Synthesis of an Oxazoline Analogue of Apratoxin A

Article abstract of DOI:10.1021/ol035332y

Equation presented. Michael addition of Me₂Cu(CN)Li₂ to α,β-unsaturated lactone 7 derived from β-hydroxyl ketone 5 provides lactone 8, which is converted to alcohol 11 using Oppolzer's methodology as the key step. Connection of 11 wi

Full text of DOI:10.1021/ol035332y

ORGANIC
LETTERS
2003
Vol. 5, No. 19
3503-3506
Synthesis of an Oxazoline Analogue of
Apratoxin A
Bin Zou, Jingjun Wei, Guorong Cai, and Dawei Ma*
State Key Laboratory of Bioorganic and Natural Products Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
madw@pub.sioc.ac.cn
Received July 18, 2003
ABSTRACT
Michael addition of Me2Cu(CN)Li2 to r,â-unsaturated lactone 7 derived from â-hydroxyl ketone 5 provides lactone 8, which is converted to
alcohol 11 using Oppolzer’s methodology as the key step. Connection of 11 with the L-proline moiety and subsequent installation of an
oxazoline ring affords 16, which is coupled with tripeptide 21; subsequent macrocyclization then furnishes 4, an oxazoline analogue of apratoxin
A.
Apratoxin A (1, Figure 1) is a cyclodepsipeptide bearing both
peptide and polyketide moieties isolated from the marine
cyanobacterium Lyngbya majuscula by Moore and co-
workers.1 One year later, two other analogues, apratoxins B
and C (2 and 3, Figure 1) were discovered by further
organism collections and isolations.2 Through in vitro studies
on cytotoxicity against several human tumor cell lines,
apratoxin A was found to have IC50 values ranging from
0.36 to 0.52 nM. However, in vivo antitumor investigation
indicated that 1 was poorly tolerated in mice mainly due to
lack of selectivity for different cell lines.1 Structure-activity
relationship (SAR) studies on this compound might provide
a cure for this problem. In fact, the different cytotoxicity
pattern displayed by 1-3 has already demonstrated that their
cytotoxicity is highly dependent on their primary structures.
It was reported that 3 possesses almost identical IC50 values
for in vitro cytotoxicity in comparison with 1, but 2 is
significantly less cytotoxic than 1.2 To initiate SAR studies
we decided to explore an efficient synthesis of apratoxin A
and its analogues. Recently, Forsyth and Chen disclosed an
(1) Leusch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J.; Corbett, T.
H. J. Am. Chem. Soc. 2001, 123, 5418-5423.
(2) Leusch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. Bioorg. Med.
Chem. Lett. 2002, 10, 1973-1978.
Figure 1. Structures of apratoxins A-C and an oxazoline analogue
of apratoxin A.
10.1021/ol035332y CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/27/2003

with NaBH4 and the resultant alcohol was subjected to
elimination through its mesylate to afford allyl ether 6a. It
is notable that we initially attempted asymmetric allylboration
of trimethylacetaldehyde according to Brown’s procedure in
order to obtain allyl alcohol 6b.8 However, an inconvenient
workup operation (it was found that the desired product was
difficult to separate from the resultant isopinocampheol by
either distillation or column chromatography) made us give
up this direct synthesis. Next, ozonolysis of 6a provided an
aldehyde, which was condensed with LiCH2CO2Et at -78
°C followed by cyclization and elimination to give the R,â-
unsaturated lactone 7. Methylcupration with Me2Cu(CN)-
Li2 served as an excellent method9 for installing the C37
methyl group in a highly diastereoselective fashion to provide
Scheme 1a
1
8 in 94% yield as a single diastereomer as detected by H
NMR. After LAH reduction of the lactone 8 to produce a
diol and then protection of it with acetic chloride, selective
deprotection with K2CO3 was carried out in methanol.
Subsequent Dess-Martin oxidation of the resulting primary
alcohol delivered aldehyde 9.
For the assembly of the C33-C35 unit in the target
molecule, Oppolzer’s methodology10 for preparing anti-diols
from bornanesultam-derived boryl enolates appeared to be
particularly attractive. Toward this end, treatment of N-
propionylsultam with Et2BOTf resulted in a (Z)-enolate,
which was reacted with the aldehyde 9 under the action of
TiCl4 to afford “anti”-aldol 10 in 90% yield as the only
detectable diastereomer. It is noteworthy that 8 equiv of TiCl4
was necessary in this case to ensure the high yield. Finally,
removal of both the chiral auxiliary and the acyl group by
LAH reduction gave a triol, which was protected with DMP
to furnish 11 in 67% yield.
With the alcohol 11 in hand, our next task was to connect
it with the L-proline unit on the right side followed by
subsequent installation of an oxazoline ring bearing an R,â-
unsaturated ester side chain on the left side. As outlined in
Scheme 2, esterification of 11 with N-Fmoc-L-proline using
Yamaguchi’s procedure11 worked well to produce 12 in 90%
yield. Liberation of the diol moiety in 12 with TsOH in
methanol followed by selective oxidation of the primary
alcohol using hindered chloro oxammonium salt generated
from TEMPO/NaClO afforded an aldehyde, which was
further oxidized with NaClO2 to provide acid 13.12 In a
parallel procedure, R,â-unsaturated ester 14, prepared by a
Wittig olefination reaction of (R)-Garner aldehyde and the
corresponding ylide, was treated with 3:1 trifluoroacetic acid
and water to give liberated amine. Coupling of this amine
with the acid 13 mediated with HATU and DIPEA (diiso-
propylethylamine) furnished 15 in 90% yield. The oxazoline
a Reagents and conditions: (i) TBSCl/imidazole, DMF, rt; (ii)
NaBH4, MeOH, 0 °C; (iii) MsCl/Et3N, CH2Cl2, rt; (iv) t-BuOK,
toluene, reflux; (v) O3/Me2S; (vi) LiCH2CO2Et, THF, -78 °C; (vii)
40% HF, MeCN, rt; (viii) MsCl/Et3N, CH2Cl2, 0 °C to rt; (ix)
Me2(CuCN)Li2, ether, -78 °C; (x) LAH, THF, reflux; (xi) AcCl/
Py, CH2Cl2, 0 °C, then K2CO3, MeOH; (xii) Dess-Martin oxida-
tion; (xiii) N-propionylsultam/Et2BOTf/DIPEA, CH2Cl2, -15 °C,
then 9, TiCl4, -78 °C; (xiv) LAH, THF, reflux; (xv) DMP/PPTs.
elegant total synthesis of apratoxin A3 based on their newly
developed methodology for preparing thiazoline,4 which
prompted us to report here our result on the synthesis of 4,
an oxazoline analogue of apratoxin A. Obviously, this
compound may still have potent cytotoxicity activity as
apratoxin A, but its total synthesis may be simpler than that
of 1 because there exist more convenient methods to form
oxazolines in comparison with thiazolines.5,6
Structurally, 4 contains a novel 3,7-dihydroxy-2,5,8,8-
tetramethylnonanoic acid (Dtena) unit, which connects with
a modified serine moiety (moSer) by an oxazoline ring, and
a tetrapeptide unit possessing a high degree of methylation
at the L-proline site. We planned to synthesize 4 in a
convergent manner as illustrated in Figure 1, that is, peptide
formation between the O-Me-Tyr and moSer sites and
macrocyclization at the N-Me-IIe-Pro site.
The synthesis of enantiopure 3,7-dihydroxy-2,5,8,8-tet-
ramethylnonanoic acid (Dtena) precursor 11 is outlined in
Scheme 1. â-Hydroxyl ketone 5 (>99% ee), an aldol reaction
product of trimethylacetaldehyde with acetone catalyzed by
D-proline,7 was employed as the starting material. After
protection of 5 with TBSCl, the ketone moiety was reduced
(3) Chen, J.; Forsyth, C. J. J. Am. Chem. Soc. 2003, 125, 8734-8735.
(4) Chen, J.; Forsyth, C. J. Org. Lett. 2003, 5, 1281-1283.
(5) For reviews, see: (a) Wipf, P. Chem. ReV. 1995, 95, 2115-2134.
(b) Wipf, P.; Venkatraman S. Synlett 1997, 1-10.
(6) For recent leading references on the preparation of thiazoline and
oxazoline rings, see: (a) McKeever, B.; Pattenden, G. Tetrahedron 2003,
59, 2701-2712. (b) Yokokawa, F.; Sameshima, H.; Katagiri, D.; Aoyama,
T.; Shioiri, T. Tetrahedron 2002, 58, 9445-9458. (c) Kedrowski, B. L.;
Heathcock, C. H. Heterocycles 2002, 58, 601-634. (d) Boden, C. D. J.;
Norley, M.; Pattenden, G. J. Chem. Soc., Perkin Trans. 1 2000, 883-888.
(e) Boden, C. D. J.; Norley, M.; Pattenden, G. J. Chem. Soc., Perkin Trans.
1 2000, 875-882.
(7) (a) List, B. Synlett 2001, 1675-1685. (b) List, B.; Pojarliev, P.;
Castello, C. Org. Lett. 2001, 3, 573-575.
(8) (a) Racherla, U. S.; Brown, H. C. J. Org. Chem. 1991, 56, 401-
401. (b) Jadhav, P. K.; Bhat, K. S.; Perumal, P. T.; Brown, H. C. J. Org.
Chem. 1986, 51, 432-439.
(9) Pirkle, W. H.; Adams, P. E. J. Org. Chem. 1980, 45, 4117-4121.
(10) Oppolzer, W.; Lienard, P. Tetrahedron Lett. 1993, 34, 4321-4324.
(11) (a) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989-1993. (b) Hikota, M.; Sakurai, Y.;
Horita, K.; Yonemitsu, O. Tetrahedron Lett. 1990, 31, 6367-6370.
(12) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. J. Org. Chem. 1987,
52, 2559-2562.
3504
Org. Lett., Vol. 5, No. 19, 2003

Scheme 2a
Scheme 3a
a Reagents and conditions: (i) 2,4,6-trichlorobezoyl chloride,
DIPEA, benzene then 11, DMAP, rt; (ii) TsOH, MeOH, rt. (iii)
TEMPO, NaClO, aq NaHCO3, CH2Cl2, 0 °C; (iv) NaH2PO4/
NaClO2, t-BuOH, H2O, 2-methylbutene; (v) 14/TFA/H2O, then 13/
HATU/DIPEA, CH2Cl2, rt. (vi) DAST, CH2Cl2, -78 °C; (vii)
Pd(PPh3)4, N-methylaniline, THF, rt.
ring formation was achieved through exposure of 15 to 2.5
equiv of DAST in methylene chloride at -78 °C.13 Subse-
quent cleavage of allyl ester catalyzed by Pd(PPh3)4 under
the assistance of N-methylaniline resulted in acid 16 in 70%
yield over two steps.14 It is notable that the utilization of
N-methylaniline as the base was essential for this step
because other bases employed such as morpholine were
found to cause the simultaneous cleavage of the NR-Fmoc
protecting group during the above deprotection reaction.
For the synthesis of the highly methylated tripeptide part,
2-bromo-1-ethyl pyridinium tetrafluoroborate (BEP), an
efficient coupling reagent for hindered peptide synthesis
developed by Xu and Li, was used for peptide formation.15
As shown in Scheme 3, deprotection of 17 with diethylamine
in acetonitrile followed by amide formation with N-Fmoc-
O-methyl-L-tyrosine 18 under the assistance of BEP gave a
dipeptide, which was subjected to Pd/C-catalyzed hydro-
genolysis to afford acid 19 in 55% yield over three steps.
Cleavage of the Fmoc group in 20 and subsequent connection
of the liberated amine with 19 mediated with BEP resulted
in tripeptide 21. Next, treatment of 21 with diethylamine in
acetonitrile produced a free amine, which was coupled with
a Reagents and conditions: (i) Et2NH, MeCN, rt; (ii) 18/BEP/
DIPEA, CH2Cl2, rt; (iii) Pd/C/H2, EtOAc; (iv) 20/Et2NH, MeCN,
then 19/BEP/DIPEA, CH2Cl2, rt; (v) Et2NH, MeCN, then 16/
HATU/DIPEA; (vi) TBAF, THF; (vii) HATU/DIPEA, CH2Cl2
(0.002 M), rt.
the acid 16 to afford 22 in 59% yield. The stage was now
set for the crucial macrocyclization. We were gratified to
observe that after removal of the Fmoc and TMSE protecting
groups with TBAF in one pot, the target molecule 416 was
obtained in 45% yield by treatment the resultant amino acid
with HATU/DIPEA in a diluted methylene chloride solution.
1
(16) Selected data for 4: [R]20 -117.5 (c 0.2, CHCl3); H NMR (500
D
MHz, CDCl3) δ 7.15 (d, J ) 8.7 Hz, 2H), 6.80 (d, J ) 8.2 Hz, 2H), 6.21
(d, J ) 9.3 Hz, 1H), 6.01 (d, J ) 9.3 Hz 1H), 5.25 (d, J ) 11.5 Hz, 1H),
5.05 (td, J ) 10.2, 5.6 Hz, 1H), 4.97 (d, J ) 11.6 Hz, 1H), 4.81 (m, 1H),
4.70 (d, J ) 10.6 Hz, 1H), 4.38 (m, 1H), 4.20 (m, 1H), 3.78 (s, 3H), 3.70-
3.55 (m, 3H), 3.28 (br q, J ) 6.7 Hz, 1H), 3.15 (d, J ) 3.8 Hz, 1H), 3.10
(d, J ) 11.7 Hz, 1H), 2.85 (dd, J ) 12.7, 4.6 Hz, 1H), 2.81 (s, 3H), 2.75
(s, 3H), 2.65 (d, J ) 13.5 Hz, 1H), 2.33 (m, 1H), 2.25 (m, 1H), 2.15 (m,
1H), 2.05 (m, 1H), 1.92 (s, 3H), 1.90-1.75 (m, 3H), 1.45 (m, 1H), 1.31
(m, 1H), 1.26 (m, 1H), 1.25 (s, 3H), 1.10 (m, 1H), 1.07 (d, J ) 6.9 Hz,
3H), 1.01 (t, J ) 7.0 Hz, 3H), 0.99 (d, J ) 6.4 Hz, 3H), 0.96 (m, 1H), 0.95
(d, J ) 7.6 Hz, 3H), 0.87 (s, 9H); ESI-MS m/z 824 (M + H)+; HRMS
calcd for C45H70N5O9 (M + H)+ 824.5168, found 824.5160.
(13) Lafargue, P.; Guenot, P.; Lellouche, J.-P. Heterocycles 1995, 41,
947-957.
(14) Ciommer, M.; Kunz, H. Synlett 1991, 593-595.
(15) Li, P.; Xu, J.-C. Tetrahedron 2000, 56, 8119-8131.
Org. Lett., Vol. 5, No. 19, 2003
3505

In conclusion, we have described here an efficient route
to synthesize an oxazoline analogue of apratoxin A. Notable
elements include a highly diastereoselective assembly of
the Dtena moiety and subsequent installation of the oxa-
zoline ring bearing an R,â-unsaturated ester side chain. In
addition, the success in macrocyclization at the N-Me-IIe-
Pro site demonstrates again that this site is a suitable macro-
lactamization site for synthesizing apratoxins and their
analogues.3 Extension of these investigations to the total
synthesis of apratoxin A and other analogues, as well as the
biological evaluation of the synthesized compounds, is being
actively pursued in our laboratory and will be reported in
due course.
Acknowledgment. The authors are grateful to the Chinese
Academy of Sciences, National Natural Science Foundation
of China (Grant 20242003), and Science and Technology
Commission of Shanghai Municipality (Grant 02JC14032)
for their financial support. We thank Professor Tao Ye of
The Hong Kong Polytechnic University for helpful discussion
and for providing information about their results in this field.
Supporting Information Available: Experimental pro-
cedures and characterizations for compounds 6a, 7-13, 15,
16, 19, 21, 22, and 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL035332Y
3506
Org. Lett., Vol. 5, No. 19, 2003