Total synthesis of (±)-glabridin

Article abstract of DOI:10.1080/00397911.2013.821616

An efficient formal synthesis of (±)-glabridin was accomplished in 10 steps from resorcinol using Raney Ni to reduce carbon-carbon double bonds in α,β-unsaturated carbonyl compound as the key step.

Full text of DOI:10.1080/00397911.2013.821616

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Total Synthesis of (±)-Glabridin
Wen-Hua Ji a , Qian-Shan Gao a , Yun-Liang Lin a , Hong-Mei Gao a ,
Xiao Wang a & Yan-Ling Geng a
a Shandong Analysis and Test Center, Shandong Academy of
Sciences , Jinan , China
Published online: 27 Dec 2013.
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To cite this article: Wen-Hua Ji , Qian-Shan Gao , Yun-Liang Lin , Hong-Mei Gao , Xiao Wang
& Yan-Ling Geng (2014) Total Synthesis of (±)-Glabridin, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:4, 540-546, DOI:
10.1080/00397911.2013.821616
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Synthetic Communications1, 44: 540–546, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.821616
TOTAL SYNTHESIS OF (ꢀ)-GLABRIDIN
Wen-Hua Ji, Qian-Shan Gao, Yun-Liang Lin, Hong-Mei Gao,
Xiao Wang, and Yan-Ling Geng
Shandong Analysis and Test Center, Shandong Academy of Sciences,
Jinan, China
GRAPHICAL ABSTRACT
Abstract An efficient formal synthesis of (ꢀ)-glabridin was accomplished in 10 steps from
resorcinol using Raney Ni to reduce carbon–carbon double bonds in a,b-unsaturated
carbonyl compound as the key step.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Chemoselective reduction; glabridin; resorcinol; total synthesis
INTRODUCTION
In 1970s, glabridin was first isolated as an isoflavan from the root of a
Glycyrrhiza glabra licorice.[1] The extract of the root of licorice is a Chinese herbal
medicine that is used as demulcents and expectorants to treat allergic inflammation.
Glabridin has been identified as responsible for the antioxidative effect and other
activities shown in licorice.[2] Additionally, further research showed that glabridin
could be used to efficiently inhibit the tyrosinase-dependent melanin biosynthesis,
suggesting that it may serve as candidates for skin-lightening agents.[2e]
Although it has attracted considerable attention, only two total syntheses of
(ꢀ)-glabridin have been reported, and Nahm by Yoo[3] and by Kenichi and
Shingo[4]. In this article, we report our synthetic studies toward (ꢀ)-glabridin using
Received April 21, 2013.
Address correspondence to Wen-Hua Ji, Shandong Analysis and Test Center, Shandong Academy
of Sciences, Keyuan Road No. 19, Jinan, Shandong 250014, China. E-mail: jwh519@163.com
540

TOTAL SYNTHESIS OF (ꢀ)-GLABRIDIN
541
Scheme 1. Retrosynthetic approach of (ꢀ)-glabridin.
the chemoselective reduction of conjugated olefins in a,b-unsaturated carbonyl com-
pound as the key step. The retrosynthetic sequence is represented in Scheme 1.
RESULTS AND DISCUSSION
At first, we obtained 2,4-dimethoxyphenylacetic acid (3) from resorcinol (2)
through a four-step reaction as described in the literature.[5–7] In the next step, the
preparation of the isoflavone 4 was easily achieved on the basis of the reported using
BF3 ꢁ Et2O as the catalyst and solvent[8] (Scheme 2).
With available isoflavone 4 in hand, our next task was to reduce both carbonyl
group and carbon–carbon double bonds in 4. Initially, isoflavane 5 could be easily
obtained in a single step according to Goto et al.’s method[8] in 87% yield. However,
when isoflavane 5 was treated with 3-methyl-2-butenal through methods as described
in the literature,[3,9] the major product 7 along with a trace amount of the target
product 6 was isolated (Scheme 3).
Therefore, a stepwise reduction of 4 had to be conducted. The chemoselective
reduction of conjugated olefins in a,b-unsaturated carbonyl compounds have been
reported.[10] After screening several reducing reagents (e.g., Pd=C, Raney Ni,
InCl3=NaBH4, Na2S2O4), Raney Ni in dimethylformamide (DMF) was found to
be a good choice for reducing reagent to produce 8 in a 91% yield (Table 1). Com-
pound 8 was then reacted with 3-methyl-2-butenal in the presence of phenylboronic
acid to afford 9 and 10 with a 84% yield and a 15:1 ratio. The isomers 9 and 10 could
be isolated by column chromatography (Scheme 3).
Unfortunately, the methods to reduce the carbonyl group in 9 using Zn(Hg)=
HCl[11] and NH2NH2=NaOH[12] were met with failure because the carbon–carbon
Scheme 2. Reagents and conditions: (a) Me2SO4, NaOH=H2O, 0 ꢂC to 70 ꢂC, 3 h, 85%; (b) CH3COCl,
AlCl3=CH2Cl2, 0 ꢂC to rt, 1 h, 88%; (c) sulfur, morpholine, 130 ꢂC, 12 h; (d) NaOH, H2O, reflux,12 h,
62% over two steps; (e) resorcinol, BF3 ꢁ Et2O, 100 ꢂC, 10 min; (f) (1) DMF, 20 ꢂC to 55 ꢂC, 20 min; (2)
MeSO2Cl, 80 ꢂC, 3 h, 65% over two steps.

542
W.-H. JI ET AL.
Scheme 3. Reagents and conditions: (a) H2, Raney nickel, 1 atm, rt, 30 min, 91% (b) H2, 5% Pd=C (con-
=
taining about 50% water), AcOH–EtOH(1:9), rt, 12 h, 87%; (c) (CH3)2C CHCHO (1.5eq), phenylboronic
acid (1.2eq), toluene=HOAc, reflux, 12 h, 87%; (d) LiAlH4 (10eq), THF, rt ro reflux, 6 h, 75%; (e)
BBr3(5eq), CH2Cl2, ꢃ78 ꢂC to rt, 2 h, 84%.
double bonds could not be tolerated under the high-temperature condition. Then we
examined other reducing reagents (LiAlH4,[13] LiAlH4=AlCl3,[14] NaBH4=AlCl3,[15]
NaBH4=TFA[16]) and the reduction was accomplished in acceptable yield using
LiAlH4 (10eq) in THF (Table 2). Removal of both methyl ether groups[17] led to
(ꢀ)-glabridin (1) in 84% yield (Scheme 3). Finally, we tried to separate racemic
Table 1. Reduction of 4
Isolated yield (%)
Entry
Conditions
5
8
11
1
2
3
4
5
6
7
H2, 5% (w=w) 10% Pt=C, AcOH, 100 ꢂC,12 h
H2, 5% (w=w) 5% Pt=C, AcOH, 100 ꢂC,12 h
Na2S2O4, PTK, NaHCO3, toluene=H2O, 110 ꢂC,4 h
0.5 equiv InCl3, 1 equiv NaBH4, MeOH, 22 ꢂC,5 h
H2, 200% (w=w) Raney Ni, DMF, 22 ꢂC, 30 min
Li-NH3, ꢃ30 ꢂC, 2 h
77
65
—
—
—
—
21
33
—
—
42
57
Trace
—
—
91
75
—
1 equiv NaBH4, MeOH, 22 ꢂC, 10 h
Messy

TOTAL SYNTHESIS OF (ꢀ)-GLABRIDIN
Table 2. Reduction of 9
543
Entry
Conditions
Isolated yield (%)
1
2
3
4
5
6
7
Zn(Hg)=HCl, 120 ꢂC, 24 h
NH2NH2=NaOH, 200 ꢂC, 5 h
LiAlH4, THF, rt to reflux, 6 h
Messy
Messy
75
LiAlH4=AlCl3, THF, rt to reflux, 7 h
NaBH4=AlCl3, MeOH, rt to reflux, 12 h
NaBH4=TFA, MeOH, rt to reflux, 24 h
NaBH4, MeOH rt to reflux, 24 h
66
10
—
Trace
mixtures (i.e., compound 8 or 1) by the Chiralpak OD-H chiral column (250 mm ꢄ
4.6 mm ꢄ 5 mm) from Japan. Racemic mixtures were performed with a mobile phase
consisted of n-hexane: isopropanol (80:20, v:v), at flow rate of 1.0 mL minꢃ1, and the
UV detector wavelength was set at 282 nm. Unfortunately we failed.
In conclusion, we have synthesized (ꢀ)-glabridin from the easily available
resorcinol in 10 steps with a 14% overall yield. This procedure provide a practical
synthesis of (ꢀ)-glabridin. Efforts to complete an asymmetric synthesis of Glabridin
are in progress.
EXPERIMENTAL
NMR spectra were in CDCl3 or CD3SOCD3 (1H at 600 MHz and 13C at
125 MHz). Column chromatography was performed on silica gel (300–400 mesh).
All chemicals were purchased from Sigma Aldrich. Unless otherwise noted, all re-
agents were obtained commercially and used without further purification. DMF was
dried by CaH2. Compounds 4 and 5 were prepared as per reported procedures.[5–8]
Synthesis of 7, 9, and 10: General Procedure
A solution of 5 or 8 (0.02 mol), aldehyde (0.03 mol), phenylboronic acid
(0.024 mol), and glacial HOAc (130 mL) in anhydrous toluene (100 mL) was refluxed
for 12 h under N2 in an apparatus fitted with a Dean–Stark trap. The mixture was
cooled and concentrated in vacuo. The residue was purified by flash chromatography
on silica gel to give the major product 7 or 9 and 10.
Compound 7
1
Yield: 87%. Mp: 143–144 ꢂC. Rf 0.35 (20:1 hex–EtOAc). H NMR (CDCl3,
600 MHz): d 7.06 (d, 1H, J ¼ 8.4 Hz), 6.77 (s, 1H), 6.57 (s, 1H), 6.51 (d, 1H,
J ¼ 8.4 Hz), 6.33 (d, 1H, J ¼ 10.2 Hz), 6.19 (s, 1H), 5.54 (d, 1H, J ¼ 9.6 Hz), 4.16

544
W.-H. JI ET AL.
(d, 1H, J ¼ 10.2 Hz), 3.94 (t, 1H, J ¼ 10.2 Hz), 3.81 (s, 6H), 3.51–3.60 (m, 1H), 2.89
(dd, 1H, J ¼ 11.4 and 15.0 Hz), 2.76 (dd, 1H, J ¼ 15.6 and 3.6 Hz), 1.39 (s, 6H); 13C
NMR (CDCl3, 125 MHz): d 159.64, 158.24, 155.01, 152.29, 128.30, 127.51, 126.92,
121.92, 121.85, 114.79, 114.56, 104.18, 104.06, 98.66, 75.99, 70.11, 55.34, 31.55,
30.34, 27.91; ESI-MS: m=z (%) ¼ 353 (100) [M þ Hþ]. Anal. calcd. for C22H24O4:
C, 74.98; H, 6.86. Found: C, 74.88; H, 6.92.
Compound 9
1
Yield: 78%. Mp: 137–139 ꢂC. Rf 0.39 (6:1 hex–EtOAc). H NMR (CDCl3,
600 MHz): d 7.79 (d, 1H, J ¼ 9.0 Hz), 7.00 (d, 1H, J ¼ 8.4 Hz), 6.62 (d, 1H, J ¼
10.2 Hz), 6.45–6.49 (m, 3H), 5.59 (d, 1H, J ¼ 10.2 Hz), 4.58 (t, 1H, J ¼ 11.4 Hz),
4.52 (dd, 1H, J ¼ 11.4 and 5.4 Hz), 4.24 (dd, 1H, J ¼ 11.4 and 5.4 Hz), 3.83 (s,
3H), 3.77 (s, 3H), 1.47 (s, 3H), 1.44 (s, 3H); 13C NMR (CDCl3, 125 MHz): d
191.55, 160.46, 159.13, 158.43, 157.95, 130.69, 128.74, 128.49, 116.02, 115.80,
115.46, 110.88, 109.18, 104.59, 99.09, 77.40, 71.24, 55.49, 53.36, 47.18, 28.37,
28.01; ESI-MS: m=z (%) ¼ 367 (100) [M þ Hþ].
Compound 10
1
Yield: 5.3%. Mp: 153–154 ꢂC. Rf 0.32 (6:1 hex–EtOAc). H NMR (CDCl3,
600 MHz): d 7.62 (s, 1H), 7.00 (d, 1H, J ¼ 7.8 Hz), 6.49 (s, 1H), 6.46 (d, 1H, J ¼ 7.8 Hz),
6.32–6.35 (m, 2H), 5.59 (d, 1H, J ¼ 9.6 Hz), 4.55 (t, 1H, J ¼ 10.8 Hz), 4.46 (dd, 1H,
J ¼ 10.8 and 4.8 Hz), 4.23 (dd, 1H, J ¼ 10.8 and 4.8 Hz), 3.79 (s, 3H), 3.76 (s, 3H),
1.45 (s, 6H); 13C NMR (CDCl3, 125 MHz): d 191.57, 163.49, 160.46, 159.86, 158.40,
130.66, 129.34, 125.41, 121.28, 116.06, 115.90, 114.54, 104.58, 104.07, 99.11, 77.79,
71.12, 55.48, 55.36, 47.34, 28.54, 28.52; ESI-MS: m=z (%) ¼ 367 (100) [M þ Hþ]; Anal.
calcd. for C22H224O5: C, 72.12; H, 6.05. Found: C, 72.06; H, 6.14.
3-(2,4-Dimethoxyphenyl)-7-hydroxychroman-4-one (8)
Raney nickel (5.8 g, 0.1 mol) was added to a solution of 4 (2.98g, 0.01mol) in
DMF (100mL). The reaction mixture was further stirred at room temperature for
30min under a hydrogen atmosphere. The mixture was filtrated, and the filtrate was
concentrated in vacuo. The residue was recrystallized from EtOAc at ꢃ5 ꢂC to give pure
8 (2.73 g, 91% yield). Mp: 181–183 ꢂC. Rf 0.31 (2:1 hex–EtOAc). 1H NMR (DMSO-d6,
600 MHz): d 10.57 (s, 1H), 7.69 (d, 1H, J ¼ 8.4 Hz), 6.69 (d, 1H, J ¼ 8.4 Hz), 6.59 (d, 1H,
J ¼ 2.4 Hz), 6.53 (dt, 1H, J ¼ 8.4 and 1.2 Hz), 6.48 (dd, 1H, J ¼ 8.4 and 2.4 Hz), 6.35 (m,
1H), 4.53 (t, 1H, J ¼ 10.8 Hz), 4.42 (dd, 1H, J ¼ 10.8 and 5.4 Hz), 4.16 (dd, 1H, J ¼ 10.8
and 5.4 Hz), 3.75 (s, 3H), 3.72 (s, 3H); 13C NMR (DMSO-d6, 125 MHz): d 190.8, 164.8,
163.7, 160.4, 158.6, 131.1, 129.4, 116.5, 114.5, 111.0, 105.3, 102.8, 99.2, 70.8, 56.0, 55.6,
47.1; ESI-MS: m=z (%) ¼ 323 (100) [Mþ Naþ].
2’,4’-Dimethylglabridin (6)
A solution of 9 (3.66 g, 0.01 mol) in ether (40 mL) was added dropwise to a sol-
ution of LiAlH4 (3.8 g, 0.1 mol) in ether (150 mL) at 20 ꢂC and then the mixture was

TOTAL SYNTHESIS OF (ꢀ)-GLABRIDIN
545
boiled under reflux for 6 h. The solution was cooled. The 50 mL of EtOAc was added
dropwise. The solid was removed, filtrate was concentrated, and residue was purified
by flash chromatography on silica gel (EtOAc–hexane) to give 6 (2.64 g, 75% yield).
1
Mp: 97–99 ꢂC. Rf 0.39 (20:1 hex–EtOAc). H NMR (CDCl3, 600 MHz): d 7.02 (d,
1H, J ¼ 8.4 Hz), 6.82 (d, 1H, J ¼ 8.4 Hz), 6.64 (d, 1H, J ¼ 9.6 Hz), 6.45–6.48 (m,
2H), 6.36 (d, 1H, J ¼ 7.8 Hz), 5.55 (d, 1H, J ¼ 9.6 Hz), 4.34 (dd, 1H, J ¼ 7.2 and
1.8 Hz), 3.97 (t, 1H, J ¼ 10.8 Hz), 3.80 (s, 3H), 3.79 (s, 3H), 3.53–3.57 (m, 1H),
2.95 (dd, 1H, J ¼ 15.6 and 11.4 Hz), 2.82 (dd, 1H, J ¼ 15.6 and 3.0 Hz), 1.42
(s, 3H), 1.40 (s, 3H); 13C NMR (CDCl3, 125 MHz): d159.64, 158.29, 151.82,
149.78, 129.17, 128.51, 127.54, 121.86, 116.98, 114.53, 109.86, 108.57, 104.08,
98.67, 75.53, 70.20, 55.34, 55.32, 31.47, 30.58, 27.78, 27.49; ESI-MS: m=z
(%) ¼ 353 (100) [M þ Hþ].
(ꢀ)-Glabridin (1)
A solution of boron tribromide (1.0 M in CH2Cl2, 0.05 mol) was added to a
stirred solution of 6 (3.52 g, 0.01 mol) in CH2Cl2 (200 mL) at ꢃ78 ꢂC. The reaction
mixture was further stirred at room temperature for 2 h. Then the mixture was
poured into an aqueous solution of saturated NaHCO3 (100 mL) and extracted with
EtOAc (3 ꢄ 100 mL). The combined organic layers were washed with brine (100 mL)
and dried (Na2SO4). Removal of the solvent in vacuo followed by purification on
silica gel provided (ꢀ)-glabridin (1) (2.72 g, 84%). Mp: 227–229 ꢂC. Rf 0.41 (100:9
1
CH2Cl2–MeOH). H NMR (DMSO-d6, 600 MHz): d 9.39 (s, 1H), 9.11 (s, 1H),
6.86 (d, 1H, J ¼ 7.8 Hz), 6.83 (d, 1H, J ¼ 8.4 Hz), 6.54 (d, 1H, J ¼ 9.6 Hz), 6.33 (s,
1H), 6.29 (d, 1H, J ¼ 8.4 Hz), 6.19 (d, 1H, J ¼ 8.4 Hz), 5.64 (d, 1H, J ¼ 10.2 Hz),
4.23 (d, 1H, J ¼ 10.2 Hz), 3.93 (t, 1H, J ¼ 10.2 Hz), 3.29 (t, 1H, J ¼ 10.2 Hz), 2.89
(dd appeared t, 1H, J ¼ 11.4 Hz), 2.69 (dd, 1H, J ¼ 16.2 and 4.2 Hz), 1.76 (s, 6H);
13C NMR (DMSO-d6, 125 MHz): d 157.31, 156.28, 151.65, 149.67, 129.76, 129.62,
127.99, 117.85, 116.86, 115.17, 109.99, 108.53, 106.70, 102.92, 75.65, 70.17, 31.31,
30.41, 27.77, 27.66; ESI-MS: m=z (%) ¼ 347 (100) [M þ Naþ].
SUPPORTING INFORMATION
1
Full experimental detail and H and 13C NMR spectra can be found via the
Supplementary Content section of this article’s Web page.
ACKNOWLEDGMENT
This work was supported by a research grant from Shandong Analysis and
Test Center.
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