The total synthesis and reassignment of stereochemistry of dragonamide

Article abstract of DOI:10.1016/j.tet.2005.09.040

The first total synthesis of dragonamide is reported. The synthesis has led to a reassignment of the configuration at the stereogenic centre on the alkyne-bearing fragment of the molecule.

Full text of DOI:10.1016/j.tet.2005.09.040

Tetrahedron 61 (2005) 11132–11140
The total synthesis and reassignment of stereochemistry
of dragonamide
c,
Hongliang Chen,a,c Yaqing Feng,a Zhengshuang Xub,c, and Tao Ye
*
*
aSchool of Chemical Engineering, Tianjin University, Tianjin 300072, China
bLaboratory of Chemical Genomics, The Shenzhen Graduate School of Peking University, Shenzhen 518055, China
cDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China
Received 20 July 2005; revised 9 September 2005; accepted 12 September 2005
Available online 30 September 2005
Abstract—The first total synthesis of dragonamide is reported. The synthesis has led to a reassignment of the configuration at the stereogenic
centre on the alkyne-bearing fragment of the molecule.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
The marine cyanobacteria have provided chemists and
biologists with a plethora of natural products, with a wide
array of structures and functional groups.1–4 They have been
a fertile source for new bioactive molecules, and many
possess potent activity, across a broad spectrum of targets.
Their diversity in both biological activity and in chemical
complexity has made these secondary metabolites the focus
of much work in recent years. We have had a great deal of
interest in products isolated from marine cyanobacteria,5
and given its cytotoxicity against several cell lines it was
decided to undertake the synthesis of dragonamide 1
(Scheme 1 and Fig. 1).
Dragonamide 1 was isolated from the marine cyano-
bacterium Lyngbya majuscula, collected at Boca del
Drago Beach, Panama, and its structure was determined.6
Scheme 1.
It is a structurally unusual natural product insofar as it has
the 2-substituted alkynoate unit. The stereochemistry of this
unit was assigned by degradation of the peptide chain,
isolation and hydrogenation of the acid and correlation to
previous work (centre marked * is (R) as reported). We
would now like to amend the stereochemistry of the
alkynoate fragment.
Figure 1.
2. Results and discussion
Keywords: Stereochemistry; Dragonamide; Tetrapeptide.
*
In our retrosynthetic analysis, dragonamide 1 was decon-
structed into two parts: the moya 3 and the protected
tetrapeptide 4 (Scheme 1).
Corresponding authors. Tel.: C86 75526035351 (Z.X.); tel.: C852
27664173; fax: 852 22641912 (T.Y.);
e-mail addresses: xuzs@szpku.edu.cn; bctaoye@inet.polyu.edu.hk
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.040

H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
11133
2.1. Synthesis of tetrapeptide 4
2.2. Synthesis of moya 3
The synthesis started with (S)-valine, which was converted
into its t-butyl ester using tert-butyl acetate and perchloric
acid (Scheme 2).7 It was then successively protected as its
2-nitrobenzenesulfonamide 5, by treating it with o-nitro-
benzenesufonyl chloride in the presence of base.8 This
method of protection was chosen as it allows the nitrogen to
be alkylated under mild conditions, and it is readily
removable. The alkylation was performed by treating 5
with methyl iodide in DMF with potassium carbonate as
base,9 to furnish 6. Once the alkylation had been performed,
6 was treated with TFA to produce the free acid 7, which
was readily converted into its acyl chloride 8 by reacting it
with oxalyl chloride and DMF in dichloromethane.
This fragment was synthesized via the route shown in
Scheme 4. The monoprotection of 1,5-pentanediol 14 was
effected by treatment with sodium hydride and benzyl
bromide in THF,11 and Swern oxidation of the remaining
alcohol functionality produced aldehyde 15. This aldehyde
was readily olefinated using the commercially available
(carbethoxymethylene) triphenylphosphorane to furnish the
unsaturated ester in high yield. Hydrogenation using H2, Pd/
C in the presence of Na2CO3 in methanol afforded ester
16.12 After hydrolysis with LiOH in aqueous THF, 16 was
converted into the acyl chloride with oxalyl chloride and
DMF in dichloromethane, followed by treatment with (R)-4-
benzyl-2-oxazolidinone, DMAP and triethylamine to give
imide 17.13 The a-methylation was smoothly accomplished
following the methodology of Evans,14–15 to give the
R-configuration at the newly formed centre in 18. The chiral
auxiliary was removed by treating 18 with hydrogen
peroxide in aqueous THF, followed by acidification to
give an excellent yield of the acid,16 which was esterified by
reacting it with freshly prepared diazomethane to give ester
19. Cleavage of the benzyl protecting group through
catalytic hydrogenation with Pd/C, followed by Swern
oxidation produced aldehyde 20. This was converted to the
alkyne 21 in reasonable yield using the Ohira–Bestmann
reagent 22,17–18 Saponification of alkyne 21 liberated the
free acid 3.
Scheme 2. Reagents and conditions: (i) AcOBut, HClO4, 91%;
(ii) 2-nitrobenzenesufonyl chloride, Et3N, CH2Cl2, 91%; (iii) CH3I, K2CO3,
DMF, 77%; (iv) TFA–CH2Cl2; (v) (COCl)2, CH2Cl2, DMF.
(S)-phenylalanine was treated in an analogous manner to
(S)-valine above and was converted into, sequentially, its
t-butyl ester, 2-nitrobenzenesulfonamide and N-methyl
derivative 9, with all steps proceeding smoothly (Scheme 3).
The sulfonamide group was readily removed at this stage by
treating 9 with mercaptoacetic acid in DMF under basic
conditions, to give amino ester 10.10
Scheme 4. Reagents and conditions: (i) NaH, BnBr, THF, 83%;
(ii) (COCl)2, DMSO, Et3N, CH2Cl2, 81%; (iii) Ph3PZCHCO2Et, CH2Cl2,
91%; (iv) H2, Pd/C, Na2CO3, MeOH, 77%; (v) LiOH, THF–H2O, 89%;
(vi) (a) (COCl)2, DMF, CH2Cl2; (b) (R)-4-benzyl-2-oxazolidinone, DMAP,
Et3N, CH2Cl2, 85% (over two steps); (vii) NaHMDS, MeI, THF, 87%;
(viii) LiOH, H2O2, THF–H2O, 91%; (ix) CH2N2, Et2O, 99%; (x) H2, Pd/C,
MeOH, 99%; (xi) (COCl)2, DMSO, Et3N, CH2Cl2, 84%; (xii) Ohira–
Bestmann reagent 22, K2CO3, MeOH, 78%; (xiii) LiOH, THF–H2O, 92%.
Scheme 3. Reagents and conditions: (i) AcOBut, HClO4, 90%;
(ii) o-nitrobenzenesufonyl chloride, Et3N, CH2Cl2, 93%; (iii) CH3I, K2CO3,
DMF, 66%; (iv) LiOH, HSCH2COOH, DMF, 90%; (v) 8, Et3N, CH2Cl2,
93%; (vi) repeat iv and v, 69%; (vii) repeat iv and v, 79% over two steps.
Fragments 8 and 10 were coupled under mild conditions to
yield the protected dipeptide 11. Compound 11 was then
subjected to the same conditions for removal of the
2-nitrobenzenesulfonamide group as compound 9 and the
product was coupled with 8 to give tripeptide 12. Further
iteration of this deprotection/coupling procedure led to
compound 13 being obtained. These completed the main
peptidic portion of the molecule. Our attention then turned
towards the novel substituted alkynoate fragment.
2.3. Completion of the total synthesis of 1
With fragments 13 and 3 in hand, we next focused on the
completion of the synthesis. The sulfonamide was cleaved
by treating 13 with mercaptoacetic acid under basic
conditions to form the free amine 4, and it was then coupled
with the acid 3 activated by BOPCl to produce 23 in

11134
H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
moderate yield. The final two steps involved cleavage of the
t-butyl ester of 23 using TFA and formation of the primary
amide, which was achieved by activation with PyBOP and
treatment with ammonia,19–20 to produce dragonamide 1, or
so it was assumed (Scheme 5).
from reagents, and all reaction vessels were oven-dried.
Solvents were distilled prior to use: THF from Na/
benzophenone; dichloromethane, DMF, triethylamine and
diisopropylethylamine from CaH2; methanol from iodine
and magnesium. NMR spectra were recorded on Bruker AV
400 MHz spectrometers. Chemical shifts are reported in
parts per million (ppm), relative to the signals due to the
solvent. Data are reported as follows: chemical shift,
multiplicity (sZsinglet, dZdoublet, tZtriplet, qZquartet,
mZmultiplet, brZbroad), coupling constants and inte-
gration. High resolution mass spectra were obtained using a
Finnigan MAT 95 mass spectrometer, while ESI mass
spectra were obtained with a MicroMass Q-Tof-2TM
spectrometer. Optical rotations were recorded on a Perkin
Elmer 343 Polarimeter. TLC was carried out using
precoated sheets (Merck silica gel 60-F254, 0.2 mm)
which, after development, were visualized at 254 nm, and/
or staining in para-anisole, ninhydrin or phosphomolybdic
acid solution followed by heating. Flash column chroma-
tography was performed using the indicated solvents (with
RfZ0.15–0.20 for the desired component) on E. Merck
silica gel 60 (230–400 mesh ASTM).
4.2. Synthesis of tetrapeptide
4.2.1. L-Valine tert-butyl ester. To a solution of L-valine
(5.89 g, 50.0 mmol) in tert-butyl acetate (120 mL) at 0 8C,
was added HClO4 (6.5 mL, 75 mmol) slowly. The reaction
mixture was stirred at room temperature for 12 h then
washed with H2O (250 mL) and 1.0 mol/L hydrochloric
acid (150 mL). The resultant aqueous solution was adjusted
to pH 9 by addition of 10% K2CO3 solution, and then
extracted with dichloromethane (3!50 mL). The combined
organic phases were dried with anhydrous sodium sulfate,
filtered and concentrated to give an oil. This was purified by
flash chromatography on silica gel, using ethyl acetate–
hexane (1/1) as eluent, to give the title compound (7.9 g,
91%) as a yellow oil. [a]2D0 C20.6 (c 5.3, EtOAc); 1H NMR
(CDCl3, 400 Hz) d ppm: 0.92 (d, JZ7.0 Hz, 3H), 1.00 (d,
JZ7.0 Hz, 3H), 1.50 (s, 9H), 1.54 (s, 2H), 2.01–2.03 (m,
1H), 3.18 (d, JZ4.9 Hz, 1H); 13C NMR (CDCl3, 100 Hz) d
ppm: 16.4, 18.6, 27.4, 31.5, 59.6, 79.8, 174.1; HR-ESIMS
calcd for C9H20NO2 [MCH]C 174.1494, found 174.1509.
Scheme 5. Reagents and conditions: (i) LiOH, HSCH2COOH, DMF; (ii) 3,
BOPCl, DIPEA, CH2Cl2, 36% (two steps); (iii) TFA, CH2Cl2; (iv), PyBOP,
DIPEA, THF, then NH3 and NH4OH, 32%.
The material we had made had the same stereochemistry as
that reported in the isolation paper; however, both the NMR
and optical rotation data of the synthetic material did not
match. The main source of doubt about the true structure
was the configuration of the novel alkynoate fragment, and
it was here that we focused our attention.
Using exactly the same procedure, but this time with (S)-4-
benzyl-2-oxazolidinone it was possible to make the
enantiomer of the substituted alkynoic acid, (S)-3. This
was used to complete the synthesis in exactly the same
manner, and it was indeed found that the data for the newly
synthesized material was an excellent match for the
literature data on dragonamide. Consequently, we can now
report that the correct structure for dragonamide is
(S,S,S,S,S) (Fig. 1).
4.2.2. N-o-Nitro-benzosulfonyl-L-valine tert-butyl ester 5.
To a solution of L-valine tert-butyl ester (6.3 g, 36.0 mmol)
in dichloromethane (120 mL) at 0 8C, was added triethyl-
amine (6.0 mL, 39.6 mmol) and o-nitrobenzene-sulfonyl
chloride (10.5 g, 45.0 mmol). The reaction mixture was
stirred at room temperature overnight and then washed with
H2O (3!15 mL) and the organic extracts were dried
(Na2SO4) and concentrated under reduced pressure. The
crude residue was purified by flash chromatography on
silica gel, using ethyl acetate–hexane (1/4) as eluent, to
produce the title compound (11.7 g, 91%) as a yellow oil.
3. Conclusion
The marine cyanobacteria secondary metabolite dragon-
amide has been synthesized. As a result, the configuration of
the moya fragment has been amended.
1
[a]2D0 K108.2 (c 1.4, CHCl3); H NMR (CDCl3, 400 Hz) d
ppm: 0.94 (d, JZ6.9 Hz, 3H), 1.03 (d, JZ6.7 Hz, 3H), 1.20
(s, 9H), 2.11–2.17 (m, 1H), 3.88 (q, JZ5.0 Hz, 1H), 6.06 (d,
JZ9.8 Hz, 1H), 7.70–7.73 (m, 1H), 7.86–7.94 (m, 2H),
8.06–8.08 (m, 1H); 13C NMR (CDCl3, 100 Hz) d ppm: 17.3,
19.0, 27.6, 31.6, 62.6, 82.35, 125.5, 130.4, 132.8, 133.5,
134.4, 136.5, 169.6; HR-ESIMS calcd for C15H22N2O6SNa
[MCNa]C 381.1096, found 381.1121.
4. Experimental
4.1. General
All non-aqueous reactions were run under an inert
atmosphere (nitrogen) with rigid exclusion of moisture

H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
11135
4.2.3. N-o-Nitro-benzosulfonyl-N-methyl-L-valine tert-
butyl ester 6. K2CO3 (7.7 g, 55.2 mmol) was added to a
solution of 5 (9.9 g, 27.6 mmol) in DMF (60.0 mL) at 0 8C
under a protective flow of N2. After 10 min, iodomethane
(7.0 mL, 110.4 mmol) was introduced via a syringe. The
reaction mixture was then stirred at room temperature
overnight prior to being quenched by the addition of
saturated aqueous NH4Cl (20 mL). The mixture was
extracted with benzene–ethyl acetate (1/3, v/v) (3!
80 mL). The combined organic extracts were washed with
0.5 mol/L HCl (2!50 mL) and brine (2!50 mL), dried
(Na2SO4) and concentrated. The residue was subjected to
flash chromatography on silica gel, using ethyl acetate–
hexane (1/3) as eluent, affording the title compound (7.9 g,
77%) as a yellow oil. [a]2D0 K12.3 (c 3.5, CH3CO2C2H5); 1H
NMR (CDCl3, 400 Hz) d ppm: 0.98 (d, JZ6.8 Hz, 3H), 1.02
(d, JZ6.6 Hz, 3H), 1.31 (s, 9H), 2.17–2.23 (m, 1H), 3.08 (s,
3H), 4.04 (d, JZ10.1 Hz, 1H), 7.63–7.66 (m, 1H), 7.70–
7.76 (m, 2H), 8.04–8.06 (m, 1H); 13C NMR (CDCl3,
100 Hz) d ppm: 19.0, 19.1, 27.5, 27.8, 30.6, 65.5, 81.7,
123.7, 130.8, 131.4, 132.8, 133.4, 147.9, 168.8; HR-ESIMS
calcd for C16H24N2O6SNa [MCNa]C 395.1253, found
395.1270.
a 41.9 mmol scale using the same procedures for compound
6, with a yield of 66%. [a]2D0 C88.5 (c 3.5, CHCl3); H
1
NMR (CDCl3, 400 Hz) d ppm: 1.35 (s, 9H), 3.00 (dd, JZ
9.2, 14.2 Hz, 1H), 3.06 (s, 3H), 3.36 (dd, JZ6.5, 14.2 Hz,
1H), 4.88 (dd, JZ6.5, 9.2 Hz, 1H), 7.21–7.27 (m, 5H),
7.53–7.57 (m, 1H), 7.61–7.63 (m, 2H), 7.73–7.75 (m, 1H);
13C NMR (CDCl3, 100 Hz) d ppm: 27.3, 30.4, 35.4, 61.0,
81.9, 123.6, 126.5, 128.1, 128.7, 130.2, 131.4, 132.2, 133.2,
136.1, 147.5, 168.5; HR-ESIMS calcd for C20H24N2O6SNa
[MCNa]C 443.1253, found 443.1254.
4.2.8. N-Methyl-L-phenylalanine tert-butyl ester 10.
Compound 9 (550 mg, 1.3 mmol) was dissolved in DMF
(15 mL) at 0 8C and LiOH$H2O (570 mg, 13.1 mmol) was
added, followed by mercaptoacetic acid (0.19 mL, 2.6 mol).
After 2 h, the reaction mixture was poured into a saturated
aqueous solution of NH4Cl (5 mL) and extracted with
benzene–ethyl acetate (1/3, v/v) (3!20 mL). The combined
organic phase was washed with saturated NaHCO3 (2!
60 mL) and brine (2!60 mL), then dried (Na2SO4) and
concentrated. The residue was purified by flash chromato-
graphy on silica gel, using ethyl acetate–hexane (1/3) as
eluent, to produce the title compound (280 mg, 90%) as a
1
yellow oil. [a]D20 C22.7 (c 3.3, CH3CO2C2H5); H NMR
4.2.4. (2S)-3-Methyl-2-(o-nitro-benzenesulfonyl-methyl-
amino)-butyryl chloride 8. To a solution of 6 (980 mg,
2.6 mmol) in dichloromethane (10 mL) at 0 8C was added
TFA (5.0 mL, 67.3 mmol) dropwise and with stirring at
0 8C. The reaction was monitored by TLC and after the
starting material had disappeared, the reaction mixture was
concentrated to give a dark red oil (7).
(CDCl3, 400 Hz) d ppm: 1.24 (s, 9H), 1.82 (s, 1H), 2.24 (s,
3H), 2.72–2.75 (m, 1H), 2.82 (dd, JZ6.4, 13.5 Hz, 1H),
3.17–3.21 (m, 1H), 7.04–7.15 (m, 5H); HR-ESIMS calcd for
C14H22NO2 [MCH]C 236.1651, found 236.1652.
4.2.9. Ns-(S)-MeVal-(S)-MePhe-O-tert-Bu 11. To a
solution of 10 (280 mg, 1.2 mmol) in dichloromethane
(3 mL) at 0 8C was added Et3N (0.84 mL, 6 mmol) and 8
(503 mg, 1.5 mmol) dissolved in dichloromethane (3 mL).
The reaction mixture was stirred at room temperature
overnight, and was washed with 5% HCl (2!30 mL),
saturated NaHCO3 (2!30 mL), brine (2!30 mL), dried
(Na2SO4) and then concentrated. The crude oil was purified
by flash chromatography on silica gel, using ethyl acetate–
hexane (1/2) as eluent, to give the title compound (0.58 g,
To a solution of 7 in dichloromethane (10 mL) at 0 8C was
added (COCl)2 (0.56 mL, 6.5 mmol) and DMF (20 mL,
0.3 mmol). The reaction was monitored by TLC and after
the starting material had disappeared, the reaction mixture
was concentrated to give a brown-yellow oil (8).
4.2.5. L-Phenylalanine tert-butyl ester. This compound
was prepared in an analogous manner to compound L-valine
tert-butyl ester on a 50.0 mmol scale. The yield for the title
compound was 90%. [a]2D0 C25.1 (c 2.8, EtOAc); 1H NMR
(CDCl3, 400 Hz) d ppm: 1.31 (s, 11H), 2.70 (dd, JZ7.7,
13.5 Hz, 1H), 2.89 (dd, JZ5.8, 13.5 Hz, 1H), 3.47 (dd, JZ
5.8, 7.7 Hz, 1H), 7.09–7.19 (m, 5H); 13C NMR (CDCl3,
100 Hz) d ppm: 27.7, 41.1, 56.1, 80.7, 126.4, 128.2, 129.2,
137.4, 174.1; HR-ESIMS calcd for C13H20NO2 [MCH]C
222.1494, found 222.1490.
1
92.7%) as a yellow oil. [a]2D0 K108.2 (c 1.4, CHCl3); H
NMR (CDCl3, 400 Hz) d ppm: 0.66 (d, JZ6.6 Hz, 3H), 0.98
(d, JZ6.3 Hz, 3H), 1.48 (s, 9H), 2.24–2.31 (m, 1H), 2.98
(dd, JZ7.1, 14.3 Hz, 1H), 3.02 (s, 3H), 3.12 (s, 3H), 3.34
(dd, JZ5.9, 14.3 Hz, 1H), 4.53 (d, JZ10.7 Hz, 1H), 5.31
(dd, JZ5.9, 7.1 Hz, 1H), 7.21–7.37 (m, 5H), 7.59–7.72 (m,
3H), 7.91–7.93 (m, 1H); 13C NMR (CDCl3, 100 Hz) d ppm:
18.6, 18.6, 27.9, 28.8, 30.6, 32.8, 34.6, 59.1, 59.3, 81.9,
124.0, 126.5, 128.4, 129.0, 129.8, 131.6, 133.2, 133.3,
137.2, 148.0, 169.6, 170.6; HR-ESIMS calcd for C26H35-
N3O7SNa [MCNa]C 556.2093, found 556.2007.
4.2.6. N-o-Nitro-benzosulfonyl-L-phenylalanine tert-
butyl ester. This compound was prepared according to
the procedures for compound 5 on a 45.1 mmol scale, with a
yield of 93%. [a]D20 C88.7 (c 3.4, CHCl3); 1H NMR (CDCl3,
400 Hz) d ppm: 1.19 (s, 9H), 3.07–3.09 (m, 2H), 4.31–4.36
(m, 1H), 6.06 (d, JZ9.0 Hz, 1H), 7.15–7.24 (m, 5H), 7.65–
7.70 (m, 1H), 7.82–7.84 (m, 2H), 7.89–8.00 (m, 1H); 13C
NMR (CDCl3, 100 Hz) d ppm: 27.4, 39.3, 58.0, 82.6, 125.1,
127.0, 128.3, 129.3, 130.2 132.9, 133.5, 135.0, 136.7, 147.4,
169.1; HR-ESIMS calcd for C19H22N2O6SNa [MCNa]C
429.1096, found 429.1108.
4.2.10. (S)-MeVal-(S)-MePhe-O-tert-Bu. To a solution of
11 (6.68 g, 12.5 mmol) in DMF (120 mL) at 0 8C was added
solid LiOH$H2O (5.42 g, 125.2 mmol) and mercaptoacetic
acid (1.78 mL, 25.0 mol). The reaction mixture was stirred
at room temperature for 2 h, quenched by the addition of a
saturated NH4Cl (20 mL) and extracted with benzene–ethyl
acetate (1/3, v/v) (3!120 mL). The combined organic
extracts were washed with saturated NaHCO3 (2!100 mL)
and brine (2!100 mL), dried (Na2SO4) and concentrated to
give the desired secondary amine as a yellow oil (4.18 g,
96%), which was used in next step without further
purification.
4.2.7. N-o-Nitro-benzosulfonyl-N-methyl-L-phenyl-
alanine tert-butyl ester 9. Compound 9 was prepared on

11136
H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
4.2.11. Ns-(S)-MeVal-(S)-MeVal-(S)-MePhe-O-tert-Bu
12. To a solution of (S)-MeVal-(S)-MePhe-O-tert-Bu
(4.18 g, 12 mmol) in dichloromethane (30 mL) at 0 8C
was added Et3N (8.4 mL, 60 mmol), followed by a solution
of 8 (5.02 g, 15 mmol) in dichloromethane (30 mL). The
reaction mixture was stirred at room temperature overnight
and then concentrated to dryness. The residue was dissolved
in EtOAc (150 mL) and washed with 5% HCl (2!60 mL),
saturated NaHCO3 (2!60 mL) and brine (2!60 mL).
After being dried (Na2SO4), this solution was concentrated
to a crude oil, which was subjected to chromatographic
purification on silica gel, using ethyl acetate–hexane (1/2) as
eluent. The title compound was obtained as a yellow oil
(5.60 g, 69.2%, over two steps). [a]2D0 K513.8 (c 2.7,
5.44 (dd, JZ11.8, 4.7 Hz, 1H), 7.14–7.32 (m, 5H), 7.58–
7.60 (m, 1H), 7.65–7.71 (m, 2H), 7.93–7.95 (m, 1H); 13C
NMR (CDCl3, 100 Hz) d ppm: 17.6, 18.1, 18.6, 18.6, 19.6,
19.7, 25.6, 27.0, 27.7, 28.0, 29.9, 30.5, 30.8, 30.9, 31.8,
34.2, 57.8, 57.9, 58.1, 59.7, 81.9, 124.0, 126.7, 128.4, 128.7,
130.1, 131.5, 133.0, 133.5, 137.0, 147.9, 169.6, 169.9,
170.4, 171.4; HR-ESIMS calcd for C38H57N5O9SNa [MC
Na]C 782.3775, found 782.3776.
4.3. Synthesis of moya
4.3.1. 5-Benzyloxy-pentan-1-ol. To a flask charged with
NaH (80% in mineral oil, 0.96 g, 32.0 mmol) at 0 8C, was
added pentane-1,5-diol (14) (11.0 mL, 105.0 mmol) slowly.
After stirring for 6 h at room temperature, the reaction
mixture was cooled in an ice-water bath and benzyl bromide
(5.0 mL, 42.0 mmol) was added dropwise via a syringe and
stirring was continued overnight. The reaction was
quenched by slow addition of saturated NH4Cl (30 mL)
and the mixture was extracted with ether (3!30 mL). The
combined organic extracts were washed with brine (60 mL),
dried (Na2SO4) and concentrated. The residue was purified
by flash chromatography on silica gel, using ethyl acetate–
hexane (1/2) as eluent, to provide the title compound (6.8 g,
83%) as a clear oil. 1H NMR (CDCl3, 400 Hz) d ppm: 1.47–
1.54 (m, 2H), 1.59–1.66 (m, 2H), 1.68–1.75 (m, 2H), 3.52–
3.55 (m, 2H), 3.60–3.64 (m, 2H), 4.16 (s, 1H), 4.54 (s, 2H),
7.31–7.41 (m, 5H); 13C NMR (CDCl3, 100 Hz) d ppm: 22.0,
29.0, 32.0, 61.8, 70.0, 72.5, 127.1, 127.3, 128.0, 138.1; HR-
ESIMS calcd for C12H19O2 [MCH]C 195.1385, found
195.1369.
1
CHCl3); H NMR (CDCl3, 400 Hz) d ppm: 0.50 (d, JZ
6.7 Hz, 3H), 0.55 (d, JZ6.7 Hz, 3H), 0.76 (d, JZ6.8 Hz,
3H), 0.88 (d, JZ6.6 Hz, 3H), 1.39 (s, 9H), 2.03–2.08 (m,
1H), 2.24–2.27 (m, 1H), 2.74 (s, 3H), 2.85 (s, 3H), 2.88 (dd,
JZ11.2, 15.0 Hz, 1H), 3.06 (s, 3H), 3.30 (dd, JZ5.0,
15.0 Hz, 1H), 4.34 (d, JZ10.6 Hz, 1H), 5.06 (d, JZ
10.7 Hz, 1H), 5.36 (dd, JZ5.0, 11.2 Hz, 1H), 7.10–7.18 (m,
5H), 7.51–7.54 (m, 1H), 7.59–7.63 (m, 2H), 7.84–7.85 (m,
1H); 13C NMR (CDCl3, 100 Hz) d ppm: 17.5, 18.4, 18.5,
19.6, 27.4, 27.8, 28.4, 30.0, 30.5, 31.7, 34.1, 57.7, 59.4,
64.0, 81.6, 123.8, 126.4, 128.2, 128.4, 129.7, 131.4, 132.6,
133.4, 136.9, 147.7, 169.4, 170.1, 170.7; HR-ESIMS calcd
for C32H46N4O8SNa [MCNa]C 669.2934, found 669.2908.
4.2.12. (S)-MeVal-(S)-MeVal-(S)-MePhe-O-tert-Bu. To a
solution of 12 (370 mg, 0.57 mmol) in DMF (3 mL) at 0 8C
was added solid LiOH$H2O (0.25 g, 5.7 mmol), followed
by mercaptoacetic acid (81 mL, 1.14 mol). The reaction
mixture was stirred at room temperature for 2 h and was
then quenched by pouring it into saturated NH4Cl (5 mL)
and extracting with benzene–ethyl acetate (1/3, v/v) (3!
5 mL). The combined organic extracts were washed with
saturated NaHCO3 (2!10 mL), brine (2!10 mL), dried
(Na2SO4) and concentrated to give the title compound as a
yellow oil, which was used in next steps without further
purification.
4.3.2. 5-Benzyloxy-pentanal 15. To a solution of (COCl)2
(0.86 mL, 10.0 mmol) in dichloromethane (20 mL) at
K78 8C was added DMSO (1.42 mL, 20 mmol) in
dichloromethane (5 mL). After 10 min, a pre-cooled
solution of 5-benzyloxy-pentan-1-ol (0.97 g, 5 mmol) in
dichloromethane (5 mL) was added dropwise. The reaction
was kept at K78 8C for 1 h before Et3N (5.6 mL,
39.8 mmol) was added. The reaction mixture was allowed
to warm to K60 8C within 1 h then quenched by slow
addition of saturated NH4Cl. The volatiles were removed
under reduced pressure and the remaining solution was
extracted with ethyl acetate (3!50 mL). The combined
organic extracts were washed with saturated NaHCO3
(60 mL), brine (60 mL), dried (Na2SO4) and concentrated.
The residue was purified by flash chromatography on silica
gel, using ethyl acetate–hexane (1/5) as eluent, to give the
title compound (0.78 g, 81%) as a clear oil. 1H NMR
(CDCl3, 400 Hz) d ppm: 1.52–1.57 (m, 2H), 1.59–1.65 (m,
2H), 2.32–2.36 (m, 2H), 3.38 (t, JZ6.1 Hz, 2H), 4.38 (s,
2H), 7.17–7.24 (m, 5H), 9.64 (s, 1H); 13C NMR (CDCl3,
100 Hz) d ppm: 18.8, 29.0, 43.5, 69.6, 72.8, 127.4, 127.5,
128.3, 138.3, 202.4; HR-ESIMS calcd for C12H16O2Na
[MCNa]C 215.1048, found 215.1077.
4.2.13. Ns-(S)-MeVal-(S)-MeVal-(S)-MeVal-(S)-MePhe-
O-tert-Bu 13. To a solution of (S)-MeVal-(S)-MeVal-(S)-
MePhe-O-tert-Bu (1.64 g, 3.6 mmol) in dichloromethane
(6 mL) at 0 8C was added Et3N (3 mL, 21.4 mmol),
followed by addition of a solution of 8 (1.34 g, 4 mmol)
in dichloromethane (6 mL). The reaction mixture was
stirred at room temperature overnight and was then
concentrated to dryness. The residue was dissolved in
EtOAc (150 mL) and was successively washed with 5%
HCl (2!30 mL), saturated NaHCO3 (2!30 mL) and brine
(2!30 mL), dried (Na2SO4) and concentrated. The
resultant residue was purified by flash chromatography on
silica gel, using ethyl acetate–hexane (1/1) as eluent, to
produce the title compound (2.14 g, 79%, over the last two
1
steps) as a yellow oil. [a]2D0 K288.6 (c 1.3, CHCl3); H
NMR (CDCl3, 400 Hz) d ppm: 0.58 (d, JZ6.6 Hz, 3H), 0.67
(d, JZ6.8 Hz, 3H), 0.70 (d, JZ6.5 Hz, 3H), 0.83 (d, JZ
6.6 Hz, 3H), 0.87 (d, JZ6.8 Hz, 3H), 0.91 (d, JZ6.5 Hz,
3H), 1.46 (s, 9H), 2.18–2.29 (m, 3H), 2.60 (s, 3H), 2.87 (s,
3H), 2.92 (dd, JZ11.8, 14.9 Hz, 1H), 3.15 (s, 3H), 3.17 (s,
3H), 3.37 (dd, JZ4.7, 14.9 Hz, 1H), 4.53 (d, JZ10.7 Hz,
1H), 5.02 (d, JZ10.8 Hz, 1H), 5.08 (d, JZ10.6 Hz, 1H),
4.3.3. 7-Benzyloxy-hept-2-enoic acid ethyl ester. (Carb-
ethoxymethylene)triphenylphosphorane (3.69 g, 10.6 mmol)
was dissolved in dichloromethane (50 mL) at 0 8C and to
this was added aldehyde (15) (1.94 g, 10.1 mmol) in
dichloromethane (10 mL), and the reaction was stirred at
room temperature overnight. The triphenylphosphine oxide
was removed simply by filtering the reaction mixture

H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
11137
through a pad of silica gel. The filtrate was concentrated to
dryness, and the residue was purified by flash chromato-
graphy on silica gel, using ethyl acetate–hexane (1/5) as
eluent, to give the title compound (2.42 g, 91%) as a clear
(4.61 g, 26.0 mmol) and DMAP (0.16 g, 1.29 mmol) were
dissolved in dichloromethane (80 mL). The solution was
cooled to 0 8C and Et3N (5.48 mL, 39 mmol) added,
followed by a solution of the above acylchloride in
dichloromethane (10 ml). The reaction was stirred at room
temperature for 12 h and concentrated under reduced
pressure and the residue was dissolved in EtOAc
(100 mL). The solution was successively washed with 3%
HCl (100 mL), saturated NaHCO3 (100 mL) and brine
(100 mL), dried (Na2SO4) and concentrated. The residue,
after being purified by flash chromatography on silica gel,
using ethyl acetate–hexane (1/5) as eluent, provided the
desired compound (3.48 g, 84.9%) as a clear oil. [a]D20
1
oil. H NMR (CDCl3, 400 Hz) d ppm: 1.17 (t, JZ7.15 Hz,
3H), 1.43–1.57 (m, 4H), 2.07–2.13 (m, 2H), 3.36 (t, JZ
6.2 Hz, 2H), 4.07 (q, JZ7.1 Hz, 2H), 4.38 (s, 2H), 5.69–
5.74 (m, 1H), 6.82–6.89 (m, 1H), 7.15–7.25 (m, 5H); 13C
NMR (CDCl3, 100 Hz) d ppm: 14.0, 24.5, 29.0, 31.7, 59.8,
69.6, 72.7, 121.3, 127.3, 127.3, 128.1, 138.3, 148.6, 166.3;
HR-ESIMS calcd for C16H23O3 [MCH]C 263.1647, found
263.1636.
1
K91.1 (c 2.6, CHCl3); H NMR (CDCl3, 400 Hz) d ppm:
4.3.4. 7-Benzyloxy-heptanoic acid methyl ester 16.
7-Benzyloxy-hept-2-enoic acid ethyl ester (0.64 g,
2.4 mmol) and Na2CO3 (0.77 g, 7.3 mmol) were placed in
a 100 mL round-bottom flask and dissolved in methanol
(50 mL). The flask was flushed with nitrogen. A catalytic
amount of (10%) Pd/C was added; the flask was sealed
tightly and then purged with hydrogen. The reaction mixture
was stirred vigorously under a hydrogen atmosphere for 4 h,
then filtered through a plug of celite and concentrated. The
resultant residue was dissolved in ethyl acetate (150 mL)
and washed with brine (30 mL). The solution was then dried
(Na2SO4) and concentrated to leave a crude oil, which was
purified by flash chromatography on silica gel, using ethyl
acetate–hexane (1:10) as eluent, to produce the title
1.33–1.35 (m, 4H), 1.55–1.64 (m, 4H), 2.68 (dd, JZ9.6,
13.3 Hz, 1H), 2.82–2.89 (m, 2H), 3.21 (dd, JZ2.8, 13.3 Hz,
1H), 3.40 (t, JZ6.5 Hz, 2H), 4.06–4.12 (m, 2H), 4.42 (s,
2H), 4.56–4.60 (m, 1H), 7.11–7.27 (m, 10H); 130C NMR
(CDCl3, 100 Hz) d ppm: 24.1, 25.9, 28.9, 29.5, 35.4, 37.9,
55.1, 66.1, 70.3, 72.8, 127.3, 127.4, 127.6, 128.3, 128.9,
129.4, 135.3, 138.6, 153.4, 173.3; HR-ESIMS calcd for
C24H30NO4 [MCH]C 396.2175, found 396.2166.
4.3.7. (20R),4R]-3-(7-Benzyloxy-2-methyl-1-oxoheptyl)-
4-(benzyl)-2-oxazolidinone 18. To NaHMDS (sodium
bis-(trimethylsilyl)amide) (5.8 mL, 9.9 mmol, 1.71 M sol-
ution in THF) in THF (70 mL) at K78 8C was added a pre-
cooled solution of 17 (3.25 g, 8.22 mmol) in THF (10 mL).
After 45 min, iodomethane (2.56 mL, 41.12 mmol) was
added slowly via a syringe. After stirring at !78 8C for 4 h,
the reaction mixture was quenched by adding saturated
NH4Cl (50 mL). Volatiles were removed under reduced
pressure and the aqueous layer was extracted with
dichloromethane (3!50 mL). The combined organic
extracts were successively washed with 5% KHSO4
(100 mL), saturated Na2S2O3 (100 mL) and brine
(100 mL), dried (Na2SO4) and concentrated. The residue
was purified by flash chromatography on silica gel, using
ethyl acetate–hexane (1/5) as eluent, to give the title
compound (2.92 g, 87%) as a clear oil. [a]2D0 K75.1 (c 1.4,
1
compound (0.47 g, 77%) as a clear oil. H NMR (CDCl3,
400 Hz) d ppm: 1.15–1.26 (m, 4H), 1.43–1.51 (m, 4H), 2.13
(t, JZ7.5 Hz, 2H), 3.29 (t, JZ6.5 Hz, 2H), 3.47 (s, 3H),
4.32 (s, 2H), 7.06–7.17 (m, 5H); 13C NMR (CDCl3, 100 Hz)
d ppm: 24.3, 25.4, 28.4, 29.1, 33.3, 50.6, 69.7, 72.2, 126.8,
126.9, 127.7, 138.2, 173.2; HR-ESIMS calcd for C15H23O3
[MCH]C 251.1647, found 251.1654.
4.3.5. 7-Benzyloxy-heptanoic acid. To a solution of 16
(0.47 g, 1.9 mmol) in THF–water (8 mL/2 mL) at 0 8C was
added solid LiOH$H2O (130 mg, 2.9 mmol). The reaction
mixture was stirred at room temperature for 12 h then
extracted with EtOAc (5 ml). The organic layer was
discarded. The aqueous phase was acidified to pH 2 with
5%KHSO4 in the presence of EtOAc (20 mL). The layers
were separated and the aqueous layer was further extracted
with EtOAc (3!20 mL). The combined organic extracts
was washed with brine (50 mL), dried (Na2SO4) and
concentrated to give the title compound (0.39 g, 89%) as a
1
CHCl3); H NMR (CDCl3, 400 Hz) d ppm: 1.24 (d, JZ
6.9 Hz, 3H), 1.33–1.48 (m, 5H), 1.61–1.68 (m, 2H), 1.77–
1.81 (m, 1H), 2.79 (dd, JZ9.5, 13.3 Hz, 1H), 3.24–3.27 (m,
1H), 3.48 (t, JZ6.5 Hz, 2H), 3.71–3.76 (m, 1H), 4.13–4.16
(m, 2H), 4.51 (s, 2H), 4.65–4.68 (m, 1H), 7.21–7.36 (m,
10H); 13C NMR (CDCl3, 100 Hz) d ppm: 17.2, 26.0, 26.9,
29.4, 33.0, 37.4, 37.6, 55.0, 65.7, 70.1, 72.6, 127.1, 127.2,
127.4, 128.1, 128.7, 129.2, 135.1, 138.5, 152.8, 176.9; HR-
ESIMS calcd for C25H32NO4 [MCH]C 410.2331, found
410.2316.
1
clear oil. H NMR (CDCl3, 400 Hz) d ppm: 1.23–1.29 (m,
4H), 1.45–1.53 (m, 4H), 2.18 (t, JZ7.5 Hz, 2H), 3.33 (t, JZ
6.6 Hz, 2H), 4.36 (s, 2H), 7.10–7.20 (m, 5H), 10.87 (s, 1H);
13C NMR (CDCl3, 100 Hz) d ppm: 24.3, 25.5, 28.5, 29.2,
33.7, 69.9, 72.5, 127.2, 127.4, 128.0, 138.2, 179.4; HR-
ESIMS calcd for C14H21O3 [MCH]C 237.1491, found
237.1482.
4.3.8. (2R)-7-Benzyloxy-2-methyl-heptanoic acid. To a
solution of 18 (2.7 g, 6.5 mmol) in THF–water (80 mL/
25 mL) at 0 8C was added H2O2 (5.3 mL, 52.2 mmol) and
after 10 min, LiOH$H2O (0.56 g, 13.1 mmol) was added.
The reaction mixture was stirred at room temperature for
12 h and then cooled to 0 8C before Na2SO3 (6.9 g,
53.3 mol) was added. Stirring was continued for another
30 min, then the mixture was washed with EtOAc and the
organic extracts were discarded. The aqueous solution was
covered with EtOAc (100 mL) and acidified to pH 2 with
5% KHSO4, the layers were separated and the aqueous layer
was further extracted with EtOAc (3!100 mL). The
4.3.6. (4R)-3-(7-Benzyloxy-1-oxoheptyl)-4-(benzyl)-2-
oxazolidinone 17. To the solution of 7-benzyloxy-hepta-
noic acid (2.45 g, 10.4 mmol) in dichloromethane (80 mL)
at 0 8C, was added (COCl)2 (4.47 mL, 52.1 mmol), followed
by DMF (100.5 mL, 1.3 mmol). The reaction was monitored
with TLC and after the starting material had been
consumed, the mixture was concentrated to provide the
corresponding acylchloride as a dark red oil. Meanwhile, in
another reaction vessel, (R)-4-benzyl-2-oxazolidinone

11138
H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
combined organic extracts were washed with brine
(200 mL), dried (Na2SO4) and concentrated to give the
title compound (1.49 g, 91%) as a clear oil. [a]2D0 K5.4 (c
0.6, CHCl3); 1H NMR (CDCl3, 400 Hz) d ppm: 1.14 (d, JZ
7.0 Hz, 3H), 1.33–1.44 (m, 5H), 1.56–1.70 (m, 3H), 2.37–
2.46 (m, 1H), 3.42–3.45 (m, 2H), 4.47 (s, 2H), 7.21–7.31
(m, 5H), 10.62 (s, 1H); 13C NMR (CDCl3, 100 Hz) d ppm:
16.8, 26.0, 26.9, 29.4, 33.4, 39.2, 70.2, 72.7, 127.4, 127.6,
128.3, 138.4, 183.0; HR-ESIMS calcd for C15H23O3 [MC
H]C 251.1647, found 251.1647.
ethyl acetate–hexane (1/5) as eluent, to give the title
compound (0.76 g, 84%) as a yellow oil. [a]2D0 K24.9 (c 1.4,
CHCl3); H NMR (CDCl3, 400 Hz) d ppm: 1.13 (d, JZ
7.0 Hz, 3H), 1.28–1.33 (m, 2H), 1.34–1.45 (m, 1H), 1.57–
1.69 (m, 3H), 2.40–2.46 (m, 3H), 3.65 (s, 3H), 9.74 (t, JZ
1.7 Hz, 1H); 13C NMR (CDCl3, 100 Hz) d ppm: 17.0, 212.8,
26.7, 33.4, 39.2, 43.6, 51.5, 177.0, 202.4; HR-ESIMS calcd
for C9H16O3Na [MCNa]C 195.0997, found 195.1028.
1
4.3.12. (2R)-2-Methyl-oct-7-ynoic acid methyl ester 21.
To a solution of diethyl 1-diazo-2-oxopropylphosphonate
(4.3 g, 19.7 mmol) in methanol (50 mL) at 0 8C was added
K2CO3. After stirring for 30 min at this temperature, a
solution of 20 (0.67 g, 3.9 mmol) in methanol (40 mL) was
added slowly and stirring was continued at 0 8C for 1 h then
room temperature overnight. The reaction was quenched by
adding brine (20 mL), the mixture was concentrated and
extracted with dichloromethane (3!100 mL). The com-
bined organic extracts were dried (Na2SO4) and concen-
trated. The residue was purified by flash chromatography on
silica gel, using ethyl acetate–hexane (1:10) as eluent, to
give the title compound (0.51 g, 78%) as a yellow oil. [a]D20
4.3.9. (2R)-7-Benzyloxy-2-methyl-heptanoic acid methyl
ester 19. (2R)-7-benzyloxy-2-methyl-heptanoic acid
(1.49 g, 5.9 mmol) in ether (20 mL) was treated with freshly
prepared diazomethane (large excess) and the reaction was
monitored by TLC. When all of the acid had been
consumed, a flow of nitrogen was used to bubble the excess
CH2N2 into a solution of acetic acid. When this was
complete, the residue was diluted with ethyl acetate
(120 mL), washed with dilute acetic acid, water and brine,
dried and concentrated under reduced pressure. The residue
was subjected to flash chromatography on silica gel, using
ethyl acetate–hexane (1/5) as eluent, to provide the title
compound (1.56 g, 99%) as a clear oil. [a]2D0 K9.3 (c 0.8,
1
K13.3 (c 0.7, CHCl3); H NMR (CDCl3, 400 Hz) d ppm:
1.15 (d, JZ7.0 Hz, 3H), 1.37–1.49 (m, 3H), 1.51–1.56 (m,
2H), 1.63–1.70 (m, 1H), 1.94 (t, JZ2.6 Hz, 1H), 2.16–2.21
(m, 2H), 2.42–2.47 (m, 1H), 3.67 (s, 3H); 13C NMR (CDCl3,
100 Hz) d ppm: 17.0, 18.1, 26.2, 28.2, 33.1, 39.2, 51.4, 68.3,
84.2, 177.1; HR-ESIMS calcd for C10H17O2 [MCH]C
169.1229, found 169.1234.
1
CHCl3); H NMR (CDCl3, 400 Hz) d ppm: 1.15 (d, JZ
7.0 Hz, 3H), 1.28–1.45 (m, 5H), 1.59–1.68 (m, 3H), 2.42–
2.47 (m, 1H), 3.46 (t, JZ6.6 Hz, 2H), 3.66 (s, 3H), 4.50 (s,
2H), 7.27–7.35 (m, 5H); 13C NMR (CDCl3, 100 Hz) d ppm:
17.0, 26.0, 27.0, 29.5, 33.7, 39.3, 51.4, 70.2, 72.8, 127.4,
127.5, 128.3, 138.5, 177.2; HR-ESIMS calcd for
C16H24O3Na [MCNa]C 287.1623, found 287.1595.
4.3.13. (2R)-2-Methyl-oct-7-ynoic acid 3. To a solution of
21 (0.11 g, 0.64 mmol) in THF–water (2 mL/0.5 mL) at
0 8C was added LiOH$H2O (0.08 g, 1.92 mmol). The
reaction mixture was stirred at room temperature for 12 h
and then extracted with EtOAc (30 mL). The organic layer
was discarded. The aqueous solution was covered with
EtOAc (20 mL) and acidified to pH 2 with 5%KHSO4, the
layers were separated and the aqueous phase was further
extracted with EtOAc (2!20 mL). The combined organic
extracts were washed with brine (50 mL), dried (Na2SO4)
and concentrated to give the title compound (91 mg, 92%)
as a clear oil. [a]D20 K12.9 (c 0.9, CHCl3); 1H NMR (CDCl3,
400 Hz) d ppm: 1.18 (d, JZ7.0 Hz, 3H), 1.40–1.57 (m, 5H),
1.66–1.73 (m, 1H), 1.93 (t, JZ2.6 Hz, 1H), 2.17–2.21 (m,
2H), 2.42–2.50 (m, 1H), 10.81 (s, 1H); 13C NMR (CDCl3,
100 Hz) d ppm: 16.7, 18.2, 26.2, 28.2, 32.8, 39.2, 68.3, 84.2,
183.3; HR-ESIMS calcd for C9H15O2 [MCH]C 155.1072,
found 155.1091.
4.3.10. (2R)-7-Hydroxy-2-methyl-heptanoic acid methyl
ester. Compound 19 (1.56 g, 5.92 mmol) was dissolved in
methanol (50 mL), a catalytic amount of (10%) Pd/C was
added, and the reaction mixture was stirred vigorously
under a hydrogen atmosphere for 4 h. The reaction mixture
was filtered through a plug of celite and concentrated to give
the title compound (1.03 g, 99%) as a clear oil. [a]2D0 K15.2
(c 0.8, CHCl3);1H NMR (CDCl3, 400 Hz) d ppm: 1.15 (d,
JZ7.0 Hz, 3H), 1.26–1.47 (m, 5H), 1.52–1.59 (m, 2H),
1.62–1.71 (m, 1H), 2.42–2.48 (m, 1H), 2.79 (s, 1H), 3.60 (t,
JZ6.6 Hz, 2H), 3.67 (s, 3H); 13C NMR (CDCl3, 100 Hz) d
ppm: 16.4, 25.1, 26.4, 31.8, 33.1, 38.7, 50.8, 61.4, 176.7;
HR-ESIMS calcd for C9H19O3 [MCH]C 175.1334, found
175.1349.
4.3.11. (2R)-2-Methyl-7-oxo-heptanoic acid methyl ester
20. To a solution of (COCl)2 (0.9 mL, 10.5 mmol) in
dichloromethane (20 mL) at K78 8C was added DMSO
(1.49 mL, 21.0 mmol) in dichloromethane (5 mL). After
10 min, a pre-cooled solution of (2R)-7-hydroxy-2-methyl-
heptanoic acid methyl ester (0.91 g, 5.3 mmol) in dichloro-
methane (5 mL) was added dropwise. The reaction was kept
at K78 8C for 1 h before Et3N (5.9 mL, 42.1 mmol) was
added. The reaction mixture was warmed up to K60 8C
during 1 h, then quenched by slow addition of a saturated
aqueous solution of NH4Cl. The volatiles were removed
under vacuum and the remaining solution was extracted
with ethyl acetate (3!50 mL). The combined organic
extracts were washed with NaHCO3 (50 mL), brine
(50 mL), dried (Na2SO4) and concentrated. The residue
was purified by flash chromatography on silica gel, using
4.4. Completion of the total synthesis of 1
4.4.1. (S)-MeVal-(S)-MeVal-(S)-MeVal-(S)-MePhe-O-
tert-Bu. To a solution of 13 (1.28 g, 1.68 mmol) in DMF
(13 mL) at 0 8C was added solid LiOH$H2O (0.73 g,
16.8 mmol) followed by mercaptoacetic acid (239 mL,
3.36 mol). The reaction mixture was stirred at room
temperature for 2 h, quenched by pouring it into saturated
NH4Cl (8 mL) then extracted with benzene–ethyl acetate
(1/3, v/v) (3!10 mL). The combined organic extracts were
washed with saturated NaHCO3 (2!20 mL), brine (2!
20 mL), dried (Na2SO4) and concentrated to give the title
compound as a yellow oil, which was used in next step
without further purification.

H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
11139
4.4.2. (2R)-2-Methyl-oct-7-ynoyl-(S)-MeVal-(S)-MeVal-
(S)-MeVal-(S)-MePhe-O-tert-Bu 23. Acid moiety 3
(0.25 g, 1.63 mmol) and (S)-MeVal-(S)-MeVal-(S)-MeVal-
(S)-MePhe-O-tert-Bu (0.92 g, 1.6 mmol) were dissolved in
dichloromethane (20 mL). After the solution was cooled to
0 8C, BOPCl (0.84 g, 3.20 mmol) was added with stirring.
After 5 min, DIPEA (1.06 mL, 6.4 mmol) was introduced
via a syringe. The reaction was stirred at room temperature
for 16 h and then concentrated under reduced pressure. The
residue was dissolved in EtOAc (20 mL) and the solution
was successively washed with saturated NH4Cl (2!
20 mL), saturated NaHCO3 (2!20 mL), brine (2!
20 mL), dried (Na2SO4) and concentrated. The crude
residue was purified by flash chromatography on silica
gel, using ethyl acetate–hexane (1/2) as eluent, to provide
the desired compound (0.41 g, 36%) as a yellow oil. [a]D20
K294.8 (c 1.1, CHCl3); 1H NMR (CDCl3, 400 Hz) d ppm:
0.66 (d, JZ6.8 Hz, 3H), 0.71 (d, JZ6.7 Hz, 3H), 0.74 (d,
JZ6.4 Hz, 3H), 0.83 (d, JZ4.0 Hz, 3H), 0.84 (d, JZ
3.6 Hz, 3H), 0.90 (d, JZ6.3 Hz, 3H), 1.07 (d, JZ6.8 Hz,
3H), 1.36–1.42 (m, 3H), 1.46 (s, 9H), 1.49–1.54 (m, 2H),
1.72–1.76 (m, 1H), 1.92 (t, JZ2.6 Hz, 1H), 2.16–2.20 (m,
2H), 2.23–2.34 (m, 3H), 2.47 (s, 3H), 2.70–2.71 (m, 1H),
2.86 (s, 3H), 2.91 (dd, JZ11.8, 14.8 Hz, 1H), 2.98 (s, 3H),
3.00 (s, 3H), 3.36 (dd, JZ4.6, 14.8 Hz, 1H), 4.98 (d, JZ
10.7 Hz, 1H), 5.04 (d, JZ10.6 Hz, 1H), 5.20 (d, JZ
10.8 Hz, 1H), 5.46 (dd, JZ11.8, 4.6 Hz, 1H), 7.13–7.31 (m,
5H); 13C NMR (CDCl3, 100 Hz) d ppm:17.3, 17.5, 17.9,
18.2, 18.3, 19.3, 19.6, 19.7, 26.7, 26.9, 27.0, 27.0, 28.0,
28.4, 29.6, 30.1, 30.2, 31.7, 33.3, 34.2, 36.2, 57.9, 58.1 (3C),
68.3, 81.8, 84.3, 126.7, 128.4, 128.7, 137.1, 169.6, 169.6,
169.8, 170.9, 176.8; HR-ESIMS calcd for C41H67N4O6
[MCH]C 711.5061, found 711.5027.
(s, 3H), 2.99 (s, 3H), 3.02–3.08 (m, 1H), 3.26 (dd, JZ5.4,
15.1 Hz, 1H), 4.94 (d, JZ10.7 Hz, 1H), 5.09 (d, JZ
10.6 Hz, 1H), 5.19 (d, JZ10.7 Hz, 1H), 5.54 (s, 1H), 5.58
(dd, JZ11.2, 5.4 Hz, 1H), 6.02 (s, 1H), 7.15–7.32 (m, 5H);
13C NMR (CDCl3, 100 Hz) d ppm: 17.3, 17.5, 17.9 (2C),
18.2, 19.4, 19.6, 20.0, 26.7, 26.8, 26.9, 27.0, 28.4, 29.6,
30.2, 30.2, 30.6, 30.9, 33.3, 36.2, 56.3, 57.9, 58.0, 58.4,
68.3, 84.3, 126.9, 128.6, 128.7, 136.6, 169.7, 170.9, 171.2,
171.9, 176.8; HR-ESIMS calcd for C37H60N5O5 [MCH]C
654.4594, found 654.4580.
4.4.4. (2S)-2-Methyl-oct-7-ynoyl-(S)-MeVal-(S)-MeVal-
(S)-MeVal-(S)-MePhe-O-tert-Bu. [a]2D0 K282.6 (c 1.8,
1
CHCl3); H NMR (CDCl3, 400 Hz) d ppm: 0.66 (d, JZ
6.8 Hz, 3H), 0.72–0.75 (m, 6H), 0.81 (d, JZ7.0 Hz, 3H),
0.83 (d, JZ6.9 Hz, 3H), 0.90 (d, JZ6.4 Hz, 3H), 1.12 (d,
JZ6.7 Hz, 3H), 1.30–1.34 (m, 3H), 1.46 (s, 9H), 1.49–1.55
(m, 2H), 1.62–1.78 (m, 1H), 1.94 (t, JZ2.7 Hz, 1H), 2.14–
2.16 (m, 2H), 2.18–2.35 (m, 3H), 2.46 (s, 3H), 2.69–2.74
(m, 1H), 2.86 (s, 3H), 2.91 (dd, JZ11.8, 14.8 Hz, 1H), 2.97
(s, 3H), 2.98 (s, 3H), 3.36 (dd, JZ4.6, 14.8 Hz, 1H), 4.99 (d,
JZ10.7 Hz, 1H), 5.04 (d, JZ10.6 Hz, 1H), 5.20 (d, JZ
10.8 Hz, 1H), 5.46 (dd, JZ11.8, 4.6 Hz, 1H), 7.13–7.31 (m,
5H)
4.4.5. (2S)-2-Methyl-oct-7-ynoyl-(S)-MeVal-(S)-MeVal-
(S)-MeVal-(S)-MePhe-NH2. [a]2D0 K237.2 (c 2.4,
CHCl3); lit.6 [a]D K260.8 (c 2.6, CH2Cl2); 1H NMR
(CDCl3, 400 Hz) d ppm: 0.63 (d, JZ6.7 Hz, 3H), 0.67 (d,
JZ6.5 Hz, 3H), 0.68 (d, JZ6.8 Hz, 3H), 0.76 (d, JZ
6.7 Hz, 3H), 0.79 (d, JZ6.4 Hz, 3H), 0.85 (d, JZ6.4 Hz,
3H), 1.08 (d, JZ6.7 Hz, 3H), 1.25–1.33 (m, 3H), 1.42–1.51
(m, 2H), 1.56–1.78 (m, 1H), 1.89 (t, JZ2.5 Hz, 1H), 2.10–
2.14 (m, 2H), 2.16–2.32 (m, 3H), 2.40 (s, 3H), 2.63–2.70
(m, 1H), 2.90 (s, 3H), 2.92 (s, 3H), 2.94 (s, 3H), 2.95–2.98
(m, 1H), 3.22 (dd, JZ5.3, 15.1 Hz, 1H), 4.91 (d, JZ
10.7 Hz, 1H), 5.05 (d, JZ10.5 Hz, 1H), 5.15 (d, JZ
10.8 Hz, 1H), 5.32 (s, 1H) 5.54 (dd, JZ11.0, 5.3 Hz, 1H),
5.97 (s, 1H), 7.11–7.30 (m, 5H); 13C NMR (CDCl3, 100 Hz)
d ppm: 17.3, 17.6, 17.9 (2C), 18.2, 19.4, 19.6, 20.0, 26.7,
26.9, 27.0, 27.0, 28.3, 29.6, 30.1, 30.3, 30.6, 33.1, 33.5,
36.1, 56.2, 57.8 (2C), 58.4, 68.3, 84.1, 126.9, 128.5, 128.6,
136.5, 169.6, 170.7, 171.2, 171.7, 176.7
4.4.3. (2R)-2-Methyl-oct-7-ynoyl-(S)-MeVal-(S)-MeVal-
(S)-MeVal-(S)-MePhe-NH2 1. Compound 23 (135 mg,
0.19 mmol) in dichloromethane (16 mL) was treated with
TFA (7 mL) at 0 8C and the reaction was monitored by TLC.
After all the starting material was consumed the reaction
mixture was concentrated to leave a dark red oil, which was
subsequently dissolved in THF (15 mL) and cooled to 0 8C.
PyBOP (198 mg, 0.38 mmol) was added under a protective
flow of nitrogen and after 5 min DIPEA (94.2 mL,
0.57 mmol) was introduced via syringe. The reaction
mixture was then stirred for 20 min at 0 8C and 30 min at
room temperature. After cooling to 0 8C again, it was
exposed to NH3 for 25 min and then aqueous ammonia
(29 mL, 0.38 mmol) was added. Stirring was continued for
20 min at room temperature, and the reaction was quenched
with brine (10 mL). All volatiles were removed under
reduced pressure and the residue was extracted with EtOAc
(3!20 mL), the combined organic extracts were dried
(Na2SO4) and concentrated. The crude product was
subjected to chromatographic purification, using ethyl
acetate–hexane (2/1) as eluent, to give compound 1
(39 mg, 32%, over two steps) as a white solid. [a]D20
K639.6 (c 3.2, CHCl3); 1H NMR (CDCl3, 400 Hz) d ppm:
0.66 (d, JZ6.8 Hz, 3H), 0.71 (d, JZ6.6 Hz, 3H), 0.73 (d,
JZ6.6 Hz, 3H), 0.83 (d, JZ6.7 Hz, 3H), 0.84 (d, JZ
6.5 Hz, 3H), 0.88 (d, JZ6.4 Hz, 3H), 1.07 (d, JZ6.8 Hz,
3H), 1.32–1.40 (m, 3H), 1.49–1.55 (m, 2H), 1.74–1.76 (m,
1H), 1.92 (t, JZ2.6 Hz, 1H), 2.16–2.19 (m, 2H), 2.23–2.34
(m, 3H), 2.44 (s, 3H), 2.69–2.72 (m, 1H), 2.94 (s, 3H), 2.96
Acknowledgements
Support from The Shenzhen Graduate School of Peking
University and The Hong Kong Polytechnic University was
gratefully received.
References and notes
1. Burja, A. M.; Banaigs, B.; Abou-Mansour, E.; Burgess, J. G.;
Wright, P. C. Tetrahedron 2001, 57, 9347–9377.
2. Gerwick, W. H.; Tan, L. T.; Sitachitta, N. In Alkaloids, Vol.
57; Academic: San Diego, 2001; pp 75–184.
3. Fusetani, N. Drugs from the Sea; Karger: Basel, 2000.
4. Luesch, H.; Harrigan, G. G.; Goetz, G.; Horgen, F. D. Curr.
Med. Chem. 2002, 9, 1791–1806.

11140
H. Chen et al. / Tetrahedron 61 (2005) 11132–11140
5. Xu, Z.; Peng, Y.; Ye, T. Org. Lett. 2003, 5, 2821–2824.
6. Jimenez, J. I.; Scheuer, P. J. J. Nat. Prod. 2001, 64, 200–203.
7. Hu, J.; Miller, M. J. J. Am. Chem. Soc. 1997, 119, 3462–3468.
8. Bowman, W. R.; Coghlan, D. R. Tetrahedron 1997, 53,
15787–15798.
13. Wipf, P.; Kim, Y.; Fritch, P. C. J. Org. Chem. 1993, 58,
7195–7203.
14. Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem. Soc.
1982, 104, 1737–1739.
15. Evans, D. A.; Wu, L. D.; Wiener, J. J. M. J. Org. Chem. 1999,
64, 6411–6417.
`
9. Farras, J.; Ginesta, X.; Sutton, P. W. Tetrahedron 2001, 57,
7665–7674.
16. Evans, D. A.; Britton, T. C.; Ellman, J. A. Tetrahedron Lett.
1987, 28, 6141–6144.
10. Shin, I.; Park, K. Org. Lett. 2002, 4, 869–872.
11. McDougal, P. G.; Rico, J. G.; Oh, Y.-I.; Condon, B. D. J. Org.
Chem. 1986, 51, 3388–3390.
17. Ohira, S. Synth. Commun. 1989, 19, 561–564.
18. Mu¨ller, S.; Liepold, B.; Roth, G. J.; Bestmann, H. J. Synlett
1996, 521–522.
12. In the presence of 3 equiv of sodium carbonate, the benzyl
group on the terminal hydroxy group of compound 15 was
retained under a catalytic hydrogenation condition. See: Cook,
G. R.; Beholz, L. J.; Stille, J. R. J. Org. Chem. 1994, 59,
3575–3584.
19. McLaughlin, M.; Mohareb, R. M.; Rapoport, H. J. Org. Chem.
2003, 68, 50–54.
20. Parsons, R. L.; Heathcock, C. H. Tetrahedron Lett. 1994, 35,
1383–1384.