Enantioselective synthesis of n-protected α-amino acid hydrazides

Article abstract of DOI:10.1055/s-0028-1088013

A new, mild, general, and efficient synthesis of N-protected α-amino acid hydrazides is described. This two-step preparation uses N-aminophthalimide as protected hydrazine to prepare N-protected α-amino acid hydrazide precursors, and subsequent dephthaloy

Full text of DOI:10.1055/s-0028-1088013

1180
PAPER
Enantioselective Synthesis of N-Protected a-Amino Acid Hydrazides
E
nantioselective Synth
a
e
sis of
N
-P
t
rotecte
h
d
a
-Amino
A
i
cid
H
y
e
drazides u Bibian, Sarah El-Habnouni, Jean Martinez, Jean-Alain Fehrentz*
Institut des Biomolécules Max Mousseron, UMR 5247, CNRS , Universités Montpellier 1 – Montpellier 2, Faculté de Pharmacie,
B.P. 14491, 15 Avenue Charles Flahault, 34093 Montpellier Cedex 5, France
Fax 33(4)67548654; E-mail: jean-alain.fehrentz@univ-montp1.fr
Received 30 October 2008
R1
R1
R1
Abstract: A new, mild, general, and efficient synthesis of N-pro-
tected a-amino acid hydrazides is described. This two-step prepara-
tion uses N-aminophthalimide as protected hydrazine to prepare N-
protected a-amino acid hydrazide precursors, and subsequent de-
phthaloylation with an aminomethyl polystyrene resin yields N-
protected a-amino acid hydrazides. It has the advantages of avoid-
ing the use of the toxic hydrazine reagent and being compatible with
the most commonly used N-protecting groups. This strategy is par-
ticularly interesting in the case of N-(9-fluorenylmethoxycarbonyl)-
protected amino acids. Within the limits of chiral HPLC detection,
no epimerization is apparent.
a
b
OMe
NHNH2
OH
R2HN
R2HN
R2HN
O
O
O
R1 = amino acid side chain
2 = Boc or Cbz
R
Scheme 1 Hydrazinolysis of N-protected amino acid methyl esters.
Reagents and conditions: (a) MeI (5 equiv), DBU, MeCN, reflux, 6 h;
(b) H2NNH2·H2O (3 equiv), MeOH, r.t., overnight.
In our attempt to develop a general method for the synthe-
sis of N-protected a-amino acid hydrazides, we looked at
different strategies in which protected hydrazines were
used as key reagents. Quibell6 described the acylation of
(benzyloxycarbonyl)-protected hydrazine with N-protect-
ed valine and lysine esters. In a second step, the benzyl-
oxycarbonyl (Cbz) protecting group was removed by
hydrogenolysis. In the case of N-Fmoc protection, stan-
dard hydrogenolysis conditions can lead to partial or total
deprotection of the Fmoc group.6,7 This last method has
three major limitations: during (benzyloxycarbonyl)hy-
drazine acylation, a small amount of the Fmoc group was
aminolyzed; the workup is very long and difficult; and it
has been described, to our knowledge, only for Fmoc-va-
line and Fmoc-lysine(Boc).
Key words: amino acids, hydrazides, epimerization, protecting
groups, aminomethyl resins
Hydrazides have been studied for their biological1 and
chemical properties. This functional group can be used in
peptide synthesis as a carboxy protecting group2 or for hy-
drazide peptide preparations. Hydrazides can also be used
as precursors in heterocyclic chemistry.3,4 Our study fo-
cused on the synthesis of a-amino acid hydrazides that can
be obtained from a-amino acids. The use of a-amino acids
introduces a chiral center in the target molecules that can
be conserved. Additionally, a-amino acids need orthogo-
nal protection to potentially reactive groups.
Stravropoulos8 reported the solid-phase synthesis of small
hydrazide peptides. We applied this method to the synthe-
sis of Fmoc-phenylalanine hydrazide. This reaction, for
which trityl resin is used, proceeds in good yield and re-
sults in good purity, and workup is easy. The limitations
are related to the cleavage conditions that are not compat-
ible with the tert-butoxycarbonyl (Boc) protecting group,
as well as the cost of this strategy.
Several strategies have been described for the synthesis of
N-protected a-amino acid hydrazides. There are mainly
two synthetic pathways, namely hydrazinolysis of esters
and acylation of protected hydrazines.
The most commonly employed method is hydrazinolysis
of alkyl esters, mainly methyl esters (Scheme 1). This re-
action needs strong conditions and employs toxic re-
agents, including hydrazine monohydrate. In the case of
N-protected a-amino acids, the yields range from low to
medium. In our study, the major problem was the use of
hydrazine which did not allow the 9-fluorenylmethoxy-
carbonyl (Fmoc) group to be used for amine protection.
The Stravropoulos and Quibell studies convinced us that
the use of a protected hydrazide was a good way for de-
veloping a general multigram synthesis of N-protected a-
amino acid hydrazides in solution (especially with the
Fmoc protecting group). The choice of a protected hydra-
zine, easily deprotected under mild conditions with re-
spect to Boc, Cbz, and Fmoc protecting groups, is very
important for the synthesis of a-amino acid hydrazides.
Brosse9 described a two-step synthesis of isoniazide in
which N-aminophthalimide was used as a protecting
group: the procedure consisted of activation of isonicotin-
ic acid with 1,1¢-carbonyldiimidazole (CDI) and coupling
with N-aminophthalimide followed by dephthaloylation.
The yields reported by Brosse are good, but, to apply this
strategy for a-amino acid hydrazides, the use of 1,1¢-car-
Another example of this strategy has been described by
Krysin5 and concerns hydrazinolysis of trimethylsilyl es-
ters of N-protected amino acids. The hydrazinolysis step
was achieved with anhydrous hydrazine. The reported
yields are good but the main problem is the use of anhy-
drous hydrazine, which is difficult to handle and incom-
patible with the presence of N-Fmoc protection.
SYNTHESIS 2009, No. 7, pp 1180–1184
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Advanced online publication: 16.03.2009
DOI: 10.1055/s-0028-1088013; Art ID: P12408SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Enantioselective Synthesis of N-Protected a-Amino Acid Hydrazides
Table 2 Isolated Yields for the Coupling Stepa
1181
bonyldiimidazole under strong conditions (for a-amino
acids) can induce epimerization, and the use of amine or
hydrazine during the dephthaloylation process is not com-
patible with the Fmoc protecting group. Nevertheless, we
decided to explore this strategy to adapt it to our goal.
N-Protected residueb C(O)CHR1NHR2
Cbz(L)Phe
2
Yieldc (%)
2a
2b
2c
2d
2e
2f
2g
2h
2i
71d
56d
39d
81d
83
Fmoc(L)Phe
The first challenge was the activation of the carboxylic ac-
id; Brosse commented on the difficulty of the coupling
step (mainly due to the poor reactivity of the NH2 group
of N-aminophthalimide). For the coupling of N-protected
amino acid 1 with N-aminophthalimide, we tried different
activating reagents widely used in peptide synthesis and
known to avoid epimerization (Table 1, see also
Scheme 2).
Boc(L)Tyr(Dzb)
Boc(L)Trp
Boc(D)Trp
Boc(L)Lys(Cbz)
Fmoc(L)Asp(t-Bu)
BocGly
100
98
n.d.e
29
Table 1 Conditions and Reagents Used for the Coupling Stepa
Boc(L)Orn(Cbz)
Fmoc(L)Orn(Boc)
Fmoc(L)Ile
Activating agent
IBCF (1 equiv)
PyBOP (1 equiv)
HBTU (1 equiv)
DIC (1 equiv)
Conditions
2j
2k
2l
98
NMM, THF, –10 °C to r.t.
DIEA, 0 °C to r.t.
DIEA, 0 °C to r.t.
DMAP (0.01 equiv), 0 °C to r.t.
0 °C to r.t.
65d
100
Fmoc(L)Pro
a See also footnote c of Table 1.
b The ‘N-protected residue’ corresponds to the C(O)CHR1NHR2 moi-
ety.
EDACb (2 equiv)c
c All final compounds were identified by LCMS.
d Compounds were characterized by 1H and 13C NMR spectroscopy.
See experimental section.
a Reaction conditions: 1 (1 equiv), PhthNNH2 (1.1 equiv), activating
agent.
b EDAC = 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydro-
chloride.
e n.d. = not determined.
c EDAC (1.4 equiv) was generally used, but for some difficult reac-
tions EDAC (2 equiv) was used, and is generally best used. DCC is
similarly effective, but results in a difficult workup.
synthesize N-Fmoc a-amino acid hydrazides. Therefore
we decided to use a supported reagent to avoid partial
Fmoc cleavage. Two kinds of supported reagents on poly-
styrene resin were tested, namely resin substituted with
aminomethyl (X = CH2) and N¢-methylhydrazino (X =
NMe; Scheme 3). The most efficient resin was found to be
the aminomethyl resin. During the dephthaloylation, no
Fmoc deprotection was observed by LCMS analysis of the
crude product. The yields of the dephthaloylated a-amino
acid hydrazides 4 are collected in Table 3.
Symmetrical anhydrides obtained by carbodiimide re-
agents {e.g., 1-[3-(dimethylamino)propyl]-3-ethylcarbo-
diimide hydrochloride (EDAC)} seemed to give the best
results (Table 1, Scheme 2). It allowed us to get the corre-
sponding N-protected hydrazides in good to excellent
yields (Table 2).
EDAC
O
R1
(2 equiv)
CH2Cl2
R1
O
H
R3
X
N
H2NN
R2HN
N
R2HN
CO2H
3 h at 0 °C
then r.t.
overnight
NHR2
NH R1
O
O
HN
O
H2NXR3
O
O
O
O
N
H
CH2Cl2
r.t., overnight
1 (2 equiv)
1 equiv
2
N
NHR2
N
O
H
O
R1
R1 = amino acid side chain
2 = Cbz or Boc or Fmoc
2
R
Scheme 2 Coupling of N-protected amino acids 1 with N-amino-
phthalimide in the presence of a carbodiimide activating reagent
O
O
R3
NHR2
N
X
The second step, that is, deprotection (or dephthaloyla-
tion) of 2 gave more difficulties. Known methods for the
dephthaloylation include the use of primary amines (e.g.,
methylamine) or unsubstituted or monosubstituted hydra-
zines (e.g., phenylhydrazine, methylhydrazine).9 These
two procedures were not compatible with our attempt to
H2NHN
R1
R1 = amino acid side chain
R
R
O
2 = Boc or Fmoc or Cbz
3 = polystyrene
3
4
X = CH2, NMe
Scheme 3 Dephthaloylation of 2 with supported reagent
Synthesis 2009, No. 7, 1180–1184 © Thieme Stuttgart · New York

1182
M. Bibian et al.
PAPER
Table 3 Isolated Yields Obtained for the Dephthaloylation Step
O
O
aq MeNH2
r.t., 3 d
Phthalimide 2
Product 4
Yielda (%)
88
N
H2N
N Me
O
dioxane
2a
2b
2c
2d
2e
2f
4a
4b
4c
4d
4e
4f
O
5
3
83b
51b
70b
72
Scheme 5 Resin recycling
a dioxane/methylamine mixture. Under these conditions,
the reaction proceeded at room temperature.
In conclusion, we reported a new method for the synthesis
of a-amino acid hydrazides using nontoxic N-amino-
phthalimide. This strategy could be applied to the more
currently used protecting groups in peptide chemistry.
The use of mild conditions and very selective cleavage of
the hydrazine protecting group brought us closer to the
possibility of the synthesis of hydrazide peptides in solu-
tion. Moreover, no or little epimerization was detected un-
der these conditions, and the resin used for the removal of
the N-phthalimide protecting group could be regenerated
and recycled for use in further deprotection cycles.
74
2g
2h
2i
4g
4h
4i
72b
89
68b
51b
90
2j
4j
2k
2l
4k
4l
82
a All final compounds were identified by LCMS.
b Compounds were characterized by 1H and 13C NMR spectroscopy;
see experimental section.
Solvents and other chemicals were obtained commercially and used
as received. NMR spectra were recorded on a Bruker AMX-300
spectrometer. Column chromatography was performed on silica gel
(230–400 mesh).
We then checked the optical purity of the obtained hy-
drazides. For this purpose, two Trp enantiomer deriva-
tives were synthesized, and the optical purity was
monitored by chiral HPLC (Scheme 4).10 Under our ex-
perimental conditions, no epimerization could be ob-
served within the limits of chiral HPLC detection. In
N-Protected Hydrazides 2a–l; General Procedure
The protected amino acid 1 (2 equiv) and EDAC (2 equiv) were dis-
solved in the minimum amount of CH2Cl2 at 0 °C. The mixture was
stirred at 0 °C for 2 h. PhthNNH2 (1 equiv) was added and the mix-
ture was stirred at 0 °C for 1 h and then at r.t. overnight. Distillation
of the solvent under reduced pressure gave a yellow-white solid
which was dissolved in EtOAc and washed successively with 1 M
addition, the specific rotation of 4d and 4e was also mea-
20
sured: [a]D +11.7 and –11.5, respectively (c 1.00, KHSO4, sat. aq NaHCO3, and brine. The organic layer was then
dried (Na2SO4), filtered, and concentrated in vacuo. The residue
MeOH).
was precipitated with Et2O to yield compound 2. If necessary, the
In the synthesis described above, the aminomethyl resin
residue was purified by column chromatography (silica gel, hex-
can be regenerated upon treatment of 3 with aqueous
methylamine to give the aminomethylated resin
(Scheme 5), which can be used again in the dephthaloyl-
ation step. Mitchell11 carried out the dephthaloylation
with hydrazine monohydrate. More recently, Adams12 re-
investigated this process using aqueous methylamine, a
less toxic reagent. For resin regeneration, we chose to use
ane–EtOAc, 3:7 to 7:3).
Cbz(L)-Phenylalanine-phthalimide (2a)
1H NMR (300 MHz, DMSO-d6): d = 2.91 (dd, J = 11, 14 Hz, 1 H,
CH2b Phe), 3.19 (dd, J = 4, 14 Hz, CH2b Phe), 4.57 (m, 1 H, CHa
Phe), 4.98 (d, J = 4 Hz, 2 H, CH2 Cbz), 7.23–7.39 (m, 10 H, CH ar-
omatics Cbz, Phe), 7.76 (d, J = 9 Hz, 1 H, NH Cbz), 7.97 (m, 4 H,
CH aromatics Phth), 10.99 (s, 1 H, NH hydrazide).
13C NMR (75 MHz, DMSO-d6): d = 37.6 (CH2b Phe), 54.7 (CHa
Phe), 65.3 (CH2 Cbz), 123.7 (C4C7 Phth), 126.4 (C4 Phe), 127.3
H
N
H
N
H
N
L-Trp
OH
O
H
N
NHNH2
BocHN
N
BocHN
BocHN
NH2
CH2Cl2
O
O
2d
EDAC, CH2Cl2
O
O
H
N
O
H
N
4d
D-Trp
H
NNH2
O
N
chiral HPLC analysis
O
H
N
OH
BocHN
BocHN
N
O
O
NHNH2
BocHN
O
2e
4e
O
Scheme 4 Synthesis of Trp enantiomers for chiral HPLC analysis
Synthesis 2009, No. 7, 1180–1184 © Thieme Stuttgart · New York

PAPER
Enantioselective Synthesis of N-Protected a-Amino Acid Hydrazides
1183
(C2C6 Phe), 127.6 (C4 Cbz), 128.1–128.2–129.2 (C2C3C5C6 Cbz,
Phe), 129.5 (C8C9 Phth), 135.2 (C5C6 Phth), 136.9 (C1 Cbz), 137.5
(C1 Phe), 155.8 (CO Cbz), 164.9 (C1C3 Phth), 171.2 (CO hy-
drazide).
13C NMR (75 MHz, DMSO-d6): d = 10.7 (Cd Ile), 15.1 (CH3g Ile),
24.3 (CH2g Ile), 36.5 (CHb Ile), 46.6 (CH Fmoc), 57.3 (CHa Ile),
65.8 (CH2 Fmoc), 120.0 (C1C8 Fmoc), 123.7 (C4C7 Phth), 125.4
(C2C7 Fmoc), 127.0 (C4C5 Fmoc), 127.6 (C3C6 Fmoc), 129.4 (C8C9
Phth), 135.2 (C5C6 Phth), 140.7, 143.6, 143.9 (C4a/b, C8a/b Fmoc),
156.0 (CO Fmoc), 165.0 (C1C3 Phth), 170.0 (CO hydrazide).
Fmoc(L)-Phenylalanine-phthalimide (2b)
1H NMR (300 MHz, DMSO-d6): d = 2.93 (dd, J = 11, 13 Hz, 1 H,
CH2b Phe), 3.19 (dd, 1 H, 3, 14 Hz, CH2b Phe), 4.17 (m, 3 H,
CH2CH Fmoc), 4.56 (m, 1 H, CHa Phe), 7.19–7.34 (m, 6 H,
H2H3H4H5H6 Phe, NH Fmoc), 7.40 (m, 4 H, H3H6H2H7 Fmoc), 7.65
(m, 2 H, H4H5 Fmoc), 7.88 (d, J = 7 Hz, 2 H, H1H8 Fmoc), 7.97 (m,
4 H, H4H5H6H7 Phth), 11.00 (s, 1 H, NH hydrazide).
Compounds 4a–l; General Procedure
Aminomethylated polystyrene resin (3 equiv; 1.1 mmol/g, 100–200
mesh) was conditioned for 10 min at r.t. in CH2Cl2. Then 2 (1 equiv)
was added, and the mixture was slowly stirred for 24 h at r.t. The
mixture was filtered and the solvent was evaporated in vacuo to give
product 4 as a white solid.
13C NMR (75 MHz, DMSO-d6): d = 38.9 (CH2b Phe), 46.5 (CHCH2
Fmoc), 54.6 (CHa Phe), 65.8 (CHCH2 Fmoc), 120.0 (C1C8 Fmoc),
123.7 (C4C7 Phth), 125.3 (C2C7 Fmoc), 126.4 (C4 Phe), 127.0 (C4C5
Fmoc), 127.6 (C3C6 Fmoc), 128.1, 129.2 (C2C3C5C6 Phe), 129.5
Fmoc(L)-Phenylalanine-NHNH2 (4a)
1H NMR (300 MHz, DMSO-d6): d = 2.85 (dd, J = 10 Hz, 1 H, CH2b
Phe), 2.93 (dd, J = 13 Hz, 1 H, CH2b Phe), 4.20 (m, 6 H, CH2CH
Fmoc, NHNH2, CHa Phe), 7.20 (d, J = 6 Hz, 1 H, NH Fmoc), 7.24–
7.35 (m, 7 H, H2H3H4H5H6 Phe, H2H7 Fmoc), 7.41 (t, J = 7 Hz, 2 H,
H3H6 Fmoc), 7.64 (dd, J = 7, 12 Hz, 2 H, H4H5 Fmoc), 7.88 (d, J =
7 Hz, 2 H, H1H8 Fmoc), 9.21 (s, 1 H, NHNH2).
(C8C9 Phth), 135.2 (C5C6 Phth), 137.6 (C1 Phe), 140.6–143.6 (C4a/b
,
C8a/b Fmoc), 155.8 (CO Fmoc), 164.9 (C1C3 Phth), 171.2 (CO hy-
drazide).
Boc(L)-Tyrosine(DCB)-phthalimide (2c)
1H NMR (300 MHz, DMSO-d6): d = 1.31 (s, 9 H, CH3 Boc), 2.82
(dd, J = 9, 14 Hz, 1 H, CH2b Tyr), 3.09 (dd, J = 3, 14 Hz, 1 H, CH2b
Tyr), 4.42 (m, 1 H, CHa Tyr), 5.20 (s, 2 H, CH2 DCB), 6.98–7.10
(dd, J = 8, 9 Hz, 2 H, H3H5 Tyr, NH Boc), 7.34 (d, J = 8 Hz, 2 H,
H2H6 Tyr), 7.44–7.57 (m, 3 H, H3H4H5 DCB), 7.92–8.00 (m, 4 H,
H4H5H6H7 Phth), 10.85 (s, 1 H, NH hydrazide).
13C NMR (75 MHz, DMSO-d6): d =37.7 (CH2b Phe), 46.5 (CHCH2
Fmoc), 54.9 (CHa Phe), 65.6 (CHCH2 Fmoc), 120.0 (C3C6 Fmoc),
125.2, 125.3 (C2C7 Fmoc), 126.2 (C4 Phe), 127.0 (C4C5 Fmoc),
127.5 (C3C6 Fmoc), 128.0, 129.1 (C2C3C5C6 Phe), 138.0 (C1 Phe),
140.6, 143.7 (C4a/4b, C8a/8b Fmoc), 155.6 (CO Fmoc), 170.7 (CO hy-
drazide).
13C NMR (75 MHz, DMSO-d6): d = 28.1 (CH3 Boc), 36.8 (CH2b
Tyr), 54.4 (CHa Tyr), 64.8 (CH2 DCB), 78.2 (C quat Boc), 114.2
(C3C5 Tyr), 123.7 (C4C7 Phth), 128.7 (C3C5 DCB), 129.5 (C8C9
Phth), 130.2 (C1 Tyr), 130.3 (C2C6 Tyr), 131.4 (C4 DCB), 135.2
(C5C6 Phth), 136.0 (C1 DCB), 155.2 (CO Boc), 157.1 (C4 Tyr),
164.9 (C1C3 Phth), 171.3 (CO hydrazide).
Boc(L)-Tyrosine(DCB)-NHNH2 (4c)
1H NMR (300 MHz, DMSO-d6): d = 1.30 (s, 9 H, CH3 Boc), 2.65–
2.73 (dd, J = 10, 13 Hz, 1 H, CH2b Tyr), 2.81–2.87 (1 H, dd, J = 4,
14 Hz, CH2b Tyr), 4.08 (m, 1 H, CHa Tyr), 4.30 (sl, 2 H, NH2 hy-
drazide), 5.18 (s, 2 H, CH2 DCB), 6.83 (d, 1 H, 8 Hz, NH Boc), 6.95
(d, J = 8 Hz, 2 H, H3H5 Tyr), 7.19 (d, J = 8 Hz, 2 H, H2H6 Tyr), 7.43–
7.57 (m, 3 H, H3H4H5 DCB), 9.09 (s, 1 H, NH hydrazide).
Boc(L)-Tryptophan-phthalimide (2d)
1H NMR (300 MHz, DMSO-d6): d = 1.32 (s, 9 H, CH3 Boc), 3.03
(dd, J = 14, 10 Hz, 1 H, CH2 b Trp), 3.28 (m, 1 H, CH2 b Trp), 4.50
(m, 1 H, CHa Trp), 6.98 (t, J = 8 Hz, 1 H, H5 Trp), 7.02 (d, J = 7 Hz,
1 H, H4 Trp), 7.08 (t, J = 7 Hz, 1 H, H6 Trp), 7.23 (s, 1 H, H2 Trp),
7.35 (d, J = 8 Hz, 1 H, H7 Trp), 7.68 (d, J = 8 Hz, 1 H, NH Boc),
7.97 (m, 4 H, H4 H5 H6 H7 Phth), 10.86 (m, 2 H, NH indole Trp, NH
hydrazide).
13C NMR (75 MHz, DMSO-d6): d = 28.1 (CH3 Boc), 37.0 (CH2b
Tyr), 54.6 (CHa Tyr), 64.8 (CH2 DCB), 77.8 (C quat Boc), 114.1
(C3C5 Tyr), 128.7 (C3C5 DCB), 130.2 (C1 Tyr), 130.6 (C2C6 Tyr),
131.4 (C4 DCB), 131.7 (C2C6 Tyr), 136.0 (C1 DCB), 155.0 (CO
Boc), 156.9 (C4 Tyr), 170.9 (CO hydrazide).
Boc(L)-Tryptophan-NHNH2 (4d)
1H NMR (300 MHz, DMSO-d6): d = 1.31 (s, 9 H, CH3 Boc), 2.89
(dd, J = 9, 14 Hz, 1 H, CH2b Trp), 3.02 (dd, J = 5, 14 Hz, 1 H, CH2
b Trp), 4.17 (dd, J = 8 Hz, 1 H, CHa Trp), 4.29 (s, 2 H, NH2 hy-
drazide), 6.72 (d, J = 8 Hz, 1 H, NHBoc), 6.97 (t, J = 8 Hz, 1 H, H5
Trp), 7.06 (t, J = 7 Hz, 1 H, H6 Trp), 7.13 (s, 1 H, H2 Trp), 7.32 (d,
J = 8 Hz, 1 H, H4 Trp), 7.59 (d, J = 8 Hz, 1 H, H7 Trp), 9.12 (s, 1 H,
NH hydrazide), 10,78 (s, 1 H, NH Trp).
13C NMR (75 MHz, DMSO-d6): d = 27.9 (CH2 b Trp), 28.1 (CH3
Boc), 53.5 (CH a Trp), 78.2 (C quat Boc), 109.6 (C3 Trp), 111.3 (C7
Trp), 118.2 (C4 Trp), 118.4 (C5 Trp), 120.8 (C6 Trp), 123.7 (C4C7
Phth), 123.9 (C2 Trp), 127.3 (C9 Trp), 129.5 (C8C9 Phth), 135.2
(C5C6 Phth), 136.0 (C8 Trp), 155.1 (CO Boc), 165.0 (C1C3 Phth),
171.6 (CO hydrazide).
Boc-Glycine-phthalimide (2h)
13C NMR (75 MHz, DMSO-d6): d = 28.7 (CH3 Boc), 39.2 (CH2b
Trp), 53.7 (CHa Trp), 77.8 (C quat Boc), 110.1 (C3 Trp), 111.2 (C7
Trp), 118.1 (C4 Trp), 118.4 (C5 Trp), 120.7 (C6 Trp), 127.3 (C9 Trp),
136.0 (C8 Trp), 155.0 (CO Boc), 171.3 (CO hydrazide).
1H NMR (300 MHz, DMSO-d6): d = 1.39 (s, 9 H, CH3 Boc), 3.83
(d, J = 6 Hz, 2 H, CH2a Gly), 7.13 (t, J = 6 Hz, 1 H, NH Boc), 7.95
(m, 4 H, H4H5H6H7 Phth), 10.65 (s, 1 H, NH hydrazide).
13C NMR (75 MHz, DMSO-d6): d = 27.8 (CH3 Boc), 41.5 (CH2 a
Gly), 78.2 (C quat Boc), 123.7 (C4C7 Phth), 129.4 (C8C9 Phth),
135.2 (C5C6 Phth), 155.7 (CO Boc), 165.0 (C1C3 Phth), 169.0 (CO
hydrazide).
Fmoc(L)-Aspartic Acid(t-Bu)-NHNH2 (4g)
1H NMR (300 MHz, DMSO-d6): d = 1.37 (s, 9 H, CH3 t-Bu), 2.64
(m, 2 H, CH2b Asp), 4.24 (m, 6 H, CH2CH Fmoc, NHNH2, CHa
Asp), 7.33 (s, 2 H, H2H7 Fmoc), 7.42 (s, 2 H, H3H6 Fmoc), 7.57 (d,
J = 7 Hz, 1 H, NH Fmoc), 7.71 (s, 2 H, H4H5 Fmoc), 7.89 (d, J = 6
Hz 2 H, H1H8 Fmoc), 9.11 (s, 1 H, NHNH2).
Fmoc(L)-Isoleucine-phthalimide (2k)
1H NMR (300 MHz, DMSO-d6): d = 0.88 (t, J = 7 Hz, 3 H, CH3d
Ile), 1.02 (d, J = 6 Hz, 3 H, CH3g Ile), 1.23, 1.56 (m, 2 H, CH2g Ile),
1.84 (m, 1 H, CHb Ile), 4.17–4.33 (m, 4 H, CHCH2 Fmoc, CHa Ile),
7.34 (m, 2 H, H3H6 Fmoc), 7.44 (m, 2 H, H2H7 Fmoc), 7.72 (d, J =
10 Hz, 1 H, NH Fmoc), 7.77 (m, 2 H, H4H5 Fmoc), 7.92 (m, 6 H,
H4H5H6H7 Phth, H1H8 Fmoc), 10.88 (s, 1 H, NH hydrazide).
13C NMR (75 MHz, DMSO-d6): d = 27.6 (CH3 t-Bu), 37.5 (CH2b
Asp), 46.6 (CHa Asp), 50.3 (CH Fmoc), 65.7 (CH2 Fmoc), 80.1 (C
quat Boc), 120.0 (C1C8 Fmoc), 125.2 (C2C7 Fmoc), 127.0 (C3C6
Fmoc), 127.6 (C4C5 Fmoc), 140.6 (C4a/b Fmoc), 143.7 (C8a/b Fmoc),
155.6 (CO Fmoc), 169.2, 169.6 (CO t-Bu, hydrazide).
Synthesis 2009, No. 7, 1180–1184 © Thieme Stuttgart · New York

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M. Bibian et al.
PAPER
Boc(L)-Ornithine(Cbz)-NHNH2 (4j)
References
1H NMR (300 MHz, DMSO-d6): d = 1.37–1.58 (m, 14 H, CH3 Boc,
CH2g, CH2b Orn), 2.97 (m, 2 H, CH2d Orn), 3.85 (m, J = 6, 8 Hz, 1
H, CHa Orn), 4.24 (s, 2 H, NH2 hydrazide), 5.00 (s, 2 H, CH2 Cbz),
6.74 (d, J = 8 Hz, 1 H, NH Boc), 7.20 (m, 1 H, NH Cbz), 7.33 (m,
5 H, H2H3H4H5 Cbz), 8.98 (s, 1 H, NH hydrazide).
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13C NMR (75 MHz, DMSO-d6): d = 26.0 (CH2 g Orn), 28.1 (CH3
Boc), 29.5 (CH2b Orn), 39.9 (CH2g Orn), 52.6 (CHa Orn), 65.1
(CH2 Cbz), 77.9 (Cq Boc), 127.6 (C2C6C4 Cbz), 128.3 (C3C5 Cbz),
137.2 (C1 Cbz), 155.1, 156.0 (CO Cbz, Boc), 171.3 (CO hy-
drazide).
Resin Recycling
The methylphthalimide polystyrene was suspended in 1,4-dioxane
and stirred until fully swollen. A 40% aq soln of MeNH2 was added,
and the mixture was stirred at r.t. for 3 d. The suspension was fil-
tered and the beads were washed with 1,4-dioxane–H2O (4:1), 1,4-
dioxane, DMF, CH2Cl2, and MeOH. After air-drying, the resin
could be reused without any further treatment.
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(10) Chiral HPLC: Chiracel OD column, 1 mL/min, hexane–
i-PrOH, 80:20 with 0.1% TFA, monitoring at l = 280 nm.
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Synthesis 2009, No. 7, 1180–1184 © Thieme Stuttgart · New York