A concise and stereoselective synthesis of squalamine

Article abstract of DOI:10.1021/ol035062j

(Matrix presented) A short and highly stereoselective synthesis of the novel steroid squalamine (1) was accomplished in nine steps from easily available methyl chenodeoxylcholanate 2. Our synthesis featured improved dehydrogenation of 4 followed by conjugate reduction to construct the trans AB-ring system and efficient asymmetric isopropylation of aldehyde 6 to introduce the C-24R-hydroxyl group.

Full text of DOI:10.1021/ol035062j

ORGANIC
LETTERS
2003
Vol. 5, No. 18
3257-3259
A Concise and Stereoselective
Synthesis of Squalamine
,†
Dong-Hui Zhang,† Feng Cai,† Xiang-Dong Zhou,‡ and Wei-Shan Zhou*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China, and Department of Chemistry,
The Third Military Medical UniVersity, Chongqing 400038, China
zhws@pub.sioc.ac.cn
Received June 12, 2003
ABSTRACT
A short and highly stereoselective synthesis of the novel steroid squalamine (1) was accomplished in nine steps from easily available methyl
chenodeoxylcholanate 2. Our synthesis featured improved dehydrogenation of 4 followed by conjugate reduction to construct the trans AB-
ring system and efficient asymmetric isopropylation of aldehyde 6 to introduce the C-24R-hydroxyl group.
Squalamine (Figure 1) is the first member of a class of natural
aminosterols isolated by Zasloff and co-workers from the
as a new chemotherapeutic approach in the treatment of late
stage lung cancer and ovarian cancer.3 The novel structural
features and remarkable antitumor activity have provided the
impetus for several synthesis studies. Two lengthy syntheses
(17 steps) of an equal mixture of C24-stereoisomers were
reported, starting from 3â-acetoxy-5-cholenic acid4 and 3â-
hydroxy-5-cholenic acid, respectively.5 Stereoselective syn-
theses of squalamine were also achieved from stigmasterol6
or methyl 3-keto-5R-chenodeoxy cholanate.7 All these
syntheses required many steps and were impractical for the
(2) Sills, A. K., Jr.; Williams, J. I.; Tyler, B. M.; Epstein, D. S.; Sipos,
E. P.; Davis, J. D.; McLane, M. P.; Pitchford, S.; Cheshire, K.; Gannon, F.
H.; Kinney, W. A.; Chao, T. L.; Donowitz, M.; Laterra, J.; Zasloff, M.;
Brem, H. Cancer Res. 1998, 58, 2784-2792.
Figure 1. Squalamine 1.
(3) (a) Williams, J. I.; Weitman, S.; Gonzalez, C. M.; Jundt, C. H.; Marty,
J.; Stringer, S. D.; Holroyd, K. J.; McLane, M. P.; Chen, Q.; Zasloff, M.;
Von Hoff, D. D. Clin. Cancer Res. 2001, 7, 724-733. (b) Bhargava, P.;
Marshall, J. L.; Dahut, W.; Rizvi, N.; Trocky, N.; Williams, J. I.; Hait, H.;
Song, S.; Holroyd, K. J.; Hawkins, M. J. Clin. Cancer Res. 2001, 7, 3912-
3919. (c) Li, D.; Williams, J. I.; Pietras, R. J. Oncogene 2002, 21, 2805-
2814.
(4) Moriarty, R. M.; Tuladhar, S. M.; Guo, L.; Wehrli, S. Tetrahedron
Lett. 1994, 35, 8103-8106.
(5) Pechulis, A. D.; Bellevue, F. H., III; Cioffi, C. L.; Trapp, S. G.; Fojtik,
J. P.; McKitty, A. A.; Kinney, W. A.; Frye, L. L. J. Org. Chem. 1995, 60,
5121-5126.
stomach of the dogfish shark squalus acanthias in 1993.1
This important compound has been shown to possess
antiangiogenic and antitumor activity2 and was developed
† Chinese Academy of Sciences.
‡ The Third Military Medical University.
(1) (a) Moore, K. S.; Wehrli, S.; Roder, H.; Rogers, M.; Forrest, J. N.,
Jr.; McCrimmon, D.; Zasloff, M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90,
1354-1358. (b) Stone, R. Science 1993, 259, 1125.
10.1021/ol035062j CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/15/2003

large-scale synthesis of squalamine. To provide adequate
supplies for clinical trials, the pursuit of a more efficient
synthesis of squalamine became necessary. Recently, great
advance on the synthesis of squalamine has been achieved,
which reduced the number of steps from 16 to 11 by utilizing
biotransformation methodology to introduce the 7R-hydroxyl
group.8 In connection with our previous work, herein, we
wish to describe a new concise route to squalamine starting
from easily available material, methyl chenodeoxylcholanate
2, with efficient construction of the trans AB-ring system
and highly setereoselective introduction of the C-24R-
hydroxyl group. This concise synthesis shortened the route
to nine steps and, to our best knowledge, is the shortest
synthetic route to squalamine so far described.
Scheme 1. Short Synthesis of Squalamine 1 from Methyl
Chenodeoxylcholanate 2a
As depicted in Scheme 1, our synthesis commenced with
methyl chenodeoxycholanate 2, which is easily available in
China. Ester 2 was a very appealing starting material for
the synthesis of squalamine because it contained the requisite
7R-hydroxyl groups. Thus, 2 was selectively oxidized to the
7R-hydroxy-3-one 3 in 92% yield, using freshly prepared
silver carbonate on Celite according to the literature.9
Compound 3 was stirred with chloromethyl methyl ether and
N,N-diisopropylethylamine in methylene chloride to afford
7R-methoxy-methyl ether 4 in 91% yield.
The next step is the dehydrogenation of 4. Recently,
Nicoloau et al.10 have reported a new method for the
synthesis of R,â-unsaturated carbonyl compounds utilizing
the cheap and nontoxic IBX (o-iodoxybenzoic acid) as the
oxidizing reagent. This IBX-based method is superior to the
conventional protocols relying on highly toxic selenium
reagents. Unfortunately, complex products were formed when
4 was exposed to 4.0 equiv of IBX at 85 °C in DMSO
according to the standard conditions10 (Table 1, entry 1).
Careful analysis of the NMR spectrum of these products
showed that the acid sensitive protecting group of methoxyl
methyl in substrate 4 could not survive these conditions.
When the reaction temperature was decreased to room
temperature, none of the desired product was obtained
accompanied by complete recovering of substrate 4. Surpris-
ingly, the methoxyl methyl ether of 4 was stable under this
condition. Thus, we tried to effect this reaction at room
temperature.11 Because it has been reported10 that addition
of a catalytic amount of p-TsOH tended to significantly
a Reagents and conditions: (a) Ag2CO3 on Celite, toluene, reflux,
92%; (b) MOMCl, iPr2NEt, cat. NaI, CH2Cl2, reflux, 91%; (c) 2.0
equiv of IBX, 30 mol % of TFA, DMSO, rt, 24 h, 87%; (d) Li,
ammonia, THF, -78 °C for 1 h then quenching with anhydrous
NH4Cl, 73%; (e) 20 mol % of ligand 9, 2.2 equiv of iPr2Zn, toluene,
0 °C, 4 h, 84% yield, 99% de; (f) PPTS, tBuOH, reflux, 96%.
(6) (a) Moriarty, R. M.; Enache, L. A.; Kinney, W. A.; Allen, C. S.;
Canary, J. W.; Tuladhar, S. M.; Guo, L. Tetrahedron Lett. 1995, 36, 5139-
5142. (b) Zhang, X.; Rao, M. N.; Jones, S. R.; Shao, B.; Feibush, P.;
Mcguigan, M.; Tzodikov, N.; Feibush, B.; Sharkansky, I.; Snyder, B.; Mallis,
L. M.; Sarkahian, A.; Wilder, S.; Turse, J. E.; Kinney, W. A.; Tham, F. S.
J. Org. Chem. 1998, 63, 8599-8603.
(7) Zhou, X.-D.; Cai, F.; Zhou, W.-S. Tetrahedron 2002, 58, 10293-
10299.
accelerate the reaction at 85 °C, we surmised the reaction
might be triggered at room temperature by addition of
p-TsOH. Indeed, in the presence of 0.3 equiv of p-TsOH,
substrate 4 was smoothly oxidized to the desired R,â-
unsaturated carbonyl compound 5 in 72% isolated yield by
using 2.0 equiv of IBX at room temperature (entry 3).
Furthermore, it was found that by using TFA (trifluoroacetic
acid) instead of p-TsOH, the isolated yield of the product
was increased to 87% (entry 4).
(8) Kinney, W. A.; Zhang, X.; Williams, J. I.; Johnston, S.; Michalak,
R. S.; Deshpande, M.; Dostal, L.; Rosazza, J. P. N. Org. Lett. 2000, 2,
2921-2922.
(9) Tohma, M.; Mahara, R.; Takeshita, H.; Kurosawa, T.; Ikegawa, S.;
Nittono, H. Chem. Pharm. Bull. 1985, 33, 3071-3073.
(10) (a) Nicolaou, K. C.; Zhong, Y.-L.; Baran, P. S. J. Am. Chem. Soc.
2000, 122, 7596-7597. (b) Nicolaou, K. C.; Montagnon, T.; Baran, P. S.;
Zhong, Y. L. J. Am. Chem. Soc. 2002, 124, 2245-2258.
(11) It was reported that addition of N-methylmorpholine tended to
decrease the reaction temperature. (See: Nicoloau, K. C.; Montagnon, T.;
Baran, P. S. Angew. Chem., Int. Ed. 2002, 41, 993-996.) However, we
found this condition had no effect on the substrate 4.
3258
Org. Lett., Vol. 5, No. 18, 2003

pylation of 6 seemed very favorable. Indeed, the asymmetric
addition reaction of this aldehyde 6 with diisopropylzinc14
smoothly afforded the isopropylated adduct 7 in 82% isolated
yield, in the presence of 20 mol % of ligand 9, (-)-(N,N)-
di-n-butylamino-1-pheneylpropane-1-ol ((-)-DBNE). The
diastereomeric ratio of the product 7 was determined to be
99.5:0.5 by HPLC analysis and the absolute configuration
was established at the level of 8 by comparing with the
authentic sample from our previous work.7 After removal
of the methoxyl methyl group in 7, the advanced intermediate
8 was obtained in 96% isolated yield. This key intermediate,
which contained two hydroxyl groups at C-7 and C-24 in
the proper orientation, was used to conveniently produce the
natural product squalamine (1) in 33% overall yield in three
steps according to our previous published work.7
In summary, a short and highly stereoselective synthesis
of squalamine has been accomplished in nine steps and 14%
overall yield from easily available methyl chenodeoxyl-
cholanate 2, by taking advantage of the improved mild
dehydrogenation reaction as a key step to construct the trans
AB-ring system and utilizing a highly stereoselective iso-
propylation to introduce the C-24R-hydroxyl group. Our
current efforts are focused on applying this remarkably
practical strategy to provide adequate supplies for clinical
trials and synthesizing analogues of squalamine for exploring
their biological activities.
Table 1. Dehydrogenation of 4 with IBX in DMSO
entrya
acid
none
none
p-TsOH
TFAd
mol %
temp (°C)
yieldb (%)
c
1
2
3
4
0
0
30
30
85
25
25
25
-
0
72
87
a Reactions 1 and 2 were performed with 4.0 equiv of IBX in DMSO,
reactions 3 and 4 were performed with 2.0 equiv of IBX in DMSO. b Isolated
yields were calculated after purification by silica gel chromatography.
c Complex products. d TFA ) trifluoroacetic acid.
The following step involved the saturation of the 4,5-
double bond of 5 with lithium in liquid ammonia. This
method was commonly used to afford the trans AB-ring
junction.12 Exposure of 5 to lithium in liquid ammonia at
-78 °C for 1 h followed by quenching with anhydrous NH4-
Cl smoothly delivered 6 in 73% isolated yield. Surprisingly,
at some point during the course of the reduction event, the
C-24 methyl ester was simultaneously reduced to the
aldehyde group. Thus, two functional groups were success-
fully transformed in only one step. The next crucial step was
the introduction of the required C-24R-hydroxyl group, by
asymmetric isopropylation of aldehyde 6.13 Although 6
contained two carbonyl groups, the C-24 aldehyde carbonyl
group is more active toward nucleophilic attack than the C-3
ketone carbonyl group. Consequently, the prospects for
effecting a chemical selective and stereoselective isopro-
Acknowledgment. We thank the National Nature Science
Foundation of China for the financial support of this work
(20272068), Ms. Qun-yan Pan for her assistance in this study,
and Professor Byung Tae Cho (Department of Chemistry,
Hallym University, Hallym, South Korea) for providing the
procedure for the preparation of diisopropylzinc.
Supporting Information Available: Spectroscopic and
analytical data for 4, 5, 6, 7, and 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
(12) Starr, J. E. In Steroid Reactions; Djerassi, C., Ed.; Holden-Day.
Inc.; San Francisco, CA, 1963; Chapter 7, pp 300-307.
(13) For a similar example, see: Okamoto, M.; Tabe, M.; Fujii, T.;
Tanaka, T. Tetrahedron: Asymmetry 1995, 6, 767-778.
OL035062J
(14) Diisopropylzinc is now available from Aldrich Inc.
Org. Lett., Vol. 5, No. 18, 2003
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