ORGANIC LETTERS
Enantiodifferentiating Photoisomerization of (Z,Z)-1,3-Cyclooctadiene Included and Sensitized by Naphthoyl-Curdlan
2011 Vol. 13, No. 7 1856–1859
Gaku Fukuhara,* Mami Imai, Cheng Yang, Tadashi Mori, and Yoshihisa Inoue* Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
[email protected];
[email protected] Received February 8, 2011
ABSTRACT
6-O-(2-Naphthoyl)curdlan was newly synthesized as a sensitizing polysaccharide host to examine the chiroptical properties, supramolecular complexation, and photochirogenic behavior with (Z,Z)-1,3-cyclooctadiene (1ZZ). The enantiodifferentiating photoisomerization of 1ZZ included and sensitized by this polysaccharide host gave a highly strained chiral (E,Z)-isomer in up to 8.7% enantiomeric excess (ee) in solution and 11.7% ee in the solid state, which are the highest values ever reported for a supramolecular photochirogenesis of 1EZ.
Photochirogenesis,1 which opens a channel to optically active compounds via thermally forbidden routes, is one of the most challenging goals in current photochemistry. A supramolecular approach to photochirogenesis is advantageous due to the dual stereocontrol in both ground and excited states, and is also intriguing from an entropic point of view, as the enantiodifferentiation occurs in an inherently low entropic (well-organized) environment.1b,c,2 In a nonsupramolecular regime, the enantiodifferentiating photoisomerization of (Z,Z)-1,3-cyclooctadiene (1ZZ) (Scheme 1) sensitized by optically active arene(poly)carboxylates was examined to give chiral (E,Z)-isomer (1EZ) in modest enantiomeric excess (ee) of 17.6% with (-)(1) For reviews, see: (a) Griesbeck, A. G.; Meierhenrich, U. J. Angew. Chem., Int. Ed. 2002, 41, 3147. (b) Inoue, Y.; Ramamurthy, V., Eds. Chiral Photochemistry; Marcel Dekker: New York, 2004. (c) M€uller, C.; Bach, T. Aust. J. Chem. 2008, 61, 557. (d) Hoffmann, N. Chem. Rev. 2008, 108, 1052. (2) For representative papers, see: (a) Bach, T.; Bergmann, H.; Grosch, B.; Harms, K. J. Am. Chem. Soc. 2002, 124, 7982. (b) Fukuhara, G.; Mori, T.; Wada, T.; Inoue, Y. J. Org. Chem. 2006, 71, 8233. (c) Fukuhara, G.; Mori, T.; Inoue, Y. J. Org. Chem. 2009, 74, 6714. (d) Albrecht, D.; Vogt, F.; Bach, T. Chem.-Eur. J. 2010, 16, 4284. 10.1021/ol2003644 r 2011 American Chemical Society Published on Web 03/01/2011
menthyl mellitate and 1.2% with di-(-)-menthyl 1,4naphthalenedicarboxylate.3 These are highly contrasting to the much higher ee values reported for cycloheptene4 and cyclooctene,5 proving the difficulty of achieving high ee for 1EZ and the necessity to invent a new methodology.
Scheme 1. Enantiodifferentiating Photoisomerization of (Z,Z)1,3-Cyclooctadiene
(3) Inoue, Y.; Tsuneishi, H.; Hakushi, T.; Tai, A. J. Am. Chem. Soc. 1997, 119, 472. (4) Hoffmann, R.; Inoue, Y. J. Am. Chem. Soc. 1999, 121, 10702. (5) Inoue, Y.; Ikeda, H.; Kaneda, M.; Sumimura, T.; Everitt, S. R. L.; Wada, T. J. Am. Chem. Soc. 2000, 122, 406.
More recently, supramolecular approaches were applied to this photochirogenic reaction. Thus, the photoisomerization of 1ZZ was mediated by chiral sensitizing hosts, naphthalene-appended molecular clips,6 and cyclodextrins,7 to give 1EZ in low 0.7% and 4.6% ee, respectively. These results revealed that achieving high ee for 1EZ is an equally difficult task even in supramolecular photochirogenesis. Nevertheless, this approach is worth expanding from such simple clips and macrocycles to polymeric biomolecules, where the host structures are anticipated to be more dynamically manipulated by various environmental factors. Curdlan (Cur) is a linear glucan composed of (1f3)linked β-D-glucose units and is known to form a triple helical structure.8 The most intriguing feature of Cur is the ability to reversibly denature/renature by simply changing the solvent from water or aqueous acidic solution to DMSO or aqueous alkaline solution.8,9 Thus, Cur host is expected to provide an induced chiral environment fitted to the size and shape of the target guest through reversible complexation equilibrium with dynamic conformational changes. In this study, we newly synthesized a sensitizer-appended Cur, 6-O-(2-naphthoyl)Curdlan (Nap-Cur), as a sensitizing host, elucidated its chiroptical properties and complexation behavior with 1ZZ, and then investigated the enantiodifferentiating photoisomerization of 1ZZ included and sensitized by Nap-Cur.
The chiroptical properties of Nap-Cur were examined in DMSO and aqueous solutions. The CD spectrum of NapCur measured in DMSO showed an extremely weak negative Cotton effect at the 1La band of Nap centered at 280 nm (Figure 1, black line; the wavelength region
