Synthesis of 1-deoxysphingosine derivatives with conformationally restricted pyrrolidinediol head groups

Article abstract of DOI:10.1021/ol052839v

A family of cyclic 1-deoxysphingolipid derivatives of structure 4 has been designed and synthesized, which may serve as tumorigenesis suppressors for various cancers. Compound 4 is a second-generation analogue developed from sphingosine (1), in which a hydroxyl substituent is moved from C1 to C5 and a methylene is added for conformational rigidity between the C2-nitrogen substituent and C4. The synthetic chemistry for pyrrolidine ring closure at C3-C4 features ring-closing metathesis followed by hydroboration-oxidation.

Full text of DOI:10.1021/ol052839v

ORGANIC
LETTERS
2006
Vol. 8, No. 4
649-652
Synthesis of 1-Deoxysphingosine
Derivatives with Conformationally
Restricted Pyrrolidinediol Head Groups
Ann M. Dougherty,† Frank E. McDonald,*,† Dennis C. Liotta,*,† Steven J. Moody,‡
David C. Pallas,‡ Carrie D. Pack,§ and Alfred H. Merrill§
Departments of Chemistry and Biochemistry and the Winship Cancer Institute, Emory
UniVersity, Atlanta, Georgia 30322, and School of Biology, Georgia Institute of
Technology, Atlanta, Georgia 30332
fmcdona@emory.edu; dliotta@emory.edu
Received November 23, 2005
ABSTRACT
A family of cyclic 1-deoxysphingolipid derivatives of structure 4 has been designed and synthesized, which may serve as tumorigenesis
suppressors for various cancers. Compound 4 is a second-generation analogue developed from sphingosine (1), in which a hydroxyl substituent
is moved from C1 to C5 and a methylene is added for conformational rigidity between the C2-nitrogen substituent and C4. The synthetic
chemistry for pyrrolidine ring closure at C3−C4 features ring-closing metathesis followed by hydroboration-oxidation.
Sphinoglipids are natural products found in most cell
membranes and are structurally characterized by a long
carbon chain “sphingoid” base that is derivatized with amide-
linked fatty acids and various polar headgroups.1 Sphingoid
bases have additional functions as cellular mediators and
protein kinase C (PKC) inhibitors,2 affecting the growth,
differentiation, migration, and apoptosis of cells. Extensive
research efforts have resulted in several synthetic approaches
to sphingoid bases3 and structural analogues, encouraged in
part by recent discoveries regarding the anticancer activity
of sphingolipids. In 2003 Menaldino et al. reported that
1-deoxysphingoid bases of general structure 3 (Figure 1)
were growth inhibitory and cytotoxic at concentrations up
to 10-fold lower than for sphingosine (1) and up to 50-fold
more potent than the corresponding N-acylated ceramide 2
in HT29 and DU145 cell lines.4 As the primary alcohol of
sphingosine is phosphorylated in vivo (resulting in undesired
mitogenic/anti-apoptotic activity), the design of 1-deoxy-
analogues 3 prevents phosphorylation, and by moving the
hydroxyl group to the 5-position, lipophilicity of 3 is similar
to that of sphingosine (1). To minimize N-acylation activity,
we have further modified the 1-deoxysphingoid lead structure
3 to cyclic pyrrolidinediol 4, which also provides confor-
† Department of Chemistry.
‡ Department of Biochemistry and Winship Cancer Institute.
§ Georgia Institute of Technology.
(1) Biochemistry, 2nd ed.; Voet, D., Voet, J. G., Eds.; Wiley: New York,
1995.
(2) (a) Hannun, Y. A.; Loomis, C. R.; Merrill, A. H., Jr.; Bell, R. M. J.
Biol. Chem. 1986, 261, 12604. (b) Merrill, A. H., Jr.; Sereni, A. M.; Stevens,
V. L.; Hannun, Y. A.; Bell, R. M.; Kinkade, J. M., Jr. J. Biol. Chem. 1986,
261, 12610.
(3) (a) Garner, P.; Park, J. M.; Malecki, E. J. Org. Chem. 1988, 53, 11061.
(b) Herold, P. HelV. Chim. Acta 1988, 71, 354. (c) Nimkar, S.; Menaldino,
D.; Merrill, A. H.; Liotta, D. Tetrahedron Lett. 1997, 38, 7687.
(4) (a) Menaldino, D. S.; Bushnev, A.; Sun, A.-M.; Liotta, D. C.;
Symolon, H.; Desai, K.; Dillehay, D. L.; Peng, Q.; Wang, E.; Allegood, J.;
Trotman-Pruett, S.; Sullards, M. C.; Merrill, A. H. Pharmacol. Res. 2003,
47, 373. (b) For second-generation synthesis of aminodiol compounds 3:
Wiseman, J. M.; McDonald, F. E.; Liotta, D. C. Org. Lett. 2005, 7, 3155.
10.1021/ol052839v CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/21/2006

of diastereomers (Scheme 1). The minor diastereomer was
separated from 8 by careful silica gel chromatography.8
Acidic cleavage9 of the chiral auxiliary and Cbz-protection
of nitrogen provided compound 9.
Scheme 1. Enantioselective Synthesis of
N-Cbz-2-amino-3-butene (9)
Figure 1. Structures of sphingosine (1), ceramide (2), and
1-deoxyanalogues 3 and 4.
mational restriction of the polar groups. This communication
describes the synthesis and biological evaluation of several
stereoisomers of 4, prepared in highly convergent fashion.
We envisioned that the cyclic pyrrolidinediol analogue 4
could be prepared from the functionalized dihydropyrrole
5, which in turn would arise from ring-closing metathesis5
of the diallylamine 6 (Figure 2).
Several approaches were explored for preparation of the
fragment bearing carbons 4 and 5. The best route involved
asymmetric epoxidation of 11 to 12 (Scheme 2), followed
Scheme 2. Stereoselective Synthesis of Allylic Bromide (14)
Figure 2. Retrosynthesis for cyclic 1-deoxysphinganine.
We initially planned to prepare 2-amino-3-butene from the
amino acid L-alanine, but racemization occurred under all
conditions attempted via R-aminoaldehyde intermediates.6
Therefore, enantioselective synthesis of 2-amino-3-butene
was accomplished by sodium borohydride reduction of the
chiral sulfinimide7 derived from methyl vinyl ketone (7),
providing sulfinamine 8 as the major product of a 7:1 mixture
by LDA elimination10 to give the allylic diol 13. Differentia-
tion of the primary alcohol as the bromide and protection of
secondary alcohol as the silyl ether afforded chiral non-
racemic synthon 14 for carbon-nitrogen coupling. The
enantiomer of 14 was likewise prepared beginning with
epoxidation of 11 with D-DIPT.
(5) Kirkland, T. A.; Grubbs, R. H. J. Org. Chem. 1997, 62, 7310.
(6) (a) Albeck, A.; Persky, R. J. Org. Chem. 1994, 59, 653. (b) McKillop,
A.; Taylor, R. J. K.; Watson, R. J.; Lewis, N. Synthesis 1994, 31. (c) Takai,
K.; Hotta, Y.; Oshima, K.; Nozaki, H. Tetrahedron Lett. 1978, 27, 2417.
(d) Nishizawa, R.; Saino, T.; Takita, T.; Suda, H.; Aoyagi, T.; Umezawa,
H. J. Med. Chem. 1977, 20, 510.
(7) (a) Borg, G.; Cogan, D. A.; Ellman, J. A. Tetrahedron Lett. 1999,
40, 6709. (b) Cogan, D. A.; Liu, G.; Ellman, J. Tetrahedron 1999, 55, 8883.
(c) Ellman, J. A.; Owens, T. D.; Tang, T. P. Acc. Chem. Res. 2002, 35,
984. (d) Weix, D. L.; Ellman, J. A. Org. Lett. 2003, 5, 1317. (e) Zhou, P.;
Chen, B.; Davis, F. A. Tetrahedron 2004, 60, 8003.
(8) We also explored addition of vinylmagnesium bromide to the
sulfinimide derived from acetaldehyde, but this reaction proceeded with
poor diastereoselectivity, reaching a maximum of 3:1 dr favoring compound
8 when 8 equiv of CH2dCHMgBr was used.
(9) Liu, G.; Cogan, D. A.; Ellman, J. A. J. Am. Chem. Soc. 1997, 119,
9913.
(10) (a) Kang, S. H.; Jun, H. S. Chem. Commun. 1998, 1929. (b) Hao,
J. L.; Aiguade, J.; Forsyth, C. J. Tetrahedron Lett. 2001, 42, 821. (c) Gao,
Y.; Klunder, J. M.; Hanson, R. M.; Masamune, H.; Ko, S. Y.; Sharpless,
K. B. J. Am. Chem. Soc. 1987, 109, 5765.
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Org. Lett., Vol. 8, No. 4, 2006

Reaction of carbamate 9 with sodium hydride11 and
N-alkylation of each enantiomer of allylic bromide 14
provided the dienes 15 and 16 (Scheme 3), which each
Scheme 4. Stereoselectivity of Hydroboration of 19 vs 20 and
Completion of Target Compound Syntheses
Scheme 3. Coupling of 9 and 14 and Ring-closing Metathesis
to Diastereomeric Dihydropyrrolidines 17 and 18
expected, diastereomer 19 produced a single pyrrolidinediol
diastereomer 21, arising from stereoinduction from the C5-
hydroxyl (Still-Barrish model),15 reinforcing the steric effect
from the methyl substituent attached at C2. In contrast,
diastereomer 20 gave a separable mixture of diastereomers
22 and 23, consistent with opposing effects of stereoinduction
from C2 and C5 chiral allylic postions.16
underwent ring-closing metathesis to dihydropyrroles 17 and
18 in excellent yield, using the Hoveyda metathesis catalyst.12
A variety of conditions were explored for the introduction
of the C3 alcohol via anti-Markovnikov hydration, with the
best results obtained with hydroboration13 of the alcohols
19 and 20 with either borane-dimethyl sulfide or thexyl-
borane followed by alkaline hydrogen peroxide oxidation and
hydrogenolytic removal of the N-Cbz protective group.14 As
After hydrogenolysis of the N-Cbz protective group,
pyrrolidinediol stereoisomers 24-26 and their enantiomers
Table 1. Biological Evaluation of Pyrrolidinediol Analogs 24-26 and ent-24-26
a In ethanol solution. b In 1:1 ethanol/ethyl acetate solution, with serum-starved cells. c ND ) not determined, due to insufficient sample. d For these
analyses, the Hill slope of the toxicity curve was too steep to provide an exact value.
Org. Lett., Vol. 8, No. 4, 2006
651

(prepared from ent-8 following the same synthetic route)
were evaluated for cytotoxicity in DU-145 (human prostate
carcinoma) and HT-29 (human colon carcinoma) cell lines
(Table 1).17,18 We were pleased to observe that pyrrolidinediol
compounds 25 and ent-25 exhibited cytotoxicity against DU-
145 cells in the same range as acyclic (2S,3S,5S)-3, and the
compounds 25, ent-25, 26, and ent-26 were uniformly more
cytotoxic against HT-29 cells in comparison with D-erythro-
sphingosine (1).19 The range of biological activities among
the various stereoisomers of cyclic structures 24-26 is
relatively small, and thus further structure-activity studies
on other cyclic aminodiols 24-26 as well as first-generation
aminodiols 3 are warranted.
Acknowledgment. This research was supported by the
National Institutes of Health, through the National Coopera-
tive Drug Discovery Grant program U19 CA 87525 and R01
CA 57327 to D.C.P.
Supporting Information Available: Experimental pro-
cedures and characterization data for compounds 8-26. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(11) For example, see: Briot, A.; Bujard, M.; Gouverneur, V.;
Mioskowski, C. Eur. J. Org. Chem. 2002, 139.
(12) (a) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H J.
Am. Chem. Soc. 2000, 122, 8168. (b) Kinderman, S. S.; Doodeman, R.;
van Beijma, J. W.; Russcher, J. C.; Tjen, K. C. M. F.; Kooistra, T. M.;
Mohaselzadeh, H.; van Maarseveen, J. H.; Hiemstra, H.; Schoemaker, H.
E.; Rutjes, F. P. J. T AdV. Synth. Catal. 2002, 344, 736. (c) The Grubbs
second-generation catalyst Cl2(PCy3)(IMes)RudCHPh also provided me-
tathesis products 17 and 18, but a longer reaction time was required and it
proved difficult to remove all traces of the Ru catalyst from the products.
(13) (a) Brown, H. C.; Vara Prasad, J. V. N.; Gupta, A. K. J. Org. Chem.
1986, 51, 4296. (b) Lutjens, H.; Knochel, P. Tetrahedron: Asymmetry 1994,
5, 1161.
OL052839V
(17) DU-145 cell cytotoxicity assays were also conducted under serum-
starved conditions (protocol b, Table 1) to minimize artifacts of protein
binding, but results are also reported without serum deprivation (protocol
a), since serum removal can affect sphingolipid metabolism: Colombaioni,
L.; Frago, L. M.; Varela-Nieto, I.; Pesi, R.; Garcia-Gil, M. Neurochem.
Int. 2002, 40, 327.
(14) Other hydroboration reagents, such as 9-BBN, or hydrosilylation
directed by the C5-alcohol gave no reaction and recovery of substrate. The
silyl ether compounds 17 and 18 were either unreactive or gave unsatisfac-
tory yields under all hydroboration conditions attempted.
(15) Still, W. C.; Barrish, J. C. J. Am. Chem. Soc. 1983, 105, 2487.
(16) After our synthetic work was completed, we learned of another study
involving hydroboration of dihydropyrrole-substituted with oxygenated
substituents: Cren, S.; Wilson, C.; Thomas, N. R. Org. Lett. 2005, 7, 3521.
(18) Compounds 24-26 exhibited poor solubility in ethanol solvent
vehicles, therefore experiments were conducted in a 1:1 ethanol/ethyl acetate
mixture as well as in ethanol. Control experiments indicated that addition
of ethyl acetate was neither toxic to nor inhibited the growth of DU-145
cells.
(19) N-Cbz compounds 21-23 and ent-21-23 were also evaluated, as
well as N-acetyl derivatives of 24-26 and ent-24-26, and were generally
less potently cytotoxic than the cyclic amines 24-26 and enantiomers.
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