Synthesis of palau'amide and its diastereomers: confirmation of its stereostructure

Article abstract of DOI:10.1016/j.tetlet.2009.10.059

Four diastereomers of palau'amide (1-4), a cytotoxic cyclodepsipeptide, were synthesized. The ¹H NMR spectrum of 1 was identical to that of natural palau'amide. This established the complete stereostructure of palau'amide.

Full text of DOI:10.1016/j.tetlet.2009.10.059

Tetrahedron Letters 50 (2009) 7343–7345
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of palau’amide and its diastereomers: confirmation of its
stereostructure
*
Hirokazu Sugiyama, Atsushi Watanabe, Toshiaki Teruya , Kiyotake Suenaga
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama 223-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Four diastereomers of palau’amide (1–4), a cytotoxic cyclodepsipeptide, were synthesized. The 1H NMR
spectrum of 1 was identical to that of natural palau’amide. This established the complete stereostructure
of palau’amide.
Article history:
Received 8 September 2009
Revised 12 October 2009
Accepted 13 October 2009
Available online 31 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Palau’amide (1) is a 24-membered cyclodepsipeptide that was
isolated in 2003 by Moore and co-workers from a species of the
marine cyanobacterium Lyngbya collected at Palau. It has been
shown to exhibit potent cytotoxicity against KB cells with an IC50
value of 13 nM (Fig. 1).1 The absolute configurations of all but
one (C37) of the nine chiral centers were determined by NMR anal-
ered. To determine the stereochemistry of palau’amide, we began
by synthesizing 1. We describe here the synthesis of four possible
diastereomers of palau’amide with reference to C37 and C38.
ysis of 1 and the a-methoxyphenylacetic acid derivatives and chi-
ral HPLC analysis of the acid hydrolysate of 1. The stereochemistry
of C37 was proposed based on NMR analysis of 1, including NOE
data and theoretical calculations. In 2005, Dawei Ma and co-work-
ers achieved a total synthesis of the proposed structure of palau’a-
mide (1),2 however the NMR data of synthetic 1 were not identical
to those of natural 1. Recently, synthesis of the C33–C44 fragment
of palau’amide was reported by Mohapatra and Nayak.3 It is gener-
ally difficult to determine the stereochemistries of conformational-
ly flexible systems. Since the stereochemistry of C39 was clearly
Me
N
O
Me
HN
O
N
N
H
H
O
OCH2CCl3
O
O
Fmoc
1
2
3
4
(37S, 38R)
(37R, 38R)
(37S, 38S)
(37R, 38S)
N
Me
CO2H
OH
5
6
determined by NMR analysis of the
a-methoxyphenylacetic acid
esters,4 the stereochemistries of C37 and C38 should be reconsid-
OTBS
W
X
38
HO2C
37
Y
Z
Me
N
O
7a
7b
7c
7d
W = H, X = OTES, Y = Me, Z = H
W = OTES, X = H, Y = Me, Z = H
W = H, X = OTES, Y = H, Z = Me
W = OTES, X = H, Y = H, Z = Me
Me
HN
O
N
N
H
H
O
N
O
O
O
Me
OH
1
O
O
28
38
44
33
37
O
OTES
O
OTBS
1 (37S, 38R)
Figure 1. Structure of palau’amide.
MeO
H
Y
Z
9a Y = Me, Z = H
9c
8
* Corresponding author. Tel./fax: +81 45 566 1819.
E-mail address: suenaga@chem.keio.ac.jp (K. Suenaga).
Y = H, Z = Me
Present address: Faculty of Education, University of the Ryukyus, 1 Senbaru,
Nishihara, Okinawa 903-0213, Japan.
Scheme 1. Retrosynthesis of palau’amide and its diastereomers with regard to C37
and C38.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.059

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H. Sugiyama et al. / Tetrahedron Letters 50 (2009) 7343–7345
A retrosynthetic analysis of palau’amide and its diastereomers
Inversion of the stereochemistry of a hydroxyl group of 13, silyla-
tion of a hydroxyl group, and hydrolysis of a methyl ester afforded
carboxylic acid 7a (see Supplementary data). On the other hand,
manipulation of the protecting groups provided carboxylic acid
7b. The stereochemistry of the newly formed hydroxyl group at
C37 was determined based on the 13C chemical shifts of the aceto-
nide methyls (dC 30.1 and 19.6 ppm)8 of the derived acetonide 17a.
The synthesis of carboxylic acids 7c and 7d began with an Evans
aldol reaction9 between imide 14 and 5-hexynal to afford hydroxy
imide 15 as a single diastereomer (Scheme 3). Transamidation and
protection of a hydroxyl group of 15 followed by reduction of the
amide provided aldehyde 9c. The vinylogous Mukaiyama aldol
reaction between 9c and 8 gave alcohols 16c and 16d (dr = 8:1),
which were transformed into 7c and 7d, respectively, in the same
manner as described above. The stereochemistry of C37 in 7b, 7c,
and 7d was determined by 13C NMR analysis of the derived aceto-
nides, respectively.
(1–4) is shown in Scheme 1. A key step in the synthesis of palau’a-
mide is closure of the 24-membered ring. We planned to construct
the cyclic structure by macrolactamization at the N-methylala-
nine-isoleucine site. The precursor of macrocyclization was syn-
thesized from pentapeptide 5, N-Fmoc-N-Me-L-Ala (6), and
protected carboxylic acids (7a–d). Carboxylic acids (7a–d) could
be synthesized by a vinylogous Mukaiyama aldol reaction5 be-
tween 2-methyl-1-triethylsiloxy-1-methoxy-1,3-butadiene (8)6
and aldehydes 9a and 9b.
The synthesis of pentapeptide 5 was carried out in a stepwise
manner starting from N-Boc-N-methylglycine benzyl ester, and 5
was obtained in 64% overall yield (Scheme 2).
The synthesis of protected carboxylic acids 7a and 7b began
with a Roush crotylboration reaction7 between boronate 10 and
6-trimethylsilyl-5-hexynal to afford homoallylic alcohol 12 as a
single diastereomer (Scheme 3). Silylation of a hydroxyl group of
12, removal of a TMS group, re-silylation of a partially deprotected
hydroxyl group, and oxidative cleavage of an olefin moiety pro-
vided aldehyde 9a. The vinylogous Mukaiyama aldol reaction be-
The condensation of carboxylic acids 7a, 7b, 7c, and 7d with
pentapeptide 5 followed by selective desilylation afforded alcohols
18a, 18b, 18c, and 18d, respectively (Scheme 4). Esterification of
tween 9a and
8
gave alcohol 13 as
a
single diastereomer.
18a, 18b, 18c, and 18d with N-Fmoc-N-Me-L-Ala (6), removal of
Me
N
O
g, h
a, b
Ph
c, d
e, f
5
Boc
OBn
CO2Na
CO2H
NHBoc
Me
OH
N
Boc
CO2H
O
HCl•H2N
H
OCH2CCl3
Scheme 2. Synthesis of pentapeptide 5. Reagents and conditions: (a) TFA, CH2Cl2, 0 °C, 1.5 h; (b) DEPC Et3N, DMF, rt, 16 h, 94% in two steps; (c) TFA, CH2Cl2, 0 °C, 1.5 h; (d)
DEPC Et3N, DMF, rt, 17 h, 94% in two steps; (e) TFA, CH2Cl2, 0 °C, 1.5 h; (f) EDCIꢀHCl, HOBt, Et3N, DMF, rt, 17 h, 75% in two steps; (g) H2, 5% Pd–C, EtOH, rt, 5.5 h; (h) EDCIꢀHCl,
HOBt, Et3N, DMF, rt, 12.5 h, 96% in two steps.
O
B
MeO2C
MeO2C
O
OH OTBS
OH
TMS
b - f
a
g
10
MeO2C
9a
37
+
TMS
13
12
OHC
j, k
h - k
7a
11
7b
O
O
O
O
OH
OTBS
W X
O
N
m- o
g
MeO2C
l
37
O
N
9c
Bn
14
Bn
16c W = H, X = OH
16d W = OH, X = H
15
+
j, k
j, k
7d
OHC
7c
O
O
MeO2C
37
17a
Scheme 3. Synthesis of protected carboxylic acids. Reagents and conditions: (a) MS 4 Å, toluene, ꢁ78 °C, 3 h, quant.; (b) TBSCl, imidazole, DMF, rt, 3 h, 94%; (c) Bu4NF, THF, rt,
1 h; (d) TBSCl, imidazole, DMF, rt, 2.5 h, 100% in two steps; (e) OsO4, NMO, acetone–H2O, rt, 75 min; (f) NaIO4, acetone–H2O, rt, 1.5 h, 56% in two steps (recovered diol 28%); (g)
8, BF3ꢀEt2O, CH2Cl2–Et2O, –40 °C, 4.5 h; then aq HCl, (13) 80%, (16c) 81% based on recovered starting material (br sm), (16d) 10% br sm; (h) Dess–Martin periodinane, CH2Cl2,
rt, 1 h, quant; (i) NaBH4, MeOH, ꢁ78 °C, 1.5 h, 83%; (j) LiOH, H2O, MeOH, 40 °C, 17 h; (k) TESCl, imidazole, DMF, rt, 4.5 h; (7a) 90% in two steps, (7b), 72% in two steps, (7c) 83%
in two steps, (7d) 41% in two steps; (l) Bu2BOTf, Et3N, CH2Cl2, ꢁ78 °C, 2 h?rt, 1 h, quant; (m) Me2AlN(Me)OMe, THF, toluene, ꢁ10 °C, 4 h, 79%; (n) TBSOTf, 2,6-lutidine, CH2Cl2,
0 °C, 4 h, 99%; (o) DIBAL, THF, hexane, ꢁ78 °C, 3 h, 91%.

H. Sugiyama et al. / Tetrahedron Letters 50 (2009) 7343–7345
7345
Me
N
O
6
Me
HN
O
N
N
H
H
1 (37S, 38R) (14% in 5 steps)
2 (37R, 38R) (15% in 5 steps)
3 (37S, 38S) (13% in 5 steps)
4 (37R, 38S) (12% in 5 steps)
7a
7b
7c
7d
O
O
OCH2CCl3
a, b
c - g
O
O
+
5
OTBS
W
X
38
37
O
Y
Z
18a W = H, X = OH, Y = Me, Z = H (59% in 2 steps, brsm)
18b W = OH, X = H, Y = Me, Z = H (61% in 2 steps, brsm)
18c W = H, X = OH, Y = H, Z = Me (63% in 2 steps)
18d W = OH, X = H, Y = H, Z = Me (79% in 2 steps, brsm)
Scheme 4. Synthesis of palau’amide and its diastereomers. Reagents and conditions: (a) EDCIꢀHCl, DMAP, CH2Cl2, rt, 16 h; (b) AcOH, H2O, THF, rt, 6 h; (c) Fmoc-N-Me-
L-Ala
(6), 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, toluene, rt, 16 h; (d) Zn, NH4OAc, THF, rt, 3 h; (e) Et2NH, MeCN, rt 3 h; (f) EDCIꢀHCl, HOAt, Et3N, DMF–CH2Cl2 (1:10, 1 mM), rt,
45 h; (g) HFꢀpyridine, pyridine, rt, 2 h.
9. Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127.
the 2,2,2-trichloroethyl and Fmoc-protecting groups, and macro-
10. Compound 1 (synthetic): ½a D28
ꢂ
ꢁ16 (c 0.22, MeOH); 1H NMR (500 MHz, CDCl3) d
lactamization provided proposed palau’aminde and its diastereo-
mers (1–4)10 after desilylation, respectively. Among the 1H NMR
spectra of the four synthetic diastereomers, 1, 2, 3, and 4, that of
1 was identical to that of natural palau’amide, except for exchange-
able protons (see Supplementary data). Furthermore, the 13C NMR
spectrum of synthetic 19 was identical to that of natural palau’a-
mide (see Supplementary data). Based on these findings, the com-
plete stereostructure of palau’amide was determined, as shown in
formula 1.
In summary, the synthesis of four possible diastereomers of pa-
lau’amide with regard to C37 and C38 was achieved. Among the 1H
NMR spectra of the four synthetic diastereomers (1–4), that of 1
was identical to that of natural palau’amide. This established the
complete stereostructure of palau’amide.
8.05 (d, J = 10.1 Hz, 0.5H), 7.81 (d, J = 9.6 Hz, 0.5H), 7.32–7.27 (m, 1.5H), 7.22–
7.18 (m, 2H), 7.15 (t, J = 6.9 Hz, 0.5H), 7.14 (d, J = 7.1 Hz, 1H), 7.10 (d, J = 9.6 Hz,
0.5H), 6.84 (dd, J = 6.6, 6.2 Hz, 0.5H), 6.55 (br d, J = 10.2 Hz, 0.5H), 6.21 (d,
J = 8.6 Hz, 0.5H), 5.53 (dd, J = 8.4, 3.7 Hz, 0.5H), 5.45 (dd, J = 10.8, 5.4 Hz, 0.5H),
5.27 (dt, J = 11.8, 3.3 Hz, 0.5H), 5.23 (dd, J = 10.4, 6.0 Hz, 0.5H), 5.15 (dd, J = 9.6,
4.3 Hz, 0.5H), 5.09 (dq, J = 10.1, 6.6 Hz, 0.5H), 4.95 (d, J = 17.2 Hz, 0.5H), 4.86 (t,
J = 9.7 Hz, 0.5H), 4.82 (m, 0.5H), 4.68 (dd, J = 9.9, 4.0 Hz, 0.5H), 4.60 (dq, J = 8.6,
7.1 Hz, 0.5H), 4.50 (q, J = 6.9 Hz, 0.5H), 4.06 (d, J = 18.1 Hz, 0.5H), 3.61 (m, 0.5H),
3.59 (q, J = 7.1 Hz, 0.5H), 3.52 (m, 0.5H), 3.34 (s, 1.5H), 3.20 (m, 0.5H), 3.20 (d,
J = 18.1 Hz, 0.5H), 3.20 (s, 1.5H), 3.12 (dd, J = 14.9, 10.8 Hz, 0.5H), 3.11–3.07 (m,
1.0H), 3.06–3.00 (m, 1H), 2.97 (s, 1.5H), 2.96 (s, 1.5H), 2.88 (m, 0.5H), 2.60 (s,
1.5H), 2.51 (s, 1.5H), 2.40 (m, 0.5H), 2.26–2.21 (m, 2H), 2.20 (m, 0.5H), 1.98 (m,
0.5H), 1.95 (t, J = 2.5 Hz, 1H), 1.97–1.91 (m, 1.0H), 1.88 (br s, 1.5H), 1.87 (m,
0.5H), 1.84 (m, 0.5H), 1.81 (m, 0.5H), 1.78 (m, 0.5H), 1.75 (m, 1.0H), 1.74 (br s,
1.5H), 1.74–1.68 (m, 1.5H), 1.65–1.60 (m, 1.5H), 1.57 (m, 0.5H), 1.51 (d,
J = 7.1 Hz, 1.5H), 1.49–1.41 (m, 2.5H), 1.39 (d, J = 6.9 Hz, 1.5H), 1.25 (m, 0.5H),
1.20 (d, J = 6.6 Hz, 1.5H), 0.983 (t, J = 7.5 Hz, 1.5H), 0.975 (d, J = 7.0 Hz, 1.5H),
0.94 (d, J = 6.6 Hz, 3H), 0.91 (d, J = 6.5 Hz, 1.5H), 0.903 (d, J = 6.5 Hz, 1.5H),
0.898 (d, J = 6.7 Hz, 3.0H), 0.86 (t, J = 7.4 Hz, 1.5H), 0.83 (d, J = 7.0 Hz, 1.5H),
0.76 (d, J = 7.1 Hz, 1.5H); 13C NMR (125 MHz, CDCl3) (rotamer 1) d 173.0, 171.3,
171.2, 170.1, 169.6, 168.6, 168.4, 139.8, 135.9, 129.0, 128.8, 128.4, 127.4, 84.1,
75.6, 73.4, 71.7, 68.9, 60.1, 53.7, 52.2, 51.12, 44.5, 42.1, 40.7, 38.2, 37.2, 36.6,
34.9, 33.9, 29.9, 28.5, 24.7, 24.45, 24.42, 23.1, 21.7, 18.0, 16.2, 15.0, 13.7, 12.5,
10.8, 10.4; (rotamer 2) d 173.3, 172.9, 170.6, 169.7, 169.5, 168.2, 166.5, 138.9,
136.8, 130.6, 129.5, 128.0, 126.4, 84.4, 76.7, 72.2, 71.8, 68.7, 55.7, 54.6, 53.2,
51.08, 44.9, 42.4, 40.9, 38.5, 35.8, 35.5, 31.6, 31.5, 30.9, 27.7, 24.49, 24.13,
23.39, 23.1, 21.8, 18.3, 16.54, 16.50, 14.2, 13.5, 12.3, 12.3; HRESIMS m/z calcd
for C46H69O10N5 (M+H)+ 852.5123, found 852.5065.
Acknowledgments
We thank Professor Philip Williams (University of Hawaii) for
providing NMR data of natural palau’amide. This work was sup-
ported by a Grant-in-Aid for Scientific Research and a Grant for Pri-
vate Education Institute Aid from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and Keio Gijuku
Academic Development Funds.
Compound 2: 1H NMR (400 MHz, CDCl3) (major rotamer) d 7.27–7.17 (m, 5H),
6.94 (tq, J = 6.4, 1.2 Hz, 1H), 5.57 (q, J = 6.8 Hz, 1H) 5.36 (m, 1H), 5.05 (dd,
J = 3.4, 4.0 Hz, 1H), 4.93 (dd, J = 5.2, 6.4 Hz, 1H), 4.78 (d, J = 17.6 Hz, 1H), 4.20
(dt, J = 5.0, 5.6 Hz, 1H), 3.77 (q, J = 6.0 Hz, 1H), 3.63–3.44 (m, 2H), 3.23 (s, 3H),
3.10 (s, 3H), 3.00 (m, 2H), 2.94 (s, 3H), 2.39 (dd, J = 5.6, 6.4 Hz, 2H), 2.20 (dt,
J = 2.4, 6.8 Hz, 2H), 2.17 (m, 1H), 1.92 (t, J = 2.4 Hz, 1H), 1.85 (d, J = 1.2 Hz, 3H),
1.66–1.19 (m, 10H), 1.46 (d, J = 6.8 Hz, 3H), 1.37 (d, J = 6.8 Hz, 3H), 1.24 (d,
J = 7.0 Hz, 3H), 1.16 (d, J = 6.0 Hz, 3H), 0.95 (t, J = 5.4 Hz, 3H), 0.94–0.85 (m, 6H).
Signals due to three protons (NH ꢃ 2, OH) were not observed. The ratio of
rotamers was ca. 13:3:1.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.10.059.
Compound 3: 1H NMR (400 MHz, CDCl3) (major rotamer) d 8.05 (d, J = 9.8 Hz,
1H), 7.27–7.12 (m, 6H), 6.46 (d, J = 9.8 Hz, 1H), 5.54 (dd, J = 3.2, 4.0 Hz, 1H),
5.20 (q, J = 6.4 Hz, 1H), 5.09 (d, J = 5.2 Hz, 1H), 4.96 (d, J = 17.2 Hz, 1H), 4.65 (m,
1H), 4.41 (q, J = 7.6 Hz, 1H), 4.20 (dd, J = 6.0, 6.0 Hz, 1H), 3.58 (m, 1H), 3.23–
3.12 (m, 2H), 3.19 (s, 3H), 3.06 (d, J = 17.2 Hz, 1H), 2.57 (s, 3H), 2.50 (s, 3H), 2.33
(dd, J = 4.0, 4.0 Hz, 2H), 2.23–2.21 (m, 2H), 1.99 (m, 1H), 1.86 (t, J = 4.0 Hz, 1H),
1.72 (s, 3H), 1.66–1.06 (m, 10H), 1.39 (d, J = 6.8 Hz, 3H), 1.17 (d, J = 6.4 Hz, 3H),
0.94 (d, J = 6.4 Hz, 3H), 0.90–0.74 (m, 12H). A signal due to OH was not
observed. The ratio of rotamers was ca. 13:1.
References and notes
1. Williams, P. G.; Yoshida, W. Y.; Quon, M. K.; Moore, R. E.; Paul, V. J. J. Nat. Prod.
2003, 66, 1545.
2. Zou, B.; Long, K.; Ma, D. Org. Lett. 2005, 7, 4237.
3. Mohapatra, D. K.; Nayak, S. Tetrahedron Lett. 2008, 49, 786.
4. Seco, J. M.; Latypov, S. K.; Quiñoá, E.; Riguera, R. J. Org. Chem. 1997, 62, 7569.
5. (a) Mukaiyama, T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc. 1974, 96, 7503; (b)
Paterson, I.; Smith, J. D.; Ward, R. A. Tetrahedron 1995, 51, 9413.
6. Suenaga, K.; Mutou, T.; Shibata, T.; Itoh, T.; Fujita, T.; Takada, N.; Hayamizu, K.;
Takagi, M.; Irifune, T.; Kigoshi, H.; Yamada, K. Tetrahedron 2004, 60, 8509.
7. Roush, W. R.; Ando, K.; Powers, D. B.; Palkowitz, A. D.; Halterman, R. L. J. Am.
Chem. Soc. 1990, 112, 6339.
Compound 4: 1H NMR (400 MHz, CDCl3) (major rotamer) d 7.50–7.16 (m, 5H),
6.54 (t, J = 7.6 Hz, 1H), 5.33 (m, 2H), 5.10 (dd, J = 3.4, 6.8 Hz, 1H), 4.94 (m, 1H),
4.20 (m, 1H), 4.12 (dd, J = 7.6, 7.6 Hz, 1H), 3.76–3.47 (m, 3H), 3.22 (s, 3H), 3.20–
2.79 (m, 2H), 3.09 (s, 3H), 2.61 (s, 3H), 2.33 (dd, J = 7.6, 7.6 Hz, 2H), 2.21 (dt,
J = 4.0, 8.0 Hz, 2H), 2.05–1.94 (m, 2H), 1.78 (s, 3H), 1.66–1.07 (m, 13H), 1.37 (d,
J = 7.6 Hz, 3H), 0.99–0.78 (m, 15H). Signals due to three protons (NH ꢃ 2, OH)
were not observed. The ratio of rotamers was ca. 15:1.
8. (a) Rychnovsky, S. D.; Skalitzky, D. J. Tetrahedron Lett. 1990, 31, 945; (b) Evans,
D. A.; Rieger, D. L. Tetrahedron Lett. 1990, 31, 7099; (c) Rychnovsky, S. D.;
Rogers, B.; Yang, G. J. Org. Chem. 1993, 58, 3511.