Synthesis of orthogonally protected optically pure ketopiperazine, diketopiperazine, ketodiazepane, and 3-aminopyrrolidone building blocks for peptidomimetic combinatorial chemistry

Article abstract of DOI:10.1016/j.tet.2008.12.046

A simple and convenient synthesis of orthogonally protected multi-tethered, optically pure 2-ketopiperazine, diketopiperazine, 2-ketodiazepane and 3-aminopyrrolidone scaffolds for Fmoc combinatorial chemistry has been developed. It utilizes accessible chi

Full text of DOI:10.1016/j.tet.2008.12.046

Tetrahedron 65 (2009) 1389–1396
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Synthesis of orthogonally protected optically pure ketopiperazine,
diketopiperazine, ketodiazepane, and 3-aminopyrrolidone building blocks
for peptidomimetic combinatorial chemistry
,
a
,
,
Gary Gellerman a , Eran Hazan b, Marina Kovaliova b, Amnon Albeck b, Shimon Shatzmiler a
*
a Department of Biological Chemistry, Ariel University Center of Samaria, PO Box 3, 40700 Ariel, Israel
b Department of Chemistry, Bar-Ilan University, Ramat Gan, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2008
Received in revised form 23 November 2008
Accepted 11 December 2008
Available online 24 December 2008
A simple and convenient synthesis of orthogonally protected multi-tethered, optically pure 2-ketopi-
perazine, diketopiperazine, 2-ketodiazepane and 3-aminopyrrolidone scaffolds for Fmoc combinatorial
chemistry has been developed. It utilizes accessible chiral amino acid precursors, sequentially applying
reductive alkylation, dipeptide coupling and regioselective ring formation as key steps. These scaffolds
are expansion of our ‘pool of privileged building blocks’ and can introduce valuable drug-like properties
in three independent directions to any medicinally relevant piperazine-, diazepane- and pyrrolidone-
based motif by ‘around-the-scaffold’ drug optimization. The synthetic routes reported in this work are
general and applicable for the preparation of a diverse library of scaffolds, controlling chirality, arm
position and length as well as the nature of functional moieties at the arms for further diversification in
three independent directions. In addition, these building blocks have a wide application scope in
managing fast and efficient multi-cyclic optimization processes in the combinatorial chemistry and drug
design fields.
Keywords:
‘Around-the-scaffold’ drug optimization
Orthogonal protection
Boc/Fmoc strategy
Reductive alkylation
Solid Phase Organic Chemistry (SPOC)
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
homogenous chemistry.2,3 Yamamoto and coworkers have de-
scribed the utilization of 3,4,5-trifluorobenzeneboronic acid as an
Piperazines, diazepanes, and their keto analogs are amongst the
most important backbones in today’s drug discovery. In recent
years combinatorial chemistry around these core structures has
become an important tool for generation of new lead structures in
drug discovery processes and for providing access to diverse
chemical entities with novel structures and properties.1 These
templates are therefore defined in medicinal chemistry as ‘privi-
leged scaffolds’dmolecular backbones with versatile binding
properties representing a frequently occurring binding motif, and
providing potent and selective ligands for a wide range of biological
targets.2
active amidation catalyst in the formation of chiral but symmetrical
diketopiperazines (DKPs),3c while Brase and coworkers have
employed MeOPCl2 in a stoichiometric amount as a coupling re-
agent in the synthesis of symmetrical and non-symmetrical chiral
DKPs containing five-membered ring amino acids.3d Other ap-
proaches rely on stereospecific alkylation of DKP lithium enolates.3e,f
This particular method involves necessary bulky groups on the DKP
structure, which restricts the fictionalization of the scaffold. How-
ever, the majority of the methodologies are not stereospecific and
complex mixtures of stereoisomers are generated.4
One of the most fascinating methods is multicomponent re-
actions (MCRs) that seem to be particularly well suited to assemble
piperazines, introducing extremely high diversity ‘around the
scaffold’.4 On the other hand, it, too, is not stereospecific with
regard to the carbons in the piperazine template, and therefore it
generates mixtures of stereoisomers.
The high number of positive hits revealed in biological screens
with the above mentioned scaffolds urged chemists to develop
different synthetic methods that allow fast and efficient building of
these heterocyclic systems on solid support as well as by
We have recently introduced a new strategy to overcome this
obstacle and to prepare the ‘privilege’ backbone in optically pure
form, bearing various tethers with orthogonally protected groups,
which could be further manipulated via Solid Phase Organic
Chemistry (SPOC).5 Such an approach preserves the important
chiral centers and the related diversity in potential hits. It was
applied in a facile and convenient synthesis of ketopiperazine and
Abbreviations: Alloc, allyloxycarbonyl; Boc, tert-butyloxycarbonyl; Cbz, benzyl-
oxycarbonyl; DCM, dichloromethane; DKP, diketopiperazine; Fmoc, 9-fluo-
renylmethoxycarbonyl; NMM, N-methyl morpholine; PE, petrol ether; SPOC, Solid
Phase Organic Chemistry.
* Corresponding author. Tel.: þ972 3 937 1442; fax: þ972 3 906 6634.
E-mail address: garyg@ariel.ac.il (G. Gellerman).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.12.046

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G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
libraries with high-resolution coverage of medicinal space around
these privileged motifs.
P2
P2
X
P2
X
O
X
O
O
n(H2C)
(CH2)n
(CH2)n
The methodology presents some unique features: we describe
representative syntheses of several scaffolds, demonstrating the
potential of our approach to diversify the piperazine, diazepane,
and pyrrolidone templates; the ring formation process can provide
unusual macrocyclic systems; the tethered amines are orthogonally
protected while a carboxylic group is used without protection,
ready for loading on the resin for further SPOC manipulations.
N
HN
HN
m(H2C)
Y
N
N
N
3
(CH2)k
m(H2C)
1 Y
P
P3
P
Z
O
P3
(CH2)m
P1
Y
1
P
I
II
III
2
P
NH
P2
X
O
2. Results and discussion
X,Y,Z = NH, CO2
O
n(H2C)
N
N
P1,2,3 = Cbz, Boc, Alloc, Fmoc, H
k,m,n = 1-4
*
N
n(H2C)
X
2.1. Optically pure Alloc/Fmoc orthogonally protected
ketopiperazine 1
COOH
3
P
(CH2)m
Y
1
P1
P
V
IV
Our preliminary studies on the synthesis of 2-ketopiperazine
5
and DKP building blocks encouraged us to prepare additional
Scheme 1. General structures of orthogonally protected optically pure keto- and
Fmoc/Alloc protected analogs of this type under optimized condi-
tions. For 2-ketopiperazines, we successfully employed a simple
reductive alkylation reaction7 between the optically pure orthog-
onally protected ornithinal 6 and optically pure (S)-glutamic acid
diester (Scheme 2) to furnish the reduced dipeptide intermediate 7
(Scheme 2) for Fmoc chemistry.
diketopiperazine, 2-ketodiazepane, and 3-aminopyrrolidone building blocks.
DKP scaffolds of types I and II (Scheme 1) tethered with orthogo-
nally protected arms ready for Fmoc and Boc SPOC ‘around the
scaffold’.
In this article, we report on the expansion of our repertoire of
‘privileged’ core structures, describing novel syntheses of 2-keto-
piperazine, 2-ketodiazepane, and 3-aminopyrrolidone scaffolds of
the general structures III, IV, and V, respectively (Scheme 1), and
the preparation of additional DKP and 2-ketopiperazine scaffolds of
types I and II. Noteworthy, I differs from III by positioning of the
arms at the piperazine template, further emphasizing the versa-
tility of our approach for diversification of the chosen core. These
chiral building blocks can be applied in ‘around-the-scaffold’
modification strategy and efficient multi-cyclic optimization pro-
cesses by SPOC in Fmoc (Fmoc/Alloc/Cbz protection) mode, in-
troducing valuable physico-chemical properties in three
independent diversity points and yielding compounds amenable to
the Lipinski rule of five and ADMETox requirements. Being small
and relatively constrained scaffolds with three hydrophilic groups
like amine and carboxyl, structures III–V, in the same manner as
their antecedents I and II, comprise a favorable source for genera-
tion of new bioavailable compounds.6 Furthermore, being chiral
and controllable in the length and the nature of the side arms, our
scaffolds will likely yield piperazine, diazepane, and pyrrolidone
In light of this need, we initially synthesized ornithinal 6 car-
rying
starting from the commercially available (S)-ornithine counterpart.
Thus, Boc- -Orn(Alloc)-OH was converted into the corresponding
a-Boc and u-Alloc orthogonal protecting groups (Scheme 2),
L
hydroxamate (Weinreb amide) by reaction with N,O-dimethylhy-
i
droxylamine hydrochloride8 in the presence of BuOCOCl/NMM.
Subsequent reduction of the Weinreb amide with LiAlH4 at rt in dry
THF9 led to the formation of aldehyde 6 (78% yield for the two
steps). (S)-Glutamic acid diester was then reductively alkylated
with 6 in methanol containing 1% acetic acid in the presence of
7,10
NaBCNH3
to form pseudodipeptide 7 (84% yield). The pseudo-
dipeptide, with the reduced amide bond [CH2NH], was protected by
Fmoc-Cl in DCM in the presence of TEA, yielding fully protected
pseudodipeptide 8 (Scheme 2),11 which after acidic tBu/Boc removal
was submitted to cyclization toward the ketopiperazine structure.
Thus, the deprotected dipeptide intermediate from 8 was cyclized
under optimized diketopiperazine cyclization conditions by reflux
in 2-butanol12 in the presence of 2 equiv of NMM in order to release
the
a
-amine from its TFA salt. Apparently, the absence of toluene
CO2tBu
CO2tBu
H N CO Me
(S)
2
2
a,b
(78%)
(S)
Boc-HN
CHO
(s)
Boc-HN
(S)
Boc-HN
CO2H
(S)
c
N
H
CO2Me
(84%)
(CH2)3
(CH2)3
(CH2)3
NH-Alloc
6
NH-Alloc
7
NH-Alloc
CO2H
CO2tBu
CO2Me
3
(S)
4
N
Fmoc
O
e,f
d
Boc-HN
(s)
(81%)
(88%)
(S)
(R)
N
HN
1
6
(CH2)3
Fmoc
(CH2)3
NH-Alloc
NH-Alloc
1
8
i
Scheme 2. Synthesis of optically pure Alloc/Fmoc orthogonally protected ketopiperazine 1. (a) NH(OMe)Me, BuOCOCl, NMM, THF, rt, 18 h; (b) LiAlH4, THF, rt; (c) NaBH3CN, AcOH,
MeOH, 1:99, rt; (d) Fmoc-Cl, TEA, DCM, 0 ꢀC to rt, overnight; (e) TFA, DCM, 1:1, 0 ꢀC to rt, 2 h; (f) TEA, 2-butanol, reflux, 18 h.

G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
1391
CO2H
Boc-HN (S)
NH-Alloc
(CH2)3
NH-Alloc
(H2C)3
(CH2)4
HO2C
HN
3
(S)
4
N
O
O
CO2H
(S)
H2N
CO2Me
(s)
(S)
CO2Me
NH-Fmoc Boc-HN
a
c,d
(68%)
(S)
(S)
6
N
CO2Me
HN
O
b
(75%)
1
(76%)
(CH2)3
(CH2)3
CO2H
(CH2)4
(CH2)4
NH-Fmoc
2
NH-Alloc
NH-Alloc
9
NH-Fmoc
10
Scheme 3. Synthesis of optically pure Alloc/Fmoc orthogonally protected diketopiperazine 2. (a) HCOCO2H, NaBH3CN, AcOH, MeOH, 1:99, rt, 24 h; (b) HATU, TEA, DMF, 60 ꢀC, 6 h;
(c) 4 N HCl in dioxane, 0 ꢀC, 24 h; (d) NMM, 2-butanol, reflux, 10 h.
afforded ketopiperazine building block 1 in better yields (81%) after
using 4 N HCl in dioxane, and after evaporation of the solvent the
resulted hydrochloride was subjected to the ring closure attempts
without further purification.
purification by flash chromatography (5% MeOH/EtOAc).5,13,14
Noteworthy, 10 already possesses the required orthogonal pro-
tection before the final cyclization to the corresponding 2,5-dike-
topiperazine, significantly simplifying the entire synthetic route.
Finally, after refluxing in 2-butanol in the presence of an equivalent
amount of NMM, the crude desired diketopiperazine was obtained.
Due to its inability to precipitate upon continuous cooling, the
crude compound was purified by flash chromatography (5% MeOH/
EtOAc), to give pure 2 in 68% yield.17 A small amount of a DKP
2.2. Fmoc/Alloc orthogonally protected optically pure
diketopiperazine (DKP) 2
Advancing to our goal, namely the formation of a pool of scaf-
folds for ‘around-the-scaffold’ combinatorial chemistry, we pre-
pared a non-symmetrical diketopiperazine (DKP) scaffold, similarly
amenable to our library generation concept. The synthetic route is
simple and applicable for the synthesis of chiral DKPs with various
arm positioning, length, and nature of functional moieties at the
end of the arms. In this particular case our chiral DKP building block
2 bears AllocNH propyl (generated from Orn), FmocNH butyl
(generated from Lys), and methylene carboxylic acid (generated
from glyoxylic acid) arms.
byproduct, without the 6-u-Fmoc protection, was also isolated
(iPrOH/AcOH/H2O (90:8:2), 7%). This side reaction was previously
observed5 and attributed to the rather vigorous conditions for
obtaining the Fmoc protected DKPs. Due to the insignificant yield of
the byproduct in this particular case, it was not subjected to Fmoc
reprotection.
Analogous to ketopiperazines, the most convenient mode to
initiate the synthesis was a straightforward reductive alkylation of
accessible precursor H-(S)-Orn(Alloc)-OMe15 with glyoxylic acid,
which constructed a side chain bearing a carboxyl functionality in
2.3. Optically pure Fmoc/Alloc protected ketopiperazines
3 and ketodiazepane 4
the final DKP scaffold. Thus, the Na-carboxymethyl
u-protected
ornithine 9 (Scheme 3) was prepared from these precursors using
NaCNBH3 in 1% of AcOH in methanol in 75% yield (estimated by
HPLC). Compound 9, with secondary amine, was then subjected to
the difficult coupling with Boc-(S)-Lys(Fmoc)-OH. Upon heating for
6 h at 60 ꢀC with HATU in DMF,5,16 dipeptide 10 was afforded in 58%
(Scheme 3). Subsequently, the obtained dipeptide was deprotected
Our approach for the synthesis of 2-ketopiperazines 3a,b and
2-ketodiazepane 4 building blocks is also based on a simple
reductive alkylation reaction between the optically pure ortho-
gonally protected lysinal 11 or ornitinal 6 and glycine tert-butyl
ester (Scheme 4). This convenient and useful reaction is a key step
in the synthesis of almost all of our intermediates toward the
Scheme 4. Synthesis of Alloc/Boc and Alloc/Fmoc orthogonally protected ketopiperazines 3 and ketodiazepane 4. (a) Na(AcO)3BH, DCE, 4 Å MS, N2, 0 ꢀC, overnight; (b)
ClCO(CH2)mBr, aq EtOAC, NaHCO3, 0 ꢀC, 20 min; (c) CsCO3, DMF, 65 ꢀC, 2 h; (d) TFA/DCM, 1 h; (e) Fmoc-Cl, DIPA, DCM, 1:1, 0 ꢀC, overnight.

1392
G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
S
H2N
CO2Me
(S)
S
H
N
CO2Me
(s)
a
b
(s)
Boc-HN
(CH2)4
(84%)
O
Boc-HN
I-
CO H
(S)
2
(CH2)4
NH-Cbz
15
NH-Cbz
S
Boc-HN
Fmoc-HN
d,e
(81%)
(s)
(s)
N
H
N
CO2H
N
CO2H
(S)
(S)
CO2Me
(s)
c
(s)
Boc-HN
O
O
(55% from 15)
O
(CH2)4
NH-Cbz
(CH2)4
(CH2)4
16
5a
5b
NH-Cbz
NH-Cbz
Scheme 5. Synthesis of Cbz/Boc and CBz/Fmoc orthogonally protected aminopyrrolidone 5. (a) DIC, HOBT; (b) CH3I; (c) NaH, DMF/DCM 1:1; (d) HCO2H, 98%; (e) Fmoc-Cl, DIEA,
DCM.
‘privileged scaffolds’, nicely reflected in the yields of the final
compounds.
the possibilities for neuroprotection after stroke and their use as
antiepileptic agents.18 This information encouraged us to develop
synthesis for aminopyrrolidone building blocks suitable for Fmoc
combinatorial chemistry.
An intramolecular cyclative alkylation route, similar to that used
for the synthesis of 3 and 4, was applied to the synthesis of the
2-aminopyrrolidone scaffold 5 (Scheme 5). Protected methionine
dipeptide 15 was chosen as a starting material since the methylthio
function can potentially be converted into a leaving group. This
dipeptide was prepared by a standard method from (S)-Boc-me-
thionine and (S)-lysine-(Cbz) methyl ester in optically pure form
(84% yield). Sulfonium salt 16 was formed by alkylation with methyl
Lysinal 11 was synthesized similar to 6 (Scheme 2) starting
from the commercially available doubly protected (S)-lysine.5
Glycine tert-butyl ester was then reductively alkylated with 11
and 6 in dichloroethane in the presence of NaB(OAc)3H7,10 to form
dipeptides 12a (84% yield) and 12b (88% yield) correspondingly.
Other reductive conditions like NaBH3CN in 1% AcOH/MeOH led
to lower yields. The obtained pseudodipeptides with the reduced
amide bond [CH2NH] were acylated by 2-bromo acetyl, 3-bromo
propionyl, and 6-bromo hexanoyl chlorides in ethyl acetate/
NaHCO3 (aq), yielding the corresponding 13a–d (Scheme 4),11
which were subjected to the cyclization reactions toward the
ketopiperazine and related structures. Despite the presence of
two alkylating moieties on 13 (NHBoc and NHAlloc), we antici-
pated to obtain regioselective cyclization toward the thermody-
namically more favorable smaller ring, namely through the
NHBoc. Indeed, after utilizing several alkylating conditions
(heating 13 with Cs2CO3 in DMF as the best choice), regiose-
lectivity was obtained for 13a–c, yielding the desired Boc/Alloc
protected ketopiperazine 3a,b and ketodiazepane 4a in reason-
able yields (86% for 3a, 82% for 3b, and 60% for 4a). All attempts to
cyclize 13d into 10-membered 1,4-diazecan-5-one analog (14)
were unsuccessful, probably due to thermodynamic restrictions to
form large rings.
Noteworthy, scaffolds 3a,b and 4a bear Boc/Alloc protection,
suitable for Boc chemistry. Thus, we decided to also prepare the
corresponding scaffolds with orthogonal protection adapted to
Fmoc chemistry, namely Fmoc on the secondary ring amine and
Alloc on the side chain primary amines (Scheme 4). This would
demonstrate the versatility of protecting strategies that can be
applied in our structures using the above synthetic method. Stan-
dard Boc/tBu removal from 3a,b and 4a (1:1 mixture of TFA in DCM,
30 min at 0 ꢀC, then 30 min at rt) led to reddish intermediates,
which were used in the next step without purification. Finally, the
above intermediaries were Fmoc reprotected (Fmoc-Cl, in DCM,
overnight in the presence of diisopropylethyl amine), yielding the
desired ketopiperazines 3c,d and ketodiazepane 4b scaffolds in
reasonable yields (72%, 77%, and 83%, respectively) after flash
chromatography purification (5% MeOH/EtOAc).
iodide, and cyclization to the corresponding g-lactam 5a was in-
duced by sodium hydride in 1:l DMF/methylene chloride in 55%
yield from 15.4 Other bases like lithium diisopropylamide (LDA) and
dimethyaminopyridine (DMAP) in THF were not effective.19 Com-
petitive amide O-alkylation under these conditions was not ob-
served by NMR spectroscopy and HPLC. Interestingly, under these
conditions of the cyclization, the methyl ester is almost completely
hydrolyzed yielding the free acid 5a as the major isolated product.
This change is comparable to the rate of disappearance of a tran-
sient TLC spot of higher Rf observed in the cyclization reactions. The
source of this ester cleavage was previously investigated and
addressed to the presence of Na2O contamination in the commer-
cial NaH.19 In fact, acid 5a is already Boc/Cbz orthogonally protected
2-aminopyrrolidone scaffolds suitable for Boc SPOC. In the next
step we converted 5a to the corresponding Fmoc analog. Thus, 2-
aminopyrrolidone 5a was deprotected using formic acid for 2 h, and
after evaporation of the solvent the resulted residue was subjected
to Fmoc protection as for 1 and 2 without further purification. The
crude product was purified by flash chromatography (1–5% MeOH/
EtOAc), to give pure 5b in 81% yield as Fmoc/Cbz orthogonally
protected 2-aminopyrrolidone building block applicable in Fmoc
SPOC.
Development of the general synthetic approaches for protect-
able tethering of core structures with various functional arms at
various positions as well as controlling stereochemistry is a valu-
able capability for drug design by combinatorial chemistry. The
work presented in this paper contributes to this endeavor. Attempts
to facilitate cyclization into other ring systems and optimization of
the reaction conditions, including microwave assisted chemistry,20
are in progress.
2.4. Fmoc/Cbz orthogonally protected optically pure
2-aminopyrrolidone scaffold 5
3. Conclusions
The pyrrolidone (2-oxopyrrolidine) family has been the subject
of research for more than three decades. Experimental and clinical
studies first focused on their so-called nootropic effects, later came
In this paper we have introduced a simple and convenient
preparation of orthogonally protected multi-tethered, optically

G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
1393
pure ketopiperazine, DKP, 2-ketodiazepane, and 3-amino-
pyrrolidone structural elements for Boc and Fmoc combinatorial
chemistry.
These medicinal cores of the general structures I–V are ex-
pansion of our ‘pool of privileged building blocks’ useful for
‘around-the-scaffold’ modification strategy by SPOC, introducing
valuable physico-chemical properties at three independent di-
versity points.
The key step of the synthesis involves standard reductive al-
kylation between optically pure amino acid synthons that bear
suitable protecting groups for Boc solid phase synthetic method-
ologies. Subsequent ring closure in the intermediate compounds,
followed by Boc to Fmoc change of protection, afforded the desired
scaffolds for Fmoc SPOC. Being optically pure, relatively constrained
and controllable in stereochemistry and the length and the nature
of the side arms, our scaffolds generate piperazine, diazepane, and
pyrrolidone-based libraries with high-resolution coverage of me-
dicinal space around the chosen motif, improving the selectivity
properties of revealed hits. In addition, our approach enables to
manage fast and efficient multi-cyclic optimization processes
around ‘privileged’ templates for the discovery of novel peptido-
mimetic bioactive compounds.
residue was extracted with ether (4ꢁ40 mL). The combined organic
layers were washed with 1 N HCl solution (3ꢁ40 mL), satd NaHCO3
(40 mL), and brine (40 mL), and then dried (MgSO4). Evaporation of
the solvent gave crude aldehyde 4 as colorless oil (1.58 g, 88% yield),
which was used in the next step without further purification.
Rf¼0.75 (EtOAc/PE, 1:1); MS m/z 323 (MNaþ, 100); nmax (KBr): 1710,
1655, 1280 cmꢂ1 1H NMR
; d 9.05 (br s, 1H), 5.75 (m, 1H), 5.40 (dd,
J¼10, 2 Hz, 1H), 5.15 (dd, J¼16, 2 Hz, 1H), 4.52 (d, J¼5 Hz, 2H), 4.17–
4.14 (m, 2H), 2.29–2.26 (m, 2H), 1.9–1.5 (m, 2H), 1.42 (s, 9H), 1.15–
1.12 (m, 2H).
4.1.2. Boc-(S)-Orn-(Alloc)-(S)-Glu-(tBu)-OMe pseudodipeptide
diester (7)
H-(S)-Glu(tBu)-OMe (1.04 g, 5 mmol) was added to 30 mL of
MeOH/AcOH (99:1) until the solution became clear. Then aldehyde
6 (1.42 g, 5 mmol) was added in 10 mL of MeOH/AcOH (99:1), the
reaction mixture was stirred for 1 h at rt, followed by portion wise
addition of NaCNBH3 (0.39 g, 6.5 mmol). When TLC showed com-
plete conversion of the starting materials, the solvent was evapo-
rated in vacuo and the residue was used in the next step without
any purification. The yield of conversion was estimated by calcu-
20
lation of the area under a peak by HPLC (1.38 g, 84% yield). [
a
]
D
þ23
(c 1.6, CHCl3); MS m/z 502 (Mþ, 100); nmax (KBr): 1670, 1650, 1320,
4. Experimental section
4.1. General
1210 cmꢂ1 1H NMR
;
d
5.72 (m, 1H), 5.33 (dd, J¼10, 2 Hz, 1H), 5.22
(dd, J¼16, 2 Hz, 1H), 4.33 (d, J¼5 Hz, 2H), 4.26–4.22 (m, 2H), 3.61 (s,
3H), 2.94–2.91 (m, 4H), 2.23–2.18 (m, 4H), 1.9–1.5 (m, 8H), 1.50 (s,
9H), 1.46 (s, 9H), 1.17–1.14 (m, 2H).
Analytical HPLC was performed on a 250ꢁ4.2 mm Lichroprep
RP-18 column from Merck, with a 1 mL/min flow and detection at
214 nm. The eluents were triply distilled water and HPLC-grade
CH3CN (containing 0.1% TFA) or MeOH. Optical rotations were
4.1.3. Fmoc protected Boc-(S)-Orn (Alloc)-(S)-Glu(tBu)-OMe
pseudodipeptide (8)
Compound 7 (1.32 g, 2.65 mmol) was taken in 30 mL DCM. The
reaction mixture was stirred for a few minutes, cooled, and TEA
(1.5 mL, 15 mmol) was added, followed by addition of Fmoc chlo-
ride (0.59 g, 2.65 mmol). After stirring overnight, DCM (40 mL) was
added and the organic phase was washed sequentially twice with
cold 0.5 N HCl and brine, and dried over Na2SO4. After filtration, the
solvent was evaporated and the residue was chromatographed
recorded at 25 ꢀC in a 10-cm length cell and [
a]D values are given in
units of 10ꢂ1 deg cm2/g. The concentration of all the samples was
0.5%. Mass spectra were measured in the positive and negative
modes using a quadrupole mass spectrometer equipped with an
electrospray ionization source and cross-flow inlet. 1H and 13C NMR
spectra were recorded at 300 and 75 MHz, respectively, in CDCl3,
unless otherwise indicated. Assignments in the final products were
supported by 2D COSY, TOCSY, NOESY, ROESY, HMBC, and HMQC
spectroscopy. All chemical shifts are reported with respect to TMS.
Chromatography was carried out by standard flash chromatogra-
phy and TLC on silica gel (Merck 7735).
(EtOAc/PE, 1:1) to afford 1.82 g (88% yield) of pure 8 as a colorless
20
oil. Rf¼0.70 (EtOAc/PE, 1:2.5), [
a
]
þ19 (c 1.7, CHCl3); HRMS m/z
D
624.3783 (100%) (MHþꢂBoc, calculated 723.3731 for C39H53N3O10),
nmax (KBr): 1685, 1660, 1220, 1070 cmꢂ1; 1H NMR
d
7.80 (d, J¼8 Hz,
2H), 7.63 (d, J¼8 Hz, 2H), 7.44 (t, J¼8 Hz, 2H), 7.40 (t, J¼8 Hz, 2H),
5.75 (m, 1H), 5.39 (dd, J¼10, 2 Hz, 1H), 5.22 (dd, J¼16, 2 Hz, 1H),
4.52–4.49 (m, 2H), 4.45 (d, J¼5 Hz, 2H) 4.35 (t, J¼6 Hz, 1H), 4.03–
3.98 (m, 2H), 3.84 (s, 3H), 2.95–2.90 (m, 4H), 2.24–2.19 (m, 4H), 1.9–
1.5 (m, 8H), 1.55 (s, 9H), 1.42 (s, 9H), 1.13–1.10 (m, 2H). 13C NMR
4.1.1. Boc-(S)-Orn-(Alloc)-H (6)
Compound 9 was prepared by optimized literature procedure:5
to
a solution of commercial Boc-(S)-Orn-(Alloc)-OH (3.53 g,
12 mmol) in DCM (50 mL) was added N-methyl morpholine
(2.6 mL, 24 mmol). The mixture was cooled to ꢂ15 ꢀC, and isobutyl
chloroformate (1.81 g, 12 mmol) was added. After 15 min at this
temperature, N,O-dimethylhydroxylamine hydrochloride (1.02 g,
14 mmol) was added. The mixture was stirred at ꢂ15 ꢀC for 1 h,
allowed to warm to rt, and stirred for additional 3 h. The reaction
mixture was poured into H2O (40 mL), and the aqueous phase was
extracted with DCM (2ꢁ40 mL). The combined organic extracts
were dried over Na2SO4 and the solvent was removed in vacuo to
give crude hydroxamate, which after purification by flash chro-
matography (50% EtOAc/PE) yielded pure intermediate hydrox-
amate (3.80 g, 90%) as a colorless oil. Rf¼0.35 (EtOAc/PE,1:1); MS m/
d 179.1, 170.3, 168.5, 166.2, 157.0, 155.4, 143.7, 141.8, 131.8, 127.3,
127.3, 124.2, 118.3, 118.3, 78.9, 77.3, 67.5, 67.2, 65.9, 59.4, 47.3, 44.0,
42.5, 40.3, 33.2, 29.4, 29.0, 28.2, 23.6, 21.9.
4.1.4. Fmoc/Alloc orthogonally protected 2-ketopiperazine
carboxylic acid (1)
Compound 8 (1.16 g, 1.36 mmol) was carefully dissolved in ice
bath cooled TFA in DCM (1:1, 30 mL). The reaction mixture was left
to warm to rt and after 2 h the solvent was removed by repeated
evaporation with DCM (50 mL, three times) in vacuum. To the
resulting viscous residue were added 2-butanol (40 mL) and NMM
(0.14 mL, 1.36 mmol). The resulting reaction mixture was refluxed
overnight. The solvent was evaporated to afford an oily residue,
z 382 (MNaþ, 100); 1H NMR
d
5.91 (m, 1H), 5.36 (dd, J¼10, 2 Hz, 1H),
5.20 (dd, J¼16, 2 Hz, 1H), 4.48 (d, J¼5 Hz, 2H), 4.35–4.32 (m, 2H),
3.62 (s, 3H), 2.75 (s, 3H), 2.21–2.18 (m, 2H), 1.9–1.5 (m, 2H), 1.42 (s,
9H), 1.10 (m, 2H). To the solution of the hydroxamate (2.57 g,
7 mmol) in THF (100 mL) LiAlH4 (0.53 g, 14 mmol) was added in
several portions and the reaction mixture was stirred under N2 for
40 min in an ice bath. A solution of KHSO4 (1.35 g) in 30 mL H2O
was added, and THF was removed under reduced pressure. The
which after purification gave 0.77 g (81%) of 1 as a colorless oil;
20
[a
]
þ30 (c 1.7, CHCl3); HRMS m/z 521.2478 (MHþ, calculated
D
521.2162 for C30H35N3O7); nmax (KBr): 3500–3100 (br s), 1700, 1650,
1430,1120 cmꢂ1; 1H NMR (DMSO-d6):
d
7.61 (d, J¼8 Hz, 2H), 7.54 (d,
J¼8 Hz, 2H), 7.36 (t, J¼8 Hz, 2H), 7.22 (t, J¼8 Hz, 2H), 6.30 (m, 1H),
5.30 (dd, J¼12, 10 Hz, 1H), 5.24 (br d, 1H), 4.47 (s, 2H), 4.23 (t,
J¼6 Hz, 1H), 4.15–4.12 (m, 2H), 2.68–2.63 (m, 4H), 2.14–2.09 (m,

1394
G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
20
4H), 1.8–1.4 (m, 8H), 1.28–1.25 (m, 2H); 13C NMR
168.0,165.2, 154.3, 142.3, 141.4, 131.2, 129.3, 126.3, 125.2, 117.5, 117.0,
67.0, 66.5, 65.1, 46.7, 44.3, 42.7, 33.9, 28.4, 28.0, 24.6, 22.9.
d
175.7, 168.3,
(2.03 g, colorless oil, 88% yield). [
a]
þ16 (c 1.5, CHCl3); MS m/z 416
D
(Mþ, 60), 316 (MþꢂBoc, 100); nmax (KBr): 1650, 1640, 1575,
1020 cmꢂ1
;
1H NMR
d
5.70 (m, 1H), 5.37 (dd, J¼10, 2 Hz, 1H), 5.23
(dd, J¼16, 2 Hz, 1H), 4.33 (d, J¼5 Hz, 2H), 4.23–4.19 (m, 2H), 2.91–
2.86 (m, 4H), 2.11–2.08 (m, 2H), 1.9–1.5 (m, 4H), 1.51 (s, 9H), 1.48 (s,
9H), 1.18–1.15 (m, 2H).
4.1.5. N-(Methylenecarboxy)-(S)-Orn-(Alloc)-OMe (9)
Compound 9 (colorless oil) was prepared from commercial
H-(S)-Orn-(Alloc)-OMe (2.44 g, 10 mmol) and glyoxylic acid mon-
ohydrate (0.92 g, 10 mmol) by the same procedure as for 7 in 75%
yield (HPLC conversion). The residue was used in the next step
without any purification. MS m/z 289 (Mþ, 100); nmax (KBr): 1690,
4.1.10. Boc-(S)-Lys-(Alloc)-(NHCOCH2Br)-Gly-O-tBu pseudo-
dipeptide ester (13a)
A solution of 2-bromo acetyl bromide (1.2 g, 6 mmol) in EtOAc
(10 mL) was added dropwise at 0 ꢀC to a stirring mixture of 12a
(2.15 g, 5 mmol) in EtOAc and 1 N NaHCO3 (4:1, 50 mL). After 2 h at
0 ꢀC, the mixture was diluted with EtOAc (50 mL) and saturated
solution of NaHCO3 (50 mL) was added. The organic phase was
collected and washed with H2O (20 mL) and brine (20 mL), dried
over Na2SO4, and concentrated. The crude product was filtered over
a silica gel column to afford compound 12a (colorless oil), which
was used without further purification. Rf¼0.60 (EtOAc). HRMS m/z
550.205 (100.0%), 552.204 (90%) (MHþ, calculated 550.2050 and
552.2029 for C23H40BrN3O7); nmax (KBr): 1660, 1650, 1320,
1680; 1645, 1310 cmꢂ1
10 Hz, 1H), 5.18 (br d, 1H), 4.14–4.11 (m, 2H), 3.98 (s, 3H), 2.99 (br s,
2H), 2.39–2.35 (m, 2H), 1.8–1.4 (m, 3H), 1.27–1.25 (m, 2H).
;
1H NMR
d
6.34 (m, 1H), 5.38 (dd, J¼12,
4.1.6. Na-Boc-(S)-Lys-(Fmoc)-N(0)a-(CH2CO2H)-(S)-Orn-(Alloc)-
OMe (10)
Compound 10 was synthesized from 9 (4.06 g, 14 mmol) and
commercial Boc-(S)-Lys-(Fmoc)-OH (6.52 g, 14 mmol) in DMF by
heating at 60 ꢀC with HATU (6.35 g, 16.8 mmol) and DIEA (5.7 mL,
50 mmol) for 6 h. After evaporation of the solvent, the residue was
taken in DCM (100 mL) and washed twice with 1 N citric acid and
1015 cmꢂ1
;
1H NMR
d
5.90 (m, 1H), 5.46 (dd, J¼10, 2 Hz, 1H), 5.34
brine. Purification by flash chromatography (EtOAc) gave pure 10
(dd, J¼16, 2 Hz, 1H), 4.45 (s, 2H), 4.26 (d, J¼5 Hz, 2H), 4.12–4.09 (m,
2H), 2.88–2.83 (m, 4H), 2.22–2.18 (m, 4H), 1.9–1.5 (m, 6H), 1.45 (s,
9H), 1.40 (s, 9H).
20
(7.15 g, yield 76%) as colorless oil. Rf¼0.35 (EtOAc) [
a]
þ12 (c 1.0,
D
CHCl3); HRMS m/z 737.3445 (Mꢂ, calculated 738.3476 for
C38H50N4O11); nmax (KBr): 1705, 1670, 1640, 1150 cmꢂ1 1H NMR
;
d
7.83 (d, J¼8 Hz, 2H), 7.63 (d, J¼8 Hz, 2H), 7.63–7.48 (m, 9H), 6.26
4.1.11. Boc-(S)-Orn-(Alloc)-(NHCOCH2Br) Gly-O-tBu pseudo-
dipeptide ester (13b)
Compound 13b (colorless oil) was prepared in the same manner
as 13a from the corresponding 12b and 2-bromo acetyl bromide.
Rf¼0.60 (EtOAc). HRMS m/z 536.187 (75.0%), 538.189 (73%) (MHþ,
calculated 536.1893 and 538.1893 for C22H38BrN3O7); nmax (KBr):
(m, 1H), 5.42 (br d, 1H), 5.27 (br d, 1H), 4.80 (s, 2H), 4.56 (s, 2H),
4.28–4.22 (m, 4H), 3.94 (s, 3H), 2.97–2.88 (m, 6H), 2.26–2.19 (m,
4H), 1.9–1.5 (m, 10H), 1.5 (s, 9H), 1.32–1.14 (m, 8H).
4.1.7. Fmoc/Alloc orthogonally protected 2,5-DKP carboxylic acid (2)
Compound 10 (1 g, 1.38 mmol) was subjected to Boc removal in
4 N HCl dioxane (40 mL) at 0 ꢀC for 24 h, and then the solvent and
the excess HCl were removed by repeated evaporation with di-
oxane (3ꢁ20 mL). The resulting hydrochloride was cyclized into the
corresponding DKP carboxylic acid 2 in the same manner as for 1,
1660, 1340, 1030 cmꢂ1; 1H NMR
d
5.90 (m, 1H), 5.45 (dd, J¼10, 2 Hz,
1H), 5.35 (dd, J¼16, 2 Hz, 1H), 4.50 (s, 2H), 4.23 (d, J¼5 Hz, 2H),
4.12–4.09 (m, 2H), 2.75–2.68 (m, 4H), 2.17–2.14 (m, 4H), 1.9–1.5 (m,
4H), 1.47 (s, 9H), 1.40 (s, 9H).
yielding after chromatography (EtOAc) 0.56 g (68% yield) of color-
4.1.12. Boc-(S)-Orn-(Alloc)-(NHCOCH2CH2Br)-Gly-O-tBu
pseudodipeptide ester (13c)
20
less oil. Rf¼0.65 (5% MeOH/EtOAc), [
a]
þ38 (c 1.9, CHCl3); HRMS
D
m/z 593.2840 (MHþ, calculated 593.2413 for C31H36N4O8); nmax
Compound 13c (colorless oil) was prepared in the same manner
as 13a from the corresponding 12b and 3-bromo propionyl chlo-
ride. Rf¼0.60 (EtOAc). HRMS m/z 550.206 (50.0%), 552.203 (46%)
(MHþ, calculated 550.2050 and 552.2029 for C23H40BrN3O7); nmax
(KBr): 3500–3100 (br s),1695,1660,1640,1270,1090 cmꢂ1; 1H NMR
(DMSO-d6):
d
7.54 (d, J¼8 Hz, 2H), 7.43 (d, J¼8 Hz, 2H), 7.40 (t,
J¼8 Hz, 2H), 7.20 (t, J¼8 Hz, 2H), 5.98 (m, 1H), 5.55 (dd, J¼10, 12 Hz,
2H), 5.25 (br d, 2H), 5.11 (s, 2H), 4.40 (br d, 2H), 4.22 (t, J¼6 Hz, 1H),
3.88 (br s, 2H), 3.66 (m, 1H), 2.77–2.70 (m, 4H), 1.5–1.2 (m, 10H); 13C
(KBr): 1660, 1640, 1355, 1085 cmꢂ1 1H NMR
; d 5.90 (m, 1H), 5.45
(dd, J¼10, 2 Hz, 1H), 5.35 (dd, J¼16, 2 Hz, 1H), 4.23 (d, J¼5 Hz, 2H),
4.11–4.09 (m, 2H), 3.62–3.59 (m, 2H), 2.67–2.63 (m, 4H), 2.26–2.18
(m, 6H), 1.9–1.5 (m, 4H), 1.45 (s, 9H), 1.42 (s, 9H).
NMR
d 179.8, 167.1, 165.5, 164.6, 153.2, 143.5, 140.8, 132.0, 125.7,
126.4, 123.5, 119.0, 118.0, 67.0, 64.8, 63.0, 47.0, 46.5, 43.0, 41.9, 40.6,
33.2, 30.2, 27.3, 23.6.
4.1.13. Boc-(S)-Orn-(Alloc)-(NHCO(CH2)5Br)-Gly-O-tBu pseudo-
dipeptide ester (13d)
4.1.8. Boc-(S)-Lys-(Alloc)-Gly-O-tBu pseudodipeptide ester (12a)
Lysinal 11 (1.61 g, 5 mmol) and Gly-O-tBu free base (0.65 g,
5 mmol) were added under N2 atmosphere to 30 mL of dry di-
chloroethane (DCE) in the presence of activated 4 Å molecular
sieves and were stirred for 1 h at 0 ꢀC. Then NaBH(AcO)3 (1.42 g,
7 mmol) was added and the reaction mixture was stirred overnight
at 0 ꢀC. The solvent was evaporated in vacuo and the oily residue
Compound 13d (colorless oil) was prepared in the same manner
as 13a from the corresponding 12b and 6-bromo hexanoyl chloride.
Rf¼0.60 (EtOAc). HRMS m/z 592.253 (90.0%), 594.251 (85%) (MHþ,
calculated 592.253 and 594.250 for C26H46BrN3O7); nmax (KBr):
1650, 1490, 1305, 1035 cmꢂ1; 1H NMR
d
5.95 (m, 1H), 5.45 (dd, J¼10,
2 Hz, 1H), 5.35 (dd, J¼16, 2 Hz, 1H), 4.32 (d, J¼5 Hz, 2H), 4.12–4.08
(m, 2H), 3.54–3.51 (m, 2H), 2.66–2.60 (m, 4H), 2.28–2.20 (m, 6H),
1.9–1.5 (m, 10H), 1.50 (s, 9H), 1.40 (s, 9H).
was used in the next step without any purification. The yield was
20
estimated by HPLC (1.90 g, 84% yield). [
a
]
þ14 (c 2.1, CHCl3); MS
D
m/z 430 (Mþ, 50), 330 (MþꢂBoc, 100); nmax (KBr): 1650; 1630,
1115 cmꢂ1
;
1H NMR
d
5.70 (m, 1H), 5.37 (dd, J¼10, 2 Hz, 1H), 5.23
4.2. General procedure for ring closure of 13a–c to 3a,b and 4a
(dd, J¼16, 2 Hz, 1H), 4.33 (d, J¼5 Hz, 2H), 4.23–4.20 (m, 2H), 2.93–
2.88 (m, 4H), 2.12–2.09 (m, 2H), 1.9–1.5 (m, 4H), 1.51 (s, 9H), 1.48 (s,
9H), 1.17–1.14 (m, 2H).
Compound 13a, 13b or 13c (5 mmol) was dissolved in 30 mL of
dry DMF. Cs2CO3 (10 mmol) was added and the reaction mixture
was heated to 65 ꢀC under N2 atmosphere with vigorous stirring.
After 2 h at this temperature, the solvent was evaporated, the res-
idue was taken into DCM (100 mL), washed twice with 1 N citric
acid (50 mL) and brine (50 mL), and dried over Na2SO4. After
4.1.9. Boc-(S)-Orn-(Alloc)-Gly-O-tBu pseudodipeptide ester (12b)
Compound 12b was prepared in the same manner as 12a from
the corresponding ornithinal 6. The yield was estimated by HPLC

G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
1395
filtration, the solvent was evaporated and the oily residue was
purified by chromatography (EtOAc/PE, 1:1) to give the desired
product.
for
C29H33N3O7); nmax (KBr): 3500–3100 (br s), 1680, 1640,
1110 cmꢂ1; 1H NMR
d
7.78 (d, J¼8 Hz, 2H), 7.57 (d, J¼8 Hz, 2H), 7.35 (t,
J¼8 Hz, 2H), 7.26 (t, J¼8 Hz, 2H), 5.90 (m,1H), 5.60 (br s,1H), 5.33 (br
d, 2H), 5.20 (br d, 2H), 4.56 (br d, 2H), 4.40 (m,1H), 4.20–4.17 (m, 2H),
3.90 (br s, 1H), 3.66 (m, 1H), 3.21–3.17 (m, 2H), 1.5–1.2 (m, 8H); 13C
NMRd175.4,163.0,162.2,160.5,141.4,140.7,133.3,126.4,124.0,122.2,
121.1,118.4, 66.0, 65.5, 63.0, 62.3, 45.8, 45.2, 43.7, 41.3, 40.9, 31.6,19.8.
Compound 3a: 0.67 g colorless oil (86% yield), Rf¼0.8 (EtOAc/PE
20
1:1), [
a]
þ23 (c 1.8, CHCl3); HRMS m/z 470.5798 (MHþ, calculated
D
470.5717 for C23H39N3O7); nmax (KBr): 1670, 1650, 1600, 1235,
1015 cmꢂ1; 1H NMR
d
5.92 (m, 1H), 5.38 (dd, J¼12, 10 Hz, 2H), 5.20
(dd, J¼12, 10 Hz, 2H), 5.11 (br s, 2H), 4.40 (br d, 2H), 3.88 (br s, 2H),
3.66 (m, 1H), 2.71–2.67 (m, 2H), 1.8–1.2 (m, 24H).
4.3.4. Boc-(S)-Met-(S)-Lys-(Cbz)-OMe (15)
Compound 3b: 0.59 g colorless oil (82% yield), Rf¼0.8 (EtOAc/PE
(S)-Lysine-(Cbz)-methyl ester hydrochloride (1 g, 3 mmol),
HOBT (0.4 g, 3 mmol), tert-butyloxycarbonyl-(S)-methionine (0.8 g,
3 mmol), and N,N-diisopropylethylamine (1.05 mL, 6 mmol) were
dissolved in dry THF (15 mL), the solution was cooled in an ice-
water bath, and diisopropylcarbodiimide (0.4 g, 3.15 mmol) was
added. Stirring was continued for 1 h at 0 ꢀC and an additional hour
at rt. The solvent was evaporated in vacuo. A mixture of EtOAc
(15 mL) and satd NaHCO3 (7.5 mL) was added to the residue and the
organic phase was sequentially extracted with 10% citric acid in
water, satd NaHCO3, and water (7.5 mL each). The solution was
dried over anhydrous Na2SO4, filtered, and evaporated to dryness.
The residue was triturated with hexane, filtered, washed with
hexane, and dried. The crude dipeptide derivative (1.80 g,) was
purified by chromatography on basic alumina (EtOAc) (1.46 g, white
solid, 84% yield). nmax (KBr): 1670, 1650, 1420, 1090 cmꢂ1; 1H NMR
20
1:1); [
a]
þ25 (c 2.0, CHCl3); HRMS m/z 456.5501 (MHþ, calculated
D
456.5451 for C22H37N3O7); nmax (KBr): 1670, 1640, 1310 cmꢂ1 1H
;
NMR
d 5.98 (m, 1H), 5.36 (br d, 2H), 5.22 (br d, 2H), 5.10 (br s, 2H),
4.45 (br d, 2H), 3.80 (br s, 2H), 3.70 (m, 1H), 2.61–2.57 (m, 2H), 1.8–
1.2 (m, 22H).
Compound 4a: 0.42 g colorless oil (60% yield), Rf¼0.8 (EtOAc/PE,
20
1:1), [
a]
þ14 (c 2.2, CHCl3); HRMS m/z 470.5786 (MHþ, calculated
D
470.5717 for C23H39N3O7); nmax (KBr): 1670, 1650, 1170 cmꢂ1 1H
;
NMR
d
5.95 (m, 1H), 5.40 (dd, J¼12,10 Hz, 2H), 5.20 (br d, 2H), 5.11
(br s, 2H), 4.40 (br d, 2H), 3.88 (br s, 2H), 3.66 (m, 1H), 2.73–2.70 (m,
2H), 2.19–2.15 (m, 2H), 1.8–1.2 (m, 22H).
4.3. General procedure for the synthesis of 3c,d and 4b
Compound 3a, 3b or 4a (1.36 mmol) was carefully dissolved in
ice bath cooled TFA in DCM (1:1, 30 mL). The reaction mixture was
left to warm to rt and after 2 h the solvent was removed by re-
peated evaporation with DCM (50 mL, three times) in vacuum
giving viscous reddish oil, which was used in the next step without
further purification. In the next step the reddish oil was taken into
50 mL of DCM and 10 mmol of DIEA was added at 0 ꢀC. Fmoc-Cl
(1.2 mmol) was added in small portions and the reaction mixture
was left overnight at rt. Then, additional 50 mL DCM was added and
the organic phase was washed twice with 1 N citric acid, satd
NaHCO3, and brine. After drying over Na2SO4 and subsequent fil-
tration, the solvent was evaporated and the final crude product was
purified by flash chromatography.
d 8.89 (br d, 1H), 7.85 (s, 5H), 5.11 (s, 2H), 3.71 (s, 3H), 3.25–3.22 (m,
2H), 2.5 (t, J¼6 Hz, 2H), 2.1–2.3 (m, 5H), 2.1–1.1 (m, 8H), 1.42 (s, 9H).
4.3.5. Boc-Met-Lys-(Cbz)-methyl ester methylsulfonium iodide (16)
Boc-Met-Lys(Cbz)-OMe (15, 18.7 g) was dissolved in CH3I
(60 mL) and stirred at rt for 3 days. Concentration in vacuo gave an
amorphous yellowish solid (19.1 g, 95% yield). nmax (KBr): 1675,
1650, 1420, 1375, 1200, 1065 cmꢂ1
7.8 (s, 5H), 6.03 (d, J¼7 Hz, 1H), 5.37 (m, 1H), 5.11 (s, 2H), 4.7–4.3
(m, 2H), 3.71 (s, 3H), 3.3–3.0 (m, 2H), 3.27–3.24 (m, 3H), 3.24–3.21
(m, 2H), 3.1 (s, 3H), 2.1–1.1 (m, 8H), 1.42 (s, 9H).
;
1H NMR
d
8.89 (d, J¼7 Hz, 1H),
4.3.6. (S)-3-[(tert-Butoxycarbonyl)amino]-2-oxo-1-pyrrolidine-(S)-
6-[(benzyloxycarbonyl)amino]-2-heptanoic acid (5a)
4.3.1. (S)-2-(5-(Allyloxycarbonylamino)butyl)-4-(2-fluorenyl-2-
oxoethyl)-5-oxopiperazine-1-carboxylic acid (3c)
Sulfonium salt 16 (10 g, 15.3 mmol) was dissolved in 300 mL of
1:l DMF–CH2C12 under N2 and cooled to 0 ꢀC. NaH (1.5 g of a 50%
mineral oil suspension, 31.5 mmol) was added at once, and the
mixture was stirred at 0 ꢀC for 2.5 h. Ethyl acetate (100 mL) fol-
lowed by water (24 mL) was added, and the resultant solution was
left overnight at rt. The solution was concentrated in vacuo to
a small volume and partitioned between water (50 mL) and CH2Cl2
(50 mL). The phases were separated, and the aqueous phase was
acidified to pH 4 with 0.5 M citric acid. Continuous extraction with
CH2Cl2 followed by concentration in vacuo gave crystalline off-
Colorless oil (0.65 g, 72% yield), Rf¼0.35 (10% MeOH/EtOAc),
20
[
a]
þ17 (c 1.9, CHCl3); HRMS m/z 536.2780 (MHþ, calculated
D
536.2319 for C29H33N3O7); nmax (KBr): 3500–3100 (br s), 1685, 1650,
1100 cmꢂ1
;
1H NMR
d
7.80 (d, J¼8 Hz, 2H), 7.60 (br d, 2H), 7.42 (t,
J¼8 Hz, 2H), 7.30 (br t, 2H), 5.95 (m, 1H), 5.38 (br d, 2H), 5.20 (br d,
2H), 5.00 (s, 2H), 4.60–4.20 (m, 10H), 3.88 (br s, 2H), 3.69–3.64 (m,
3H), 3.20–3.17 (m, 2H), 1.5–1.2 (m, 8H); 13C NMR
d 174.0, 165.3,
160.8, 162.1, 142.5, 141.2, 131.0, 128.2, 125.0, 120.5, 120.0, 117.0, 66.0,
65.1, 64.2, 48.7, 46.9, 42.5,, 42.0, 41.0, 30.4, 29.9, 20.5.
white product (4.6 g, 58% yield): mp 137–139 ꢀC. Recrystallization
20
from EtOAc gave 4 g of white crystals: mp 141.5–143 ꢀC; [
a]
þ21
D
4.3.2. (S)-2-(5-(Allyloxycarbonylamino)propyl)-4-(2-fluorenyl-2-
oxoethyl)-5-oxopiperazine-1-carboxylic acid (3d)
(c 2.5, CHCl3); MS m/z 477 (MþꢂBoc, 100); nmax (KBr): 1670,
1660,1385, 1225 cmꢂ1 1H NMR
; d 7.8 (s, 5H), 5.11 (s, 2H), 4.54 (dd,
Colorless oil (0.68 g, 77% yield), Rf¼0.35 (10% MeOH/EtOAc),
J¼11, 6 Hz, 1H), 4.3 (br t, J¼9 Hz, 1H), 3.5–3.2 (m, 2H), 3.1 (t, J¼6 Hz,
þ18 (c 1.2, CHCl3); HRMS m/z 521.2452 (MHþ, calculated
2H), 2.1–1.1 (m, 8H), 1.42 (s, 9H).
20
[
a]
D
521.2162 for C28H31N3O7), nmax (KBr): 3500–3100 (br s), 1685, 1655,
1205 cmꢂ1; 1H NMR
d
7.70 (br d, 2H), 7.55 (br d, 2H), 7.40 (br t, 2H),
4.3.7. (S)-2-((S)-3-(((9H-Fluoren-9-yl)methoxy)carbonylamino)-2-
oxopyrrolidin-1-yl)-6-(benzyloxycarbonylamino)-2-
methylhexanoic acid (5b)
7.28 (br t, 2H), 5.90 (m, 1H), 5.30 (br d, 2H), 5.23 (br d, 2H), 5.05 (s,
2H), 4.60–4.20 (m, 10H), 3.82 (br s, 2H), 3.57–3.53 (m, 3H), 3.11–
3.08 (m, 2H), 1.5–1.2 (m, 6H); 13C NMR
d
175.1, 163.3, 162.8, 161.1,
Compound 5a (4 g, 8.2 mmol) was dissolved in 98% HCO2H
(50 mL) and the solution was kept at rt for 2 h. After removal of the
excess formic acid in vacuo, the residue was dissolved in 50 mL
of DCM at 0 ꢀC, then diisopropylethylamine (8.5 g¼10.2 mL,
65.6 mmol) and Fmoc chloride (2.12 gr¼8.2 mmol) were added,
and the resultant solution was left overnight with vigorous stirring.
The organic layer was extracted with 1 N HCl and then twice with
brine. The solution was dried over anhydrous Na2SO4 filtered and
143.2, 140.2, 130.2, 127.8, 124.0, 122.3, 121.2, 118.6, 65.0, 64.5, 63.1,
45.2, 45.0, 44.7, 43.6, 42.0, 28.5, 22.7.
4.3.3. (S)-2-(5-(Allyloxycarbonylamino)propyl)-4-(2-fluorenyl-2-
oxoethyl)-5-oxodiazepane-1-carboxylic acid (4b)
20
Colorless oil (0.71 g, 83% yield), Rf¼0.40 (3% MeOH/AtOAc), [
a]
D
þ12 (c 1.8, CHCl3); HRMS m/z 536.2643 (MHþ, calculated 536.2319

1396
G. Gellerman et al. / Tetrahedron 65 (2009) 1389–1396
Tetrahedron 2001, 57, 7431–7448; (f) Dinsmore, C. J.; Beshore, D. C. Tetrahedron
2002, 58, 3297–3312.
evaporated to dryness in vacuo. The oily residue was crystallized
20
from EtOAc affording 5.4 g (81% yield) of white crystals. [
1.4, CHCl3); HRMS m/z 586.2530 (MHþ, calculated 586.2475 for
C33H35N3O7); nmax (KBr): 1680, 1660, 1015 cmꢂ1 1H NMR (DMSO-
a
]
þ24 (c
D
3. (a) Szardenings, A. K.; Burkoth, T. S.; Campbell, D. A. Tetrahedron 1997, 53, 6573–
6593; (b) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Mol. Divers. 2000, 5, 289–
304; (c) Ishihara, K.; Ohara, S.; Yamamoto, H. J. Org. Chem. 1996, 61, 4196–4197;
(d) Friedrich, A.; Jainta, M.; Nieger, M.; Brase, S. Synlett 2007, 2127–2129; (e)
Paradisi, F.; Sandri, S. Tetrahedron: Asymmetry 2001, 12, 3319–3324; (f) Bull, S.
D.; Davies, S. G.; Vickers, R. J. New J. Chem. 2007, 31, 486–502.
4. (a) Doemling, A. Org. Chem. Highlights 2005, 1–8; (b) Chucholowski, A.; Mas-
quelin, T.; Villalgordo, J. M. CHIMIA 1996, 50, 525–530; (c) Moroder, L.; Good-
man, M.; Kolbeck, W. Biopolymers 1996, 38, 295–300.
;
d6)
d
7.8–7.2 (m, 13H), 5.11 (s, 2H), 4.54 (dd, J¼11, 6 Hz, 1H), 4.3 (br t,
J¼9 Hz, 1H), 3.5–3.2 (m, 2H), 3.1 (t, J¼6 Hz, 2H), 2.1–1.1 (m, 8H); 13C
NMR
d 176.2, 171.0, 153.9, 152.3, 138.2, 130.3, 127.4, 126.8, 126.0,
118.0, 117.6, 67.0, 66.5, 65.4, 44.2, 43.1, 41.1, 34.5, 28.9, 27.2, 21.5.
5. Gellerman, G,; Hazan, E.; Albeck, A.; Shatzmiler, S. Int. J. Pept. Res. Ther. 2008, 14,
183–192.
6. Walters, W. P.; Murko, A. A.; Murko, M. A. Curr. Opin. Chem. Biol. 1999, 3,
384–387.
7. Gellerman, G.; Elgavi, A.; Salitra, Y.; Kramer, M. J. Pept. Res. 2001, 57, 277–291.
8. Wernic, D.; Adams, J. J. Org. Chem. 1989, 54, 4224–4228.
9. Chen, Z. C.; Kende, A. S.; Colson, A. O.; Mendezandino, J. L.; Ebetino, F. H.; Bush,
R. D.; Hu, X. E. Synth. Commun. 2006, 36, 473–479.
Acknowledgements
We wish to thank Mr. Vladimir Gaisin for analytical assistance.
Supplementary data
10. Gorohovsky, S.; Shkoulev, V.; Gellerman, G. Synlett 2003, 1411–1414.
11. Gazal, S.; Gellerman, G.; Ofer, Z.; Gilon, C. J. Med. Chem. 2002, 45, 1665–1671.
12. Jung, M. E.; Rohloff, J. J. Org. Chem. 1985, 50 4909–4913.
13. Gilon, C.; Huenges, M.; Matha, B.; Gellerman, G.; Kessler, H. J. Med. Chem. 1998,
41, 919–929.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2008.12.046.
References and notes
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