Studies in multidrug resistance reversal: A rapid and stereoselective synthesis of the dihydroagarofuran ring system

Article abstract of DOI:10.1016/j.tetlet.2004.08.041

Several dihydroagarofuran esters have been reported to be effective multidrug resistance (MDR) reversing agents for both cancer cells and bacteria. We report a rapid synthesis of the dihydroagarofuran ring system from carvone in a sequence that is highlig

Full text of DOI:10.1016/j.tetlet.2004.08.041

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7193–7196
Studies in multidrug resistance reversal: a rapid and
stereoselective synthesis of the dihydroagarofuran ring system
Christopher A. Lee and Paul E. Floreancig*
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
Received 21 July 2004;revised 7 August 2004;accepted 9 August 2004
Abstract—Several dihydroagarofuran esters have been reported to be effective multidrug resistance (MDR) reversing agents for both
cancer cells and bacteria. We report a rapid synthesis of the dihydroagarofuran ring system from carvone in a sequence that is high-
lighted by a sequential conjugate addition/aldol sequence, a ring closing metathesis reaction, and a diastereoselective alkene reduc-
tion to provide an axial methyl group. The synthesis allows for differential esterification reactions as required to study the roles of
these groups in MDR reversal.
Ó 2004 Elsevier Ltd. All rights reserved.
Multidrug resistance (MDR) is an insidious develop-
ment in the treatment of cancer and infectious diseases
that renders agents of a broad range of structural classes
ineffective.1 While many causes of multidrug resistance
have been identified,2 a common source is the overex-
pression of ATP-driven pumps in cell membranes that
expel drugs before they can diffuse into the cytoplasm
and perform their functions.3 P-glycoprotein (P-gp) is
an 170kDa member of this protein class that is of partic-
ular importance in cancer chemotherapy, with an
estimated 50% of refractory cancers overexpressing
this pump.4 In their search for P-gp inhibitors, Lee
and co-workers identified5 a series of esters of the
dihydroagarofuran ring system that act synergistically
with cytotoxic agents such as paclitaxel to inhibit prolif-
eration of cancer cells that overexpress P-gp (Fig. 1).
Subsequently Gamarro and co-workers reported6 that
dihydroagarofuran esters inhibit a related ATP-driven
bacterial drug efflux pump. These observations suggest
that the dihydroagarofuran core could serve as a general
scaffolding for the development of a range of MDR-
reversing agents. In conjunction with our program in
this area we report our studies on a rapid entry into
the dihydroagarofuran ring system7 that allows for dif-
ferential peripheral functionalization as required for
the development of P-gp inhibitors and other MDR-
reversing agents.
We envisioned (Fig. 2) the tricyclic dihydroagarofuran
ring system (4) to arise from an acid-mediated ring clo-
sure of a hydroxy alkene such as 5. The decalin can be
formed from a ring closing metathesis reaction of diene
6. A conjugate addition/aldol reaction between 3-bute-
nal (7) or a functional equivalent and a carvone
O
O
O
O
O
OAc
O
OAc
O
OAc
O
OAc
N
O
O
OBz
OBz
O
OH
OAc
1
2
3
Figure 1. Dihydro-b-agarofuran multidrug reversing agents.
Keywords: Multidrug resistance;Conjugate addition/aldol;Metathesis;Acylation.
*
Corresponding author. Tel.: +1 412 624 8727;fax: +1 412 624 8611;e-mail: florean@pitt.edu
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.041

7194
C. A. Lee, P. E. Floreancig / Tetrahedron Letters 45 (2004) 7193–7196
standard conditions in nearly quantitative yield and
without stereomutation through retroaldol chemistry
to yield 14. The stereochemical outcome of the aldol
reaction cannot be explained through a Zimmerman–
Traxler transition state. We postulate that the aluminum
enolate coordinates to the C6 oxygen to disfavor pro-
ductive aldehyde coordination, making open transition
state 12 more energetically favorable. Consistent with
this explanation, exposing carvone to MeAl(Cl)SPh
and acetaldehyde smoothly provided aldol adduct 15
as a single stereoisomer, consistent with the expected
Zimmerman–Traxler model. Attempts to improve the
extent of conversion for the aldol reaction were unsuc-
cessful. Monitoring the thiolate addition by NMR
showed that the enolate and enone exist in equilibrium,
and that at prolonged reaction times the mixture reverts
exclusively to the enone, presumably due to disulfide
formation by trace amounts of O2. The use of aliphatic
sulfides did not provide higher product yields, but did
lead to starting material decomposition. Attempts to
trap the enolate as an enol silane through EvansÕ meth-
od12 (PhSSiMe3, Bu4NCN) resulted in starting material
recovery.
X
9
OR
X
9
OR
X
9
OR
1
1
3
1
7
5
7
5
3
7
5
O
OH
OH
OR'
4
OR'
OR'
5
6
10
O
1
9
+
H
O
O
OR'
7
8
9
Figure 2. Synthetic approach to the ring system.
derivative (8) and can serve to form the C1–C10 bond
while establishing the stereocenters at these sites. The
low cost of carvone (9) in homochiral form makes it
an attractive starting material for this sequence.8
Oxygenation was introduced at C6 (Scheme 1) through a
Rubottom oxidation9 of (R)-(À)-carvone. The resulting
diastereomeric mixture of alcohols 10a,b was efficiently
protected with MOMCl and i-Pr2NEt. While this
sequence is not stereoselective the inexpensive starting
material, good chemical yield, and ability to invert the
stereochemistry of the undesired isomer through a
deprotonation/kinetic reprotonation sequence provide
access to sufficient quantities of material for further
studies. To construct the C1–C10 bond and to introduce
versatile functionality at the C9 position we exposed 10b
to MeAl(Cl)SPh to generate aluminum enolate 11.10
While 3-butenal proved to be too unstable for coupling
to 11, 4-phenylselenobutanal, prepared in two steps
from c-butyrolactone,11 reacted to provide a 51% yield
(based on recovered starting material) of 13 as a 6:1
mixture of diastereomers at C1. Protection of the C1
hydroxyl group as a MOM ether proceeded under
In preparation for the ring closing metathesis reaction,
selenide oxidation and elimination was effected through
exposure of 15 to m-CPBA at À78°C followed by heat-
ing in the presence of dihydropyran and pyridine to
form 16 in excellent yield. Introducing the other olefin
was achieved with a diastereoselective addition of iso-
propenylmagnesium bromide into the hindered C5
ketone to form diene 17. We propose that the stereocon-
trol in this process can be attributed to steric shielding of
the bottom face of 16, which would be expected to be
the more reactive conformer based on the Felkin–Anh
model. Quenching this reaction with D2O resulted in
deuterium incorporation at C6, indicating that enolate
SPh
10
9
a-b
d
6
O
6
6
O
O
O
OMOM
Al Cl
Me
OMe
9
10a: β-OMOM
10b: α-OMOM
11
c
PhS
H
O
PhS
OH
1
PhS
OMOM
R
O
e
1
10
O
O
O
Al Cl
Me
OMOM
SePh
OMOM
SePh
OMe
12
13
14
PhS
OH
O
f
O
9
15
Scheme 1. Conjugate addition/aldol sequence. Reagents and conditions: (a) i. LDA, THF, TMSCl, À78°C, ii. m-CPBA, NaHCO3, CH2Cl2, iii. HF,
MeOH, 63%, anti:syn = 5.5:1;(b) MOMCl, DMAP, i-Pr2NEt, CH2Cl2, 40°C, 99%;(c) LDA, THF, À78°C, 92%, 10a:10b = 1:3;(d) Me 2AlCl, PhSH,
CH2Cl2, À50°C, then 10b, then 4-phenylselenobutanal, 51% (borsm);(e) MOMCl, DMAP, i-Pr2NEt, CH2Cl2, 40°C, 95%;(f) Me 2AlCl, PhSH,
CH2Cl2, À50°C, then 9, then CH3CHO.

C. A. Lee, P. E. Floreancig / Tetrahedron Letters 45 (2004) 7193–7196
7195
tively provided ester 21 in 56% yield. The C6 hydroxyl
group proved to be quite inert toward acylation, pre-
sumably because of steric shielding by the vinylic methyl
group. Therefore we reduced the C3–C4 olefin with
CrabtreeÕs catalyst16 and H2 in order to exploit coordi-
nation from the C5 oxygen for facial control. This reac-
tion provided 22 as a single diastereomer with the
correct stereochemistry at C4, as determined by an
NOE between the C6 hydrogen and the C4 methyl
group, thus completing the enantio- and diastereoselec-
tive synthesis of the dihydroagarofuran ring system.
While the C6 hydroxyl group still proved to be difficult
to acylate, efficient benzoylation was achieved with
BzOTf17 and pyridine in CH2Cl2 (Scheme 3).
PhS
OMOM
O
PhS
O
a
b
14
5
R
OMOM
OMOM
16
PhS
OMOM
PhS
OMOM
N
N
Mes
Mes
Ph
3
c
Cl
Cl
3
4
4
Ru
OH
OH
PCy3
MOMO
MOMO
17
18
19
Scheme 2. Construction of the B-ring. Reagents and conditions:
(a) m-CPBA, CH2Cl2, À78°C, then pyridine, DHP, reflux, 97%;(b)
isopropenyl magnesium bromide, THF, rt, 80% (borsm);(c) 18,
CH2Cl2, reflux, 94%.
We have demonstrated that the dihydroagarofuran ring
system can be prepared rapidly from carvone. Key steps
in this sequence include a conjugate addition/aldol reac-
tion to introduce functionality at C9, form the C1–C10
bond, and set the stereocenters at C1 and C10 with good
control, a diastereoselective Grignard addition to set the
stereochemistry of the tertiary alcohol at C5, a ring clos-
ing metathesis reaction to form the didehydrodecalin
ring system, and a facially selective hydrogenation to
set the stereochemistry at C4. The reactivity of the C1
and C6 hydroxyl groups is dramatically different, allow-
ing for facile differentiation in acylation reactions as re-
quired for a flexible approach to studying the roles of
the various ester groups in modulating the activity of
P-gp and other ATP-driven pumps with relevance to
multidrug resistance.
formation competes with nucleophilic addition, and rep-
rotonation is facially selective for starting material
regeneration. The use of isopropenylcerium dichloride
was not effective at increasing conversion. In considera-
tion of the sulfide group in 17 we selected the second
generation Grubbs metathesis catalyst (18)13,14 to
perform the ring closing reaction. Exposing 17 to
2.5mol% of this catalyst provided didehydrodecalin 19
in 94% yield within 15min at 40°C. H NMR analysis
of 19 allowed us to confirm the stereochemistry of the
Grignard addition and established that the A-ring exists
in a twist boat conformation (Scheme 2).
1
The twist boat conformation in 19 places the isoprop-
enyl group relatively far from the angular hydroxyl
group for facile etherification. In analogy to WhiteÕs
work on this ring system,15 triflic acid in THF was re-
quired to effect the desired ring closure and concomitant
methoxymethyl ether deprotection to form tricycle 20 in
30% yield. Weaker acids failed to effect the cyclization,
and the use of CH2Cl2 as a non-basic solvent resulted
in complete decomposition. Given the differences in
the steric environments around the C1 and C6 hydroxyl
groups, we felt that selective acylation at C1 could be
achieved. Indeed, exposure of 20 to standard acylation
conditions (Ac2O, pyridine, DMAP, CH2Cl2, rt) selec-
Acknowledgements
This work was supported by generous funding from the
University of Pittsburgh.
Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
tetlet.2004.08.041. Experimental procedures and spec-
tral data for all reaction products is available.
References and notes
PhS
6
OH
1
PhS
OAc
1
1. (a) Gottesman, M. M.;Pastan, I.;Ambudkar, S. Curr.
Opin. Genet. Dev. 1996, 6, 610;(b) Gottesman, M. M.;
Pastan, I. Annu. Rev. Biochem. 1993, 62, 385.
2. Brennan, R. G. Semin. Cell Dev. Biol. 2001, 12, 201.
3. Ambudkar, S.;Dey, S.;Hrycyna, C.;Ramachandra, M.;
Pastan, I.;Gottesman, M. M. Annu. Rev. Pharmacol.
Toxicol. 1999, 39, 361.
4. van Veen, H. W. Semin. Cell Dev. Biol. 2001, 12, 239.
5. (a) Kim, S. E.;Kim, Y. H.;Lee, J. J. J. Nat. Prod. 1998,
61, 108;(b) Kim, S. E.;Kim, H. S.;Hong, Y. S.;Kim, Y.
C.;Lee, J. J. J. Nat. Prod. 1999, 62, 697.
a
b
c
3
19
4
O
O
OH
20
OH
21
PhS
OAc
4
PhS
6
OAc
d
6
O
O
OH
OBz
22
23
´
6. Perez-Victoria, J. M.;Tincusi, B. M.;Jimenez, I. A.;
Scheme 3. Completion of the ring system. Reagents and conditions:
(a) TfOH, THF, 30%;(b) Ac 2O, py, DMAP, CH2Cl2, 56%;(c)
[Ir(cod)py(PCy3)]PF6, H2, CH2Cl2, 65% (borsm);(d) BzOTf, py,
CH2Cl2, À78 to 0°C, 100%.
Bazzocchi, I. L.;Gupta, M. P.;Castanys, S.;Gamarro, F.;
Ravelo, A. G. J. Med. Chem. 1999, 42, 4388.
7. For an excellent review on the biological activity of and
synthetic approaches to dihydroagarofuran natural

7196
C. A. Lee, P. E. Floreancig / Tetrahedron Letters 45 (2004) 7193–7196
products, see: Spivey, A. C.;Weston, M.;Woodhead, S.
Chem. Soc. Rev. 2002, 31, 43.
8. For the first use of carvone as a starting material for this
class of natural products, see: Buchi, G.;Wu est, H. J. Org.
12. Evans, D. A.;Truesdale, L. K.;Grimm, K. G.;Nesbitt,
S. L. J. Am. Chem. Soc. 1977, 99, 5009.
13. Scholl, M.;Ding, S.;Lee, C. W.;Grubbs, R. H. Org. Lett.
1999, 1, 953.
¨
¨
Chem. 1979, 44, 546.
9. Rubottom, G. M.;Gruber, J. M. J. Org. Chem. 1978, 43,
1599.
14. For an example of the use of this catalyst in the presence
of a sulfide group, see: Garbaccio, R. M.;Danishefsky,
S. J. Org. Lett. 2000, 2, 3127.
10. Itoh, A.;Ozawa, S.;Oshima, K.;Nozaki, H. Tetrahedron
Lett. 1980, 21, 361.
15. White, J. D.;Shin, H.;Kim, T.;Cutshall, N. S.
Chem. Soc. 1997, 119, 2404.
J. Am.
11. (a) Scarborough, R. M., Jr.;Smith, A. B., III. Tetrahedron
Lett. 1977, 50, 4361;(b) Dowd, P.;Kennedy, P. Synth.
Commun. 1981, 11, 935.
16. Crabtree, R. H.;Davis, M. W. J. Org. Chem. 1986, 51,
2655.
17. Brown, L.;Koreeda, M. J. Org. Chem. 1984, 49, 3875.