Bis- and Trisamides Derived From 1′-Aminoferrocene-1-carboxylic

Mar 3, 2009 - Department of Chemistry and Biochemistry, Faculty of Food Technology and Biotechnology, Pierottijeva 6, HR-10000 Zagreb, Croatia Institu...
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Organometallics 2009, 28, 2028–2037

Bis- and Trisamides Derived From 1′-Aminoferrocene-1-carboxylic Acid and r-Amino Acids: Synthesis and Conformational Analysis Mojca Cˇakic´ Semencˇic´,† Daniel Siebler,‡ Katja Heinze,*,‡ and Vladimir Rapic´*,† Department of Chemistry and Biochemistry, Faculty of Food Technology and Biotechnology, PierottijeVa 6, HR-10000 Zagreb, Croatia Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-UniVersity of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany ReceiVed December 8, 2008

Ferrocene derivatives with one or two achiral and chiral arms based on R-amino acids (Gly, L-Ala, attached to the cyclopentadienyl rings were prepared by solution-phase peptide synthesis from N-acetyl- and N-Boc-protected 1′-aminoferrocene-1-carboxylic acids (Boc ) tert-butoxycarbonyl). The conformational preference in the solid state of selected examples was elucidated by X-ray crystallography. The chiroptical properties of chiral bis- and trisamides were investigated by circular dichroism (CD) spectroscopy in solution. The conformational preference was studied by NMR and IR spectroscopy, as well as by molecular modeling (DFT). For the bisamides, a conformational library is observed in solution. Increasing the steric bulk of the amino acid side chain disfavors several energetically accessible conformers of bisamides and specific conformers can be selected by changing the environment (type of solvent; solid/solution). For the trisamides, a single conformer is highly stabilized by two intramolecular hydrogen bonds irrespective of the size of the protecting group, the size of the amino acid side chain and the medium. L-Val)

Introduction In the past decade bioorganometallic chemistrysa hybrid area between biochemistry and organometallic chemistryshas attracted much attention, and the bioorganometallics found application in asymmetric catalysis, biology, medicine, molecular biotechnology, etc.1,2 This holds especially true for bioconjugates of ferrocene, with, for example, natural amino acids and peptides.3 The favorable electrochemical properties of ferrocene have been exploited for use as redox probes in its conjugates with biomolecules. The three-dimensional structure and the biological activity of proteins is induced by their secondary structure (Rhelices, β-sheets, turns). In this context ferrocenes are recognized as molecular scaffolds with the potential to mimic natural turn inducers in proteins (e.g., Proline, Pro) as the (almost) free rotating cyclopentadienyl rings are separated by about 3.3 Å which is an appropriate distance for intramolecular hydrogen bonding between podand peptide chains.4,5 This aspect has first been realized by Herrick and co-workers, who proposed the use of ferrocene-1,1′-dicarboxylic acid (Fcd) as a turn mimetic by designing conjugates of type I (m ) n ) 1, 2; X ) Y ) OR, * To whom correspondence should be addressed. Fax: + 385-4836-082. E-mail: [email protected] (V.R.). Fax: + 49-6131-3927277. E-mail: [email protected] (K.H.). † Faculty of Food Technology and Biotechnology. ‡ University of Mainz. (1) Jaouen, G. Bioorganometallics; Wiley-VCH: New York, 2006. (2) Kraatz, H.-B.; Metzler-Nolte, N. Concepts and Models in Bioinorganic Chemistry; Wiley-VCH: New York, 2006. (3) Van Staveren, D. N.; Metzler-Nolte, N. Chem. ReV. 2004, 104, 5931– 5985. (4) Kirin, S. I.; Kraatz, H.-B.; Metzler-Nolte, N. Chem. Soc. ReV. 2006, 35, 348–354. (5) Moriuchi, T.; Hirao, T. Chem. Soc. ReV. 2004, 33, 294–301. (6) Herrick, R. S.; Jarret, R. M.; Curran, T. P.; Dragoli, D. R.; Flaherty, M. B.; Lindyberg, S. E.; Slate, R. A.; Thornton, L. C. Tetrahedron Lett. 1996, 37, 5289–5292.

Scheme 1).6 Detailed structural work has been carried out by several groups, in particular those of Hirao and Kraatz.7-13 In the solid state the structures (independent of the number and type of amino acids and nature of ester groups included) are mostly stabilized by two 10-membered rings (β-turn) spanning NH groups from the substituent of one Cp ring to CO functions of the substituent of the other Cp ring of ferrocene (these four functions are constituents of two “inner” amino acids). These intramolecular hydrogen bonds are sufficiently strong to allow spectroscopic observation of these stable “Herrick conformers” in solution. In recent work from our groups the influence of the size of amino acid side chains to the conformational preferences in “desymmetrized” bioconjugates of type I (m ) 1; n ) 0; AA ) Gly, Ala, Val; X ) NHMe; Y ) OMe, NHMe, Scheme 1) has been examined.14 These systems feature two or three NH groups as possible hydrogen-donors and three hydrogenaccepting carbonyl functions. In weakly coordinating solvents these compounds have to be described as an ensemble of interconverting conformations. Increasing the steric demand of the amino acid reduces the number of energetically accessible conformations. Larger amino acid side chains favor conformations with a 1′-NH · · · OC intramolecular hydrogen bond which (7) Nomoto, A.; Moriuchi, T.; Yamazaki, S.; Ogawa, A.; Hirao, T. J. Chem. Soc., Chem. Commun. 1998, 1963–1964. (8) Moriuchi, T.; Nomoto, A.; Yoshida, K.; Hirao, T. J. Organomet. Chem. 1999, 589, 50–58. (9) Moriuchi, T.; Nomoto, A.; Yoshida, K.; Hirao, T. Organometallics 2001, 20, 1008–1013. (10) Moriuchi, T.; Nomoto, A.; Yoshida, K.; Ogawa, A.; Hirao, T. J. Am. Chem. Soc. 2001, 123, 68–75. (11) Xu, Y.; Saweczko, P.; Kraatz, H.-B. J. Organomet. Chem. 2001, 637-639, 335–342. (12) Kraatz, H.-B.; Galka, M. Met. Ions Biol. Syst. 2001, 38, 385–409. (13) Oberhoff, M.; Duda, L.; Karl, J.; Mohr, R.; Erker, G.; Fro¨hlich, R.; Grehl, M. Organometallics 1996, 15, 4005–4011. (14) Lapic´, J.; Siebler, D.; Heinze, K.; Rapic´, V. Eur. J. Inorg. Chem. 2007, 2014–2024.

10.1021/om801163s CCC: $40.75  2009 American Chemical Society Publication on Web 03/03/2009

Bis- and Trisamides Scheme 1. Amides Derived from Ferrocene-1,1′-dicarboxylic Acid (Type I; X, Y ) OR; top) and from 1′-Aminoferrocene-1-carboxylic Acid (Type II; X ) Ac, Boc; bottom)a

Organometallics, Vol. 28, No. 7, 2009 2029 Scheme 2. Syntheses of Fca Amidesa

a Arrows point from the N- to the C- termini of the amides. The dashed hydrogen bonds shown are only formed between subunits directly attached to ferrocene (the “inner” amino acids).

stabilizes chiral secondary structures with helical ferrocene units.14 An important consequence of the symmetrical nature of complexes of type I is that only parallel peptide strands can be formed (Scheme 1). Natural peptide turns, however, will always result in antiparallel peptide strands. This feature is realized in compounds of type II, which incorporate 1′aminoferrocene-1-carboxylic acid (Fca)15 coupled to natural R-amino acids (Scheme 1). Unlike previous metallocene turn structures based on Fcd, these Fca conjugates are true organometallic turn mimetics, maintaining an antiparallel orientation of the peptide strands supported by intramolecular hydrogen bonds. Synthesis and conformational analysis of oligoamides of type II (n ) 0 - 3; m ) 1 - 4; various L- and D-amino acids, Scheme 1) were described in previous studies of our groups.16-20 A systematic study of L- and D-Ala containing Fca-derived Bocprotected oligoamides ranging from di- to pentaamides (type II, X ) Boc, R ) CH3, Scheme 1) both in solution and in the solid state has been conducted by means of CD, IR, 1H NMR (variation ratio) spectroscopy and X-ray diffraction analysis.16,17 The helical chirality of the metallocene unit is an important property of Fca amides with chiral substitutents. For example, the tetrakisamide Boc-Ala-Fca-Ala-Ala-OMe is characterized by a (P)-helical ferrocene and a positiVe Cotton effect around λmax ) 480 nm which is a characteristic spectral region of the ferrocene chromophore.16,17 The data of the Boc-tetrakisamide are in accordance with the chiroptical properties of type I conjugates derived from L-amino acids: a (P)-helical chirality (15) (a) Barisˇic´, L.; Rapic´, V.; Kovacˇ, V. Croat. Chem. Acta 2002, 75, 199–210. (b) Pavlovic´, G.; Barisˇic´, L.; Rapic´, V.; Leban, I. Acta Crystallogr. 2002, E58, m13–15. (c) Pavlovic´, G.; Barisˇic´, L.; Rapic´, V.; Kovacˇ, V. Acta Crystallogr. 2003, C59, m55–57. (d) Barisˇic´, L.; Rapic´, V.; Pritzkow, H.; Pavlovic´, G.; Nemet, I. J. Organomet. Chem. 2003, 682, 131–142. (e) Heinze, K.; Schlenker, M. Eur. J. Inorg. Chem. 2004, 2974–2988. (f) Okamura, T.; Sakauye, K.; Ueyama, N.; Nakamura, A. Inorg. Chem. 1998, 37, 6731–6736. (16) Barisˇic´, L.; Dropucˇic´, M.; Rapic´, V.; Pritzkow, H.; Kirin, S. I.; Metzler-Nolte, N. Chem. Commun. 2004, 2004–2005. (17) Barisˇic´, L.; Cˇakic´, M.; Mahmoud, K. A.; Liu, Y.; Kraatz, H.-B.; Pritzkow, H.; Kirin, S. I.; Metzler-Nolte, N.; Rapic´, V. Chem.-Eur. J. 2006, 12, 4965–4980. (18) Heinze, K.; Beckmann, M. Eur. J. Inorg. Chem. 2005, 3450–3457. (19) Barisˇic´, L.; Rapic´, V.; Metzler-Nolte, N. Eur. J. Inorg. Chem. 2006, 4019–4021. (20) Heinze, K.; Wild, U.; Beckmann, M. Eur. J. Inorg. Chem. 2007, 617–623.

a (a) 1. HOBt/EDC, CH Cl 2. AA-OMe · HCl, Et N, CH Cl , (b) 2 2 3 2 2 HCl(g)/ EtOAc, (c) 1. HOBt/EDC, CH2Cl2, 2. Boc-AA-OH, CH2Cl2, (d) AcCl, Et3N, CH2Cl2 (a denotes N-Ac and b N-Boc derivatives).

of the ferrocene core is characterized by a positive Cotton effect and, vice versa, D-amino acids containing type I conjugates show a (M)-helical conformation of the ferrocene and a mirror image CD spectrum.6-20 Conformational analysis of N-acetyl bisamides containing L-Val and L-Ile subunits instead of L-Ala [AcFca-AA-OMe, AA ) L-Val (3a), L-Ile] demonstrate that in these conjugates (P)-helical conformations also dominate in solution as judged from the positive CD signal at λmax ) 450 nm in CH2Cl2.18 Contrary to the above-described Fca- and Fcd-derived amides with L-amino acids and (P)-helical ferrocene moieties the bisamide Boc-Fca-Ala-OMe (2b) shows a (M)-helical ferrocene unit in the solid state and a negatiVe Cotton effect in the ferrocene spectral region in CH3CN solution.17 The puzzling discrepancy in the chiroptical properties of BocFca-Ala-OMe (2b)17 and Ac-Fca-AA-OMe [AA ) Val (3a), Ile]18 prompted us to perform a thorough conformational analysis of complete sets of bisamides X-Fca-AA-OMe and trisamides X-AA1-Fca-AA2-OMe (X ) Ac, Boc; AA ) Gly, Ala, Val) in order to elucidate the influence of the protecting group (Ac/Boc), the influence of the size of the amino acid side chain (R ) H, Me, iPr)14 and the medium (solid state, different solvents) on the conformational preferences in these bis- and trisamides. The synthesis and properties of the previously reported bisamide Ac-Fca-Val-OMe 3a (prepared previously by solution phase peptide synthesis methods),18 as well as of the trisamides Ac-Ala-Fca-Ala-OMe 5a (prepared previously by Solid-Phase Peptide Synthesis methods, SPPS)20 and Boc-AlaFca-Ala-OMe 5b (prepared previously by solution phase peptide synthesis methods)17 are included here for comparison (Scheme 2). The octaamide Ac-Val-Gly-Ala-Fca-Ala-Gly-Val-Leu-NH2 (prepared by SPPS techniques) also features a (P)-helical ferrocene subunit as judged from the positive Cotton effect at λmax ) 480 nm in DMSO).19 Peptidic foldamers derived from

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Figure 1. Molecular structure of bisamide 1a in the solid state and intermolecular hydrogen bonds.

Boc-[Ala-Fca]n-OMe (n ) 1-4) also realize a positive Cotton effect around λmax ) 450 nm (in CH3CN) in conjunction with (P)-helical ferrocene units.21

Results and Discussion Synthesis of Fca bisamides 1a-3a, 1b-3b and Fca trisamides 4a-6a, 4b-6b. N-Acetyl- (7, Ac-Fca) and N-Bocprotected (8, Boc-Fca) 1′-aminoferrocene-1-carboxylic acids were prepared by previously described procedures.15 Ac-Fca (7) was activated using the HOBt/EDC protocol and coupled to glycine (Gly), L-alanine (Ala) and L-valine (Val) methyl esters giving the N-acetyl bisamides 1a (90%), 2a (83%) and 3a (89%) as depicted in Scheme 2. The same R-amino acid esters were coupled in a similar fashion to activated Boc-Fca 8 giving the N-Boc-protected bisamides 1b (87%), 2b (85%) and 3b (76%), respectively. Deprotection of N-Boc bisamides 1b, 2b, and 3b was performed by action of gaseous hydrochloric acid in ethyl acetate. The resulting amine hydrochlorides were treated with an excess of triethyl amine (pH ≈ 8) and the resulting free amines were coupled to Boc-AA-OH (AA ) Gly, Ala or Val) giving 4b (67%), 5b (77%), and 6b (72%), respectively. These N-Boc protected trisamides were subjected to Boc-deprotection by HCl and N-acylation by acetyl chloride affording the N-Ac derivatives 4a (77%), 5a (85%), and 6a (81%), respectively (Scheme 2). All Ac- and Boc-protected oligoamides were fully characterized by spectroscopic and mass spectrometric techniques (see Experimental Section for low- and high resolution mass spectrometric data and Tables S1-S4 in the Supporting Information for 1H NMR and 13C NMR spectroscopic data). Molecular Structures of 1a, 4a, 5a, 6a, and 6b in the Solid State. Ferrocene conjugates 1a, 4a, 6a, and 6b were crystallized from dichloromethane to give crystals suitable for single crystal X-ray analysis while 5a was crystallized from THF (Figures 1-5). The achiral glycine derivatives 1a and 4a crystallize in the monoclinic space groups P21/c (1a) and P21/a (4a) while the chiral amides crystallize in the orthorhombic space groups C2221 (5a) and P212121 (6a, 6b). The asymmetric unit of crystals of 6a contains a molecule of dichloromethane and two independent molecules of 6a (denoted Fe1, Fe2) which differ only in the orientation of the isopropyl group of Val1 (Fe1:H16-C16-C22-H22-66.4°;Fe2:H116-C116-C122-H122 175.3°) and to only a minor extend in some other torsion angles (21) Chowdhury, S.; Schatte, G.; Kraatz, H.-B. Angew. Chem. 2006, 118, 7036–7038; Angew. Chem., Int. Ed. 2006, 45, 6882-6884. (22) (a) Angelici, G.; Falini, G.; Hofmann, H.-J.; Huster, D.; Monari, M.; Tomasini, C. Angew. Chem. 2008, 120, 8195–8198; Angew. Chem., Int. Ed. 2008, 47, 8075-8078. (b) Binder, W. H.; Smrzka, O. W. Angew. Chem. 2006, 118, 7482–7487; Angew. Chem., Int. Ed. 2006, 45, 73247328.

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(Figure 4, Table 1). The hydrogen bonding motifs of the two independent molecules Fe1 and Fe2 are identical so that the following discussion can be focused on Fe1 (Figure 4). In the solid state 1a features an intramolecular hydrogen bond from the NHFca group to the ester carbonyl group [N2H2 · · · O2C13] (Figure 1, hydrogen bond a: N · · · O 3.0 Å) giving a 9-membered ring and an intermolecular hydrogen bond from the NHGly unit to the acetyl amide carbonyl group of a neighboring molecule [N1H1 · · · O4_$1C15_$1 with symmetry code x, -y + 1/2, z + 1/2] (Figure 1, hydrogen bond b: N · · · O 2.8 Å) giving a chain of molecules along the crystallographic c axis. Thus both NH groups of 1a are involved in hydrogen bonds in the solid state. Interestingly, a fiberlike peptide material based on the organic dipeptide Boc-L-Phe-D-Oxd-OBn (Phe ) phenylalanine; Oxd ) 4-methyl-5-carboxy oxazolidin-2-one; Bn ) benzyl) which also exhibits a single intermolecular hydrogen bond has been reported very recently.22a Moreover the formation of fibers, fibrils and sheets from peptides is associated with several diseases such as Alzheimer’s or prion diseases that feature pathological changes in protein conformation.22b The achiral bis(glycinyl) derivative 4a only displays intermolecular hydrogen bonds in the solid state (Figure 2). The two substituents Ac-Gly1 and Gly2-OMe (the Gly1 part is disordered over two orientations) of the ferrocene point in nearly opposite directions with δ ) -169°. All NH groups are involved in hydrogen bonds with N · · · O distances between 2.8-3.0 Å giving a sheet-like structure in the crystallographic ab plane. All the chiral trisamides 5a, 6a, and 6b exhibit the two expected intramolecular hydrogen bonds16,17,20 NHFca · · · OCester [N2H2 · · · O2C13] (hydrogen bond a: N · · · O 2.9-3.0 Å) and NHAA2 · · · OCAc/Boc [N1H1 · · · O5C17] (hydrogen bond c: N · · · O 2.8-2.9 Å) as depicted in Figures 3-5. This gives unstrained 9- and 11-membered rings (Table 1) irrespective of the sterics of the two R-amino acid subunits AA1/AA2 and the protecting group Ac or Boc (Figures 35). These two hydrogen bonds also induce a positive ferrocene helicity of δ ≈ 50° in all compounds (Table 1) similar to the secondary structure of Boc-Ala-Fca-Ala-Ala-OMe.16,17 The individual molecules of the trisamides are connected by single intermolecular hydrogen bonds NHAA1 · · · OCFca [N3H3 · · · O1_$1C11_$1] between molecules translated along a crystallographic axis of around 9.5 Å (hydrogen bond b: N · · · O 2.8-2.9 Å). Generally, in the solid state all NH groups are involved in hydrogen bonding either in an intra- or in an intermolecular fashion. As ester units are only hydrogen acceptor groups there is a mismatch of hydrogen donor groups to hydrogen acceptor groups (NH:CO) leaving one carbonyl group without a hydrogen bond (1a: COFca; 4a: COester; 5a, 6a, 6b: COAA1). NMR and IR Spectroscopy of Fca-bisamides 1a-3a, 1b-3b and Fca-trisamides 4a-6a, 4b-6b. The solution IR spectra of all amides display signals due to hydrogen-bonded NH groups around 3270-3390 cm-1 together with signals around 3440 cm-1 assigned to free NH groups. Absorption bands around 1730 cm-1 correspond to hydrogen-bonded ester carbonyl groups and bands around 1740 cm-1 are assigned to free ester carbonyl groups. In addition several amide I and amide II bands are observed (Table 2).23 The data suggest that in CH2Cl2 solution the ester CO groups of all trisamides 4a, 4b, 5a, 5b, 6a, and 6b are engaged in hydrogen bonds irrespective of the steric bulk of the amino acid and protecting groups employed. This is compatible with the stable doubly hydrogen-bonded conformer found in the solid state of 5a, 6a, and 6b. This interpretation also underscores that a structure observed in the

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Organometallics, Vol. 28, No. 7, 2009 2031 Table 1. Selected Bond Angles (deg) of 1a, 4a, 5a, 6a, and 6b

1a 4a 5a 6a (Fe1) 6a (Fe2) 6b a

δa

O1-C11-C6-C7

C15-N2-C1-C2

C12-N1-C11-O1

C1-N2-C15-O4

8.4 -169.1 58.9 49.8 50.0 52.4

-0.7 15.4 5.3 3.0 7.1 3.9

-176.5 163.8 167.4 164.5 -179.5 179.4

2.0 -10.3 0.8 -8.2 -4.5 -3.2

2.7 -1.0 3.3 5.9 2.4 2.8

δ is defined as the relative rotation of Cp rings (0° ) fully eclipsed; 36° ) fully staggered).

Figure 2. Molecular structure of trisamide 4a in the solid state and intermolecular hydrogen bonds.

Figure 3. Molecular structure of trisamide 5a in the solid state and intermolecular hydrogen bonds.

solid state can be completely different from the structure or conformation in solution (4a in the solid state: free COester; 4a in solution: hydrogen-bonded COester). In all the bisamides examined free ester carbonyl groups are additionally present (shoulders at the absorption band) suggesting that several conformers namely with and without a hydrogen bond to COester are present in solution. The proton NMR spectra show all expected signals (Tables S1 and S2, Supporting Information). Especially important are the resonances of the different amide protons NHFca and NHAA1/ NHAA2 which are easily assigned by correlation spectroscopy (HH-COSY experiments), NOE spectroscopy and coupling patterns. NHFca and NHAla2 are engaged in intramolecular hydrogen bonds while NHAla1 only forms intermolecular hydrogen bonds at higher concentration (Figure 6) or at lower temperature (Figure 7). The same behavior is found for the Bocprotected derivative 5b, albeit association via NHAla1 appears to occur at lower temperature as compared to the acetylprotected 5a probably due to steric hindrance imposed by the tert-butyl group (Figure 7). Similar plots are obtained for the other trisamides 4a/4b and 6a/6b (see Supporting Information) and the same arguments apply. Thus, in solution, all the trisamides realize the double hydrogen intramolecular bond as the most preferred conformational motif. In CD2Cl2 or CDCl3 solution the NMR data of bisamides (chemical shifts, temperature and concentration dependence,

Figure 4. Molecular structure of trisamide 6a in the solid state and intermolecular hydrogen bonds of the two independent molecules in the unit cell (top: Fe1; bottom: Fe2). The main structural difference between Fe1 and Fe2 is highlighted by a blue ellipse.

Figure 5. Molecular structure of trisamide 6b in the solid state and intermolecular hydrogen bonds.

Figures 8-9 and Supporting Information) suggest that both amide protons are weakly involved in hydrogen bonds, however to a lesser extent than found in the trisamides. This is due to that fact that in one conformation only one intramolecular hydrogen bond can be formed thus interconverting and equilibrating species have to be assumed in solution. The most stable hydrogen bond appears to be that involving NHFca in the acetylprotected derivatives 1a, 2a, and 3a (δ ) 7.8-7.9 ppm in CDCl3). The Boc-protected bisamides feature even weaker if any hydrogen bonds in solution as judged from the chemical shifts of the amide protons (δ < 7 ppm). Also, distinct intermolecular interactions as observed for NHAA1 in the

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Figure 6. Concentration dependent 1H NMR chemical shifts of the amide protons of 5a (top) and 5b (bottom) in CDCl3. Table 2. IR Spectroscopic Data of Bis- and Trisamide (ν/ cm-1) in CH2Cl2 (c ) 10-2 M) NHfree 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b

3433 3434 3434 3433 3433 3433 3455 3454 3439 3440 3437 3433

NHasscoc

COester

amide I

amide II

3350 3342 3351 3328 3354 3335 3318, 3272 3387, 3327 3329 3372, 3328 3328 3369, 3326

1745 1723 1739 1731 1739, 1728(sh) 1717 1731 1732 1726 1729 1721 1720

1681, 1663 1661 1682, 1657 1714, 1655 1682, 1661 1659 1649 1698, 1652 1692, 1668, 1648 1696, 1647 1671, 1646 1693, 1647

1519 1533, 1520 1514 1536 1511 1532, 1509 1575, 1533 1569, 1528, 1509 1571, 1527 1567, 1525, 1502 1572, 1524 1568, 1520, 1499

trisamides (Figures 6-7) are absent for the bisamides probably due to the manifold of solution equilibria present. Several such conformers are depicted in Scheme 3. Conformer A of the bisamides represents exactly “one half” of the double hydrogen-bond motif of the trisamides, that is, the NHFca · · · OCester hydrogen bond with the two NH vectors NHFca and NHAA pointing toward each other. This conformation appears to be present in Ac-Fca-AA-OMe [AA ) L-Val (3a), 18 L-Ile]. The B conformation (Scheme 3) features a flipped NHAA vector with respect to that of A (observed in the solid state of achiral 1a, vide supra) and the C conformation features an inverted NHFca vector relative to that of conformer A and a NHAA · · · COAc/Boc hydrogen bond. The pseudo mirror image C′ is observed in the solid state of chiral 2b17 and C/C′ conformers have been suggested for the achiral bis(ferrocene) derivative Fmoc-Fca-NH-Fc.24 In the bisamides one free NH unit (bold NH group in Scheme 3) is available for intermolecular hydrogen bonding either to CdO groups of neighboring molecules (solid state) or to solvent molecules (when dissolved in hydrogen bond accepting solvents) so that it is expected that intermolecular interactions might influence or even determine the type of intramolecular hydrogen bond in the bisamides.

Figure 7. Temperature dependent 1H NMR chemical shifts of the amide protons of 5a (top) and 5b (bottom) in CD2Cl2 (c ) 2 × 10-2 m).

Figure 8. Concentration dependent 1H NMR chemical shifts of the amide protons of 2a (top) and 2b (bottom) in CDCl3.

UV/vis and CD Spectroscopy of Fca-bisamides 1a-3a, 1b-3b, and Fca-trisamides 4a-6a, 4b-6b. In the UV/vis spectra of the oligoamides in CH2Cl2 or CH2Cl2/DMSO (20% v/v) the typical ferrocene absorption is observed at around λmax ) 445 nm irrespective of the substituents attached to the cyclopentadienyl rings and the polarity or hydrogen accepting property of the solvent (Table 3). The CD spectra of the chiral trisamides 5a/5b and 6a/6b are almost superimposable and display a strong positive signal at

Bis- and Trisamides

Organometallics, Vol. 28, No. 7, 2009 2033 Scheme 3. Formal Genealogy of Experimentally Observed Conformations of Short Bisamides (With 9-Membered and 8-Membered Hydrogen Bonded Rings) in Solution and in the Solid State Derived from the Stable Conformation of Trisamidesa

Figure 9. Temperature dependent 1H NMR chemical shifts of the amide protons of 2a (top) and 2b (bottom) in CD2Cl2 (c ) 2 × 10-2 M).

a

Red arrows denote relevant NH vectors.

Table 3. UV/Vis and CD Spectroscopic Data of Chiral Bis- and Trisamides in CH2Cl2 and CH2Cl2/DMSO (20% v/v)a λmax () 2a 2b 3a 3b 5a 5b 6a 6b a

λmax (Mθ)

CH2Cl2

CH2Cl2/DMSO

CH2Cl2

CH2Cl2/DMSO

446 (700) 446 (540) 446 (565) 445 (490) 446 (600) 446 (560) 446 (425) 445 (460)

446 (555) 446 (545) 446 (520) 445 (450) 446 (545) 446 (440) 446 (400) 445 (430)

491 (-9.9) 458 (4.9) 451 (9.1) 451 (7.8) 469 (100) 468 (90) 468 (97) 468 (99)

484 (-5.1) 463 (0.3) 459 (2.6) 458 (2.2) 469 (60) 467 (51) 468 (54) 468 (49)

λmax in nm;  in M-1 cm-1; Mθ in deg M-1 cm-1; c ) 5.5 × 10-4

M.

Figure 10. CD spectra of 6a (top) and 6b (bottom) in CH2Cl2 and in CH2Cl2/DMSO (20% v/v).

λmax ) 468 nm in CH2Cl2 which is reduced in intensity to 50-60% when DMSO is added as a competitive hydrogen accepting solvent (Table 3, Figure 10, Supporting Information). Thus the (P)-helical conformation of the ferrocene unit as enforced by the two intramolecular hydrogen bonds is strongly favored in CH2Cl2 and only partially disrupted by DMSO.

The bisamides 2b, 3a/3b also display a positive Cotton effect at λmax ) 450-460 nm in CH2Cl2 which is reduced in intensity upon addition of DMSO (Table 3, Figure 11, Supporting Information). However, the absolute value is much lower than that observed for the trisamides (Table 3). Thus there appears to be only a weak biased right-handed helical conformation for 2b and 3a/3b in CH2Cl2 solution. In CH3CN 2b displays a weak negative signal suggesting that (M)-helical conformers are slightly favored in this weakly hydrogen-accepting solvent (see Supporting Information).17 A negative Cotton effect is also observed for the acetyl-derivative 2a in CH2Cl2. These data

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Figure 12. Chart of the relative energies of hydrogen-bonded conformers of chiral 2a/2b and 3a/3b. Scheme 4. Calculated Conformations of Short Bisamides with 6-Membered Hydrogen Bonded Ringsa

Figure 11. CD spectra of 2a (top) and 2b (bottom) in CH2Cl2 and in CH2Cl2/DMSO (20% v/v).

support the interpretation that several conformers are required to adequately describe the properties of Fca-containing bisamides in solution. Cyclic Voltammetry of Selected Bis- and Trisamides 1a, 1b, 2a, 3b, 4a, 4b, 6a, 6b. The electrochemical behavior of 1a, 1b, 2a, 3b, 4a, 4b, 6a, and 6b was studied by cyclic voltammetry in CH2Cl2 with nBu4NPF6 as supporting electrolyte. Under these conditions all ferrocene conjugates exhibit a reversible electrochemical one-electron oxidation to the corresponding ferricinium cations around 0.53 V versus SCE almost independently of the R-amino acids and protecting groups employed [E1/2 ) 0.52 V (1a), 0.48 V (1b), 0.53 V (2a), 0.53 V (3b), 0.53 V (4a), 0.58 V (4b), 0.55 V (6a), 0.55 V (6b)]. A representative cyclic voltammogram of 3b is depicted in the Supporting Information. It has been noted before that remote substituents barely influence the redox potential of the ferrocene core while directly bound substituents may modify the redox potential significantly.17,20,24-26 DFT Modeling of Chiral Bisamides 2a/2b and 3a/3b. A full conformational energy landscape on chiral 2a/2b and 3a/ 3b featuring an intramolecular hydrogen bond has been calculated by DFT methods (B3LYP, Lanl2DZ, gas phase, no symmetry constraints).14,15e,18,20,24,27-31 The results are depicted in Figure 12 with conformers A, B, and C shown in Scheme 3 and conformers D and E (along with their pseudo mirror images) are presented in Scheme 4. Evidently, many conformers fall within a narrow energy band of 20 kJ mol-1. The energetically accessible subpopulation A/A′, B/B′ features a hydrogen bond (23) Vass, E.; Hollo´si, M.; Besson, F.; Buchet, R. Chem. ReV. 2003, 103, 1917–1954. (24) Heinze, K.; Siebler, D. Z. Anorg. Allg. Chem. 2007, 633, 2223– 2233. (25) Heinze, K.; Schlenker, M. Eur. J. Inorg. Chem. 2005, 66–71. (26) Lu, S.; Strelets, V. V.; Ryan, M. F.; Pietro, W. J.; Lever, A. B. P. Inorg. Chem. 1996, 35, 1013–1023.

a

Red arrows denote relevant NH vectors.

to the ester carbonyl group COester (Scheme 3) which agrees with the IR data (vide supra) and the B/B′ conformation has been observed in the solid state of achiral 1a (Scheme 3, Figure 1). There is no clear trend discernible concerning the steric bulk of the protecting group Ac or Boc in 2a/2b and 3a/3b. However, (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr. T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C. and Pople, J. A. Gaussian 03, Revision B.03; Gaussian, Inc., Pittsburgh, PA, 2003. (28) Heinze, K.; Beckmann, M. J. Organomet. Chem. 2006, 691, 5576– 5584. (29) Lapic´, J.; Pavlovic´, G.; Siebler, D.; Heinze, K.; Rapic´, V. Organometallics 2008, 27, 726–735. (30) Djakovic´, S.; Siebler, D.; Cˇakic´-Semencˇic´, M.; Heinze, K.; Rapic´, V. Organometallics 2008, 27, 1447–1453. (31) Kovacˇ, V.; Radolovic´, K.; Habusˇ, I.; Siebler, D.; Heinze, K.; Rapic´, V. Eur. J. Inorg. Chem. 2009, 389–399.

Bis- and Trisamides

Organometallics, Vol. 28, No. 7, 2009 2035

the increasing size of the amino acid side chain (R ) Me or iPr) in 2 and 3 seems to destabilize certain conformations, for example, (M)-A′, (P)-B, (M)-D′, (P)-E for 3a/3b (Figure 12) and restricts the available conformational space. An analogous result has been put forward for conjugates of Fcd and R-amino acids with varying steric bulk.14 In other words, R-amino acids with small side chains (R ) H, Me) allow for many accessible conformations of Fca-bisamides while larger side chains (R ) iPr) reduce the number of accessible conformations. From such a dynamic library certain conformations can be selected (stabilized) by an appropriate choice of solvent (CH2Cl2/CH3CN) or medium (solid/solution) which modifies the free NH group (bold in Schemes 3 and 4) by intermolecular hydrogen bonding and thus changes thermodynamic preferences. The experimental results on Boc-protected trisamides of the type Boc-AA-Fca-AA-OMe (vide supra) are in full agreement with DFT-modeled conformational preferences of acetylprotected analogues reported previously.20 Thus the most stable conformation with two intramolcular hydrogen bonds (Scheme 3) is realized irrespective of the protecting group. Thus further DFT modeling studies on trisamides appear obsolete.

Conclusions In solution Fca-containing bisamides 1a/1b, 2a/2b, and 3a/ 3b form a library of conformational isomers. Some conformers are destabilized when the steric demand of the amino acid side chain increases (R ) H, Me, iPr for AA ) Gly, Ala, Val). Even changing the solvent from CH2Cl2 to CH3CN can promote some conformers over others (probably by forming hydrogen bonds to CH3CN). In the solid state intermolecular forces (hydrogen bonds) can override the preferences induced by weak intramolecular forces (hydrogen bonds). For the trisamides 4a/4b, 5a/ 5b, and 6a/6b one single conformer is highly stabilized in solution by two intramolecular hydrogen bonds irrespective of the size of the protecting group (Ac, Boc) or the size of the amino acid side chain (R ) H, Me, iPr) and a significant bias toward the right-handed helical conformation of the ferrocene unit is revealed by strong CD absorptions. This study rationalizes and confirms the division of Fcaoligoamides of type II (Scheme 1) into two well-defined subclasses: (a) short oligoamides (n ) 0, m ) 1) (with elucidation of the puzzling discrepancy in the chiroptical properties of the numerous previously described bisamides containing N-Boc or N-Ac protecting groups and diverse amino acids16-18) and (b) longer chain conjugates such as trisamides (n ) m ) 1) or tetrakisamides (n ) 1, m ) 2).16,17,20

Experimental Section General Considerations. Most synthetic procedures were carried out under argon. CH2Cl2 used for synthesis and spectroscopy was dried (P2O5), distilled over CaH2 and stored over molecular sieves (4 Å). EDC [EDC ) N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride, Aldrich] and HOBt (HOBt ) 1-hydroxybenzotriazole, Aldrich) and R-amino acids and their esters (Merck) were used as received. 1′-(Acetylamino)ferrocene-1-carboxylic acid (Ac-Fca, 7) and 1′-(tert-butoxycarbonylamino)ferrocene-1-carboxylic acid (Boc-Fca, 8) were prepared using previously described procedures.15 Products were purified by preparative thin layer chromatography on silica gel (Merck, Kieselgel 60 HF254) using CH2Cl2/EtOAc mixtures. Melting points were determined with a Gallenkamp capillary melting point apparatus MFB 595 010 and are uncorrected. IR spectra were recorded as CH2Cl2 solutions with a Bomem MB 100 mid-FTIR spectrophotometer. UV/vis and CD spectra were recorded on a Jasco-810 spectropolarimeter in CH3CN,

CH2Cl2 or CH2Cl2/DMSO solution. Cyclic voltammetry was performed on a Metrohm “Universal Mess- und Titriergefaess”, Metrohm GC electrode RDE 628, platinum electrode, SCE electrode, Princeton Applied Research potentiostat Model 273; 10-3 M in 0.1 M nBu4NPF6/CH2Cl2. Potentials are given relative to that of SCE. 1H and 13C NMR spectra were recorded on a Bruker AC 200 or on a Varian Unity Plus 400 in CD2Cl2 or CDCl3 solutions. Chemical shifts (δ/ppm) are reported with respect to residual solvent peaks as internal standards: CD2Cl2 δ(1H) ) 5.32 ppm, δ(13C) ) 53.5 ppm; CDCl3 δ(1H) ) 7.24 ppm, δ(13C) ) 77.0 ppm. EI and HR-EI mass spectra (MS) were recorded on a JEOL JMS-700. Elemental analyses were performed by the microanalytical laboratory of the chemistry department, University of Heidelberg. Computational Method. Density functional calculations were carried out with the Gaussian03/DFT27 series of programs. The B3LYP formulation of density functional theory was used employing the Lanl2DZ basis set which has been successfully applied previously for comparable systems.14,15e,18,20,24,27-31 All points were characterized as minima (Nimag ) 0) by frequency analysis. Crystal Structure Determinations. Intensity data were collected with a Nonius-Kappa CCD diffractometer using Mo-KR radiation (0.71073 Å) at 200 K. The data were processed using the standard Nonius software.32 Crystal data are compiled in Table 4. The structures were solved by direct methods and refined by full-matrix least-squares based on F2. All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were fixed at calculated positions and refined with a riding model or detected in a Fourier difference map and refined isotropically. The calculations were performed using the program SHELXS-97.33 Synthesis of Ac-Fca-AA-OMe (1a, 2a, 3a) and Boc-FcaAA-OMe (1b, 2b, 3b). Ac-Fca (7, 250 mg, 0.872 mmol) or BocFca (8, 301 mg, 0.872 mmol) was activated using EDC (200.6 mg, 1.046 mmol) and HOBt (145.6 mg, 1.046 mmol) in CH2Cl2 and AA-OMe (1.744 mmol) (obtained from AA-OMe · HCl by treatment with Et3N in CH2Cl2, pH ≈ 8) was added. The mixture was stirred for one hour at room temperature, washed with a saturated aqueous solution of NaHCO3, a 10% aqueous solution of citric acid and H2O, dried over Na2SO4 and evaporated in vacuo. TLC purification of the crude Ac-protected bisamide with CH2Cl2/EtOAc (1:1) and Boc-bisamides with CH2Cl2/EtOAc (10:1) gave orange crystals of the respective product. Ac-Fca-Gly-OMe (1a). 280 mg (90%). Mp ) 127-129 °C. MS(EI): m/z (%) ) 358 (100) [M]•+, 316 (7) [M - C2H2O]•+, 178 (31) [FeCpNHAc]•+. HRMS(EI): calcd for C16H18N2O4Fe, 358.0616; found, 358.0641. Anal. Calcd for C16H18N2O4Fe (358.18) · H2O: C, 51.08; H, 5.36; N, 7.45. Found: C, 51.67; H, 5.20; N, 6.93. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 515 mV. Ac-Fca-Ala-OMe (2a). 270 mg (83%). Mp ) 52-55 °C. MS(EI): m/z (%) ) 372 (100) [M]•+, 330 (5) [M-C2H2O]•+, 312 (3) [M - CH3COOH]•+, 270 (7) [M - AlaOMe]•+, 178 (36) [FeCpNHAc]•+. HRMS(EI): calcd for C17H20N2O4Fe, 372.0772; found, 372.0789. Anal. Calcd for C17H20N2O4Fe (372.20) · 1/2 H2O: C, 53.56; H, 5.55; N, 7.35. Found: C, 53.38; H, 5.57; N, 7.09. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 525 mV. Ac-Fca-Val-OMe (3a). 312 mg (89%). Mp ) 55-58 °C. MS(EI): m/z (%) ) 400 (100) [M]•+, 358 (2) [M - C2H2O]•+, 340 (3) [M - CH3COOH]•+, 270 (6) [M - ValOMe]•+, 178 (33) [FeCpNHAc]•+. HRMS(EI): calcd for C19H24N2O4Fe, 400.1085; found, 400.1075. Boc-Fca-Gly-OMe (1b). 315 mg (87%). Mp ) 57 °C. MS(EI): m/z (%) ) 416 (32) [M] •+, 360 (30) [M - CH2C(CH3)2]•+, 342 (80) [M - tBuOH]•+, 316 (100) [H2NFcCOGlyOMe]•+. HRMS(EI): calcd for C19H24N2O5Fe, 416.1035; found, 416.1007. Anal. Calcd (32) DENZO-SMN, Data processing software, Nonius 1998; http:// www.nonius.com. (33) Sheldrick, G. M. SHELXS-97, University of Go¨ttingen: Germany, 1997.

6b

C27H39FeN3O6 557.46 orthorhombic P212121 9.4810(19) 10.332(2) 30.099(6) 90 2948.4(10) 4 1.256 0.553 1184 0.50 × 0.50 × 0.30 4.2 to 54.9° -12 e h e 12 -13 e k e 13 -38 e l e 39 6772 6736 1.039 346 -0.055(16) R1 ) 0.0439, wR2 ) 0.1072 R1 ) 0.0561, wR2 ) 0.1167 0.612/0.312

6a 5a

Reflections collected Independent reflections Goodness-of-fit on F2 Parameters Absolute structure parameter Final R indices [I > 2σ(I)] R indices (all data) Largest diff. peak and hole [eÅ-3]

4a

C18H21FeN3O5 415.23 monoclinic P21/a 9.720(2) 12.170(2) 15.385(3) 93.50(3) 1816.5(6) 4 1.518 0.866 864 0.30 × 0.30 × 0.30 2.6 to 60.0° -13 e h e 13 -17 e k e 16 -21 e l e 21 10537 5303 1.030 305 R1 ) 0.0395, wR2 ) 0.0928 R1 ) 0.0640, wR2 ) 0.1028 0.365/0.454 Empirical formula Formula weight Crystal system Space group a [Å] b [Å] c [Å] β [deg] Volume [Å3] Z Density (calcd) [g cm-3] Abs. coefficient [mm-1] F(000) Crystal size [mm3] 2θ range for data collection Index ranges

C16H18FeN2O4 358.17 monoclinic P21/c 8.790(2) 12.198(2) 14.244(3) 99.82(3) 1504.9(5) 4 1.581 1.025 744 0.20 × 0.20 × 0.10 4.4 to 60.2° -12 e h e 12 -17 e k e 17 -20 e l e 20 8668 4420 1.028 280 R1 ) 0.0384, wR2 ) 0.0943 R1 ) 0.0569 wR2 ) 0.1042 0.365/0.612

C20H25FeN3O5 443.28 orthorhombic C2221 9.649(2) 16.192(3) 26.654(5) 90 4164.3(14) 8 1.414 0.760 1856 0.40 × 0.20 × 0.20 3.1 to 55.3° -12 e h e 12 -21 e k e 21 -34 e l e 34 4844 4828 1.048 270 0.007(13) R1 ) 0.0331, wR2 ) 0.0710 R1 ) 0.0421, wR2 ) 0.0753 0.279/0.283

C24H33FeN3O5 · 0.5 CH2Cl2 541.84 orthorhombic P212121 35.107(7) 9.5040(19) 15.552(3) 90 5189.0(18) 8 1.387 0.723 2280 0.10 × 0.05 × 0.05 2.8 to 54.9° -45 e h e 45 -12 e k e 12 -20 e l e 20 11942 11902 1.014 639 -0.019(19) R1 ) 0.0597, wR2 ) 0.1151 R1 ) 0.1370, wR2 ) 0.1429 0.738/0.715

Semencˇic´ et al.

1a

Table 4. Crystal Data for 1a, 4a, 5a, 6a, and 6b

2036 Organometallics, Vol. 28, No. 7, 2009

for C19H24N2O5Fe (416.26): C, 54.79; H, 5.81; N, 6.73. Found: C, 54.80; H, 5.90; N, 6.58. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 475 mV. Boc-Fca-Ala-OMe (2b). 318 mg (85%). Mp ) 125-127 °C. MS(EI): m/z (%) ) 430 (24) [M]•+, 374 (18) [M - CH2C(CH3)2]•+, 356 (100) [M - tBuOH]•+, 330 (82) [H2NFcCOAlaOMe]•+, 254 (25) [M - tBuOH-AlaOMe]•+, 130 (18) [COAlaOMe]•+. HRMS(EI): calcd for C20H26N2O5Fe, 430.1191; found, 430.1138. Boc-Fca-Val-OMe (3b). 302 mg (76%). Mp ) 111-114 °C. MS(EI): m/z (%) ) 458 (16) [M]•+, 402 (10) [M - CH2C(CH3)2]•+, 348 (100) [M - tBuOH]•+, 358 (68) [H2NFcCOValOMe]•+, 254 (25) [M - tBuOH-ValOMe]•+. HRMS(EI): calcd for C22H30N2O5Fe, 458.1504; found, 458.1485. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 530 mV. Synthesis of Boc-AA-Fca-AA-OMe (4b, 5b, 6b). A suspension of the appropriate Boc-bisamide 1b, 2b, or 3b (1 mmol) in ethyl acetate (10 mL) was cooled to 0 °C and treated with gaseous HCl for two hours. Thereafter, the mixture was evaporated to dryness and the resulting hydrochloride was treated with Et3N in CH2Cl2 (pH ≈ 8). The free amine was coupled with Boc-AA-OH (2 mmol) (activated by using standard EDC/HOBt method). After stirring for 2-4 h at room temperature TLC monitoring revealed the consumption of all starting material. The mixture was washed thrice with a saturated aqueous solution of NaHCO3, a 10% aqueous solution of citric acid and H2O, dried over Na2SO4 and evaporated in vacuo. TLC purification of crude products with CH2Cl2/ EtOAc (10:1) gave orange crystalline materials of the respective Boc-protected trisamides. Boc-Gly-Fca-Gly-OMe (4b). 316 mg (67%). Mp ) 69-73 °C. MS(EI): m/z (%) ) 473 (32) [M]•+, 417 (30) [M - CH2C(CH3)2]•+, 399 (100) [M - tBuOH]•+, 373 (26) [H2NGlyFcGlyOMe]•+, 342 (8) [COFcaGlyGlyOMe - H]•+, 316 (10) [H2NFcGlyOMe]•+. HRMS(EI): calcd for C21H27N3O6Fe, 473.1249; found, 473.1259. Anal. Calcd for C21H27N3O6Fe (473.31): C, 53.26; H, 5.75; N, 8.88. Found: C, 53.23; H, 5.99; N, 8.48. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 575 mV. Boc-Ala-Fca-Ala-OMe (5b). 485 mg (77%). Mp ) 58 °C. MS(EI): m/z (%) ) 501 (100) [M]•+, 445 (70) [M - CH2C(CH3)2]•+, 427 (78) [M - tBuOH]•+, 401 (22) [H2NAlaFcAlaOMe]•+, 356 (10) [COFcaAlaOMe - H]•+, 330 (36) [NH2FcCOAlaOMe]•+. HRMS(EI): calcd for C23H31N3O6Fe, 501.1562; found, 501.1592. Boc-Val-Fca-Val-OMe (6b). 403 mg (72%). Mp ) 150-153 °C. MS(EI): m/z (%) ) 557 (100) [M]•+, 501 (64) [M CH2C(CH3)2]•+, 483 (100) [M - tBuOH]•+, 457 (15) [H2NValFcCOValOMe]•+, 358 (29) [H2NFcCOValOMe]•+. HRMS(EI): calcd for C27H39N3O6Fe, 557.2188; found, 557.2245. Anal. Calcd for C27H39N3O6Fe (557.47): C, 58.15; H, 7.05; N, 7.54. Found: C, 58.37; H, 7.22; N, 7.19. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 550 mV. Synthesis of Ac-AA-Fca-AA-OMe (4a, 5a, 6a). Boc-trisamide 4b, 5b or 6b (0.4 mmol) in ethyl acetate (8 mL) was deprotected by gaseous HCl and evaporated to dryness in vacuo to give the respective yellow hydrochloride AA-Fca-AA-OMe · HCl. Acetyl chloride (2.4 mmol, 170 µL) was added dropwise to a cold solution (0 °C) of the hydrochloride and Et3N (3.2 mmol, 445 µL) in CH2Cl2 (8 mL). After stirring for one hour at 0 °C the reaction mixture was poured into water and extracted three times with CH2Cl2. The combined organic phases were washed with a saturated aqueous solution of NaCl, dried over Na2SO4 and evaporated to dryness in vacuo. The resulting crude products were purified by TLC on silicagel (EtOAc) to give orange crystals of the respective acetylprotected trisamides. Ac-Gly-Fca-Gly-OMe (4a). 128 mg (77%). Mp ) 180 °C. MS(EI): m/z (%) ) 415 (100) [M]•+, 384 (2) [M - OCH3]•+, 316 (21) [H2NFcCOGlyOMe]•+, 234 (7) [FeCpNHGlyAc - H]•+. HRMS(EI): calcd for C18H21N3O5Fe, 415.0831; found, 415.0820. Anal. Calcd for C18H21N3O5Fe (415.23): C, 52.04; H, 5.10; N, 10.12.

Bis- and Trisamides Found: C, 52.04; H, 5.24; N, 10.06. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 530 mV. Ac-Ala-Fca-Ala-OMe (5a). 150 mg (85%). Mp ) 178 °C. MS(EI): m/z (%) ) 443 (100) [M]•+, 330 (30) [H2NFcCOAlaOMe]•+. HRMS(EI): calcd for C20H25N3O5Fe, 443.1144; found, 443.1137. Anal. Calcd for C20H25N3O5Fe (443.28): C, 54.16; H, 5.69; N, 9.48. Found: C, 54.11; H, 5.74; N, 8.90. Ac-Val-Fca-Val-OMe (6a). 162 mg (81%). Mp ) 185 °C. MS(EI): m/z (%) ) 499 (100) [M]•+, 358 (30) [H2NFcCOValOMe]•+. HRMS(EI): calcd for C24H33N3O5Fe, 499.1770; found, 499.1750. Anal. Calcd for C24H33N3O5Fe (499.39) · 1/2 CH2Cl2: C, 54.33; H, 6.33; N, 7.76. Found: C, 54.17; H, 6.35; N, 7.69. CV(CH2Cl2, nBu4NPF6, vs SCE): E1/2 ) 545 mV.

Acknowledgment. We thank the Ministry of Science, Education and Sport of Croatia and the Deutsche Forschungsgemeinschaft for financial support, and the Deutscher

Organometallics, Vol. 28, No. 7, 2009 2037

Akademischer Austauschdienst DAAD for a fellowship to ˇ .S., and the research training group ”Molecular Probes” M.C for a fellowship to D.S. Supporting Information Available: Tables of 1H and 13C NMR spectroscopic data of 1a/1b, 2a/2b, 3a/3b, 4a/4b, 5a/5b, and 6a/ 6b; concentration dependent 1H NMR chemical shifts of the amide protons of 1a/1b, 3a/3b, 4a/4b, and 6a/6b in CDCl3; CD spectra of 3a/3b and 5a/5b in CH2Cl2 and in CH2Cl2/DMSO (20% v/v); CD spectra of 2b in CH2Cl2 and CH3CN; DFT calculated Cartesian coordinates of conformers of 2a, 2b, 3a, and 3b; plots showing thermal ellipsoids of 1a, 4a, 5a, 6a, and 6b; Cyclic voltammogram of 3b. This material is available free of charge via the Internet at http://pubs.acs.org. OM801163S