfonate to provide 14 in 49 % yield. Further methylation on position 3 gave 15, which, after mild reduction in the presence of 1,3-propanedithiol and trimethylamine, produced the aminoalkyl g-CD derivative 16 quantitatively.[11] Coupling of 16 with diacide 21 gave a CD dimer 17. The Hirsch–Bingel reaction was then applied to link C60 and the CD dimer to yield a single 2A-g-CD-C60 conjugate 11. C. Bingel confirmed that the bonds between two hexagons attached with malonate by 1H NMR and 13C NMR. The new bonds were formed which were considered as cyclopropane

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Scheme 1. Regioselectvie deprotection of per-O-methylated-g-CD.

Scheme 2. Preparation of 2A,3B-dihydroxyl-per-O-methylated-g-cyclodextrin (2A,3B-g-CD-diol, 10), 6-hydroxyl-per-O-methylated-g-CD (6A-g-CD-monol, 12), 2A-gCD-C60 conjugate 11, and 6A-g-CD-C60 conjugate 13.

structure.[32] The regioselectivity on C60 induced the least damage to its p-extended system (see Scheme 2). The same methodology was applied to afford 6A-g-CD-C60 conjugate 13. The alkylation on 6A-g-CD-monol 12 at 80 8C gave the azido derivative 18, which produced aminoalkyl g-CD derivative 19 after mild reduction. Dimer 20 was afforded through N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC HCl) condensation. Then, C60 was linked to dimer 20 to get 6A-g-CD-C60 conjugate 13 (see Scheme 2). Chem. Eur. J. 2017, 23, 9462 – 9466

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Having CD-C60 conjugates 11 and 13 in hand we embarked in the study of their water solubility. Conjugate 11 (27.5 mg) and conjugate 13 (3.5 mg) were found to be soluble in 50 mL water, so the solubility of conjugates 11 and 13 was estimated to be 550 mg mL@1 (0.12 m) and 70 mg mL@1 (0.015 m), respectively. The brown aqueous solutions of conjugates 11 and 13 were viscous. The absence of UV/VIS absorption at & 430 nm was previously attributed to aggregation of CD-C60 conjugates in dichloromethane (DCM) or in water.[10, 11, 26] We measured UV/

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Communication VIS absorption of 11 and 13 in DCM and water. The spectrum showed: (a) sharp peaks in the 210, 260, and 330 nm regions, which are characteristic of the C60 molecule absorption spectrum in aqueous and organic solvents; (b) peaks at & 430 nm both in DCM and water, which indicate the absence of (or minor) aggregation; (c) broad peak at & 500 nm in DCM, which is more definite than that in water. These broad peaks are noticeable in UV/VIS absorption spectra of CD-C60 conjugate in DCM and pristine C60 in n-hexane and toluene without aggregation (see Figure 2 and Figure 3).[9, 27] Thus, we assumed that conjugates 11 and 13 did not form aggregates in DCM nor in water.

spectra were recorded in aqueous solution by nanosecond laser flash photolysis. The absorption profiles of both CD-C60 conjugates exhibited the absorption band maximum at & 710 nm (see Figure 4). These features have been previously observed for different CD-C60 conjugates as well as other methanofullerene monoadducts.[9, 26, 28] The excited triplet state lifetime was & 50 ms for both conjugates 11 and 13.

Figure 4. Transient triplet-minus-singlet ground-state difference absorption spectra for 2A-g-CD-C60 conjugate 11 and 6-g-CD-C60 conjugate 13 observed by laser flash photolysis in argon-purged optically matched (ground-state absorbance A = 0.28) aqueous solutions at 1 ms after 355 nm laser pulse. Laser energy: 7 mJ. Figure 2. UV/VIS spectrum of 2A-g-CD-C60 conjugate 11 in DCM (c = 3.68 V 10@6 m) and water (c = 5.56 V 10@6 m).

Figure 3. UV/VIS spectrum of 6-g-CD-C60 conjugate 13 in DCM (c = 3.45 V 10@6 m) and water (c = 5.56 V 10@6 m).

Nevertheless, 1H NMR peaks of 11 and 13 were much broader in D2O than in CDCl3, which suggested some aggregate formation. 1H NMR-based dilution experiments were performed, but sharpening of signals could not be obtained (see the Supporting Information). We therefore conducted dynamic light scattering (DLS) studies. It showed aggregate formation, even at high dilution (114 mm). The average diameters of 11 and 13 were approximately the same, which were 155.1 nm (PdI = 0.240) and 155.6 nm (PdI = 0.233), respectively (see the Supporting Information).[29] To further characterize CD-C60 conjugates, transient triplet-minus-singlet ground-state absorption Chem. Eur. J. 2017, 23, 9462 – 9466

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Interestingly, the differential absorbance observed for 6A-gCD-C60 conjugate 13 is half the absorbance observed for 2A-gCD-C60 conjugate 11 (see Figure 4). This result suggests a lower quantum yield of triplet formation and is consistent with a higher singlet oxygen quantum yield, FD & 0.39 for 2A-g-CDC60 conjugate 11 and & 0.27 for 6A-g-CD-C60 conjugate 13 (see the Supporting Information). The previously studied g-CD-C60 conjugate 9 linked through the primary external rim showed a lower quantum yield for singlet oxygen generation & 0.12 in water.[9] Furthermore, FD of 2A-g-CD-C60 conjugate 11 was 43 times higher than that of highly water-soluble b-CD-C60 conjugate 4 (FD = & 0.009). Hence, compared to the reported conjugates 1–9, compounds 11 and 13 displayed higher singlet oxygen generation. In conclusion, we have delineated a novel and efficient access to secondary rim functionalization of g-CDs. In addition, DIBAL-H demethylation gives an access to the primary rim modification of g-CDs as well. The novel CD-C60 conjugates 11 and 13 have been synthesized. Compared to the reported CDC60 conjugates and g-CD/C60 complexes, 2A-g-CD-C60 conjugate 11 is the most water-soluble cyclodextrin-C60 conjugate reported so far. Indeed, it is eight times more water-soluble than 6Ag-CD-C60 conjugate 13, 150 times more water-soluble than native g-CD/C60 complex (0.12 m vs. 0.8 mm), and is also more water-soluble than pyridyl-g-CD/C60 complex (72.7 mm).[31] 2A-gCD-C60 conjugate 11 displayed the highest 1O2 quantum yield (0.39) among CD-C60 conjugates and thus has the greatest potential as a candidate photosensitizer for biological systems.

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Communication Acknowledgements This work was supported by the French Infrastructure for Integrated Structural Biology (FRISBI) ANR-10-INSB-05-01. We sincerely thank the China Scholarship Council (CSC) for a Ph.D. fellowship to X.Z. Financial support from the Centre National de la Recherche Scientifique (CNRS) and the Sorbonne Universit8s, UPMC are gratefully acknowledged.

Conflict of interest The authors declare no conflict of interest. Keywords: aggregation · C60 · gamma-CD · photosensitizer · secondary rim [1] L. Y. Chiang, R. B. Upasani, J. W. Swirczewski, S. Soled, J. Am. Chem. Soc. 1993, 115, 5453 – 5457. [2] J. W. Arbogast, A. P. Darmanyan, C. S. Foote, Y. Rubin, F. N. Diederich, M. M. Alvarez, S. J. Anz, R. L. Whetten, J. Phys. Chem. 1991, 95, 11 – 12. [3] T. Mashino, N. Usui, K. Okuda, T. Hirota, M. Mochizuki, Bioorg. Med. Chem. 2003, 11, 1433 – 1438. [4] S. H. Friedman, D. L. Decamp, R. P. Sijbesma, G. Srdanov, F. Wudl, G. L. Kenyon, J. Am. Chem. Soc. 1993, 115, 6506 – 6509. [5] a) M. B. Patel, U. Harikrishnan, N. N. Valand, N. R. Modi, S. K. Menon, Arch. Pharm. 2013, 346, 210 – 220; b) S. Takenaka, K. Yamashita, M. Takagi, T. Hatta, O. Tsuge, Nucleic Acids Symp. Ser. (Oxf) 1999, 42, 149 – 150. [6] a) A. W. Jensen, S. R. Wilson, D. I. Schuster, Bioorg. Med. Chem. 1996, 4, 767 – 779; M. Carini, L. Wordevic, T. D. Ros, Fullerenes in Biology and Medicine. Handbook of Carbon Nano Materials, in Medicinal and Bio-related Applications, Vol. 3, World Scientific Publishing Co. Pte. Ltd, Singapore, 2012, pp. 31 – 32. [7] a) X. Zhu, M. Sollogoub, Y. Zhang, Eur. J. Med. Chem. 2016, 115, 438 – 452; b) A. Ikeda, T. Mae, M. Ueda, K. Sugikawa, H. Shigeto, H. Funabashi, A. Kuroda, M. Akiyama, Chem. Commun. 2017, 53, 2966 – 2969; c) C. Zhou, Q. Liu, W. Xu, C. Wang, X. Fang, Chem. Commun 2011, 47, 2982 – 2984; d) M. B. Patel, U. Harikrishnan, N. N. Valand, D. S. Mehta, K. V. Joshi, S. P. Kumar, K. H. Chikhalia, L. B. George, Y. T. Jasrai, S. K. Menon, RSC Adv. 2013, 3, 8734 – 8746; e) S. Tanimoto, S. Sakai, E. Kudo, S. Okada, S. Matsumura, D. Takahashi, K. Toshima, Chem. Asian J. 2012, 7, 911 – 914; f) T. Komatsu, A. Nakagawa, X. Qu, Drug Metab. Pharmacokinet. 2009, 24, 287 – 299; g) S. Oriana, S. Aroua, J. O. Sçllner, X. J. Ma, Y. Iwamoto, Y. Yamakoshi, Chem. Commun. 2013, 49, 9302 – 9304; h) J. Shi, X. Yu, L. Wang, Y. Liu, J. Gao, J. Zhang, R. Ma, R. Liu, Z. Zhang, Biomaterials 2013, 34, 9666 – 9677; i) J. J. Wang, Z. H. Zhang, W. Wu, X. Q. Jiang, Chin. J. Chem. 2014, 32, 78 – 84; j) A. Ikeda, T. Iizuka, N. Maekubo, R. Aono, J. Kikuchi, M. Akiyama, T. Konishi, T. Ogawa, N. Ishida-Kitagawa, H. Tatebe, K. Shiozaki, ACS Med. Chem. Lett. 2013, 4, 752 – 756; k) Y. Chen, D. Zhao, Y. Liu, Chem. Commun. 2015, 51, 12266 – 12269. [8] S. Filippone, A. Rassat, C. R. Chimie 2003, 6, 83 – 86.

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[9] A. Quaranta, Y. Zhang, S. Filippone, J. Yang, P. Sinay¨, A. Rassat, R. Edge, S. Navaratnam, D. J. McGarvey, E. J. Land, M. Brettreich, A. Hirsch, R. V. Bensasson, Chem. Phys. 2006, 325, 397 – 403. [10] a) J. Yang, Y. Wang, A. Rassat, Y. Zhang, P. Sinay¨, Tetrahedron 2004, 60, 12163 – 12168; b) S. Xiao, Q. Wang, F. Yu, Y. Peng, M. Yang, M. Sollogoub, P. Sinay¨, Y. Zhang, L. Zhang, D. Zhou, Bioorg. Med. Chem. 2012, 20, 5616 – 5622. [11] S. Filippone, F. Heimann, A. Rassat, Chem. Commun. 2002, 1508 – 1509. [12] a) Z. Guan, Y. Wang, Y. Chen, L. Zhang, Y. Zhang, Tetrahedron 2009, 65, 1125 – 1129; b) Y. Chen, Y. Wang, A. Rassat, P. Sinay¨, Y. Zhao, Y. Zhang, Tetrahedron 2006, 62, 2045 – 2049; c) L. Posp&sˇil, M. Hromadov#, M. G#l, J. Bul&cˇkov#, R. Sokolov#, S. Filippone, J. Yang, Z. Guan, A. Rassat, Y. Zhang, Carbon 2010, 48, 153 – 162. [13] T. Andersson, G. Westman, O. Wennerstrçm, M. Sundahl, J. Chem. Soc. Perkin Trans. 2 1994, 1097 – 1101. [14] V. Ramamurthy, Tetrahedron 1986, 42, 5753 – 5839. [15] a) D. Armspach, D. Matt, Carbohydr. Res. 1998, 310, 129 – 133; b) D. Armspach, L. Poorters, D. Matt, B. Benmerad, F. Balegroune, L. Toupet, Org. Biomol. Chem. 2005, 3, 2588 – 2592. [16] a) A. R. Khan, P. Forgo, K. J. Stine, V. T. D’Souza, Chem. Rev. 1998, 98, 1977 – 1996; b) H. H. Baer, A. V. Berenguel, Y. Y. Shu, Carbohydr. Res. 1992, 228, 307 – 314. [17] T. Lecourt, A. Herault, A. J. Pearce, M. Sollogoub, P. Sinay¨, Chem. Eur. J. 2004, 10, 2960 – 2971. [18] B. Wang, E. Zaborova, S. Guieu, M. Petrillo, M. Guitet, Y. Bl8riot, M. M8nand, Y. Zhang, M. Sollogoub, Nat. Commun. 2014, 5, 5354 – 5360. [19] S. Volkov, L. Kumprecht, M. Budeˇsˇ&nsky´, M. Lepsˇ&k, M. Dusˇekb, T. Kraus, Org. Biomol. Chem. 2015, 13, 2980 – 2985. [20] F. Bellia, D. L. Mendola, C. Pedone, E. Rizzarelli, M. Saviano, G. Vecchio, Chem. Soc. Rev. 2009, 38, 2756 – 2781. [21] a) S. Xiao, M. Yang, P. Sinay¨, Y. Bl8riot, M. Sollogoub, Y. Zhang, Eur. J. Org. Chem. 2010, 1510 – 1516; b) S. Xiao, D. Zhou, M. Yang, P. Sinay¨, M. Sollogoub, Y. Zhang, Tetrahedron Lett. 2011, 52, 5273 – 5276. [22] B. du Roizel, J. P. Baltaze, P. Sinay¨, Tetrahedron Lett. 2002, 43, 2371 – 2373. [23] T. Murakami, K. Harata, S. Morimoto, Tetrahedron Lett. 1987, 28, 321 – 324. [24] K. Teranishi, S. Tanabe, M. Hisamatsu, T. Yamada, Biosci. Biotechnol. Biochem. 1998, 62, 1249 – 1252. [25] M. Bl#hov#, E. Bedn#rˇov#, M. Rˇezanka, J. Jindrˇich, J. Org. Chem. 2013, 78, 697 – 701. [26] R. V. Bensasson, E. Bienvenue, C. Fabre, J. M. Janot, E. J. Land, S. Leach, V. Leboulaire, A. Rassat, S. Roux, P. Seta, Chem. Eur. J. 1998, 4, 270 – 278. [27] R. V. Bensasson, E. Bienvenue, M. Dellinger, S. Leach, P. Seta, J. Phys. Chem. 1994, 98, 3492 – 3500. [28] A. Quaranta, D. J. McGarvey, E. J. Land, M. Brettreich, S. Burghardt, H. Schonberger, A. Hirsch, N. Gharbi, F. Moussa, S. Leach, H. Gçttinger, R. V. Bensasson, Phys. Chem. Chem. Phys. 2003, 5, 843 – 848. [29] K. Nobusawa, M. Akiyama, A. Ikeda, M. Naito, J. Mater. Chem. 2012, 22, 22610 – 22613. [30] D. Rong, V. T. D’Souza, Tetrahedron Lett. 1990, 31, 4275 – 4278. [31] K. Nobusawa, D. Payra, M. Naito, Chem. Commun. 2014, 50, 8339 – 8342. [32] C. Bingel, Chem. Ber. 1993, 126, 1957 – 1959. Manuscript received: February 17, 2017 Accepted manuscript online: May 26, 2017 Version of record online: June 29, 2017

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