K. Tadpetch et al. / Bioorg. Med. Chem. Lett. xxx (2016) xxx–xxx
3
OH
O
OH
O
OH
O
NaBH4
O
O
O
+
MeOH, 0 °C to rt
(reverse phase CC)
HO
O
HO
OH
HO
OH
OH
1
18
(29%)
a, b or c
2 (58%)
a, b or c
OR1
O
OR1
O
O
O
R2O
OH
R2O
19: R1 = H, R2 = Me; 68%
20: R1 = Me, R2 = Me; 64%
21: R1 = H, R2 = Et; 83%
26: R1 = H, R2 = Me; 88%
: R1 = Me, R2 = Me; 52%
: R1 = H, R2 = Et; 85%
: R1 = H, R2 = Pr; 61%
27
28
29
: R1 = H, R2 = Pr; 96%
: R1 = H, R2 = Oct; 57%
22
23
30: R1 = H, R2 = Oct; 60%
31: R1 = H, R2 = Ac; 61%
24: R1 = H, R2 = Ac; 71%
25: R1 = H; R2 = propionyl; 51%
: R1 = H; R2 = propionyl; 59%
32
Scheme 2. Reagents and conditions: (a) iodomethane or iodoethane or iodopropane, K2CO3, acetone, rt; (b) 1-bromooctane, K2CO3, acetone, reflux; (c) Ac2O, pyridine, CH2Cl2,
0 °C to rt or propionyl chloride, Et3N, THF, 0 °C to rt.
OR1
O
OH
O
OH
O
H2, Pd/C
a
O
O
O
EtOH, rt, 97%
R2O
OH
HO
OH
HO
OH
3
33: R1 = H, R2 = Me; 58%
2
34: R1 = Me, R2 = Me; 73%
OR1
O
OR1
O
H2, Pd/C
EtOH, rt
O
O
R2O
OH
R2O
OH
: R1 = H, R2 = H
26: R1 = H, R2 = Me
27: R1 = Me, R2 = Me
: R1 = H, R2 = H; 79%
35
18
36: R1 = H, R2 = Me; 99%
37: R1 = Me, R2 = Me; 93%
Scheme 3. Reagents and conditions: (a) iodomethane, K2CO3, acetone, rt.
positive controls. The cytotoxic activities of all compounds tested
were expressed as IC50 values in M as shown in Table 1.
relationship was also consistent with Chai’s observation on SAR
of RALs on kinase inhibition activity.9
l
The parent compound 1 exhibited weak cytotoxic activities
against KB and MCF-7 cells with IC50 values of 46.30 and
For cytotoxicity against MCF-7 breast cancer cells, the impor-
tance of the double bond at the 10 and 20 positions on cytotoxic
activity against this cancer cell line was still realized. All ketone
analogues (6–17) were inactive toward this cell line. Only three
b-zearalenol analogues with alkoxy substituents at the 4-position,
4-ethoxy b-zearalenol (21), 4-propoxy b-zearalenol (22) and
4-octoxy b-zearalenol (23), were active against this cancer cell line
104.72
(IC50 = 40.58
l
M, respectively, while being more cytotoxic to Vero cells
M). To the best of our knowledge, this is the first Let-
l
ter on cytotoxic activity of zearalenone toward human oral carci-
noma cells. For cytotoxic activity against KB cells, three
zearalenol derivatives, 4-propionoxy b-zearalenol (25), 4-propoxy
a
-zearalenol (29) and 4-octoxy
enhanced cytotoxicity with the IC50 values of 33.42, 35.70 and
40.87 M, respectively (entries 21, 29 and 30); however, the cyto-
a
-zearalenol (30), showed
with IC50 values of 70.23, 81.88 and 93.64 lM, respectively (entries
17–19), which was 1.1- to 1.5-fold more potent than 1. Neverthe-
less, compounds 21–23 were still cytotoxic to Vero cells although
l
toxicity of these compounds to Vero cells also increased. It should
be noted that although 4-acetoxyzearalenone (8) and 4-acetoxy
not as strong as 1. Similar to b-zearalenol analogues, five
alenol derivatives with alkoxy or alkanoyloxy side chains at the
4-position including 4-methoxy -zearalenol (26) 4-ethoxy
-zearalenol (29) and 4-octoxy
-zearalenol (30) and 4-acetoxy -zearalenol (31) exhibited
a-zear-
a-zearalenol (31) displayed similar level of cytotoxicity to that of
a
1 (IC50 = 52.86 and 51.63 M, respectively, entries 4 and 31), they
l
a
a
-zearalenol (28), 4-propoxy a
were approximately 3.3-fold less cytotoxic to Vero cells compared
a
to 1. Interestingly, 4-methoxy b-zearalenol (19) exhibited 1.2-fold
enhanced cytotoxicity against MCF-7 cells with IC50 values of
weaker cytotoxic activity (IC50 = 55.98
lM, entry 15) compared to
93.33, 61.76, 43.78, 75.22 and 89.01 M, respectively (entries 26
l
that of 1 without cytotoxicity against Vero cells. The Cl atom at
the 5-position seemed to decrease cytotoxicity in all cell lines
including Vero cells (entries 8–10). In addition, it was clear from
these results that the double bond at the 10 and 20 positions was
required for cytotoxic activity as the saturated analogues, zear-
alanones 15–17 and zearalanols 3, 33–34 and 35–37, were all inac-
tive. These results also suggested that the unprotected hydroxyl
group at the 2-position was important for cytotoxicity since 2,4-
dimethoxy analogues 7, 14, 20 and 27 were inactive against this
cancer cell line (entries 3, 10, 16 and 27). This structure–activity
and 28–31). These analogues were 1.1- to 2.4-fold more potent
that the parent 1. It is worth noting that compound 28 was 1.7-fold
more cytotoxic to MCF-7 cells compared to
1 while being
non-cytotoxic to Vero cells and compound 31 was approximately
3.3-fold less cytotoxic to Vero cells compared to 1. These results
strongly suggested that an alkoxy group at the 4-position played
an important role on cytotoxic activity against MCF-7 breast cancer
cells of zearalenol analogues. Additionally, the effect of a free
hydroxyl group at the 2-position of zearalenol derivatives on
cytotoxicity against this cancer cell line was similarly observed.