Langmuir 2005, 21, 10311-10315
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In-Plane Enyne Metathesis and Subsequent Diels-Alder Reactions on Self-Assembled Monolayers Jungkyu K. Lee,† Young Shik Chi,† Joon Sung Lee,† Yang-Gyun Kim,‡ Young Hwan Jung,† Eugene Oh,§ Sung-Bo Ko,§ Hyuk-jun Jung,§ Pill-Seong Kang,§ and Insung S. Choi*,† Department of Chemistry and School of Molecular Science (BK21), KAIST, Daejeon 305-701, Korea, Department of Biochemistry, College of Medicine, Chung-Ang University, 221 Heuksuk-dong, Dongjak-gu, Seoul 156-756, Korea, and COS Biotech, Inc., Daejeon Bio Venture Town 461-58, Jeonmin-dong, Yuseong-gu, Daejeon 305-811, Korea Received June 22, 2005. In Final Form: September 4, 2005 We report in-plane enyne metathesis and subsequent Diels-Alder reactions on self-assembled monolayers (SAMs) terminating in vinyl and acetylenyl groups on gold. After the formation of SAMs of vinyl and acetylenyl group-containing dithiols on gold, in-plane enyne metathesis of the vinyl and acetylenyl groups, leading to the formation of 1,3-diene, was achieved on the SAMs, and Diels-Alder reactions were then successfully performed with tetracyanoethylene, maleic anhydride, and maleimide. The reactions were confirmed by FT-IR spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary-ion mass spectrometry. In-plane enyne metathesis developed herein would offer a versatile platform for the functionalization of surfaces with mild reaction conditions and a high compatibility in functional groups.
Self-assembled monolayers (SAMs) of alkanethiolates on Au(111) have been a useful platform for the fundamental study of interfacial phenomena and many technologically important applications, where physical, chemical, and biological properties of surfaces are modulated permanently or dynamically by functional groups of selfassembling molecules.1-9 In addition, the utilization of SAM-based, interfacial chemical reactions in the fields of biology, (nano)biotechnology, and biomedical engineering yielded a newly emerging field, biosurface organic chemistry.10 The chemical modification of surfaces has often been achieved by separate solution synthesis of desired self-assembling molecules and a subsequent formation of SAMs (direct method). Compared with the direct method, post-modification of surfaces neither requires cumbersome synthesis nor needs to consider functional group tolerance with surface-anchoring groups (e.g., thiols for gold and silanes for silicon oxide).5,6,11-14 Although ideally more * Corresponding author. E-mail:
[email protected]. † KAIST. ‡ Chung-Ang University. § COS Biotech, Inc. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Chi, Y. S.; Lee, J. K.; Lee, S.-g.; Choi, I. S. Langmuir 2004, 20, 3024. (3) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S.-g. J. Am. Chem. Soc. 2004, 126, 480. (4) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (5) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17. (6) Mrkish, M. Curr. Opin. Chem. Biol. 2002, 6, 794. (7) Kakkar, A. K. Chem. Rev. 2002, 102, 3579. (8) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315. (9) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161. (10) Murphy, W. L.; Mercurius, K. O.; Koide, S.; Mrksich, M. Langmuir 2004, 20, 1026. (11) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051. (12) T. Lummerstorfer, T.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 3963. (13) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (14) Chi, Y. S.; Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Bull. Korean Chem. Soc. 2005, 26, 361.
suitable for surface modification, the post-modification has not fully been acknowledged because of low yields, limited applicability of reaction conditions, and little understanding of rules governing organic reactions at the surface.15 To develop a reliable reaction for modifying/ functionalizing gold surfaces via post-modification, we should consider the instability of SAMs on gold surfaces: (1) The bond between gold and thiol is not thermally stable.16-20 The desorption of thiols occurs around at 60 °C; therefore, the reaction should be performed below this temperature. (2) Because of the instability of the SAMs on gold, neither highly acidic nor highly basic conditions can be applied. Among the organic reactions, some transition metalcatalyzed reactions, such as ruthenium- and coppercatalyzed reactions, are good candidates for the postmodification of surfaces because the reactions generally do not require harsh reaction conditions and have a great tolerance in functional groups.11,12,21-23 In the course of investigating methods for directly modifying chemical properties of SAMs,22-24 we have recently applied ruthenium-catalyzed olefin cross-metathesis (CM) to the vinylterminated SAM and successfully introduced R,β-unsaturated carbonyl functionalities onto the SAM.22 During the study, one of the disadvantages in the CM on the SAM was found to be the undesirable in-plane dimerization of (15) Kwon, Y.; Mrksich, M. J. Am. Chem. Soc. 2002, 124, 806 and references therein. (16) Kim, J.-B.; Breuning, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616. (17) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Herdrick, J. L. Marcromolecules 2000, 33, 597. (18) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. J. Chem. Phys. 1993, 98, 3503. (19) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (20) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem Soc. 1989, 111, 321. (21) Samanta, D.; Faure, N.; Rondelez, F.; Sarkar, A. Chem. Commun. 2003, 1186. (22) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141. (23) Lee, J. K.; Chi, Y. S.; Choi, I. S. Langmuir 2004, 20, 3884. (24) Lee, J. K.; Kim, Y.-G.; Chi, Y. S.; Yun, W. S.; Choi, I. S. J. Phys. Chem. B 2004, 108, 7665.
10.1021/la051680s CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005
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two neighboring vinyl groups at the surface because of a proximity effect25-27 when the CM was applied to SAMs; therefore, the CM on the SAM would not be generalized to the introduction of any functional groups and would be limited to the vinyl group-containing compounds that preferentially react with the vinyl groups at the surface. As another method for introducing functional groups onto surfaces, we and others demonstrated copper-catalyzed triazole formation between acetylenyl and azido groups (“click chemistry”) on SAMs.11,12,23 In terms of the design of SAM-based organic reactions, it would be desirable to utilize the structural wellorderedness of SAMs. In other words, in-plane reactions, the reactions of self-assembled molecules at surfaces themselves caused by the proximity effect, would be more favored than other possible reactions thermodynamically and/or kinetically, as shown in the in-plane dimerization of vinyl groups at the surface.22 In addition, it would be beneficial in the introduction of organic functionalities onto surfaces if in-plane reactions generate chemically modifiable groups at surfaces. Enyne metathesis, one of the ruthenium- and molybdenum-catalyzed metathesis processes, not only offers great potential in the formation of C-C bond but proceeds under mild reaction conditions.28-33 In addition, the reaction product, 1,3-diene, could be used for various cycloadditions, leading to the introduction of useful functional groups.34-42 In this letter, we utilized proximity effect to achieve in-plane enyne metathesis reactions on the SAMs terminating in vinyl and acetylenyl groups on gold surfaces by the secondgeneration Grubbs catalyst22 and performed subsequent Diels-Alder reactions with the aim of directly introducing various functional groups onto SAMs (Scheme 1). We designed and synthesized the vinyl and acetylenyl group-containing dithiol 1 for the formation of the SAMs required for in-plane enyne metathesis because the dithiol would generate a surface presenting vinyl and acetylenyl groups in a 1:1 ratio. Compound 1 was synthesized from tri(ethylene glycol) (Scheme 2). Briefly, a substitution reaction with 11-bromo-1-undecene and the subsequent addition of thioacetic acid yielded compound 6. The acetyl group was removed by refluxing in 1 M HCl, and treatment with iodine converted the thiol to dithiol compound 8.43 The addition of propargyl bromide and subsequent addition of allyl bromide yielded compound 1. The SAMs presenting both vinyl and acetylenyl groups were formed by immersing a freshly prepared, gold-coated (with a (25) Coates, G. W.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 229. (26) Ivin, K. J.; Kenwright, A. M.; Khosravi, E. Chem. Commun. 1999, 1209. (27) Barrett, A. G. M.; Cramp, S. M.; Roberts, R. S. Org. Lett. 1999, 1, 1083. (28) Diver, S. T.; Giessert, A. J. Chem. Rev. 2004, 104, 1317. (29) Diver, S. T.; Giessert, A. J. Synthesis 2004, 466. (30) Mori, M. In Handbook of Metathesis; Grubbs, R. H., Ed.; WileyVCH: Weinheim, Germany 2003; Vol. 2, pp 176-204. (31) Lee, H.-Y.; Kim, B. G.; Snapper, M. L. Org. Lett. 2003, 5, 1855. (32) Hansen, E. C.; Lee, D. J. Am. Chem. Soc. 2004, 126, 15074. (33) Hansen, E. C.; Lee, D. J. Am. Chem. Soc. 2003, 125, 9582. (34) Poulsen, C. S.; Madsen, R. Synthesis 2003, 1. (35) Kang, B.; Kim, D.-h.; Do, Y.; Chang, S. Org. Lett. 2003, 5, 3041. (36) Banti, D.; North, M. Tetrahedron Lett. 2002, 43, 1561. (37) Harmata, M. Acc. Chem. Res. 2001, 34, 595. (38) Moreno-Manas, M.; Pleixats, R.; Santamaria, A. Synlett 2001, 1784. (39) Zheng, G.; Dougherty, T. J.; Pandey, R. K. Chem. Commun. 1999, 2469. (40) Schurer, S. C.; Blechert, S. Chem. Commun. 1999, 1203. (41) Wender, P. A.; Nuss, J. M.; Smith, D. B.; Suarez-Sobrino, A.; Vagberg, J.; Decosta, D.; Bordner, J. J. Org. Chem. 1997, 62, 4908. (42) Fringluelli, F.; Taticchi, A. Dienes in the Diels-Alder Reaction; John Wiley & Sons: New York, 1990. (43) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735.
Letters Scheme 1. Schematic Description of the Procedure
Scheme 2. Synthetic Procedure of Vinyl and Acetylenyl Group-Containing Dithiol 1a
a (a) NaH, THF; (b) CH3COSH, ABCV, THF with UV irradiation; (c) 1 M HCl, MeOH; (d) I2, EtOH; (e) BrCH2CtCH, NaH, THF; (f) BrCH2CHdCH2, NaH, THF.
titanium adhesion layer of 50 Å and thermally evaporated gold layer of 1000 Å) silicon wafer in an ethanolic solution of the vinyl and acetylenyl group-containing dithiol 1 (5 mM). After the formation of the SAMs,44 the gold substrate was rinsed with ethanol and methylene chloride several
Letters
times and dried under a stream of argon. The formation of the SAMs was confirmed by polarized infrared external reflectance spectroscopy (PIERS). The IR spectrum showed characteristic peaks at 3085 cm-1 (dCH2 antisymmetric) and 3324 cm-1 (tCH antisymmetric) (Figure 1a).22,23 Compared with the reported IR spectrum of the vinylterminated SAM on gold,22,45 we did not observe other peaks from the olefin group, such as 912 cm-1 (C-H outof-plane deformation of the dCH2 group), 993 cm-1 (CdC out-of-plane deformation), 1643 (CdC stretch), 2983 cm-1 (dCH2 symmetric), and 3007 cm-1 (dC-H stretch) presumably because of the flexibility of the tri(ethylene glycol) moiety in the SAMs.23,24 After the formation of the SAM presenting vinyl and acetylenyl groups, in-plane enyne metathesis was performed by stirring a mixture of the gold substrate and the second-generation Grubbs catalyst (0.1 mmol) in benzene (10 mL) at 50 °C for 4 h under a nitrogen atmosphere in a glovebox. After the reaction, the gold substrate was rinsed with benzene, methylene chloride, and ethanol several times and dried under a stream of argon. Because of the weak IR intensities of dCH2 and tCH antisymmetric stretching peaks, we could not positively follow the formation of the 1,3butadiene moiety at the surface by PIERS, but the negative-ion time-of-flight secondary-ion mass spectrometry (TOF-SIMS) spectrum showed a peak of the enyne metathesis product at m/z 748.7 (C40H76O8S2-; calculated value: 748.5).46 The formation of the 1,3-butadiene moiety by in-plane enyne metathesis was further confirmed by subsequent Diels-Alder reactions.47 Tetracyanoethylene 2, maleic anhydride 3, and maleimide 4 were tested as dienophiles. The Diels-Alder reaction was conducted by stirring a mixture of the 1,3-butadiene-presenting gold substrate and a dienophile (1 M) at 40 °C for 2 h in methylene chloride. The reaction was characterized by PIERS and X-ray photoelectron spectroscopy (XPS).20,24 In the IR spectra, characteristic peaks from the Diels-Alder adducts were observed: 2206 cm-1 (CtN, from 2), 1952 and 1725 cm-1 (CdO, from 3), and 1724 cm-1 (CdO, from 4) (Figure 1). The wide-scan XPS spectra showed the appearance of a nitrogen peak (at 402 eV) in the Diels-Alder adducts with 2 and 4 (Figures 2b and d). The C(1s) region of the XPS spectra further confirmed successful Diels-Alder (44) The phase separation in the SAMs of asymmetric disulfides after dissociative adsorption of the disulfides on gold was reported. See Tamada, K.; Akiyama, H.; Wei, T.-X.; Kim, S.-A. Langmuir 2003, 19, 2306; Noh, J.; Hara, M. Langmuir 2000, 16, 2045; Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. The extent of the phase separation present in mixed SAMs remains largely unexamined,13 and it was reported that homogeneously mixed SAMs were formed if thiols had similar alkyl chain lengths. See Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.-i.; Niki, K. Langmuir 2001, 17, 1599; Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91. As a control experiment, we synthesized acetylenyl- and vinyl-terminated thiols separately and formed a mixed SAM of the thiols. In-plane enyne metathesis was also successful on the mixed SAM of acetylenyl- and vinyl-terminated thiols and yielded Diels-Alder products. On the basis of the results, we presume that the SAM was formed homogeneously on gold although a detailed characterization is needed to elucidate the chemical nature of the SAM. (45) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113. (46) TOF-SIMS data were obtained by using a PHI 7200 TOFSIMS/SALI system in which the primary ion beam was 8-keV Cs+ ion. (47) Intermolecular enyne cross metathesis is generally not stereoselective and yields mixtures of E/Z stereoisomers.28 Our system could be considered to be a ring-closing enyne metathesis, where in principle the exo and endo modes of ring-closing reactions are possible and the endo mode yields mixtures of E/Z stereoisomers. The product of inplane enyne metathesis in our system was drawn as the exo product because the successful Diels-Alder reactions require E stereoisomers and sterically restricted systems, such as 5-11-membered rings, give exo products.32,33 However, we cannot completely exclude the possibility of the formation of other regioisomers and stereoisomers.
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Figure 1. PIERS spectra of (a) SAMs of vinyl and acetylenyl group-containing dithiol 1, (b) SAMs after in-plane enyne metathesis, and SAMs after Diels-Alder reactions with (c) tetracyanoethylene 2, (d) maleic anhydride 3, and (e) maleimide 4. Expanded version of PIERS spectra of SAMs between 3000 and 3400 cm-1 (f) before and (g) after in-plane enyne metathesis.
reactions: 289 eV (-COO, from 3) and 288 eV (-CON, from 4) (Figures 2f and g). Although we could not quantify the reaction yields of in-plane enyne metathesis and Diels-Alder reactions because of the weak intensity of unsaturated hydrocarbons, we concluded that the reactions proceeded in reasonably good yields because the peaks at 3085 and 3324 cm-1 disappeared in the IR spectra after the reactions. We also added ethylene gas to the in-plane enyne metathesis reaction because it was reported that ethylene not only served as a substrate for enyne metathesis but also maintained the reactivity of the ruthenium catalyst.33 In our system, the addition of ethylene gas did not facilitate but retarded the reaction of in-plane metathesis. After 4 h of reaction of in-plane metathesis in the presence of ethylene gas and subsequent Diels-Alder reactions, we observed peaks from unsaturated hydrocarbons (at 3085 and 3324 cm-1) in the IR spectra, which imply that in-plane metathesis did not proceed completely in comparison with the reaction without ethylene gas.
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Letters
Figure 2. Wide-scan XPS spectra of (a) SAMs of vinyl and acetylenyl group-containing dithiol 1 and SAMs after Diels-Alder reactions with (b) tetracyanoethylene 2, (c) maleic anhydride 3, and (d) maleimide 4. C(1s) region of the XPS spectra after DielsAlder reactions with (e) 2, (f) 3, and (g) 4.
In summary, in-plane enyne metathesis of self-assembled monolayers on gold generated a surface presenting 1,3-diene, a versatile synthon for various cycloadditions. As a proof of concept, we demonstrated Diels-Alder reactions between the 1,3-diene moiety and dienophiles. Diels-Alder reactions have been used intensively to introduce various (bio)molecules onto surfaces15 because they are addition reactions and therefore do not require stringent control of the spatial distribution of reactants at surfaces. Most of the reported methods for functionalizing surfaces rely on the coupling reactions between amino compounds and the activated surfaces, and the system described herein would broaden our selection of functional groups for the facile attachment of (bio)molecules of interest. Therefore, we believe that surfaces presenting 1,3-diene groups could be used as a versatile platform in tailoring surface properties. Experimental Section Synthesis of 1. The synthesis of 1 is outlined in Scheme 2. To a THF solution (30 mL) of triethylene glycol (7.16 g, 47.7 mmol) was added NaH (381.2 mg of a 60% suspension in oil, 9.53 mmol) at 0 °C. The resulting mixture was stirred at 0 °C for 30 min and then held at 80 °C for 2 h under an argon atmosphere. When the solution turned dark brown, 11-bromo-1-undecene (2 g, 8.58 mmol) was added to the solution, and the resulting mixture was stirred at 80 °C for 12 h. The resulting solution was cooled to room temperature and extracted with hexane (500 mL). The hexane layer was washed with deionized (DI) water, dried over MgSO4, concentrated in vacuo, and purified by column chromatography (elution with 2:1 hexane/ethyl acetate) to give product 5 in 80% yield. 1H NMR (300 MHz, CDCl3): δ 5.73 (m, 1H), 4.87 (m, 2H), 3.53 (m, 12H), 3.40 (t, 2H), 2.70 (t, 1H), 2.05 (q, 2H), 1.53 (q, 2H), 1.25 (br m, 12H). A mixture of 5 (1.0 g, 2.48 mmol), thioacetic acid (5 mL, 69.6 mmol), and 4,4′-azobis(4-cyanovaleric acid) (ABCV) (150 mg) in THF (40 mL) was irradiated by UV (253.7 nm). After 4 h, additional ABCV (150 mg) was added, and the irradiation was continued for 6 h. The mixture was concentrated in vacuo and purified by column chromatography (elution with 2:1 hexane/ ethyl acetate) to give thioacetate 6 in 90% yield. 1H NMR (300 MHz, CDCl3): δ 3.59 (m, 12H, -OCH2CH2O-), 3.40 (t, 2H, -OCH2), 2.83 (t, 2H, -CH2SCO), 2.5 (br s, 1H, -OH),
2.28 (s, 3H, -SCOCH3), 1.50 (m, 4H, -CH2CH2SCO- and -OCH2CH2-), 1.23 (br m, 14H). To a MeOH solution (20 mL) of 6 (870 mg, 2.30 mmol) was added concentrated HCl (2 mL, 23.0 mmol) at room temperature. The resulting mixture was heated to reflux for 10 h under an argon atmosphere. After the mixture was concentrated in vacuo, the resulting solution was extracted with hexane (50 mL). The hexane layer was washed with DI water, dried over MgSO4, concentrated in vacuo, and purified by flash column chromatography (elution with 4:1 hexane/ethyl acetate) to give product 7 in 90% yield. 1H NMR (300 MHz, CDCl3): δ 3.59 (m, 12H, -OCH2CH2O-), 3.40 (t, 2H, -OCH2), 2.51 (q, 2H, HS-), 2.06 (s, 1H, -OH), 1.64 (m, 2H, -CH2CH2S-), 1.57 (m, 2H, -OCH2CH2-), 1.23 (br m, 14H). Compound 7 (758 mg, 2.25 mmol) was dissolved in absolute EtOH (16 mL), and I2 (571 mg, 2.25 mmol) was added to the solution. The mixture was stirred at 70 °C for 2 h followed by concentration in vacuo to remove solvent. The resulting solid was extracted with CH2Cl2 (50 mL), and the CH2Cl2 layer was washed with saturated sodium bisulfite solution several times, dried over MgSO4, concentrated in vacuo, and purified by flash column chromatography (elution with 1:1 hexane/ethyl acetate) to give product 8 in 98% yield. 1H NMR (300 MHz, CDCl3): δ 3.60 (m, 24H, -OCH2CH2O-), 3.40 (t, 4H, -OCH2-), 2.63 (t, 4H, -CH2SSCH2-), 2.06 (br s, 2H, -OH), 1.64 (m, 4H, -CH2CH2S-), 1.57 (m, 4H, -OCH2CH2-), 1.23 (br m, 28H). To a THF solution (15 mL) of compound 8 (550 mg, 0.81 mmol) was added NaH (33 mg of a 60% suspension in oil, 0.75 mmol) at 0 °C. The resulting mixture was stirred at 0 °C for 30 min and then at room temperature for 30 min followed by the addition of propargyl bromide (178 mg, 1.50 mmol) under an argon atmosphere. After 1 h, the resulting solution was extracted with hexane (500 mL), and then the hexane layer was washed with DI water, dried over MgSO4, concentrated in vacuo, and purified by column chromatography (elution with 4:1 hexane/ethyl acetate) to give product 9 in 40% yield. 1H NMR (300 MHz, CDCl3): δ 4.12 (s, 4H, -CtCCH2-), 3.60 (m, 24H, -OCH2CH2O), 3.40 (t, 4H, -OCH2), 2.60 (t, 4H,-CH2SSCH2-), 2.41 (s, 1H, HCtC-), 1.90 (br s, 1H, -OH), 1.64 (m, 4H, -CH2CH2S-), 1.57 (m, 4H, -OCH2CH2-), 1.23 (br m, 28H). To a THF solution (5 mL) of compound 9 (54 mg, 0.076 mmol) was added NaH (6 mg of a 60% suspension in oil, 0.15 mmol) at 0 °C. The resulting mixture was stirred at 0 °C for 30 min and then at room temperature for 30 min followed by the addition of allyl bromide (178 mg, 0.15 mmol) under an argon atmosphere. After 1 h, the resulting solution was extracted with hexane (10
Letters mL), and then the hexane layer was washed with DI water, dried over MgSO4, concentrated in vacuo, and purified by column chromatography (elution with 6:1 hexane/ethyl acetate) to give product 1 in 60% yield. 1H NMR (300 MHz, MeOD): δ 5.90 (m, 1H, -CHd), 5.10 (m, 2H, dCH2), 4.12 (s, 2H, -CtCCH2-), 3.98 (d, 2H, -CH2CHd), 3.60 (m, 24H, -OCH2CH2O-), 3.40 (t, 4H, -OCH2), 2.80 (s, 1H, HCtC-), 2.63 (t, 4H,-CH2SSCH2-), 1.64 (m, 4H, -CH2CH2S-), 1.57 (m, 4H, -OCH2CH2-), 1.23 (br m, 28H).
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Acknowledgment. This work was supported by the Korea Research Foundation (KRF-2004-015-C00301). We thank Dr. Won of the Korea Basic Science Institute for XPS analysis and Dr. Yeonhee Lee and Ms. Moonhee Kwon of the Korea Institute of Science and Technology for TOFSIMS analysis. LA051680S
