Resolution of racemic rhododendrol by lipase-catalyzed enantioselective acetylation

Article abstract of DOI:10.1081/SCC-120018756

Both (R)- and (S)-enantiomers of rhododendrol were prepared in high enantiomeric exess by lipase from Pseudomonas cepacia (Amano PS)-catalyzed acetylation of racemic 1 with vinyl acetate at room temperature. Especially, in the case of using acetonitrile as the solvent, by-products 4 and 5 were minimized.

Full text of DOI:10.1081/SCC-120018756

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Resolution of Racemic Rhododendrol by Lipase-
Catalyzed Enantioselective Acetylation
Yoshifumi Yuasa a , Shiroshi Shibuya b & Yoko Yuasa b
a Technical Engineering Department , Fine Chemical Division , Takasago International
Corporation , Hiratsuka, Kanagawa, Japan
b School of Pharmacy , Tokyo University of Pharmacy and Life Science , Hachioji, Tokyo,
Japan
Published online: 16 Aug 2006.
To cite this article: Yoshifumi Yuasa , Shiroshi Shibuya & Yoko Yuasa (2003) Resolution of Racemic Rhododendrol by Lipase-
Catalyzed Enantioselective Acetylation, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 33:9, 1469-1475, DOI: 10.1081/SCC-120018756
To link to this article: http://dx.doi.org/10.1081/SCC-120018756
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 9, pp. 1469–1475, 2003
Resolution of Racemic Rhododendrol
by Lipase-Catalyzed
Enantioselective Acetylation
Yoshifumi Yuasa,1 Shiroshi Shibuya,2 and Yoko Yuasa2,
*
1Technical Engineering Department, Fine Chemical
Division, Takasago International Corporation,
Hiratsuka, Kanagawa, Japan
2School of Pharmacy, Tokyo University of Pharmacy
and Life Science, Hachioji, Tokyo, Japan
ABSTRACT
Both( R)- and (S )-enantiomers of rhododendrol were prepared in
high enantiomeric exess by lipase from Pseudomonas cepacia
(Amano PS)-catalyzed acetylation of racemic 1 withvinyl acetate
at room temperature. Especially, in the case of using acetonitrile as
the solvent, by-products 4 and 5 were minimized.
Key Words: Rhododendrol; Lipase; Kinetic resoluton; Enantio-
selectivity.
*Correspondence: Yoko Yuasa, School of Pharmacy, Tokyo University of
Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo, 192-0392,
Japan; E-mail: yuasay@ps.toyaku.ac.jp.
1469
DOI: 10.1081/SCC-120018756
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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1470
Yuasa, Shibuya, and Yuasa
(S )-(þ)-Rhododendrol; (S )-(þ)-4-(40-hydroxyphenyl)-2-butanol 1a,
which has hepato-protective activity,[1] has been isolated from
Rhododendron maximum,[2] or Acer nikoense.[3–5] Furthermore, (R)-(À)-
rhododendrol 1b and its glucoside have been isolated from Rododendron
chrysanthem.[6] Recently, rhododendrol and its glucoside have been used
as a melanin-inhibitor and in skin-lightening cosmetics.[7] The absolute
configurations of 1a and 1b have been determined by X-ray crystallo-
graphic analysis and a comparison of their optical rotations[4,8,9] (Fig. 1).
The enantioselective synthesis of 1a from 4-(40-hydroxyphenyl)-
2-butanone 2 by enzymatic reduction using baker’s yeast has been
reported by Das et al.[10] However, a useful asymmetric synthesis of 1b
has not been reported so far.
On the other hand, Burgess and Jennings[11] reported that 4-phenyl-
2-butanol similar to 1 could be resolved by lipase AK. Aiming at target
compound 1a and 1b, we made a plan to use the inexpensive lipases as a
chiral catalyst. Enzymes, especially lipases are now well established as
valuable catalysts in organic syntheses. Moreover, the enzymatic
resolution mediated by lipase is attractive because these enzymes are
commercially available at a low cost. Kinetic resolution is one of the
most useful and convenient methods. In the light of the above considera-
tions, we investigate the enzymatic reaction of phenol-containing alkyl
alcohol 1 by lipase which hardly had been reported. We report here the
enantioselective and simultaneous syntheses of 1a and 1b (Sch. 1).
We first investigated the enzymatic resolution of 1, which was
obtained from the reduction of 2 by NaBH4, withsome lipases including
Candida rugosa lipase (MY), Pseudomonas cepacia lipase (PS),
Pseudomonas fluorescense lipase (AK), and Alcaligenes sp. lipase
(QLG). The amount of vinyl acetate as the acetate donor and lipase
were almost fixed at a 1.5 mol equivalent and a 0.5 mass equivalent
withrespect to 1 in diisopropyl ether (i-Pr2O) at room temperature,
respectively. The results of the kinetic resolution by transesterification
are shown in Table 1. Kinetic transesterification of (S )-(þ)-alcohol 1a
was found to proceed slower than the (R)-(À)-enantiomer under most of
Figure 1.

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Resolution of Racemic Rhododendrol
1471
Scheme 1.
Table 1. Kinetic resolution of (ꢀ)-1 by lipase-catalyzed acetylation.a
By-product
4 %c 5 %c
Alcohol Acetate
Time 1a %c
3 %c
(%ee)d (%ee)d (%ee)d
Run
Lipaseb
Solvent
(h)
(%ee)d
1
2
3
4
5
6
7
Lipase MY
Lipase AK
Lipase AK
i-Pr2O
i-Pr2O
CH3CN
72
24
72
48
72
96
72
93 (—)
5 (—)
2
—
20
25 (99) 32 (46) 21
36 (99) 45 (41) 10
8
Lipase QLG CH3CN
54 (53) 40 (85)
86 (10) 12 (—)
4
1
2
—
Lipase PS
Lipase PS
Lipase PS
THF
i-Pr2O
CH3CN
42 (78) 42 (98) 9 (98)
49 (79) 48 (98)
7 (78)
1
2
aAll reactions were carried out at room temperature.
bFour enzymes were commercially available (MEITO, AMANO).
cProduct ratio was determined by GC.
dThe enantiomeric excess (%ee) was determined by HPLC on CHIRALCEL
OD-H after hydrolysis with NaOH aqueous solution. The chromatogram showed
two signals with tr ¼ 35.6 min and 40.4 min assignable to 1b and 1a, respectively.
the lipases. Among the lipases, lipase PS showed high enantioselectivities
for this substrate. For lipase MY, transesterification proceed at slower
rate (run 1). Whereas for lipase AK and lipase QLG, acetylated 4 and 5
were obtained as by-products (runs 2, 3, and 4).

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1472
Yuasa, Shibuya, and Yuasa
We then investigated the proper choice of solvent. The influence of
solvent in the enantioselectivity for enzyme-catalyzed transesterifications
is well documented. In general, enzyme selectivity was good enoughfor
practical use in all of the solvents. However, on using tetrahydrofuran
(THF) as the solvent, reaction rate and enantiomeric excess did not give
satisfactory result (run 5). The other solvents were also examined, non-
polar solvents (for example, hexane, benzene, and toluene) showed poor
selectivity and reactivity (data were not shown). Acetonitrile (CH3CN)
gave the best result with regard to enantioselectivity and minimal
by-products 4 (2%) and 5 (1%) (run 7).
Enantiomeric excess of the acetylated product 3 was determined by
hydrolysis with NaOH/H2O–MeOH to afford 1b. The optical purities of
1a and 1b were directly measured by HPLC on CHIRALCEL OD-H
columm. As by-products, the optically active diacetate 4 and monoacetate
5 were isolated by column chromatography. The absolute configuration
of these compounds was also determined by hydrolysis with NaOH/
H2O–MeOH to afford 1b and 1a, respectively. The recrystallization
of 1a (run 6) with78%ee from CHCl afforded 1a with99%ee in
3
54% yield.
In conclusion, we succeed in the preparation of the optically active
products (R)-(À)- and (S )-(þ)-rhododendrol with exceedingly high
enantiomeric excess by means of enzymatic kinetic resolution. When
lipase PS was used in acetonitrile, transesterification of (Æ)-1 proceeded
in good chemical and optical yield. This method is suitable for large-scale
synthesis of (R)-(À)- and (S )-(þ)-rhododendrol.
EXPERIMENTAL SECTION
All reagents and solvents were obtained from commercial sources and
used without further purification. Lipase MY and QLG were obtained
from Meito Co., Ltd. Lipase AK and PS were obtained from Amano
Pharm. Co., Ltd. Melting points: Yanagimoto micromelting apparatus,
1
uncorrected values. IR: Jasco IR-810. H NMR: (TMS as internal stan-
dard): Bruker AM-400 (400 MHz). 13C NMR (TMS as internal standard):
Bruker AM-400 (100 MHz). MS: Hitachi M-80A mass spectrometer at
70 eV. Optical rotations: Jasco DIP-4 digital polarimeter. GLC: Shimadzu
GC-14A withan FID detector (column, Neutrabond-1, df ¼ 0.15 mm,
0.25 mm ID X 30 m; carrier gas N2, 0.1 MPa, oven temperature,
100–200ꢀC programmed at 10ꢀC/min; injection temperature, 250ꢀC,
detector temperature, 250ꢀC); 1 14.49 min, 5 16.79 min, 3 17.66 min,
4 19.71 min. HPLC: Hitachi L-6000 [column, CHIRALCEL OD-H,

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Resolution of Racemic Rhododendrol
1473
4.6 mm ID Â 250 mm; eluent: 2-propanol/n-hexane ¼ 5:95, flow rate,
1 mL/min; detector, Hitachi L-4000 (254 nm)]; 1b tr 35.6 min, 1a tr
40.4 min. Column chromatography: Merck Kieselgel 60 Art-N: 7734.
Synthesis of Rhododendrol 1
To 4-(40-hydroxyphenyl)-2-butanone 2 (200 g, 1.22 mol) in EtOH
(500 mL) were added small portions of NaBH4 (34.8 g, 0.92 mol) at
room temperature. After the reagent was completely added, the mixture
was stirred for 2 h and then concentrated at reduced pressure. To the
residue was added water (200 mL), 10% HCl (250 mL), and AcOEt
(500 mL), and then the organic layer was extracted, washed with brine,
and dried withanyhdrous MgSO 4. The solvent was removed under
reduced pressure to give crude 1 (214 g). The product was recrystallized
from 5% aqueous EtOH solution (1600 mL) at 15ꢀC to give pure 1
(183 g, 90%). M.p. 71ꢀC {lit.[12] 72ꢀC}.
Enzymatic Resolution of Rhododendrol 1 for
Determination of Product
To the lipase PS (1.0 g, 0.5 mass equiv.) were added i-Pr2O (30 mL),
vinyl acetate (1.5 g, 1.5 mol equiv.), and 1 (2.0 g). The solution was stirred
at room temperature for 96 h and the course of the reaction was followed
by GC. The product ratios of 1a, 3, 4, and 5 were 42/42/9/7. The solution
was filtered and the volatiles were removed under reduced pressure to
give an oily product (2.15 g). The crude product was chromatographed on
silica gel, then eluted with 5–20% AcOEt/hexane to afford three acetates
and an alcohol.
First eluted acetate: (R)-(þ)-Rhododendrol diacetate, 4-(40-acetoxy-
24
phenyl)-2-butyl acetate 4 (0.2 g) was obtained as a viscous oil. ½ꢀꢁD þ6.1ꢀ
(c 1.64, EtOH). IR (neat) 2940, 1745, 1510 cmÀ1. 1H NMR (CDCl3) 1.24
(3H, d, J 8.5), 1.65–1.83 (1H, m), 1.88–1.93 (1H, m), 2.03 (3H, s), 2.28
(3H, s), 2.60–2.69 (1H, m), 4.91–4.97 (1H, m), 6.99 (2H, d, J 8.5), 7.17
(2H, d J 8.5). 13C NMR (CDCl3) 20.70, 21.78, 21.97, 31.90, 38.18, 71.15,
122.09, 129.88, 139.79, 149.51, 170.30 and 171.43. EIMS: (m/z) 250 (Mþ),
208, 190, 148, 133, 107, 43. HRMS calcd. for [C14H18O4]: 250.2938.
Found: [M]þ 250.2939.
4 was hydrolyzed with aqueous NaOH/MeOH at room temperature
for 3hto give a ( R)-(À)-rhododendrol 1b. The optical purity was 98%ee
by HPLC.

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1474
Yuasa, Shibuya, and Yuasa
Second eluted acetate: (R)-(þ)-Rhododendrol monoacetate,
4-(40-hydroxyphenyl)-2-butyl acetate 3 (0.85 g) was obtained as a viscous
24
oil. ½ꢀꢁD þ2.12ꢀ (c 1.42, EtOH). IR (neat) 3390, 2970, 1715, 1600,
1
1520 cmÀ1. H NMR (CDCl3) 1.24 (3H, d, J 6.3), 1.76–1.84 (1H, m),
1.86–1.92 (1H, m), 2.04 (3H, s), 2.53–2.60 (1H, m), 4.90–4.94 (1H, m),
6.75 (2H, d, J 8.5), 7.02 (2H, d J 8.5). 13C NMR (CDCl3) 20.67, 22.04,
31.53, 38.41, 71.45, 115.95, 130.03, 134.03, 154.67, 171.97. EIMS: (m/z)
208 (Mþ), 148, 133, 107, 43. HRMS calcd. for [C12H16O3]: 208.2566.
Found: [M]þ 208.2566.
3 was hydrolyzed with aqueous NaOH/MeOH at room temperature
24
for 3 hto give a ( R)-(À)-rhododendrol 1b. ½ꢀꢁD À19.4ꢀ (c 1.03, EtOH).[13]
The optical purity was 98%ee by HPLC.
Third eluted acetate: (S )-(þ)-Rhododendrol monoacetate, 4-(40-
24
acetoxyphenyl)-2-butanol 5 (0.2 g) was obtained as a viscous oil. ½ꢀꢁD
þ14.43ꢀ (c 1.32, EtOH). IR (neat) 3390, 2930, 1755, 1510 cmÀ1
.
1H NMR (CDCl3) 1.22 (3H, d, J 8.5), 1.71–1.78 (1H, m), 2.28 (3H, s),
2.63–2.67 (1H, m), 2.69–2.76 (1H, m), 3.79–3.84 (1H, m), 6.99 (2H, d,
J 8.5), 7.19 (2H, d, J 8.5). 13C NMR (CDCl3) 21.78, 24.31, 32.14, 41.41,
68.02, 122.03, 129.96, 140.31, 149.40, 170.37. EIMS: (m/z) 208 (Mþ),
166, 148, 133, 107, 43. HRMS calcd. for [C12H16O3]: 208.2566. Found:
[M]þ 208.2568.
5 was hydrolyzed with aqueous NaOH/MeOH at room temperature
for 3 hto give a ( S )-(þ)-rhododendrol 1a. The optical purity was 79%ee
by HPLC.
Final eluted alcohol: (S )-(þ)-Rhododendrol 1a (0.87 g) was obtained
as a solid. The optical purity was 79%ee by HPLC. ½ꢀꢁD þ12.72ꢀ (c 1.1,
24
EtOH).[13] The solid was recrystallizated from CHCl3 and the optical
24
purity was 99%ee by HPLC. M.p. 78–78.5ꢀC; ½ꢀꢁD þ16.9ꢀ (c 1.06,
EtOH).[13] IR (KBr) 3350, 2930, 1610, 1590, 1520 cmÀ1 1H NMR
.
(CDCl3) 1.23 (3H, d, J 7.9), 1.68–1.79 (3H, m), 2.58–2.71 (2H, m),
3.81–3.85 (1H, m), 5.25 (1H, brs), 6.75 (2H, d, J 8.5), 7.06 (2H, d,
J 8.5). 13C NMR (CDCl3) 24.25, 31.89, 41.67, 68.36, 115.96, 130.65,
134.65, 154.49. EIMS: (m/z) 166 (Mþ), 148, 133, 107.
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Resolution of Racemic Rhododendrol
1475
4. Kubo, M.; Nagai, M.; Inoue, T. Chem. Pharm. Bull. 1983, 31, 1917.
5. Fujita, T.; Hatamoto, H.; Iwasaki, T.; Takafuji, S. Pytochemistry
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Tripatni, S.C.; Boll, P.M.; Larsen, S. J. Chem. Soc., Perkin Trans. 1
1991, 2687.
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Phytochemistry 1993, 33, 1529.
11. Burgess, K.; Jennings, L.D. J. Am. Chem. Soc. 1991, 113, 6129.
12. Zempe, G.; Bognar, R.; Boskovitz, L. Chem. Ber. 1944, 77B, 784.
´ ´
13. The [ꢀ]D-values of different samples of the aglycone reported
by different researchgroups are:
À20ꢁ (c 1.25, EtOH) for
(À)-rhododendrol (betuligenol) from T. baccata; À18.5ꢁ (c 3.88,
EtOH) for (À)-rhododendrol (betuligenol) B. alba, þ20ꢁ (c 1.0,
EtOH) for (þ)-rhododendrol from A. nikoense MAXIM; and
þ17.1ꢁ (c 2.0, EtOH) for (þ)-rhododendrol (d-betuligenol) from
R. maximum. See Ref.[10]
Received in Japan May 6, 2002

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