9494
J. Org. Chem. 1996, 61, 9494-9502
Design, Synthesis, and Evaluation of Bile Acid-Based Molecular Tweezers Lawrence J. D’Souza and Uday Maitra* Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India Received April 24, 1996 (Revised Manuscript Received September 30, 1996X)
A family of bile acid-based molecular tweezers (7-9) has been constructed readily from simple precursors. Binding experiments with various electron deficient aromatic compounds showed that tweezer 8 binds trinitrofluorenone 10e with an association constant of 220 M-1 in CDCl3. Singlecrystal X-ray analysis of compound 8 shows aromatic-aromatic interactions producing a twodimensional lattice of pyrene units. Tweezer 8 was immobilized on Merrifield resin, and binding studies have shown that these data compare well with those of the solution state studies. Introduction The past decade has witnessed an explosive growth in research involving various aspects of supramolecular chemistry.1 A large number of molecular receptors (including chiral receptors)2-4 of varying sizes, shapes, and functionalities have been synthesized, and their interaction with guests has been assessed. The design of all such receptors is based on the fundamental molecular interactions exhibited by biological systems, particularly enzymes.5 Since the binding of a substrate by an enzyme is the first step in catalysis, the primary goal in molecular recognition research has been toward the development of selective synthetic receptors. Such studies have not only advanced our knowledge in understanding fundamental molecular interactions but have also led to the design of novel molecular devices, including sensors.6 There has also been considerable interest in designing molecular units which self-assemble in solution and in the solid state.7 Many organized solid state structures are expected to have interesting material X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) (a) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (b) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1991, 30, 1417. (c) Rebek, J., Jr. Angew. Chem., Int. Ed. Engl. 1990, 29, 245. (d) Frontiers in Supramolecular Organic Chemistry; Schneider, H.-J., Durr, H., Eds.; VCH: Weinheim, 1991 and references therein. (2) Wilcox, C. S. Chem. Soc. Rev. 1993, 383 and references therein. (3) (a) Maitra, U.; Bag, B. G. J. Org. Chem. 1994, 59, 6114. (b) Voyer, N.; Lamothe, J. Tetrahedron 1995, 34, 9241. (c) Kikuchi, Y.; Kobayashi, K.; Aoyama, Y. J. Am. Chem. Soc. 1992, 114, 1351. (d) Breslow, R. Science 1982, 218, 532. (e) Schneider, H.-J. Angew. Chem., Int. Ed. Engl. 1993, 32, 848. (f) Rebek, J., Jr.; Askew, B.; Nemeth, D.; Parris, K. J. Am. Chem. Soc. 1987, 109, 2432. (g) Garcia-Tellado, F.; Albert, J.; Hamilton, A. D. J. Chem. Soc., Chem. Commun. 1991, 1761. (4) For representative examples, see: (a) Davis, A. P. Chem. Soc. Rev. 1993, 243. (b) Bonar-Law, R. P.; Sanders, J. K. M. J. Am. Chem. Soc. 1995, 117, 259. (c) Bonar-Law, R. P.; Mackay, L. G.; Walter, C. J.; Marvaud, V.; Sanders, J. K. M. Pure Appl. Chem. 1994, 66, 803. (d) Maitra, U.; Balasubramanian, S. J. Chem. Soc., Perkin Trans. 1 1995, 83. (e) Bonar-Law, R. P.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1995, 3085. (5) (a) Schneider, H.-J. Angew. Chem., Int. Ed. Engl. 1991, 30, 1417. (b) Fersht, A. Enzyme Structure and Mechanism, 2nd ed.; Freeman and Co.: New York, 1985; p 293. (6) (a) Sandanayake, K. R. A. S.; Imazu, S.; James, T. D.; Mikami, M.; Shinkai, S. Chem. Lett. 1995, 139. (b) Zhao, S.; Luong, J. H. T. J. Chem. Soc., Chem. Commun. 1995, 663. (c) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (d) Sandanayake, K. R. A. S.; Shinkai, S. Angew. Chem., Int. Ed. Engl. 1994, 33, 2207. (e) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Sacchi, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 1975. (f) Vance, D. H.; Czarnik, A. W. J. Am. Chem. Soc. 1994, 116, 9397. (g) Chawla, H. M.; Srinivas, K. J. Chem. Soc., Chem. Commun. 1994, 2593. (h) Hamasaki, K.; Ikeda, H.; Nakamura, A.; Ueno, A.; Toda, F.; Suzuki, I.; Osa, T. J. Am. Chem. Soc. 1993, 115, 5035. (i) Teixidor, F.; Flores, M. A.; Escriche, L.; Vinas, C.; Casabo, J. J. Chem. Soc., Chem. Commun. 1994, 963. (j) Marsella, M. J.; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 12214.
S0022-3263(96)00752-9 CCC: $12.00
properties, which can possibly be tailor-made through proper and optimized molecular design. Although diverse types of molecular hosts have been designed during the past decade, one of the simplest concepts has been that of a molecular tweezer. First developed by Whitlock8 and Zimmerman,9 this class of molecules was shown to form sandwich complexes with aromatic guests through π-π interaction. Whitlock's tweezers were flexible, and apart from π-π interaction, hydrophobic interaction also played an important role in their tight binding to aromatic (bis-phenol)carboxylates in water. The tweezers constructed by Zimmerman, however, were more rigid and showed exceptionally high association constants with guests such as polynitroaromatics and 9-alkylated adenines in chloroform. Among the variety of molecular scaffolds which have been employed in supramolecular design, bile acids have recently attracted the attention of a number of research groups for their ready availability and unique structural features.4 The array of hydroxyl groups lining one surface of bile acids provides opportunities for the attachment of appropriate molecular units to generate structures with properties determined by the nature and arrangement of these units. Most of the interesting chemistry of the bile acids before the 1990s have come from their ability to form inclusion complexes and clathrates with organic guests.10 We have been interested in building novel structures with bile acids and decided to utilize the hydroxyl groups to design a series of hosts in which structural and/or electronic perturbations could be made easily. Although a variety of host structures could be envisaged, we decided to construct a series molecular tweezers as the first step.11 (7) Sessler, J. S. New J. Chem. 1991, 15, 153. (8) (a) Chen, C. W.; Whitlock, H. W. J. Am. Chem. Soc. 1978, 100, 4921. (b) Nedar, K. M.; Whitlock, H. W. J. Am. Chem. Soc. 1990, 112, 7269. (9) (a) Zimmerman, S. C.; Zeng, Z.; Wu, W.; Reichert, D. E. J. Am. Chem. Soc. 1991, 113, 183. (b) Zimmerman, S. C.; Wu, W.; Zeng, Z. J. Am. Chem. Soc. 1991, 113, 196. (c) For further references see: Zimmerman, S. C. Bioorganic Chemistry Frontiers; Springer-Verlag: New York, 1991; Vol. 2, p 33. (d) Blacker, A. J.; Jazwinski, J.; Lehn, J.-M. Helv. Chem. Acta 1987, 70, 1. (10) (a) Biro, R. P.; Chang, H. C.; Tang, T. P.; Shochet, N. R.; Lahav, M.; Leiserowitz, L. Pure Appl. Chem. 1980, 52, 2693. (b) Giglio, E. Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 2, p 207. (c) Mik, K.; Kasai, N.; Shibakami, M.; Takemoto, K.; Mujata, M. J. Chem. Soc., Chem. Commun. 1991, 1757. (d) Caira, M. R.; Nassimbeni, L. R.; Scott, J. L. J. Chem. Soc., Chem. Commun. 1993, 612. (11) Preliminary communication: Maitra, U.; D’Souza, L. J. J. Chem. Soc., Chem. Commun. 1994, 2793.
© 1996 American Chemical Society
Bile Acid-Based Molecular Tweezers
J. Org. Chem., Vol. 61, No. 26, 1996 9495
Figure 1. (a) Schematic representations of a bile acid-based molecular tweezer (b) O3-O12 distances in bile acids 1-3, from PCMODEL.
Objectives The construction of bile acid-based molecular tweezers, involving the linkage of the rigid bile acid backbone with rigid π-surfaces, presents a general strategy by which a variety of semirigid receptors can be synthesized easily. Since a small degree of flexibility is present in these tweezers, detailed studies on a family of such molecules can lead to a better understanding of the geometrical requirements of binding. In spite of the success of Zimmerman’s approach, variation of electron density of the aromatic arms required considerable synthetic effort. Our modular approach for the construction of semirigid systems, in which “sticky” arms are attached to a template through spacers by a simple reaction, is synthetically much straightforward. We initially planned to synthesize molecular tweezers based on π-π interactions.8,11,12 Bile acids have a rigid backbone: the C-3 and C-12 OH groups are ca. 6-6.5 Å apart, and they are approximately parallel. We reasoned that these would meet the geometric requirements for synthesizing bile acidbased tweezers which can be easily made by attaching flat π-surfaces such as pyrene units on the 3- and 12hydroxyl groups. We also felt that the 7-OH position can be used subsequently, after appropriate functionalization, as an additional handle to interact with a bound guest (Figure 1a). We have used both cholic and deoxycholic acids to synthesize a family of molecular tweezers, which are described in the following section. Results and Discussion Modeling Studies. Analysis of the PCMODEL13 -minimized structures of cholic (1), 7-deoxycholic (2), and 7-ketocholic (3) acids revealed that the hydroxyl groups at the 3- and 12-positions are ca. 5.9-6.2 Å apart (O-O distance) (Figure 1b). The two C-O vectors, however, (12) (a) Heinemann, U.; Saenger, W. Nature 1982, 229, 27. (b) Ibid. Pure Appl. Chem. 1985, 57, 417. (c) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag: New York, 1984; p 132. (d) Dahl, T. Acta Chem. Scand. 1994, 48, 95. (e) Stynes, D. V. Inorg. Chem. 1994, 33, 5022. (f) Burly, S. K.; Petsko, G. A. Adv. Protein Chem. 1988, 39, 125. (g) Burly, S. K.; Petsko, G. A. Science 1985, 229, 23. (h) Burly, S. K.; Petsko, G. A. J. Am. Chem. Soc. 1986, 108, 7995. (13) PCMODEL version 4.0 and PCMODEL for Windows v. 5.13 were purchased from Serena Software, Bloomington. In general the minimizations were first carried out without hydrogens. Subsequently hydrogen atoms were put, and the resulting structure was minimized. The entire operation was repeated several times with different initial geometries until consistent results were obtained.
are not exactly parallel; rather they diverge away from the steroid. This suggested that the attachment of two large and flat aromatic surfaces to these two hydroxyl groups will lead to the formation of a deep cleft, thereby generating a new class of molecular tweezers in which structural variations could be accomplished in a straightforward manner. Synthesis. 1-Acetylpyrene (5a) was prepared from pyrene (4) using Ac2O/AcOH and anhyd ZnCl2 in 83% yield.14 Compound 5a was oxidized with NaOCl in aq pyridine to acid 5b in 75% yield, which was converted to acid chloride 5c using either thionyl chloride or oxalyl chloride in benzene (Scheme 1). Methyl deoxycholate 2a was converted to 6 in 62% yield by careful esterification using Oppenauer conditions (CaH2, toluene, BnEt3N+Cl-, reflux) with pyrene-1-carbonyl chloride (5c) (Scheme 1). With the use of an excess of 5c, and at a higher temperature, compound 7 was obtained in 86% yield from methyl deoxycholate 2a. The C-7 hydroxyl group of 1 was selectively oxidized with NBS/aq NaHCO3 to the corresponding ketone,15 which upon esterification with methanolic HCl afforded methyl 7-ketocholate (3a) in 77% yield. Following the same esterification procedure as above, compound 8 was prepared from 3a in 86% yield. The C-7 hydroxy derivative 9 was prepared in 89% yield by the reduction of the C-7 keto group of 8 with NaBH4 in MeOH/THF (Scheme 2). A mixture of steroidal ester 6, along with 5d, was used as a control for all spectroscopic studies, including association constant measurements. Fluorescence Spectra. Pyrene and its derivatives show intense fluorescence emission even at very low concentrations.16 Our systems with two pyrenes placed at a distance of ca. 6-7 Å clearly are good substrates for studying their fluorescence properties. As the distance between the two pyrenes was close enough to sandwich a guest (which was apparent from InsightII calcula(14) Vollmann, H.; Becker, H.; Corell, M.; Streeck, H. Liebigs Ann. Chem. 1937, 1, 531, 108. (15) Fieser, L. F.; Rajagopalan, S. J. Am. Chem. Soc. 1950, 72, 5530; 1951, 73, 4133. (16) (a) Iyoda, T.; Morimoto, M.; Kawasaki, N.; Shimadzu, T. J. Chem. Soc., Chem. Commun. 1991, 1480. (b) Jin, T.; Ichikawa, K.; Koyama, T. J. Chem. Soc., Chem. Commun. 1992, 499. (c) Aoki, I.; Sakaki, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1992, 730. (d) Ueno, A.; Suzuki, I.; Osa, T. J. Chem. Soc., Chem. Commun. 1988, 1373. (e) Sandanayake, K. R. A. S.; James, T. D.; Shinkai, S. Chem. Lett. 1995, 503. (f) Aoki, I.; Harada, T.; Sakaki, T.; Kawahara, Y.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1992, 1341.
9496 J. Org. Chem., Vol. 61, No. 26, 1996
D’Souza and Maitra Scheme 1
Scheme 2
tions),17 fluorescence experiments were expected to give additional information in support of the proximity of the pyrenes in our tweezers. A strong intramolecular excimer emission was observed for all the tweezers (1.04 µM in 3% CHCl3/CH3CN), whereas, with the control run using 6 and 5d each at 1.04 µM, only monomer emission was observed (Figure 2). This variation in emission behavior results in an important noticeable difference: Hosts 7-9 in CHCl3 appear greenish, while a mixture of 6 and 5d (control) at identical concentrations appears bluish when held in sunlight. These experiments suggested that the long-wave emission resulted from the (17) InsightII minimization calculations showed that all three tweezers have a distance of separation of 5-6 Å (data not shown).
interaction of the pyrene units from the same steroid unit and not through intermolecular interaction. Fluorescence Spectra in the Presence of a Guest. Synergistic interaction between the two pyrenes in the hosts was further evident from fluorescence-quenching experiments. Figure 3 shows the fluorescence spectrum of host 7 (0.67 mM) with octafluoronaphthalene (10b; 33 mM) in CHCl3. In this case the guest fluorescence was completely quenched in the presence of the host, and there was an increase in the monomer emission; however, there was no significant change in the intensity of excimer emission. This provides evidence that the guest sandwiches between the two pyrenes thereby increasing the monomer emission. Figure 3 also shows the fluores-
Bile Acid-Based Molecular Tweezers
Figure 2. Fluorescence spectra (λex ) 355 nm) in 3% chloroform-acetonitrile: (a) 7, (b) 8, (c) 9, and (d) 6, each at 1.04 µmol dm-3, and (e) 5d plus 6, each at 1.04 µmol dm-3.
Figure 3. Fluorescence spectra (λex ) 355 nm) in chloroform: (a) 7 at 0.67 mmol dm-3, (b) 7 at 0.67 mmol dm-3 plus 10b at 33 mmol dm-3, (c) 10b at 33 mmol dm-3, (d) 5d plus 6, each at 0.67 mmol dm-3, and (e) 5d plus 6, each at 0.67 mmol dm-3 plus 10b at 33 mmol dm-3.
cence spectra for the controls (each at identical concentration as 7) in the presence of guest 10b. Although the fluorescence of the guest was quenched, there was no significant increase in the monomer emission. This clearly shows that the controls act as independent entities, and the quenching observed is due to the stacking of donor-acceptor groups which is a predominant interaction in organic solvents. Absorption Spectra. UV-visible spectroscopic studies on these compounds indicated that the two pyrene
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moieties of compounds 7-9 did not interact in the ground state, since the spectral pattern of these compounds roughly matched the absorption of the control (i.e., a mixture of 6 and 5d). When a CHCl3 solution of any of the hosts was mixed with trinitrofluorenone 10e, trinitrobenzene 10d, and 3,5-dinitrobenzonitrile (10c), deep red, red, and yellowish-red solutions were obtained, respectively. Tweezer 8 in the presence of trinitrofluorenone 10e as the guest showed a well-defined chargetransfer band, as observed in the difference spectrum.18 These observations support the existence of donoracceptor interactions in our systems. NMR Titration Experiments. Encouraged by the modeling work and fluorescence/UV experiments, we proceeded to use NMR spectroscopy as the tool to assess the binding strengths of our tweezers with different guests in CDCl3. Before detailed NMR titration studies were initiated with 7-9, an important control experiment was done. A mixture of 5d and 6 (each at 0.031 M in CDCl3) and 1,3,5-trinitrobenzene (10d; 0.06 M) was prepared in CDCl3, and the NMR spectrum was recorded. This spectrum, when compared with that of a mixture of 5d and 6, showed that under these conditions there were no significant upfield shifts of the aromatic signals, suggesting the lack of any interaction under these conditions. Interestingly, NMR titration experiments with trinitrofluorenone 10e with the controls under conditions as stated above could not be carried out because of the poor solubility of the former in CDCl3. But this 'problem' was not encountered with tweezers 7-9 since all the guest (up to a maximum of 1.5 equiv) went into solution. This observation confirms that the hosts indeed increased the solubility of the guest (upon binding). All NMR titration experiments were performed in CDCl3 at 25 °C. Guests were usually titrated up to 70% saturation limits. Almost all aromatic 1H NMR resonances of the hosts moved upfield (by varying degrees), but the aromatic region between δ 7 and 8.3 was quite complicated for analysis. However, one of the pyrene doublets (δ 8.91 in host 8) moved considerably upfield and was well-separated from all other resonances, and this signal was monitored during the titration. The chemical shifts (∆δ) were analyzed using a nonlinear curve-fitting program.19 All experiments were repeated at least twice. When an NMR titration experiment was carried on methyl pyrene-1-carboxylate (5d) with guest 10c, the estimated association constant was
