Total synthesis of (-)-CP2-disorazole C1

Article abstract of DOI:10.1021/ol2015994

The total synthesis of a bis-cyclopropane analog of the antimitotic natural product (-)-disorazole C₁ was accomplished in 23 steps and 1.1% overall yield. A vinyl cyclopropane cross-metathesis reaction generated a key (E)-alkene segment of the

Full text of DOI:10.1021/ol2015994

ORGANIC
LETTERS
2011
Vol. 13, No. 15
4088–4091
Total Synthesis of (ꢀ)-CP2-Disorazole C1
Chad D. Hopkins,† John C. Schmitz,#,§ Edward Chu,‡,§ and Peter Wipf*,†,§
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260,
United States, VACT Cancer Research Laboratory, VACT Healthcare System,
West Haven, Connecticut 06516, United States, Division of Hematology/Oncology,
Department of Medicine and University of Pittsburgh Cancer Institute, and Molecular
Therapeutics Drug Discovery Program, Pittsburgh, Pennsylvania 15232, United States
pwipf@pitt.edu
Received June 14, 2011
ABSTRACT
The total synthesis of a bis-cyclopropane analog of the antimitotic natural product (ꢀ)-disorazole C1 was accomplished in 23 steps and 1.1%
overall yield. A vinyl cyclopropane cross-metathesis reaction generated a key (E)-alkene segment of the target molecule. IC50 determinations of
(ꢀ)-CP2-disorazole C1 in human colon cancer cell lines indicated low nanomolar cytotoxic properties. Accordingly, this synthetic bioisostere
represents the first biologically active disorazole analog not containing a conjugated diene or polyene substructure element.
The disorazoles compromise a family of ∼30 closely
related polyketide microtubule disruptors isolated since
1994 from the fermentation broth of the myxobacterium
range against a variety of transformed cell lines, including
multidrug resistant cells.3 To date, only (ꢀ)-disorazole C1
has yielded to total synthesis,4 although several simplified
analogs and segments have been reported.5 As part of our
investigations of structureꢀactivity relationships (SAR) of
biologically active natural products,2d,6 we designed a
€
Sorangium cellulosum by Jansen, Reichenbach, Hofle and
co-workers.1,2 Members of the disorazole class have dis-
played anticancer activity in the nano- and picomolar
† Department of Chemistry, University of Pittsburgh.
# VACT Cancer Research Laboratory, VACT Healthcare System.
§ Molecular Therapeutics Drug Discovery Program Pittsburgh.
‡ Division of Hematology/Oncology, Department of Medicine and Uni-
versity of Pittsburgh Cancer Institute.
(4) Wipf, P.; Graham, T. H. J. Am. Chem. Soc. 2004, 126, 15346.
(5) (a) Schaeckel, R.; Hinkelmann, B.; Sasse, F.; Kalesse, M. Angew.
Chem., Int. Ed. 2010, 49, 1619. (b) Niess, B.; Hartung, I. V.; Haustedt,
L. O.; Hoffmann, H. M. R. Eur. J. Org. Chem. 2006, 1132. (c) Hearn,
B. R.; Arslanian, R. L.; Fu, H.; Liu, F.; Gramajo, H.; Myles, D. C. J.
Nat. Prod. 2006, 69, 148. (d) Haustedt, L. O.; Panicker, S. B.; Kleinert,
M.; Hartung, I. V.; Eggert, U.; Niess, B.; Hoffmann, H. M. R. Tetra-
hedron 2003, 59, 6967. (e) Hartung, I. V.; Eggert, U.; Haustedt, L. O.;
Niess, B.; Schaefer, P. M.; Hoffmann, H. M. R. Synthesis 2003, 1844. (f)
Hartung, I. V.; Niess, B.; Haustedt, L. O.; Hoffmann, H. M. R. Org.
Lett. 2002, 4, 3239. (g) Hillier, M. C.; Price, A. T.; Meyers, A. I. J. Org.
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€
(1) Jansen, R.; Irschik, H.; Reichenbach, H.; Wray, V.; Hofle, G.
Liebigs Ann. Chem. 1994, 759.
(2) For reviews of disorazole chemistry and biology, see: (a) Hopkins,
C. D.; Wipf, P. Nat. Prod. Rep. 2009, 26, 585. (b) Weissman, K. J.;
Mueller, R. Nat. Prod. Rep. 2010, 27, 1276. (c) Kretschmer, M.; Menche,
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Chem. 2007, 79, 753.
(6) (a) Wang, Z.; McPherson, P. A.; Raccor, B. S.; Balachandran, R.;
Zhu, G.; Day, B. W.; Vogt, A.; Wipf, P. Chem. Biol. Drug Des. 2007, 70,
75. (b) Powis, G.; Wipf, P.; Lynch, S. M.; Birmingham, A.; Kirkpatrick,
D. L. Mol. Cancer Ther. 2006, 5, 630. (c) Wipf, P.; Minion, D. J.; Halter,
R. J.; Berggren, M. I.; Ho, C. B.; Chiang, G. G.; Kirkpatrick, L.;
Abraham, R.; Powis, G. Org. Biomol. Chem. 2004, 2, 1911. (d) Wipf, P.;
Fritch, P. C.; Geib, S. J.; Sefler, A. M. J. Am. Chem. Soc. 1998, 120, 4105.
(7) Examples documenting the lability of trienes include: (a) Burke,
C. P.; Haq, N.; Boger, D. L. J. Am. Chem. Soc. 2010, 132, 2157. (b)
Hicks, J. D.; Roush, W. R. Org. Lett. 2008, 10, 681. (c) Trost, B. M.;
Frederiksen, M. U.; Papillon, J. P. N.; Harrington, P. E.; Shin, S.;
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M.; Satake, M.; Tachibana, K. Org. Lett. 2002, 4, 2981.
(3) (a) Lazo, J. S.; Reese, C. E.; Vogt, A.; Vollmer, L. L.; Kitchens,
€
C. A.; Gunther, E.; Graham, T. H.; Hopkins, C. D.; Wipf, P. J.
Pharmacol. Exp. Ther. 2010, 332, 906. (b) Xu, F. L.; Rbaibi, Y.;
Kiselyov, K.; Lazo, J. S.; Wipf, P.; Saunders, W. S. BMC Chem. Biol.
2010, 10, 1. (c) Tierno, M. B.; Kitchens, C. A.; Petrik, B.; Graham, T. H.;
Wipf, P.; Xu, F. L.; Saunders, W. S.; Raccor, B. S.; Balachandran, R.;
Day, B. W.; Stout, J. R.; Walczak, C. E.; Ducruet, A. P.; Reese, C. E.;
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r
10.1021/ol2015994
Published on Web 07/08/2011
2011 American Chemical Society

cyclopropane analog of (ꢀ)-disorazole C1 (i.e., CP2-disor-
azole C1), with the goal to replace the labile7 (E,Z,Z)-triene
subunitwithanisosteric and biologically similarly effective
moiety. The choice of absolute configuration for the cis-
cyclopropanes in CP2-disorazole C1 reflected the stereochem-
istry observed for the epoxide present in disorazole A1.2a
The vinyl cyclopropane 5 was prepared in 74% yield in a
Wittig condensation from known aldehyde 45d (Scheme 3).
Scheme 3. Cross-Metathesis and Synthesis of Aldehyde 10
Scheme 1. Retrosynthetic Approach for CP2-Disorazole C1 1
Cross-metathesis reaction of alkenes 5 and 310 in the pre-
senceof asecondgeneration ruthenium catalyst11 provided
alkene 6 in 70% yield as an inseparable 10:1 mixture of
E/Z-stereoisomers. O-Methylation of 6 with silver oxide12
and methyl iodide, followed by enzymatic hydrolysis13
with pig liver esterase (PLE) and subsequent coupling of
the carboxylic acid with serine methyl ester, yielded amide
8. A two-step cyclodehydration/oxidation procedure14 led
to oxazole 9. Oxidative deprotection of the PMB ether
followed by further oxidation of the resulting primary alcohol
under DessꢀMartin conditions afforded the desired aldehyde
10 in excellent yield.
Retrosynthetically, the C2-symmetrical macrodiolide
CP2-disorazole C1 (1) could be constructed from two key
fragments, 15 and 9 (Scheme 1). Diene 15 is derived from
β-hydroxy ester 12 which can be obtained from an asym-
metric Mukaiyama aldol reaction of crotonaldehyde. By
taking advantage of a vinyl cyclopropane cross-metathesis
reaction,8 our plan was to access alkene 9 from the readily
available cyclopropane 5 and the oxazole precursor 3. This
approach was realized in excellent yields beginning with
thestraightforwardpreparationofester3intwostepsfrom
the known9 alkyne 2 (Scheme 2).
After considerable experimentation, we found that
Kiyooka’s conditions were best suited for the asym-
metric Mukaiyama aldol reaction between the sterically
(8) Nicola, T.; Brenner, M.; Donsbach, K.; Kreye, P. Org. Process
Res. Dev. 2005, 9, 513.
ꢀ
(9) (a) Salit, A.-F.; Meyer, C.; Cossy, J.; Delouvrie, B.; Hennequin, L.
Tetrahedron 2008, 64, 6684. (b) Salit, A.-F.; Meyer, C.; Cossy, J. Synlett
2007, 934.
(10) Attempts at performing the cross-metathesis of 5 with the methyl
ether of 3 or the oxazole analog of 3 afforded complex mixtures.
(11) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18.
(12) Stronger bases led to a rapid β-elimination of the C6 methyl
ether.
Scheme 2. Synthesis of Ester 3
(13) Mahato, T. K.; Babu, S.; Basak, A. Biotechnol. Lett. 1993, 15,
1147. Alternative conditions, including LiOH, KOH, TMSOK, (Bu3Sn)2O,
and Me3SnOH, led to significant β-elimination at C6.
(14) Phillips, A. J.; Uto, Y.; Wipf, P.; Reno, M. J.; Williams, D. R.
Org. Lett. 2000, 2, 1165.
Org. Lett., Vol. 13, No. 15, 2011
4089

hindered silyl ketene acetal 11 and crotonaldehyde
(Scheme 4).5a,15
Scheme 4. Synthesis of Hydroxy Ester Segment 18
The R,R-dimethylated aldol product 12 was obtained in
79% yield and 93% ee as determined by chiral HPLC
analysis. Protection of the secondary alcohol in 12 with
3,4-dimethoxybenzyl (DMB) bromide16 followed by ester
reduction with LAH and oxidation of the primary alcohol
under Swern conditions provided aldehyde 13. Among
contemporary allylation protocols,17 we found that the
DuthalerꢀHafner allylation18 of aldehyde 13 gave alcohol
14 in the highest (87%) yield and with a satisfactory dr =
4.8:1.0, favoring the desired anti-configuration of the 1,
3-diol. The minor isomer was present in diminishing
amounts in subsequent intermediates until it was finally
completely removed in the Wittig reaction of 17 and 10.
At first, the advancement of homoallylic alcohol 14 to
the desired phosphonium salt 17 proved problematic.
Upon formation of the alkyl iodide at C12, an in situ
SN2-displacement of the iodide by the electron-rich DMB
ether at C16 afforded the corresponding tetrahydropyran
as the major product.19 Although small quantities of the
desired alkyl iodide were isolated, immediate formation of
the tetrahydropyran occurred upon heating. Conse-
quently, TES protection of the alcohol at C14 followed
by the exchange of the DMB with a TBS ether at C16 was
used to access the orthogonally silyl-protected diol 15
(Scheme 4). Stepwise JohnsonꢀLemieux oxidation fol-
lowed by NaBH4 reduction afforded the primary alcohol
16. Iodination and phosphonium salt generation at ele-
vated temperature in a sealed vessel proceeded now with-
out incident to furnish 17 in good overall yield. The Wittig
condensation of the freshly formed phosphonium salt with
aldehyde 10 in a vigorously degassed THF solution yielded
86% of the desired Z-alkene as a single stereoisomer by 1H
NMR analysis. Finally, selective TES deprotection with
PPTs in the presence of the allylic TBS ether proceeded in
65% yield to afford the advanced segment 18.20
15-membered macrolactone arising from the direct lacto-
nization at the C14 hydroxyl group, i.e. the formation of
the cyclic monomer.21 To circumvent this undesired reac-
tion, a stepwise approach was adopted (Scheme 5). Hydro-
lysis of ester 9 followed by DCC-mediated coupling to the
secondary alcohol 18 provided the bis-cyclopropane 19 in
89% yield. Oxidative PMB deprotection with DDQ and
subsequent DessꢀMartin oxidation of the resulting pri-
mary alcohol led to the requisite cyclopropyl aldehyde in
good yield. Condensation of this aldehyde and the ylide
generated from phosphonium salt 17 after vigorous degas-
sing afforded the seco-dimer 20 in 74% yield as a single
alkene stereoisomer.
Selective TES ether deprotection of 20 revealed the
secondary alcohol at C140 (Scheme 5). With all functional
groups properly installed, the stage was now set for the final
lactone formation. Selective saponification of the methyl
ester in the presence of the internal ester linkage of 20 with
barium hydroxide followed by Shiina macrolactonization22
with 2-methyl-6-nitrobenzoic anhydride (MNBA) and
DMAP provided the desired 30-membered macrocycle in
55% yield over the two steps. Finally, double TBS ether
deprotection in the presence of HF-pyridine proceeded in
64% yield to the desired (ꢀ)-CP2-disorazole C1.
Originally, our end game strategy had envisioned the
direct cyclo-dimerization of the carboxylic acid corre-
sponding to 18. However, all efforts to realize this cyclo-
dimerization resulted in the exclusive formation of the
(15) Kiyooka, S.; Kaneko, Y.; Komura, M.; Matsuo, H.; Nakano,
M. J. Org. Chem. 1991, 56, 2276.
(16) Use of the more electron rich 3,4-DMB ether suppressed over-
oxidation of the allylic alcohol at C16 during DDQ deprotection.
(17) Lu, Z.; Ma, S. Angew. Chem., Int. Ed. 2008, 47, 258.
(18) (a) Hafner, A.; Duthaler, R. O.; Marti, R.; Rihs, G.; Rothe-
Streit, P.; Schwarzenbach, F. J. Am. Chem. Soc. 1992, 114, 2321. (b)
Cossy, J.; Willis, C.; Bellosta, V.; BouzBouz, S. J. Org. Chem. 2002, 67,
1982.
(19) Jarosz, S.; Gajewska, A.; Luboradzki, R. Tetrahedron: Asym-
metry 2008, 19, 1385.
(20) NOE analysis confirmed the cis-cyclopropane stereochemistry.
(21) Varying mixtures of the cyclo-monomer 21 and its correspond-
ing C16 TBS-protected derivative were isolated, and their structures
confirmed by MALDI-TOF MS. Even upon attempted stepwise cou-
pling of the two monomers, 21 was formed by an apparent in situ
deprotection of the TES group of the acid component.
(22) (a) Shiina, I.; Kubota, M.; Oshiumi, H.; Hashizume, M. J. Org.
Chem. 2004, 69, 1822. (b) Schweitzer, D.; Kane, J. J.; Strand, D.;
McHenry, P.; Tenniswood, M.; Helquist, P. Org. Lett. 2007, 9, 4619.
4090
Org. Lett., Vol. 13, No. 15, 2011

Table 1. Cytotoxic Activity of 1 and Cyclic Monomer 21 in
Human Colon Cancer Cell Linesa,b
Scheme 5. Segment Condensation and Shiina Macrolactoniza-
tion
cell
1
21
vincristine
IC50 [nM]c
line
IC50 [nM]c
IC50 [μM]c
RKO
28.0 ( 9.2
28.3 ( 11.6
49.5 ( 25.0
>50
>50
>50
18.6 ( 7.6
35.2 ( 11.9
68.0 ( 16.3
HCT116
H630
a Vincristine is used as a reference compound.23 b Cell proliferation
and viability were quantified by the WST-1 assay (Roche Applied
Science; Indianapolis, IN). c Values represent the mean ( SD from 4
to 5 independent determinations.
Insummary, wehave developed anefficient routeto(ꢀ)-
CP2-disorazole C1 1, a bis-cyclopropane analog of the
antimitotic natural product disorazole C1.24 The target
agent was obtained in 23 steps and 1.1% overall yield for
the longest linear sequence. It contains 10 stereogenic
carbons, 4 more than disorazole C1, and 6 double bonds,
2 less than the natural product. Most importantly, by
replacing the central cis-alkenes at C9ꢀC10/C90-C100 of
disorazole C1 with cis-cyclopropane rings, 1 no longer has
a labile conjugated (E,Z,Z)-triene; yet, it shows nearly the
same potent level of low nanomolar in vitro cytotoxicity in
human colon cancer cell lines as the natural myxobacter-
ium metabolite. Further biological studies of 1 will be
reported in due course.
Acknowledgment. This work was supported in part by
the NCI (CA78039) and the NSF (CHE-0910560). The
authors thank Mr. Tian-min Chen (VACT Cancer Re-
search Laboratory, West Haven, CT) for biological assays,
and Profs. John Lazo (University of Virginia, Charlottes-
ville, VA), Billy Day (University of Pittsburgh, Pittsburgh,
PA), and Andreas Vogt (University of Pittsburgh, Pitts-
burgh, PA) for stimulating discussions and collaborative
studies on antitubulin agents.
Table 1 illustrates our preliminary analysis of the
antiproliferative properties of this novel disorazole
analog. In three human colon cancer cell lines, 1 dis-
played low IC50 values ranging from 25 to 50 nM and,
thus, proved to be only slightly less active than its
parent, (ꢀ)-disorazole C1.3aꢀd,23 Also, 1 was equipo-
tent to the clinically used anticancer vinca alkaloid
vincristine in these colon cancer cells. In contrast,
cyclomonomer 21 was inactive at concentrations up
to 50 μM in all three cell lines, thereby serving as a
negative control and confirming the importance of the
3-D architecture of the disorazoles.3d
Supporting Information Available. Experimental pro-
cedures and spectral data for all new compounds,
including copies of 1H and 13C NMR spectra. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(23) Disorazole C1 was not available for a direct comparison in these
preliminary assays. Ref. 3c reports the IC50 of disorazole C1 and
vincristine in HCT116 cells as 1.09 ( 0.41 and 5.62 ( 0.33 nM,
respectively. Unsurprisingly, there is some variability in the common
standard vincristine in these two assays. However, in both batches of
HCT116 cells, disorazole C1 and 1 are more potent than vincristine by a
factor of 5 and 1.2, respectively, making disorazole C1 ca. 4-fold more
active than its cyclopropane analog 1.
(24) For cyclopropane analogs of natural products, see: (a) Cachoux,
F.; Isarno, T.; Wartmann, M.; Altmann, K.-H. Synlett 2006, 1384. (b)
Wipf, P.; Reeves, J. T.; Balachandran, R.; Day, B. W. J. Med. Chem.
2002, 45, 1901.
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