Total synthesis of dolastatin 10 through ruthenium-catalyzed asymmetric hydrogenations

Article abstract of DOI:10.1016/j.tet.2007.03.036

A total synthesis of dolastatin 10, a potent inhibitor of microtubule assembly, which displayed remarkable antineoplastic activity, is reported. Our synthetic approach was based upon ruthenium-promoted asymmetric hydrogenation of β-keto esters derived from (S)-Boc-proline and (S)-Boc-isoleucine for the construction of the two key units: (2R,3R)-Boc-dolaproine (Dap) and (3R)-Boc-dolaisoleucine (Dil).

Full text of DOI:10.1016/j.tet.2007.03.036

Tetrahedron 63 (2007) 6115–6123
Total synthesis of dolastatin 10 through ruthenium-catalyzed
asymmetric hydrogenations
a
Celine Mordant, Sebastien Reymond, Hitoshi Tone, Damien Lavergne, Ridha Touati,
a
a
a
b
ꢀ
ꢀ
Bechir Ben Hassine,b Virginie Ratovelomanana-Vidala, and Jean-Pierre Genet
a,
*
*
a
´
Laboratoire de Syntheꢁse Selective Organique et Produits Naturels—UMR CNRS 7573, Ecole Nationale Superieure de Chimie de
´
Paris, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
b
´
´
Laboratoire de Syntheꢁse Organique Asymetrique et Catalyse Homogeꢁne—01UR1201, Faculte des Sciences de Monastir,
avenue de l’Environnement, 5019 Monastir, Tunisia
Received 8 February 2007; revised 1 March 2007; accepted 5 March 2007
Available online 12 March 2007
Abstract—A total synthesis of dolastatin 10, a potent inhibitor of microtubule assembly, which displayed remarkable antineoplastic activity,
is reported. Our synthetic approach was based upon ruthenium-promoted asymmetric hydrogenation of b-keto esters derived from (S)-Boc-
proline and (S)-Boc-isoleucine for the construction of the two key units: (2R,3R)-Boc-dolaproine (Dap) and (3R)-Boc-dolaisoleucine (Dil).
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
O
N
N
H
N
N
N
S
The marine natural product dolastatin 10 originally isolated
from the sea hare Dolabella auricularia and structurally
defined in 1987 by Pettit and co-workers1 is a member of a
whole family of cytotoxic antimitotic peptides.2
O
O
O
O
O
N
Dap
Dov
Val
Doe
OEt
Dil
dolaisoleucine dolaproine
dolastatin 10
Total syntheses of dolastatin 103–10 have been reported as
well as the preparation of the three unique key units:
(2R,3R,4S)-dolaproine (Dap),11–16 (3R,4S,5S)-dolaisoleu-
cine (Dil)17–22 and (S)-dolaphenine (Doe)23 fragments. In
our retrosynthetic approach, the target compound dolastatin
10 was envisioned as potentially being derived from the
union of the two key subunits, dolaisoleucine (Dil) and do-
laproine (Dap), which could be prepared by catalytic hydro-
genation24 of b-keto esters promoted by ruthenium-catalysts
(Scheme 1). To the best of our knowledge, no synthetic
approach towards dolastatin 10 was based upon asymmetric
hydrogenations as key steps to furnish the key intermediates
dolaisoleucine (Dil) and dolaproine (Dap). In our continuous
interest25 in the synthesis of challenging bioactivemolecules,
we report in this paper full experimental details regarding
the total synthesis of dolastatin 10 by using ruthenium-
SYNPHOS26 catalyzed asymmetric hydrogenations.
1
OEt
N
Boc
HN
Boc
OH
O
OH
O
9
3
H , Ru-SYNPHOS
2
DKR
H , Ru-SYNPHOS
2
O
OEt
O
OEt
O
N
Boc
PPh2
PPh2
HN
Boc
O
O
O
O
O
8
2
(S)-SYNPHOS®
(S)-Boc-isoleucine
(S)-Boc-proline
Scheme 1. Retrosynthetic analysis of dolastatin 10.
isoleucine in reaction either with carbonyldiimidazole and
the magnesium enolate of ethyl hydrogen malonate, which
proceeded in 62% yield or with monoethyl monopotassium
malonate, magnesium chloride and triethylamine in THF
with an increased yield of 88%. Next, the amine hydrochlo-
ride salt 3 was prepared quantitatively from the correspond-
ing b-keto ester 2 in a single step with gaseous HCl in
ethanol at 0 ꢀC for 2 h. Under these conditions, no work
2. Results and discussion
Our approach began with the synthesis of b-keto ester 2
(Scheme 2) from the commercially available (S)-Boc-
* Corresponding authors. E-mail addresses: virginie-vidal@enscp.fr; jean-
pierre-genet@enscp.fr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.036

6116
C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
Table 1. Asymmetric hydrogenation of b-keto esters 2 and 3 derived from
Boc-Ile
up was necessary and simple evaporation of the solvent
afforded the required b-keto ester 3.
Entry
Substratea
PꢁP
Timeb
drc (3R)/(3S)
1
2
3
4
5
2
2
3
3
3
(S)-SYNPHOS
(R)-SYNPHOS
(S)-MeO-BIPHEP
(R)-MeO-BIPHEP
(S)-SYNPHOS
72
66
24
24
24
92:8
2:98
94:6
8:92
98:2
a or b
OH
OEt
BocHN
c
BocHN
O
O
O
2
a
OEt
O
Reactions were conducted on 1 mmol scale.
Reaction time was not optimized.
H2N
.HCl
b
c
O
Diastereomeric ratio was measured by 1H NMR (400 MHz).
3
Scheme 2. Reagents and conditions: (a) carbonyldiimidazole, THF, 0 ꢀC,
then Mg(O2CCH2CO2Et)2, THF/Et2O, rt, 96 h, 62%; (b) carbonyldiimid-
azole, THF, 0 ꢀC, then KO2CCH2CO2Et, NEt3, MgCl2, THF, rt, 39 h,
88%; (c) HCl gas, EtOH, 0 ꢀC, quant.
a, b
OEt
OEt
BocN
Me OMe
6
BocHN
O
O
OH
4
Afterwards, hydrogenation reactions of keto esters 2 and 3
were conducted under mild conditions (12 bar, 50 ꢀC in
EtOH) with both Ru-SYNPHOS and Ru-MeO-BIPHEP cat-
alysts as shown in Scheme 3. Under these standard sets of
conditions, complete conversions were observed in all cases,
at a substrate/catalyst ratio (S/C)¼100. The hydrogenation
of 2 was first performed by using the in situ generated
[RuBr2((S)-SYNPHOS)] catalyst prepared by using our con-
venient procedure,27 leading to good enantiofacial discrimi-
nation (Table 1, entry 1, dr 92:8 (3R)-4/(3S)-5). Comparable
results were obtained when (R)-SYNPHOS ligand was used
to perform the hydrogenation (entry 2, dr 2:98 (3R)-4/(3S)-
5). Based on our previous results,28 we studied the hydro-
genation of the hydrochloride salt 3 (entries 3–5).
c
OH
BocN
O
Me OMe
7
Scheme 4. Reagents and conditions: (a) LiHMDS, HMPA, THF, ꢂ78 ꢀC,
25 min; (b) MeOTf, ꢂ20 ꢀC, 25 min, 74%; (c) NaOH aq, EtOH, 0 ꢀC to
rt, 16 h, 81%.
Ru-SYNPHOS catalyst prepared by mixing [RuCl2(p-
cymene)]2 with (S)-SYNPHOS (Scheme 5). An unseparable
mixture of syn (2R,3R)-9 and anti (2S,3R)-9 diastereoiso-
mers was isolated in a dr 2:1 with 55% yield after chromato-
graphy on silica gel.
By using the in situ generated [RuBr2((S)-MeO-BIPHEP)]
catalyst, a comparable diastereoselectivity (entry 3, dr 94:6
(3R)-4/(3S)-5) was observed in a shorter reaction time
(24 h). Finally, we were pleased to find that an excellent dia-
stereoselectivity (entry 5, dr 98:2 (3R)-4/(3S)-5) was reached
by using in situ prepared [RuBr2((S)-SYNPHOS)]. The syn-
thesis of Boc-(3R)-Dil 7 was completed in two subsequent
steps (Scheme 4). First, N-methylation and O-methylation
of 4 afforded the corresponding compound 6 (74% yield),
which was treated with sodium hydroxide to furnish the cor-
responding (3R,4S,5S)-dolaisoleucine (Dil) 7 (81% yield)
whose spectroscopic data were in agreement with those
reported in the literature.6,7,21
2S
2R
3R
a
3R
OEt
OEt
OEt
+
N
Boc
N
Boc
N
Boc
OH
O
O
O
OH
O
8
(2R,3R)-9
(2S,3R)-9
2R
c
b
3R
2R
OEt
3R
OH
N
Boc
N
Boc
OMe O
OMe O
10
N-Boc-(2R,3R)-Dap-11
Scheme 5. Reagents and conditions: (a) [Ru]/(PꢁP)¼3 mol %, [RuCl2(p-
cymene)]2/(S)-SYNPHOS, H2, 130 bar, 50 ꢀC, 117 h, 55% yield; (b)
LiHMDS, HMPA, THF, ꢂ78 ꢀC, 25 min, then MeOTf, ꢂ20 ꢀC, 15 min,
45% yield; (c) LiOH, EtOH/H2O, overnight, 59% yield.
Next, the synthesis of Dap11–16 subunit 11 started with the
hydrogenation reaction of b-keto ester 8 through dynamic
kinetic resolution (DKR).29 The latter was performed under
newly optimized conditions at 50 ꢀC under 130 bar of hydro-
gen pressure for 117 h in the presence of 3 mol % of in situ
The syn (2R,3R)-9 and anti (2S,3R)-9 b-hydroxy esters were
subjected to the next synthetic steps of O-methylation with
LiHMDS in HMPA–THF and MeOTf. At this point, the
syn (2R,3R)-10 and anti (2S,3R)-10 were separated and the
[RuBr2(P*P)]
OEt
3S
3R
OEt
OEt
PHN
PHN
O
PHN
H2, 12 bar, 50 °C
EtOH
O
OH
O
O
OH
O
100% conv.
4
5
2 NHP = NHBoc
3 NHP = NH2.HCl
O
O
PPh2
PPh2
MeO
MeO
PPh2
PPh2
P*P =
O
®
(S)-MeO-BIPHEP
(S)-SYNPHOS
Scheme 3.

C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
6117
OMe
.HCl
syn (2R,3R)-10 was isolated with 45% yield. After saponifi-
cation of (2R,3R)-10 with LiOH, the naturally occurring N-
Boc-(2R,3R)-Dap 11 was obtained with 59% yield in an
enantiomerically pure form.
OMe
Me
HN
N
Me
Ph
Ph
COCl
12
pyridine, CH2Cl2
0 °C,2 h - r.t., 12 h
77%
O
13
S
Li
S
The C-terminal Doe unit N-Boc-(S)-dolaphenine ((S)-2-
phenyl-1-(2-thiazolyl)ethylamine) 18 was prepared accord-
ing to a modified procedure reported by Hamada and
co-workers.23b Phenylacetyl chloride 12 was condensed
with N-methoxy-N-methylamine hydrochloride in a mixture
of pyridine/CH2Cl2 to give the amide 13 with 77% yield.
Reaction of 13 with thiazolyllithium 14 obtained from 2-
bromothiazole and n-butyllithium led to the benzyl thiazolyl
ketone 15 (Scheme 6).
N
14
Ph
N
THF,-78 °C, 30 min
-10 °C, 2 h
56%
O
15
Scheme 6.
The asymmetric reduction of 15 was carried out by using
Brown’s reagent (Ipc2BCl) (Scheme 7) affording the (R)-
alcohol 16 with 60% yield and enantiomeric excess >99%
measured by HPLC analysis after recrystallization.
)2BCl
1-
S
S
R
Et2O, - 10 °C
Ph
Ph
N
N
2- NaOH, 0 °C
3- H2O2, 10 °C to r.t.
The alcohol 16 was then treated under classical Mitsunobu
reaction in the presence of triphenylphosphine, diethyl azo-
dicarboxylate and diphenyl phosphorazidate to lead to the
azide compound 17, which was successively treated with tri-
phenylphosphine, aqueous ammonia and Boc2O to obtain
optically pure Boc-(S)-dolaphenine 1823 (44%, two steps).
O
15
OH
16
81% yield, e.e. 92%
60% yield, e.e. >99%
(after recrystallization)
O
PhO
PhO
S
S
P N3
1- PPh3, 50 °C, 2 h
Ph
S
S
Having prepared the three key units required for the synthe-
sis of dolastatin 10, the construction of 1 started from the C-
terminal Doe unit N-Boc-(S)-dolaphenine. Thus, diethyl
phosphorocyanidate (DEPC, (C2H5O)2P(O)CN) and tri-
fluoroacetic acid were mainly used for the coupling and de-
protection (Scheme 8). Trifluoroacetate salt of dolaphenine
N
Ph
N
2- NH4OH
PPh3, DEAD
THF, 0 °C to r.t.,
48 h
NHBoc
18
N3
3- Boc2O, dioxane
0 °C to r.t., 60 h
17
44% (2 steps)
e.e. >99%
Scheme 7.
S
H
N
2R
3R
OMe O
4S
DEPC (1.3 eq.), NEt3 (2.7 eq.)
DMF, 0 °C, 2h - r.t., 14-24h
S
1- CF3CO2H, CH2Cl2, 1.5h
2R
OH
4S
N
N
Boc
N
Boc
3R
2-
DEPC (1.2eq.), NEt3 (2.7eq.)
DMF, 0 °C, 2h - r.t., 14h
OMe O
Ph
S
N-Boc-Dap-Doe 20
11
79%
S
Ph
N
-
+
CF3CO2
NH2
BocN
CO H
3R
2
19
Me OMe
7
5S
O
2R
H
N
N
S
1- CF3CO2H, CH2Cl2, 3h
4S
N
4S
S
3R
S
BocHN
2R
O
H
N
N
O
3R
OMe
S
N
3R
O
N
2- BroP (3.1 eq.), iPr2NEt (7 eq.)
CH2Cl2, 0 °C, 2h , r.t., 72h
O
3R
OMe
S
Me
72%
N-Boc-Dil-Dap-Doe 21
O
Me
O
Ph
Me
BocN
O
55%
Me
S
BocHN
OH
N-Boc-Val-Dil-Dap-Doe 22
N-Boc-Val
5S
4S
O
H
1- CF3CO2H, CH2Cl2, 4h
4S
3R
S
N
N
Me
2R
2-DEPC (1.3 eq.)
N-methylmorpholine (6 eq.)
DMF, 0 °C, 38h
H
N
S
Me
N
N
3R
S
N
S
O
O
O
Me
O
Me
O
Me
52%
Dolastatin 10
OH
S
N
O
Dov
Scheme 8.

6118
C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
19, prepared in dichloromethane from 18 and TFA, was con-
densed with 11 to afford N-Boc-Dap-Doe 20 with 79% yield.
temperature and dissolved in degassed acetone (1 mL). To
this suspension was added at room temperature a 0.15 N
methanolic HBr solution (147 mL, 0.022 mmol) and the mix-
ture was stirred at 25 ꢀC for 30 min. After evaporation of the
solvent under vacuum, a solution of b-keto ester (2 mmol) in
EtOH (2 mL) was added to the ruthenium catalyst.
The resulting mixture was placed under the desired
hydrogen pressure and temperature for 24 h. After removal
of the solvent, the residue was purified by flash chro-
matography on silica gel to afford the b-hydroxyesters
4 and 5.
Attachment of N-Boc-Dil 7 with the dipeptide N-Boc-Dap-
Doe 20 was accomplished by using trifluoroacetic acid and
DEPC to afford N-Boc-Dil-Dap-Doe 21 with 72% yield.
Next, coupling of N-Boc-valine with tripeptide fragment
N-Boc-Dil-Dap-Doe 21 was carried out by using TFA
and (trisdimethylamino)phosphonium hexafluorophosphate
(BroP) in the presence of diisopropylethylamine to provide
N-Boc-Val-Dil-Dap-Doe 22 with 55% yield. Last coupling
of (S)-dolavaline with N-Boc-Val-Dil-Dap-Doe 22 fragment
in the presence of TFA and diethyl phosphorocyanidate
provided the naturally occurring dolastatin 10 with spectro-
scopic data in agreement with those reported previously for
this compound.1,6,7
4.3. Ethyl (4S,5S)-4-(N-tert-butoxycarbonylamino)-
5-methyl-3-oxoheptanoate 2
Procedure A:
a
solution of (S)-N-Boc-isoleucine
(86.5 mmol, 20 g) in tetrahydrofuran (120 mL) was cooled
to 0 ꢀC, and N,N0-carbonyldiimidazole (95.15 mmol,
1.1 equiv, 15.43 g) was added in small portions under vigor-
ous stirring. After evolution of gas, the mixture was stirred at
room temperature for 4 h, then cooled to ꢂ10 ꢀC. In a 2 L
three-necked reactor equipped with a mechanical stirrer
and argon inlet, a solution of ethyl hydrogen malonate
(173 mmol, 2 equiv, 22.85 g) in tetrahydrofuran (120 mL)
was cooled to ꢂ10 ꢀC. A solution of isopropylmagnesium
bromide in diethyl ether (346 mmol, 4 equiv, 169 mL of
a 2.05 M solution) was added dropwise, the temperature of
the reaction mixture being kept below 5 ꢀC. The resulting
slurry was stirred at room temperature for 3 h, then recooled
again to ꢂ10 ꢀC before adding dropwise the solution of imi-
dazolide, the temperature of the reaction mixture being kept
below 5 ꢀC. The homogeneous reaction mixture was vigor-
ously stirred at room temperature for 96 h. The reaction
mixture was quenched at 0 ꢀC with 10% citric acid and
acidified to pH 3 (500 mL). The mixture was extracted
with ethyl acetate/toluene (4:1). The combined organic
layers were washed with water (200 mL), saturated aqueous
sodium hydrogen carbonate (2ꢁ150 mL) and saturated
aqueous sodium chloride (150 mL), dried over sodium
sulfate and concentrated under reduced pressure to give
a yellow oil. The residue was purified by silica gel column
chromatography using cyclohexane/ethyl acetate (9:1) as
eluent to give the b-keto ester 2 (16 g, 62% yield) as
a pale yellow oil.
3. Conclusion
In summary, a total synthesis of dolastatin 10 has been
achieved. In this approach, three stereogenic centres were
successfully set by catalytic hydrogenation reactions of
b-keto esters by using ruthenium-SYNPHOS complexes
with a good control of the stereochemistry of the natural
product. This versatile approach could be applied to the
synthesis of analogues.
4. Experimental
4.1. General methods
Dichloromethane was distilled from calcium hydride, and
tetrahydrofuran and diethyl ether from sodium/benzophe-
none. Acetone for the catalyst preparation was distilled
over potassium carbonate. Other solvents were used without
any purification. Triethylamine was distilled from potassium
hydroxide. All air and/or water sensitive reactions were car-
ried out under an argon atmosphere unless otherwise noted.
1H NMR spectra were recorded on an Avance 300 at
300 MHz or an Avance 400 at 400 MHz; 13C NMR spectra
were recorded on an Avance 300 at 75 MHz or an Avance
400 at 100 MHz. Chemical shifts (d) are reported in parts
per million downfield relative to internal Me4Si. Coupling
constants (J) are reported in hertz and refer to apparent
peak multiplicities (recorded as s, singlet; d, doublet; t, trip-
let; q, quadruplet; qu, quintet; o, octet; m, multiplet; and br,
broad). Mass spectra were determined on a Nermag R10-
10C instrument. Ionization was obtained by chemical ioni-
zation with ammonia (DCI/NH3) or by electrospray (on an
API 3000 PE Sciex instrument). Optical rotations were mea-
sured on a Perkin–Elmer 241 polarimeter at 589 nm (sodium
lamp). GC analyses of compounds 9 were performed on
a Agilent 6850 series equipped with an HP01 column capil-
lary column (30 m, B 0.25 mm): 70–210 ꢀC, 5 ꢀC/min, flow:
4 mL/min (He).
Procedure B:
a
solution of (S)-N-Boc-isoleucine
(27.4 mmol, 6.34 g) in tetrahydrofuran (50 mL) was cooled
to 0 ꢀC, and N,N0-carbonyldiimidazole (27.4 mmol, 4.45 g)
was added in small portions under vigorous stirring. After
evolution of gas, the mixture was stirred at room temperature
for 3 h and then cooled in an ice bath. To a suspension of
monoethyl malonate potassium salt (60.0 mmol, 10.22 g)
in THF (100 mL) at 5 ꢀC was added Et3N (86 mmol,
13.4 mL) followed by anhydrous MgCl2 (75 mmol, 7.15 g).
The mixture was stirred at room temperature for 3 h, then
cooled to 0 ꢀC and the above solution of the activated ester
previously prepared in THF was added dropwise over
35 min. The mixture was allowed to stir for 39 h at room
temperature, quenched with aqueous citric acid and ex-
tracted with ethyl acetate. The organic layers were washed
with saturated NaHCO3 solution and brine, dried (Na2SO4)
and concentrated in vacuo to give the b-keto ester 2
(7.30 g, 88%) as a pale yellow oil.
4.2. General procedure for asymmetric hydrogenation
of b-keto esters 2 and 3
(R) or (S)-SYNPHOSÒ (7.7 mg, 0.012 mmol) and (COD)-
Ru(2-methylallyl)2 (3.2 mg, 0.01 mmol, commercially
available from Acros) were placed in a round-bottomed
tube, degassed by three vacuum/argon cycles at room

C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
6119
1H NMR (CDCl3, 200 MHz, 24 ꢀC): d 5.04 (m, 1H), 4.32 (m,
1H), 4.22 (q, J¼7.1 Hz, 2H), 3.53 (s, 2H), 1.7–2.0 (m, 1H),
1.44 (s, 9H), 1.28 (t, J¼7.1 Hz, 3H), 1.0–1.3 (m, 2H), 0.98
(d, J¼6.8 Hz, 3H), 0.90 (t, J¼7.3 Hz, 3H). 13C NMR
(CDCl3, 50 MHz, 24 ꢀC): d 202.1, 166.5, 155.5, 79.7,
64.0, 61.1, 47.0, 36.0, 28.0, 23.9, 15.8, 13.8, 11.3. MS
(DCI, NH3): m/z 319 (100%, [M+NH4]+), 302 (42%,
[M+H]+), 263 (96%, [MꢂC4H8+NH4]+), 246 (25%,
[MꢂC4H8+H]+). [a]2D1 ꢂ41 (c 1.0, EtOH).
1H), 1.51 (m, 1H), 1.47 (s, 9H), 1.27 (t, J¼7.2 Hz, 3H),
1.10 (m, 1H), 0.99 (d, J¼6.9 Hz, 3H), 0.91 (t, J¼7.1 Hz,
13
3H). C NMR (CDCl3, 75 MHz, 24 ꢀC): d (conformers)
172.1 and 171.8, 156.4 and 156.3, 79.7 and 79.2, 78.4,
60.6 and 60.5, 57.8, 57.6, 37.7 and 37.0, 34.9, 34.4, 28.4,
25.8, 16.2 and 16.1, 14.2, 11.3. MS (DCI, NH3): m/z 332
(100%, [M+H]+), 293 (3%, [MꢂC4H8+NH4]+), 276 (7%,
[MꢂC4H8+H]+). [a]2D1 ꢂ16 (c 1.4, MeOH).
4.6. (3R,4S,5S)-4-(N-tert-Butoxycarbonyl-N-methyl-
amino)-3-methoxy-5-methyl-heptanoic acid (3R)-7
4.4. Ethyl (3R,4S,5S)-4-(N-tert-butoxycarbonylamino)-
3-hydroxy-5-methyl-heptanoate (3R)-4
To an ice-cooled solution of ethyl (3R,4S,5S)-4-(N-tert-but-
oxycarbonylamino)-3-methoxy-5-methyl-heptanoate (3R)-6
(1.18 mmol, 390 mg) in ethanol (3 mL) was added 1 N so-
dium hydroxide (1.24 mmol, 1.05 equiv, 1.24 mL). The mix-
ture was then stirred overnight at room temperature. The
resulting solution was acidified to pH 4 with 1 N aqueous hy-
drochloric acid and then extracted (3ꢁ20 mL) with ethyl ace-
tate. The combined organic layers were washed with 1 M
aqueous potassium hydrogen sulfate, then saturated aqueous
sodium chloride, dried over sodium sulfate and concentrated
under reduced pressure to give the desired N-Boc-dolaisoleu-
cine (3R)-7 as a colourless viscous oil (290 mg, 81% yield).
1H NMR (CDCl3, 400 MHz, 54 ꢀC): d 3.80–4.02 (m,
2H), 3.41 (s, 3H), 2.70 (br s, 3H), 2.54 (m, 2H), 1.78 (m,
1H), 1.49 (m, 1H), 1.45 and 1.44 (s, 9H), 1.08 (m, 1H),
0.96 (d, J¼6.8 Hz, 3H), 0.89 (t, J¼6.8 Hz, 3H). 13C
NMR (CDCl3, 75 MHz, 24 ꢀC): d (conformers) 176.4,
156.6 and 156.3, 80.1 and 79.6, 78.2, 60.2 (br), 57.7 and
57.6, 37.1 and 36.8, 35.0, 34.5, 28.4, 25.9 and 25.8, 16.2
and 16.1, 11.3. MS (DCI, NH3): m/z 304 (100%, [M+H]+),
265 (12%, [MꢂC4H8+NH4]+), 248 (23%, [MꢂC4H8+H]+).
[a]D21 ꢂ13 (c 0.95, MeOH) ([a]2D3 (lit.)6 ꢂ10.5 (c 0.97,
MeOH)).
A solution of ethyl (4S,5S)-4-(N-tert-butoxycarbonyl-
amino)-5-methyl-3-oxo-heptanoate 2 (0.5 mmol, 150 mg)
in absolute ethanol (2 mL) was degassed by three vacuum–
argon cycles at room temperature and added via cannula
to the catalyst [Ru(S)-SYNPHOSÒBr2] (0.05 mmol pre-
pared according to the general procedure). The Schlenk ves-
sel was then placed under argon in a 250 mL stainless steel
autoclave. The argon atmosphere was replaced with hydro-
gen by three cycles of pressurizing and the pressure adjusted
to 12 bar. The autoclave was heated at 50 ꢀC and stirring
was maintained for 72 h. After cooling, the reaction mixture
was concentrated under reduced pressure to afford the crude
b-hydroxy ester as a brown oil. The crude product was
purified by silica gel column chromatography using cyclo-
hexane/ethyl acetate (9:1) as eluent to give the b-hydroxy
ester (3R)-4 (125 mg, 83% yield) as a white solid. 1H
NMR analysis of the crude product showed a dr 92:8,
(3R)-4/(3S)-5 diastereoisomers. Mp 50 ꢀC. 1H NMR
(CDCl3, 300 MHz, 24 ꢀC): d 4.41 (d, J¼9.8 Hz, 1H, N),
4.15 (q, J¼7.1 Hz, 2H), 3.98 (m, 1H), 3.55 (m, 1H), 2.55
(dd, J¼2.8, 16.3 Hz, 1H), 2.43 (dd, J¼8.9, 16.3 Hz, 1H),
1.79 (m, 1H), 1.53 (m, 1H), 1.28 (s, 9H), 1.26 (t,
J¼7.1 Hz, 3H), 0.99 (m, 1H), 0.92 (d, J¼6.8 Hz, 3H), 0.90
13
(t, J¼7.1 Hz, 3H). C NMR (CDCl3, 75 MHz, 24 ꢀC):
4.7. Ethyl (2R,3R,4S)-3-(N-tert-butoxycarbonyl-20-
pyrrolidinyl)-3-hydroxy-2-methyl-propanoate
Boc-(2R,3R)-9
d 173.2, 156.4, 79.5, 69.0, 60.7, 58.9, 38.3, 34.6, 28.3,
23.3, 16.2, 14.1, 11.7. MS (DCI, NH3): m/z 304 (100%,
[M+H]+), 265 (14%, [MꢂC4H8+NH4]+), 248 (51%,
[MꢂC4H8+H]+). [a]2D1 +10 (c 1.02, MeOH) ([a]2D4 (lit.)6
+10.3 (c 0.98, MeOH)).
A solution of ethyl (4S)-3-(N-tert-butoxycarbonyl-20-pyrro-
lidinyl)-3-oxo-2-methyl-propanoate 8 (3 mmol, 897 mg) in
absolute ethanol (6 mL) was degassed by three vacuum–
argon cycles at room temperature and added via cannula
to the mixture (3 mol %) [RuCl2(p-cymene)]2 (27 mg) and
(S)-SYNPHOSÒ (63 mg). The Schlenk vessel was then
placed under argon in a 250 mL stainless steel autoclave.
The argon atmosphere was replaced with hydrogen by three
cycles of pressurizing and the pressure adjusted to 130 bar.
The autoclave was heated at 50 ꢀC and stirring was main-
tained for 117 h. After cooling, the reaction mixture was
concentrated under reduced pressure to afford the crude b-
hydroxy ester as a brown oil. The crude product was purified
by silica gel column chromatography using cyclohexane/
ethyl acetate (9:1) as eluent to give the b-hydroxy ester
(500 mg, 55% yield) as a slightly yellow oil. GC analysis
showed a 2:1 mixture of (2R,3R)-9/(2S,3R)-9 diastereoiso-
4.5. Ethyl (3R,4S,5S)-4-(N-tert-butoxycarbonyl-N-
methylamino)-3-methoxy-5-methyl-heptanoate (3R)-6
To a solution of LiHMDS (1 M in THF, 5.2 mmol, 2.6 equiv,
5.2 mL) and HMPA (6 mmol, 3 equiv, 1.05 mL) in tetra-
hydrofuran (3 mL) at ꢂ78 ꢀC was added a solution of ethyl
(3R,4S,5S)-4-(N-tert-butoxycarbonylamino)-3-hydroxy-5-
methyl-heptanoate (3R)-4 (2 mmol, 606 mg) in tetrahydro-
furan (6 mL). The stirring was maintained for 25 min at
ꢂ78 ꢀC before the addition of MeOTf (12 mmol, 6 equiv,
1.36 mL) at ꢂ20 ꢀC. The reaction was then quenched with
saturated aqueous ammonium chloride at ꢂ10 ꢀC. After
extraction with ethyl acetate, the organic layer was washed
with saturated aqueous sodium chloride, dried over magne-
sium sulfate and concentrated under reduced pressure. The
residue was purified by silica gel column chromatography
using petroleum ether/ethyl acetate (9:1) as eluent to give
1
mers (tR (2R,3R) 21.8 min, tR (2S,3R) 21.9 min). H NMR
(CDCl3, 300 MHz, 24 ꢀC): d 5.01 (br s, 1H), 4.14 (q,
J¼7.1 Hz, 2H), 3.99 (app t, J¼4.9 Hz, 1H), 3.95 (m, 1H),
3.50 (m, 1H), 3.25 (m, 1H), 2.54 (m, 1H), 1.71–1.97 (m,
4H), 1.46 (s, 9H), 1.25 (m, 6H). 13C NMR (CDCl3,
75 MHz, 24 ꢀC): d broad peaks (conformers) 175.6, 155
1
(3R)-6 (493 mg, 74% yield) as a colourless oil. H NMR
(CDCl3, 400 MHz, 54 ꢀC): d 4.17 (q, J¼7.1 Hz, 2H), 3.89
(m, 2H), 3.38 (s, 3H), 2.72 (s, 3H), 2.50 (m, 2H), 1.80 (m,

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C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
(br), 79.9 (br), 73.8 (br), 60.5, 59.4, 47.3, 42.1, 28.5, 25.2,
24.3, 14.6, 14.1. MS (DCI, NH3): m/z 302 (100%,
[M+H]+), 263 (4%, [MꢂC4H8+NH4]+), 246 (13%,
[MꢂC4H8+H]+). [a]D21 ꢂ47.7 (c 1.0, CHCl3).
4.8. Ethyl (2R,3R,4S)-3-(N-tert-butoxycarbonyl-20-pyr-
rolidinyl)-3-methoxy-2-methyl-propanoate (2R,3R)-10
4.10. N-Methoxy-N-methylphenylacetamide 13
To an ice-cooled solution of N,O-dimethylhydroxylamine
hydrochloride (51.2 mmol, 1.1 equiv, 5 g) and 2-phenyl-
acetyl chloride (46.5 mmol, 1 equiv, 6.15 mL) in dichloro-
methane (200 mL) was added pyridine (102.3 mmol,
2.2 equiv, 8.3 mL). After 2 h at 0 ꢀC, the reaction mixture
was stirred overnight at room temperature. After evaporation
of the solvents, the residue was diluted with a 1:1 mixture of
dichloromethane and diethyl ether, filtered on a Celite pad
and then washed with saturated aqueous sodium chloride
(2ꢁ70 mL), dried over sodium sulfate and concentrated
under reduced pressure. The crude product was purified by
silica gel column chromatography using cyclohexane/ethyl
acetate (9:1 to 5:5) as eluent to give the amide 13 (6.40 g,
77% yield) as a slightly yellow oil. 1H NMR (CDCl3,
300 MHz, 54 ꢀC): d 7.25–7.32 (m, 5H), 3.77 (s, 2H), 3.60
A solution of ethyl (2R,3R,4S)-3-(N-tert-butoxycarbonyl-20-
pyrrolidinyl)-3-hydroxy-2-methyl-propanoate Boc-(2R,3R)-
9 (2 mmol, 602 mg) in tetrahydrofuran (6 mL) was added to
a solution of LiHMDS (1 M in THF, 2.8 mmol, 1.4 equiv,
2.8 mL) in HMPA (3.2 mmol, 1.6 equiv, 557 mL) and tetra-
hydrofuran (3.2 mL) at ꢂ78 ꢀC. The stirring was maintained
for 25 min at ꢂ78 ꢀC before the addition of MeOTf
(6.0 mmol, 3.0 equiv, 679 mL) at ꢂ20 ꢀC. The mixture was
stirred at ꢂ20 ꢀC for an additional 15 min. The reaction
was then quenched with saturated aqueous ammonium chlo-
ride. After extraction with ethyl acetate, the organic layer
was washed with saturated aqueous sodium chloride, dried
over sodium sulfate and concentrated under reduced pres-
sure. The residue was purified by silica gel column chroma-
tography using petroleum ether/ethyl acetate (9:1) as eluent
to give (2R,3R)-10 (282 mg, 45% yield) as a colourless oil.
1H NMR (CDCl3, 300 MHz, 24 ꢀC): d 4.14 (br q, 2H,
J¼7.1 Hz), 3.70–3.95 (m, 2H), 3.53 (m, 1H), 3.41 (s, 3H),
3.22 (m, 1H), 2.47 (m, 1H), 1.80–2.05 (m, 3H), 1.65–1.77
(m, 1H), 1.49 (m, 9H), 1.25 (t, J¼7.1 Hz, 3H), 1.23 (d,
J¼7.2 Hz, 3H). 13C NMR (CDCl3, 75 MHz, 24 ꢀC):
d 174.6, 154.4 (br), 83.4, 79.6 (br), 61.0 (br), 60.4, 59.6
(br), 46.6 (br), 43.1, 28.5, 26.2 (br), 24.0 (br), 14.2, 13.6.
MS (DCI, NH3): m/z 333 (7%, [M+NH4]+), 316 (100%,
[M+H]+), 277 (7%, [MꢂC4H8+NH4]+), 260 (34%,
[MꢂC4H8+H]+). [a]D21 ꢂ50.8 (c 1.2, CHCl3).
13
(s, 3H), 3.19 (s, 3H). C NMR (CDCl3, 75 MHz, 24 ꢀC):
d 172.2, 134.9, 129.3, 128.5, 126.7, 61.3, 39.4, 32.2.
4.11. 2-Phenyl-1-(2-thiazolyl)ethanone 15
To a solution of n-butyllithium (2.5 M in hexane, 40.2 mmol,
1.2 equiv, 16.1 mL) and TMEDA (40.2 mmol, 1.2 equiv,
6.48 mL) in tetrahydrofuran (80 mL) at ꢂ78 ꢀC was added
dropwise 2-bromothiazole (40.2 mmol, 1.2 equiv, 3.62 mL).
The reaction mixturewas stirred at ꢂ78 ꢀC for 2 h prior to the
addition of a solution of N-methoxy-N-methylphenylaceta-
mide 13 (33.5 mmol, 6 g) in tetrahydrofuran (40 mL). After
0.5 h at ꢂ78 ꢀC and 2 h at ꢂ10 ꢀC, the reaction mixture was
quenched with 1 M aqueous potassium hydrogen sulfate.
After extraction with diethyl ether (2ꢁ200 mL), the com-
bined organic layers were washed with saturated aqueous
sodium chloride, dried over sodium sulfate and concentrated
under reduced pressure to give the crude product, which was
purified by silica gel column chromatography using cyclo-
hexane/ethyl acetate (95:5 to 9:1) as eluent to give the ketone
4.9. (2R,3R,4S)-3-(N-tert-Butoxycarbonyl-20-
pyrrolidinyl)-3-methoxy-2-methylpropanoic
acid Boc-(2R,3R)-dolaproine 11
1
15 as a white solid (5.07 g, 75% yield). H NMR (CDCl3,
300 MHz, 54 ꢀC): d 8.04 (d, J¼3.0 Hz, 1H), 7.68 (d,
J¼3.0 Hz, 1H), 7.26–7.39 (m, 5H), 4.47 (s, 2H). 13C NMR
(CDCl3, 75 MHz, 24 ꢀC): d 191.0, 166.7, 144.8, 133.6,
129.9, 128.6, 127.0, 126.7, 44.8.
To an ice-cooled solution of ethyl (2R,3R,4S)-3-(N-tert-
butoxycarbonyl-20-pyrrolidinyl)-3-methoxy-2-methyl-prop-
anoate 10 (0.78 mmol, 247 mg) in a mixture of ethanol
(5 mL) and water (1 mL) was added lithium hydroxide
monohydrate (2.35 mmol, 3 equiv, 99 mg). The mixture
was then stirred overnight at room temperature. After evap-
oration of the solvent, the residue was diluted with dichloro-
methane and washed with water. The aqueous layer was
acidified to pH 4 by adding 1 N aqueous hydrochloric acid
and then extracted with ethyl acetate followed by dichloro-
methane. The combined organic layers were dried over
sodium sulfate and concentrated under reduced pressure to
give the desired product 11 as a colourless viscous oil
4.12. (S)-2-Phenyl-1-(2-thiazolyl)ethanol 16
To an ice-cooled solution of (+)-di-iso-pinocamphenylchlo-
roborane (44.33 mmol, 3 equiv, 14.22 g) in diethyl ether
(10 mL) was added a solution of 2-phenyl-1-(2-thiazolyl)-
ethanone 15 (14.78 mmol, 3 g) in diethyl ether (60 mL).
The reaction mixture was stirred for 23 h at 0 ꢀC before
the addition of 10% aqueous sodium hydroxide. Then 30%
aqueous hydrogen peroxide was added at 10 ꢀC and the
resulting mixture was stirred for 5 h at room temperature.
After dilution with water, the aqueous layer was saturated
with potassium carbonate and extracted with diethyl ether
three times. The combined organic layers were then washed
with saturated aqueous sodium chloride, dried over magne-
sium sulfate and concentrated under reduced pressure to give
the crude alcohol as a white solid. The residue was purified
by silica gel column chromatography using cyclohexane/
ethyl acetate (8:2) as eluent to give the alcohol 16 as a white
solid (2.45 g, 81% yield, ee 92%). The pure product was re-
crystallized in a 1:1 mixture of diethyl ether and n-hexane as
1
(132 mg, 59% yield). H NMR (CDCl3, 400 MHz, 24 ꢀC):
d 4.11 (m, 1H), 3.62 (m, 1H), 3.47 (s, 3H), 3.44–3.51 (m,
1H), 3.06 (m, 1H), 2.65 (m, 1H), 1.89–1.96 (m, 2H), 1.76–
1.88 (m, 2H), 1.47 (s, 9H), 1.26 (d, J¼6.9 Hz, 3H). 13C
NMR (CDCl3, 75 MHz, 24 ꢀC): d 179.8 (br), 154.3 (br),
83.0, 79.9, 61.2, 59.4, 46.6, 42.8, 28.5, 26.1, 24.0, 13.5.
MS (DCI, NH3): m/z 305 (4%, [M+NH4]+), 288 (100%,
[M+H]+), 249 (14%, [MꢂC4H8+NH4]+), 232 (30%,
[MꢂC4H8+H]+). HRMS (DCI+), m/z calcd for
C14H26O5N: 288.1811, found: 288.1804. [a]2D4 ꢂ60.0 (c
1.03, MeOH).

C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
6121
colourless crystals (1.82 g, 60% yield, ee >99%). 1H NMR
(CDCl3, 300 MHz, 54 ꢀC): d 7.75 (d, J¼3.3 Hz, 1H), 7.22–
7.33 (m, 6H), 5.25 (dd, J¼4.2, 8.4 Hz, 1H), 3.36 (dd, J¼4.2,
13.5 Hz, 1H), 3.28 (br s, 1H, O), 3.10 (dd, J¼8.4, 13.8 Hz,
136.9, 129.7, 128.7, 127.1, 119.1, 72.8, 44.6. [a]2D4 ꢂ56.2
(c 1.6, MeOH). HPLC: Chiralcel OD-H, 95:5 hexane/iso-
propanol, 1.0 mL/min, l¼254 nm, tR (S) 14.8 min, tR (R)
16.3 min.
0.56, CHCl3) ([a]D24.5 (lit.)23b ꢂ25.5 (c 0.60, CHCl3)).
HPLC: Chiralcel OD-H, 95:5 hexane/iso-propanol,
1.0 mL/min, l¼254 nm, tR (S) 9.6 min, tR (R) 11.0 min.
13
1H). C NMR (CDCl3, 75 MHz, 24 ꢀC): d 174.4, 142.1,
4.15. General procedure for the deprotection of N-Boc
derivatives with trifluoroacetic acid
To a solution of the N-Boc amino acid or peptide in dichloro-
methane (2.5 mL/mmol) was added trifluoroacetic acid
(11.6 equiv). After being stirred at room temperature for
2–24 h, the reaction mixture was concentrated under re-
duced pressure to give the crude deprotected product that
is used immediately in the next synthetic step without any
purification.
4.13. (S)-2-Phenyl-1-(2-thiazolyl)ethylazide 17
To an ice-cooled solution of (S)-2-phenyl-1-(2-thiazolyl)-
ethanol 16 (8.29 mmol, 1.7 g) in tetrahydrofuran (80 mL)
was added triphenylphosphine (9.12 mmol, 1.1 equiv,
2.39 g) followed by diethyl azodicarboxylate (DEAD,
9.12 mmol, 1.1 equiv, 1.79 mL) and diphenyl azidophos-
phate (DPPA, 9.12 mmol, 1.1 equiv, 2.51 g). The resulting
mixture was then stirred for 60 h at room temperature. After
concentration under reduced pressure, the residue was puri-
fied by silica gel column chromatography using cyclohex-
ane/ethyl acetate (8:2) as eluent to give the azide 17 as an
orange oil in a mixture with DPPA (1.70 g, 89% yield).
This mixture was used without any other purification in
the next synthetic step. 1H NMR (CDCl3, 300 MHz,
54 ꢀC): d 7.82 (d, J¼3.2 Hz, 1H), 7.22–7.42 (m, 6H), 5.02
(dd, J¼5.1, 8.8 Hz, 1H), 3.42 (dd, J¼5.1, 13.9 Hz, 1H),
3.17 (dd, J¼8.8, 13.9 Hz, 1H).
4.16. N-Boc-Dap-Doe 20
To an ice-cooled solution of the carboxylic acid N-Boc-Dap
11 (0.252 mmol, 72.3 mg, 1 equiv) and the deprotected
(S)-Doe 19 (0.35 mmol, 1.44 equiv) in dimethylformamide
(3 mL/mmol) was added DEPC (0.328 mmol, 55 mL,
1.3 equiv) followed by triethylamine (0.68 mmol, 95 mL,
2.7 equiv). After 2 h at 0 ꢀC, the stirring was continued at
room temperature for 14 h. The reaction mixture was then
diluted with a 2:1 mixture of ethyl acetate and toluene,
washed with 1 M aqueous potassium hydrogen sulfate, wa-
ter, saturated aqueous sodium hydrogen carbonate and satu-
rated aqueous sodium chloride, dried over sodium sulfate
and concentrated under reduced pressure. The crude dipep-
tide was purified by silica gel column chromatography using
cyclohexane/ethyl acetate (5:5). N-Boc-Dap-Doe 20 was ob-
tained as a white solid (94 mg, 79% yield). 1H NMR (CDCl3,
300 MHz, 24 ꢀC): d 7.72 (d, J¼3.2 Hz, 1H), 7.13–7.27 (m,
6H), 6.42 (br d, 1H, N), 5.60 (m, 1H), 3.70–3.90 (m, 2H),
3.49 (m, 1H), 3.38 (m+s, 4H), 3.15–3.26 (m, 2H), 2.25–
2.40 (m, 1H), 1.59–1.78 (m, 4H), 1.47 (m, 9H), 1.14 (m,
4.14. (S)-Boc-Dolaphenine 18
To a solution of triphenylphosphine (8.42 mmol, 1.2 equiv,
2.21 g) in tetrahydrofuran (15 mL) was added a solution
of (S)-2-phenyl-1-(2-thiazolyl)ethylazide 17 (6.96 mmol,
1.6 g) in tetrahydrofuran (20 mL). The reaction mixture
was then heated at 50 ꢀC for 3 h. Then 28% aqueous ammo-
nium hydroxide (19.3 mL) was added and the stirring was
continued for additional 3 h at 50 ꢀC. The reaction mixture
was diluted with water (100 mL) and extracted with diethyl
ether (3ꢁ150 mL). The combined organic layers were
washed with 1 N aqueous hydrochloric acid (148 mL). The
aqueous layer was then cooled to 0 ꢀC and adjusted to pH
14 with 10% aqueous sodium hydroxide. The aqueous layer
was then extracted with dichloromethane (3ꢁ100 mL),
dried over magnesium sulfate and concentrated under re-
duced pressure. The residue was then dissolved in dioxane
(20 mL) and cooled to 0 ꢀC. Di-tert-butyl dicarbonate
(11.2 mmol, 1.6 equiv, 2.46 g) was then added and the reac-
tion mixture was stirred at room temperature overnight. Af-
ter dilution with water, the mixture was extracted twice with
diethyl ether. The combined organic layers were washed
with saturated aqueous sodium chloride, dried over sodium
sulfate and concentrated under reduced pressure to give
the crude product 18, which was purified by silica gel col-
umn chromatography using cyclohexane/ethyl acetate
(8:2) as eluent to give the N-Boc-(S)-dolaphenine 18 as
a white solid (1.12 g, 44% yield over two steps). The pure
(S)-dolaphenine was recrystallized in acetone/n-hexane
(ee>99%). Mp 82 ꢀC. 1H NMR (CDCl3, 300 MHz, 54 ꢀC):
d 7.76 (d, J¼3.3 Hz, 1H), 7.21–7.27 (m, 4H), 7.10 (m, 2H),
5.30 (br s, 1H), 3.30 (m, 2H), 1.40 (s, 9H). 13C NMR (CDCl3,
75 MHz, 24 ꢀC): d 172.0, 154.0, 142.2, 136.4, 129.4, 128.4,
126.8, 118.8, 80.1, 51.6, 41.9, 28.3. [a]2D1 ꢂ25.4 (c
13
3H). C NMR (C6D6, 100 MHz, 24 ꢀC): d (conformers)
174.0 and 173.4, 171.6 and 171.1, 154.8 and 154.4, 142.5,
137.0 and 136.6, 129.3, 128.6, 127.0 and 126.8, 118.9,
81.9, 79.8 and 79.4, 60.9 and 60.6, 59.0 and 58.7, 52.5 and
51.9, 46.9 and 46.6, 44.3 and 43.9, 41.4, 28.7, 25.7 and
25.1, 24.8 and 24.3, 14.1 and 13.9. MS (DCI, NH3): m/z
474 (100%, [M+H]+). [a]D21 ꢂ75.0 (c 1.24, MeOH) ([a]D24
(lit.)6 ꢂ76.5 (c 0.96, MeOH)).
4.17. N-Boc-Dil-Dap-Doe 21
To an ice-cooled solution of the carboxylic acid (N-Boc-Dil,
1.1 equiv) and the deprotected dipeptide Dap-(S)-Doe 20
(1 equiv) in dimethylformamide (3 mL/mmol) was added
DEPC (1.2 equiv) followed by triethylamine (2.7 equiv). Af-
ter being stirred at 0 ꢀC for 2 h, the stirring was continued at
room temperature for 14 h. The reaction mixture was then
diluted with a 2:1 mixture of ethyl acetate and toluene,
washed with 1 N aqueous potassium hydrogen sulfate, wa-
ter, saturated aqueous sodium hydrogen carbonate and satu-
rated aqueous sodium chloride, dried over sodium sulfate
and concentrated under reduced pressure. The crude dipep-
tide was purified by silica gel column chromatography using
cyclohexane/ethyl acetate (5:5). N-Boc-Dil-Dap-Doe 21
was obtained from the deprotected dipeptide Dap-(S)-Doe
20 (0.123 mmol) and N-Boc-Dil (3R)-7 (0.136 mmol,
41 mg) according to the general procedure using DEPC

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C. Mordant et al. / Tetrahedron 63 (2007) 6115–6123
(0.15 mmol, 25 mL) and triethylamine (0.33 mmol, 46 mL),
as a colourless viscous oil (58.2 mg, 72% yield). H NMR
purified by silica gel column chromatography using hex-
ane/acetone (75:25 to 5:5) as eluent to give the dolastatin
10 as a white powder (10.7 mg, 52% yield). 1H NMR
(CD2Cl2, 400 MHz, 24 ꢀC): d 7.71 (d, J¼3.3 Hz, 1H),
7.18–7.27 (m, 7H), 6.80 (br d, 1H), 5.52 (m, 1H), 4.75
(dd, J¼6.4, 9.0 Hz, 1H), 4.10 (m, 1H), 3.96 (m, 1H), 3.84
(dd, J¼1.9, 8.2 Hz, 1H), 3.31 (s, 6H), 3.18–3.40 (m, 5H),
3.01 (s, 3H), 2.23 (s, 6H), 2.21–2.44 (m, 3H), 1.58–2.14
(m, 7H), 1.26–1.40 (m, 1H), 1.08 (d, J¼7.0 Hz, 3H), 1.00
(d, J¼7.0 Hz, 3H), 0.97 (d, J¼6.7 Hz, 3H), 0.95 (d, J¼
7.2 Hz, 3H), 0.93 (d, J¼7.2 Hz, 3H), 0.90 (d, J¼6.7 Hz,
3H), 0.81 (t, J¼7.3 Hz, 3H). 13C NMR (CD2Cl2, 100 MHz,
24 ꢀC): d 173.2, 171.7, 170.8, 169.8, 142.1, 137.0, 129.0,
128.0, 126.3, 118.4, 81.2, 78.0, 76.2, 60.2, 59.1, 57.4,
53.6, 53.4, 52.3, 47.3, 44.0, 42.3, 40.7, 37.2, 32.8, 30.6,
30.2, 27.3, 25.4, 24.7, 24.2, 19.5, 19.0, 17.3, 15.3, 13.8,
10.1. ESI-MS (MS+): m/z 785.7 (100%, [M+H]+). ESI-MS
(MSꢂ): m/z 783.9 (100%, [MꢂH]ꢂ). FABMS (FAB+): m/z
785 (100%, [M+H]+). HRMS (DCI+), m/z calcd for
C42H69N6O6S: 785.4999, found: 785.4980. [a]2D4 ꢂ57.5 (c
1.07, MeOH) ([a]2D3 (lit.)6 ꢂ59.8 (c 0.035, MeOH)).
1
(CDCl3, 400 MHz, 50 ꢀC): d 7.71 (d, J¼3.2 Hz, 1H),
7.16–7.24 (m, 6H), 5.60 (m, 1H), 4.13 (m, 2H), 3.90 (m,
2H), 3.38 (s, 3H), 3.34 (s, 3H), 3.32–3.44 (m, 4H), 2.70 (s,
3H), 2.44 (m, 3H), 1.50–2.00 (m, 6H), 1.47 (m, 9H), 1.16
(d, J¼7.0 Hz, 3H), 1.12 (m, 1H), 0.99 (d, J¼6.7 Hz, 3H),
0.91 (t, J¼7.3 Hz, 3H). 13C NMR (CDCl3, 100 MHz,
24 ꢀC): d 173.9, 171.8, 170.7 and 170.2, 156.7, 142.5,
137.1, 129.5, 128.5, 126.8, 118.8, 81.8 and 81.7, 79.8 and
79.3, 78.4 (br), 60.5, 59.2 and 59.1, 58.1 and 58.0, 52.7,
47.6, 43.9 and 43.8, 41.2, 38.0 and 37.8, 34.6, 28.6, 25.9,
25.1, 24.8 and 24.7, 16.2, 14.0 and 13.9, 11.5 and 11.1.
MS (DCI, NH3): m/z 659 (100%, [M+H]+). [a]D21 ꢂ66.7 (c
1.14, MeOH) ([a]2D3 (lit.)6 ꢂ71.0 (c 0.12, MeOH)).
4.18. N-Boc-Val-Dil-Dap-Doe 22
To a solution of the deprotected tripeptide Dil-Dap-Doe 21
(0.0745 mmol, 49 mg) in dichloromethane (500 mL),
shielded from light, was added diisopropylethylamine
(0.13 mmol, 1.8 equiv, 21.5 mL) followed by N-Boc-(S)-
Val (0.149 mmol, 2 equiv, 32.3 mg). Additional diisopropyl-
ethylamine (0.13 mmol, 1.8 equiv, 21.5 mL) was added and
the resulting mixture was cooled to 0 ꢀC before the addition
of BroP (0.116 mmol, 1.56 equiv, 45.1 mg). The mixture
was allowed to come back to room temperature and stirring
was continued for 24 h. All the reactants were added once
more to the mixture and then stirred for additional 48 h.
The resulting mixture was extracted with ethyl acetate,
washed with 10% aqueous sodium carbonate, dried over so-
dium sulfate and concentrated under reduced pressure. The
crude tetrapeptide was purified by silica gel column chroma-
tography using cyclohexane/ethyl acetate (5:5 to 0:100) as
eluent to afford the desired product 22 as a colourless oil
Acknowledgements
We thank Dr. R. Schmid and Dr. M. Scalone (Hoffmann-
La Roche) for samples of (R)- and (S)-MeO-BIPHEP¼
(S)-(ꢂ)-6,60-dimethoxy-2,20-bis(diphenylphosphino)-1,10-bi-
phenyl. A fellowship from C.N.R.S./D.G.A. (C.M) is grate-
fully acknowledged. Dr. S.R. is grateful to C.N.R.S. for
a post-doctoral fellowship. We also acknowledge CMCU
ꢀ
for a grant to R.T. (ComiteMixte pour laCooperation Univer-
sitaire). This work was partially supported by a grant from
ꢀ
ꢀ
SESAME (2003–2006, E1767 ‘Developpement du pole
Chimie du Vivant a l’ENSCP: de la chimie moleculaire a la
ˆ
ꢁ
ꢀ
ꢁ
1
(31 mg, 55% yield). H NMR (CDCl3, 400 MHz, 24 ꢀC):
biotechnologie’.
d 7.72 (d, J¼3.3 Hz, 1H), 7.20 (d, J¼3.3 Hz, 1H), 7.16–
7.26 (m, 7H, N), 5.57 (br q, J¼6.0 Hz, 1H), 5.21 (d,
J¼9.5 Hz, 1H), 4.39 (br t, J¼7.9 Hz, 1H), 3.95–4.20 (m,
2H), 3.88 (dd, J¼2.0, 7.7 Hz, 1H), 3.33 (s, 3H), 3.32 (s,
3H), 3.27–3.40 (m, 4H), 2.78 and 3.01 (s, 3H), 2.30–2.45
(m, 3H), 1.54–2.02 (m, 6H), 1.42 (m, 9H), 1.12 (d,
J¼7.0 Hz, 3H), 1.15 (m, 1H), 0.98 (d, J¼6.6 Hz, 3H), 0.97
(d, J¼6.7 Hz, 3H), 0.93 (d, J¼6.7 Hz, 3H), 0.84 (t,
J¼7.3 Hz, 3H). MS (DCI, NH3): m/z 758 (100%,
[M+H]+). HRMS (DCI+), m/z calcd for C40H64N5O7S:
758.4526, found: 758.4520. [a]2D1 ꢂ57.3 (c 0.62, MeOH)
([a]D23.5 (lit.)6 ꢂ64.6 (c 0.50, MeOH)).
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