H. Zhang et al. / Tetrahedron Letters 56 (2015) 1720–1723
1721
O
OH
O
OH
O
OH
Following the generation of a bromide intermediate (S)-9, the viny-
larene (S)-10 was obtained via a Stille cross-coupling using
trimethyl(phenyl)stannane. Deprotection of (S)-10 with tetrabuty-
lammonium fluoride produced the alcohol (S)-11.3
As previously described, pancreatic tumor cells are known to be
significantly more resilient than normal human cells under nutri-
ent-deprived conditions. Therefore, we expect an ideal selective
anti-austerity agent to induce cell death only under nutrient-de-
prived conditions conferred by the absence of essential amino
acids, glucose, and serum. The survival of PANC-1 cells under nutri-
ent-deprived and nutrient-rich conditions within 24 h following
exposure to compounds 6, (S)-7, (S)-8, (S)-10, and (S)-11 is shown
in Figure 2. In this study, compounds 6, (S)-7, and (S)-10 showed no
Cl
O
Cl
O
1
4
9
6
O
O
H
O
O
O
O
O
O
O
4'
1'
6'
9'
O
1'
1'
Cl
OH
O
OH
O
O
Spiroxin A (1)
Spiroxin C (3)
Spiroxin B (2)
OH
O
OH
HO
H
Cl
1
1
O
O
cytotoxicity in either medium condition, even at 100
pound (S)-11 also did not show appreciable cytotoxicity. It induced
30% cell death only at 100 M, exhibiting no cytotoxicity at low
lM. Com-
O
O
O
O
l
O
O
concentrations. We were pleased to discover that compound
(S)-8, exhibited preferential cytotoxicity under nutrient-deprived
conditions, with an LC50 of 11 lM. In this investigation, 100 lM
of (S)-8 induced 88% cell death under nutrient-deprived conditions,
whereas no cytotoxicity was observed under nutrient-rich
conditions (Fig. 2).
1'
1'
Cl
H
OH
Spiroxin E (5)
O
OH
Spiroxin D (4)
Figure 1. Spiroxins A–E.
The preferential cytotoxicity of (S)-8 suggests a future struc-
ture–activity relationship (SAR) study to further investigate which
structural components of (S)-8 are responsible for its activity. The
significant difference in cytotoxicity between (S)-8 and (S)-7
indicates that the presence of the tert-butyldiphenylsilyl (TBDPS)
protecting group may play an important role in the selective
cytotoxic activity observed for (S)-8. Our results, however, show
that the TBDPS group alone is insufficient to solely confer selective
cytotoxicity to this series of compounds. For example, the
inactivity of (S)-10, which also contains a TBDPS protecting group,
suggests that cytotoxic activity is not necessarily directly related to
the structure of the TBDPS group. Given these results, we explored
the role of hydrophobicity in the activity observed for this series of
compounds. To this end, the octanol–water partition coefficients of
all five compounds (logP) were calculated and are listed in Table 1.
LogP is important for predicting cell membrane penetration8 and
studies have utilized logP values to estimate biological activities
of structurally-related compounds. For example, Fratello et al.
demonstrated that spiroxin A reacts with 2-mercaptoethanol and
dithiothreitol to form conjugates, indicating that its mechanism
of action may be more complex.4 To date, the biological activities
of spiroxins B–E (2–5) have not been reported.
In 2011, our laboratory reported a novel catalytic asymmetric
approach to the core structure of spiroxin A via a tandem oxida-
tion/ring-opening sequence (Scheme 1).3 Intrigued by the antitu-
mor activity of spiroxin A and with ready access to multiple
synthetic intermediates, we decided to examine whether struc-
tural components of the core tertiary naphthoquinol might them-
selves possess useful biological activity. Herein, we present the
evaluation of structural components of spiroxin A against PANC-1
cells under both nutrient-deprived and nutrient-rich conditions,
and the identification of a lead pharmacophore for future struc-
ture–activity relationship (SAR) studies.
Results and discussion
reported
a
good correlation between the cytotoxicity of
halogenated benzenes and their logP values9, whereas Sasaki
et al. re-evaluated the tumor-specific cytotoxicity of mitomycin
C, bleomycin, and peplomycin based on their logP values.10 Studies
have shown excellent correlation between ChemDraw-estimated
ClogP values and experimentally measured logP values.8,11 In this
study, ClogP estimations by ChemDrawÒ 8.0 (PerkinElmer,
Cambridge, MA, USA) and the experimentally measured values
Five compounds (6, (S)-7, (S)-8, (S)-10, (S)-11, Scheme 1) were
available in sufficient quantity to be tested in vitro for their cyto-
toxic activity against PANC-1 cells under both nutrient-rich and
nutrient-deprived conditions.7 (S)-7 was obtained by a stereoselec-
tive ring-opening of the cyclic ether 6. The protection of (S)-7 was
accomplished using tert-butyldiphenylsilyl chloride and imidazole.
Scheme 1. Synthesis of enantioenriched tertiary naphthoquinol (S)-13.
Zhang, Hong
