Asymmetric synthesis of valilactone

Article abstract of DOI:10.1134/S1070428014010199

Total synthesis of (-)-valilactone, an efficient pancreatic lipase inhibitor, was accomplished with the use of Keck allylation in the key step of construction of the target carbon skeleton.

Full text of DOI:10.1134/S1070428014010199

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2014, Vol. 50, No. 1, pp. 100–104. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © I.V. Mineeva, 2014, published in Zhurnal Organicheskoi Khimii, 2014, Vol. 50, No. 1, pp. 109–113.
Asymmetric Synthesis of Valilactone
I. V. Mineeva
Belarusian State University, pr. Nezavisimosti 4, Minsk, 220030 Belarus
e-mail: i.mineyeva@yandex.ru
Received May 15, 2013
Abstract—Total synthesis of (–)-valilactone, an efficient pancreatic lipase inhibitor, was accomplished with
the use of Keck allylation in the key step of construction of the target carbon skeleton.
DOI: 10.1134/S1070428014010199
In the past decades, much attention has been given
to natural β-lactones (oxetan-2-ones) due to their anti-
bacterial and immunomodulating activity, phytotox-
icity, and the ability to inhibit animal and human
enzymes [1–3]. Tetrahydrolipstatin (orlistat, I), tetra-
hydroesterastin (II), panclicin D (III), and valilactone
(IV) are striking representatives of esters derived from
amino acids and δ-hydroxy-β-lactones, which differ by
the acid residue and side-chain structure; all asym-
metric centers in their molecules have S configuration
[4]. Compounds I–IV and their analogs are pancreatic
lipase inhibitors and therefore may be used as weight-
reducing agents in obesity [4, 5].
with a chiral auxiliary [4], Mukayama reaction accom-
panied by lactonization [9], and preparation of anti-
1,3-diol with subsequent lactone ring closure via ac-
tivation of the hydroxy group [5].
In the present work, the starting material was
an allylic synthetic building block, methyl 3-[(tributyl-
stannyl)methyl]but-3-enoate (V), which was prepared
in 5 steps [10, 11] from ethyl 3,3-diethoxypropionic
acid (VI) (Scheme 1). Compound V was used previ-
ously in the synthesis of massoia lactone [12], dihydro-
kavain [13], and insect pheromones with methyl-
branched carbon skeleton [14].
The key step in the proposed scheme is asymmetric
allylation of hexanal (VII) with stannane V according
to Keck [15, 16] (Scheme 1). Unlike the known proce-
dure, no trifluoroacetic acid was added, and the
amount of the catalyst was reduced to 5 mol %. These
conditions ensured formation of hydroxy ester VIII in
82% yield with more than 99% enantioselectivity, as
followed from the intensity ratio of the MeO proton
signals (δ 3.47 and 3.49 ppm) in the 1H NMR spectrum
of the acylation product of VIII with (S)- and
(R)-2-methoxy-2-phenyl-3,3,3-trifluoropropanoyl
chlorides (Mosher’s acid chloride) [17].
Valilactone (IV) was isolated from the microbial
strain MG147-CF2 which is similar to Streptomyces
albolongus [6]. Compound IV reacts with serine
residues in pancreatic lipase, so that fatty acids are
retained as triglycerides and are not assimilated, which
leads to weight loss. The inhibitory activity of valilac-
tone toward pancreatic lipase is lower by 3 orders of
magnitude than that of tetrahydrolipstatin (I); however,
among lactones I–IV only tetrahydrolipstatin (I,
Xenical®) is commercially available [4, 5].
Four schemes for the total synthesis of valilactones
have been reported. These schemes utilized π-allyl
tricarbonyl iron complexes [7, 8], acyl iron complex
Protection of the hydroxy group in VIII by silyla-
tion with tert-butyl(chloro)dimethylsilane and subse-
Me
Me
H2N
O
Me
H
N
NHCHO
Me
O
NHCHO
NHCHO
Me
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
C11H23
C11H23
C7H15
C5H11
C6H13
C6H13
C10H21
C6H13
I
II
III
IV
100

ASYMMETRIC SYNTHESIS OF VALILACTONE
101
Scheme 1.
C5H11CHO (VII), (R)-BINOL
OEt
O
CH2
O
OH
CH2
O
Ti(OPr-i)4, CH2Cl2
5 Steps
25%
Bu3Sn
EtO
OEt
OMe
C5H11
OMe
VI
V
VIII
quent ozonolysis of ester IX gave protected keto ester
(X) (Scheme 2). It should be noted that silylated
δ-hydroxy-β-oxo esters are useful intermediate prod-
ucts for the synthesis of many biologically active com-
pounds, such as goniothalamin [18], antifungal drug
(6S)-5,6-dihydro-6-[(2R)-2-hydroxy-6-phenylhexyl]-
2H-pyran-2-one [19], parasorbic acid and colletodiol
[20], lactone analogs of compactin and mevinolin [21,
22], and many others [23, 24].
The structure of XIV was confirmed by comparing
its spectral parameters with those reported in [4], as
well as by the spin–spin coupling constant for trans-
oriented protons in the oxetane ring (J = 4 Hz). The
synthesis of valilactone was completed by esterifica-
tion with N-formyl-L-valine (XV) [28] via activation
with N,N′-dicyclohexylcarbodiimide (DCC) in the
presence of 4-dimethylaminopyridine (DMAP).
The overall yield of valilactone (IV) starting from
ester VI was 3.1% (14 steps) or 12.4% calculated on
allylic building block V (9 steps). The proposed
synthetic approach to valilactone is not inferior to the
other syntheses described in [4, 5, 7–9]. In particular,
the first total synthesis of IV ensured as poor yield as
0.6% in 13 steps. The procedure described in the
present work is advantageous due to experimental
simplicity and the use of inexpensive and accessible
reagents; furthermore, it requires neither expensive and
toxic catalysts nor enzymatic transformations.
The alkylation of X with hexyl bromide, followed
by nonselective reduction of ketone XI, afforded ester
XII. Diastereoisomeric silylated oxetan-2-one deriva-
tives XIII were obtained in moderate yield according
to the standard procedure for the preparation of four-
membered lactones via Adam’s lactonization [25, 26].
Removal of the tert-butyl(dimethyl)silyl protection
from XIII by the action of tetrabutylammonium
fluoride in tetrahydrofuran gave a complex mixture of
products, whereas treatment of XIII in methanol with
pyridinium p-toluenesulfonate (PPTS) smoothly pro-
duced a mixture of diastereoisomeric hydroxy lactones
which were separated by chromatography. By analogy
with tetrahydrolipstatin [27], non-targeted trans-lac-
tone was eluted first, next followed compound XIV,
and then a mixture of cis-lactones was isolated;
the yield of targeted isomer XIV was 34% .
In summary, a new approach has been developed to
the synthesis of natural oxetan-2-one derivative, vali-
lactone (IV), which is potentially efficient pancreatic
lipase inhibitor and anti-obesity drug. The proposed
scheme may be successfully applied to the preparation
of tetrahydrolipstatin (I, orlistat), tetrahydroesterastin
(II), and panclicin D (III).
Scheme 2.
(1) O3, CH2Cl2
(2) PPh3
TBSO
C5H11
CH2
O
TBSO
C5H11
O
O
TBSCl, imidazole, CH2Cl2
VIII
OMe
OMe
IX
X
TBSO
C5H11
O
O
TBSO
C5H11
OH
O
C6H13Br, K2CO3, Me2CO
NaBH4, MeOH
OMe
OMe
C6H13
C6H13
XI
XII
Me
Me
NHCHO
COOH
XV
DCC, DMAP, CH2Cl2
O
O
(1) NaOH, MeOH, H2O
(2) PhSO2Cl, pyridine
C5H5N·TsOH
MeOH
TBSO
C5H11
OH
O
O
IV
C5H11
C6H13
C6H13
XIII
XIV
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

MINEEVA
(2 mmol), was dissolved in 10 mL of anhydrous
102
EXPERIMENTAL
methylene chloride, 0.3 g (3.5 mmol) of imidazole and
0.45 g (3 mmol) of tert-butyl(dimethyl)silyl chloride
were added, and the mixture was stirred for 12 h and
treated with water (30 mL). The organic phase was
separated, the aqueous phase was extracted with
methylene chloride (3×5 mL), the extracts were com-
bined with the organic phase and dried over MgSO4,
the solvent was distilled off under reduced pressure,
and the product was isolated by chromatography using
petroleum ether–ethyl acetate (80:1) as eluent. Yield
0.58 g (88%), [α]D = +40.1° (c = 1.8, CHCl3). IR spec-
1
The H and 13C NMR spectra were recorded from
solutions in chloroform-d on a Bruker AC-400 spec-
trometer operating at 400 and 100 MHz, respectively.
The IR spectra were recorded from solutions in carbon
tetrachloride using a Bruker Vertex 70 instrument. The
optical rotations were measured on an SM-3 polarim-
eter (scale division value 0.05°) at room temperature.
Individual products were isolated by chromatography
on silica gel (70–230 mesh). All solvents were purified
and dried by standard methods and were distilled
prior to use.
1
trum, ν, cm–1: 1743, 1258, 1158, 1094. H NMR spec-
trum, δ, ppm: 0.02 s and 0.04 s (3H each, CH3Si),
0.86–0.92 m [12H, t-BuSi, CH3(CH2)4], 1.22–1.45 m
[8H, CH3(CH2)4], 2.24–2.27 m (2H, 3-CH2), 3.07 d
and 3.12 d (1H each, 2-H, J = 15.1 Hz), 3.68 br.s (3H,
CH3O), 3.74–3.82 m (1H, CHOSi), 4.95 br.s (2H,
CH2=). 13C NMR spectrum, δC, ppm: –4.5 (2C, CH3),
14.0 (CH3), 18.1 (C), 22.6 (CH2), 24.9 (CH2), 25.9
(3C, CH3), 32.0 (CH2), 36.8 (CH2), 42.1 (CH2), 43.6
(CH2), 51.7 (OCH3), 71.0 (CH), 116.5 (C4), 139.7
(C3), 171.9 (C=O). Found, %: C 65.82; H 11.02.
C18H36O3Si. Calculated, %: C 65.80; H 11.04.
Methyl (5S)-5-hydroxy-3-methylidenedecanoate
(VIII). A 0.1 M solution of titanium(IV) isopropoxide,
2.9 mL (294 μmol), was added to a mixture of 168 mg
(588 μmol) of (R)-BINOL and 1.5 g of activated 4-Å
molecular sieves (preliminarily calcined at 125–130°C
under an oil-pump vacuum) in 50 mL of anhydrous
methylene chloride, and the mixture was heated for 1 h
under reflux with vigorous stirring. The resulting
catalyst-containing mixture was cooled to –78°C,
1.76 g (12 mmol) of hexanal (VII) in 10 mL of anhy-
drous methylene chloride was added, the mixture was
stirred for 15 min, 6.15 g (15.6 mmol) of stannane V
[10] in 20 mL of anhydrous methylene chloride was
added, and the mixture was left to stand for 5 days in
a refrigerator at –25°C without stirring. The mixture
was then treated under vigorous stirring with a saturat-
ed aqueous solution of NaHCO3 (100 mL), the organic
phase was separated, the aqueous phase was extracted
with methylene chloride (3×30 mL), and the extracts
were combined with the organic phase, washed with
brine (50 mL), and dried over MgSO4. The solvent was
removed under reduced pressure, and the product was
isolated by chromatography using petroleum ether–
ethyl acetate (40:1 to 5:1) as eluent. Yield 2.10 g
(82%), [α]D = –3.9° (c = 1.6, CHCl3). IR spectrum, ν,
Methyl (5S)-5-[tert-butyl(dimethyl)silyloxy]-3-
oxodecanoate (X). A solution of 2.30 g (7.0 mmol) of
compound IX in 50 mL of methylene chloride was
cooled to –78°C, an ozone/oxygen mixture was passed
through the solution over a period of 1.5 h until
persistent blue color, and oxygen was then passed to
remove excess ozone. Triphenylphosphine, 1.84 g
(15.6 mmol), was added in portions under vigorous
stirring, and the mixture was allowed to slowly warm
up to room temperature (over a period of 3 h) and
treated with water (30 mL). The organic phase was
separated, the aqueous phase was extracted with meth-
ylene chloride (2×10 mL), the extracts were combined
with the organic phase and dried over MgSO4, the
solvent was distilled off under reduced pressure, and
the product was isolated by chromatography on silica
gel using petroleum ether–ethyl acetate (90:1) as
eluent. Yield 2.17 g (94%), [α]D = –4.0° (c = 1.0,
CHCl3). IR spectrum, ν, cm–1: 1753, 1720, 1255, 1072.
1H NMR spectrum, δ, ppm: 0.02 s and 0.06 s (3H each,
CH3Si), 0.86 br.s (9H, t-BuSi), 0.88 t [3H, CH3(CH2)4,
J = 6.6 Hz], 1.22–1.34 m [6H, CH3(CH2)3], 1.41–
1.49 m (2H, 6-H), 2.59 d.d (1H, 4-H, J = 15.2, 4.9 Hz),
2.69 d.d (1H, 4-H, J = 15.2, 6.9 Hz), 3.47 d and 3.51 d
(1H each, 2-H, J = 15.7 Hz), 3.73 br.s (3H, CH3O),
4.13–4.20 m (1H, 5-H). 13C NMR spectrum, δC, ppm:
–4.6 (2C, CH3), 14.1 (CH3), 18.5 (C), 22.6 (CH2), 26.1
1
cm–1: 3446, 1739, 1245, 1199, 1161. H NMR spec-
trum, δ, ppm: 0.88 t (3H, CH3, J = 6.9 Hz), 1.24–
1.49 m [8H, CH3(CH2)4], 2.14 d.d (1H, 4-H, J = 14.1,
9.7 Hz), 2.33 d.d (1H, 4-H, J = 14.1, 2.1 Hz), 3.07 d
and 3.14 d (1H each, 2-H, J = 15.6 Hz), 3.66–3.72 m
(1H, 5-H), 3.69 s (3H, CH3O), 5.04 s and 5.07 s (1H
each, CH2=). 13C NMR spectrum, δC, ppm: 14.0 (CH3),
22.6 (CH2), 25.4 (CH2), 31.8 (CH2), 37.1 (CH2), 41.5
(CH2), 44.7 (CH2), 52.0 (OCH3), 69.1 (CH), 117.5
(=CH2), 139.3 (C3), 172.3 (C=O). Found, %: C 67.30;
H 10.32. C12H22O3. Calculated, %: C 67.26; H 10.35.
Methyl 3-{(2S)-2-[tert-butyl(dimethyl)silyloxy]-
heptyl}but-3-enoate (IX). Ester VIII, 0.43 g
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

ASYMMETRIC SYNTHESIS OF VALILACTONE
103
(3C, CH3), 27.3 (CH2), 30.9 (CH2), 37.5 (CH2), 49.1
(CH2), 51.4 (CH2), 52.3 (CH3), 66.8 (C5), 167.4
(C=O), 171.9 (C=O). Found, %: C 61.79; H 10.34.
C17H34O4Si. Calculated, %: C 61.77; H 10.37.
methanol, 2.5 mL of 1 M aqueous sodium hydroxide
was added, and the mixture was heated for 2 h under
reflux. The mixture was cooled, neutralized with
2.5 mL of 1 M aqueous HCl, diluted with 5 mL of
water, and extracted with diethyl ether (3×5 mL). The
extracts were combined and dried over MgSO4, the
solvent was distilled off, the residue was dissolved in
10 mL of pyridine, the solution was cooled to 0°C,
0.46 g (2.6 mmol) of benzenesulfonyl chloride was
added in one portion, and the mixture was stirred for
24 h at room temperature. The mixture was treated
with water (30 mL) and extracted with diethyl ether
(3×10 mL), the combined extracts were dried over
MgSO4, the solvent was distilled off under reduced
pressure, and the product (a 1.4:1 mixture of dia-
stereoisomers) was isolated by chromatography on
silica gel using petroleum ether–ethyl acetate (100:1)
as eluent. Yield 0.31 g (80%). IR spectrum, ν, cm–1:
Methyl (2RS,5S)-5-[tert-butyl(dimethyl)silyloxy]-
2-hexyl-3-oxodecanoate (XI). A mixture of 0.66 g
(2 mmol) of keto ester X, 0.57 g (4 mmol) of calcined
K2CO3, and 0.66 g (4 mmol) of hexyl bromide in
10 mL of anhydrous acetone was heated for 1 h under
reflux with vigorous stirring. The mixture was treated
with water (20 mL) and extracted with methylene
chloride (3×10 mL), the combined extracts were dried
over MgSO4, the solvent was distilled off under
reduced pressure, and the product was isolated by
chromatography on silica gel using petroleum ether–
ethyl acetate (100:1) as eluent. Yield 0.75 g (90%). IR
spectrum, ν, cm–1: 1717, 1463, 1378, 1256, 1142,
1
1056. H NMR spectrum, δ, ppm: 0.01 s and 0.02 s
1
1823, 1100, 1073. H NMR spectrum, δ, ppm: 0.04 s
(3H each, CH3Si), 0.87 t [3H, CH3(CH2)5, J = 6.4 Hz],
0.87 br.s (9H, t-BuSi), 0.88 t [3H, CH3(CH2)4, J =
6.9 Hz], 1.26–1.43 m [14H, CH3(CH2)3, CH3(CH2)4],
1.76–1.89 m (4H, 6-H, 2-CH2), 2.57 d.d (1H, 4-H, J =
17.5, 5.1 Hz), 2.70 d.d (1H, 4-H, J = 17.5, 6.1 Hz),
3.39–3.47 m (1H, 2-H), 3.66 br.s (3H, CH3O), 4.12–
4.13 m (1H, 5-H). Found, %: C 66.64; H 11.15.
C23H46O4Si. Calculated, %: C 66.61; H 11.18.
(1.2H, CH3Si), 0.05 s (3H, CH3Si), 0.06 s (3H, CH3Si),
0.80–0.93 m [36H, CH3(CH2)5, t-BuSi, CH3(CH2)4],
1.21–1.91 m [43.2H, CH3(CH2)4, CH3(CH2)5], 2.02–
2.36 m (4.8H, 4-CH2), 3.14–3.27 m (2.4H, 3-H), 3.59–
3.66 m (0.4H, CHOSi), 3.79–3.86 m (2H, CHOSi),
4.42–4.46 m (1.4H, 4-H), 4.71–4.78 m (1H, 4-H).
Found, %: C 68.72; H 11.51. C22H44O3Si. Calculated,
%: C 68.69; H 11.53.
Methyl (2RS,3RS,5S)-5-[tert-butyl(dimethyl)-
silyloxy]-2-hexyl-3-hydroxydecanoate (XII). A solu-
tion of 0.83 g (2 mmol) of keto ester XI in 20 mL of
methanol was cooled to 0°C, 0.09 g (2.3 mmol) of
NaBH4 was added, and the mixture was kept for 0.5 h,
treated with a saturated solution of ammonium chlo-
ride (20 mL), and extracted with methylene chloride
(3×10 mL). The combined extracts were washed with
a saturated aqueous solution of NaHCO3 (50 mL) and
dried over MgSO4, the solvent was distilled off under
reduced pressure, and the product was isolated by
chromatography on silica gel using petroleum ether–
ethyl acetate (25:1) as eluent. Yield 0.73 g (87%). IR
(3S,4S)-3-Hexyl-4-[(2S)-2-hydroxyheptyl]oxetan-
2-one (XIV). Pyridinium p-toluenesulfonate, 0.05 g
(0.2 mol), was added to a solution of 0.77 g (2 mmol)
of lactone XIII in 6 mL of methanol, and the mixture
was heated for 2 h at 50°C. A few drops of triethyl-
amine were added, the mixture was evaporated under
reduced pressure, and the target diastereoisomer was
isolated by chromatography using petroleum ether–
ethyl acetate (40:1) as eluent. Yield 0.18 g (34%), [α]D =
–16.3° (c = 0.5, CHCl3). The spectral parameters of the
product were consistent with those reported in [4].
(2S,2′S,3′S)-1-(3-Hexyl-4-oxooxetan-2-yl)heptan-
2-yl (S)-N-formyl-3-methylbutanoate [IV, (–)-vali-
lactone]. A solution of 0.1 g (0.4 mmol) of lactone
XIV in 0.5 mL of anhydrous methylene chloride was
added in one portion at room temperature to a mixture
of 0.08 g (0.6 mmol) of N-formyl-L-valine (XV) [28],
0.12 g (0.6 mmol) of N,N′-dicyclohexylcarbodiimide,
and 9 mg (0.07 mmol) of 4-dimethylaminopyridine in
1 mL of anhydrous methylene chloride. After 24 h, the
mixture was treated with water (10 mL), the organic
phase was separated, the aqueous phase was extracted
with diethyl ether (3×5 mL), the extracts were com-
1
spectrum, ν, cm–1: 3436, 1738, 1256. H NMR spec-
trum, δ, ppm: 0.07 s and 0.08 s (3H each, CH3Si),
0.85–0.89 m [15H, CH3(CH2)5, t-BuSi, CH3(CH2)4],
1.20–1.31 m [14H, CH3(CH2)3, CH3(CH2)4], 1.43–
1.68 m (4H, 6-H, 2-CH2), 2.39–2.47 m (1H, 2-H),
3.46 br.s (1H, OH), 3.71 br.s (3H, CH3O), 3.81–3.92 m
(1H, 3-H), 3.96–4.06 m (1H, 5-H). Found, %: C 66.32;
H 11.59. C23H48O4Si. Calculated, %: C 66.29; H 11.61.
(3RS,4RS)-4-{(2S)-2-[tert-Butyl(dimethyl)silyl-
oxy]heptyl}-3-hexyloxetan-2-one (XIII). Hydroxy
ester XII, 0.42 g (1 mmol), was dissolved in 2.5 mL of
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

MINEEVA
104
12. Mineeva, I.V., Russ. J. Org. Chem., 2012, vol. 48,
bined with the organic phase and dried over MgSO4,
the solvent was distilled off under reduced pressure,
and the product was isolated by chromatography on
silica gel using petroleum ether–ethyl acetate (2:1) as
eluent. Yield 0.11 g (86%), [α]D = –33.5° (c = 0.9,
CHCl3). The spectral parameters of the product were
consistent with those given in [4].
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13. Mineeva, I.V., Russ. J. Org. Chem., 2013, vol. 49,
p. 712.
14. Mineeva, I.V., Russ. J. Org. Chem., 2013, vol. 49,
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15. Keck, G.E. and Yu, T., Org. Lett., 1999, vol. 1, p. 289.
16. Sanchez, C.C. and Keck, G.E., Org. Lett., 2005, vol. 7,
p. 3053.
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