Synthesis of (S)- and (R)-5-oxo-piperazine-2-carboxylic acid and its application to peptidomimetics

Article abstract of DOI:10.1021/jo901389q

(Chemical Equation Presented) A straightforward synthesis of (S)- and (R)-N-Boc-5-oxopiperazine-2-carboxylic acid is reported starting from L- or D-serine and ethyl glyoxylate. Those were evaluated as constituents in two tetrapeptides by studying their se

Full text of DOI:10.1021/jo901389q

pubs.acs.org/joc
chemists.2 In particular several six-membered-ring hetero-
Synthesis of (S)- and (R)-5-Oxo-piperazine-2-
Carboxylic Acid and Its Application to
Peptidomimetics
cyclic amino acids have been synthesized, comprising deri-
vatives of pipecolic,3 piperazine-2-carboxylic,4 1,4-thiazine-
3-carboxylic,5 1,3-thiazine-4-carboxylic,6 and morpholine-
3-carboxylic acid.7 5-Oxo-piperazine-2-carboxylic acid
(PCA, Figure 1), on the other hand, has received much less
attention, and only a few reports deal with its synthesis and
structural and biological properties.8
Karine Guitot,†,‡ Stefano Carboni,† Oliver Reiser,*,‡ and
Umberto Piarulli*,†
†
ꢀ
Dipartimento di Scienze Chimiche e Ambientali, Universita
degli Studi dell’Insubria, Via Valleggio, 11, 22100 Como,
‡
Italy, and Insitut fur Organische Chemie, Universitat
€
Regensburg, Universitatstrasse, 31, 93053 Regensburg,
Germany
€
umberto.piarulli@uninsubria.it;
oliver.reiser@chemie.uni-regensburg.de
FIGURE 1. (S)-N-Boc-5-oxo-piperazine-2-carboxylic acid 1.
Received June 30, 2009
In the frame of our studies directed toward the synthesis of
peptidomimetics with well-defined conformational proper-
ties,9 we became interested in the synthesis of 5-oxo-piper-
azinone-2-carboxylic acid (PCA), and its introduction into
peptide sequences as an inducer of secondary structures.
Herein we report a practical synthesis of (S)- and (R)-N-
Boc-5-oxo-piperazine-2-carboxylic acid 1 (Figure 1), its
introduction into peptides by a solution-phase peptide-
synthesis strategy, and a conformational analysis of two
tetrapeptide mimics incorporating a PCA residue.
Our synthetic approach (Scheme 1) started from L-serine
methyl ester hydrochloride, which was N-alkylated by using
ethyl glyoxylate in the presence of Pd/C and under an
atmosphere of H2. Following a procedure that has been
reported for the synthesis of 2,3-diamino propionic acid
starting from serine derivatives,10 the resulting alcohol 2
was transformed into the azide 3 by a Mitsunobu reaction,
using a solution of hydrazoic acid in toluene. Other meth-
odologies involving the activation of the hydroxyl group of
A straightforward synthesis of (S)- and (R)-N-Boc-5-oxo-
piperazine-2-carboxylic acid is reported starting from
L- or D-serine and ethyl glyoxylate. Those were evaluated
as constituents in two tetrapeptides by studying their
1
secondary structure by H NMR spectroscopy. In the
case of Boc-Val-(S)-PCA-Gly-Leu-OMe, two readily
interconverting conformations (in a 40%:60% ratio)
were observed, differing for the cis-trans isomerizaton
of the tertiary amide bond, while Boc-Val-(R)-PCA-Gly-
Leu-OMe displayed an equilibrium between a γ-turn and
a type II β-turn conformation.
(4) (a) Eichorn, E.; Roduit, J.-P.; Shaw, N.; Heinzmann, K.; Kiener, A.
Tetrahedron: Asymmetry 1997, 8, 2533–2536. (b) Letavic, M. A.; Barberia,
J. T.; Carty, T. J.; Hardinik, J. R.; Liras, J.; Lopresti-Morrow, L. L.;
Mitchell, P. G.; Noe, M. C.; Reeves, L. M.; Snow, S. L.; Stam, E. J.; Sweeney,
F. J.; Vaughn, M. L.; Yu, C. H. Bioorg. Med. Chem. Lett. 2003, 13, 3243–
3246. (c) Palucki, B. L.; Park, M. K.; Nargund, R. P.; Tang, R.; MacNeil, T.;
Weinberg, D. H.; Vongs, A.; Rosenblum, C. I.; Doss, G. A.; Miller, R. R.;
Stearns, R. A.; Peng, Q.; Tamvakopoulos, C.; Van der Ploeg, L. H. T.;
Partchett, A. A. Bioorg. Med. Chem. Lett. 2005, 15, 1993–1996. (d) Morley,
R. M.; Tse, H.-W.; Feng, B.; Miller, J. C.; Monaghan, D. T.; Jane, D. E.
J. Med. Chem. 2005, 48, 2627–2637. (e) Olsen, C. A.; Christensen, C.;
Nielsen, B.; Clausen, R. C.; Kristensen, J. L.; Franzyk, H. H.; Jaroszewski,
J. W. Org. Lett. 2005, 8, 3371–3374.
The design and synthesis of conformationally restricted
amino acids has been the focus of extensive research because
these compounds mimic or induce specific secondary struc-
tural features of peptides and proteins.1 Since the discovery
of the crucial role of proline in protein structures, cyclic
R-amino acids containing a heterocyclic ring have attracted
considerable attention from both synthetic and medicinal
(5) Shiraiwa, T.; Tadokoro, K.; Tanaka, H.; Namba, K.; Yokono, N.;
Shibazaki, K.; Kubo, M.; Kurokawa, H. Biosci. Biotechnol. Biochem. 1998,
62, 2382–2387.
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3131–3152.
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4254–4257. (b) Trabocchi, A.; Sladojevich, F.; Guarna, A. Chirality 2009, 21,
584–594.
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Greenstein, J. P. J. Biol. Chem. 1953, 201, 547–551. (b) Hardegger, E.;
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Kato, T.; Moriguchi, M.; Soda, K.; Nishino, N. Tetrahedron: Asymmetry 2005,
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DOI: 10.1021/jo901389q
r
Published on Web 09/29/2009
J. Org. Chem. 2009, 74, 8433–8436 8433
2009 American Chemical Society

Guitot et al.
JOCNote
SCHEME 1. Synthesis of (S)-N-Boc-5-oxo-piperazine-2-
carboxylic Acid 1
FIGURE 2. X-ray structure of methyl (S)-N-Fmoc-5-oxo-pipera-
zine-2-carboxylate 5.
(Boc-Val-(S)-PCA-Gly-Leu-OMe 6 and Boc-Val-(R)-PCA-
Gly-Leu-OMe 7) were prepared by solution phase peptide
synthesis (Boc strategy), using HATU (O-(7-azobenzotria-
zol-1-yl)-N,N,N0,N0-tetramethyluronium hexafluorophosphate)
to couple Boc-PCA to the Gly-Leu-OMe dipeptide and Boc-
Val to the PCA moiety. Similar sequences containing L- or
D-proline,13 and other proline mimics such as (S)- (R)-pipecolic
acid,11 have been studied and their preferred conformations
in solutions were determined by 1H NMR. In the latter
case both thermodynamic and kinetic aspects of the cis-
trans isomerization about pipecolic peptide bonds have been
studied.
serine were hampered by a concurrent elimination reaction
leading to the corresponding dehydroalanine derivative as
the major reaction product.
Catalytic hydrogenation of 3 with Pd/C caused reduction
of the azide to the primary amine with concurrent cyclization
to methyl 5-oxo-piperazinone-2-carboxylate 4 in quantita-
tive yield. Finally, a one-pot hydrolysis of the methyl ester
and N-Boc protection afforded N-Boc-5-oxo-piperazine-
2-carboxylic acid 1 in pure form. The synthesis of (R)-1
was analogously achieved by starting from D-serine. The
protection of the amino group of methyl (S)-5-oxo-pipera-
zinone-2-carboxylate 4 was then realized by using Fmoc-Cl in
a biphasic aqueous NaHCO3 dioxane medium in good yield.
(S)-N-Fmoc-5-oxo-piperazine-2-carboxylic acid methyl
ester 5 (Figure 2) could be crystallized from a mixture of
methanol and diethyl ether to allow an X-ray structure
analysis: the piperazinone ring adopts a half-chair confor-
mation with the methoxycarbonyl group in a pseudoaxial and
the N-protecting group in an equatorial orientation. This
arrangement, which has been reported for pipecolic acid
containing peptides,11 was found also in solution as judged
by the coupling constants for the protons on C2/C3 being
smaller than 5 Hz (typically 4.4 Hz). Theamidegroupdisplays
a planar geometry as expected with the N-H bond bisecting
the angle of the vicinal CH2 group.
To evaluate the enantiomeric purity of these derivatives,
(S)-1 and (R)-1 were then coupled to (R)-1-phenylethyla-
mine.7,12 The coupling was achieved in good yield with EDC
(N-ethyl,N0-[30-(dimethylamino)propyl]carbodiimide)/HOBT
(1-hydroxybenzotriazole)/iPr2NEt; however, the optical pur-
ity of the resulting N-Boc amides could not be established
either by HPLC or by NMR, the latter being complicated by
doubling of the signals due to rotamers caused by the N-Boc
group. Removal of the Boc protecting group revealed a single
set of NMR signals, whereas HPLC analysis of the single
amides and of the 1:1 mixture showed that in both (S)-1 and
(R)-1 cases the amides were obtained with a de greater than
95%.
A conformational study of 6 and 7 was performed in
1
CDCl3 solution by H NMR. In the case of Boc-Val-(S)-
PCA-Gly-Leu-OMe 6, the 1H NMR spectrum (5 mM)
showed two sets of signals in a 1.7:1 ratio, which were
attributed to the trans and cis tertiary amide isomers,
respectively (37% of the cis-conformer). It has been reported
that the presence of a pipecolic acid residue leads to a
significant increase in population of the cis conformer
(35-50%),11 whereas the cis amide-content in tetrapeptides
containing L-proline appears to be definitely lower (up to
21% if an aromatic amino acid, Phe or Tyr, is preceding
proline in the sequence).14 Analogously, the introduction of
a morpholine-3-carboxylic acid (Mor) in the tetrapeptide
sequence Ac-Val-(S)-Mor-Gly-Leu-OMe resulted in
a
2:3 mixture of cis and trans rotamers (40% of the cis-
conformer) at the Val-Mor amide bond.7b NOESY experi-
ments conducted at 298 K revealed the presence of intense
exchange cross peaks (EXSY) between the cis and trans PCA
containing peptides, which hampered the correct attribution
of the signals to the cis and trans conformers. This is
indicative of a rather low barrier to isomerization, as can
also be inferred from the coalescence of several backbone
(and amide) signals, which is observed by heating the sample
to 313 K. The presence of the pipecolic acid ring is reported
to increase the kinetics of cis-trans isomerizaton with
respect to proline, but with PCA an even faster isomerization
is observed. When a NOESY experiment was conduced at
273 K the exchange peaks were far less intense and contacts
for the two conformers could be assigned. The trans con-
former did not show the presence of relevant cross-peaks
indicative of a well-organized structure. This fact, together
with the relatively low chemical shift value of the NH signals
(δ e 7.0 ppm) and their high temperature coefficients (see the
1
Supporting Information for the H NMR spectra and the
determination of the Δδ/ΔT values), is indicative of a
The ability of both (S)- and (R)-PCA to act as proline
mimics in inducing turn structures when inserted into peptide
sequences was then studied. Two tetrapeptide sequences
(13) Haque, T. S.; Little, J. C.; Gellman, S. H. J. Am. Chem. Soc. 1996,
118, 6975–6985.
(14) Wu, W.-J.; Raleigh, D. P. Biopolymers 1998, 45, 381–394.
(11) Wu, W.-J.; Raleigh, D. P. J. Org. Chem. 1998, 63, 6689–6698.
(12) Souers, J. A.; Ellman, J. A. J. Org. Chem. 2000, 65, 1222–1224.
8434 J. Org. Chem. Vol. 74, No. 21, 2009

Guitot et al.
JOCNote
FIGURE 3. Hypothesized conformation of tetrapeptide Boc-Val-
(S)-PCA-Gly-Leu-OMe 6. The arrow indicates the NOE contact
between Val H-R and H-2 of the PCA moiety.
FIGURE 4. Hydrogen-bonded structures for tetrapeptide Boc-Val-
(R)-PCA-Gly-Leu-OMe 7. The arrows indicate significant NOE
contacts.
TABLE 1. 1H NMR Chemical Shifts at Room Temperature and
Temperature Coefficients for Tetrapeptide 7
NH
δ (ppm)
Δδ/ΔT (ppb/K)
In conclusion we have reported a straightforward synth-
esis of both (S)- and (R)-N-Boc-5-oxo-piperazine-2-car-
boxylic acid 1, starting from serine and ethyl glyoxylate.
The incorporation of both enantiomeric tertiary amino acids
into a model tetrapeptide was obtained by solution phase
peptide synthesis and their turn inducing abilities studied by
1H NMR spectroscopy. Interestingly Boc-Val-(S)-PCA-
Gly-Leu-OMe 6 showed two readily interconverting con-
formations (in a 40%:60% ratio), differing in the cis and
trans configuration of the tertiary amide bond. The barrier to
isomerization of the tertiary amide is unusually low with
a coalescence temperature of 313 K. On the other hand,
Boc-Val-(R)-PCA-Gly-Leu-OMe 7, which contains the R-PCA
enantiomer, showed a more defined turn conformation.
Val-NH
PCA-NH
Gly-NH
Leu-NH
5.13
6.38
7.79
6.53
-1.82
-10.25
-4.59
-2.78
random coil structure. In the case of the cis-conformer
(Figure 3), the Gly-NH appears at a remarkably deshielded
chemical shift value (δ = 8.27 ppm) and with a temperature
coefficient indicative of an equilibrium between an intramo-
lecularly bonded and a nonbonded status. The NOESY
experiments showed a rather strong contact between Val
H-R and H-2 of the PCA moiety, indicating both the cis-
conformation of the amide bond and the presence of a β-turn
structure stabilized by a 10-membered cyclic hydrogen bond.
The 1H NMR spectrum of the heterochiral Boc-Val-(R)-
PCA-Gly-Leu-OMe (7) showed a single set of signals indi-
cating the existence of a single rotamer in solution, which,
owing to a strong NOE contact between the Val H-R and the
H-6 protons of the PCA residue, was assigned to the trans
isomer. A study of the chemical shift of the amide protons
and their temperature coefficients determination (Table 1)
indicated for Gly-NH an equilibrium between hydrogen-
bonded and nonhydrogen-bonded states; a similar behavior
could be assumed for Leu-NH, although with a distinctively
lower chemical shift.
Experimental Section
(S)-2-((Ethoxycarbonylmethyl)amino)-3-hydroxypropionic Acid
Methyl Ester (2). L-Serine methyl ester hydrochloride (1.0 g,
6.45 mmol) was dissolved in methanol, then triethylamine
(902 μL, 6.48 mmol), a 50% solution of ethyl glyoxalate in toluene,
and 10% Pd/C (90 mg), were successively added, and the resulting
mixture was stirred overnight under a hydrogen atmosphere. The
suspension was filtered over a pad of Celite, and the solvent was
removed under reduced pressure. The crude product was purified
by flash chromatography on silica gel (CH2Cl2/MeOH, 98:2) to
afford the desired product as a colorless oil (902 mg, 68%).
[R]20D -27.8 (c 1.0, CHCl3); 1H NMR (CDCl3) δ 4.21-4.15 (q,
2H, J = 8.7 Hz), 3.79 (dd, 1H, J1 = 11.1 Hz, J2 = 4.5 Hz), 3.75 (s,
3H), 3.69 (dd, 1H, J1 = 5.8 Hz, J2 = 11.1 Hz), 3.52 (d, 1H, J =
17.4 Hz), 3.40 (d, 1H, J = 17.5 Hz), 2.76 (br, 1H), 1.26 (t, 3H, J =
7.1 Hz); 13C NMR δ 172.5, 172.0, 62.4, 62.3, 61.1, 52.3, 48.9, 14.1;
IR (nujol) νmax 3329, 3184, 1723, 1377, 1201, 1068. Anal. Calcd for
C8H15NO5: C 46.82, H 7.37, N 6.83. Found: C 46.74, H 7.32,
N 6.46.
(S)-3-Azido-2-((ethoxycarbonylmethyl)amino)propionic Acid
Methyl Ester (3). Triphenylphosphine (1.44 g, 5.50 mmol) was
added to a solution of 2 (807 mg, 3.93 mmol) in dry toluene
(30.0 mL) under nitrogen at room temperature. After complete
dissolution of the phosphine, HN3 (0.5 M in toluene; 15.70 mL,
7,86 mmol) was added, followed by diisopropylazodicarboxy-
late (DIAD, 1.10 mL, 5.50 mmol). After 2 h the mixture was
directly purified by flash chromatography (CH2Cl2/MeOH,
The NOESY experiments showed strong contacts of Gly
NH with H-2 and on H-6 of the PCA moiety as well as with
Leu-NH. These findings are consistent with an equilibrium
between a γ-turn and a type II β-turn conformation
(Figure 4).
A 1H NMR study in DMSO-d6 revealed two sets of signals
in a 1.7:1 ratio, which were attributed to the trans and cis
tertiary amide isomers, respectively. The NOESY experi-
ment showed intense exchange peaks and the absence of
significant long-range interactions. This behavior is in agree-
ment with other similar cases.13,15
Conformational preferences of compounds 6 and 7 were
investigated by Monte Carlo simulations without imposing
any constraint, using Spartan 06 version 1.03. The lowest
energy conformations calculated substantially reflect the
experimentally observed structures (see the Supporting
Information for a more detailed description of these
results).
99:1) to give the desired product as a colorless oil (796 mg,
1
88%). [R]20 -37.6 (c 0.5, CHCl3); H NMR (CDCl3) δ 4.19
D
(q, 2H, J = 7.1 Hz), 3.77 (s, 3H), 3.58-3.56 (m, 2H), 3.52-3.46
(m, 3H), 2.33 (br, 1H), 1.25 (t, 3H, J = 7,1 Hz); 13C NMR
(CDCl3) δ 171.9, 171.5, 60.9, 60.2, 53.0, 52.3, 49.0, 14.1; IR
(CH2Cl2) νmax 3352, 3063, 2108, 1744, 1445; HRMS (ESIþ) m/z
calcd for [C8H14N4NaO4]þ 253.O9073 [M þ Na]þ, found
253.09097.
(15) D’Elia, V.; Zwicknagl, H.; Reiser, O. J. Org. Chem. 2008, 73, 3262–
3265.
J. Org. Chem. Vol. 74, No. 21, 2009 8435

Guitot et al.
JOCNote
(S)-5-Oxo-piperazine-2-carboxylic Acid Methyl Ester (4).
Pd/C (0.112 mmol 0.1 equiv) was added to a solution of 3 (260
mg, 1.12 mmol) in MeOH (5 mL). The reaction was stirred
overnight at rt under 1.0 atm of H2. The solution medium was
then filtrated through a pad of Celite and the solvent was
evaporated under reduced pressure. The crude was purified by
flash chromatography (DCM/MeOH, 92:8) to obtain the
the reaction mixture was left stirring for 2.5 h at rt. Successively,
the mixture was partitioned between EtOAc (14.0 mL) and
water (7.0 mL), and the organic phase was washed with 1.0 M
HCl and brine and dried over Na2SO4. The organic solvent was
removed and the crude product was purified by flash chroma-
tography (DCM/MeOH, 95:5) to afford the desired compound
5 (258 mg, 87%). Mp 143-144 °C; [R]20D -11.9 (c 1.0, CHCl3);
two sets of signals were observed in the 1H and 13C NMR
spectrum due to the presence of two rotational isomers A:B (3:1
ratio) 1H NMR (CDCl3) δ 6.57 (br s,1HB), 6,54 (br s, 1HA), 5,04
(t, 1HA, J = 2.0 Hz), 4.61 (br s, 1HB),4.60-4.49 (m, HA and HB),
4.44 (dd, 1HA, J1 = 7.2 Hz, J2 = 3.6 Hz), 4.28-4.15 (m, 2HA
and 1HB), 4.12 (d, 1HA, J = 18.0 Hz), 4.03 (d, 1HB, J = 18.4
Hz), 3.84 (dd, 1HB, J1 = 5.2 Hz, J2 = 1.6 Hz), 3.81 (dd, 1HB,
J1 = 3.6 Hz, J2 = 1.6 Hz), 3.78 (s, 3HA), 3.71 (s, 3HB), 3.68 (dd,
1HA, J1 = 13.2 Hz, J2 = 4.8 Hz), 3.57 (dd, 1HA, J1 = 12.8 Hz,
J2 = 4.4 Hz); 13C NMR (CDCl3) δ 169.7 (A), 169.6 (B),
167.4 (B), 166.9 (A), 155.2 (A), 154.3 (B), 143.6 (A and B),
143.5 (B), 143.4 (A), 141.4 (A and B), 141.3 (A and B),
127.92 (A), 127.88 (B), 127.82 (B), 127.22 (A), 127.20 (A),
127.15 (B), 127.1 (B), 124.9 (A), 124.7 (B), 124.6 (B), 120.1
(A), 120.0 (B), 68.4 (A), 67.8 (B), 53.1 (A and B), 52.5 (B), 52.0
(A), 47.2 (B), 47.1 (A), 46.0 (A and B), 42.3 (A and B); IR (nujol)
νmax 3195, 1745, 1694, 1410, 1315, 1232, 1099. Anal. Calcd
for C21H20N2O5: C 66.29, H 5.30, N 7.37. Found: C 66.20, H
5.35, N 7.28.
desired product as a white paste (176 mg, 99%). [R]20 -46.3
D
(c 1.0, CHCl3); 1H NMR (CDCl3) δ 6.87 (br, 1H), 3.77 (s, 3H),
3.75 (dd, 1H, J1 = 8.0 Hz, J2 = 4.4 Hz), 3.66 (d, 1H, J = 17.1
Hz), 3.61 (ddd, 1H, J1 = 11.8 Hz, J2 = 4.4 Hz, J3 = 3.1 Hz),
3.53 (d, 1H, J = 17.4 Hz), 3.52 (ddd, 1H, J1 = 11.8 Hz, J2 = 8.0
Hz, J3 = 1.8 Hz), 2.15 (br, 1H); 13C NMR (CDCl3) δ 171.3,
169.8, 54.3, 53.0, 48.3, 44.2; IR (CH2Cl2) νmax 3406, 3354, 3211,
1744, 1678; HRMS (EI) m/z calcd for [C6H10N2O3]þ 158.06910
[M]þ, found 158.06920.
(S)-5-Oxopiperazine-1,2-dicarboxylic Acid 1-tert-Butyl Ester
(1). Compound 4 (120 mg, 0.76 mmol) was dissolved in a 1:1
dioxane/NaOH (1.0 M) solution (2.0 mL) at rt. The mixture was
cooled to 0 °C and Boc2O (331 mg, 1.52 mmol) was added. After
15 min the ice bath was removed and the reaction was stirred for
10 h at rt. The reaction was quenched by addition of water
(5.0 mL), and the aqueous phase was washed with EtOAc (2 ꢀ
3.0 mL). The aqueous solution was acidified until pH 3 mediat-
ing addition of a 1.0 M solution of KHSO4. The acid phase was
extracted with EtOAc (5 ꢀ 5.0 mL) and dried over Na2SO4. The
solvent was removed under reduced pressure and after purifica-
tion by flash chromatography (CH2Cl2/MeOH, 97:3) the pro-
duct (S)-1 was collected as a white powder (163 mg, 88%). Mp
X-ray Crystallographic Data of 5:. C21H20N2O5; MW =
380.39 g mol-1; T = 123 K; λ(Mo KR) = 1.54184 A; ortho-
rhombic, space group P21 21 21, a = 6.5176(2) A, b = 11.6690(5)
3
˚
103-104 °C; [R]20 -33.8 (c 0.5, CH3OH); two sets of signals
A, c = 23.9190(9) A, R = 90°, β = 90°, γ = 90°, V = 1819.13(12)
˚
˚
D
1
3
were observed in the H and 13C NMR spectrum due to the
presence of two rotational isomers A:B (2:1 ratio) H NMR
A , Fcalc = 1.389 mg m-3, Z = 4; μ(ΜοKR) = 0.828 mm-1, R1 =
˚
3
1
0.0283, wR2 = 0.0713, for 2679 unique data collected in the
3.70-66.5° θ ranges.
(CDCl3) δ 8.35 (d, 1HB, J = 3.2 Hz), 8.20 (d, 1HA, J = 4.0 Hz),
4.99 (d, 1HA, J = 4.0 Hz), 4.79 (d, 1HB, J = 4.0 Hz), 4.23 (d,
1HA and 1HB, J = 18.8 Hz), 4.07 (d, 1HA and 1HB, J = 18.8
Hz), 3.88 (dd, 1HA, J1 = 12.6 Hz, J2 = 3.9 Hz), 3.84 (dd, 1HB,
J1 = 17.3 Hz, J2 = 4.1 Hz), 3.65 (dd, 1HA and 1HB, J1 = 17.3
Hz, J2 = 4.9 Hz), 1.50 (s, 9HA), 1.48 (s, 9HB); 13C NMR
(CDCl3) δ 174.3 (A), 174.0 (B), 170.7 (A), 170.1 (B), 154.4
(A), 153.6 (B), 81.9 (A), 81.8 (B), 52.6 (A), 51.1 (B), 45.9 (A), 45.3
(B), 42.4 (A), 40.7 (B), 28.3 (A), 28.2 (B); IR (CH2Cl2) νmax 3408,
3230, 1707, 1647; HRMS (ESIþ) m/z calcd for [C10H16N2O5]þ
244.1059 [M þ H]þ, found 244.1060.
Acknowledgment. We thank the European Commission
(contract MEST-CT-2004-515968, “Foldamers”) for finan-
cial support and for Ph.D. Fellowships (to K.G.). Financial
support from the Deutsche Forschungsgemeinschaft
(Graduiertenkolleg Medizinische Chemie 760) is also grate-
fully acknowledged. We would also like to thank Dr. D.
Potenza (University of Milano) and Prof. Dr. C. Cabrele
(Bochum University) for helpful discussions.
(S)-5-Oxopiperazine-1,2-dicarboxylic Acid 1-(Fluoren-9-yl)
Ester 2-Methyl Ester (5). 4 (128 mg, 0.81 mmol) was dissolved
in a 2:1 water:dioxane (3.0 mL) solution and NaHCO3 (136 mg,
1.61 mmol) was added. The mixture was cooled to 0 °C and a
solution of Fmoc-Cl (209 mg, 0.81 mmol) in dioxane (1.5 mL)
was added dropwise over 15 min. The ice bath was removed and
Supporting Information Available: Synthetic schemes,
experimental procedures, and characterization of compounds
1
6-7 and H and 13C NMR of all reported compounds, con-
formational studies of compounds 6-7. This material is avai-
lable free of charge via the Internet at http://pubs.acs.org.
8436 J. Org. Chem. Vol. 74, No. 21, 2009