A convergent synthesis of the immunosuppressant FTY720 employing aqueous Wittig chemistry

Article abstract of DOI:10.1016/j.tetlet.2011.08.100

A short, convergent synthesis of the immunosuppressant FTY720 is described involving the use of 4-hydroxymethylbenzaldehyde as a pivotal intermediate. A double Wittig strategy was developed to connect this dual-functional aldehyde with an alkyl-tether and to a readily available TRIS-derivative leading to an efficient synthesis of the target molecule.

Full text of DOI:10.1016/j.tetlet.2011.08.100

Tetrahedron Letters 52 (2011) 5672–5675
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Tetrahedron Letters
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A convergent synthesis of the immunosuppressant FTY720
employing aqueous Wittig chemistry
⇑
Janice Calzavara, James McNulty
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Canada L8S 4M1
a r t i c l e i n f o
a b s t r a c t
Article history:
A short, convergent synthesis of the immunosuppressant FTY720 is described involving the use of 4-
hydroxymethylbenzaldehyde as a pivotal intermediate. A double Wittig strategy was developed to con-
nect this dual-functional aldehyde with an alkyl-tether and to a readily available TRIS-derivative leading
to an efficient synthesis of the target molecule.
Received 16 August 2011
Accepted 17 August 2011
Available online 24 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
FTY720
Immunosuppressant
Multiple sclerosis
Wittig reaction
Aqueous chemistry
Introduction
one of which (S1P1 receptor subtype) inhibits antigen-activated
lymphocyte egression. FTY720 has also been implicated in sensitiz-
FTY720 1 (Fig. 1 and 1, R = H), also known as Fingolimod, has
shown impressive activity, initially as an immunosuppressant in
the survival of transplanted tissues (e.g., kidney, liver, heart).1–3
FTY720 was first prepared as a synthetic analogue of myriocin 2,
a naturally occurring (fungal) palmitoyl transferase inhibitor that
inhibits the de novo synthesis of ceramide.4 FTY720 is also structur-
ally related to the natural mediator lipid sphingosine 3. The biolog-
ical activity of FTY720 is essentially connected to modulation of
those functions normally mediated through the analogous sphin-
gosine adducts. Phosphorylation of sphingosine 3 (R = H) via sphin-
gosine kinases (SphK1 or SphK2) yields sphingosine-1-phosphate
(S1P) (Fig. 1 and compound 3, R ¼ PO3ꢀ2),5–7 a lipid that has
emerged as a central regulator in mammalian biology.8 In a similar
fashion, FTY720 is phosphorylated (SphK2) in vivo to give the ac-
tive S enantiomer FTY720-P (Fig. 1 and 1, R ¼ PO3ꢀ2). Biological
activity is associated with binding of the phosphorylated adducts
to an expanding array of S1P receptors eliciting varied responses.
Related to the immunosuppressant activity, FTY720 has also re-
cently been investigated for the treatment of multiple sclerosis
(MS) due to its ability to inhibit the autoimmune disintegration
of the myelin sheath.1,6,9 These findings have culminated in the ap-
proval of FTY720 (trade name Gilenya) as an orally administered
agent that has shown impressive clinical efficacy in treating relaps-
ing–remitting MS.9 This activity is now linked to a complex inter-
play of agonism and antagonism of S1P receptors by FTY720-P,9d
ing prostate cancer cells to radiotherapy through the inhibition of
SphK1,10 as well as in promoting proteosomal degradation of pros-
tate, breast and other cancer cell lines.11 FTY720 and structural
analogues have been shown to be pan-antagonists of S1P GPCR sig-
nalling.12 Downstream events mediated through the binding of S1P
to GPCRs are recognized as highly significant in the signalling net-
work that regulates the survival and proliferation of cancer cells.8
SphK1 has been labelled an ‘oncogene’ and is known to be upreg-
ulated in several solid tumors.13
While several syntheses of FTY720 have been reported,7,14–16
new biologically active analogues continue to be discovered12
and, in conjunction with this impressive array of selective biolog-
ical activity, the development of a short efficient route became of
interest. Of the reported syntheses of FTY720,7,15,16 two make use
of the inexpensive, readily available amine trishydroxymethylami-
no methane (Scheme 1, TRIS).14 Both of these prior syntheses dif-
ferentiate the hydroxyl groups of TRIS through the intermediacy
of the N-Boc protected acetonide and convert the remaining pri-
mary alcohol to the aryl/alkyl side chain using Pd-mediated
cross-coupling and Julia-olefination strategies. We have recently
been investigating various aspects of aqueous Wittig olefination
reactions with trialkylphosphine derived semi-stabilized ylides.17
It occurred to us that the dual functional benzylic alcohol/aldehyde
618 could potentially serve as a pivotal building block for the syn-
thesis of FTY720 and analogues in conjunction with a suitable
intermediate derived from TRIS. We have recently shown that olef-
ination on the aldehyde of 6 can be carried out without protection
of the 4-hydroxymethyl substituent under classic Wittig condi-
⇑
Corresponding author. Tel.: +1 905 525 9140x27393; fax: +1 905 522 2509.
E-mail address: jmcnult@mcmaster.ca (J. McNulty).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.100

J. Calzavara, J. McNulty / Tetrahedron Letters 52 (2011) 5672–5675
5673
NH2
HO
RO
FTY720 (FingolimodTM) 1 (R=H)
OH
O
OH
O
O
H
4
6
O
R
OH
9
NH2
O
H
NH2
Myriocin 2
Sphingosine 3 (R=H)
Figure 1. FTY720 and related molecules.
tions,18c which in this case should lead to the alkene 8. The second
advantage that could be gained using 6 is the direct conversion of
the benzylic alcohol in 8 to the benzylic phosphonium salt 4 using
our recently reported trialkylphosphine hydrobromide protocol.17c
We thus envisioned a convergent route to FTY720 involving an
aqueous Wittig reaction between the ylide derived of 4 (Fig. 2)
and aldehyde 5 that would proceed via 9. Hydrogenation and glo-
bal Boc/benzylidene deprotection would lead to FTY720 as the sole
product. In this Letter, we report the successful, rapid, convergent
synthesis of FTY720 from the bifunctional aldehyde 6 involving an
aqueous Wittig olefination reaction with TRIS-derived aldehyde 5
as the critical point of assembly.
The overall convergent synthesis of FTY720 from aldehyde 6
and TRIS is shown in Scheme 1. The benzylidene acetal derivative
of TRIS 7 has been previously prepared in a three step sequence
from tris-(hydroxymethyl)-nitromethane through benzylidene for-
mation,19b nitro-reduction and N-protection.19a In our hands, this
derivative was readily accessed in two steps from TRIS via N-pro-
tection with Boc2O and benzylidene formation using benzaldehyde
dimethylacetal. The mono-alcohol 7 was isolated by crystallization
as a mixture of cis- and trans-isomers in 81% yield (50:50 to 60:40
ratio). The two isomers were readily separable on chromatography
and structures independently assigned.16a Swern oxidation was
normally conducted on the mixture giving the cis- and trans-iso-
mers of the fully protected aldehyde 5. The three steps required
to prepare 5 from TRIS can be conducted on a gram scale, requiring
only one silica-gel chromatographic purification to obtain 5 in 59%
overall yield as a colourless, amorphous solid. Aldehyde 5 proved
to be very stable and could be stored for weeks without noticeable
deterioration. The tail section of FTY720 was synthesized from the
key building block 4-(hydroxymethyl)benzaldehyde 6 employing
the double Wittig strategy. A standard (Z)-selective Wittig reaction
using the non-stabilized ylide derived from triphenyl(heptyl)phos-
phonium bromide was employed for addition of the alkenyl side-
chain. Protection of the hydroxyl group on 6 was not necessary;
the olefinic benzyl alcohol 8 was isolated in 87–88% yield as a
10:1 mixture of (Z):(E)-olefins. The benzylic alcohol 8 was now
subjected to our recently described one-step protocol17c yielding
the triisobutylphosphonium salt 4 in quantitative yield. In this pro-
cess, the use of a tertiary phosphine hydrobromide salt allows di-
rect conversion of a benzylic alcohol to the Wittig salt without
the requirement of formation of the intermediate benzylic bromide
(or other activating group). The salt 4 was obtained in quantitative
yield and was now subjected to an aqueous Wittig reaction with
aldehyde 5 under our recently reported microwave conditions17e
(30 min at 70 °C) using aqueous lithium hydroxide. The mixture
of stereoisomers of 9 was obtained in high yield and was flushed
through a short plug of silica to remove polar impurities giving 9
in 76% yield from the reaction of 4 and 5. The aqueous Wittig reac-
tion can also be conducted with standard thermal heating but is
slower (64% isolated yield over 18 h at 70 °C). While the aqueous
Wittig olefination protocol is (E)-selective, compound 9 was ob-
served to be composed of four stereoisomers, cis/trans plus
(E):(Z), observable by 600 MHz NMR spectroscopy. The mixture
O
NH2
HN
O
b
OH
a
O
HO
O
OH
TRIS
OH
O
7
O
HN
O
O
O
HN
O
O
f
e
HN
O
O
P(iBu)3
Br
O
O
+
O
5
O
10
4
7
5
9
5
H2O
g
d
OH
c
OH
NH2
O
OH
5
8
6
OH
FTY720
Scheme 1. Convergent synthesis of FTY720 from TRIS. Reagents and conditions: (a) (1) Boc2O, DMF, 24 h, (2) PhCH(OMe)2, PTSA(cat), 24 h, 81% over both steps; (b) (COCl)2,
DMSO, ꢀ78 °C to rt, 4 h, 79%; (c) (1) (Ph)3P–C7H15 + Brꢀ, LiHMDS, 0 °C, 1 h, (2) compound 6, THF, ꢀ78 °C to rt, 18 h, 88%; (d) (iBu)3PH+ Brꢀ, 110 °C, 18 h. Quant; (e) LiOH, H2O,
MW, 70 °C, 30 min, 76%; (f) H2, Pd/C(cat), THF, rt; (g) HCl, MeOH/DCM, rt, 92% over both steps.

5674
J. Calzavara, J. McNulty / Tetrahedron Letters 52 (2011) 5672–5675
NH2
Acknowledgments
HO
HO
We thank NSERC for the financial support of this work and Mr.
D. McLeod for valuable contributions.
FTY720
Supplementary data
Boc
NH
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.08.100.
O
O
Ph
9
References and notes
1. Mandala, S.; Hajdu, R.; Bergstrom, J.; Quackenbush, E.; Xie, J.; Milligan, J.;
Thornton, R.; Shei, G.-J.; Card, D.; Keohane, C.; Rosenbach, M.; Hale, J.; Lynch, C.
L.; Rupprecht, K.; Parsons, W.; Rosen, H. Science 2002, 296, 346–349.
2. Fujita, T.; Matsumoto, N.; Uchida, S.; Kohno, T.; Shimizu, T.; Hirose, R.; Yanada,
K.; Kurio, W.; Watabe, K. Bioorg. Med. Chem. Lett. 2000, 10, 337–339.
3. (a) Brinkmann, V.; Davis, M. D.; Heise, C. E. J Biol Chem. 2002, 277, 21453–
21457; (b) Suzuki, S. Transplant. Proc. 1999, 31, 2779–2782.
O
Br
P(iBu)3
HN
O
O
O
5
5
O
4
4. Pyne, S.; Pyne, N. J. Biochem. J. 2000, 349, 385–402.
5. Evindar, G.; Satz, A. L.; Bernier, S. G.; Kavarana, M. J.; Doyle, E.; Lorusso, J.;
Taghizadeh, N.; Halley, K.; Hutchings, A.; Kelley, M. S.; Wright, A. D.; Saha, A. K.;
Hannig, G.; Morgan, B. A.; Westlin, W. F. Bioorg. Med. Chem. Lett. 2009, 19,
2315–2319.
6. Albert, R.; Hinterding, K.; Brinkmann, V.; Guerini, D.; Müller-Hartwieg, C.;
Knecht, H.; Simeon, C.; Streiff, M.; Wagner, T.; Welzenbach, K.; Zécri, F.;
Zollinger, M.; Cooke, N.; Francotte, E. J. Med. Chem. 2005, 48, 5373–5377.
7. Balasubramaniam, S.; Annamalai, S.; Aidhen, I. Synlett 2007, 2841–2846.
8. Fyrst, H.; Saba, J. D. Nature Chem. Biol. 2010, 6, 489–497.
NH2
OH
OH
TRIS
OH
O
HO
6
Figure 2. Retrosynthetic analysis of FTY720.
9. (a) Brinkmann, V. Br. J. Pharmacol. 2009, 158, 1173–1182; (b) Kappos, L.; Radue,
E. W.; O’Connor, P.; Polman, C.; Hohlfeld, R.; Calabresi, P.; Selmaj, K.;
Agoropoulou, C.; Leyk, M.; Zhang-Auberson, L.; Burtin, P. N. Engl. J. Med.
2010, 362, 387–401; (c) Cohen, J. A.; Barkhof, F.; Comi, G.; Hartung, H. P.;
Khatri, B. O.; Montalban, X.; Pelletier, J.; Capra, R.; Gallo, P.; Izquierdo, G.; Tiel-
Wilck, K.; De Vera, A.; Jin, J.; Stites, T.; Wu, S.; Aradhye, S.; Kappos, L. N. Engl. J.
Med. 2010, 362, 402–415; (d) Choi, J. W.; Gardell, S. E.; Herr, D. R.; Rivera, R.;
Lee, C. W.; Noguchi, K.; Teo, S. T.; Yung, Y. C.; Lu, M.; Kennedy, G.; Chun, J. Proc.
Natl. Acad. Sci. 2011, 108, 751–756; (e) Brinkmann, V.; Billich, A.; Baumruker, T.;
Heining, P.; Schmouder, R.; Francis, G.; Arahye, S.; Burtin, P. Nat. Rev. Drug Disc.
2010, 9, 883–897; (f) Strader, C. R.; Pearce, C. J.; Oberlies, N. H. J. Nat. Prod. 2011,
74, 900–907.
10. Pchejetski, D.; Bohler, T.; Brizuela, L.; Sauer, L.; Doumerc, N.; Golzio, M.;
Salunkhe, V.; Teissie, J.; Malavaud, B.; Waxman, J.; Cuvillier, O. Am. Assoc.
Cancer Res. 2010, 9, 8651–8661.
11. Tonelli, F.; Lim, K. G.; Loveridge, C.; Long, J.; Pitson, S. M.; Tigyi, G.; Bittman, R.;
Pyne, S.; Pyne, N. J. Cell. Signalling 2010, 22, 1536–1542.
was hydrogenated over Pd/C to give the doubly reduced product 10
as the cis/trans isomers only. Hydrolysis of the benzylidene acetal
and removal of the Boc-protecting group was now performed in
a mixture of dichloromethane and hydrochloric acid in methanol,
from which the HCl salt and subsequently free base of FTY720
could readily be isolated. The last two steps involve four separate
transformations on the molecule, three of which eradicate all ste-
reochemical complexities. Intermediate 10 was readily isolatable
and fully characterized, however, these two steps were conducted
sequentially and FTY720 isolated in 92% overall yield from 9.
From an overall strategic synthesis viewpoint, the convergent
fusion of benzylic alcohol 8 and TRIS-derived aldehyde 5 fuses
the two essential ‘halves’ of the molecule in an efficient and
straightforward process. The salt 4 is prepared in quantitative
yield from 8 and is dissolved in water for the aqueous Wittig
reaction with 5, giving direct access to 9 as the mixture of stereo-
isomers. Hydrogenation and deprotection of 9 are carried out
sequentially, involving four separate functional group intercon-
versions that result in unravelling all stereoisomeric complica-
tions inherent in 9. The process provides for the synthesis of
FTY720 in 45% yield and only five steps from TRIS, or 62% yield
and three steps from the aldehyde 6. This compares favourably
to the method of Kim et al.,7,9f that provides FTY720 in 64% yield
but requires seven linear steps from TRIS. The readily available
bifunctional aldehyde 6 proved to be an ideal building block for
the rapid elaboration to FTY720 via the double Wittig strategy
and this synthesis should be readily amenable to the preparation
of truncated or functionalized analogues of FTY720 through their
incorporation onto 6.
12. Valentine, W. J.; Kiss, G. N.; Liu, J.; E, S.; Gotoh, M.; Murakami-Murofushi, K.;
Pham, T. C.; Baker, D. L.; Parrill, A. L.; Lu, X.; Sun, C.; Bittman, R.; Pyne, N. J.;
Tigyi, G. Cell. Signalling 2010, 22, 1543–1553.
13. Bektas, M.; Johnson, S. P.; Poe, W. E.; Bigner, D. D.; Friedman, H. S. Cancer
Chemother. Pharmaol. 2009, 64, 1053–1058.
14. For prior syntheses of FTY720 employing TRIS see Ref. 7 and: Kim, S.; Lee, H.;
Lee, M.; Lee, T. Synthesis 2006, 753–755.
15. For other syntheses of FTY720 see: (a) Adachi, K.; Kohara, T.; Nakao, N.; Arita,
M.; Chiba, K.; Mishina, T.; Sasaki, S.; Fujita, T. Bioorg. Med. Chem. Lett. 1995, 5,
853–865; (b) Kiuchi, M.; Adachi, K.; Kohara, T.; Minoguchi, M.; Hanano, T.;
Aoki, Y.; Mishina, T.; Arita, M.; Nakao, N.; Ohtsuki, M.; Hoshino, Y.; Teshima, K.;
Chiba, K.; Sasaki, S.; Fujita, T. J. Med. Chem. 2000, 43, 2946–2961; (c) Durand, P.;
Peralba, P.; Sierra, F.; Renaut, P. Synthesis 2000, 4, 505–506; (d) Kalita, B.; Barua,
N. C.; Bezbarua, M.; Bez, G. Synlett 2001, 1411–1414; (e) Hinterding, K.;
Cottens, S.; Albert, R.; Zecri, F.; Buehlmayer, P.; Spanka, C.; Brinkmann, V.;
Nussbaumer, P.; Ettmayer, P.; Hoegenauer, K.; Gray, N.; Pan, S. Synthesis 2003,
1667–1670; (f) Hale, J. J.; Yan, L.; Neway, W. E.; Hajdu, R.; Bergstrom, J. D.;
Milligan, J. A.; Shei, G.-J.; Chrebet, G. L.; Thornton, R. A.; Card, D.; Rosenbach,
M.; Rosen, H.; Mandala, S. Bioorg. Med. Chem. 2004, 12, 4803–4807; (g) Seidel,
G.; Laurich, D.; Fürstner, A. J. Org. Chem. 2004, 69, 3950–3952; (h) Sugiyama, S.;
Arai, S.; Kiriyama, M.; Ishii, K. Chem. Pharm. Bull. 2005, 53, 100–102; (i) Lu, X.;
Bittman, R. Tetrahedron Lett. 2006, 47, 825–827; (j) Matsumoto, N.; Hirose, R.;
Sasaki, S.; Fujita, T. Chem. Pharm. Bull. 2008, 56, 595–597.
16. For syntheses of analogues of FTY720 see (a) Ko, R. Y. Y.; Chu, J. C. K.; Chiu, P.
Tetrahedron 2011, 67, 2542–2547 (Note: the formation and analysis of the
obtained cis/trans isomers is not reported by these authors); (b) Li, Z.; Bittman,
R. J. Org. Chem. 2007, 72, 8376–8382; (c) Kiuchi, M.; Adachi, K.; Tomatsu, A.;
Chino, M.; Takeda, S.; Tanaka, Y.; Maeda, Y.; Sato, N.; Mitsutomi, N.; Sugahara,
K.; Chiba, K. Bioorg. Med. Chem. 2005, 13, 425–432; (d) Hale, J. J.; Neway, W.;
Mills, S. G.; Hajdu, R.; Keohane, C. A.; Rosenbach, M.; Milligan, J.; Shei, G.-J.;
Chrebet, G.; Bergstrom, J.; Card, D.; Koo, G. C.; Koprak, S. L.; Jackson, J. J.; Rosen,
H.; Mandala, S. Bioorg. Med. Chem. Lett. 2004, 14, 3351–3355; (e) Hinterding, K.;
Albert, R.; Cottens, S. Tetrahedron Lett. 2002, 43, 8095–8097.
In conclusion, a short efficient synthesis of FTY720 from the
inexpensive readily available starting material TRIS and the bifunc-
tional aldehyde 6 is reported. The synthesis employs an aqueous
Wittig reaction at the point of convergence providing rapid entry
to 9 as a mixture of stereoisomers. Hydrogenation and acidic
hydrolysis of 9 yield FTY720 as the sole product in high yield. Alde-
hyde 6 proved to be a pivotal intermediate to access FTY720
employing the double Wittig strategy. Preparation and assessment
of structural analogues of FTY720 employing this method are cur-
rently under investigation.
17. (a) Das, P.; McLeod, D.; McNulty, J. Tetrahedron Lett. 2011, 52, 199–201; (b)
McNulty, J.; McLeod, D. Synlett 2011, 717–721; (c) McNulty, J.; Das, P.; McLeod,
D. Chem. Eur. J. 2010, 16, 6756–6760; (d) McNulty, J.; Das, P. Tetrahedron Lett.

J. Calzavara, J. McNulty / Tetrahedron Letters 52 (2011) 5672–5675
5675
2010, 51, 3197–3199; (e) McNulty, J.; Das, P. Eur. J. Org. Chem. 2009, 24, 4031–
4035; (f) McNulty, J.; Das, P. Tetrahedron Lett. 2009, 50, 5737–5740.
2010, 8, 1181–1187; (c) McNulty, J.; McLeod, D. Tetrahedron Lett. 2011, 52,
5467–5470.
18. (a) Loim, N. M.; Kelbyscheva, E. S. Russ. Chem. Bull. 2004, 53, 2080–2085; (b)
Simpson, M. G.; Pittelkow, M.; Watson, S. P.; Sanders, J. K. M. Org. Biomol. Chem.
19. (a) Schmidt, U.; Riedl, B. Synthesis 1993, 815–818; (b) Scattergood, A.; MacLean,
A. L. J. Am. Chem. Soc. 1949, 71, 4153–4154.