α-Amino-β-hydroxy-γ-lactam for constraining peptide Ser and Tnr residue conformation

Article abstract of DOI:10.1021/ol1000582

α-Amino-β-hydroxy-γ-lactam 1 is a peptide mimic in which the Ser/Thr residue ω-, ψ-, and χ-dihedral angle geometry all are constrained by the 5-membered lactam ring. Lactams 1 were made by employing N-(Fmoc)oxiranylglycine 3 as a bis-electrophile in TFE with cat. BzOH to sequentially alkylate and acylate a variety of amino acid derivatives in one pot. Solid-phase synthesis of β-hydroxy-γ-lactam 8, an analogue of the IL-1 modulator 101.10, was achieved using this method for studying Ser/Thr geometry.

Full text of DOI:10.1021/ol1000582

ORGANIC
LETTERS
2010
Vol. 12, No. 8
1652-1655
r-Amino-ꢀ-hydroxy-γ-lactam for
Constraining Peptide Ser and Thr
Residue Conformation
Daniel J. St-Cyr, Andrew G. Jamieson, and William D. Lubell*
De´partement de Chimie, UniVersite´ de Montre´al, C.P. 6128, Succursale Centre-Ville,
Montre´al, Que´bec H3C 3J7, Canada
lubell@chimie.umontreal.ca
Received January 10, 2010
ABSTRACT
r-Amino-ꢀ-hydroxy-γ-lactam 1 is a peptide mimic in which the Ser/Thr residue ω-, ψ-, and ꢁ-dihedral angle geometry all are constrained by
the 5-membered lactam ring. Lactams 1 were made by employing N-(Fmoc)oxiranylglycine 3 as a bis-electrophile in TFE with cat. BzOH to
sequentially alkylate and acylate a variety of amino acid derivatives in one pot. Solid-phase synthesis of ꢀ-hydroxy-γ-lactam 8, an analogue
of the IL-1 modulator 101.10, was achieved using this method for studying Ser/Thr geometry.
Serine and threonine play important roles in peptide
activity and secondary structure. For example, the phos-
phorylation and glycosylation of the ꢀ-hydroxyl group of
these amino acid residues in proteins is vital for cellular
signaling and function.1 Moreover, hydrogen bonding to
the side-chain hydroxyl group may stabilize peptide
secondary structure. Constrained Ser and Thr analogues
are attractive targets for exploring the impact of their
conformation on peptide biology.2 For example, 3-hy-
droxyproline mimics Ser and Thr with constrained φ- and
ꢁ-dihedral angles (Figure 1). The ꢀ-turn inducing ability
of 3-hydroxyproline and its occurrence in bioactive
peptides underscores the importance of this structural
motif.3-5
Complementing the conformational effects of ꢀ-hydroxy-
proline, R-amino-ꢀ-hydroxy-γ-lactam would constrain the
C-terminal amide and ψ- and ꢁ-dihedral angles (Figure 1).6
Specifically, the side-chain gauche (+) and (-) isomers of
Ser/Thr are locked in by the lactam, which in ꢁ space,7
(3) For ꢀ-turns in 3-Hyp peptides: Chakraborty, T. K.; Srinivasu, P.;
Rao, R. V.; Kumar, S. K.; Kunwar, A. C. J. Org. Chem. 2004, 69, 7399.
(4) For example, empedopeptin: (a) Elhammer, A. P.; Stachelhaus, T.
U.S. Patent 2009124539, 2009. Pneumocandin B0: (b) Schwartz, R. E.;
Sensin, D. F.; Joshua, H.; Wilson, K. E.; Kempf, A. J.; Goklen, K. A.;
Kuehner, D.; Gailliot, P.; Gleason, C.; White, R.; Inamine, E.; Bills, G.;
Salmon, P.; Zitano, L. J. Antibiot. 1992, 45, 1853. Telomycin: (c) Sheehan,
J. C.; Mania, D.; Nakamura, S.; Stock, J. A.; Maeda, K. J. Am. Chem. Soc.
1968, 90, 462. Revised structure: (d) Katrukha, G. S.; Maevskaya, S. N.;
Silaev, A. B.; Lomonosov, M. V. Bioorg. Khim. 1977, 3, 422. Cyclothia-
lidine: (e) Nakada, N.; Shimada, H.; Hirata, T.; Aoki, Y.; Kamiyama, T.;
Watanabe, J.; Arisawa, M. Antimicrob. Agents Chemother. 1993, 37, 2656.
Mauritine K: (f) Singh, A. K.; Pandey, M. B.; Singh, V. P.; Pandey, V. B.
J. Indian Chem. Soc. 2007, 84, 781. Plusbactin A3: (g) Wohlrab, A.; Lamer,
R.; VanNieuwenhze, M. S. J. Am. Chem. Soc. 2007, 129, 4175.
(5) Cyclopeptide natural products review: Pomilio, A. B.; Battista, M. E.;
Vitale, A. A. Curr. Org. Chem. 2006, 10, 2075.
(1) (a) Glycosylation review: Steen, P. V. D.; Rudd, P. M.; Dwek, R. A.;
Opdenakker, G. Crit. ReV. Biochem. Mol. Biol. 1998, 33, 151. (b)
Phosphorylation review: Pinna, L. A.; Ruzzene, M. Biochim. Biophys. Acta
1996, 1314, 191.
(2) (a) Jenkins, C. L.; Bretsher, L. E.; Guzei, I. A.; Raines, R. T. J. Am.
Chem. Soc. 2003, 125, 6422. (b) Rao, M. H. V. R.; Pinyol, E.; Lubell,
W. D. J. Org. Chem. 2007, 72, 736.
(6) Toniolo, C. Int. J. Pept. Protein Res 1990, 35, 287, and references
therein.
10.1021/ol1000582  2010 American Chemical Society
Published on Web 03/12/2010

employing dioxooxathiazinane 2 to alkylate and acylate
amines, such as the N-terminal of a resin-bound peptide chain
to yield γ-lactam 1d (Figure 2).10 In considering the
construction of Agl’s ꢀ-hydroxy counterpart 1e, Rapoport’s
use of N-(Cbz)oxiranylglycine as a building block in alkaloid
synthesis (i.e., pentostatin/coformysin aglycons11 and mito-
mysin analogues)12 inspired the application of this bis-
electrophile for the synthesis of peptide mimics 1e bearing
the R-amino-ꢀ-hydroxy-γ-lactam moiety.
The utility of Fmoc protection compelled the synthesis of
N-(Fmoc)oxiranylglycine methyl ester (2S,2′S)-3.13 The
higher boiling 2,4-dichlorotoluene, instead of xylenes, for
pyrolysis of N-(Fmoc)Met(O)-OMe gave the vinylglycine
precursor in 2 h instead of 2-3 days.13,14 Epoxidation gave
3 as a 4:1 mixture of diastereomers, from which a 9:1 mixture
was isolated by flash chromatography15,16 and used subse-
quently to give mixtures of lactams 1, which were separated
by flash chromatography.16,17
Epoxide 3 reacted with Ala-OBn to produce lactam 1f in
10% yield (Scheme 1).18 Little improvement was obtained
Figure 1. Constraint of ꢁ-dihedral angles in 3-hydroxyproline and
R-amino-ꢀ-hydroxy-γ-lactam mimics of Ser/Thr residues.
Scheme 1. Initial γ-Lactam Synthesis
complements the gauche (+) and trans isomers available to
ꢀ-hydroxyproline, contingent on stereochemistry.8
R-Amino-ꢀ-hydroxy-γ-lactams have been investigated as
N-methyl-D-aspartate receptor agonists (i.e., 1a, Figure 2),9a
in attempts to yield lactam 1f using acid catalysis.19 Epoxide
ring opening was accelerated using fluorinated alcohol
solvents.20 In 2,2,2-trifluoroethanol (TFE), N-(Fmoc)oxira-
nylglycine 3 and Ala-OBn reacted at 80 °C affording
γ-lactam 1f in 65% yield within 12 h (Figure 3). With the
(9) (a) Leeson, P. D.; Williams, B. J.; Rowley, M.; Moore, K. W.; Baker,
R.; Kemp, J. A.; Priestley, T.; Foster, A. C.; Donald, A. E. Bioorg. Med.
Chem. Lett. 1993, 3, 71. (b) Okumura, K.; Inoue, K.; Fukamizu, M. Jpn.
Patent 46041305 19711206, 1971. (c) Scholz, D.; Hecht, P.; Schmidt, H.;
Billich, A. Monatsh. Chem. 1999, 130, 1283. (d) Scholz, D.; Billich, A.;
Charpiot, B.; Ettmayer, P.; Lehr, P.; Rosenwirth, B.; Schreiner, E.; Gstach,
H. J. Med. Chem. 1994, 37, 3079. (e) Alvarez-Ibarra, C.; Csa´ky¨, A. G.;
Mart´ınez-Santos, E.; Quiroga, M. L.; Tejedor, J. L. Tetrahedron 1999, 55,
3041. (f) Limberg, G.; Lundt, I.; Zavilla, J. Synthesis 1999, 1, 178. (g)
Sauer, S.; Schumacher, A.; Barbosa, F.; Giese, B. Tetrahedron Lett. 1998,
39, 3685. (h) Farran, D.; Toupet, L.; Martinez, J.; Dewynter, G. Org. Lett.
2007, 9, 4833. (i) Leban, J. J.; Colson, K. L. J. Org. Chem. 1996, 61, 228.
(j) Almeida, J. F.; Grande, M.; Moran, J. R.; Anaya, J.; Mussons, L.;
Caballero, C. Tetrahedron: Asymmetry 1993, 4, 2483. (k) Cottrell, I. F.;
Davis, P. J.; Moloney, M. G. Tetrahedron: Asymmetry 2004, 15, 1239.
(10) (a) Jamieson, A. G.; Boutard, N.; Beauregard, K.; Bodas, M. S.;
Ong, H.; Quiniou, C.; Chemtob, S.; Lubell, W. D. J. Am. Chem. Soc. 2009,
131, 7917. (b) Boutard, N.; Jamieson, A. G.; Ong, H.; Lubell, W. D. Chem.
Biol. Drug Des. 2010, 75, 40.
Figure 2. Precedence for R-amino-ꢀ-hydroxy-γ-lactams in me-
dicinal chemistry.9 Recently reported lactam synthesis with sulfa-
midate 210 and proposed synthesis with epoxide 3.11
antiinflammatory agents (1b),9b and HIV-protease inhibitors
(1c);9d however, methodology is lacking for the assembly
of this motif on amino acid residues.9
We have recently demontrated that the parent R-amino-
γ-lactam (Agl) residue can be introduced into peptides by
(11) Truong, T. V.; Rapoport, H. J. Org. Chem. 1993, 58, 6090.
(12) Shaw, K. J.; Luly, J. R.; Rapoport, H. J. Org. Chem. 1985, 50,
4515.
(13) N-(Fmoc)vinylglycine-OMe precursor: Organ, M. G.; Xu, J.;
N’Zemba, B. Tetrahedron Lett. 2002, 43, 8177.
(7) Hruby, V. J.; Li, G.; Haskell-Luevano, C.; Shenderovich, M.
Biopolymers 1997, 43, 219.
(14) Vinylglycine synthesis: (a) Afzali-Ardakani, A.; Rapoport, H. J.
Org. Chem. 1980, 45, 4817. (b) Carrasco, M.; Jones, R. J.; Kamel, S.;
Rapoport, H. Org. Synth. 1992, 70, 29. (c) Meffre, P.; Vo-Quang, L.; Vo-
Quang, Y.; Le Goffic, F. Synth. Commun. 1989, 19, 3457. (d) Review:
Berkowitz, D. B.; Charette, B. D.; Karukurichi, K. R.; McFadden, J. M.
(8) An astute reviewer noted that the actual ground state conformation
of the Figure 1 structures will likely be intermediate between the idealized
Newman-projection staggered and eclipsed confomers due to the constraint
of the five-membered ring.
Tetrahedron: Asymmetry 2006, 17, 869
.
Org. Lett., Vol. 12, No. 8, 2010
1653

by the reversible ring opening of the oxiranyl moiety.15,21
Moreover, when Val-OMe reacted with 3 under standard
reaction conditions, HPLC analysis of the crude revealed that
ca. 10% epimer was incorporated into the corresponding
γ-lactam product 1g.
The hydroxy group was further elaborated (Scheme 2).
Phosphorylated dipeptide 6 was made from alcohol 1g using
Scheme 2.
Phosphorylation and Bromination of γ-Lactama
a Double-headed arrow represents NOESY correlations.
POCl3 and 2,6-lutidine, followed by a methanol quench.
Dehydroxybromination of 1g with PPh3Br2 occurred with
inversion, providing access to lactam 7. The stereochemistry
of 7 was assigned by examining the relative intensity of the
magnetization transfer between the lactam R-proton and the
other ring hydrogens.17
Lactam dipeptide has been employed in solid-phase
synthesis of peptide mimics.22 A more modular approach
was examined to install directly R-amino-ꢀ-hydroxy-γ-lactam
onto the N-terminal of solid-supported peptide. Peptide
101.10 (rytvela) is an allosteric modulator of the interleukin
1 (IL-1) receptor, which has potential clinical applications
Figure 3. Amino acid scope in dipeptide synthesis. Key: (a) epoxide
(2S, 2′S)-3 (50 µmol, 9: 1 mixture with (2S, 2′R)-3), 4 (150-180
µmol), BzOH (15 µmol), and TFE (0.3 mL) were heated at 80 °C
until TLC showed that 3 was consumed (2-24 h); (b) 40 °C; (c)
(2R, 2′R)-3 used; (d) 2.5 equiv of BzOH; (e) Glu(OMe)-OMe gave
1o and 5.
(15) The enantiomeric purity of 3 was ascertained by chiral SFC
chromatography to be of >96%. The major diasteriomer was assigned by
conversion of (2S,2′S)-N-(Cbz)oxiranylglycine methyl ester into 3 under
hydrogenative conditions: Dzubeck, V.; Schneider, J. P. Tetrahedron Lett.
2000, 41, 9953.
more sterically encumbered Val-OMe as substrate, however,
the reaction required 2.5 days at 80 °C. Monitoring (1H
NMR, TLC, HPLC-MS) revealed rapid formation and
buildup of linear intermediate from epoxide opening, sug-
gesting annulation was the slower step. In TFE, catalytic
benzoic acid (0.3 equiv) promoted γ-lactam formation within
1 day (1g, Figure 3). The TFE/catalytic BzOH combination
proved effective with a variety of R-amino esters (e.g., 1h-j,
Figure 3). The nucleophilic phenol of unprotected Tyr-OMe
was tolerated (1k). ꢀ- and γ-amino ester substrates, benzyl
ꢀ-alaninate and methyl m-aminobenzoate, gave, respectively,
63% and 49% yields of 1l and 1m. Lower reaction temper-
ature (40 °C) mitigated Fmoc deprotection using Gly-OBn
to make 1n. The methyl ester side chain of dimethyl
glutamate competed in the annulation to 1o producing
pyroglutamate 5. Enantiomeric (2R,2′R)-3 reacted with
D-Val-OMe providing access to (2R,3′R,4′S)-1g.
(16) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923.
(17) See the Supporting Information for details.
(18) (2S, 2′S)-N-(Cbz)oxiranylglycine methyl ester has been reported
to form lactams on reactions with H2NOBn and Gly; see ref 9a and: Tashiro,
T.; Fushiya, S.; Nozoe, S. Chem. Pharm. Bull. 1988, 36, 893.
(19) For example, BF3·Et2O, 34%; TsOH·H2O, 46%.
(20) (a) Philippe, C.; Milcent, T.; Crousse, B.; Bonnet-Delpon, D. Org.
Biomol. Chem. 2009, 7, 2026. (b) Das, U.; Crousse, B.; Kesavan, V.; Bonnet-
Delpon, D.; Be´gue´, J.-P. J. Org. Chem. 2000, 65, 6749. (c) Westermaier,
M.; Mayr, H. Chem.sEur. J. 2008, 14, 1638. (d) Review: Be´gue´, J.-P.;
Bonnet-Delpon, D.; Crousse, B. Synlett 2004, 1, 18.
(21) (2S,2′S)-N-(Cbz)oxiranylglycine methyl ester ꢀ-eliminated to afford
3-(Cbz-amino)furan-2(5H)-one.12 Reversible ꢀ-elimination may explain
racemization.
(22) Galaud, F.; Demers, A.; Ong, H.; Lubell, W. D., Understanding
Biology Using Peptides; Blondelle, S. E., Ed.; Springer: New York, 2006;
pp 188-189.
(23) (a) Quiniou, C.; Sapieha, P.; Lahaie, I.; Hou, X.; Brault, S.;
Beauchamp, M.; Leduc, M.; Rihakova, L.; Joyal, J.-S.; Nadeau, S.; Heveker,
N.; Lubell, W. D.; Sennlaub, F.; Gobeil, F., Jr.; Miller, G.; Pshezhetsky,
A. V.; Chemtob, S. J. Immunol. 2008, 180, 6977. (b) Chemtob, S.; Quiniou,
C.; Lubell, W. D.; Beauchamp, M.; Hansford, K. A. US Patent No. 20,060-
094,663, 2006. (c) Quiniou, C.; Kooli, E.; Joyal, J.-S.; Sapieha, P.; Sennlaub,
F.; Lahaie, I.; Zhuo, S.; Hou, X.; Hardy, P.; Lubell, W.; Chemtob, S. Semin.
Perinatol. 2008, 32, 325.
The configurational lability of 3 was examined by heating
to 80 °C for 1 day, revealing 3% epimerization of the
R-center and 3% racemization, which may be rationalized
1654
Org. Lett., Vol. 12, No. 8, 2010

in inflammation.23 It was chosen as a challenging target
because the Thr to be replaced in lactam 8 preceded a
sterically encumbered D-Val residue (Scheme 3). Synphase
equally produced 0.5 mg of the diasteriomeric (3R,4S)-lactam
counterpart.
In the context of solid-supported peptide synthesis,
elaboration of the hydroxy group may allow mimicry with
other Ser/Thr residues, attached to carbohydrate, phospho-
nate, sulfate, ester, and ether moieties. Oxiranylglycine 3 has
thus proven effective for the synthesis of R-amino-ꢀ-
hydroxy-γ-lactams in the context of structure-activity
relationships of Ser/Thr-containing peptides.
Scheme 3
.
Solid-Phase Synthesis of Constrained Peptide
Mimics
Acknowledgment. Financial support from the Natural
Sciences and Engineering Research Council of Canada, le
Fond Que´be´cois de la Recherche sur la Nature et les
Technologies, and the Canadian Institutes of Health Research
Team Grant Program (funding no. CTP79848) in G-Protein
Coupled Receptor Allosteric Regulation is gratefully ac-
knowledged. We thank the following current/former mem-
bers of the De´partement de Chimie, Universite´ de Montre´al:
Dr. Alexandra Fu¨rto¨s, Marie-Christine Tang, and Karine
Venne for assistance in LC-MS and HRMS analyses, Jad
Tannous for conducting SFC analyses, Dr. Phan viet Minh
Tan, Dr. Ce´dric Malveau, and Sylvie Bilodeau for assistance
with NMR spectroscopy, and Dr. Luisa Ronga for help with
solid-phase peptide synthesis.
Note Added in Proof. In the course of review, the
following publication was reported in which a complemen-
tary method featuring N-(Cbz)oxiranylglycine was employed
to make dipeptide building blocks that were inserted into
longer peptides: Sicherl, F.; Cupido, T.; Albericio, F. Chem.
Commun. 2010, 46, 1266.
lantern-supported vela peptide 9 was reacted with 3 in TFE
at 60 °C for 4 days, followed by 1 day in CH2Cl2/BzOH.
Lactam 11 was assessed to be of 56% purity after TES/TFA/
H2O cleavage of a sliver of the lantern, followed by
HPLC-MS analysis; linear alkylation product 10 was the
major impurity (9% conversion). After Fmoc group removal
with 20% piperidine/DMF, the remaining residues were
added using standard solid-phase peptide synthesis.24 Cleav-
age of the peptide from the support gave a 16:5:1 mixture
of closely eluting isomers. The purity of the major isomer
was assessed at 28%, from which 0.6 mg of 96.7% pure
isomer assigned as 8 (0.4% yield overall) was isolated along
with mixed fractions. Using (2R,2′R)-3, the above synthesis
Supporting Information Available: Full details on the
preparation and characterization of synthetic products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL1000582
(24) (a) Fields, G. B.; Noble, R. L. Int. J. Pept. Protein Res. 1990, 35,
161. (b) Lubell, W. D.; Blankenship, J. W.; Fridkin, G.; Kaul, R. 21.11
Peptides. In Science of Synthesis, Volume 21: Three Carbon-Heteroatom
Bonds: Amides and DeriVatiVes; Peptides; Lactams; Weinreb, S. M., Ed.;
Thieme: Stuttgart, 2005.
Org. Lett., Vol. 12, No. 8, 2010
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