Preparation and biological evaluation of key fragments and open analogs of scleritodermin A

Article abstract of DOI:10.1016/j.tet.2010.05.040

The synthesis of key fragments of scleritodermin A, their assembly, and their biological evaluation as cytotoxic and anthelmintic were performed.Highlights of the synthetic route include formation of the a-ketoamide linkage and use of stereocontrolled reactions.Open analogs of this natural product were obtained using a convergent strategy.

Full text of DOI:10.1016/j.tet.2010.05.040

Tetrahedron 66 (2010) 5384e5395
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Preparation and biological evaluation of key fragments and open analogs of
scleritodermin A
Diver Sellanes a, Francisco Campot a, Ivana Núñez a, Gerardo Lin b, Pablo Espósito b, Sylvia Dematteis b,
,
Jenny Saldaña c, Laura Domínguez c, Eduardo Manta a, Gloria Serra a
*
a Cátedra de Química Farmacéutica (DQO), Facultad de Química, UdelaR, Gral. Flores 2124, CC 1157, Montevideo, Uruguay
b Cátedra de Inmunología, Facultad de Química, UdelaR, Gral. Flores 2124, CC 1157, Montevideo, Uruguay
c Cátedra de Farmacología, Facultad de Química, UdelaR, Gral. Flores 2124, CC 1157, Montevideo, Uruguay
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of key fragments of scleritodermin A, their assembly, and their biological evaluation as
Received 8 April 2010
Received in revised form
10 May 2010
Accepted 11 May 2010
Available online 15 May 2010
cytotoxic and anthelmintic were performed. Highlights of the synthetic route include formation of the a-
ketoamide linkage and use of stereocontrolled reactions. Open analogs of this natural product were
obtained using a convergent strategy.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Scleritodermin A analogs
Key fragments
a
-Ketoamide
Azoles
1. Introduction
The macrocyclic compound, scleritodermin A (Fig. 1), was isolated
by Schmidt et al. from the marine sponge Scleritoderma nodosum,
The role of natural products in drug discovery has undergone
many changes during the past two decades from a decline in the
participation by pharmaceutical companies by the 1990s to a re-
naissance in the last years.1 The expectations from combinatorial
libraries in drug screening have not been fulfilled and more than
60% of the prescription drugs are of natural product origin.2 As
a consequence, the natural product-inspired drug discovery and
development has received renewed attention in recent years.3
A large number of natural products, in particular from the marine
environment, contain thiazole or oxazole heterocycles. In many
cases, promising antiproliferative and anthelmintic activities have
been identified for these compounds.4 Chemotherapy has been
a long standing effective instrument for battling parasitic infections
in both human and veterinary medicine. It has an important role, not
only in the treatment of individual patients but also, in conjunction
with public health and vector-control measures, in reducing the
transmission of parasitic infections. Unfortunately, resistance to the
therapeutic arsenal for the control of helminth infections repre-
sented by the benzimidazole, levamizole, and avermectina class of
broad-spectrum drugs has become a common and global phenom-
enon.5 The loss of income combined with the loss of productivity
represents a serious economical problem for countries in South
America, Australia, the UK, Holland, and South Africa.
showing significant cytotoxic activity against a panel of human tumor
cells lines (IC50<2
m
M).6 The proposed structure of scleritodermin A
(1) incorporates a novel conjugated thiazole moiety 2-(1-amino-
2-p-hydroxyphenylethane)-4-(4-carboxy-2,4-dimethyl-2Z,4E-prop-
adiene)-thiazole (ACT),
L-proline,
L-serine, and the unusual amino
acids keto-allo-isoleucine and O-methyl-N-sulfo-
D-serine.
Diverted total synthesis is the most versatile approach to ana-
logs preparation.7 Danishefsky et al. defined this term as the design
of a synthetic route to a given natural product that allows for the de
novo introduction of improved structural characteristics en route to
the target molecule. The benefit of this approach is that the analogs
are usually easier to prepare than the natural product.
Our interest on the synthetic studies of marine natural products
with anthelmintic or cytotoxic activity,8 encouraged us to embark
on a general program for the diverted total synthesis of scler-
itodermin A and analogs. In 2007, our group reported the prepa-
ration of the C1eN15 fragment,9 and recently, Nan et al. reported the
total synthesis of the originally proposed and revised structures of
scleritodermin A, 1 and 2, Figure 1.10
The present studies allowed us to obtain different key fragments
that have been tested for cytotoxic and antihelminthic activity. This
information could be essential in order to achieve one of the main
aims of analogs synthesis to reduce molecular complexity while
maintaining or even improving biological activity. There are many
* Corresponding author. E-mail address: gserra@fq.edu.uy (G. Serra).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.05.040

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5385
OH
OH
(Z)
S
N
N
O
O
NH
NH
NH
O
S
O
O
O
(E)
O
O
O
(S)
H2N
O
(R)
N
N
O
O
HN
N
HN
OCH3
O
N
OCH3
O
O
O
1
2
NHSO3Na
NHSO3Na
Scleritodermin A (revised)
Scleritodermin A (originally proposed)
Figure 1. Originally proposed and revised structures of scleritodermin A.
examples of simplified compounds derived from natural products
with high biological activity. Danishefsky et al. reached two-
migrastatin core derivatives through diverted total synthesis,
whose IC50 in migration assays were, remarkably, reduced by three
orders of magnitude relative to migrastatin.7 In addition, during
their total synthesis of Mycobactin S12, Miller et al. encountered
that a small fragment, exhibited similar antituberculosis activity to
the whole product.11 This exemplifies the importance of generating
information on the minimal structural requirement for bioactivity.
In the present study we propose the synthesis of scleritodermin
We herein describe the synthesis of different key fragments of
analogs of scleritodermin A, their assembly, and their biological
evaluation. Open analogs of scleritodermin A were obtained using
a convergent strategy that allows for structural variations of char-
acteristic parts of the natural product.
2. Results and discussion
2.1. Chemistry
A fragments containing a thiazole or oxazole ring,
L-allo-Ile or
L-Ile,
Our retrosynthetic analysis is shown in Scheme 1. Disconnection
E,E or E,Z configurations of the double bonds and different
at the estergroupof the macrolactonegenerates the open analog (3).
peptides.
Cleavage at the
a-ketoamide bond and at the amide bond between
OBn
OH
Open Analogs
3
2
N
N
O
O
E or Z
1
O
CO2Et
NH
NH
NH
S
X
O
O
O
O
O
OH
NH
N
3
N
O
NHR
O
HN
N
OCH3
R or S
R = Boc or Boc-
D-Ser(Me)-Pro-
O
1 or 2
O
X = S or O
NHSO3Na
Ph3P
O
OBn
CN
O
OH
E or Z
NH
N
O
+
N
NHR
R or S
CO2Et
BocHN
X
A Fragments
PPh3
R1
OR'
NHBoc
O
O
NC
R2
O
O
N
O
O
NHBoc
NHBoc
+
+
N
BnO
MeO
Boc-Ser(R')-Pro-OBn
Boc-
D
-Ser(Me)-Pro-OMe
R1= Et, R2= H
R1= H, R2= Et
R' = H or Ac
C Fragments
B Fragments
D Fragment
Scheme 1. Retrosynthetic analysis for scleritodermin A and analogs.

5386
D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
L
-Ile or
L
-allo-Ile and
L
-Proline produce different key fragments: A,
We investigated different conditions for the last coupling re-
action (Scheme 4). When 13 and the ylide ethyl 2-(triphenylphos-
phoranylidene)propionate were refluxed in benzene, the E isomer
(17) was obtained exclusively. Then, we used the modification of
Still/Gennari for the HWE reaction,13 and the fragments 16 and 17
were obtained in ratio, 80:20 using K2CO3 or 97:3 using KN(TMS)2
as base. Similarly, the isomers 18 and 19 were obtained in ratio,
92:8 using KHMDS or 99:1 using LiHMDS as base. The configuration
ketophosphorane fragments B, and the peptides C and D.
2.1.1. Synthesis of A fragments. The synthesis of these fragments
was based on the methodology previously described by us for the
synthesis of ACT fragment.9
Firstly, we obtained thiazole
4 and oxazole 5 by cyclo-
dehydration of a
b
-hydroxyamide or thioamide derived from L-
OBn
OBn
OBn
i) (Boc)2O, NaOH, H2O, t-BuOH
iii) DAST, CH2Cl2,
ii) H-Ser-OMe.HCl, DCC,
HOBT.H2O, Et3N, CH2Cl2
-78ºC
CO2Me
OH
O
N
96 % (two steps)
COOH
87 %
N
H
CO2Me
O
NHBoc
NH2
6
7
NHBoc
v) a) DAST, -78ºC, CH2Cl2
iv) H2S, Et3N,
b) DBU(-40ºC), BrCCl3 (0ºC)
80%
OBn
92 %
MeOH
OBn
OBn
v) a) DAST, -78ºC, CH2Cl2
b) DBU(-40ºC), BrCCl3 (0ºC)
CO2Me
N
CO2Me
OH
N
S
80%
O
N
H
S
CO2Me
5
NHBoc
4
NHBoc
NHBoc
8
Scheme 2. Synthesis of thiazole 4 and oxazole 5 from Tyr and Ser.
serine and
L
-tyrosine (Scheme 2). The thiolysis of oxazoline 7 in
of the double bonds in the obtained fragments 16 and 18 was
confirmed by NOE correlation between: (1) H7 of the ring and the
methyl group attached to C4 and (2) H3 and the methyl group at-
tached to C2.
H2S/MeOH is high-yielding, and offers an alternative to the thio-
nation of peptides using Lawesson’s reagent.12
Then, the ester groups of 4 or 5 were converted to the corre-
sponding aldehydes with LiAlH4 and activated MnO2 (Scheme 3).
OBn
OBn
OBn
OBn
CHO
CO2Me
i) LiAlH4,
Et2O, 0ºC
CH2OR
CHO
ii) MnO2,
CH2Cl2, rt
X=S 80%
X=O 87%
(two steps)
N
N
N
iii) Ph3PC(CHO)CH3,
Benzene, reflux
N
X
X
X
X
X=S 92%
NHBoc
NHBoc
NHBoc
NHBoc
X=O quant.
9 X=S, R = H
10 X=O, R= H
4 X=S
5 X=O
11 X=S
12 X=O
13 X=S
14 X=O
76%
iv) Myristic acid, EDCI,
HOBt,TEA,CH2Cl2
15, X=S, R = Myr
Scheme 3. Synthesis of E alkenes 13 and 14.
The Wittig reaction between the aldehydes 11 or 12 and 2-
(triphenylphosphoranylidene)-propianaldehyde rendered exclu-
sively the E alkenes 13 and 14 in excellent yield.
2.1.2. Synthesis of B fragments. Protection of the amine group of
L-isoleucine or
L-allo-isoleucine with Boc and further reaction of
their carboxylic acid with the (triphenylphosphoranylidene)
OBn
OBn
OBn
i) Ph3P=C(CH3)CO2Et,
H3
4
H3'
4
2 CO2Et
2
CHO
Benzene, Reflux, 100% (E >99) or
ii) (CF3CH2O)2P(O)CH(CH3)CO2Et,
K2CO3, 18-c-6, Toluen, -20º,
100% (Z: E, 80: 20) or
H5'
H5
S
CO2Et
+
N
N
N
H7
H7'
iii) (CF3CH2O)2P(O)CH(CH3)CO2Et,
KHMDS, 18-c-6, THF, -78ºC,
92% (Z: E, 97: 3)
S
S
NHBoc
13
NHBoc
16
NHBoc
17
OBn
OBn
OBn
H3
4
H3'
4
2 CO2Et
CHO
2
H5
H5'
iv) (CF3CH2O)2P(O)CH(CH3)CO2Et,
KHMDS, 18-c-6, THF, -78ºC,
89% (Z:E, 92:8) or
CO2Et
+
N
N
N
O
H7
H7'
v) (CH3CH2O)2P(O)CH(CH3)CO2Et,
LiHMDS, THF, -78ºC,
O
O
NHBoc
14
85% (E: Z, 99:1)
NHBoc
18
NHBoc
19
Scheme 4. Synthesis of fragments A.

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5387
HO
O
HO
O
PPh3
R1
i) Boc2O, NaOH,
H2O, t-BuOH
ii)Ph3P=CHCN EDCI,
DMAP, CH2Cl2
O
NC
R2
NH2
NHBoc
73% (two steps)
R2
R2
R1
R1
NHBoc
R1= Et, R2= H:
R1= H, R2= Et
20 R1= Et, R2= H
21 R1= H, R2= Et
Scheme 5. Synthesis of fragments B.
O
O
i) SOCl2, BnOH,
HO
BnO
0ºC to RT
82%
NH
NH2Cl
22
H-Pro-OH
H-Pro-OBn.HCl
O
O
NHBoc
OR
iii) DCC, HOBt,
TEA, CH2Cl2
R1O
N
54%
O
O
HO
HO
ii) Boc2O, NaOH,
H2O, t-BuOH
24; R1 = Bn, R = H
25; R1 = Bn, R = Ac
iv) Ac2O, TEA, 0ºC, 99%
v) H2, MeOH, quant.
NHBoc
NH2
v) H2, MeOH,
quant.
quant.
23
OH
OH
H-Ser-OH
26; R1 = H, R = Ac
27; R1 = H, R = H
Boc-Ser-OH
Scheme 6. Synthesis of fragments C.
acetonitrile ylide using EDCI as coupling reagent, allowed us to
CH2Cl2 and LiOH 2 M/THF, respectively (Scheme 8). Dipeptides 32
and 31 were coupled using different conditions. The best result was
obtained using PyBrOPas coupling agent (70% yield). Hydrogenation
of the resulting ester (33) rendered the tetrapeptide 34 in 98% yield.
obtain ketophosporanes 20 and 21 (Scheme 5).10
2.1.3. Synthesis of C fragments. The synthesis of C fragments was
performed from
L-serine and L-proline, (Scheme 6). First, the car-
boxylic acid of -proline was protected as its methyl ester and then,
L
2.1.6. Coupling between B and C fragments. Tripeptide 35 was
obtained in 88% yield from 21 and 26 using EDCI/HOBt or HBTU as
coupling reagents (Scheme 9).
it was coupled to Boc-Ser. However, using a variety of conditions
the basic hydrolysis of the dipeptide Boc-Ser-Pro-Me, was low
yielding. In contrast, when we changed the protecting group to
a benzyl ester, hydrogenation of the protected dipeptide allowed us
to obtain 26 and 27 in excellent yields.
2.1.7. Synthesis of C1eN15 fragments analogs. For the construction
of the
a-ketoamide function, we used the procedure described by
O
O
HO
MeO
i) SOCl2, MeOH,
0ºC to rt
NH
NH2Cl
28
quant.
O
O
NHBoc
OMe
H-Pro-OH
H-Pro-OMe.HCl
RO
iv) DCC, HOBt,
TEA, CH2Cl2
N
72%
ii) Boc2O, NaOH,
H2O, t-BuOH
iii) Me2SO4, t-BuLi,
THF
Boc-D-Ser(Me)-Pro-OMe
O
O
HO
HO
30, R = Me
NHBoc
NH2
v) LiOH 2M, THF
83%
57% (two steps)
29
OMe
Boc-D-Ser(Me)-OH
OH
-Ser-OH
31, R = H
H-
D
Scheme 7. Synthesis of fragments D.
2.1.4. Synthesis of D fragments. The synthesis of the D fragments
was performed from -serine and -proline as shown in Scheme 7.
Wasserman in the synthesis of cyclotheonamide A and B.14 Ozo-
nolysis of the cyanoketophosporanes 20 and 21, generated the
D
L
In contrast with our result for the Boc-Ser-Pro-Me dipeptide, 31
was obtained from 30 in good yield using basic hydrolysis. We
conclude that the hydroxyl group on serine must be protected in
order to successfully carry out this reaction.
highly electrophilic not isolables a,b-ketonitriles (Scheme 10).
Products 16 or 18 were deprotected using TFA/CH2Cl2 and then
added, without purification, to the corresponding reaction mixture
to obtain 36 and 37 in 49% and 58% yield, respectively. The struc-
tures of 36 and 37 were confirmed by the presence of a character-
istic 13C signal at 194.5 ppm or at 196.6 ppm, respectively,
assignable to the keto function.
2.1.5. Coupling between C and D fragments. The Boc group of pep-
tide 24 and the methyl ester of 30 were deprotected using TFA/

5388
D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
i) TFA:CH2Cl2
BnO
BnO
(1:1)
N
N
quant.
O
O
OH
NHBoc
OH
O
O
NH2
24
32
Boc-Ser-Pro-OBn
H-Ser-Pro-OBn
iii) PyBrOP, Et3N,
RO
OH
O
CH2Cl2, 70% or
EDCI, HOBt, TEA,
CH2Cl2, 66% or
DCC, HOBt, TEA,
CH2Cl2, 58%;
N
O
O
HN
N
MeO
HO
ii) LiOH 2M,
THF
N
N
33, R = Bn
O
O
O
O
O
NHBoc
O
NHBoc
O
83%
O
iv) H2, MeOH
98%
NHBoc
30
31
34, R = H
Boc-
D
-Ser(Me)-Pro-OMe
Boc-
D-Ser(Me)-Pro-OH
Scheme 8. Synthesis of tetrapeptide 34.
PPh3
PPh3
O
O
NC
HO
HBTU, TEA, CH2Cl2,
88% or
OAc
NC
N
NH
O
OAc
EDCI, HOBt, TEA, CH2Cl2
88%
N
NH2
O
O
NHBoc
O
26
21
35
NHBoc
Boc-Ser(Ac)-Pro-OH
Scheme 9. Synthesis of cyano ketophosphorane tripeptide 35.
Scheme 10. Synthesis of C1eN15 fragment analogs.
2.1.8. Synthesis of open analogs of scleritodermin A. Product 36 was
deprotected in 4 M HCl/dioxane and then coupled without purifi-
cation to 34 using HBTU. The open analog product of scleritodermin
A isomer 38 was obtained in 66% yield (Scheme 11).
than the parent natural product. Therefore, we have selected the
other synthethized A fragment, product 18 that has been obtained
in few steps in order to prepare a new analog, (Scheme 12). Ozo-
nolysis of 35 generated the a,b-ketonitrile. Product 18 was depro-
One of the advantages of Diverted Total Synthesis is the possi-
bility of obtaining simplified analogs that are easier to synthesize
tected in 4 M HCl/dioxane and then added to the reaction mixture
to obtain the open and simplified analog 39.
OBn
OBn
N
O
NH
S
EtO
O
N
O
O
NH2
NH
NH
S
EtO
OH
O
O
Derivative
O
HBTU, Et3N, CH2Cl2
66%
of 36
N
O
O
HN
O
HO
O
N
OH
O
HN
O
N
NHBoc
OMe
O
38
N
NHBoc
34
OMe
Scheme 11. Synthesis of the open analog of scleritodermin A 38.

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5389
Cl
Scheme 12. Synthesis of the open analog of scleritodermin A isomer 39.
2.2. Biological evaluation
example,11 is inactive (GI50>500) and 12 shows a micromolar level
GI50. In contrast, thiazole 9 is more active than oxazole 10.
Compounds 40 and 41 were obtained in order to study the in-
Growth inhibition properties against HCT-15, colon cancer cells
were investigated using the sulfurhodamine B (SRB)-assay and
mitomycin as control.15 The anthelmintic effect on the parasitic
stage (L4) of Nippostrongylus brasiliensis was evaluated using the
protocol of Gordon et al. and albendazole as positive control.16 The
results are summarized in Table 1.
fluence of the
a,b-ketoamide function coupled to the deprotected
derivatives of compounds 4 and 5, respectively. According with the
results, the introduction of the
toxic activity.
a,b-ketoamide improves the cyto-
Comparing the results for 36, 37, 38, and 39, we can conclude
The results of cytotoxic activity show that the type of substituent
on the C4-position of the thiazole or oxazole has different effects. For
that the presence of tetrapeptide Boc-
proves the cytotoxic activity.
D-Ser(Me)-Pro-Ser-Pro im-
Table 1
Anthelmintic effect and cytotoxic activity of the obtained products
Compound
Anthelmintic
LC50
M)a
HCT-15
GI50
M)b
Compound
Anthelmintic LC50
(
m
M)a
HCT-15
GI50
M)b
(
m
(
m
(m
OBn
OBn
CO2Me
CO2Me
N
N
500ꢀ38
340ꢀ26
10.0ꢀ0.8
630ꢀ30
15ꢀ1
>2000
5
S
O
4
NHBoc
NHBoc
OBn
OBn
CHO
CHO
N
N
>513
170ꢀ13
7.6ꢀ0.4
12
11
S
O
NHBoc
NHBoc
OBn
OBn
OH
OH
N
N
10.3ꢀ0.4
500ꢀ38
54ꢀ6
10
9
S
O
NHBoc
NHBoc
OBn
OBn
CO2Me
CO2Me
N
N
40
41
S
O
O
O
300ꢀ23
54ꢀ8
13ꢀ1
20ꢀ7
HN
O
HN
O
NHBoc
NHBoc
(continued on next page)

5390
Table 1 (continued)
D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
Compound
Anthelmintic
LC50
M)a
HCT-15
GI50
M)b
Compound
Anthelmintic LC50
(
m
M)a
HCT-15
GI50
M)b
(
m
(
m
(m
OBn
OBn
CO2Et
CO2Et
CO2Et
CO2Et
N
N
100ꢀ8
30ꢀ2
40ꢀ3
d
1600ꢀ400
>3000
21ꢀ2
300ꢀ23
135ꢀ34
228ꢀ86
23ꢀ1
16
18
S
O
NHBoc
NHBoc
OBn
OBn
N
N
70ꢀ5
19
17
S
O
NHBoc
NHBoc
OBn
OBn
CHO
CHO
N
N
15ꢀ1
14
13
S
O
NHBoc
NHBoc
OBn
O
NC
CO2Myr
H
N
Ph3P
N
OAc
N
>422
170ꢀ13
>391
15
O
35
S
O
NHBoc
NHBoc
OBn
OBn
CO2Et
CO2Et
N
N
36
37
S
O
O
O
20ꢀ1
>500
30ꢀ2
192ꢀ32
HN
O
HN
O
NHBoc
NHBoc
OBn
N
OBn
N
O
NH
NH
S
EtO
OH
O
O
O
O
38
NH
NH
9.6ꢀ0.7
34ꢀ1
16ꢀ1
>246
O
EtO
N
O
O
O
HN
O
N
O
OAc
NHBoc
O
N
NHBoc
OMe
a
Control: albendazole (ABZ), LC50¼0.34ꢀ0.02
mM.
mM.
b
Control: mitomycine (Mit), GI50¼1.6ꢀ0.4

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5391
The compound with the highest antihelminthic activity is the
open analog of 1, compound 38 (LC50¼9.6 M). The open and sim-
plified analog 39 also exhibits considerable activity (LC50¼16 M).
The -ketoamide function has different effects in the anthel-
mintic activity. For example, compounds 4 and 40 and 5 and 41
have the same order of activity. In contrast, compounds 36 and 37
are more active as anthelmintic than 16 and 18, respectively.
provided 6 (3.3 g, 7.0 mmol) (96% two steps) as a colorless oil.
m
Rf¼0.45 (EtOAc/n-hexanes, 2:1); [
a
]
25 þ12.3 (c1.0, CHCl3); IR (film)
D
m
3887, 1741, 1687, 1660, 1512, 1242, 1176, 1024 cmꢃ1; 1H NMR (CDCl3,
a,
b
400 MHz)
d
1.41 (s, 9H), 2.97 (dd, J¼7.6, 14.0 Hz,1H), 3.08 (dd, J¼6.0,
14.0 Hz, 1H), 3.44 (sa, 1H), 3.75 (s, 3H), 3.92 (s, 2H), 4.38 (m, 1H),
4.64 (m, 1H), 5.04 (s, 2H), 5.27 (d, J¼7.8 Hz, 1H), 6.92 (d, J¼8.7 Hz,
2H), 7.10 (d, J¼8.0 Hz, 1H), 7.14 (d, J¼8.7 Hz, 2H), 7.41 (m, 5H); 13C
NMR (CDCl3, 100 MHz) d 28.7, 37.8, 53.0, 55.3, 56.4, 63.2, 70.4, 80.8,
3. Conclusion
115.4, 127.8, 128.3, 129.0, 129.2, 130.8, 137.4, 156, 158.3, 171.0, 172.3;
HRMS (ESI) m/z calcd for C25H32N2O7 472.2210, found 472.2226.
Different key fragments of scleritodermin A and open analogs
werepreparedinverygoodyield. Themethodologythatinvolves the
4.1.2. (S)-Methyl
2-((S)-2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
use of cyanoketophosporanes derivatives allowed us to obtain
a
-
carbonylamino)ethyl)-4,5-dihydrooxazole-4-carboxylate (7). Dieth-
ylaminosulfur trifluoride (0.91 mL, 6.88 mmol) was added
dropwise to a cold (ꢃ78 ꢂC) solution of 6 in CH2Cl2 (50 mL). After
stirring for 1 h at ꢃ78 ꢂC, anhydrous K2CO3 (0.50 g, 6.88 mmol) was
added in one portion and the mixture was allowed to warm to room
temperature. The reaction was poured into saturated aqueous
NaHCO3, and the biphasic mixture was extracted with CH2Cl2. The
combined organic extracts were dried with MgSO4, filtered, and
concentrated in vacuo. Purification of the residue by flash chro-
matography (SiO2, EtOAc/n-hexane, 2:1) led to the desired oxazo-
line 7 (2.46 g, 87%) as a colorless oil. Rf¼0.67 (EtOAc/n-hexane, 2:1);
ketoamides in very good yield (42e58%) compared with the pre-
vious reported by Nan et al. for the synthesis of scleritodermin A.10
All the key fragments that would be used for the synthesis of the
revised structure of scleritodermin A were obtained. The developed
synthetic strategies could be applied in order to obtain even more
structurally diverse molecules.
Some of the products showed cytotoxic activity on HCT-15 cells.
Some of the most active products as anthelmintic are the open
analogs of scleritodermin A, 38 and 39.
The biological evaluation revealed some differences that will
serve as the basis for further preparations of structurally simplified,
bioselective derivatives of this natural product.
[a
]
25 þ5.7 (c1.0, CHCl3); IR (film) 2976, 1743, 1716, 1662, 1512, 1367,
D
1244,1174,1016 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.43 (s, 9H), 3.09
(m, 2H), 3.76 (s, 3H), 4.45 (dd, J¼10.3, 8.4 Hz, 1H), 4.60 (t, J¼8.4 Hz,
1H),4.68 (m, 1H), 4.74 (m, 1H), 5.05 (s, 2H), 5.15 (sa, 1H), 6.90 (d,
J¼8.6 Hz, 2H), 7.06 (d, J¼8.6 Hz, 2H), 7.41 (m, 5H); 13C NMR (CDCl3,
4. Experimental
4.1. General
100 MHz)
d 28.7, 38.4, 50.2, 53.0, 68.2, 70.4, 70.4, 80.1, 115.1, 127.8,
128.3,128.5,129.0,131.0,137.5,156.2,158.3,169.8,171.3; HRMS (ESI)
Melting points were determined on a Gallenkamp capillary
melting point apparatus and are uncorrected. IR spectra were
recorded on a Shimadzu FTIR 8101A spectrophotometer. NMR
spectra were recorded on a Bruker Avance DPX-400. Chemical shifts
are related to TMS as an internal standard. High resolution mass
spectra (HRMS) were obtained on a micro Q-TOF (Bruker Daltonics)
and on VG AutoSpecQ mass spectrometers. Flash column chroma-
m/z calcd for C25H30N2O6 (Mþ) 454.2104, found 454.2108.
4.1.3. (S)-Methyl
2-((S)-3-(4-(benzyloxy)phenyl)-2-(tert-butoxy-
carbonylamino) propanethioamido)-3-hydroxypropanoate (8). A
solution of 7 (88 mg, 0.19 mmol) in 14 mL of MeOH/Et3N (1:1) was
saturated with H2S and stirred at room temperature overnight.
CAUTION! H2S is a toxic gas and has to be handled in a ‘well-vented
hood’. Excess H2S, MeOH, and Et3N were removed by evaporation in
vacuo through a solution of bleach, and the residue was chroma-
tographed on SiO2 (EtOAc/n-hexane, 1:1) to yield 8 (87 mg, 92%) as
a yellow oil. Rf¼0.40 (EtOAc/n-hexane, 1:1); IR (film) 2978, 1741,
tography was carried out with Silica gel 60 (J.T. Baker, 40 mm average
particle diameter). All reactions and chromatographic separations
were monitored by TLC, conducted on 0.25 mm Silica gel plastic
sheets (Macherey/Nagel, PolygramÒ SIL G/UV 254). TLC plates were
analyzed under 254 nm UV light, by iodine vapor, p-hydrox-
ybenzaldehyde spray or ninhydrine spray. All solvents were purified
according to literature procedures.17 All reactions werecarried out in
dry, freshly distilled solvents under anhydrous conditions unless
otherwise stated. Yields are reported for chromatographically and
spectroscopically (1H and 13C NMR) pure compounds.
1689, 1512, 1242, 1174, 1022 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d 1.43
(s, 9H), 3.14 (d, 2H, J¼6.8 Hz), 3.77 (s, 3H), 3.96 (m, 1H), 4.14 (m, 1H),
4.59 (m, 1H), 5.06 (s, 2H), 5.17 (m, 1H), 5.26 (sa, 1H), 5.36 (sa, 1H),
6.94 (m, 2H), 7.19 (m, 2H), 7.39 (m, 5H); 13C NMR (CDCl3, 100 MHz)
d
28.7, 41.1, 53.2, 60.0, 61.8, 62.9, 70.4, 81.1, 115.4, 127.8, 128.4, 129.0,
129.1, 130.7, 137.4, 156.1, 158.3, 170.2, 205.0; HRMS (ESI) m/z calcd
for C25H32N2O6S (Mþ) 488.1981, found 488.1977.
4.1.1. Boc-Tyr(Bn)-SerOMe (6). A solution of O-benzyl-L-tyrosine
(1.96 g, 7.2 mmol) in a mixture of t-BuOH (20 mL), water (50 mL),
and NaOH (578 mg, 14.5 mmol) was stirred and cooled in an ice-
water bath. Di-tert-butyl pyrocarbonate (1.58 g, 7.2 mmol) was
added and stirring was continued at room temperature overnight.
The solution was acidified with 5% HCl. The aqueous phase was
extracted with ethyl acetate (3ꢁ40 mL). The ethyl acetate was
pooled and dried over anhydrous MgSO4 and evaporated in vacuo.
4.1.4. (S)-Methyl
2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
carbonylamino)ethyl)thiazole-4-carboxylate (4). Diethylaminosulfur
trifluoride (0.43 mL, 3.3 mmol) was added dropwise to a cold
(ꢃ78 ꢂC) solution of 8 (1.44 g, 2.95 mmol) in CH2Cl2. After 30 min,
DBU (1.58 mL, 10.6 mmol) was added dropwise to the reaction mix-
ture at ꢃ40 ꢂC, followed by bromotrichloromethane (1.04 mL,
10.6 mmol) at 0 ꢂC. The reaction was stirred at room temperature for
8 h and then quenched with saturated aqueous sodium bicarbonate.
The mixture was extracted with CH2Cl2, and the combined organic
layers were dried with MgSO4, filtered, and concentrated. Purification
by flash chromatography(SiO2, EtOAc/n-hexane,1:2)gavethedesired
thiazole 4 as a yellow oil (1.10 g, 80%). Rf¼0.48 (EtOAc/n-hexane,1:1);
N-Boc-O-benzyl-
L-tyrosine (2.46 g, 6.6 mmol) was used without
further purification.
L-Serine methyl ester hydrochloride (1.03 g,
6.6 mmol), 1-hydroxy-benzotriazole monohydrate (895 mg,
6.6 mmol), Boc-O-benzyl-tyrosine (2.46 g, 6.6 mmol), and triethyl-
amine (1.34 g, 13.2 mmol) were dissolved in dry CH2Cl2 (30 mL).
The solution was cooled in an ice-water bath and stirred while DCC
(1.37 g, 6.6 mmol) was added. Stirring was continued for 1 h at 0 ꢂC
and an additional hour at room temperature. The precipitated DCU
was removed by filtration and the solvent was evaporated in vacuo.
Flash column chromatography (SiO2, EtOAc/n-hexane, 2:1)
[a]
25 ꢃ7.8 (c1.0, CHCl3); IR (film) 3348, 2361, 1716, 1512, 1244, 1221,
D
1167 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.41 (s, 9H), 3.28 (m, 2H), 3.97
(s, 3H), 5.04 (s, 2H), 5.26 (sa, 2H), 6.88 (d, J¼8.6 Hz, 2H), 7.01 (d,
J¼8.6 Hz, 2H), 7.38 (m, 5H), 8.07 (s, 1H); 13C NMR (CDCl3, 100 MHz)
d
28.7, 41.1, 52.8, 54.5 70.4, 80.7,115.4,127.9,128.4,128.8,129.0,130.9,

5392
D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
137.4, 147.3, 155.3, 158.3, 162.2, 173.8; HRMS (ESI) m/z calcd for
130.5, 137.0,151.4,154.0, 157.8,173.5,174.0. HRMS (ESI) m/z calcd for
C
C25H28N2O5S (Mþ) 468.1719, found 468.1732.
38H54N2NaO5S ((MþNa)þ) 673.3646, found 673.3663.
4.1.5. (S)-Methyl 2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxycarbonyl-
amino)ethyl)oxazole-4-carboxylate (5). According to the procedure
for the conversion of 8 into 4, 6 (1,37 g, 2,9 mmol) was transformed
4.1.8. (S)-tert-Butyl 2-(4-(benzyloxy)phenyl)-1-(4-hidroxymethylox-
azol-2-yl)ethylcarbamate (10) and (S)-tert-butyl 2-(4-(benzyloxy)
phenyl)-1-(4-formyloxazol-2-yl)ethylcarbamate (12). According to
the procedure for the conversion of 4 into 11, 5 (118 mg, 3.1 mmol)
was transformed into 12 (236 mg, 87%). Compound 10: Rf¼0.10
(EtOAc/n-hexane, 1:2); IR (film) 742, 1024, 1170, 1244, 1510, 1699,
into 5 (1.05 g, 80%). Rf¼0.64 (EtOAc/n-hexane, 2:1); [
a
]
25 ꢃ2.0 (c1.0,
D
CHCl3); IR (film) 3368, 2980, 1724, 1693, 1583, 1512, 1246, 1167,
1003 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d 1.42 (s, 9H), 3.18 (m, 2H), 3.92
(s, 3H), 5.03 (s, 2H), 5.20 (m, 2H), 6.87 (d, J¼8.5 Hz, 2H), 6.96 (d,
2374, 2976, 3354 cmꢃ1 1H NMR (CDCl3, 400 MHz)
; d 1.42 (s, 9H),
J¼8.5 Hz, 2H), 7.39 (m, 5H), 8.14 (s, 1H); 13C NMR (CDCl3, 100 MHz)
3.18 (m, 2H), 4.56 (s, 2H), 5.05 (s, 2H), 5.15 (m, 1H), 5.30 (m, 1H),
d
28.7, 39.9, 50.7, 52.6, 70.4, 80.6,115.4,127.8,128.2128.3,129.0,130.7,
6.87 (d, J¼8.6 Hz, 2H), 6.96 (d, J¼8.6 Hz, 2H), 7.41 (m, 5H), 7.51 (s,
133.7, 137.4, 144.2, 156.2, 158.4, 161.9, 165.3; HRMS (ESI) m/z calcd for
1H); 13C NMR (CDCl3, 100 MHz)
d 28.7, 39.9, 50.7, 57.0, 70.4, 80.4,
C25H28N2O6 (Mþ) 452.1947, found 452.1942.
115.3, 127.8, 128.3, 128.7, 129.0, 130.7, 135.4, 137.4, 140.7, 154.0,
158.3, 164.7; HRMS (ESI) m/z calcd for C24H28N2O5Na ((MþNa)þ)
447.1890, found 447.1872. Compound 12: Rf¼0.7 (EtOAc/n-hexane,
1:1); IR (film) 696, 810, 1047, 1165, 1246, 1512, 1691, 3092,
4.1.6. (S)-tert-Butyl 2-(4-(benzyloxy)phenyl)-1-(4-hydroxymethylth-
iazol-2-yl)ethylcarbamate (9) and (S)-tert-butyl 2-(4-(benzyloxy)
phenyl)-1-(4-formylthiazol-2-yl)ethylcarbamate (11). LiAlH4 (17.1 mg,
0.45 mmol) was added in several portions to a solution of ester 4
(211 mg, 0.45 mmol) in dry diethyl ether (5 mL) at 0 ꢂC. The reaction
mixture was stirred overnight at room temperature. The solution
was quenched with 20% NaOH solution (20 mL). The mixture was
extracted with EtOAc, and the combined organic layers were dried
over MgSO4 and the solvent was evaporated under reduced pressure
to obtain the alcohol 9 that was used in the next step without further
purification.
3362 cmꢃ1 1H NMR (CDCl3, 400 MHz)
; d 1,43 (s, 9H), 3,20 (m, 2H),
5.03 (s, 2H), 5.20 (m, 2H), 6.87 (d, J¼8.5 Hz, 2H), 6.96 (d, J¼8.5 Hz,
2H), 7.39 (m, 5H), 8.18 (s, 1H), 9,90 (s, 1H); 13C NMR (CDCl3,
100 MHz)
d 28.7, 39.7, 50.7, 70.4, 80.7, 115.5, 127.8, 128.4, 128.5,
129.0, 130.7, 137.4, 141.1, 144.8, 155.3, 158.4, 165.8, 184.1; HRMS (ESI)
m/z calcd for C24H26N2NaO5 ((MþNa)þ) 445.1734, found 445.1725.
4.1.9. (S,E)-tert-Butyl 2-(4-(benzyloxy)phenyl)-1-(4-(2-methyl-3-oxo-
prop-1-enyl)thiazol-2-yl)ethylcarbamate (13). A solution of aldehyde
MnO2 (304 mg, 3.5 mmol) was added in five portions over 5 h to
a stirred solution of alcohol 9 (154 mg, 0.35 mmol) in CH2Cl2 (10 mL)
at room temperature. The resulting reaction mixture was stirred for
15 h until TLC indicated that the reaction was completed. The MnO2
was removed by filtration, washed with CH2Cl2, and the combined
organic portions were concentrated in vacuo. Purification by flash
chromatography (SiO2, EtOAc/n-hexane, 1:2) gave the desired al-
dehyde 11 as a colorless oil (123 mg, 80%). Compound 9: Rf¼0.19
(EtOAc/n-hexane, 2:3); IR (film) 3346, 1688. 1514, 1267, 1248,
11 (1.26 g, 2.88 mmol) and
(a-formylethylidene)triphenyl
phosphorane (1.1 g, 3.46 mmol) in benzene (30 mL) was stirred at
reflux for 5 h. The reaction mixture was stirred until monitoring by
TLC indicated that all starting material had been consumed. The
residue obtained after evaporation of the solvent was purified by
chromatography (SiO2, EtOAc/n-hexane, 1:2) to afford the
saturated aldehyde 13 (1.27 g, 92%). Rf¼0.36(EtOAc/n-hexane,1:3); IR
(film) 2977, 1681, 1679, 1512, 1244, 1164 cmꢃ1 1H NMR (CDCl3,
400 MHz)
a,b-un-
;
d
1.44(s, 9H),2.24(s, 3H), 3.28(d, J¼6.2 Hz,2H), 5.05(s, 2H),
1167 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.42 (s, 9H), 2.69 (s,1H), 3.25
5.20(sa,1H),5.28(sa,1H),6.89(d, J¼8.6 Hz,2H),7.04(d,J¼8.6 Hz, 2H),
(m, 2H), 4.77 (s, 2H), 5.04 (s, 2H), 5.28 (m, 2H), 6.88 (d, J¼8.6 Hz, 2H),
7.27 (s, 1H), 7.40 (m, 5H), 7.56 (s, 1H), 9.61 (s, 1H); 13C NMR (CDCl3,
7.01 (d, J¼8.6 Hz, 2H), 7.08 (s, 1H), 7.41 (m, 5H); 13C NMR (CDCl3,
100 MHz)
d 11.5, 28.7, 41.1, 54.5, 70.5, 80.5, 115.4, 123.3, 127.8, 128.4,
100 MHz)
d
28.7, 41.5, 54.5, 61.4, 70.4, 80.8, 114.8, 115.5, 127.8, 128.3,
128.9, 129.0, 130.9, 137.4, 139.2, 141.2, 152.3, 155.0, 158.3, 173.0, 195.7;
HRMS (ESI) m/z calcd for C27H30N2O4S (Mþ) 479.2005, found
479.2000.
129.0, 129.1,130.8, 130.8, 137.4,155.3, 156.7,158.2, 174.0; HRMS (ESI)
m/z calcd for C24H28N2O4SNa ((MþNa)þ) 463.1662, found 463.1672.
Compound 11: Rf¼0.58 (EtOAc/n-hexane, 2:3); IR (film) 3343, 1701,
1512,1367,1246,1174 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.44 (s, 9H),
4.1.10. (S,E)-tert-Butyl 2-(4-(benzyloxy)phenyl)-1-(4-(2-methyl-3-
oxoprop-1-enyl)oxazol-2-yl)ethylcarbamate (14). According to the
procedure for the conversion of 11 into 13, 12 (1.1 g, 2.6 mmol) was
transformed into 14 (1.04 g, 100%). Rf¼0.71 (EtOAc/n-hexane, 1:1);
IR (film) 696, 1026, 1111, 1167, 1246, 1514, 1693, 1713, 2076 cmꢃ1; 1H
3.28 (m, 2H), 5.05 (s, 2H), 5.28 (m, 2H), 6.89 (d, J¼8.6 Hz, 2H), 7.03 (d,
J¼8.6 Hz, 2H), 7.36 (m, 5H), 8.08 (s, 1H), 10.03 (s, 1H); 13C NMR
(CDCl3, 100 MHz)
d 28.7, 41.0, 54.5, 70.5, 80.8, 115.5, 127.8, 128.2,
128.4,128.6,129.0,131.1,137.4,141.2,155.3,158.4,174.5,185.0; HRMS
(ESI) m/z calcd for C24H26N2O4S (Mþ) 439.1692, found 439.1662.
NMR (CDCl3, 400 MHz) d 1,45 (s, 9H), 2.10 (s, 3H), 3.20 (m, 2H), 5.05
(s, 2H), 5.13 (m, 2H), 6.89 (d, 2H, J¼8.5 Hz), 7.00 (d, 2H, J¼8.5 Hz),
4.1.7. (S)-(2-(2-(4-(Benzyloxy)phenyl)-1-(tert-butoxy-
7.08 (s, 1H), 7.36 (m, 5H), 7,85 (s, 1H), 9,56 (s, 1H); 13C NMR (CDCl3,
carbonylamino)ethyl)thiazol-4-yl)methyl myristate (15). Myristic
100 MHz) d 11.5, 28.7, 39.6, 50.6, 70.4, 80.7 115.4, 127.8, 128.4, 128.5,
acid (6.6 mg, 0.03 mmol), TEA (4
m
L, 0.03 mmol), 1-hydroxy-ben-
129.0, 130.0, 130.8, 137.4, 137.8, 139.5, 140.1, 155.3, 158.3, 164.6,
194.6; HRMS (ESI) m/z calcd for C27H30N2O5 ((MþH)þ) 463.2227,
found 463.2218; ((MþNa)þ) 485.2047, found 485.2049.
zotriazole monohydrate (4.2 mg, 0.03 mmol), EDCI (8.0 mg,
0.04 mmol) were dissolved in dry CH2Cl2 (0.5 mL). The solution was
cooled in an ice-water bath and stirred while alcohol 9 (12.5 mg,
0.03 mmol) was added. Stirring was continued for 12 h at room
temperature. The solution was quenched with water. The mixture
was extracted with CH2Cl2, and the combined organic layers were
dried over MgSO4 and the solvent was evaporated in vacuo. Puri-
fication by flash chromatography (SiO2, CHCl3) gave the desired
ester 15 (14 mg, 76%). Rf¼052 (EtOAc/n-hexane, 1:3); 1H NMR
4.1.11. (S,2Z,4E)-Ethyl 5-(2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
carbonylamino)ethyl)thiazol-4-yl)-2,4-dimethylpenta-2,4-dienoate
(16). To a solution of (CF3CH2O)2 P(O)CH(CH)3CO2Et (505 mg,
1.46 mmol) and 18-crown-6 (1.9 g, 7.3 mmol) in CH2Cl2 (50 mL) at
ꢃ78 ꢂC was added KHMDS (2.92 mL, 1.46 mmol). The reaction
mixture was stirred at ꢃ78 ꢂC, and then aldehyde 13 (700 mg,
1.46 mmol) was added. The mixturewas stirred at ꢃ78 ꢂC for 4 h and
satd NH4Cl was added, the product was extracted with CH2Cl2. The
CH2Cl2 extracts were dried with MgS04 and evaporated, and the
residuewas purified byflash chromatography (EtOAc/n-hexane,1:6)
to afford 16 as a yellow oil (757 mg, 92%) (Z/E, 97:3). Rf¼0.56 (EtOAc/
(CDCl3, 400 MHz)
d
0.90 (t, J¼6.0 Hz, 3H), 1.27 (m, 20H), 1.43 (s, 9H),
1.66 (m, 2H), 2.40 (m, 2H), 3.24 (d, J¼8.0 Hz, 2H), 5.05 (s, 2H), 5.19
(m, 4H), 6.89 (d, J¼8.6 Hz, 2H), 7.01 (d, J¼8.6 Hz, 2H), 7.17 (s, 1H),
7.40 (m, 5H); 13C NMR (CDCl3, 100 MHz)
d 14.1, 24.9, 28.3,
29.2e29.7, 34.2, 40.9, 54.0, 61.6, 70.0, 80.0, 114.8, 117.2, 127.5e128.6,

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5393
n-hexane, 1:3); IR (film) 2924, 1749, 1716, 1508, 1265, 1014,
740 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
(28 mg, 0.23 mmol), and EDCI(484 mg, 2.52 mmol) inCH2Cl2 (30 mL)
was stirred at room temperature for 4 h. The solvent was evaporated,
and the residue was purified by chromatography on silica gel. The
fraction eluted with EtOAc/n-hexane (1:2) was collected. Removal of
the solvents gave the product 20 (857 mg, 73%). Rf¼(EtOAc/n-hexane,
d
1.29 (t, J¼7.2 Hz, 3H), 1.44 (s,
9H), 2.05 (d, J¼1.4 Hz, 3H), 2.19 (s, 3H), 3.26 (m, 2H), 4.23 (q, J¼7.2 Hz,
2H), 5.04 (s, 2H), 5.25 (m, 2H), 6.30 (s, 1H), 6.50 (br s, 1H), 6.88 (d,
J¼8.7 Hz, 2H), 7.03 (d, J¼8.7 Hz, 2H), 7.4 (s,1H), 7.41 (m, 5H); 13C NMR
(CDCl3, 100 MHz)
d
14.5, 17.8, 22.1, 28.7, 41.3, 54.3, 61.1, 70.4, 80.4,
1:2); 1H NMR (CDCl3, 400 MHz)
d
0.75 (d, 3H, J¼6.86 Hz), 1.03 (t, 3H,
115.3,117.3,124.9,127.8,128.3,128.9,129.3,129.6,130.9,136.4,137.5,
138.2, 153.6, 155.4, 158.2, 169.2, 170.6; HRMS (ESI) m/z calcd for
C32H38N2O5S ((MþH)þ) 563.2580, found 563.2600.
J¼7.4 Hz),1.28 (m,1H),1.44 (s, 9H),1.56 (m,1H), 2.17 (m,1H), 4.92 (m,
1H), 5.12(m,1H), 7.64 (m,15H);13CNMR(CDCl3,100 MHz)
d
12.3,14.2,
27.6, 28.8, 38.4, 47.6 (d, J¼125.5 Hz), 59.4, 79.2, 121.5, 123.6 (d, C2,
J¼93.0 Hz), 129.5, 133.5, 134.1, 156.5, 195.5.
4.1.12. (S,2E,4E)-Ethyl 5-(2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
carbonylamino)ethyl)thiazol-4-yl)-2,4-dimethylpenta-2,4-dienoate
(17). A solution of aldehyde 13 (50 mg, 0.1 mmol) and Ph3P]C(Me)
CO2Et (45 mg, 0.13 mmol) in benzene (5 mL) was stirred at reflux. The
reactionmixturewasstirreduntil monitoringbyTLC indicated thatall
starting material had been consumed. The residue obtained after
evaporation of the solvent was purified by chromatography (SiO2,
EtOAc/n-hexane, 1:2) to afford the ester 17 (54 mg, 100%). Rf¼0,56
(EtOAc/n-hexane, 1:2); IR (film) 2978, 2930, 1711, 1699,
4.1.16. Boc-Ser-Pro-OBn (24). L-Proline benzyl ester hydrochloride
(1.29 g, 5.34 mmol), 1-hydroxy-benzotriazole monohydrate (0.79 g,
5.88 mmol), Boc-Ser-OH (1.1 g, 5.34 mmol), and TEA (1.5 mL,
10.8 mmol) were dissolved in dry CH2Cl2 (25 mL). The solution was
cooled in an ice-water bath and stirred while DCC (1.2 g,
5.88 mmol) was added. Stirring was continued for 1 h at 0 ꢂC and
12 h at room temperature. The precipitated DCU was removed by
filtration and the solvent was evaporated in vacuo. Flash column
chromatography (SiO2, EtOAc/n-hexane, 3:1) provided 24 (1.13 g,
54%) as a colorless oil. Rf¼0.5 (EtOAc/n-hexane, 3:1); 1H NMR
;
1610, 1516, 1390, 1253, 1111, 1026, 752 cmꢃ1 1H NMR (CDCl3,
400 MHz)
d
1.32 (t, J¼7.2 Hz, 3H),1.44 (s, 9H), 2.14 (s, 3H), 2.33 (s, 3H),
3.28 (m, 2H), 4.24 (q, J¼7.2 Hz, 2H), 5.04 (s, 2H), 5.26 (m, 2H), 6.63
(s,1H), 6.89 (d, J¼8.6 Hz, 2H), 7.04 (d, J¼8.6 Hz, 2H), 7.1 (s,1H), 7.30 (s,
(CD3OD, 400 MHz) d 1.45 (s, 9H), 2.05 (m, 3H), 2.26 (m, 1H), 3,14 (m,
1H), 3.8 (m, 4H), 4,65 (m, 2H), 5,12 (d, J¼16.0 Hz, 1H), 5,25 (d,
1H), 7.41 (m, 5H); 13C NMR (CDCl3, 100 MHz)
d 14.4, 14.6, 19.3, 28.7,
J¼16.0 Hz, 1H), 5.50 (sa, 1H), 7.38 (m, 5H); 13C NMR (CD3OD,
41.3, 54.4, 61.3, 70.4, 80.5, 115.3,118.2,127.1, 127.8, 128.2, 128.4, 129.0,
131.0, 133.8, 136.0, 137.5, 143.3, 153.6, 155.4, 158.2, 169.7, 171.4; HRMS
(ESI) m/z calcd for C32H38N2O5S ((MþH)þ) 563.2574, found 563.2568.
100 MHz) d 24.8, 28.3, 28.9, 47.2, 53.3, 59.1, 64.2, 67.3, 80.0, 128.1,
128.5, 128.7, 135.3, 155.6, 170.2, 172.2.
4.1.17. Boc-Ser-Pro-OH (27). The ester 24 (3.74 mg, 0.95 mmol) was
dissolved in MeOH (10 mL). Pd/C 10% (37 mg) catalyst was added.
The reaction was exposed to an atmosphere of H2 overnight. The
catalyst was removed by filtration. Removal of the solvents gave the
product 27 (300 mg, 100%). Rf¼0.1 (EtOAc/n-hexane, 3:1); 1H NMR
4.1.13. (S,2Z,4E)-Ethyl 5-(2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
carbonylamino)ethyl)oxazol-4-yl)-2,4-dimethylpenta-2,4-dienoate
(18). According to the procedure for the conversion of 13 into 16, 14
(940 mg, 2.03 mmol) was transformed into 18 (984 mg, 89%) (Z/E,
92:8). Rf¼0.4 (EtOAc/n-hexane, 1:3); IR (film) 696, 1024, 1111, 1172,
(CD3OD, 400 MHz) d 1.41 (s, 9H), 2.05 (m, 3H), 2.30 (m, 1H), 3.74 (m,
1244, 1367, 1512, 1720, 2980 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.30
4H), 4,76 (m, 2H); 13C NMR (CD3OD, 100 MHz)
47.4, 55.0, 53.8, 60.1, 82.1, 158.0, 172.0, 176.4.
d 25.1, 28.0, 29.3,
(t, 3H, J¼7.2 Hz), 1,44 (s, 9H), 2.03 (m, 6H), 3.19 (m, 2H), 4.22 (q, 2H,
J¼7.2 Hz), 5.04 (s, 2H), 5.16 (m, 2H), 6.25 (m, 2H), 6.87 (d, 2H,
J¼8.5 Hz), 6.97 (d, 2H, J¼8.5 Hz), 7.39 (m, 5H), 7.52 (s, 1H); 13C NMR
4.1.18. Boc-Ser(Ac)-Pro-OBn (26). To
a solution of alcohol 24
(CDCl3, 100 MHz)
d
14.9, 18.0, 22.0, 28.7, 39.8, 50.6, 61.1, 70.4, 80.4,
(533 mg, 1.36 mmol) in TEA (10 mL), acetic anhydride (10 mL) was
added at 0 ꢂC. The mixture was stirred overnight at room tempera-
ture. HCl (1 M) was added until pH 3 and the mixture was extracted
with EtOAc. The combined organic layers were dried (MgSO4) and
the solvent was evaporated in vacuo. Flash chromatography (EtOAc/
n-hexane, 1:1) afforded 25 (582 mg, 99%). Rf¼0.5 (EtOAc/n-hexane,
115.3, 121.2, 127.8, 128.3, 128.7, 128.9, 129.4, 130.8, 136.5, 137.5, 137.7,
139.5, 138.6, 142.5, 158.2, 163.3, 170.4. HRMS (ESI) m/z calcd for
C32H38N2NaO6, ((MþNa)þ) 569.2628, found 569.2568.
4.1.14. (S,2E,4E)-Ethyl 5-(2-(2-(4-(benzyloxy)phenyl)-1-(tert-butoxy-
carbonylamino)ethyl)oxazol-4-yl)-2,4-dimethylpenta-2,4-dienoate
(19). To a solution of (CH3CH2O)2 P(O)CH(CH)3CO2Et (312 mg,
1.31 mmol) in THF (10 mL) at ꢃ78 ꢂC was added LiHMDS (1.2 mL,
1.31 mmol). The reaction mixture was stirred at ꢃ78 ꢂC, and then
aldehyde 14 (55 mg, 1.31 mmol) was added. The mixture was stirred
at ꢃ78 ꢂC for 0.5 h and satd NH4Cl was added, the product was
extracted with EtOAc. The EtOAc extracts were dried with MgSO4
and evaporated, and the residue was purified by flash chromatog-
raphy (EtOAc/n-hexane, 1:6) to afford 19 as a yellow oil (55 mg, 85%).
Rf¼0.4 (EtOAc/n-hexane, 1:3); IR (film) 2980, 2932, 1711, 1512, 1248,
1:1); 1H NMR (CDCl3, 400 MHz)
d 1.44 (s, 9H),1.97e2.2 (m, 6H), 2.23
(m,1H), 3,72 (m,1H), 3.84 (m,1H), 3.98 (dd, J¼8.0,12.0 Hz,1H), 4.36
(dd, J¼4.0, 12.0 Hz, 1H), 4.6 (m, 1H), 4.8 (m, 1H), 5.11 (d, J¼12.0 Hz,
1H), 5.19 (d, J¼12.0 Hz,1H), 5.45 (m,1H), 7.31e7.39 (m, 5H); 13C NMR
(CDCl3, 100 MHz)
d 20.7, 24.9, 28.3, 29.0, 47.2, 51.2, 59.4, 64.7, 67.0,
79.9, 128.1, 128.3, 128.6, 135.5, 155.6, 167.9, 170.6, 171.3.
4.1.19. Boc-
chloride (1.36 g, 8.2 mmol), 1-hydroxy-benzotriazole monohydrate
(1.11 g, 8.2 mmol), Boc- -Ser(Me)-OH (1.64 g, 7.5 mmol), and trie-
D-Ser(Me)-Pro-OMe (30). L-Proline methyl ester hydro-
D
1172, 1022, 754 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
1.34 (t, J¼7.2 Hz,
thylamine (2.2 mL, 16 mmol) were dissolved in dry CH2Cl2 (20 mL).
The solution was cooled in an ice-water bath and stirred while DCC
(1.69 g, 8.2 mmol) was added. Stirring was continued for 1 h at 0 ꢂC
and 12 h at room temperature. The precipitated DCU was removed
by filtration and the solvent was evaporated in vacuo. Flash column
chromatography (SiO2, EtOAc/CH2Cl2, 1:2) provided 30 (1.8 g, 72%)
as an oil. Rf¼0.4 (EtOAc/CH2Cl2, 1:2); IR (film) 2978, 1747, 1711, 1649,
3H), 1.44 (s, 9H), 2.07 (s, 3H), 2.17 (s, 3H), 3,22 (m, 2H), 4,22 (q,
J¼7,2 Hz, 2H), 5.04 (s, 2H), 5.13 (m, 2H), 6.25 (s,1H), 6.90 (d, J¼8.6 Hz,
2H), 7.04 (d, J¼8.6 Hz, 2H), 7.25 (s, 1H), 7,41 (m, 5H), 7.59 (s, 1H); 13C
NMR (CDCl3, 100 MHz)
d 14.6, 14.7, 19.5, 28.7, 39.8, 50.6, 61.1, 70.4,
80.4, 115.3, 123.3, 127.8, 127.9, 128.3, 128.7, 128.9, 130.8, 136.2, 136.9,
137.5, 138.5, 142.5, 154.0, 158.3, 163.5, 169.1. HRMS (ESI) m/z calcd for
C32H38N2NaO6, ((MþNa)þ) 569.2628, found 569.2575.
1437, 1390, 1172 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d 1.45 (s, 9H), 1.99
(m, 3H), 2.23 (m, 1H), 3.34 (s, 3H), 3.51 (m, 2H), 3.60 (m, 2H), 3,76 (s,
4.1.15. (Triphenylphosphoranyliden)-(2S)-((tert-butoxi-
3H), 4.48 (dd, J¼8.7, 4.0 Hz, 1H), 4.67 (m, 1H), 5.36 (d, J¼7.9 Hz, 1H);
carbonylamino)-3R-methyl-pentanoyl)acetonitrile (20)10. A solution
13C NMR (CDCl3, 100 MHz)
d 25.1, 28.7, 29.5, 47.5, 51.2, 52.6, 59.4,
of carboxylic acid Boc-
nylphosphoranylidene)acetonitrile (759 mg, 2.52 mmol), 4-DMAP
L
-allo-Ile-OH (529 mg, 2.29 mmol), (triphe-
59.7, 73.7, 80.2, 155.5, 169.9, 172.8; HRMS (ESI) m/z calcd for
C15H26N2NaO6 ((MþNa)þ) 353.1683, found 353.1687.

5394
D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
4.1.20. Boc-
D
-Ser(Me)-Pro-Ser-Pro-OBn (33). Acid 31 (1.41 mmol),
30.0, 36.7, 41.4, 52.5, 58.8, 61.1, 70.4, 79.9, 115.3, 117.5, 124.2, 127.8,
128.3, 128.9, 129.3, 129.6, 130.8, 136.4, 137.5, 137.9, 153.6, 152.4,
158.4, 167.4, 169.2, 170.2, 194.5; HRMS (ESI) m/z calcd for
TEA (1.41 mmol), and amine 32 (1.41 mmol) were dissolved in dry
CH2Cl2 (10 mL). The solution was cooled in an ice-water bath and
stirred while PyBroP (657 mg, 1.41 mmol) was added. Stirring was
continued for 12 h at room temperature. The solution was
quenched with 1 M HCl. The mixture was extracted with CH2Cl2,
and the combined organic layers were dried over MgSO4 and the
solvent was evaporated in vacuo. Purification by flash chromatog-
raphy (SiO2, CHCl3/MeOH, 10:1) gave the desired tetrapeptide 33
(580 mg, 70%). Rf¼0.5 (CHCl3/MeOH, 10:1); 1H NMR (CDCl3,
C
39H49N3O7S (Mþ) 703.3291, found 703.3247.
4.1.23. (2Z,4E)-Ethyl 5-(2-((S)-2-(4-(benzyloxy)phenyl)-1-((3S,4R)-
3-(tert-butoxycarbonylamino)-4-methyl-2-oxohexanamido)ethyl)-
oxazol-4-yl)-2,4-dimethylpenta-2,4-dienoate (37). Compound 18
(0.36 mmol) was dissolved in CH2Cl2 (2 mL) and TFA (1 mL) was
added at 0 ꢂC. The reaction mixture was stirred until TLC indicated
that all starting material had been consumed. The solvent was
evaporated and the residue was dissolved with saturated aqueous
sodium bicarbonate. The mixture was extracted with EtOAc, and
the combined organic layers were dried with MgSO4, filtered, and
concentrated to afford the amine as a yellow oil.
400 MHz) d 1.43 (s, 9H), 1.97e2.3 (m, 8H), 3.38 (s, 3H), 3.56 (m, 3H),
3.74 (m, 5H), 4.48 (m, 1H), 4.71 (m, 2H), 4.86 (m, 1H), 5.11 (d,
J¼12.0 Hz,1H), 5.19 (d, J¼12.0 Hz,1H), 5.43 (m,1H), 7.28 (s,1H), 7.36
(m, 5H); 13C NMR (CDCl3, 100 MHz)
d 25.2, 28.7, 28.8, 29.2, 30.1,
47.5, 47.8, 52.0, 53.3, 59.5, 59.7, 60.1, 63.7, 67.5, 73.0, 80.4, 128.6,
128.8, 129.0, 135.8, 155.0, 170.0, 171.1, 171.8, 172.3. HRMS (ESI) m/z
calcd for C29H42N4NaO9 ((MþNa)þ) 613.2850, found 613.2867.
A solution of 21 (238 mg, 0.46 mmol) in CH2Cl2 (15 mL) was
ozonized at ꢃ78 ꢂC until the color of the solution remained blue
(or yellow-blue). After the solution was purged with N2 to remove
the excess O3, the amine derivative of 18 (159 mg, 0.36 mmol) was
introduced at ꢃ78 ꢂC. The reaction was stirred at ꢃ78 ꢂC for 2 h
and then quenched with HCl 1 M and extracted with AcOEt. The
combined organic layers were dried with MgSO4, filtered, and
concentrated in vacuo. Purification by flash chromatography (SiO2,
EtOAc/n-hexane, 1:4) gave the desired product 37 as a yellow oil
(141 mg, 58%). Rf¼0.4 (EtOAc/n-hexane, 1:4); IR (film) 1514, 1242,
4.1.21. (Triphenylphosphoranyliden)-(2S,3S)-2-((S)-1-((S)-3-acetoxy-
2-(tert-butoxycarbonylamino)propanoyl)pyrrolidine-2-carboxamido)-
3-methyl-pentanamidoacetonitrile (35). Acid 26 (0.61 mmol), TEA
(0.19 mL, 0.68 mmol), amine 21 (0.68 mmol) were dissolved in dry
CH2Cl2 (10 mL). The solution was cooled in an ice-water bath and
stirred while HBTU (254 mg, 0,67 mmol) was added. Stirring was
continued for 12 h at room temperature. The solution was quenched
with 1 M HCl. The mixture was extracted with DCM, and the com-
bined organic layers were dried over MgSO4 and the solvent was
evaporated in vacuo. Purification by flash chromatography (SiO2,
CHCl3/MeOH, 10:1) gave the desired tripeptide 35 (450 mg, 88%).
Rf¼0.4 (CHCl3/methanol, 10:1); IR (film) 2968, 2876, 2177, 1743, 1647,
1172, 1115, 1020, 698 cmꢃ1 1H NMR (CDCl3, 400 MHz)
; d 0.85 (t,
J¼7.4 Hz, 3H), 0.90 (d, J¼6.7 Hz, 3H), 0.95 (m, 1H), 1.31 (m, 4H), 1.45
(s, 9H), 2.04 (m, 7H), 3.21 (m, 2H), 4.22 (q, J¼7.4, 12.0 Hz, 2H, ), 5.01
(m, 4H), 5.40 (m, 1H), 6.25 (s, 2H), 6.86 (d, J¼8.0 Hz, 2H), 6.96 (d,
J¼8.0 Hz, 2H), 7.43 (m, 6H), 7.54 (s, 1H); 13C NMR (CDCl3, 100 MHz)
1585, 1439, 1242, 1109, 754 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
0.81
d 11.4, 14.1, 16.1, 17.6, 21.6, 24.3, 28.3, 37.3, 38.8, 48.9, 60.0, 60.7,
(m, 3H), 0.97 (m, 3H), 1.45 (s, 9H), 1.75 (m, 2H), 1.97 (m, 1H), 2.03 (s,
3H), 2.12e2.22 (m, 4H), 3.65 (m, 1H), 3.79 (m, 1H), 3.9 (m, 1H), 4.12
(m, 1H), 4.53 (m, 1H), 4.77 (m, 1H), 5.03 (m, 1H), 5.40 (m, 1H), 6.88
70.0, 80.4, 114.9, 120.4, 127.4, 127.6, 128.0, 128.6, 129.2, 130.7, 136.4,
136.6, 136.9, 137.3, 138.5, 154.0, 158.4, 159.1, 161.6, 170.0, 196.6;
HRMS (ESI) m/z calcd for C39H49N3NaO8 ((MþNa)þ) 688.3592,
found 688.3606.
(m, 1H), 7.50e7.60 (m, 15H); 13C NMR (CDCl3, 100 MHz)
d 11.7, 16.1,
20.7, 23.8, 25.1, 28.1, 28.3, 38.7, 47.5, 50.8, 59.5, 60.4, 64.4, 79.9, 120.8,
122.9 (J¼93.0 Hz), 129.1 (J¼10.0 Hz), 133.2 (J¼2.9 Hz), 133.6
(J¼10.3 Hz), 155.2, 168.7, 170.1,170.7,193.9 (J¼4.0 Hz); HRMS (ESI) m/
z calcd for C41H49N4NaO7P ((MþNa)þ) 763.3231, found 763.3268.
4.1.24.
(2Z,4E)-Ethyl
5-(2-((S)-2-(4-(benzyloxy)phenyl)-1-
((3S,4R)-3-((S)-1-((S)-2-((S)-1-((R)-2-(tert-butoxycarbonylamino)-3-me-
thoxypropanoyl)pyrrolidine-2-carboxamido)-3-hydroxypropanoyl)
pyrrolidine-2-carboxamido)-4-methyl-2-oxohexanamido)ethyl)
thiazol-4-yl)-2,4-dimethylpenta-2,4-dienoate (38). Compound 36
(0.025 mmol) was dissolved in 4 M HCl in dioxane. The reaction
mixture was stirred until TLC indicated that all starting material
had been consumed. The solvent was evaporated in vacuo and
the amine was used without further purification.
4.1.22. (2Z,4E)-Ethyl 5-(2-((S)-2-(4-(benzyloxy)phenyl)-1-((3S,4R)-
3-(tert-butoxycarbonylamino)-4-methyl-2-oxohexanamido)ethyl)
thiazol-4-yl)-2,4-dimethylpenta-2,4-dienoate (36). Compound 16
(128 mg, 0.23 mmol) was dissolved in CH2Cl2 (2 mL) and TFA (1 mL)
was added at 0 ꢂC. The reaction mixture was stirred until TLC in-
dicated thatall starting material had been consumed. The solvent was
evaporated and the residue was dissolved with saturated aqueous
sodium bicarbonate. The mixture was extracted with EtOAc, and the
combined organic layers were dried with MgSO4, filtered, and con-
centrated to afford the amine as a yellow oil (103 mg).
Acid 34 (0.028 mmol), TEA (0.05 mmol), amine (0.025 mmol)
were dissolved in dry CH2Cl2 (5 mL). The solution was cooled in an
ice-water bath and stirred while HBTU (11 mg, 0,03 mmol) was
added. Stirring was continued for 12 h at room temperature. The
solution was quenched with HCl 1 M. The mixture was extracted
with DCM, the combined organic layers were dried over MgSO4 and
the solvent was evaporated in vacuo. Purification by flash chro-
matography (SiO2, CHCl3/MeOH, 10:1) gave the desired analog 38
(13.2 mg, 66%). Rf¼0.4 (AcOEt/methanol,10:1); IR (film) 2978, 2932,
A solution of 20 (126 mg, 0.24 mmol) in CH2Cl2 (5 mL) was
ozonized at ꢃ78 ꢂC until the color of the solution remained blue (or
yellow-blue). After the solution was purged with N2 to remove the
excess O3, the amine derivative of 16 (103 mg, 0.22 mmol) was
introduced at ꢃ78 ꢂC. The reaction was stirred at ꢃ78 ꢂC for 2 h and
then quenched with HCl 1 M and extracted with AcOEt. The com-
bined organic layers were dried with MgSO4, filtered, and con-
centrated in vacuo. Purification by flash chromatography (SiO2,
EtOAc/n-hexane, 1:2) gave the desired product 36 as a yellow oil
(76 mg, 49%). Rf¼0.45 (EtOAc/n-hexane, 1:2); IR (film) 2926, 1716,
1745, 1647, 1512, 1452, 1167, 754 cmꢃ1 1H NMR (CDCl3, 400 MHz)
;
d
0.67 (m, 3H), 0.92 (m, 3H), 1.1 (m, 1H), 1.26 (t, J¼7.2 Hz, 3H), 1.32
(m,1H),1.42 (s, 9H), 1.67e2.10 (m, 9H), 2.02 (s, 3H), 2.14 (s, 3H), 3.22
(m, 2H), 3.31 (s, 3H), 3.46e3.82 (m, 8H), 4.19 (q, J¼7.2 Hz, 2H), 4.44
(m, 1H), 4.62 (m, 2H), 4.84 (m, 1H), 4.99 (s, 2H), 5.32 (m, 1H), 5.32
(m, 1H), 5.45 (m, 1H), 6.27 (s, 1H), 6.47 (s, 1H), 6.85 (m, 2H), 7.01 (m,
1699, 1508, 1248, 1016, 748 cmꢃ1; 1H NMR (CDCl3, 400 MHz)
d
0.68
3H), 7.16e7.39 (m, 8H);13C NMR (CDCl3, 100 MHz)
d 11.7, 14.1, 14,2,
(d, J¼6.9 Hz, 3H), 1.00 (t, J¼6.9 Hz, 3H), 1.31 (m, 5H), 1.44 (s, 9H),
1.96 (m, 1H), 2.06 (s, 3H), 2.20 (s, 3H), 3.29 (m, 2H), 4.23 (q,
J¼6.9 Hz, 2H), 5.03 (m, 3H), 5.20 (m, 1H), 5.50 (m, 1H), 6.31 (s, 1H),
6.50 (s, 1H), 6.88 (d, 2H, J¼8.7 Hz), 7.03 (m, 3H), 7.42 (m, 5H), 7.67
17.4, 21.7, 25.0, 25.6, 28.3, 40.1, 47.5, 49.1, 51.9, 53.1, 53.2, 57.4, 59.0,
59.8, 60.2, 60.5, 63.2, 69.5, 72.2, 81.2, 114.7, 117.2, 123.8, 127.5, 128.0,
128.6, 130.1, 137.1, 153.5, 156.9, 157.8, 170.2, 171.3, 197.8; HRMS (ESI)
m/z calcd for C56H75N7O13SNa ((MþNa)þ) 1108.5036, found
1108.4997; calcd for ((MþH)þ) 1086.5222, found 1086.5101.
(m, 1H); 13C NMR (CDCl3, 100 MHz)
d 12.0, 14.1, 14.5, 17.8, 22.0, 28.6,

D. Sellanes et al. / Tetrahedron 66 (2010) 5384e5395
5395
4.1.25.
(2Z,4E)-Ethyl 5-(2-((S)-1-((3S,4S)-3-((S)-1-((S)-3-acetoxy-2-
Supplementary data
(tert-butoxycarbonylamino)propanoyl)pyrrolidine-2-carboxamido)-4-
methyl-2-oxohexanamido)-2-(4-(benzyloxy)phenyl)ethyl)oxazol-4-yl)-2,
4-dimethylpenta-2,4-dienoate (39). Compound 18 (0.23 mmol) was
dissolved in CH2Cl2 (3 mL) and TFA (1 mL) was added at 0 ꢂC. The
reaction mixture was stirred until TLC indicated that all starting ma-
terial had been consumed. The solvent was evaporated and the resi-
due was dissolved with saturated aqueous sodium bicarbonate. The
mixture was extracted with EtOAc, and the combined organic layers
were dried with MgSO4, filtered, and concentrated to afford the amine.
A solution of 35 (206 mg, 0.28 mmol) in CH2Cl2 (15 mL) was
ozonized at ꢃ78 ꢂC until the color of the solution remained blue (or
yellow-blue). After the solution was purged with N2 to remove the
excess O3, the amine (0.23 mmol) was introduced at ꢃ78 ꢂC. The
reaction was stirred at ꢃ78 ꢂC for 2 h and then quenched with HCl
1 M and extracted with AcOEt. The combined organic layers were
dried with MgSO4, filtered, and concentrated in vacuo. Purification
by flash chromatography (CHCl3/MeOH, 10:1) gave the desired
product 39 (80 mg, 42%). Rf¼0.6 (CHCl3/MeOH, 10:1); 1H NMR
Supplementary data for this article can be found in the online
version, at doi:10.1016/j.tet.2010.05.040. These data include MOL
files and InChIKeys of the most important compounds described in
this article.
References and notes
1. Molinski, T. F.; Dalisay, D. S.; Lievens, S. L.; Saludes, J. P. Nat. Rev. Drug Discov.
2009, 8, 69.
2. Newman, D. J.; Cragg, G. M. J. Nat. Prod. 2007, 70, 461.
3. Paterson, I.; Anderson, E. A. Science 2005, 310, 451.
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(m, 1H), 1.22 (m, 1H), 1.27 (t, 3H, J¼7.2 Hz), 1.45 (s, 9H), 1.89 (m, 1H),
2.00e2.20 (m, 3H) 2.00 (s, 3H), 2.06 (s, 3H), 2.19 (s, 3H), 2.3 (m, 1H),
3.2 (m, 2H), 3.7 (m, 2H), 4.06 (dd, J¼7.6, 11.1 Hz, 1H), 4.22 (q, J¼7,2,
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5.02 (s, 2H), 5.19 (dd, J¼4.0, 7.6 Hz, 1H), 5.38 (dd, J¼5.6, 14.0 Hz, 1H),
5.46 (d, J¼8.4 Hz,1H), 6,25 (s,1H), 6.27 (s,1H), 6.85 (d, J¼8.0 Hz,1H,),
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Acknowledgements
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A Laboratory
This work was supported by grants from DICyT (Dirección de
Innovación, Ciencia y Tecnología para el Desarrollo) and NIH/FIRCA
(Fogarty International Research Collaboration Award).
The authors acknowledge a doctoral fellowship from PEDECIBA
(Programa de Desarrollo de Ciencias Básicas) (D.S.).
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