Preparation of penta-azole containing cyclopeptides: challenges in macrocyclization

Article abstract of DOI:10.1016/j.tet.2007.06.103

Herein is described the synthesis of several analogs of the natural product IB-01211 from concatenated azoles, via a biomimetic pathway based on cyclization-oxidation of serine containing peptides combined with the Hantzsch synthesis. The macrocyclization

Full text of DOI:10.1016/j.tet.2007.06.103

Tetrahedron 63 (2007) 9862–9870
Preparation of penta-azole containing cyclopeptides:
challenges in macrocyclization
a
Delia Hernandez, Estela Riego, Andres Francesch, Carmen Cuevas,
a
b
b
ꢀ
ꢀ
Fernando Albericioa,c, and Mercedes Alvarez
a,d,
ꢀ
*
*
aBarcelona Science Park, Josep Samitier 1-5, E-08028 Barcelona, Spain
bPharma Mar, Avda Reyes Catoꢀlicos 1, E-28770 Colmenar Viejo, Madrid, Spain
cDepartment of Organic Chemistry, University of Barcelona, E-08028 Barcelona, Spain
dLaboratory of Organic Chemistry, Faculty of Pharmacy, University of Barcelona, E-08028 Barcelona, Spain
Received 11 April 2007; revised 22 June 2007; accepted 28 June 2007
Available online 10 July 2007
Abstract—Herein is described the synthesis of several analogs of the natural product IB-01211 from concatenated azoles, via a biomimetic
pathway based on cyclization–oxidation of serine containing peptides combined with the Hantzsch synthesis. The macrocyclization of rigid
peptide compounds 1 and 2 to give IB-01211 and its epimer 12b was explored, and the results are compared here to those previously obtained
for the macrocyclization of more flexible structures in the syntheses of YM-216391, telomestatin, and IB-01211. Lastly, the preliminary
results of anti-tumor activity screening of the synthesized analogs are discussed.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
to Thermoactinomyces genus.10,11 It is strongly cytotoxic
against several tumor cell lines,12 and contains four oxazoles
and one thiazole.
Directly linked 2,4-azoles are found in natural products that
have interesting biological activities and fascinating struc-
tures. Numerous bis- and trisoxazoles, as well as a few ox-
azole–thiazoles, have been isolated from marine organisms,
whereas linked thiazole-containing natural products have
generally been obtained from microorganisms.1 Marine
organism secondary metabolites such as ulapualides,2
halichondramides,3 kabiramides,3 mycalolides,4 halishig-
amides,5 and jaspisamides,6 all contain a trisoxazole frag-
ment. These compounds show a broad range of unusual
biological activities.7
Concatenated azoles have been prepared following several
strategies,1c including cyclodehydration of peptides contain-
ing serine, threonine or cysteine followed by oxidation of the
azoline to azole; the classical Hantzsch synthesis allowing
a one-pot synthesis of the azole ring; Pd(0) catalyzed
cross-coupling reaction—despite difficulties in the prepara-
tion of several precursors; sequential [3+2] cycloaddition of
an appropriate rhodium carbene with nitriles, a new route to
polyoxazoles from an ulapualide fragment; sequential Chan-
type rearrangements of tertiary amides for the preparation of
trisoxazoles; and finally, iterative oxazole assembly via
base-promoted cyclization of alkynyl glycine derivatives
prepared from the corresponding a-chloroglycinates by re-
action with alkynyl dimethyl-aluminum reagents. The total
syntheses of YM-216391,13 telomestatin,14 and IB-0121115
have recently been described. The three procedures possess
a common feature, a macrocyclization of flexible precursors
(Scheme 1).
Telomestatin, a potent telomerase inhibitor isolated from
Streptomyces anulatus 3533-SV48 that interacts specifically
with the human telomeric intramolecular G-quadruplex
without affecting DNA polymerases or reverse transcrip-
tases, contains a novel macrocyclic structure comprised of
seven linked oxazoles and one thiazoline unit. A related cy-
clopeptide, YM-216391, containing only four oxazoles and
one thiazole, has been isolated from Streptomyces nobilis.9
A more recently discovered macrocyclic peptide, IB-01211
(Fig. 1), has been isolated from the marine-derived micro-
organism strain ES7-008, which is phylogenetically close
The key step in the aforementioned synthesis of IB-01211 is
macrocyclization by Hantzsch formation of the thiazole
ring. Alternatively, the macrocyclization could be envisaged
through formation of an amide bond between penta-azole
peptides 1 and 2 as the last synthetic step (see Fig. 2).
Thus, with the aim of synthesizing IB-01211 and related
* Corresponding authors. Tel.: +34 93 403 7086; fax: +34 93 403 7126;
e-mail addresses: albericio@pcb.ub.es; malvarez@pcb.ub.es
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.06.103

D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
9863
O
O
S
O H
N
N
H
O
N
N
N
N
N
O
HN
O
N
N
N
N
O
O
O
NH
N
O
N
N
S
O
O
O
O
O
H
N
Ph
YM-216391
N
H
Telomestatin
O
HN
O
O
N
N
Ph
O
N
O
N
N
S
O
IB-01211
Figure 1. Natural compounds with 2,4-concatenated azoles.
S-tBu
intermediate 1 was thought to be more favorable than that
with 2, which involves a coupling reaction of a hindered
a-amine and a poorly reactive carboxylic group. A less con-
vergent strategy through amide bond formation between the
hindered Ile and Val residues was rejected. The common
precursors for both azole peptides are the penta-azole 3
and the dipeptide 4. Finally, assembly of the middle thiazole
present in compound 3 was planned from appropriately
functionalized bisoxazoles 5 and 6.
O
O
O
O
O
N
H
N
HN
NH
O
N
N
O
N
H2N
O
N
N
CO2H
Ph
N
O
O
N
O
HO2C
N
H
N
NH2
N
O
O
OH
S
OH
2.1. Synthesis of penta-azole 3
Preparation of 3 was attempted by transformation of peptide
7a into thioamide 7b, followed by cyclization and oxidation
(Scheme 2A).16 Reaction of the acid 5a and the amine 6a
using the general procedure described in Section 5 for
peptide bond formation afforded 7a in good yield.17 The
bisoxazole derivative 5a was obtained by cyclization of the
proper Ser peptide followed by oxidation of the resulting
oxazolines, as previously described by our group.15 How-
ever, reaction of 7a with the Lawesson reagent to produce
the thioamide 7b gave a complex mixture from which no
product could be isolated.
YM-216391
Telomestatin
O
OH
H
N
N
H
O
N
O
O
O
HN
Ph
N
IB-01211
N
O
O
N
S
O
H2N
Br
After exploring a broad range of reagents and conditions to
afford the thiazole moiety, including the use of a cysteine-
containing building block,18 we took a major shift in strat-
egy: a classical Hantzsch synthesis using a thioamide and
an a-bromoketone (Scheme 2B).
Scheme 1. Macrocyclization in the syntheses of YM-216391, tetomestatin,
and IB-01112.
derivatives, we studied the macrocyclization of penta-azole
containing peptides. We describe here the preparation of sev-
eral open chain IB-01211 derivatives, including subsequent
macrocyclization studies and anti-tumor activity screening.
Bisoxazole 10, containing the protected a-bromoketone res-
idue for the Hantzsch synthesis, was obtained by two se-
quential oxazole-ring formations in order to minimize the
amount of by-product resulting from water elimination,
and also because formation of a conjugated carbon–carbon
double bond is favored by the presence of the phenyl ring.
Cyclization and oxidation of dipeptide 819 afforded an ox-
azole with a protected amino ethanol substituent at position
2 of the ring, which was then deprotected with 95% trifluoro-
acetic acid (TFA) to give the amino alcohol 9. Compound 9
was used in the following amide bond formation by reaction
with bromopyruvic acid dimethyl acetal. Subsequent ring
closure and oxidation then provided 10. Elimination of the
acetal protecting group of 10 by treatment with formic
2. Results and discussion
The retro-synthetic strategy used for this work is shown in
Figure 2. Disconnection of the amide bonds between the
D-Val and the aminoethylidene (disconnection a), or between
the D-Allo-Ile and the phenyloxazolcarbonyl (disconnection
b), affords the penta-azole peptides 1 or 2, respectively. Both
peptides possess a tert-butyl protected alcohol, which can
be readily transformed into the exocyclic methylidene
present in the natural compound. The strategy involving

9864
D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
O
H
N
N
a
H
O
HN
b
O
O
N
N
Ph
O
N
O
N
N
S
O
IB-01211
a
b
O
R1O2C
NHR2
O
H
HN
N
N
H
t-BuO
t-BuO
NHBoc
N
O
S
HN
O
O
CO2Me
Ph
N
O
N
Ph
N
N
N
O
O
O
O
N
N
N
N
S
1
2
O
O
NHBoc
t-BuO
O
CO2Me
Ph
N
O
N
MeO2C
N
N
O
O
H
NHBoc
N
N
3
4
S
O
Y
X
t-BuO
CO2Me
R
N
N
N
N
BocHN
Ph
O
O
O
O
5
R = CO2Me
6
X = NHBoc, Y = O-tertBu
5a R = CO2H
5b R = CSNH2
6a X = NH2, Y = OH
6b X = O, Y = Br
Figure 2. Retro-synthetic analysis of IB-01211.
acid at reflux gave the bromoketone 6b in quantitative yield.
Penta-azole 3 was obtained in 62% yield by Hantzsch thi-
azole synthesis using the bisoxazolethioamide 5b and the
bis-oxazolyl a-bromomethyl ketone 6b. It was characterized
by H NMR, whereby the four singlets in the aromatic
region, and the five aromatic protons, were taken as repre-
sentative signals.
and epimeric peptides 4 and 4a in hand, we considered
two options for macrocyclization, both of which implied
preparation of peptides 1 and 2 (Scheme 3). The macrocyc-
lization studies were performed with 4a, as it is the cheaper
of the two peptides.
1
Compound 1a was prepared in good yield by methyl ester
hydrolysis of 3 with LiOH, followed by condensation with
N-deprotected-4a using EDC$HCl and HOBt as activating
agents, and N,N-diisopropylethylamine (DIEA). In parallel,
2a was also obtained with good yields, by N- and O-depro-
tection of 3 with 95% TFA, followed by condensation with
the acid obtained in the saponification of 4a, as described
above. C- and N-deprotection of the linear precursors 1a
(R1¼Me, R2¼Boc) and 2a (R1¼Me, R2¼Boc) were ob-
tained in situ by methyl ester hydrolysis with LiOH followed
2.2. Macrocyclization reaction
Peptides 4 and 4a were synthesized in solution from Boc–D-
allo-Ile–OH and H–Val–OMe, and from Boc–D-Ile–OH and
H–Val–OMe, respectively, using EDC$HCl/HOBt and
DIEA as coupling reagents. The N-deprotected and C-depro-
tected peptides were obtained by TFA treatment and by
methyl ester hydrolysis, respectively. With penta-azole 3

D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
9865
Ot-Bu
NHBoc
A
CO2Me
CO2Me
Ph
Ph
O
NHBoc
N
N
N
O
t-BuO
N
O
N
EDC, HOBt, DIEA,
CH2Cl2
N
H
N
O
N
N
O
CO2H
H2N
HO
+
O
43%
O
O
X
HO
7a X = O
7b X = S
5a
6a
B
1. BrCH2C(OMe)2CO2H,
1. DAST, THF, then Pyr
2. DBU-CCl4, MeCN,
Pyr
CO2Me
Ph
t-BuO
O
EDC, HOBt, DIEA, CH2Cl2
2. DAST, CH2Cl2, then K2CO3
3. DBU-CCl4, MeCN, Pyr
N
H
N
CO2Me
Ph
H2N
HO
N
OMe
BocHN
OMe
Ph
3. 95% TFA
O
N
MeO
Br
O
O
25% 3 step
50%
HO
O
10
8
9
NHBoc
t-BuO
Ot-Bu
S
N
CO2Me
Ph
NHBoc
N
O
O
N
N
CO2Me
Ph
NH2
HCO2H, reflux
91%
O
Br
5b
O
O
N
O
N
NaHCO3, THF, TFAA, lutidine
62%
O
N
6b
O
N
N
O
S
3
Scheme 2. Synthesis of penta-azole 3.
by TFA treatment. Several macrocyclization trials were
performed from C- and N-deprotected 1a (R1¼R2¼H) using
the following activating agents: 1-[bis(dimethylamino)-
methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium hexafluoro-
phosphate 3-oxide (HATU)/N-methylmorpholine (NMM),
HATU/DIEA, (benzotriazol-1-yloxy)tris(pyrrolidino)phos-
phonium hexafluorophosphate (PyBOP)/HOBt/DIEA, and
(7-azabenzotriazol-1-yloxy)tris(pyrrolidino)phosphonium
hexafluorophosphate (PyAOP)/DIEA. No traces of cyclized
The peptide-heterocycle 1 (R1¼Me, R2¼Boc) was obtained
by condensation of the free carboxylic acid of 3 with the free
amine of 4, using the same reaction conditions described
above for 1a. Similar conditions were also used for the
methyl ester hydrolysis of 1, conversion of the resulting
acid into the pentafluorophenyl ester followed by selective
N-deprotection. Macrocyclization was then attempted using
DIEA and CuSO4 in a highly dilute THF solution. Unfortu-
nately, no trace of the cyclized product was found, and only
the pentafluorophenyl ester 13 of the open chain peptide-
heterocycle was obtained.
1
compound were detected by either H NMR or MS. Like-
wise, no sign of 12b was detected when the same cyclization
conditions were applied to the methylidene derivative 11
(R1¼R2¼H), obtained by dehydration of 2a (R1¼Me,
R2¼Boc) with mesyl chloride (MsCl) in Et3N followed by
deprotection. Finally, macrocyclization of 1a (R1¼R2¼H)
was achieved using pentafluorophenol (Pfp-OH) as an acti-
vating agent. Thus, methyl ester hydrolysis of 1a (R1¼Me,
R2¼Boc), conversion of the resulting acid into the penta-
fluorophenyl ester, N-deprotection,20 and finally, macrocyc-
lization by treatment with DIEA in a highly diluted THF
solution gave the cyclic peptide 12a (Scheme 3).
3. Biological activity
The cytotoxicity of the concatenated azoles was evaluated
against a panel of three human tumor cell lines: A-549
lung carcinoma NSCL, HT-29 colon carcinoma, and MDA-
MB-231 breast adenocarcinoma. A conventional colorimet-
ric assay was run to estimate GI50 values (i.e., the drug
concentration at which 50% of cell growth is inhibited after
72 h of continuous exposure to the test molecule), in which
IB-01211 was for comparison.
The exocyclic methylidene of 12b was prepared by O-
deprotection and dehydration of 12a using mesyl chloride
as an activating agent and N,N,N-triethylamine (TEA) in
The results obtained are shown in Table 1. A decrease of ac-
tivity of compounds 3, 1, and 13 in relation with IB-01211
has been observed. Penta-azole 3 possesses activity at micro-
molar (mM) concentration in the three cell lines, whereas 1
possesses the same activity only in A-549 and 13 is inactive.
The natural compound, IB-01211, shows activity in the three
lines at nanomolar concentration. None of the peptides with
1
THF. It was characterized by H NMR, whereby the two
methylidene singlets at 6.06 and 6.70 ppm, and the four sin-
glets of the azole rings (7.96, 8.20, 8.27, and 8.30 ppm),
were taken as representative signals.21 As the yield of this
macrocyclization was poor (less than 10%), we decided to
test cyclization using a copper salt template.

9866
D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
O
H
N
R
O
N
H
NHR2
N
O
HO2C
N
H
O
NH2
CO2R1
Ph
N
HO
NHBoc
O
C-deprotected-4a
O
H
N
CO2Me
Ph
N
N
R
O
O
N
H
EDC, HOBt, DIEA, CH2Cl2
50%
N
N
N
O
HN
O
N
O
N
O
O
S
O
N
N
2a R = CH2OH
11 R = CH2
MsCl, TEA, THF
quant. yield
N
Ph
S
O
O
N
O
N
N
1. LiOH, THF, H2O, MeOH
2. PfpOH, EDC, DIEA,
THF, DMF
3. 30% TFA, CH2Cl2
4. DIEA, THF 9%
95% TFA, CH2Cl2
S
O
3
12a: R= CH2OtBu 1. 95% TFA
12b: R= CH2
2. MsCl, TEA, THF
90%
LiOH, THF, H2O, MeOH
100%
O
NHBoc
MeO2C
N
H
t-BuO
R1O2C
O
NH2
O
N-deprotected-4a
EDC, HOBt, DIEA, CH2Cl2
70%
HN
NHR2
CO2H
N
t-BuO
Ph
N
HN
N
O
N
O
N
O
O
N
O
Ph
N
N
O
O
S
N
N
O
S
O
O
R1O2C
O
C6F6O2C
NH2
1a
HN
MeO2C
N
H
HN
NHR2
tBuO
NH2
t-BuO
HN
N
O
HN
N
N-deprotected-4
O
O
N
1. LiOH, THF/H2O/MeOH
2. PfpOH, EDC, DIEA, THF, DMF
3. 30% TFA, CH2Cl2
O
N
EDC, HOBt, DIEA,
Ph
Ph
CH2Cl2
N
70%
N
O
O
O
O
N
N
4. DIEA, CuSO4, THF
N
N
S
S
O
37%
O
1
13
Scheme 3. Macrocyclization of 1 and 2.
Table 1. Cell growth inhibition (GI50) of synthetized azoles
heterocycles 1, 1a, 2a, 12a, 12b, and 13. Attempts at macro-
cyclization using the amino acids from the deprotection of 1
and 2 only led to a small amount of cyclized product, under-
scoring the need to change the synthetic strategy. Likewise,
macrocyclization of N- and C-deprotected 1 using a copper
chelating group also failed. While 1 and 2 possess an almost
planar polyheterocyclic system of penta-azoles that separate
the reactive groups the shown precursor of IB-01211
possesses bigger conformational freedom, which made the
macrocyclization easier.
Compound
Cytotoxicity (GI50, mM)
HT-29b
A-549a
MDA-MB-231c
IB-01211
1e
0.06
4.92
9.84
n.a
5.98
n.ad
n.ad
n.ad
0.10
n.a
0.069
n.a
n.a
1ae
6.78
10.1
4.55
n.ad
n.ad
n.ad
2ae
3
12a
12b
13
n.a
6.26
n.ad
n.ad
n.ad
a
The open chain polyazoles with the same stereocenter con-
figuration as the natural product inhibit growth of human
carcinoma cells (GI50) at micromolar concentrations,
whereas the epi-analogs obtained with D-Ile are inactive.
A-549 lung carcinoma NSCL.
HT-29 colon carcinoma cells.
MDA-MB-231 breast adenocarcinoma.
n.a¼not active at 10 mg/mL.
R1¼Me, R2¼Boc.
b
c
d
e
D-Ile, 2a, 12a, and 12b, has notable activity. These results
demonstrate the importance of the configuration of the
stereocenter to the activity of these compounds.
5. Experimental section
5.1. General
€
Melting points (mp) were determined in a Buchi Melting
4. Conclusions
Point B540 in open capillaries and are uncorrected. Re-
versed-phase analytical HPLC was performed on a Waters
Alliance separation module 2695 using a Waters Xterra
Herein we have reported preparation of the functionalized
polyazoles 6a, 6b, 7a, the penta-azole 3, and the peptide-
´
MS C18 column (1504.6 mm, 5 mm) and a Waters 996

D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
9867
PDA with a photodiode array detector with MeCN (0.036%
TFA) and H2O (0.045% TFA). Sonication was performed in
a Branson ultrasound bath. Polarimetry studies were per-
temperature for 5 h. The TFA was then removed under
reduced pressure, and the crude material was used for subse-
quent chemistry without further purification.
1
formed in a Perkin–Elmer 241 polarimeter. H NMR and
13C NMR spectra were recorded on a Varian Mercury
400 MHz and Varian 500 MHz spectrometers. Multiplicity
of the carbons was assigned with DEPT and gHSQC exper-
iments. Usual abbreviations for off-resonance decoupling
have been used here: (s) singlet, (d) doublet, (t) triplet, and
(q) quartet. The same abbreviations have also been used
5.2.3.2. Method B. Selective elimination of N-Boc
protecting group: a solution of N-Boc protected compound
(27 mmol) in TFA–CH2Cl2 (1 mL, 3:7) was stirred at room
temperature for 1 h. The TFA and the solvent were removed,
and the crude material was used for subsequent chemistry
without further purification.
1
for the multiplicity of signals in H NMR, plus: (m) multi-
plet, (dd) double doublet, (br s) broad singlet, and (br d)
broad doublet. Spectra were referenced to appropriate resid-
ual solvent peaks (CDCl3 and DMSO-d6). CIMS were mea-
sured in a Hewlett-Packard model 5890A with ammonia
(NH3). MALDI-TOF and ES-MS were performed in a Per-
Septive Biosystems Voyager DE RP, using an ACH matrix,
for the former, and a Waters alliance 2795 HPLC equipped
with a 2487 UV–vis detector and coupled to a ZQ electro-
spray mass detector, for the latter. The samples were run
with MeCN (0.07% HCO2H) and H2O (0.1% HCO2H).
HRMS were performed on a Bruker Autoflex high resolution
mass spectrometer by the Mass Spectrometry Service of the
University of Santiago de Compostela.
5.2.4. Peptide 1 (R1=Me, R2=Boc). Coupling of the free car-
boxylic acid of 3 to the free amine of 4 using the general pro-
cedure for peptide formation provided 1 (70%) as a white
solid, mp (MeCN) 186–188 ꢀC. [a]D +4.0 (c 0.56, CHCl3).
1H NMR (CDCl3, 400 MHz) d 0.88–1.08 (m, 12H); 1.12 (s,
9H); 1.22–1.34 (m, 1H); 1.48 (s, 9H); 1.52–1.62 (m, 1H);
2.11–2.25 (m, 2H); 3.70 (s, 3H); 3.74 (dd, J¼4.4 and
9.2 Hz, 1H); 4.57 (dd, J¼5.2 and 8.6 Hz, 1H); 4.65 (dd, J¼
5.6 and 8.8 Hz, 1H); 5.06–5.14 (m, 1H); 5.62 (br s, 1H);
6.56 (br s, 1H); 7.42–7.53 (m, 3H); 7.81 (br s, 1H); 8.24 (s,
1H); 8.29 (s, 1H); 8.33–8.38 (m, 2H); 8.42 (s, 1H); 8.47 (s,
1H). 13C NMR (CDCl3, 100 MHz) d 11.6 (q); 14.7 (q);
17.8 (q); 18.9 (t); 27.3 (q); 28.3 (q); 29.7 (t); 31.2 (d); 37.1
(d); 50.1 (d); 52.1 (q); 57.1 (d); 57.2 (t); 62.9 (t); 73.7 (s);
80.2 (s); 122.0 (d); 126.6 (s); 128.3 (d); 128.5 (d); 129.6
(s); 129.9 (s); 130.2 (d); 131.0 (s); 136.2 (d); 136.3 (s); 139.3
(d); 143.4 (d); 151.9 (s); 153.0 (s); 155.3 (s); 155.6 (s);
158.2 (s); 161.2 (s); 161.3 (s); 164.8 (s); 171.0 (s); 172.0
(s). MS (MALDI): m/z 937.33 (M+23, 100). HRMS m/z calcd
for C45H54N8NaO11S (M+Na) 937.3525, found 937.3527.
Syntheses of compounds 4 ([a]D +22.3 (c 0.56, CHCl3)), 5a,
5b, 8, 9, and 10 have previously been reported.15
5.2. Peptide bond formation. Sample procedure
5.2.1. D-Boc–Ile–L-Val–OMe (4a). D-Boc–Ile–OH$1/2 H2O
(795 mg, 3.31 mmol), EDC$HCl (698 mg, 3.64 mmol),
HOBt (491 mg, 3.64 mmol), and DIEA (1.21 mL,
7.11 mmol) were added to a solution of L-H–Val–OMe$HCl
(550 mg, 3.31 mmol) and dry CH2Cl2 (28 mL) at 0 ꢀC. The
mixture was stirred at room temperature for 20 h. The or-
ganic solution was washed with 5% NaHCO3 and NH4Cl,
dried, and concentrated to give the title compound (1.02 g,
90%) as a white solid, mp 105–107 ꢀC. [a]D +16.6 (c 0.7,
CHCl3). 1H NMR (CDCl3, 400 MHz) d 0.88–0.93 (m,
12H); 1.09–1.21 (m, 1H); 1.42 (s, 9H); 1.49–1.55 (m, 1H);
1.86 (m, 1H); 2.11–2.20 (m, 1H); 3.72 (s, 3H); 3.92–3.95
(m, 1H); 4.51–4.55 (m, 1H); 5.04–5.06 (d, 6.8 Hz, 1H);
6.37–6.39 (d, 6.8 Hz, 1H). 13C NMR (CDCl3, 100 MHz)
d 11.31 (q); 15.5 (q); 17.71 (q); 18.9 (q); 24.78 (t); 28.27
(q); 31.20 (d); 36.91 (d); 52.09 (q); 57.01 (d); 59.37 (d);
79.88 (s); 155.73 (s); 171.55 (s); 172.08 (s). MS (CI): m/z
345 (M+1, 100), 289 (M, 29), 245 (M, 43). HRMS m/z calcd
for C17H32N2NaO5 (M+Na) 367.2203, found 367.2206.
5.2.5. Peptide 1a (R1=Me, R2=Boc). Coupling of the free
carboxylic acid of 3 to the free amine of 4a using the general
procedureforpeptide formation provided1a(70%) asawhite
solid, mp (MeCN) 188–190 ꢀC. [a]D +3.4 (c 0.64, CHCl3).
1H NMR (CDCl3, 400 MHz) d 0.87–1.02 (m, 12H); 1.03–
1.06 (m, 1H); 1.11 (s, 9H); 1.21–1.33 (m, 1H); 1.47 (s,
9H); 1.60–1.75 (m, 1H); 2.07–2.24 (m, 1H); 3.69 (s, 3H);
3.71–3.76 (m, 1H); 3.84–3.91 (m, 1H); 4.47–4.61 (m, 2H);
5.04–5.14 (m, 1H); 5.60 (br s, 1H); 6.50 (br s, 1H); 7.42–
7.53 (m, 3H); 7.82 (br s, 1H); 8.23 (s, 1H); 8.28 (s, 1H);
8.32–8.38 (m, 2H); 8.4 (s, 1H); 8.47 (s, 1H). 13C NMR
(CDCl3, 100 MHz) d 11.4 (q); 15.7 (q); 17.6 (q); 19.0 (q);
25.1 (t); 27.3 (q); 28.3 (q); 31.2 (d); 36.9 (d); 50.1 (d); 52.1
(q); 57.2 (d); 58.1 (d); 62.9 (t); 73.7 (s); 80.2 (s); 122.0 (d);
126.7 (s); 128.4 (d); 128.5 (d); 129.7 (s); 130.0 (s); 130.2
(s); 131.1 (s); 136.2 (d); 136.4 (s); 139.3 (d); 143.4 (d);
151.8 (s); 153.1 (s); 155.3 (s); 155.6 (s); 158.2 (s); 161.1
(s); 161.2 (s); 164.8 (s); 170.7 (s); 171.9 (s). MS (MALDI):
m/z 937.31 (M+Na, 100). HRMS m/z calcd for
C45H54N8NaO11S (M+Na) 937.3525, found 937.3529.
5.2.2. Hydrolysis of methyl esters. LiOH (2 M, 9 mmol)
was added to a solution of methyl ester (3 mmol) in THF–
H2O–MeOH (50:6:0.2, 14 mL), and the reaction mixture
was stirred at room temperature for 1 h. The pH was brought
to 3 by addition of 1 M HCl, and then the solution was ex-
tracted with EtOAc. The organic layer was dried over
MgSO4 and concentrated to afford the acid as a white solid.
5.2.6. Peptide-heterocycle 2a (R1=Me, R2=Boc). The free
amino alcohol, obtained by N- and O-deprotection of 3 using
95% TFA (2 mL), was coupled to the acid of 4a following the
general procedure for peptide formation to provide 2a (50%)
as a white solid, mp (MeCN) 224–226 ꢀC. 1H NMR (DMSO,
400 MHz) d 0.76–0.88 (m, 12H); 0.95–1.10 (m, 1H); 1.37 (s,
9H); 1.45–1.46 (m, 1H); 1.63–1.72 (m, 1H); 1.94–2.06 (m,
1H); 3.57–3.63 (m, 1H); 3.76–3.84 (m, 1H); 3.86 (s, 3H);
3.97–4.07 (m, 1H); 4.11–4.17 (m, 1H); 5.14–5.21 (m, 1H);
5.30–5.36 (br s, 1H); 6.77 (br s, 1H); 7.55–7.61 (m, 3H);
5.2.3. Deprotection with TFA.
5.2.3.1. Method A. Elimination of N-Boc and O-t-Bu
protecting groups: 95% TFA (5 mL) was added to a solution
of the di-protected (N-Boc and O-t-Bu) compound
(2.38 mmol), and the solution was stirred at room

9868
D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
7.73–7.80 (br s, 1H); 8.06–8.11 (m, 2H); 8.67 (s, 1H); 9.03 (s,
1H); 9.08 (s, 1H); 9.2 (s, 1H). 13C NMR (CDCl3, 100 MHz)
d 10.8 (q); 15.3 (q); 17.9 (q); 19.0 (q); 29.0 (t); 29.8 (q); 36.27
(d); 40.6 (d); 50.2 (d); 52.0 (q); 57.7 (d); 58.7 (d); 60.5 (t);
69.7 (s); 78.8 (s); 123.4 (d); 126.1 (s); 127.2 (s); 128.2
(2d); 128.5 (2d); 129.8 (s); 129.9 (s); 130.6 (d); 135.4 (d);
139.1 (s); 141.2 (d); 141.3 (s); 141.5 (s); 142.6 (d); 152.8
(s); 154.3 (s); 155.2 (s); 157.4 (s); 160.4 (s); 161.6 (s);
163.6 (s); 164.1 (s); 172.6 (s). MS (MALDI): m/z 881.07
(M+Na, 78), 783 (95), 759 (100). HRMS m/z calcd for
C41H46N8NaO11S (M+Na) 881.2899, found 881.2886.
452 mmol) and formic acid (3.5 mL) was refluxed for 2 h,
and then cooled to room temperature. The organic solution
was poured into an aqueous solution of NaHCO3 and ex-
tracted with CH2Cl2. The organic layer was dried over
MgSO4 and concentrated to give 6b (164 mg, 91%) as a yel-
low oil. 1H NMR (CDCl3, 400 MHz) d 3.98 (s, 3H); 4.68 (s,
2H); 7.51–7.53 (m, 3H); 8.13–8.16 (m, 2H); 8.58 (s, 1H). 13C
NMR (CDCl3, 100 MHz) d 30.3 (t); 52.5 (q); 126.1 (s); 127.8
(s); 128.5 (2d); 128.6 (2d); 130.9 (d); 131.7 (s); 142.5 (d);
151.3 (s); 155.9 (s); 156.1 (s); 162.1 (s); 178.7 (s). MS
(ES): m/z 408.1 (MBr79+18, 61), 410.1 (MBr81+18, 62),
391 (MBr79, 62), 393 (MBr81, 63), 313 (100). HRMS m/z
calcd for C16H12BrN2O5 390.9924, found 390.9911.
5.2.7. Methyl 20-{2-[20-(2-tert-butoxy-1-tert-butoxycar-
bonyl-aminoethyl)-[2,40]bis-oxazol-4-yl]thiazol-4-yl}-5-
phenyl[2,40]bis-oxazolyl-4-carboxylate (3). Bromoketone
6b (64 mg, 0.16 mmol) was added to a suspension of 5b
(45 mg, 0.11 mmol) and NaHCO3 (29 mg, 0.35 mmol) in
THF (2 mL). The mixture was stirred for 8 h at room temper-
ature, at which point it was filtered over alumina and washed
with CH2Cl2–MeOH (4:1). The organic layer was concen-
trated to give a crude material, which was dissolved in dry
THF (2 mL) and cooled to ꢁ10 ꢀC. Lutidine (0.10 mL,
0.87 mmol) and TFA (46 mL, 0.33 mmol) were then added
to the solution, and the reaction mixture was stirred at
room temperature overnight. Concentration in vacuo gave
a brown residue, which was purified by silica gel chromato-
graphy. Elution with CH2Cl2–EtOAc (9:1) gave 3 (47.8 mg,
62%) as a white solid, mp (MeCN) 216–218 ꢀC. [a]D ꢁ12.2
(c 0.51, CHCl3). 1H NMR (CDCl3, 400 MHz) d 1.11 (s, 9H);
1.47 (s, 9H); 3.73 (dd, J¼4.4 and 9.2 Hz, 1H); 3.86–389 (m,
1H); 3.98 (s, 3H); 5.06–5.12 (m, 1H); 5.58–5.60 (d, J¼
8.4 Hz, 1H); 7.49–7.52 (m, 3H); 8.17–8.20 (m, 2H); 8.22
(s, 1H); 8.28 (s, 1H); 8.46 (s, 1H); 8.49 (s, 1H). 13C NMR
(CDCl3, 100 MHz) d 27.29 (q); 28.32 (q); 29.69 (q); 50.14
(s); 52.43 (d); 62.94 (t); 73.75 (s); 121.92 (d); 126.46 (s);
127.68 (s); 128.44 (d); 128.66 (d); 129.95 (s); 130.63 (s);
131.0 (s); 136.17 (d); 136.4 (s); 139.30 (s); 139.37 (d);
143.51 (d); 153.03 (s); 155.61 (s); 158.01 (s); 161.17 (s);
162.35 (s); 164.79 (s). MS (FAB): m/z 720.1 (M+18, 65),
589 (M+1, 100). HRMS m/z calcd for C34H34N6NaO9S
(M+Na) 725.200, found 725.1993.
5.2.10. Peptide 7a. The free carboxylic acid of 5 (40 mg,
0.10 mmol) was coupled to 6a (33 mg, 0.10 mmol) using
the general procedure for peptide formation to provide 7a
(31 mg, 43%) as a yellow solid, mp (MeCN) 98–100 ꢀC.
[a]D ꢁ13.4 (c 0.35, CHCl3). H NMR (CDCl3, 400 MHz)
1
d 1.09 (s, 18H); 1.45 (s, 9H); 3.6–3.70 (m, 1H); 3.79–3.84
(m, 1H); 3.93 (s, 3H); 4.09–4.13 (m, 1H); 4.28–4.32 (m,
1H); 5.01–5.09 (m, 1H); 5.55–5.59 (m, 1H); 7.43–7.48 (m,
4H); 7.95 (d, J¼8.0 Hz, 1H); 8.07–8.10 (m, 2H); 8.20 (s,
1H); 8.26 (s, 1H); 8.33 (s, 1H). 13C NMR (CDCl3,
100 MHz) d 27.2 (q); 28.2 (q); 48.5 (d); 50.1 (d); 52.4 (q);
62.9 (t); 63.2 (t); 73.7 (s); 80.3 (s); 126.6 (s); 127.8 (s);
128.7 (d); 128.8 (d); 129.9 (s); 130.9 (d); 136.5 (s); 139.5
(d); 140.2 (d); 141.8 (d); 153.0 (s); 153.6 (s); 155.6 (s);
155.8 (s); 155.9 (s); 160.5 (s); 160.8 (s); 163.2 (s); 164.2
(s); 171.3 (s). MS (MALDI-TOF) 745 (M+K); 729.24
(M+Na, 100); 707.64 (60); 674 (60). HRMS m/z calcd for
C34H38N6NaO11 (M+Na) 729.2491, found 729.2480.
5.2.11. Peptide 12a. A solution of the carboxylic acid
(125 mg, 190 mmol) obtained from 1a in dry THF–DMF
(120 and 5 mL) was cooled to 0 ꢀC. EDC$HCl (190 mg,
99 mmol), DIEA (167 mL, 99 mmol), and Pfp-OH (194 mg,
1.05 mmol) were then added, and the mixture was stirred
at room temperature for 20 h. The solvents were removed
in vacuo, and the residue was diluted with CH2Cl2 and
washed with 5% aqueous NaHCO3 and aqueous NH4Cl.
The organic solution was dried and concentrated, and the
residue was diluted with TFA–CH2Cl2 (6 and 14 mL). The
solution was stirred for 1 h at room temperature, and then
the TFA was removed, and the crude material was dissolved
in THF (250 mL). DIEA (479 mL, 2.82 mmol) was added,
and the mixture was stirred for 96 h at room temperature.
The solvents were removed, and the residue was then
washed with MeOH. The residue obtained after removing
the solvent was purified by preparative HPLC. A gradient
of H2O (0.045% TFA)–MeCN (0.036% TFA) from 6:4 until
1:9 in 15 min gave a white solid (10 mg, 9%, rt¼7.47 min),
5.2.8. Methyl 20-(2-tert-butoxy-1-tert-butoxycarbonyl-
amino-ethyl)-5-phenyl[2,40]bis-oxazolyl-4-carboxylate
(6). The free amine of 9 was coupled with Boc–L-Ser(t-Bu)–
OH following the general procedure for peptide formation.
Cyclization using DAST/K2CO3 and oxidation with DBU-
CCl4 in Pyr and ACN as solvents gave a crude, which was
purified by column chromatography on silica gel. Elution
with hexane–EtOAc (3:1) gave 6 (50%) as a solid, mp
1
(MeCN) 66–68 ꢀC. H NMR (CDCl3, 400 MHz) d 1.1 (s,
9H); 1.47 (s, 9H); 3.72 (dd, J¼4.0 and 9.2 Hz, 1H); 3.83–
3.89 (m, 1H); 3.96 (s, 3H); 5.02–5.12 (m, 1H); 5.6 (br s,
1H); 7.45–7.51 (m, 3H); 8.10–8.15 (m, 2H); 8.33 (s, 1H).
13C NMR (CDCl3, 100 MHz) d 27.24 (q); 28.30 (q); 50.10
(d); 52.32 (q); 62.89 (t); 73.70 (s); 80.13 (s); 126.50 (s);
127.60 (s); 128.40 (2d); 128.56 (2d); 129.80 (s); 130.50
(s); 139.41 (d); 153.31 (s); 155.34 (s); 162.35 (s); 164.50
(s). MS (ES) m/z 486.53 (M+1, 100). HRMS m/z calcd for
C25H32N3O7 486.2235, found 486.2222.
1
mp (MeCN) 206–208 ꢀC. [a]D ꢁ48.3 (c 0.35, DMSO). H
NMR (DMSO-d6, 500 MHz) d 0.82–0.95 (m, 12H); 1.02–
1.06 (m, 2H); 1.22 (s, 9H); 1.49–1.57 (m, 1H); 1.85–1.91
(m, 1H); 3.85–3.94 (m, 1H); 3.97–4.04 (m, 1H); 4.17–
4.24 (m, 1H); 4.62–4.71 (m, 1H); 5.16–5.21 (m, 1H);
5.28–5.35 (m, 1H); 7.53–7.58 (m, 3H); 8.01 (br s, 1H);
8.03 (br s, 1H); 8.27–8.37 (m, 2H); 8.49 and 8.62 (2br s,
1H); 8.73 (s, 1H); 9.04 (s, 1H); 9.12 (s, 1H); 9.23 (s, 1H);
10.17 (br s, 1H). 13C NMR (DMSO-d6, 125 MHz) d 11.3
(q); 14.2 (q); 15.6 (q); 18.7 (q); 18.8 (q); 24.5 (t); 38.1 (d);
50.5 (d); 56.5 (d); 57.7 (d); 60.9 (t); 67.8 (d); 123.9 (d);
5.2.9. Methyl 20-(2-bromoacetyl)-5-phenyl[2,40]bis-
oxazolyl-4-carboxylate (6b). A mixture of 10 (200 mg,

D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
9869
128.4 (2d); 128.9 (2d); 130.5 (d); 138.0 (d); 141.8 (d); 141.9
(d). MS (MALDI): m/z 782.19 (M, 40).
and the Barcelona Science Park. We gratefully acknowledge
PharmaMar S.L. for performing the preliminary biological
tests. D.H. thanks the Ministerio de Educacioꢀn y Ciencia
for a doctoral fellowship, and E.R. thanks the Principado
de Asturias for a postdoctoral fellowship.
5.2.12. Peptide 12b. Peptide 12a (9 mg, 0.0114 mmol) was
deprotected with 95% TFA (1 mL). The crude was dissolved
in dry THF (1 mL), and the solution was cooled to 0 ꢀC, TEA
(15.82 mL, 0.114 mmol) and MsCl (4.4 mL, 0.057 mmol)
were added dropwise. The resulting solution was stirred for
2 h at 0 ꢀC, then washed with NH4Cl and water, dried, and
concentrated. The solvents were removed and the residue
was washed with MeCN to give a white solid (7.4 mg,
90%), mp (MeCN) 147–149 ꢀC. [a]D +12.4 (c 0.15,
CHCl3). 1H NMR (CDCl3, 500 MHz) d 0.79–0.92 (m,
12H); 0.99–1.09 (m, 2H); 1.41–1.45 (m, 1H); 1.52–1.55
(m, 1H); 3.65–3.75 (m, 2H); 3.78–3.89 (m, 1H); 6.06 (s,
1H); 6.70 (s, 1H); 7.43–7.52 (m, 4H); 7.96 (s, 1H); 8.20 (s,
1H); 8.27 (s, 1H); 8.30 (s, 1H); 8.31 (br s, 1H); 8.36–8.37
(m, 2H); 8.47 (br s, 1H). 13C NMR (CDCl3, 125 MHz)
d 11.29 (q); 13.9 (q); 15.2 (q); 18.7 (q); 25.2 (t); 30.2 (d);
30.7 (d); 53.6 (d); 63.2 (d); 111.7 (t); 119.5 (d); 127.9 (d);
128.4 (2d); 130.2 (2d); 137.7 (d); 139.0 (d); 139.4 (d). MS
(MALDI): m/z 749.3 (M+Na, 100), 765.3 (M+K, 47).
References and notes
1. Recent revisions about the chemistry and properties can be
found in: (a) Roy, R. S.; Gehring, A. M.; Milne, J. C.;
Belshaw, P. J.; Walsh, C. T. Nat. Prod. Rep. 1999, 16, 249; (b)
Yeh, V. S. C. Tetrahedron 2004, 60, 11995; (c) Riego, E.;
ꢀ
ꢀ
Hernandez, D.; Albericio, F.; Alvarez, M. Synthesis 2005, 1907.
2. Roesener, J. A.; Scheuer, P. J. J. Am. Chem. Soc. 1986, 108,
846; For absolute stereochemistry of ulapualide A, see:
Allingham, J. S.; Tanaka, J.; Marriott, G.; Rayment, I. Org.
Lett. 2004, 6, 597.
3. Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Koseki, K.; Noma,
M.; Noguchi, H.; Sankawa, U. J. Org. Chem. 1989, 54, 1360.
4. (a) Fusetani, N.; Yasumuro, K.; Matsunaga, S.; Hashimoto, K.
Tetrahedron Lett. 1989, 30, 2809; (b) Rashid, M. A.; Gustafson,
K. R.; Cardeilina, J. H., II; Boyd, M. R. J. Nat. Prod. 1995, 58,
1120; (c) Matsunaga, S.; Nogata, Y.; Fusetani, N. J. Nat. Prod.
1998, 61, 663; (d) Matsunaga, S.; Sugawara, T.; Fusetani, N.
J. Nat. Prod. 1998, 61, 1164; (e) Phuwapraisirisan, P.;
Matsunaga, S.; van Soest, R. W. M.; Fusetani, N. J. Nat.
Prod. 2002, 65, 942.
5. Kobayashi, J.; Tsuda, M.; Fuse, H.; Sasaki, T.; Mikami, Y.
J. Nat. Prod. 1997, 60, 150.
6. Kobayashi, J.; Murata, O.; Shigemori, H.; Sasaki, T. J. Nat.
Prod. 1993, 56, 787.
7. Michael, J. P.; Pattenden, G. Angew. Chem., Int. Ed. Engl. 1993,
32, 1.
8. (a) Shin-ya, K.; Wierzba, K.; Matsuo, K.; Ohtani, T.; Yamada, Y.;
Furihata, K.; Hayakawa, Y.; Seto, H. J. Am. Chem. Soc. 2001,
123, 1262; (b) Kim, M.-Y.; Vankayalapati, H.; Shin-ya, K.;
Wierza, K.; Hurley, L. H. J. Am. Chem. Soc. 2002, 124, 2098.
9. Hayata, A.; Takebashi, Y.; Nagai, K.; Hiramoto, M. Jpn. Kokai
TokkyoKohoJP11180997-A, 1999;Chem. Abstr. 1999, 131,101.
10. Romero, F.; Malet, L.; Can˜edo, M. L.; Cuevas, C.; Reyes, F.
WO 2005/000880 A2, 2005.
11. Same structure was proposed for Merchercharmycin A isolated
from a marine-derived Thermoactinomices sp. by Kanoh, K.;
Matsuo, Y.; Adachi, K.; Imagawa, H.; Nishizawa, M.;
Shizuri, Y. J. Antibiot. 2005, 58, 289.
5.2.13. Peptide 13. A solution of carboxylic acid (150 mg,
0.166 mmol) obtained from 1 in dry THF–DMF (10 and
2 mL) was cooled to 0 ꢀC. EDC$HCl (0.22 mg, 1.16 mmol),
DIEA (0.19 mL, 1.16 mmol), and Pfp-OH (0.229 mg,
1.24 mmol) were then added, and the reaction mixture was
stirred at room temperature for 20 h. The solvents were re-
moved in vacuo, and the residue was diluted with CH2Cl2,
and then washed with 5% aqueous NaHCO3 and aqueous
NH4Cl. The organic solution was dried and concentrated,
and the residue was diluted in TFA–CH2Cl2 (1 and 3 mL).
The solution was stirred for 1 h at room temperature, and
then the TFAwas removed. The crude material was dissolved
in THF (300 mL). DIEA (0.28 mL, 1.66 mmol) and CuSO4
(132 mg, 0.83 mmol) were added, and the mixture was
stirred for 72 h at room temperature. The solvents were re-
moved, and the crude was purified by silica gel chromato-
graphy (9:1 CH2Cl2–MeOH) to give 13 (58.8 mg, 37%) as
a yellow solid, mp (MeCN) 140–142 ꢀC. [a]D +7.7 (c 0.39,
CHCl3). 1H NMR (CDCl3, 400 MHz) d 0.94–1.09 (m,
12H); 1.13 (s, 9H); 1.20–1.32 (m, 1H); 1.51–1.61 (m, 1H);
2.08–2.26 (m, 1H); 2.32–2.43 (m, 1H); 3.77–3.83 (m, 1H);
3.96–4.01(m, 1H);4.55–4.65(m, 1H); 4.82–4.91 (m, 1H); 5.38–
5.45(m, 1H);6.67(d, J¼8.4 Hz, 1H); 6.81 (d, J¼8.4 Hz, 1H);
7.42–7.47 (m, 3H); 8.24 (s, 1H); 8.29–8.30 (m, 2H); 8.31 (s,
1H); 8.40 (s, 1H); 8.47 (s, 1H). 13C NMR (CDCl3, 100 MHz)
d 11.4 (q); 14.7 (q); 17.6 (q); 18.9 (q); 26.2 (t); 27.3 (q); 30.9
(d); 36.4 (d); 48.9 (d); 57.2 (d); 57.5 (d); 61.8 (t); 74.3 (s);
122.3 (d); 125.1 (s); 126.4 (s); 126.5 (s); 128.3 (2d); 128.5
(2d); 128.7 (s); 129.4 (s); 130.1 (s); 130.3 (d); 130.9 (s);
131.5 (s); 136.3 (s); 136.4 (d); 139.3 (d); 139.8 (d); 143.3
(s); 151.8 (s); 153.2 (s); 155.1 (s); 156.8 (s); 157.2 (s);
158.2 (s); 161.0 (s); 161.5 (s); 162.0 (s); 167.7 (s); 171.5
(s). 19F NMR (CDCl3, 400 MHz) d 84.0 (s). MS (MALDI):
m/z 1006.7 (M+K, 100).
ꢀ
ꢀ
ꢀ
12. Can˜edo, M. L.; Martınez, M.; Sanchez, J. M.; Fernandez-
ꢀ
ꢀ
Puentes J. L.; Malet, L.; Perez J.; Romero, F.; Garcıa, L. F.
Fourth Eur. Conference on Marine Natural Products, Paris,
2005; poster 54.
13. Deeley, J.; Pattenden, G. Chem. Commun. 2005, 797.
14. Doi, T.; Yoshida, M.; Shin-ya, K.; Takahashi, T. Org. Lett.
2006, 8, 4165. A penta-azole related to telomestatin has been
recently described by Marson, M. C.; Saadi, M. Org. Biomol.
Chem. 2006, 4, 3892.
ꢀ
˜
15. Hernandez, D.; Vilar, G.; Riego, E.; Canedo, L. M.; Cuevas, C.;
ꢀ
Albericio, F.; Alvarez, M. Org. Lett. 2007, 9, 809.
ꢀ ꢀ
€ €
ꢀ
ꢀ
16. (a) Kaleta, Z.; Tarkanyi, G.; Gomory, A.; Kalman, F.; Nagy, T.;
Soos, T. Org. Lett. 2006, 8, 1093; (b) Sowinski, J. A.; Toogood,
ꢀ
Acknowledgements
P. L. J. Org. Chem. 1996, 61, 7671.
17. 7a was obtained as unique stereoisomer as indicated in its H
and C NMR maintaining the configuration of starting L-Ser
used in the preparation of 5a and 6a.
This study was partially supported by CICYT (BQU 2003-
00089 and BQU2006-03794), Generalitat de Catalunya,

9870
D. Hernaꢀndez et al. / Tetrahedron 63 (2007) 9862–9870
18. S-trityl cysteine containing peptide was treated with TiCl4 and
Ph3PO/Tf2O for a concomitant removal of the trityl group and
cyclization following the methods developed by Kelly and co-
workers: (a) Raman, P.; Razavi, H.; Kelly, J. W. Org. Lett. 2000,
2, 3289; (b) You, S.-L.; Razavi, H.; Kelly, J. W. Angew. Chem.,
Int. Ed. 2003, 42, 83; (c) You, S.-L.; Kelly, J. W. J. Org. Chem.
2003, 68, 9506.
described in Ref. 15. The three stereocenters of 8 were lost in
the bisoxazole 10.
20. Selective deprotection of the N-Boc in front of O-t-Bu group
was afforded by treatment with a solution of TFA in CH2Cl2
(30:70) for 1 h at room temperature.
21. Assays of coelution in the HPLC of the natural product and
the obtained macrocyclic peptide demonstrated that both
compounds were different. A sample of IB-01211 was kindly
supplied by PharmaMar S.L.
19. The peptide 8 was prepared as a stereoisomer mixture from
N–Boc–O–t-Bu–L-Ser–OH and D,L-PhSer–OMe as it is