Synthesis and structure revision of tyroscherin, a growth inhibitor of IGF-1-dependent tumor cells

Article abstract of DOI:10.1016/j.tetlet.2008.09.141

Synthesis of the proposed structure of tyroscherin, a growth inhibitor of IGF-1-dependent cancer cells, was succeeded by one-pot Julia coupling. However, spectral data of the synthetic compound were not identical with those of natural tyroscherin. The stereochemistry of tyroscherin was revised to be 2S,3R,8R,10R by syntheses of stereoisomers.

Full text of DOI:10.1016/j.tetlet.2008.09.141

Tetrahedron Letters 49 (2008) 7042–7045
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and structure revision of tyroscherin, a growth inhibitor
of IGF-1-dependent tumor cells
b
Ryo Katsuta a, Chié Shibata a, Ken Ishigami a, Hidenori Watanabe a, , Takeshi Kitahara
*
a Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
b School of Pharmacy, Teikyo Heisei University, 2289 Uruido, Ichihara City, Chiba 290-0193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of the proposed structure of tyroscherin, a growth inhibitor of IGF-1-dependent cancer cells,
was succeeded by one-pot Julia coupling. However, spectral data of the synthetic compound were not
identical with those of natural tyroscherin. The stereochemistry of tyroscherin was revised to be
2S,3R,8R,10R by syntheses of stereoisomers.
Received 5 September 2008
Revised 20 September 2008
Accepted 25 September 2008
Available online 30 September 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Tyroscherin
Inhibitor of IGF-1-dependent growth
Structure revision
One-pot Julia coupling
Recently, mechanism-based drugs have been remarkably no-
ticed, since they will potentially provide selective treatments for
various diseases such as infections and cancers. Insulin-like growth
factor (IGF) plays a key role in human cancer progression,1 and
selective inhibitors of its signal transduction are thought to provide
a selective treatment against IGF-dependent tumor cells. In 2004,
Hayakawa et al. isolated tyroscherin from the mycelium of Pseud-
allescheria sp. as a potent and selective inhibitor of IGF-1-depen-
dent growth of MCF-7 human breast cancer cell.2 We started the
synthesis of tyroscherin with the intention of further research on
its biological activity and structure–activity relationship.
Herein, we report the synthesis of the proposed structure of
tyroscherin (1). However, spectral data of synthetic 1 were not
identical with those of natural tyroscherin. The stereochemistry
of tyroscherin is revised to be 2S,3R,8R,10R, as shown in Figure 1,
by syntheses of stereoisomers.
ded amino ketone 4. This ketone was subjected to stereoselective
reduction3 to give syn-amino alcohol 5, whose stereochemistry
was confirmed by NOE experiment after conversion into cyclic car-
bamate 11. After protection and deprotection, syn-amino alcohol 5
was converted to the PT-sulfone 7 under Mitsunobu condition.4
Then one-pot Julia coupling5,6 of sulfone 7 and known aldehyde
87 gave (E)-olefin 9 selectively. Though N-methylation of the com-
pound 9 did not proceed directly, it was succeeded in good yield
after replacement of the protecting group. Finally, deprotection
of 10 gave desired compound, the proposed structure of tyroscher-
in (1). However, the 1H NMR spectral data of the synthetic com-
pound 18 were not identical with those reported for natural
tyroscherin.2 Chemical shifts of 1-H, 2-H, and 3-H were much
different between natural tyroscherin and synthetic 1 as shown
in Table 1. Hayakawa et al. have determined the relative configura-
tion at C-2 and C-3 by analysis of 1H–1H and 1H–13C coupling con-
stants,2,9 after determination of the absolute configuration at C-3
by modified Mosher method.10 We supposed the correct relative
stereochemistry of natural compound to be 2,3-anti. To ascertain
the correct structure of natural tyroscherin, we started synthesis
of 2,3-anti-stereoisomers of 1.
The synthesis of the proposed structure (1) is shown in Scheme
1.
D-Tyrosine was protected in a usual manner to give ester 3. C3
elongation of ester 3 via the corresponding Weinreb amide affor-
HO
HO
HNMe
HNMe
Synthesis of 2,3-anti-isomers is shown in Scheme 2. Ester 12,
derived from L-tyrosine, was subjected to N-methylation using
3
R
S
2
8
10
R
R
OH
Proposed structure of tyroscherin (1)
OH
NaH and was subsequently converted to Weinreb amide 13. During
this sequence, partial racemization was observed, and enantio-
meric purity of 13 was determined to be 33% ee by chiral HPLC
(Chiralcel OD, hex/i-PrOH = 19:1). This compound was reduced to
aldehyde, which was reacted with siloxypropyllithium to afford a
mixture of anti- and syn-amino alcohols. After protection of
Revised structure of tyroscherin (2)
Figure 1. Proposed and revised structures of tyroscherin.
* Corresponding author. Tel.: +81 3 5841 5119; fax: +81 3 5841 8019.
E-mail address: ashuten@mail.ecc.u-tokyo.ac.jp (H. Watanabe).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.141

R. Katsuta et al. / Tetrahedron Letters 49 (2008) 7042–7045
7043
MOMO
MOMO
MOMO
a, b, c
d, e
f
g, h, i
NBn2
CO2Me
NBn2
NBn2
-Tyrosine
D
OTBS
m
OTBS
OH
O
3
4
5
MOMO
Boc
MOMO
Boc
MOMO
Boc
HN
HN
HN
j, k, l
OTBS
SO2PT
OTBS
OTBS
Ph
PT =
OTBS
9
6
7
N
N
OHC
N
N
8
O
HO
MOMO
Boc
HNMe
MeN
MOMO
n, o, p
q
HN
O
OTBS
OH
Proposed structure of tyroscherin (1)
H H
NOE
OEE
10
11
Scheme 1. Synthesis of the proposed structure of tyroscherin (1). Reagents and conditions: (a) SOCl2, MeOH, reflux, quant.; (b) BnBr, DIPEA, DMF, 0 °C to rt, 97%; (c) MOMCl,
K2CO3, CH3CN, 0 °C to rt, 95%; (d) MeNHOMeꢀHCl, i-PrMgBr, THF, ꢁ20 °C to rt, 93%; (e) TBSO(CH2)3I, t-BuLi, ether, ꢁ78 °C to rt, 86%; (f) NaBH4, MeOH, EtOH, ꢁ20 °C, 99%, single
isomer; (g) TBSOTf, 2,6-lutidine, CH2Cl2, 0 °C to rt, 78%; (h) H2, Pd–C, EtOH, EtOAc, rt, 92%; (i) (Boc)2O, THF, rt, 94%; (j) Dowex-50, MeOH, rt, 71% (and 25% of diol); (k) PTSH,
DEAD, PPh3, THF, 0 °C to rt, 99%; (l) H2O2, (NH4)6Mo7O24ꢀ4H2O, EtOH, 0 °C to rt, 85%; (m) KHMDS, THF, ꢁ78 °C to rt, 70% (based on recovery); (n) Dowex-50, MeOH, rt, 96%; (o)
CH2@CHOEt, PPTS, CH2Cl2, rt, quant.; (p) NaH, MeI, THF, reflux, quant.; (q) HCl, MeOH, rt, 99%.
Table 1
Selected 1H NMR (500 MHz, CDCl3) data and specific rotations for natural tyroscherin and synthetic stereoisomers
1H NMR (ppm, multiplicity, Hz)
½aꢂD (c O.35, MeOH)
1-Ha
1-Hb
2-H
3-H
12-H
8-Me
10-Me
Natural tyroscherin1
2.86
dd, 14.0, 7.0
2.91
dd, 14.0, 7.0
3.34
ddd, 7.0, 7.0, 3.0
3.83
dt, 10.0, 3.0
0.84
t, 7.0
0.91
d, 6.5
0.82
d, 6.5
ꢁ21
Proposed structure (1)
2.87
2.96
3.22
3.62
0.85
0.89
0.81
+11
dd, 13.8, 8.3
dd, 13.8,6.0
ddd, 8.3,6.0, 4.7
ddd, 6.8, 5.3,4.7
t, 7.0
d, 6.2
d, 6.5
Mixture of anti-isomers
(Scheme 2) Major (16)
Minor (17)
2.86
dd, 14.7, 7.9
2.86
2.93
dd, 14.7, 6.7
2.91
3.32–3.38
m, overlapped
3.32–3.38
3.81–3.87
m, overlapped
3.81–3.87
0.84
t, 7.3
0.84
t, 7.3
0.92
d, 6.8
0.91
0.81
d, 6.5
0.82
—
—
dd, 14.7, 7.9
dd, 14.7,7.0
m, overlapped
m, overlapped
d, 6.8
d, 6.5
(2R,3S,8S,10S)-isomer (17)
2.86
2.91
3.34
3.83
0.84
0.91
0.82
+20
dd, 14.7, 7.9
dd, 14.7, 7.0
ddd, 7.9, 7.0, 3.0
ddd, 9.4, 3.6, 3.0
t, 7.3
d, 6.8
d, 6.5
Boc
Boc
MOMO
MOMO
Boc
Boc
MOMO
f, g, h
MOMO
d, e
a, b, c
HN
MeN
MeN
MeN
+
L-Tyrosine
i, j, k
N
CO2Me
O
3OTBS
3OTBS
O
TBSO
TBSO
14b
12
13
14a
(33% e.e.)
MOMO
Boc
HO
HO
+
l, m
HNMe
OH
HNMe
MeN
SO2PT
MOMO
OH
(2R,3S)-isomer (17)
OTBS
15
(2S,3R)-isomer (16)
2 : 1
O
O
MOMO
MeN
O
MeN
O
OTBS
OTBS
H H
H H
NOE
18a
18b
NOE
Scheme 2. Synthesis of mixture of (2S,3R)-isomer (16) and (2R,3S)-isomer (17). Reagents and conditions: (a) SOCl2, MeOH, reflux; (b) NaOH, H2O, then (Boc)2O, 0 °C, THF; (c)
MOMCl, DIPEA, CH2Cl2, 0 °C to rt, 99% in three steps; (d) NaH, MeI, THF, reflux; (e) MeNHOMeꢀHCl, i-PrMgBr, THF, ꢁ20 °C to rt, 83% in two steps; (f) DIBAL, Et2O, 0 °C, 89%; (g)
TBSO(CH2)3I, t-BuLi, Et2O, ꢁ78 °C to rt, 79%; (h) TBSOTf, 2,6-lutidine, CH2Cl2, 0 °C to rt, 96%, dr = 7:1; (i) Dowex-50, MeOH, H2O, rt, 97%; (j) DEAD, PPh3, PTSH, THF, 0 °C to rt; (k)
H2O2, (NH4)6Mo7O24ꢀ4H2O, EtOH, 0 °C to rt, 62% in 2 steps; (l) KHMDS, 8, THF, ꢁ78 °C to rt, 39%; (m) TFA, THF, MeOH, H2O, 50 °C, quant.
hydroxy group, anti- and syn-isomers were separated by silica gel
chromatography (14a:14b = 7:1). The stereochemistries at C-3
were determined by NOE experiments after conversion to cyclic
carbamates 18a and 18b, respectively. In a similar manner to the
synthesis of 1, the anti-isomer 14a was converted to inseparable
mixture of (2S,3R)-isomer (16) and (2R,3S)-isomer (17) (16:17 =
2:1), which came from low enantiomeric purity of 13 (33% ee).
1H NMR spectrum of the mixture (16 and 17) showed that signals
of the major component were not identical with those of natural
tyroscherin, while signals of the minor component were observed
at quite similar chemical shifts to those of natural tyroscherin2
(Fig. 2 and Table 1).
Thus, we were confident that the minor component 17 had the
same relative configuration as natural tyroscherin. Hayakawa et al.

7044
R. Katsuta et al. / Tetrahedron Letters 49 (2008) 7042–7045
1-Ha (dd) 1-Hb (dd)
8-Me (d)
10-Me (d)
12-H (t)
Natural Tyroscherin
Mixture of anti-isomers
[16 (major) + 17 (minor)]
(2R,3S,8S,10S)-isomer (17)
Figure 2. 1H NMR signals of natural and synthesized compounds (1-H and Me groups).
The stereoselective synthesis of (2S,3R,8R,10R)-isomer (2) is
MOMO
Boc
MOMO
Boc
shown in Scheme 4. Ester 12 was converted to Weinreb amide,
and N-methylation was followed by LAH reduction to give alde-
hyde ent-19. Enantiomeric purity of ent-19 was determined to be
>99% ee by chiral HPLC. Reaction of the aldehyde with sil-
oxypropyllithium in THF was followed by protection as TBS ether
to give the anti-isomer 14a (14a:14b = 13:1, separated by silica
gel chromatography). The anti-isomer 14a was converted to the
corresponding sulfone 15, and one-pot Julia coupling of 15 and
ent-8 afforded trans-olefin 20 selectively. Finally, deprotection of
20 gave 2 successfully as colorless crystals. Its specific rotation
a, b, c
HN
MeN
CO2Me
CHO
19
ent-12
(97% e.e.)
Similarly
HO
HNMe
30% in 7 steps
OH
17
Scheme 3. Synthesis of (2R,3S,8S,10S)-isomer (17). Reagents and conditions: (a)
MeNHOMe ꢀ HCl, i-PrMgBr, THF, ꢁ20 °C to rt, 82%; (b) NaH, MeI, DMF, ꢁ20 °C, 76%;
(c) DIBAL, ether, 0 °C, 97% ee.
was ½a 2D5
ꢁ21 (c 0.35, MeOH), and its melting point and spectro-
ꢂ
scopic data of 215 were fully identical to those of natural tyrosch-
erin.2 From all of these results, the stereochemistry of natural
tyroscherin is determined to be 2S,3R,8R,10R as shown in Figure 1.
In summary, we succeeded in the first synthesis of tyroscherin
have determined the absolute configuration at C-3 by modified
Mosher method, as already mentioned, and the absolute configura-
tions at C-8 and C-10 by degradation studies.2 Because several fun-
gal metabolites, such as squalestatin S1,11 TMC-171,12 and lunatoic
acids,13 have been reported to have similar (S,S)-dimethylalkyl
chain, we assumed that tyroscherin has the same stereochemist-
ries at C-8 and C-10 as these compounds. So, we started the stere-
(and its stereoisomers), in 14.4% overall yield from L-tyrosine,
and revised the absolute configuration of tyroscherin to be
2S,3R,8R,10R. Our work is under way to improve some steps of
the synthesis, and to submit these isomers to further biological
assay. Results will be reported in a full account.
oselective synthesis of 17 (Scheme 3). Ester ent-12, derived from D-
Acknowledgments
tyrosine, was converted to the corresponding Weinreb amide and
was subjected to subsequent N-methylation.14 Consequently, ent-
13 was obtained with almost no racemization. Reduction of the
amide gave aldehyde 19, whose enantiomeric purity was deter-
mined to be 97% ee by chiral HPLC (Chiralpak AD–H, hex/i-
PrOH = 19:1). In the same manner as Scheme 2, the aldehyde 19
was transformed into 17. After recrystallization, (2R,3S,8S,10S)-iso-
mer (17) was obtained in a diastereomerically pure form. 1H NMR
spectrum of 17 was completely identical with that of natural tyro-
scherin as we expected (Fig. 2 and Table 1), but unfortunately, 17
showed opposite sign of specific rotation to that of natural tyrosch-
The authors sincerely thank Professor Y. Hayakawa, Tokyo Uni-
versity of Science, for a kind gift of the spectral charts of natural
tyroscherin. This work was partly supported by a Grant-in-Aid
for Scientific Research from the Japanese Ministry of Education,
Culture, Sports, Science and Technology.
References and notes
1. Baserga, R.; Hongo, A.; Rubini, M.; Prisco, M.; Valentinis, B. Biochem. Biophys.
Acta 1997, 1332, F105–F126.
2. Hayakawa, Y.; Yamashita, T.; Mori, T.; Nagai, K.; Shin-ya, K.; Watanabe, H. J.
Antibiot. 2004, 57, 634–638.
3. Reetz, M. T.; Drewes, M. W.; Lennick, K.; Schmitz, A.; Holdgrün, X. Tetrahedron:
Asymmetry 1990, 1, 375–378.
erin {17: ½a 2D4
ꢂ
+20 (c 0.35, MeOH), natural tyroscherin: ½a D24
ꢁ21 (c
ꢂ
0.35, MeOH)2}. From these results, we finally concluded that the
correct stereostructure of natural tyroscherin must be 2, an enan-
tiomer of 17.
MOMO
Boc
MOMO
Boc
MOMO
Boc
OH
a, b, c
d, e
f, g, h
HN
MeN
MeN
CO2Me
CHO
OTBS
12
ent-19
(>99% e.e.)
14a
Boc
MOMO
Boc
MOMO
HO
j
i
MeN
MeN
HNMe
OH
SO2PT
OHC
OTBS
OTBS
15
20
ent-8
Revised structure of tyroscherin (2)
Scheme 4. Synthesis of (2S,3R,8R,10R)-isomer (2). Reagents and conditions: (a) MeNHOMeꢀHCl, i-PrMgBr, THF, ꢁ20 °C to rt, 73%; (b) NaH, MeI, DMF, ꢁ20 °C, 95%; (c) LiAlH4,
ether, 0 °C, >99% ee; (d) I(CH2)3OTBS, t-BuLi, THF, ꢁ78 °C; (e) TBSOTf, 2,6-lutidine, CH2Cl2, 0 °C to rt, 59% in three steps, dr = 13:1; (f) Dowex-50, MeOH, H2O, rt, 82%; (g) PTSH,
DEAD, PPh3, THF, 0 °C to rt; (h) (NH4)6Mo7O24ꢀ4H2O, H2O2, EtOH, 0 °C to rt, 62% in two steps; (i) KHMDS, THF, ꢁ78 °C to rt, 70%; (j) TFA, THF, MeOH, H2O, 50 °C, quant.

R. Katsuta et al. / Tetrahedron Letters 49 (2008) 7042–7045
7045
4. Mitsunobu, O.; Yamada, M.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1967, 40, 935–
939.
11. Sidebottom, P. J.; Highcock, R. M.; Lane, S. J.; Procopiou, P. A.; Watson, N. S. J.
Antibiot. 1992, 45, 648–658.
5. Blakemore, P. R.; Cole, W. J.; Kocien´ ski, P. J.; Morley, A. Synlett 1998, 26–28.
6. Blakemore, P. R. J. Chem. Soc., Perkin Trans. 1 2002, 2563–2585.
7. Organ, M. G.; Bilokin, Y. V.; Bratovanov, S. J. Org. Chem. 2002, 67, 5176–5183.
12. Kohno, J.; Asai, Y.; Nishino, M.; Sakurai, M.; Kawano, K.; Hiramatsu, H.;
Kameda, M.; Kishi, N.; Okuda, T.; Komatsubara, S. J. Antibiot. 1999, 52, 1114–
1123.
8. Analytical and spectral data of synthesized 1: n2D51:4714.
½
a 2D5
ꢂ
+11 (c 0.35,
13. Nukina, M. T.; Marumo, S. Tetrahedron Lett. 1977, 18, 2603–2606.
14. Li, M.; Zhou, P.; Roth, H. F. Synthesis 2007, 55–60.
CH3OH). 1H NMR (500 MHz, CD3OD) d (ppm) 0.81 (3H, d, J = 6.5 Hz, 10-Me),
0.85 (3H, t, J = 7.0 Hz, 12-H), 0.89 (3H, d, J = 6.2 Hz, 8-Me), 0.97 (1H, ddd,
J = 13.0, 8.6, 4.2 Hz, 9-Ha), 1.13 (1H, m, 11-Ha), 1.2-1.35 (2H, m, 10-H, 11-Hb),
1.21 (1H, ddd, J = 13.0, 9.4, 4.7 Hz, 9-Hb), 1.5-1.6 (2H, m, 4-H), 1.95–2.15 (3H, m,
5-H, 8-H), 2.67 (3H, s, N-Me), 2.87 (1H, dd, J = 13.8, 8.3 Hz, 1-Ha), 2.96 (1H, dd,
J = 13.8, 6.0 Hz, 1-Hb), 3.22 (1H, ddd, J = 8.3, 6.0, 4.7 Hz, 2-H), 3.63 (1H, ddd,
J = 6.8, 5.3, 4.7 Hz, 3-H), 5.16 (1H, dd, J = 15.1, 8.1 Hz, 7-H), 5.27 (1H, dt, J = 15.1,
6.5 Hz, 6-H), 6.77 (2H, quasi d, J = 8.1 Hz, 30-H, 50-H), 7.11 (2H, quasi d,
J = 8.1 Hz, 20-H, 60-H). 13C NMR (125 MHz, CD3OD) d (ppm) 11.7, 19.3, 22.3,
29.3, 31.1, 31.8, 33.1, 34.1, 35.4, 35.7, 45.5, 65.9, 68.7, 116.8, 127.6, 128.3,
15. Analytical and spectral data of synthesized 2: mp 122–126 °C. ½a D25
ꢁ21 (c 0.35,
ꢂ
CH3OH). 1H NMR (500 MHz, CD3OD) d (ppm) 0.82 (3H, d, J = 6.5 Hz, 10-Me),
0.84 (3H, t, J = 7.3 Hz, 12-H), 0.91 (3H, d, J = 6.8 Hz, 8-Me), 0.99 (1H, ddd,
J = 13.5, 9.7, 4.8 Hz, 9-Ha), 1.13 (1H, m, 11-Ha), 1.22 (1H, ddd, J = 13.5, 9.7,
4.8 Hz, 9-Hb), 1.25–1.35 (2H, m, 10-H, 11-Hb), 1.45–1.6 (2H, m, 4-H), 1.99 (1H,
m, 5-Ha), 2.1–2.25 (2H, m, 5-Hb, 8-H), 2.62 (3H, s, N-Me), 2.86 (1H, dd, J = 14.7,
7.9 Hz, 1-Ha), 2.91 (1H, dd, J = 14.7, 7.0 Hz, 1-Hb), 3.34 (1H, ddd, J = 7.9, 7.0,
3.0 Hz, 2-H), 3.83 (1H, ddd, J = 9.4, 3.6, 3.0 Hz, 3-H), 5.22 (1H, dd, J = 15.5,
8.3 Hz, 7-H), 5.33 (1H, dt, J = 15.5, 6.7 Hz, 6-H), 6.78 (2H, quasi d, J = 8.5 Hz, 30-
H, 50-H), 7.10 (2H, quasi d, J = 8.5 Hz, 20-H, 60-H). 13C NMR (125 MHz, CD3OD) d
(ppm) 11.7, 19.4, 22.3, 29.9, 31.1, 32.4, 33.1, 33.2, 35.8, 45.6, 66.8, 68.7, 116.9,
131.4, 138.6, 157.9. IR (KBr)
m
= 3382, 2958, 1681, 1517, 1202 cmꢁ1. ESI-TOFMS
m/z calcd for C21H36NO2 [M+H]+ 334.2741, found 334.2773.
9. Morvai, M.; Nagy, T.; Kocsis, Á.; Szabó, L. F.; Podányi, B. Magn. Reson. Chem.
2000, 38, 343–359.
127.6, 128.4, 131.3, 138.8, 158.0. IR (KBr)
m
= 3239, 2961, 1671, 1203, 1185,
1146 cmꢁ1
334.2773.
.
ESI-TOFMS m/z calcd for C21H36NO2 [M+H]+ 334.2741, found
10. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092–4096.