Syntheses of hydrazino peptides and conjugates

Article abstract of DOI:10.1002/ejoc.201201731

(α-Benzyloxycarbonyl-aminoacyl)benzotriazolides (Cbz = benzyloxycarbonyl) underwent a coupling reaction with α-hydrazino acids under microwave irradiation to form hybrid hydrazino dipeptides (42-71 %). Chiral acylations of β-N-Cbz-α-hydrazino acylbenzotriazolides were successfully carried out with N-, S-, O-, and C-nucleophiles in yields of 49-88 %. Benzyloxycarbonyl-protected hybrid hydrazino dipeptides and benzyloxycarbonyl-protected hydrazino aminoacyl conjugates with N-, S-, O-, and C-nucleophiles were prepared by using benzotriazole methodology. Copyright

Full text of DOI:10.1002/ejoc.201201731

FULL PAPER
DOI: 10.1002/ejoc.201201731
Syntheses of Hydrazino Peptides and Conjugates
Siva S. Panda,[a] Claudia El-Nachef,[a] Kiran Bajaj,[a] and Alan R. Katritzky*[a,b]
Keywords: Synthetic methods / Acylation / Microwave chemistry / Peptides / Conjugates
(α-Benzyloxycarbonyl-aminoacyl)benzotriazolides (Cbz
=
Cbz-α-hydrazino acylbenzotriazolides were successfully car-
ried out with N-, S-, O-, and C-nucleophiles in yields of 49–
88%.
benzyloxycarbonyl) underwent a coupling reaction with α-
hydrazino acids under microwave irradiation to form hybrid
hydrazino dipeptides (42–71%). Chiral acylations of β-N-
Introduction
The replacement of one or more α-amino acids by β-
amino acid units is a well-known technique in the search
for pharmacologically active peptides.[1] Further replace-
ment of the C-α and/or C-β atom of the β-amino acid con-
stituent by a hetero atom is another attractive extension of
the β-peptide concept. The replacement of the C-β atom by
a nitrogen can be accomplished by incorporating one or
more α-hydrazino acid units (H2Nβ-NαH-CH(R)-COOH)[2]
into a peptide, which produces hydrazino peptides, that is,
peptide analogues in which one or more of the peptidic
bond(s) [-HN-CO-CH(R)-] is replaced by one or more hy-
drazidic bond(s) [HN-NH-CH(R)-].[3] The vitamin B6 an-
tagonist linatine (1) and the antibiotic negamycin (2) are
naturally occurring peptides that contain such a α-hydraz-
ino acid moiety (see Figure 1).[4,5]
Unlike the corresponding amino acids [NH2CH(R)-
COOH], α-hydrazino acids [H2N-NH-CH(R)-COOH] can
inhibit the enzymes that metabolize amino acids.[6] Peptide
analogs that contain an N-terminal achiral hydrazino acetic
acid residue display anticancer properties[7] and are inter-
mediates in the preparation of lipopeptides.[8] Hydrazino
peptides are also building blocks in the syntheses of antivi-
ral peptidomimetics,[9] substrates for hydrazone chemical
ligation,[10] and solvent gelators.[11] Peptides that contain an
N-alkyl-, N-aryl-, or N-acyl-hydrazino group undergo reac-
tion with peptide aldehydes to give high molecular weight
conjugates.[12]
Figure 1. Linatine (1) and negamycin (2).
Structurally, a hydrazone fragment in a peptide chain in-
duces a “hydrazino turn”,[13,14] which folds the peptide
backbone locally by way of well-defined intramolecular bi-
furcated hydrogen bonding. Quantum and molecular me-
chanics calculations[2] reveal that oligomers of α-hydrazino
acids adopt a wide variety of secondary structures that are
characteristic of foldamers.[15]
Published preparations of hydrazino acids 8 include
(i) the Hofmann rearrangement of hydantoic acids 3,[6,17]
(ii) the hydrogenation of diacyl hydrazino acid 4,[18,19]
(iii) the amination of chiral α-amino acids with N-alkoxy-
carbonyl-3-phenyloxaziridines 5,[20] (iv) the asymmetric hy-
drogenation of N-acylhydrazones derived from α-keto acids
6,[21] and (v) the synthesis from α-bromo acids 7 (see
Scheme 1).[22–24] Cheguillaume et al. reported the synthesis
of protected hydrazine peptides from protected hydrazine
and esters of bromoacetate.[25] Killian et al. incorporated
hydrazinophenylalanine into modified peptide and protein
analogues through ribosomes.[26] Despite their interesting
biological properties and diverse methods for their prepara-
tion, hydrazino peptides have not been widely studied,
probably because of the difficulties associated with their
synthesis, which include tedious reaction procedures,[18] low
yields of the employed aminating agents,[20] and incomplete
chiral characterization of the products.[23]
[a] Center for Heterocyclic Compounds, Department of Chemistry,
University of Florida,
Gainesville, FL 32611-7200, USA
E-mail: katritzky@chem.ufl.edu
Homepage: http://www.ufl.edu/
[b] Department of Chemistry, King Abdulaziz University,
Jeddah, 21589 Saudi Arabia
Homepage: http://www.kau.edu.sa
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201731.
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Syntheses of Hydrazino Peptides and Conjugates
Table 1. Preparation of α-bromo acids 7a–7d.
Product
R
M.p. Yield
[α]2D2
(c = 1, MeOH)
[°C]
[%]
7a
7b
7c
7d
CH3
CH2CH(CH3)2
CH2Ph
oil
oil
oil
oil
68
67
70
60
+41
+17
+20
+20
CH(CH3)2
Table 2. Preparation of α-hydrazino acids 8a–8d.
Product M.p.
[°C]
Yield
[%]
[α]2D2
Ref.[17] [α]D
(6 n HCl)
8a
8b
8c
8d
214–215[16] 48
–32 (c = 1)
–12 (c = 1)
–22 (c = 1)
–3 (c = 1)
–26.5 (c = 1.2)
–13.2 (c = 1)
–15.8 (c = 0.5)
–17.1 (c = 0.8)
214–217
43
203–206[24] 45
232–235[21] 40
Preparation of Hybrid α-Hydrazino Dipeptides 11a–11f
The coupling reaction between α-hydrazino acids 8a–8d
and Cbz-protected α-aminoacylbenzotriazoles 10a–10f[30]
under microwave irradiation (70 °C and 65 W) for 15 min
afforded new hydrazino hybrid dipeptides 11a–11f (42–
71%). Comparatively low yields were obtained in case of
compounds 11b and 11c as a result of purifying these com-
pounds by column chromatography, but no side products
were obtained after this step except for some unreacted
starting material (see Schemes 3 and 4 and Table 3).
Scheme 1. Synthesis of unprotected α-hydrazino acids.
We recently disclosed the efficient benzotriazole-medi-
ated syntheses of α-aminoxy acids and peptides.[27,28] Fol-
lowing an analogous pathway, we herein report an alternate
route to synthesize chirally pure α-hydrazino acids.[22] This
was accomplished by using microwave irradiation in the dis-
placement reaction of α-bromo acids with hydrazine hy-
drate followed by the solution-phase conversion into chi-
rally pure α-hydrazino hybrid dipeptides and α-hydrazino
acid conjugates through N-, O-, S-, and C-acylation reac-
tions.
Results and Discussion
Preparation of α-Hydrazino Acids 8a–8d
α-Hydrazino acids 8a–8d were prepared through the nu-
cleophilic substitution of the corresponding α-bromo acids
7a–7d[29] with hydrazine hydrate at 70 °C under microwave
irradiation for 15–20 min. According to the literature pro-
cedure, the displacement reaction took place at 20 °C in
24 h, and the crucial step was the purification and
recrystallization of the desired α-hydrazino acid. By chang-
ing the procedure and carrying out to the reaction under
microwave conditions, the reaction took less time, and the
products were isolated in fairly good yields (see Scheme 2
and Tables 1 and 2). The chiral purity of 8a–8d was con-
firmed by the products formed from coupling 8a–8d with
benzyloxycarbonyl-protected (Cbz = benzyloxycarbonyl) α-
(aminoacyl)benzotriazoles 10a–10f as described below.
Scheme 3. Preparation of hybrid α-hydrazino dipeptides 11a–11f
(Bt = benzotriazolyl, THF = tetrahydrofuran).
Scheme 2. Preparation of α-hydrazino acids 8a–8d (MW = micro-
wave).
Scheme 4. Preparation of hybrid α-hydrazino dipeptide (11d + 11dЈ).
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FULL PAPER
Table 3. Preparation of dipeptides 11a–11f that contain one hydraz-
The coupling of α-hydrazino valine 8d with the benzotri-
azole derivative of Cbz-protected phenylalanine 10c to pre-
pare compound 11b did give a product that displayed dupli-
cate peaks in the NMR spectra. This suggests incomplete
inversion of the chirality, and, thus, 8d was accompanied by
enantiomer 8dЈ in a ratio of 2:1 (as inferred by integrations
ino acid unit.
Product
R
R1
M.p.
[°C]
Yield
[%]
11a
11b
11c
11d
CH3
CH(CH3)2
CH2CH(CH3)2 (CH2)4NH(Cbz)
CH2Ph
CH(CH3)2
CH2Ph
140–141
126–128
93–95
113–114
145–150
72–73
61
42
45
68
71
60
50
1
in the H NMR spectrum of 11).
However, the chiral integrity of compounds 11a and 11c–
11f was supported by the NMR spectroscopic data. No-
CH3
CH3
CH2S(Bz)
11d + 11dЈ CH2Ph
1
tably, there was no duplication of peaks in the H and 13C
11e
11f
CH2Ph
CH2CH(CH3)2 CH2(3-indolyl)
81–83
NMR spectra of 11a and 11c–11f. For further proof, we
Figure 2. 13C NMR spectra of compound (a) 11d; (b) 11d + 11dЈ.
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Syntheses of Hydrazino Peptides and Conjugates
coupled compound 8c with the racemate (10a + 10aЈ) to Chiral Acylation of Compounds 13a–13c
give the expected mixture of diastereomers (11d + 11dЈ, see
Protected hydrazino acids 12a–12c were then converted
Scheme 4). The 13C NMR spectrum displayed the duplica-
tion of all of the carbonyl peaks, which were originally sing-
lets in the spectrum of chirally pure isomer 11d (see Fig-
ures 2, a and b). The chirality of all the compounds was
confirmed by the optical rotation and chiral HPLC analy-
sis.
into their benzotriazolides 13a–13c, which were highly hy-
groscopic and used without further purification for the re-
action with nucleophiles. Under the appropriate conditions,
benzotriazolides 13a–13c of the β-N-Cbz-hydrazino acids
underwent successful acylations with N-, O-, S-, and C-nu-
cleophiles to give products with retention of chirality. Dur-
ing the acylation, we did not observe any evidence of oligo-
merization, which indicates that the free NH of hydrazine
group was not affected under these reaction conditions (see
Scheme 6 and Table 5).
Preparation of β-N-Cbz-hydrazino Acids
β-N-Cbz-hydrazino acids 12a–12c were prepared in good
yields (68–92%) by treating the unprotected α-hydrazino
acids with Cbz-Bt[31] at 20 °C in the presence of triethyl-
amine (see Scheme 5 and Table 4).
Scheme 5. Preparation of β-N-Cbz-hydrazino acids 12a–12c.
Table 4. Preparation of N-Pg-hydrazino acids 12a–12c (Pg = pro-
tecting group).
Product
R
M.p. [°C]
Yield [%]
12a
12b
12c
CH3
CH2CH(CH3)2
CH2Ph
110–112
119–120
158–160
68
74
92
Scheme 6. Chiral acylation of compounds 13a–13c.
Table 5. N-, O-, S-, and C-acylations utilizing compounds 13a–13c.
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1
m.p. 250 °C. H NMR (D2O): δ = 3.47 (dd, J = 4.5, 0.9 Hz, 1 H),
2.25–2.12 (m, 1 H), 1.00 (d, J = 6.9 Hz, 6 H) ppm. 13C NMR (D2O/
CD3OD): δ = 147.7, 72.1, 29.5, 19.0 ppm. C5H12N2O2 (132.16):
calcd. C 45.44, H 9.15, N 21.20; found C 45.82, H 9.44, N 21.11.
Conclusions
In conclusion, an efficient general method for the prepa-
ration of hybrid α-hydrazino peptides was developed by tre-
ating N-(Pg-α-aminoacyl)benzotriazolides with α-hydrazine
acids. Similarly, α-hydrazino acid conjugates were prepared
by treating β-N-Cbz-α-hydrazino aminoacylbenzotriazol-
ides with nucleophiles without any evidence of oligomeriza-
tion. All of the hydrazino derivatives were obtained under
mild reaction conditions in good yields and with no detect-
able racemization.
General Procedure for the Synthesis of Cbz-Protected α-(Amino-
acyl)benzotriazoles (10a–10f): The compounds were synthesized by
following our established procedure.[28]
(S)-Benzyl [1-(1H-Benzo[d][1,2,3]triazol-1-yl)-3-(benzylthio)-1-oxo-
propan-2-yl]carbamate [Cbz-L-Cys(Bz)-Bt, 10f]: White microcrys-
1
tals (0.54 g, 84%); m.p. 92–93 °C. H NMR (CDCl3): δ = 8.24 (d,
J = 8.1 Hz, 1 H), 8.13 (d, J = 8.1 Hz, 1 H), 7.67 (t, J = 7.7 Hz, 1
H), 7.53 (t, J = 7.5 Hz, 1 H), 7.40–7.31 (br. s, 5 H), 7.16–7.10 (m,
5 H), 5.98–5.89 (m, 2 H), 5.13 (br. s, 2 H), 3.71 (s, 2 H), 3.17 (dd,
J = 14.1, 5.1 Hz, 1 H), 3.03 (dd, J = 14.3, 6.8 Hz, 1 H) ppm. 13C
NMR (CDCl3): δ = 170.1, 155.9, 146.2, 137.1, 136.1, 131.1, 129.0,
128.7, 128.4, 128.3, 127.4, 126.8, 126.1, 120.6, 115.1, 114.5, 67.6,
53.9, 36.4, 33.8 ppm. C24H22N4O3S (446.52): calcd. C 64.56, H
4.97, N 12.55; found C 64.41, H 5.03, N 12.71.
Experimental Section
General Remarks: Melting points were recorded with a capillary
point apparatus equipped with a digital thermometer. The NMR
spectroscopic data were recorded in D2O, CF3CO2D, CDCl3 or
[D6]DMSO with Mercury, Gemini NMR spectrometers that oper-
ated at 300 MHz for 1H NMR (with TMS as an internal standard)
and 75 MHz for 13C NMR. Elemental analyses were performed
with a Carlo–Erba EA1108 instrument. All microwave-assisted re-
actions were carried out with a single-mode cavity Discover Micro-
wave Synthesizer (CEM Corporation, NC). The reaction mixtures
were transferred into a 10 mL glass pressure microwave tube
equipped with a magnetic stir bar. The tube was sealed with a sili-
con septum, and the reaction mixture was subjected to microwave
irradiation (Discover mode; run time: 60 s; PowerMax-cooling
mode).
General Procedure for the Synthesis of α-Hydrazino Hybrid Dipept-
ides 11a–11f: A dried thick-walled Pyrex tube containing a small
stir bar was charged with Cbz-protected aminoacyl benzotriazole
(1.0 equiv.), α-hydrazino acid (1.0 equiv.), and a catalytic amount
of triethylamine dissolved in THF (3 mL). The reaction mixture
was exposed to microwave irradiation (65 W) at 70 °C (internal
probe) for 15 min. The reaction mixture was cooled until the tem-
perature dropped below 30 °C (approximately 10 min). The solvent
was removed under reduced pressure, and the residue was washed
with diethyl ether and then recrystallized with dichloromethane
(DCM). In a few cases, the residue was subjected to a silica gel
column (DCM/methanol) to remove the benzotriazole and the hy-
brid dipeptide.
General Procedure for the Synthesis of α-Bromo Acids 7a–7c: The
compounds were synthesized by following our established pro-
cedure.[29]
(S)-2-(2-((S)-2-(((Benzyloxy)carbonyl)amino)-3-methylbutanoyl)-
hydrazinyl)propanoic Acid (Cbz-L-Val-NH-L-Ala-OH, 11a): White
microcrystals (0.12g, 61 %); m.p. 140–141 °C. 1H NMR ([D6]-
DMSO): δ = 9.42 (s, 1 H), 7.40–7.25 (m, 6 H), 5.02 (br. s, 2 H),
3.83–3.75 (m, 1 H), 3.55–3.45 (m, 1 H), 1.90–1.82 (m, 1 H), 1.16
(d, J = 6.9 Hz, 3 H), 0.85–0.79 (m, 6 H) ppm. 13C NMR ([D6]-
DMSO): δ = 174.4, 170.3, 156.0, 137.1, 128.3, 127.7, 127.6, 65.4,
58.8, 57.1, 30.2, 19.1, 18.4, 16.1 ppm. C16H23N3O5 (337.37): calcd.
C 56.96, H 6.87, N 12.45; found C 56.64, H 7.05, N 12.08.
General Procedure for the Synthesis of α-Hydrazino Acids 8a–8d:
Hydrazine hydrate (4.7 equiv.) was dissolved in ethanol (3 mL), and
the solution of α-bromo acid 7a–7c in ethanol (1 equiv.) was added
dropwise with water cooling. The reaction mixture was irradiated
under MW at 70 °C (internal probe) and 50 W for 15–20 min. The
resulting solid was washed with diethyl ether, dried under vacuum,
and recrystallized to give the corresponding α-hydrazino acids.
(S)-2-(2-((S)-2-(((Benzyloxy)carbonyl)amino)-3-phenylpropanoyl)-
hydrazinyl)-3-methylbutanoic Acid (Cbz-L-Phe-NH-D/L-Val-OH,
(S)-2-Hydrazinylpropanoic Acid (8a): Recrystallized (ethanol/water)
to give white microcrystals (0.33 g, 48%), m.p. 214–215 °C; ref.[16]
m.p. 186–188 °C. 1H NMR (D2O): δ = 3.80–3.73 (m, 1 H), 1.48 (d,
J = 7.2 Hz, 3 H) ppm. 13C NMR (D2O/[D6]DMSO): δ = 175.8,
61.7, 15.4 ppm.
11b): White powder (0.09 g, 42%); 126–128 °C. 1H NMR ([D6]-
DMSO): δ = 9.53 (d, J = 6.3 Hz, 1 H), 7.53 (t, J = 9.2 Hz, 1 H),
7.37–7.15 (m, 10 H), 4.97–4.87 (m, 2 H), 4.25–4.12 (m, 1 H), 3.17
(d, J = 5.1 Hz, 1 H), 3.05 (d, J = 4.8 Hz, 1 H), 2.93–2.81 (m, 1 H),
2.80–2.67 (m, 1 H), 1.95–1.80 (m, 1 H), 0.96–0.87 (m, 6 H) ppm.
13C NMR ([D6]DMSO): δ = 173.6, 170.2, 155.7, 137.8, 137.0,
129.2, 128.3, 128.1, 127.7, 127.5, 126.3, 67.9, 65.2, 54.8, 37.6, 29.3,
18.7 ppm. C22H27N3O5 (413.47): calcd. C 63.91, H 6.58, N 10.16;
found C 63.67, H 6.80, N 10.55.
(S)-2-Hydrazinyl-4-methylpentanoic Acid (8b): Recrystallized (eth-
anol/water) to give white microcrystals (0.32 g, 43%); m.p. 214–
1
217 °C. H NMR (D2O): δ = 3.64 (t, J = 6.8 Hz, 1 H), 1.78–1.59
(m, 3 H), 0.93 (d, J = 6.3 Hz, 6 H) ppm. 13C NMR (D2O/
[D6]DMSO): δ = 176.4, 65.1, 25.9, 23.4, 23.3 ppm. C6H14N2O2
(146.19): calcd. C 49.30, H 9.65, N 19.16; found C 49.29, H 10.08,
N 19.01.
(S)-2-(2-((S)-2,6-Bis(((benzyloxy)carbonyl)amino)hexanoyl)-
hydrazinyl)-4-methylpentanoic Acid [Cbz-L-Lys(Z)-NH-L-Leu-OH,
(S)-2-Hydrazinyl-3-phenylpropanoic Acid (8c): Recrystallized (hot
1
11c]: White microcrystals (0.09 g, 45%); m.p. 93–95 °C. H NMR
([D6]DMSO): δ = 9.36 (s, 1 H), 7.39–7.26 (m, 12 H), 7.22 (t, J =
5.7 Hz, 1 H), 5.05–4.91 (m, 4 H), 3.96–3.83 (m, 1 H), 3.46–3.37 (m,
1 H), 3.00–2.90 (m, 2 H), 1.78–1.66 (m, 1 H), 1.55–1.44 (m, 2 H),
1.43–1.15 (m, 6 H), 0.87 (t, J = 6.8 Hz, 6 H) ppm. 13C NMR ([D6]-
DMSO): δ = 174.7, 171.0, 156.1, 155.9, 137.3, 137.0, 128.4, 127.8,
65.4, 65.2, 60.7, 53.3, 31.7, 29.1, 24.4, 22.8, 22.7, 22.4 ppm.
C28H38N4O7 (542.63): calcd. C 61.98, H 7.06, N 10.32; found C
62.07, H 7.31, N 10.46.
water) to give white microcrystals (0.35 g, 45%), m.p. 203–206 °C;
1
ref.[24] m.p. 191–194 °C. H NMR (D2O): δ = 7.42–7.18 (m, 5 H),
3.88 (t, J = 6.3 Hz, 1), 3.19 (dd, J = 14.4, 5.7 Hz, 1 H), 3.11 (dd,
J = 14.6, 7.1 Hz, 1 H) ppm. 13C NMR (CF3CO2D): δ = 179.1,
135.7, 132.1, 131.5, 131.2, 64.3, 38.3 ppm. C9H12N2O2 (180.21):
calcd. C 59.99, H 6.71, N 15.54; found C 60.33, H 6.95, N 15.73.
2-Hydrazinyl-3-methylbutanoic Acid (8d): Recrystallized (hot water)
to give white microcrystals (0.29 g, 40%), m.p. 232–235 °C; ref.[22]
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Syntheses of Hydrazino Peptides and Conjugates
(S)-2-(2-((S)-2-(((Benzyloxy)carbonyl)amino)propanoyl)hydrazinyl)–
(S)-2-(2-((Benzyloxy)carbonyl)hydrazinyl)-4-methylpentanoic Acid
(Cbz-NH- -Leu-OH, 12b): White powder (0.28 g, 74%); m.p. 119–
microcrystals (0.16 g, 68 %); m.p. 113–114 °C. 1H NMR ([D6]- 120 °C. 1H NMR ([D6]DMSO): δ = 8.56 (br. s, 1 H), 7.40–7.27 (m,
3-phenylpropanoic Acid (Cbz-
L
-Ala-NH-
L
-Phe-OH, 11d): White
L
DMSO): δ = 9.45 (br. s, 1 H), 7.45 (d, J = 7.2 Hz, 1 H), 7.40–7.32
(m, 5 H), 7.30–7.16 (m, 6 H), 5.06–4.96 (m, 2 H), 4.10–3.98 (m, 1
H), 3.68 (t, J = 6.3 Hz, 1 H), 2.87 (d, J = 6.0 Hz, 2 H), 1.17 (d, J
= 8.4 Hz, 3 H) ppm. 13C NMR ([D6]DMSO): δ = 173.0, 171.6,
155.6, 137.5, 137.0, 129.3, 128.3, 128.1, 127.7, 126.3, 65.3, 63.4,
5 H), 5.07 (d, J = 12.6 Hz, 1 H), 5.01 (d, J = 12.6 Hz, 1 H), 3.46
(t, J = 6.8 Hz, 1 H), 1.81–1.66 (m, 1 H), 1.38 (t, J = 6.9 Hz, 2 H),
0.88–0.84 (m, 6 H) ppm. 13C NMR ([D6]DMSO): δ = 174.8, 156.8,
136.9, 128.3, 127.8, 127.6, 65.4, 61.0, 39.4, 24.3, 22.6, 22.3 ppm.
C
14H20N2O4 (280.32): calcd. C 59.98, H 7.19, N 9.99; found C
48.7, 36.0, 18.2 ppm. C20H23N3O5 (385.42): calcd. C 62.33, H 6.01, 59.91, H 7.37, N 9.99.
N 10.90; found C 62.09, H 6.14, N 10.84.
(S)-2-(2-((Benzyloxy)carbonyl)hydrazinyl)-3-phenylpropanoic Acid
(2S)-2-(2-(2-(((Benzyloxy)carbonyl)amino)propanoyl)hydrazinyl)-
3-phenylpropanoic Acid (Cbz- -Ala-NH- -Phe-OH, 11d + 11dЈ):
(Cbz-NH-L-Phe-OH, 12c): White powder (0.32 g, 92%); m.p. 158–
160 °C. 1H NMR ([D6]DMSO): δ = 8.70 (br. s, 1 H), 7.40–7.13 (m,
11 H), 5.04 (s, 2 H), 3.74 (t, J = 6.3 Hz, 1 H), 2.86 (d, J = 6.3 Hz,
2 H) ppm. 13C NMR ([D6]DMSO): δ = 173.2, 156.9, 137.6, 136.9,
129.3, 128.3, 128.0, 127.8, 127.7, 126.2, 65.5, 52.1, 36.0 ppm.
L
L
White powder (0.17 g, 71 %); m.p. 145–150 °C. 1H NMR ([D6]-
DMSO): δ = 9.64 (br. s, 1 H), 7.48 (d, J = 7.2 Hz, 1 H), 7.40–7.16
(m, 11 H), 5.01 (br. s, 2 H), 4.10–3.98 (m, 1 H), 3.74 (t, J = 6.6 Hz,
0.5 H), 3.69 (t, J = 6.3 Hz, 0.5 H), 2.90 (d, J = 6.6 Hz, 2 H), 1.18
(d, J = 6.9 Hz, 3 H) ppm. 13C NMR ([D6]DMSO): δ = 171.8, 172.7,
171.6, 171.3, 155.6, 137.4, 137.3, 137.0, 129.3, 128.3, 128.1, 127.7,
127.1, 126.3, 65.4, 63.6, 63.4, 48.8, 35.9, 36.0, 18.2 ppm.
C20H23N3O5 (385.42): calcd. C 62.33, H 6.01, N 10.90; found C
62.09, H 5.93, N 10.74.
C
17H18N2O4 (314.34): calcd. C 64.96, H 5.77, N 8.91; found C
65.02, H 5.70, N 8.95.
General Procedure for N-Acylation: Synthesis of Compounds 14a
and 14b: The N-nucleophile (1 equiv.) and triethylamine (1.5 equiv.)
were dissolved in THF (5 mL). The benzotriazole intermediate (13a
or 13b, 1 equiv.) was added to the solution, and the mixture was
stirred at room temperature for 4 h. The mixture was acidified with
HCl (6 n solution), and the resulting solution was concentrated and
then diluted with ethyl acetate. The organic layer was washed with
HCl (6 n solution), dried with anhydrous MgSO4, filtered, and
evaporated to give the desired compound.
(S)-2-(2-((S)-2-(((Benzyloxy)carbonyl)amino)-3-(benzylthio)-
propanoyl)hydrazinyl)-3-phenylpropanoic Acid [Cbz-
L
L-Cys(Bz)-NH-
-Phe-OH, 11e]: White solid (0.14 g, 60 %); m.p. 72–73 °C. 1H
NMR ([D6]DMSO): δ = 9.89 (s, 1 H), 7.63 (d, J = 8.1 Hz, 1 H),
7.37–7.19 (m, 16 H), 5.06–5.04 (m, 2 H), 4.37–4.21 (m, 1 H), 3.76
(s, 2 H), 3.73–3.71 (m, 1 H), 2.94–2.90 (m, 2 H), 2.72–2.63 (m, 1
H), 2.59–2.50 (m, 1 H) ppm. 13C NMR ([D6]DMSO): δ = 172.7,
169.2, 155.8, 138.3, 137.3, 136.9, 129.3, 128.9, 128.3, 128.1, 127.8,
127.7, 126.8, 126.4, 65.5, 63.5, 52.8, 35.9, 35.0, 33.0 ppm. HRMS
[ESI(+)-TOF]: calcd. for C27H29N3O5NaS [M + Na]+ 530.1720;
found 530.1745.
(S)-Benzyl 2-(1-((3-(1H-Imidazol-1-yl)propyl)amino)-1-oxopropan-
2-yl)hydrazinecarboxylate (14a): Oil (0.14 g, 71 %). 1H NMR
(CD3OD): δ = 7.53 (s, 1 H), 7.24–7.16 (m, 5 H), 6.99 (s, 1 H), 6.84
(s, 1 H), 4.96 (br. s, 2 H), 3.82 (t, J = 6.9 Hz, 2 H), 3.35 (q, J =
6.9 Hz, 1 H), 3.00 (t, J = 6.5 Hz, 2 H), 1.77 (t, J = 6.8 Hz, 2 H),
1.17 (d, J = 6.9 Hz, 3 H) ppm. 13C NMR (CD3OD): δ = 176.8,
159.5, 138.6, 138.1, 129.6, 129.2, 129.1, 129.0, 120.7, 67.9, 61.6,
45.5, 37.1, 31.9, 31.1, 17.3 ppm. HRMS [ESI(+)-TOF]: calcd. for
C17H23N5O3 [M + H]+ 346.1874; found 346.1890.
(S)-2-(2-((S)-2-(((Benzyloxy)carbonyl)amino)-3-(1H-indol-3-yl)-
propanoyl)hydrazinyl)-4-methylpentanoic acid (Cbz-L-Trp-NH-L-
Leu-OH, 11f): White microcrystals (0.11 g, 50%); m.p. 81–83 °C.
1H NMR ([D6]DMSO): δ = 10.79 (s, 1 H), 9.54 (s, 1 H), 7.62 (d, J
= 7.8 Hz, 1 H), 7.38 (d, J = 8.4 Hz, 1 H), 7.33–7.21 (m, 7 H), 7.13–
7.12 (m, 1 H), 7.04 (t, J = 7.1 Hz, 1 H), 6.95 (t, J = 7.4 Hz, 1 H),
4.96–4.86 (m, 2 H), 4.27–4.22 (m, 1 H), 3.46–3.33 (m, 1 H), 3.00
(dd, J = 14.6, 4.2 Hz, 1 H), 2.88 (dd, J = 14.4, 9.6 Hz, 1 H), 1.80–
1.65 (m, 1 H), 1.40 (t, J = 6.9 Hz, 2 H), 0.87 (d, J = 6.6 Hz, 3 H),
0.85 (d, J = 6.9 Hz, 3 H) ppm. 13C NMR ([D6]DMSO): δ = 174.6,
170.8, 155.7, 137.0, 136.0, 128.3, 127.7, 127.5, 127.2, 123.8, 120.8,
118.5, 118.2, 111.3, 110.0, 65.2, 60.7, 54.1, 28.0, 24.4, 22.6,
22.4 ppm. HRMS [ESI(–)-TOF]: calcd.for C25H30N4O5 [M – H]+
465.2143; found 465.2164.
(S)-Benzyl 2-(1-Morpholino-1-oxo-3-phenylpropan-2-yl)hydrazine-
carboxylate (14b): Oil (0.13 g, 69%). 1H NMR ([D6]DMSO): δ =
10.15 (br. s, 1 H), 8.70 (br. s, 1 H), 7.36–7.10 (m, 10 H), 5.14–5.05
(m, 2 H), 4.05–4.01 (m, 1 H), 3.54–3.28 (m, 8 H), 2.98–2.79 (m, 2
H) ppm. 13C NMR ([D6]DMSO): δ = 169.6, 156.7, 137.7, 136.9,
129.3, 128.5, 128.4, 127.9, 127.8, 126.3, 66.6, 65.9, 65.8, 41.9, 36.9,
30.5 ppm. HRMS [ESI(+)-TOF]: calcd. for C21H25N3O4Na [M +
Na]+ 406.1737; found 406.1731.
General Procedure for O-Acylation: Synthesis of Compound 14c: A
dried thick-walled Pyrex tube containing a small stir bar was
charged with the benzotriazole intermediate 13c (1 equiv.). The O-
nucleophile (1.5 equiv.) and 4-(dimethylamino)pyridine (DMAP,
0.1 equiv.) were dissolved in THF (3 mL). The reaction mixture
was exposed to microwave irradiation (50 W) at 60 °C (internal
probe) for 1 h. The mixture was cooled through a custom-made
system until the temperature fell below 30 °C (approximately
10 min). The reaction mixture was quenched with water, and the
resulting solution was extracted with EtOAc. The extracts were
washed with a 10% Na2CO3 aqueous solution and then dried with
MgSO4. The solvent was removed under reduced pressure, and the
residue was subjected to a silica gel column (EtOAc/hexane) to give
compound 14c.
General Procedure for the Synthesis of Cbz-Hydrazino Acids 12a–
12c: To a solution of the hydrazino acid (1.0 equiv.) in water (2 mL)
were added Et3N (2.0 equiv.) and a solution of Cbz-Bt (1.0 equiv.)
in acetonitrile (10 mL). The reaction mixture was stirred at room
temperature for 3–4 h (monitored by TLC). The mixture was then
acidified with HCl (4 n solution, 2 mL). Acetonitrile was removed
under reduced pressure, and the resulting crude product was sub-
jected to column chromatography (hexanes/ethyl acetate) to give
the desired product.
(S)-2-(2-((Benzyloxy)carbonyl)hydrazinyl)propanoic Acid (Cbz-NH-
L
-Ala-OH, 12a): White powder (0.31 g, 68%); m.p. 110–112 °C. 1H
NMR ([D6]DMSO): δ = 8.63 (br. s, 1 H), 7.91 (br. s, 1 H), 7.40–
7.28 (m, 5 H), 5.04 (s, 2 H), 3.60–3.51 (m, 1 H), 1.13 (d, J = 6.0 Hz, Benzyl 2-((S)-1-(((1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl)oxy)-
3 H) ppm. 13C NMR ([D6]DMSO): δ = 174.5, 156.9, 136.9, 128.3,
127.8, 127.7, 65.5, 16.0 ppm. HRMS [ESI(+)-TOF]: calcd. for
C11H14N2O4Na [M + Na]+ 261.0846; found 261.0840.
1-oxo-3-phenylpropan-2-yl)hydrazinecarboxylate (14c): White pow-
der (0.11 g, 49%); m.p. 112–114 °C. 1H NMR ([D6]DMSO): δ =
7.33–7.18 (m, 11 H), 6.40 (br. s, 1 H), 5.14–5.00 (m, 2 H), 4.75–
Eur. J. Org. Chem. 2013, 4156–4162
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4161

S. S. Panda, C. El-Nachef, K. Bajaj, A. R. Katritzky
FULL PAPER
4.60 (m, 1 H), 4.00–3.92 (m, 1 H), 3.10–2.91 (m, 2 H), 1.82–1.60
(m, 4 H), 1.46–1.22 (m, 2 H), 1.08–0.96 (m, 1 H), 0.87–0.77 (m, 8
H), 0.68 (d, J = 7.2 Hz, 3 H) ppm. 13C NMR ([D6]DMSO): δ =
172.4, 156.8, 136.4, 136.1, 129.4, 129.3, 128.7, 128.4, 128.2, 127.1,
75.4, 67.3, 64.2, 47.0, 40.8, 37.1, 34.3, 31.5, 26.5, 23.5, 22.2, 20.9,
16.5 ppm. HRMS [APCI(+)-TOF]: calcd. for C27H37N2O4 [M +
H]+ 453.2748; found 453.2744 (APCI = atmospheric pressure
chemical ionization).
Kenan Foundation, and King Abdulaziz University, Jeddah, Saudi
Arabia for their financial support.
[1]
[2]
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General Procedure for S-Acylation: Synthesis of Compounds 14d and
14e: The mercapto nucleophile (1 equiv.) and triethylamine
(1.5 equiv.) were dissolved in THF (5 mL). Benzotriazole interme-
diate (13b or 13c, 1 equiv.) was added to the solution, and the mix-
ture was stirred at room temperature for 4 h and then acidified with
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(S)-Benzyl 2-(1-(Benzylthio)-4-methyl-1-oxopentan-2-yl)hydrazine-
1
carboxylate (14d): Oil (0.12 g, 61%). H NMR (CDCl3): δ = 7.44–
7.20 (m, 10 H), 6.48 (br. s, 1 H), 5.15 (d, J = 12.0 Hz, 2 H), 5.08
(d, J = 12.0 Hz, 1 H), 4.09 (s, 2 H), 3.90–3.78 (m, 1 H), 1.88–1.73
(m, 1 H), 1.49 (t, J = 6.8 Hz, 2 H), 1.10–0.85 (m, 6 H) ppm. 13C
NMR (CDCl3): δ = 203.0, 157.1, 137.5, 135.9, 129.0, 128.8, 128.6,
128.4, 127.4, 69.2, 67.6, 41.0, 32.9, 25.1, 23.3, 22.2 ppm. HRMS
[ESI(+)-TOF]: calcd. for C21H26N2O3SNa: [M + Na]+ 409.1556;
found 409.1572.
[10]
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[12]
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(S)-Benzyl 2-(1-((2-Methoxy-2-oxoethyl)thio)-1-oxo-3-phenylprop-
1
an-2-yl)hydrazinecarboxylate (14e): Oil (0.12 g, 63 %). H NMR
([D6]DMSO): δ = 7.45–7.20 (m, 11 H), 6.68–6.60 (m, 1 H), 5.12–
4.96 (m, 2 H), 4.20–4.08 (m, 1 H), 3.71 (s, 3 H), 3.65 (s, 2 H) 3.14–
3.06 (m, 1 H), 2.84–2.78 (m, 1 H) ppm. 13C NMR ([D6]DMSO): δ
= 201.0, 169.3, 156.8, 135.7, 135.2, 129.3, 128.8, 128.6, 128.4, 128.2,
127.4, 70.4, 67.4, 52.8, 37.9, 30.7 ppm. HRMS [ESI(+)-TOF]:
calcd. for C20H22N2O5NaS [M + Na]+ 425.1142; found 425.1162.
H. Gustafsson, Acta Chem. Scand. 1975, B29, 93–98.
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General Procedure for C-Acylation: Synthesis of Compound 14f: A
dried thick-walled Pyrex tube containing a small stir bar was
charged with benzotriazole intermediate 13c (1 equiv.), the N-nu-
cleophile (1 equiv.) dissolved in THF (5 mL), and N,N-diisopropyl-
ethylamine (DIPEA, 1.5 equiv.). The reaction mixture was exposed
to microwave irradiation (50 W) at 60 °C (internal probe) for
30 min. The mixture was cooled through an inbuilt system until the
temperature fell below 30 °C (approximately 10 min). The reaction
mixture was quenched with water, and the resulting solid was fil-
tered and washed with 10% Na2CO3 and then water to give the
desired compound.
[21] M. J. Burk, J. E. Martinez, J. E. Feaster, N. Cosford, Tetrahe-
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2007, 80, 2165–2168.
(S)-Benzyl 2-(4,4-Dicyano-3-oxo-1-phenylbutan-2-yl)hydrazinecarb-
oxylate (14f): Light orange solid (0.15 g, 88%); m.p. 233–235 °C.
1H NMR ([D6]DMSO): δ = 8.49 (br. s, 1 H), 7.38 (br. s, 5 H), 7.23–
7.14 (m, 5 H), 5.07 (s, 2 H), 4.04 (t, J = 4.7 Hz, 1 H), 3.01–2.86
(m, 2 H) ppm. 13C NMR ([D6]DMSO): δ = 188.3, 169.0, 155.0,
136.2, 135.8, 129.7, 128.4, 128.0, 127.8, 126.3, 114.9, 67.3, 66.9,
33.9 ppm. C20H18N4O3 (362.39): calcd. C 66.29, H 5.01, N 15.46;
found C 66.10, H 4.84, N 15.17.
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8690–8694.
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR and CHN/HRMS spectra for all com-
pounds.
[28] S. Biswas, S. R. Tala, A. Kalatozishvili, A. R. Katritzky, ARKI-
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Acknowledgments
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Received: December 21, 2012
The authors thank Dr C. D. Hall for useful suggestions and check-
ing the English. We also thank the University of Florida, The
Published Online: May 16, 2013
4162
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 4156–4162