ORGANIC LETTERS
Synthesis and Characterization of a Series of Homooligopeptide Peroxyesters
2004 Vol. 6, No. 16 2753-2756
Fernando Formaggio,*,†,‡ Marco Crisma,‡ Laura Scipionato,† Sabrina Antonello,† Flavio Maran,† and Claudio Toniolo†,‡ Department of Chemistry, UniVersity of PadoVa, 35131 PadoVa, Italy, and Institute of Biomolecular Chemistry, CNR, PadoVa Unit, 35131 PadoVa, Italy
[email protected] Received May 26, 2004
ABSTRACT
A homologous series of stable Nr-phthaloyl peptide peroxyesters based on r-aminoisobutyric acid residues was prepared. In each of the six oligomers synthesized, the chain of r-amino acids is separated from the peroxyester function by a β-amino acid.
Peroxyesters are extensively exploited as initiators for the radical polymerization of vinyl compounds1 and represent useful sources for kinetics studies in radical chemistry, as they exhibit fast homolytic decomposition in the presence of nucleophiles and by thermolysis or photolysis.2 Peroxyesters are also used as oxidants in the Kharasch-Sosnovsky, copper-catalyzed, allylic oxidation reaction.3 Peroxyesters from R-amino acids have already been prepared in good yields, although their synthesis has been limited to the modification of (i) the R-COOH group of R-amino acid derivatives bis-acylated at the R-NH2 function (phthaloyl or succinoyl R-amino acids)4 and (ii) the β- or †
University of Padova. ‡ Institute of Biomolecular Chemistry. (1) Sheppard, C. S. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Klingsbergs, A., Piccininni, R. M., Salvatore, A., Baldwin, T., Eds.; Wiley: New York, 1988; Vol. 11, pp 1-21. (2) (a) Bartlett, P. D.; Simons, D. M. J. Am. Chem. Soc. 1960, 82, 17531756. (b) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Thang, S. H.; Suyama, S. J. Org. Chem. 2000, 65, 16-23. (c) Kim, S. S.; Tuchkin, A.; Kim, C. S. J. Org. Chem. 2001, 66, 7738-7740. (d) Engel, P. S.; Ying, Y.; He, S. Macromolecules 2003, 36, 3821-3825. (e) Sawaki, Y. In Organic Peroxides; Ando, W., Ed.; Wiley: New York, 1992; pp 426-477. (3) Andrus, M. B.; Lashley, J. C. Tetrahedron 2002, 58, 845-866. 10.1021/ol049028w CCC: $27.50 Published on Web 07/16/2004
© 2004 American Chemical Society
the γ-COOH group of an NR-acylated R-ester of the Asp or the Glu residue,5 respectively. In the past few years, the study of intramolecular electron transfer (ET) reactions in donor(D)-bridge-acceptor(A) molecules has provided relevant information on how electrons are transferred through bonds and through space (solvent). We have recently investigated intramolecular ET reactions within systems in which A undergoes reductive cleavage of a σ-bond (dissociative electron transfer, DET).6 More specifically, as illustrated in Figure 1A, the phthaloyl (Pht) imido moiety of the CR-tetrasubstituted R-amino acid (4) Ru¨chardt, C.; Hamprecht, G. Chem. Ber. 1968, 101, 3957-3962. (5) (a) Spantulescu, M. D.; Jain, R. P.; Derksen, D. J.; Vederas, J. C. Org. Lett. 2003, 5, 2963-2965. (b) Jain, R. P.; Vederas, J. C. Org. Lett. 2003, 5, 4669-4672. (6) (a) Maran, F.; Wayner, D. D. M.; Workentin, M. S. AdV. Phys. Org. Chem. 2001, 36, 85-166. (b) Antonello, S.; Formaggio, F.; Moretto, A.; Toniolo, C.; Maran, F. J. Am. Chem. Soc. 2001, 123, 9577-9584. (c) Moretto, A.; De Zotti, M.; Scipionato, L.; Formaggio, F.; Crisma, M.; Toniolo, C.; Antonello, S.; Maran, F.; Broxterman, Q. B. HelV. Chim. Acta 2002, 85, 3099-3112. (d) Antonello, S.; Crisma, M.; Formaggio, F.; Moretto, A.; Taddei, F.; Toniolo, C.; Maran, F. J. Am. Chem. Soc. 2002, 124, 11503-11513. (e) Antonello, S.; Formaggio, F.; Moretto, A.; Toniolo, C.; Maran, F. J. Am. Chem. Soc. 2003, 125, 2874-2875.
predominant formation of an acyl-pyridinium intermediate11 (in addition to a smaller amount of symmetrical anhydride),4,12 which, in the presence of tBuOOH (tert-butyl hydroperoxide), would eventually afford the NR-protected R-aminoacyl peroxyester. Any type of -COOH activation of NR-protected Aib homopeptides overwhelmingly generates the corresponding oxazol-5(4H)-one13 (Figure 2). In the presence of DMAP/ t BuOOH, this reactive intermediate was expected to afford the desired peptide peroxyester.
Figure 1. (A) DET of Pht-Aib-OOtBu. (B) Chemical structures of the NR-phthaloyl peroxyester (I) and dialkylperoxide (II) Aib homopeptide series and the NR-phthaloyl peroxyester Aib/1,2-Chx peptide series (III).
Aib (R-aminoisobutyric acid), which is easily reduced to its radical anion, was selected as D. The acceptor A was the -Aib-OOtBu tert-butyl peroxyester function, a well-characterized dissociative-type acceptor. Interestingly, we found that the DET rate of Pht-Aib-OOtBu is several orders of magnitude lower than the adiabatic limit. We also tackled the problem of the distance dependency of the intramolecular DET process. To achieve this goal and to control the electronic interaction between the D and A redox centers, we decided to exploit rigid, 310-helical peptide bridges.7 Accordingly, since Aib homooligomers are well-known to fold into this ordered secondary structure,8 we planned the synthesis of the peptide peroxyester series of type I (Figure 1B). In the published syntheses of Pht-Aib-OOtBu (I, n ) 1), activation of the COOH group and coupling of Pht-Aib-OH9 was achieved via the corresponding acid chloride and pyridine,4 in higher yields with 1-[3-(dimethylamino)propyl]3-ethylcarbodiimide (EDC) and 4-(dimethylamino)pyridine (DMAP),6b or with the acid chloride and DMAP.10 It is reasonable to assume that all these methods lead to the (7) (a) Toniolo, C.; Benedetti, E. Trends Biochem. Sci. 1991, 16, 350353. (b) Bolin, K. A.; Millhauser, G. L. Acc. Chem. Res. 1999, 32, 10271033. (c) Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C.; Broxterman, Q. B.; Kaptein, B. Biopolymers (Pept. Sci.) 2004, 76, 162-176. (8) (a) Marshall, G. In Intra-Science Chemistry Report; Kharasch, N., Ed.; Gordon and Breach: New York, 1971; pp 305-316. (b) Karle, I. L.; Balaram, P. Biochemistry 1990, 29, 6747-6756. (c) Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C. Biopolymers (Pept. Sci.) 2001, 60, 396419. (9) Leibfritz, D.; Haupt, E.; Dubischar, N.; Lachmann, H.; Oekonomopulos, R.; Jung, G. Tetrahedron 1982, 38, 2165-2181. (10) Moretto, A. Ph. D. Thesis, University of Padova, Padova, Italy, 2001. 2754
Figure 2. Proposed mechanism for the synthesis and decomposition of the peroxyester from the homotripeptide of the R-amino acid Aib (with formation of the related dialkyl peroxide).
Instead, this reaction yielded the series of peptide dialkyl peroxides of type II (Figure 1B) as the major products.6c,14 Similarly, dialkylperoxides were obtained when different types of NR-monoacylated Aib and MeAib (NR-methylated Aib) derivatives were treated under the same conditions.6c,14 In our proposed mechanism6c (Figure 2), the unstable peroxyester fragments heterolytically with loss of CO2 and formation of an iminium salt, as described by Ru¨chardt and Hamprecht.4 In the final step, the iminium ion is attacked by a second molecule of tBuOOH that ultimately produces the dialkyl peroxide derivative. On the basis of literature data4,5 and our own results,6b-e we reasoned that any destabilization of the critical iminium ion intermediate (e.g., (11) Scriven, E. F. V. Chem. Soc. ReV. 1983, 12, 129-161. (12) Valle, G.; Toniolo, C.; Jung, G. Liebigs Ann. Chem. 1986, 18091822. (13) (a) Jones, D. S.; Kenner, G. W.; Preston, J.; Sheppard, R. C. J. Chem. Soc. 1965, 6227-6239. (b) Bru¨ckner, H. In Chemistry of Peptides and Proteins; Ko¨nig, W. A., Woelter, W., Eds.; Attempto: Tu¨bingen, Germany, 1989; Vol. 4, pp 79-86. (14) Scipionato, L. Chemistry Degree Thesis, University of Padova, Padova, Italy, 2002. Org. Lett., Vol. 6, No. 16, 2004
Table 1. Syntheses and Isolated Yields of NR-Acylated Amino Acid and Peptide Peroxyesters
NR-acylated amino acid or peptide free acid PhAc-trans-(RS)-1,2-Chx-OH PhAc-cis-(RS)-1,4-Chx-OH Pht-trans-(RS)-1,2-Chx-OH Pht-(Aib)n-trans-(RS)-1,2-Chx-OH n)1 n)2 n)3 n)4 n)5 n)6 a
NR-acylated amino acid or peptide peroxyester PhAc-trans-(RS)-1,2-Chx-OOtBu PhAc-cis-(RS)-1,4-Chx-OOtBu Pht-trans-(RS)-1,2-Chx-OOtBu Pht-(Aib)n-trans-(RS)-1,2-Chx-OOtBu (III) n)1 n)2 n)3 n)4 n)5 n)6
yield (%) 94 65 70 88 85 75 78 81 66
AA ) amino acid.
by bis-acylation of the R-amino acid) or any chemical modification of the peptide C-terminal amino acid leading to a less stable iminium ion (e.g., when the C-terminal unit of the peptide chain is a β-, γ-, or a δ-amino acid) should have predominantly afforded a peroxyester compound. Our hypothesis was initially validated by the preparation of stable peroxyesters from NR-monoacylated (phenylacetylated, PhAc), conformationally restricted 2(or 4)-aminocyclohexane carboxylic acid (Chx)-based β- or δ-amino acids (throughout this work both chiral β- and δ-amino acids were used in their racemic forms). The NR-monoacylated δ-amino acid derivative PhAc-cis-(RS)-1,4-Chx-OH afforded the corresponding peroxyester in 65% isolated yield (Table 1), while from the NR-monoacylated β-amino acid derivative PhAc-trans-(RS)-1,2-Chx-OH the peroxyester was obtained in 94% yield.15 Not surprisingly, the reaction of the NR-bisacylated β-amino acid derivative Pht-trans-1,2-Chx-OH also proceeded smoothly (70% isolated yield). Formation of the peroxyesters was achieved from an iced CH2Cl2 solution of the corresponding free acids (1 equiv) in the presence of EDC hydrochloride (2 equiv). After a few minutes, DMAP (4 equiv) and a 5.5 M solution of tBuOOH in decane (6 equiv) were added and the resulting solution was refluxed for 2 h. The solvent was evaporated, and the product purified by flash chromatography. The NR-blocked amino acid peroxyesters were characterized by melting-point determination, TLC in three different elution systems, solidstate FT-IR absorption, 1H and 13C NMR spectrometries, and ESI-TOF mass spectrometry (see Supporting Information). In particular, the informative CdO stretching mode (IR spectrum) of the tert-butyl peroxyester function is found near 1770 cm-1, while the typical carbonyl carbon and quaternary carbon signals (13C NMR spectrum) of the same function are seen near 170.2 and 83.4 ppm, respectively. In two cases, PhAc-cis-(RS)-1,4-Chx-OOtBu and PhActrans-(RS)-1,2-Chx-OOtBu, the correctness of the chemical (15) Usual precautions regarding peroxides were followed for the preparation of all peroxyesters. Org. Lett., Vol. 6, No. 16, 2004
structures was further confirmed by X-ray diffraction analysis (Figure 3).16 Encouraged by these promising findings, we decided to synthesize the peptide series Pht-(Aib)n-trans-(RS)-1,2-ChxOOtBu (n ) 1-6) (III) following the experimental protocol described above. In all cases, the peroxyesters were obtained in good isolated yields (66-88%) (Figure 3). Unlike peptide dialkyl peroxides II, the peroxyesters (III) are stable compounds and could be stored for months without decomposition. All peroxyesters III were extensively characterized. The chemical shift of the tert-butyl protons in the 1H NMR spectrum proved to be particularly useful for rapidly discriminating the peptide esters (from which the peptide free acid starting materials were prepared by acidolysis) from the corresponding peroxyesters. The δ values of the free acids were observed at 1.33-1.42 ppm, while those of the latter compounds were seen at 1.21-1.28 ppm. A preliminary conformational analysis, carried out by FTIR absorption (see Supporting Information) and monodimensional 1H NMR (Figure 4), indicated that in the threedimensional structure supporting solvent CDCl3 the 310helical conformation, known to be overwhelmingly preferred by NR-protected (Aib)n homopeptide esters,17 is the most largely populated in the corresponding peroxyesters III as well. The typical NMR plots of 310-helical peptides as a function of temperature or of the addition of a perturbing agent (e.g., the hydrogen-bond acceptor solvent dimethylsulfoxide, DMSO) exhibit only two NH protons, N(1)H and N(2)H, sensitive to the environmental change. However, in (16) CCDC 238021 and 238022 contain the supplementary crystallographic data for the structures of PhAc-trans-(RS)-1,2-Chx-OOtBu and PhAc-cis-(RS)-1,4-Chx-OOtBu, respectively. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax (int.) +44-1223/336-033; e-mail
[email protected]]. (17) Toniolo, C.; Bonora, G. M.; Barone, V.; Bavoso, A.; Benedetti, E.; Di Blasio, B.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C. Macromolecules 1985, 18, 895-902. 2755
Figure 3. X-ray diffraction structures of PhAc-cis-(RS)-1,4-Chx-OOtBu (left) and PhAc-trans-(RS)-1,2-Chx-OOtBu (right). For each amino acid derivative, the three-dimensional structure of only one of the two enantiomers is reported. Also, only one of the two molecules (molecule A) in the asymmetric unit of the latter compound is shown (for the conformationally similar molecule B, see Supporting Information).
the present peptide series, the N(1) atom, being bis-acylated by the phthaloyl group, lacks the hydrogen atom.
the phthaloyl side of the so-formed carboxylate anion, the charge of which induces a negative shift of the peak.
Figure 4. (I) Temperature coefficients of the NH proton chemical shifts in the 1H NMR spectrum of Pht-(Aib)5-trans-(RS)-1,2-ChxOOtBu in CDCl3 solution. The N(6)H proton refers to that of the trans-1,2-Chx residue. (II) Variation of the NH proton chemical shifts of the same peptide upon addition of 10% DMSO (v/v) to the CDCl3 solution. Peptide concentration: 5 mM.
Figure 5. Background-subtracted cyclic voltammetry curves for the reduction of 1 mM C6H11-CO-OOtBu (curve a), Pht-(Aib)2(RS)-1,2-Chx-OtBu (curve b), and Pht-(Aib)2-(RS)-1,2-Chx-OOtBu (curve c) in CH CN/0.1 M nBu NClO . Glassy carbon electrode, 3 4 4 0.2 V s-1, 25 °C.
Cyclic voltammetry of peroxyesters III reflects typical features that are better appreciated by comparing the curve of III (n ) 2) with those of the corresponding model donor (the tert-butyl ester) and model acceptor (C6H11-CO-OOt Bu) molecules. Figure 5b, which shows the curve of the tert-butyl ester corresponding to Pht-(Aib)2-(RS)-1,2-ChxOOtBu, illustrates that indeed the phthaloyl moiety can be reduced reversibly and with little associated reorganization energy. The reduction of the peroxyester acceptor, on the other hand, is typical of a slow irreversible DET (Figure 5a).6d Finally, Figure 5c shows that electron injection into Pht-(Aib)2-(RS)-1,2-Chx-OOtBu involves the phthaloyl end of the peptide, which acts as an electron-transfer antenna. The peak is irreversible because of rapid intramolecular DET to the peroxyester end with formation of the corresponding carboxylate. The second peak is the reversible reduction of 2756
Preliminary results of the intramolecular DET to the peroxyester function have already indicated that the synthesized compounds III provide a particularly convenient series to investigate the role of the rigid, 310-helical -(Aib)nhomooligopeptide bridges in mediating the electron tunneling from D to A. In this context, the good chemical stability of peroxyesters III will be exploited to test the solvent effect on these DET reactions. Acknowledgment. The authors are grateful to MIUR (Ministry of Education, University, and Research) for financial support. Supporting Information Available: Experimental procedures and characterization details for all newly synthesized compounds. This material is available free of charge via the Internet at http://pubs.acs.org. OL049028W Org. Lett., Vol. 6, No. 16, 2004
