Total synthesis of bryostatin 1

Article abstract of DOI:10.1021/ja110198y

Bryostatin 1 is a marine natural product that is a very promising lead compound because of the potent biological activity it displays against a variety of human disease states. We describe herein the first total synthesis of this agent. The synthetic rout

Full text of DOI:10.1021/ja110198y

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pubs.acs.org/JACS
Total Synthesis of Bryostatin 1
Gary E. Keck,* Yam B. Poudel, Thomas J. Cummins, Arnab Rudra, and Jonathan A. Covel
Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States
S Supporting Information
b
ABSTRACT: Bryostatin 1 is a marine natural product that
is a very promising lead compound because of the potent
biological activity it displays against a variety of human
disease states. We describe herein the first total synthesis of
this agent. The synthetic route adopted is a highly con-
vergent one in which the preformed, heavily functionalized
pyran rings A and C are united by “pyran annulation”, the
TMSOTf-promoted reaction between a hydroxyallylsilane
appended to the A-ring fragment and an aldehyde contained
in the C-ring fragment, with concomitant formation of
the B ring. Further elaborations of the resulting very highly
functionalized intermediate include macrolactonization and
selective cleavage of just one of five ester linkages present.
ryostatin 1 is a now well-known natural product originally
B
isolated by Pettit and co-workers from the marine organism
Bugula neritina.1 Since that time, other members of this family
have been isolated, and some 20 members are now known
(Figure 1).2 It has also been established that the true source of
the bryostatins is not actually B. neritina but rather a bacterial
symbiont.3
Figure 1. Bryostatin structures.
Interest in the bryostatins, and bryostatin 1 in particular, has
been intense because of the wide range of potent bioactivity
associated with bryostatin 1. Bryostatin 1 has shown activity
against a range of cancers and has also shown synergism with
established oncolytic agents such as Taxol.4 This has led to the
use of bryostatin 1 in numerous clinical trials for cancer, despite
the absence of any renewable supply for this compound at
present. In addition, bryostatin 1 has shown promising activity
relevant to a number of other diseases and conditions, including
diabetes,5 stroke,6 and Alzheimer's disease.7 A clinical trial for
Alzheimer's disease is commencing.8 This wide range of promis-
ing potential indications for bryostatin 1 becomes more under-
standable when it is recognized that at least one mechanism for
function of this agent involves activity on protein kinase C
(PKC) isozymes and other proteins containing C1 domains.9
These signaling proteins are known to regulate some of the most
critical cellular processes and properties, including proliferation,
differentiation, motility and adhesion, inflammation, and apoptosis.10
In view of this backdrop, it is not surprising that synthetic
activity directed toward the bryostatins has been intense. What is
surprising, perhaps, is that bryostatin 1 itself has not been pre-
viously synthesized, while other members of the family have been
prepared. Previous total syntheses include those of bryostatin 7
(2, by Masamune), bryostatin 2 (3, by Evans), bryostatin 3 (4, by
Yamamura), and bryostatin 16 (5, by Trost).11 In addition, Hale
has described a formal synthesis of bryostatin 7, and Trost has
recently reported a synthesis of C20-epi-bryostatin 7.11
Another very important aspect of bryostatin synthesis has
been the area of analogue synthesis, originally introduced by
Wender and co-workers.12 This group targeted analogues in which
the B-ring pyran was replaced by an acetal linkage for ease of
synthesis. Recent work in our laboratory directed at establishing
structure-activity relationships for bryostatin 1 has led to the
synthesis of a number of bryopyrans that are close structural
analogues of the bryostatin natural products using methodology
developed explicitly for this purpose.13 The pyran annulation
process has been used to prepare the analogues Merle 23 (6) and
Merle 27 (7)14, which display phorbol ester-like biology in U937
leukemia cells, as well as analogues Merle 28 (8) and Merle 30
(9), which are predominantly bryo-like in the same differential
assay (Figure 2).15 In addition, Wender has prepared the bryo-
pyran analogues 10 and 11 using the intramolecular variant of the
pyran annulation reaction as well as numerous other analogues
incorporating the previously mentioned scaffold variation in
which the B-ring pyran is replaced by an acetal.16
Received: November 12, 2010
Published: December 22, 2010
r
2010 American Chemical Society
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The work directed at Merle 23 and 27 in comparison with that
directed toward Merle 28 and 30 afforded an opportunity to
compare two convergent strategies for the union of A-ring and
C-ring fragments with concomitant formation of the B ring. On
the basis of this previous experience, a strategy of combining an
A-ring hydroxyallylsilane and a C-ring aldehyde was selected for
an attempted synthesis of bryostatin 1.
The synthesis of the fully functionalized C-ring fragment used
a new approach that was more convergent than the one utilized
previously for the Merle analogues (Scheme 1). Carboxylic acid
12 (available in four steps) was esterified with the previously
reported homoallylic alcohol 13 [prepared from commercial
(R)-isobutyl lactate in six steps].13b Selective oxidative function-
alization at the terminus of the less hindered alkene followed by
Wittig chain extension afforded 16, which was then utilized in a
Rainier metathesis reaction to construct glycal 17 in 80% yield.17
Oxidative functionalization of the glycal afforded methoxyketone
18 in 66% yield over two steps. Aldol condensation with methyl
glyoxylate then afforded enoate 19 in 82% isolated yield.18 Luche
reduction gave an intermediate C20 alcohol that was found to be
unstable with respect to storage or purification by column chroma-
tography. Immediate acetylation of the alcohol after isolation
gave the C20 acetate derivative 20 in 84% yield for the two steps
of reduction and acylation. This was converted to aldehyde 21 in
91% yield by TBS removal and Ley oxidation using TPAP and
NMO.
The A-ring hydroxyallylsilane 22 was prepared in three steps
via allylation of the corresponding previously reported aldehyde.19
The crucial pyranannulationreaction betweentheA-ringhydroxy-
allylsilane 22 and the C-ring aldehyde 21 then provided the
tricyclic intermediate 23 in 61% isolated yield (Scheme 2). This
represents one of the most complex examples of such a pyran
annulation to date. The major byproduct was a spirocylic struc-
ture formed via intramolecular cyclization of the silane at the C9
position of 22. However, attempts to suppress this side reaction
by increasing the amount of aldehyde or the concentration of the
reaction were not successful.
Hydrolysis of the thiolester to reveal the carboxylic acid was
attempted next. Selective hydrolysis using various reagents such
as LiOH/H2O2, AgNO3/H2O, or Hg(OCOCF3)2 was compli-
cated because of the competitive hydrolysis of other esters, hydro-
lysis of the methyl ketals, or oxymercuration of the olefins. After
considerable experimentation, it was discovered that this hydro-
lysis could be carried out selectively using LiOH/H2O2 when the
C3 hydroxyl was free. The exact origin of the selectivity is unclear,
but a decrease in the steric hindrance around the thiol ester
and activation of the C1 carbonyl due to hydrogen bonding
with the C3 alcohol seems to be the most reasonable explanation.
To obtain the free alcohol, the BPS group was removed using
HF py under buffered conditions. The presence of methanol
during the silyl deprotection was necessary to avoid any hydrolysis
3
Figure 2. Representative bryopyran analogues.
Scheme 1. Synthesis of C-Ring Aldehyde 21 Using the Rainier Metathesis Reaction
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Scheme 2. Synthesis of Bryostatin 1 via Pyran Annulation
of the methyl ketals at C19 and particularly C9. Treatment of the
β-hydroxy thiol ester with LiOH/H2O2 in aqueous THF then
cleanly afforded the β-hydroxycarboxylic acid. Since the free
hydroxyl group at the C3 position was found to interfere with
the subsequent macrolactonization, it was protected as the TES
ether. Following removal of the PMB group using DDQ, the
resulting seco acid was subjected to Yamaguchi macrolactoniza-
tion conditions,20 which afforded macrolactone 26 in 71% yield
over two steps.
At this point, the regioselective oxidative cleavage of the
C13-C30 olefin of 26 could be achieved by either ozonolysis
or reaction with OsO4/NaIO4. However, a protocol involving
Sharpless asymmetric dihydroxylation followed by periodate
oxidation was found to be superior to the other conditions and
provided ketone 27 in 81% yield.21 An asymmetric Horner-
Wadsworth-Emmons reaction on the ketone using Fuji's chiral
BINOL phosphonate 28 provided a 4:1 Z/E mixture R,β-
unsaturated methyl esters in favor of the desired isomer.22 The
geometric isomers were easily separated using preparative thin-
layer silica gel chromatography.
We were pleased to find that when bisacetate 29 was exposed
to K2CO3/MeOH, selective methanolysis of the C20 acetate
occurred in just 45 min, providing the desired C20 alcohol. The
regioselectivity observed in this reaction could best be explained
by activation of the C20 acetate due to the inductive effects
associated with the neighboring groups. Since the C20 alcohol
was again found to be unstable, it was immediately esterified by
reaction with (2E,4E)-octa-2,4-dienoic anhydride to give 30, a
protected version of bryostatin 1, in 71% yield from bisacetate 29.
When protected bryostatin 1 derivative 30 was subjected to
global deprotection using LiBF4 in acetonitrile/water at 80 °C,23
two methyl ketals, the TES ether, and the BOM group were all
removed without cleavage of any of the five esters present,
providing bryostatin 1 in 72% yield. It should be noted that
similar deprotection of the bisacetate 29 would provide bryo-
statin 7. In addition, it is apparent that introduction of other side
chains at the C20 position as described above would lead to the
synthesis of other naturally occurring bryostatins, such as bryo-
statins 9 and 15, as well as new bryostatin analogues.
The analytical data for synthetic bryostatin 1 were found to be
in excellent agreement with those of a sample of natural bryo-
statin 1 on the basis of several criteria, such as thin-layer chromato-
graphy, 1H NMR, 13C NMR, and 13C distortionless enhancement
by polarization transfer (DEPT) spectroscopy, high-resolution
mass spectrometry, optical rotation, and LC-MS. As previously
1
reported in the literature and observed by us, the H NMR
spectra of the bryostatins are highly concentration dependent
and very sensitive to trace water or D2O.24 A change of just 2-fold
in concentration gave observable changes in the proton NMR
spectra. Thus, the richly detailed 13C NMR spectra (all carbons
observed) of synthetic and natural samples serve as a more
reliable fingerprint.
In summary, the total synthesis of bryostatin 1 has been
achieved by the convergent union of the A-ring hydroxyallyl-
silane 22 with the C-ringaldehyde 21. An almost perfect balancein
convergence was realized, as 21 was prepared in 18 steps while 22
was made in 17 steps. After union of these two fragments, an
additional 11 steps afforded bryostatin 1. This first total synthesis
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of bryostatin 1 was thus accomplished in 30 steps for the longest
linear sequence from commercially available (R)-isobutyl lactate.
(13) (a) Keck, G. E.; Covel, J. A.; Schiff, T.; Yu, T. Org. Lett. 2002, 4,
1189–1192. (b) Keck, G. E.; Truong, A. P. Org. Lett. 2005, 7, 2149–
2152. (c) Keck, G. E.; Truong, A. P. Org. Lett. 2005, 7, 2153–2156.
(14) (a) Keck, G. E.; Kraft, M. B.; Truong, A. P.; Li, W.; Sanchez,
C. C.; Kedei, N.; Lewin, N.; Blumberg, P. M. J. Am. Chem. Soc. 2008, 130,
6660–6661. (b) Keck, G. E.; Li, W.; Kraft, M. B.; Kedei, N.; Lewin, N. E.;
Blumberg, P. M. Org. Lett. 2009, 11, 2277–2280.
(15) (a) Keck, G. E.; Poudel, Y. B.; Welch, D. S.; Kraft, M. B.;
Truong, A. P.; Stephens, J. C.; Kedei, N.; Lewin, N. E.; Blumberg, P. M.
Org. Lett. 2009, 11, 593–596. (b) Keck, G. E.; Poudel, Y. B.; Rudra, A.;
Stephens, J. C.; Kedei, N.; Lewin, N. E.; Peach, M. L.; Blumberg, P. M.
Angew. Chem., Int. Ed. 2010, 49, 4580–4584.
(16) (a) Wender, P. A.; DeChristopher, B. A.; Schrier, A. J. J. Am.
Chem. Soc. 2008, 130, 6658–6659. (b) For an overview of bryostatin
analogue synthesis by the Wender group, see: Wender, P. A.; Baryza,
J. L.; Hilinski, M. K.; Horan, J. C.; Kan, C.; Verma, V. A. In Drug
Discovery Research: New Frontiers in the Post-Genomic Era; Huang, Z., Ed.;
Wiley: Hoboken, NJ, 2007; pp 127-162.
(17) Iyer, K.; Rainier, J. D. J. Am. Chem. Soc. 2007, 129, 12604–
12605.
(18) Similar aldols have been used previously by Evans (ref 11b),
Wender (ref 16b), and us (refs 14 and 15). The yield in the present
instance is noteworthy, however.
(19) Keck, G. E.; Welch, D. S.; Poudel, Y. B. Tetrahedron Lett. 2006,
47, 8267–8270.
(20) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
spectral data. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
keck@chem.utah.edu
’ ACKNOWLEDGMENT
Financial support of this research by the National Institutes of
Health through Grant GM-28961 is gratefully acknowledged. We
thank Dr. Peter Blumberg of the NCI for supplying an authentic
sample of bryostatin 1 for comparison purposes.
’ REFERENCES
(1) Pettit, G. R.; Herald, C. L.; Doubek, D. L.; Herald, D. L. J. Am.
Chem. Soc. 1982, 104, 6846–6848.
(2) For reviews of the bryostatins and bryostatin analogues, see:
(a) Hale, K. J.; Hummersome, M. G.; Manaviazar, S.; Frigerio, M. Nat.
Prod. Rep. 2002, 19, 413–453. (b) Hale, K. J.; Manaviazar, S. Chem.—
Asian J. 2010, 5, 704–754.
Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
(21) Sharpless, K. B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.;
Hartung, J.; Jeong, K. S.; Kwong, H. L.; Morikawa, K.; Wang, Z. M. J. Org.
Chem. 1992, 57, 2768–2771.
(22) (a) Tanaka, K.; Ohta, Y.; Fuji, K. Tetrahedron Lett. 1993, 34,
4071–4074. (b) For a previous use of the reagent in bryostatin synthesis,
see refs 11b and 11c.
(3) Sudek, S.; Lopanik, N. B.; Waggoner, L. E.; Hildebrand, M.;
Anderson, C.; Liu, H.; Patel, A.; Sherman, D. H.; Haygood, M. G. J. Nat.
Prod. 2007, 70, 67–74.
(23) Lipschutz, B. H.; Harvey, D. F. Synth. Commun. 1982, 14,
267–277.
(24) Chmurny, G. N.; Koleck, M. P. Magn. Reson. Chem. 1991, 29,
366–374.
(4) (a) Schwartz, G. K.; Shah, M. A. J. Clin. Oncol. 2005, 23, 9408–
9421. (b) Wang, S.; Wang, Z.; Dent, P.; Grant, S. Blood 2003, 101, 3648–
3657.
(5) Way, K. J.; Katai, N.; King, G. L. Diabetic Med. 2001, 18,
945–959.
(6) Sun, M.-K.; Hongpaisan, J.; Nelson, T. J.; Alkon, D. L. Proc. Natl.
Acad. Sci. U.S.A. 2008, 105, 13620–13625.
(7) (a) Etcheberrigaray, R.; Tan, M.; Dewachter, I.; Kuipꢀeri, C.; Van
der Auwera, I.; Wera, S.; Qiao, L.; 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) Alkon, D. L.; Sun, M.-K.; Nelson, T. J. Trends
Pharmacol. Sci. 2007, 28, 51–60.
(8) For further information, see: http://www.brni.org.
(9) (a) Dell'Aquilla, M. L.; Harold, C. L.; Kamano, Y.; Pettit, G. R.;
Blumberg, P. M. Cancer Res. 1988, 48, 3702–3708.
(10) (a) Reyland, M. E.; Insel, P. A.; Messing, R. O.; Dempsey, E. C.;
Newton, A. C.; Mochly-Rosen, D.; Fields, A. P. Am. J. Physiol.: Lung Cell.
Mol. Physiol. 2000, 279, 429–438. (b) Griner, E. M.; Kazanietz, M. G.
Nat. Rev. Cancer 2007, 7, 281–294. (c) Roffey, J.; Rosse, C.; Linch, M.;
Hibbert, A.; McDonald, N. Q.; Parker, P. J. Curr. Opin. Cell Biol. 2009,
21, 1–12.
(11) (a) 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. (b) 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.
(c) Ohmori, K.; Ogawa, Y.; Obitsu, T.; Ishikawa, Y.; Nishiyama, S.;
Yamamura, S. Angew. Chem., Int. Ed. 2000, 39, 2290–2294. (d) Manaviazar,
S.; Frigerio, M.; Bhatia, G. S.; Hummersone, M. G.; Aliev, A. E.; Hale, K. J.
Org. Lett. 2006, 8, 4477–4480. (e) Trost, B. M.; Dong, G. Nature 2008,
456, 485–488. (f) Trost, B. M.; Dong, G. J. Am. Chem. Soc. 2010, 132,
16403–16416. (g) For other synthetic work directed toward the bryo-
statins, see ref 2.
(12) Wender, P. A.; DeBrabander, J.; Harran, P. G.; Jimenez, J.-M.;
Koehler, M. F. T.; Lippa, B.; Park, C.-M.; Shiozaki, M. J. Am. Chem. Soc.
1998, 120, 4534–4535.
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