Enantioselective synthesis of (+)-(S)-7,8-dihydrokavain and (4R,6R)-4-hydroxy-6-(2-phenylethyl)tetrahydro-2H-pyran-2-one, lactone analog of compactin and mevinolin

Article abstract of DOI:10.1134/S1070428013050138

A simple and efficient asymmetric synthesis was performed of (+)-(S)-7,8-dihydrokanian and (4R,6R)-4-hydroxy-6-(2-phenylethyl)tetrahydro-2H- pyran-2-one involving Keck allylation in the key stage of building up the carbon scaffold of the target molecules.

Full text of DOI:10.1134/S1070428013050138

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 5, pp. 712−716. © Pleiades Publishing, Ltd., 2013.
Original English Text © I.V. Mineeva, 2013, published in Zhurnal Organicheskoi Khimii, 2013, Vol. 49, No. 5, pp. 729−733.
Enantioselective Synthesis of (+)-(S)-7,8-Dihydrokavain
and (4R,6R)-4-Hydroxy-6-(2-phenylethyl)tetrahydro-
2Н-pyran-2-one, Lactone Analog of Compactin and Mevinolin
I. V. Mineeva
Belarussian State University, Minsk, 220030 Belarus
e-mail: i.mineyeva@yandex.ru
Received September 14, 2012
Abstract—Asimple and efficient asymmetric synthesis was performed of (+)-(S)-7,8-dihydrokanian and (4R,6R)-
4-hydroxy-6-(2-phenylethyl)tetrahydro-2Н-pyran-2-one involving Keck allylation in the key stage of building up
the carbon scaffold of the target molecules.
DOI: 10.1134/S1070428013050138
Diverse structure lactones attract attention of research-
ers due to their versatile biological activity: antibacterial,
fungicidal, anti-HIV, antiphlogistic, and pheromone
[1–4]. For instance, (+)-(S)-7,8-dihydrokanian (I) is one
of numerous lactones contained in the roots, stalks, and
shoots of a tropical plant of type Kava (Piper mythisticum)
growing on islands Fiji, Vanuatu, and Hawaii [4].
Dihydrokanian is a myorelaxant, it exhibits sedative,
anxiolytic, anticonvulsant, anesthetic, and fungicidal
actions [5], and also inhibits the factor of tumor necrosis
and therefore is useful in the treatment of the so-called
TNF-α-induced diseases [6].
O
O
O
O
Ph
OMe
Ph
OH
I
II
It is possible to obtain the mixtures of kavalactones
from plant extracts, but in individual state in the enantio-
merically pure form they can be obtained only syntheti-
cally [20]. Several protocols were published describing
the preparation both of racemic [21–27] and optically
active dihydrokanian [20, 28–34]. The described proce-
dures include the use of high pressure and Noyori catalyst
[28], Diels–Alder reaction involving Brassard diene in
the presence of chiral catalysts [29, 34], asymmetric aldol
reaction with N-acetylthiasolidinethione [30], sonochemi-
cal Blaise reaction [31], asymmetric deprotonation of
О-alkylcarbamates in the presence of (–)-sparteine [32],
palladium-catalyzed Cosford cross-coupling [20], and
also reduction by lipases [33]. The application of these
methods is restricted by expensive reagents, stages of
enzymatic reactions, and the multistage processes.
Mevinolin, a metabolite of fungi Aspergillus sp., is
a strong antihypercholesterolemic agent, i.e., it inhibits
the 3-hydroxy-3-methylglutaryl-coenzyme А reductase
on the route of the cholesterol synthesis in cells [7]. Com-
pactin, pravastatin, and simvastatin, structurally similar
to mevinolin, are used as drugs to decrease the content of
fatty acids and cholesterol in blood, and also to reduce the
level of the high density lipoproteins in order to prevent
the development of atherosclerosis [7, 8]. The biologi-
cal properties of mevinolin and compactin appear due to
the presence of a lactone fragment [7, 9]. Therefore the
investigations were focused on the synthesis of versatile
lactone derivatives with two asymmetric carbon atoms
in the (R,R)-configuration having an aromatic fragment
in the side chain, for instance (4R,6R)-4-hydroxy-6-(2-
phenylethyl)tetrahydro-2Н-pyran-2-one (II) [8, 10–19].
In the synthesis of the lactone fragment of compactin
and mevinolin diverse approaches were applied, in
particular, using (S)-malic [10, 35, 36], L-glutamic [37,
38], tartaric acid [39], carbohydrates [12, 40–51], sulfox-
ides [17], asymmetric syntheses including Diels–Alder
712

ENANTIOSELECTIVE SYNTHESIS OF (+)-(S)-7,8-DIHYDROKAVAIN
713
heteroreaction [52–56], diastereoselective aldol reac-
tions [57–59], asymmetric epoxidation [11, 15, 60–64],
enantioselective reduction of prochiral β,δ-diketoesters
[39, 65–68] and 1,3-diketones [15], reactions of enanti-
oselective deprotonation of prochiral compounds with
chiral bases [18], reduction of δ-hydroxy-β-ketoesters
with alkyl- and arylboranes [8, 17, 36, 59, 65, 67–74],
microbiological reduction (in particular, by the baker’s
yeast) [10, 67, 75, 76], biocatalyzed lactonization [14,
73], and the other methods [77–79].
(III) prepared from available ethyl 3,3-diethoxypropio-
nate in 5 preparative stages in a 25% yield [80, 81].
The key stage was asymmetric allylation of an avail-
able aldehyde IV [82] with stannane III under Keck
reaction conditions [83, 84] in the presence of 10 mol%
of BINOL as catalyst over 5 days. Hydroxyester V was
obtained in 80% yield and the enantioselectivity exceed-
ing 99% as was established from the 1Н NMR spectrum
by the comparison of integral intensities of methoxy
group signals (3.48 and 3.51 ppm) in the acylation
product of compound V with (S)- and (R)-2-methoxy-2-
phenyl-3,3,3-trifluoropropanoyl chloride (Mosher’s acid
chloride) [85].
Here we report on the stereoselective synthesis of
lactones I, II utilizing as the С5-allyl synthetic building
block methyl 3-[(tributylstannyl)methyl]but-3-enoate
O
OH
O
O
(R)-BINOL, Ti(OPr-i)4, CH2Cl2
+
SnBu3
Ph
H
Ph
OMe
MeO
80%
V
IV
III
The ozonolysis of compound V followed by treat-
ing the reaction mixture with zinc powder in acetic acid
led to the formation of ketoester VI whose keeping in a
methanol solution in the presence of sodium carbonate
furnished ketolactone VII. The long stirring of the latter
in acetone in the presence of potassium carbonate and
dimethyl sulfate resulted in the target dihydrokanian
(I) in an overall yield 47% with respect to stannane III
O
(1) O3, CH2Cl2
Me2SO4, K2CO3,
(CH3)2CO
Na2CO3,
MeOH
(2) Zn, CH3CO2H_H2O
OH
O
O
O
I
V
86%
81%
85%
Ph
OMe
Ph
O
VI
VII
(5 successive stages).
as was established by 1Н NMR spectrum of its acylation
product with Mosher’s (S)-acid [85]. The ozonolysis of
compound VIII followed by treating the reaction mixture
with zinc powder in acetic acid led to the formation of
ketoester IX that was reduced with sodium borohydride
At the replacement of the catalyst (R)-BINOL for
(S)-BINOL the reaction of the asymmetric allylation
proceeded without complication giving hydroxyester
VIII in 78% yield and the enantioselectivity over 99%,
OH
O
(S)-BINOL, Ti(OPr-i)4, CH2Cl2
+
IV
III
Ph
O
OMe
78%
Ph
VIII
(1) O3, CH2Cl2
(2) Zn, CH3CO2H_H2O
(1) Ti(OPr-i)4, NaBH4, THF
OH
O
(2) TsOH, PhH
OMe
II
78%
65%
IX
in the presence of titanium(IV) isopropoxide along pro-
cedure [14]. The reduction occurred with a high selec-
tivity (3R,5R–3S,5R, 95:5) affording predominantly the
3R,5R-syn-diastereomer. The minor isomer was separated
by chromatography. The intermediate diol at keeping
in the presence of acid cleanly converted into target
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 5 2013

MINEEVA
714
hydroxylactone II in an overall yield 40% with respect
to compound III (5 successive stages).
It should be noted in conclusion that in this publica-
tion a convenient and simple protocol is described of the
synthesis of several natural δ-lactones proceeding from
the same initial available allylstannane III.
added, and the reaction mixture was maintained for 24
h at room temperature. After adding 0.25 ml (1.8 mmol)
of Et3N the reaction mixture was evaporated. The reac-
tion product was isolated by chromatography (eluent
petroleum ether–ethyl acetate, 15:1–3:1). Yield 0.29 g
(65%), colorless crystals, mp 106–108°С, [α]D +47.8°
(c 0.9, CHCl3). Spectra of compound obtained were
consistent with those published in [8, 17].
EXPERIMENTAL
Methyl 3-[(2S)-2-hydroxy-4-phenylbutyl]but-
3-enoate (V). To a mixture of 1.03 g (3.6 mmol) of
(R)-BINOL and 2.3 g of activated molecular sieves 4 Å
(calcined in a vacuum of an oil pump at 125–130°С for
12 h) in 75 ml of anhydrous СH2Сl2 was added 18 ml
(1.8 mmol) of 0.1 М solution of Ti(OPr-i)4 in CH2Cl2 and
0.5 ml (5 μmol) 0.01 М solution of СF3CO2H in CH2Cl2,
and the mixture was boiled at vigorous stirring for 1 h.
To the catalyst thus prepared was added at cooling to
–78°С while stirring 2.41 g (18 mmol) of aldehyde IV
in 18 ml of anhydrous CH2Cl2 and 15 min later, 9.23 g
(23.4 mmol) of stannane III [80] in 40 ml of anhydrous
CH2Cl2, and the reaction mixture was left standing with-
out stirring in a freezer at –25°С for 5 days. The reaction
mixture was treated with a saturated water solution of
NaHCO3 (200 ml). The organic layer was separated,
the reaction product was extracted from the water layer
into CH2Cl2 (3 × 50 ml), the combined organic solutions
were washed with brine (100 ml) and dried with МgSO4.
After distilling off the solvent at a reduced pressure the
reaction product was isolated by chromatography (eluent
petroleum ether–ethyl acetate, 40 : 1–5 : 1). Yield 3.57 g
(80%), [α]D –14.0° (c2.2, CHCl3). IR spectrum, cm–1:
1H and 13С NMR spectra were registered on a spec-
trometer BrukerAC 400 at operating frequencies 400 and
100 МHz respectively from solutions in deuterochloro-
form. IR spectra were recorded on a spectrophotometer
Bruker Vertex 70 from solutions in tetrachloromethane.
Melting point was measured by capillary method on an
Apotec instrument. The optical rotation was determined
on a polarimeter СМ-3 (value of the scale division 0.05°)
at room temperature. The chromatographic isolation
of individual substances was performed on silica gel
(70–230 mesh). All solvents prior to use were dried by
common methods and were distilled.
(6S)-4-Methoxy-6-(2-phenylethyl)-5,6-dihydro-
2Н-pyran-2-one [(S)-(+)-7,8-dihydrokanian] (I). To
a solution of 0.44 g (2 mmol) of lactone VII in 4 ml of
acetone was added 0.5 g (4 mmol) of dimethyl sulfate and
0.55 g (4 mmol) of K2СО3, and the mixture was stirred
for 12 h till the completion of the reaction. After adding
40 ml of saturated water solution of NaHCO3 the reaction
product was extracted from the water layer by CHCl3 (3 ×
15 ml), the combined organic extracts were dried with
МgSO4.After distilling off the solvent at a reduced pressure
the reaction product was isolated by chromatography on
silica gel (eluent petroleum ether–ethyl acetate, 2 : 1).
Yield 0.39 g (86%), colorless crystals, mp 56–57°С,
[α]D +29.0° (c 1.1, CHCl3). Spectra of compound obtained
were consistent with those published in [31].
1
3457, 1738, 1260, 1164. Н NMR spectrum, δ, ppm:
1.68–1.75 m (2Н, СН2СН2СНОН), 2.10–2.20 m (1Н,
PhСН2), 2.25–2.33 m (1Н, PhСН2), 2.58–2.66 m (1Н,
СНСН2С), 2.71–2.82 m (1Н, СНСН2С), 2.98 d (1Н,
СН2СО, J 15.6 Hz), 3.06 d (1Н, СН2СО, J 15.6 Hz),
3.61–3.68 m (1H, СНOH), 3.62 s (3Н, CH3O), 4.99 s
(1Н, CH2=), 5.01 s (1Н, CH2=), 7.10–7.23 m (5Н, Ph
ЯMР 13С NMR spectrum, δ, ppm: 32.0, 38.7, 41.4, 44.8,
52.0, 68.3, 117.9, 125.8, 128.3, 128.4, 139.0, 142.0, 172.4.
Found, %: C 72.59; H 8.07. C15H20O3. Calculated, %:
С72.55; H 8.12.
(4R,6R)-4-Hydroxy-6-(2-phenylethyl)tetra-hydro-
2Н-pyran-2-one (II). At –78°С to a solution of 0.5 g (2
mmol) of ketone IX in 20 ml of THF was added 0.6 ml
(2 mmol) of Ti(OPr-i)4. After 30 min was added in one
portion 0.38 g (10 mmol) of NaBH4 , and the mixture was
vigorously stirred for 1.5 h. The mixture was diluted with
60 ml of saturated water solution of NH4Cl and it was
warmed to room temperature. The reaction product was
extracted into Et2O (3 × 30 ml) and the combined organic
extracts were dried with МgSO4. After distilling off the
solvent at a reduced pressure the residue was dissolved in
10 ml of benzene, 50 mg (0.3 mmol) of TsOH·2Н2О was
Methyl (5S)-5-hydroxy-3-oxo-7-phenyl-heptanoate
(VI). Through a solution of 1 g (4 mmol) of ester V in
30 ml of CH2Cl2 at –78°С was passed a mixture of О3/
О2 for 1 h till the appearance of a stable blue color, and
the oxygen was passed to remove excess ozone.At vigor-
ous stirring 0.45 g (8 mmol) of zinc powder and 6 ml of
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 5 2013

ENANTIOSELECTIVE SYNTHESIS OF (+)-(S)-7,8-DIHYDROKAVAIN
715
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concn. CH3CO2H in 10 ml of water was added by por-
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