S. Fiorito et al. / Bioorg. Med. Chem. Lett. 24 (2014) 454–457
455
containing naphthoquinones are widely used in Southeast Asia and
South America to treat malignant and parasitic diseases. In this
context, lapachol (2-hydroxy-3-isopentenyl-1,4-naphthoqui-
O
O
R1
1
none) represents one of the best described examples of a research
topic in natural product chemistry. Its phytochemical and pharma-
R2
1
3
cological properties has been recently exhaustively reviewed.
Entry
R1
R2
In the current work, we characterized the in vitro growth inhib-
itory activity of 17 selected 1,4-naphthoquinones, including 1 and
its structurally related natural and semi-synthetic derivatives. The
in vitro IC50 growth inhibitory concentration was determined for
each compound in a panel of six human cancer cell lines exhibiting
different levels of resistance to pro-apoptotic stimuli using the
MTT colorimetric assay. The lack of a reference drug is justified
upon the fact that practically all, except PC3 cells, cancer cell lines
have been selected to be resistant to the most common therapeu-
tically used chemotherapeutics.
Compounds were synthesized following three criteria: (a) ester-
ification of the hydroxyl function of lapachol 1 with acids having
different lengths of the carbon skeleton (C2, C12, C14, C16, C18
either saturated or monounsaturated); it is thought that such a
chemical modification may allow these highly lipophilic deriva-
tives to permeate the cell membrane and release the active portion
inside the cell upon cleavage by esterases; (b) decrease in polarity
of 1 by etherification of the OH function with 1–5 carbons chains or
its transformation into –Cl; (c) replacement of the isopentenyl
chain in position 3 of lapachol with other allylic or benzylic moie-
ties. The effects of lawsone 17 was also studied to highlight the role
of the C-side chain of lapachol on the biological activity. The chem-
ical structures of the compounds under study are illustrated in
Figure 1.
1
2
-OH
-OAc
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
isopentenyl
geranyl
3
-OCO(CH ) CH
2
2
2
2
10
3
3
3
3
4
-OCO(CH
-OCO(CH
-OCO(CH
)
)
)
12CH
5
14CH
16CH
6
7
-OCO(CH
2
)
7
CH=CH(CH
-OCH3
-OCH CH
-OCH CH
-OCH
2
)
7
CH
3
8
9
2
3
10
2
2
CH
3
1
1
1
2
2
CH=CH
2
-OCH
2
CH=C(CH
-Cl
)
3 2
13
14
15
16
17
-OH
-OH
-OH
-OH
styryl
benzyl
H
The main natural sources of lapachol 1 and lawsone 17 have
been described previously.1
3,14
The compounds lapachol methyl
Figure 1. Illustration of the chemical structures studied. The 17 compounds studied
belong to three chemical groups, that is, esters of lapachol 1 (compounds 2–7
Table 1), ethers or halogen derivatives of lapachol 1 (compounds 8–13; Table 1),
compounds with a different C-side chain (compounds 14–16; Table 1).
ether 8 has been previously extracted from Rubia tinctorum L.
1
5
(
Rubiaceae), 3-geranyllawsone 14 has been isolated from the
roots of Conospermum teretifolium R. Br. and Conospermum brownii
1
6
Meisn. (Proteaceae), and finally 2-chloro-3-dimethylallyl-1,4-
naphthoquinone 13 has been extracted from twigs and leaves of
Avicennia germinans L. (Avicenniaceae).
O
O
O
O
Lapachol 1 and lawsone 17 were commercially available, while
the synthesis of lapachol acetate 2 and 2-chloro-3-dimethylallyl-
OCOR3
OH
3
R COCl
1
,4-naphthoquinone 13 were accomplished according to the
3 2
Et N, Et O
18,19
procedure described previously.
All other esters (laurate, myr-
r.t., 30 min.
istate, palmitate, stearate, and oleate) were obtained by reaction of
lapachol 1 with acetic anhydride or the respective acyl chloride in
Et O in the presence of Et N at room temperature for 30 min. The
2 3
Scheme 1.
yields of the desired adducts were in the range 97–99% (Scheme 1).
O-alkylation of lapachol 1 to obtain ethers 8–12 was carried out
in acetone at 80 °C for 1 h using methyl iodide, ethyl iodide, n-pro-
pyl iodide, allyl bromide, or 3,3-dimethylallyl bromide respectively
O
O
O
O
OR4
OH
4
R X
in the presence of K
2
CO
3
as the base, followed by acid-base work-
K CO
2
3
up and crystallization from n-hexane. The respective adducts were
obtained in 47–90% yields (Scheme 2).
acetone, 80 °C
1h
Finally compounds 14–16, bearing a C-side chain structurally
different with respect to lapachol 1, were synthesized starting from
lawsone 17 using geranyl bromide, styryl bromide or the combina-
X = -Br, -I
Scheme 2.
2 3
tion benzyl chloride/KI as alkylating agents and K CO as the base
in DMF at 150 °C for 2 h, followed by acid-base work-up and crys-
tallization from n-hexane. The respective adducts were obtained in
lapachol display weak or no (IC50 >100 lM) inhibitory activity.
The same pattern has been recorded for products resulting from
the substitution of the C-side chain of lapachol with benzyl or sty-
ryl moieties like in 15 and 16. The retention of a terpenyl side chain
in position 3 of the naphthoquinone ring like in 3-geranyllawsone
14 resulted in a decrease of activity especially against U373 and
SKMEL-28 cell lines. 3-Chlorodeoxylapachol 13, that was previ-
ously reported to exert in vitro growth inhibitory effects on Col2
(human colon cancer), KB (human oral epidermoid carcinoma),
LNCaP (human hormone-dependent prostate cancer), Lu1 (human
3
7% (for the two bromides) and 38% (in the case of use of benzyl
chloride) yields (Scheme 3).
Chemical stability of esters 2–7 was investigated by incubation
of each synthesized compound in the medium used to perform
pharmacological assays for 72 h. After this period in every case
the percentage of recovery of each ester was >95%.
The data show that of the three chemical groups under study,
esters, with the only exception of the acetate 2, and ethers of