Synthesis of 6-deoxymollugins and their inhibitory activities on tyrosinase

Article abstract of DOI:10.1007/s12272-013-0240-1

A series of 6-deoxymollugins were prepared five steps from benzaldehyde and its derivatives via phenylboronic acid-catalyzed chromenylation as a key step. Their inhibitory activities against tyrosinase from mushroom were evaluated to show that the parent,

Full text of DOI:10.1007/s12272-013-0240-1

Arch. Pharm. Res.
DOI 10.1007/s12272-013-0240-1
RESEARCH ARTICLE
Synthesis of 6-deoxymollugins and their inhibitory activities
on tyrosinase
•
Jing Lu Liang Umair Javed Seung Ho Lee
•
•
•
Jae Gyu Park Yurngdong Jahng
Received: 24 June 2013 / Accepted: 31 August 2013
Ó The Pharmaceutical Society of Korea 2013
Abstract A series of 6-deoxymollugins were prepared five
steps from benzaldehyde and its derivatives via phenylbo-
ronic acid-catalyzed chromenylation as a key step. Their
inhibitory activities against tyrosinase from mushroom were
evaluated to show that the parent, methyl 2,2-dimethyl-2H-
benzo[h]chromene-5-carboxylate (9a) showed best and
promising inhibitory activity at IC50 = 18.3 lM.
related protein-2 (TYRP-2), are involved in the process of
forming melanin pigments from tyrosine (Park et al. 2004).
Especially tyrosinase participates in two rate limiting steps
of melanin synthesis, which are a hydroxylation of tyrosine
and an oxidation L-2,4-dihydroxyphenylalanine (L-DOPA)
(Halaban et al. 2002). Proper inhibition of tyrosinase,
therefore, can be an effective tool for the treatment of such
dermatological disorders (Seo et al. 2003; Okombi et al.
2006).
Keywords 6-Deoxymollugin ꢀ Chromenylation ꢀ
Tyrosinase ꢀ 4-Hydroxynaphthoic acid
A series of tyrosinase inhibitors such as azelaic acid
(Lemic-Stoicevic et al. 1995), arbutin (Maeda and Fukuda
1996; Nihei and Kubo 2003), hydroquinones (Sanchez and
Vazquez 1982), kojic acid and its derivatives (Saruno et al.
1979), hydroxystilbenes represented by resveratrol (Sa-
tooka and Kubo 2012), methyl gentisate (Curto et al. 1999),
N-phenylthioureas (Sawant et al. 2010), oxadiazoles (Khan
et al. 2006a), oxazolones (Khan et al. 2006b), and tetrak-
etone derivatives (Okombi et al. 2006) are developed as
potential candidates for skin-decolorizing cosmetic agents.
However, only few of these are successfully launched in
practice mainly due to undesirable side effects.
Introduction
Light especially ultraviolet (UV) irradiation on skin may
result in skin damages such as sun-tanning, photoaging,
and even photocarcinogenesis. Such damages can be pro-
tected by melanin endogenetically produced by a series of
combined enzymatic and chemical reactions (Cullen 1998;
Urabe et al. 1998). Accumulation of melanin at any specific
sites of skin, however, resulted in many dermatological
disorders including acquired hyperpigmentation such as
melasma, freckles, postinflammatory melanoderma, solar
lentigo, and age spot (Lerner et al. 1949; Kuzukami et al.
1993). Three melanocyte-specific enzymes, tyrosinase,
tyrosinase-related protein-1 (TYRP-1) and tyrosinase-
Our interest in searching tyrosinase inhibitors from nat-
ural sources have led to find mollugin (methyl 6-hydroxy-
2,2-dimethyl-2H-benzeno[h]chromene-5-carboxylate, 1) as
an strong inhibitor of tyrosinase. Mollugin was originally
isolated from the herbal medicine Rubiaceae family such as
Putoria calabrica, (Gonzalez et al. 1974) Gallium mollugo
L. (Schildknecht et al. 1976a), and later from various Rubia
species such as R. cordifolia and R. akane (Itokawa et al.
1983), and R. tinctorum (Marec et al. 2001).
J. L. Liang ꢀ U. Javed ꢀ S. H. Lee ꢀ Y. Jahng (&)
College of Pharmacy, Yeungnam University,
Gyeongsan 712-749, Korea
Continuing study on mollugin revealed that mollugin
inhibited cell adhesion between angioendothelial cells and
lymphocytes thus having potentials for cosmetic additives
with hair growth stimulant activity (Shibuya et al. 1999) and
anti-aging activity (Murase et al. 1996; Murase et al. 1997).
e-mail: ydjahng@ynu.ac.kr
J. G. Park
Pohang Technopark, Pohang 790-834, Korea
123

J. L. Liang et al.
OH
O
CO2CH3
O
O
O
O
OH
OH
OH
HO
HO
HO
OH
mollugin (1)
arbutin
kojic acid
Although unique structure and intriguing biological
properties of mollugin led to develop several total synthetic
procedures (Schildknecht and Straub 1976b; Heide and
Leistner 1981; Lumb and Trauner 2005; Habonimana et al.
2006; Claessens et al. 2006; Jung et al. 2007) systematic
studies on specified biological properties were not yet
pursued. We herein described a synthesis 6-deoxymollu-
gins and their inhibitory activity on tyrosinase.
TMS. Chemicals and solvents were commercial reagent
grade and used without further purification. The derivatives
of 2-(E)-benzylidenesuccinic acids, 6b and 6d (El-Abbady
et al. 1961), and 4-hydroxynaphthoic acids, 7b (Beech and
Beech and Legg 1949), 7d (El-Abbady et al. 1961) and 7j
(Sieglitz and Jordanides 1967) have been reported previ-
ously. Mollugin was prepared by employing previously
reported methods (Jung et al. 2007). Elemental analyses
were taken on a Hewlett-Packard Model 185B elemental
analyzer. UV absorption experiment was performed from a
JASCO-V550 spectrophotometer.
Materials and methods
Melting points were determined using a Fischer-Jones
melting points apparatus and are not corrected. IR spectra
were obtained using a Perkin-Elmer 1330 spectrophotom-
eter. NMR spectra were obtained using a Bruker-250
2-(E)-benzylidenesuccinic acid (6a) (general
procedure)
A solution of benzaldehyde (15 g, 141 mmol) and dimethyl
succinate (25.76 g, 176 mmol) in t-BuOH (20 mL) was
added drop wise within 3 h to a refluxing mixture of t-BuOK
spectrometer 250 MHz for 1H NMR and 62.5 MHz for 13
C
NMR and are reported as ppm from the internal standard
Table 1 Physical properties of 8 and 9 and their inhibitory activities against tyrosinase
7
6
5
2
R
CO2
8
X
9
4
3
10
O
=
=
8 R
9 R
H
H
C
3
Entry
X
Yield (%)
mp (oC)
IC50 (lM)a
Entry
X
Yield (%)
mp (oC)
IC50 (lM)
8a
H
79
79
82
85
85
80
84
80
81
84
192
211
218
216
195
204
202
209
219
212
[50
[50
[50
[50
[50
[50
[50
[50
[50
[50
1.5
9a
9b
9c
9d
9e
9f
H
95
93
96
92
92
95
90
91
90
94
40
18.3
29.0
32.8
45.7
26.5
28.9
34.2
32.3
44.6
47.3
[50
8b
7-Cl
8-Cl
9-Cl
7-Br
8-Br
9-Br
7-CH3
8-CH3
9-CH3
7-Cl
8-Cl
9-Cl
7-Br
8-Br
9-Br
7-CH3
8-CH3
9-CH3
88
8c
82
8d
111
98
8e
8f
92
8g
9g
9h
9i
113
84
8h
8i
78
8j
9j
99
Mollugin
Arbutin
a
10
96–97
180
The IC50 value was expressed as the mean of 50 % inhibitory concentration of triplicate determination
123

Synthesis of 6-deoxymollugins and their inhibitory activities
2-(E)-(4-Bromophenyl)methylidenesuccinic acid (6g)
(17.4 g, 155 mmol) and t-BuOH (100 mL). Reflux was
continued for another 3 h, and t-BuOH was removed in
vacuo. The residue obtained was dissolved in 1 N aq. HCl
(100 mL), and the aqueous solution was extracted with
EtOAc (3 9 25 mL). The combined organic layers were
dried (Na2SO4) and concentrated in vacuo to yield an oily
product as methyl 2-(E)-benzylidenesuccinate (5a). 1H
NMR (250 MHz, CDCl3) d 7.91 (s, 1H, –CH =), 7.43–7.30
(m, 5H, phenyl H’s), 3.82 (s, 3H, OCH3), 3.57 (s, 2H, –CH2–).
13C NMR (62.5 MHz, CDCl3) d 177.11, 167.85, 142.59,
134.65, 129.04, 128.96, 128.67, 125.20, 52.38, 33.50. This oil
was re-dissolved in MeOH (60 mL), a 15 % solution of
NaOH in MeOH (100 mL, 375 mmol) was added and the
mixture was refluxed for 12 h. The resulting suspension was
concentrated under reduced pressure, and the residue dis-
solved in water (150 mL). The solution was washed with
EtOAc (3 9 100 mL), acidified with concentrated HCl and
extracted with EtOAc (3 9 50 mL). The organic extracts
were dried (Na2SO4) and concentrated in vacuo to afford a
solid which was crystallized from EtOAc/hexane to give 6a
(20.4 g, 73 %) as a white crystalline solid: mp 174 °C. [lit.
1
White needles (72 %): mp 180 °C. H NMR (DMSO-d6,
250 MHz) d 12.63 (s, 2H, 2COOH), 7.68 (s, 1H, Ar–CH),
7.63 (dm, 2H, J = 8.3 Hz, H30 and H50), 7.35 (dm, 2H,
J = 8.3 Hz, H20 and H60), 3.34 (s, 2H, CH2–COOH). 13C
NMR (DMSO-d6, 62.5 MHz) d 172.07, 168.40, 138.85,
134.22, 131.77, 131.10, 128.21, 122.26, 33.47.
2-(E)-(2-Methylphenyl)methylidenesuccinic acid (6h)
1
White needles (65 %): mp 186 °C. H NMR (DMSO-d6,
250 MHz) d 12.45 (s, 2H, 2COOH), 7.78 (s, 1H, Ar–CH),
7.27–7.17 (m, 4H, Ar), 3.20 (s, 2H, CH2–COOH), 2.24 (s,
3H, Ar–CH3). 13C NMR (DMSO-d6, 62.5 MHz) d 171.97,
168.10, 139.20, 136.41, 134.27, 129.99, 128.53, 128.02,
127.79, 125.77, 33.27, 19.31.
2-(E)-(3-Methylphenyl)methylidenesuccinic acid (6i)
1
White needles (67 %): mp 171 °C. H NMR (DMSO-d6,
1
250 MHz) d 12.50 (s, 2H, 2COOH), 7.70 (s, 1H, Ar–CH),
7.36–7.30 (m, 1H, Ar), 7.21 (m, 3H, Ar), 3.37 (s, 2H, CH2–
COOH), 2.32 (s, 3H, Ar-CH3). 13C NMR (DMSO-d6,
62.5 MHz) d 172.11, 168.45, 139.98, 137.85, 134.84,
129.59, 129.47, 128.55, 127.18, 125.98, 33.40, 20.94.
(Caro et al. 2003) mp 174–176 °C]. H NMR (DMSO-d6,
250 MHz) d 12.57 (s, 2H, 2COOH), 7.73 (s, 1H, Ar–CH),
7.47–7.37 (m, 5H, Ar), 3.36 (s, 2H, CH2-COOH). 13C NMR
(DMSO-d6, 62.5 MHz) d 172.42, 168.72, 140.19, 135.11,
129.24, 129.12, 128.95, 127.53, 33.64.
2-(E)-(4-Methylphenyl)methylidenesuccinic acid (6j)
2-(E)-(3-Chlorophenyl)methylidenesuccinic acid (6c)
1
1
White needles (70 %): mp 211 °C. H NMR (DMSO-d6,
White needles (71 %): mp 189 °C. H NMR (DMSO-d6,
250 MHz) d 12.45 (s, 2H, 2COOH), 7.70 (s, 1H, Ar–CH),
7.30 (d, 2H, J = 8.3 Hz, H30, H50), 7.25 (d, 2H,
J = 8.3 Hz, H20, H60), 3.40 (s, 2H, CH2-COOH), 2.33 (s,
3H, Ar–CH3). 13C NMR (DMSO-d6, 62.5 MHz) d 172.04,
168.49, 139.85, 138.54, 132.01, 129.23, 129.00, 126.45,
33.37, 20.82.
250 MHz) d 12.47 (br s, 2H, 2COOH), 7.70 (s, 1H, Ar–
CH), 7.46–7.45 (m, 3H, Ar), 7.37–7.34 (m, 1H, Ar), 3.34
(s. 2H, CH2–COOH). 13C NMR (DMSO-d6, 62.5 MHz) d
172.17, 168.39, 138.54, 137.29, 133.55, 130.74, 128.98,
128.79, 128.75, 127.70, 33.67.
2-(E)-(2-Bromophenyl)methylidenesuccinic acid (6e)
4-Hydroxy-2-naphthoic acid (7a)
1
White needles (65 %): mp 189 °C. H NMR (DMSO-d6,
2-(E)-Benzylidenesuccinic acid 6a (206 mg, 1.0 mmol)
was dissolved in conc. H2SO4 (464 mg) and stirred at room
temperature for 8 h. The reaction mixture was carefully
poured over cold water (1 mL) and allowed to crystallize
overnight at 5 °C. The resulting crystals were filtered,
washed with water, dried in vacuo and recrystallized from
95 % EtOH to give 144 mg (77 %) of 7a: mp 223–224 °C.
[lit. (Cason 1941) 225–226 °C]. IR (KBr) 3447, 1656,
250 MHz) d 12.55 (br. s, COOH), 7.72 (dd, 1H, J = 7.8,
1.0 Hz, H3), 7.68 (s, 1H, Ar–CH), 7.77 (dd, 1H, J = 7.8,
1.0 Hz, H6), 7.38–7.30 (m, 2H, H4 and H5), 3.23 (s, 2H, CH2–
COOH). 13C NMR (DMSO-d6, 62.5 MHz) d 172.07, 168.03,
138.98, 135.09, 130.81, 130.09, 129.16, 128.08, 123.56, 33.51.
2-(E)-(3-Bromophenyl)methylidenesuccinic acid (6f)
1
1579 cm-1. H NMR (DMSO-d6) d 10.48 (s, 1H, OH),
1
8.17–8.13 (dt, 1H, J = 8.5, 2.0 Hz, H5), 8.05 (s, 1H, H4),
7.99 (dt, 1H, J = 8.5, 2.0 Hz, H8), 7.56–7.51 (m, 2H, H6,
H7), 7.37 (s, 1H, H3) 13C NMR (DMSO-d6) d 167.82,
153.53, 133.67, 129.19, 128.76, 127.17, 127.12, 126.84,
122.19, 121.44, 107.11.
White needles (69 %): mp 163 °C. H NMR (DMSO-d6,
250 MHz) d 7.70 (s, 1H, Ar–CH), 7.60–7.57 (m, 2H, Ar–H),
7.41–7.39 (m, 2H, Ar–H), 3.35 (s, 2H, CH2-COOH). 13CNMR
(DMSO-d6, 62.5 MHz) d 171.99, 168.16, 138.40, 137.38,
131.51, 131.41, 130.81, 128.75, 127.87, 121.93, 33.43.
123

J. L. Liang et al.
1H, J = 8.4, 1.8 Hz, H6), 7.31 (d, 1H, J = 1.2 Hz, H3). 13C
NMR (DMSO-d6, 62.5 MHz): d167.72, 152.79, 136.63, 131.71,
129.06, 128.93, 127.60, 126.82, 121.18, 120.98, 106.99, 21.64.
6-Chloro-4-hydroxy-2-naphthoic acid (7b)
Pale yellow solid (65 %): mp 298 °C [lit. (Beech and Legg
1949) mp 302–304 °C]. Unreported spectral data are as
1
follows: H NMR (DMSO-d6, 250 MHz) d 10.72 (br. s,
7-Methyl-4-hydroxy-2-naphthoic acid (7i)
1H), 8.08 (dd, 1H, J = 2.3 Hz, H5), 8.07 (s, 1H, H1), 8.05
(d, 1H, J = 8.8 Hz, H8), 7.56 (dd, 1H, J = 8.8, 2.2 Hz,
H7), 7.40 (d. 1H. J = 1.3 Hz, H3). 13C NMR (DMSO-d6,
62.5 MHz) d 167.54, 152.83, 132.07, 132.05, 131.54,
129.39, 127.64, 127.43, 121.24, 121.10, 108.29.
Pale yellow solid (63 %): mp 239 °C. 1H NMR (DMSO-d6,
250 MHz) d 10.32 (br s, 1H, OH), 8.06 (d, 1H, J = 8.5 Hz,
H5), 7.94 (s, 1H, H1), 7.75 (s, 1H, H8), 7.40 (d, 1H,
J = 8.5 Hz, H7), 7.30 (s, 1H, H3). 13C NMR (DMSO-d6,
62.5 MHz): d 167.67, 153.22, 136.13, 133.68, 128.96,
128.72, 127.69, 124.83, 121.86, 120.52, 106.25.
7-Chloro-4-hydroxy-2-naphthoic acid (7c)
1
Pale yellow solid (57 %): mp 290 °C. H NMR (DMSO-d6,
2,2-Dimethyl-2H-benzo[h]chromene-5-carboxylic acid
(8a) (general procedure for phenylboronic acid-
mediated chromenylation)
250 MHz): d 13.07 (br s, 1H, COOH), 10.69 (s, 1H, OH), 8.15
(d, 1H, J = 2.3 Hz, H8), 8.13 (d, 1H, J = 8.4 Hz, H5), 8.03 (s,
1H, H1), 7.56 (dd, 1H, J = 8.4, 2.0 Hz, H7), 7.36 (s, 1H, H3).
13C NMR (DMSO-d6, 62.5 MHz) d 167.55, 153.75, 134.61,
131.88, 130.23, 127.73, 127.55, 125.12, 124.61, 120.56, 107.69.
To a mixture of 4-hydroxy-2-naphthoic acid (7a, 940 mg,
5 mmol), 3-methyl-2-butenal (505 mg, 6 mmol), phenylbo-
ronic acid (732 mg, 6 mmol), and propionic acid (0.1 mL) in
toluene (50 mL) was refluxed using Dean-Stark trap for 24 h.
The reaction mixture was cooled to room temperature and
diluted with EtOAc (50 mL). The organic layer was sepa-
rated, washed with brine (3 9 50 mL), dried, and evaporated
in vacuo. The resultant residue was purified by column
chromatography. The early fractions were recrystallized from
pet. hexane:EtOAc to afford 8a (1.0 g, 79 %) as yellow-green
flakes: mp 192 °C. 1H NMR (CDCl3, 250 MHz) d 8.24(s, 1H,
H6), 8.22 (d, 1H, J = 8.0 Hz, H10), 7.84 (d, 1H, J = 7.8 Hz,
H7), 7.59–7.49 (m, 2H, H8, H9), 7.41 (d, 1H, J = 10.0 Hz,
6-Bromo-4-hydroxy2-naphthoic acid (7e)
1
Pale yellow solid (58 %): mp 305 °C. H NMR (DMSO-d6,
250 MHz): d12.99 (br s, 1H, COOH), 10.71 (s, 1H, OH), 8.29 (d,
1H, J = 2.3 Hz, H5), 8.07 (s, 1H, H1), 7.99 (d, 1H, J = 8.5 Hz,
H8), 7.68 (d, 1H, J = 8.8, 2.3 Hz, H7), 7.42 (s, 1H, H3). 13
C
NMR (DMSO-d6, 62.5 MHz): d167.34, 152.51, 132.01, 131.33,
129.93, 129.31, 127.61, 124.16, 121.08, 120.52, 108.10.
7-Bromo-4-hydroxy-2-naphthoic acid (7f)
H4), 5.75 (d, 1H, J = 10.0 Hz, H3), 1.53 (s, 6H, CMe2). 13
C
Pale yellow solid (55 %): mp 283 °C. 1H NMR (DMSO-d6,
250 MHz): d 12.99 (br s, 1H, COOH), 10.65 (s, 1H, OH),
8.31 (d, 1H, J = 1.8 Hz, H8), 8.08 (d, 1H, J = 8.9 Hz,
H5), 8.03 (s, 1H, H1), 7.68 (dd, 1H, J = 8.9, 2.0 Hz, H6),
7.39 (d, 1H, J = 1.0 Hz H3). 13C NMR (DMSO-d6,
62.5 MHz): d 167.36, 153.63, 134.87, 130.75, 129.98,
129.86, 125.12, 124.47, 120.45, 120.31, 107.61.
NMR (CDCl3, 62.5 MHz) d 172.70, 149.36, 132.39, 129.80,
129.00, 128.02, 127.57, 127.03, 125.51, 123.21, 122.25,
120.88, 114.76, 76.08, 27.57.
7-Chloro-2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylic acid (8b)
The same procedure described for 8a was applied to 8-chloro-4-
hydroxy-2-naphthoic acid (7d) (El-Abbady et al. 1961) to afford
8b (320 mg, 79 %) as yellow-green flakes: mp211 °C. 1HNMR
(CDCl3, 250 MHz) d 8.62 (s, 1H, H6), 8.16 (d, 1H, J = 8.3 Hz,
H10), 7.57 (d, 1H, J = 6.8 Hz, H8), 7.44 (t, 1H, J = 7.9 Hz,
H9), 7.40 (d, 1H, J = 10.0 Hz, H4), 5.78 (d, 1H, J = 10.0 Hz,
H3), 1.52 (s, 6H, CMe2). 13C NMR (CDCl3, 62.5 MHz) d
171.92, 149.50, 133.05, 130.62, 129.80, 128.63, 127.71, 127.27,
124.13, 121.51, 121.38, 120.56, 115.53, 77.21, 27.53.
8-Bromo-4-hydroxy-2-naphthoic acid (7g)
1
Pale yellow solid (61 %): mp 289 °C. H NMR (DMSO-d6,
250 MHz) d12.94 (br s, 1H, COOH), 10.82 (s, 1H, OH), 8.27 (d,
1H, J = 1.8 Hz, H1), 8.22 (d, 1H, J = 8.4 Hz, H7), 7.94 (dd,
1H, J = 7.4, 0.9 Hz, H5), 7.51–7.45 (m, 2H, H6 and H3). 13
C
NMR (DMSO-d6, 62.5 MHz): d 167.21, 153.96, 131.76,
131.21, 130.08, 127.93, 127.52, 122.90, 122.37, 119.27, 107.94.
6-Methyl-4-hydroxy-2-naphthoic acid (7h)
8-Chloro-2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylic acid (8c)
1
Pale yellow solid (60 %), mp 257 °C. H NMR (DMSO-d6,
250 MHz) d 812.79 (br. s, COOH), 10.34 (s, 1H, OH), 7.98 (s,
1H, H5), 7.91 (s, 1H, H1), 7.87 (d, J = 8.4 Hz, H8), 7.37 (dd,
The same procedure described for 8a was applied to
7-chloro-4-hydroxy-2-naphthoic acid (7c) to afford 8c
123

Synthesis of 6-deoxymollugins and their inhibitory activities
1
1
(330 mg, 82 %) as yellow-green flakes: mp 218 °C. H
NMR (CDCl3, 250 MHz) d 8.16 (d, 1H, J = 9.0 Hz, H10),
8.11 (s, 1H, H6), 7.80 (d, 1H, J = 2.0 Hz, H7), 7.47 (dd,
1H, J = 9.0, 2.0 Hz, H9), 7.37 (d, 1H, J = 10.0 Hz, H4),
5.76 (d, 1H, J = 10.0 Hz, H3), 1.52 (s, 6H, CMe2). 13C
NMR (CDCl3, 62.5 MHz) d 171.29, 149.51, 133.04 (2
C’s), 131.17, 128.76, 127.49, 125.67, 124.28, 124.12 (2
C’s), 120.56, 115.11, 77.21, 27.60.
(245 mg, 84 %) as yellow-green flakes: mp 246 °C. H
NMR (CDCl3, 250 MHz): d 8.35 (s, 1H, H10), 8.15 (s, 1H,
H6), 7.68 (d, 1H, J = 8.8 Hz, H7), 7.54 (dd, 1H, J = 8.8,
1.8 Hz, H8), 7.36 (d, 1H, J = 10.0 Hz, H4), 5.76 (d, 1H,
J = 10.0 Hz, H3), 1.52 (s, 6H, CMe2).
2,2,7-Trimethyl-2H-benzo[h]chromene-5-carboxylic
acid (8h)
9-Chloro-2,2-dimethyl-2H-benzo[h]chromene-5-
The same procedure described for 8a was applied to
8-methyl-4-hydroxy-2-naphthoic acid (7j) to afford 8h
carboxylic acid (8d)
1
(340 mg, 80 %) as yellow-green flakes: mp 209 °C. H
The same procedure described for 8a was applied to
6-chloro-4-hydroxy-2-naphthoic acid (7b) to afford 8d
NMR (CDCl3, 250 MHz) d 8.40 (s, 1H, H6), 8.11 (d, 1H,
J = 8.3 Hz, H10), 7.44 (t, 1H, J = 7.8 Hz, H9), 7.42 (d,
1H, J = 10.3 Hz, H4), 7.33 (d, 1H, J = 6.8 Hz, H8), 5.75
(d, 1H, J = 10.0 Hz, H3), 2.70 (s, 3H, CH3), 1.52 (s, 6H,
CMe2). 13C NMR (CDCl3, 62.5 MHz): d 172.66, 149.71,
135.72, 131.66, 129.95, 127.84, 127.81, 127.79, 122.65,
121.76, 120.89, 120.47, 114.63, 76.04, 27.52, 19.44.
1
(345 mg, 85 %) as yellow-green flakes: mp 216 °C. H
NMR (CDCl3, 250 MHz): d 8.18 (s, 1H, H6), 8.18 (d, 1H,
J = 2.0 Hz, H10), 7.76 (d, 1H, J = 8.8 Hz, H7), 7.42 (dd,
1H, J = 8.8, 2.0 Hz, H8), 7.38 (d, 1H, J = 10.3 Hz, H4),
5.77 (d, 1H, J = 10.0 Hz, H3), 1.52 (s, 6H, CMe2). 13C
NMR (CDCl3, 62.5 MHz) d 171.96, 148.52, 130.52 (2
C’s), 130.19, 128.50, 128.03, 127.95, 125.01, 121.41,
124.21, 120.55, 115.69, 77.21, 27.60.
2,2,8-Trimethyl-2H-benzo[h]chromene-5-carboxylic
acid (8i)
7-Bromo-2,2-dimethyl-2H-benzo[h]chromene-5-
The same procedure described for 8a was applied to
7-methyl-4-hydroxy-2-naphthoic acid (7i) to afford 8i
carboxylic acid (8e)
1
(345 mg, 81 %) as yellow-green flakes: mp 227 °C. H
The same procedure described for 8a was applied to
8-bromo-4-hydroxy-2-naphthoic acid (7g) to afford 8e
NMR (CDCl3, 250 MHz): d 8.14 (s, 1H, H6), 8.12 (d, 1H,
J = 9.8 Hz, H10), 7.59 (s, 1H, H7), 7.39 (d, 1H,
J = 10.0 Hz, H4), 7.38 (dd, 1H, J = 8.5, 1.5 Hz, H9), 5.71
(d, 1H, J = 10.0 Hz, H3), 2.50 (s, 3H, CH3), 1.51 (s, 6H,
CMe2). 13C NMR (CDCl3, 62.5 MHz): d 172.14, 149.42,
136.91, 132.74, 130.29, 129.37, 127.96, 125.81, 124.86,
123.21, 122.15, 120.96, 114.13, 76.01, 27.56, 21.63.
1
(250 mg, 85 %) as yellow-green flakes: mp 195 °C. H
NMR (CDCl3, 250 MHz) d 8.58 (s, 1H, H6), 8.21 (d, 1H,
J = 8.5 Hz, H10), 7.77 (d, 1H, J = 7.3 Hz, H8), 7.39 (d,
1H, J = 10.3 Hz, H4), 7.37 (t, 1H, J = 7.9 Hz, H9), 5.78
(d, 1H, J = 10.0 Hz, H3), 1.52 (s, 6H, CMe2).
8-Bromo-2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylic acid (8f)
2,2,9-Trimethyl-2H-benzo[h]chromene-5-carboxylic
acid (8j)
The same procedure described for 8a was applied to
7-bromo-4-hydroxy-2-naphthoic acid (7f) to afford 8f
The same procedure described for 8a was applied to
6-methyl-4-hydroxy-2-naphthoic acid (7h) to afford 8j
1
1
(235 mg, 80 %) as yellow-green flakes: mp 204 °C. H
(350 mg, 83 %) as yellow-green flakes: mp 246 °C. H
NMR (CDCl3, 250 MHz): d 8.07 (s, 1H, H6), 8.06 (d, 1H,
J = 8.5 Hz, H10), 7.95 (d, 1H, J = 1.8 Hz, H7), 7.58 (dd,
1H, J = 9.1, 1.9 Hz, H9), 7.34 (d, 1H, J = 10.0 Hz, H4),
5.75 (d, 1H, J = 10.0 Hz, H3), 1.50 (s, 6H, CMe2). 13C
NMR (CDCl3, 62.5 MHz): d 172.05, 149.46, 133.36,
131.16, 130.75, 130.19, 125.78, 124.46, 124.11, 124.04,
121.24, 120.59, 115.17, 76.36, 27.52.
NMR (CDCl3, 250 MHz): d 8.20 (s, 1H, H6), 7.98 (s, 1H,
H10), 7.73 (d, 1H, J = 8.5 Hz, H7), 7.41 (d, 1H,
J = 10.0 Hz, H4), 7.33 (d, 1H, J = 8.3 Hz, H8), 5.74 (d,
1H, J = 10.0 Hz, H3), 2.54 (s, 3H, CH3), 1.52 (s, 6H,
CMe2). 13C NMR (CDCl3, 62.5 MHz): d 171.73, 148.76,
138.32, 130.61, 129.69, 129.32, 128.88, 127.68, 125.43,
122.06, 121.13, 120.95, 114.88, 75.95, 27.50, 22.13.
9-Bromo-2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylic acid (8g)
Methyl 2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylate (9a)
The same procedure described for 8a was applied to
6-bromo-4-hydroxy-2-naphthoic acid (7e) to afford 8g
To a solution of 2,2-dimethyl-2H-benzo[h]chromene-5-
carboxylic acid (8a, 254 mg, 1.0 mmol) in CH3OH
123

J. L. Liang et al.
(50 mL) was slowly added conc. H2SO4 (0.5 mL) and
resulting mixture was refluxed for 10 h and poured to
water. The white precipitate formed was collected and
washed with CH3OH and water to give 9a (80 mg, 95 %)
as yellow flakes after recrystallization from hexane:EtOAc
111 °C. 1H NMR (CDCl3, 250 MHz) d 8.16 (dd, 1H,
J = 2,1, 0.7 Hz, H10), 8.00 (s, 1H, H6), 7.71 (d, 1H,
J = 8.8 Hz, H7), 7.38 (dd, 1H, J = 8.7, 2.1 Hz, H8), 7.27
(d, 1H, J = 10.1 Hz, H4), 5.73 (d, 1H, J = 10.1 Hz, H3),
3.93 (s, 3H, OCH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz) d 167.41, 148.38, 133.73, 130.66, 130.34,
130.27, 127.80, 127.57, 124.88, 123.66, 121.36, 120.56,
115.50, 76.42, 52.17, 27.61. Anal. Calcd for C17H15ClO3:
C, 67.44; H, 4.99. Found C, 68.54; H, 4.82.
1
(1:1): mp 40 °C. H NMR (CDCl3, 250 MHz) d 8.19 (d,
1H, J = 8.1 Hz, H10), 8.05 (s, 1H, H6), 7.78 (d, 1H,
J = 8.4 Hz, H7), 7.55–7.42 (m, 2H, H8, H9), 7.28 (d, 1H,
J = 10.1 Hz, H4), 5.71 (d, 1H, J = 10.1 Hz, H3), 3.93 (s,
3H, CO2CH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz) d 167.73, 149.15, 132.44, 129.66, 128.71,
127.52, 126.99, 126.88, 124.52, 124.01, 122.16, 120.81,
114.55, 76.04, 52.15, 27.54. Anal. Calcd for C17H16O3: C,
76.10; H, 4.99. Found C, 74.98; H, 5.06.
Methyl 7-bromo-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9e)
The same procedure described for 9a was applied to
7-bromo-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9e (71 mg, 92 %) as yellow-green flakes: mp
Methyl 7-chloro-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9b)
1
98 °C. H NMR (CDCl3, 250 MHz) d 8.38 (s, 1H, H6),
8.19 (d, 1H, J = 8.3 Hz, H10), 7.75 (d, 1H, J = 7.3 Hz,
H8), 7.33 (t, 1H, J = 8.3 Hz, H9), 7.28 (d, 1H,
J = 10.3 Hz, H4), 5.75 (d, 1H, J = 9.8 Hz, H3), 3.96 (s,
3H, OCH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz) d 167.46, 149.29, 131.07, 130.90, 130.50,
128.19, 127.60, 125.77, 123.59, 122.71, 122.06, 120.50,
115.30, 76.49, 52.33, 27.56. Anal. Calcd for C17H15BrO3:
C, 58.81; H, 4.35. Found C, 59.41; H, 4.21.
The same procedure described for 9a was applied to
7-chloro-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9b (66 mg, 93 %) as yellow-green flakes: mp
1
88 °C. H NMR (CDCl3, 250 MHz): d 8.41 (s, 1H, H6),
8.13 (d, 1H, J = 8.3 Hz, H10), 7.53 (dd, 1H, J = 7.4,
1.1 Hz, H8), 7.40 (t, 1H, J = 7.9 Hz, H9), 7.28 (d, 1H,
J = 10.3 Hz, H4), 5.75 (d, 1H, J = 10.3 Hz, H3), 3.95 (s,
3H, OCH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz): d 167.47, 149.31, 132.75, 130.45, 129.83,
128.09, 127.20, 127.11, 125.51, 121.32, 120.49, 120.03,
115.30, 76.45, 52.31, 27.54. Anal. Calcd for C17H15ClO3:
C, 67.44; H, 4.99. Found C, 67.67; H, 4.85.
Methyl 8-bromo-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9f)
The same procedure described for 9a was applied to
8-bromo-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9f (63 mg, 95 %) as yellow flakes: mp
92 °C. 1H NMR (CDCl3, 250 MHz) d 8.06 (d, 1H,
J = 8.8 Hz, H10), 7.93 (s, 1H, H6), 7.93 (d, 1H,
J = 2.5 Hz, H7), 7.56 (dd, 1H, J = 8.9, 1.9 Hz, H9), 7.25
(d, 1H, J = 10.0 Hz, H4), 5.73 (d, 1H, J = 10.3 Hz, H3),
3.93 (s, 3H, OCH3), 1.49 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz): d 167.35, 149.34, 133.52, 130.72, 130.54,
130.07, 125.74, 125.33, 124.19, 122.71, 121.11, 120.56,
115.00, 76.38, 52.26, 27.57. Anal. Calcd for C17H15BrO3:
C, 58.81; H, 4.35. Found C, 58.33; H, 4.43.
Methyl 8-chloro-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9c)
The same procedure described for 9a was applied to
8-chloro-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9c (73 mg, 96 %) as yellow flakes: mp
82 °C. 1H NMR (CDCl3, 250 MHz) d 8.14 (d, 1H,
J = 9.0 Hz, H10), 7.93 (s, 1H, H6), 7.76 (d, 1H,
J = 2.0 Hz, H7), 7.44 (dd, 1H, J = 9.0, 2.0 Hz, H9), 7.27
(d, 1H, J = 10.3 Hz, H4), 5.73 (d, 1H, J = 10.0 Hz, H3),
3.94 (s, 3H, OCH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz) d 167.37, 149.31, 133.14, 132.83, 129.96,
128.24, 127.24, 125.84, 125.14, 124.05, 124.76, 120.57,
114.89, 76.38, 52.22, 27.60. Anal. Calcd for C17H15ClO3:
C, 67.44; H, 4.99. Found C, 68.36; H, 4.80.
Methyl 9-bromo-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9g)
The same procedure described for 9a was applied to
9-bromo-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9g (57 mg, 90 %) as yellow flakes: mp
113 °C. 1H NMR (CDCl3, 250 MHz) d 8.34 (dd, 1H,
J = 2,0, 0.7 Hz, H10), 7.99 (s, 1H, H6), 7.63 (d, 1H,
J = 8.8 Hz, H7), 7.51 (dd, 1H, J = 8.8, 2.0 Hz, H8), 7.27
(d, 1H, J = 10.0 Hz, H4), 5.73 (d, 1H, J = 10.3 Hz, H3),
3.93 (s, 3H, OCH3), 1.51 (s, 6H, CMe2). 13C NMR (CDCl3,
Methyl 9-chloro-2,2-dimethyl-2H-benzo[h]chromene-
5-carboxylate (9d)
The same procedure described for 9a was applied to
9-chloro-2,2-dimethyl-2H-benzo[h]chromene-5-carboxylic
acid to afford 9d (55 mg, 92 %) as yellow flakes: mp
123

Synthesis of 6-deoxymollugins and their inhibitory activities
Methyl 2,2-dimethyl-3,4-dihydro-2H-
benzo[h]chromene-5-carboxylate (10)
62.5 MHz) d 167.39, 148.28, 130.83, 130.35, 130.29 (two
C’s), 127.90, 125.01, 124.66, 123.69, 122.07, 120.53,
115.51, 76.44, 52.19, 27.61. Anal. Calcd for C17H15BrO3:
C, 58.81; H, 4.35. Found C, 59.33; H, 4.24.
Method A
A mixture of 9a (134 mg, 0.5 mmol) and 5 % Pd/C
(25 mg) in CH3OH(15 mL) was stirred in a H2 atmosphere
for 5 h at 50 psi. The reaction mixture was filtered through
Celite. Evaporation of the solvent gave pale yellow oily
material which was flash column chromatographed on sil-
ica gel eluting with CH2Cl2 to give an thick oil (130 mg,
96 %). 1H NMR (CDCl3, 250 MHz) d 8.21 (dd, 1H,
J = 7.8, 1.0 Hz, H10), 8.03 (s, 1H, H6), 7.78 (dd, 1H,
J = 7.8, 1.3 Hz, H7), 7.52 (td, 1H, J = 7.8, 1.3 Hz, H9),
7.45 (td, 1H, J = 7.8, 1.3 Hz, H8), 3.92 (s, 3H, OCH3),
3.16 (t, 2H, J = 6.8 Hz, H3), 1.87 (t, 2H, J = 6.8 Hz, H4),
1.41 (s, 6H, CMe2). 13C NMR (CDCl3, 62.5 MHz) d
168.24, 149.42, 131.59, 130.25, 128.41, 127.45, 126.24,
123.17, 122.72, 121.84, 114.13, 74.25, 51.93, 32.70, 26.84,
21.59. Anal. Calcd for C17H18O3: C, 75.53; H, 6.71. Found
75.68; H, 6.68.
Methyl 2,2,7-trimethyl-2H-benzo[h]chromene-5-
carboxylate (9h)
he same procedure described for 9a was applied to 2,2,7-
trimethyl-2H-benzo[h]chromene-5-carboxylic acid to
1
afford 9h (63 mg, 91 %) as yellow flakes: mp 84 °C. H
NMR (CDCl3, 250 MHz): d 8.19 (d, 1H, J = 0.5 Hz, H6),
8.09 (d, 1H, J = 8.3 Hz, H10), 7.41 (t, 1H, J = 7.6 Hz,
H9), 7.30 (t, 1H, J = 6.8 Hz, H8), 7.28 (d, 1H,
J = 10.0 Hz, H4), 5.72 (d, 1H, J = 10.0 Hz, H3), 3.97 (s,
3H, OCH3), 2.68 (s, 3H, CH3), 1.50 (s, 6H, CMe2). 13C
NMR (CDCl3, 62.5 MHz): d 167.98, 149.52, 135.39,
131.74, 129.77, 127.67, 127.23 (two C’s), 124.14, 120.84,
120.39, 120.22, 114.36, 76.00, 52.12, 27.53. Anal. Calcd
for C18H18O3: C, 78.57; H, 6.43. Found C, 79.67; H, 6.22.
Methyl 2,2,8-trimethyl-2H-benzo[h]chromene-5-
carboxylate (9i)
Method B
The same procedure, described above for Method A, was
applied to 9b (151 mg, 0.5 mmol) to give thick oil
(132 mg, 98 %). The spectral data were identical to those
obtained above from Method A.
The same procedure described for 9a was applied to 2,2,8-
trimethyl-2H-benzo[h]chromene-5-carboxylic acid to
afford 9i (58 mg, 90 %) as yellow-green flakes: mp 78 °C.
1H NMR (CDCl3, 250 MHz): d 8.09 (d, 1H, J = 8.5 Hz,
H10), 7.96 (s, 1H, H6), 7.55 (s, 1H, H7), 7.35 (d, 1H,
J = 8.3 Hz, H9), 7.27 (d, 1H, J = 10.3 Hz, H4), 5.69 (d,
1H, J = 10.0 Hz, H3), 3.93 (s, 3H, OCH3), 2.48 (s, 3H,
CH3), 1.49 (s, 6H, CMe2). 13C NMR (CDCl3, 62.5 MHz): d
167.84, 149.19, 136.72, 132.75, 129.80, 129.22, 127.69,
125.21, 124.55, 123.42, 122.04, 120.89, 113.89, 75.94,
52.09, 27.51. Anal. Calcd for C18H18O3: C, 78.57; H, 6.43.
Found C, 79.46; H, 6.29.
Inhibitory activity on tyrosinase from mushroom (Yang
and Robb 2005)
The activity of tyrosinase from mushroom (Sigma-Aldrich)
was spectrophotometrically determined by evaluating the
formation of dopachrome at 30 °C in a 50 mM phosphate
buffer (pH 6.0) by employing previously reported method.
An enzyme solution (50 lL) was added to a cuvette con-
taining 50 mM phosphate buffer (1.2 mL) and 10 mM L-
DOPA (0.8 mL), the solution was immediately mixed. The
increase in absorbance at 475 nm (indicating the formation
of dopachrome) was recorded with a JASCO-V550 spec-
trophotometer, which was compared to that of a blank
cuvette. One enzyme unit was defined as the volume of
enzyme solution producing a rate of absorbance change of
one per minute.
Methyl 2,2,9-trimethyl-2H-benzo[h]chromene-5-
carboxylate (9j)
The same procedure described for 9a was applied to 2,2,9-
trimethyl-2H-benzo[h]chromene-5-carboxylic acid to
afford 9j (67 mg, 94 %) as yellow-green flakes: mp 99 °C.
1H NMR (CDCl3, 250 MHz): d 8.02 (s, 1H, H6), 7.96 (d,
1H, J = 0.7 Hz, H10), 7.69 (d, 1H, J = 8.3 Hz, H7),
7.33–7.26 (m, 1H, H8), 7.29 (d, 1H, J = 10.0 Hz, H4),
5.70 (d, 1H, J = 10.3 Hz, H3), 3.92 (s, 3H, OCH3), 2.52 (s,
3H, CH3), 1.50 (s, 6H, CMe2). 13C NMR (CDCl3,
62.5 MHz): d 167.80, 148.59, 137.71, 130.72, 129.52,
129.17, 128.60, 127.16, 123.98, 123.58, 121.05, 120.93,
114.86, 75.90, 52.03, 27.51, 22.05. Anal. Calcd for
C18H18O3: C, 78.57; H, 6.43. Found C, 78.97; H, 6.38.
Inhibition of melanin production (Siegrist and Eberle
1986)
B-16 mouse melanoma cell lines were obtained from ATCC
(Manassas, USA) and cultured in Dulbecco’s modified
Eagle’s medium, supplemented with 10 % fetal bovine
serum, 143 M/mL benzylpenicillin potassium and 10 mg/
123

J. L. Liang et al.
mL streptomycin sulfate at 37 °C under the 5 % CO2
atmosphere. The B-16 cells were seeded at a density of
2.5 9 103 cells per well in 96-well culture plates, and
incubated at 37 °C under the 5 % CO2 atmosphere for 24 h.
The cells were then treated with the samples and 100 lM
IBMX for 72 h, with the amounts of melanin in the culture
media were spectrophotometrically measured at 405 nm.
We, therefore, hydrolyzed the Stobbe adducts (5) to their
corresponding free acids, 2-(substituted-phenyl)meth-
ylidenesuccinic acids (6) (Doulut et al. 1993; Caro et al.
2003) in 67–73 % yields. A direct Friedel–Crafts acylation
of 6 with H2SO4 at room temperature led a series of
expected (substituted)-4-hydroxy-2-naphthoic acids (7)
(Cason, 1941), which were then subjected to phenylbo-
ronic acid-catalyzed intermolecular chromenylation (Mur-
phy et al. 1992; Pettigrew et al. 2005) with 3-methyl-2-
butenal to afford the corresponding 2H-naphtho[1,2-b]py-
rans 8 in 79–85 % yields. Subsequent esterification of 8
with CH3OH to afford a series of 6-deoxymollugin
derivatives (9) in over 90 % yields (Table 1). It should be
noted that 2,2-dimethyl-2H-pyranylation of 2-(E)-(3-
substituted-phenyl)methylidenesuccinic acids (6) afforded
two isomeric 4-hydroxy-2-naphthoic acids such as 8-sub-
stituted-4-hydroxy-2-naphthoic acids and 10-substituted-4-
Results and discussion
Chemistry
The 6-deoxymollugin derivatives were prepared 5 steps
from benzaldehyde and its derivatives. The Stobbe reaction
of (substituted)benzaldehydes (2) and dimethyl succinate
afforded 2-(E)-(substituted-phenyl)methylidenesuccinic acid
mono methyl esters (3) (El-Abbady et al. 1961; El-Assal
et al. 1963). Although previous reports claimed that com-
pounds 3 were readily cyclized with Ac2O in the presence of
anhydrous NaOAc to lead a series of methyl (substituted)-4-
acetoxy-2-naphthoates (4a) (El-Abbady et al. 1961), all the
attempts for direct Friedel–Crafts acylation were failed
(Jahng 2013).
1
hydroxy-2-naphthoic acids, of which the H NMR spectra
showed a ratio of 8.5–9.0:1 in crude stage. Although
careful chromatography provided the major products
8-substituted-4-hydroxy-2-naphthoic acids as pure state,
the other congeners would only be obtained as mixtures.
Thus, it was not able to prepare the compounds with a
substituent at C10.
CO2CH3
CO2CH3
CO2CH3
CHO
Friedel-Craft
X
CO2CH3
R1
R1
R1
t-BuOK/t-BuOH
acylation
CO2H
OR
2
3
4a R = Ac
4b R = H
O
OCH3
OCH3
8
1
CO2H
i)
7
CO2H
CO2H
conc. H2SO4
O
R1
6
2
R1
3
t-BuOK/t-BuOH
r.t., 8 h
4
5
OH
ii) NaOH
iii) H3O+
7
6
7
6
5
2
CO2H
O
CO2CH3
8
R1
9
R1
CH3OH
H2SO4
H
4
3
10
PhB(OH)2
O
O
8
9
123

Synthesis of 6-deoxymollugins and their inhibitory activities
Additional homologous alkyl esters such as ethyl,
propyl and isopropyl esters of 8a were prepared by
simple Fischer esterification, of which the inhibitory
activities against tyrosinase were, however, not improved
at all. Although catalytic hydrogenation of 9a afforded
corresponding series of 6-deoxydihydromollugin analogue
(10) in quantitative yield, catalytic hydrogenation of 9
with halogens as a substituent (9b–g) suffered from hy-
drogenolysis to afford unsubstituted 10 in fairly good
yields.
showed the promising activity, no significant increase of
activity was observed upon the introduction of substituent
on benzene ring.
To prove a potential for the skin-decolorizing effect, the
inhibitory activity of 9a, the most potent congener, on
melanin synthesis in mouse melanoma B-15 cells was
evaluated by previously reported method (Siegrist and
Eberle 1986) showing strong activity at 10 lM. It should
be noted that all the compounds prepared did not show any
significant toxicity against selected human cancer cell lines
such as breast adenocarcinoma (MCF7), prostate tumor
(DU-145), colorectal adrenocarcinoma (HCT60), and
chronic myelogenous leukemia (HL60) up to 70 lM, while
mollugin (1) showed strong cytotoxicity against several
selected human cancer cell lines (Itokawa et al. 1993, Son
et al. 2008).
COOCH3
CO2CH3
R
H2
10% Pd-C
O
O
In conclusion, a series of 6-deoxymollugins were pre-
pared in 5-steps from benzaldehyde and its derivatives in
fairly good yields. The inhibitory activity of the com-
pounds prepared against tyrosinase were evaluated to
show that the parent, methyl 2,2-dimethyl-2H-
benzo[h]chromene-5-carboxylate (9a) showed best inhib-
itory activity (IC50 = 18.3 lM) with no significant cyto-
toxicity implying potential lead for skin-decolorizing
agent.
9a-g
10
It should be noted that attempts to convert 4-hydro-
xynaphthalene-2-carboxylic acid (7) and their methyl ester
(11) to corresponding 1,4-dihydroxynaphtalene-2-car-
boxylic acids and their methyl esters (12), possible syn-
thetic precursors for substituted mollugins, by ceric
ammonium nitrate (Jacob et al. 1976) or PhI(OCOCF3)2-
mediated oxidation (Andrews et al. 1996) were failed. All
attempts to prepare the derivatives of mollugin were, thus,
failed.
O
O
OH
OH
CO2R
CO2R
COOR
Na2S2O4
CAN
R1
R1
R1
X
or
PhI(OCOCF3)3
OH
7 R = H
12
11 R = CH3
Acknowledgments Financial support from Yeungnam University
Grant is gratefully acknowledged.
Biology
In vitro inhibitory activity of the compounds prepared on
tyrosinase from mushroom was evaluated by measuring the
formation of dopachrome employing previously reported
method (Yang and Robb 2005). The results were summa-
rized in Table 1. Although the parent 6-deoxymollugin
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