Λ-Type Regioregular Oligothiophenes: Synthesis and Second-Order

Λ-Type Regioregular Oligothiophenes: Synthesis and Second-Order NLO Properties Samia Zrig,†,‡ Guy Koeckelberghs,‡ Thierry Verbiest,*,‡ Bruno Andrioletti,*,† Eric Rose,*,† Andre´ Persoons,‡ Inge Asselberghs,‡ and Koen Clays‡ Laboratoire de chimie organique (UMR CNRS 7611), Institut de chimie mole´ culaire (FR 2769), UniVersite´ Pierre et Marie Curie-Paris 6, case 181, 4 place Jussieu, F-75252 Paris cedex 05, France, and Laboratory for Molecular Electronics and Photonics, Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 D, B-3001, Belgium

[email protected]; [email protected]; [email protected] ReceiVed May 9, 2007

The synthesis of a new series of Λ-type, D-Π-A regioregular oligothiophenes is described. The simultaneous presence of the chiral centers and the Λ-type structure disfavored the formation of centro-symmetrical dimeric assemblies. Hence, enhanced first hyperpolarizabilities βHRS were measured in comparison with those of the corresponding monomers. In 1964, Rentzepis et al. and Heilmeier et al. first described the use of organic molecules for the generation of optical second harmonics (SHG).1 These unprecedented results pioneered the amazing development of organic nonlinear optic chromophores (NLO-phores).2 Indeed, as compared to the inorganic analogues, organic NLO-phores exhibit high versatilities and processabilities together with a low refractive index.3 Considering the high academic and industrial impact of the second-order NLO phenomenon, a special emphasis was dedicated to the preparation of efficient second-order NLO-phores. Among the plethora of organic NLO-phores, oligothiophenes end-capped with donor (D) and acceptor (A) moieties, D-Π-A type molecules, are particularly promising because of a low HOMO-LUMO gap favoring a high electron mobility, a decent transparency, and a † ‡

Universite´ Pierre et Marie Curie-Paris 6. Katholieke Universiteit Leuven.

(1) (a) Rentzepis, P. M.; Pao, Y.-H. Appl. Phys. Lett. 1964, 5, 156158. (b) Heilmeier, G. H.; Ockman, N.; Braunstein, R. J.; Kramer, D. A. Appl. Phys. Lett. 1964, 5, 229-230. (2) (a) Prasard, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (b) Nonlinear Optics of Organic and Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press: Boca Raton, FL, 1997. (3) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546.

high stability and processability. However, the D-Π-A structure presents some drawbacks. First, the transparency-efficiency tradeoff is difficult to settle with conventional D-Π-A systems because the desirable βHRS increase is associated with a bathochromatic shift of the electronic transition.4 Moreover, most one-dimensional (1D) NLO-phores favor the formation of a centrosymmetric arrangement in the crystal because of dipole-dipole intermolecular interactions and exhibit no SHG response. Concerning phase matching conditions, the optimal molecular orientation of the 1D NLO-phore in the crystal does not allow one to recover more than 38% of the microscopic nonlinear optical response at the macroscopic scale.5 Different approaches such as the use of electric field poling or the preparation of Langmuir-Blodgett (LB) thin films were proposed to limit the natural antiparallel dipolar interaction. However, those strategies suffer from inherent relaxation or limitations due to the tedious preparation of multilayered LB films. The preparation of chiral oligothiophenes was also proposed.6 An alternative approach that can rule out the formation of centrosymmetric supramolecular arrays involves the preparation of Λ-type molecules that consists in the pre-organization of the NLO-phore by pairing two chromophores in a predetermined two-dimensional configuration (2D).7 Additionally, Λ-type molecules exhibit large off-diagonal β-tensor components beneficial for the observation of large phase matched secondorder NLO responses,5,8 and high thermal stability.9 Accordingly, following preliminary work that had been carried out in our groups,10 we designed a new series of regioregular11 Λ-type oligothiophenes (bi-, quater-, and sexithiophene) end-capped with a thermally stable diphenylamino-donating group, and an imino group, respectively. Despite a modest accepting character, we retained the versatile imine function because it allows the easy preparation of a large variety of (4) (a) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. ReV. 1994, 94, 195-242. (b) Zyss, J.; Ledoux, I.; Bertault, K.; Toupet, E. Chem. Phys. 1991, 150, 125-135. (c) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.; Peyghambarian, N. Nature 1997, 388, 845-851. (5) (a) Yamamato, H.; Katogi, S.; Watanabe, T.; Sato, H.; Miyata, S.; Hosomi, T. Appl. Phys. Lett. 1992, 60, 935-937. (b) Kuo, W.-J.; Hsiue, G.-H.; Jeng, R.-J. Macromolecules 2001, 34, 2373-2384. (6) See, for instance: Kinbinger, A. F. M.; Shenning, A. P. H. J.; Goldoni, F.; Feast, W. J.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 1820-1821. Shenning, A. P. H. J.; Kinbinger, A. F. M.; Biscari, F.; Cavallini M.; Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Lecle`re, P.; McDonnell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002, 124, 12691275. (7) Selected examples: (a) Ostroverkhov, V.; Petschek, R. G.; Singer, K. D.; Twieg, R. J. Chem. Phys. Lett. 2001, 340, 109-115. (b) Coe, B. J.; Harris, J. A.; Brunschwig, B. S.; Garin, J.; Orduna, J. J. Am. Chem. Soc. 2005, 127, 3284-3285. (c) Moylan, C. R.; Ermer, S.; Lovejoy, S. M.; McComb, I.-H.; Leung, D. S.; Wortmann, R.; Krdmer, P.; Twieg, R. J. J. Am. Chem. Soc. 1996, 118, 12950-12955. (d) Van Elshocht, S.; Verbiest, T.; Kauranen, M.; Ma, L.; Cheng, H.; Musick, K. Y.; Pu, L.; Persoons, A. Chem. Phys. Lett. 1999, 309, 315-320. (e) Langeveld-Voss, B. M. W.; Beljonne, D.; Shuai, Z.; Janssen, R. A. J.; Meskers, S. C. J.; Meijer, E. W.; Bre´das, J.-L. AdV. Mater. 1998, 10, 1343-1348. (8) Yang, M. L.; Champagne, B. J. Phys. Chem. A 2003, 107, 39423951. (9) Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599-12600. (10) Loire, G.; Prim, D.; Andrioletti, B.; Rose, E.; Persoons, A.; Sioncke, S.; Vaissermann, J. Tetrahedron Lett. 2002, 43, 6541-6544. (11) For other regioregular oligothiophenes, see, for instance: Sakurai, S.; Goto, H.; Yashima, E. Org. Lett. 2001, 3, 2379-2382.

10.1021/jo070888a CCC: $37.00 © 2007 American Chemical Society

Published on Web 06/27/2007

J. Org. Chem. 2007, 72, 5855-5858

5855

FIGURE 1. General structure of the Λ-type chromophores. SCHEME 1.

Preparation of Dibromobithiophene 1

analogues. Among the parameters we could possibly investigate, we decided to evaluate the contribution of the θ angle value defined by the two chromophores, the nature of the imine function (aryl vs aliphatic), and the number of thiophenes to the overall second-order NLO response (Figure 1). To maximize the efficiency of the synthetic approach, we adopted a convergent strategy based on the exploitation of the key dibromobithiophene 1. The choice of 1 relies on two different considerations. First, the two C8 alkyl chains that functionalize the central “tail to tail” bithiophene are required for ensuring a good solubility of the oligothiophene moiety in most organic solvent without disrupting the overall molecular orbital overlap. 1 was readily prepared by dimerization of 2-bromo-3-octylthiophene using the PdCl2(PhCN)2/AgF system as catalyst and DMSO as solvent (Scheme 1). In comparison with the classical two step Stille coupling/bromination generally proposed for the preparation of oligothiophenes, use of Mori’s conditions12 (PdCl2(PhCN)2, AgF, DMSO) allowed the efficient one-step preparation of 1 in 96% yield. In parallel, the donating 2-(N,N-diphenylamino)thiophene brick 2 was prepared according to Hartmann’s procedure13 knowing that neither nucleophilic14 nor palladium-catalyzed15 amination of thiophene were successful in our hands. Undoubtedly, the absence of an activating electron-withdrawing group appended on the heterocycle was responsible for the lack of reactivity. 2 was further quantitatively converted to the corresponding 5-tri-n-butylstannyl analogue 3 after deprotonation with stoichiometric amounts of n-butyllithium followed by quenching with chlorotri-n-butylsilane. The preparation of the accepting imino moiety required the preparation of the corresponding formyl oligothiophene. Stille coupling between 3 and the commercially available 5-bromothiophen-2-carboxaldehyde afforded 416 in 74% yield (Scheme 2). (12) Masui, K.; Ikegami, H.; Mori, A. J. Am. Chem. Soc. 2004, 126, 5074-5075. (13) Heyde, C.; Zug, I.; Hartmann, H. Eur. J. Org. Chem. 2000, 19, 3273-3278. (14) Prim, D.; Kirsch, G. Tetrahedron 1999, 55, 6511-6526. (15) Driver, M. S.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 36093612.

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The key step for the preparation of the quater- and sexithiophenes 8 and 13 (Scheme 2) consisted in the desymmetrization of the oligomer. As the incorporation of an already formylated thiophene on the oligothiophene chain was low yielding and hardly reproducible, we decided to carry out the preliminary coupling of a thiophene subunit followed by a subsequent formylation. The optimized conditions for the preparation of terthiophene 6 involved a Suzuki coupling between 1 and 1.14 equiv of the boronic ester 5. Using Pd(PPh3)4 as catalyst and K2CO3aq as base, 6 was isolated in 45% yield. Using the same conditions, the symmetrical quaterthiophene 9 was isolated in 86% yield by condensation of 1 with 2.5 equiv of 5. After double bromination of 9, an iterative procedure allowed the preparation of quinquethiophene 11 from 10 in 35% yield. Subsequently, ter- and quinquethiophene 6 and 11 were efficiently formylated using a classical VilsmeierHaack procedure in 91% and 100% yield, respectively. Finally, a Stille coupling using Pd2(dba)3-AsPh3 as catalyst afforded the unsymmetrical quater- and sexithiophenes 8 and 13 in 95% and 89% yield, respectively. Oligothiophenes 4, 8, and 13 were fully characterized using the usual techniques (see Experimental Section). Ultimately, the Λ-type structures were readily prepared by condensation of two equivalents of formyl D-Π-A oligothiophenes 4, 8, and 13 with stoichiometric amounts of (R)(+)-1,1′-binaphthyl-2,2′diamine 14 or (1R, 2R)-(-)-1,2-diaminocyclohexane 15 in the presence of 4 Å molecular sieves, respectively. The same experimental procedure involving aniline 16 and cyclohexylamine 17 afforded the corresponding linear analogues (Table 1). Preliminary NLO experiments carried out in THF in the presence of triethylamine indicate that our strategy to enhance the nonlinear optical response in the Λ-shaped molecules was successful. In Table 1, we listed the first hyperpolarizability βHRS, measured by hyper-Rayleigh scattering (HRS) at 800 nm,17 for several compounds. In all samples, no multiphoton fluorescence was observed. Importantly, we used βHRS instead of the traditional βzzz value in a Cartesian coordinate system.18 The reason is two-fold: while it is very straightforward to determine βzzz from βHRS values for 1D dipolar molecules, it is not the case for the two-dimensional chromophores used in this study. For such molecules, there are several nonvanishing tensor components whose values are highly dependent on the choice of the coordinate system. Furthermore, it is experimentally very difficult to deduce the value of all of these components from HRS measurements. Therefore, we opted for βHRS, which is a relatively simple combination of all of the tensor components contributing to the nonlinear optical response. As βHRS can be considered as the “overall” nonlinear response of the molecules, an increase in βHRS value of the Λ-shaped dimeric molecules as compared to that of the monomeric compounds will reflect an increase in magnitude of one of the hyperpolarizability components and/or the contribution of new hyperpolarizability components that are not present in the monomeric system. The results listed in Table 1 reveal a general trend. Despite the fluctuations due to the rather large 10-20% experimental (16) Bedworth, P. V.; Cai, Y.; Jen, A.; Marder, S. R. J. Org. Chem. 1996, 61, 2242-2246. (17) Olbrechts, G.; Strobbe, R.; Clays, K; Persoons, A. ReV. Sci. Instrum. 1998, 69, 2233-2241. (18) Hendrickx, E.; Clays, K.; Persoons, A. Acc. Chem. Res. 1998, 31, 675-683.

SCHEME 2.

Syntheses of the D-Π-A Oligothiophenes 4, 8, and 13

TABLE 1. Synthesis and Properties of the Imine NLOphores

entry 1 2 3 4 5 6 7 8 9 10 11 12 a

aldehyde

amine

iminea

βHRS (10-30 esu)

4 4 4 4 8 8 8 8 13 13 13 13

14 15 16 17 14 15 16 17 14 15 16 17

18 (32) 19 (12) 20 (29) 21 (29) 22 (86) 23 (86) 24 (65) 25 (75) 26 (47) 27 (46) 28 (57) 29 (82)

190 122 126 110 185 335 ND 125 1535 262 277 257

Yields are given in parentheses; ND ) not determined.

error, λmax is very close to the frequency doubled, it is clear that βHRS of dimeric compounds is generally significantly higher than that of the corresponding monomeric compounds. Furthermore, it has been shown that it should be possible to determine the dihedral angle19 between the chromophores in bisdipolar compounds of the type measured here, provided that some assumptions are made. For example, one needs to assume that the monomeric compound has only one hyperpolarizability component βzzz and that off-diagonal tensor components do not contribute to the nonlinear optical response of the dimeric compound. Using these assumptions, βHRS can be expressed solely in terms of βzzz for the monomer or βzzz for the dimer, and the relation between the two is given19 by βzzz ) 2(cos(θ/ 2))3βzzz with θ being the (full) dihedral angle between the two monomeric chromophore parts. In this way, we calculated the angle to be 82° for compound 18, 112° for compound 19, and (19) Deussen, H.-J.; Hendrichx, E.; Boutton, C.; Krog, D.; Clays, K.; Bechgaard, K.; Persoons, A.; Bjørnholm, T. J. Am. Chem. Soc. 1996, 118, 6841-6852.

119° for compound 27. This is in good agreement with the expected values of 90° for 1,1′-binaphthyl compounds and 109° for diaminocyclohexane derivatives (19 and 27). In conclusion, we have developed an efficient strategy leading to a new type of Λ-shaped D-Π-A oligothiophenes. The convergent synthetic approach presented here afforded a new series of chromophores whose second-order nonlinear properties were estimated. Preliminary βHRS measurements proved the efficiency of the strategy developed here, as the βHRS values of the Λ-shaped molecules generally exceed those of the corresponding monomers. Work is currently in progress to elucidate the independent contributions to the overall NLO response. Experimental Section 5,5′-Dibromo-4,4′-dioctyl-2,2′-bithiophene, 1. A two-neck roundbottom flask shed from light was charged with AgF (2.11 g, 16.78 mmol), 2-bromo-3-octylthiophene (2.3 g, 8.39 mmol), Pd(PhCN)2Cl2 (96 mg, 0.251 mmol), and DMSO (30 mL). The reaction was allowed to proceed for 16 h at 30 °C. The black reaction mixture was filtered over celite and extracted with AcOEt (200 mL). The filtrate was washed with water (2 × 150 mL) and brine (100 mL), before being dried over Na2SO4, filtered, and concentrated. The crude product was purified by silica gel column chromatography (cyclohexane) to afford 1 as a yellow oil (2.21 g, 96%). 1H (CDCl3): 6.77 (s, 2H, H3Th), 2.51 (t, J ) 7.70 Hz, 4H, H1Oct), 1.57 (m, 4H, H2Oct), 1.29 (m, 20H, H3-7Oct), 0.88 (t, J ) 6.80 Hz, 6H, H8Oct). 13C (CDCl3): 143.3 (C4Th), 136.5 (C2Th), 124.8 (C3Th), 108.2 (C5Th), 32.2, 30.1, 30.0, 29.9, 29.7, 29.6, 29.0, 14.5 (COct). MS (MALDI-TOF) m/z (relative intensity) calcd for [C24H36Br2S2]+: 548.0605. Found: 469 (100) [M - Br]+, 548 (10) [M]+. Anal. Calcd for C24H36Br2S2: C, 52.56; H, 6.62. Found: C, 52.71; H, 6.87. Typical Procedure for the Suzuki-type Couplings: Synthesis of 5′′-Bromo-3′,4′′-dioctyl-2,2′:5′,2′′-terthiophene, 6. 2-(4,4,5,5Tetramethyl-1,3,2-dioxaborane)-thiophene 5 (3.06 g, 14.5 mmol), 5,5′-dibromo-4,4′-dioctyl-2,2′-bithiophene 1 (6.98 g, 12.7 mmol), and Pd(PPh3)4 (820 mg, 0.71 mmol) were dissolved in THF (120 mL) before aqueous K2CO3 (2M, 32 mL, 63.7 mmol) was added. The reaction mixture was brought to reflux for 36 h. After being cooled to room temperature, the mixture was poured in water (100 mL) and extracted with CH2Cl2 (3 × 100 mL). The organic layers were combined, washed with water (100 mL), dried over Na2SO4, J. Org. Chem, Vol. 72, No. 15, 2007 5857

filtered, and concentrated in vacuo. The crude product was purified by silica gel column chromatography (cyclohexane) to afford 6 as a yellow oil (3.16 g, 45%). 1H (CDCl3): 7.31 (dd, J ) 5.25 Hz, 1.14 Hz, 1H, H5Th), 7.12 (dd, J ) 3.66 Hz, 1.14 Hz, 1H, H3Th), 7.06 (dd, J ) 5.25 Hz, 3.66 Hz, 1H, H4Th), 6.92 (s, 1H, H4′Th), 6.84 (s, 1H, H3′Th), 2.71 (t, J ) 7.82 Hz, 2H, H1′Oct), 2.53 (t, J ) 7.82 Hz, 2H, H1Oct), 1.62 (m, 4H, H2,2′Oct), 1.30 (m, 20H, H3-7,3′-7′Oct), 0.88 (t, J ) 6.84 Hz, 6H, H8,8′Oct). 13C (CDCl3): 143.3 (C4′′Th), 140.6 (C3′Th), 136.9 (C5′Th), 136.1 (C2′′Th or C2′Th), 134.8 (C2′′Th or C2′Th), 130.0 (C2Th), 127.8 (C4Th), 126.6 (C3′′Th), 126.2 (C4′Th), 125.8 (C5Th), 124.6 (C3Th), 107.9 (C5′′Th), 32.2, 32.2, 31.7, 30.0, 29.9, 29.9, 29.7, 29.7, 29.6, 29.5, 28.9, 23.0, 23.0, 14.5 (COct). MS (MALDI-TOF) m/z calcd for [C28H39BrS3]+: 552.1377. Found: 552.0930 ([M]+). Typical Procedure for the Vilsmeier-Haack Formylation: Synthesis of 5′′-Bromo-3′,4′′-dioctyl-2,2′:5′,2′′-terthiophen-5carboxaldehyde, 7. DMF (1.83 mL, 19.6 mmol) was added slowly to POCl3 (1.52 mL, 19.6 mmol) at 0 °C. After 15 min without stirring, a white solid was formed and a solution of 5′′-bromo3′,4′′-dioctyl-2,2′:5′,2′′-terthiophene 6 (1.08 g, 1.96 mmol) in dichloroethane (60 mL) was added. The yellow reaction mixture was stirred and brought to reflux for 12 h. The solution turned red. After the mixture was cooled to room temperature, an aqueous solution of AcONa (2 M, 20 mL, 40 mmol) was added and the biphasic mixture was brought to reflux for 2 h. The mixture was poured in water (50 mL) and extracted with CH2Cl2 (3 × 100 mL). The organic layers were combined, washed with water (100 mL), dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by silica gel column chromatography (cyclohexane/CH2Cl2: 6/4) to afford 7 as a yellow oil (1.02 g, 91%). 1H (CDCl3): 9.88 (s, 1H, HCHO), 7.70 (d, J ) 4.0 Hz, 1H, H4Th), 7.21 (d, J ) 4.0 Hz, 1H, H3Th), 6.95 (s, 1H, H4′Th), 6.95 (s, 1H, H3′′Th), 2.78 (t, J ) 7.7 Hz, 2H, H1Oct), 2.54 (t, J ) 7.7 Hz, 2H, H1′Oct), 1.63 (m, 4H, H2-2′Oct), 1.27 (m, 20H, H3-7,3′-7′Oct), 0.88 (t, J ) 5.8 Hz, 6H, H8,8′Oct). 13C (CDCl3): 182.9 (CCHO), 146.3 (C5Th), 143.6 (C3′Th), 143.4 (C4′′Th), 142.5 (C2Th), 137.2 (C2′Th), 137.0 (C5′Th), 136.2 (C2′′Th), 128.2 (C4Th), 127.3 (C3Th), 126.3 (C4′Th), 125.4 (C3′′Th), 109.0 (C5′′Th), 32.2, 30.6, 30.1, 30.0, 29.9, 29.7, 29.7, 29.6, 27.2, 23.0, 14.5 (COct). MS (MALDI-TOF) m/z (relative intensity) calcd for [C29H39BrOS3]+: 578.1346. Found: 578.1341 (85) [M]+, 579.1372 (31) [M + H]+, 580.1322 (100) [M]+. IR νCO (cm-1): 1664.4. Typical Procedure for the Stille-type Couplings: Synthesis of 3′,4′′-Dioctyl-5′′′-N,N-diphenylamino-2,2′:5′,2′′:5′′,2′′′-quater-

5858 J. Org. Chem., Vol. 72, No. 15, 2007

thiophen-5-carboxaldehyde, 8. Triphenylarsine (282 mg, 0.92 mmol) was added to a solution of Pd2dba3 (211 mg, 0.23 mmol) in THF (10 mL). After being stirred for 30 min, the purple solution turned yellow. Separately, 5′′-bromo-3′,4′′-dioctyl-2,2′:5′,2′′-terthiophen-5-carboxaldehyde 7 (2.67 g, 4.61 mmol) and 2-(N,Ndiphenylamino)-5-tri-n-butylstannylthiophene 3 (3.42 g, 5.07 mmol) were dissolved in THF (60 mL), and the Pd(AsPh3)4 catalyst prepared above was added. The reaction mixture was refluxed for 24 h. After being cooled to room temperature, the mixture was poured in water (60 mL) and extracted with CH2Cl2 (3 × 50 mL). The organic layers were combined, washed with water (100 mL), dried over Na2SO4, filtered, and concentrated in vacuo. The expected compound 8 (3.11 g, 90%) was isolated as a red oil after silica gel column chromatography (cyclohexane/CH2Cl2: 1/1). 1H (CDCl3): 9.88 (s, 1H, HCHO), 7.70 (d, J ) 4.04 Hz, 1H, H4Th), 7.30 (dd, J ) 7.33 Hz, 7.32 Hz, 4H, H3Ph), 7.21 (d, J ) 4.04 Hz, 1H, H3Th), 7.19 (d, J ) 7.33 Hz, 4H, H2Ph), 7.06 (dd, J ) 7.32 Hz, 2H, H4Ph), 7.01 (s, 1H, H4′Th), 6.99 (1H, s, H3′′Th), 6.91 (d, J ) 3.79 Hz, 1H, H3′′′Th), 6.63 (d, J ) 3.79 Hz, 1H, H4′′′Th), 2.79 (t, J ) 7.84 Hz, 2H, H1Oct), 2.70 (t, J ) 7.84 Hz, 2H, H1′Oct), 1.65 (m, 4H, H2Oct), 1.27 (m, 20H, H3-7Oct), 0.88 (t, J ) 5.80 Hz, 6H, H8Oct). 13C (CDCl ): 182.9 (C 3 CHO), 152.0 (C5Th), 147.9 (C1Ph), 146.6 (C2Th), 143.5 (C2′Th), 143.1 (C3′Th), 142.3 (C4′′Th), 140.3 (C5′Th), 137.6 (C2′′′Th), 137.3 (C4Th), 133.7 (C5′′Th), 131.6 (C2′′′Th), 129.6 (C3Ph), 128.5 (C5′′′Th), 127.6 (C4′Th), 127.1 (C3′′Th), 126.1 (C3Th), 124.8 (C3′′′Th), 123.6 (C4Ph), 123.1 (C2Ph), 121.1 (C4′′′Th), 32.2, 30.8, 30.5, 30.5, 30.2, 30.0, 29.9, 29.7, 29.6, 23.0, 14.5 (COct). MS (MALDITOF) m/z calcd for [C45H51NOS4]+: 749.2853. Found: 749.2848 [M]+. IR νCO (cm-1): 1665.5. UV λmax () in CH2Cl2: 432 nm (29 000 M-1 cm-1).

Acknowledgment. We thank the KU Leuven (GOA) and the CNRS for financial support. G.K. and I.A. acknowledge a fellowship from the fund for scientific research-Flanders FWOV. S.Z. acknowledges a fellowship from the KU Leuven GOA 2006/03. Supporting Information Available: Experimental procedures and complete characterization of all compounds; copies of the 1H and 13C spectra of compounds 1, 6-13. This material is available free of charge via the Internet at http://pubs.acs.org. JO070888A