A direct transformation of O-silyl groups into O-trichloroacetates. A novel synthetic approach to protein kinase C ligands: 1-Oleoyl-2-acetyl- and 1-hexadecyl-2-acetyl-sn-glycerols

Article abstract of DOI:10.1016/j.tetlet.2005.08.015

A fluoride ion-promoted direct esterification of tert-butyldimethylsilyl- (TBDMS), or triisopropylsilyl (TIPS)-protected glycerol derivatives by means of trichloroacetic anhydride (TCAA), followed by removal of the trichloroacetyl transient protection, provides a new, efficient entry to stereochemically pure 1-oleoyl-2-acetyl- and 1-O-hexadecyl-2-acetyl-sn-glycerols.

Full text of DOI:10.1016/j.tetlet.2005.08.015

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6855–6859
A direct transformation of O-silyl groups into
O-trichloroacetates. A novel synthetic approach to protein kinase C
ligands: 1-oleoyl-2-acetyl- and 1-hexadecyl-2-acetyl-sn-glycerols
a,
b
b,c,
*
*
Stephan D. Stamatov, Martin Kullberg and Jacek Stawinski
a
Department of Chemical Technology, University of Plovdiv, 24 Tsar Assen Street, Plovdiv 4000, Bulgaria
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden
b
c
Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland
Received 1 July 2005; revised 28 July 2005; accepted 2 August 2005
Available online 16 August 2005
Abstract—A fluoride ion-promoted direct esterification of tert-butyldimethylsilyl- (TBDMS), or triisopropylsilyl (TIPS)-protected
glycerol derivatives by means of trichloroacetic anhydride (TCAA), followed by removal of the trichloroacetyl transient protection,
provides a new, efficient entry to stereochemically pure 1-oleoyl-2-acetyl- and 1-O-hexadecyl-2-acetyl-sn-glycerols.
Ó 2005 Elsevier Ltd. All rights reserved.
1
. Introduction
Despite the apparent structural simplicity, synthesis of
unsymmetrical 1,2-diacylglycerols has repeatedly been
shown to be a complicated task since incorporation
of two different substituents at the primary versus
secondary glycerol position calls for an extensive use
of protecting groups, which must be removable under
conditions that do not provoke acyl scrambling and
It has only recently become apparent that 1,2-diacyl-sn-
glycerols (DAG) and their 1-O-alkyl-2-acyl-sn-isosteres
ALAG), when transiently generated within cells from
various carrier-molecules (e.g., phospholipids, triglycer-
ides, etc.), can modulate a broad spectrum of protective
or physiopathological processes in living organisms via
(
in the case of unsaturated derivatives, are also compat-
1
5,10
signal transduction pathways. Due to this, the afore-
ible with the presence of double bonds.
Unfortu-
mentioned classes of diglycerides (DG) emerged as
synthetic targets of considerable importance in carbohy-
nately, problems often arise at the end of synthesis as
most of the protecting systems available to date
2
3
4,5
11
12
9,13
drate, nucleic acid, and lipid research, relevant to
(e.g., levulinate, benzyl, trityl,
2,2,2-trichloroeth-
6
14
15
rational drug design. Growing interest in lipid media-
oxycarbonyl,
9-phenylxanthen-9-yl,
etc.) require
tors and their conjugates caused an increasing demand
for configurationally pure 1-oleoyl-2-acetyl- and 1-O-
hexadecyl-2-acetyl-sn-glycerol as valuable precursors to
deblocking conditions that either preclude admission
to conjugates with unsaturated or acid/base-sensitive
replacements, or trigger undesirable molecular rear-
5
7
,8
various bioactive natural products and specific effec-
rangements (e.g., acyl migration, racemization, etc.)
after exposure of a free hydroxyl group of the glycerol
8
tors of protein kinase C (PKC). The latter aspect is of
5
,16
particular importance for developing new biochemical
and medicinal diagnostics and in structure–activity rela-
tionship studies of potential therapeutics targeting
backbone.
The growing recognition of the common protection–
deprotection protocols as sources of acute preparative
9
PKC.
5
,11,17
complications,
turned our attention to silyl groups
as alternative protecting groups. Although organosil-
icon derivatives have found numerous applications in
Keywords: Triethylamine tris(hydrofluoride); Trichloroacetic anhy-
dride; 1-Oleoyl-2-acetyl-sn-glycerol; 1-O-Hexadecyl-2-acetyl-sn-gly-
cerol; Diglycerides; Protein kinase C ligands.
1
8
the synthesis of complex organic compounds, they
have not been exploited to any significant extent in
glycerolipid chemistry since the cleavage conditions
*
Corresponding authors. Tel.: +46 8 16 24 85; fax: +46 8 15 49 08;
e-mail: js@organ.su.se
1
9
20
are frequently detrimental to acid-,
base-,
or
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.015

6856
S. D. Stamatov et al. / Tetrahedron Letters 46 (2005) 6855–6859
2
1
oxidation/reduction-susceptible substrates,
or pro-
80 °C for 2h. This produced quantitatively and in a
1
2
0
mote an acyl migration process. A reagent recom-
mended for the deprotection of terminal TBDMS–
ethers of DAG without acyl migration, N-bromosuc-
cinimide, proved to be incompatible with unsaturated
fatty acid residues due to its powerful brominating prop-
erties. Another reagent, pyridinium p-toluenesulfonate
in CH Cl –CH OH (2:1 v/v) significantly suppressed
highly chemo- and regiospecific manner (>99%,
H
1
3
and C NMR spectroscopy) trichloroacetates 4 and 5,
which were isolated in 90–93% yields after flash column
silica gel chromatography. The obtained trichloroacetyl
derivatives 4 and 5 can either be subjected directly to
subsequent transformations, or stored for several
months (ꢀ20 °C, under argon) without detectable alter-
2
2
2
3
2
2
3
1
13
the acyltropy (acid, base, or heat initiated migration of
ations of their spectral characteristics ( H and C NMR
spectroscopy).
2
4
25
an acyl group ) during desilylation of ALAG, but
was shown to be impractically slow (reaction time: up
to 14 days at 0 °C).
Regarding the scope and limitations of this particular
chemistry, some additional observations are pertinent.
The rate of replacement of the silyl groups by a trichloro-
acetyl moiety was not appreciably influenced by
electronic features or the type of molecular fragments
present in 1–3 (e.g., unsaturated acyl, various alkyl groups).
Trimethylsilyl derivatives underwent esterification at
comparable rates to those of TBDMS and TIPS ethers.
The reactions in organic solvents were considerably
One should also note that none of the above methodol-
ogies provides direct access to the DG needed after
deprotection. Instead, these procedures involve work-
up and a painstaking chromatography5
,22,25
to separate
the target compounds from by-products. However, such
additional operations often contribute to side-reactions
5
,24
or decomposition of labile acylglycerols.
slower (e.g., in CHCl ; reaction time ꢁ48 h) or produced
3
In this letter, we report that treatment of 1-oleoyl-2-acet-
yl-3-O-tert-butyldimethylsilyl- 1, 1-oleoyl-2-acetyl-3-O-
triisopropylsilyl- 2, or 1-O-hexadecyl-2-acetyl-3-O-tert-
butyldimethylsilyl-sn-glycerols 3 with trichloroacetic
anhydride in the presence of triethylamine tris(hydrofluo-
ride) (TEAÆ3HF) effects a direct transformation between
silyl and trichloroacetyl protecting groups. Since the re-
moval of a trichloroacetyl moiety can be carried out
quantitatively and without additional purification, thus
eliminating drawbacks inherent to deprotection of their
silyl-counterparts, this may constitute a novel strategy
for the synthesis of configurationally homogeneous pro-
tein kinase C ligands, namely 1-oleoyl-2-acetyl- 6 and
complex reaction mixtures (e.g., reactions in THF). The
rate of trichloroacetylation remained practically within
the same range of magnitude upon replacement of
TEAÆ3HF by tetra-n-butylammonium fluoride (TBAFÆ
1
13
3H O). In the latter case, however, H and C NMR
2
analysis revealed that regioselectivity was eroded due
to acyl migration (5–10%), triggered probably by the
substantial amounts of free trichloroacetic acid gener-
ated from TCAA in the presence of water. To suppress
this process, a larger excess of TCAA (up to 15 equiv)
was required. Although trifluoroacetic anhydride could
be used as a substitute for TCAA, the trifluoroacetyl
derivatives were less convenient to work with as they
decomposed during column chromatography.
1
-O-hexadecyl-2-acetyl-sn-glycerols 7 (Scheme 1), or
related compounds.
It was interesting to note that for TBDMS derivatives 1
and 3 the reactions could be carried out without fluoride
ion present, by using tetra-n-butylammonium trichloro-
acetate with 2equiv of TCAA. Unfortunately, these
To implement this protocol, reaction conditions for a
direct trichloroacetylation of compounds 1–3 (Scheme
1
, step A) bearing two silyl protecting groups of most
1
8
common use in general organic synthesis, TBDMS
and TIPS, were investigated employing different sol-
vents, ratio of reactants, etc. The best results were
achieved when compounds 1–3 were treated under
argon with neat TCAA (9.0 equiv) and TEAÆ3HF
reactions were rather slow (e.g., in CHCl at rt, ca.
3
2 4 h;ꢁ90% conversion) and failed for TIPS derivatives
(<5% conversion after 24 h; TLC).
On the basis of the above data we can tentatively formu-
late a mechanism which involves an electrophilic attack
(
2.0 equiv) in a tightly stoppered glass ampoule at
OCOC17H33
OCOC17H33
OCOC17H33
AcO
, 2
H
OSi
AcO
H
AcO
AcO
H
OCOCCl3
OH
6
1
4
Step A
Yields: 90 - 93%
Step B
Yields: 100%
OC16H33
H
OC16H33
H
OC16H33
H
AcO
AcO
OSi
OCOCCl3
OH
3
5
7
1
, 3 Si = TBDMS
Si = TIPS
2
3 3
Scheme 1. Reagents and conditions: (step A) Et NÆ3HF (2.0 equiv)/(CCl CO)2O (9.0 equiv), no solvent, 80 °C/2h; (step B) pyridine (50 equiv)/
MeOH (500 equiv), THF, rt/3 h.

S. D. Stamatov et al. / Tetrahedron Letters 46 (2005) 6855–6859
OAc OAc
6857
OAc
F-
RO
O
O Si
CCl3
RO
O
Si
RO
O
CCl3
O
O
CCl3
Cl3C
O
O
A
R = acyl or alkyl
Scheme 2.
by TCAA to form an intermediate of type A, followed
by a nucleophilic attack by the fluoride ion on the silicon
thus eliminating potential sources for acyl migration and
other side-reactions; (iii) the method introduces silyl and
trichloroacetyl groups as versatile protecting functional-
ities in the synthesis of bioactive lipid mediators; (iv) it
makes use of commercially available reactants and can
easily be scaled up.
(
Scheme 2). This rationalizes the replacement of the silyl
protection by trichloroacetyl without migration of the
acetyl group. Since no bond breaking takes place at
the stereogenic C2-center, the transformation is sup-
posed to be stereospecific and to occur with retention
of configuration. The observed exclusive formation of
trifluoroacetates 4 or 5 with defined stereochemistry
2. Experimental
(
see Experimental) and the lack of an intramolecular
acyl scrambling, are in agreement with this hypothesis.
2.1. Starting substrates 1–3
An alternative mechanism comprising a nucleophilic
attack by a trichloroacetate anion on silicon, seems to
be less likely as fluoride ion is orders of magnitude more
effective as a nucleophile for silicon then carboxylates.
This is also consistent with the reactions carried out in
the absence of fluoride (see above).
Obtained in two steps via consecutive silylation and
2
6
acylation of chiral monoglycerides according to stan-
2
2,27
or as described elsewhere.28 The
dard procedures
synthesized compounds have been characterized by
1
13
H- and C NMR spectroscopy, and their purity
1
(>99%) assessed by H NMR.
As a final stage of this investigation, the trichloroace-
tates 4 and 5 were converted into 2-acetylglycerols 6
and 7, respectively (Scheme 1, step B). Due to the high
electrophilicity of the carbonyl center of trichloroacetyl
derivatives, this could be carried out selectively even in
the presence of an acetyl group, by treating compounds
2.1.1. 1-Oleoyl-2-acetyl-3-O-tert-butyldimethylsilyl-sn-
glycerol 1. Colorless oil; Rf (pentane–toluene–
20
EtOAc = 40:50:10,
v/v/v) = 0.62;
½aꢂ +9.75
(c
D
11.05, CHCl ). Found: C, 67.88; H, 11.08%. C H O Si
3
29 56
5
(512.84) requires C, 67.92; H, 11.01%.
4
and 5 in THF with pyridine (50 equiv) and methanol
2.1.2. 1-Oleoyl-2-acetyl-3-O-triisopropylsilyl-sn-glycerol
2. Colorless oil; Rf (pentane–toluene–EtOAc =
(
500 equiv) at room temperature. The reactions were
20
quantitative (completion within 3 h) and after removal
of the volatile products, diglycerides 6 and 7 of purity
>
40:50:10, v/v/v) = 0.61; ½aꢂ +11.28 (c 9.87, CHCl3).
D
Found: C, 69.20; H, 11.30%. C H O Si (554.92)
3
2
62
5
1
13
99% ( H and C NMR spectroscopy) could be ob-
requires C, 69.26; H, 11.26%.
tained without any additional purification.
2.1.3. 1-O-Hexadecyl-2-acetyl-3-O-tert-butyldimethylsi-
In conclusion, we have developed an efficient synthetic
strategy based on a novel transformation of silyl ethers
into trichloroacetyl derivatives to afford functionalized
glyceride synthons 4 and 5 as convenient storage forms
lyl-sn-glycerol 3. Colorless oil; R (pentane–toluene–
f
20
EtOAc = 40:50:10,
v/v/v) = 0.58;
½aꢂ +4.51
(c
D
14.60, CHCl ). Found: C, 68.67; H, 11.87%. C H O Si
3
27 56
4
(472.82) requires C, 68.59; H, 11.94%.
(
protection of 3-hydroxyl groups as trichloroacetyl
esters should prevent scrambling of the acyl moiety) or
intermediate frameworks from which protein kinase C
ligands (e.g., 6 and 7) can be retrieved directly without
recourse to any additional purification techniques. The
method seems to be general and will probably be appli-
cable to the synthesis of other lipid derivatives.
2.2. General procedure for the direct conversion of silyl
ethers 1–3into trichloroacetates 4 and 5 (step A)
A mixture of the starting silyl ether 1–3 (1.00 mmol),
trichloroacetic anhydride (1.644 mL, 9.00 mmol) and
triethylamine tris(hydrofluoride) (0.326 mL, 2.00 mmol)
was kept under argon, in a pressure-proof glass ampoule
at 80 °C (bath) for 2h. The system was taken in toluene–
ethyl acetate (98:2, v/v; 5 mL) and the target trichloro-
acetyl derivative 4 or 5 was isolated in pure state (purity
The main features of this new protocol are: (i) highly
regioselective and quantitative generation under mild
conditions of 1,2-diacyl- 6 and 1-alkyl-2-acyl-3-sn-glyce-
rols 7 bearing unsaturated or saturated alkyl chain; (ii)
the produced compounds 6 and 7 are of high purity,
which alleviates problems of their additional processing,
1
>99%, H NMR spectroscopy) by flash column chroma-
tography (silica gel; mobile phase: toluene–ethyl ace-
tate = 98:2, v/v).

6858
S. D. Stamatov et al. / Tetrahedron Letters 46 (2005) 6855–6859
2
(
.2.1.
obtained from 1) 4. Yield: 0.506 g (93%, colorless
oil); R (pentane–toluene–EtOAc = 40:50:10, v/v/v) =
1-Oleoyl-2-acetyl-3-trichloroacetyl-sn-glycerol
Weber, N. Prog. Biochem. Pharmacol. 1988, 22, 48–57;
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3
f
20
266, 4661–4664.
0.56; ½aꢂ ꢀ0.40 (c 7.18, CHCl ). Found: C, 55.00; H,
7
5
D
3
2
3
. VonMinden, H. M.; Morr, M.; Milkereit, G.; Heinz, E.;
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.70; Cl, 19.73%. C H O Cl (543.95) requires C,
25 41 6 3
5.20; H, 7.60; Cl, 19.55%.
2
001, 1379–1382.
2
.2.2. 1-Oleoyl-2-acetyl-3-trichloroacetyl-sn-glycerol
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(
obtained from 2) 4. Yield: 0.489 g (90%, colorless
20
oil); ½aꢂ ꢀ0.42( c 9.43, CHCl ); all other physicochem-
D
3
ical and spectral characteristics are identical with those
of the previous product.
2
.2.3. 1-O-Hexadecyl-2-acetyl-3-trichloroacetyl-sn-gly-
5
. Paltauf, F.; Hermetter, A. Prog. Lipid Res. 1994, 33, 239–
28.
cerol (obtained from 3) 5. Yield: 0.464 g (92%, colorless
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3
f
6
. Arrese, E. L.; Wells, M. A. J. Lipid Res. 1997, 38, 68–76;
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81–82; Marquez, V. E.; Nacro, K.; Benzaria, S.; Lee, J.;
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20
0.54; ½aꢂ ꢀ5.81 (c 11.81, CHCl ). Found: C, 54.82; H,
8
5
D
3
.27; Cl, 21.00%. C H O Cl (503.93) requires C,
23 41 5 3
4.82; H, 8.20; Cl, 21.11%.
2
6
.3. General procedure for the synthesis of diglycerides
and 7 (step B)
7
. Nacro, K.; Bienfait, B.; Lewin, N. E.; Blumberg, P. M.;
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5.0 mL), a mixture of pyridine (4.0 mL, 50 mmol) and
(
6
55; Nacro, K.; Sigano, D. M.; Yan, S.; Nicklaus, M. C.;
methanol (20.3 mL, 500 mmol) was added and the reac-
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vents were evaporated under reduced pressure (bath
temp. 50 °C) and the residue was kept under high vacu-
um at room temperature for 2–3 h to give the unpro-
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tected diglyceride 6 or 7 (purity >99%, H NMR
3
463–3466.
spectroscopy).
8
. Slater, S. J.; Seiz, J. L.; Stagliano, B. A.; Cook, A. C.;
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4
.3.1. 1-Oleoyl-2-acetyl-sn-glycerol (obtained from 1 via
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4
0, 6085–6092.
9
. Strawn, L. M.; Martell, R. E.; Simpson, R. U.; Leach, K.
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f
20
½
aꢂ ꢀ5.42( c 5.07, CHCl ). Found: C, 70.01; H,
D
3
1
0.60%. C H O (398.58) requires C, 69.31; H, 10.62%.
23 42 5
2
4
.3.2. 1-Oleoyl-2-acetyl-sn-glycerol (obtained from 2 via
) 6. Yield (calculated on 4): 0.398 g (100%, colorless
20
oil); ½aꢂ ꢀ5.33 (c 9.50, CHCl ); all other physicochem-
D
3
1
ical and spectral characteristics are identical with those
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1
671–1678.
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5
.3.3. 1-O-Hexadecyl-2-acetyl-sn-glycerol (obtained from
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1
1
from pentane); R (toluene–EtOAc = 80:20, v/v) =
f
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2
D
0
29
20
0
.23; ½aꢂ ꢀ5.44 (c 2.39, CHCl ); lit. ½aꢂD ꢀ11.1 (c 0.4,
3
CHCl ).
3
1
1
1
4. Pfeiffer, F. R.; Miao, C. K.; Weisbach, J. A. J. Org. Chem.
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970, 35, 221–224.
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