Efficient synthetic access to a new family of highly potent bryostatin analogues via a prins-driven macrocyclization strategy

Article abstract of DOI:10.1021/ja8015632

The step-economical synthesis of a new class of bryostatin analogues that contain the complete oxycarbocyclic core ring system of the bryostatin natural products is reported. These agents are convergently assembled via a highly efficient, functional-group-tolerant, and stereoselective Prins-driven macrocyclization. These tetrahydropyranyl B-ring analogues are among our most potent and efficacious analogues to date, exhibiting nanomolar and picomolar activities in protein kinase C affinity assays as well as in cellular antiproliferation assays. Copyright

Full text of DOI:10.1021/ja8015632

Published on Web 05/02/2008
Efficient Synthetic Access to a New Family of Highly Potent Bryostatin
Analogues via a Prins-Driven Macrocyclization Strategy
Paul A. Wender,* Brian A. DeChristopher, and Adam J. Schrier
Departments of Chemistry and Chemical and Systems Biology, Stanford UniVersity, Stanford, California 94305-5080
Received March 1, 2008; E-mail: wenderp@stanford.edu
Bryostatin 1, the prototypical member of a class of structurally
complex macrolides isolated from the marine bryozoan Bugula neritina,
exhibits remarkable biological activity, including potent antineoplastic
activity, restoration of apoptotic function, immune system stimulation,
and reversal of multidrug resistance.1 Recent clinical trials using
bryostatin in combination with other agents have shown significant
promise for the treatment of certain cancers.2 The exceptional potency
of bryostatin is such that only ∼1.5 mg is needed for a full eight-
week clinical treatment cycle.
Recently and quite significantly, this natural product has also been
shown to enhance cognition and memory in animals,3 suggesting its
potential use in the treatment of neurological disorders, including
Alzheimer’s disease, depression, and other cognitive impairments.4
Bryostatin’s activities are thought to arise in part from its modulation
of protein kinase C (PKC), a family of C1 domain containing kinases
implicated in a variety of biological processes and disease states.5 Other
C1 domain proteins have also been suggested as possible targets.6
There are three overarching and interconnected problems that have
impeded clinical advancement of the bryostatin lead. It is naturally
scarce and difficult to isolate. Second, synthetic routes are not as yet
amenable to the step-economical production of the natural product.7,8
Finally, it is difficult to modify as needed to access analogues for both
mechanistic and clinical studies. As a result of these combined
problems, the vast majority of preclinical and clinical work has been
done only on bryostatin 1; very few analogues have been prepared
and even fewer have been advanced preclinically. Facile and scalable
access to analogues is critically needed as bryostatin is not optimized
for clinical use.
Given that the activity of a molecule can often be attributed to a
subset of its functionalities, functionally superior yet structurally simpler
molecules amenable to practical synthesis can be designed to address
the above-noted bryostatin problems.1a The success of this function-
oriented synthesis strategy has been demonstrated by simplified
analogue 4,9 which is more active than bryostatin but is prepared in
40 fewer steps.
Analogue 4 has now been moved into in vitro and in vivo preclinical
evaluation. Because of the enormity of this effort, it has become
increasingly important to define whether the hydropyranyl B-ring of
the natural product offers any advantage in efficacy or off-target
selectivity over the dioxane B-ring common thus far to all potent
analogues. To address this need, we have targeted analogues 1-3,
which incorporate the complete oxycarbocyclic core ring system of
the bryostatins. We report herein a highly efficient, step-economical
route to this new analogue class based on a novel Prins-driven
macrocyclization strategy.
Our approach to dioxane 4 involves an acetalization-driven mac-
rocyclization procedure that proceeds putatively through the formation
of an oxocarbenium ion (Figure 1a) and its capture by a suitably
situated oxygen nucleophile. We reasoned that the targeted hydro-
pyranyl systems might be formed by positioning a suitable carbon
nucleophile in place of the nucleophilic oxygen (Figure 1b), setting
the stage for a Prins-driven macrocyclization.10 The Prins reaction has
Figure 1. Bryostatin 1, new analogues 1-3, representative B-ring dioxane
analogue 4, and strategies for their synthesis.
found much use in the construction of hydropyranyl ring systems, but
its adaptation to a process that concomitantly closes a macrocycle is
rare.11 This strategy gains added significance in the case of bryostatin
as all total syntheses have relied on an often demanding Julia reaction12
in combination with a lactonization to produce the macrocycle.
Attempts to replace the Julia reaction with milder processes have thus
far encountered difficulties.13
The synthesis of analogues 1-3 was planned to proceed from a
common intermediate 13, the product of a Prins-driven macrocycliza-
tion of intermediate 12. The latter would, in turn, be convergently
assembled from recognition domain 109 and acid 9. The synthesis of
9 started with the known intermediate 5 (available in seven steps),14
which was protected as its triethylsilyl ether (Scheme 1) and was
converted via a Bunnelle reaction15 in 69% yield to the corresponding
allylsilane 7. Intermediate 7 was debenzylated using lithium naph-
thalenide, affording alcohol 8 in 93% yield. Oxidation of 8 cleanly
provided the acid 9, which was subsequently esterified with recognition
domain 10 using Yamaguchi’s conditions.16 The triethylsilyl ether was
then removed with pyridinium para-toluenesulfonate to reveal mac-
rocyclization precursor 12.
Gratifyingly, upon exposure to TMS-OTf,17 intermediate 12 was
cleanly converted, with complete diastereoselectivity and excellent yield
(93%), into bryopyran 13. Notably, this constitutes a rare and
exceptionally high yielding example of a Prins-driven macrocyclization
and one of the most complex examples of any Prins cyclization.
Bryostatin analogue 1 was revealed in 77% yield upon exposure of
13 to HF•pyridine.
The conversion of bryopyran 13 into enoates 2 and 3 was next
explored (Scheme 2). The C13 exocyclic olefin in 13 was chemose-
lectively cleaved with ozone (1 equiv) to afford ketone 14 in 88%
yield. Treatment of 14 with the sodium anion of trimethyl phospho-
noacetate gave an E:Z mixture of the corresponding enoates in 87%
9
6658 J. AM. CHEM. SOC. 2008, 130, 6658–6659
10.1021/ja8015632 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Binding and Growth Inhibitory Data
Scheme 1. Synthesis of Bryostatin Analogue 1a
ID
PKC Ki (nM)
K562 (nM)d
MV411 (nM)d
a
1
2
3
4
2.3 ( 0.6
4 ( 1
0.47 ( 0.09
15 ( 2
1.4 ( 0.7
0.17 ( 0.05
0.4 ( 0.2
1.4 ( 0.3
1.63 ( 0.08
b
2.5 ( 0.1
a
0.9 ( 0.2
a,c
3.1 ( 0.3
a Average of two experiments. b Average of four experiments. c See
Supporting Information for discussion. d EC50 values are an average of
three experiments. All error bars indicate standard error of the mean.
Acknowledgment. Support of this work provided by the NIH
(CA31845) is gratefully acknowledged. We also thank Professor
Daria Mochly-Rosen for access to assay materials.
Supporting Information Available: Experimental procedures,
spectral data, and assay protocol. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) (a) Wender, P. A.; Baryza, J. L.; Hilinski, M. K.; Horan, J. C.; Kan, C.;
Verma, V. A. Beyond Natural Products: Synthetic Analogues of Bryostatin
1. In Drug DiscoVery Research: New Frontiers in the Post-Genomic Era;
Huang, Z., Ed.; Wiley-VCH: Hoboken, NJ, 2007; pp 127-162. (b) Hale,
K. J.; Hummersone, M. G.; Manaviazar, S.; Frigerio, M. Nat. Prod. Rep.
2002, 19, 413–453.
(2) (a) For an important combination trial, see: Dowlati, A.; Lazarus, H. M.;
Hartman, P.; Jacobberger, J. W.; Whitacre, C.; Gerson, S. L.; Ksenich, P.;
Cooper, B. W.; Frisa, P. S.; Gottlieb, M.; Murgo, A. J.; Remick, S. C.
Clin. Cancer Res. 2003, 9, 5929–5935. (b) For a review of clinical
performance, see: Kortmansky, J.; Schwartz, G. K. Cancer InVest. 2003,
21, 924–936. (c) For ongoing studies, see http://clinicaltrials.gov/.
(3) Hongpaisan, J.; Alkon, D. L. Proc. Natl. Acad. Sci. U.S.A. 2007, 104,
19571–19576.
a Reagents and conditions: (a) TES-Cl, imidazole, CH2Cl2, rt, 97%; (b) (i)
CeCl3, TMSCH2MgCl, THF, -78 °C f rt, 12 h, (ii) SiO2, CH2Cl2, rt, 69%;
(c) Li0, naphthalene, THF, -25 °C, 93%; (d) TPAP (10 mol %), NMO (3
equiv), powdered 4Å MS, CH2Cl2, rt; (e) NaClO2, NaH2PO4, 2-methyl-2-butene,
2:1 t-BuOH:H2O, 0 °C, 89% from 8; (f) 9, 2,4,6-trichlorobenzoyl chloride,
Et3N, toluene, then 10, DMAP, rt; (g) PPTS (30 mol %), 1:4 H2O:THF, rt,
71% from 9; (h) TMS-OTf, Et2O, -78 f 0 °C, 93%; (i) HF•py, THF, rt, 77%.
Scheme 2. Synthesis of Bryostatin Analogues 2 and 3a
(4) Etcheberrigaray, R.; Tan, M.; Dewachter, I.; Kuiperi, 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.
(5) Dempsey, E. C.; Newton, A. C.; Mochly-Rosen, D.; Fields, A. P.; Reyland,
M. E.; Insel, P. A.; Messing, R. O. Am. J. Physiol. Lung Cell Mol. Physiol.
2000, 279, L429–L438.
(6) Kazanietz, M. G. Mol. Pharmacol. 2002, 61, 759–767.
(7) (a) Bryostatin 7: 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) Formal syntheses of bryo 7: Manaviazar, S.; Frigerio,
M.; Bhatia, G. S.; Hummersone, M. G.; Aliev, A. E.; Hale, K. J. Org.
Lett. 2006, 8, 4477–4480. (c) Bryostatin 2: Evans, D. A.; Carter, P. H.;
Carreira, E. M.; Charrette, A. B.; Prunet, J. A.; Lautens, M. J. Am. Chem.
Soc. 1999, 121, 7540–7552. (d) Bryostatin 3: Ohmori, K.; Ogawa, Y.;
Obitsu, T.; Ishikawa, Y.; Nishiyama, S.; Yamamura, S. Angew. Chem.,
Int. Ed. 2000, 39, 2290–2294.
(8) For a promising biosynthetic approach, see: (a) 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.
(9) Wender, P. A.; Baryza, J. L.; Bennett, C. E.; Bi, F. C.; Brenner, S. E.;
Clarke, M. O.; Horan, J. C.; Kan, C.; Lacote, E.; Lippa, B.; Nell, P. G.;
Turner, T. M. J. Am. Chem. Soc. 2002, 124, 13648–13649.
(10) For an impressive intermolecular Prins approach to the bryopyran core,
see: Keck, G. E.; Truong, A. P. Org. Lett. 2005, 7, 2153–2156.
(11) (a) Schulte-Elte, K. H.; Hauser, A.; Ohloff, G. HelV. Chim. Acta 1979, 62,
2673–2680. (b) Custar, D. W.; Zabawa, T. P.; Scheidt, K. A. J. Am. Chem.
Soc. 2008, 130, 804–805. (c) Woo, S. K.; Kwon, M. S.; Lee, E. Angew.
Chem., Int. Ed. 2008, in press.
(12) All bryostatin syntheses require g14 operations after their respective A-,
B-, and C-ring fragments have been conjoined. See ref 7.
(13) For an example of this issue, see: Trost, B. M.; Yang, H.; Thiel, O. R.;
Frontier, A. J.; Brindle, C. S. J. Am. Chem. Soc. 2007, 129, 2206–2207.
(14) Wender, P. A.; Mayweg, A. V. W.; VanDeusen, C. L. Org. Lett. 2003, 5,
277–279.
(15) Narayanan, B. A.; Bunnelle, W. H. Tetrahedron Lett. 1987, 28, 6261–6264.
(16) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull. Chem.
Soc. Jpn. 1979, 52, 1989–1993.
(17) (a) Mekhalfia, A.; Marko, I. E.; Adams, H. Tetrahedron Lett. 1991, 32,
4783–4786. (b) For the formation of 2,6-disubstituted pyrans: Keck, G. E.;
Covel, J. A.; Schiff, T.; Yu, T. Org. Lett. 2002, 4, 1189–1192.
(18) Alternatively, a 1:3 E:Z selectivity is obtained with the sodium salt of a
(R)-BINOL-derived phosphonate ester: Tanaka, K.; Ohta, Y.; Fuji, K.
Tetrahedron Lett. 1993, 34, 4071–4074.
(19) 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.
JA8015632
a Reagents and conditions: (a) O3, CH2Cl2, -78 °C, then thiourea, CH2Cl2:
MeOH, rt, 88%; (b) trimethyl phosphonoacetate, NaHMDS, THF, 0 °C, 87%
combined yield, 1:1.05 E:Z; (c) HF•py, THF, rt, 83% combined yield, 32%
isolated 2, 37% isolated 3.
combined yield.18 This mixture was desilylated using HF•pyridine to
reveal the C13-enoate analogues 2 and 3 in 32 and 37% yield,
respectively.
In accord with our pharmacophoric hypothesis,19 analogues 1, 2,
and 3 were found to be potent ligands for PKC, with Ki values of 1.6,
2.5, and 0.9 nM, respectively. Given these potent Ki values, we were
motivated to test the antileukemic activity of analogues 1-3 in vitro.
Significantly, these compounds exhibit nanomolar and subnanomolar
EC50 values for growth inhibition of K562 human erythroleukemia
cells and MV411 B-myelomonocytic leukemia cells (Table 1).
In summary, the first members of a new class of potent bryostatin
analogues containing the complete bryostatin oxycarbocyclic ring
system have been synthesized using a novel Prins-driven macrocy-
clization. Given the remarkable functional group tolerance and
efficiency of this process, it is an attractive strategy for the convergent
synthesis of tetrahydropyran containing targets. Significantly, initial
in vitro assays indicate that these analogues are our most active
compounds made to date, exceeding the antiproliferative activity of
our most potent dioxane analogue 4 by up to 2 orders of magnitude.
Further studies on these promising leads will be reported in due course.
9
J. AM. CHEM. SOC. VOL. 130, NO. 21, 2008 6659