Synthesis of a new turn mimic bearing a β-lactam moiety

Article abstract of DOI:10.1002/1099-0690(200208)2002:16<2686::AID-EJOC2686>3.0.CO;2-1

A new turn mimic derived from PLG (prolyl-leucyl-glycine amide) containing a β-lactam in the turn area has been prepared. The β-lactam moiety was furnished by treating an Fmoc-protected leucine-derived diazo ketone 2 with a benzylidene -protected glycine ester in a photochemically induced Staudinger-type reaction. The trans -substituted β-lactams 3a/b are formed in 70% yield (dr 70:30). Separation of the isomers, deprotection and attachment of Fmoc-prohne using the pentafluorphenyl ester activation protocol yielded the protected peptidomimetic 4 in 93% yield. Deprotection and amidation resulted in formation of the target substrate 1 in 82% yield but with a low purity. Better results were obtained using Nsc ({2-[(4-nitrophenyl)sulfonyl]jethoxy}-carbonyl) as protection for proline. It could be cleaved yielding a spectroscopically pure product 1 whose structure was elucidated by X-ray crystallographic analysis. It shows an open turn conformation, i.e., the turn is not stabilized by a hydrogen bond between the termini of the turn region. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0690(200208)2002:16<2686::AID-EJOC2686>3.0.CO;2-1

SHORT COMMUNICATION
Synthesis of a New Turn Mimic Bearing a β-Lactam Moiety
Thomas C. Maier,[a] Wolfgang U. Frey,[a] and Joachim Podlech*[a]
Dedicated to Prof. Dr. Dieter Seebach on the occasion of his 65th birthday
Keywords: Peptidomimetics / Turn mimetics / β-Lactams / Protective groups
A new turn mimic derived from PLG (prolyl-leucyl-glycine
amide) containing a β-lactam in the turn area has been pre-
pared. The β-lactam moiety was furnished by treating an
and amidation resulted in formation of the target substrate 1
in 82% yield but with a low purity. Better results were ob-
tained using Nsc ({2-[(4-nitrophenyl)sulfonyl]ethoxy}-
carbonyl) as protection for proline. It could be cleaved
yielding a spectroscopically pure product 1 whose structure
was elucidated by X-ray crystallographic analysis. It shows
an open turn conformation, i.e., the turn is not stabilized by
a hydrogen bond between the termini of the turn region.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
2002)
Fmoc-protected leucine-derived diazo ketone
2 with a
benzylidene-protected glycine ester in a photochemically in-
duced Staudinger-type reaction. The trans-substituted β-lac-
tams 3a/b are formed in 70% yield (dr 70:30). Separation of
the isomers, deprotection and attachment of Fmoc-proline
using the pentafluorphenyl ester activation protocol yielded
the protected peptidomimetic 4 in 93% yield. Deprotection
Introduction
Peptides showing a turn conformation[1Ϫ4] are of major
relevance since, in most cases, the turn regions are respons-
ible for their biological activity. Well-known representatives
showing
a
turn conformation are, inter alia,
somatostatin[5Ϫ7] and oxytocin.[8] Substrates containing ri-
gid frameworks, i.e., turn mimics, can be prepared by re-
placement of the turn region with other, mostly cyclic,
templates.[9Ϫ11]
Johnson et al. have incorporated β-lactams into peptide
chains (A) to mimic the β-turn-adopting tripeptide
HϪProϪLeuϪGlyϪNH2 (PLG, Figure 1)[12Ϫ15] and found
that the products show a similar conformation and compar-
able biological activity through selectively enhancing the af-
finity of dopamine agonists to dopamine receptors. Further
examples of peptidomimetics containing higher lactam
moieties have been reported as well.[14,16Ϫ19]
Figure 1. Targeted peptidomimetics
influence on the formation of the turn conformation. Here
we describe as a first example the construction of the PLG-
derived turn mimic 1.
For the synthesis of the peptidomimetics, we started with
suitably protected activated amino acids, which were al-
lowed to react with diazomethane to yield the respective
diazo ketones.[20Ϫ22] The latter were used in a photochemic-
ally induced Wolff reaction leading to ketenes which, in the
presence of imines, furnished β-lactams (Scheme 1).[23Ϫ29]
N-Peptidyl-substituted β-lactams are directly accessible
when these imines have been prepared by condensation of
an amino acid or a peptide derivative with an aldehyde.[26]
Results and Discussion
Starting from α-amino acids, we have developed a β-lac-
tam synthesis for use in the preparation of similar turn
mimics B (Figure 1). Molecular modeling suggested that the
additional alkylidene moiety between the four-membered
lactam ring and the amide group should not have a negative
Although dipeptidic diazo ketones may be used in this
protocol,[26] the yields are significantly higher when the tri-
peptide analogues were assembled by conventional peptide
coupling. The Fmoc-leucine-derived diazo ketone 2 used
[a]
Institut für Organische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax: (internat.) ϩ 49-711/685-4269
E-mail: joachim.podlech@po.uni-stuttgart.de
2686
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0816Ϫ2686 $ 20.00ϩ.50/0 Eur. J. Org. Chem. 2002, 2686Ϫ2689

Synthesis of a New Turn Mimic Bearing a β-Lactam Moiety
SHORT COMMUNICATION
with polymer-supported TAEA (purification by filtration).
The latter reagent has been described before,[40] but has, to
the best of our knowledge, not previously been used for
a base-induced removal of protecting groups. Subsequent
amidation with methanolic ammonia completed the reac-
tion sequence.
Scheme 1. Synthesis of the β-lactam moiety
here was prepared according to a procedure published by
Seebach et al.;[22] the imine was prepared by condensation
of glycine methyl ester with benzaldehyde.[30,31] In order to
achieve high yields of about 70% in the photochemically
induced β-lactam synthesis, it was necessary to use an imine
containing no residual benzaldehyde and to perform the re-
action at low temperature (Ϫ30 °C). The trans-substituted
Scheme 2. Assembly of peptidomimetic 1; a) HNEt2, THF, room
temp., 4.5 h; b) R1ϪProϪOPfp (R1 ϭ Fmoc, Nsc), THF, room
temp., 2 h, 93Ϫ96% (two steps); c) piperidine, MeOH/NH3, 0 °C
to room temp., 16 h, 82%; d) TAEA or polymer-supported TAEA,
CH2Cl2, 0.5 h, room temp., then extraction with phosphate buffer
β-lactams 3a/b (dr 70:30) formed in this reaction were sep- or filtration; e) MeOH/NH3, 0 °C to room temp., 16 h, 90% (two
steps)
arated by medium pressure chromatography (MPLC, ethyl
acetate/light petroleum ether); both isomers were Ϫ separ-
ately Ϫ used for the preparation of turn mimics, although
The thus accessible tripeptide analog 1 was spectroscop-
ically pure without further purification, and a single recrys-
only the reaction of the major isomer 3a is discussed here.
tallization (from iPrOH) gave crystals suitable for an X-ray
On the basis of semi-empirical calculations (PM3), both
crystallographic analysis (Figure 2).[41] Surprisingly, the
isomers should lead to the formation of a stable turn con-
substrate has a turn conformation even though no stabiliz-
ing central hydrogen bond is present. This so-called open
formation.
In the further course of the reaction sequence, the Fmoc
turn conformation[42] is in fact stabilized by a hydrogen
group was cleaved with diethylamine, and peptide coupling
bond between the β-lactam carbonyl group (O1 in Figure 2)
and the neighboring NH moiety (N2). The resulting six-
membered twist-boat-like ring is not only present in the
solid state but also in solution in CDCl3, as has been deter-
mined by NMR spectroscopy (by inspection of the respect-
ive coupling constants). This stabilization is only possible
as a result of the additional alkylidene group between the
nitrogen atom and the β-lactam ring. The angle between
with an Fmoc-protected proline pentafluorophenyl es-
ter[32,33] led to the protected tripeptide analog 4 in 93% yield
(two steps). Cleavage of the Fmoc group and amidation of
the C-terminus were achieved with methanolic ammonia
containing piperidine. The target turn mimic 1 was formed
in 82% yield (crude material, determined by NMR spectro-
scopy), although purification by chromatographic methods
was not feasible. Separation of the by-products formed dur-
incoming and leaving peptide strands (i.e. the vectors at
ing Fmoc cleavage [dibenzofulvene (DBF), polymerized
DBF and a DBF amine adduct which is present in an equi-
librium][34] was possible only with high losses during crys-
tallization. When tris(2-aminoethyl)amine (TAEA)[35] was
used as a base instead of piperidine, extraction with phos-
phate buffer was possible, although the gelatinous DBF
polymer could not be removed by this method. Accord-
ingly, we changed our strategy and used the {2-[(4-
nitrophenyl)sulfonyl]ethoxy}carbonyl group (Nsc) for N-
terminal protection, a protecting group that has been used
only recently for solid-phase peptide synthesis even though
it has been known for much longer.[34,36Ϫ38] The Nsc group
is again cleaved by amine bases; the vinyl sulfone formed
thereby is not prone to polymerization and the formation
of the amine adduct is fast, irreversible and quantitative.
The tripeptide analog 5 protected in this way was prepared
from 3a by attaching an Nsc-protected amino acid, which
is easily prepared by published procedures or can be ob-
tained from commercial sources.[34,36,39] The final cleavage
of the protecting group was either achieved with TAEA
(purification of the product by extraction with buffer) or the usual turn numbering)
Figure 2. X-ray crystal structure of the turn mimic 1 (labels are
due to the X-ray crystallographic processing, they do not indicate
Eur. J. Org. Chem. 2002, 2686Ϫ2689
2687

T. C. Maier, W. U. Frey, J. Podlech
SHORT COMMUNICATION
H, aryl-CH), 7.86 (d, J ϭ 9.8 Hz, 1 H, NHLeu) ppm. 13C NMR
(CDCl3, 125 MHz): δ ϭ 21.5, 23.2 (C-δLeu), 25.0 (C-γLeu), 26.2 (C-
γPro), 30.6 (C-βPro), 42.4 (C-βLeu), 43.5 (C-αGly), 45.1 (C-αLeu), 47.1
(C-δPro), 59.5 (C-4), 60.6 (C-αPro), 65.6 (C-3), 126.5, 128.7, 129.1
(aryl-C), 136.8 (aryl-Cipso), 168.5, 169.5, 176.5 (CϭO) ppm. MS
which the peptide chain enters and leaves the turn mimetic)
is 185.3°, demonstrating the almost perfect turn conforma-
tion of the substrate.[43]
(EI, 70 eV): m/z (%) ϭ 386 [Mϩ], 70 [C4H7NHϩ]. HRMS (EI,
Conclusion
12
70 eV):
C
21
1H3014N416O3: calcd. 386.2317; found 386.2317.
C21H30N4O3 (386.5): calcd. C 65.26, H 7.82, N 14.50; found C
64.95, H 7.78, N 14.33.
Most published turn mimetics do not allow for a flexible
positioning of side chains in the turn area. The substrates
B presented here carry up to three residues which perfectly
mimic a peptide in the turn region. This provides for further
flexibility in the construction and optimization of possibly
biologically active compounds.[44,45] The fact that no central
hydrogen bond is needed to stabilize the turn conformation
gives further possibilities for variation. It is feasible that
the peptide backbone, which usually supplies the hydrogen
bonds, is not even essential in the termini of the turn region.
Investigations to elucidate the scope of the accessible turn
mimics and to incorporate this and similar substrates in
longer peptide chains are now in progress.
Acknowledgments
The support of Prof. Dr. V. Jäger and the help of Dr. R. E. Dunmur
in the final preparation of the manuscript is gratefully acknow-
ledged. This work was supported by the Fonds der Chemischen
Industrie, the Deutsche Forschungsgemeinschaft, the Landesgradu-
iertenförderung Baden-Württemberg (stipend to T. C. M.), BASF
AG and Degussa AG (donation of chemicals).
[1]
J. A. Smith, L. G. Pease, Rev. Biochem. 1980, 8, 315Ϫ399.
[2]
J. S. Richardson, Adv. Protein Chem. 1981, 34, 168Ϫ339.
[3]
G. D. Rose, L. M. Gierasch, J. A. Smith, Adv. Protein Chem.
1985, 37, 1Ϫ109.
Methods Org. Chem. (Houben-Weyl), vol. E22 (‘‘Synthesis of
[4]
Peptides and Peptidomimetics’’) (Eds.: M. Goodman, A. Felix,
L. Moroder, C. Toniolo), Thieme, Stuttgart, 2001.
K. Hallenga, G. van Binst, A. Scarso, A. Michel, M. Knappen-
Experimental Section
[5]
Synthesis of Turn Mimic 1. Method A: The protected peptide ana-
log 5 (30 mg, 45 µmol) in CH2Cl2 (5 mL) was added to polymer-
bound TAEA (1.00 g, 700 µmol) and stirred for about 72 h at room
temp. (the reaction was monitored by TLC). The mixture was then
filtered through a sintered-glass frit and the solvent was removed
in a rotary evaporator. The resulting clear oil was chilled to 0 °C,
cold methanolic NH3 (2 mL, ca. 7.5 ) was added and the mixture
was stirred for 16 h whilst warming to room temp. The volatile
material was removed in a rotary evaporator yielding a clear oil
which slowly crystallized. Yield: 16 mg, 92%. Method B: The pro-
tected peptide analog 5 (30 mg, 0.045 mmol) and TAEA (135 µL,
900 µmol) in CH2Cl2 (1 mL) were stirred for about 30 min at room
temp.; CH2Cl2 was then added and the solution was extracted with
brine (three times), buffer (once; 13.1 g Na2HPO4, 40.7 g
NaH2PO4·2H2O in 200 mL H2O) and brine (once), and then dried
with MgSO4. The solvent was removed in a rotary evaporator giv-
ing a clear oil which was chilled to 0 °C. Cold methanolic NH3
(2 mL, ca. 7.5 ) was added and the mixture was stirred for 16 h
whilst warming to room temp. The volatile material was removed
in a rotary evaporator yielding a clear oil which slowly crystallized.
Yield: 14 mg, 81%.
berg, C. Dremier, J. Brison, J. Dirkx, FEBS Lett. 1980, 119,
47Ϫ52.
[6]
D. F. Veber, R. M. Freidinger, D. S. Perlow, W. J. Paleveda, Jr.,
F. W. Holly, R. G. Strachan, R. F. Nutt, B. H. Aryson, C.
Homnick, W. C. Randall, M. S. Glitzer, R. Saperstein, R.
Hirschmann, Nature 1981, 292, 55Ϫ58.
J. Rivier, J. Spiess, M. Thorner, W. Vale, Nature 1982, 300,
276Ϫ278.
D. W. Urry, R. Walter, Proc. Natl. Acad. Sci. U. S. A. 1971,
68, 956Ϫ958.
R. M. Freidinger, D. F. Veber, D. S. Perlow, J. R. Brooks, R.
[7]
[8]
[9]
Saperstein, Science 1980, 210, 656Ϫ658.
[10]
J. B. Ball, P. F. Alewood, J. Mol. Recognit. 1990, 3, 55Ϫ64.
[11]
See as well: G. Müller, G. Hessler, H. Y. Decornez, Angew.
Chem. Int. Ed. 2000, 39, 894Ϫ896.
L. K. Srivastava, S. B. Bajwa, R. L. Johnson, R. K. Mishra, J.
Neurochem. 1988, 50, 960Ϫ968.
K.-L. Yu, G. Rajakumar, L. K. Srivastava, R. K. Mishra, R.
L. Johnson, J. Med. Chem. 1988, 31, 1430Ϫ1436.
U. Sreenivasan, R. K. Mishra, R. L. Johnson, J. Med. Chem.
1993, 36, 256Ϫ263.
R. K. Mishra, S. Chiu, P. Chiu, C. P. Mishra, Meth. Find.
[12]
[13]
[14]
[15]
Exptl. Clin. Pharmacol. 1983, 5, 203Ϫ233.
S. Hanessian, G. McNaughton-Smith, H.-G. Lombart, W. D.
[16]
1: Colorless crystals, m.p. 199Ϫ201 °C. [α]2D0 ϭ Ϫ36.5 (c ϭ 1,
CHCl3). IR (KBr): ν˜ ϭ 3418, 3253, 2940, 1755, 1670, 1635, 1520
cmϪ1. 1H NMR (CDCl3, 500 MHz): δ ϭ 0.86 (d, J ϭ 6.7 Hz, 3 H,
H-δLeu), 0.91 (d, J ϭ 6.6 Hz, 3 H, H-δLeu), 1.32 (ddd, Jgem ϭ 13.9,
J ϭ 9.2, J ϭ 4.7 Hz, 1 H, H-βLeu), 1.56 (dseptd, J ϭ 9.2, J ϭ 6.6,
J ϭ 5.1 Hz, 1 H, H-γLeu), 1.65Ϫ1.76 (m, 2 H, H-γPro), 1.78 (ddd,
Lubell, Tetrahedron 1997, 53, 12789Ϫ12854.
[17]
P. Gillespie, J. Cicariello, G. L. Olson, Biopolymers 1997, 43,
191Ϫ217.
[18]
T. Hoffmann, H. Lanig, R. Waibel, P. Gmeiner, Angew. Chem.
Int. Ed. 2001, 40, 3361Ϫ3364.
[19]
´
´
E. Alonso, F. Lopez-Ortiz, C. del Pozo, E. Peralta, A. Macıas,
´
Jgem ϭ 13.9, J ϭ 10.7, J ϭ 5.1 Hz, 1 H, H-βLeu), 1.88 (dddd, Jgem ϭ
J. Gonzalez, J. Org. Chem. 2001, 66, 6333Ϫ6338.
[20]
[21]
[22]
J. Podlech, D. Seebach, Liebigs Ann. 1995, 1217Ϫ1228.
J. Podlech, D. Seebach, Helv. Chim. Acta 1995, 78, 1238Ϫ1246.
G. Guichard, S. Abele, D. Seebach, Helv. Chim. Acta 1998,
81, 187Ϫ206.
12.9, J ϭ 7.5, J ϭ 5.8, J ϭ 5.7 Hz, 1 H, H-βPro), 2.15 (dddd, Jgem ϭ
12.9, J ϭ 9.2, J ϭ 7.5, J ϭ 7.5 Hz, 1 H, H-βPro), 2.23 (br. s, 1 H,
NHPro), 2.90 (ddd, Jgem ϭ 10.0, J ϭ 6.4, J ϭ 5.9 Hz, 1 H, H-δPro),
3.00 (ddd, Jgem ϭ 10.0, J ϭ 7.1, J ϭ 7.0 Hz, 1 H, H-δPro), 3.09 (dd,
J ϭ 4.1, J ϭ 2.2 Hz, 1 H, H-3), 3.26 (d, Jgem ϭ 17.1 Hz, 1 H, H-
[23]
[24]
[25]
[26]
[27]
J. Podlech, Synlett 1996, 582Ϫ584.
J. Podlech, M. R. Linder, J. Org. Chem. 1997, 62, 5873Ϫ5883.
J. Podlech, S. Steurer, Synthesis 1999, 650Ϫ654.
M. R. Linder, J. Podlech, Org. Lett. 1999, 1, 869Ϫ871.
J. Podlech, M. R. Linder, T. C. Maier, Targets Heterocycl. Syst.
2000, 4, 269Ϫ291.
αGly), 3.82 (dd, J ϭ 9.2, J ϭ 5.5 Hz, 1 H, H-αPro), 4.27 (d, Jgem
ϭ
17.1 Hz, 1 H, H-αGly), 4.45 (dddd, J ϭ 10.7, J ϭ 9.8, J ϭ 4.7, J ϭ
4.1 Hz, 1 H, H-αLeu), 4.60 (d, J ϭ 2.2 Hz, 1 H, H-4), 5.68, 6.47 (br.
s, 2 H, CONH2), 7.28Ϫ7.31 (m, 2 H, aryl-CH), 7.32Ϫ7.40 (m, 3
2688
Eur. J. Org. Chem. 2002, 2686Ϫ2689

Synthesis of a New Turn Mimic Bearing a β-Lactam Moiety
SHORT COMMUNICATION
R. Ramage, L. Jiang, Y.-D. Kim, K. Shaw, J.-L. Park, H.-J.
Kim, J. Pept. Sci. 1999, 5, 195Ϫ200.
C. G. J. Verhart, G. I. Tesser, Recl. Trav. Chim. Pays-Bas 1988,
107, 621Ϫ626.
[28]
[38]
[39]
[40]
[41]
M. R. Linder, J. Podlech, Org. Lett. 2001, 3, 1849Ϫ1851.
M. R. Linder, W. U. Frey, J. Podlech, J. Chem. Soc., Perkin
[29]
Trans. 1 2001, 2566Ϫ2577.
F. Texier-Boullet, Synthesis 1985, 679Ϫ681.
First attempts have been made to remove the phenyl substitu-
[30]
J. R. Booth, J. C. Hodges, J. Am. Chem. Soc. 1997, 119,
4882Ϫ4886.
[31]
ent, either by using electron-rich phenyl groups which can be
degradated oxidatively,[29] or by performing the β-lactam syn-
thesis with microwave-induced heating instead of irradiation,
which allows the introduction of substituents other than
phenyl.[28]
L. Kisfaludy, I. Schön, Synthesis 1983, 89, 325Ϫ327.
U. Schmidt, F. Stäbler, J. Chem. Soc., Chem. Commun. 1992,
1353Ϫ1354.
CCDC-173332 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK [Fax: (internat.) ϩ 44-1223/336-033;
E-mail: deposit@ccdc.cam.ac.uk].
J. L. Crawford, W. N. Lipscomb, C. G. Schellman, Proc. Natl.
Acad. Sci. U. S. A. 1973, 70, 538Ϫ542.
The angle between the vector of the N-terminal NϪH bond
and the C-terminal CϪN bond was calculated. In a perfect
turn these vectors should be co-planar and show an angle of
180°.
G. L. Olson, D. R. Bolin, M. P. Bonner, M. Bös, C. M. Cook,
D. C. Fry, B. J. Graves, M. Hatada, D. E. Hill, M. Kahn, V. S.
Madison, V. K. Rusiecki, R. Sarabu, J. Sepinwall, G. P. Vinc-
ent, M. E. Voss, J. Med. Chem. 1993, 36, 3039Ϫ3049.
K. Burgess, Acc. Chem. Res. 2001, 34, 826Ϫ835.
Received April 9, 2002
[32]
[33]
[42]
[43]
[34]
J. Podlech, M. Gurrath, G. Müller, E. Lohof, in Methoden Org.
Chem. (Houben-Weyl), vol. E22 (‘‘Synthesis of Peptides and
Peptidomimetics’’) (Eds.: M. Goodman, A. Felix, L. Moroder,
C. Toniolo), Thieme, Stuttgart, 2001, pp. 41Ϫ165.
[35]
[44]
[45]
L. A. Carpino, D. Sadat-Aalaee, M. Beyermann, J. Org. Chem.
1990, 55, 1673Ϫ1675.
V. V. Samukov, A. N. Sabirov, P. I. Pozdnyakov, Tetrahedron
[36]
Lett. 1994, 35, 7821Ϫ7824.
V. V. Samukov, A. N. Sabirov, P. I. Pozdnyakov, H.-J. Kim, Y.-
[37]
D. Kim (Hyundai Pharm. Ind. Co., Ltd., S. Korea) WO Patent
9817638 A1, 1998.
[O02197]
Eur. J. Org. Chem. 2002, 2686Ϫ2689
2689