The synthesis of 1,3-diamidophospholipids

Article abstract of DOI:10.1016/j.tetlet.2010.07.140

A straightforward synthesis of a small library of 1,3-diamidophospholipids is presented using readily available, cheap reagents and introducing a simple phosphoramidate protecting group strategy.

Full text of DOI:10.1016/j.tetlet.2010.07.140

Tetrahedron Letters 51 (2010) 5382–5384
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The synthesis of 1,3-diamidophospholipids
⇑
Illya A. Fedotenko, Pierre-Leonard Zaffalon, France Favarger, Andreas Zumbuehl
Department of Organic Chemistry, University of Geneva, 1211 Geneva, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A straightforward synthesis of a small library of 1,3-diamidophospholipids is presented using readily
available, cheap reagents and introducing a simple phosphoramidate protecting group strategy.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 23 June 2010
Revised 20 July 2010
Accepted 23 July 2010
Available online 7 August 2010
Keywords:
Nonnatural phospholipids
Phospholipid synthesis
Phosphatidyl ethanolamine
Amine-bearing phospholipids
Phospholipid nomenclature
Lipids are at the very heart and beginning of life on Earth and
owing to the tremendous efforts in lipidomics we are beginning
to understand why hundreds of structurally different lipids can
be found in a single cell.1–3 This variety originates in the modular
platform found by nature: Held together by glycerol, various fatty
acids and phosphate head groups can be chosen to build ever dif-
ferent phospholipids.4 Exchanging the natural glycerol for nonnat-
ural molecules such as diaminopropanol could lead to
phospholipids with new and interesting physical properties not
found in natural systems.
Historically, the synthesisofnonnaturalphospholipidscontaining
amides at the interfacehas been motivated by increasing the stability
of liposomes through additional hydrogen bonds and resistance to
phospholipase A2 cleavage.5 As a simplified version of the natural
sphingomyelin, 1,2-dimyristoylamido-l,2-deoxy-sn-glycero-3-phos-
phatidylcholine was found to significantly ease the reconstitution
of membrane proteins into liposomes.5 Such carriers showed a
potential for antitumor and anti-HIV agents.6,7 These studies were
carried out using racemic mixtures, and only later was enantiopure
material synthesized.8 However, significant advances in liposomal
stability were only achieved after moving to the symmetrical
1,3-diamido phospholipids carrying fluorinated alkyl chains.9–11
The synthesis of a 1,3-diamido phospholipid with hydrogenated
alkyl chains had not been reported yet, therefore this symmetrical
phospholipid seemed both an attractive chemical challenge and a
versatile platform for biophysical and biomedical studies. The
appearance of such scaffolds in the recent literature prompted us
to communicate our own efforts in this field.12,13
Presently, there is a lack of a clear nomenclature for nonnatural
phospholipids. The current IUPAC-recommended nomenclature for
natural phospholipids is based on the Fischer projection of the
glycerol placing the substituent on the secondary alcohol in an L-
position.14,15 Consecutive numbering of the glycerol carbon centers
leads to names such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phos-
phocholine abbreviated as POPC and 3-palmitoyl-2-oleoyl-sn-gly-
cero-1-phosphocholine for its enantiomer. Recently, Szoka and
Huang included the nature of the chemical linkage branching off
the glycerol into a modified nomenclature.16 Based on these two
recommendations, we propose a two-letter code that unambigu-
ously identifies the chemical linker. This code would follow the
standard trivial abbreviation of fatty acids (P for palmitoyl, O for
oleoyl, etc.) and would, for example, lead to Pes-Oes-PC being the
natural ester phospholipid POPC and PC-Oes-Pes being its enantio-
mer. Pad-PE-Pad would be the 1,3-palmitoylamido-l,3-deoxy-
sn-glycero-2-phosphatidyl-ethanolamine (1c) reported in this
communication. Besides ‘‘ad” for amide, ‘‘an” might stand for
amine, ‘‘et” for ether etc.
Our goal was to find an efficient synthesis based on readily
available reagents. We opted for introducing the fatty acid chains
late in the synthesis since we found 1,3-dipalmitoylamido-2-pro-
panol to be virtually insoluble in any solvent.13 Furthermore, such
an approach would also allow us to rapidly access various lipids of
different chain lengths.
Unfortunately, the general methods for phospholipid chemistry
that we tried were not applicable to the less reactive secondary
alcohol found in our scaffold, a problem that was also reported in
earlier syntheses of diamidophospholipids.9 Although being
reactive, reagent cost and oxidation problems kept us away from
using phosphoramidites.
⇑
Corresponding author. Tel.: +41 22 379 6719; fax: +41 22 379 3215.
E-mail address: andreas.zumbuehl@unige.ch (A. Zumbuehl).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.140

I. A. Fedotenko et al. / Tetrahedron Letters 51 (2010) 5382–5384
5383
HO
O
P
Cl
NHBoc
O
P
H2N
OH
1) Et3N, CH2Cl2
POCl3, Et3N
Et2O, 70%
Cl
O
O
O
Cl
Cl
Cl
NHBoc
2) aq. NH4OH, 53%
Cl
Cl
O
Cl
2
3
4
O
P
H2N
O
P
O
H2, Pd/C
NaN3, 100 °C
DMSO, 72%
O
O
H2N
N3
NHBoc
H2N
H2N
NHBoc
MeOH, 73%
N3
m
5
6
O
O
P
O
O
O
P
O
O
H2N
O
O
O
Cl
m
N
NHBoc
N
NH3
HCl
Dioxane
H
m
H
HN
HN
Et3N, CH2Cl2
7a)
7b)
7c)
7d)
1a)
1b)
1c)
1d)
m = 9: 46%
m = 11: 43%
m = 13: 41%
m = 15: 44%
m = 9: 93%
m = 11: 93%
m = 13: 99%
m = 15: 19%
O
O
m
m
Scheme 1. Synthesis of the symmetrical phospholipids Lad-PE-Lad (1a), Mad-PE-Mad (1b), Pad-PE-Pad (1c), and Sad-PE-Sad (1d).
O
P
O
O
P
O
O
O
O
O
O
O
N
NH3
N
NH3
H
H
HN
HN
O
O
1a
1c
O
O
O
O
P
O
P
O
O
O
O
O
N
NH3
N
NH3
H
H
HN
HN
O
O
1b
1d
Figure 1. A small library of 1,3-diamidophospholipids containing various acyl chain lengths.
We finally identified phosphorodichloridate 3 as a versatile
platform for our needs (see Scheme 1). The corresponding phos-
phate has been introduced as a flame retardant additive in polymer
synthesis and low-yielding syntheses of isomeric mixtures of 3
have been reported.17–19
Starting from 1,3-dichloropropanol 2 a substitution of POCl3 led
to phosphorodichloridate 3 that could be readily purified by distil-
lation (see Scheme 1). Any primary alcohol could then be used for a
second substitution on the activated phosphorous compound. The
obvious choice was a protected ethanolamine.20 The resulting
disubstituted phosphorochloridate could not be hydrolyzed di-
rectly into the phosphate. This problem could be circumvented
by converting the phosphorochloridate to the corresponding phos-
phoramidate 4. This simple protecting group was now readily
removable under the same conditions used for BOC deprotection.
To our knowledge, this was the first time that this strategy was ap-
plied to the synthesis of a phospholipid.
Amide bond formation with long chain acyl chlorides led to the
protected diamido phospholipids 7a–d: Reaction with lauryl
chloride led to Lad-PE(Boc)-Lad (7a); reaction with myristoyl chlo-
ride led to Mad-PE(Boc)-Mad (7b); reaction with palmitoyl
chloride led to Pad-PE(Boc)-Pad (7c); and reaction with stearoyl
chloride led to Sad-PE(Boc)-Sad (7d). Treatment with concentrated
hydrochloric acid both hydrolyzed the BOC protecting group as
well as the amidophosphate yielding the title compounds 1a–d
in six linear steps.
In conclusion, we have reported a straightforward synthesis of
several symmetrical 1,3-diamido phospholipids requiring no strin-
gent exclusion of either air or water. The reported structures might
serve as an interesting and flexible platform for further studies in
biophysics and related fields. Work in this direction is currently
ongoing.
Acknowledgments
The dichloropropyl phosphoramidate 4 was transformed into
the diazide 5 by substitution with sodium azide and further re-
duced to the diamine 6.
Diamine 6 is an interesting intermediate, allowing the pursuit
of several different synthetic directions. Here, the double substitu-
tion with acyl chains of different length led to a rapid access of a
small library of 1,3-diamidophospholipids (see Fig. 1).
The authors thank the University of Geneva and the Swiss Na-
tional Science Foundation for the financial support and acknowl-
edge the contributions of the Mass Spectrometry platform at the
Faculty of Sciences, University of Geneva, for mass spectrometry
services and A. Pinto for NMR measurements.

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I. A. Fedotenko et al. / Tetrahedron Letters 51 (2010) 5382–5384
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Supplementary data (experimental procedures and full spectro-
scopic data for all compounds) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2010.07.140.
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