Convergent assembly of highly potent analogues of bryostatin 1 via pyran annulation: Bryostatin look-alikes that mimic phorbol ester function

Article abstract of DOI:10.1021/ja8022169

Highly potent bryostatin analogues which contain the complete bryostatin core structure have been synthesized using a pyran annulation approach as a key strategic element. The A ring pyran was assembled using a pyran annulation reaction between a C₁-C₈ hydroxy allylsilane and an aldehyde comprising C₉-C₁₃. This pyran was transformed to a new hydroxy allylsilane and then coupled with a preformed C ring aldehyde subunit in a second pyran annulation, with concomitant formation of the B ring. This tricyclic intermediate was elaborated to bryostatin analogues which displayed nanomolar to subnanomolar affinity for PKC, but displayed properties indistinguishable from a phorbol ester in a proliferation/attachment assay. Copyright

Full text of DOI:10.1021/ja8022169

Published on Web 05/02/2008
Convergent Assembly of Highly Potent Analogues of Bryostatin 1 via Pyran
Annulation: Bryostatin Look-Alikes That Mimic Phorbol Ester Function
Gary E. Keck,*,† Matthew B. Kraft,† Anh P. Truong,†,§ Wei Li,† Carina C. Sanchez,†, Noemi Kedei,‡
Nancy E. Lewin,‡ and Peter M. Blumberg‡
Department of Chemistry, UniVersity of Utah, 315 South 1400 East, RM 2020, Salt Lake City, Utah 84112, and
LCBG, Center for Cancer Research, NCI, National Institutes of Health, Besthesda, Maryland 20892
Received March 28, 2008; E-mail: keck@chem.utah.edu
The bryostatins are a family of complex macrolactone natural
products, originally isolated from the marine bryozoan Bugula
neritina by Pettit and co-workers.1 Bryostatin 1 (1, Figure 1) has
shown promising anticancer activity that has led to some 80 clinical
trials which are either complete or ongoing.2 Although bryostatin
1 has shown limited utility as a single agent, it has displayed
remarkable synergy with a number of established chemotherapeutic
agents.3 Other recent studies have revealed fascinating effects of
bryostatin on memory and have suggested potential for therapeutic
use in Alzheimer’s disease.4
Figure 1. Structures of bryostatin 1 and a bryologue.
The mode of action of bryostatin 1 is itself a subject of intense
research activity, but it is well-known to have exceptionally high
The preparation of these analogues is shown in Scheme 1. The
affinity for protein kinase C (PKC) isozymes.5 Unfortunately,
A-ring intermediate 5 was prepared by a pyran annulation reaction9a
therapeutic development of bryostatin has been hampered by the
between the known9c hydroxyallylsilane 4 and aldehyde 3. The ester
limited and nonrenewable supply of this marine natural product.
moiety was then used to fashion a second hydroxyallylsilane for
Although several total syntheses of bryostatins have been reported,
the next pyran annulation by application of the Bunnelle reaction.10
these require truly monumental levels of effort.6
Pyran annulation between this new hydroxyallylsilane 5 and C-ring
aldehyde 6 then provided tricyclic intermediate 7 in 84% isolated
yield. Elaboration at C1 to give the required carboxylic acid 8 was
accomplished via selective deprotection of the BPS group followed
As reported in an important series of papers, Wender and co-workers
have prepared structurally simplified analogues of bryostatin which
rival or exceed the activity of bryostatin 1 itself in terms of affinity
for PKC isozymes and cytotoxicity toward certain cell lines. One of
the most potent of these, compound 2, is representative and can be
seen to have deleted all substitution on the A- and B-rings; in addition,
the B-ring pyran has been replaced by an acetal (1,3-dioxane) subunit
for ease of synthesis.7
by sequential application of the Parikh-Doering and Pinnick
oxidations.11 Removal of the sole TBS group at C25 was then
followed by a highly efficient Yamaguchi macrolactonization to
afford the desired tricylic macrolactone 9 in 87% isolated yield for
the two steps.
We have also established a synthetic program in this area which
has as an important subgoal the identification of those structural features
which are required both for PKC binding and also for biological
function as a bryo 1 mimic. Thus, of the agents known to bind to and
activate PKCs, only the bryostatins act as functional antagonists of a
subset of biological responses.8 Toward this end, we have reported
the development of powerful new synthetic methodology designed
specifically for this problem as well as very flexible strategies for its
implementation.9 Application of our pyran annulation methodology
to the synthesis of a bryostatin analogue which contains the complete
tricyclic macrocyclic framework of bryostatin 1 has been reported.9cWe
report herein: (1) a more convergent, second generation synthesis of
the tricylic macrolactone core via our pyran annulation approach, (2)
elaboration of this material to analogues with very high affinity for
PKC, and (3) preliminary biological characterization of these materials
that suggests a biological profile more akin to that of the tumor-
promoting phorbol 12-myristate 13-acetate (PMA) than to that
exhibited by bryostatin 1.
Introduction of the C21 enoate functionality along the lines
previously established by Evans6b proved difficult. Although the
aldol condensation with freshly prepared methyl glyoxalate could
be accomplished quite readily using LDA, elimination from the
resulting ꢀ-hydroxy ketone proved exceedingly difficult with this
substrate. Ultimately, a new procedure for this very demanding
reaction was devised which involved treatment with carbonyl
diimidazole in the presence of (iPr)2NEt.12,13 This very cleanly
afforded the desired enoate 10. Luche reduction of the C20 ketone
gave the desired alcohol, which was immediately acylated to give
protected versions of analogues 11-13. Removal of protecting
groups commenced by removal of the PMB group with DDQ.
Finally, global deprotection of the remaining groups could be
accomplished without incident and in essentially quantitative yield
in all three cases using the LiBF4 conditions originally developed
by Lipshutz14 for acetal hydrolysis and previously used successfully
on the macrolactone core structure.9c
All three of these materials proved to have higher affinity for
PKCR than does bryostatin (bryo 1 Ki ) 1.35 nM, Ki values given
in Scheme 1). Each of these analogues has also been screened for
function by examination of proliferation and attachment of U937
leukemia cells.15 In this assay, phorbol esters inhibit proliferation
and induce attachment. Bryostatin has a much reduced effect and
† University of Utah.
‡ National Institutes of Health.
§ Present address: Elan Corporation, South San Francisco, CA 94080.
Present address: Novartis Institutes for BioMedical Research, Cambridge, MA
02139.
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6660 J. AM. CHEM. SOC. 2008, 130, 6660–6661
10.1021/ja8022169 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Preparation of Analogues 11-13 via Pyran Annulationa
a Key: (a) TMSOTf, Et2O,-78 °C, 96%; (b) TsOH, MeOH, rt, 92%; (c) TMSCl, Et3N, CH2Cl2, 99%; (d) TMSCH2MgCl, CeCl3, THF, 81%; (e) TBAF, AcOH,
DMF, 90%; (f) DMSO, SO3 ·Py, (iPr)2NEt, 93%; (g) NaClO2, 2-methyl-2-butene, tBuOH, KH2PO4, H2O, 99%; (h) HF ·Py, THF, Py; (i) 2,4,6-Cl3PhCOCl, Et3N,
THF, then DMAP, tol, 40 °C, 87% over 2 steps; (j) LDA, THF, -78 °C, then methyl glyoxate, 76% plus 19% recovered ketone; (k) CDI, DMAP, (iPr)2NEt,
CH2Cl2, 75%; (l) NaBH4, CeCl3, MeOH, -40 °C; (m) (PhCO)2O, DMAP, CH2Cl2, 91% over 2 steps, dr ) 6:1; (n) DDQ, pH 7 buffer, CH2Cl2; (o) LiBF4,
CH3CN/H2O (20:1), 80 °C, quantitative.
Acknowledgment. Dedicated to Professor E. J. Corey on the
occasion of his 80th birthday. Financial support was provided by the
NIH through Grant GM28961 and through the Intramural Research
Program, CCR, NCI, NIH.
Supporting Information Available: Experimental procedures, assay
results, and spectral data. This material is available free of charge via
the Internet at http://pubs.acs.org.
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It is clear that the fingerprint displayed here by 13 is virtually
identical to that of the tumor-promoting phorbol ester PMA and
distinctly different from that of bryostatin 1. This may have
significant implications regarding the projected use of such
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determine, through synthesis, how substitution at each of these sites
impacts function are in progress. Further biological characterizations
of these highly potent materials will be provided in due course.
(12) Details of this process will be reported separately.
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(16) Results for all analogues in the U937 attachment and proliferation assays
are similar and are provided in the Supporting Information.
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