Synthesis of 1-thio-phytosphingolipid analogs by microwave promoted reactions of thiols and aziridine derivatives

Article abstract of DOI:10.1016/j.tetlet.2012.02.066

A practical and versatile method for the synthesis of 1-thio- phytosphingolipid analogs through regioselective nucleophilic ring-opening reactions of phytosphingosine aziridine derivatives with thiols is described. The reactions were carried out with N-ac

Full text of DOI:10.1016/j.tetlet.2012.02.066

Tetrahedron Letters 53 (2012) 2137–2139
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Tetrahedron Letters
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Synthesis of 1-thio-phytosphingolipid analogs by microwave promoted
reactions of thiols and aziridine derivatives
⇑
Anna Alcaide, Amadeu Llebaria
Research Unit on Bioactive Molecules (RUBAM), Departament de Química Biomèdica, Institut de Química Avançada de Catalunya (IQAC–CSIC), Jordi Girona 18-26,
Barcelona 08034, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A practical and versatile method for the synthesis of 1-thio-phytosphingolipid analogs through regiose-
lective nucleophilic ring-opening reactions of phytosphingosine aziridine derivatives with thiols is
described. The reactions were carried out with N-acylaziridines and a variety of thiol compounds. Micro-
wave irradiation highly improved the yield of the ring-opening reaction and the intermediate N-acyl
adducts were converted into 1-S-phytosphingolipid analogs, such as phytoceramide and phytosphingo-
sine derivatives.
Received 21 December 2011
Revised 10 February 2012
Accepted 14 February 2012
Available online 21 February 2012
Keywords:
Ring-opening
Aziridines
Thiols
Ó 2012 Elsevier Ltd. All rights reserved.
Microwave
Sphingolipids
Sphingolipids are essential biomolecules for the physiological
cell function. In addition to its structural role in cell membranes,
these lipids have also crucial functions in signal transduction and
cell regulation.1–3 Glycosphingolipids having a phytoceramide scaf-
fold have been the object of attention since the discovery of their
tivity to aziridine and it was easily deprotected by the reaction
with thiolates.13 Although some examples of ring-opening reac-
tions of N-nosylaziridines with thiolates have been described,13 it
was considered that the ring-opening reactions of N-nosyl aziri-
dines with thiols might not be of general application for the prep-
aration of thio-phytosphingolipid analogs. Therefore, a different
nitrogen activating group should be introduced in aziridine 2 to
obtain the desired thioether sphingolipids. Different nitrogen sub-
stituents have been reported to activate aziridines for thiol open-
ing. Since in our case most of the desired modifications in the
sphingolipid framework contain N-acyl groups, it was considered
that N-acylaziridines would be an optimal solution because this
would directly lead to ceramides avoiding the formation and cleav-
age steps of other groups and increasing the synthetic efficiency.
Moreover we thought that this approach could be expanded to
the synthesis of phytosphingosine derivatives having a free amino
group if a benzyl carbamate was used in the aziridine intermediate
as amino protecting group.
immunostimulant activity.4 Particularly
a-galactosylceramides,
such as the natural agelasphins, the synthetic KRN7000, and other
synthesized thioglycloside analogs have been studied for their
effect on NKT cell activation.5–7 As a consequence of the important
biological roles of sphingolipids, the synthesis of their analogs and
the development of chemical inhibitors of sphingolipid enzymes
are the object of current interest.8–11
In this context, we are interested in the development of a syn-
thetic methodology for the obtention of 1-thio-phytosphingolipid
analog libraries at milligram scale to find active modulators of
sphingolipid metabolism. To achieve this aim, we envisaged the
preparation of thiosphingolipid analogs through nucleophilic
ring-opening reactions of an aziridine sphingolipid with thiols.
The main advantage of this strategy would be its high versatility,
allowing the obtention of different structural analogs from a com-
mon aziridine precursor by simply changing the sulfur nucleophile.
It was previously described that the synthesis of N-nosylaziri-
dine derivatives from phytosphingosine hydrochloride 1, is useful
for obtaining 1,2-diaminophytosphingolipids by reaction with
nitrogen nucleophiles.12 This sulfonyl group conferred a high reac-
The described synthesis of aziridine 212 from phytosphingosine
hydrochloride 1 started with the protection of the amino function-
ality of 1 as an azide group with trifluoromethanesulfonyl azide
(TfN3) to afford azido-phytosphingosine 3, followed by the selec-
tive protection of the primary alcohol with t-butyldiphenylsilyl
chloride (TBDPSCl) to get diol 4 in direct analogy with the reported
synthesis of
D-erythro-sphingosine and D-ribo-phytosphingosine
from phytosphingosine.14 Then, conversion of the 3,4-vicinal diol
4 into its cyclic isopropylidene acetal in the presence of 2,2-dime-
thoxypropane (DMP) and p-toluenesulfonic acid (pTSA) to give 5
⇑
Corresponding author.
E-mail address: alsqob@cid.csic.es (A. Llebaria).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.066

2138
A. Alcaide, A. Llebaria / Tetrahedron Letters 53 (2012) 2137–2139
Table 1
and the subsequent deprotection of the primary alcohol with tetra-
butylammonium fluoride (TBAF) afforded intermediate 6.15 Trans-
formation of the alcohol function of 6 in a good leaving group after
mesylation reaction in the presence of methanesulfonyl chloride
(MsCl) reached aziridine precursor 7, which afforded aziridine
derivative 2 after Staudinger reduction of 7 with in situ intramolec-
ular cyclization.12
Then, the N-acylaziridines were prepared from known aziridine
2,12 by the reaction with octanoyl chloride or benzylchloroformate
to give aziridines 8 and 9, respectively (Scheme 1). Ring-opening
reactions of 8 and 9 with mercaptans followed by diol deprotec-
tion, would lead to the thio-phytosphingolipid analogs.
Literature reports for nucleophilic opening of acylaziridines
with thiols are known but limited.16–19 We decided to explore
the reaction of thiophenol and aziridine 8, according to the proce-
dure16 reported by Galonic et al. for the conjugation of thiols with
N-acylaziridine-containing peptides. When aziridine 8 was reacted
at room temperature with thiophenol (1.2 mol-equiv) in the pres-
ence of DBU (0.1 mol-equiv) in MeCN, only partial consumption of
the starting aziridine 8 was observed after 12 h of reaction. It was
necessary to increase the amount of DBU (1 mol-equiv) and to
warm the mixture to 85 °C for 3 days to afford the ring-opened
product in a 55% yield. Attempts to optimize the reaction by mod-
ifications on the relative amount of reagents, solvent, or other
parameters were unsuccessful. However, when the reaction was
performed under microwave irradiation (MW), its rate was
impressively improved over the thermal reaction. The reaction
time reduced from 4 days to 5 min and the yield in 10a increased
from 55% to 90% (Table 1, entry 1). In all cases, the nucleophilic at-
tack was giving exclusively the product resulting from reaction at
the less hindered aziridine carbon. Under MW conditions, the acyl-
ated products 10a–10j could be smoothly obtained from starting
aziridines 8 and 9 and different thiols.20 Moreover, the acidic cleavage
of isopropylidene acetal in direct analogy with previously reported
diol deprotections in phytoceramide synthetic intermediates21 gave
the final thio-phytoceramide analogs 11a–11j (Table 1) in two steps
from aziridine derivatives.22
Ring-opening reaction of N-acylaziridines with thiols under microwave irradiation for
the synthesis of phytoceramides
O
O
OH
C14H29
O
RSH
O
CSA
R
R
S
S
N
X
C14H29
O
NH C14H29
O
NH OH
O
MeOH, rt
DBU, MeCN
100 ºC, MW
X
X
10a-10j
11a-11j
8
9
X = C7H15
X = OBn
Entry
1
R-SH
Aziridine
Time (min)
Yielda (%)
Yieldb (%)
SH
SH
8
5
10a: 90
11a: 60
2
3
8
8
80
20
10b: 66
10c: 60
11b: 81
11c: 75
OH
Cl
SH
SH
EtO
N
4
5
8
8
60
50
10d: 87
10e: 53
11d: —
11e: 95
N
H
SH
O
SH
6
7
8
8
8
9
50
5
10f: 56
10g: 80
10h: 87
11f: 98
11g: 68
11h: 90
SH
SH
SH
5
SH
9
9
9
15
20
10i: 75
11i: 77
11j: 97
10
10j: 82c
6
a
b
c
Isolated yields of ring-opening reactions.
Isolated yields of diol deprotection reactions.
1.6 mol-equiv of RSH were used.
(Table 1, entry 7) was used the corresponding thiosphingolipid 10g
was obtained in a good yield, this requiring an additional closed
reaction flask inside the microwave vessel to avoid thiol losses
due to its volatility. Other thiols such as 4-chlorobenzylthiol (Table 1,
entry 3), 2-naphthalenethiol, (Table 1, entry 9) and octyl mercaptan
The reactivity of aziridines 8 and 9 (Table 1, entries 1 and 8)
with thiophenol was similar, giving the corresponding adducts in
high yields with short reaction times. A variety of thiols can be
used including aliphatic and aromatic ones. When t-butyl mercaptan
TfN3
OH
OH
OH
TBDPSCl
Imidazole, DMAP
C14H29
OH
C14H29
OH
C14H29
OH
CuSO4, K2CO3
HO
HO
TBDPSO
N3
N3
NH2
MeOH, DCM, rt
95%
DMF, rt
85%
1
HCl
3
4
O
O
O
MsCl, TEA
O
DMP, pTSA
TBAF
O
O
C14H29
TBDPSO
HO
MsO
THF, rt
75%
THF
0 ºC to rt
99%
C14H29
C14H29
N3
DCM, rt
91%
N3
N3
7
6
5
O
O
C7H15
Cl
O
N
O
TEA, DCM
-10 ºC to rt
C14H29
C7H15
81%
87%
8
O
PPh3, DIPEA
O
HN
C14H29
THF:H2O 9:1
60 ºC
2
83%
O
BnO
TEA, DCM
0 ºC to rt
Cl
O
O
N
O
C14H29
OBn
9
Scheme 1. Synthesis of aziridine derivative 2 from phytosphingosine hydrochloride 1 and acylation reactions with octanoyl chloride and benzylchloroformate.

A. Alcaide, A. Llebaria / Tetrahedron Letters 53 (2012) 2137–2139
12. Harrak, Y.; Llebaria, A.; Delgado, A. Eur. J. Org. Chem. 2008, 4647–4654.
2139
OH
OH
C14H29
R
R
C14H29
13. Maligres, P. E.; See, M. M.; Askin, D.; Reider, P. J. Tetrahedron Lett. 1997, 38,
5253–5256.
14. Kim, S.; Lee, S.; Lee, T.; Ko, H.; Kim, D. J. Org. Chem. 2006, 71, 8661–8664.
15. Fan, Q-H.; Ni, N-T.; Li, Q.; Zhang, L-H.; Ye, X-S. Org. Lett. 2006, 8, 1007–1009.
16. Galonic, D. P.; van der Donk, W. A.; Gin, D. Y. J. Am. Chem. Soc. 2004, 126,
12712–12713.
40% aq. KOH
MeOH, 100ºC
S
S
O
NH OH
OBn
OH
NH2
(70%)
(57%)
11h
11i
12h
R =
R =
R =
12i R =
17. Galonic, D. P.; Ide, N. D.; van der Donk, W. A.; Gin, D. Y. J. Am. Chem. Soc. 2005,
127, 7359–7369.
18. Wu, J.; Hou, X-L.; Dai, L-X. J. Chem. Soc. Perkin Trans. 1 2001, 1314–1317.
19. For references related to catalytic desymmetrization of meso-aziridine
derivatives with thiols see: (a) Zhang, Y.; Kee, C. W.; Lee, R.; Fu, X.; Soh, J. Y.-
T.; Loh, E. M. F.; Huang, K.-W.; Tan, C.-H. Chem. Commun. 2011, 47, 3897–3899;
(b) Larson, S. E.; Baso, J. C.; Li, G.; Antilla, J. C. Org. Lett. 2009, 11, 5186–5189; (c)
Wang, Z.; Sun, X.; Ye, S.; Wang, W.; Wang, B.; Wu, J. Tetrahedron: Asymmetry
2008, 19, 964–969; (d) Peruncheralathan, S.; Henze, M.; Schneider, C.
Tetrahedron Lett. 2007, 48, 6743–6746.
Scheme 2. Carbamate cleavage of diol 11h and 11i.
(Table 1, entry 10) needed longer reaction times than thiophenol
and produced 10c, 10i, and 10j, respectively in moderate to high
yields. Other mercaptans such as 2-mercaptobenzyl alcohol (Table 1,
entry 2), 5-ethoxy-2-mercaptobenzimidazole (Table 1, entry 4),
furfuryl mercaptan, (Table 1, entry 5) and cyclohexanethiol (Table 1,
entry 6) required even longer reaction times for starting aziridine
consumption. In general, extended reaction times were detrimen-
tal for reaction yields.
20. Typical procedure for microwave-enhanced nucleophilic ring-opening reaction
of acylated aziridine 9 with thiophenol to afford 10h: In a 10 mL vessel, a
solution of aziridine derivative
9
(24.9 mg, 0.053 mmol) and thiophenol
L) was prepared. After, DBU (9 L,
(350 L, 0.066 mmol) in MeCN (500
l
l
l
0.060 mmol) was added and the mixture was stirred for 3 min under nitrogen.
The vessel was sealed with a septum and placed into the microwave cavity. The
microwave source was then turned on. Constant microwave irradiation
(150 W, 689.5 kPa, 100 °C) as well as simultaneous air-cooling were used
during the entire reaction time. The evolution of the reaction was monitored by
TLC and total consumption of the starting aziridine was achieved after
irradiating for 5 min. After cooling the reaction mixture to room
temperature, the solvent was removed under reduced pressure to yield the
crude product. Flash chromatographic purification (silica gel, hexane-ethyl
The final diol deprotection in acidic MeOH was achieved in
moderate to high yields, except for compound 11d that decom-
posed to a mixture of products under these conditions.
It is noteworthy that diols 11h, 11i afforded thio-phytosphingo-
sines 12h, 12i (Scheme 2)23 by the cleavage of carbamate group in
the presence of 40% aq KOH in MeOH, according to a related litera-
ture procedure.24 Catalytic hydrogenolysis was not effective for the
benzylcarbamate cleavage, presumably due to catalyst poisoning by
the sulfur functionality. The N-acylation of these sphingosine deriva-
tives represents an alternative way to obtain thio-ceramides increasing
the versatility of the aziridine route toward 1-S-phytosphingolipids.
In summary, we synthesized 1-thio-phytosphingolipid analogs
in moderate to good yields by means of regioselective nucleophilic
ring-opening reactions of acylated aziridine derivatives 8 and 9
followed by acetal deprotection. This synthetic methodology led us
to obtain phytosphingosine analogs by benzylcarbamate cleavage,
which would afford other ceramide analogs by subsequent acyla-
tion of the resulting amino group. This approach is a short and flex-
ible route for the synthesis of a variety of 1-thio-phytosphingolipids
by changing the thiol in the ring-opening step or the acylating agent
in the acylation of phytosphingosine analogs.
acetate 9:1) afforded adduct 10h as a colorless oil (26.9 mg, 87%). ½a D20
ꢁ18.7 (c
ꢀ
1.06, CHCl3). IR (film):
m = 3360, 2920, 2852, 1692, 1525, 1237, 1220,
1039 cmꢁ1 1H NMR (400 MHz, CDCl3) d 7.36 (d, J = 7.5 Hz, 2H), 7.32–7.23 (m,
.
5H), 7.22–7.15 (m, 2H), 7.12 (t, J = 7.2 Hz, 1H), 5.03 (d, A part of a AB system,
JAB = 12.2 Hz, 1H), 4.96 (d, B part of a AB system, JAB = 12.2 Hz, 1H), 4.94 (d,
J = 10.2 Hz, 1H), 4.08–3.98 (m, 2H), 3.97–3.87 (m, 1H), 3.22 (dd, A part of a AB
system, JAB = 13.9 Hz, J = 3.0 Hz, 1H), 3.09 (dd,
B part of a AB system,
JAB = 13.9 Hz, J = 6.8 Hz, 1H), 1.43–1.28 (m, 3H), 1.35 (s, 3H), 1.28–1.14 (m,
26H), 0.82 (t, J = 6.7 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 155.7 (C@O), 136.4
(C), 136.0 (C), 130.7 (CH), 129.1 (CH), 128.6 (CH), 128.3 (CH), 128.1 (CH), 126.8
(CH), 108.2 (C), 77.9 (CH), 77.8 (CH), 67.0 (CH2), 50.8 (CH), 37.5 (CH2), 32.1
(CH2), 29.9 (CH2), 29.8 (2CH2), 29.7 (CH2), 29.6 (CH2), 29.5 (CH2), 29.1 (CH2),
27.8 (CH3), 26.7 (CH2), 25.5 (CH3), 22.8 (CH2), 14.3 (CH3). HRMS calculated for
C
35H54NO4S: 584.3774 [M+H]+; found: 584.3752. Data for compounds 10a–10j
reported in Table 1 are collected in the Supplementary data.
21. Kratzer, B.; Mayer, T. G.; Schmidt, R. R. Eur. J. Org. Chem. 1998, 291–298.
22. Typical procedure for diol deprotection reaction of ring-opened product 10h to
afford 11h: To a solution of ring-opened product 10h (61.7 mg, 0.11 mmol) in
MeOH (10 mL), was added CSA (52.0 mg, 0.22 mmol). The mixture was stirred
at room temperature overnight. Then, the solvent was removed under reduced
pressure to give a crude that was purified by flash chromatography (silica gel,
hexane–ethyl acetate 7:3) and pure diol 11 h was isolated as a white solid
Acknowledgments
(53.9 mg, 90%). ½a D20
ꢀ
ꢁ31.1 (c 1.43, CHCl3). IR (film):
m
.
= 3439, 3338, 3066,
3037, 2958, 2914, 2847, 1689, 1527, 1342–1244 cmꢁ1
1H NMR (500 MHz,
CDCl3) d 7.41–7.30 (m, 7Har), 7.26 (t, J = 7.5 Hz, 2Har), 7.19 (t, J = 7.3 Hz, 1Har),
5.27 (d, J = 7.5 Hz, 1H (NH)), 5.06 (s, 2H (CH2–O)), 4.00–3.92 (m, 1H (CH–N)),
3.71–3.63 (m, 2H (CH–O (3) and CH–O (4)), 3.38 (dd, A part of a AB system,
This work was supported by MICINN (CTQ2011-29549-C02-01),
CSIC and Generalitat de Catalunya (2009-SGR-1072). A.A. thanks
Ministerio de Educación (Spain) for a predoctoral fellowship.
JAB = 13.6 Hz, J = 4.0 Hz, 1H (CH2–S)), 3.27 (dd,
B part of a AB system,
JAB = 13.6 Hz, J = 7.8 Hz, 1H (CH2–S)), 1.78 (br s, 2H (2OH)), 1.65–1.55 (m,
1H), 1.51–1.42 (m, 1H), 1.41–1.35 (m, 1H), 1.35–1.18 (m, 23H), 0.88 (t,
J = 6.9 Hz, 3H (CH3)). 13C NMR (101 MHz, CD3OD) d 158.4 (C@O), 138.4 (Car),
138.2 (Car), 130.5 (CHar), 129.9 (CHar), 129.4 (CHar), 128.9 (CHar), 128.7 (CHar),
126.9 (CHar), 77.6 (CH–O), 73.2 (CH–O), 67.4 (CH2–O), 53.9 (CH–N), 35.8 (CH2–
S), 33.6 (CH2), 33.1 (CH2), 30.8 (3CH2), 30.5 (CH2), 26.8 (CH2), 23.7 (CH2), 14.4
(CH3). HRMS calculated for C32H49NO4NaS: 566.3280 [M+Na]+; found:
566.3270. mp: 107–109 °C. Data for compounds 11a–11j reported in Table 1
are collected in the Supplementary data.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.02.066. These data
include MOL files and InChiKeys of the most important compounds
described in this article.
23. Typical procedure for carbamate cleavage of product 11h to afford aminodiol
12h: To a solution of carbamate 11h (19.1 mg, 0.035 mmol) in MeOH (500
lL),
References and notes
was added 40% aq KOH (240 L, 0.18 mmol). The mixture was heated to 100 °C
l
to give complete conversion after 24 h. After, the solvent was removed in vacuo
to give crude, which was purified by flash chromatography (silica gel, hexane–
ethyl acetate 2:8 + 1% aq NH3) to afford aminodiol 12h as a colorless oil
1. Gault, C. R.; Obeid, L. M.; Hannun, Y. A. Adv. Exp. Med. Biol. 2010, 688, 1–23.
2. Pitson, S. M. Trends Biochem. Sci. 2011, 36, 97–107.
3. Hannun, Y. A.; Obeid, L. M. Nat. Rev. Mol. Cell Biol. 2008, 9, 139–150.
4. Chang, Y-J.; Huang, J-R.; Tsai, Y-C.; Hung, J-T.; Wu, D.; Fujio, M.; Wong, C-H.; Yu,
A. L. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10299–10304.
5. Dere, R. T.; Zhu, X. Org. Lett. 2008, 10, 4641–4644.
6. Blauvelt, M. L.; Khalili, M.; Jaung, W.; Paulsen, J.; Anderson, A. C.; Wilson, S. B.;
Howell, A. R. Bioorg. Med. Chem. Lett. 2008, 18, 6374–6376.
7. Zhu, X.; Dere, R. T.; Jiang, J. Tetrahedron Lett. 2011, 52, 4971–4974.
8. Brodesser, S.; Kolter, T. J. Lipids 2011, 724015–724024.
9. Arenz, C. Cell Physiol. Biochem. 2010, 26, 1–8.
(10.1 mg, 70%). ½a D20
ꢀ
D ꢁ29.2 (c 0.98, CHCl3). IR (film):
m = 3351, 3276, 3060,
2920, 2850, 1736, 1584, 1468 cmꢁ1
.
1H NMR (500 MHz, CD3OD) d 7.43 (d,
J = 7.5 Hz, 2Har), 7.30 (t, J = 7.7 Hz, 2Har), 7.19 (t, J = 7.4 Hz, 1Har), 3.55–3.47 (m,
2H (CH–O (4) and A part of a CH2–S AB system)), 3.42 (dd, J = 8.3, 4.5 Hz, 1H
(CH–O (3)), 3.21–3.14 (m, 1H (CH–N)), 2.84 (dd, B part of a AB system,
JAB = 13.9 Hz, J = 10.4 Hz, 1H (CH2–S)), 1.78–1.71 (m, 1H), 1.60–1.50 (m, 1H),
1.38–1.27 (m, 24H), 0.90 (t, J = 6.9 Hz, 3H (CH3)). 13C NMR (101 MHz, CD3OD) d
136.8 (Car), 130.4 (CHar), 130.1 (CHar), 127.3 (CHar), 76.5 (CH–O (3)), 73.9 (CH–
O (4)), 53.3 (CH–N), 36.4 (CH2–S), 35.3 (CH2), 33.1 (CH2), 30.9 (CH2), 30.8
(2CH2), 30.5 (CH2), 26.5 (CH2), 23.8 (CH2), 14.5 (CH3). HRMS calculated for
10. Delgado, A.; Casas, J.; Llebaria, A.; Abad, J. L.; Fabriàs, G. ChemMedChem 2007, 2,
580–606.
11. Delgado, A.; Casas, J.; Llebaria, A.; Abad, J. L.; Fabriàs, G. Biochim. Biophys. Acta
2006, 1758, 1957–1977.
C
24H44NO2S: 410.3093 [M+H]+; found: 410.3087. Data for compound 12i
reported in Scheme 2 are collected in the Supplementary data.
24. Angle, S. R.; Arnaiz, D. O. Tetrahedron Lett. 1989, 30, 515–518.