Benzotriazole-mediated syntheses of depsipeptides and oligoesters

Article abstract of DOI:10.1021/jo200174j

Reactions of O-Pg(α-hydroxyacyl)benzotriazoles with (a) unprotected α-hydroxycarboxylic acids, (b) amino acids, and (c) amines afforded, respectively, chirally pure (a) oligoesters, (b) depsidipeptides, and (c) amide conjugates (yields 52-94%). N-Pg(α-Ami

Full text of DOI:10.1021/jo200174j

ARTICLE
pubs.acs.org/joc
Benzotriazole-Mediated Syntheses of Depsipeptides and Oligoesters
Ilker Avan,‡,§ Srinivasa R. Tala,‡ Peter J. Steel,|| and Alan R. Katritzky*,‡
‡Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
§Department of Chemistry, Anadolu University, 26470, Eski-sehir, Turkey
Department of Chemistry, University of Canterbury, Christchurch, New Zealand
S Supporting Information
b
ABSTRACT: Reactions of O-Pg(R-hydroxyacyl)benzotriazoles
with(a) unprotectedR-hydroxycarboxylicacids, (b) aminoacids,
and (c) amines afforded, respectively, chirally pure (a) oligoe-
sters, (b) depsidipeptides, and (c) amide conjugates (yields
52ꢀ94%). N-Pg(R-Aminoacyl)benzotriazoles reacted with R-
hydroxycarboxylic acids to yield depsidipeptides (47ꢀ87%). N-
Pg(depsidipeptidoyl)benzotriazoles, obtained from depsidipep-
tides, gave depsitripeptides (yields 55ꢀ78%) on reaction with
amino acids and R-hydroxycarboxylic acids. O-Acylation of R-
hydroxycarboxylic acids with N-Pg(R-aminoacyl)benzotriazoles
followed by deprotection produced unprotected depsides useful
for the preparation of depsitripeptides.
’ INTRODUCTION
Methods for the preparation of depsipeptides include the use,
alone or in combination, of coupling reagents such as DIC/
DMAP,11 DCC/DMAP;12 EDCl/DMAP,13 or PyBrop/DIEA14
or Yamaguchi conditions (TCBC, DIPEA, DMAP).15 The study
of several coupling reagents by Riguera et al.11 showed variable
yields (2ꢀ92%) and coupling times (2ꢀ20 h), with the best
results (92%, 2 h) obtained by DIC in the presence of DMAP.
N-Acylbenzotriazoles are convenient intermediates to prepare
longer peptide conjugates since they are stable, mostly crystalline
compounds and easy to handle. N-Fmoc(R-aminoacyl)benzo-
triazoles were used to synthesize “difficult” peptide sequences on
SPPS in good yields.16 Recently, we were able to prepare N-
Pg(tri- or tetrapeptidoyl)benzotriazoles as advantageous coupling
reagents for synthesizing tetra-, penta-, and hexapeptides in so-
lution phase.17 These N-acylbenzotriazoles are also advantageous
for N-, O-, C-, and S-acylation, especially while the corresponding
acid chlorides are unstable or difficult to prepare.18 Cysteine-
containing S-Pg(R-aminoacyl)di- and tripeptides were prepared
from N-Pg(R-aminoacyl)benzotriazoles via S-acylation without
racemization and underwent chemical ligations to generate native
peptides.19 Moreover, we previously demonstrated the utility of
N-Pg(R-aminoxyacyl)benzotriazoles for synthesizing aminoxy
peptides in peptidomimetics.20 We now describe a new and mild
method for the efficient preparation of depsipeptides via both N-
and O-acylation. In the present work, O-Pg(R-hydroxyacyl)-
benzotriazoles were prepared and their synthetic utility was
shown in the synthesis of both depsipeptides via N-acylation
and chiral oligoesters via O-acylation without causing
Depsipeptides, analogues of peptides, contain both amino
acids and hydroxy acids linked by amide and ester bonds. Natural
depsipeptides can exhibit significant biological activities (antimi-
crobial, antifungal, anti-inflammatory) and possess therapeutic
properties including anticancer and anti-HIV activity.1
Cyclic depsipeptide romidepsin (FR228) is an antineoplastic
antitumor agent as histone deacetylase inhibitor,2 originally ex-
tracted from the bacterium Chromobacterium violaceum. Romi-
depsin (FK228) is an FDA-approved anticancer drug marketed
under the trade name Istodax for treatment of cutaneous T-cell
lymphoma.3 Cytotoxic depsipeptides dolastine-10 and didemnin
B were subjected to extensive phase II studies revealing their
antitumor activities.4 Callipeltins and papuamide A show pro-
mising inhibitory activity against HIV.1,5 The cyclic depsipeptide
valinomycin is a natural ionophore selective for potassium;6a
nonactin is another natural ionophore.6b
Depsipeptides contain at least one ester bond in place of an
amide link in their structure. The isosteric replacement of the
H-bond donor group (NꢀH) by the O atom of an ester causes
depsipeptides to have poorer H-bonding. The decreased reso-
nance of esters relative to amides leads to lower rotational barriers
for cisꢀtrans isomerization in depsipeptides than in native peptide
conjugates and also induces more flexible structures.7 Replacing an
aminoacidresiduewithanR-hydroxy acid causes minimal structural
perturbation in R-helix8 and β-sheet9 peptide structures. Quantum
mechanical calculations on the conformational properties of an
alanine dipeptide show this depsidipeptide has a strong preference
to have polyproline II (PPII) helical conformation by decreasing
R-helix and β-sheet structures.10
Received: January 24, 2011
Published: March 31, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of Depsiamides 3aꢀd
Table 1. O-Pg(r-Hydroxyacyl)benzotriazoles 2aꢀe
Table 3. Depsidipeptides 5aꢀe
entry
Pg: protecting group
R1
2, yielda (%)
entry
Pg
R1
amino acid, 4
5, yield (%)
5a, 86
a
Z
CH2Ph (Phe)
2a, 76
2b, 86
2c, 72
2d, 80
2e, 78
a
b
c
d
e
f
Z
Z
CH2Ph (Phe), 2a L-Ala-OH, 4a
CH2Ph (Phe), 2a DL-Ala-OH, (4aþ4a0) (5aþ5a0), 80
b
c
Z
CH2CH(CH3)2 (Leu)
CH2CH(CH3)2 (Leu)
CH2Ph (Phe)
Fmoc
Fmoc
THP
Fmoc CH2Ph (Phe), 2c L-Met-OH, 4b
Fmoc CH2Ph (Phe), 2c L-Glu-OH, 4c
Fmoc CH2Ph (Phe), 2c L-Ala-OH, 4a
Fmoc CH2Ph (Phe), 2c DL-Ala-OH, (4aþ4a0) (5dþ5d0), 94
Fmoc CH2Ph (Phe), 2c L-Phe-OH, 4d
5b, 74
5c, 82
5d, 87
d
e
CH(CH3)2 (Val)
a Isolated yield.
g
h
5e, 86
Fmoc CH2Ph (Phe), 2c DL-Phe-OH, (4dþ4d0) (5eþ5e0), 80
Table 2. O-Pg(r-Hydroxyacyl)amides 3aꢀd
entry
2
amine
3, yielda (%)
and 5.95 min. Compound 3b has a single retention-time
peak in chiral HPLC at 10.02 min, while its corresponding
diastereomeric pair (3bþ3b0) showed two peaks at 7.81 and
10.03 min (MeOH (100%), flow rate 0.5 mL/min, detection at
254 nm).
a
b
c
d
e
f
Z-(O-Phe)-Bt, 2a
Z-(O-Phe)-Bt, 2a
Z-(O-Leu)-Bt, 2b
Z-(O-Leu)-Bt, 2b
Z-(O-Leu)-Bt, 2b
F-(O-Leu)-Bt, 2c
L-R-methylbenzylamine
DL-R-methylbenzylamine
L-R-methylbenzylamine
DL-R-methylbenzylamine
p-methoxyaniline
3a, 88
(3aþ3a0), 76
3b, 65
(3bþ3b0), 56
3c, 71
Preparation of Depsipeptides. Coupling of O-Pg(R-hydro-
xyacyl)benzotriazoles 2 with the appropriate natural amino acids
in aqueous acetonitrileꢀdiisopropylethylamine (DIEA) at 10 °C
for 0.5ꢀ2 h yielded depsidipeptides 5aꢀe (74ꢀ94%) (Scheme 2
and Table 3). In an attempt to show the retention of chirality,
diastereoisomeric analogues of 5a, 5d, and 5e were prepared
from DL-amino acids, and chiral HPLC analyses were performed
using Chirobiotic T, (S,S) Whelk-O1, and Chiracel OD-H. Diaster-
eoisomeric separations for (5aþ5a0), (5dþ5d0), and (5eþ5e0)
were not observed on HPLC analysis using a variety of solvent
system and flow rates; however, the absence of racemization in the
depsidipeptide conjugates (5aþ5a0) and (5dþ5d0) was deduced
from the 1H NMR, where the methyl signal showed two separated
doublets split in the DL-alanine moiety. While 5a has a clear
doublet at 1.27 ppm (J = 7.2 Hz), (5aþ5a0) has two separated
doublets at 1.24 and 1.33 ppm (J = 7.2 Hz). A similar result was
observed for 5d and (5dþ5d0). Spectral data for 5a, 5d, and their
diastereoisomers (5aþ5a0) and (5dþ5d0) are available in the
Supporting Information. In previous studies, we showed retention
of chirality on N-acylation with N-acylbenzotriazoles for aminoxy
hybrid-peptides20 and peptides.18g
L-R-methylbenzylamine
3d, 67
a Isolated yield.
Scheme 2. Preparation of Depsipeptides 5aꢀe via
N-Acylation
racemization. Unprotected depsidipeptides (depsides) were read-
ily obtained from N-Pg(R-aminoxyacyl)benzotriazoles and used
for the preparation of longer depsipeptide conjugates.
’ RESULTS AND DISCUSSION
Preparation of Depsiamides 3aꢀd. O-Pg(R-Hydroxya-
cyl)benzotriazoles 2aꢀe (72ꢀ86%) were prepared by treatment
of the corresponding carboxylic acids 1aꢀe with 4 equiv of
1H-benzotriazole and 1 equiv of SOCl2 in THF at 10 °C for
4 h (Scheme 1, Table 1). Reaction of O-Pg(R-hydroxyacyl)-
benzotriazoles 2aꢀd with amines and amino acids affords the
corresponding amide derivatives 3aꢀd (56ꢀ88%) (Scheme 1
and Table 2). Retention of the original chirality in the products
was confirmed by chiral HPLC analysis using a (S,S) Welk-O1
column (MeOH (100%), flow rate 1.0 mL/min, detection at
254 nm). The diastereomer 3a showed a single retention-time
peak in chiral HPLC at 5.93 min, while its corresponding
diastereomeric mixture (3aþ3a0) showed two peaks at 4.24
Compound 5b was treated with SOCl2 and 1H-benzotriazole
in THF to obtain benzotriazole-activated conjugate 6 (Scheme 3).
O-Fmoc(depsidipeptidoyl)benzotriazole 6 was treated with
L-methylbenzylamine to afford the corresponding amide derivative
7 in 52% yield (Scheme 3).
N-Pg(R-Aminoacyl)benzotriazoles 9aꢀd on treatment with
R-hydroxycarboxylic acids 10 in the presence of DMAP in THF
afforded depsidipeptides 11aꢀd (Scheme 4). The use of DMAP,
which acts as a base and also activates the carboxyl group via the
acyl pyridinium salt,21 is advantageous for O-acylation. More-
over, in our preliminary results, when DBU, TEA, and DIEA were
used for O-acylation, O-acylated products showed an increase
in racemization since the coupling reaction needs a longer time.
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Scheme 3. Preparation of Amide Derivative 7
Scheme 4. Preparation of Depsidipeptides 11 and 13 via O-Acylation
Scheme 5. Preparation of Depsitripeptides 15a, 15b, and 16
The enantiopurity of product depsidipeptides 11a and (11aþ11a0)
was supported by chiral HPLC analysis using a Chirobiotic T
column (MeOH (100%), flow rate 0.5 mL/min, detection at
254 nm.). Compound 11a showed a single retention-time peak in
chiral HPLC analysis at 6.46 min, while the corresponding diaster-
eomixture (11aþ11a0) showed two peaks at 6.33 and 6.84 min.
Similarly, compound 13 was derived from the tert-butyl ester of
R-hydroxycarboxylate 12b in 87% yield (Scheme 4). Compounds
11b and 11c were isolated as the dicyclohexylamine salts.
N-Cbz(Depsidipeptidoyl)benzotriazoles 14a and 14b were
obtained by treatment of 11a and 11b with benzotriazole in the
presence of SOCl2 in THF. Reaction of 14a and 14b with amino
acids 4a and 4b in CH3CNꢀwater in the presence of DIEA
Figure 1. X-ray crystal structure of Z-Leu-(O-Phe)-Met-OH 15a.
Hydrogen atoms are not shown.
afforded depsitripeptides 15a and 15b, while treatment with R-
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Scheme 6. Preparation of Oligoesters 17aꢀc via O-Acylation
Scheme 7. Synthesis of Unprotected Depsides 19aꢀd
Scheme 8. Preparation of Depsitripeptides 20aꢀc
hydroxycarboxylic acid 10c in THF in the presence of DMAP
gave depsitripeptide 16 by O-acylation (Scheme 5).
Table 4. Unprotected Depsidipeptides 19aꢀd
entry
R1
R2
19, yield (%)
The structure and absolute configuration of 15a was unam-
biguously established by X-ray crystallography (Figure 1).
Preparation of Chiral Oligoesters 17aꢀc. Oligoesters 17a
and 17b were prepared by the reaction of Z-(O-Phe)-Bt 2a with
unprotected R-hydroxy acids 10b and 10d in THF in the
presence of DMAP for 4ꢀ6 h (75ꢀ85%). In the case of Z-(O-
Leu)-Bt 2b, under the same conditions, the coupling reaction was
completed in 10 h. Based on 1H NMR studies, we observed some
racemization due to the longer reaction time. To overcome this
problem, the coupling reaction was carried out using the tert-
butyl ester of R-hydroxy acid 12a. Once 18 was formed, it was
deprotected using a mixture of TFA/CH2Cl2 (1:1) to afford 17c
in 69% overall yield (Scheme 6).
a
H
CH2Ph
19a, 48
19b, 36
19c, 62
19d, 56
b
c
CH3
H
CH2CH(CH3)2
CH2CH(CH3)2
CH2CH(CH3)2
d
CH2Ph
preparation, the functions and applications of dipeptides have
been poorly examined compared with proteins or amino acids.22
We now describe a new route to prepare unprotected depsidi-
peptides under mild conditions and a further manipulation to
obtain depsitripeptides (Schemes 7 and 8). Boc-protected amino
acids 8eꢀg were treated with 1H-benzotriazole in DCC-cou-
pling conditions to obtain N-Boc(R-aminoacyl)benzotriazoles
9eꢀg, which were reacted with R-hydroxy acids 10a and 10b in
THF in the presence of DMAP to obtain Boc-protected
Unprotected Depsides 19. L-R-Dipeptides (dipeptides) are
useful intermediates to attain longer peptide analogues; never-
theless, because of the lack of an efficient process for dipeptide
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Table 5. Depsitripeptides 20aꢀc
entry
128.5, 128.8, 129.6, 134.9, 135.4, 154.7, 174.4. Anal. Calcd for C17H16O5: C,
67.99; H, 5.37. Found: C, 67.71; H, 5.50.
R3
R1
R2
20, yield (%)
Z-(O-Leu)-OH (1b): colorless oil (62%); [R]2D3 = ꢀ29.2 (c 1.5,
1
CH3OH); H NMR (CDCl3) δ 0.95 (dd, J = 6.0 Hz, 3.3 Hz, 6 H),
a
b
c
CH2Ph
H
CH2CH(CH3)2
CH2CH(CH3)2
CH2CH(CH3)2
20a, 71
20b, 56
20c, 68
1.62ꢀ1.76 (m, 1H), 1.78ꢀ1.91 (m, 2H), 5.00 (dd, J = 9.6 Hz, 3.9 Hz,
1H), 5.20 (s, 2H), 7.30ꢀ7.40 (m, 5H); 13C NMR (CDCl3) δ 21.6, 23.1,
24.7, 39.9, 70.9, 73.4, 128.5, 128.8, 135.0, 154.9, 176.2. Anal. Calcd for
C14H18O5: C, 63.14; H, 6.81. Found: C, 62.93; H, 7.14.
CH2CH2SCH3
CH2CH2SCH3
CH2Ph
H
depsidipeptides 11dꢀg. Then, without isolation of 11dꢀg, Boc-
deprotection was conducted by HCl(g) in dry CH2Cl2 for 4ꢀ6 h
to yield unprotected depsidipeptides 19aꢀd as hydrochloride
salts (Scheme 7, Table 4).
Unprotected depsidipeptides 19a and 19d were reacted with
N-Pg(R-aminoacyl)benzotriazoles 9c and 9d in the presence of 2
equiv of DIEA in MeCNꢀH2O (7:1) to afford N-Pg-depsitri-
peptides 20aꢀc (Scheme 8, Table 5).
Syntheses of 1c and 1d. A solution of pyridine (8.2 mmol) in THF
(8 mL) was added to a stirred solution of Fmoc-Cl (8.0 mmol) and R-
hydroxycarboxylic acids 10 (8.2 mmol) at ꢀ10 °C in THF (30 mL). The
reaction was allowed to reach room temperature and stirred overnight.
The reaction was monitored by TLC [EtOAcꢀhexanes (1:4)]. The
mixture was taken into a separation funnel and washed with 2 N HCl
(3 ꢁ 20 mL) and brine (20 mL). After the mixture was dried over
MgSO4, the solvent was removed under reduced pressure. The crude
was purified by column chromatography [EtOAcꢀhexanes (1:4)] to
afford 1c,d as white microcrystals.
’ CONCLUSION
F-(O-Leu)-OH(1c):27.white microcrystals (1.76 g, 62%); mp 112ꢀ113 °C;
[R]2D3 = ꢀ8.9 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 0.99 (dd, J = 7.2 Hz,
6.6 Hz, 6H), 1.66ꢀ1.78 (m, 1H), 1.80ꢀ1.98 (m, 2H), 4.26ꢀ4.40 (m,
2H), 4.53 (dd, J = 9.9 Hz, 6.9 Hz, 1H), 5.02 (dd, J = 9.6 Hz, 3.6 Hz, 1H),
7.28ꢀ7.36 (m, 2H), 7.36ꢀ7.44 (m, 2H), 7.62 (t, J = 6.6 Hz, 2H), 7.76 (d,
J = 7.8 Hz, 2H); 13C NMR (CDCl3) δ 21.6, 23.2, 24.8, 39.8, 46.9, 70.6,
73.8, 120.3, 125.3, 125.4, 127.4, 128.1, 141.5, 143.2, 143.5, 154.9, 176.2.
Anal. Calcd for C21H22O5: C, 71.17; H, 6.26. Found: C, 71.26; H, 6.22.
F-(O-Phe)-OH (1d): white microcrystals (1.6 g, 51%); mp 101ꢀ103 °C;
[R]2D3 = ꢀ12.4 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 3.18 (dd, J = 14.4
Hz, 9.0 Hz, 1H), 3.30 (dd, J = 14.4 Hz, 3.9 Hz, 1H), 4.20ꢀ4.34 (m, 2H),
4.38ꢀ4.46 (m, 1H), 5.21 (dd, J = 8.7 Hz, 4.1 Hz, 1H), 7.20ꢀ7.42 (m,
9H), 7.54 (dd, J = 7.5 Hz, 3.3 Hz, 2H), 7.73 (d, J = 7.5 Hz, 2H), 9.35 (br s,
1H); 13C NMR (CDCl3) δ 37.4, 46.7, 70.6, 75.8, 120.2, 125.3, 125.4,
127.4, 127.5, 128.1, 128.8, 129.6, 135.5, 141.4, 143.2, 143.6, 154.6, 175.1.
Anal. Calcd for C24H20O5: C, 74.21; H, 5.19. Found: C, 74.02; H, 5.42.
THP-(O-Val)-OH (1e) was prepared according to the literature
method:24 colorless oil (77%); [R]D23 = ꢀ27.1 (c 1.5, CHCl3).
General Preparation of O-Pg(r-hydroxyacyl)benzotriazole (2).
Thionyl chloride (1.2 mmol) was added to a solution of benzotriazole
(4.16 mmol) in freshly distilled CH2Cl2 (10 mL) at 5 °C and the reaction
mixture stirred for 20 min at the same temperature. O-(Pg)Hydro-
xycarboxylic acid 1 (1.0 mmol) dissolved in CH2Cl2 (2 mL) was added
dropwise to the mixture. After being stirred for 3 h at 10 °C, the reaction
mixture was allowed to warm to room temperature. After 1 h, the white
precipitate was filtered off and discarded. The solution was diluted with
more CH2Cl2 (10 mL), washed with saturated Na2CO3 solution (3 ꢁ
10mL) and then saturatedbrinesolution, andfinallydriedoveranhydrous
Na2SO4. The solvent was evaporated under reduced pressure to afford
O-Pg(hydroxyacyl)benzotriazole 2.
In conclusion, novel O-Pg(R-hydroxyacyl)benzotriazoles have
been prepared and used for the synthesis of unprotected depsides,
depsipeptides, depsitripeptides, and chiral oligoesters by O-acyla-
tion of unprotected R-hydroxy acids in dry THF solvent or N-
acylation of unprotected amino acids in aqueous condition in good
yields under mild conditions. The retention of chirality of depsi-
peptides in O-acylation and N-acylation reactions was examinedby
chiral HPLC, which revealed no racemization. In addition, novel
unprotected depsides were prepared in good yields, and their
synthetic utility for the preparation of longer depsipeptide analo-
gues was demonstrated. These synthetic methodologies are ex-
pected to be useful for the syntheses of biologically active longer
and cyclic depsipeptides.
’ EXPERIMENTAL SECTION
Melting points are uncorrected. 1H (300 MHz) and 13C (75 MHz)
NMR spectra were recorded on 300 MHz apparatus in CDCl3 or
DMSO-d6 with TMS as internal standard. The data are reported as
follows: chemical shift in parts per million (ppm, δ units) and spinꢀspin
coupling J (Hz). DMF was dried and distilled over CaH2, whereas THF
was used after distillation over Na/benzophenone. Unprotected amino
acids 4 and N-(protected)-amino acids 8 were purchased from com-
mercial sources. N-(Acylbenzotriazoles) 9aꢀd were prepared by our
previously reported methods.18b,c The elemental analysis of 2c deviates
from the calculated value. This compound was further characterized by the
preparation of 3d. L-R-Hydroxycarboxylic acids 10,23 THP-L-O-(Val)-OH
1e,24 and 12a,b25 were prepared using literature methods.
Syntheses of O-Cbz- and O-Fmoc-Protected r-Hydroxy-
carboxylic Acids Analogues 1aꢀe. Syntheses of 1a and 1b.
Pyridine (18 mmol) was added to a stirred solution of R-hydroxycar-
boxylic acids 10 (18 mmol) in THF (30 mL) dropwise at 4 °C in 10 min.
After the mixture was stirred 10 min in the cold, benzyl chloroformate
(21.7 mmol) was added at same temperature over 30 min. Then the
mixture was stirred overnight at room temperature. The solvent was
removed and the residue taken into EtOAc (50 mL), washed with 2 N
HCl (3 ꢁ 15 mL) and brine (15 mL), and dried over MgSO4. The
solvent was evaporated under reduced pressure. The crude was purified
with flash column chromatography (EtOAcꢀhexanes (15%)) to afford
1a,b as a colorless oil.
Z-(O-Phe)-Bt (2a): white microcrystals (76%); mp 97ꢀ98 °C;
[R]2D3 = ꢀ4.3 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 3.34 (dd, J = 14.1
Hz, 9.0 Hz, 1H), 3.50 (dd, J = 14.1 Hz, 3.6 Hz, 1H), 5.15 (s, 2H), 6.49
(dd, J = 9.0 Hz, 3.6 Hz, 1H), 7.20ꢀ7.42 (m, 10H), 7.55 (t, J = 7.8 Hz,
1H), 7.69 (t, J = 7.7 Hz, 1H), 8.15 (d, J = 8.1 Hz, 1H), 8.26 (d, J =
8.1 Hz, 1H); 13C NMR (CDCl3) δ 38.0, 70.6, 114.5, 120.6, 126.9,
127.6, 128.5, 128.8, 129.7, 131.1, 131.2, 134.8, 135.2, 146.1, 154.7,
168.4. Anal. Calcd for C23H19N3O4: C, 68.82; H, 4.77; N, 10.47.
Found: C, 68.88; H, 4.77; N, 10.39.
Z-(O-Leu)-Bt (2b): colorless oil (86%); [R]2D3 = ꢀ61.6 (c 1.5, CHCl3);
1H NMR (CDCl3) δ 0.98 (d, J = 6.3 Hz, 3H), 1.08 (d, J = 6.3 Hz,
3H), 1.82ꢀ2.10 (m, 3H), 5.12 (s, 2H), 6.33 (dd, J = 10.2 Hz, 3.0 Hz,
1H), 7.30ꢀ7.42 (m, 5H), 7.54 (t, J = 7.2 Hz, 1H), 7.69 (t, J = 6.9 Hz,
1H), 8.15 (d, J = 8.1 Hz, 1H), 8.27 (d, J = 8.1 Hz, 1H); 13C NMR
(CDCl3) δ 21.5, 23.4, 25.2, 40.4, 70.6, 74.9, 114.6, 120.6, 126.8,
128.5, 128.8, 131.0, 131.3, 134.9, 146.1, 154.9, 169.6. Anal. Calcd
Z-(O-Phe)-OH (1a): colorless oil (56%); [R]2D3 = ꢀ25.6 (c 1.5,
CH3OH) [lit.26 [R]D23 = ꢀ18.2 (c 1.2, CHCl3)]; 1H NMR (CDCl3) δ
3.15 (dd, J = 14.3, 8.7 Hz, 1H), 3.26 (dd, J = 14.4, 4.2 Hz, 1H),
5.10ꢀ5.20 (m, 2H), 5.19 (dd, J = 8.4, 3.9 Hz, 1H), 5.40 (br s, 1H),
7.20ꢀ7.40 (m, 10H); 13C NMR (CDCl3) δ 37.4, 70.4, 75.8, 127.4,
4888
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ARTICLE
for C20H21N3O4: C, 65.38; H, 5.76; N, 11.44. Found: C, 65.24; H,
6.10; N, 11.07.
[R]2D3 = ꢀ66.0 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 0.93 (d, J = 6.3 Hz,
6H), 1.47 (d, J = 6.9 Hz, 3H), 1.72ꢀ1.82 (m, 3H), 5.10 (dd, J = 7.2 Hz,
5.4 Hz, 1H), 5.16 (s, 2H), 6.31 (d, J = 7.8 Hz, 1H), 7.20ꢀ7.44 (m, 10H);
13C NMR (CDCl3) δ 21.8, 22.0, 23.3, 24.7, 41.1, 48.6, 70.5, 126.3, 127.6,
128.6, 128.9, 129.0, 134.9, 142.7, 154.2, 169.2. Anal. Calcd for
C22H27NO4: C, 71.52; H, 7.37; N, 3.79. Found: C, 71.15; H, 7.67;
N, 3.54.
Benzyl ((2S)-4-methyl-1-oxo-1-((1-phenylethyl)amino)pen-
tan-2-yl)carbonate (3bþ3b0): diastereoisomeric mixture; white mi-
crocrystals (56%); mp 38ꢀ42 °C; 1H NMR (CDCl3) δ 0.84ꢀ0.98 (m,
6H), 1.38ꢀ1.52 (m, 3H), 1.66ꢀ1.82 (m, 3H), 5.04ꢀ5.26 (m, 4H),
6.24ꢀ6.36 (m, 1H), 7.20ꢀ7.42 (m, 10H); 13C NMR (CDCl3) δ 21.8,
22.03, 23.3, 24.7, 41.1, 48.6, 70.5, 126.3, 127.6, 128.6, 128.7, 128.9, 134.9,
142.7, 154.2, 169.1. Anal. Calcd for C22H27NO4: C, 71.52; H, 7.37; N,
3.79. Found: C, 71.20; H, 7.52; N, 3.60.
(S)-Benzyl (1-((4-methoxyphenyl)amino)-4-methyl-1-ox-
opentan-2-yl)carbonate (3c): white microcrystals (71%); mp
117ꢀ118 °C; [R]2D3 = ꢀ53.3 (c 1.2, CHCl3); 1H NMR (CDCl3)
δ 0.96 (dd, J = 6.0 Hz, 3.3 Hz, 6H), 1.72ꢀ1.86 (m, 3H), 3.79 (s, 3H),
5.20ꢀ5.28 (m, 3H), 6.85 (d, J = 6.9 Hz, 2H), 7.36ꢀ7.42 (m, 7H), 7.71
(s, 1H); 13C NMR (CDCl3) δ 22.1, 23.2, 24.7, 41.3, 55.7, 70.7, 77.0,
114.4, 122.2, 128.7, 129.0, 129.1, 130.1, 134.8, 154.2, 157.0, 167.9. Anal.
Calcd for C21H25NO5: C, 67.91; H, 6.78; N, 3.77. Found: C, 67.83; H,
6.98; N, 3.67.
(9H-Fluoren-9-yl)methyl ((2S)-4-methyl-1-oxo-1-((1-phe-
nylethyl)amino)pentan-2-yl)carbonate (3d): white microcrys-
tals (67%); mp 155ꢀ158 °C; [R]2D3 = ꢀ54.6 (c 1.5, CH3OH); 1H NMR
(CDCl3) δ 0.95 (dd, J = 6.3 Hz, 3.0 Hz, 6H), 1.51 (d, J = 6.9 Hz, 3H),
1.66ꢀ1.82 (m, 3H), 4.18ꢀ4.26 (m, 1H), 4.38ꢀ4.54 (m, 2H),
5.06ꢀ5.20 (m, 2H), 6.31 (d, J = 7.5 Hz, 1H), 7.22ꢀ7.34 (m, 7H),
7.41 (t, J = 7.5 Hz, 2H), 7.52ꢀ7.58 (m, 2H), 7.77 (d, J = 7.8 Hz, 2H); 13C
NMR (CDCl3) δ 21.8, 22.0, 23.4, 24.7, 41.2, 46.9, 48.7, 70.4, 120.3,
125.1, 126.3, 127.4, 127.7, 128.2, 128.9, 141.5, 143.2, 143.3, 154.3, 169.1.
Anal. Calcd for C29H31NO4: C, 76.12; H, 6.83; N, 3.06. Found: C, 76.13;
H, 6.79; N, 2.71.
General Preparation of Depsidipeptides 5 and 15. The
unprotected amino acids 4 (1.5 mmol) and DIEA (1.5 mmol) were
dissolved in the minimum amount of water. Acetonitrile (3 mL) was
added to the solution, which was cooled to 10 °C. A solution of O-
Pg(hydroxyacyl)benzotriazole 2 or 14 (1 mmol) in acetonitrile (4 mL)
was added dropwise over 10 min at 10 °C and stirred for 0.5ꢀ2 h at
10 °C. The reaction mixture was monitored by TLC [EtOAcꢀhexanes
(1:2)]. After completion of reaction, the solvent was evaporated, EtOAc
(20 mL) was added, and the mixture was washed with 4 N HCl solution
(3 ꢁ 15 mL) and brine (15 mL) and dried over anhydrous MgSO4. The
solvent was evaporated under reduced pressure to give crude product 5
or 15.
Z-L-(O-Phe)-L-Ala-OH (5a). The crude product was recrystallized from
CH2Cl2ꢀhexanes to give white microcrystals (86%): mp 137ꢀ139 °C;
[R]2D3 = ꢀ17.6 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 1.27 (d, J = 7.2 Hz,
3H), 3.16 (dd, J = 14.4 Hz, 6.0 Hz, 1H), 3.26 (dd, J = 14.4 Hz, 4.2 Hz, 1H),
4.50ꢀ4.60 (m, 1H), 5.11 (d, J = 12.0 Hz, 1H), 5.17 (d, J = 12.0 Hz, 1H),
5.33 (t, J = 5.7 Hz, 1H), 6.48 (d, J = 7.5 Hz, 1H), 7.10ꢀ7.18 (m, 2H),
7.20ꢀ7.30 (m, 3H), 7.30ꢀ7.40 (m, 5H); 13C NMR (CDCl3) δ 18.1, 37.9,
47.9, 70.6, 77.5, 127.3, 128.6, 128.7, 128.9, 129.0, 129.9, 134.8, 135.2, 153.7,
168.7, 176.4. Anal. Calcd for C20H21N1O6: C, 64.68; H, 5.70; N, 3.77.
Found: C, 64.52; H, 5.82; N, 3.36.
F-(O-Leu)-Bt (2c): colorless oil (72%); [R]2D3 = ꢀ79.2 (c 1.5, CHCl3);
1H NMR (CDCl3) δ 1.03 (d, J = 5.1 Hz, 3H), 1.12 (d, J = 5.1 Hz, 3H),
1.86ꢀ2.28 (m, 3H), 4.28ꢀ4.42 (m, 2H), 4.50ꢀ4.58 (m, 1H), 6.33 (d,
J = 10.8 Hz, 1H), 7.28ꢀ7.46 (m, 4H), 7.54 (t, J = 7.2 Hz, 1H), 7.60ꢀ7.80
(m, 5H), 8.15 (d, J = 8.4 Hz, 1H), 8.27 (d, J = 8.4 Hz, 1H); 13C NMR
(CDCl3) δ 21.5, 23.4, 25.3, 40.3, 46.9, 70.8, 75.0, 114.6, 120.3, 120.6,
125.4, 125.5, 126.8, 127.4, 128.2, 131.0, 131.3, 141.5, 143.2, 143.5, 146.2,
155.0, 169.6. Anal. Calcd for C27H25N3O4: C, 71.19; H, 5.53; N, 9.22.
Found: C, 70.35; H, 5.58; N, 9.75.
F-(O-Phe)-Bt (2d): white microcrystals (80%); mp 132ꢀ134 °C;
[R]2D3 = ꢀ12.4 (c 1.5, CHCl3); 1H NMR (CDCl3) δ 3.37 (dd, J = 14.1
Hz, 9.3 Hz, 1H), 3.55 (dd, J = 14.1 Hz, 3.6 Hz, 1H), 4.20ꢀ4.36 (m, 2H),
4.38ꢀ4.50 (m, 1H), 6.49 (dd, J = 9.3 Hz, 3.6 Hz, 1H), 7.20ꢀ7.44 (m,
9H), 7.46ꢀ7.60 (m, 3H), 7.60ꢀ7.76 (m, 3H), 8.14 (d, J = 8.1 Hz, 1H),
8.25 (d, J = 8.1 Hz, 1H); 13C NMR (CDCl3) δ 38.0, 46.7, 70.8, 76.8,
114.5, 120.2, 120.6, 125.3, 125.5, 126.9, 127.4, 127.6, 128.1, 128.9, 129.7,
131.1, 131.2, 135.4, 141.4, 143.1, 143.4, 146.1, 154.6, 168.3. Anal. Calcd
for C30H23N3O4: C, 73.61; H, 4.74; N, 8.58. Found: C, 73.47; H, 5.01;
N, 8.40.
THP-(O-Val)-Bt (2e): Diastereisomeric pair, colorless oil (78%);
[R]2D3 = ꢀ82.4 (c 1.5, CHCl3); H NMR (CDCl3) δ 0.99 (d, J = 6.9
1
Hz, 1.5H), 1.05 (d, J = 6.9 Hz, 1.5H), 1.10 (d, J = 6.9 Hz, 1.5H), 1.18 (d,
J = 6.9 Hz, 1.5H), 1.40ꢀ2.00 (m, 6H), 2.30ꢀ2.42 (m, 0.5 H), 2.44ꢀ2.54
(m, 0.5H), 3.16ꢀ3.26 (m, 0.5H), 3.50ꢀ3.60 (m, 0.5H), 3.68ꢀ3.78 (m,
0.5H), 3.86ꢀ3.96 (m, 0.5H), 4.72ꢀ4.80 (m, 1H), 5.30 (d, J = 6.9 Hz, 0.5
H), 5.67 (d, J = 4.5 Hz, 0.5 H), 7.48ꢀ7.58 (m, 1H), 7.63ꢀ7.72 (m, 1H),
8.10ꢀ8.18 (m, 1H), 8.30ꢀ8.38 (m, 1H); 13C NMR (CDCl3) δ 17.2,
18.2, 18.8, 19.1, 19.8, 20.1, 25.2, 25.6, 30.5, 32.2, 62.4, 63.1, 77.9, 82.3,
96.8, 102.0, 114.6, 114.8, 120.3, 120.4, 126.4, 126.6, 130.5, 130.7, 131.3,
146.2, 172.2. Anal. Calcd for C16H21N3O3: C, 63.35; H, 6.98; N, 13.85.
Found: C, 63.39; H, 7.20; N, 13.30.
General Preparation of O-Pg(r-hydroxyacyl) Amides
(3aꢀd). Amine (1.2 equiv) and pyridine (1.2 molar equiv) in THF
(2 mL) were added to a stirred solution of O-Pg(hydroxyacyl)benzotriazole
2 (1 molar equiv) in THF (4 mL) dropwise at 10 °C, and the mixture was
stirred for 2 h at room temperature. After evaporation of THF, EtOAc
(15 mL) was added to the solution, which was washed with 2 N HCl (2 ꢁ
10 mL), saturated Na2CO3 solution (3 ꢁ 10 mL), and brine (10 mL) and
dried over anhydrous Na2SO4. Evaporation of the solvent under reduced
pressure gave the crude amides 3aꢀd.
Benzyl ((S)-1-Oxo-3-phenyl-1-(((S)-1-phenylethyl)amino)pro-
pan-2-yl)carbonate (3a). The crude product was recrystallized from
diethyl etherꢀhexanes to give white microcrystals (88%): mp 121ꢀ122 °C;
[R]2D3 = ꢀ12.9 (c 1.5, CH3OH); 1H NMR (CDCl3) δ 1.28 (d, J = 6.9 Hz,
3H), 3.18 (dd, J = 14.4 Hz, 6.3Hz, 1H), 3.28 (dd, J = 14.1 Hz, 4.5 Hz, 1H),
5.00ꢀ5.10 (m, 1H), 5.10 (d, J = 5.7 Hz, 2H), 5.29 (dd, J = 6.0 Hz, 4.8 Hz,
1H), 6.12 (d, J = 7.8 Hz, 1H), 7.12ꢀ7.40 (m, 15H); 13C NMR (CDCl3) δ
21.5, 38.0, 48.5, 70.5, 77.9, 126.3, 127.2, 127.6, 128.6, 128.7, 128.8, 128.9,
129.0, 130.0, 134.8, 135.4, 142.5, 153.7, 167.6. Anal. Calcd for
C25H25N1O4: C, 74.42; H, 6.25; N, 3.47. Found: C, 74.36; H, 6.44;
N, 3.34.
Benzyl ((2S)-1-oxo-3-phenyl-1-((1-phenylethyl)amino)pro-
pan-2-yl) carbonate (3aþ3a0): diastereoisomeric mixture; white
microcrystals (76%); mp 88ꢀ92 °C; 1H NMR (CDCl3) δ 1.28 (d, J =
6.9 Hz, 2H), 1.37 (d, J = 6.9 Hz, 1H), 3.10ꢀ3.22 (m, 2H), 5.00ꢀ5.16 (m,
3H), 5.26ꢀ5.36 (m, 1H), 6.10ꢀ6.18 (m, 1H), 7.00ꢀ7.40 (m, 15H); 13C
NMR (CDCl3) δ 21.5, 21.6, 37.8, 38.0, 48.5, 70.5, 77.9, 126.3, 127.1,
127.2, 127.5, 127.6, 128.6, 128.7 (2C) 128.8, 128.9, 129.0, 130.0, 134.8,
135.4, 142.5, 153.7, 167.6. Anal. Calcd for C25H25NO4: C, 74.42; H, 6.25;
N, 3.47. Found: C, 74.25; H, 6.45; N, 3.35.
Z-L-(O-Phe)-DL-Ala-OH (5aþ5a0). The crude product was recrystal-
lized from CH2Cl2ꢀhexanes to give white microcrystals (80%): diaster-
eoisomeric mixture; mp 135ꢀ137 °C; 1H NMR (CDCl3) δ 1.24 (d, J =
7.2 Hz, 1.5H), 1.33 (d, J = 7.2 Hz, 1.5H), 3.02ꢀ3.18 (m, 1H), 3.20ꢀ3.28
(m, 1H), 4.46ꢀ4.56 (m, 1H), 5.04ꢀ5.14 (m, 2H), 5.24ꢀ5.34 (m, 1H),
6.47 (d, J = 7.2 Hz, 1H), 7.08ꢀ7.16 (m, 2H), 7.17ꢀ7.24 (m, 3H),
7.26ꢀ7.38 (m, 5H); 13C NMR (CDCl3) δ 17.9, 18.1, 37.9, 38.1, 47.9,
Benzyl ((S)-4-methyl-1-oxo-1-(((S)-1-phenylethyl)amino)pen-
tan-2-yl)carbonate (3b): white microcrystals (65%); mp 66ꢀ67 °C;
4889
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ARTICLE
48.0, 70.6, 127.3, 128.6, 128.9, 129.0, 129.8, 130.0, 134.8, 135.2, 135.3,
153.7, 168.6, 169.1, 176.5. Anal. Calcd for C20H21N1O6: C, 64.68; H,
5.70; N, 3.77. Found: C, 64.78; H, 5.80; N, 3.65.
(m, 4H), 4.10ꢀ4.46 (m, 3H), 4.82ꢀ4.94 (m, 1H), 5.22ꢀ5.32 (m, 1H),
6.46ꢀ6.52 (m, 1H), 6.84ꢀ6.88 (m, 1H), 7.00ꢀ7.04 (m, 1H), 7.12ꢀ7.34
(m, 11H), 7.38ꢀ7.44 (m, 2H), 7.46ꢀ7.54 (m 2H), 7.76 (d, J = 7.8 Hz,
2H); 13C NMR (CDCl3) δ 37.4, 37.6, 37.7, 38.1, 46.7, 46.8, 52.6, 52.9,
70.5, 70.6, 77.5, 77.9, 120.3, 125.3, 127.3, 127.4, 127.5, 128.2, 128.7 (2C),
128.9, 129.4, 129.5, 129.8, 130.1, 135.1, 135.3, 135.5, 141.5 (2C), 143.1,
143.2 (2C), 143.4, 153.7, 153.9, 168.6, 169.2, 175.2 (C). Anal. Calcd for
C33H29N1O6: C, 74.00; H, 5.46; N, 2.62. Found: C, 73.66; H, 5.76;
N, 2.31.
F-L-(O-Phe)-L-Met-OH (5b). The crude product was recrystallized from
CH2Cl2ꢀhexanes to give white microcrystals (74%): mp 130ꢀ132 °C;
[R]2D3 = ꢀ16.9 (c 1.0, CH3OH); H NMR (CDCl3) δ 1.84ꢀ1.96 (m,
1
1H), 1.98 (s, 3H), 2.00ꢀ2.12 (m, 1H), 2.15ꢀ2.28 (m, 2H), 3.19 (d,
J = 4.8 Hz, 2H), 4.21 (t, J = 6.9 Hz, 1H), 4.40 (dd, J = 10.5 Hz, J = 6.9 Hz,
1H), 4.50 (dd, J = 10.8 Hz, 7.5 Hz, 1H), 4.64ꢀ4.80 (m, 1H), 5.34 (t,
J = 4.8, 1H), 6.78 (d, J = 7.8 Hz, 1H), 7.08ꢀ7.14 (m, 2H), 7.20ꢀ7.36 (m,
5H), 7.36ꢀ7.44 (m, 2H), 7.56(t, J = 7.2 Hz, 2H), 7.76 (d, J = 7.5 Hz, 2H);
13C NMR (CDCl3) δ 15.4, 29.7, 30.9, 37.7, 46.9, 51.4, 70.5, 77.5, 120.3,
125.2, 127.4, 128.2, 128.6, 130.1, 135.1, 141.5, 143.1, 143.4, 153.7, 168.8,
175.4. Anal. Calcd for C29H29N1O6S: C, 67.03; H, 5.63; N, 2.70. Found:
C, 67.25; H, 5.93; N, 2.50.
F-L-(O-Phe)-L-Met-Bt (6). Compound 6 was prepared according to
the given procedure for 2. The crude product was recrystallized from
CH2Cl2ꢀhexanes to give white microcrystals (80%): mp 160ꢀ162 °C;
[R]2D3 = ꢀ56.9 (c 1.5, CHCl3); H NMR (CDCl3) δ 1.96 (s, 3H),
1
2.02ꢀ2.18 (m, 1H), 2.22ꢀ2.40 (m, 3H), 3.25 (d, J = 5.1 Hz, 2H), 4.28
(t, J = 6.9 Hz, 1H), 4.47 (dd, J = 10.2 Hz, 6.9 Hz, 1H), 4.55 (dd, J = 10.5
Hz, 6.9 Hz, 1H), 5.39 (t, J = 4.8, 1H), 5.98ꢀ6.08 (m, 1H), 7.10 (d, J = 8.4
Hz, 1H), 7.14ꢀ7.46 (m, 9H), 7.52 (t, J = 6.9 Hz, 1H), 7.60ꢀ7.70 (m,
3H), 7.78 (d, J = 7.5 Hz, 2H), 8.13 (d, J = 8.1 Hz, 1H), 8.19 (d, J = 8.4 Hz,
1H); 13C NMR (CDCl3) δ 15.4, 29.9, 32.0, 37.7, 46.9, 52.5, 70.6, 77.7,
114.5, 120.3, 120.6, 125.3, 126.9, 127.4, 127.5, 128.2, 128.7, 130.1, 131.3,
131.2, 135.2, 141.6, 143.2, 143.4, 146.2, 153.8, 168.6, 170.5. Anal. Calcd
for C35H32N4O5S: C, 67.72; H, 5.20; N, 9.03. Found: C, 67.48; H, 5.23;
N, 8.85.
F-L-(O-Phe)-L-Glu-(OMe)-OH (5c). The crude product was recrystal-
lized from EtOAcꢀhexanes to give white microcrystals (82%): mp
155ꢀ157 °C; [R]2D3 = ꢀ23.8 (c 1.0, CH3OH); 1H NMR (DMSO-d6) δ
1.76ꢀ1.90 (m, 1H), 1.98ꢀ2.10 (m, 1H), 2.28ꢀ2.38 (m, 2H), 2.94 (dd,
J = 14.4 Hz, 9.6 Hz, 1H), 3.08 (dd, J = 14.7 Hz, 3.9 Hz, 1H), 3.55 (s, 3H),
4.20ꢀ4.32 (m, 2H), 4.34ꢀ4.48 (m, 2H), 5.07 (dd, J = 9.6 Hz, 3.6 Hz,
1H), 7.20ꢀ7.36 (m, 6H), 7.41 (t, J = 7.8 Hz, 2H), 7.50ꢀ7.60 (m, 2H),
7.89 (d, J = 7.8 Hz, 2H), 8.51 (d, J = 8.1 Hz, 1H), 12.82 (br s, 1H); 13C
NMR (DMSO-d6) δ 26.3, 29.7, 37.2, 46.2, 51.0, 51.4, 69.0, 76.9, 120.3,
125.1, 126.8, 127.2, 127.9, 128.4, 129.3, 136.6, 140.8, 143.1, 143.4, 153.8,
168.6, 172.7, 172.8. Anal. Calcd for C30H29N1O8: C, 67.79; H, 5.50; N,
2.63. Found: C, 67.43; H, 5.47; N, 2.57.
F-L-(O-Phe)-L-Ala-OH (5d). The crude product was recrystallized from
EtOAcꢀhexanes to give white microcrystals (87%): mp 195ꢀ196 °C;
[R]2D3 = ꢀ30.0 (c 1.5, CH3OH); 1H NMR (DMSO-d6) δ 1.29 (d, J = 7.2
Hz, 3H), 3.92 (dd, J = 14.7 Hz, 9.9 Hz, 1H), 3.10 (dd, J = 14.4 Hz, 3.3 Hz,
1H), 4.20ꢀ4.30 (m, 2H), 4.32ꢀ4.44 (m, 2H), 5.05 (dd, J = 9.6 Hz, 3.3 Hz,
1H), 7.20ꢀ7.36 (m, 6H), 7.38ꢀ7.46 (m, 2H), 7.50ꢀ7.60 (m, 2H), 7.89
(d, J = 7.8 Hz, 2H), 8.52 (d, J = 7.5 Hz, 1H), 12.6 (br s, 1H); 13C NMR
(DMSO-d6) δ 17.2, 37.3, 46.1, 47.5, 69.0, 76.8, 120.3, 125.0, 125.1, 126.7,
127.2, 127.8, 128.3, 129.3, 136.6, 140.8, 143.1, 143.3, 153.8, 168.2, 173.8.
Anal. Calcd for C27H25N1O6: C, 70.58; H, 5.48; N, 3.05. Found: C, 70.20;
H, 5.51; N, 2.86.
(9H-Fluoren-9-yl)methyl ((S)-1-(((S)-4-(Methylthio)-1-oxo-1-(((S)-
1-phenylethyl)amino)butan-2-yl)amino)-1-oxo-3-phenylpropan-2-yl)-
carbonate Monohydrate (7). Compound 7 was prepared according to
the given procedure for 3. The crude product was recrystallized from
EtOAcꢀhexanes to give white microcrystals (52%): mp 176ꢀ178 °C;
[R]2D3 = ꢀ60.0 (c 1.5, CHCl3); 1H NMR (CDCl3) δ 1.45 (d, J = 6.9 Hz,
3H), 1.78ꢀ1.88 (m, 2H), 1.94 (s, 3H), 2.08ꢀ2.18 (m, 1H), 2.24ꢀ2.34
(m, 1H), 3.18ꢀ3.22 (m, 2H), 4.24 (t, J = 7.2 Hz, 1H), 4.38ꢀ4.56 (m,
3H), 5.00ꢀ5.10 (m, 1H), 5.29 (t, J = 5.4 Hz, 1H), 6.56 (d, J = 8.1 Hz, 1H),
6.92 (d, J = 7.5 Hz, 1H), 7.14ꢀ7.20 (m, 2H), 7.22ꢀ7.38 (m, 10H),
7.40ꢀ7.46 (m, 2H), 7.59 (t, J = 7.2 Hz, 2H), 7.78 (d, J = 6.9 Hz, 2H); 13C
NMR (CDCl3) δ 15.1, 22.1, 30.0, 30.8, 37.9, 46.9, 49.4, 52.1, 70.6, 120.3,
125.3, 126.2, 127.5, 127.7, 128.2, 128.7, 128.9, 130.0, 135.3, 141.5, 143.1,
143.3 154.1, 168.6, 169.4. Anal. Calcd for C37H38N2O5S.H2O: C, 69.35;
H, 6.29; N, 4.37. Found: C, 69.09; H, 6.15; N, 4.25.
General Preparation of Depsidipeptides (11). DMAP (0.75
mmol) in dry THF (2 mL) was added to a stirred solution of N-Pg(R-
aminoacyl)benzotriazole 9 (0.5 mmol) and R-hydroxycarboxylic acid
(0.75 mmol) in dry THF (10 mL) at 4 °C. Then the reaction mixture
was stirred for 4ꢀ6 h at room temperature until shown to be completed
by TLC [EtOAcꢀhexanes (1:2)]. The solvent was evaporated under
reduced pressure, and the residue was dissolved in diethyl ether (25 mL),
washed with 3 N HCl (4 ꢁ 5 mL), water (3 ꢁ 10 mL), and brine (5 mL),
and dried over anhydrous MgSO4. The solvent was evaporated under
reduced pressure to give crude product 11.
F-L-(O-Phe)-DL-Ala-OH (5dþ5d0). The crude product was recrystal-
lized from EtOAcꢀhexanes to give white microcrystals (94%): diaster-
eoisomeric mixture; mp 203ꢀ205 °C; 1H NMR (DMSO-d6) δ 1.23 (d,
J = 7.2 Hz, 1.5 H), 1.29 (d, J = 7.2 Hz, 1.5 H), 2.88ꢀ3.00 (m, 1H),
3.02ꢀ3.16 (m, 1H), 4.20ꢀ4.30 (m, 2H), 4.32ꢀ4.48 (m, 2H), 5.02ꢀ5.12
(m, 1H), 7.20ꢀ7.38 (m, 6H), 7.38ꢀ7.46 (m, 2H), 7.50ꢀ7.60 (m, 2H),
7.89 (d, J = 7.5 Hz, 2H), 8.52 (d, J = 7.5 Hz, 1H), 12.65 (br s, 1H); 13
C
NMR (DMSO-d6) δ 17.2, 17.4, 37.5, 46.2, 47.5, 69.0, 76.8, 76.9, 120.3,
125.0, 125.1, 126.7, 127.2, 127.9, 128.3, 129.3, 129.4, 136.5, 136.7, 140.8,
143.1, 143.4, 153.8, 168.0, 168.3, 173.8. Anal. Calcd for C27H25N1O6: C,
70.58; H, 5.48; N, 3.05. Found: C, 70.27; H, 5.55; N, 2.89.
Z-L-Leu-L-(O-Phe)-OH (11a). The residue was purified by column
chromatograph [EtOAcꢀhexanes, (from 15 to 30%)] to obtain a sticky
oil (76%): [R]2D3 = ꢀ21.0 (c 2.0, CHCl3); 1H NMR (CDCl3) δ
0.76ꢀ1.05 (m, 6H), 1.40ꢀ1.55 (m, 1H), 1.55ꢀ1.80 (m, 2H), 3.13
(dd, J = 13.8 Hz, 7.8 Hz, 1H), 3.26 (dd, J = 14.4 Hz, 4.2 Hz, 1H),
4.30ꢀ4.40 (m, 1H), 5.00ꢀ5.20 (m, 2H), 5.31 (dd, J = 8.4 Hz, 5.1 Hz,
1H), 7.18ꢀ7.52 (m, 10H); 13C NMR (CDCl3) δ 21.9, 23.0, 24.8, 37.2,
41.6, 52.5, 67.3, 73.2, 127.3, 128.2, 128.4, 128.7, 129.5, 135.5, 136.3,
156.3, 172.5, 173.7. Anal. Calcd for C23H27N1O6: C, 66.81; H, 6.58; N,
3.39. Found: C, 66.54; H, 6.87; N, 3.29.
F-L-(O-Phe)-L-Phe-OH (5e). The crude product was recrystallized from
EtOAcꢀhexanes to give white microcrystals (86%): mp 165ꢀ167 °C;
[R]2D3 = ꢀ13.4 (c 1.0, CH3OH); 1H NMR (CDCl3) δ 2.90 (dd, J = 13.8
Hz, 5.7 Hz, 1H), 3.04 (dd, J = 13.8 Hz, 5.4 Hz, 1H), 3.10ꢀ3.18 (m, 2H),
4.13 (t, J = 7.5 Hz, 1H), 4.28 (dd, J = 10.5 Hz, 6.9 Hz, 1H), 4.38 (dd, J =
10.5 Hz, 7.8 Hz, 1H), 4.82ꢀ4.90 (m, 1H), 5.22ꢀ5.30 (m, 1H), 6.46 (d,
J = 7.2 Hz, 1H), 6.80 (dd, J = 5.7 Hz, 2.1 Hz, 2H), 7.08ꢀ7.50 (m, 15H),
7.73 (d, J = 7.8, 2H); 13C NMR (CDCl3) δ 37.6, 37.7, 46.8, 52.6, 70.4,
77.5, 120.3, 125.3, 127.4, 127.5 (2C), 128.2 (2C), 128.7, 128.8, 129.5,
130.1, 135.1, 135.3, 141.5, 143.1, 143.4, 153.7, 168.5, 175.0. Anal. Calcd
for C33H29N1O6: C, 74.00; H, 5.46; N, 2.62. Found: C, 73.85; H, 5.51;
N, 2.45.
Z-L-Leu-DL-(O-Phe)-OH (11aþ11a0). The crude was purified by
column chromatography [EtOAcꢀhexanes, (from 15 to 30%)] to obtain
a sticky oil (78%): diastereoisomeric mixture; 1H NMR (CDCl3)
δ 0.76ꢀ1.00 (m, 6H), 1.30ꢀ1.53 (m, 2H), 1.56ꢀ1.76 (m, 1H),
3.04ꢀ3.20 (m, 1H), 3.20ꢀ3.36 (m, 1H), 3.34ꢀ3.44 (m, 1H),
5.02ꢀ5.18 (m, 3H), 5.33 (dd, J = 9.9 Hz, 3.6 Hz, 1H), 7.18ꢀ7.40
F-L-(O-Phe)-DL-Phe-OH (5eþ5e0). The crude product was recrystal-
lized from EtOAcꢀhexanes to give white microcrystals (80%): diaster-
eoisomeric mixture; mp 151ꢀ153 °C; 1H NMR (CDCl3) δ 2.90ꢀ3.30
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(m, 10H); 13C NMR (CDCl3) δ 21.9, 22.8, 23.0, 24.7, 24.8, 37.2, 41.4,
41.6, 52.5, 52.6, 67.4, 73.2, 73.4, 127.4, 128.3, 128.4, 128.7, 129.5, 129.6,
135.5, 135.8, 136.3, 156.2, 156.3, 172.5, 173.7. Anal. Calcd for
C23H27N1O6: C, 66.81; H, 6.58; N, 3.39. Found: C, 66.40; H, 6.75;
N, 3.60.
Z-L-Leu-L-(O-Phe)-Bt (14a). Compound 14a was prepared according
to the given procedure for 2. The yellowish oil (83%) was used without
purification.
Z-L-Ala-L-(O-Phe)-Bt (14b). Compound 14b was prepared according
to the given procedure for 2. The yellowish oil (78%) was used without
purification.
Z-L-Ala-L-(O-Phe)-N(Cy)2 (11b). The crude was taken into in diethyl
ether (6 mL) in a test tube, and dicyclohexylamine (110 mg, 0.60 mmol)
was added dropwise. Hexane (2 mL) was added, and the mixture was
kept at room temperature until it gave white crystals. Then the crystals
were collected and washed with excess of hexanes to give pure Z-L-Ala-
L-(OPhe)-N(Cy)2 11b as white crystals (76%): mp 155ꢀ156 °C;
[R]2D3 = ꢀ25.9 (c 1.2, CH3OH); 1H NMR (CDCl3) δ 1.00ꢀ1.25 (m,
7H), 1.30ꢀ1.45 (m, 6H), 1.59 (s, 2H), 1.73 (s, 4H), 1.91 (s, 4H),
2.80ꢀ2.95 (m, 2H), 3.04 (dd, J = 14.4 Hz, 9.9 Hz, 1H), 3.24 (dd, J = 14.4
Hz, 3.6 Hz, 1H), 4.20ꢀ4.40 (m, 1H), 4.95ꢀ5.10 (m, 3H), 5.30ꢀ5.41
(m, 1H), 7.10ꢀ7.40 (m, 10H); 13C NMR (CDCl3) δ 25.0, 25.3, 29.0,
29.1, 38.2, 50.0, 52.7, 67.0, 77.0, 126.5, 128.5, 128.7, 129.4, 136.6, 138.4,
155.6, 172.2, 173.6. Anal. Calcd for C32H44N2O6: C, 69.54; H, 8.02; N,
5.07. Found: C, 69.25; H, 8.21; N, 4.90.
Z-L-Phe-L-(O-Leu)-N(Cy)2 (11c). The crude was taken into diethyl
ether (6 mL) in a test tube, and dicyclohexylamine (110 mg, 0.60 mmol)
was added dropwise. Hexane (2 mL) was added, and the mixture was
kept at room temperature until it gave white crystals. Then the crystals
were collected and washed with excess of hexanes to give pure Z-L-Phe-
L-(O-Leu)-N(Cy)2 11c as white crystals (47%): mp 127ꢀ128 °C;
[R]2D3 = ꢀ34.5 (c 1.2, CH3OH); 1H NMR (CDCl3) δ 0.90 (t, J = 6.2
Hz, 6H), 1.06ꢀ1.86 (m, 19H), 1.90ꢀ2.06 (m, 4H), 2.82ꢀ3.00 (m, 2H),
3.09 (dd, J = 14.4 Hz, 6.6 Hz, 1H), 3.31 (dd, J = 13.5 Hz, 5.7, 1H),
4.62ꢀ4.74 (m, 1H), 4.90ꢀ5.16 (m, 3H), 5.29 (d, J = 7.8 Hz, 1H),
7.18ꢀ7.40 (m, 10H); 13C NMR (CDCl3) δ 21.9, 23.6, 25.0, 25.1, 25.3,
29.5, 38.4, 41.0, 52.8, 55.2, 67.0, 75.1, 127.0, 128.3, 128.5, 128.7, 129.9,
136.6, 155.8, 171.2, 175.0. Anal. Calcd for C35H50N2O6: C, 70.68; H,
8.47; N, 4.71. Found: C, 70.78; H, 8.92; N, 4.75.
Z-L-Leu-L-(O-Phe)-L-Met-OH (15a). Compound 15a was prepared
according to the given procedure for 5: white microcrystals (78%); mp
116ꢀ118 °C; [R]2D3 = ꢀ37.4 (c 1.5, CHCl3); 1H NMR (CDCl3)
δ 0.80ꢀ0.98 (m, 6H), 1.40ꢀ1.52 (m, 1H), 1.52ꢀ1.70 (m, 2H),
1.84ꢀ2.16 (m, 4H), 2.22 (q, J = 7.2 Hz, 2H), 3.08ꢀ3.25 (m, 2H),
4.25ꢀ4.30 (m, 1H), 4.50ꢀ4.62 (m, 1H), 4.90ꢀ5.04 (m, 1H), 5.05ꢀ5.20
(s, 2H), 5.40ꢀ5.50 (m, 1H), 6.40ꢀ6.65 (br s, 2H), 7.00ꢀ7.40 (m,
10H); 13C NMR (CDCl3) δ 15.3, 21.7, 23.0, 24.9, 29.9, 30.7, 37.6, 40.5,
51.6, 53.0, 67.6, 74.9, 127.3, 128.2, 128.6, 128.8, 129.9, 135.6, 136.0,
156.9, 169.5, 171.3, 174.8. Anal. Calcd for C28H36N2O7S: C, 61.75; H,
6.66; N, 5.14. Found: C, 61.73; H, 6.62; N, 5.06.
Z-L-Ala-L-(O-Phe)-L-Trp-OH (15b). Compound 15b was prepared
according to the given procedure for 5: white microcrystals (63%);
mp 193ꢀ195 °C; [R]2D3 = ꢀ87.3 (c 1.5, CH3OH); 1H NMR (DMSO-
d6) δ 1.20 (d, J = 7.2 Hz, 3H), 2.85ꢀ3.20 (m, 4H), 4.00ꢀ4.15 (m, 1H),
4.51 (q, J = 7.2 Hz, 1H), 4.85ꢀ5.05 (m, 2H), 5.10ꢀ5.25 (m, 1H),
6.90ꢀ7.40 (m, 11H), 7.48 (d, J = 7.2 Hz, 1H), 7.23 (d, J = 6.9 Hz, 1H),
8.22 (d, J = 7.8 Hz, 1H), 10.87 (s, 1H), 12.71 (br s, 1H); 13C NMR
(DMSO-d6) δ 16.7, 26.9, 37.2, 49.1, 52.6, 65.5, 74.0, 109.3, 111.3, 118.2,
118.4, 120.9, 123.6, 126.5, 127.2, 127.8, 127.8, 128.1, 128.3, 129.3, 136.0,
136.5, 136.8, 155.9, 168.3, 172.0, 172.8. Anal. Calcd for C31H31N3O7: C,
66.77; H, 5.60; N, 7.54. Found: C, 66.49; H, 5.48; N, 7.47.
Z-L-Leu-L-(O-Phe)-L-(O-Ile)-OH (16). Compound 16 was prepared
according to the given procedure for 11: colorless oil (55%); [R]D23
=
1
ꢀ24.2 (c 1.5, CHCl3); H NMR (CDCl3) δ 0.80ꢀ1.00 (m, 12H),
1.20ꢀ1.40 (m, 1H), 1.40ꢀ1.60 (m, 2H), 1.60ꢀ1.78 (m, 2H),
1.94ꢀ2.08 (m, 1H), 3.14 (dd, J = 14.4 Hz, 8.1 Hz, 1H), 3.30 (dd, J =
14.7 Hz, 4.2 Hz, 1H), 4.36ꢀ4.48 (m, 1H), 5.00ꢀ5.12 (m, 3H),
5.14ꢀ5.40 (m, 2H), 7.10ꢀ7.40 (m, 10H), 8.56 (s, 1H); 13C NMR
(CDCl3) δ 11.7, 15.4, 21.8, 23.1, 24.5, 24.8, 36.7, 37.2, 41.8, 52.4, 67.3,
73.4, 127.3, 128.3, 128.4, 128.7 (2C), 129.5, 129.6, 135.6, 136.4, 156.2,
169.0, 172.7, 174.2. Anal. Calcd for C29H37NO8: C, 66.02; H, 7.07; N,
2.65. Found: C, 65.79; H, 7.53; N, 2.42.
Z-L-Met-L-(OPhe)-OH (11d). The crude was recrystallized from
CHCl3ꢀhexanes to give white microcrystals (54%): mp 114ꢀ116 °C;
[R]2D3 = ꢀ18.6 (c 2.0, CHCl3); 1H NMR (CDCl3) δ 1.88ꢀ2.00 (m, 1H),
2.03 (s, 3H), 2.06ꢀ2.20 (m, 1H), 2.44ꢀ2.58 (m, 2H), 3.14 (dd, J = 14.4
Hz, 7.8 Hz, 1H), 3.26 (dd, J = 14.7 Hz, 4.5 Hz, 1H), 4.50ꢀ4.60 (m, 1H),
5.09 (s, 2H), 5.32 (dd, J = 7.8 Hz, 3.9 Hz, 1H), 5.42 (d, J = 7.8 Hz, 1H),
7.18ꢀ7.40 (m, 10H); 13C NMR (CDCl3) δ 15.4, 29.7, 31.9, 37.2, 53.1,
67.4, 73.4, 127.5, 128.3, 128.5, 128.7, 129.5, 135.3, 136.2, 156.1, 171.6,
173.7. Anal. Calcd for C22H25N1O6S: C, 61.24; H, 5.84; N, 3.25. Found:
C, 60.98; H, 5.85; N, 3.08.
General Preparation of Oligoesters 17. O-Pg(R-Hydroxya-
cyl)benzotriazole 2 (0.5 mmol) in dry THF (2 mL) was added to a
stirred solution of DMAP (0.6 mmol) and R-hydroxycarboxylic acid (0.6
mmol) in dry THF (10 mL) at 4 °C. Then the reaction mixture was
stirred for 4ꢀ6 h at room temperature until it was completed, as
indicated by monitoring with TLC [EtOAcꢀhexanes (1:2)]. The
solvent was evaporated under reduced pressure. The residue was
dissolved in EtOAc (15 mL), washed with 3 N HCl (4 ꢁ 5 mL) and
brine (5 mL), and dried over anhydrous MgSO4. The solvent was
evaporated under reduced pressure to give crude oily product 17a,b. The
solution of crude in CH2Cl2 (1 mL) was loaded onto a short silica
column (1 cm diameter ꢁ 2 cm length) and eluted with EtOAcꢀ
hexanes (1:1). Evaporation of solvent gave pure 17a,b.
Synthesis of tert-Butyl L-r-Hydroxycarboxlates 12a and
12b. Compounds 12a and 12b were prepared using procedure reported
by Yang et al.4 (Scheme S1, Supporting Information).
Z-L-Met-L-(O-Leu)-OBu-t (13). DMAP (0.105 g, 0.85 mmol) was
added to a stirred solution of Z-L-Met-Bt (0.32 g, 0.83 mmol) and H-(O-
Leu)-OBu-t 12b (0.16 g, 0.83 mmol) in THF (10 mL) at 4 °C. Then the
reaction mixture was stirred for 5 h at room temperature until it was
completed, which was monitored by TLC (EtOAcꢀhexanes (1:2)).
Then the solvent was evaporated under reduced pressure. The residue
was taken into EtOAc (25 mL) and washed with 3 N HCl (3 ꢁ 10 mL),
saturated Na2CO3 (2 ꢁ 10 mL), and brine (10 mL). The mixture was
dried over MgSO4, and the solvent was evaporated to give Z-L-Met-
L-(O-Leu)-OBu-t 13 as a colorless oil (87%): [R]2D3 = ꢀ15.3 (c 1.2,
CH3OH); 1H NMR (CDCl3) δ 0.93 (dd, J = 8.4 Hz, 6.6 Hz, 6H), 1.44
(s, 9H), 1.54ꢀ1.84 (m, 3H), 1.92ꢀ2.32 (m, 4H), 2.28ꢀ2.30 (m, 1H),
2.54ꢀ2.70 (m, 2H), 4.57 (q, J = 5.1 Hz, 1H), 4.92 (dd, J = 8.4 Hz, 3.9 Hz,
1H), 5.10 (s, 2H), 5.46 (d, J = 8.1 Hz, 1H), 7.28ꢀ7.42 (m, 5H); 13C
NMR (CDCl3) δ 15.5, 21.9, 23.2, 24.8, 28.1, 30.0, 32.4, 39.8, 53.2, 67.3,
72.7, 82.4, 127.6, 128.3, 128.4, 128.7, 136.4, 156.1, 169.3, 171.9. Anal.
Calcd for C23H35N1O6S1: C, 60.90; H, 7.78; N, 3.09. Found: C, 60.99;
H, 8.39; N, 3.23.
Z-L-(O-Phe)-L-(O-Val)-OH (17a): colorless oil (85%); [R]2D3 = ꢀ30.0
(c 1.5, CHCl3); 1H NMR (CDCl3) δ ; 0.98 (t, J = 6.9 Hz, 6H),
2.24ꢀ2.36 (m, 1H), 3.14 (dd, J = 14.7 Hz, 9.0 Hz, 1H), 3.32 (dd, J = 14.7
Hz, 3.9 Hz, 1H), 5.00 (d, J = 4.2 Hz, 1H), 5.10 (d, J = 12.3 Hz, 1H), 5.16
(d, J = 12.3 Hz, 1H), 5.23 (dd, J = 9.0 Hz, 3.6 Hz, 1H), 7.20ꢀ7.38
(m, 10H); 13C NMR (CDCl3) δ 17.1, 18.8, 30.3, 37.4, 70.3, 76.0, 77.1,
127.3, 128.5, 128.7, 128.8, 129.6, 135.0, 135.8, 154.7, 169.2, 174.5. Anal.
Calcd for C22H24O7 : C, 65.99; H, 6.04. Found: C, 65.61; H, 6.23.
3
Z-L-(O-Phe)-L-(O-Leu)-OH (17b): colorless oil (75%); [R]2D3 = ꢀ34.8
(c 1.5, CHCl3); 1H NMR (CDCl3) δ 0.93 (dd, J = 8.1 Hz, 5.7 Hz, 6H),
1.64ꢀ1.86 (m, 3H), 3.12 (dd, J = 14.4, J = 9.3 Hz, 1H), 3.31 (dd, J = 14.7
Hz, 3.6 Hz, 1H), 5.04ꢀ5.22 (m, 4H), 7.20ꢀ7.38 (m, 10H); 13C NMR
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(CDCl3) δ 21.7, 23.1, 24.8, 37.3, 39.7, 70.3, 71.5, 76.1, 127.3, 128.5,
128.7, 128.8, 129.7, 135.0, 135.8, 154.7, 169.2, 175.3; HRMS (ESI) calcd
for C23H26O7 [M þ Na]þ 437.1571, found 437.1575.
170.8. Anal. Calcd for C9H18ClNO4: C, 45.10; H, 7.57; N, 5.84. Found:
C, 45.16; H, 7.84; N, 5.63.
Gly-L-(O-Leu) HCl (19c): white hydroscopic microcrystals (62%);
3
Z-L-(O-Leu)-L-(O-Phe)-OBu-t (18). DMAP (0.05 g, 0.41 mmol) was
added to a stirred solution of Z-(O-Leu)-Bt 2b (0.15 g, 0.41 mmol) and
H-(O-Phe)-OBu-t 12a (0.091 g, 0.41 mmol) in THF (10 mL) at 4 °C.
Then the reaction mixture was stirred for 5 h at room temperature until it
was completed, as monitored by TLC (EtOAcꢀhexanes (1:2)). Then
the solvent was evaporated under reduced pressure. The residue was
taken into EtOAc (15 mL) and washed with saturated 3 N HCl (3 ꢁ
5 mL), saturated Na2CO3 (2 ꢁ 5 mL), and brine (5 mL). After dried
over MgSO4, the solvent was evaporated to give pure Z-L-(O-Leu)-L-(O-
Phe)-OBu-t 18 as a colorless oil (86%): [R]2D3 = ꢀ39.0 (c 1.5, CHCl3);
1H NMR (CDCl3) δ 0.93 (d, J = 6.3 Hz, 6H), 1.36 (s, 9H), 1.64ꢀ1.86
(m, 3H), 3.14 (d, J = 5.7 Hz, 2H), 5.00 (dd, J = 9.9, J = 4.2 Hz, 1H),
5.14ꢀ5.20 (m, 3H), 7.20ꢀ7.40 (m, 10H); 13C NMR (CDCl3) δ 21.6,
23.2, 24.6, 28.1, 37.4, 39.9, 70.24, 74.1, 82.7, 127.2, 128.5 (2C), 128.8
(2C), 129.7, 135.2, 135.8, 154.8, 168.0, 169.9. Anal. Calcd for
C27H34O7: C, 68.92; H, 7.28. Found: C, 69.14; H, 7.62.
mp 65ꢀ70 °C; [R]2D3 = ꢀ37.5 (c 1.5, CH3OH); 1H NMR (DMSO-d6) δ
0.90 (t, J = 6.9 Hz, 6H), 1.52ꢀ1.82 (m, 3H), 3.87 (d, J = 9.0 Hz, 2H), 4.96
(dd, J = 9.6 Hz, 3.6 Hz, 1H), 8.58 (br s, 3H), 13.20 (br s, 1H); 13C NMR
(DMSO-d6) δ 21.5, 22.9, 24.1, 39.1, 71.9, 167.5, 170.8. Anal. Calcd for
C8H16ClNO4: C, 42.58; H, 7.15; N, 6.21. Found: C, 42.96; H, 7.15; N, 6.15.
L-Phe-L-(O-Leu) HCl (19d): white microcrystals (56%); mp
3
139ꢀ140 °C; [R]2D3 = ꢀ24.4 (c 1.5, CH3OH); 1H NMR (DMSO-d6)
δ 0.84 (dd, J = 5.1 Hz, 5.1 Hz, 6H), 1.45ꢀ1.65 (m, 3H), 3.19 (d, J = 6.6
Hz, 2H), 4.35 (t, J = 6.3 Hz, 1H), 4.84ꢀ5.00 (m, 1H), 7.20ꢀ7.42
(m, 5H), 9.00 (br s, 3H); 13C NMR (DMSO-d6) δ 21.4, 22.8, 23.9, 35.6,
52.9, 72.1, 127.1, 128.4, 129.7, 135.1, 168.9, 170.8; HRMS (ESI) calcd
for C15H22ClNO4 [M þ H]þ 280.1543, found 280.1544.
General Procedure for the Preparation of Depsitripep-
tides (20). The HCl salt of depside 19 (0.40 mmol) and DIEA
(0.80 mmol) were dissolved in the minimum amount of cold water.
Acetonitrile (3 mL) was added and the solution cooled to 10 °C. A solution
of N-Pg(R-aminoacyl)benzotriazoles 9 (0.20 mmol) in acetonitrile (3 mL)
was added dropwise over 10 min at 10 °C and stirred for 0.5ꢀ2 h at 10 °C.
The reaction mixture was monitored by TLC [EtOAcꢀhexanes (1:2)].
After completion of reaction, the solvent was evaporated. EtOAc (15 mL)
was added, and the mixture was washed with 4 N HCl solution (3 ꢁ 10 mL)
and brine (10 mL) and dried over anhydrous MgSO4. The solvent was
evaporated under reduced pressure to give crude product. The solution of
crude in CH2Cl2 (1 mL) was loaded onto a short silica column (1 cm
diameter ꢁ 2 cm length) and eluted with EtOAc. The solvent was removed
under reduced pressure to give pure depsitripeptide 20aꢀc.
Z-L-(O-Leu)-L-(O-Phe)-OH (17c). The solution of Z-L-(O-Leu)-L-(O-
Phe)-OBu-t 18 (0.10 g, 0.21 mmol) in CH2Cl2ꢀTFA [6 mL, (1:1)] was
stirred for 2 h at 4 °C. Then the volatile part was removed under reduced
pressure. The residue oil which was taken into EtOAc (10 mL) was
washed with 1 N HCl (3 ꢁ 5 mL) and brine (5 mL). After the mixture
was dried over MgSO4, the solvent was removed to give oily crude. The
solution of crude in CH2Cl2 (1 mL) was loaded onto a short silica
column (1 cm diameter ꢁ 2 cm length) and eluted with EtOAcꢀ
hexanes (1:1). Evaporation of solvent gave pure Z-L-(O-Leu)-L-(O-
Phe)-OH 17c as a colorless oil (80%): [R]2D3 = ꢀ38.9 (c 1.5, CHCl3); 1H
NMR (CDCl3) δ 0.90 (d, J = 6.3 Hz, 6H), 1.56ꢀ1.84 (m, 3H), 3.14 (dd,
J = 14.4 Hz, 7.8 Hz, 1H), 3.25 (dd, J = 14.4 Hz, 4.5 Hz, 1H), 4.97 (dd,
J = 9.3 Hz, 4.2 Hz, 1H), 5.16 (s, 2H), 5.36 (dd, J = 7.8 Hz, 4.5 Hz, 1H),
6.02 (s, 1H), 7.20ꢀ7.42 (m, 10H); 13C NMR (CDCl3) δ 21.7, 23.1,
24.5, 37.2, 39.7, 70.3, 73.0, 74.1, 127.5, 128.5, 128.7, 128.8, 129.6, 135.1,
135.3, 154.9, 169.8, 173.8. Anal. Calcd for C23H24O7: C, 66.65; H, 6.32.
Found: C, 66.95; H, 6.54.
General Preparation of Unprotected Depsides (19). DMAP
(6.0 mmol) was added to a stirred solution of N-Boc(R-aminoa-
cyl)benzotriazole 9 (5.0 mmol) and R-hydroxycarboxylic acid 10 (0.6
mmol) in THF (10 mL) at 4 °C. The reaction mixture was stirred for 6 h
at room temperature until the reaction was complete by TLC [EtOAcꢀ
hexanes (1:2)]. The solvent was evaporated under reduced pressure, and
the residue was taken into EtOAc (25 mL), washed with saturated citric
acid solution (3 ꢁ 15 mL) and brine (10 mL), and dried over MgSO4.
The solvent was evaporated under reduced pressure to yield the crude
product as an oil. The crude was dissolved in 5.0 N HCl(g) in dry EtOAc
(20 mL), and the solution was stirred for 15 min at room temperature. A
white precipitate was formed, filtered off, and discarded. The solvent was
removed. The residue was dissolved in dry CH2Cl2 (25 mL). Dry HCl
gas was bubbled into the flask for 4ꢀ6 h at room temperature while
stirring. The precipitate which formed was collected and washed with
dry CH2Cl2 to give pure unprotected depside as hydrochloride salt 19.
Z-L-Phe-Gly-L-(O-Leu)-OH (20a): white hydroscopic microcrystals
(71%); mp 52ꢀ55 °C; [R]2D3 = ꢀ15.7 (c 1.2, CHCl3); 1H NMR
(CDCl3) δ 0.91 (t, J = 6.9 Hz, 6H), 1.60ꢀ1.84 (m, 3H), 2.84ꢀ3.00
(m, 1H), 3.02ꢀ3.22 (m, 1H), 3.90ꢀ4.20 (m, 2H), 4.40ꢀ4.60 (m, 1H),
4.90ꢀ5.10 (m, 3H), 5.72 (d, J = 8.1 Hz, 1H), 6.93 (br s, 1H), 7.10ꢀ7.34
(m, 10H), 7.95 (br s, 1H); 13C NMR (CDCl3) δ 21.7, 23.2, 24.8, 38.6,
39.7, 41.3, 56.2, 67.4, 72.0, 127.2, 128.1, 128.4, 128.7, 128.8, 129.5, 136.2,
136.3, 156.5, 169.5, 172.1, 173.7. Anal. Calcd for C25H30N2O7: C, 63.82;
H, 6.43; N, 5.95. Found: C, 63.76; H, 6.53; N, 5.90.
Z-L-Met-L-Phe-L-(O-Leu)-OH (20b): white microcrystals (56%); mp
140ꢀ142 °C; [R]2D3 = ꢀ36.3 (c 1.2, CH3OH); 1H NMR (CDCl3) δ 0.93
(dd, J = 7.5 Hz, 6.3 Hz, 6H), 1.64ꢀ1.96 (m, 5H), 2.01 (s, 3H), 2.49 (t, J =
6.9 Hz, 2H), 3.05 (dd, J = 14.1 Hz, 7.5 Hz, 1H), 3.28 (dd, J = 14.1 Hz, 5.4
Hz, 1H), 4.34 (q, J = 7.2 Hz, 1H), 4.82ꢀ4.94 (m, 1H), 5.04ꢀ5.14 (m, 3H),
5.64 (d, J = 7.5 Hz, 1H), 6.77 (d, J = 7.2 Hz, 1H), 7.14ꢀ7.28 (m, 5H),
7.30ꢀ7.38 (m, 5H); 13C NMR (CDCl3) δ 15.2, 21.7, 23.2, 24.8, 30.0, 31.6,
37.7, 39.8, 53.3, 53.7, 67.4, 71.9, 127.4, 128.3, 128.5, 128.8 (2C), 129.6,
135.8, 136.2, 156.3, 171.1, 171.5, 174.1. Anal. Calcd for C28H36N2O7S: C,
61.75; H, 6.66; N, 5.14. Found: C, 61.53; H, 7.02; N, 5.35.
Z-L-Met-Gly-L-(O-Leu)-OH (20c): white microcrystals (68%); mp
1
113ꢀ115 °C; [R]2D3 = ꢀ22.5 (c 1.5, CH3OH); H NMR (CDCl3)
δ 0.94 (dd, J = 8.4 Hz, 6.3 Hz, 6H), 1.64ꢀ1.90 (m, 3H), 1.90ꢀ2.00 (m,
1H), 2.00ꢀ2.16 (m, 1H), 2.06 (s, 3H), 2.56 (t, J = 7.2 Hz, 2H), 3.95 (dd,
J = 18.0 Hz, 3.9 Hz, 1H), 4.28 (dd, J = 17.4 Hz, 5.7 Hz, 1H), 4.40ꢀ4.50
(m, 1H), 5.00ꢀ5.20 (m, 3H), 5.92 (d, J = 8.4 Hz, 1H), 7.15 (br s, 1H),
7.30ꢀ7.40 (m, 5H), 8.20 (br s, 1H); 13C NMR (CDCl3) δ 15.4, 21.7,
23.2, 24.8, 30.0, 32.0, 39.7, 41.3, 53.9, 67.6, 72.0, 128.3, 128.5, 128.7,
136.1, 156.7, 169.6, 172.3, 173.5. Anal. Calcd for C21H30N2O7S: C,
55.49; H, 6.65; N, 6.16. Found: C, 55.71; H, 6.85; N, 5.95.
Gly-L-(O-Phe) HCl (19a): white microcrystals (48%); mp 150ꢀ155 °C;
3
1
[R]2D3 = ꢀ31.4 (c 1.5, CH3OH); H NMR (DMSO-d6) δ 3.09 (dd,
J = 14.4 Hz, 7.8 Hz, 1H), 3.20 (dd, J = 14.7 Hz, 4.5 Hz, 1H), 3.79 (s, 2H),
5.25 (dd, J = 7.5 Hz, 4.2 Hz, 1H), 7.20ꢀ7.40 (m, 5H), 8.57 (br s, 3H);
13C NMR (DMSO-d6) δ 36.4, 39.4, 74.0, 126.9, 128.5, 129.5, 136.1,
167.4, 169.8. Anal. Calcd for C11H14ClNO4: C, 50.88; H, 5.43; N, 5.19.
Found: C, 50.41; H, 5.93; N, 5.53.
L-Ala-L-(O-Leu) HCl (19b): white microcrystals (36%); mp
3
165ꢀ168 °C; [R]2D3 = ꢀ25.8 (c 1.5, CH3OH); 1H NMR (DMSO-d6)
δ 0.91 (t, J = 6.6 Hz, 6H), 1.48 (d, J = 7.2 Hz, 3H), 1.58ꢀ1.82 (m, 3H),
4.14 (q, J = 7.2 Hz, 1H), 4.98 (dd, J = 8.7 Hz, 3.6 Hz, 1H), 8.69 (br s,
3H); 13C NMR (DMSO-d6) δ 15.8, 21.5, 22.9, 24.1, 47.7, 71.8, 170.0,
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra of
S
b
compounds and chiral HPLC chromatograms; X-ray structure of
4892
dx.doi.org/10.1021/jo200174j |J. Org. Chem. 2011, 76, 4884–4893

The Journal of Organic Chemistry
ARTICLE
15a (CIF). This material is available free of charge via the In-
ternet at http://pubs.acs.org.
Narindoshvili, T.; Chung, A.; Khashab, N. M. Chem. Biol. Drug Des.
2008, 72, 182–188. (c) Katritzky, A. R.; Khashab, N. M.; Yoshioka, M.;
Haase, D. N.; Wilson, K. R.; Johnson, J. V.; Chung, A.; Haskell-Luevano,
C. Chem. Biol. Drug Des. 2007, 70, 465–468.
’ AUTHOR INFORMATION
(17) Katritzky, A. R.; Abdelmajeid, A.; Tala, S. R.; Amine, M. S.
Unpublished results
Corresponding Author
*E-mail: katritzky@chem.ufl.edu.
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’ ACKNOWLEDGMENT
We thank Dr. C. D. Hall and Mr. Z. Wang for helpful
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