Enantioselective transesterification of (±)-6-benzyloxy-2,5,7,8- tetramethyl-3,4-dihydro-2H-1-benzopyran-2-ylmethanol catalyzed by the Amano PS lipase in the ionic liquid [bmim]PF6

Article abstract of DOI:10.1007/s11172-010-0367-x

(S)-(-)-6-Benzyloxy-2,5,7,8-tetramethyl-3,4-dihydro-2H-1-benzopyran-2- ylmethanol, a synthon for the design of natural α-tocopherol, was obtained by kinetically selective acetylation of the corresponding racemic alcohol in the presence of the Amano PS lipase from Burkholderia cepacia in the ionic liquid 1-butyl-3-methylimidazolinium hexafluorophosphate ([bmim]PF₆).

Full text of DOI:10.1007/s11172-010-0367-x

Russian Chemical Bulletin, International Edition, Vol. 59, No. 11, pp. 2129—2132, November, 2010
2129
Enantioselective transesterification of ( )ꢀ6ꢀbenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ
3,4ꢀdihydroꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀylmethanol
catalyzed by the Amano PS lipase in the ionic liquid [bmim]PF6
R. V. Shafikov, A. Yu. Spivak, and V. N. Odinokov
Institute of Petrochemistry and Catalysis, Russian Academy of Sciences,
141 prosp. Oktyabrya, 450075 Ufa, Russian Federation.
Fax: +7 (347) 284 2750. Eꢀmail: ink@anbr.ru
(S)ꢀ(–)ꢀ6ꢀBenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀylmethanol,
a synthon for the design of natural αꢀtocopherol, was obtained by kinetically selective acetylaꢀ
tion of the corresponding racemic alcohol in the presence of the Amano PS lipase from Burkꢀ
holderia cepacia in the ionic liquid 1ꢀbutylꢀ3ꢀmethylimidazolinium hexafluorophosphate
([bmim]PF6).
Key words: (S)ꢀ(–)ꢀ6ꢀbenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀylꢀ
methanol, αꢀtocopherol, enantioselective acetylation, Amano PS lipase, ionic liquid.
(S)ꢀ(–)ꢀ6ꢀBenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdiꢀ
hydroꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀylmethanol ((S)ꢀchromanylꢀ
methanol, (S)ꢀ1) and debenzylated (S)ꢀchromanylmethꢀ
anol (S)ꢀ2 are key synthons for the preparation of natural
tocols: αꢀtocopherol and αꢀtocotrienol.1—6 In early studꢀ
ies, chromanylmethanols (S)ꢀ1 and (S)ꢀ2 were obtained
from (S)ꢀchromaneꢀ2ꢀcarboxylic acid isolated in turn from
the corresponding racemic acid with (S)ꢀαꢀmethylbenzylꢀ
amine as an enantioselective agent.7,8
per year.26 Stereodivergent syntheses of (S)ꢀchromanylꢀ
methanols 1 and 2 via biocatalytic transesterification of
chromanylmethanols assisted by lipases has been studꢀ
ied.3,4,27,28 However, the syntheses were carried out in
organic etherꢀtype solvents, which are undesirable for use
in largeꢀscale production. In the last few years, ionic liqꢀ
uids have attracted considerable attention as environmenꢀ
tally safe media for biocatalytic transformations.29 For
instance, it has been found that they enhance the catalytic
activity, stereoselectivity, and stability of lipases in resoluꢀ
tion of racemic alcohols.30
In the present work, we propose an efficient route to
(S)ꢀ(–)ꢀ6ꢀbenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ
2Hꢀ1ꢀbenzopyranꢀ2ꢀylmethanol (S)ꢀ1 via lipaseꢀcatalyzed
selective acetylation of the corresponding racemic alcohol
1 in the ionic liquid 1ꢀbutylꢀ3ꢀmethylimidazolinium
hexafluorophosphate ([bmim]PF6) containing the Amano PS
lipase from Burkholderia cepacia (Scheme 1). Earlier,
2ꢀmethylchromane derivatives have never been subjected
to transesterification in ionic liquids.
Various reported9—15 synthetic routes to compounds
(S)ꢀ1 and (S)ꢀ2 start from optically active acyclic comꢀ
pounds and chiral auxiliary reagents. In the last few deꢀ
cades, homochiral chromane building blocks for tocoꢀ
pherols and tocotrienols have been actively synthesized by
asymmetric epoxidation of allylic alcohols and the Sharpꢀ
less dihydroxylation of enynes in combination with crossꢀ
coupling reactions of iodoarenes with chiral diols in the
presence of palladium complexes as catalysts.16—19 An efꢀ
ficient and versatile synthetic strategy for the design of
homochiral chromanes that involves palladiumꢀcatalyzed
asymmetric allylic alkylation of phenols with allyl carꢀ
bonates has been developed by various research teams.20—22
With the chiral ligands and catalysts found, the target
chromanes have been obtained with >97% ee.23,24 Howꢀ
ever, despite their high enantioselectivity, the aforesaid
asymmetric catalysis methods for the synthesis of homoꢀ
chiral chromanes are not used commercially so far. Bioꢀ
catalytic transformations provide a promising alternative
to chemical processes.25 Immobilized hydrolytic enzymes
(lipases) are known to be employed in commercial proꢀ
duction of vitamins, drugs, and biologically active food
supplements, with a total output over 10 000 tons
Results and Discussion
Racemic chromanylmethanol ( )ꢀ1 was prepared in
three steps from commercial chromanꢀ2ꢀylcarboxylic acid
(Trolox) ( )ꢀ3 as described earlier.4 Out of the tested acꢀ
cessible and relatively inexpensive enzymes (the lipases
from Burkholderia cepacia (Amano PS), Candida cylindrꢀ
acea (CCL), and Hog pancreas (PPL)), we selected the
Amano PS lipase for its highest catalytic activity and
enantioselectivity in the ionic liquid [bmim]PF6. Partial
acylation of alcohol ( )ꢀ1 with succinic anhydride in
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2075—2078, November, 2010.
1066ꢀ5285/10/5911ꢀ2129 © 2010 Springer Science+Business Media, Inc.

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Shafikov et al.
Scheme 1
Reagents and conditions: a. 1) TsOH, MeOH; 2) BnCl, K2CO3, DMF; 3) LiAlH4, Et2O; b. AcOCH=CH2/Amano PS, [bmim]PF6,
20 °C, 24 h; c. MeONa/MeOH, 0.5 h; d. H2, Pd(20%)—C, AcOEt; e. (COCl)2, DMSO, Et3N, CH2Cl2, –70 °C; f. (R)ꢀ(+)ꢀαꢀ
Methoxyꢀαꢀphenylꢀαꢀtrifluoromethylacetyl chloride ((R)ꢀMTPAꢀCl), Pyꢀd5, CDCl3, 24 h.
[bmim]PF6 at both 0 and 20 °C was virtually nonꢀstereoꢀ
selective: the specific rotations of the products were withꢀ
in the measurement error. However, using vinyl acetate as
an acylating reagent (substrate : enzyme = 1.5 : 1 (w/w),
20 °C, 24 h, 39% conversion), we obtained acetate (S)ꢀ4
(35% yield, [α]D20 3.6° (c 1.3, CHCl3)) and nonconsumed
alcohol (R)ꢀ1 (59% yield, [α]D20 1.1° (c 1.6, CHCl3)); the
signs and values of their specific rotations correspond to
those of their optically pure samples.3,4
Hydrolysis of acetate (S)ꢀ4 gave optically pure
(S)ꢀchromanylmethanol ([α]D20 –2.2° (c 0.7, CHCl3)).4,5
The enantiomer excess (ee) of alcohol (S)ꢀ1 was deterꢀ
mined from the 1H NMR spectra of diastereomeric Mosher
esters (SR)ꢀ6 and (RR)ꢀ6 prepared from alcohol (S)ꢀ1 and
acid chloride (R)ꢀMTPAꢀCl. The 1H NMR spectra of esꢀ
ters (SR)ꢀ6 and (RR)ꢀ6 were recorded in deuterated solꢀ
vents (pyridine, chloroform, and toluene) in a narrow range
from δ 2.5 to 5.0 to reduce the effect of intense signals.
According to the signal intensity ratio for the protons of
the oxymethylene group OCH2 in esters (SR)ꢀ6 (δ 4.02 or
4.27, 2J = 11.2 Hz) and (RR)ꢀ6 (4.08 or 4.18, 2J = 11.2 Hz),
the ratio of diastereomeric Mosher esters (SR)ꢀ6 and
(RR)ꢀ6 in the final mixture is 96 : 4. Thus, the diastereoꢀ
mer excess de of ester (SR)ꢀ6 and, consequently, the enanꢀ
tiomer excess ee of alcohol (S)ꢀ1 are 92%. The absolute
configuration and optical purity of compound (S)ꢀ1 were
confirmed by its Swern oxidation into aldehyde (S)ꢀ5
([α]D20 11.3° (c 0.9, CHCl3)); data5 for optically pure (S)ꢀ5:
25
[α]D 11.9° (CHCl3). Hydrogenolysis of alcohol (S)ꢀ1
with Pd(20%)—C in ethyl acetate gave compound (S)ꢀ2
in 90% yield ([α]D 1.5° (c 1.4, EtOH)); data3 for optiꢀ
20
cally pure (S)ꢀ2: [α]D23 1.6° (EtOH).
A study of the partial acetylation of chromanylmethanol
( )ꢀ1 under the action of the Amano PS lipase in diisoproꢀ

Enantioselective transesterification
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 11, November, 2010 2131
(S)ꢀ4 was 70 mg (35%), Rf 0.74 (hexane—ethyl acetate, 3 : 1),
pyl ether or in its mixture with [bmim]PF6 revealed a
favorable effect of the ionic liquid on the stereoselectivity
of the biocatalyst. For instance, the specific rotations of
alcohol (S)ꢀ1 obtained in [bmim]PF6 and in Pri2O are
20
m.p. 38—40 °C, [α]D 3.6° (c 1.3, CHCl3) (for optically pure
22
acetate (S)ꢀ4, cf. Ref. 4: m.p. 39 °C, [α]D 4.0° (CHCl3)). The
yield of residual alcohol (R)ꢀ1 was 107 mg (59%), Rf 0.61 (hexꢀ
20
ane—ethyl acetate, 3 : 1), m.p. 67—69 °C, [α]D 1.1° (c 1.6,
20
20
[α]D –2.2° (c 0.7, CHCl3) and [α]D –1.4° (c 0.9,
CHCl3), respectively. In mixtures of these solvents for
[bmim]PF6—Pri2O = 3 : 1, 1 : 1, and 1 : 3 (v/v), the speꢀ
cific rotations of compound (S)ꢀ1 are –2.0, –1.9, and
–1.5°, respectively (data4 for optically pure (S)ꢀ1: [α]D
–2.36° (CHCl3)).
22
CHCl3) (for (R)ꢀ1, cf. Ref. 3: m.p. 72—74 °C, [α]D 1.1°
(CHCl3)). The IR and 1H and 13C NMR spectra of compounds
(S)ꢀ4 and (R)ꢀ1 are identical with those reported earlier.3 The
suspension of the Amano PS lipase in [bmim]PF6 was separated
20
and reused three times.
(S)ꢀ(–)ꢀ6ꢀBenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ
1ꢀbenzopyranꢀ2ꢀylmethanol (S)ꢀ1. Metallic sodium (4 mg,
0.17 mmol) was added to a solution of compound (S)ꢀ4 (70 mg,
0.19 mmol) in MeOH (4 mL). The reaction mixture was stirred
for 0.5 h and then neutralized with 5% HCl. The product was
extracted with EtOAc (3×7 mL) and the combined extracts were
concentrated. The residue was chromatographed on SiO2 (4 g)
with light petroleum as an eluent. The yield of compound (S)ꢀ1
was 58 mg (93%), colorless oil, Rf 0.61 (hexane—ethyl acetate,
In addition, the ionic liquid [bmim]PF6 containing
the Amano PS lipase can be reused at least three times with
no noticeable reduction in the catalytic activity of the
lipase or in the enantioselectivity of the reaction (i.e., the
biocatalyst is stable under these conditions).
To sum up, enantioselective transesterification of
( )ꢀ6ꢀbenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀylmethanol catalyzed by the Amano PS
lipase in the ionic liquid [bmim]PF6 affords the target
(S)ꢀ(–)ꢀenantiomer with high ee; both the biocatalyst and
the solvent can be reused.
20
3 : 1), [α]D –2.2° (c 0.7, CHCl3) (for optically pure alcohol
(S)ꢀ1, cf. Ref. 4: [α]D23 –2.36° (CHCl3)). The IR and 1H NMR
spectra of compound (S)ꢀ1 are identical with those reportꢀ
ed earlier.4
(S)ꢀ(+)ꢀ6ꢀHydroxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀylmethanol (S)ꢀ2. The catalyst Pd(20%)—C
(40 mg) was added to a solution of compound (S)ꢀ1 (85 mg,
0.26 mmol) in anhydrous AcOEt (5 mL). The mixture was stirred
under hydrogen for 6 h. After the reaction was completed (moniꢀ
toring by TLC in hexane—ethyl acetate, 3 : 1), the catalyst was
filtered off and washed with AcOEt. The filtrate was concentratꢀ
ed and the residue was chromatographed on SiO2 (4 g) with light
petroleum as an eluent. The yield of compound (S)ꢀ2 was 55 mg
Experimental
1
H and 13C NMR spectra were recorded on a Bruker Avanceꢀ
400 instrument (400.13 (1H) and 100.62 MHz (13C)) in CDCl3.
Chemical shifts δ are referenced to Me4Si. The reaction prodꢀ
ucts were analyzed by HPLC on a Hewlett—Packard 1050 chroꢀ
matograph (Cꢀ18 column (250×4.6 mm, Zorbax), elution rate
1 mL min–1, CH3COCN—H2O (80 : 20) + 1% Et3N) fitted with
a UV detector at a wavelength of 254 nm). IR spectra were
recorded on a Specord 75 IR spectrophotometer (Carl Zeiss,
Jena) in KBr pellets. The specific rotations measured on a Perꢀ
20
(90%), Rf 0.42 (hexane—ethyl acetate, 3 : 1), [α]D 1.5° (c 1.4,
23
EtOH) (for optically pure alcohol (S)ꢀ2, cf. Ref. 3: [α]D 1.6°
(EtOH)). The IR and 1H and 13C NMR spectra of compound
(S)ꢀ2 are identical with those reported earlier.3
kin—Elmerꢀ141 polarimeter are expressed in deg mL g–1 dm–1
the concentration of the solution is cited in g (100 mL)–1. Meltꢀ
ing points were determined on a Boetius hot stage. TLC was
carried out on SiO2 plates (Silufol); spots were visualized in
a solution of anisaldehyde in ethanol acidified with H2SO4.
The specific activities of the lipases from Candida cylindrꢀ
acea (CCL, Fluka) and Hog pancreas (PPL, Fluka) were 3.85
and 20.6 unit mg–1, respectively. The Amano PS lipase from
Burkholderia cepacia (Aldrich) was used. Racemic chromanylꢀ
methanol ( )ꢀ1 was prepared from commercial chromanꢀ2ꢀylꢀ
carboxylic acid (Trolox) ( )ꢀ3 as described earlier.4 The syntheꢀ
sis of the ionic liquid [bmim]PF6 followed a known procedure.31
(S)ꢀ(+)ꢀ2ꢀAcetoxymethylꢀ6ꢀbenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ
3,4ꢀdihydroꢀ2Hꢀ1ꢀbenzopyran (S)ꢀ4 and (R)ꢀ(+)ꢀ6ꢀbenzyloxyꢀ
2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀbenzopyranꢀ2ꢀylmethanol
(R)ꢀ1. Equimolar amounts of vinyl acetate (0.05 mL) and the
Amano PS lipase (120 mg) were added to a solution of chromꢀ
anylmethanol ( )ꢀ1 (180 mg, 0.55 mmol) in [bmim]PF6
(2.8 mL). The reaction mixture was stirred with a magnetic stirꢀ
ring bar at 20 °C. The course of the reaction was monitored by
TLC (hexane—ethyl acetate, 3 : 1) and HPLC. After the given
conversion (39%) was achieved, the products were extracted
with Et2O (3×10 mL) and the combined extracts were concenꢀ
trated in vacuo at 40 °C. The residue was chromatographed on
SiO2 (8 g) with light petroleum as an eluent. The yield of acetate
;
(S)ꢀ6ꢀBenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀcarbaldehyde (S)ꢀ5. Oxalyl chloride (0.11 mL,
1.32 mmol) was added at –70 °C to a solution of DMSO (198 mg,
2.54 mmol) in CH2Cl2 (2 mL). The mixture was stirred for 0.5 h.
Then a solution of compound (S)ꢀ1 (50 mg, 0.16 mmol) in
CH2Cl2 (2 mL) was added and stirring was continued at –70 °C
for 1 h. Triethylamine was added and the mixture was stirred at
–70 °C for 0.5 h and then at 0 °C for 0.5 h. The reaction mixture
was diluted with water (10 mL) and the product was extracted
with ethyl acetate (3×10 mL). The combined organic layers were
dried over MgSO4 and concentrated under reduced pressure.
The residue was chromatographed on SiO2 (5 g) with hexane as
an eluent. The yield of compound (S)ꢀ5 was 46 mg (93%), Rf 0.67
(hexane—ethyl acetate, 3 : 1), m.p. 57—59 °C, [α]D20 11.3° (c 0.7,
CHCl3) (for optically pure aldehyde (S)ꢀ5, cf. Ref. 5: m.p. 56 °C,
[α]D 11.9° (CHCl3)). The IR and 1H NMR spectra of comꢀ
25
pound (S)ꢀ5 are identical with those reported earlier.4
(S)ꢀ6ꢀBenzyloxyꢀ2,5,7,8ꢀtetramethylꢀ3,4ꢀdihydroꢀ2Hꢀ1ꢀ
benzopyranꢀ2ꢀylmethyl (R)ꢀαꢀmethoxyꢀαꢀphenylꢀαꢀtrifluoromeꢀ
thylacetate (SR)ꢀ6. (R)ꢀαꢀMethoxyꢀαꢀphenylꢀαꢀtrifluoromethꢀ
ylacetyl chloride (3 mg, 0.012 mmol) was added to a solution of
alcohol (S)ꢀ1 (2 mg, 0.006 mmol) in C6D5N (0.1 mL) and CDCl3
(0.1 mL). The reaction mixture was stirred at ~20 °C for 24 h and
then diluted with toluene (0.6 mL) and C6D5CD3 (0.1 mL) for
1
recording H NMR spectra. It follows from the signal intensity

2132
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 11, November, 2010
Shafikov et al.
ratio for CH2—O (96 : 4) in diastereomers (SR)ꢀ6 (δ 4.01 (d) or
4.27 (d), 2J = 11.2 Hz) and (RR)ꢀ6 (δ 4.07 (d) or 4.18 (d),
2J = 11.2 Hz) that the diastereomer excess de and optical purity
of (SR)ꢀ6 is 92%.
15. G. Solladie, G. Moine, J. Am. Chem. Soc., 1984, 106, 6097.
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Chem. Lett., 1985, 561.
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Eur. J. Org. Chem., 1999, 1075.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 10ꢀ03ꢀ00105).
18. L. F. Tietze, K. Sommer, J. Zinngrebe, F. Stecker, Angew.
Chem., Int. Ed., 2004, 43, 2.
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Eur. J., 2006, 12, 8770.
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Received May 28, 2010