Enantioselective synthesis of natural biologically active ivaide A: 1,3- di-(R)-β-hydroxy-glyceride glycerol

Article abstract of DOI:10.1016/S0957-4166(99)00222-0

The natural 1,3-di-β-hydroxy-glyceride glycerol ivaide A 1 from Ajuga iva has been synthesised by diacylation of dihydroxyacetone with 3- hydroxyhexadecanoic acid, followed by reduction to the corresponding glycerol derivative. The enantiomerically pure (R)-β-hydroxahexadecanoic ester intermediate 6 was obtained by a coupling reaction of an ethylacetoacetate dianion and the corresponding bromoalkyl, followed by the known reduction of the resulting β-ketoester 5 by fermenting baker's yeast.

Full text of DOI:10.1016/S0957-4166(99)00222-0

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Tetrahedron: Asymmetry 10 (1999) 2381–2386
Enantioselective synthesis of natural biologically active ivaide A:
1,3-di-(R)-β-hydroxy-glyceride glycerol
H. B. Jannet,a A. Al Mourabit,b,∗ A. Gateau-Olesker,b C. Marazano b and Z. Mighri a
aLaboratoire de Chimie des Substances Naturelles et de Synthèse Organique, Faculté des Sciences, 5000 Monastir, Tunisie
bInstitut de Chimie des Substances Naturelles du C.N.R.S., 91190 Gif-sur-Yvette, France
Received 18 May 1999; accepted 3 June 1999
Abstract
The natural 1,3-di-β-hydroxy-glyceride glycerol ivaide A 1 from Ajuga iva has been synthesised by diacylation
of dihydroxyacetone with 3-hydroxyhexadecanoic acid, followed by reduction to the corresponding glycerol
derivative. The enantiomerically pure (R)-β-hydroxyhexadecanoic ester intermediate 6 was obtained by a coupling
reaction of an ethylacetoacetate dianion and the corresponding bromoalkyl, followed by the known reduction of
the resulting β-ketoester 5 by fermenting baker’s yeast. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Ivaides A, B and C are novel homologous compounds isolated from the leaves of Ajuga iva (Labiatae)
as a mixture of 1,3-diacyl-glycerol derivatives.1 The unseparated ivaides A 1, B 2 and C 3 differ only by
having different lengths of the fatty acid residues: 3-hydroxytetradecanoic and 3-hydroxyhexadecanoic
acids (Fig. 1).
Preliminary biological tests show interesting antifeedant activity of the mixture of compounds 1, 2
and 3 towards Spodoptera littoralis larvae1 and antibacterial activities against Escherichia coli, Staphyl-
ococcus aureus, Salmonella typhimurium and Pseudomonas aeruginosa. Moreover, these compounds
show a very high chelating power toward lithium cations. All their fragments on LSIMS and MS/MS
Figure 1.
∗
Corresponding author. Fax: (33) 01 69 07 72 47; e-mail: ali.almourabit@icsn.cnrs-gif.fr
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00222-0

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spectra retain the lithium cation. These properties motivated us to synthesise one of them in order to
confirm its structure and to continue biological and chemical studies on pure specimens. Optically active
3-hydroxyalkanoic esters exist widely in biological systems.2 Free 30-(R)-hydroxypalmitic acid which
corresponds to the fatty acid part of ivaide A 1 is also produced in high yield by the yeast NRRL
Y-6954.3 In this paper, we describe a total synthesis of glycerol 1,3-((R)-30-hydroxy)hexadecanoate
corresponding to the ivaide A 1 (C35H68O7). Among the steps investigated for the preparation of this
diglyceride was access to selectively 1,3-diacylated glycerol via the O-protected β-hydroxyhexadecanoic
acid. Diacylation of dihydroxyacetone followed by its reduction to the glycerol derivative was successful.
The β-hydroxyhexadecanoic acid was prepared by baker’s yeast reduction of the corresponding 3-
oxopalmitate as described by others.4
2. Results and discussion
3-Oxopalmitate 5 was prepared in one step as reported by Beijer5 and coworkers using Weiler’s
efficient alkylation of the dianion of a β-ketoester.6 Generation of a dianion of ethylacetoacetate and
treatment with 1-bromododecane give the monoalkylated precursor 5 in 72% yield after purification by
flash chromatography. The reduction of the β-ketoester by baker’s yeast at room temperature resulted in
the formation of the (R)-β-hydroxyester 6 in 43% (>97% ee).2 In order to avoid any secondary reaction
and/or trans-esterification in the rest of our synthesis steps, the β-hydroxyl function was protected as
tert-butyldimethylsilylether 7 in 85% yield. Base hydrolysis of 7 in dioxane afforded the free acid 8 in
75% yield after precipitation from heptane (Scheme 1).
Scheme 1.
Diacylation of 1,3-dihydroxypropan-2-one7,8 with 8 activated by 1,3-dicyclo-hexylcarbodiimide
in the presence of N,N-dimethylaminopyridine provides the C2-symmetric 1,3-((R)-30-protected hy-
droxy)hexadecanoate 9 in 72% yield. Compound 9 was reduced8 with NaBH4 in ethanol at 0°C to 10 in
98% yield. Deprotection of hydroxyl was carried out in 40% aqueous HF and gave the desired glycerol
1,3-((R)-30-hydroxy)hexadecanoate 1 in 72% yield (Scheme 2). NMR and MS data of synthetic ivaide A
were in agreement with those reported for the natural product.1
The enantiomeric purity was examined by comparison of compound 11 obtained by hydrolysis
of β-hydroxyester 6 (Scheme 3). The specific rotation determined for 11 [α]20 −14 (c 1, CHCl3)
D
corresponding to the (R) configuration was in agreement with the one reported in the literature.4a
The synthesised compound 1 also showed interesting antibacterial activities as indicated above. Further
chemical investigation and biological assays are currently in progress.

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H. B. Jannet et al. / Tetrahedron: Asymmetry 10 (1999) 2381–2386
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Scheme 2.
Scheme 3.
3. Experimental
3.1. General methods
Chemicals were purchased from commercial sources and used without further treatment except
1
when specified. THF was freshly distilled from sodium benzophenone under nitrogen. H and 13C
NMR spectra were recorded using Bruker AC-200 (200 MHz) and AC-300 (300 MHz) spectrometers
with tetramethylsilane or solvent (CDCl3, CD3OD) as internal standard (δ ppm). A ZabSpect/T mass
spectrometer was used in the LSIMS experiment. MS 9 (CI) was used for HRMS. IR spectra were
recorded on a Bio-Rad win-IR PRO spectrometer. Melting points were determined on a Büchi 510
apparatus using capillary tubes.
3.2. 3-Oxo-hexadecanoic acid ethyl ester 5
To a solution of sodium hydride (1.2 g, 50 mmol) in THF, distilled ethylacetoacetate 4 (4.16 g, 32
mmol) was added dropwise. The resulting mixture was stirred for 30 min at room temperature and then
cooled at −78°C. A solution of n-butyllithium in hexane (16.1 mL, 35.2 mmol) was added dropwise.
After stirring for an additional 1 h at 0°C, 1-bromododecane (19.1 mmol) in THF (8 mL) was added and
the mixture was stirred for 12 h. Ethanol (15 mL) was added slowly at room temperature. The resulting
solution was filtered through a Celite pad, concentrated in vacuo and purified by chromatography on silica
gel (heptane:EtOAc, 9:1) to give the β-ketoester 5 (4.1 g, 72%) as a solid: mp 41–42°C; 1H NMR (200
MHz, CDCl3) δ 0.87 (t, J=6.5 Hz, 3H, -(CH2)12-CH3), 1.25 (m, 21H, -(CH2)9-CH3 and O-CH2-CH3),
1.55 (m, 4H, -CO-CH2-CH2-CH2-), 2.50 (t, J=7.6 Hz, 3H, -CO-CH2-CH2-), 3.42 (s, 2H, -CO-CH2-
CO2-), 4.20 (q, J=6.5 Hz, 2H, CH3-CH2-CO2-); 13C NMR (75 MHz, CDCl3) δ 14.3, 29.4, 43.1, 49.4,
167.3, 203.0. LSIMS m/z (rel. intensity) 321 [M+Na]+ (48), 299 [M+H]+ (100), 211 (67), 115 (31);
HRMS calcd for C18H34O3 (M++1): 299.2587; found: 299.2587.

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3.3. 3-Hydroxyhexadecanoic acid ethyl ester 6
To a mixture of distilled water (950 mL), baker’s yeast (84 g) and saccharose (105 g), a solution of
β-ketoester 5 (4.0 g, 13.4 mmol) in ethanol (120 mL) was added at 30°C. After 24 h stirring at room
temperature, baker’s yeast was removed by filtration and the filtrate was extracted with ethyl acetate (250
mL×5). The combined organic layers were dried over anhydrous MgSO4 and concentrated in vacuo.
Purification by silica gel column chromatography (heptane:EtOAc, 8:2) gave (R)-β-hydroxyester 6 (1.73
g, 43%, >97% ee) as a solid: mp 47–48°C; [α]20 −19 (c 1, CHCl3); H NMR (200 MHz, CDCl3) δ
1
D
0.85 (t, J=6.4 Hz, 3H, -(CH2)12-CH3), 1.25 (m, 21H, -(CH2)9-CH3 and O-CH2-CH3), 1.43 (m, 4H,
-CO-CH2-CH2-CH2-), 2.35 (dd, JAB=17.1 Hz, JAX=8.5 Hz, 1H, H2a), 2.48 (dd, JAB=17.5 Hz, JBX=4.3
Hz, 1H, H2b), 2.93 (br s, 1H, OH), 3.96 (m, 1H, -CHOH-), 4.15 (q, J=7.7 Hz, 2H, CH3-CH2-CO2-);
13C NMR (75 MHz, CDCl3) δ 13.9, 14.0, 29.4, 36.4, 41.2, 60.5, 68.0, 173.0. LSIMS m/z (rel. intensity)
323 [M+Na]+ (31), 301 [M+H]+ (100), 283 (66); HRMS calcd for C18H36O3 (M++1): 301.2729; found:
301.2728.
3.4. 3-(tert-Butyldimethylsilanyloxy)hexadecanoic acid ethyl ester 7
β-Hydroxyester 6 (1.5 g, 5 mmol) was added to a mixture of tert-butyldimethylchlorosilane (1.13 g,
7.5 mmol) and imidazole (1.02 g, 15 mmol) in dry DMF (7 mL) at room temperature. After 15 h stirring,
the solvent was removed in vacuo. Chromatography of the residue on silica gel (heptane:EtOAc, 9:1)
gave (R)-β-hydroxyester 7 (1.76 g, 85%) as an oil: [α]20 −18 (c 1, CHCl3); 1H NMR (200 MHz, CDCl3)
D
δ 0.01 (s, 3H, CH3a-Si-), 0.03 (s, 3H, CH3b-Si-), 0.83 (m, 12H, tBu-Si- and -(CH2)12-CH3), 4.1 (m, 3H,
-O-CH2-CH3 and -CHOH-). LSIMS m/z (rel. intensity) 415 [M+H]+ (10), 357 (100); HRMS calcd for
C24H51O3Si (M++1): 415.3636; found: 415.3607.
3.5. 3-(tert-Butyldimethylsilanyloxy)hexadecanoic acid 8
To the protected (R)-β-hydroxyester 7 (1.6 g, 3.86 mmol) dissolved in dioxane (45 mL), NaOH 1N
(45 mL) was added at room temperature. The mixture was stirred for 24 h, concentrated under reduced
pressure and the residue was precipitated from heptane to give (R)-β-hydroxyhexadecanoic acid 8 (1.1
g, 75%) as a semisolid: [α]20 −16 (c 1, CHCl3); 1H NMR (200 MHz, CDCl3) δ 0.06 (s, 3H, CH3a-Si-),
D
0.07 (s, 3H, CH3b-Si-), 0.89 (m, 12H, tBu-Si- and -(CH2)12-CH3), 1.27 (m, 18H, -(CH2)9-CH3), 1.54
(m, 4H, -Si-O-CH-CH2-CH2-CH2-), 2.51 (m, 2H, -Si-O-CH-CH2-CO2H), 4.11 (m, 1H, -CHO-Si-); 13
C
NMR (75 MHz, CDCl3) δ 14.1, 18.0, 22.7, 25.2, 25.7, 29.3, 37.2, 41.7, 69.5, 176.3. LSIMS m/z (rel.
intensity) 393 [M+Li]+ (45), 387 [M+H]+ (91), 369 (18), 329 (36); HRMS calcd for C22H47O3Si (M++1):
387.3294; found: 387.3276.
3.6. 3-(tert-Butyldimethylsilanyloxy)hexadecanoic acid 3-[3-(tert-butyldimethylsilanyloxy)hexadeca-
noyloxy]-2-oxo-propyl ester 9
To a solution of 8 (1.0 g, 2.6 mmol) in dry CH3CN (9 mL), was added 1,3-dihydroxypropan-2-one
(126 mg, 1.4 mmol), DCC (535 mg, 2.6 mmol) and DMAP (32 mg, 2.6 mmol) at room temperature
and the mixture was stirred for 18 h. The reaction mixture was then filtered and concentrated in vacuo.
Chromatography of the residue on silica gel (heptane:EtOAc, 6:4) gave 9 (833 mg, 72%) as an oil: [α]20
D
1
−20 (c 1, CHCl3); H NMR (200 MHz, CDCl3) δ 0.05 (s, 3H, CH3a-Si-), 0.08 (s, 3H, CH3b-Si-), 0.87
(m, 12H, tBu-Si- and -(CH2)12-CH3), 1.25 (m, 18H, -(CH2)9-CH3), 1.53 (m, 2H, -CH2-(CH2)9-CH3),