ABC synthesis and antitumor activity of a series of Annonaceous acetogenin analogs with a threo, trans, threo, trans, threo-bis-tetrahydrofuran core unit

Article abstract of DOI:10.1016/j.bmcl.2007.02.033

Side-chain analogs of Annonaceous acetogenins with a threo, trans, threo, trans, threo-bis-tetrahydrofuran core unit have been prepared and tested for cytotoxicity against HCT-116 human colon cancer cells.

Full text of DOI:10.1016/j.bmcl.2007.02.033

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2434–2437
ABC synthesis and antitumor activity of a series of Annonaceous
acetogenin analogs with a threo, trans, threo, trans,
threo-bis-tetrahydrofuran core unit
James A. Marshall,a,* Jesse J. Sabatinia and Frederick Valerioteb
aDepartment of Chemistry, University of Virginia, Charlottesville, VA 22904, USA
bJosephine Ford Cancer Center, Detroit, MI 48202, USA
Received 18 January 2007; revised 10 February 2007; accepted 13 February 2007
Available online 15 February 2007
Abstract—Side-chain analogs of Annonaceous acetogenins with a threo, trans, threo, trans, threo-bis-tetrahydrofuran core unit have
been prepared and tested for cytotoxicity against HCT-116 human colon cancer cells.
Ó 2007 Elsevier Ltd. All rights reserved.
First described in 19821, the class of natural products
known as Annonaceous acetogenins now number more
than 350 members.2 All are isolated from members of
the plant family Annonaceae, and many show potent
antitumor, antimicrobial, antimalarial, pesticidal, and
antifeedant activities. Biogenetically derived from poly-
oxygenated C32 or C34 fatty acids and a branched pro-
pionate moiety at C2 as part of a butenolide terminus,
the most active compounds generally possess one or
two tetrahydrofuran rings flanked by hydroxyl substitu-
ents near the center of the fatty acid chain (Fig. 1).
biological activity than the stereochemistry of the bis-
tetrahydrofuran core.3 However, this postulate has not
been much explored because few examples of natural
Annonaceous acetogenins differing only in spacer chain
length are known. Furthermore, most synthetic studies
have focused on relatively few of the natural acetogenins
or core stereoisomers of natural acetogenins.5
We recently reported a highly convergent bidirectional
route to acetogenins with a threo, trans, threo, trans,
threo core unit by a modular approach involving Grub-
bs cross-metathesis methodology for attachment of the
aliphatic side chains to a readily prepared core unit.6
This route seemed ideally suited for a preliminary exam-
ination of the influence of side chains on bioactivity in
The biological activity of these compounds stems from
their inhibition of the terminal electron transfer step of
cell membrane mitochondrial Complex I. It is postu-
lated that the oxygenated central segment of the bis-tet-
rahydrofuran acetogenins acts as a hydrophilic anchor
to the glycerol backbone region of liposomal mem-
branes with the appended aliphatic arms extending into
the lipid bilayer.3 These arms are thought to control the
positioning of the butenolide terminus within the bilayer
and its proximity to the active site of Complex I.4 Since
the requisite hairpin conformation of the membrane-
bound acetogenin can be realized with a variety of core
stereoisomers it is further postulated that the length of
the butenolide spacer chain has a greater influence on
acetogenin analogs
stereochemistry.
with
a
common
core
Our overall plan, as summarized in Figure 2, consisted of
three stages starting with a synthesis of the aforementioned
Me
OH
O
1
34
CH
15
24
O
4
( )
O
7
3
H
H
H H
O
OH
HO
Me
threo, trans, threo, trans, threo
Keywords: Acetogenins; Cross-metathesis; Antitumor activity;
Cytotoxicity.
Figure 1. Asimicin. A typical threo, trans, threo, trans, threo bis-
tetrahydrofuran Annonaceous acetogenin derived from a C34 fatty
acid.
*
Corresponding author. Tel.: +1 4349247997; e-mail: jam5x@
virginia.edu
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.02.033

J. A. Marshall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2434–2437
2435
O
First Cycle
1.Grubbs II (82%)
OH
1
OEt
R CH=CH
2
EtO
2. Swern oxidation-
Horner Emmons
1
1
B
R
R
2
1
O
O
O
O
O
H
H
H
H
H
H
H
2
H
1. AD-mix β
homologation (74%)
OMOM
O
MOMO
OMOM
2
MOMO
Grubbs II
A
2. pyridine
3. MOMCl
ABB
O
OMs
1. AD-mix α
OEt
EtO
2
2. MsCl, Et N
1
3
R
4. LiAlH
4
5. Dess-Martin
oxidation
R
R
MsO
O
O
3 (73%)
R
2
R CH=CH
H
H
H
H
H
2
CC
OMOM
OMOM
MOMO
C
+
+
+
2
ABC
R
1
R
1
Wittig
1
R
2
R
Grubbs II
BC
2
2
R
O
R
O
O
O
+
H
H
H
H
2
O
H
H
H
H
O
2
OR
OMOM
H
H
H
R O
ABB
MOMO
A (84%)
MOMO
1
ACC
4 R = CO Et, R = H (65%)
5 R = CO Et, R = MOM (90%)
2
1
2
Second Cycle
ACC + ABB + CC + BC
2
Grubbs II
1
2
6 R = CH OH, R = MOM (81%)
2
ABC
1
2
7 R = CHO, R = MOM
Figure 2. ABC plan for the synthesis of Annonaceous acetogenin
analogs with C2 core symmetry.
Scheme 1.
compounds and possible derivatives might have higher
water solubility than their alkyl counterparts.
core unit A which would then be elaborated by a Grubbs
cross-metathesis reaction with a terminal alkene precursor
B of one of the side chains to form the symmetric diene
ABB.7 This ’type III’ diene would then be subjected to a sec-
ond cross metathesis with a terminal alkene precursor C of
the second side chain to yield the unsymmetrical diene
ABC. Final elaboration of this intermediate to the targeted
acetogenin analog would then require only hydrogenation
and global deprotection. An excess of the initialterminal al-
kene B would be employed to minimize the potentially
competing oligomerization of the diene A. The expected
byproducts of the second cross metathesis, an equilibrium
mixture of ACC, BC, CC, and recovered ABB, can be
resubjected to cross metathesis thereby generating addi-
tional quantities of ABC. For our initial studies we focused
on functional variations in the nonbutenolide side chain to
evaluate the flexibility of the approach. Additional studies
on systematic chain length variations of the butenolide
chain are planned for a future date.
The core tetrahydrofuran segment A was prepared as
previously described starting from 4-penten-1-ol (1
Scheme 1).6 Dimerization with the Grubbs II catalyst7
followed by in situ Swern oxidation and Horner–Em-
mons phosphonate homologation afforded the triene
diester 2 which was selectively dihydroxylated with the
Sharpless AD-mix a reagent. Conversion to the dimesy-
late 3 then dihydroxylation of the conjugated double
bonds with AD-mix b led to a tetraol, which was heated
in pyridine to effect conversion to the bis-tetrahydrofu-
ran diester 4. Protection of the diol with MOMCl fol-
lowed by reduction of the diester 5 with lithium
aluminum hydride afforded the diol 6. Oxidation to dial-
dehyde 7 and Wittig homologation of dialdehyde 7com-
pleted the sequence to diene A in an overall yield of 18%.
The terminal alkene precursors to side chain B1(1-de-
cene) and B2 (1-octene) are commercially available and
those of B3 and B4 are the TBS ethers of commercially
available 5-hexen-1-ol and 10-undecen-1-ol.
As the B components of the present analog synthesis, we
chose readily available alkenes as these would be em-
ployed in excess for the initial cross metathesis
(Fig. 3). For the C components we mainly used the hy-
droxy butenolide C1 because 4-hydroxy substituted ace-
togenins show some of the highest reported activity
against tumor cells. In these initial studies we were more
interested in exploring the versatility of the synthetic
methodology than devising a series of closely related
(R = Me, Et, Pr, Bu, etc.) series of analogs. We were
particularly interested in the previously unknown termi-
nal alcohol analogs derived from B3 and B4 as these
Our synthesis of the 4-hydroxy butenolide side-chain
precursor 17, outlined in Scheme 2, is an improved ver-
sion of our previous sequence. Asymmetric dihydroxyla-
tion of 1,11-dodecadiene (9), easily prepared from 10-
undecenal (8) by Wittig homologation, afforded the diol
10 of 92% enantiomeric purity in 60% yield.8 Selective
tosylation then TBS protection and treatment with
NaI then led to the iodide 13. Alkylation of White’s
+
OR
-
AD-mix β
Ph PCH
I
3
3
X
CHO
8
( )
( )
( )
8
8
1
BuLi, Et O
2
(92%)
(60%, 92:8)
R = CH (CH ) , CH (CH ) , TBSO(CH ) , TBSO(CH ) ,
3
2 7
3
2 5
2 4
2 9
8
9
10 R = H, X = OH
4
2
3
1
B
B
B
B
11 R = H, X = OTs (88%)
12 R = TBS, X = OTs (94%)
13 R = TBS, X = I (93%)
Me
Me
TBSO
OTBS
1. m-CPBA
1. LDA, THF
OTBS
2
O
O
R =
O
O
O
( )
2. toluene,
reflux (91%)
2. 13 (88%)
( )
7
( )
8
8
( )
8
PhS
R
O
1
2
O
O
O
C
C
O
15 R = SPh
16 R = SOPh
17
14
Figure 3. Terminal alkenes R1CH@CH2 and R2CH@CH2 employed in
cross metathesis.
Scheme 2.

2436
J. A. Marshall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2434–2437
Me
O
1. m-CPBA
1. LDA, THF-HMPA
2.CH =CH(CH ) I
OH
O
MOMCl,
CH Cl
O
O
( )
( )
7
7
PhS
2. toluene
reflux (91%)
R
PhS
2
2 9
( )
9
O
O
O
O
14
O
H
H
H
H
2
2
(90%)
O
OH
26 R = HO(CH )
20
HO
EtN(i-Pr)
18 R = SPh
19 R = SOPh
2
Me
O
2 6
OH
MOMO
Scheme 3.
O
O
H
H
H
H
O
OH
HO
31
lactone 149 with this iodide proceeded in 88% yield. The
homologated lactone 15 was converted to the sulfoxide
16 which underwent elimination in refluxing toluene to
the butenolide 17 in 91% yield.5b The butenolide precur-
sor 20 of side chain C2 was prepared analogously
(Scheme 3).
Scheme 6.
40% yield and recovered approximately 20% of the ABB
starting material, 20% of the ABB double exchange
product, and 10% each of the dimeric CC and cross
BC products. Typically, more than 90% of the total
material could be accounted for. We have previously
shown that recycling of the recovered materials results
in additional quantities of the ABC product along with
the others in a comparable ratio.6 This option was not
examined in the present cases.
We next addressed coupling of the core unit A with the
various B and C side chain alkenes by cross metathesis
(Scheme 4). In our preliminary report of this methodol-
ogy, in which the unfunctionalized alkenes 1-decene and
1-octene were employed in the initial cross metathesis,
we found that the reaction proceeded smoothly with
5 mol% of the Grubbs II catalyst. However, with the
oxygenated alkenes related to B3 and B4 of the current
study, this protocol was not successful owing to exten-
sive migration of the terminal double bonds and catalyst
deactivation. Fortunately, this side reaction could be
shut down through use of added p-benzoquinone to
intercept the responsible RuH intermediate.10 However,
it was necessary to increase catalyst loading to 35 mol%.
These same conditions were successfully employed for
the cross-metathesis reactions as summarized in Scheme
5. In each case we obtained the ABC products in about
Reduction of the disubstituted alkenes and global
deprotection of the ABC intermediates was smoothly ef-
fected by sequential treatment with diimide and methan-
olic HCl (Scheme 5).6 The terminal OH derivative 26
was chain-extended by selective reaction with MOMCl
leading to the MOM derivative 31 (Scheme 6). We had
hoped that replacing two of the methylene groups of
the side chain with oxygen would increase water solubil-
ity, but this was not the case. Ether 31 exhibited no mea-
surable solubility in water.
Table 1 summarizes cytotoxicity data for the acetogenin
analogs 26–31 against HCT-116 colon cancer cells.5b
Notably all analogs show IC50values in the nanomolar
range. Of the terminal hydroxy analogs 26, 27, and 28
the latter, lacking a C4 OH, is the least active in keeping
with expectation based on reported activities of natural
acetogenins. Interestingly, the truncated analog 30 of
asimicin (29) is some 20 times more active than its nat-
ural counterpart. Although an exact comparison is not
possible owing to differing chain lengths, the compara-
ble activity exhibited by the terminal hydroxy com-
pounds 26 and 27 and the related alkyl compounds 30
and 29 suggests that the added terminal OH group does
not markedly affect cytotoxicity. While enhanced water
solubility was not observed with the hydroxylated com-
pounds, it may be possible to realize that goal through
conversion of the OH to a water soluble derivative that
could be cleaved in vivo.
1
R CH=CH
2,
1
1
R
R
O
O
O
O
2
H
H
H
H
H
H
H
H
Grubbs II
OMOM
MOMO
1,4-benzoquinone
OMOM
MOMO
A
ABB E,E:E,Z = 10:1
1
21 R = TBSO(CH ) (87%)
2 4
1
22 R = TBSO(CH ) (87%)
2 9
Me
Me
O
R
O
2
R
1
( )
n
R
( )
O
O
O
n
H
H
H
H
n
O
OMOM
MOMO
CH Cl Grubbs II
1,4-benzoquinone
ABC
2
2
1
2
yield, %
compound
R
R
23
24
25
TBSO(CH )
7
7
6
TBSO
41%
40%
41%
2 4
TBSO(CH )
TBSO
H
2 9
TBSO(CH )
2 9
Scheme 4.
Me
O
1. p-TsNHNH , NaOAc
2
2
R
H O, DME
2
1
R
( )
O
O
n
2.6M HCl THF, MeOH
Table 1. Cytotoxicity against HCT-116 colon cancer cells
H
H
H
H
O
OMOM
MOMO
ABC
Compound
IC50 (ng/mL)
IC50 (nM)
1
2
yield, %
n
compound
R
R
Me
O
26
27
2.2
0.39
22
3.8
0.60
35
OH
OH
H
26
27
28
29
HO(CH )
9
9
8
9
9
(91)
(90)
(91)
2 6
2
R
HO(CH )
2 11
1
R
28
HO(CH )
2 11
O
H H
( )
n
O
H
29a
30a
31
5.5
0.30
9.9
8.8
0.50
16
a
H
O
(80)
OH
OH
CH (CH )
OH
3
2 9
HO
a
(80)
30
a
CH (CH )
3
2 7
Reference 6
a Ref. 6.
Scheme 5.

J. A. Marshall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2434–2437
2437
5. Recent examples (a) Takahashi, S.; Nakata, T. J. Org.
Chem 2002, 67, 5739; (b) Marshall, J. A.; Piettre, A.;
Paige, M. A.; Valeriote, F. J. Org. Chem. 2003, 68, 1771;
(c) Nattrass, G. L.; Diez, E.; McLachlan, M. M.; Dixon,
D. J.; Ley, S. V. Angew. Chem., Int. Ed. 2005, 44, 580; (d)
Tinsley, J. M.; Roush, W. R. J. Am. Chem. Soc. 2005, 127,
10818; (e) Keum, G.; Hwang, C. H.; Kang, S. B.; Kim, Y.;
Lee, E. J. Am. Chem. Soc. 2005, 127, 10396; (f) Das, S.; Li,
L.-S.; Abraham, S.; Chen, Z.; Sinha, S. C. J. Org. Chem.
2005, 70, 5922; (g) Crimmins, M. T.; Zhang, Y.; Diaz, F.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2007.02.033.
References and notes
1. Jolad, S. D.; Hoffmann, J. J.; Schram, K. H.; Cole, J. R.
J. Org. Chem. 1982, 47, 3151.
A. Org. Lett. 2006, 8, 2369; (h) Bandur, N. G.; Bruckner,
¨
D.; Hoffmann, R. W.; Koert, U. Org. Lett. 2006, 8, 3829;
(i) Zhao, H.; Gorman, S. T.; Pagenkopf, B. L. Org. Lett.
2006, 8, 4379; (j) Wysocki, L. M.; Dodge, M. W.; Voight,
E. A.; Burke, S. D. Org. Lett. 2006, 8, 5637.
6. Marshall, J. A.; Sabatini, J. J. Org. Lett. 2006, 8,
3557.
7. (a) Grubbs, R. H. Tetrahedron 2004, 60, 7117; (b)
Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs,
R. H. J. Am. Chem. Soc. 2003, 125, 11360.
8. Curran, D. P.; Zhang, Q.; Richard, C.; Lu, H.; Gudipati,
V.; Wilcox, C. S. J. Am. Chem. Soc. 2006, 71, 3559.
9. White, J. D.; Somers, T. C.; Reddy, G. N. J. Org. Chem.
1992, 57, 4991.
2. Reviews (a) Alali, F. Q.; Liu, X.-X.; McLaughlin, J. L.
J. Nat. Prod. 1999, 62, 504; (b) Zafra-Polo, M. C.; Figadere,
B.; Gallardo, T.; Tormo, J. R.; Cortes, D. Phytochemistry
1998, 48, 1087; (c) Cave, A.; Figadere, B.; Laurens, A.;
Cortes, D.. In Progress in the Chemistry of Organic Natural
Products: Acetogenins from Annonaceae; Hertz, W., Ed.;
Springer-Verlag: New York, 1997; Vol. 70, pp 81–288; (d)
Zeng, L.; Ye, Q.; Oberlies, N. H.; Shi, G.; Gu, Z.-M.; He,
K.; McLaughlin, J. L. Nat. Prod. Rep. 1996, 13, 275.
3. Miyoshi, H.; Ohshima, M.; Shimada, H.; Akagi, T.;
Iwamura, H.; McLaughlin, J. L. Biochim. Biophys. Acta
1998, 443.
4. Shimada, H.; Grutzner, J. B.; Kozlowski, J. F.; McLaugh-
lin, J. L. Biochemistry 1998, 37, 854.
10. Hong, S. H.; Sanders, D. P.; Lee, C. W.; Grubbs, R. H.
J. Am. Chem. Soc. 2007, 127, 17160.