2
D.S. Huang et al. / Bioorganic & Medicinal Chemistry Letters xxx (2017) xxx–xxx
would be tolerated at the
Suzuki coupling, the secondary alcohol in
a
-position (Scheme 3). Prior to the
O
a-bromopironetin ana-
OMe OH O 1
4
log 2d was protected as the silyl ether. The coupling between vinyl
halide 14 and phenylboronic acid proceeded in moderate yield
under mild conditions to give intermediate 15. We completed
the synthesis of analog 13 following removal of the TBS protecting
group with BF3ÁEt2O.
Me
Me Me
Me
(–)-Pironetin (1)
The antiproliferative activities of the new analogs were evalu-
ated in drug-sensitive OVCAR5 and A2780 ovarian cancer cell lines
Fig. 1. Structure of pironetin.
(Table 1). The addition of any group at the
resulted in decreased antiproliferative activity. The addition of a
methyl group to the -position of pironetin (entry 4) resulted in
an approximate 200-fold decrease in activity. The -chloro and
-fluoro analogs (entries 3 and 5) showed similar GI50 values even
though these halogens along with the methyl group have different
electronic properties. The -phenyl analog 13 was found to be
inactive (entry 7). These results suggest that the decreased activity
of the -functionalized analogs could be due to the steric proper-
ties of the group at the -position instead of the electronic proper-
ties of the various groups.
Although the -functionalized pironetin analogs exhibited
decreased biological activity relative to the natural product, -bro-
mopironetin 2d had unique activity (entry 6). In the OVCAR5 cell
lines, the dose response curve for -bromopironetin 2d showed
biphasic character (Fig. 2). For this biphasic curve, the first inflec-
tion point occurs at a concentration approximately 3-times the
GI50 of pironetin (Table 1 entry 6); while the second point occurs
at double-digit micromolar concentrations. However, a biphasic
dose response curve was not observed when A2780 cell lines with
treated with analog 2d or with pironetin or the other analogs
(Fig. 2). Another interesting aspect for the dose-response curves
a-position of pironetin
synthesized via a selective olefination reaction of aldehyde 4. A
variety of olefination conditions have been reported for the synthe-
sis of tri-substituted olefins carrying an alkoxide, halide, or alkyl
a
a
a
moiety at the a
-position.30–35 An intermediate such as 4 could be
synthesized from aldehyde 5, which has been employed in previ-
a
ous syntheses of pironetin analogs in our group.18
The synthesis of analogs 2 began with the diastereoselective
aldol reaction between aldehyde 6 and thiazolidinethione 7 under
conditions similar to those reported by Crimmins23 and Marco20
for the synthesis of pironetin and related analogs (Scheme 2). Fol-
lowing protection of the secondary alcohol as the TBS ether, the
chiral auxiliary was cleaved with DIBAL-H to provide aldehyde
10. We performed the desired olefination of aldehyde 10 with flu-
orine containing phosphonate ester 11a, because the selective ole-
fination with aryl phosphonate esters containing functional groups
at the 2-position has been reported previously.30,33 The olefination
between aldehyde 10 and phosphonate ester 11a proceeded in 63%
yield to provide intermediate 12a. We subsequently carried out the
olefination reaction with the methyl, chlorine, and bromine con-
taining phosphonate esters 11b–11d to evaluate the SAR at the
a
a
a
a
a
a-position of pironetin. While the olefination with 2-methyl phos-
of a-bromopironetin 2d, was the percentage of cells remaining at
phonate ester 11b proceeded in high yield, we observed incom-
plete conversion for reactions with halogen-containing
phosphonate esters 11c and 11d. We completed the synthesis of
analogs 2 following the deprotection and lactonization of interme-
diates 12 under acidic conditions.
the high drug concentrations. In the dose-response curves of tubu-
lin-binding agents, paclitaxel, pironetin and related analogs in the
A2780 cell line, the dose-response curve plateaus at approximately
10–20% of the control population at the higher tested drug concen-
trations. In the dose response curves of a-bromopironetin 2d in the
We also sought to synthesize analogs containing an aryl group
at this position, since Michael acceptors containing electron-defi-
A2780 cell line, the highest doses of the analog resulted in <10% of
the control population; this was significantly lower than other
evaluated tubulin-binding agents paclitaxel and pironetin (Fig. 3).
cient aromatic groups at the
reversible covalent inhibitors.16 While we proposed a series of ana-
logs containing different aromatic groups at the -position via a
Suzuki coupling with -bromopironetin analog 2d, we initially
synthesized -phenyl analog 13 to determine if an aromatic group
a-position have been reported to be
In summary, we synthesized a-functionalized pironetin analogs
a
to evaluate structure-activity relationships of the C2 position since
substitution at this position could potentially decrease the natural
product’s off-target covalent adduct formation. The analogs
a
a
Scheme 1. Retrosynthesis of a-functionalized pironetin analogs 2 (R = EWG group, PG = protecting group).