Conformationally restricted peptide mimetics by ring-closing olefin metathesis

Article abstract of DOI:10.1055/s-0032-1316758

Elegant chemical methodology restricting the backbone flexibility of biologically active peptides has attracted growing interest. A practical synthetic strategy is presented to access ten-membered lactam peptide mimetics. Employing a ring-closing olefin metathesis as the key reaction step, the cyclic olefin moiety was obtained with cis configuration. Conformational investigations were performed with two model peptides. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0032-1316758

2682▌
FEATURE ARTICLE
feature article
Conformationally Restricted Peptide Mimetics by Ring-Closing Olefin
Metathesis
Conformationally Restricted Peptide Mimetics
Satish Wakchaure, Jürgen Einsiedel, Reiner Waibel, Peter Gmeiner*
Department of Chemistry and Pharmacy, Emil Fischer Center, Friedrich Alexander University, Schuhstraße 19, 91052 Erlangen, Germany
Fax +49(9131)8522585; E-mail: peter.gmeiner@medchem.uni-erlangen.de
Received: 25.05.2012; Accepted after revision: 27.06.2012
tostatin led to selective somatostatin sst5 receptor li-
Abstract: Elegant chemical methodology restricting the backbone
gands.8 Grubbs and co-workers were successful in
flexibility of biologically active peptides has attracted growing in-
covalently cross-linking two O-allylserine residues in the
terest. A practical synthetic strategy is presented to access ten-mem-
bered lactam peptide mimetics. Employing a ring-closing olefin
positions i and i+4 of a peptide, thus, inducing the forma-
metathesis as the key reaction step, the cyclic olefin moiety was ob- tion of a stable, short helix.9 Verdine and co-workers de-
tained with cis configuration. Conformational investigations were
performed with two model peptides.
veloped a ‘peptide stapling’ strategy in which an all-
hydrocarbon cross-link was generated within natural pep-
Key words: peptide mimetics, cyclic olefins, ring-closing metathe-
sis, conformational investigations, medicinal chemistry
tides by RCM of inserted α,α-disubstituted nonproteogen-
ic amino acids bearing olefinic side chains.10 This strategy
resulted in a binding affinity to the target protein resulting
from the human double minute oncogene (hDM2), that
In the last few decades, there has been growing interest in was increased by three orders of magnitude when com-
the therapeutic use of peptide drugs.1 Major achievements pared to the wild-type peptide.10a Another attractive meth-
in solid-phase peptide synthesis, large-scale preparation, od to stabilize α-helix structures has been described as an
and purification techniques have been developed to pro- RCM-based ‘hydrogen bond surrogate’ (HBS) approach.
duce efficiently this intricate substance class in a compet- Here, the formation of helices is enforced by completely
itive manner.2
replacing the H-bonded –HN–C(=O)–HN– motif, which
is originally formed by the N-terminal amide carbonyl
group of amino acid i and the NH of amino acid i+4, by
using the covalent isostere –CH2–C=C–CH2–N– as a con-
straint element.11
However, the use of native peptides is frequently limited
by proteolytic degradation, unfavorable pharmacokinetic
properties,1a or low target selectivity.3 Therefore, research
has focused on the exploration of peptide mimetics that al-
low disadvantages associated with natural sequences to be Besides its application for cyclopeptide mimetics, RCM
circumvented, and, thus, to exploit fully their therapeutic has also proved superior in the synthesis of a large number
potential in the treatment of severe diseases. Besides the of lactam-bridged dipeptide surrogates. Depending on the
incorporation of structurally modified amino acids, ele- size of the ring and the position of the olefin, distinct di-
gant chemical strategies restricting the peptide backbone hedral angles of the peptide backbone can be adjusted.
flexibility have attracted growing interest.3–5 If the bioac- Figure 1 indicates the positions of the peptide backbone
tive conformation has been adjusted properly by suitable that have been used to attach the alkene-based, conforma-
constraint elements, the number of degrees of conforma- tionally rigidizing elements. Grubbs and co-workers dem-
tional freedom and, thus, binding-associated loss of entro- onstrated that the nitrogen atoms of the first and the
py can be decreased, thus increasing target affinity. following amino acid (N  N+1) could be connected by
Moreover, off-target binding may be reduced, metabolic RCM to give lactam derivatives of type I.12 Liskamp and
stability will be enhanced, and an increase of lipophilicity co-worker were able to broaden successfully the scope of
originating from hydrocarbon-based constraint elements this concept to amino acids different from glycine.13 For-
may ameliorate cell penetration through membrane barri- mal bridging Cα of an amino acid with the backbone nitro-
ers.
gen of the following amino acid (Cα  N+1) leads to the
formation of Freidinger lactams.14 Upon ring-closing ole-
fin metathesis, monocyclic seven-membered ‘dehydro-
Freidinger lactams’ were synthesized by Grubbs’ labora-
tory (type IIIa)12 and by Piscopio et al.15 We further ex-
ploited the potential of the concept to access eight-, nine-
and ten-membered congeners and investigated the result-
ing conformational properties.16 Furthermore, RCM was
successfully used for the synthesis of β-peptide mimick-
ing analogues.17 Proline-derived fused lactams of type
IIIb offer the possibility to adjust gradually the Ψi+1 dihe-
dral angle from 140–170°, a range frequently adopted by
prolyl residues in protein cystal structures.18 Moreover,
Grubbs’ ring-closing olefin metathesis (RCM)6 has
emerged as a versatile tool that facilitates the conforma-
tional rigidization of peptides.7 This highly orthogonal
strategy has been exploited to synthesize peptide macro-
cycles. Replacing disulfide moieties, RCM has become a
very useful tool for the formation of ‘dicarba’ peptides.
The synthesis of carba-analogous derivatives of soma-
SYNTHESIS 2012, 44, 2682–2694
Advanced online publication: 06.08.2012
DOI: 10.1055/s-0032-1316758; Art ID: SS-2012-T0470-FA
© Georg Thieme Verlag Stuttgart · New York
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2683
spirocyclic proline derivatives of type IIIc were designed col.23 To complement this, Lubell’s group developed an
and constructed by RCM resulting in highly potent type II EPC-strategy leading to a complete series of eight-, nine-
β-turn nucleating scaffolds.19 It is worthy of note that the and ten-membered scaffolds.24 The laboratory of
principle of proline annulation via positions 1 and 5 of the Katzenellenbogen demonstrated the synthesis of the
pyrrolidine ring was successfully demonstrated by the trans-configured azacyclodecenone of type IVc, which
groups of Moeller, Wagner and Schmalz.20,21 The Cα  has been described as the first peptide mimetic that
Cα+1 linkage principle is realized in compounds of type showed type I β-turn inducing properties.25
IV. A highly efficient type VI β-turn mimetic was ob-
A proline-based tripeptide mimetic following the con-
tained when the fused proline derivative IVa was synthe-
junction principle Cα  N+2 was recently designed in our
sized via the RCM approach.22 Nine-membered lactam
laboratory.26 We followed the idea to replace the turn sta-
derivatives of type IVb were obtained in racemic form
bilizing side chain of aspartate or asparagine by a covalent
taking advantage of a tandem Ugi reaction/RCM proto-
Biographical Sketches█
Satish Wakchaure was position as a research assis- dia. He started his Ph.D. in
born in 1979, Sangamner, tant. From 2006–2007, he 2010 under the supervision
India. He received his Mas- worked as an industrial of Prof. Peter Gmeiner at the
ter’s degree in Organic placement student at Glaxo- University of Erlangen-
Chemistry from the Univer- SmithKline Pharmaceuti- Nürnberg. He is currently
sity of Pune, India in 2005. cals, UK. From 2008 to focused on the synthesis of
In 2006, he moved to the 2009, he held an appoint- peptide mimetics.
National Chemical Labora- ment at GlaxoSmithKline
tory, Pune, India, holding a Pharmaceuticals, Thane, In-
Jürgen Einsiedel was born at the University of Erlan- scientist position. His re-
in 1969 in Nürnberg. He re- gen-Nürnberg
concerned search interests are neuro-
ceived his degree in phar- the EPC synthesis of dopa- tensin receptor ligands,
macy (1994) at the mine receptor ligands start- peptide mimetics, and solid-
University of Erlangen- ing from amino acids. He is phase-supported
Nürnberg. His Ph.D. (2000) currently holding a senior synthesis.
peptide
Reiner Waibel was born in the group of Prof. Hans structure elucidation of un-
1951 in Singen. He received Achenbach, leading to a known compounds and con-
his diploma in chemistry in Ph.D. degree in 1983. He is formational analysis of
Freiburg in 1979. His Ph.D. currently working in the lab- peptides and peptide mimet-
work was concerned with oratory of Prof. Peter ics.
isolation and structure eluci- Gmeiner. His scientific in-
dation of natural products in terest is mainly focused on
Peter Gmeiner was born in bilitation in 1992, he was organic synthesis, and phar-
1959 in Vohenstrauß. He re- appointed as a Professor of macological investigation
ceived his Ph.D. in 1986 Pharmaceutical Chemistry of bioactive molecules
from the University of Mu- at the University of Bonn. when class I G-protein cou-
nich. From 1987 to 1988 he Since October 1996, he has pled receptors (GPCRs) are
was a postdoc at the Univer- held the chair of Full Profes- addressed as allosteric tar-
sity of California in Berke- sor of Pharmaceutical/Me- get proteins. Within these
ley, USA. Peter Gmeiner dicinal Chemistry at the studies, the synthesis of
subsequently returned to University of Erlangen- peptide mimetics is of par-
Munich as a research associ- Nürnberg. Peter Gmeiner’s ticular interest.
ate. Upon receiving his ha- research spans the design,
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2684
S. Wakchaure et al.
FEATURE ARTICLE
O
R1
H
O
N
O
N
O
O
O
O
N
N
N
HN
N
N
R2
N
O
Boc
OMe
NH
N
H
N
O
R2
H
R1
O
O
O
HN
R3
I
II
N => N+1
Cα => N+2
Bn
O
O
O
NH
H
N
N
H
Cα+1
Cα
N+1
N
N
N
N+2
O
H
H
O
N
N
HN
OMe
( )
0,1
O
Boc
O
O
HN
Cα => N+1
Cα => Cα+1
IIIa
IVa
( )
( )
0–2
H
N
0,1
( )1,2
( )1,2
R3
N
O
O
R1
O
N
O
H
N
R1
N
N
Boc
O
N
N
H
N
H
O
O
N
H
O
H
O
O
R4
O
O
HN
R2
O
R2
IIIb
IVb
( )0,1
( )
H
N
1–3
N
N
O
HN
Boc
Boc
O
O
HN
R
OMe
O
IIIc
IVc
Figure 1 Secondary-structure-inducing peptide mimetic scaffolds synthesized via Grubbs olefin metathesis
alkene element leading to the cis-configured, ten-mem- Our initial investigations were directed to the incorpora-
bered scaffold of type II. Hence, a highly efficient type I tion of glycine and N-methylglycine replacing the cyclic
β-turn mimetic simulating an Asx turn via the HBS prin- amino acid proline (Scheme 1). We employed the tert-bu-
ciple was realized. Complementary to the concept of J. A. tyl esters of glycine (1a) and N-methylglycine (1b) and
Katzenellenbogen,25 peptide mimetics of type II offer the performed acylation with (S)-Fmoc-allylglycine, which
possibility to present individual amino acid side chains was activated by using the mixed anhydride method.27
putatively forming attractive contacts to biological recep- Thus, we obtained the dipeptides 2a,b, which were readily
tors.
deprotected with trifluoroacetic acid. Utilizing HATU-
promoted activation of 2a,b, we were able to couple the
carboxylic acids with N-allylglycine tert-butyl ester (3a)28
to furnish the dienes 4a,b in high yield. RCM employing
the Grubbs second-generation catalyst (A) led to the for-
mation of a ten-membered ring resulting in production of
the lactams 5a,b. In contrast to the more effective forma-
tion of 5b (50%), the metathesis step to obtain the glycine
derivative 5a only worked with low conversion of the
starting material (20%). Removal of the catalyst during
workup was done taking advantage of the water-soluble
phosphine, tris(hydroxymethyl)phosphine.29 With the
help of 1H NMR spectroscopy it was possible to establish
the geometry of the double bonds of both cyclic olefins
5a,b, when the cis configuration was determined. For lac-
tam 5a, it was possible to obtain resolved olefin protons
employing acetone-d6 as the solvent, allowing the cis-spe-
cific coupling constant of 10 Hz to be determined. For 5b,
we were able to assess the cis configuration at a later stage
of the synthesis. To approach a model tetrapeptide mimet-
Based on the conformationally restricted peptide mimetic
of type II, we envisaged an exploration of the scope and
limitations of the synthetic approach when we intended to
exchange the reverse-turn nucleator proline by individual
open-chain amino acids (Figure 2). We were intrigued by
question of whether the resulting diene precursors devoid
of the rigid proline still allowed the formation of ten-
membered cis-alkene ring systems of type V.
O
R1
O
R1
R3
O
O
N
N
N
HN
R2
R2
N
HN
O
O
HN
O
HN
O
II
V
Figure 2 Substitution of proline leads to peptide mimetics of type V
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2685
R3
O
O
R2
O
N
1. TFA–CH2Cl2 (1:1)
r.t., 2.5 h
2. HATU, DIPEA
NMP, r.t., 24 h
O
R2
R2
OH
Ot-Bu
N
R1
Ot-Bu
O
NH
Ot-Bu
N
Fmoc
HN
NH R1
O
HN
R1
O
R3
ClCO2i-Bu, NMM
Et 3N, THF, DMF
–15 °C to 5 °C, 12 h
Fmoc
Fmoc
1a: R1 = H, R2 = H
1b: R1 = Me, R2 = H
1c: R1 = H, R2 = Me
O
2a: R1 = H, R2 = H (91%)
2b: R1 = Me, R2 = H (80%)
2c: R1 = H, R2 = Me (81%)
4a: R1 = R2 = R3 = H (86%)
HN
4b: R1 = Me, R2 = R3 = H (83%)
Ot-Bu
4c: R1 = H, R2 = Me, R3 = Bn (70%)
3a: R3 = H
3b: R3 = Bn
PCy3
Ph
Cl
Ru
Cl
Mes
Mes
N
O
N
O
N
O
O
N
O
O
N
A (15 mol%)
CH2Cl2
1. Et2NH
CH2Cl2, r.t., 2 h
TFA–CH2Cl2 (1:1)
r.t., 2.5 h
N
HN
X
N
Me
Me
O
N
Ot-Bu
reflux, 24 h
Me
O
R1
O
2. Ac2O, DIPEA
r.t., 12 h
HN
HN
HN
Fmoc
Fmoc
O
MeNH2·HCl
HATU, DIPEA
CH2Cl2, r.t., 12 h
5a: R1 = H (20%)
5b: R1 = Me (50%)
6: X = OH (85%)
7: X = NHMe (crude)
8 (25%)
Scheme 1 Synthesis of conformationally restricted peptide mimetics
ic, the tert-butyl ester derivative 5b was subjected to acid- lylalanine tert-butyl ester, which resulted in the formation
ic cleavage with trifluoroacetic acid to give the carboxylic of dipeptide 9b11 at room temperature.
acid 6. Applying HATU as the activating reagent, peptide
coupling was performed with methylamine hydrochloride
to obtain the intermediate 7. Fmoc deprotection using di-
In the presence of piperidine, the N-methyl-derivative 9d
was deprotected; flash chromatography gave 10d, which
was characterized by NMR spectroscopy. Observing a mi-
ethylamine and subsequent acylation with acetic anhy-
nor fraction of a side product (represented by a second set
dride to gave rise to the model peptide 8.
of proton resonances), we assumed that a minor fraction
The prolyl residue should be also replaced by alanine as a of the dipeptide had cyclized. Acylation with (S)-Fmoc-
prototype of an α-substituted amino acid and coupled to allylglycine gave the cyclization precursor 11d in 93%
phenylalanine. According to the pathway described for yield. To avoid ‘head-to-tail’ cyclization, Fmoc deprotec-
the preparation of 8, peptide coupling with alanine tert- tion of 9a–c was performed with the volatile diethylamine
butyl ester (1c) and (S)-Fmoc-allylglycine gave the pro- to give the crude dipeptides 10a–c. Residues of diethyl-
tected dipeptide 2c. HATU-promoted coupling of 2c with amine were removed by washing with aqueous ammoni-
N-allylphenylalanine tert-butyl ester (3b)30 furnished a um chloride solution and the crude material was directly
mixture of diastereomers 4c. This side reaction could not transformed into the cyclization precursors 11a–c.
be suppressed by employing alternative coupling reagents
including triphosgene, carbodiimides (DCC or EDC),
Ring-closing metathesis of the dienes 11a–c was accom-
plished employing Grubbs II catalyst (A) to afford the ex-
pected ten-membered, exclusively cis-configured lactams
phosphonium salts like PyBOP, isobutyl chloroformate,
or COMU.31 Obviously, activation of the dipeptide 2c led
12a–c in 50–60% yield. Interestingly, for the conversion
to epimerization of the N-acylalanine methine via the for-
of the diene 11d into the lactam 12d, the reaction time was
mation of an oxazolone intermediate.27 In order to circum-
significantly lower and the cyclization worked with only
vent this problem, we decided to change the reaction
5 mol% of catalyst.
sequence for the construction of the precursor diene
Using trifluoroacetic acid, deprotection of the tert-butyl
ester functions of 12a–d gave the carboxylic acid building
(Scheme 2).
Thus, starting from the N-allylamino acid derivatives 3b,
blocks 13a–d in 70–91% yield. It is noteworthy that in the
3c,32 3d,26 HATU-promoted acylation with Fmoc-alanine
case of the tyrosine derivative 12c, additional cleavage of
or Fmoc-N-methylalanine at elevated temperatures gave
the tert-butyl ether was observed. Conversion of 13d into
the dipeptides 9a,31 and 9c,d. Apparently, higher activa-
the model peptide 14 was accomplished by activation of
tion energy is crucial due to the bulkier benzyl residues of
the carboxylic acid, aminolysis, concomitant Fmoc-cleav-
the phenylalanine derivatives. This is in accordance with
age applying a methylamine solution in tetrahydrofuran,
and subsequent acetylation of the amine function.
the peptide coupling of the less sterically demanding N-al-
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2686
S. Wakchaure et al.
FEATURE ARTICLE
O
O
OH
OH
R1
O
N
NHFmoc
R2
Fmoc
O
R1
Ot-Bu
for 11a, 11c:
DIC, HOBt, DIPEA,THF
r.t., 12 h
O
N
for 9a–c: R = H
for 9d: R = Me
Ot-Bu
R1
HN
N
N
O
R2
O
Ot-Bu
N
O
for 11b, 11d:
HATU, DIPEA, NMP, r.t., 2 d
HATU, DIPEA, CH2Cl2
for 9a, 9c, 9d: 42 °C, 2 d
for 9b: r.t., 8 h
R2
R3
HN
Fmoc
9a: R1 = Bn, R2 = H, R3 = Fmoc (81%)
3b : R1 = Bn
3c : R1 = Me
11a: R1 = Bn, R2 = H (61%)
9b: R1 = Me, R2 = H, R3 = Fmoc (90%)
9c: R1 = 4-t-BuOC6H4CH2, R2 = H, R3 = Fmoc (80%)
9d: R1 = Bn, R2 = Me, R3 = Fmoc (91%)
for 10a–c:
Et2NH, CH2Cl2
r.t., 3.5 h
11b: R1 = Me, R2 = H (45%)
3d : R1 = 4-t-BuOC6H4CH2
11c: R1 = 4-t-BuOC6H4CH2, R2 = H (51%)
11d: R1 = Bn, R2 = Me (93%)
for 10d:
piperidine
CH2Cl2, r.t., 2 h
10a: R1 = Bn, R2 = H, R3 = H (crude)
10b: R1 = Me, R2 = H, R3 = H (crude)
10c: R1 = 4-t-BuOC6H4CH2, R2 = H, R3 = H (crude)
10d: R1 = Bn, R2 = Me, R3 = H (81%)
R1
Grubbs II (A)
O
R1
O
CH2Cl2 (for 12b–d)
or toluene (for 12a)
reflux, 2.5 or 24 h
Ot-Bu
X
N
O
TFA–CH2Cl2 (1:1)
N
O
r.t., 2.5 h (13a,c,d)
N
R2
O
O
N
R2
TFA, 0 °C, 1.5 h (13b)
HN
Fmoc
HN
Y
13a: R1 = Bn, R2 = H, X = OH, Y = Fmoc (90%)
13b: R1 = Me, R2 = H, X = OH, Y = Fmoc (82%)
13c: R1 = 4-HOC6H4CH2, R2 = H, X = OH, Y = Fmoc (70%)
13d: R1 = Bn, R2 = Me, X = OH, Y = Fmoc (91%)
14: R1 = Bn, R2 = Me, X = NHMe, Y = Ac (59%)
12a: R1 = Bn, R2 = H (70%)
12b: R1 = Me, R2 = H (60%)
1. a) HATU, DIPEA, THF,
r.t., b) MeNH2, THF,
r.t., 12 h
2. Ac2O, DIPEA, DMAP
CH2Cl2, r.t., 6 h
12c: R1 = 4-t-BuOC6H4CH2, R2 = H (57%)
12d: R1 = Bn, R2 = Me (60%)
Scheme 2 Alternative reaction sequence for the synthesis of conformationally restricted peptide mimetics
Detailed NMR spectroscopic studies revealed the config-
O
H
uration and the conformational properties of the Gly-Gly-
derived peptide mimetic 8 and its Ala-Phe analogue 14
(Figure 3). Both compounds exhibited very similar NMR
spectroscopic properties. The most striking features were
the low-field-shifted signals of the H3α protons and the
small coupling constants for the coupling of H10β (N-al-
lylic methylene protons) and the vicinally positioned ole-
finic proton (H9) indicating a dihedral angle close to 90°.
Me
N 4
Me
6
Me
3
5
N
H
H
H
H
H
2
O
O
8
7
9
~90°
N1
H
10
H
H
O
H
H
H
HN
Me
A strong NOE between the protons of the N-methyl group
in position 4 and H6 demonstrated the s-trans-configura-
tion of the amide bond connecting N4 and C5. The olefin-
ic double bond was established to be cis by significant
NOEs between the olefinic protons and diagnostic 3J cou-
pling constants (8: J = 10.5 Hz; 14: J = 11.4 Hz).
Figure 3 Conformation of the model peptide 14 experimentally veri-
fied by NOE investigations (schematic presentation)
and one of the olefinic protons established their close
proximity and a conformation that prevents a hydrogen
bond between the C-terminal N–H and the carbonyl oxy-
gen in position 5 and, thus, β-turn formation. Interesting-
ly, similar coupling constants for all resolved ring proton
resonances of the tert-butyl ester derivatives 12a–d re-
vealed the same ring folding principle as was shown by
the model peptides 8 and 14.
Neither for compound 8 nor for 14 could any appreciable
amount of amide rotamers be detected. However, the
NMR signals of 8 appeared broadened at ambient temper-
ature indicating some conformational flexibility. In con-
trast, the signals of the alanine derivative 14 appeared
only slightly broadened, suggesting a more rigid ring sys-
tem. NOE measurements (Figure 3) demonstrated very
similar structures for both 8 and 14. A transannular NOE
between the protons of the N-methyl group in position 4
In conclusion, we were able to successfully extend the
RCM-based synthetic strategy to access ten-membered
lactam peptide mimetics of type V. It was feasible to re-
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2687
stirring. After 15 min, a soln of glycine tert-butyl ester hydrochlo-
ride (1a·HCl, 476 mg, 2.84 mmol) and Et3N (287 mg, 396 μL, 2.84
mmol) in DMF (7 mL) was added and the resulting mixture was
stirred for 12 h at 5 °C. After 12 h, the soln was filtered, H2O (15
mL) was added to the filtrate, and extraction was performed with
Et2O (3 × 30 mL). The combined organic layers were washed with
5% citric acid (2 × 20 mL), H2O (25 mL), and brine (25 mL), and
dried (Na2SO4). The solvent was removed in vacuo and flash chro-
matography (n-hexane–EtOAc, 60:40) was performed to give 2a
(972 mg, 91%) as a pale yellow foam; HPLC (220 nm) system M1:
tR = 21.1 min (99%), system A1: tR = 15.8 min (97%).
place the proline unit of scaffold II by glycine or alanine
and its N-methyl analogues, leading to a set of novel
building blocks that are suited for the Fmoc-supported in-
corporation into peptides. Careful NOE studies with the
model peptides 8 and 14 revealed conformational proper-
ties not allowing a β-turn-like conformation.
All chemicals and solvents were obtained from commercial sources
and used as received. All reactions carried out under an N2 atmo-
sphere except cleavage of protecting groups. TLC: silica gel 60
F254 aluminum plates (UV, KMnO4, I2, and ninhydrin detection).
Flash chromatography: 60 μm silica gel. Solvents were removed by
rotary evaporation under reduced pressure. IR spectroscopy: FT/IR
spectrophotometer, film on NaCl plates. NMR spectra: Bruker Ad-
vance 600 or a Bruker AM 360 at 300 K unless otherwise noted, rel-
ative to TMS. HPLC-MS analyses were carried out on an analytical
HPLC system with a VWL detector, coupled to a Bruker Esquire
2000–mass spectrometer with electron spray or atmospheric pres-
sure chemical ionization (ESI or APCI, respectively). ESI-TOF
high mass accuracy and resolution experiments were performed on
a Bruker maXis MS in the laboratories of the Chair of Bioinorganic
Chemistry (Prof. Dr. Ivana Ivanović-Burmazović), Department of
Pharmacy and Chemistry, Friedrich-Alexander University of Erlan-
gen Nuremberg. Purities of the products were assessed using an Ag-
ilent 1200 analytical HPLC equipped with a Zorbax Eclipse XDB-
C8 column (4.6 × 150 mm, 5 μm, flow rate: 0.5 mL/min) and a di-
ode array detector employing the following gradient systems
(MeOH = A, MeCN = B): M1: MeOH–H2O + 0.1% TFA; gradient:
0–10 min: 40–70% A, 10–20 min: 70–100% A, 20–25 min: 100%
A. M2: MeOH–H2O + 0.1% HCO2H; gradient: 0–2 min: 30% A, 2–
20 min: 30–100% A, 20–25 min: 100% A. M3: MeOH–H2O + 0.1%
HCO2H; gradient: 0–2 min: 5% A, 2–25 min: 5–100% A, 25–27
min: 100% A. M4: MeOH–H2O + 0.1% HCO2H; gradient: 0–2 min:
30% A, 2–30 min: 30–100% A, 30–32 min: 100% A. M5: MeOH–
H2O + 0.1% HCO2H; gradient: 0–22 min: 30–80% A, 22–25 min:
80–100% A, 25–28 min: 100% A. M6: MeOH–H2O + 0.1%
HCO2H; gradient: 0–20 min: 60–70% A, 20–22 min: 70–100% A,
22–24 min: 100% A. M7: MeOH–H2O + 0.1% HCO2H; gradient:
0–22 min: 50–100% A, 22–25 min: 100% A. M8: MeOH–H2O +
0.1% HCO2H; gradient: 0–20 min: 30–100% A, 20–25 min: 100%
A. M9: MeOH–H2O + 0.1% HCO2H; gradient: 0–5 min: 50–70%
A, 5–10 min: 70–100% A, 10–15 min: 100% A. M10: MeOH–H2O;
gradient: 0–2 min: 50% A, 2–12 min: 50–100% A, 12–15 min:
100% A. A1: MeCN–H2O + 0.1% TFA; gradient: 0–10 min: 40–
70% B, 10–20 min: 70–100% B, 20–25 min: 100% B. A2: MeCN–
H2O + 0.1% HCO2H; gradient: 0–20 min: 30–100% B, 20–25 min:
100% B. A3: MeCN–H2O + 0.1% HCO2H; gradient: 0–2 min: 5%
B, 2–25 min: 5–100% B, 25–27 min: 100% B. A4: MeCN–H2O +
0.1% HCO2H; gradient: 0–22 min: 50–100% B, 22–25 min: 100%
B. A5: MeCN–H2O + 0.1% HCO2H; gradient: 0–20 min: 30–80%
B, 20–23 min: 80–100% B, 23–26 min: 100% B. A6: MeCN–H2O
+ 0.1% HCO2H; gradient: 0–25 min: 20–70% B, 25–27 min: 70–
100% B, 27–29 min: 100% B.
[α]D21 –9.4 (c 0.7, CHCl3).
IR (NaCl): 1735, 1718, 1701, 1667 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.76 (m, 2 H,), 7.55–7.61 (m, 2
H), 7.39 (m, 2 H), 7.29–7.34 (m, 2 H), 6.44 (br s, 1 H), 5.76 (br s, 1
H), 5.26–5.29 (m, 1 H), 5.13–5.19 (m, 2 H), 4.37–4.48 (m, 2 H),
4.25–4.32 (m, 1 H), 4.21–4.25 (m, 1 H), 3.84–4.00 (m, 2 H), 2.46–
2.64 (m, 2 H), 1.47 (s, 9 H).
13C NMR (90 MHz, CDCl3): δ = 170.9, 168.6, 156.0, 143.8, 141.3,
132.7, 127.7, 127.1, 125.0, 120.0, 119.4, 82.5, 67.2, 63.7, 54.2,
47.2, 42.0, 36.8, 28.0.
MS (ESI): m/z = 473.2 [M + Na]+.
tert-Butyl {[(S)-2-(9H-Fluoren-9-ylmethoxycarbonylami-
no)pent-4-enoyl]methylamino}acetate (2b)
Following the typical procedure for 2a using Fmoc-allylglycine
(1000 mg, 2.96 mmol), NMM (299 mg, 325 μL, 2.96 mmol), and
isobutyl chloroformate (485 mg, 460 μL, 3.55 mmol) in THF (5
mL) and sarcosine tert-butyl ester hydrochloride (1b·HCl, 645 mg,
3.55 mmol) and Et3N (359 mg, 495 μL, 3.55 mmol) in DMF (7 mL).
Flash chromatography (n-hexane–EtOAc 70:30) furnished 2b
(1100 mg, 80%) as a pale yellow foam; HPLC (220 nm) system M1:
tR = 21.6 min (98%), system A1: tR = 17.4 min (98%).
[α]D25 +2.6 (c 1.9, CHCl3).
IR (NaCl): 1740, 1720, 1650 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.76 (m, 2 H), 7.56–7.62 (m, 2 H),
7.39 (m, 2 H), 7.31 (m, 2 H), 5.71–5.85 (m, 1 H), 5.63–5.70 (m, 1
H), 5.09–5.20 (m, 2 H), 4.80 (ddd, J = 6.9, 6.9, 6.9 Hz, 0.75 H), 4.59
(ddd, J = 6.9, 6.9, 6.9 Hz, 0.25 H), 4.35–4.41 (m, 1 H), 4.30 (d,
J = 17.0 Hz, 0.75 H), 4.28–4.35 (m, 1 H), 4.18–4.25 (m, 1.25 H),
3.93 (d, J = 18.1 Hz, 0.25 H), 3.76 (d, J = 17.0 Hz, 0.75 H), 3.14 and
3.00 (2 × s, 3 H), 2.58 (ddd, J = 13.9, 6.9, 6.9 Hz, 0.75 H), 2.52
(ddd, J = 13.9, 6.9, 6.9 Hz, 0.25 H), 2.42 (ddd, J = 13.9, 6.9, 6.9 Hz,
0.75 H), 2.37 (ddd, J = 13.9, 6.9, 6.9 Hz, 0.25 H), 1.44 and 1.47
(2 × s, 9 H); rotamers were observed.
13C NMR (90 MHz, CDCl3): δ = 172.0, 171.9, 167.9, 167.8, 155.9,
144.1, 144.0, 141.4, 132.7, 132.6, 127.8, 127.2, 125.3, 120.1, 119.1,
119.0, 83.0, 82.2, 67.2, 52.3, 50.6, 50.5, 50.4, 47.3, 37.6, 37.3, 36.5,
35.3, 28.2, 28.1; rotamers were observed.
MS (ESI): m/z = 487.3 [M + Na]+.
tert-Butyl (S)-2-{[(S)-2-(9H-Fluoren-9-ylmethoxycarbonylami-
no)pent-4-enoyl]amino}propanoate (2c)
The following abbreviations are used N,N′-diisopropylcarbodiimide
(DIC), 1-hydroxybenzotriazole (HOBt), 2-(1H-7-azabenzotriazol-
1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU), 1-
ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC), (benzotri-
Following the typical procedure for 2a, using Fmoc-allylglycine
(985 mg, 2.92 mmol), NMM (295 mg, 320 μL, 2.92 mmol), and iso-
butyl chloroformate (478 mg, 454 μL, 3.50 mmol) in THF (5 mL)
and alanine tert-butyl ester hydrochloride (1c·HCl, 636 mg, 3.50
mmol) and Et3N (354 mg, 488 μL, 3.50 mmol) in DMF (7 mL). Af-
ter filtration and concentration of the filtrate, H2O (20 mL) was add-
ed and immediate crystallization of the dipeptide was observed.
After filtration and subsequent washings with H2O and dilute aq
NaHCO3, the material was dried (1099 mg, 81%). This material was
used for the following reactions without further purification; mp
102 °C; HPLC (220 nm) system M1: tR = 21.7 min (97%), system
A1: tR = 16.9 min (96%).
azol-1-yloxy)tripyrrolidinophosphonium
hexafluorophosphate
(PyBOP), {[(1-cyano-2-ethoxy-2-oxoethylidene)amino]oxy}(di-
methylamino)(morpholino)carbenium hexafluorophosphate (CO-
MU).
tert-Butyl {[(S)-2-(9H-Fluoren-9-ylmethoxycarbonylami-
no)pent-4-enoyl]amino}acetate (2a); Typical Procedure
To a cooled soln (–15 °C) of Fmoc-allylglycine (800 mg, 2.37
mmol) and NMM (240 mg, 261 μL, 2.37 mmol) in THF (5 mL), iso-
butyl chloroformate (388 mg, 368 μL, 2.84 mmol) was added with
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2688
S. Wakchaure et al.
FEATURE ARTICLE
[α]D25 –4.1 (c 0.7, CHCl3).
IR (NaCl): 1732, 1708, 1661, 1540 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.78 (m, 2 H), 7.58–7.62 (m, 2 H),
7.42 (m, 2 H), 7.31–7.35 (m, 2 H), 6.50 (d, J = 7.0 Hz, 2 H), 5.71–
5.85 (m, 1 H), 5.33–5.41 (m, 1 H), 5.15–5.21 (m, 2 H), 4.42–4.49
(m, 2 H), 4.35–4.42 (m, 1 H), 4.22–4.30 (m, 2 H), 2.49–2.61 (m, 2
H), 1.47 and 1.48 (2 × s, 9 H); rotamers were observed.
13C NMR (150 MHz, CDCl3): δ = 170.84, 170.79, 168.3, 168.1,
167.8, 167.5, 155.9, 143.9, 143.7, 141.3, 132.7, 132.2, 131.5, 127.7,
127.1, 125.1, 119.9, 119.3, 118.7, 118.2, 83.1, 82.2, 67.1, 54.2,
50.3, 49.8, 48.4, 48.0, 47.1, 41.14, 41.11, 37.2, 28.02, 27.98; rotam-
ers were observed.
MS (ESI): m/z = 570.3 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C31H37N3NaO6:
570.2580; found: 570.2574.
13C NMR (150 MHz, CDCl3): δ = 171.4, 170.3, 156.0, 143.7, 141.3,
132.7, 127.7, 127.1, 125.1, 120.0, 119.3, 82.5, 67.1, 54.1, 49.3,
48.8, 37.2, 27.9, 18.5; rotamers were observed.
tert-Butyl [Allyl({[(S)-2-(9H-fluoren-9-ylmethoxycarbonylami-
no)pent-4-enoyl]methylamino}acetyl)amino]acetate (4b)
Following the typical procedure for 4a. Cleavage of the tert-butyl
ester: 2b (1250 mg, 2.69 mmol), CH2Cl2 (10 mL), and TFA–CH2Cl2
(1:1, 10 mL). Peptide coupling: crude carboxylic acid derivative, N-
allylglycine tert-butyl ester (3a, 921 mg, 5.38 mmol), HATU (1534
mg, 4.04 mmol), and DIPEA (695 mg, 921 μL, 5.38 mmol) in NMP
(25 mL). Flash chromatography (n-hexane–EtOAc, 70:30) afforded
4b (1254 mg, 83%) as a colorless oil; HPLC (220 nm) system M2:
tR = 22.6 min (99%), system A2: tR = 19.4 min (99%).
MS (ESI): m/z = 487.3 [M + Na]+.
tert-Butyl (S)-2-(Allylamino)propanoate (3c)32
Allyl bromide (2.38 g, 1.7 mL, 19.8 mmol) was slowly added to a
soln of alanine tert-butyl ester hydrochloride (1000 mg, 5.51 mmol)
and DIPEA (1.82 g, 2.5 mL, 17.93 mmol) in MeCN (50 mL) at r.t.
After 3 h, the mixture was filtered, H2O (50 mL) was added to the
filtrate, and extraction was performed with Et2O (3 × 50 mL). The
combined organic layers were washed with H2O (50 mL) and brine
(50 mL), and dried (Na2SO4). The solvent was removed in vacuo at
low temperature and flash chromatography (n-hexane–EtOAc,
70:30) was performed to give 3c (815 mg, 80%) as a colorless oil;
HPLC (220 nm) system M2: tR = 24.1 min (98%), system A2:
tR = 24.9 min (98%).
[α]D25 +3.0 (c 1.8, CHCl3).
IR (NaCl): 1789, 1700, 1735, 1685, 1654 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.75 (m, 2 H), 7.52–7.62 (m, 2 H),
7.39 (m, 2 H), 7.31 (m, 2 H), 5.70–5.90 (m, 2 H), 5.51–5.57 and
5.64–5.70 (m, 1 H), 5.07–5.34 (m, 4 H), 4.78–4.85 (m, 0.8 H), 4.40–
4.67 (m, 1.2 H), 4.35–4.41 (m, 1 H), 4.26–4.35 (m, 1 H), 4.19–4.24
(m, 1 H), 4.03–4.16 (m, 1 H), 3.81–4.03 (m, 4 H), 3.20, 3.17, 3.00,
and 2.99 (4 × s, 3 H), 1.48, 1.46, and 1.45 (4 × s, 9 H); rotamers
were observed.
13C NMR (90 MHz, CDCl3): δ = 172.4, 172.0, 168.3, 168.2, 168.0,
167.9, 155.9, 155.8, 144.0, 143.9, 141.3, 132.6, 132.1, 127.7, 127.1,
125.3, 120.0, 118.9, 118.5, 117.9, 83.0, 82.1, 67.2, 50.8, 50.6, 50.1,
49.4, 49.2, 48.3, 47.3, 37.4, 36.8, 28.2, 28.1; rotamers were ob-
served.
[α]D24 –26.2 (c 0.5, CHCl3).
IR (NaCl): 3335, 1730, 1644, 1643 cm–1.
1H NMR (600 MHz, CDCl3) δ = 5.88 (dddd, J = 17.2, 10.2, 6.0, 6.0
Hz, 1 H), 5.18 (dddd, J = 17.2, 3.0, 1.4, 1.4 Hz, 1 H), 5.08 (dddd,
J = 10.2, 3.0, 1.4, 1.4 Hz, 1 H), 3.26 (dddd, J = 13.6, 6.0, 1.4, 1.4
Hz, 1 H), 3.23 (q, J = 7.0 Hz, 1 H), 3.15 (dddd, J = 13.6, 6.0, 1.4,
1.4 Hz, 1 H), 1.47 (s, 9 H), 1.26 (d, J = 7.0 Hz, 3 H).
13C NMR (90 MHz, CDCl3): δ = 175.3, 136.6, 116.4, 81.0, 56.7,
50.7, 28.3, 19.3.
MS (ESI): m/z = 186.0 [M + H]+.
HRMS (ESI-TOF): m/z [M + H]+ calcd for C10H20NO2: 186.1494;
MS (ESI): m/z = 584.4 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C32H39N3NaO6:
584.2737; found: 584.2728.
found: 186.1487.
tert-Butyl (S)-2-(Allyl{(S)-2-[(S)-2-(9H-fluoren-9-ylmethoxy-
carbonylamino)pent-4-enoylamino]propanoyl}amino)-3-phen-
ylpropanoate (4c)
tert-Butyl [Allyl({[(S)-2-(9H-fluoren-9-ylmethoxycarbonylami-
no)pent-4-enoyl]amino}acetyl)amino]acetate (4a); Typical Pro-
cedure
Following the typical procedure for 4a. Cleavage of the tert-butyl
ester: 2c (682 mg, 1.47 mmol), CH2Cl2 (5 mL), and TFA–CH2Cl2
(1:1, 5 mL). Peptide coupling: crude carboxylic acid derivative, N-
allyl-(S)-phenylalanine tert-butyl ester (3b, 578 mg, 2.21 mmol),
HATU (840 mg, 2.21 mmol), and DIPEA (286 mg, 378 μL, 2.21
mmol) in NMP (15 mL). After stirring for 24 h at 40 °C, a second
portion of 3a (192 mg, 0.735 mmol) and HATU (280 mg, 0.735
mmol) in NMP (5 mL) were added to the mixture and stirring was
continued at 40 °C for a further 24 h. Flash chromatography (n-hex-
ane–EtOAc, 50:50) afforded a mixture of diastereomers of 4c (670
mg, 70%) as a pale yellow oil. Since this reaction affords a mixture
of diastereomers, NMR data is given in the protocol that furnished
4c as a pure isomer (see 11a); HPLC (220 nm) system M1: tR (peak
1) = 23.5 min (42%) and tR (peak 2) = 23.7 min (58%), system A1:
tR (peak 1) = 20.6 min (43%) and tR (peak 2) = 20.8 min (57%).
Compound 2a (1028 mg, 2.28 mmol) was dissolved in CH2Cl2 (10
mL) and TFA–CH2Cl2 (1:1, 10 mL) was added. After stirring at r.t.
for 3 h, the mixture was concentrated and thoroughly dried in vacuo
to give the crude carboxylic acid, which was used in the following
reaction without further purification. The carboxylic acid thus ob-
tained and HATU (1300 mg, 3.42 mmol) were dissolved in NMP
(15 mL) followed by the addition of DIPEA (589 mg, 780 μL, 4.56
mmol) at r.t. After 15 min of stirring, a soln of N-allylglycine tert-
butyl ester (3a, 781 mg, 4.56 mmol) in NMP (5 mL) was added to
the mixture. After 24 h, the mixture was diluted with H2O (25 mL)
and extracted with Et2O (4 × 25 mL). The combined organic layers
were washed with aq 5% citric acid (25 mL), H2O (25 mL), and
brine (25 mL), and dried (Na2SO4), and the solvent was removed in
vacuo. The product was purified by flash chromatography (n-hex-
ane–EtOAc, 50:50) to give 4a (1083 mg, 86%) as a colorless oil;
HPLC (220 nm) system M1: tR = 21.6 min (96%), system A2:
tR = 18.7 min (96%).
IR (NaCl): 1729, 1643, 1517 cm–1.
MS (ESI): m/z = 674.4 [M + Na]+.
[α]D25 –9.8 (c 0.9, CHCl3).
IR (NaCl): 1954, 1914, 1844, 1739, 1649 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.76 (m, 2 H), 7.56–7.61 (m, 2 H),
7.39 (m, 2 H), 7.31 (m, 2 H), 7.03 and 7.00 (2 × s, 1 H), 5.67–5.83
(m, 2 H), 5.40 (br s, 1 H), 5.09–5.28 (m, 4 H), 4.30–4.45 (m, 3 H),
4.21–4.25 (m, 1 H), 4.15–4.20 (m, 1 H), 3.98–4.10 (m, 3 H), 3.84–
3.95 (m, 2 H), 2.47–2.62 (m, 2 H), 1.47 and 1.46 (2 × s, 9 H); rota-
mers were observed.
tert-Butyl [(Z)-(S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-yl]acetate
(5a); Typical Procedure
To a soln of diene 4a (215 mg, 0.37 mmol) in CH2Cl2 (120 mL) a
soln of Grubbs II catalyst (A; 33.4 mg, 0.039 mmol, 10 mol%) in
CH2Cl2 (10 mL) was added. After refluxing for 6 h, a second portion
of Grubbs II catalyst (A; 16.7 mg, 0.019 mmol, 5 mol%) in CH2Cl2
(5 mL) was added and reflux was continued. After 24 h, the solvent
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2689
was removed in vacuo and flash chromatography (n-hexane–EtOAc
50:50) was performed. Removal of the residual ruthenium complex
was done by dissolving the purified material and P(CH2OH)3 (287
mg, 2.32 mmol) in CH2Cl2 (10 mL), adding silica gel (50 mg) and
stirring for 15 min. Subsequent filtration furnished 5a (44.2 mg,
20%) as a pale yellow oil; HPLC (254 nm) system M2: tR = 21.0
min (97%), system A2: tR = 15.9 min (99%).
[α]D26 –26.6 (c 0.5, MeOH).
IR (NaCl): 3354, 2927, 1686, 1639 cm–1.
1H NMR (600 MHz, CDCl3, 240 K): δ = 7.81 (m, 2 H), 7.68 (m, 2
H), 7.40 (m, 2 H), 7.32 (m, 2 H), 5.58–5.80 (m, 3 H), 4.52 (dd,
J = 12.5, 2.5 Hz, 1 H), 4.33 (dd, J = 10.5, 7.0 Hz, 1 H), 4.31 (dd,
J = 10.5, 7.0 Hz, 1 H), 4.18–4.26 (m, 2 H), 4.05 (dd, J = 16.0, 10.5
Hz, 1 H), 3.75 (br d, J = 16.2 Hz, 1 H), 3.56 (br d, J = 16.2 Hz, 1 H),
3.34 (d, J = 15.5 Hz, 1 H), 2.58 (ddd, J = 13.0, 12.5, 8.5 Hz, 1 H),
2.26 (ddd, J = 13.0, 8.0, 2.5 Hz, 1 H), 3.09 (s, 3 H).
[α]D24 –8.3 (c 0.9, CHCl3).
IR (NaCl): 1736, 1720, 1671, 1655 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.74–7.79 (m, 2 H, ArFmoc), 7.53–
7.60 (m, 2 H, ArFmoc), 7.37–7.45 (m, 2 H, ArFmoc), 7.29–7.35 (m, 2
H, ArFmoc), 6.22 (br s, 1 H, NHFmoc or H4), 5.67–5.90 (m, 3 H,
H9/H8, H3), [in acetone-d6, resonances of the olefin protons are the
following: 5.84 (ddd, J = 10.0, 9.5, 9.5 Hz, 1 H, H9), 5.73 (ddd,
J = 10.0, 8.5, 6.2 Hz, 1 H, H8)], 5.05–5.21 (m, 1 H, H4 or NHFmoc),
4.34–4.46 (m, 2 H, Fmoc-CH2), 4.19–4.25 (m, 1 H, CHFmoc), 3.88–
4.13 (m, 4 H, H3′, NCH2CO, H6), 3.67–3.78 (m, 1 H, H10), 3.44–
3.53 (m, 1 H, H10′), 2.39–2.53 (m, 2 H, H7/H7′), 1.48 (s, 9 H, t-Bu).
MS (ESI): m/z = 500.2 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C26H27N3NaO6:
500.1798; found: 500.1778
2-[(Z)-(S)-6-(Acetylamino)-4-methyl-2,5-dioxo-3,4,5,6,7,10-
hexahydro-1,4-diazecin-1(2H)-yl]-N-methylacetamide (8)
To a suspension of 6 (100 mg, 0.20 mmol) and HATU (160 mg,
0.42 mmol) in CH2Cl2 (5 mL), DIPEA (27.1 mg, 36 μL, 0.21 mmol)
was added. After stirring for 15 min at r.t., a soln of MeNH2·HCl
(14.3 mg, 0.21 mmol) and DIPEA (72 μL, 0.42 mmol) in CH2Cl2 (5
mL) was added. After 1 h, a second portion of MeNH2·HCl (14.3
mg, 0.21 mmol) and DIPEA (36 μL, 0.21 mmol) in CH2Cl2 (5 mL)
was added. The mixture was stirred for 8 h at r.t. and concentrated
to obtain the crude N-methylacetamide 7. Subsequently, a soln of
15% Et2NH in CH2Cl2 (2 mL) was added to the residue. After stir-
ring for 2 h at r.t., addition of a soln of Ac2O (216 mg, 0.2 mL, 2.1
mmol) and DIPEA (271 mg, 0.36 mL, 2.1 mmol) in CH2Cl2 (4 mL)
was performed and stirring was continued for 12 h at r.t. The mix-
ture was poured into aq NH4Cl soln (3 mL) and extracted with EtO-
Ac (3 mL). The aqueous layer was concentrated without heating and
the resulting solid was purified by using flash chromatography
(CH2Cl2–MeOH–25% NH4OH, 90:5:5) to furnish 8 (16.2 mg, 25%)
as a pale yellow oil; HPLC (220 nm) system M10: tR = 5.9 min
(95%), system A5: tR = 12.4 min (96%).
MS (ESI): m/z = 542.2 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C29H33N3NaO6:
542.2267; found: 542.2262.
tert-Butyl [(Z)-(S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
4-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-
yl]acetate (5b)
Following the typical procedure for 5a using diene 4b (200 mg, 0.36
mmol) in CH2Cl2 (120 mL) and a soln of Grubbs II catalyst (A; 36.1
mg, 0.054 mmol, 15 mol%) in CH2Cl2 (30 mL). Flash chromatog-
raphy (n-hexane–EtOAc, 50:50) and removal of residual ruthenium
complex using P(CH2OH)3 (268 mg, 2.20 mmol) and silica gel (36
mg) in CH2Cl2 (10 mL) afforded 5b (95 mg, 50%) as a pale yellow
oil; HPLC (220 nm) system M1: tR = 20.1 min (99%), system A1:
tR = 14.1 min (96%).
[α]D26 –42.8 (c 0.2, CHCl3).
IR (NaCl): 1750, 1700, 1674, 1670 cm–1.
[α]D25 –10.4 (c 2.5, CHCl3).
IR (NaCl): 1740, 1722, 1649 cm–1.
1H NMR (600 MHz, CDCl3, 240 K): δ = 6.62 (d, J = 7.7 Hz, 1 H,
NHAc), 6.41 (q, J = 4.5 Hz, 1 H, NHCH3), 5.71–5.80 (m, 2 H,
H8/H9), 5.67 (d, J = 16.0 Hz, 1 H, H3), 4.94 (ddd, J = 12.0, 7.7, 3.3
Hz, 1 H, H6), 4.30 (d, J = 14.8 Hz, 1 H, NCH2CO), 4.07 (dd,
J = 16.3, 10.1 Hz, 1 H, H10α), 3.77 (d, J = 14.8 Hz, 1 H, NCH2CO),
3.64 (d, J = 16.3 Hz, 1 H, H10β), 3.49 (d, J = 16.0 Hz, 1 H, H3),
3.06 (s, 3 H, NCH3), 2.83 (d, J = 4.5 Hz, 1 H, NHCH3), 2.51 (ddd,
J = 12.6, 12.0, 8.2 Hz, 1 H, H7), 2.43 (ddd, J = 12.6, 7.7, 3.2 Hz, 1
H, H7′), 2.05 (s, 3 H, COCH3); a minor rotamer was additionally ob-
served.
1H NMR (600 MHz, acetone-d6, 250K): δ = 7.65 (d, J = 7.1 Hz, 1
H, NHAc), 7.42 (br s, 1 H, NHCH3), 5.80 (ddd, J = 11.3, 10.5, 2.0
Hz, 1 H, H9), 5.73 (ddd, J = 10.5, 9.5, 9.5 Hz, 1 H, H8), 5.53 (d,
J = 15.5 Hz, 1 H, H3), 4.71 (ddd, J = 12.2, 7.1, 2.7 Hz, 1 H, H6),
4.28 (d, J = 16.4 Hz, 1 H, NCH2CO), 4.05 (dd, J = 16.2, 11.3 Hz, 1
H, H10α), 3.75 (d, J = 16.4 Hz, 1 H, NCH2CO), 3.56 (d, J = 16.2
Hz, 1 H, H10β), 3.26 (d, J = 15.5 Hz, 1 H, H3′), 3.10 (s, 3 H, NCH3),
2.67 (d, J = 4.7 Hz, 3 H, NHCH3), 2.24 (ddd, J = 12.8, 9.5, 2.7 Hz,
1 H, H7′), 2.52 (ddd, J = 12.8, 12.2, 9.5 Hz, 1 H, H7), 1.92 (s, 3 H,
COCH3).
1H NMR (600 MHz, CDCl3): δ = 7.77 (d, J = 7.5 Hz, 2 H, ArFmoc),
7.59 (dd, J = 7.5, 1.8 Hz, 2 H, ArFmoc), 7.40 (d, J = 7.5 Hz, 2 H,
ArFmoc), 7.32 (d, J = 7.5 Hz, 2 H, ArFmoc), 5.80 (d, J = 8.4 Hz, 1 H,
NHFmoc), 5.65–5.75 (m, 2 H, H9/H8), 5.61 (d, J = 15.3 Hz, 1 H, H3),
4.70 (ddd, J = 11.6, 8.4, 2.6 Hz, 1 H, H6), 4.40 (dd, J = 10.6, 7.4 Hz,
1 H, Fmoc-CH2), 4.37 (dd, J = 10.6, 7.4 Hz, 1 H, Fmoc-CH2), 4.22
(dd, J = 7.4, 7.4 Hz, 1 H, CHFmoc), 4.14 (dd, J = 15.5, 10.8 Hz, 1 H,
H10α), 4.02 (d, J = 17.0 Hz, 1 H, NCH2CO), 3.94 (d, J = 17.0 Hz,
1 H, NCH2CO), 3.46–3.49 (m, 1 H, H10β), 3.44 (d, J = 15.3 Hz, 1
H, H3′), 3.09 and 3.03 (2 × s, 3 H, NCH3), 2.58 (ddd, J = 12.6, 11.6,
8.4 Hz, 1 H, H7), 2.41 (ddd, J = 12.6, 7.8, 2.6 Hz, 1 H, H7′), 1.48 (s,
9 H).
13C NMR (150 MHz, CDCl3): δ = 173.1, 168.6, 168.0, 155.6, 144.0,
143.8, 141.5, 141.4, 130.0, 127.9, 127.2, 125.2, 124.0, 120.2, 82.5,
82.3, 67.24, 67.19, 51.3, 51.1, 49.0, 48.1, 47.5, 47.3, 36.7, 33.9,
28.2.
MS (APCI): m/z = 534.3 [M + H]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C30H35N3NaO6:
556.2423; found: 556.2418.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C14H22N4NaO4:
333.1539; found: 333.1529.
[(Z)-(S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-4-methyl-
2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-yl]acetic
Acid (6)
tert-Butyl (S)-2-[(S)-N-Allyl-2-(9H-fluoren-9-ylmethoxycarbon-
ylamino)propanamido]-3-phenylpropanoate (9a);31 Typical
Procedure
To a soln of Fmoc-alanine (1630 mg, 5.22 mmol) and HATU (2980
mg, 7.83 mmol) in CH2Cl2 (150 mL), DIPEA (675 mg, 890 μL, 5.22
mmol) was added at r.t. After 25 min of stirring, a soln of 3b (2050
mg, 7.83 mmol) in CH2Cl2 (30 mL) was added and stirring was con-
tinued at 42 °C for 2 d. Thereafter, the mixture was washed with
The cleavage of the tert-butyl ester was performed using 5b (112
mg, 0.21 mmol) dissolved in CH2Cl2 (1 mL) and employing TFA–
CH2Cl2 (1:1, 2 mL). The crude carboxylic acid derivative thus ob-
tained was purified by flash chromatography (CH2Cl2–MeOH–
AcOH, 75:20:5) to afford 6 (85 mg, 85%) as a pale yellow oil;
HPLC (220 nm) system M9: tR = 11.3 min (95%), system A5:
tR = 12.4 min (96%).
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2690
S. Wakchaure et al.
FEATURE ARTICLE
IR (NaCl): 1727, 1650, 1646, 1608 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.73–7.78 (m, 2 H), 7.57–7.61 (m,
2 H), 7.38–7.42 (m, 2 H), 7.29–7.32 (m, 2 H), 7.02–7.06 (m, 2 H),
6.90–6.94 (m, 2 H), 5.73 (d, J = 7.6 Hz, 1 H), 5.53 (dddd, J = 17.8,
10.5, 6.2, 5.0 Hz, 1 H), 5.12 (d, J = 17.8 Hz, 1 H), 5.11 (d, J = 10.5
Hz, 1 H), 4.54 (dq, J = 7.6, 6.8 Hz, 1 H), 4.31–4.36 (m, 2 H), 4.19–
4.24 (m, 1 H), 4.00–4.05 (m, 1 H), 3.78 (dd, J = 16.8, 5.0 Hz, 1 H),
3.18–3.29 (m, 3 H), 1.42 (s, 9 H), 1.32 (s, 9 H), 1.30 (d, J = 6.8 Hz,
3 H); a minor rotamer was additionally observed.
H2O (3 × 50 mL) and brine (2 × 50 mL), dried (Na2SO4), and con-
centrated in vacuo. Flash chromatography (n-hexane–EtOAc,
60:40) afforded 9a (2114 mg, 81%) as a pale yellow foam; HPLC
(220 nm) system M1: tR = 23.6 min (98%), system A1: tR = 21.5
min (96%)
[α]D25 –72.7 (c 1.1, CHCl3).
IR (NaCl): 1729, 1650, 1498 cm–1.
1H NMR (360 MHz, CDCl3): δ = 7.76 (d, J = 7.5 Hz, 2 H), 7.59 (d,
J = 7.5 Hz, 2 H), 7.40 (dd, J = 7.5, 7.5 Hz, 2 H), 7.31 (dd, J = 7.5,
7.5 Hz, 2 H), 7.13–7.30 (m, 6 H), 5.68 (d, J = 7.9 Hz, 1 H), 5.59
(dddd, J = 17.3, 10.5, 6.1, 5.0 Hz, 1 H), 5.15 (d, J = 17.3 Hz, 1 H),
5.13 (d, J = 10.5 Hz, 1 H), 4.57 (dq, J = 7.9, 6.8 Hz, 1 H), 4.32–4.38
(m, 2 H), 4.20–4.25 (m, 1 H), 4.19 (dd, J = 10.1, 5.2 Hz, 1 H), 3.81
(dd, J = 16.8, 5.0 Hz, 1 H), 3.32 (dd, J = 14.3, 5.2 Hz, 1 H), 3.23
(dd, J = 14.3, 10.1 Hz, 1 H), 1.42 and 1.37 (2 × s, 9 H), 1.31 and
1.01 (2 × d, each J = 6.8 Hz, 3 H); a minor rotamer was additionally
observed.
13C NMR (90 MHz, CDCl3) δ = 172.5, 169.1, 155.5, 154.0, 143.9,
143.8, 141.3, 133.0, 132.9, 129.8, 127.7, 127.1, 125.2, 124.4, 119.9,
118.8, 81.6, 78.4, 66.9, 62.5, 52.0, 47.2, 47.1, 33.9, 28.8, 27.8, 19.4.
MS (ESI): m/z = 649.4 [M + Na]+.
tert-Butyl (S)-2-{(S)-N-Allyl-2-[(9H-Fluoren-9-ylmethoxycar-
bonyl)methylamino]propanamido}-3-phenylpropanoate (9d)
Following the typical procedure for 9a using Fmoc-N-methylala-
nine (1075 mg, 3.31 mmol), HATU (2517 mg, 6.62 mmol), and DI-
PEA (428 mg, 567 μL, 3.31 mmol) in CH2Cl2 (100 mL) and 3b
(1300 mg, 4.96 mmol) in CH2Cl2 (20 mL). Flash chromatography
(n-hexane–EtOAc, 70:30) furnished 9d (1710 mg, 91%) as a pale
yellow foam.
13C NMR (90 MHz, CDCl3): δ = 172.7, 169.3, 155.6, 144.1, 144.0,
141.5, 138.2, 133.1, 129.5, 128.6, 127.8, 127.2, 126.9, 125.3, 120.1,
118.6, 81.8, 67.1, 62.3, 51.9, 47.34, 47.29, 34.9, 28.1, 27.9, 19.5.
MS (ESI): m/z = 577.3 [M + Na]+.
[α]D24 –124.5 (c 1.8, CHCl3).
IR (NaCl): 1733, 1695, 1655, 1597 cm–1.
tert-Butyl (S)-2-[(S)-N-Allyl-2-(9H-fluoren-9-ylmethoxycarbon-
ylamino)propanamido]propanoate (9b)11
Following the typical procedure for 9a using Fmoc-alanine (1008
mg, 3.24 mmol), HATU (1232 mg, 3.24 mmol), and DIPEA (419
mg, 555 μL, 3.24 mmol) in CH2Cl2 (100 mL) and 3c (300 mg, 1.62
mmol) in CH2Cl2 (15 mL) for 8 h at r.t. Washing of the mixture with
aq NaHCO3 (2 × 25 mL), H2O (3 × 50 mL), and brine (2 × 50 mL).
Flash chromatography (n-hexane–EtOAc, 60:40) yielded 9b (698
mg, 90%) as a pale yellow foam; HPLC (220 nm) system M2:
tR = 22.9 min (99%), system A2: tR = 20.4 min (99%).
1H NMR (360 MHz, CDCl3): δ = 7.70–7.83 (m, 2 H), 7.51–7.62 (m,
2 H), 7.36–7.43 (m, 2 H), 7.28–7.33 (m, 2 H), 7.11–7.34 (m, 5 H),
5.88 (dddd, J = 17.0, 10.9, 5.7, 5.7 Hz, 0.3 H), 5.50 (dddd, J = 17.5,
10.3, 7.0, 4.4 Hz, 0.7 H), 5.20 (dd, J = 17.0, 1.4 Hz, 0.3 H), 5.13 (dd,
J = 10.3, 1.4 Hz, 0.3 H), 5.07 (d, J = 17.5 Hz, 0.7 H), 5.04 (q,
J = 6.8 Hz, 0.7 H), 4.98 (d, J = 10.3 Hz, 0.7 H), 4.53 (q, J = 6.8 Hz,
0.3 H), 4.32–4.44 (m, 2 H), 4.16–4.23 (m, 1 H), 4.09 (dd, J = 10.0,
5.4 Hz, 1 H), 3.87 (dd, J = 16.7, 4.4 Hz, 1 H), 3.28 (dd, J = 14.0, 5.4
Hz, 1 H), 3.23 (dd, J = 14.0, 10.0 Hz, 1 H), 3.18 (dd, J = 16.7, 7.0
Hz, 1 H), 2.79 and 2.73 (2 × s, 3 H), 1.44 and 1.41 (2 × s, 9 H), 1.28
and 0.84 (2 × d, each J = 6.8 Hz, 3 H); rotamers were observed.
[α]D25 –33.3 (c 0.6, CHCl3).
IR (NaCl): 1650, 1510, 1526 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.76 (d, J = 7.5 Hz, 2 H), 7.57–
7.62 (m, 2 H), 7.40 (dd, J = 7.5, 7.5 Hz, 2 H), 7.28–7.33 (m, 2 H),
5.96 (d, J = 7.5 Hz, 0.2 H), 5.91 and 5.82 (2 × dddd, J = 17.5, 10.0,
5.3, 5.3 Hz, 1 H), 5.72 (d, J = 7.5 Hz, 0.8 H), 5.28 (d, J = 17.5 Hz,
0.8 H), 5.26 (d, J = 10.0 Hz, 0.8 H), 5.15 (d, J = 17.5 Hz, 0.2 H),
5.13 (d, J = 10.0 Hz, 0.2 H), 4.77 (q, J = 7.2 Hz, 0.8 H), 4.73 and
4.62 (2 × dq, J = 7.5, 6.8 Hz, 1 H), 4.42 (q, J = 7.2 Hz, 0.2 H), 4.31–
4.38 (m, 2 H), 4.18–4.24 (m, 1.2 H), 4.04 (dd, J = 17.9, 5.3 Hz, 0.8
H), 3.88 (dd, J = 17.9, 5.3 Hz, 0.8 H), 3.68 (dd, J = 15.9, 5.3 Hz, 0.2
H), 1.47 (d, J = 6.8 Hz, 0.6 H), 1.44 and 1.42 (2 × s, 9 H), 1.39 and
1.38 (2 × d, each J = 7.2 and 6.8 Hz, respectively, 5.4 H); a minor
rotamer was additionally observed.
13C NMR (150 MHz, CDCl3): δ = 171.0, 169.2, 155.9, 144.0, 143.9,
143.80, 143.78, 141.4, 138.2, 136.8, 134.4, 133.6, 129.5, 129.3,
128.6, 128.5, 127.7, 127.1, 127.0, 126.9, 126.6, 125.05, 125.02,
124.99, 124.93, 120.04, 119.99, 117.7, 116.9, 82.3, 81.5, 68.0, 67.7,
62.1, 61.8, 51.5, 51.2, 50.9, 50.2, 47.4, 47.3, 35.8, 34.7, 29.1, 28.7,
28.0, 27.9, 14.8, 14.7.
MS (ESI): m/z = 591.3 [M + Na]+.
tert-Butyl (S)-2-{Allyl[(S)-2-(methylamino)propanoyl]amino}-
3-phenylpropanoate (10d)
To a soln of 9d (500 mg, 0.88 mmol) in CH2Cl2 (9 mL), piperidine
(1.6 mL) was added. After stirring for 2 h at r.t., the solvent was re-
moved under reduced pressure at low temperature. The residue was
purified by flash chromatography (CH2Cl2–MeOH, 96:4) furnish-
ing 10d (247 mg, 81%) as a pale yellow oil; HPLC (220 nm) system
M2: tR = 15.6 min (98%), system A2: tR = 9.3 min (98%).
13C NMR (150 MHz, CDCl3): δ = 173.3, 170.7, 155.6, 155.4, 144.1,
144.0, 141.4, 134.0, 127.8, 127.2, 125.3, 120.1, 117.8, 82.9, 81.7,
67.1, 55.7, 54.5, 48.5, 47.5, 47.4, 47.3, 46.0, 28.1, 28.0, 20.0, 19.5,
15.7, 14.8.
MS (ESI): m/z = 501.3 [M + Na]+.
[α]D22 –64.6 (c 0.6, CHCl3).
IR (NaCl): 1732, 1651, 1597 cm–1.
tert-Butyl (S)-2-[(S)-N-Allyl-2-(9H-Fluoren-9-ylmethoxycarbo-
nylamino)propanamido]-3-(4-tert-butoxyphenyl)propanoate
(9c)
1H NMR (360 MHz, CDCl3): δ = 7.23–7.31 (m, 2 H), 7.19–7.23 (m,
1 H), 7.15–7.19 (m, 2 H), 5.61 (dddd, J = 17.2, 10.4, 5.7, 5.2 Hz, 1
H), 5.20 (dddd, J = 17.2, 1.5, 1.5, 1.5 Hz, 1 H), 5.11 (dddd, J = 10.4,
1.5, 1.5, 1.5 Hz, 1 H), 4.26 (dd, J = 10.7, 5.0 Hz, 1 H), 3.76 (dd,
J = 17.4, 5.2, 1.5, 1.5 Hz, 1 H), 3.33 (dd, J = 14.2, 5.0 Hz, 1 H), 3.27
(q, J = 7.0 Hz, 1 H), 3.23 (dd, J = 14.2, 10.7 Hz, 1 H), 3.14 (dd,
J = 17.4, 5.7, 1.5, 1.5 Hz, 1 H), 2.21 (s, 3 H), 1.46 (s, 9 H), 1.18 (d,
J = 7.0 Hz, 3 H).
13C NMR (150 MHz, CDCl3): δ = 176.0, 169.5, 138.2, 133.7, 129.3,
128.4, 126.6, 117.6, 81.6, 62.4, 55.4, 51.2, 34.8, 28.0, 19.7.
MS (ESI): m/z = 347.2 [M + H]+.
Following the typical procedure for 9a using Fmoc-alanine (150
mg, 0.48 mmol), HATU (183 mg, 0.48 mmol), and DIPEA (62 mg,
82 μL, 0.48 mmol) in CH2Cl2 (50 mL) and 3d (80 mg, 0.24 mmol)
in CH2Cl2 (10 mL) for 24 h at 42 °C. The mixture was washed with
aq NaHCO3 (2 × 20 mL), H2O (20 mL), and brine (20 mL). Flash
chromatography (n-hexane–EtOAc, 60:40) yielded 9c (225 mg,
80%) as a pale yellow foam; HPLC (220 nm) system M1: tR = 24.2
min (99%), system A1: tR = 23.1 min (98%).
[α]D24 –85.0 (c 0.2, CHCl3).
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2691
tert-Butyl (S)-2-(Allyl{(S)-2-[(S)-2-(9H-fluoren-9-ylmethoxy-
carbonylamino)pent-4-enoylamino]propanoyl}amino)-3-phen-
ylpropanoate (11a); Typical Procedure
81.7, 67.2, 54.3, 48.2, 47.2, 46.2, 37.4, 28.0, 19.2, 14.6; a minor ro-
tamer was additionally observed.
MS (ESI): m/z = 598.4 [M + Na]+.
Compound 9a (102 mg, 0.18 mmol) was dissolved in a soln of 32%
Et2NH in CH2Cl2 (5 mL). After stirring for 3.5 h at r.t., CH2Cl2 (20
mL) was added and the soln was washed with aq sat. NH4Cl soln
(3 × 20 mL), dried (Na2SO4), and concentrated in vacuo at low tem-
perature. The resulting material 10a was dried in vacuo and used in
the following coupling procedure without further purification. To a
soln of Fmoc-allylglycine (152 mg, 0.45 mmol) and HOBt (61 mg,
0.45 mmol) in THF (3 mL), DIPEA (58 mg, 77 μL, 0.45 mmol) and
DIC (57 mg, 70 μL, 0.45 mmol) and, after 25 min a soln of the crude
amine 10a in THF (6 mL) were added. After 12 h, the soln was di-
luted with H2O (20 mL) and extracted with Et2O (4 × 20 mL). The
combined organic layers were washed with H2O (20 mL) and brine
(20 mL), dried (Na2SO4), and concentrated in vacuo. Flash chroma-
tography (n-hexane–EtOAc, 70:30) gave 11a (72 mg, 61%) as a
pale yellow oil; HPLC (220 nm) system M2: tR = 23.8 min (96%),
system A2: tR = 21.9 min (98%).
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C33H41N3NaO6:
598.2893; found: 598.2890.
tert-Butyl (S)-2-(Allyl{(S)-2-[(S)-2-(9H-fluoren-9-ylmethoxy-
carbonylamino)pent-4-enoylamino]propanoyl}amino)-3-(4-
tert-butyloxyphenyl)propanoate (11c)
Following the typical procedure for 11a. Fmoc deprotection: 9c
(105 mg, 0.17 mmol), 32% Et2NH in CH2Cl2 (5 mL), 3 h at r.t.;
workup CH2Cl2 (10 mL) and aq sat. NH4Cl (3 × 15 mL). Coupling:
Fmoc-allylglycine (145.0 mg, 0.43 mmol), DIC (54 mg, 72 μL, 0.43
mmol), HOBt (58 mg, 0.43 mmol), DIPEA (56 mg, 74 μL, 0.43
mmol), and the crude primary amine 10c in THF (8 mL). Flash
chromatography (n-hexane–EtOAc, 70:30) gave 11c (59 mg, 51%)
as a pale yellow foam; HPLC (220 nm) system M1: tR = 24.0 min
(99%), system A1: tR = 22.2 min (94%).
[α]D23 –51.9 (c 0.6, CHCl3).
IR (NaCl): 1730, 1643, 1550 cm–1.
[α]D22 –80.3 (c 1.1, CHCl3).
IR (NaCl): 1729, 1644, 1526 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.73–7.79 (m, 2 H), 7.55–7.62 (m,
2 H), 7.38–7.43 (m, 2 H), 7.29–7.35 (m, 2 H), 7.00–7.05 (m, 2 H),
6.87–6.95 (m, 3 H), 5.67–5.78 (m, 1 H), 5.46–5.55 (m, 1 H), 5.38
(d, J = 7.6 Hz, 1 H), 5.04–5.19 (m, 4 H), 4.68–4.75 (m, 1 H), 4.40–
4.47 (m, 1 H), 4.30–4.37 (m, 1 H), 4.19–4.28 (m, 1 H), 4.00–4.08
(m, 1 H), 3.72–3.81 (m, 1 H), 3.14–3.34 (m, 3 H), 2.44–2.58 (m, 2
H), 1.41 and 1.38 (2 × s, 9 H), 1.32 and 1.38 (2 × s, 9 H), 1.28 (d,
J = 6.8 Hz, 3 H); a minor rotamer was additionally observed.
13C NMR (150 MHz, CDCl3): δ = 172.0, 169.8, 169.0, 155.9, 154.1,
143.9, 143.8, 141.3, 132.9, 132.8, 132.6, 129.8, 127.7, 127.1, 125.1,
124.4, 120.0, 119.4, 118.6, 81.7, 78.5, 67.1, 62.3, 54.3, 51.9, 47.2,
45.9, 37.3, 33.9, 28.8, 27.9, 27.8, 19.1; a minor rotamer was addi-
tionally observed.
1H NMR (360 MHz, CDCl3): δ = 7.73–7.79 (m, 2 H), 7.55–7.62 (m,
2 H), 7.37–7.43 (m, 2 H), 7.29–7.35 (m, 2 H), 7.24–7.29 (m, 2 H),
7.19–7.24 (m, 1 H), 7.10–7.18 (m, 2 H), 6.86 (d, J = 6.9 Hz, 1 H),
5.63–5.80 (m, 1 H), 5.48–5.62 (m, 1 H), 5.38 (d, J = 7.5 Hz, 1 H),
5.08–5.19 (m, 4 H), 4.73 (dq, J = 7.9, 6.8 Hz, 1 H), 4.44 (dd,
J = 10.5, 7.5 Hz, 1 H), 4.31–4.38 (m, 1 H), 4.21–4.30 (m, 1 H), 4.18
(dd, J = 10.2, 5.1 Hz, 1 H), 3.81 (dd, J = 16.4, 5.0 Hz, 1 H), 3.31
(dd, J = 14.2, 5.1 Hz, 1 H), 3.25 (dd, J = 16.4, 6.0 Hz, 1 H), 3.20
(dd, J = 14.2, 10.2 Hz, 1 H), 2.44–2.58 (m, 2 H), 1.43 and 1.40
(2 × s, 9 H), 1.31 and 1.00 (2 × d, each J = 6.9 Hz, 3 H); a minor ro-
tamer was additionally observed.
13C NMR (150 MHz, CDCl3): δ = 173.1, 172.4, 169.8, 169.0, 155.8,
143.9, 143.8, 141.3, 137.9, 136.8, 133.6, 132.8, 132.6, 129.3, 128.8,
128.5, 127.7, 127.1, 126.7, 125.1, 120.0, 119.4, 118.6, 117.3, 83.0,
81.7, 67.1, 62.3, 62.1, 54.2, 51.7, 47.2, 45.9, 37.4, 34.5, 27.9, 27.8,
19.1, 19.0; a minor rotamer was additionally observed.
MS (ESI): m/z = 746.5 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C43H53N3NaO7:
746.3781; found: 746.3776.
MS (ESI): m/z = 674.4 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C39H45N3NaO6:
tert-Butyl (S)-2-[Allyl-((S)-2-{[(S)-2-(9H-fluoren-9-ylmethoxy-
carbonylamino)pent-4-enoyl]methylamino}propanoyl)amino]-
3-phenylpropanoate (11d)
674.3206; found: 674.3201.
Following the typical procedure for 4a using Fmoc-allylglycine
(1110.0 mg, 3.28 mmol), HATU (2076.0 mg, 5.46 mmol), DIPEA
(424 mg, 561 μL, 3.28 mmol), and the secondary amine 10d (945.0
mg, 2.73 mmol) in NMP (100 mL) for2 d at r.t. Flash chromatogra-
phy (n-hexane–EtOAc, 60:30) gave 11d (1690 mg, 93%) as a pale
yellow foam; HPLC (220 nm) system M1: tR = 23.8 min (99%),
system A1: tR = 21.8 min (99%).
tert-Butyl (S)-2-(Allyl{(S)-2-[(S)-2-(9H-fluoren-9-ylmethoxy-
carbonylamino)pent-4-enoylamino]propanoyl}amino)pro-
panoate (11b)
Following the typical procedure for 11a. Fmoc deprotection: 9b
(100 mg, 0.21 mmol), 32% Et2NH in CH2Cl2 (5 mL), 3 h at r.t.;
workup used CH2Cl2 (10 mL) and aq sat. NH4Cl (3 × 15 mL). Cou-
pling: Fmoc-allylglycine (192 mg, 0.57 mmol), DIC (72 mg, 88 μL,
0.57 mmol), HOBt (77 mg, 0.57 mmol), DIPEA (74 mg, 98 μL, 0.57
mmol), and the crude primary amine 10b in THF (10 mL). Flash
chromatography (n-hexane–EtOAc, 65:35) gave 11b (54 mg, 45%)
as a pale yellow foam; HPLC (220 nm) system M2: tR = 23.0 min
(99%), system A2: tR = 19.9 min (99%).
[α]D22 –110.8 (c 1.0, CHCl3).
IR (NaCl): 1726, 1680 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.73–7.78 (m, 2 H), 7.53–7.60 (m,
2 H), 7.37–7.41 (m, 2 H), 7.20–7.35 (m, 5 H), 7.09–7.16 (m, 2 H),
5.66–5.75 (m, 1 H), 5.45–5.56 (m, 2 H), 5.38 (q, J = 6.9 Hz, 1 H),
5.06–5.24 (m, 4 H), 4.66 (ddd, J = 8.0, 8.0, 5.2 Hz, 1 H), 4.36–4.42
(m, 1 H), 4.29–4.34 (m, 1 H), 4.26 (dd, J = 10.3, 5.2 Hz, 1 H), 4.19–
4.28 (m, 1 H), 4.18–4.22 (m, 1 H), 3.76 (dd, J = 16.6, 4.8 Hz, 1 H),
3.32 (dd, J = 16.6, 7.3 Hz, 1 H), 3.30 (dd, J = 14.1, 5.2 Hz, 1 H),
3.24 (dd, J = 14.1, 10.3 Hz, 1 H), 3.72–3.81 (m), 2.42–2.48 (m, 1
H), 2.20–2.28 (m, 2 H), 1.46 and 1.45 (2 × s, 9 H), 1.28 and 0.79
(2 × d, J = 6.8 Hz, 3 H); a minor rotamer was additionally observed.
13C NMR (150 MHz, CDCl3): δ = 171.2, 170.8, 169.2, 155.8, 143.9,
143.8, 141.3, 134.3, 133.6, 132.7, 132.4, 129.7, 129.3, 128.7, 128.5,
127.7, 127.1, 125.2, 125.1, 120.0, 119.0, 118.3, 81.7, 67.0, 65.9,
61.3, 53.4, 51.0, 49.3, 48.8, 47.8, 47.2, 37.2, 36.3, 33.9, 30.3, 28.0,
15.3, 14.7; a minor rotamer was additionally observed.
[α]D23 –83.8 (c 0.3, CHCl3).
IR (NaCl): 1732, 1638, 1540 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.74–7.81 (m, 2 H), 7.56–7.61 (m,
2 H), 7.37–7.42 (m, 2 H), 7.29–7.34 (m, 2 H), 6.84 (d, J = 6.4 Hz, 1
H), 5.85–5.93 (m, 1 H), 5.67–5.77 (m, 1 H), 5.34–5.40 (m, 1 H),
5.21–5.28 (m, 1.6 H), 5.10–5.17 (m, 2.4 H), 4.73–4.83 (m, 2 H),
4.39–4.45 (m, 1 H), 4.31–4.38 (m, 1 H), 4.19–4.29 (m, 2 H), 3.99–
4.06 (m, 1 H), 3.82–3.90 (m, 1 H), 2.42–2.58 (m, 1 H), 1.43 (s, 9 H),
1.36 (2 × d, each J = 6.4 Hz, 6 H); a minor rotamer was additionally
observed.
13C NMR (90 MHz, CDCl3): δ = 172.8, 170.5, 169.9, 155.9, 143.9,
143.8, 141.4, 133.8, 132.6, 127.7, 127.1, 125.1, 120.0, 119.3, 117.6,
MS (ESI): m/z = 688.4 [M + Na]+.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2692
S. Wakchaure et al.
FEATURE ARTICLE
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C40H47N3NaO6:
688.3363; found: 688.3357.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C31H37N3NaO6:
570.2580; found: 570.2566.
tert-Butyl (S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbo-
nylamino)-3-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-di-
azecin-1(2H)-yl]-3-phenylpropanoate (12a); Typical Procedure
To a soln of diene 11a (70 mg, 0.11 mmol) in toluene (120 mL) a
soln of Grubbs II catalyst (A; 4.7 mg, 0.05 mmol, 5 mol% ) in tolu-
ene (10 mL) was added. After refluxing for 1 h, a second portion of
A (4.7 mg, 0.05 mmol, 5 mol%) was added and reflux was contin-
ued under constant N2 flow through the mixture. After 2.5 h, the sol-
vent was removed in vacuo and flash chromatography (n-hexane–
EtOAc, 50:50) afforded 12a (46.8 mg, 70%) as a pale yellow oil;
HPLC (220 nm) system M4: tR = 31.0 min (96%), system A2:
tR = 20.0 min (95%).
tert-Butyl (S)-3-(4-tert-Butoxyphenyl)-2-[(Z)-(3S,6S)-6-(9H-
fluoren-9-ylmethoxycarbonylamino)-3-methyl-2,5-dioxo-
3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-yl]propanoate (12c)
Following the typical procedure for 5a using diene 11c (100 mg,
0.14 mmol) in CH2Cl2 (125 mL) and a soln of Grubbs II catalyst (A;
18.2 mg, 0.021 mmol, 15 mol%) in CH2Cl2 (25 mL). Flash chroma-
tography (n-hexane–EtOAc, 50:50) afforded 12c (55 mg, 57%) as a
brown oil; HPLC (220 nm) system M2: tR = 23.8 min (99%), sys-
tem A2: tR = 21.8 min (99%).
[α]D24 –83.8 (c 1.4, CHCl3).
IR (NaCl): 1734, 1700, 1600 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.74–7.78 (m, 2 H, ArFmoc), 7.56–
7.61 (m, 2 H, ArFmoc), 7.37–7.42 (m, 2 H, ArFmoc), 7.28–7.35 (m, 2
H, ArFmoc), 6.97–7.06 (m, 2 H, ArTyr), 6.87–6.96 (m, 2 H, ArTyr),
6.26 (d, J = 7.8 Hz, 1 H, NHFmoc), 5.87 (d, J = 7.6 Hz, 1 H, H4),
5.78–5.84 (m, 1 H, H9 or H8), 5.68–5.75 (m, 1 H, H8 or H9), 4.73
(dq, J = 7.6, 6.7 Hz, 1 H, H3), 4.33–4.41 (m, 2 H, Fmoc-CH2), 4.18–
4.23 (m, 1 H, CHFmoc), 3.85 (ddd, J = 10.1, 7.8, 2.1 Hz, 1 H, H6),
3.64 (dd, J = 10.6, 4.5 Hz, 1 H, CH Tyr), 3.52 (dd, J = 16.1, 11.3
Hz, 1 H, H10α), 3.32 (dd, J = 13.9, 10.6 Hz, 1 H, Tyr-CH2), 3.26
(dd, J = 13.9, 4.5 Hz, 1 H, Tyr-CH2), 2.56 (d, J = 16.1 Hz, 1 H,
H10β), 2.23–2.37 (m, 2 H, H7/H7′), 1.48 (s, 9 H, Boc-t-Bu), 1.39
(d, J = 6.7 Hz, 3 H, Ala-CH3), 1.34 (s, 9 H, Ot-Bu); a minor rotamer
was additionally observed.
13C NMR (90 MHz, CDCl3): δ = 170.3, 169.3, 168.8, 155.2, 154.1,
143.8, 143.7, 141.3, 133.2, 131.3, 131.2, 129.5, 127.7, 127.1, 125.3,
125.1, 124.5, 119.9, 82.1, 78.6, 67.2, 66.6, 53.4, 49.0, 48.2, 47.3,
33.9, 32.9, 29.0, 28.2, 18.1.
MS (ESI): m/z = 718.5 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C41H49N3NaO7:
[α]D22 –75.0 (c 1.0, CHCl3).
IR (NaCl): 1725, 1649, 1534 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.70–7.78 (m, 2 H, ArFmoc), 7.52–
7.62 (m, 2 H, ArFmoc), 7.36–7.42 (m, 2 H, ArFmoc), 7.27–7.33 (m, 4
H, ArFmoc, ArPhe), 7.19–7.25 (m, 1 H, ArPhe), 7.05–7.15 (m, 2 H,
ArPhe), 6.16 (d, J = 7.8 Hz, 1 H, NHFmoc), 5.86 (d, J = 7.2 Hz, 1 H,
H4), 5.83 (ddd, J = 11.3, 10.8, 2.2 Hz, 1 H, H9), 5.72 (ddd, J = 10.8,
10.2, 8.0 Hz, 1 H, H8), 4.74 (dq, J = 7.2, 6.7 Hz, 1 H, H3), 4.34–
4.41 (m, 2 H, Fmoc-CH2), 4.18–4.23 (m, 1 H, CHFmoc), 3.85 (ddd,
J = 10.2, 7.8, 2.8 Hz, 1 H, H6), 3.60 (dd, J = 10.5, 4.4 Hz, 1 H,
CHPhe), 3.54 (dd, J = 16.2, 11.3 Hz, 1 H, H10α), 3.37 (dd, J = 13.8,
10.5 Hz, 1 H, Phe-CH2), 3.31 (dd, J = 13.8, 4.4 Hz, 1 H, Phe-CH2),
2.58 (d, J = 16.2 Hz, 1 H, H10β), 2.25–2.36 (m, 2 H, H7/H7′), 1.49
(s, 9 H, t-Bu), 1.40 (d, J = 6.7 Hz, 3 H, Ala-CH3); a minor rotamer
was additionally observed.
13C NMR (150 MHz, CDCl3): δ = 170.4, 169.3, 168.7, 155.2, 143.9,
143.7, 141.33, 141.31, 138.3, 131.2, 129.1, 128.8, 127.8, 127.7,
126.8, 125.3, 125.1, 120.1, 82.0, 67.1, 66.3, 53.2, 49.0, 48.1, 47.2,
34.5, 32.8, 28.0, 17.9; a minor rotamer was additionally observed.
718.3468; found: 718.3463.
MS (ESI): m/z = 646.4 [M + Na]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C37H41N3NaO6:
646.2893; found: 646.2888.
tert-Butyl (S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbo-
nylamino)-3,4-dimethyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-
diazecin-1(2H)-yl]-3-phenylpropanoate (12d)
Following the typical procedure for 5a using diene 11d (320 mg,
0.48 mmol) in CH2Cl2 (200 mL) and a soln of Grubbs II catalyst (A;
20.0 mg, 0.024 mmol, 5 mol%) in CH2Cl2 (25 mL) with reflux for
8 h. Flash chromatography (n-hexane–EtOAc, 50:50) afforded 12d
(183 mg, 60%) as a brown oil; HPLC (220 nm) system M2:
tR = 23.1 min (98%), system A2: tR = 20.8 min (97%).
[α]D22 –97.5 (c 0.4, CHCl3).
IR (NaCl): 1728, 1647, 1600 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.70–7.78 (m, 2 H, ArFmoc), 7.54–
7.60 (m, 2 H, ArFmoc), 7.37–7.41 (m, 2 H, ArFmoc), 7.28–7.33 (m, 4
H, ArFmoc, ArPhe), 7.21–7.25 (m, 1 H, ArPhe), 7.05–7.13 (m, 2 H,
ArPhe), 5.77 (d, J = 8.3 Hz, 1 H, NHFmoc), 5.72 (ddd, J = 11.4, 10.8,
1.3 Hz, 1 H, H9), 5.63 (ddd, J = 10.8, 10.2, 8.0 Hz, 1 H, H8), 5.28
(q, J = 6.8 Hz, 1 H, H3), 4.60 (ddd, J = 11.3, 8.3, 2.8 Hz, 1 H, H6),
4.34–4.40 (m, 2 H, Fmoc-CH2), 4.18–4.23 (m, 1 H, CHFmoc), 3.65
(dd, J = 10.3, 5.0 Hz, 1 H, CHPhe), 3.53 (dd, J = 16.4, 11.4 Hz, 1 H,
H10α), 3.34 (dd, J = 13.9, 10.3 Hz, 1 H, Phe-CH2), 3.29 (dd,
J = 13.9, 5.0 Hz, 1 H, Phe-CH2), 2.89 (s, 3 H, NCH3), 2.52 (d,
J = 16.4 Hz, 1 H, H10β), 2.40 (ddd, J = 12.7, 11.3, 10.2 Hz, 1 H,
H7), 2.29 (ddd, J = 12.7, 8.0, 2.8 Hz, 1 H, H7′), 1.53 and 1.49 (2 × s,
9 H, t-Bu), 1.35 (d, J = 6.8 Hz, 3 H, Ala-CH3); a minor rotamer was
additionally observed.
tert-Butyl (S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbo-
nylamino)-3-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-di-
azecin-1(2H)-yl)]propanoate (12b)
Following the typical procedure for 5a using diene 11b (80 mg, 0.14
mmol) in CH2Cl2 (200 mL) and a soln of Grubbs II catalyst (A; 6.0
mg, 0.007 mmol, 5 mol%) in CH2Cl2 (10 mL). After 2 h a second
portion of A (6.0 mg, 0.007 mmol, 5 mol%) in CH2Cl2 (10 mL) was
added. Flash chromatography (n-hexane–EtOAc, 30:70) afforded
12b (48.1 mg, 60%) as brown oil; HPLC (220 nm) system M8:
tR = 21 min (94%), system A2: tR = 17 min (98%).
[α]D22 –2.4 (c 0.6, CHCl3).
IR (NaCl): 3326, 2934, 1715, 1644 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.77 (d, J = 7.5 Hz, 2 H, ArFmoc),
7.60 (d, J = 7.5 Hz, 2 H, ArFmoc), 7.41 (dd, J = 7.5, 7.5 Hz, 2 H,
ArFmoc), 7.32 (dd, J = 7.5, 7.5 Hz, 2 H, ArFmoc), 6.18 (d, J = 8.8 Hz,
1 H, H4), 5.93 (d, J = 7.7 Hz, 1 H, NHFmoc), 5.90 (ddd, J = 11.4,
11.4, 2.8 Hz, 1 H, H9), 5.83 (ddd, J = 11.4, 10.0, 7.8 Hz, 1 H, H8),
4.96 (dq, J = 8.8, 6.8 Hz, 1 H, H3), 4.36–4.42 (m, 2 H, Fmoc-CH2),
4.20–4.24 (m, 1 H, CHFmoc), 3.97 (dd, J = 16.1, 11.4 Hz, 1 H,
H10α), 3.96 (q, J = 6.8 Hz, 1 H, CHAla), 3.93 (ddd, J = 10.6, 7.7, 2.8
Hz, 1 H, H6), 3.51 (d, J = 16.1 Hz, 1 H, H10β), 2.49 (ddd, J = 13.3,
10.6, 10.4 Hz, 1 H, H7), 2.44 (ddd, J = 13.3, 7.8, 2.8 Hz, 1 H, H7′),
1.47 (d, J = 6.8 Hz, 3 H, Ala-CH3), 1.46 (s, 9 H, t-Bu), 1.38 (d,
J = 6.8 Hz, 3 H, Ala-CH3); a minor rotamer was additionally ob-
served.
13C NMR (150 MHz, CDCl3): δ = 172.9, 169.0, 168.6, 155.4, 143.8,
143.7, 141.3, 138.3, 130.0, 129.1, 128.7, 127.7, 127.1, 126.7, 125.1,
123.5, 120.0, 81.8, 67.0, 66.1, 51.7, 49.0, 48.9, 47.1, 34.5, 34.2,
32.0, 28.1, 13.6.
13C NMR (150 MHz, CDCl3): δ = 170.2, 169.9, 169.5, 155.3, 143.9,
143.7, 141.4, 132.2, 127.8, 127.1, 125.8, 125.1, 120.1, 81.6, 67.1,
58.8, 53.4, 48.2, 47.2, 46.6, 32.9, 28.0, 18.1, 14.3.
MS (ESI): m/z = 660.4 [M + Na]+.
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
Conformationally Restricted Peptide Mimetics
2693
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C38H43N3NaO6:
MS (ESI): m/z = 606.3 [M + Na]+.
660.3050; found: 660.3042.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C33H33N3NaO7:
606.2216; found: 606.2115.
(S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
3-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-
yl)]-3-phenylpropanoic Acid (13a); Typical Procedure
Acidic ester cleavage was performed by dissolving 12a (50 mg,
0.09 mmol) in CH2Cl2 (0.2 mL) and employing TFA–CH2Cl2 (1:3,
1.3 mL) After stirring at r.t. for 3 h, the mixture was concentrated
and flash chromatography (CH2Cl2–MeOH–AcOH, 74:25:1) gave
the carboxylic acid 13a (41.2 mg, 90%) as a yellow foam; HPLC
(220 nm) system M7: tR = 14.8 min (95%), system A6: tR = 16.3
min (95%).
(S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
3,4-dimethyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-
1(2H)-yl]-3-phenylpropanoic Acid (13d)
Following the typical procedure for 13a using 12d (733 mg, 1.15
mmol) in CH2Cl2 (20 mL) and TFA–CH2Cl2 (1:1, 16 mL) with stir-
ring at r.t. for 3 h. The mixture was concentrated and thoroughly
dried in vacuo to give the crude carboxylic acid 12d (610 mg, 91%)
as a white solid; mp 225 °C; HPLC (220 nm) system M7: tR = 19.0
min (97%), system A4: tR = 10.4 min (96%).
[α]D26 +13.0 (c 1.7, DMSO).
IR (NaCl): 3319, 1689, 1654, 1617, cm–1.
[α]D26 –128.2 (c 1, DMSO).
IR (NaCl): 3315, 1726, 1706, 1649, 1613 cm–1.
1H NMR (600 MHz, CD3OD): δ = 7.77–7.82 (m, 2 H), 7.61–7.68
(m, 2 H), 7.36–7.41 (m, 2 H), 7.33–7.36 (m, 2 H), 7.28–7.35 (m, 6
H), 7.22–7.27 (m, 1 H), 5.73–5.86 (m, 1 H), 5.50–5.58 (m, 1 H),
4.36–4.46 (m, 2 H), 4.27–4.33 (m, 1 H), 4.18–4.25 (m, 2 H), 3.76–
3.83 (m, 1 H), 3.50–3.69 (m, 2 H), 3.23–3.28 (m, 1 H), 3.12–3.18
(m, 1 H), 2.49–2.61 (m, 1 H), 2.35–2.44 (m, 1 H), 1.38–1.40 (m, 3
H); a minor rotamer was additionally observed.
1H NMR (600 MHz, CDCl3): δ = 7.73–7.78 (m, 2 H), 7.53–7.60 (m,
2 H), 7.38–7.42 (m, 2 H), 7.29–7.34 (m, 4 H), 7.24–7.28 (m, 1 H),
7.12–7.15 (m, 2 H), 5.81 (d, J = 8.4 Hz, 1 H), 5.60–5.70 (m, 2 H),
5.33 (q, J = 6.7 Hz, 1 H), 4.70 (ddd, J = 11.7, 8.4, 2.9 Hz, 1 H),
4.34–4.41 (m, 2 H), 4.19–4.22 (m, 1 H), 3.79 (dd, J = 10.7, 4.3 Hz,
1 H), 3.58 (dd, J = 16.4, 10.5 Hz, 1 H, H10α), 3.44 (dd, J = 13.9,
10.7 Hz, 1 H), 3.38 (dd, J = 13.9, 4.3 Hz, 1 H), 2.87 (s, 3 H), 2.57
(d, J = 16.4 Hz, 1 H), 2.41 (ddd, J = 12.7, 11.7, 7.8 Hz, 1 H), 2.29
(ddd, J = 12.7, 7.6, 2.9 Hz, 1 H), 1.36 (d, J = 6.8 Hz, 3 H); a minor
rotamer was additionally observed.
13C NMR (90 MHz, DMSO-d6): δ = 173.6, 171.3, 168.8, 155.5,
143.8, 140.7, 138.5, 129.3, 129.2, 127.6, 127.1, 126.3, 125.3, 123.3,
120.1, 65.6, 63.4, 50.5, 48.7, 48.2, 46.6, 33.9, 31.3, 30.7, 13.6.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C34H35N3NaO6:
604.2423; found: 604.2425.
HRMS (ESI-TOF): m/z [M – H]– calcd for C33H32N3O6: 566.2291;
found: 566.2278.
(S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
3-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-
yl]propanoic Acid (13b)
Acidic ester cleavage was performed with 12b (30.3 mg, 0.055
mmol) using TFA (1.0 mL) After stirring at 0 °C for 1.5 h, the mix-
ture was concentrated and flash chromatography (CH2Cl2–MeOH–
H2O, 93:6:1) to give the carboxylic acid 13b (22.1 mg, 82%) as a
yellow foam; HPLC (220 nm) system M6: tR = 13.4 min (98%),
system A5: tR = 13.0 min (96%).
(S)-2-[(Z)-(3S,6S)-6-(Acetylamino)-3,4-dimethyl-2,5-dioxo-
3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-yl]-N-methyl-3-
phenylpropanamide (14)
[α]D26 –15.2 (c 0.4, MeOH).
To a soln of carboxylic acid 13d (100 mg, 0.17 mmol) and HATU
(130 mg, 0.34 mmol) in THF (4.0 mL), DIPEA (44 mg, 58 μL, 0.34
mmol) was added. After stirring for 30 min at r.t., 2 M MeNH2 in
THF soln (127 μL, 0.51 mmol) was added. After 12 h, a second por-
tion of a 2 M MeNH2 in THF soln (2.0 mL) was added and stirring
was continued for 1 h at r.t. The mixture was concentrated in vacuo
and the crude secondary amine was disolved in CH2Cl2 (2.0 mL). N-
Acetylation was performed by successively adding DIPEA (659
mg, 873 μL, 5.1 mmol), DMAP (0.1 mg, 0.005 mmol), and Ac2O
(521 mg, 482 μL, 5.1 mmol) and stirring for 12 h at r.t. The mixture
was diluted with aq sat. NH4Cl soln (3.0 mL) and extracted with
CH2Cl2 (3 × 5.0 mL). The combined organic layers were dried
(Na2SO4) and the solvent was removed in vacuo. The product was
purified by flash chromatography (CH2Cl2–MeOH–NH4OH 25%,
90:5:5) to give 14 (42.1 mg, 59%) as a pale yellow oil; HPLC (220
nm) system M5: tR = 14.6 min (98%), system A3: tR = 14.5 min
(95%).
IR (NaCl): 3326, 2934, 1715, 1644 cm–1.
1H NMR (600 MHz, CD3OD, 240 K): δ = 7.76–7.84 (m, 2 H), 7.62–
7.75 (m, 2 H), 7.37–7.43 (m, 2 H), 7.29–7.35 (m, 2 H), 5.87 (ddd,
J = 11.5, 11.5, 3.0 Hz, 1 H), 5.72 (ddd, J = 11.5, 10.5, 9.6 Hz, 1 H),
4.86 (q, J = 7.0 Hz, 1 H), 4.28–4.35 (m, 2 H), 4.17–4.25 (m, 1 H),
4.00 (q, J = 7.0 Hz, 1 H), 3.84 (dd, J = 16.0, 11.5 Hz, 1 H), 3.73 (dd,
J = 12.0, 1.6 Hz, 1 H), 3.55 (br d, J = 16.0 Hz, 1 H), 2.50 (ddd,
J = 12.5, 12.0, 9.6 Hz, 1 H), 2.20 (ddd, J = 12.5, 10.5, 1.6 Hz, 1 H),
1.41 (d, J = 7.0 Hz, 3 H), 1.24 (d, J = 7.0 Hz, 3 H).
MS (APCI): m/z = 492.3 [M + H]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C27H29N3NaO6
514.1954; found: 514.1943.
(S)-2-[(Z)-(3S,6S)-6-(9H-Fluoren-9-ylmethoxycarbonylamino)-
3-methyl-2,5-dioxo-3,4,5,6,7,10-hexahydro-1,4-diazecin-1(2H)-
yl]-3-(4-hydroxyphenyl)propanoic Acid (13c)
Following the typical procedure for 13a using 12c (50 mg, 0.072
mmol) in CH2Cl2 (0.7 mL) and TFA–CH2Cl2 (1:1, 1.4 mL) with
stirring at r.t. for 3 h. The mixture was concentrated and thoroughly
dried in vacuo to give the crude carboxylic acid 13c (29.3 mg, 70%)
as a yellow foam; HPLC (220 nm) system M7: tR = 15.7 min (94%),
system A5: tR = 13.8 min (97%).
[α]D22 –99.6 (c 1.2, CHCl3).
IR (NaCl): 2945, 1633, 1541 cm–1.
1H NMR (600 MHz, CDCl3): δ = 7.28–7.33 (m, 2 H, ArPhe), 7.23–
7.27 (m, 1 H, ArPhe), 7.13–7.17 (m, 2 H, ArPhe), 6.46 (d and br s,
J = 7.8 Hz, 2 H, NHAc and NHCH3), 5.78 (ddd, J = 11.5, 11.4, 2.7
Hz, 1 H, H9), 5.66 (ddd, J = 11.4, 10.6, 8.1 Hz, 1 H, H8), 5.28 (q,
J = 6.8 Hz, 1 H, H3), 4.83 (ddd, J = 11.2, 7.8, 3.1 Hz, 1 H, H6), 4.15
(br s, 1 H, CHPhe), 3.60 (dd, J = 16.3, 11.5 Hz, 1 H, H10α), 3.45 (dd,
J = 13.2, 9.8 Hz, 1 H, Phe-CH2), 3.21 (dd, J = 13.2, 6.0 Hz, 1 H,
Phe-CH2), 2.96 (d, J = 16.4 Hz, 1 H, H10β), 2.84 (s, 3 H, NCH3),
2.82 (d, J = 4.9 Hz, 3 H, NHCH3), 2.37 (ddd, J = 12.5, 11.2, 10.6
Hz, 1 H, H7), 2.30 (ddd, J = 12.5, 8.1, 3.1 Hz, 1 H, H7′), 1.99 (s, 3
H, COCH3), 1.33 (d, J = 6.8 Hz, 3 H, Ala-CH3); a minor rotamer
was additionally observed.
[α]D26 –66.7 (c 1.2, MeOH).
IR (NaCl): 3498, 1683, 1646, 1515 cm–1.
1H NMR (600 MHz, CD3OD): δ = 7.76–7.81 (m, 2 H), 7.57–7.68
(m, 2 H), 7.35–7.41 (m, 2 H), 7.27–7.33 (m, 2 H), 6.91–6.98 (m, 2
H), 6.67–6.73 (m, 2 H), 5.72–5.83 (m, 1 H), 5.56–5.65 (m, 1 H),
4.60–4.67 (m, 1 H), 4.32–4.39 (m, 2 H), 4.17–4.23 (m, 1 H), 3.65–
3.96 (m, 2 H), 3.38–3.48 (m, 1 H), 3.17–3.29 (m, 2 H), 2.49–2.77
(m, 1 H), 2.28–2.38 (m, 1 H), 2.06–2.23 (m, 1 H), 1.27–1.33 (m, 3
H); a minor rotamer was additionally observed.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2682–2694

2694
S. Wakchaure et al.
FEATURE ARTICLE
13C NMR (150 MHz, CDCl3): δ = 173.3, 170.74, 170.73, 169.2,
137.1, 130.3, 128.9, 127.1, 124.1, 66.9, 52.2, 48.9, 47.4, 34.8, 33.8,
31.9, 26.5, 23.3, 13.7; a minor rotamer was additionally observed.
MS (APCI): m/z = 415.4 [M + H]+.
HRMS (ESI-TOF): m/z [M + Na]+ calcd for C22H30N4O4Na:
(10) (a) Bernal, F.; Tyler, A. F.; Korsmeyer, S. J.; Walensky, L.
D.; Verdine, G. L. J. Am. Chem. Soc. 2007, 129, 2456.
(b) Moellering, R. E.; Cornejo, M.; Davis, T. N.; Del Bianco,
C.; Aster, J. C.; Blacklow, S. C.; Kung, A. L.; Gilliland, D.
G.; Verdine, G. L.; Bradner, J. E. Nature (London) 2009,
462, 182.
437.2165; found: 437.2156.
(11) Chapman, R. N.; Dimartino, G.; Arora, P. S. J. Am. Chem.
Soc. 2004, 126, 12252.
(12) Miller, S. J.; Blackwell, H. E.; Grubbs, R. H. J. Am. Chem.
Soc. 1996, 118, 9606.
Acknowledgment
(13) Reichwein, J. F.; Liskamp, R. M. J. Eur. J. Org. Chem. 2000,
2335.
(14) Freidinger, R. M. J. Org. Chem. 1985, 50, 3631.
(15) Piscopio, A. D.; Miller, J. F.; Koch, K. Tetrahedron 1999,
55, 8189.
We thank Prof. Ivana Ivanović-Burmazović and Leanne Nye (De-
partment of Chemistry and Pharmacy, Chair of Bioinorganic Che-
mistry, Friedrich Alexander University Erlangen-Nürnberg) for
high resolution ESI-TOF mass spectra.
(16) Hoffmann, T.; Waibel, R.; Gmeiner, P. J. Org. Chem. 2003,
68, 62.
(17) Hoffmann, T.; Gmeiner, P. Synlett 2002, 1014.
(18) Einsiedel, J.; Lanig, H.; Waibel, R.; Gmeiner, P. J. Org.
Chem. 2007, 72, 9102.
(19) Bittermann, H.; Boeckler, F.; Einsiedel, J.; Gmeiner, P.
Chem.–Eur. J. 2006, 12, 6315.
(20) Beal, L. M.; Liu, B.; Chu, W.; Moeller, K. D. Tetrahedron
2000, 56, 10113.
(21) (a) Grossmith, C. E.; Senia, F.; Wagner, J. Synlett 1999,
1660. (b) Zaminer, J.; Brockmann, C.; Huy, P.; Opitz, R.;
Reuter, C.; Beyermann, M.; Freund, C.; Mueller, M.;
Oschkinat, H.; Kuehne, R.; Schmalz, H.-G. Angew. Chem.
Int. Ed. 2010, 49, 7111.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
n
nfomartit
References
(1) (a) Vlieghe, P.; Lisowski, V.; Martinez, J.; Khrestchatisky,
M. Drug Discovery Today 2010, 15, 40. (b) Sato, A. K.;
Viswanathan, M.; Kent, R. B.; Wood, C. R. Curr. Opin.
Biotechnol. 2006, 17, 638.
(2) Bray, B. L. Nat. Rev. Drug Discovery 2003, 2, 587.
(3) Hruby, V. J. Drug Discovery Today 1997, 2, 165; and
references cited therein.
(4) See, for example: Giannis, A.; Kolter, T. Angew. Chem. Int.
Ed. 1993, 32, 1244.
(22) Hoffmann, T.; Waibel, R.; Gmeiner, P. Angew. Chem. Int.
Ed. 2001, 40, 3661.
(5) See, for example: (a) Kahn, M.; Chen, B. Tetrahedron Lett.
1987, 28, 1623. (b) Hinds, M. G.; Welsh, J. H.; Brennand, D.
M.; Fisher, J.; Glennie, M. J.; Richards, N. G. J.; Turner, D.
L.; Robinson, J. A. J. Med. Chem. 1991, 34, 1777.
(c) Hanessian, S.; McNaughton-Smith, G.; Lombart, H.-G.;
Lubell, W. D. Tetrahedron 1997, 53, 12789. (d) Souers, A.
J.; Virgilio, A. A.; Rosenquist, A.; Fenuik, W.; Ellman, J. A.
J. Am. Chem. Soc. 1999, 121, 1817. (e) Freidinger, R. M.
J. Med. Chem. 2003, 46, 5553. (f) Stepien, A.; Loska, R.;
Cmoch, P.; Stalinsky, K. Synlett 2005, 83. (g) Wang, W.;
Xiong, C.; Hruby, V. J. Tetrahedron Lett. 2001, 42, 3159.
(h) Pawar, V. G.; De Borggraeve, W. M.; Maes, V.; Tourwé,
D. A.; Compernolle, F.; Hoornaert, G. J. Tetrahedron Lett.
2005, 46, 1707. (i) Albericio, F.; Arvidson, P. I.; Bisetty, K.;
Giralt, E.; Govender, T.; Jali, S.; Kongsaeree, P.; Kruger, H.
G.; Prabpai, S. Chem. Biol. Drug Des. 2008, 71, 125. (j) Ko,
E.; Burgess, K. Org. Lett. 2011, 13, 980.
(23) Banfi, L.; Basso, A.; Guanti, G.; Riva, R. Tetrahedron Lett.
2003, 44, 7655.
(24) Kaul, R.; Surprenant, S.; Lubell, W. D. J. Org. Chem. 2005,
70, 3838; corrigendum: J. Org. Chem. 2005, 70, 4901.
(25) Fink, B. E.; Kym, P. R.; Katzenellenbogen, J. A. J. Am.
Chem. Soc. 1998, 120, 4334.
(26) Pinsker, A.; Einsiedel, J.; Haerterich, S.; Waibel, R.;
Gmeiner, P. Org. Lett. 2011, 13, 3502.
(27) Anderson, G. W.; Zimmerman, J. E.; Callahan, F. M. J. Am.
Chem. Soc. 1967, 89, 5012.
(28) Compound 3a is described in: Senokuchi, K.; Nakai, H.;
Nakayama, Y.; Odagaki, Y.; Sakaki, K.; Kato, M.;
Maruyama, T.; Miyazaki, T.; Ito, H.; Kamiyasu, K.; Kim, S.-
i.; Kawamura, M.; Hamanaka, N. J. Med. Chem. 1995, 38,
2521; however a protocol for the preparation is not given.
We obtained 3a when following the procedure for the
synthesis of N-allylglycine ethyl ester described in ref. 13
starting from tert-butyl bromoacetate and allylamine.
(29) Maynard, H. D.; Grubbs, R. H. Tetrahedron Lett. 1999, 40,
4137.
(30) Henchey, L. K.; Porter, J. R.; Ghosh, I.; Arora, P. S.
ChemBioChem 2010, 11, 2104.
(31) El-Faham, A.; Albericio, F. J. Pept. Sci. 2010, 16, 6.
(32) Compound 3c is described in ref. 11, however, we preferred
the synthesis via direct allylation of alanine tert-butyl ester
with allyl bromide.
(6) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413.
(7) De Vega, M. J. P.; García-Aranda, M. I.; González-Muniz,
R. Med. Res. Rev. 2011, 31, 677.
(8) Di Cianni, A.; Carotenuto, A.; Brancaccio, D.; Novellino,
E.; Reubi, J. C.; Beetschen, K.; Papini, A. M.; Ginanneschi,
M. J. Med. Chem. 2010, 53, 6188.
(9) Grubbs, R. H.; Blackwell, H. E. Angew. Chem. Int. Ed. 1998,
37, 3281.
Synthesis 2012, 44, 2682–2694
© Georg Thieme Verlag Stuttgart · New York