Total synthesis and initial biological evaluation of new B-ring-modified bryostatin analogs

Article abstract of DOI:10.1021/ol0620904

The total synthesis and preliminary biological evaluation of the first bryostatin analogs (bryologs) to incorporate B-ring substitution are reported. Asymmetric syntheses of two new polyketide "spacer" domains are described, one exploiting the pseudosymme

Full text of DOI:10.1021/ol0620904

ORGANIC
LETTERS
2006
Vol. 8, No. 23
5299-5302
Total Synthesis and Initial Biological
Evaluation of New B-Ring-Modified
Bryostatin Analogs
Paul A. Wender,*,†,‡ Joshua C. Horan,† and Vishal A. Verma†
Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080, and
Department of Molecular Pharmacology, Stanford UniVersity School of Medicine,
Stanford UniVersity, Stanford, California 94305
wenderp@stanford.edu
Received August 24, 2006
ABSTRACT
The total synthesis and preliminary biological evaluation of the first bryostatin analogs (bryologs) to incorporate B-ring substitution are
reported. Asymmetric syntheses of two new polyketide “spacer” domains are described, one exploiting the pseudosymmetry of the C1 C13
−
region. These fragments are convergently joined to the “recognition” domain through a remarkably versatile macrotransacetalization process.
The resulting new analogs exhibit potent nanomolar or picomolar affinity to protein kinase C (PKC), comparable to or better than that found
for bryostatin.
Bryostatin 1 (Figure 1) is a complex, marine-derived
macrolactone1 that is currently in phase I and II clinical trials
for the treatment of cancer.2 This compound exhibits a unique
range of biological activities including induction of apoptosis,
reversal of multidrug resistance, and immune system modu-
lation.3 Of special therapeutic importance, bryostatin 1 has
been shown to enhance the overall efficacy of other oncolytic
agents, suggesting its potential use in combination therapy.4
A study reported earlier this year, for example, showed that
bryostatin in combination with paclitaxel produced a superior
response rate in patients with untreated, advanced gastric or
gastroesophageal junction adenocarcinoma compared to
paclitaxel alone.5 Bryostatin also exhibits activity that could
have therapeutic implications for Alzheimer’s disease,
including an ability to improve memory and learning in
animal models.6
Unfortunately, bryostatin is not readily obtained from its
source organism.7 This scarcity has hampered clinical studies,
(4) (a) Mohammad, R. M.; Wall, N. R.; Dutcher, J. A.; Al-Katib, A. M.
Clin. Cancer Res. 2000, 6, 4950-4956. (b) Wang, S.; Wang, Z.; Boise, L.
H.; Dent, P.; Grant, S. Leukemia 1999, 13, 1564-1573.
(5) Ajani, J. A.; Jiang, Y.; Faust, J.; Chang, B. B.; Ho, L.; Yao, J. C.;
Rousey, S.; Dakhil, S.; Cherny, R. C.; Craig, C.; Bleyer, A. InVest. New
Drugs 2006, 24, 353-357.
(6) (a) Etcheberrigaray, R.; Tan, M.; Dewachter, I.; Kuiperi, C.; Van
der Auwera, I.; Wera, S.; Qiao, L. X.; Bank, B.; Nelson, T. J.; Kozikowski,
A. P.; Van Leuven, F.; Alkon, D. L. Proc. Natl. Acad. Sci. U.S.A. 2004,
101, 11141-11146. (b) Sun, M. K.; Alkon, D. L. Eur. J. Pharmacol. 2005,
512, 43-51. (c) Alkon, D. L.; Epstein, H.; Kuzirian, A.; Bennett, M. C.;
Nelson, T. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16432-16437.
† Department of Chemistry.
‡ Department of Molecular Pharmacology.
(1) (a) Pettit, G. R.; Herald, C. L.; Doubek, D. L.; Herald, D. L.; Arnold,
E.; Clardy, J. J. Am. Chem. Soc. 1982, 104, 6846-6848. (b) Hale, K. J.;
Hummersone, M. G.; Manaviazar, S.; Frigerio, M. Nat. Prod. Rep. 2002,
19, 413-453.
(2) For current information, see http://clinicaltrials.gov.
(3) (a) Kortmansky, J.; Schwartz, G. K. Cancer InVest. 2003, 21, 924-
936. (b) Mutter, R.; Wills, M. Bioorg. Med. Chem. 2000, 8, 1841-1860.
10.1021/ol0620904 CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/20/2006

analogs 1 and 2 (Figure 1) are representative. Analog 1
exhibits in vitro and in vivo biological activities comparable
to or better than bryostatin 1 in various assays.11 Analog 2,
which lacks the A-ring found in bryostatin 1, represents the
simplest analog reported to date that maintains high binding
affinity.10 The availability of these analogs in quantity and
the ability to tune them for performance has now enabled
several mode of action and preclinical studies to move
forward.
Bryostatin’s activity is thought to arise from its binding
to the regulatory domain of certain proteins, including kinases
such as protein kinase C (PKC). Since this domain is found
in only a small subset of the human kinome, targeting this
domain could lead to selective kinase regulation.12 In
addition, unlike many ligands that serve as kinase inhibitors
at the ATP binding site, binding to the C1 domain can result
in inhibition or activation. This “gain of function” activity
has many basic and therapeutic ramifications. A purpose of
our ongoing studies in this area is to design agents that would
bind to the C1 domain and offer, as needed, selective
regulation of kinase isoforms. Bryostatin binds to two
subclasses of PKC, the conventional and novel isoforms.13
It has been hypothesized that the “recognition” domain is
responsible for direct interaction with the binding pocket,10
and as such, functionality on the A- and B-rings of bryostatin
could be responsible for selectivity between these two classes
of PKC. Previous work has shown that the B-ring could be
replaced with a 5- or 6-membered dioxolane ring while
maintaining high potency and affecting selectivity.14 Recent
efforts have been directed at a new family of analogs
incorporating B-ring functionalities at C13 that could be
diversified to probe for potency and selective binding to PKC
isoforms. An ester was chosen in order to mimic the B-ring
ester of bryostatin, and terminal olefins were chosen in order
to diversify analogs through late-stage cross-metathesis. We
describe herein the synthesis of four new spacer domains
and their incorporation into the synthesis of the first
bryostatin analogs (3-6) derivatized at C13.
Figure 1. Bryostatin 1 and synthetic analogs.
research on its mode of action, and access to clinically
superior structural or functional analogs. While three impres-
sive total syntheses of the bryostatins have been completed,
each is over 70 steps and thus not able as yet to provide
sufficient material to impact the clinical supply.8 More
importantly, bryostatin 1 is produced in nature for uses other
than human therapy and is therefore not an optimized
therapeutic agent. Prompted by these considerations and the
unique activity profile of bryostatin, our group initiated a
program directed at the design and synthesis of simplified
bryostatin analogs that can be produced in a practical, step-
economical fashion and tuned for optimal performance in
the clinic.9 This function-oriented design and synthesis
approach10 has produced several promising leads of which
The spacer domains of analogs 5 and 6 are pseudo-C2-
symmetric with respect to the axis bisecting the A-ring
oxygen.15 This pseudosymmetry was exploited to efficiently
and step economically synthesize B-ring analogs lacking the
A-ring (Scheme 1). Toward this end, the Blaise reaction16
proceeded in high yield to join 2 equiv of acetate 7 to
symmetric ether 8 to produce the symmetric bis-â-keto ester
9. This diketone smoothly underwent a double Noyori
asymmetric reduction,17 selectively producing only one
(7) Schaufelberger, D. E.; Koleck, M. P.; Beutler, J. A.; Vatakis, A. M.;
Alvarado, A. B.; Andrews, P.; Marzo, L. V.; Muschik, G. M.; Roacch, J.;
Ross, J. T.; Lebherz, W. B.; Reeves, M. P.; Eberwein, R. M.; Rodgers, L.
L.; Testerman, R. P.; Snader, K. M.; Forenza, S. J. Nat. Prod. 1991, 54,
1265-1270.
(8) (a) Evans, D. A.; Carter, P. H.; Carreira, E. M.; Charette, A. B.;
Prunet, J. A.; Lautens, M. J. Am. Chem. Soc. 1999, 121, 7540-7552. (b)
Kageyama, M.; Tamura, T.; Nantz, M. H.; Roberts, J. C.; Somfai, P.;
Whritenour, D. C.; Masamune, S. J. Am. Chem. Soc. 1990, 112, 7407-
7408. (c) Ohmori, K.; Ogawa, Y.; Obitsu, T.; Ishikawa, Y.; Nishiyama, S.;
Yamamura, S. Angew. Chem., Int. Ed. 2000, 39, 2290-2294. For other
work on the bryostatins see: Keck, G. E.; Welch, D. S.; Vivian, P. K. Org.
Lett. 2006, 8, 3667-3670. Voight, E. A.; Seradj, H.; Roethle, P. A.; Burke,
S. D. Org. Lett. 2004, 6, 4045-4048. Hale, K. J.; Frigerio, M.; Manaviazar,
S. Org. Lett. 2003, 5, 503-505. Cho, C.-W.; Krische, M. J. Org. Lett. 2006,
8, 891-894 and references therein.
(9) (a) Wender, P. A.; Cribbs, C. M.; Koehler, K. F.; Sharkey, N. A.;
Herald, C. L.; Kamano, Y.; Pettit, G. R.; Blumberg, P. M. Proc. Natl. Acad.
Sci. U.S.A. 1988, 85, 7197-7201. (b) Wender, P. A.; De Brabander, J.;
Harran, P. G.; Jimenez, J. M.; Koehler, M. F. T.; Lippa, B.; Park, C. M.;
Siedenbiedel, C.; Pettit, G. R. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 6624-
6629. (c) For other efforts toward bryostatin analogs, see: Keck, G. E.;
Truong, A. P. Org. Lett. 2005, 7, 2153-2156. Hale, K. J.; Frigerio, M.;
Manaviazar, S.; Hummersone, M. G.; Fillingham, I. J.; Barsukov, I. G.;
Damblon, C. F.; Gescher, A.; Robert, G. C. K. Org. Lett. 2003, 5, 499-
502. Ball, M.; Bradshaw, B. J.; Dumeunier, R.; Gregson, T. J.; MacCormick,
S.; Omori, H.; Thomas, E. J. Tetrahedron. Lett. 2006, 47, 2223-2227 and
references cited therein.
(10) Wender, P. A.; Baryza, J. L.; Brenner, S. E.; Clarke, M.O.; Craske,
M. L.; Horan, J. C.; Meyer, T. Curr. Drug DiscoV. Tech. 2004, 1, 1-11.
(11) Wender, P. A.; Baryza, J. L.; Bennett, C. E.; Bi, F. C.; Brenner, S.
E.; Clarke, M. O.; Horan, J. C.; Kan, C.; Lacoˆte, E.; Lippa, B. S.; Nell, P.
G.; Turner, T. M. J. Am. Chem. Soc. 2002, 124, 13648-13649.
(12) Manning, G.; Whyte, D. B.; Martinez, R.; Hunter, T.; Sudarsanam,
S. Science 2002, 298, 1912-1934.
(13) For a review on PKC, see: Newton, A. C. Chem. ReV. 2001, 101,
2353-2364.
(14) Wender, P. A.; Verma, V. A. Org. Lett. 2006, 8, 1893-1896.
(15) Wender, P. A.; Horan, J. C. Org. Lett. 2006, 8, 4581-4584.
(16) Hannick, S. M.; Kishi, Y. J. Org. Chem. 1983, 48, 3833-3835.
(17) Noyori, R.; Ohkuma, T.; Kitamura, M.; Takaya, H.; Sayo, N.;
Kumobayashi, H.; Akutagawa, S. J. Am. Chem. Soc. 1987, 109, 5856-
5858.
5300
Org. Lett., Vol. 8, No. 23, 2006

Scheme 1. Synthesis of Spacer Domains 12 and 13
Scheme 2. Synthesis of Spacer Domains 18 and 20
detectable isomer, which was subsequently silyl protected.
Desymmetrization via monoreduction of one tert-butyl ester
to the alcohol was followed by oxidation to the aldehyde.
Brown’s allylation18 and subsequent protection furnished 11.
This intermediate was then taken on to spacer domain 12
by cleavage of the tert-butyl ester. This synthesis provided
the completed spacer domain in 29% overall yield over 8
steps.
Separately, intermediate 11 was also converted to a second
spacer domain through a three-step sequence. Oxidative
cleavage of the terminal olefin and conversion to the allyl
ester was followed by selective deprotection of the tert-butyl
ester to give completed spacer domain 13 in 31% overall
yield over 10 steps.
The synthesis of the spacer domains for analogs 3 and 4
began with silyl protection of known hydroxy ester 1419
(seven steps from commercially available methyl glutaryl
chloride) followed by reduction and reoxidation to provide
aldehyde 15 (Scheme 2). Asymmetric allylation was then
used to set the C13 stereocenter giving a homoallylic alcohol
that was silylated to provide 16.18 The configuration of the
newly set secondary alcohol was confirmed by analysis of
the corresponding C11/C13 acetonide using Rychnovsky’s
method.20
To avoid reduction of the newly installed allyl group, the
C1 benzyl group of 16 was deprotected using dissolving
metal conditions. Interestingly, these conditions also partially
reduced the phenyl substituent of the C3 TBDPS group,
which, however, was readily reoxidized with DDQ to provide
17.21 The low yield for this two-step procedure was due to
extensive migration of the TBDPS group from C3 to the
more stable C1 position during the reduction step. Oxidation
of the newly revealed primary C1 alcohol to the carboxylic
acid completed the synthesis of spacer domain 18.22
Intermediate 16 was separately subjected to oxidative
cleavage to reveal a carboxylic acid. Hydrogenolysis of the
C1 benzyl ether provided 19, which was then esterified with
allyl alcohol. In the final step, the C1 alcohol was oxidized
to the carboxylic acid to provide completed spacer domain
20.
Each of the four spacer domains was coupled individually
to the existing recognition domain 2111 using Yamaguchi’s
esterification procedure (Scheme 3).23 The macrocycles were
closed and the silyl protecting groups removed in a remark-
ably general one-step, mild, and diastereoselective macro-
transacetalization, providing completed analogs 3-6. The
(18) Brown, H. C.; Jadhav, P. K. J. Am. Chem. Soc. 1983, 105, 2092-
(21) Toyota, M.; Hirota, M.; Hirano, H.; Ihara, M. Org. Lett. 2000, 2,
2031-2034.
(22) Zhao, M. Z.; Li, J.; Mano, E.; Song, Z. G.; Tschaen, D. M.;
Grabowski, E. J. J.; Reider, P. J. J. Org. Chem. 1999, 64, 2564-2566.
(23) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989-1993.
2093.
(19) Wender, P. A.; Mayweg, A. V. W.; VanDeusen, C. L. Org. Lett.
2003, 5, 277-279.
(20) Rychnovsky, S. D.; Rogers, B. N.; Richardson, T. I. Acc. Chem.
Res. 1998, 31, 9-17.
Org. Lett., Vol. 8, No. 23, 2006
5301

Scheme 3. Convergent Assembly of Analogs 3-6
Figure 2. PKC binding affinities for B-ring bryologs.
newly formed C15 stereocenter in each analog was set under
thermodynamic control affording only the cis-diequatorial
dioxolane B-ring.
the basis for probing differences in the isoform surfaces in
contact with the B-ring region of bryostatin and the bryologs.
Studies on diversification of these new leads through cross-
metathesis and further biological evaluation are currently
underway.
These new analogs exhibit potent nanomolar or picomolar
binding affinities to PKC when tested in a competition
binding assay against the known PKC ligand phorbol 12,13-
dibutyrate (Figure 2). Significantly, analogs 5 and 6 exhibit
binding potencies superior to analog 2. As the first C13
modified bryologs, it is especially noteworthy that 3 and 4
exhibit potency on par with if not better than bryostatin.
A series of potent B-ring analogs of bryostatin have been
synthesized from four new spacer domains through a
convergent esterification/macrotransacetalization strategy.
The new bryologs, the first bearing B-ring substitution,
exhibit single-digit nanomolar or picomolar affinity to PKC,
on par or better than bryostatin, indicating that the B-ring
can be modified while maintaining or improving potency.
Relative to the parent analog 2, the side chain in analogs 5
and 6 improves potency. This adds support to the hypothesis
that B-ring, and more broadly, spacer domain functionality
can be exploited to achieve higher potency and possibly
isozyme selectivity. These analogs can be efficiently gener-
ated in a scalable, step economical synthesis and provide
Acknowledgment. Support of this work through a grant
(CA31845) provided by the NIH is gratefully acknowledged.
V.A.V. is supported by an Amgen Graduate Fellowship. We
thank the Daria Mochly-Rosen group (Department of Mo-
lecular Pharmacology, Stanford University) for their support
with biological studies. HRMS analyses for some compounds
were performed at UCSF.
Supporting Information Available: Experimental condi-
tions and spectral data for compounds reported in this paper.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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