Streamlined syntheses of (-)-dictyostatin, 16-desmethyl-25,26- dihydrodictyostatin, and 6- epi -16-desmethyl-25,26-dihydrodictyostatin

Article abstract of DOI:10.1021/ja103537u

The dictyostatins are a promising class of potential anti-cancer drugs because they are powerful microtubule-stabilizing agents, but the complexity of their chemical structures is a severe impediment to their further development. On the basis of both synt

Full text of DOI:10.1021/ja103537u

Published on Web 06/14/2010
Streamlined Syntheses of (-)-Dictyostatin,
16-Desmethyl-25,26-dihydrodictyostatin, and
6-epi-16-Desmethyl-25,26-dihydrodictyostatin
Wei Zhu,† Mar´ıa Jime´nez,† Won-Hyuk Jung,† Daniel P. Camarco,‡
Raghavan Balachandran,§ Andreas Vogt,‡ Billy W. Day,*,†,§ and Dennis P. Curran*,†
Departments of Chemistry, Pharmacology & Chemical Biology, and Pharmaceutical Sciences,
UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261
Received April 26, 2010; E-mail: bday@pitt.edu; curran@pitt.edu
Abstract: The dictyostatins are a promising class of potential anti-cancer drugs because they are powerful
microtubule-stabilizing agents, but the complexity of their chemical structures is a severe impediment to
their further development. On the basis of both synthetic and medicinal chemistry analyses, 16-desmethyl-
25,26-dihydrodictyostatin and its C6 epimer were chosen as potentially potent yet accessible dictyostatin
analogues, and three new syntheses were developed. A relatively classical synthesis involving vinyllithium
addition and macrocyclization gave way to a newer and more practical approach based on esterification
and ring-closing metathesis reaction. Finally, aspects of these two approaches were combined to provide
a third new synthesis based on esterification and Nozaki-Hiyama-Kishi reaction. This was used to prepare
the target dihydro analogues and the natural product. All of the syntheses are streamlined because of their
high convergency. The work provided several new analogues of dictyostatin, including a truncated
macrolactone and a C10 E-alkene, which were 400- and 50-fold less active than (-)-dictyostatin,
respectively. In contrast, the targeted 16-desmethyl-25,26-dihydrodictyostatin analogues retained almost
complete activity in preliminary biological assays.
Introduction
After resting in the shadow of the important natural product
(+)-discodermolide for about a decade,1 (-)-dictyostatin (1)
has recently emerged as a potentially useful anti-cancer agent
in its own right. Pettit isolated dictyostatin in tiny quantities
from a marine sponge in 1994, showed its potent anti-cancer
activity, and provided its constitution.2 Several years later,
Wright isolated enough sample to garner information about the
mechanism of action3 and to collect a full battery of NMR
spectra in collaboration with Paterson. These spectra were the
basis for a complete assignment of configuration in 20044 that
was confirmed almost immediately by simultaneous reports of
total syntheses by Paterson5 and our group.6
These total syntheses provided precious samples and thereby
paved the way for a detailed biological characterization of
dictyostatin.7 It is one of the most potent microtubule-stabilizing
agents known, inducing the assembly of purified tubulin even
more rapidly than paclitaxel. It inhibits the binding of paclitaxel,
discodermolide, and epothilone B to microtubules. It is a
powerful anti-proliferative agent in several standard and pacli-
taxel-resistant cancer cell lines.
† Department of Chemistry.
‡ Department of Pharmacology & Chemical Biology.
§ Department of Pharmaceutical Sciences.
(1) (a) Kingston, D. G. I. J. Nat. Prod. 2009, 72, 507–515. (b) Gunasekera,
S. P.; Wright, A. E. In Anticancer Agents from Natural Products;
Cragg, G. M., Kingston, D. G. I., Newman, D. J., Eds.; CRC Taylor
Francis: New York, NY, 2005; pp 171-189. (c) Paterson, I.; Anderson,
E. A. Science 2005, 310, 451–453.
There has also been a lively interplay between synthesis and
structure-activity relationship (SAR) studies in the dictyostatin
(2) (a) Pettit, G. R.; Cichacz, Z. A.; Gao, F.; Boyd, M. R.; Schmidt, J. M.
J. Chem. Soc., Chem. Commun. 1994, 1111–1112. (b) Pettit, G. R.;
Cichacz, Z. A. U.S. Patent 5,430,053, 1995; 8 pp.
(3) (a) Wright, A. E.; Cummins, J. L.; Pomponi, S. A.; Longley, R. E.;
Isbrucker, R. A. PCT Int. Appl. WO20010162239, 2001; 30 pp. (b)
Isbrucker, R. A.; Cummins, J.; Pomponi, S. A.; Longley, R. E.; Wright,
A. E. Biochem. Pharmacol. 2003, 66, 75–82.
(6) Shin, Y.; Fournier, J. H.; Fukui, Y.; Bru¨ckner, A. M.; Curran, D. P.
Angew. Chem., Int. Ed. 2004, 43, 4634–4637.
(7) (a) Madiraju, C.; Edler, M. C.; Hamel, E.; Raccor, B. S.; Balachandran,
R.; Zhu, G.; Giuliano, K. A.; Vogt, A.; Shin, Y.; Fournier, J. H.; Fukui,
Y.; Bru¨ckner, A. M.; Curran, D. P.; Day, B. W. Biochemistry 2005,
44, 15053–15063. (b) Buey, R.; Barasoain, I.; Jackson, E.; Meyer,
A.; Giannakakou, P.; Paterson, I.; Mooberry, S.; Andreu, J. M.; Diaz,
J. F. Chem. Biol. 2005, 12, 1269–1279.
(4) Paterson, I.; Britton, R.; Delgado, O.; Wright, A. E. Chem. Commun.
2004, 632–633.
(5) Paterson, I.; Britton, R.; Delgado, O.; Meyer, A.; Poullennec, K. G.
Angew. Chem., Int. Ed. 2004, 43, 4629–4633.
9
10.1021/ja103537u  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 9175–9187 9175

A R T I C L E S
Zhu et al.
class. The original syntheses have been refined and improved,8
new total syntheses by Phillips9 and Ramachandran10 have
appeared, and partial syntheses of key fragments have provided
additional new options.11 Building on these foundations, our
group12 and Paterson’s13 have made an assortment of analogues
and stereoisomers of dictyostatin either by modifications of
existing synthetic routes or by introducing new routes. A clear
outline of the dictyostatin SAR has emerged,14 and several
potent analogues with potential advantages over dictyostatin
itself have been identified. Most importantly, in the first in vivo
studies of any dictyostatin, synthetic analogue 6-epi-dictyostatin
was more effective than paclitaxel in treating mice bearing
human breast cancer xenografts.15
An important next step in the field would be to advance
dictyostatin, 6-epi-dictyostatin, or another analogue farther down
the road of preclinical development toward possible clinical
trials. The current syntheses of dictyostatin are, however, at the
outer limit of today’s technology for scale-up, and the active
analogues of dictyostatin are not that much simpler to make
than is the parent. So, there is a need for streamlined syntheses
Figure 1. High-level plan of the vinyllithium approach.
(8) Review: Florence, G. J.; Gardner, N. M.; Paterson, I. Nat. Prod. Rep.
2008, 25, 342–375.
of the class and for discovery of analogues with biological
profiles comparable to that of dictyostatin but that are easier to
make. Here we report advances toward both of these objectives.
Based on SAR and metabolic lability analyses,12f,16 we
identified 16-desmethyl-25,26-dihydrodictyostatin and its 6-epimer
as potential analogues of dictyostatin that might be active yet
considerably easier to make. We have developed three new,
streamlined routes to this class of compounds that are based on
vinyllithium addition, ring-closing metathesis (RCM), and
Nozaki-Hiyama-Kishi (NHK) coupling. Along the way, we
implemented a straightforward esterification method to make
the problematic O22-C1 bond. In contrast to related macro-
lactonization reactions, no isomerization of the adjacent C2-C3
alkene was observed. To culminate the synthesis studies, the
NHK approach was used to make (-)-dictyostatin itself. This
is the first synthesis in which all of the carbon atoms of the
molecule are built into the three main fragments so that none
are introduced after fragment couplings begin. The increased
convergency makes this the shortest current synthesis of
dictyostatin. Finally, both of the new analogues exhibit interest-
ing results in preliminary biological assays.
(9) O’Neil, G. W.; Phillips, A. J. J. Am. Chem. Soc. 2006, 128, 5340–
5341.
(10) Ramachandran, P. V.; Srivastava, A.; Hazra, D. Org. Lett. 2007, 9,
157–160.
(11) (a) O’Neil, G. W.; Phillips, A. J. Tetrahedron Lett. 2004, 45, 4253–
4256. (b) Kangani, C. O.; Bru¨ckner, A. M.; Curran, D. P. Org. Lett.
2005, 7, 379–382. (c) Baba, V. S.; Das, P.; Mukkanti, K.; Iqbal, J.
Tetrahedron Lett. 2006, 47, 7927–7930. (d) Jagel, J.; Maier, M. E.
Synlett 2006, 693–696. (e) Prusov, E.; Rohm, H.; Maier, M. E. Org.
Lett. 2006, 8, 1025–1028. (f) Shaw, S. J.; Zhang, D.; Sundermann,
K. F.; Myles, D. C. Synth. Commun. 2006, 36, 1735–1743. (g) Gennari,
C.; Castoldi, D.; Sharon, O. Pure Appl. Chem. 2007, 79, 173–180.
(h) Monti, C.; Sharon, O.; Gennari, C. Chem. Commun. 2007, 4271–
4273. (i) Moura-Letts, G.; Curran, D. P. Org. Lett. 2007, 9, 5–8. (j)
Saibaba, V.; Sampath, A.; Mukkanti, K.; Iqbal, J.; Das, P. Synthesis
2007, 2797–2802. (k) Sharon, O.; Monti, C.; Gennari, C. Tetrahedron
2007, 63, 5873–5878. (l) Zanato, C.; Pignataro, L.; Hao, Z.; Gennari,
C. Synthesis 2008, 2158–2162. (m) Dias, L. C.; Lima, D. J. P.;
Gonc¸alves, C. C. S.; Andricopulo, A. D. Eur. J. Org. Chem. 2009,
10, 1491–1494. (n) Dilger, A. K.; Gopalsamuthiram, V.; Burke, S. D.
J. Am. Chem. Soc. 2007, 129, 16273–16277.
(12) (a) Shin, Y.; Choy, N.; Turner, T. R.; Balachandran, R.; Madiraju,
C.; Day, B. W.; Curran, D. P. Org. Lett. 2002, 4, 4443–4446. (b)
Shin, Y.; Fournier, J. H.; Balachandran, R.; Madiraju, C.; Raccor, B. S.;
Zhu, G.; Edler, M. C.; Hamel, E.; Day, B. W.; Curran, D. P. Org.
Lett. 2005, 7, 2873–2876. (c) Fukui, Y.; Bru¨ckner, A. M.; Shin, Y.;
Balachandran, R.; Day, B. W.; Curran, D. P. Org. Lett. 2006, 8, 301–
304. (d) Jung, W. H.; Harrison, C.; Shin, Y.; Fournier, J. H.;
Balachandran, R.; Raccor, B. S.; Sikorski, R. P.; Vogt, A.; Curran,
D. P.; Day, B. W. J. Med. Chem. 2007, 50, 2951–2966. (e) Shin, Y.;
Fournier, J.-H.; Bru¨ckner, A.; Madiraju, C.; Balachandran, R.; Raccor,
B. S.; Edler, M. C.; Hamel, E.; Sikorski, R. P.; Vogt, A.; Day, B. W.;
Curran, D. P. Tetrahedron 2007, 63, 8537–8562. (f) Raccor, B. S.;
Vogt, A.; Sikorski, R. P.; Madiraju, C.; Balachandran, R.; Montgom-
ery, K.; Shin, Y.; Fukui, Y.; Jung, W.-H.; Curran, D. P.; Day, B. W.
Mol. Pharmacol. 2008, 73, 718–726.
Results and Discussion
Analogue Design and Vinyllithium Addition Route. The new
analogues targeted in this work are 16-desmethyl-25,26-dihy-
drodictyostatin (2b) and its 6-epimer, 2a (see Figure 1). These
were selected for several reasons. First, removal of the C16
methyl group deletes an isolated stereocenter, thereby simplify-
ing the synthesis. 16-Desmethyldictyostatin is a known com-
pound with an interesting biological profile.12b,13c Second,
metabolism16 and SAR work in the discodermolide area17
suggested that modification of the terminal C25-C26 alkene
of dictyostatin, while retaining good activity, might be possible.
Reduction of this alkene provides a dihdyro analogue that is
not much simpler than the parent. We felt, however, that
(13) (a) Paterson, I.; Gardner, N. M. Chem. Commun. 2007, 49–51. (b)
Paterson, I.; Gardner, N. M.; Poullenneca, K. G.; Wright, A. E. Bioorg.
Med. Chem. Lett. 2007, 17, 2443–2447. (c) Paterson, I.; Gardner,
N. M.; Guzma´n, E.; Wright, A. E. Bioorg. Med. Chem. Lett. 2008,
18, 6268–6272. (d) Paterson, I.; Gardner, N. M.; Poullennec, K. G.;
Wright, A. E. J. Nat. Prod. 2008, 71, 364–369. (e) Paterson, I.; Naylor,
G. J.; Wright, A. E. Chem. Commun. 2008, 4628–4630. (f) Paterson,
I.; Gardner, N. M.; Guzma´n, E.; Wright, A. E. Bioorg. Med. Chem.
2009, 17, 2282–2289. (g) Paterson, I.; Gardner, N. M.; Naylor, G. J.
Pure Appl. Chem. 2009, 81, 169–180. (h) Paterson, I.; Naylor, G. J.;
Fujita, T.; Guzma´n, E.; Wright, A. E. Chem. Commun. 2010, 261–
263.
(16) Fan, Y.; Schreiber, E. M.; Day, B. W. J. Nat. Prod. 2009, 72, 1748–
1754.
(17) (a) Choy, N.; Shin, Y.; Nguyen, P. Q.; Curran, D. P.; Balachandran,
R.; Madiraju, C.; Day, B. W. J. Med. Chem. 2003, 46, 2846–2864.
(b) Minguez, J. M.; Kim, S.-Y.; Giuliano, K. A.; Balachandran, R.;
Madiraju, C.; Day, B. W.; Curran, D. P. Bioorg. Med. Chem. 2003,
11, 3335–3357. (c) Smith, A. B., III; Xian, M. Org. Lett. 2005, 7,
5229–5232.
(14) Canales, A.; Matesanz, R.; Gardner, N. M.; Andreu, J. M.; Paterson,
I.; D´ıaz, J. F.; Jime´nez-Barbero, J. Chem.sEur. J. 2008, 14, 7557–
7569.
(15) Eiseman, J. L.; Bai, L.; Jung, W.-H.; Moura-Letts, G.; Day, B. W.;
Curran, D. P. J. Med. Chem. 2008, 51, 6650–6653.
9
9176 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
Scheme 1. Synthesis of Top Fragment 5, C18-C26
were already in hand through an efficient six-step route based
on cross-metathesis.11i All three fragments were readily made
on a multigram scale.
The initial synthesis of combined top-middle fragment 3
(C10-C26) is summarized in Scheme 3. HWE coupling of
fragments 5 and 6 gave enone 13 in 91% yield. The R,ꢀ-alkene
of enone 13 was selectively reduced with Stryker’s reagent in
95% yield.22 Stereoselective reduction of the ketone with Li(t-
BuO)3AlH provided the alcohol as an 85:15 mixture of C19
epimers in 83% yield. After column chromatography to remove
the minor epimer, alcohol 14 was then protected as the TBS
ether in quantitative yield, and the primary TBS group was
removed with a solution of HF ·pyridine in pyridine/THF. The
resulting alcohol was subsequently treated with Dess-Martin
reagent to give aldehyde 15, which was immediately subjected
to Wittig reaction23 to install the vinyl iodide functionality in
top-middle fragment 3 in 66% yield over two steps.
Scheme 2. Synthesis of Middle Fragment 6, C11-C17
The more convergent route to top-middle fragment 3,
featuring the complete middle fragment 7, is summarized in
Scheme 4. First, Marshall’s palladium-catalyzed addition reac-
tion of the allenylzinc reagent derived from (R)-2-mesyloxy-
3-butyne to aldehyde 1624 gave alcohol 17 (82%, dr >95:5).25
After protection of alcohol 17 as the TBS ether, the acetylene
was deprotonated, iodinated, and reduced with o-nitrobenzene-
sulfonyl hydrazide26 (NBSH) to provide Z-alkenyl iodide 18
(95%). Deprotection of primary TBS ether (97%) followed by
Dess-Martin oxidation furnished the new middle fragment 7
(90%). This was then coupled to fragment 5 to provide top-
middle fragment 3 (via enone 19 and alcohol 20) in good overall
yield by using the same sequence of steps as in Scheme 3. Note
that only four steps are required to produce top-middle fragment
3 from the start of the fragment coupling (fragments 5 + 7) in
Scheme 4, whereas seven steps are needed to produce it from
the same point (fragments 5 + 6) in Scheme 3.
synthetic opportunities to make such molecules could be greatly
expanded because dictyostatin has five alkenes and a sixth is
present during the synthesis. Due to its exposure and therefore
high reactivity, the C25-C26 monosubstituted alkene of the
terminal diene complicates reactions such as metathesis or
hydrogenation that are used to make or manipulate other alkenes.
Such complications have been avoided by late introduction of
one (or both) of the two dienes, but this sacrifices convergency.
Finally, the terminal double bond of discodermolide is a
metabolic soft spot,16 and other dihydro analogues of dictyostatin
at C10,C11 and C2,C3 are biologically active.13d,g
Coupling of the bottom fragments to top-middle fragment 3
and completion of the syntheses of 16-desmethyl-25,26-dihy-
drodictyostatin (2b) and its 6-epimer 2a are summarized in
Scheme 5. Yields and selectivities for both epimers were
comparable (see data in the scheme), so we focus here only on
the (6S)-epimer (the “a” series). Lithium-iodine exchange with
top-middle fragment 3 followed by addition of aldehyde 4a
provided a mixture of three compounds that were separated by
flash chromatography to provide C-9 epimers 21a-r (29%) and
21a-ꢀ (19%), together with the de-iodinated byproduct 22
(30%).
We adopted the strategy shown in Figure 1 to make the new
analogues 2a,b. Three main fragmentsstop (C18-C26, 5),
middle (C11-C17, 6 or C10-C17, 7), and bottom (C1-C9,
4)swere targeted. The commonly used Horner-Wadsworth-
Emmons (HWE) reaction18 first joins the top and middle
fragments to give 3, while the vinyllithium addition later
appends the bottom fragment. We have studied related vinyl-
lithium approaches,12d and Ramachandran has used a vinylzinc
addition as a key step in his synthesis.10 We initially used known
middle fragment 6 because it was available from prior work;
however, it lacks a carbon atom of the target (C10) and therefore
does not meet our objective of maximum convergency. We
therefore later directly incorporated vinyl iodide 7 as the middle
fragment to increase the convergency.
The top (C18-C26) fragment 5 was readily made in five steps
from the well-known intermediate 819 as summarized in Scheme
1. Briefly, Z-selective Wittig reaction to give 9, then desilylation
and Swern oxidation, provided an aldehyde (10) that was
converted to the ketophosphonate 5 in the usual way. Full details
of these and all reactions to make new intermediates and
products are provided in the Supporting Information. Similarly,
initial middle fragment 6 (Scheme 2) was made from homoal-
lylic alcohol 1120 by cross-metathesis with crotonaldehyde21
followed by reduction. Both epimers of the bottom fragment 4
The established end-game was then followed with C9
R-epimer 21a-r to complete the synthesis. Protection of the
alcohol with TBSOTf (23a, 91%), followed by deprotection of
the PMB group with DDQ, provided C21 alcohol 24a in 85%
yield. The C1 methyl ester was then hydrolyzed by using KOH
(21) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 11360–11370.
(22) (a) Mahoney, W. S.; Brestensky, D. M.; Stryker, J. M. J. Am. Chem.
Soc. 1988, 110, 291–293. The alkene was also reduced under nickel
boride conditions (NiCl2, NaBH4), but the target product was mixed
with a byproduct tentatively assigned as the C20 epimer. These were
not separable by flash chromatography, even in subsequent steps.
(23) White, J. D.; Jensen, M. S. J. Am. Chem. Soc. 1995, 117, 6224–6233.
(24) Paterson, I.; Tudge, M. Tetrahedron 2003, 59, 6833–6850.
(25) Marshall, J. A.; Yanik, M. M.; Adams, N. D.; Ellis, K. C.; Chobanian,
H. R. Org. Synth. 2005, 81, 157–170.
(18) Paterson, I.; Yeung, K.-P.; Sunil, J. B. Synlett 1993, 774–776.
(19) Curran, D. P.; Furukawa, T. Org. Lett. 2002, 4, 2233–2235.
(20) Roush, W. R.; Palkowitz, A. D.; Palmer, M. J. J. Org. Chem. 1987,
52, 316–318.
(26) (a) Myers, A. G.; Zheng, B.; Movassaghi, M. J. Org. Chem. 1997,
62, 7507. (b) Fuwa, H.; Goto, T.; Sasaki, M. Org. Lett. 2008, 10,
2211–2214.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9177

A R T I C L E S
Zhu et al.
Scheme 3. Initial Synthesis of Top-Middle Fragment 3, C10-C26
Scheme 4. Improved Synthesis of Top-Middle Fragment 3, C10-C26
Scheme 5. End-game of the Vinyllithium Route
or TMSOK27 (which gave a better yield) to produce a seco-
acid. Macrolactonization of the seco-acid using the Shiina
reagent (2,6-methylnitrobenzoyl anhydride)28 in toluene gave
the crude protected macrolactone in 93% yield as a 60:40
mixture of the target (2Z,4E)-dienyl lactone and its (2E,4E)-
isomer (not shown). Global deprotection of the isomer mixture
with HCl followed by isomer separation provided the target
analogue 2a (6 mg, 49% over three steps). Likewise, the (6R)-
epimer 2b was prepared on a 1.6 mg scale by a similar sequence
of reactions starting from top-middle fragment 3 and bottom
fragment 4b. Again, (6R)-epimer 2b had to be separated from
its (2E,4E)-isomer.
This route is considerably shorter than other syntheses of
dictyostatin, needing only 10 steps from the start of the fragment
coupling to the end. The three fragments are also made in 10
steps or fewer, for a longest linear sequence of about 20 steps.
On the down side, the key vinyllithium addition was low
yielding and did not scale well,29 and the formation of substantial
(27) Denmark, S. E.; Regens, C. S.; Kobayashi, T. J. Am. Chem. Soc. 2007,
129, 2774–2776.
(28) (a) Shiina, I.; Ibuka, R.; Kubota, M. Chem. Lett. 2002, 286–287. (b)
Shiina, I.; Kubota, M.; Ibuka, R. Tetrahedron Lett. 2002, 43, 7535–
7359. (c) Shiina, I.; Oshiumi, H.; Hashizume, M.; Yamai, Y. S.; Ibuka,
R. Tetrahedron Lett. 2004, 45, 543–547. (d) Shiina, I.; Kubota, M.;
Oshiumi, H.; Hashizume, M. J. Org. Chem. 2004, 69, 1822–1830.
9
9178 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
Scheme 6. Bimolecular Esterification by Shiina Coupling and
through an Acid Chloride
25. The strategy is attractive because it could require as few as
eight steps from the beginning of the fragment couplings to the
target. Four of those steps (the top/middle coupling sequence,
Scheme 4) were already well established and reliable. But, in
addition to the chemoselectivity risk, there is also a risk
regarding stereoselectivity. The previous RCM approaches to
the C10-C11 Z-alkene used a temporary 10-membered lactone
to ensure Z-selectivity in the RCM.9,11b Here, there is no such
insurance. In part because we already had available both
fragments 22 and 27, we decided to take on the RCM challenge
uninsured.
E/Z-Isomerization during macrolactonization has been a
persistent problem in our and other syntheses of dictyostatins.6,12
The use of the Shiina reagent28 can sometimes suppress this
isomerization, so we first tried esterification under the preferred
conditions for macrolactonization. Alcohol 28 (1 equiv) and acid
27 (2 equiv) were reacted with the Shiina reagent (4 equiv) in
the presence of excess Et3N and DMAP (Scheme 6). An 81%
yield of ester 26a was obtained, but this was a 58:42 mixture
of target (2Z,4E)- and isomerized (2E,4E)-dienyl esters. Ap-
parently, the alkene isomerization is not associated with
macrolactonization per se.
Figure 2. RCM route to the C10,C11 alkene.
amounts of (2E,4E)-isomer of the final macrolactonization was
a persistent problem. (The E,E-isomers are not very active.12d-f
)
Because the final products 2a,b showed promising preliminary
biological activity (see below), we decided to pursue further
improvements to the synthesis.
Esterification/Ring-Closing Metathesis Routes. Previous routes
to dictyostatin and analogues almost all used macrolactonization
as a penultimate step to close the ring. In the lone exception to
date, O’Neil and Phillips closed the ring at the C2-C3 alkene
by an intramolecular Still-Gennari reaction.9 In recent years,
the traditional late-stage lactonization strategy for making
macrolactones has received competition from a strategy involv-
ing esterification followed by late-stage ring-closing metathe-
sis.30 This strategy is challenging for dictyostatin itself because
chemoselectivity issues with the C10-C11 alkene and the two
dienes arise. We11b and Phillips introduced RCM strategies to
join bottom and middle fragments at the C10-C11 alkene, and
Phillips carried his intermediate through to make dictyostatin.9
In such approaches, however, both dienes have to be constructed
after the initial fragment couplings.
On the basis of an understanding of cross-metathesis chem-
istry of dienyl esters,11i,31 we hypothesized that the simple tactic
of saturating the C25-C26 terminal alkene would enable late-
stage RCM reactions to make molecules like targets 2a,b.
Accordingly, we drew up the strategy shown in Figure 2
involving fragments 27 and 28. The top-middle fragment 28
(C11-C26) comes from deprotection of the PMB group of
alkene 22 and is coupled with fragment 27 (C1-C10) to
assemble the complete carbon skeleton of diene 26a. Finally,
RCM builds the C10-C11 bond and completes the macrocycle
Roush has suggested that related isomerizations are induced
by the acylation catalyst,32 here DMAP, but Yamaguchi and
Shiina reactions in this series do not work if the DMAP is
omitted. Accordingly, we transformed acid 27 to the corre-
sponding acid chloride 29 with the Ghosez reagent (1-choro-
1-(dimethylamino)-2-methyl-2-propene).33 Alcohol 28 (1 equiv)
was deprotonated with NaHMDS, and then crude acid chloride
29 (1 equiv) was added. After standard workup and chroma-
tography, the Z,E-isomer 26a was obtained in 75% isolated yield
along with 5% of recovered 28. There was no evidence for
1
formation of the E,E-isomer in the H NMR spectrum of the
crude product 26a.
The acid chloride esterification method provided general
access to the late-stage compounds needed for both this RCM
(29) Jung, W.-H. Ph.D. Thesis, University of Pittsburgh, 2007 (http://
etd.library.pitt.edu/ETD/available/etd-03252008-125826/).
(32) Roush, W. R.; Blizzard, T. A. J. Org. Chem. 1984, 49, 1772–1783.
(33) Devos, A.; Remion, J.; Frisque-Hesbain, A. M.; Colens, A.; Ghosez,
L. J. Chem. Soc., Chem. Commun. 1979, 1180–1181.
(30) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012–3043.
(31) Funk, T. W.; Efskind, J.; Grubbs, R. H. Org. Lett. 2005, 7, 187–190.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9179

A R T I C L E S
Zhu et al.
Scheme 7. RCM Reaction of Diene 26a Provides Truncated
Products by RRCM
We ascribe the outcome of this RCM reaction to the initial
formation of ruthenium carbene on C10 rather than C11, as
shown in Figure 4. Instead of reacting with the terminal alkene
at C11 to afford the 22-membered ring product, the carbene
intermediate 33 reacted with the internal alkene at C5 to give
a new ruthenium carbene (34) with extrusion of cyclohexene
30. Carbene 34 then reacted with the terminal alkene to furnish
the 16-membered ring product 31. This failure nonetheless
encouraged us because the C23-C24 alkene was a passive
spectator in the process and because the structure of the side
products suggested that both terminal alkenes could participate
in RCM reactions.
In a sense, the ring-contracted product 31 was formed by a
so-called “relay ring-closing metathesis” (RRCM) reaction.34
We therefore first attempted to derail this RRCM reaction with
a better RRCM option by designing substrate 26c, which was
readily prepared as shown in Scheme 8. Wittig reaction of
aldehyde 15 and the ylide derived from 35 provided alkene 36
in 91% yield. Deprotection with DDQ gave alcohol 37 (92%),
which was esterified as above to give diene 26c. Assuming that
the initial carbene is generated at the new terminal alkene of
diene 26c, this should be relayed in turn to a carbene at C11
(not C10), so the extrusion of cyclohexene 30 should be
suppressed. It was not, however.
Again, several conditions were explored, but the RCM
reactions of diene 26c produced no major new products, giving
instead two products that we already had in hand (Scheme 9).
Starting from diene 26c, we observed mostly product 26a, in
which the newly introduced side chain was simply clipped off.
We also desilylated 26c to tetraol 38, but this gave similar
results. Truncated product 39 was formed at short reaction times,
followed by growth of its derived (doubly truncated) product
32. In a typical experiment with diene 38 (Grubbs II, dichlo-
romethane, reflux, 14 h or Grubbs II, toluene, reflux), we isolated
3935 and the 16-membered macrolactone 32 in 30% and 36%
yields, respectively. On the basis of these results, we suppose
Figure 3. Esters made by the acid chloride route (with comparison data
for the Shiina route in parentheses).
approach and the subsequent Nozaki-Kishi approach. Six
additional esters were made by this method, as summarized in
Figure 3. In four more cases (compounds 26b-e), the acid
chloride method was compared with the Shiina method for yields
and stereoselectivity. The Shiina method uniformly provided
excellent yields (89-94%) but poor stereoselectivities (53:47
to 67:33). Furthermore, the alkene isomers were not separable
at this point. In contrast, the acid chloride provided lower total
yields of ester (62-71%), but there was no alkene isomerization
in any case. The moderate yields were offset by recovery of
some amounts of the more complex starting alcohol. (We did
not attempt to recover the acid 27.) The acid chloride method
was clearly superior because it provided a higher absolute yield
of the Z,E-isomer and because it was much easier to separate
the Z,E-isomer of 26 from the starting alcohol than from its
E,E-isomer produced in the Shiina coupling. Furthermore,
several of the acid chloride couplings in Figure 3 were only
conducted once or twice on small scale, so prospects for
optimization to increase conversion and yield remain.
We next examined the RCM reaction of diene 26a under
several conditions by varying the solvent, catalyst, and tem-
perature. The target 22-membered ring product was not,
however, detected in any case. In a typical experiment (Scheme
7), treatment of diene 26a with 10% Grubbs II catalyst in
refluxing toluene followed by cooling, concentration, and
chromatography provided cyclohexene 30 and 16-membered
ring product 31 in comparable yields (61% and 52%, respec-
tively). The silyl groups of macrolactone 31 were removed with
HCl/MeOH to provide the ring-contracted dictyostatin analogue
32 that was used for structure elucidation (see data in Supporting
Information) and biological testing (see below).
(34) Hoye, T. R.; Jeffrey, C. S.; Tennakoon, M. A.; Wang, J.; Zhao, H.
J. Am. Chem. Soc. 2004, 126, 10210–10211.
(35) The sample of 39 isolated from this reaction still contained some
impurities, but the structure assignment of 39 as the main component
was clear from 1H NMR spectroscopy and MS analysis (ESI m/z 548,
[M + Na]+).
9
9180 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
Figure 4. Suggested pathway for formation of truncated RCM products.
Scheme 8. Synthesis of RRCM Precursor 26c
two TBS groups. The RRCM contraction reaction that took place
with 26a should now be prevented (because the extruded product
would be a trans-bicyclo[3.3.0] ring). The acetal 40 was easily
made from the bis-silyl ether by desilylation and acetonide
formation (see Supporting Information). The esterification
reaction to make 26d was already shown in Figure 3.
Initial reactions of 26d with Grubbs II catalyst provided little
or no conversion. Speculating that this might be caused by the
increased Lewis basicity of the acetal oxygens compared to the
prior silyl ethers,36 we added Lewis acids to interfere with
potential chelate formation of ruthenium carbene intermediates.
Indeed, treatment of 26d with 10% Grubbs II catalyst and 25%
Ti(OiPr)4 cleanly produced a new 22-membered ring macrocycle
42 in 72% isolated yield. Analysis of a set of 2D NMR spectra
of 42 provided assignments for all of the protons, and the
coupling constant J10,11 was extracted. Its 15 Hz magnitude
showed that the RCM reaction produced exclusively the 10E-
isomer. Indeed, desilylation of 42 did not produce 2a but gave
instead its stereoisomer 43. Paterson has recently reported 10,11-
dihydrodictyostatin,13d and 43 is the first 10,11-E isomer of the
dictyostatin family. The dihdyro analogue is active, but the
E-isomer is not (see below).
On the basis of what we learned from this series of RCM
reactions, we redesigned the RCM approach to form the C4-C5
E-alkene via 26e, as shown in Figure 5. In the syntheses of the
bottom fragments above, we routinely used a related cross-
metathesis reaction. This early-stage precedent suggests that a
late-stage application could work if other pathways for carbene
that the initial metathesis was initiated at the terminal alkene
and then relayed to C11 as expected. This carbene (not shown)
then suffered cross-metathesis to give 39 rather than RCM to
give the target macrolactone. In turn, 32 is a secondary reaction
product derived from 39.
Noting the demonstration that RCM reactions could be
initiated at C10, we prepared precursor 26d (Scheme 10) with
a dioxane acetal protecting group on O7 and O9 in place of the
(36) Fu¨rstner, A.; Langemann, K. J. Am. Chem. Soc. 1997, 119, 9130–
9136.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9181

A R T I C L E S
Zhu et al.
Scheme 9. Typical RCM Reaction of 26c
Scheme 10. Synthesis and RCM Reaction of 26d
(C10-C26) and addition of aldehyde 46 (C5-C9) provided a
mixture of epimeric alcohols at C9 that was not easily separated.
This product was subjected to Dess-Martin oxidation to afford
the enone 47 in 52% yield for two steps. After DDQ depro-
tection of the bis-PMB ether, the enone was diastereoselectively
reduced with Evans-Saksena conditions37 to give the 1,3-anti
triol 48 (90% yield, dr 75:25). Triol 48 was then protected with
2,2-dimethoxypropane to give the acetonide 49 in 94% yield.
The esterification reaction between acetonide 49 and (2Z,4E)-
hexa-2,4-dienoyl chloride38 gave RCM precursor 26e in 63%
yield (90% BRSM). The RCM reaction was run with Ti(OiPr)4
as additive, and product 50 was isolated in 65% yield. Global
deprotection of macrolactone 50 with HF ·pyridine gave 6-epi-
16-desmethyl-25,26-dihydrodictyostatin 2a (70%), whose ana-
1
lytical and spectroscopic data (Rf, H NMR spectra, and co-
injection on chiral HPLC) matched those of the previous
synthetic sample, thus confirming the structure and stereochem-
ical outcome of this RCM reaction.
Because top-middle fragment 3 is an intermediate in both
the RCM and vinyllithium synthesis of 2, it is instructive to
compare and contrast the approaches. Recall that 3 is already
four steps in from the start of the fragment couplings. The
vinyllithium route requires six more steps to provide 2, whereas
the RCM route requires eight. The yields are about the same (8
and 9%, respectively). The RCM route is apparently less
convergent because what was the “bottom fragment” in prior
approaches (C1-C9) is now introduced in two pieces rather
than one. The two “extra” steps in RCM route are, however,
misleading because we separated the epimeric C9 alcohols
directly in Scheme 1 but added the steps of oxidation and
reduction to get the target epimer in Scheme 11. Therefore, the
step count of the two routes is effectively the same.
reaction are not available. On the surface, substrate 26e has
major design problems since initiation of the RCM reaction at
three different places (C2, C10, and C24) could be followed
by rapid extrusion of a cyclohexene and a truncated carbene.
We expected the reaction of 26e to initiate at C5, however, so
the lone extrusion reaction to derail is the RRCM reaction
starting there (which would form the same product 30 as in
Scheme 7, though in the reverse direction). We again initiated
this path by using the trans-acetal protecting group.
(37) (a) Evans, D. A.; Chapman, K. T.; Carreira, E. M. J. Am. Chem. Soc.
1988, 110, 3560–3578. (b) Saksena, A. K.; Magiaracina, P. Tetrahe-
dron Lett. 1983, 24, 273–276.
(38) For the synthesis of the corresponding acid, see: Smith, A. B., III;
Safonov, I. G.; Corbett, R. M. J. Am. Chem. Soc. 2002, 124, 11102–
11113.
The synthesis and RCM reaction of 26e are shown in Scheme
11. Generation of the vinyllithium reagent from iodide 3
9
9182 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
Figure 5. RCM route to the C4-C5 alkene.
Scheme 11. Successful RCM Route to 2a
Furthermore, while the vinyllithium route is already maximally
convergent (assuming no changes to the basic fragment assembly
strategy), the successful RCM route is not. The convergency and
step count of this synthesis could be considerably improved by
appending the fragment 46 (C5-C9) to the middle fragment (see
7) and the dienyl ester fragment (C1-C4) to the top fragment (see
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9183

A R T I C L E S
Zhu et al.
5) prior to the fragment couplings. This could provide a very
efficient synthesis based on two (rather than three) main fragments
with as few as six steps for the end game.
Nozaki-Hiyama-Kishi Route. Although the RCM route has
many attractive features coupled with potential for further
improvement, it still probably cannot directly accommodate the
C25-C26 terminal alkene present in both the natural product
dictyostatin and the preclinical candidate 6-epi-dictyostatin.
Accordingly, we pursued a third approach based on macrocy-
clization by Nozaki-Hiyama-Kishi reaction39,40 that is a hybrid
of the first two approaches. Like the RCM approach, the
esterification is done before making the macrocycle. Like the
vinyllithium approach, the macrocycle is made from an alkenyl
iodide and an aldehyde. Maier proposed a related approach and
prepared key fragments but did not yet describe the NHK
reaction.11d,e
The plan for the NHK approach to dihydro analogues 2a,b
is summarized in Figure 6. Cyclization of alkenyl iodide 52
should produce alcohol 51, which is very similar to the usual
penultimate intermediate 25 (one fewer TBS group). In turn,
52 is made by esterification of 54 (C10-C26) and 55 (C1-C9),
followed by selective desilylation and oxidation. An attractive
feature of the plan is the small number of steps needed from
the beginning of the fragment coupling to the end of the
synthesis (only five). The key concern is the viability and
stereoselectivity of the proposed NHK reaction. Myles and co-
workers used a bimolecular NHK reaction in their synthesis of
discodermolide.41 We had previously explored a bimolecular
NHK approach to dictyostatin with limited success.42 The model
addition reactions succeeded, but the stereoselectivity at C9 was
very low.
Figure 6. Plan for the NHK route to 2.
are contained in the Supporting Information. Under the best
conditions identified, 52a was reacted with excess chromium
chloride and 4,4′-di-tert-butyl-2,2′-dipyridyl (15 equiv each) and
NiCl2(dppf) (0.2 equiv) in THF at room temperature. After
workup and flash chromatography, macrocycle 51a was isolated
in 53% yield as a single stereoisomer. The configuration of NHK
product 51a was confirmed by global deprotection as usual to
provide 2a, which was identical to the previous two samples.
A similar sequence of reactions in the C6-(R) series starting
from 53b provided NHK precursor 52b in comparable yields.
This time, however, the NHK reaction was not as stereoselective,
and a 75:25 mixture of 51b and its C9 epimer (not shown) was
produced. The mixture was desilylated, and the target C9 epimer
2b was isolated in 47% yield after careful purification. Appar-
ently the stereoselectivity of the NHK reaction is modulated
by the configuration at C6; however, even in the less selective
(R) series, the stereoselectivity is still better than in related
bimolecular model reactions.42
The NHK route is both convergent and efficient compared
to the previous two routes. Starting from the same intermediate
3, the NHK approach provides 2a in six steps in 15.7% overall
yield (six steps, 8.3% yield for vinyllithium addition approach;
eight steps, 9% yield for RCM approach, respectively). In
addition, we expected that dictyostatin analogues with the
C25-C26 diene could be made by this route. To demonstrate
this, we synthesized dictyostatin itself to culminate the work.
The new synthesis of dictyostatin is summarized in Schemes
13 and 14. The combined top-middle fragment 63 (C10-C26)
was prepared from alcohol 56, a readily available intermediate
from our first synthesis of dictyostatin (Scheme 13).6 The TBS
group in 56 was removed with HCl to give a diol that was
protected as bis-TES ether 57 (86% yield, two steps). Removal
of the PMB group with DDQ gave the corresponding alcohol
(90% yield), which was oxidized with SO3 ·pyridine, DMSO,
The fragment coupling and execution of the NHK reaction
are shown in Scheme 12. Common intermediate 3 was first
deprotected with DDQ to give alcohol 54 in 94% yield, and
then 54 was coupled with the acid chloride 55a (prepared from
the corresponding acid 53a and the Ghosez reagent) to afford
the ester 26b (62%, 78% BRSM). The primary TBS ether was
removed with HF ·pyridine, and the resulting alcohol was
oxidized to 52a (92%) with the Dess-Martin reagent. Epimer
52b was prepared by a similar sequence of reactions in
comparable yields.
The key intramolecular NHK reaction40 of 52a was then
examined under various conditions, and details of this survey
(39) (a) Okude, Y.; Hirano, S.; Hiyama, T.; Nozaki, H. J. Am. Chem. Soc.
1977, 99, 3179–3181. (b) Takai, K.; Tagashira, M.; Kuroda, T.;
Oshima, K.; Utimoto, K.; Nozaki, H. J. Am. Chem. Soc. 1986, 108,
6048–6050. (c) Fu¨rstner, A. Chem. ReV. 1999, 99, 991–1046.
(40) For recent examples of intramolecular Nozaki-Hiyama-Kishi reac-
tions, see: (a) Takao, K.-I.; Hayakawa, N.; Yamada, R.; Yamaguchi,
T.; Morita, U.; Kawasaki, S.; Tadano, K.-I. Angew. Chem., Int. Ed.
2008, 47, 3426–3429. (b) Marshall, J. A.; Eidam, P. M. Org. Lett.
2008, 10, 93–96. (c) Pelphrey, P. M.; Bolstad, D. B.; Wright, D. L.
Synlett 2007, 17, 2647–2650. (d) Matsuda, M.; Yamazaki, T.;
Fuhshuku, K.-I.; Sugai, T. Tetrahedron 2007, 63, 8752–8760. (e)
Venkatraman, L.; Salomon, C. E.; Sherman, D. H.; Fecik, R. A. J.
Org. Chem. 2006, 71, 9853–9856. (f) Suzuki, K.; Takayama, H. Org.
Lett. 2006, 8, 4605–4608. (g) Bian, J.; Van Wingerden, M.; Ready,
J. M. J. Am. Chem. Soc. 2006, 128, 7428–7429. (h) Namba, K.; Kishi,
Y. J. Am. Chem. Soc. 2005, 127, 15382–15383. (i) Araki, K.; Saito,
K.; Arimoto, H.; Uemura, D. Angew. Chem., Int. Ed. 2004, 43, 81–
84. (j) Njardarson, J. T.; Biswas, K.; Danishefsky, S. J. Chem.
Commun. 2002, 23, 2759–2761. (k) Pilli, R. A.; Victor, M. M.
Tetrahedron Lett. 1998, 39, 4421–4424.
(41) (a) Harried, S. S.; Yang, G.; Strawn, M. A.; Myles, D. C. J. Org.
Chem. 1997, 62, 6098–6099. (b) Harried, S. S.; Lee, C. P.; Yang, G.;
Lee, T. I. H.; Myles, D. C. J. Org. Chem. 2003, 68, 6646–6660.
(42) Moura-Letts, G. Ph.D. Thesis, University of Pittsburgh, 2007 (http://
etd.library.pitt.edu/ETD/available/etd-11162007-001904/).
9
9184 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
Scheme 12. Synthesis of 2a,b by the Nozaki-Hiyama-Kishi Route
Scheme 13. NHK Route to Dictyostatin: Synthesis of the Top-Middle Fragment 63, C10-C26
and TEA (Parikh-Doering conditions) to provide the aldehyde
58 (80% yield). Next, Wittig olefination (ICH3PPh3, I2, BuLi,
and NaHMDS)43 gave a (Z)-vinyl iodide, which was directly
treated with dichloroacetic acid to give a primary alcohol (48%
yield, two steps). Oxidation under Parikh-Doering conditions
provided the C10-C17 aldehyde 59 (70% yield).
give the TES ether (99%), followed by DDQ deprotection to
afford fragment 63 (92%).
To start the end game, acid 6444 was converted to the acid
chloride 65, and this was coupled with alcohol 63 as usual to
form the ester 26g (71%, 82% BRSM). DDQ deprotection of
PMB ether 26g gave a primary alcohol, which was oxidized to
aldehyde 66 (71% over two steps) by the Dess-Martin reagent.
Treatment of 66 under optimal NHK reaction conditions gave
macrocyclized product 67 in 55% combined yield as two
separable C9 isomers in 78:22 in favor of the R isomer. Global
deprotection of the major isomer provided natural dictyostatin
1 in 77% yield, identical with previous natural and synthetic
The usual C18-C26 phosphonate 6012c was coupled with
aldehyde 59, mediated by Ba(OH)2, to produce the enone 61
(88% yield). Again following established conditions, the
C17-C18 alkene was reduced with the Stryker’s reagent to give
a ketone (77% yield), which was reduced with Li(t-BuO)3AlH
to afford two C19 epimers in a 78:22 ratio favoring the C19
ꢀ-epimer 62 (70%). Alcohol 62 was protected with TESOTf to
(44) The synthesis of acid 64 is described in the Supporting Information
(43) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 2173–2174.
of ref 12d.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9185

A R T I C L E S
Zhu et al.
Scheme 14. NHK Route to Dictyostatin: End-game Steps
paclitaxel-resistant subclones 1A9/PTX10 and 1A9/PTX22, as
well as for microtubule-stabilizing activity in HeLa human
cervical cancer cells. The side products 9-epi-dictyostatin (68),
6-epi-10,11-E-16-desmethyl-25,26-dihydrodictyostatin (43), and
the 16-member macrolactone 32 were also examined. As shown
in Table 1, both 2a and2b caused microtubule bundling in HeLa
cells and retained submicromolar growth inhibitory activity
against 1A9 cells. In results that parallel those obtained with
(-)-dicytostatin, the 1A9/PTX10 cells were cross-resistant to
25,26-dihydro derivatives, but the 1A9/PTX22 cells were not.
This suggests that this saturation of the terminal alkene does
not alter the mode of binding to tubulin. Interestingly, both
analogues appeared to be much less susceptible to cross-
resistance in both 1A9/PTX10 and 1A9/PTX22 than was 16-
desmethyldictyostatin.
Conclusions
In summary, we have developed three new routes to the
dictyostatin class of natural products. A relatively classical route
based on vinyllithium addition and macrocyclization was first
used to make 16-desmethyl-25,26-dihydrodictyostatin 2b and
its 6-epimer 2a. That route is short and highly refined, but the
two key reactions are problematicsthe vinyllithium addition
was low-yielding and not stereoselective, while the macrolac-
tonization was high-yielding but marred by a nagging isomer-
ization problem at the C2-C3 alkene.
Several new approaches to the 25,26-dihydro series of
analogues based on esterification and ring-closing metathesis
(RCM) were investigated. Synthesis of a bottom fragment acid
chloride and esterification occurred in good yield and without
isomerization at the C2-C3 alkene. This solution to the
isomerization problem proved quite general and opened the door
to both RCM and Nozaki-Hiyama-Kishi (NHK) approaches.
RCM approaches to make the C10-C11 bond were problematic,
with formation of truncated products or E-isomers prevailing.
In contrast, an approach to make the C4-C5 E-alkene succeeded
well, avoiding pitfalls like formation of truncated products and
isomerization of the C2-C3 Z-alkene. This RCM approach
already shows excellent potential for making diverse 25,26-
dihydrodictyostatins, and room remains for improvement with
increased convergency.
Finally, the discovery of practical esterification conditions
allowed us to acylate intermediates from the initial vinyllithium
approach and then close the macrocycle on the acylated products
by an intramolecular NHK reaction. Though modest yields (up
to about 50%) in the key NHK reaction have been obtained at
this point, the route is attractive because it is highly convergent
and because there is moderate to excellent stereoselectivity for
samples. Global deprotection of the minor C9 isomer provided
9-epi-dictyostatin 68 (81%), whose spectra matched well those
reported by Paterson.13b This synthetic route provides natural
dictyostatin over a 22-step linear sequence with a 3.3% overall
yield.
Preliminary Biological Testing Results. The target compounds
6-epi-16-desmethyl-25,26-dihydrodictyostatin (2a) and 16-des-
methyl-25,26-dihydrodictyostatin (2b) were tested in comparison
to 16-desmethyldictyostatin and paclitaxel for anti-proliferative
activity against 1A9 human ovarian carcinoma cells and their
Table 1. Cell-Based Activities of New Analogues in Comparison to Paclitaxel and 16-Desmethyldictyostatin
GI50,b nM + SD (fold resistance)
test agent
MDECb for tubulin polymer increase, nM ( SD (N)
1A9
1A9/PTX10
1A9/PTX22
2b
2a
43.2 ( 10.3 (3)
54.2 ( 3.1 (3)
74.4 ( 42.5 (4)
>5000 (2)
285 ( 18
241 ( 7
2817 ( 37 (10)
4090 ( 21 (17)
2160 ( 160 (27)
35000 ( 0.1 (2)
2610 ( 210 (3)
470 ( 70 (1146)
3.2 ( 2.4 (4.6)
4.5 ( 0.3 (5)
445 ( 27 (1.6)
193 ( 4 (0)
9-epi-dictyostatin
32
43
16-desmethyl-dictyostatin
(-)-dictyostatin
6-epi-dictyostatin
paclitaxel
79 ( 1
160 ( 45 (2)
17000 ( 0.2
820 ( 10
0.41 ( 0.52
0.69 ( 0.80
0.85 ( 0.03
0.71 ( 0.11
23000 ( 0.2 (1.4)
1370 ( 170 (1.7)
5.6 ( 4.7 (14)
1.3 ( 1.0 (1.9)
0.81 ( 0.17 (0)
51 ( 9 (72)
713.0 ( 145.3 (3)
25 ( 9 (3)
12.8 ( 3.7 (4)
9.1 ( 6.3 (9)
5.2 ( 0.4 (3)
64 ( 8 (70)
a Minimum detectable effective concentration of the test agent in HeLa cells after 21 h of continuous exposure. b Fifty percent growth inhibitory
concentration after 72 h of continuous exposure to the test agent (N ) 4).
9
9186 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Streamlined Syntheses of Dictyostatins
A R T I C L E S
formation of a C9 secondary alcohol stereocenter. The NHK
route was used to produce both C6 epimers of 25,26-dihydrod-
ictyostatin and dictyostatin itself.
Both of the new 6-epimers of 25,26-dihydrodictyostatin
exhibit considerable activity in preliminary biological assays.
The saturation of the potentially troublesome 25,26-terminal
alkene also opens new options for synthetic manipulations.
Accordingly, the further investigation of the medicinal chemistry
of this series of analogues is an attractive line of research.
The synthetic work marks significant advances toward the
practical synthesis of dictyostatin and analogues for both drug
discovery and development purposes. The “three fragment”
approaches are all based on related fragments that can be made
in 10 steps or fewer on large scale and that can be coupled,
refunctionalized, and deprotected in less than 10 steps. Accord-
ingly, the longest linear sequences of steps are 20 or less, and
the total numbers of steps (from three commercial starting
materials) are 36-40. All the approaches maximize convergency
by building the complete carbon skeleton into the fragments;
no carbons are added after the fragment couplings begin. The
NHK approach to dictyostatin is the first synthesis of the natural
product to meet this target, and it is currently the shortest
synthesis of the natural product.
Acknowledgment. We thank the National Institutes of Health
for support through research grant CA078039 and for a grant to
purchase an NMR spectrometer.
Supporting Information Available: Detailed descriptions of
the synthesis and characterization of all new compounds, along
with copies of key NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
JA103537U
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9187