New synthetic methodology for the construction of 7-substituted farnesyl diphosphate analogs

Article abstract of DOI:10.1021/ol201069x

Through the use of a 1,2-metalate rearrangement, six 7-substituted farnesol analogs were generated in a concise manner. This new synthetic route allowed us to quickly prepare several diverse farnesyl diphosphate analogs with interesting biological activit

Full text of DOI:10.1021/ol201069x

ORGANIC
LETTERS
2011
Vol. 13, No. 14
3576–3579
New Synthetic Methodology for the
Construction of 7-Substituted Farnesyl
Diphosphate Analogs
Andrew T. Placzek and Richard A. Gibbs*
Department of Medicinal Chemistry and Molecular Pharmacology and
Center for Cancer Research, Purdue Univesity, West Lafayette, Indiana 47907,
United States
rgibbs@purdue.edu
Received April 23, 2011
ABSTRACT
Through the use of a 1,2-metalate rearrangement, six 7-substituted farnesol analogs were generated in a concise manner. This new synthetic route
allowed us to quickly prepare several diverse farnesyl diphosphate analogs with interesting biological activities against mammalian protein-
farnesyl transferase.
Isoprenoid diphosphates playvital roles in many cellular
processes, such as cholesterol biosynthesis, protein preny-
lation, and as substrates for sesquiterpene cyclases in
plants. The post-translational prenylation of proteins is a
required event in the targeting and function of many
biologically important proteins. Farnesyl protein transfer-
ase (FTase) catalyzes the attachment of a 15 carbon
isoprenoid moiety from (farnesyl diphosphate) onto over
60 targets bearing a carboxyl-terminal Ca1a2X sequence,
including many signaling proteins that play a key role in
cancer.1 With the high prevalence of farnesylated proteins
found in cancer, there is a need to characterize the biolo-
gical activities of farnesylated proteins. The overall goal of
thisprojectis todevisenew chemicaltools thatwillenablea
method to distinguish between the biological activities of
different farnesylated proteins. The ability to differentiate
the biological impact of one farnesylated protein over
another may lead to the development and design of more
powerful and robust anticancer therapeutics.
of the incoming peptide substrate.2 Based on the crystal
structure and the known conformational changes in
the FTase active site, we evaluated several 7-substituted
FPP compounds against a library of CaaX-containing
peptides.3,4 The biochemical screening revealed several
7-substituted FPP analogs that selectively farnesylated
certain CaaX-box-containing peptides but not others.4
Based on these previous results, there is a need for a larger
and more diverse library of 7-substituted FPP compounds.
Previously we used the synthetic methodology for the
synthesis of 7-substituted FPP analogs developed by Raw-
at and Gibbs.3 This synthesis was highlighted by two
consecutive rounds of vinyl triflate-mediated chain-elon-
gation sequences to successfully complete 7-substituted
FPP compounds in 10 linear steps. While successful, this
route has several drawbacks. It is linear, with the diversity
element (the 7-substituent) installed early in the route.
Second, the triflimide reagent needed for each isoprenoid
homologation step is expensive and utilized in excess. To
facilitate an increase in the size and diversity of the 7-sub-
stituted FPP library we developed a new approach that
would reduce the number of linear steps. This methodology
The crystal structure of FTase reveals that FPP adopts a
conformation in the enzyme that leads to an interaction
between the 7 position of the isoprenoid and the a2 residue
(1) Basso, A. D.; Kirschmeier, P.; Bishop, W. R. J. Lipid Res. 2006,
(3) Rawat, D. S.; Gibbs, R. A. Org. Lett. 2002, 4, 3027–3030.
(4) Krzysiak, A. J.; Rawat, D. S.; Scott, S. A.; Pais, J. E.; Handley,
M.; Harrison, M. L.; Fierke, C. A.; Gibbs, R. A. ACS Chem. Biol. 2007,
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(2) Reid, T. S.; Terry, K. L.; Casey, P. J.; Beese, L. S. J. Mol. Biol.
2004, 343, 417–433.
r
10.1021/ol201069x
Published on Web 06/23/2011
2011 American Chemical Society

would also allow us to access other centrally modified
farnesyl diphosphate analogs.
production of the higher order alkenylcuprate 6, a versatile
intermediate for the synthesis of 7-substituted FPP ana-
logs. The coupling of 6 with a variety of electrophiles
(SnBu3Cl, I2, TMS-propargyl bromide, and allyl bromide)
was achieved by recooling the solution of alkenylcuprate
(6) to 0 °C and adding in the appropriate electrophiles.
This led to the formation of 4-substituted homogeraniol
derivatives (7aÀe) in moderate yields (42À62%). Despite
the modest yields, utilization of this synthetic transforma-
tion is beneficial in the synthesis of farnesol derivatives
because it allows for the transformation of readily avail-
able starting materials into advanced synthetic intermedi-
ates in one step. We next focused on introducing various
substituents into iodide 7a and stannane 7b that would
eventually become the 7-substituent on the corresponding
farnesyl diphosphate analog.
After an extensive investigation of various synthetic
approaches that could leadtothe synthesis of 7-substituted
FPP compounds, we focused on a route that utilizes
substituted dihydrofuran molecules for installing the
7-substituents into the farnesyl structure. The synthesis
of trisubstituted olefins from a Ni-(0)-catalyzed coupling
of 2,3-dihydrofurans with Grignard reagents was first
reported by Wenkert and colleagues5 and more extensively
studied by Kocienski.6 Kocienski and colleagues reported
a copper(I)-catalyzed coupling of Grignard reagents and
organolithiums with 5-lithio-2,3-dihydrofuran results in
trisubstituted olefins7,8 in a straightforward and stereose-
lective manner. Therefore, we applied the synthesis of
4-homogeraniol derivatives to the generation of substi-
tuted farnesyl analogs.
The facile nature of the SuzukiÀMiyaura reaction and
the commercial availability of a large library of organo-
boranes prompted us to examine their cross coupling of
organoboranes with 7a. We used the standard SuzukiÀ
Miyaura coupling conditions11 in which 2-thienyllboronic
Scheme 1. Synthesis of 4-Homogeraniol Derivatives
Scheme 2. Pd-Catalyzed Coupling Reactions with 4-Substituted
Homogeraniol Derivatives
To begin the synthesis of 4-homogeraniol derivatives
(Scheme 1), we first prepared homoprenyl iodide (2) from
cyclopropyl methyl ketone in a 75% yield.9 Subsequent
lithium-halogen exchange, followed by the addition of
CuCN, resulted in 3. With 3 in hand, we generated
5-lithio-2,3-dihydrofuran (5) from the action of t-BuLi
on 2,3-dihydrofuran (4). Attempts to replicate the 1,
2-metalate rearrangement resulting from the addition of 3 to
5, as reported by Kocienski and colleagues8 for the synth-
esis of 4-homogeraniol derivatives, were largely unsuccess-
ful because of the decomposition of organocuprate 3. The
problemwas resolvedwhendimethylsulfide was added asa
cosolvent, which presumably stabilizes the organocuprate,
and as a result the expected 1,2-metalate rearrangement
took place.10 The 1,2-metalate rearrangement led to the
acid (8) was successfully coupled to vinyl iodide 7a to result
in 9a in a 56% yield (Scheme 2). Similar SuzukiÀMiyaura
couplings should allow for the future installment of diverse
aromatic and vinyl substituents at the 7-position of FPP.
Once the methodology was developed to enable the synth-
esis of 7-substituted FPP compounds with aryl and vinyl
moieties at the 7-position, we next examined methodology
that would enable us to install alkyl moieties at the 7-posi-
tion. A Negishi coupling was envisioned to replace the iodide
of 7a with a variety of commercially available or readily
prepared alkyl zinc reagents.12 The organozinc reagent
10 was prepared through a lithium-halogen exchange of
(5) Wenkert, E.; Michelotti, E. L.; Swindell, C. S.; Tingoli, M. J. Org.
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Org. Lett., Vol. 13, No. 14, 2011
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1-iodo-4-trimethylsily-1-butyne with t-BuLi, followed by the
addition of a zinc chloride solution in THF. With 13 in hand,
we employed a Negishi cross coupling procedure,12 to join 7a
and the organozinc 10 resulting in the synthesis of the alkyl
substituted 4-homogeraniol derivative 9b. The vinyl stan-
nane 7b would serve to provide 4-substituted homogeraniol
derivatives via the Stille reaction with commercially available
vinyl and aryl halides. One substituent we wished to install at
Scheme 3. Conversion of 4-Homogeraniol Derivatives into
Their Corresponding 7-Substituted FPP Analogs
Figure 1. Structures and substrate activity of 7-substituted FPP
analogs. (a) Structures of synthesized 7-substituted dipho-
sphates. (b) Substrate activity of 7-substituted FPP analogs with
dansyl-GCVLS.
was unsuccessful with the harsh conditions employed for
constructing 10. The slightly acidic proton or the conjuga-
tion of the thiophene to the double bond of the homoger-
aniol backbone may have caused unwanted side reactions
with t-BuLi, thus preventing the preparation of 14d. In
order to circumvent this problem, we followed a procedure
developed by Huo to convert unactivated halides into the
corresponding organozincs without the use of strong
bases,14 which resulted in the preparation of the organo-
zinc intermediate 14d. The rest of the organozinc deriva-
tives (14aÀc and 14eÀf) were generated as previously
described for the construction of 10 (Scheme 2). Vinyl
iodide 1515,16 was then added to the organozinc solution of
14aÀf, followed by the addition of Pd(PPh3)4 at room
temperature resulting in the synthesis of 7-substituted-
farnesol derivatives (16aÀf). With the 7-substituted farne-
sol derivatives synthesized we next converted them into
their corresponding diphosphates (17aÀf) by employing a
CoreyÀKim chlorination17 of the allylic alcohol, followed
by displacement of the resulting chloride with tetrabuty-
lammonium diphosphate (Scheme 3).18 Previously, our
laboratory has shown that 7-substituted FPP analogs can
display diverse biochemical activity with FTase and a
the 7-position of FPP was the 1,3-dimethylpropenyl moiety.
Using CsF as an additive, we successfully coupled 7b to 11
using Pd(PPh3)4, in DMF to produce 9a in a 50% yield
(Scheme 2).
Once the synthesis of the 4-substituted homogeraniol
derivatives (7cÀe and 9aÀc (Schemes 1 and 2)) was
complete, we performed the final steps necessary to gen-
erate 7-substituted FPP analogs (Scheme 3). Transforma-
tionof the homogeraniol derivatives 10cÀe and 11aÀc into
the resulting iodides 13aÀf was accomplished in good
yields.13 With the alkyl iodides 13aÀf in hand, we next
focused our attention on converting the alkyl iodides into
their corresponding organozinc reagents. To achieve this,
we employed two different procedures depending on the
substituent at the four position of our homogeraniol
derivatives. We successfully employed the same strategy
to prepare the organozinc derivatives 14aÀc, eÀf as were
generated as previously described for the construction of
10 (Scheme 2). Preparation of organozinc derivative 14d
(13) Lange, G. L.; Gottardo, C. Synth. Commun. 1990, 20, 1473–
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3578
Org. Lett., Vol. 13, No. 14, 2011

variety of CaaX-peptides.4 The 7-substituted FPP com-
pounds synthesized in this study (17aÀf, Figure 1a) were
initially evaluated for their biological activity in an in vitro
fluorescence-based assay4 with FTase and the CaaX-pep-
tide, dansyl-GCVLS (H-Ras). Among the 7-substituted
compounds assayed for their in vitro biological activity,
compounds 17 a,b,e,f were found to behave as efficient
substrates (Figure 1b). In addition to the substrate activity
of the newly synthesized 7-substituted compounds, one
compound (17c) was found to act as a weak inhibitor with
an IC50 of 25 μM against FTase. Of particular importance
are the substrate activities of the two alkynyl FPP com-
pounds 17e and 17f with dansyl-GCVLS. Both com-
pounds are efficient substrates as demonstrated in the
fluorescence assay and in an HPLC confirmation assay
(data not shown). These compounds were designed with
the goal to be used as chemical tools for the tagging of
prenylated proteins via “click chemistry”.19,20 This strat-
egy has proved valuable for the dissection of the cellular
biology of various ligands with protein targets,21À24 in-
cluding FTase.25,26
The synthetic approaches developed in this study for the
synthesis of 7-substituted FPP analogs have been used to
develop several diverse FPP analogs with biological activ-
ities. The original synthetic plan3 for these compounds
required eleven steps for the completion of 7-allylfarnesyl
diphosphate (17a; overall yield =7%). The newly devel-
oped strategy allows for the rapid and efficient synthesis of
7-substituted FPP analogs; specifically, 7-allyl analog 17a
was prepared in six steps with a 12% overall yield. In
addition to significantly shortening the route, we have also
provided methodology that allows for the construction of
FPP analogs unachievable by previous methods.
Acknowledgment. This work was funded by NIH
Grants R01 CA78819 (RAG) and P30 CA21328 (Purdue
University Center for Cancer Research). We thank Dr.
Amanda J. Krzysiak (Purdue University) for assistance
with the FTase assays and Professor Carol A. Fierke
(Department of Chemistry, University of Michigan) for
assistance in obtaining FTase.
Supporting Information Available. Spectral data (1H
NMR, 13C NMR, LRMS, HRMS) for all newly synthe-
sized compounds; detailed experimental procdedures for
the synthesis of 2, 7a, 9aÀc, 13a, 15, 16a, 16d, and 17a. 1H
and 13C NMR spectra for all compounds excluding
diphosphates (17aÀf). This material is available free of
charge via the Internet at http://pubs.acs.org.
(24) Baskin, J. M.; Prescher, J. A.; Laughlin, S. T.; Agard, N. J.;
Chang, P. V.; Miller, I. A.; Lo, A.; Codelli, J. A.; Bertozzi, C. R. Proc.
Natl. Acad. Sci. U.S.A. 2007, 104, 16793–7.
(25) DeGraw, A. J.; Palsuledesai, C.; Ochocki, J. D.; Dozier, J. K.;
Lenevich, S.; Rashidian, M.; Distefano, M. D. Chem. Biol. Drug Des.
2010, 76, 460–71.
(26) Labadie, G. R.; Viswanathan, R.; Poulter, C. D. J. Org. Chem.
2007, 72, 9291–7.
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