Synthesis of novel chiral amino acids possessing a porphyrin moiety

Article abstract of DOI:10.1016/S0957-4166(99)00101-9

Phenylalanine derivatives bearing a porphyrin moiety at the para- position were prepared in an enantiomerically pure form. The synthetic nonnatural aromatic amino acid reacted with amines and acids to give novel functionalized peptides without loss of the

Full text of DOI:10.1016/S0957-4166(99)00101-9

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1029–1032
Synthesis of novel chiral amino acids possessing a porphyrin
moiety
Hitoshi Tamiaki ∗ and Motoki Onishi
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu,
Shiga 525-8577, Japan
Received 25 February 1999; accepted 9 March 1999
Abstract
Phenylalanine derivatives bearing a porphyrin moiety at the para-position were prepared in an enantiomerically
pure form. The synthetic nonnatural aromatic amino acid reacted with amines and acids to give novel functionalized
peptides without loss of the enantiomeric purity. © 1999 Elsevier Science Ltd. All rights reserved.
Unusual amino acids are useful for the development of artificial proteins as well as elucidation of the
structure and function of naturally occurring proteins.1,2 Many unusual amino acids have been reported3
and some of them are commercially available. Porphyrins and their related compounds are important
as cofactors of proteins, for example, in respiratory and photosynthetic apparatus. Several amino acids
bearing a porphyrin moiety at the side chain are described in the literature4,5 and all of them utilize the
functional groups in the side chain of natural amino acids for the linkage: the porphyrin moiety was
covalently bonded with the amino acids through amido, ester or ether bonds. These bonds might react
with some reagents to give decomposed products and/or to induce removal of the porphyrin moiety from
peptide chains. Moreover, the functional linkages would interact with some other functional groups in
peptides, for example, by hydrogen bonding, to affect the molecular conformations. To overcome these
problems, we designed a direct introduction of a porphyrin unit into peptide chains without any use
of the above functional groups. During preparation of this communication, Hyslop, Therien and their
colleagues6 reported an α-amino acid linked with a porphyrin moiety through a C_C bond, although
the chirality at the α-position was not confirmed. Here we report the synthesis of chiral phenylalanine
derivatives directly substituted with a porphyrin moiety at the para-position.
N-Acetyl-(p-hydroxymethyl)phenylalanine (D,L-2) was prepared from methyl p-toluate according to
the reported procedure.7 Methylation of the carboxylic acid and oxidation of the hydroxymethyl group
in D,L-2 gave N,C-protected (p-formyl)phenylalanine D,L-3 in 70% yield (see Scheme 1). BF3-catalyzed
condensation of D,L-3 (1 equiv.) and 3,5-di-t-butylbenzaldehyde (3 equiv.) with pyrrole (4 equiv.) in
∗
Corresponding author. Fax: +81-77-561-2659; e-mail: tamiaki@se.ritsumei.ac.jp
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00101-9

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chloroform and successive oxidation by p-chloranil (Lindsey’s method8) gave symmetrical 5,10,15,20-
terakis(3,5-di-t-butylphenyl)porphyrin (8, 16%) and the desired N-acetyl-(p-porphyrinyl)phenylalanine
methyl ester D,L-4 (Ac–Phe(p-Por)–OMe, 12%) which were separated by flash column chromato-
graphy on silica gel (0–5%, Et2O–CH2Cl2). Boc protection of the imino group in D,L-4 follow-
ed by LiOH hydrolysis of acetyl amido and methyl ester groups9 afforded N-t-butoxycarbonyl-(p-
porphyrinyl)phenylalanine D,L-1 (48%). In spite of the presence of the large π-conjugated macrocycle in
the molecule, both amino and carboxylic acid groups in the synthetic Phe(p-Por) are reactive for several
reagents. Using conventional methods for solution phase peptide synthesis, a diastereomeric mixture
of tripeptide, Boc–L-Leu–D,L-Phe(p-Por)–L-Ala–OMe (9) was prepared as shown in Scheme 2. Yields
of the coupling and deprotection steps were moderate, >80–90%. Boc–Phe(p-Por)–OH (1) was highly
soluble in ordinary organic solvents except in the case of alcohols, including methanol, because of the
presence of six sterically bulky t-butyl groups in the molecule:4 the solubilities of 1 were >0.1 M in
both dichloromethane and 1-methyl-2-pyrrolidinone. Therefore, 1 can be used for the preparation of
various oligopeptides in the solid phase as well as in solution. Visible and fluorescence spectra of 1 were
identical to those of 8 in dilute solutions (ca. 10−6 M), e.g., hexane, dichloromethane and tetrahydrofuran,
indicating that the porphyrin moiety did not interact with functional groups of the phenylalanine part
intramolecularly.
Scheme 1. Synthesis of N-t-butoxycarbonyl-(p-porphyrinyl)phenylalanine (Boc–Phe(p-Por)–OH, 1). (i) CH2N2/Et2O;
C5H5NH+ClCrO−3 (=PCC)/CH2Cl2; (ii) pyrrole (4 equiv.), 3,5-tBu2C6H3CHO (3 equiv.), BF3·Et2O/CHCl3; p-chloranil; (iii)
acylase/H2O (pH=8); (iv) Boc2O/1,4-dioxane, H2O, aq 1 M NaOH; (v) Boc2O, (p-Me2N)C5H4N(=DMAP)/THF; (vi) aq 2 M
LiOH

H. Tamiaki, M. Onishi / Tetrahedron: Asymmetry 10 (1999) 1029–1032
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Scheme 2. Synthesis of tripeptide 9. EDC=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, HOBt=1-hydroxybenzotriazole
Figure 1. High performance liquid chromatograms of D,L-1 (A, separation ratio (Rs)=1.8) and L-1 (B) by a chiral-phase column
(SUMICHIRAL OA-4500, 4.6 œ×250 mm, hexane:CH2ClCH2Cl:MeOH:CF3COOH=93:5:2:0.2, 1.0 ml/min) and of D,L-5 (C,
Rs=14) and L-5 (D) by a chiral-phase column (SUMICHIRAL OA-6100, 4.6 œ×150 mm, 2 mM CuSO4 in H2O:CH3CN=95:5,
1.0 ml/min)
Racemic D,L-1 was fully separated by chiral-phase HPLC (see Fig. 1A). Although each enantiomer
of 1 was obtained pure after the HPLC separation, the amount of the isolated enantiomers was small
and the absolute configuration could not be determined. Hence, an alternative route was used to obtain
L-1 as follows (see also Scheme 1). Racemic 2 was treated with acylase2 to give enantiomerically pure
L-(p-hydroxymethyl)phenylalanine (L-5, 40%): the remaining N-acetyl compound 2 (55%) was D-rich.
The enantiomeric excess of the kinetically resolved L-5 was confirmed by analysis of the chiral HPLC
(Fig. 1C, D). The unnatural L-α-amino acid L-5 was protected by Boc2O and CH2N2 and oxidized
by PCC to give 6 (53%).10 Similarly with the synthesis of D,L-4 (vide supra) 6 was converted to
the corresponding porphyrin 7 (6%) without deprotection of the Boc group. After hydrolysis of 7 by
treatment of LiOH, the resulting 1 (76%) was analyzed by chiral HPLC (Fig. 1B), which showed
that the sample was enantiomerically pure; the rapidly moving band (48.4 min) was attributed to
L-1 and the second band (51.8 min) to D-1.11 Reactions of enantiomerically pure L-1 (vide supra)
1
afforded a diastereomerically pure tripeptide 9 from the H NMR analysis. These results indicate that
no racemization occurred during the several synthetic procedures described above.
In conclusion, enantiomerically pure amino acid–porphyrin dyad, L-1 was easily prepared and should
be useful in the construction of novel functionalizing peptides including electron/energy transfer systems.

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H. Tamiaki, M. Onishi / Tetrahedron: Asymmetry 10 (1999) 1029–1032
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas (A)
‘Creation of Delocalized Electronic Systems’ (No. 10146252) from the Ministry of Education, Science,
Sports and Culture, Japan.
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11. L-1: dark purple solid; vis (CH2Cl2), λmax=648 (0.01), 592 (0.01), 551 (0.02), 517 (0.03), 420 nm (1.00); fluorescence
(CH2Cl2, λexc=420 nm) λem=653 (1.0), 718 nm (0.5); 1H NMR (20% CD3OD–CDCl3) δ=8.71 (8H, br, β-H of pyrrole),
7.97 (2H, d, J=8 Hz, 2,6-H of meso-phenylene), 7.89 (6H, br-s, 2,6-H of meso-phenyl), 7.61 (3H, m, 4-H of meso-phenyl),
7.46 (2H, d, J=8 Hz, 3,5-H of meso-phenylene), 4.41 (1H, br-t, α-H of Phe), 3.41 (2H, m, β-H of Phe), 1.34, 1.32
(18H+36H, s, 3,5-tBu of meso-phenyl), 1.29 (9H, s, tBu of Boc). MS (FAB) found: m/z 1137, calcd for C76H91N5O4: M+,
1137.