Synthesis and anticancer evaluation of naphthoquinone esters with 2′-cyclopentyl and 2′-cyclohexyl substituents

Article abstract of DOI:10.1271/bbb.90946

Twelve novel naphthoquinone esters containing cyclopentyl and cyclohexyl substituents at C-2′ of the propyl chain were synthesized by starting from 1-hydroxy-2-naphthoic acid via alkylation with cyclopentyl ester and cyclohexyl ester. They were evaluated

Full text of DOI:10.1271/bbb.90946

Biosci. Biotechnol. Biochem., 74 (6), 1205–1214, 2010
Synthesis and Anticancer Evaluation of Naphthoquinone Esters
with 20-Cyclopentyl and 20-Cyclohexyl Substituents
y
Ngampong KONGKATHIP,y Narathip PRADIDPHOL, Komkrit HASITAPAN, and Boonsong KONGKATHIP
Natural Products and Organic Synthesis Research Unit (NPOS), Department of Chemistry and Center
for Innovation in Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand
Received December 21, 2009; Accepted February 24, 2010; Online Publication, June 7, 2010
[doi:10.1271/bbb.90946]
Twelve novel naphthoquinone esters containing cyclo-
pentyl and cyclohexyl substituents at C-20 of the propyl
chain were synthesized by starting from 1-hydroxy-2-
naphthoic acid via alkylation with cyclopentyl ester and
cyclohexyl ester. They were evaluated for cytotoxicity
against three cancer cell lines (human epidermoid
carcinoma (KB), human cervical carcinoma (HeLa),
and human hepatocellular carcinoma (HepG2)). In
comparison to naphthoquinone esters with the 20,20-
dimethyl group, the naphthoquinones with a 20-cyclo-
pentyl substituent showed stronger activity than those
with a 20-cyclohexyl substituent, but less than that with
the 20,20-dimethyl group. This work provides new
information about the effect of 20-position substituents
on the cytotoxicity of naphthoquinone ester analogues.
position of the propyl chain showed better activity, this
prompted us to synthesize naphthoquinone esters with
more rigid cyclopentyl and cyclohexyl groups, hoping
that those naphthoquinones with a cyclopentyl or cyclo-
hexyl group at the 20-position would show good activity.
We therefore investigated the synthesis of naphthoqui-
none esters with a 20-cyclopentyl and 20-cyclohexyl
substituent on the propyl side chain. Twelve new
naphthoquinone esters (7–18) (Fig. 2) were synthesized
and their cytotoxicity evaluated against KB, HeLa, and
HepG2 cancer cell lines.
Results and Discussion
Chemistry
Naphthoquinone alcohols 32 and 33 were synthesized
in nine steps (Scheme 1), starting from 1-hydroxy-2-
naphthoic acid,1) by alkylating naphthyl methyl bromide
19 with cyclopentyl 20 and cyclohexyl esters 21 to
respectively afford esters 22 and 23. Demethylation-
cyclization of 22 with aluminium chloride (AlCl3)
provided cyclic ester 24. Reduction of corresponding
cyclic ester 24 led to diol 26 which was oxidized by
treating with Fremy’s salt11) to give naphthoquinone 28.
Oxidation of quinone 28 with 2,3-dichloro-5,6-dicyano-
benzoquinone (DDQ) in the presence of para-toluene
sulfonic acid (p-TsOH) provided rhinacanthone deriva-
tive 30 which was treated with 1% aqueous sodium
hydroxide to afford naphthoquinone alcohol 32. The
yields in all steps involved were very high. Naphtho-
quinone alcohol 33 could be synthesized from ester 23
by the same method as that shown in Scheme 1 in a high
yield. Four naphthoquinone esters with a 20-cyclopentyl
substituent (7, 9, 10, and 12) and another four with a
20-cyclohexyl substituent (13, 15, 16, and 18) were thus
obtained by respectively coupling naphthoquinone alco-
hols 32 and 33 with benzoic acid, naphthoic acid or their
derivatives by using carbonyldiimidazole (CDI) as a
condensing agent (Scheme 2). Demethylation of naph-
thoquinone methyl ethers 9, 15, 12, and 18 by using
boron tribromide respectively afforded naphthoquinone
esters 8, 14, 11, and 17 in good yields (Scheme 3).
It was found in the preparation of 32 and 33, after a
work-up by neutralizing with 1% hydrochloric acid,
extraction with dichloromethane, and removal of the
Key words: naphthoquinone ester; rhinacanthin deriva-
tive; cytotoxicity; cancer cell lines
Rhinacanthins1) (1,4-naphthoquinone esters) and
rhinacanthone2) were isolated from the Thai herb,
Rhinacanthus nasutus,3–5) which is used for treating
cancer.6) An aqueous extract of the roots of R. nasutus
has recently been examined in mice for its chemo-
preventive potential against colonic neoplasms induced
by azoxymethane combined with dextran sodium sul-
fate.7) Liposomal formulations of rhinacanthins have
been studied and showed strong antiproliferative activity
against HeLaS3 cells.8) Moreover, apoptosis induction
by rhinacanthones9) and rhinacanthins10) has also been
investigated.
We have already reported the synthesis of anticancer
rhinacanthone (1,2-pyranonaphthoquinone (1)) and its
derivatives, 1,2-furanonapthoquinones (2), 1,4-pyrano-
naphthoquinones (3), 1,4-furanonaphthoquinones (4),2)
and of rhinacanthins or 1,4-naphthoquinone esters1) (5
and 6) with various substituents at the 20-position of the
propyl chain (Fig. 1). We have found that 20-substituents
effected beneficial anticancer activity. Natural rhinacan-
thins3,4) and synthetic rhinacanthins or naphthoquinone
esters1) with 20,20-dimethyl substituents showed more
potent cytotoxicity against KB, HeLa and HepG2 cancer
cell lines than those with 20-monomethyl and no
substituents at the 20-position.
Since we have found that more rigidity at the 20-
y
To whom correspondence should be addressed. Ngampong KONGKATHIP, Tel: +66-2-562-5444 ext. 2139; Fax: +66-2-5793955; E-mail:
fscinpk@ku.ac.th; Boonsong KONGKATHIP, Tel: +66-2-562-5444 ext. 2235; Fax: +66-2-5793955; E-mail: fscibsk@ku.ac.th
Abbreviations: KB, human epidermoid carcinoma; HeLa, human cervical carcinoma; HepG2, human hepatocellular carcinoma; AlCl3, aluminium
chloride; DDQ, 2,3-dichloro-5,6-dicyanobenzoquinone; p-TsOH, para-toluenesulfonic acid; CDI, carbonyldiimidazole; MTT, 3-(4,5-dimethylthiazol-
2-yl)-2,5-diphenyltetrazolium bromide

1206
N. KONGKATHIP et al.
R1
R1
R2
R2
O
O
O
O
R1
R2
R1
R2
O
O
O
O
O
O
O
O
3
4
1
2
Rhinacanthone
R1,R 2 = CH3
O
O
R3
O
O
R3
1'
1'
3'
3'
2'
2'
O
O
R1 R2
OH
R1 R2
OH
O
O
5
6
5a: R1, R2 = H, R3 = H
5b: R1 = CH3, R2 = H, R3 = H
5c: R1, R2 = CH3, R3 = OH or OMe
5d: R1, R2 = CH3, R3 = H
6a: R1, R2 = H, R3 = H
6b: R1 = CH3, R2 = H, R3 = H
6c: R1, R2 = CH3, R3 = OH or OMe
6d: R1, R2 = CH3, R3 = H
Fig. 1. 1,2- and 1,4-Anticancer Naphthoquinones.
O
O
O
R
O
O
O
R
O
O
OH
OH
10: R = H
11: R = OH
7: R = H
8: R = OH
12: R = OMe
9: R = OMe
O
O
O
O
R
O
O
O
O
R
OH
OH
16: R = H
17: R = OH
18: R = OMe
13: R = H
14: R = OH
15: R = OMe
Fig. 2. 20-Cyclopentyl and 20-Cyclohexyl Naphthoquinone Esters.
solvent, that crude naphthoquinone alcohol 33 was
reversely cyclized to rhinacanthone derivative 31,
whereas only a slight change of 32 to cyclic form 30
was observed. In addition, when crude product 33 was
left at room temperature, it was completely converted to
rhinacanthone 31. This was due to the rigidity and strain
imparted by the cyclopentyl and especially the cyclo-
hexyl group at the C-20 position of the molecule. The
hydroxyl group of the propyl chain closely approached
the carbonyl group at the C-1 position of the naphtho-
quinone nucleus which caused cyclic formation to occur
very easily by the mechanism shown in Scheme 4. As a
result, esterification of these two compounds gave lower
yields of the ester products than those of the 20,20-
dimethyl analogues.1) Naphthoquinone alcohols 32 and
33 should therefore be freshly prepared from rhinacan-
thones 30 and 31, respectively. Similarly, the orientation
of the naphthoquinone ester products appeared folded
due to the same resulting effects of rigidity and strain.
Biological activities
Twelve new synthetic naphthoquinone esters with 20-
cyclopentyl (7–12) and 20-cyclohexyl (13–18) substitu-
ents were evaluated by the MTT colorimetric method12)
for their cytotoxicity against human cancer cell lines KB
(human epidermoid carcinoma), HeLa (human cervical
carcinoma), and HepG2 (human hepatocellular carcino-
ma) (Table 1), and their activities were compared to
previous naphthoquinone esters1) with the 20,20-dimethyl
substituent (Table 2).
Table 1 demonstrates that naphthoquinone ester 7
(with a 20-cyclopentyl on the propyl side chain) showed
more potent cytotoxicity against KB cells (IC50
¼
3:53 ꢀ 0:27 mM) than that of 20-cyclohexyl ester 13
(IC50 ¼ 11:45 ꢀ 2:18 mM), but was less potent than
rhinacanthin M (5d) with a 20,20-dimethyl substituent1)
(IC50 ¼ 1:53 ꢀ 0:34 mM). Naphthoquinone 10 with a
20-cyclopentyl (IC50 ¼ 5:63 ꢀ 1:50 mM) also showed
more potent cytotoxicity against KB cell lines than that
of 20-cyclohexyl ester 16 (IC50 ¼ 9:42 ꢀ 0:88 mM) and

Synthesis and Anticancer Naphthoquinone Esters
1207
Scheme 1. Synthesis of 20-Cyclopentyl and 20-Cyclohexyl Naphthoquinone Alcohols 32 and 33.
Reagents and conditions: (a) LDA, THF, ꢁ78 ꢂC, 96% for 22, 87% for 23; (b) AlCl3, PhCl, reflux, 86% for 24, 81% for 25; (c) LiAlH4, THF,
0 ꢂC, 90% for 26, 89% for 27; (d) Fremy’s salt, rt, 91% for 28, 87% for 29; (e) DDQ, p-TsOH, PhH, reflux, 99% for 30, 84% for 31; (f) 1% aq.
NaOH, reflux.
Scheme 2. Synthesis of Naphthoquinone Esters 7, 9, 10, 12, 13, 15, 16, and 18.
Reagents and conditions: (a) 1% aq. NaOH, reflux; (b) CDI, THF, rt.
was less potent than 20,20-dimethyl one (6d) (IC50
¼
moiety also affected the activity. With respect to the
naphthoquinone benzoate esters (7–9 and 13–15), the
OH and OMe groups on the benzoate part of 20-
cyclopentyl naphthoquinone esters 8 and 9 conferred
less activity against all three tested cancer cell lines than
the one with no C-1 substitution (7) which corresponds
to the analogues of the 20,20-dimethyl naphthoquinone
esters in our previous work.1) In contrast, an OH group
1:59 ꢀ 0:37 mM).1) With similar evaluation for the HeLa
and HepG2 cancer cell lines, the 20,20-dimethyl naph-
thoquinone esters (5d and 6d) showed stronger activity
than the 20-cyclopentyl (7 and 10) and 20-cyclohexyl (13
and 16) esters (Table 2).
The presence of a hydroxyl (OH) or methoxyl (OMe)
group at the C-1 position on the benzene or naphthalene

1208
N. KONGKATHIP et al.
Scheme 3. Preparation of Naphthoquinone Esters 8, 11, 14, and 17 by the Demethylation Reaction.
Scheme 4. Proposed Mechanism for the Reverse Cyclization of 32 and 33.
on the C-1 benzoate part of 20-cyclohexyl naphtho-
quinone ester 14 effected better activity against all three
tested cancer cells than the C-1 OMe substituent (15) or
without the C-1 substituent (13) and no toxicity toward
Vero cells (IC50 ¼ 72:54 ꢀ 2:0 mM). With regard to the
naphthoquinone naphthoate esters (10–12 and 16–18),
the cyclopentyl series (10–12) compounds with an OH
group at the C-1 position on the naphthalene moiety
showed stronger activity against all three tested cancer
cell lines than C-1 OMe and without the C-1 substituent.
In contrast, the cyclohexyl series compounds with an
OH or OMe group at the C-1 position (17 and 18)
showed less activity against KB cancer cells than that
without the C-1 substituent (16), but exhibited slightly
better activity against HeLa cancer cells.
The results of the cytotoxicity tests on the cancer cell
lines revealed that the 20-cyclopentyl (7–12) and 20-
cyclohexyl (13–18) compounds showed less activity
than the 20,20-dimethyl naphthoquinone esters (5d, and
6d).1) This indicates that the C-20 substituents (cyclo-
pentyl and cyclohexyl), which are more rigid than the
two methyl groups, did not effect better anticancer
activity. It could therefore be said that the 20,20-dimethyl
group on the propyl side chain resulted in the most
potent activity of this naphthoquinone ester series.
Interestingly, these analogues contained the same sub-
stituents as rhinacanthins found in the Rhinacanthus
nasutus plant.
However, naphthoquinones 14, 15, 17, and 18 showed
good inhibition (IC50 ¼ 9{15 mM) against KB and Hela
cancer cells, but no toxicity toward Vero cells
(IC50 ¼ 51{98 mM). We conclude from the structure-
activity relationships that those naphthoquinones with a
20,20-dimethyl or 20-cyclohexyl group offer strong
potential for developing new anticancer drugs. The
naphthoquinone amide analogues will also be inves-
tigated by our group, hoping that these compounds too
will show good anticancer activity.
Conclusion
Twelve novel 20-cyclopentyl and 20-cyclohexyl naph-
thoquinone esters were synthesized in moderate to high
yields, and most of them showed good inhibition against
KB and HeLa cancer cell lines, although weaker than the
rhinacanthins (20,20-dimethyl naphthoquinone esters). In
addition, the 20-cyclohexyl naphthoquinones showed
weaker anticancer activity than the 20,20-dimethyl
naphthoquinones, although not being toxic to normal
Vero cells. These results will be valuable for further
chemical modifications to naphthoquinone esters and
naphthoquinone amides with a 20,20-dimethyl or 20-

Synthesis and Anticancer Naphthoquinone Esters
1209
Table 1. Cytotoxicity of Naphthoquinone Esters 5d,1) 6d,1) and 7–18 against Human Carcinoma Cell Lines (KB, HeLa, and HepG2) and Vero
Cell Lines
Cancer cell lines,I IC50 (mM)II
Vero cellsI
IC50 (mM)II
Naphthoquinone ester
KB
HeLa
HepG2
O
O
O
O
1:53 ꢀ 0:34
3:02 ꢀ 0:58
1:17 ꢀ 0:21
7:04 ꢀ 0:52
15:99 ꢀ 0:79
57:80 ꢀ 6:71
16:35 ꢀ 0:52
6:40 ꢀ 0:72
5:48 ꢀ 0:68
17:93 ꢀ 2:67
4:85 ꢀ 1:14
48.64
OH
5dIII
O
O
O
O
1:59 ꢀ 0:37
3:53 ꢀ 0:27
5:41 ꢀ 0:59
4:23 ꢀ 0:10
5:63 ꢀ 1:50
3:68 ꢀ 0:18
65:48 ꢀ 1:47
11:45 ꢀ 2:18
1:30 ꢀ 0:21
16:90 ꢀ 5:61
83:66 ꢀ 7:58
18:31 ꢀ 1:36
83:09 ꢀ 4:97
21:47 ꢀ 0:99
63:76 ꢀ 2:59
23:24 ꢀ 2:20
1.50
OH
6dIII
O
O
O
O
13:73 ꢀ 0:66
15:25 ꢀ 6:30
24:24 ꢀ 3:02
16:19 ꢀ 5:40
7:29 ꢀ 0:78
6:42 ꢀ 1:09
3:51 ꢀ 0:57
OH
O
O
7
O
OH
OH
O
8
O
O
OMe
O
OH
O
9
O
O
O
O
O
OH
O
O
10
OH
O
OH
OH
O
O
11
OMe
O
O
12
O
O
O
O
OH
OH
O
O
13
OH
O
14
O
9:56 ꢀ 0:10
11:42 ꢀ 0:14
14:75 ꢀ 4:57
72:54 ꢀ 2:00
O
O
O
OMe
11:76 ꢀ 0:46
9:42 ꢀ 0:88
13:81 ꢀ 0:11
14:91 ꢀ 0:99
15:58 ꢀ 2:24
10:63 ꢀ 1:79
72:27 ꢀ 5:04
50:16 ꢀ 10:25
50:58 ꢀ 6:55
98:58 ꢀ 6:08
3:89 ꢀ 0:64
51:01 ꢀ 5:70
OH
O
15
O
O
O
OH
O
16
O
O
OH
O
OH
OH
O
O
17
O
OMe
O
11:35 ꢀ 0:06
10:22 ꢀ 0:17
62:73 ꢀ 1:84
58:14 ꢀ 7:31
O
18
IKB, human epidermoid carcinoma. HeLa, human cervical carcinoma. HepG2, human hepatocellular carcinoma. Vero cells, African green monkey kidney cells.
IIThe results are the mean of three replicated determinations ꢀ SD.
IIIThe cytotoxicity results are from J. Med. Chem., 47, 4427–4438 (2004).

1210
N. KONGKATHIP et al.
Table 2. Comparison of the Cytotoxicity of Naphthoquinone Esters with Various Substituents at the 20-Position of the Propyl Side Chain
IC50 (mM)II of naphthoquinone esters
Cancer cell
linesI
20-cyclopentyl
20-cyclohexyl
20,20-dimethyl
7
10
13
16
5 d
6 d
KB
HeLa
HepG2
3:53 ꢀ 0:27
7:04 ꢀ 0:52
16:90 ꢀ 5:61
5:63 ꢀ 1:50
16:35 ꢀ 0:52
83:09 ꢀ 4:97
11:45 ꢀ 2:18
17:93 ꢀ 2:67
23:24 ꢀ 2:20
11:35 ꢀ 0:06
10:22 ꢀ 0:17
62:73 ꢀ 1:84
1:53 ꢀ 0:34
3:02 ꢀ 0:58
4:85 ꢀ 1:14
1:59 ꢀ 0:37
1:17 ꢀ 0:21
1:30 ꢀ 0:21
IKB, human epidermoid carcinoma. HeLa, human cervical carcinoma. HepG2, human hepatocellular carcinoma.
IIThe results are the mean of three replicated determinations ꢀ SD.
added at ꢁ78 ꢂC to the reaction mixture, and stirring was continued for
30 min more. A solution of bromide 19 (0.30 g, 1.2 mmol) and
hexamethylphosphoramide (HMPA; 0.4 ml, 2.3 mmol) in dry THF
(2 ml) was added dropwise to the reaction mixture at the same
temperature. After stirring for 2 h at ꢁ78 ꢂC, the reaction mixture was
quenched with a saturated ammonium chloride solution and then
extracted with diethyl ether (3 ꢃ 30 ml). The combined organic phases
were washed with water and brine, dried over anhydrous sodium
sulfate, filtered and concentrated in vacuo. The residue was purified by
flash column chromatography, eluting with 1% v/v ethyl acetate-
hexane, to yield desired product 23 (0.325 g, 87%) as a colourless
solid, mp 74–75 ꢂC. IR ꢁmax (KBr) cmꢁ1: 2946, 2925, 2850 (CH2,
CH3), 1727 (C=O), 1451, 1367 (C=C), 1197, 1130, 1081 (C–O).
NMR ꢀH (CDCl3, 400 MHz): 1.15–1.40 (4H, m, 2 ꢃ CH2), 1.58–1.67
(4H, m, 2 ꢃ CH2), 2.75 (2H, m, CH2), 3.02 (2H, s, CH2Ar), 3.68 (3H,
s, CO2CH3), 3.90 (3H, s, OCH3), 7.16 (1H, d, J ¼ 8:4 Hz, ArH), 7.50
(2H, m, ArH), 7.55 (1H, d, J ¼ 8:4 Hz, ArH), 7.84 (1H, dd, J ¼ 8:4
and 0.7 Hz, ArH), 8.10 (1H, dd, J ¼ 8:4 and 0.7 Hz, ArH). NMR ꢀC:
24.14 (2 ꢃ CH2), 26.38 (CH2), 34.92 (2 ꢃ CH2), 41.82 (CH2), 49.70
(C), 52.13 (CH3), 62.26 (CH3), 122.90 (CH), 123.88 (CH), 126.23
(CH), 126.40 (C, CH), 128.51 (C), 128.58 (CH), 130.12 (CH), 134.85
(C), 155.20 (C), 177.34 (C=O). EIMS m=z: 312 (Mþ, 22), 171 (100),
157 (3), 156 (15), 141 (14). Anal. Found: C, 76.96; H, 7.85. Calcd. for
cyclohexyl substituent on the propyl side chain to
develop future anticancer drugs. We consider that the
rhinacanthin derivatives could be an additional choice
for developing anticancer drugs.
Experimental
General remarks. Melting point (mp) data were determined by
Fisher John apparatus and are uncorrected. The IR spectra were
recorded by a Perkin-Elmer FTIR 2000 system, and mass spectral data
were obtained with a GC-MS-QP-5050A device. 1H and 13C nuclear
magnetic resonance spectra were recorded by a Varian Unity INOVA
400 instrument. Chemical shifts (ꢀ) are given in parts per million
down-field from tetramethylsilane (TMS) as an internal standard, and
coupling constants are given in Hertz (Hz). The following abbrevia-
tions are used: br ¼ broad, s ¼ singlet, d ¼ doublet, t ¼ triplet, m ¼
multiplet, dd ¼ doublet of doublets, td ¼ triplet of doublets, dt ¼
doublet of triplets. Elemental analyses were performed on compounds
7–18 and 22–31 with a Perkin Elmer PE2400 series II elemental
analyzer at the Scientific and Technological Research Equipment
Centre of Chulalongkorn University in Bangkok, Thailand and are
within ꢀ0:4% of the theoretical values. Column chromatography was
performed with Merck 9385 flash silica gel.
C20H24O3: C, 76.89; H, 7.74.
3,4-Dihydro-2-spirocyclopentylbenzo[H]chromen-2-one (24).
A
Methyl-1-((1-methoxynaphthalene-2-yl)methyl)cyclopentanecarbox-
ylate (22). 1 M n-butyl lithium (8 ml, 7.96 mmol) was added dropwise
to a stirred solution of diisopropylamine (1.12 ml, 7.96 mmol) in dry
THF (15 ml) at 0 ꢂC under nitrogen. After stirring for 20 min,
cyclopentanecarboxylic acid methyl ester 20 (0.613 g, 4.78 mmol)
was slowly added to the reaction mixture at ꢁ78 ꢂC, and stirring was
continued for 30 min. A solution of bromide 19 (1.00 g, 3.98 mmol)
and hexamethylphosphoramide (HMPA; 1.38 ml, 7.96 mmol) in dry
THF (5 ml) was then added dropwise to the reaction mixture at the
same temperature. After stirring for 2 h at ꢁ78 ꢂC, the reaction mixture
was quenched with a saturated ammonium chloride solution and
extracted with diethyl ether (3 ꢃ 30 ml). The combined organic phases
were washed with water and brine, dried over anhydrous sodium
sulfate, filtered and concentrated in vacuo. The resulting residue was
purified by flash column chromatography, eluting with 1% v/v ethyl
acetate-hexane, to yield desired product 22 (1.141 g, 96%) as a white
solid, mp 77–78 ꢂC. IR ꢁmax (KBr) cmꢁ1: 1729 (C=O), 1569, 1452,
1195 (C=C), 1081 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.52–1.67
(6H, m, 3 ꢃ CH2), 1.94–2.07 (2H, m, CH2), 3.10 (2H, s, ArCH2), 3.59
(3H, s, CO2CH3), 3.80 (3H, s, OCH3), 7.10 (1H, d, J ¼ 8:38 Hz, ArH),
7.32–7.46 (2H, m, ArH), 7.43 (1H, d, J ¼ 8:38 Hz, ArH), 7.70 (1H, d,
J ¼ 8:09 Hz, ArH), 7.98 (1H, d, J ¼ 8:09 Hz, ArH). NMR ꢀC: 24.4
(2 ꢃ CH2), 35.6 (2 ꢃ CH2), 37.6 (CH2), 51.7 (C), 55.5 (CH3), 61.6
(CH3), 122.2 (CH), 123.6 (CH), 125.6 (C, CH), 125.8 (CH), 127.0 (C),
127.9 (CH), 128.6 (CH), 134.1 (C), 154.3 (C), 178 (C=O). EIMS m=z:
299 (½M þ Hꢄþ, 21), 298 (Mþ, 19), 171 (21), 170 (25), 169 (100), 159
(22). Anal. Found: C, 76.37; H, 7.41%. Calcd. for C19H22O3: C, 76.48;
H, 7.43%.
mixture of compound 22 (0.180 g, 0.603 mmol) and aluminium
chloride (0.202 g, 1.51 mmol) in dry chlorobenzene (5 ml) was refluxed
for 3.5 h. The solution was cooled to room temperature, poured into
10% hydrochloric acid (35 ml), and then extracted with diethyl ether
(3 ꢃ 30 ml). The combined organic phase was washed with water,
dried over anhydrous sodium sulfate, filtered and concentrated in
vacuo. The chlorobenzene was removed by distilling under reduced
pressure. The resulting residue was purified by flash column
chromatography, eluting with 1% v/v ethyl acetate-hexane to afford
desired product 24 (0.131 g, 86%) as a white solid, mp 109–110 ꢂC. IR
ꢁmax (KBr) cmꢁ1: 1752 (C=O), 1601, 1381, 1230 (C=C), 1084 (C–O).
NMR ꢀH (CDCl3, 400 MHz): 1.56–1.63 (2H, m, CH2), 1.71–1.90 (4H,
m, 2 ꢃ CH2), 2.20–2.27 (2H, m, CH2), 3.05 (2H, s, ArCH2), 7.25 (1H,
d, J ¼ 8:4 Hz, ArH), 7.49–7.57 (2H, m, ArH), 7.59 (1H, d, J ¼ 8:4 Hz,
ArH), 7.83 (1H, m, ArH), 8.26 (1H, m, ArH). NMR ꢀC: 25.4
(2 ꢃ CH2), 36.2 (2 ꢃ CH2), 37.1 (CH2), 47.5 (C), 117.2 (C), 121.0
(CH), 123.3 (C), 123.6 (CH), 125.8 (CH), 126.2 (CH), 126.3 (CH),
127.5 (CH), 127.5 (CH), 133.4 (C), 146.5 (C=O), 173.8 (C=O). EIMS
m=z: 253 (½M þ 1ꢄþ, 100), 235 (38), 224 (41), 169 (61), 157 (45).
Anal. Found: C, 80.68; H, 6.56%. Calcd. for C17H16O2: C, 80.93;
H, 6.39%.
3,4-Dihydro-2-spirocyclohexylbenzo[H]chromen-2-one (25). A mix-
ture of compound 23 (1.00 g, 3.2 mmol) and aluminium chloride
(0.128 g, 9.6 mmol) in dry chlorobenzene (35 ml) was refluxed for 3 h.
The solution was cooled to room temperature, poured into 10%
hydrochloric acid (35 ml), and then extracted with diethyl ether
(3 ꢃ 30 ml). The combined organic phase was washed with water,
dried over anhydrous sodium sulfate, filtered and concentrated in
vacuo. The chlorobenzene was removed by distilling under reduced
pressure. The resulting residue was purified by flash column
chromatography, eluting with 5% v/v CH2Cl2:hexane to afford desired
product 25 (0.69 g, 81%) as a colourless amorphous powder, mp 106–
107 ꢂC. IR ꢁmax (KBr) cmꢁ1: 2939, 2843 (CH2), 1753 (C=O), 1444,
Methyl-1-((1-methoxynaphthalene-2-yl)methyl)cyclohexanecarbox-
ylate (23). 1 M n-butyl lithium (2.4 ml, 2.4 mmol) was added dropwise
to a stirred solution of diisopropylamine (0.34 ml, 2.4 mmol) in dry
THF (5 ml) at 0 ꢂC under nitrogen. After stirring for 20 min, cyclo-
hexanecarboxylic acid methyl ester 21 (0.256 g, 1.8 mmol) was slowly

Synthesis and Anticancer Naphthoquinone Esters
1211
1379 (C=C), 1244, 1112, 1086 (C–O). NMR ꢀH (CDCl3, 400 MHz):
1.40–1.90 (10H, m, 5 ꢃ CH2), 2.98 (2H, s, CH2Ar), 7.17 (1H, d,
J ¼ 8:7 Hz, ArH), 7.39–7.48 (2H, m, ArH), 7.51 (1H, d, J ¼ 8:7 Hz,
ArH), 7.74 (1H, dd, J ¼ 7:2 and 1.6 Hz, ArH), 8.16 (1H, dd, J ¼ 8:2
and 1.4 Hz, ArH). NMR ꢀC: 21.46 (2 ꢃ CH2), 25.66 (CH2), 32.42
(2 ꢃ CH2), 34.99 (C), 40.15 (CH2), 115.87 (C), 121.09 (CH), 123.07
(C), 123.69 (CH), 125.92 (CH), 126.25 (CH), 126.33 (CH), 127.51
(CH), 133.42 (C), 145.99 (C), 173.06 (C=O). EIMS m=z: 267
(½M þ 1ꢄþ, 100), 238 (23), 169 (26), 157 (13). Anal. Found: C,
80.81; H, 6.86%. Calcd. for C18H18O2: C, 81.17; H, 6.81%.
31.71 (CH2), 33.41 (2 ꢃ CH2), 36.06 (C), 66.30 (CH2), 126.14 (CH),
126.77 (C), 127.15 (CH), 133.44 (C), 134.26 (CH), 134.32 (CH),
137.59 (CH), 150.29 (C), 185.03 (C=O), 187.87 (C=O). EIMS m=z:
271 (½M þ 1ꢄþ, 16), 269 (100), 251 (42), 189 (8), 155 (24), 81 (6).
Anal. Found: C, 75.18; H, 6.43%. Calcd. for C17H18O3: C, 75.53;
H, 6.71%.
2-((1-(Hydroxymethyl)cyclohexyl)methyl)naphthalene-1,4-dione (29).
A solution of Fremy’s salt (3.468 mg, 12.94 mmol) in water (129 ml)
was added to a stirred solution of naphthol 27 (0.50 g, 1.85 mmol) in
methanol:dimethylformamide (3:1; 32 ml), before adding
a 1 M
2-((1-(Hydroxymethyl)cyclopentyl)methyl)naphthalene-1-ol (26). A
solution of lactone 24 (1.385 g, 5.49 mmol) in dry THF (5 ml) was
added dropwise to a stirred and ice-cooled suspension of lithium
aluminium hydride (0.833 g, 21.95 mmol) in dry THF (20 ml). After
stirring for 3 h at room temperature, the reaction mixture was quenched
with ethyl acetate and water, and then extracted with diethyl ether
(3 ꢃ 40 ml). The combined organic phase was successively washed
with water and brine, dried over anhydrous sodium sulfate, filtered and
concentrated in vacuo. The residue was purified by flash column
chromatography, eluting with 2% v/v ethyl acetate:hexane to give
product 26 (1.266 g, 90%) as a white solid, mp 131–132 ꢂC. IR ꢁmax
(KBr) cmꢁ1: 3421 (OH), 1570, 1466, 1383, 1317 (C=C), 1076, 1033
(C–O). NMR ꢀH (CDCl3, 400 MHz): 1.27–1.82 (8H, m, 4 ꢃ CH2), 2.80
(2H, s, ArCH2), 3.29 (2H, s, CH2OH), 7.11 (1H, d, J ¼ 8:4 Hz, ArH),
7.25 (1H, d, J ¼ 8:4 Hz, ArH), 7.36 (2H, m, ArH), 7.68 (1H, m, ArH),
8.22 (1H, m, ArH). NMR ꢀC: 25.0 (2 ꢃ CH2), 35.5 (2 ꢃ CH2), 37.0
(CH2), 49.5 (C), 68.3 (CH2), 118.8 (C), 119.7 (CH), 123.2 (CH), 125.5
(CH), 126.1 (C), 126.3 (CH), 127.8 (CH), 131.0 (CH), 134.3 (CH),
151.7 (C). EIMS m=z: 256 (Mþ, 7), 171 (20), 157 (100), 141 (17), 128
(75), 115 (17). Anal. Found: C, 79.32; H, 7.55%. Calcd. for C17H20O2:
C, 79.65; H, 7.86%.
aqueous sodium acetate solution (3.7 ml). After stirring for 5 h at
room temperature, the reaction mixture was extracted with diethyl
ether (3 ꢃ 20 ml). The combined organic phase was successively
washed with water and brine, dried over anhydrous sodium sulfate,
filtered and concentrated in vacuo. The residue was purified by flash
column chromatography, using 10% v/v ethyl acetate:hexane to
provide quinone 29 (0.457 g, 87%) as a pale yellow powder, mp
136–137 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3225 (OH), 2922, 2856 (CH2), 1659,
1627 (C=O), 1590, 1450, 1332, 1310 (C=C), 1021 (C–O). NMR ꢀH
(acetone-d6, 400 MHz): 1.20–1.60 (10H, m, 5 ꢃ CH2), 2.65 (2H, d,
J ¼ 0:8 Hz, CH2C=CH), 3.37 (2H, d, J ¼ 5:3 Hz, CH2OH), 6.89 (1H,
t, J ¼ 0:8 Hz, CH2C=CHCO), 7.82 (1H, d, J ¼ 3:3 Hz, ArH), 7.83
(1H, d, J ¼ 3:3 Hz, ArH), 8.00 (1H, m, ArH), 8.07 (1H, m, ArH). NMR
ꢀC: 22.20 (2 ꢃ CH2), 31.74 (CH2), 33.42 (2 ꢃ CH2), 39.06 (C), 70.14
(CH2), 123.57 (CH), 126.62 (C), 129.1 (CH), 132.34 (C), 133.75 (CH),
136.11 (CH), 137.89 (CH), 162.31 (C), 180.09 (C=O), 181.22 (C=O).
EIMS m=z: 285 (½M þ 1ꢄþ, 100), 253 (53), 239 (7), 173 (24), 157 (19),
129 (3). Anal. Found: C, 76.18; H, 7.33%. Calcd. for C18H20O3: C,
76.03; H, 7.09%.
3,4-Dihydro-3,3-spirocyclopentyl-2H-naphtho[1,2-b]pyran-5,6-dione
(30). A mixture of quinone 28 (0.775 g, 2.87 mmol), dichlorodicyano-
benzoquinone (DDQ; 0.951 g, 4.19 mmol) and p-toluenesulfonic acid
monohydrate (0.143 g, 0.75 mmol) in dry benzene (10 ml) was stirred
under reflux for 1 h. The reaction mixture was cooled to room
temperature and filtered, washed with dichloromethane and then
concentrated in vacuo. The residue was filtered through aluminium
oxide to remove DDQ and purified by flash column chromatography,
using 2% v/v ethyl acetate:hexane to give desired product 30 (0.761 g,
99%) as an orange solid, mp 137–138 ꢂC. IR ꢁmax (KBr) cmꢁ1: 1694,
1641 (C=O), 1604, 1576, 1390 (C=C), 1174 (C–O). NMR ꢀH (CDCl3,
400 MHz): 1.40–1.78 (8H, m, 4 ꢃ CH2), 2.40 (2H, s, ArCH2), 4.01
(2H, s, OCH2), 7.44 (1H, m, ArH), 7.60 (1H, m, ArH), 7.76 (1H, m,
ArH), 8.01 (1H, m, ArH). NMR ꢀC: 25.2 (2 ꢃ CH2), 31.5 (CH2), 36.0
(2 ꢃ CH2), 39.6 (C), 76.1 (CH2), 114.2 (C), 124.4 (CH), 128.9 (CH),
130.2 (C), 131.0 (CH), 132.3 (C), 135.1 (CH), 163.0 (C), 179.2 (C=O),
180.0 (C=O). EIMS m=z: 269 (½M þ 1ꢄþ, 100), 251 (23), 233 (4), 189
(6), 159 (8), 81 (7). Anal. Found: C, 76.34; H, 6.36%. Calcd. for
2-((1-(Hydroxymethyl)cyclohexyl)methyl)naphthalene-1-ol (27). A
solution of lactone 25 (0.69 g, 2.59 mmol) in dry diethyl ether (5 ml)
was added dropwise to a stirred and ice-cooled suspension of lithium
aluminium hydride (0.197 g, 5.18 mmol) in dry diethyl ether (10 ml).
After stirring for 3 h at room temperature, the reaction mixture was
quenched with ethyl acetate and water, and then extracted with diethyl
ether (3 ꢃ 20 ml). The combined organic phase was successively
washed with water and brine, dried over anhydrous sodium sulfate,
filtered and concentrated in vacuo. The residue was purified by flash
column chromatography, eluting with 5% v/v ethyl acetate:hexane to
give product 27 (0.623 g, 89%) as a colourless amorphous powder, mp
126–127 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3413 (OH), 3047 (CH-aromatic),
2922, 2856 (CH2, CH3), 1570, 1457, 1376, 1273 (C=C), 1072, 1015
(C–O). NMR ꢀH (CDCl3, 400 MHz): 1.30–1.65 (10H, m, 5 ꢃ CH2),
2.78 (2H, s, CH2Ar), 3.31 (2H, s, CH2OH), 7.11 (1H, d, J ¼ 8:4 Hz,
ArH), 7.26 (1H, d, J ¼ 8:4 Hz, ArH), 7.38 (2H, m, ArH), 7.69 (1H, m,
ArH), 8.25 (1H, m, ArH). NMR ꢀC: 21.64 (2 ꢃ CH2), 26.47 (CH2),
33.16 (2 ꢃ CH2), 35.50 (CH2), 38.81 (C), 67.43 (CH2), 117.22 (C),
118.84 (CH), 122.52 (CH), 124.85 (CH), 125.54 (C), 125.59 (CH),
127.12 (CH), 130.45 (CH), 133.62 (C), 151.20 (C). EIMS m=z: 270
(Mþ, 11), 253 (26), 239 (5), 169 (3), 157 (100), 129 (22). Anal. Found:
C, 80.00; H, 8.20%. Calcd. for C18H22O2: C, 79.96; H, 8.20%.
C17H16O3: C, 76.10; H, 6.01%.
3,4-Dihydro-3,3-spirocyclohexyl-2H-naphtho[1,2-b]pyran-5,6-dione
(31). A mixture of quinone 29 (1.641 g, 5.77 mmol), dichlorodicyano-
benzo-quinone (DDQ; 1.571 g, 6.92 mmol) and p-toluenesulfonic acid
monohydrate (0.11 g, 0.58 mmol) in dry benzene (40 ml) was stirred
under reflux for 1 h. The reaction mixture was cooled to room
temperature and filtered, washed with dichloromethane and then
concentrated in vacuo. The residue was filtered through aluminium
oxide to remove DDQ and purified by flash column chromatography,
using 4% v/v ethyl acetate:hexane to give desired product 31 (1.369 g,
84%) as an orange amorphous powder, mp 131–132 ꢂC. IR ꢁmax (KBr)
cmꢁ1: 2923, 2848 (CH2), 1697, 1642 (C=O), 1598, 1568, 1452, 1373
(C=C), 1174, 1085 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.30–1.60
(10H, m, 5 ꢃ CH2), 2.41 (2H, s, CH2Ar), 4.07 (2H, s, CH2O), 7.51
(1H, td, J ¼ 7:8 and 1.2 Hz, ArH), 7.64 (1H, td, J ¼ 7:8 and 1.4 Hz,
ArH), 7.79 (1H, dd, J ¼ 7:8 and 0.8 Hz, ArH), 8.08 (1H, dd, J ¼ 7:8
and 1.0 Hz, ArH). NMR ꢀC: 21.28 (2 ꢃ CH2), 26.17 (CH2), 28.98 (C),
30.72 (CH2), 33.22 (2 ꢃ CH2), 76.15 (CH2), 113.03(C), 124.08 (CH),
128.68 (CH), 129.88 (C), 130.68 (CH), 131.92 (C), 134.82 (CH),
162.32 (C), 179.09 (C=O), 179.68 (C=O). EIMS m=z: 283 (½M þ 1ꢄþ,
100), 265 (5). Anal. Found: C, 76.24; H, 6.62%. Calcd. for C18H18O3:
C, 76.57; H, 6.43%.
2-((1-(Hydroxymethyl)cyclopentyl)methyl)naphthalene-1,4-dione (28).
A solution of Fremy’s salt (9.192 g, 34.30 mmol) in water (300 ml) was
added to a stirred solution of naphthol 26 (1.256 g, 4.90 mmol) in
methanol:dimethylformamide (3:1; 74 ml), before adding
a 1 M
aqueous sodium acetate solution (7.58 ml). After stirring for 7 h at
room temperature, the reaction mixture was extracted with diethyl
ether (3 ꢃ 50 ml). The combined organic phase was successively
washed with water and brine, dried over anhydrous sodium sulfate,
filtered and concentrated in vacuo. The residue was purified by flash
column chromatography, using 10% v/v ethyl acetate:hexane to
provide quinone 28 (1.205 g, 91%) as a pale yellow powder, mp
143–144 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3405 (OH), 2942, 2871 (CH2), 1640,
1621 (C=O), 1588, 1462, 1327, 1314 (C=C), 1017 (C–O). NMR ꢀH
(acetone-d6, 400 MHz): 1.44–1.85 (8H, m, 4 ꢃ CH2), 2.65 (2H, d,
J ¼ 0:8 Hz, CH2C=CH), 3.16 (2H, d, J ¼ 5:3 Hz, CH2OH), 6.88 (1H,
t, J ¼ 0:8 Hz, CH2C=CHCO), 7.74–7.79 (2H, m, ArH), 8.07 (1H, m,
ArH), 8.13 (1H, m, ArH). NMR ꢀC: 22.16 (2 ꢃ CH2), 26.82 (CH2),

1212
N. KONGKATHIP et al.
(3), 155 (4), 81 (3). Anal. Found: C, 76.28; H, 5.66%. Calcd. for
Synthesis of the 20-cyclopentyl substituent naphthoquinone ester
derivatives General procedure. A solution of 30 (0.10 g, 0.37 mmol) in
a 1% aqueous sodium hydroxide solution (0.022 g, 0.56 mmol) was
refluxed for 1 h. The reaction mixture was cooled to room temperature,
acidified with 1% hydrochloric acid and extracted with ethyl acetate
(3 ꢃ 15 ml). The combined organic phases were successively washed
with water and brine, dried over anhydrous sodium sulfate and filtered,
and the filtrate was concentrated in vacuo. The crude product was used
in the further steps without purification.
C
28H24O5: C, 76.35; H, 5.49%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
pentyl)methyl-1-methoxynaphthalene-2-carboxylate (12). The crude
residue was purified by flash column chromatography, eluting with
5% v/v ethyl acetate:hexane to provide desired product 12 (0.047 g,
28% yield in 2 steps) as a yellow gum. Product 12 was then
recrystallized with hexane:dichloromethane to give 12 as a yellow
amorphous solid, mp 113–114 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3368 (OH),
1701, 1646 (C=O), 1594, 1457, 1371, 1340, 1278 (C=C), 1242, 1157,
1085 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.63–1.85 (8H, m,
4 ꢃ CH2), 2.88 (2H, s, ArCH2), 4.00 (3H, s, OCH3), 4.21 (2H, s,
OCH2), 7.33 (1H, m, ArH), 7.41 (2H, m, ArH), 7.44 (1H, s, OH), 7.52
(2H, m, ArH), 7.74 (1H, m, ArH), 7.75 (1H, m, ArH), 7.81 (1H, dd,
J ¼ 7:66 Hz and J ¼ 0:96 Hz, ArH), 7.86 (1H, dd, J ¼ 7:66 Hz and
J ¼ 0:96 Hz, ArH), 8.11 (1H, m, ArH). NMR ꢀC: 24.6 (2 ꢃ CH2), 31.1
(CH2), 35.6 (2 ꢃ CH2), 48.6 (C), 63.7 (CH3), 71.5 (CH2), 119.6 (C),
122.8 (C), 123.6 (CH), 123.8 (CH), 125.9 (CH), 126.5 (CH), 126.78
(CH), 126.83 (CH), 127.9 (CH), 128.3 (CH), 128.6 (C), 129.4 (C),
132.5 (CH), 133.0 (C), 134.6 (CH), 136.8 (C), 154.2 (C), 158.2 (C),
166.4 (C=O), 181.5 (C=O), 185.5 (C=O). EIMS m=z: 469 (½M ꢁ 1ꢄþ,
43), 350 (7), 269 (100), 251 (22), 185 (5), 81 (4). Anal. Found: C,
74.27; H, 5.31%. Calcd. for C29H26O6: C, 74.03; H, 5.57%.
A solution of carbonyldiimidazole (CDI; 0.133 g, 0.82 mmol) in dry
THF (3 ml) was added to a stirred solution of carboxylic acid
(0.63 mmol) in THF (2 ml) at room temperature. A solution of crude
product 32 in THF (2 ml) was added after 3 h and then the mixture was
stirred at room temperature for 48 h. The reaction mixture was
quenched with saturated ammonium chloride and extracted with
dichloromethane (3 ꢃ 20 ml). The combined organic layers were
successively washed with brine and water, dried over anhydrous
sodium sulfate and filtered, and the filtrate was concentrated in vacuo.
The residue was purified by flash column chromatography.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalen-3-yl)methyl)cyclo-
pentyl)methyl benzoate (7). The crude residue was purified by flash
column chromatography, eluting with 5% v/v ethyl acetate:hexane to
provide desired product 7 (0.10 g, 69% yield in 2 steps) as a yellow
gum. Product 7 was then recrystallized with hexane:dichloromethane
to give product 7 as a yellow amorphous solid, mp 111–112 ꢂC. IR ꢁmax
(KBr) cmꢁ1: 3407 (OH), 1713, 1664, 1642 (C=O), 1590, 1364 (C=C),
1276, 1224 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.41–1.82 (8H, m,
4 ꢃ CH2), 2.85 (2H, s, ArCH2), 4.14 (2H, s, OCH2), 7.20 (2H, m,
ArH), 7.33–7.52 (1H, m, ArH), 7.60–7.72 (2H, m, ArH), 7.86 (2H, m,
ArH), 7.94 (2H, m, ArH), 8.32 (1H, s, OH). NMR ꢀC: 24.4 (2 ꢃ CH2),
30.7 (CH2), 35.0 (2 ꢃ CH2), 48.2 (C), 70.7 (CH2), 122.5 (C), 125.9
(CH), 127.0 (CH), 128.1 (2 ꢃ CH), 129.4 (2 ꢃ CH), 130.4 (C), 132.6
(CH), 132.8 (CH), 133.0 (C), 134.8 (CH), 135.3 (C), 154.2 (C), 166.6
(C=O), 181.2 (C=O), 185.1 (C=O). EIMS m=z: 390 (Mþ, 9), 269
(100), 251 (34), 189 (5), 155 (20), 81 (5). Anal. Found: C, 73.44; H,
5.59%. Calcd. for C24H22O5: C, 73.83; H, 5.68%.
Synthesis of the 20-cyclohexyl substituent naphthoquinone ester
derivatives General procedure. A solution of compound 31 (0.10 g,
0.354 mmol) in a 1% aqueous sodium hydroxide solution (0.021 g,
0.531 mmol) was refluxed for 1 h. The reaction mixture was cooled to
room temperature after completing the reaction, neutralized with acetic
acid to pH 7, and then extracted with dichloromethane (3 ꢃ 15 ml).
The combined organic phases were successively washed with water
and brine, dried over anhydrous sodium sulfate, filtered and then
concentrated in vacuo. Intermediate 33 was obtained after concen-
tration together with a small amount of cyclized product 31 (monitored
by TLC).
A solution of carbonyldiimidazole (CDI; 0.086 g, 0.53 mmol) in dry
THF (5 ml) was added to a stirred solution of carboxylic acid
(0.43 mmol) in THF (2 ml) at room temperature. A solution of the
mixture of compound 31 and naphthoquinone alcohol intermediate 33
in THF (2 ml) was added after 3 h and then the mixture was stirred at
room temperature for 45 h. The reaction mixture was quenched with
saturated ammonium chloride and extracted with dichloromethane
(3 ꢃ 15 ml). The combined organic layers were successively washed
with brine and water, dried over anhydrous sodium sulfate and filtered,
and the filtrate was concentrated in vacuo. The residue was purified by
flash column chromatography.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
pentyl)methyl-2-methoxybenzoate (9). The crude residue was purified
by flash column chromatography, eluting with 5% v/v ethyl
acetate:hexane to provide desired product 9 (0.063 g, 40% yield in 2
steps) as a yellow gum. Product 9 was then recrystallized with
hexane:dichloromethane to give 9 as a yellow amorphous solid, mp
105–106 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3369 (OH), 1694, 1649 (C=O),
1595, 1460, 1350, 1305 (C=C), 1265, 1213, 1132 (C–O). NMR ꢀH
(CDCl3, 400 MHz): 1.52–1.74 (8H, m, 4 ꢃ CH2), 2.80 (2H, s, ArCH2),
3.73 (3H, s, OCH3), 4.05 (2H, s, OCH2), 6.75 (2H, m, ArH), 7.28 (1H,
m, ArH), 7.50–7.61 (2H, m, ArH), 7.67 (1H, m, ArH), 7.92 (2H, m,
ArH). NMR ꢀC: 24.3 (2 ꢃ CH2), 30.7 (CH2), 35.0 (2 ꢃ CH2), 48.2 (C),
55.7 (CH3), 70.5 (CH2), 111.8 (CH), 119.9 (CH), 120.1 (C), 122.7 (C),
125.8 (CH), 126.9 (CH), 129.3 (C), 131.7 (CH), 132.7 (CH), 132.9 (C),
133.3 (CH), 134.7 (CH), 154.0 (C), 159.0 (C), 166.3 (C=O), 181.2
(C=O), 185.2 (C=O). EIMS m=z: 389 (3), 350 (6), 269 (100), 251
(21), 189 (7), 81 (7). Anal. Found: C, 71.62; H, 5.89%. Calcd. for
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl benzoate (13). The crude residue was purified by flash
column chromatography, eluting with 3% v/v ethyl acetate:hexane to
provide desired product 13 (0.033 g, 22% yield in 2 steps) as a yellow
gum and cyclized product 31 (0.017 g). The desired product was then
recrystallized with hexane:dichloromethane to give product 13 as a
yellow amorphous needle, mp 117–118 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3371
(OH), 3067 (CH-aromatic), 2928, 2852 (CH2), 1715, 1664, 1650
(C=O), 1594, 1451, 1371, 1274 (C=C), 1116, 1025 (C–O). NMR ꢀH
(CDCl3, 400 MHz): 1.40–1.65 (10H, m, 5 ꢃ CH2), 2.83 (2H, s,
CH2Ar), 4.24 (2H, s, OCH2), 7.42 (1H, s, OH), 7.44 (1H, m, ArH),
7.64 (2H, m, ArH), 7.70 (2H, m, ArH), 7.90 (2H, d, J ¼ 7:6 Hz, ArH),
8.00 (2H, m, ArH). NMR ꢀC: 21.72 (2 ꢃ CH2), 26.01 (CH2), 31.76
(CH2), 33.50 (2 ꢃ CH2), 39.42 (C), 69.40 (CH2), 122.15 (C), 125.93
(CH), 126.97 (CH), 128.12 (2 ꢃ CH), 129.27 (C), 129.34 (2 ꢃ CH),
130.37 (C), 132.59 (CH), 132.75 (CH), 133.04 (C), 134.82 (CH),
154.17 (C), 166.52 (C=O), 181.09 (C=O), 185.05 (C=O). EIMS m=z:
404 (Mþ, 100), 296 (22), 284 (76), 257 (42), 175 (9). Anal. Found: C,
74.44; H, 6.01%. Calcd. for C25H24O5: C, 74.24; H, 5.98%.
C
25H24O6: C, 71.41; H, 5.75%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
pentyl)methyl-2-naphthoate (10). The crude residue was purified
by flash column chromatography, eluting with 5% v/v ethyl
acetate:hexane to provide desired product 10 (0.104 g, 66% yield in
2 steps) as a yellow gum. Product 10 was then recrystallized with
hexane:dichloromethane to give 10 as a yellow amorphous solid, mp
165–166 ꢂC. IR ꢁmax (KBr) cmꢁ1: 1715, 1656 (C=O), 1595, 1456,
1371 (C=C), 1276, 1115 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.55–
1.74 (8H, m, 4 ꢃ CH2), 2.82 (2H, s, ArCH2), 4.08 (2H, s, OCH2), 7.20
(2H, m, ArH), 7.37 (2H, m, ArH), 7.53–7.61 (3H, m, ArH), 7.85 (2H,
m, ArH), 7.94 (2H, m, ArH). NMR ꢀC: 24.6 (2 ꢃ CH2), 30.9 (CH2),
35.3 (2 ꢃ CH2), 48.5 (C), 70.9 (CH2), 122.7 (C), 126.2 (CH), 127.2
(CH), 128.4 (2 ꢃ CH), 129.5 (C), 129.6 (2 ꢃ CH), 130.6 (C), 132.9
(CH), 133.0 (CH), 133.2 (C), 135.1 (CH), 154.4 (C), 166.9 (C=O),
181.4 (C=O), 185.4 (C=O). EIMS m=z: 440 (Mþ, 88), 269 (100), 251
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl-2-methoxybenzoate (15). The crude residue was purified
by flash column chromatography, eluting with 1% v/v ethyl
acetate:hexane to provide desired product 15 (0.112 g, 69% yield in

Synthesis and Anticancer Naphthoquinone Esters
1213
2 steps) as a yellow gum and cyclized product 31 (0.037 g). The
desired product was then recrystallized with hexane:dichloromethane
to give product 15 as a yellow amorphous solid, mp 115–116 ꢂC. IR
ꢁmax (KBr) cmꢁ1: 3365 (OH), 3062 (CH-aromatic), 2929, 2863 (CH2),
1716, 1660, 1648 (C=O), 1598, 1460, 1372, 1252 (C=C), 1084, 1024
(C–O). NMR ꢀH (CDCl3, 400 MHz): 1.25–1.60 (10H, m, 5 ꢃ CH2),
2.73 (2H, s, CH2Ar), 3.73 (3H, s, OCH3), 4.13 (2H, s, OCH2), 6.74
(2H, m, ArH), 7.26 (1H, m, ArH), 7.35 (1H, s, OH), 7.55 (2H, m, ArH),
7.65 (1H, dd, J ¼ 1:8 and 8.0 Hz, ArH), 7.91 (2H, m, ArH). NMR ꢀC:
21.74 (2 ꢃ CH2), 26.04 (CH2), 31.72 (CH2), 33.42 (2 ꢃ CH2), 39.41
(C), 55.70 (CH3), 69.34 (CH2), 111.77 (CH), 119.90 (CH), 120.09 (C),
122.34 (C), 125.81 (CH), 126.88 (CH), 129.26 (C), 131.71 (CH),
132.64 (CH), 133.01 (C), 133.26 (CH), 134.64 (CH), 154.07 (C),
159.00 (C), 166.22 (C=O), 181.08 (C=O), 185.08 (C=O). EIMS m=z:
433 (½M ꢁ 1ꢄþ, 7), 297 (12), 265 (8). Anal. Found: C, 71.75; H, 5.84%.
Calcd. for C26H26O6: C, 71.87; H, 6.03%.
purified by flash column chromatography, eluting with 7% v/v ethyl
acetate:hexane to afford product 8 (0.033 g, 86%) as a yellow
amorphous solid, mp 91–92 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3354 (OH),
1666 (C=O), 1593, 1486, 1459, 1365, 1300 (C=C), 1214, 1159, 1090
(C–O). NMR ꢀH (CDCl3, 400 MHz): 1.55–1.84 (8H, m, 4 ꢃ CH2), 2.82
(2H, s, ArCH2), 4.11 (2H, s, OCH2), 6.55 (1H, m, ArH), 6.84 (1H,
m, ArH), 7.27 (1H, m, ArH), 7.38 (1H, s, OH-quinone), 7.59
(3H, m, ArH), 7.95 (1H, m, ArH), 7.97 (1H, m, ArH), 10.70 (1H, s,
OH-phenol). NMR ꢀC: 24.6 (2 ꢃ CH2), 30.9 (CH2), 35.3 (2 ꢃ CH2),
48.4 (C), 71.2 (CH2), 112.8 (C), 117.7 (CH), 119.1 (CH), 122.4 (C),
126.2 (CH), 127.2 (CH), 129.5 (C), 129.8 (CH), 133.1 (CH), 133.2 (C),
135.2 (CH), 135.6 (CH), 154.4 (C), 161.7 (C), 170.3 (C=O), 181.4
(C=O), 185.4 (C=O). EIMS m=z: 405 (½M ꢁ 1ꢄþ, 5), 352 (7), 269
(100), 251 (16), 233 (3), 189 (4), 81 (5). Anal. Found: C, 70.77; H,
5.28%. Calcd. for C24H22O6: C, 70.92; H, 5.46%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl-2-hydroxybenzoate (14). 1 M boron tribromide in dichloro-
methane (0.11 ml, 0.115 mmol) was added dropwise to a stirred solution
of methyl ether 15 (0.05 g, 0.115 mmol) in dry dichloromethane (2.5 ml)
at ꢁ78 ꢂC, and the solution was stirred for 30 min at the same
temperature. Water was then added to the reaction mixture and it was
extracted with dichloromethane (3 ꢃ 10 ml). The combined organic
phases were washed with water, dried over anhydrous sodium sulfate,
filtered and concentrated in vacuo to afford a crude residue. This crude
residue was purified by flash column chromatography, eluting with
4% v/v ethyl acetate:hexane to afford product 14 (0.042 g, 87%) as a
yellow amorphous solid, mp 99–100 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3329
(OH), 2920, 2851 (CH2), 1675, 1659, 1615 (C=O), 1591, 1487, 1338,
1212 (C=C), 1156, 1090 (C–O). NMR ꢀH (CDCl3, 400 MHz): 1.30–
1.70 (10H, m, 5 ꢃ CH2), 2.74 (2H, s, CH2Ar), 4.12 (2H, s, OCH2), 6.51
(1H, m, ArH), 6.82 (1H, m, ArH), 7.24 (1H, m, ArH), 7.37 (1H, br s,
OH-quinone), 7.55 (3H, m, ArH), 7.93 (2H, m, ArH), 10.68 (1H, s,
OH-phenol). NMR ꢀC: 21.65 (2 ꢃ CH2), 25.96 (CH2), 31.67 (CH2),
33.45 (2 ꢃ CH2), 39.30 (C), 69.76 (CH2), 112.53 (C), 117.43 (CH),
118.84 (CH), 121.84 (C), 125.98 (CH), 126.93 (CH), 129.23 (C), 129.55
(CH), 132.83 (CH), 132.98 (C), 134.92 (CH), 135.34 (C), 154.21 (C),
161.39 (C), 170.01 (C=O), 181.05 (C=O), 185.06 (C=O). EIMS m=z:
419 (½M ꢁ 1ꢄþ, 2), 297 (3), 283 (100), 265 (11). Anal. Found: C, 71.26;
H, 5.70%. Calcd. for C25H24O6: C, 71.41; H, 5.75%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl-2-naphthoate (16). The crude residue was purified
by flash column chromatography, eluting with 1% v/v ethyl
acetate:hexane to provide desired product 16 (0.069 g, 43% yield in
2 steps) as a yellow gum and cyclized product 31 (0.019 g). The
desired product was then recrystallized with hexane:dichloromethane
to give 16 as a yellow amorphous needle, mp 179–180 ꢂC. IR ꢁmax
(KBr) cmꢁ1: 3383 (OH), 3055 (CH-aromatic), 2931, 2848 (CH2),
1713, 1651 (C=O), 1594, 1458, 1374, 1275 (C=C), 1196, 1024 (C–O).
NMR ꢀH (CDCl3, 400 MHz): 1.40–1.70 (10H, m, 5 ꢃ CH2), 2.86 (2H,
s, CH2Ar), 4.30 (2H, s, OCH2), 7.41 (1H, dt, J ¼ 1:2 and 7.6 Hz, ArH),
7.42 (1H, s, OH), 7.48 (1H, m, ArH), 7.56 (1H, m, ArH), 7.69 (1H, d,
J ¼ 8:8 Hz, ArH), 7.73 (1H, d, J ¼ 8:0 Hz, ArH), 7.80 (1H, d,
J ¼ 8:4 Hz, ArH), 7.85 (2H, dd, J ¼ 1:2 and 7.6 Hz, ArH), 7.91 (2H,
m, ArH), 8.37 (1H, s, ArH). NMR ꢀC: 22.06 (2 ꢃ CH2), 26.38 (CH2),
32.34 (CH2), 33.98 (2 ꢃ CH2), 39.77 (C), 70.12 (CH2), 122.61 (C),
125.35 (CH), 126.14 (CH), 126.74 (CH), 127.09 (CH), 127.85 (C),
127.93 (CH), 128.24 (CH), 128.37 (CH), 129.41 (C), 129.58 (CH),
131.01 (CH), 132.55 (C), 132.79 (CH), 133.22 (C), 134.94 (CH),
135.60 (C), 154.41 (C), 166.99 (C=O), 181.41 (C=O), 185.43 (C=O).
EIMS m=z: 454 (Mþ, 12), 297 (10), 283 (100), 265 (38). Anal. Found:
C, 76.30; H, 5.90%. Calcd. for C29H26O5: C, 76.63; H, 5.77%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl-1-methoxynaphthalene-2-carboxylate (18). The crude
residue was purified by flash column chromatography, eluting with
1% v/v ethyl acetate:hexane to provide desired product 18 (0.057 g,
33% yield in 2 steps) as a yellow gum and cyclized product
31 (0.021 g). The desired product was then recrystallized with
hexane:dichloromethane to give 18 as a yellow amorphous solid, mp
124–125 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3355 (OH), 2970, 2922, 2857 (CH2),
1709, 1647 (C=O), 1589, 1451, 1371, 1271 (C=C), 1125, 1000 (C–O).
NMR ꢀH (CDCl3, 400 MHz): 1.25–1.67 (10H, m, 5 ꢃ CH2), 2.73 (2H,
s, CH2Ar), 3.89 (3H, s, OCH3), 4.22 (2H, s, OCH2), 7.18 (1H, td,
J ¼ 7:6 and 1.2 Hz, ArH), 7.29 (1H, d, J ¼ 8:7 Hz, ArH), 7.30 (1H, td,
J ¼ 7:6 and 1.2 Hz, ArH), 7.37 (1H, s, OH), 7.45 (2H, m, ArH), 7.63
(1H, d, J ¼ 8:7 Hz, ArH), 7.67 (2H, d, J ¼ 7:7 Hz, ArH), 7.75 (1H, dd,
J ¼ 8:7 and 1.2 Hz, ArH), 8.02 (1H, d, J ¼ 8:2 Hz, ArH). NMR ꢀC:
21.75 (2 ꢃ CH2), 26.08 (CH2), 32.01 (CH2), 33.69 (2 ꢃ CH2), 39.39
(C), 63.39 (CH3), 70.02 (CH2), 119.36 (C), 122.25 (C), 123.31 (CH),
123.57 (CH), 125.54 (CH), 126.23 (CH), 126.48 (CH), 126.55 (CH),
127.64 (CH), 128.07 (CH), 128.31 (C), 129.04 (C), 132.07 (CH),
132.81 (CH), 134.24 (CH), 136.57 (C), 153.89 (C), 157.92 (C), 166.08
(C=O), 181.13 (C=O), 185.17 (C=O). EIMS m=z: 484 (Mþ, 32), 433
(½M ꢁ 1ꢄþ, 100), 297 (12), 283 (71), 265 (7). Anal. Found: C, 74.55; H,
5.59%. Calcd. for C30H28O6: C, 74.36; H, 5.82%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
pentyl)methyl-1-hydroxynaphthalene-2-carboxylate (11). 1 M boron
tribromide in dichloromethane (0.17 ml, 0.174 mmol) was added
dropwise to a stirred solution of methyl ether 12 (0.041 g, 0.087 mmol)
in dry dichloromethane (5 ml) at ꢁ78 ꢂC, and the solution was stirred
for 30 min at the same temperature. Cold water was then added to the
reaction mixture, and mixture was extracted with dichloromethane
(3 ꢃ 30 ml). The combined organic phases were washed with water,
dried over anhydrous sodium sulfate, filtered, and the filtrate was
concentrated in vacuo. The residue was purified by flash column
chromatography, eluting with 7% v/v ethyl acetate-hexane to afford
product 11 (0.028 g, 70%) as a yellow needle, mp 105–106 ꢂC. IR ꢁmax
(KBr) cmꢁ1: 3424, 3358 (oh), 1659 (c=o), 1594, 1459, 1395, 1369,
1338 (c=c), 1253, 1214, 1157 (C–O). NMR ꢀH (CDCl3, 400 MHz):
1.56–1.83 (8H, m, 4 ꢃ CH2), 2.84 (2H, s, ArCH2), 4.17 (2H, s, OCH2),
6.92 (1H, d, J ¼ 8:38 Hz, ArH), 7.37 (1H, s, OH-quinone), 7.41–7.53
(5H, m, ArH), 7.60 (1H, d, J ¼ 8:38 Hz, ArH), 7.89 (2H, m, ArH), 8.29
(1H, m, ArH), 11.87 (1H, s, OH-naphthol). NMR ꢀC: 24.6 (2 ꢃ CH2),
31.1 (CH2), 35.4 (2 ꢃ CH2), 48.5 (C), 71.3 (CH2), 106.0 (C), 118.5
(CH), 122.5 (C), 124.0 (CH), 124.2 (CH), 124.9 (C), 125.8 (CH), 126.1
(CH), 127.1 (CH), 127.6 (CH), 129.42 (C), 129.43 (CH), 132.9 (CH),
133.1 (C), 134.9 (CH), 137.2 (C), 154.3 (C), 160.8 (C), 171.1 (C=O),
181.4 (C=O), 185.4 (C=O). EIMS m=z: 454 (½M ꢁ 2ꢄþ, 8), 350 (3),
269 (100), 251 (18), 232 (3), 189 (7), 81 (7). Anal. Found: C, 73.93; H,
5.14%. Calcd. for C28H24O6: C, 73.67; H, 5.30%.
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
pentyl)methyl-2-hydroxybenzoate (8). 1 M boron tribromide in dichloro-
methane (0.11 ml, 0.11 mmol) was added dropwise to a stirred solution
of methyl ether 9 (0.04 g, 0.1 mmol) in dry dichloromethane (5 ml) at
ꢁ78 ꢂC, and the solution was stirred for 30 min at the same temper-
ature. Cold water was then added to the reaction mixture which was
extracted with dichloromethane (3 ꢃ 30 ml). The combined organic
phases were washed with water, dried over anhydrous sodium sulfate
and filtered, and the filtrate was concentrated in vacuo. The residue was
(1-((1,4-Dihydro-2-hydroxy-1,4-dioxonaphthalene-3-yl)methyl)cyclo-
hexyl)methyl-1-hydroxynaphthalene-2-carboxylate (17). 1 M boron
tribromide in dichloromethane (0.11 ml, 0.115 mmol) was added
dropwise to a stirred solution of methyl ether 18 (0.056 g, 0.115 mmol)
in dry dichloromethane (2.5 ml) at ꢁ78 ꢂC, and the solution was stirred
for 30 min at the same temperature. Water was then added to the

1214
N. KONGKATHIP et al.
reaction mixture which was then extracted with dichloromethane
(3 ꢃ 10 ml). The combined organic phases were washed with water,
dried over anhydrous sodium sulfate, filtered and concentrated in vacuo
to afford a crude residue. This crude residue was purified by flash
column chromatography, eluting with 3% v/v ethyl acetate:hexane to
afford product 17 (0.037 g, 68%) as a yellow amorphous needle, mp
103–104 ꢂC. IR ꢁmax (KBr) cmꢁ1: 3277 (OH), 2923, 2850 (CH2), 1668,
1638 (C=O), 1595, 1459, 1394, 1351, 1270 (C=C), 1164, 1089 (C–O).
NMR ꢀH (CDCl3, 400 MHz): 1.40–1.70 (10H, m, 5 ꢃ CH2), 2.68 (2H, s,
CH2Ar), 4.22 (2H, s, CH2O), 6.87 (1H, m, ArH), 7.36 (1H, s,
OH-quinone), 7.34–7.44 (3H, m, ArH), 7.43 (1H, d, J ¼ 8:8 Hz, ArH),
7.49 (1H, m, ArH), 7.58 (1H, m, ArH), 7.85 (2H, m, ArH), 8.27 (1H, m,
ArH), 11.84 (1H, s, OH-naphthol). NMR ꢀC: 21.71 (2 ꢃ CH2), 26.01
(CH2), 31.76 (CH2), 33.59 (2 ꢃ CH2), 39.38 (C), 69.91 (CH2), 105.79
(C), 118.30 (CH), 121.95 (C), 123.77 (CH), 123.95 (CH), 124.64 (C),
125.61 (CH), 125.82 (CH), 126.79 (CH), 127.32 (CH), 129.17(C),
129.20 (CH), 132.60 (CH), 132.91(C), 134.63 (CH), 136.97 (C), 154.13
(C), 160.55 (C), 170.82 (C=O), 181.08 (C=O), 185.07 (C=O). EIMS
m=z: 470 (Mþ, 32), 415 (100). Anal. Found: C, 74.08; H, 5.45%. Calcd.
for C29H26O6: C, 74.03; H, 5.57%.
Acknowledgments
This work was supported by the Thailand Research
Fund (TRF) of Thailand. Both N.P. and K.H. are Ph.D.
students under the Royal Golden Jubilee PhD program
of TRF. Financial support from the Center for Innova-
tion in Chemistry (PERCH-CIC) is also gratefully
acknowledged.
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