Efficient asymmetric epoxidation of α,β-unsaturated ketones using a soluble triblock polyethylene glycol-polyamino acid catalyst

Organic Letters

Volumn 3, Issue 5, 2001, Pages 683-686

  Flood, Robert W ^a   Geller, Thomas P ^a   Petty, Sarah A ^a   Roberts, Stanley M ^a   Skidmore, John ^a   Volk, Martin ^a  

^a UNIVERSITY OF LIVERPOOL   (United Kingdom)

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EID: 0001028390     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol007005l     Document Type: Article

References (34)

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5. (b) Allen, J. V.; Drauz, K.-H.; Flood, R. W.; Roberts, S. M.; Skidmore, J. Tetrahedron Lett. 1999, 40, 5417-5420.
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12. Kricheldorf, H. R.; Müller, D. Int. J. Biol. Macromol. 1983, 5, 171-178.
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14. Takagi, R.; Shiraki, A.; Manabe, T.; Kojima, S.; Ohkata, K. Chem. Lett. 2000, 366-367.
15. Some THF insoluble product was present. As this was also shown to be a good catalyst for asymmetric epoxidation presumably it arose from polymerization of the L-leucine NCA by adventitious water.
16. Determined by the % N in the copolymers. The amino-PEG is assumed to be fully bifunctionalized. Obviously, the polyleucine units on each end of the copolymers are not necessarily the same length.
17. 2 at rt for 20 min. The solution was filtered, and 1.7 mL of the filtrate was added to a vial containing copolymer including polyleucine (17 mg) and chalcone (17 mg). A second aliquot of oxidant and base in THF (1.7 mL) was added after 4 h.
18. Bentley, P. A.; Cappi, M. W.; Flood, R. W.; Roberts, S. M.; Smith, J. A. Tetrahedron Lett. 1998, 38, 9297-9300.
19. R refers to the solid support, see Supporting Information.
20. 2 and DBU were added.
21. By mass the PLL:chalcone ratio varied from ca. 1:5 (for the 6-mer) to 1:2 (for the 18-mer) and was always 1:1 in the experiments with the soluble polymer. More importantly, the ratio (number of polypeptide chains): chalcone was not varied significantly using this protocol.
22. 2 in a mixture of TBME and THF. Catalysts 1-4 gave significantly higher ees of epoxide 6 when tested under this protocol.
23. -1 and subtracting any residual water vapor lines.
24. (a) Arrondo, J. L. R.; Muga, A.; Castresana J.; Goñi, F. M. Prog. Biophys. Mol. Biol. 1993, 59, 23-56.
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26. (c) Surewicz, W. K.; Mantsch, H. H.; Chapman, D. Biochemistry 1993, 32, 389-394.
27. Compounds 8 and 9 show no catalytic activity, but were prepared to provide short reference polyleucine samples unable to form a well-defined secondary structure, in particular unable to form an α-helix. Their preparation is geiven in the Supporting Information.
28. The amide I spectra of all compounds were fitted simultaneously to the sum of a variable number of Lorentzian bands, using a nonlinear least-squares (Levenberg-Marquardt) fitting routine. The same Lorentzian band centre frequencies and widths, but different relative amplitudes, were used for all spectra. It was found that a minimum of five Lorentzians was needed to obtain a satisfactory fit. The use of six or more Lorentzians did not significantly improve the quality of the fit.
29. Chirgadze, Y. N.; Shestopalov, B. V.; Venyaminov, S. Y. Biopolymers 1973, 12, 1337-1351.
30. Chirgadze, Y. N.; Brazhnikov, E. V. Biopolymers 1974, 13, 1701-1712.
31. Nevskaya, N. A.; Chirgadze, Yu. N. Biopolymers 1976, 15, 637-648.
32. Graff, D. K.; Pastrana-Rios, B.; Venyaminov, S. Y.; Prendergast, F. G. J. Am. Chem. Soc. 1997, 119, 11282-11294.
33. These conclusions are in general agreement with those obtained in a recent publication on insoluble oligo-L-leucines of varying chain lengths, where it was shown that the efficiency of oligo-L-leucine as a Juliá-Colonna catalyst is related to its α-helical structure: Takagi, R.; Manabe, T.; Shiraki, A.; Yoneshige, A.; Hiraga, Y.; Kojima, S.; Ohkata, K. Bull. Chem. Soc. Jpn. 2000, 73, 2115-2121. Insoluble oligo-L-leucines show a significant contribution of β-sheet structure, which is thought to be formed upon aggregation of the insoluble polymer chains. No significant aggregation is observed for the soluble poly-L-leucines investigated here.
34. -1 are needed to obtain a satisfactory fit (Table 2). Although these wavelengths are characteristic of β-sheet structures (see refs 19, 22), at the present moment it is not clear whether these bands indicate a minor population of β-sheet structures for amino-PEG bound polyleucine or are due to some artifact, possibly related to the insoluble product found during catalyst synthesis. A more detailed investigation of these minor contributions is currently in preparation. The main conclusion, namely, that PEG-bound polyleucine is found to adopt a predominantly α-helical structure, is not affected by these minor bands.