4(2)-Methoxyphenyl glycerol ethers in the synthesis of nonracemic di-O,O-acylglycerols

Article abstract of DOI:10.1007/s11172-008-0328-9

Effective methods for the synthesis of nonracemic 4-and 2-methoxyphenyl glycerol ethers from nonracemic 3-chloropropanediols and by direct resolution of the racemate, respectively, were developed. Some existing discrepancies related to the to chiroptical properties of their derivatives were eliminated. Both ethers were used to synthesize nonracemic 3-O-aryloxy-1,2-di-O',O'-palmitoyl glycerols.

Full text of DOI:10.1007/s11172-008-0328-9

Russian Chemical Bulletin, International Edition, Vol. 57, No. 11, pp. 2320—2323, November, 2008
2320
4(2)ꢀMethoxyphenyl glycerol ethers
in the synthesis of nonracemic diꢀO,O ꢀacylglycerols
Z. A. Bredikhina, V. G. Novikova, Yu. Ya. Efremov, D. R. Sharafutdinova, and A. A. Bredikhin
A. E. Arbuzov Institute of Organic & Physical Chemistry,
Kazan Scientific Center of the Russian Academy of Sciences,
8 ul. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 2253. Eꢀmail: baa@iopc.knc.ru
Effective methods for the synthesis of nonracemic 4ꢀ and 2ꢀmethoxyphenyl glycerol ethers
from nonracemic 3ꢀchloropropanediols and by direct resolution of the racemate, respectively,
were developed. Some existing discrepancies related to the to chiroptical properties of
their derivatives were eliminated. Both ethers were used to synthesize nonracemic 3ꢀOꢀaryloxyꢀ
1,2ꢀdiꢀO´,O″−palmitoyl glycerols.
Key words: nonracemic glycerolipids; chiroptical properties, 4ꢀ and 2ꢀmethoxyphenyl
glycerol ethers, spontaneous resolution of racemates.
Glycerol ethers and esters R1OCH2CH(OR2)CH2OR3
occur in living systems as structural materials and particiꢀ
pants of important metabolic processes.1 As a rule, these
compounds are chiral and are represented in living organꢀ
isms by a single enantiomer; therefore, the synthesis of
nonracemic glycerolipids and their derivatives have long
attracted the interest of chemists.2
Scheme 1
To prepare such compounds, nonracemic 3ꢀOꢀ(4ꢀmethoxyꢀ
phenyl)ꢀsnꢀglycerol ((R)ꢀ1) was proposed as the chiral
platform. Through intermediate 1,2ꢀdiꢀO,O´ꢀpalmitoylꢀ
3ꢀO″ꢀ(4ꢀmethoxyphenyl)ꢀsnꢀglycerol ((S)ꢀ2) and subseꢀ
quent removal of the phenoxyl fragment, this compound
was converted into nonracemic 1,2ꢀdiꢀO,O´ꢀpalmitoylꢀ
snꢀglycerol ((S)ꢀ3) (Scheme 1).3 Diol 1 was used in
the same reaction sequence4 (without reference to the
original source); however, Vilcheze et al.3 described
the stereochemistry of transformations as (R)ꢀ(–)ꢀ1 →
→ (S)ꢀ(+)ꢀ2 → (S)ꢀ(–)ꢀ3, whereas according to Kamal
et al.,4 specific rotation of the initial and final products
changes in the sequence (R)ꢀ(–)ꢀ1 → (S)ꢀ(–)ꢀ2 →
→ (S)ꢀ(+)ꢀ3 (the data for aryloxypropanediols refer to
solutions in alcohols and those for diacylated derivatives
refer to chloroform solutions).
DCC is dicyclohexylcarbodiimide,
DMAP is 4ꢀ(N,Nꢀdimethylamino)pyridine
the reagent (asymmetric dihydroxylation was described in
detail5). Simultaneously, it was reported6 that (R)ꢀ1 has a
positive specific rotation [α]D25 +7.35 (c 1.5, MeOH),
while negative [α]D25 –7.4 value (c 1.16, MeOH) was
ascribed to 1ꢀOꢀ(4ꢀmethoxyphenyl)ꢀsnꢀglycerol, i.e., the
antipode (S)ꢀ1 obtained using ADꢀmixꢀβ (see Ref. 6).
According to published data,4 Pseudomonas cepacia
lipaseꢀcatalyzed partial esterification of racemic 1ꢀchloroꢀ
3ꢀ(4ꢀmethoxyphenoxy)propanꢀ2ꢀol (racꢀ4) resulted in
When using compounds like 2 and 3 to model the
properties of neutral glycerolipids, it is necessary to know
the exact relationship between the absolute configuration
and chiroptical properties for the reactants and final
products. Unfortunately, the data available from the
literature are contradictory even for diol 1.
Previously,3 enantiomer (R)ꢀ(–)ꢀ1, [α]D26 –7.58
(c 4.0, MeOH) was obtained by asymmetric dihydroxyꢀ
lation of 4ꢀmethoxyphenyl allyl ether using ADꢀmixꢀα as
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2275—2278, November, 2008.
1066ꢀ5285/08/5711ꢀ2320 © 2008 Springer Science+Business Media, Inc.

Scalemic diacylglycerols
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008 2321
Scheme 2
(R)ꢀ(–)ꢀ4 and (S)ꢀ(+)ꢀ2ꢀacetoxyꢀ1ꢀchloroꢀ3ꢀ(4ꢀmethoxyꢀ
phenoxy)propane ((S)ꢀ5), which were converted in two
steps into diols (+)ꢀ1 and (–)ꢀ1, respectively. A positive
specific rotation was attributed to (S)ꢀ3ꢀOꢀ(4ꢀmethoxyꢀ
phenyl)ꢀsnꢀglycerol, which is again selfꢀcontradictory, as
3ꢀOꢀ(4ꢀmethoxyphenyl)ꢀsnꢀglycerol has (R)ꢀconfiguraꢀ
tion according to the Cahn—Ingold—Prelog rules.
The development of enantioselective hydrolysis of epiꢀ
chlorohydrin7 provided a route to enantiopure 3ꢀchloroꢀ
propaneꢀ1,2ꢀdiols (6) whose configuration is well known.
To eliminate the above contradiction, we obtained both
enantiomers of diol 1 directly from 4ꢀmethoxyphenol and
chlorohydrins 6; the reaction does not affect the chiral
center and, hence, (R)ꢀ6 is converted into enantiomer
(R)ꢀ(–)ꢀ1, and (S)ꢀ6 gives enantiomer (S)ꢀ(+)ꢀ1 (Scheme 2).
The 3ꢀ(4ꢀmethoxyphenoxy)propaneꢀ1,2ꢀdiol samples
thus obtained with known configuration were introduced
into reactions shown in Scheme 1; their stereochemistry
can be described by the sequence (R)ꢀ(–)ꢀ1 → (S)ꢀ(+)ꢀ2
→ (S)ꢀ(–)ꢀ3 and (S)ꢀ(+)ꢀ1 → (R)ꢀ(–)ꢀ2 → (R)ꢀ(+)ꢀ3,
which coincides with the results of Vilcheze et al.3 and
disproves the results of Kamal et al.4
For ultimate assertion of the established configuraꢀ
tional relations, we implemented the same reaction
sequence starting from 3ꢀ(2ꢀmethoxyphenoxy)propaneꢀ
1,2ꢀdiol (7), which is known in racemic form as the drug
guaifenesin.8 Since nonracemic diols 7 have specific rotaꢀ
tions close to those of diols 1 as regards both the sign
and the magnitude, it was reasonable to expect that this
situation would be reproduced for diacyl derivatives.
Guaifenesin 7 belongs to a rare group of chiral comꢀ
pounds capable of spontaneous resolution into enantioꢀ
mers upon crystallization.9 Using this feature, we
developed an efficient method for the preparation of pure
enantiomers of 7 in considerable amounts10 and conꢀ
verted them into nonracemic lipid like dipalmitates 8
(Scheme 3).
The experiments confirmed that, as for 4ꢀmethoxy
derivatives, the specific rotations of the starting diols and
final dipalmitate have different signs. Pure enantiomers 2
Scheme 3
i. Resolution by entrainment.

2322
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008
Bredikhina et al.
spontaneous resolution on crystallization using the procedure
we developed previously.10 Compound (R)ꢀ7, m.p. 97—99 °C,
[α]D20 –9.4 (c 1.0, MeOH), ee 99.5% (HPLC; hexane—propanꢀ
2ꢀol—diethylamine (80 : 20 : 0.1) as the mobile phase, 1.0 mL
min–1, tR = 9.9 min). Compound (S)ꢀ7: m.p. 97—99 °C,
[α]D20 +9.5 (c 1.0, MeOH), ee 99.7% (HPLC, the same conꢀ
ditions, tR = 17.4 min).
(R)ꢀ1,2ꢀDipalmitoyloxyꢀ3ꢀ(4ꢀmethoxyphenoxy)propane
(R)ꢀ2 was prepared by analogy with published procedures3,4
from diol (S)ꢀ1 (0.3 g, 1.5 mmol). This gave 0.84 g (82%) of
(R)ꢀ2, m.p. 64—66 °C, [α]D20 –11.5 (c 0.9, CHCl3) (cf. Ref. 4:
m.p. 64—65 °C, [α]D20 +11.5 (c 1, CHCl3, ee 95%)). 1H NMR,
δ: 0.89 (t, 6 H, CH3, J = 6.8 Hz); 1.27 (br.s, 48 H, CH2);
1.61—1.65 (m, 4 H, CH2CH3); 2.32, 2.35 (both t, 2 H each,
C(O)CH2, J = 7.6 Hz); 3.78 (s, 3 H, OCH3); 4.07 (d, 2 H, CH2O,
J = 5.2 Hz); 4.30 (dd, 1 H, CH2O, J = 12.0 Hz, J = 6.0 Hz);
4.44 (dd, 1 H, CH2O, J = 12.0 Hz, J = 3.9 Hz); 5.36—5.39 (m,
1 H, CHO); 6.82—6.86 (m, 4 H, Ar). MS, m/z (Irel (%)): 675
(2.4), 674 (5.1), 553 (10.0), 552 (47.0), 551 (100.0), 550 (1.1),
436 (8.1), 419 (7.6), 313 (11.3), 163 (21.5), 124 (8.6), 95 (4.6),
85 (9.2), 83 (5.4), 71 (14.1), 57 (22.3), 43 (9.0).
and 8 and their analogs with other acyl residues can be
used for modeling the physical and chemical behavior of
triglycerides. They have similar characteristics as regards
this purpose but nonracemic 2ꢀmethoxy derivatives
are cheaper and more accessible. The removal of the
4ꢀmethoxyphenyl unit on treatment with cerium ammoꢀ
nium nitrate in aqueous acetonitrile proceeds rather comꢀ
pletely, which allows one to prepare other nonracemic
triglycerides after removal of this protective group.
Unfortunately the 2ꢀmethoxyphenyl group is not removed
so easily under the same conditions, diesters 3 being
formed with the same stereochemical outcome but in only
5—10% yield. The low yield is apparently due to lower
stability of 2ꢀquinones formed in the reaction compared
to the 4ꢀanalogs. An increase in the reaction temperature
or replacement of solvent by chloroform results in more
extensive and less selective hydrolysis giving a poorly
separable mixture of monoꢀ and disubstituted glycerol
derivatives.
(S)ꢀ1,2ꢀDipalmitoyloxyꢀ3ꢀ(4ꢀmethoxyphenoxy)propane
(S)ꢀ2 was prepared similarly to (R)ꢀ2 from diol (R)ꢀ1. M.p. 63—
65 °C, [α]D20 +10.6 (c 1, CHCl3) (cf. Ref. 3: m.p. 63.5—64 °C,
[α]D20 +11.3 (c 6.8, CHCl3); Ref. 4: m.p. 64—65 °C, [α]D20 –10.7
(c 1.1, CHCl3, ee 90%).
Experimental
The 1H and 13C NMR spectra were recorded on a
Bruker Avanceꢀ600 instrument operating at 600.13 MHz (1H),
150.864 MHz (13C) in CDCl3. The proton chemical shifts
are referred to Me4Si. Optical rotation was measured on a
Perkin—Elmer 341 polarimeter. The specific rotation is
expressed in (deg mL) (g dm)–1 and solution concentrations are
in g (100 ml)–1. The melting points were determined on a Boetius
hot stage. Enantiomeric composition was determined by HPLC
on a Shimadzu LCꢀ20AD chromatograph using a Chiralcel OD
column (0.46×25 cm) and UV detector (275 nm) at 40 °C.
The EI mass spectra (ionization energy 60 eV, electron
emission current 0.5 mA; direct sample injection into the ion
source, initial evaporator temperature 20 °C with gradual heating
to 300 °C) and CI mass spectra (pentane as the reagent gas) were
run on a МATꢀ212 mass spectrometer. The composition of
molecular ions and the major fragment ions was determined
at instrument resolution of 10000 by coinciding the peaks of the
analyzed ion and the reference compound (perfluorinated
kerosene).
(R)ꢀ1,2ꢀDipalmitoyloxyꢀ3ꢀ(2ꢀmethoxyphenoxy)propane
(R)ꢀ8. DMAP (0.93 g, 7.6 mmol), palmitic acid (1.94 g, 7.6 mmol),
and DCC (3.11 g, 15.1 mmol) were added to a solution of diol
(S)ꢀ7 (0.7 g, 3.5 mmol) in dichloromethane (55 mL). The
reaction mixture was stirred at room temperature for 64 h. After
completion of the reaction, the precipitate was filtered off, the
mother liquor was concentrated under reduced pressure, and
the residue was purified by column chromatography (silica gel,
μ 40/100, column 400×24 mm, chloroform—hexane, 2 : 3,
as the eluent) to give 2.18 g (92%) of (R)ꢀ8 as white powder,
m.p. 57—59 °C, [α]D20 –10.5 (c 1, CHCl3), ee 99.2% (HPLC,
27 °C, hexane—2ꢀpropanol as the mobile phase (97.5 : 2.5),
0.4 mL min–1; tR = 46 min). MS, m/z (Irel (%)): 675 (1.0), 674
(2.1), 553 (9.3), 552 (45.8), 551 (100.0), 550 (1.8), 436 (4.0),
419 (8.0), 367 (2.0), 313 (9.1), 239 (6.3), 163 (9.8), 124 (4.1), 85
(8.5), 83 (5.5), 71 (12.1), 57 (15.7), 43 (6.3). Composition of the
most typical fragment ion, m/z: 551, [C35H65O4]+, Mexp 551.5030,
Mcalc 551.5039. 1H NMR, δ: 0.87 (t, 6 H, CH3, J = 6.5 Hz);
1.25 (br.s, 48 H, CH2); 1.57—1.62 (m, 4 H, CH2CH3); 2.30,
2.31 (both t, 2 H each, C(O)CH2, J = 7.9 Hz); 3.83 (s, 3 H,
OCH3); 4.13—4.18 (m, 2 H, CH2O); 4.31 (dd, 1 H, CH2O,
J = 12.0 Hz, J = 6.0 Hz); 4.47 (dd, 1 H, CH2O, J = 12.0 Hz,
J = 3.8 Hz); 5.37—5.40 (m, 1 H, CHO); 6.87—6.96 (m, 4 H,
Ar). 13C NMR, δ: 15.53, 24.13, 26.36, 30.55, 30.59, 30.74, 30.80,
30.93, 31.08, 31.11, 31.13, 31.15, 33.38, 35.58, 35.72, 57.41,
63.94, 69.51, 71.24, 114.06, 117.0, 122.40, 123.89, 149.59,
151.70, 174.43, 174.74.
Commercial (Alfa Aesar) racemic epichlorohydrin, 3ꢀchloroꢀ
propaneꢀ1,2ꢀdiol (racꢀ6), and guaifenesin (racꢀ7) were used;
(R)ꢀ and (S)ꢀ3ꢀchloroꢀ1,2ꢀpropanediols (R)ꢀ6 and (S)ꢀ6 were
prepared by the Jacobsen kinetic enantioselective hydrolysis
of racemic epichlorohydrin.7
(R)ꢀ and (S)ꢀ3ꢀ(4ꢀMethoxyphenoxy)propaneꢀ1,2ꢀdiols
(R)ꢀ and (S)ꢀ1 were prepared by the procedures similar to
published ones.10,11 The reaction of (S)ꢀ6 ([α]D20 +6.3 (c 4.7,
H2O)) (1.73 g, 0.016 mol) and 4ꢀmethoxyphenol (1.62 g,
0.013 mol) gave 1.67 g (65 %) of (S)ꢀ1, m.p. 76—78 °C (from
CCl4) (cf. Ref. 4: m.p. 76—78 °C), [α]D20 +7.8 (c 0.9, MeOH),
ee 99.9% (HPLC, hexane—propanꢀ2ꢀol—diethylamine (80 : 20 : 0.1)
as the mobile phase, 1.0 mL min–1, tR = 25.2 min). Similarly,
(R)ꢀ6 ([α]D20 –6.1 (c 5.0, H2O)) was converted into diol (R)ꢀ1,
m.p. 75—78 °C (cf. Ref. 3: m.p. 78 °C), [α]D20 –7.2 (c 0.5,
MeOH), ee 99.1% (HPLC, the same conditions, tR = 33.3 min).
(R)ꢀ and (S)ꢀ3ꢀ(2ꢀMethoxyphenoxy)propaneꢀ1,2ꢀdiols
(R)ꢀ and (S)ꢀ7 were converted from racemic guaifenesin by
(S)ꢀ1,2ꢀDipalmitoyloxyꢀ3ꢀ(2ꢀmethoxyphenoxy)propane
(S)ꢀ8 was prepared similarly to (R)ꢀ8 from diol (R)ꢀ7,
m.p. 57—59 °C, [α]D20 +10.5 (c 1, CHCl3), ee 99.0% (HPLC, the
same conditions, tR = 52 min).
(R)ꢀ1,2ꢀDipalmitoyloxypropanꢀ3ꢀol (R)ꢀ3 was prepared
by analogy with the known procedure3 from dipalmitate (R)ꢀ2
(0.28 g, 0.415 mmol) and cerium ammonium nitrate (0.53 g,
0.969 mmol) in a 4 : 1 acetonitrile—water mixture (6 mL). The

Scalemic diacylglycerols
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008 2323
reaction mixture was stirred at room temperature for 72 h to give
0.21 g (89%) of product (R)ꢀ3; m.p. 66—67 °C; [α]D20 +3.4
(c 1.0, CHCl3) (cf. Ref. 4: m.p. 67 °C, [α]D20 –2.7 (c 1, CHCl3),
ee 97%). 1H NMR, δ: 0.90 (t, 6 H, CH3, J = 6.8 Hz); 1.26 (br.s,
48 H, CH2); 1.62—1.66 (m, 4 H, CH2CH3); 2.04 (t, 1 H, OH,
J = 6.7 Hz); 2.34, 2.36 (both t, 2 H each, C(O)CH2, J = 7.3 Hz);
3.74 (m, 2 H, CH2OH); 4.25 (d, 2 H, CH2O, J = 5.2 Hz); 4.33
(dd, 1 H, CH2O, J = 12.0 Hz, J = 5.7 Hz); 4.44 (dd, 1 H, CH2O,
J = 12.0 Hz, J = 4.5 Hz); 5.08—5.11 (m, 1 H, CHO). MS, m/z
(Irel (%)): 568 (1.6), 551 (18.0), 550 (49.6), 372 (6.4), 354 (27.6),
331 (29.7), 314 (24.1), 313 (95.9), 312 (36.6), 311 (7.3), 299
(47.0), 239 (100.0), 185 (8.8), 171 (5.4), 154 (7.6), 129 (27.5),
117 (29.0), 116 (25.6), 112 (22.3), 111 (11.0), 109 (14.7), 98
(56.2), 97 (23.7), 95 (25.9), 85 (40.3), 84 (38.4), 83 (38.5), 81
(21.7), 74 (7.0), 73 (6.9), 71 (63.7), 70 (10.7), 69 (49.1), 68
(8.6), 67 (17.5), 59 (5.7), 58 (6.4), 57 (99.4), 56 (16.7), 55 (62.2),
43 (94.8), 42 (13.1), 41 (52.3). Composition of the molecular
ion, m/z 568, [C35H68O5]+, Mexp 568.5063, Mcalc 568.5067
and most characteristic fragment ion, m/z: 550, [C35H66O4]+,
Mexp 550.4960, Mcalc 550.4961.
References
1. Yu. A. Ovchinnikov, Bioorganicheskaya Khimiya [Bioorganic
Chemistry], Prosveshchenie, Moscow, 1987, 815 pp; A. E.
Stepanov, Yu. M. Krasnopol´skii, V. I. Shvets, Fiziologicheski
Aktivnye Lipidy [Physiologically Active Lipids], Nauka,
Moscow, 1991, 136 pp.
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V. I. Shvets, Khimiya Lipidov [Chemistry of Lipids], Khimiya,
Moscow, 1983, 296 pp; J. H. P. Tyman, Synthesis in Lipid
Chemistry, SpringerꢀVerlag, New York, 1996, 242 pp; Lipid
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(S)ꢀ1,2ꢀDipalmitoyloxypropanꢀ3ꢀol (S)ꢀ3 was prepared similarly
to (R)ꢀ3 from dipalmitate (S)ꢀ2, m.p. 63—65 °C, [α]D20 –2.9
(c 1.0, CHCl3) (cf. Ref. 3: m.p. 66—67 °C, [α]D20 –2.69 (c 3.9,
CHCl3); Ref. 4: m.p. 66—67 °C, [α]D20 +3.5 (c 1, CHCl3)).
Oxidative dearylation of (R)ꢀ8. Cerium ammonium nitrate
(0.72 g, 1.34 mmol) was added at 0 °C to a suspension of
dipalmitate (R)ꢀ8 (0.38 g, 0.56 mmol) in a 4 : 1 acetonintrile—
water mixture (8 mL). The reaction mixture was stirred for 120 h
at ~20 °C. Ethyl acetate (100 mL) and brine (100 mL) were
added, and the organic layer was washed with a saturated solution
of NaHCO3 (2×100 mL), dried with anhydrous Na2SO4, and
concentrated in vacuo. The crude product was purified by column
chromatography on silica gel (column 500×12 mm, elution
with a mixture of ethyl acetate and light petroleum ether (4 : 96,
then 10 : 90, 20 : 80)) to give 0.023 g (7%) of product (R)ꢀ3,
m.p. 64—65 °C, [α]D20 +3.1 (c 0.5, CHCl3).
6. H. S. Byun, E. R. Kumar, R. Bittman, J. Org. Chem., 1994,
59, 2630.
7. S. E. Schaus, B. D. Brandes, J. F. Larrow, M. Tokunaga,
K. B. Hansen, A. E. Gould, M. E. Furrow, E. N. Jacobsen,
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Their Synonyms, 8th ed., WileyꢀVCH, Weinheim, 2001, 2327;
The Merck Index, 14th ed., Ed. M. J. O’Neil, Merck and Co.,
Inc., Whitehouse Station, NJ, USA, 2006, 4555.
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L. V. Konoshenko, Tetrahedron Asymmetry, 2007, 18, 1964.
The authors are grateful to A. V. Pashagin for HPLC
analysis of some samples. This work was supported
by the Russian Foundation for Basic Research (Projects
No. 06ꢀ03ꢀ32508 and No. 07ꢀ03ꢀ12070).
Received November 9, 2007;
in revised form July 18 , 2008