Synthesis and anticancer activity of sclerophytin-inspired hydroisobenzofurans

Article abstract of DOI:10.1016/j.bmcl.2009.10.079

Three structurally related sets of hydroisobenzofuran analogs of sclerophytin A were prepared in three or four steps from (S)-(+)-carvone via an aldol-cycloaldol sequence. The most potent members of each set of analogs exhibited IC₅₀'s of 1-3 μ

Full text of DOI:10.1016/j.bmcl.2009.10.079

Bioorganic & Medicinal Chemistry Letters 19 (2009) 6898–6901
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and anticancer activity of sclerophytin-inspired hydroisobenzofurans
T. David Bateman a, Aarti L. Joshi a, Kwangyul Moon a, Elena N. Galitovskaya b, Meenakshi Upreti b,
a,
Timothy C. Chambers b, , Matthias C. McIntosh
*
*
a Department of Chemistry and Biochemistry, 119 Chemistry Building, University of Arkansas, Fayetteville, AR 72701, United States
b Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, 4301 West Markham St., Little Rock, AR 72205, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Three structurally related sets of hydroisobenzofuran analogs of sclerophytin A were prepared in three or
Received 4 September 2009
Revised 15 October 2009
Accepted 19 October 2009
Available online 22 October 2009
four steps from (S)-(+)-carvone via an aldol-cycloaldol sequence. The most potent members of each set of
analogs exhibited IC50’s of 1–3
dose assay for analog 6h revealed a GI50 = 0.148
l
M in growth inhibitory assays against KB3 cells. The NCI 60-cell line 5-
M and LC50 = 9.36 M for the RPMI-8226 leukemia cell
l
l
line, and a GI50 = 0.552
l
M and LC50 = 26.8
l
M for the HOP-92 non-small cell lung cancer cell line.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Anticancer
Cycloaldol
Sclerophytin
Eunicellin
Analog
The 2,11-cyclized cembranoids are a class of diterpenoids iso-
lated from a variety of marine sources that exhibit a wide range
of biological activities.1,2 Sclerophytin A, for example, was reported
to exhibit growth inhibitory activity against the murine L1210 leu-
kemia cell line with an IC50 = 1.0 ng/mL (Fig. 1).3,4
A considerable amount of synthetic effort has been directed to-
ward the synthesis of sclerophytin A and related cembranoids.5,6
Total syntheses of these complex targets typically require in excess
of 20 steps from commercially available starting materials. The
assembling the hydroisobenzofuran core structure from (S)-(+)-
carvone.8 Following the same procedure, intermolecular aldol reac-
tion of carvone and aryl aldehydes 2a–f gave anti-aldol adducts
3a–f with varying levels of diastereoselectivity (1:1–10:1 anti:syn)
(Scheme 1). Etherification of alcohols 3a–f generally proceeded in
good yields to give glycolate esters 4a–f. Cycloaldolization under
the influence of KHMDS afforded hydroisobenzofurans 5a–f as sin-
gle diastereomers based on 1H NMR analysis. The stereochemical
assignments were made by analogy to earlier examples whose
structure was determined by X-ray crystallographic analysis.8a
A series of acyl derivatives of ester 5a were prepared to evaluate
the effect of the carboxyl substituent on activity (Scheme 2). Esters
5a.i–iii were prepared by transesterification of ethyl ester 5a in the
presence of excess alcohol and Bu3SnOAc catalyst, while carboxylic
acid 5a.iv was prepared by saponification.
In the course of optimizing the cycloaldol reaction of glycolate
4a to ester 5a, we found that diene 6a was formed in significant
amounts if the reaction mixture was not rapidly quenched with
HOAc after addition of KHMDS (Scheme 3). For methyl glycolates
4g and 4h, the diene was the only cyclized product isolated from
the reaction mixture.
The dienes were presumably formed by in situ lactonization of
the aldol intermediates to form b-lactones 7, which underwent
unusually facile loss of CO2 to give the dienes (Scheme 3).9 Since
the dienes also constituted novel scaffolds, we included several
in the subsequent assays (vide infra).
majority of these diterpenoids possess
a cis-fused hydro-
isobenzofuran core structure.1,2 We hypothesized that seco analogs
containing the hydroisobenzofuran core might exhibit some of the
same anticancer activities as the parent compounds. We desig-
nated sclerophytin A as the nominal target of the analoging study
and named the resulting compounds ‘sclerologs’.7 As part of the
design principle, we sought to construct novel scaffolds that would
readily lend themselves to diversification. To this end, we chose
aryl groups as C2 substituents and an ester as the C9 substituent
(Scheme 1, cf. Fig. 1). In order to keep the sclerolog syntheses as
short as possible, we elected to retain the isopropenyl alkene. It
seems unlikely that the steric and/or electronic differences be-
tween the isopropenyl and isopropyl groups would have a signifi-
cant effect on anticancer activity.
In our own studies toward the total synthesis of selected 2,11-
cyclized cembranoids, we developed a three-step protocol for
Sclerophytin A and all other hydroisobenzofuran-containing
2,11-cyclized cembranoids possess a C10 stereocenter in the al-
kane, rather than alcohol, oxidation state (Scheme 3, cf. Fig. 1).1,2
* Corresponding authors.
E-mail addresses: ChambersTimothyC@uams.edu (T.C. Chambers), mcintosh@
uark.edu (M.C. McIntosh).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.10.079

T. David Bateman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6898–6901
6899
OH
CO2R
O
OH
H
OH
RO
HO
HO
O
O
O
O
O2C7H15
H
a
Ar
Ar
O
H
H
O
O
Ar
H
H
H
H
and/or
H
H
O
9
2
O
HO
OH
H
H
10
H
H
H
6a, 15%
6e, 33%
6g, 41%
6h, 45%
4a R=Et, Ar=2-Br-phenyl
4e R=Me, Ar=1-naphthyl
4g R=Me, Ar=2-furyl
5
H
sclerophytin A
vigulariol
briarellin E
O
O
4h R=Me, Ar=2-F-phenyl
Figure 1. Representative hydroisobenzofuran-containing 2,11-cyclized cembra-
O
noids.
Ar
H
- CO2
O
O
OH
H
7
a
b
Ar
Scheme 3. Proposed sequence for formation of dienes 6. Reagents and conditions:
(a) (1) KHMDS, THF, À78 °C; (2) HOAc.
1
3a-f 28-77%
EtO2C
O
CO2R
b
a
Ar
RO2C
HO
5a-d
O
H
O
O
9
2
H
c
Ar
O
Ar
H
8a-d, 55-67%
5a-f 59-81%
Br
Br
EtO2C
EtO2C
H
4a-d R=Et, 77-83%
4e,f R=Me, 25, 86%
O
O
c
10
H
H
Scheme 1. Sclerolog synthesis via an aldol-cycloaldol sequence. Reagents and
conditions: (a) (1) LDA, THF, À78 °C; (2) 2a–f; (3) HOAc; (b) Ag2O, BrCH2CO2R, DMF,
2,6-lutidine, rt; (c) (1) KHMDS, THF, À78 °C; (2) HOAc.a2a–d: 2-bromo-, 3-bromo-,
2,3-dichloro-, 2,4-dichlorobenzaldehyde, respectively; 2e: 1-naphthaldehyde; 2f:
pyridine-2-carboxaldehyde.
N
NHTs
9a, 92%
10a, 68%
Scheme 4. Oxidative–reductive rearrangement sequence. Reagents and conditions:
(a) PCC, silica gel, CH2Cl2; (b) TsNHNH2, EtOH, HOAc; (c) (1) catecholborane, CHCl3,
0 °C; (2) NaOAc–3H2O, reflux.
We therefore prepared analog 10a to more closely mimic the nat-
ural structures. We have previously reported the reduction of alco-
hols similar to 5a–d in
a
three-step oxidative/reductive
rearrangement process (Scheme 4).8a,10 Enones 8a–d were pre-
pared by oxidative rearrangement of the corresponding 3° alco-
hols.11 Conversion of enone 6a to tosyl hydrazone 9a was
followed by reductive transposition to afford cis-fused hydro-
isobenzofuran 10a. As the enones also constituted a novel struc-
tural class, we included them in the biological assays.
The human KB-3 carcinoma cell line was used to perform the
MTT colorimetric assays.12–14 Cells were treated with increasing
concentrations of compounds to assess their growth inhibitory
properties. IC50 values (concentration of the drug required to re-
Several notable features become apparent upon examination of
the assay data. Firstly, for the 2-Br esters 5a and 5a.i–iii (entries 1–
4), the smaller the alcohol moiety of the ester, the lower the IC50
,
that is, Me < Et < cyclopropylmethyl < cyclopentylmethyl. Sec-
ondly, two or more members of each class (alcohols 5, dienes 6
and enones 8) possess single digit micromolar activity. Reduction
of C10 from the alcohol to the alkane oxidation state significantly
diminished activity (5a vs 10a)
The essentially equal potency of the alcohols and dienes is
intriguing and initially tempted us to speculate that the esters
might undergo lactonization and decarboxylation to the dienes un-
der the assay conditions (cf. Scheme 3). This notion is supported by
the substantial difference in activity between C10–OH ester 5a,
which can undergo lactonization, and the corresponding C10
reduction product 10a, which cannot. However, the 1-napthyl
methyl ester 5e is at least 10-fold more active than the correspond-
ing diene 6e. Furthermore, enones 8a–d exhibit essentially equal
potency to the C10–OH methyl and ethyl esters, and also cannot
undergo b-lactone formation. Further studies will be needed to
determine whether the alcohols, dienes and enones are inhibiting
growth by the same or different mechanisms of action.
duce cell viability by 50%) ranged from 1 to >100 lM (Table 1).
O
O
Br
Br
EtO
HO
RO
HO
O
O
a or b
H
H
5a
5a.i R=Me, 97%
5a.ii R=cyclopropylmethyl, 84%
5a.iii R=cyclopentylmethyl, 91%
5a.iv R=H, 95%
The most active representatives of the compound classes (alco-
hol 5d, diene 6h and enone 8d) were submitted to NCI’s Develop-
mental Therapeutics program for 60-cell line screening (see
Supplementary data for complete results). Single dose assays
Scheme 2. Variation of the C9 ester moiety. Reagents: (a) Bu3SnOAc, ROH; (b) LiOH,
THF/MeOH/H2O.

6900
T. David Bateman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6898–6901
Table 1
Table 1 (continued)
Inhibition of KB-3 cell survival assessed by MTT viability assaya
Entry
Compound
IC50 (lM)
Entry
Compound
IC50 (lM)
MeO2C
HO
Br
EtO2C
O
O
HO
9
10
1
20
H
H
5e
5a
Br
MeO2C
HO
MeO2C
HO
O
O
N
2
5
H
10
100
H
5a.i
5f
Br
O2C
Br
O
O
HO
3
50
11
4
H
H
5a.ii
6a
Br
O2C
HO
O
O
O
4
>100
12
>100
H
H
H
5a.iii
6e
6g
Br
HO2C
HO
O
O
13
2
5
>100
H
5a.iv
F
Br
EtO2C
HO
O
O
O
6
3
14
1
H
H
5b
6h
Cl
Cl
EtO2C
HO
Br
EtO2C
O
7
5
H
15
70
H
O
5c
8a
Cl
EtO2C
HO
Br
EtO2C
O
O
O
Cl
16
30
8
3
H
H
8b
5d

T. David Bateman et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6898–6901
6901
anticancer activity. Further SAR studies and investigations into
their mechanism of action are pending.
Table 1 (continued)
Entry
Compound
IC50 (lM)
Cl
Cl
EtO2C
Acknowledgments
O
We thank the NIH (CA75577, RR15569, CA125602, CA109821)
and the Arkansas Biosciences Institute for support of this work;
the NCI Developmental Therapeutics Program (http://dtp.can-
cer.gov) for 60-cell line testing; and John Beutler, NCI, for helpful
discussions.
17
5
H
O
8c
Supplementary data
Cl
EtO2C
O
O
Cl
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.10.079.
18
3
H
8d
References and notes
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589.
Br
EtO2C
H
O
19
>100
H
10a
a
Cells were treated with increasing concentration of compounds and MTT via-
bility assays were performed after 96 h. Values are the means of triplicate assays
and are expressed as mean relative to untreated controls (see Supplementary data
for details and representative concentration curves).
revealed no significant activity for alcohol 5d. Enone 8d exhibited
significant differential activity against the RPMI-8226 leukemia
and the PC-3 prostate cancer cell lines. Diene 6h was the most ac-
tive, possessing significant differential activity against the entire
leukemia panel and the NCI-H522 non-small cell lung cancer cell
line. Subsequent 5-dose testing of 6h revealed a GI50 = 0.148
and LC50 = 9.36 M for the RPMI-8226 leukemia cell line, and a
GI50 = 0.552 M and LC50 = 26.8 M for the HOP-92 non-small cell
lung cancer line.
lM
l
l
l
The results described herein suggest that the three novel hydro-
isobenzofuran-containing scaffolds 5, 6 and 8 exhibit promising