Synthesis of the first members of a new class of biologically active bryostatin analogues [2]

Full text of DOI:10.1021/ja9727631

4534
J. Am. Chem. Soc. 1998, 120, 4534-4535
Synthesis of the First Members of a New Class of
Biologically Active Bryostatin Analogues
Paul A. Wender,* Jef De Brabander, Patrick G. Harran,
Juan-Miguel Jimenez, Michael F. T. Koehler, Blaise Lippa,
Cheol-Min Park, and Makoto Shiozaki
Department of Chemistry, Stanford UniVersity
Stanford, California 94305-5080
ReceiVed August 7, 1997
The bryostatins are a novel family of emerging cancer
chemotherapeutic candidates isolated from marine bryozoa on the
basis of their significant activity against murine P388 lymphocytic
leukemia.1 These macrolactones have been shown to exhibit
remarkable and unique activities,2 leading to the recent entry of
bryostatin 1 (Figure 1) into Phase II clinical trials for the treatment
of melanoma, non-Hodgkins lymphoma, and renal cancer.3 While
their molecular mode of action is not known, the bryostatins
potently inhibit the binding of the tumor-promoting phorbol esters
to protein kinase C (PKC) and stimulate enzymatic activity both
in vitro and in vivo.4 However, they induce only a subset of
phorbol ester responses and block those actions of phorbol esters
which they themselves do not initiate, most notably tumor
promotion.5 Efforts to identify the structural basis for these and
related activities and to develop more effective clinical candidates
have been hampered by the low natural abundance1b of the
bryostatins and difficulties associated with their modification. As
an alternative approach to these goals, we describe here the first
class of simplified, synthetic bryostatin analogues which exhibit
a high affinity for PKC and potent growth inhibitory activity
against several human cancer cell lines.
Figure 1.
Scheme 1a
Computational studies,6 limited structure-activity data,1b,6,7 and
analogy to diacylglycerol, the endogenous activator of PKC,
suggest that the binding of bryostatin to PKC could be attributed
to substituents at C1, C19, and C26 (boxed in Figure 1), whose
orientations are remotely controlled by a lipophilic spacer (shaded
in Figure 1). Macrocycles of the general structure 1 were
designed to test this hypothesis. These systems retain the putative
recognition domain of the bryostatins but incorporate a simplified
spacer domain to facilitate their synthesis. This design allows
access to 1 through a novel, convergent esterification-mac-
rotransacetalization strategy involving coupling of the recognition
domain (2) with variable spacers (3), an approach which has
potential for the creation of analogue libraries.
a (a) 4, 2 equiv of LDA, THF, -78 °C, 1 h then 5, -78 °C, 30 min,
98%. (b) cat. pTsOH, toluene, room temperature (rt), 6a (41%), 6b
(49%). (c) 6a, NaBH4, CeCl3‚7H2O, MeOH, -20 °C. (d) m-CPBA,
NaHCO3, 2:1 CH2Cl2/MeOH, 71%, two steps. (e) PhCOCl, DMAP,
CH2Cl2, -10 °C; Dess-Martin periodinane, rt, 90%. (f) SmI2, THF,
MeOH, -78 °C, 95%. (g) LDA, OHCCO2Me, THF, -78 °C, 90%
based on recovered 8. (h) ClSO2Me, Et3N, CH2Cl2, -10 °C. (i) DBU,
THF, rt, 78%, two steps. (j) NaBH4, CeCl3‚7H2O, MeOH, -20 °C. (k)
C7H15CO2H, 2,4,6-trichlorobenzoyl chloride, Et3N, toluene, rt, 93%,
two steps. (l) HF/pyridine, THF, rt. (m) Dess-Martin periodinane,
CH2Cl2, rt, 86%, two steps. (n) allyl-BEt2, Et2O, -10 °C. (o) Ac2O,
DMAP, CH2Cl2, 95%, two steps. (p) cat. OsO4, NMO, THF/H2O. (q)
Pb(OAc)4, Et3N, PhH; DBU, rt, 80%, two steps. (r) DDQ, CH2Cl2,
H2O, 79%. (s) HF, CH3CN, H2O, rt, g95%.
Our first objective in this study was the synthesis of the
bryostatin C-ring and its attendant functionality (C15-C27).8
Scheme 1 depicts a first-generation sequence which has readily
(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) The bryostatin
family of macrolactones currently numbers 20: Pettit, G. R. J. Nat. Prod.
1996, 59, 812-821.
(2) (a) Kraft, A. S.; Woodley, S.; Pettit, G. R.; Gao, F.; Coll, J. C.; Wagner,
F. Cancer Chemother. Pharmacol. 1996, 37, 271-278. (b) Szallasi, Z.; Du,
L.; Levine, R.; Lewin, N. E.; Nguyen, P. N.; Williams, M. D.; Pettit, G. R.;
Blumberg, P. M. Cancer Res. 1996, 56, 2105-2111 and references therein.
(3) Current information on the scope and status of bryostatin clinical trials
can be found on the NCI web page at: http://cancernet.nci.nih.gov/prot/
protsrch.shtml.
(4) (a) Kraft, A. S.; Smith, J. B.; Berkow, R. L. Proc. Natl. Acad. Sci.
U.S.A. 1986, 83, 1334-1338. (b) Berkow, R. L.; Kraft, A. S. Biochem.
Biophys. Res. Commun. 1985, 131, 1109-1116. (c) Ramsdell, J. S.; Pettit,
G. R.; Tashjian, A. H., Jr. J. Biol. Chem. 1986, 261, 17073-17080.
(5) Gschwendt, M.; Fu¨rstenberger, G.; Rose-John, S.; Rogers, M.; Kittstein,
W.; Pettit, G. R.; Herald, C. L.; Marks, F. Carcinogenesis 1988, 9, 555-562.
(6) (a) 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., in press. (b) 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.
(7) Pettit, G. R.; Sengupta, D.; Blumberg, P. M.; Lewin, N. E.; Schmidt,
J. M.; Kraft, A. S. Anti-Cancer Drug Des. 1992, 7, 101-113.
delivered gram quantities of the target fragment 15. Condensation
of the dienolate of 49 with aldehyde 510 followed by acid-catalyzed
(8) Bryostatin fragment syntheses: (a) De Brabander, J.; Vandewalle, M.
Pure Appl. Chem. 1996, 68, 715-718. (b) Kalesse, M.; Eh, M. Tetrahedron
Lett. 1996, 37, 1767-1770. (c) Lampe, T. F. J.; Hoffmann, H. M. R.
Tetrahedron Lett. 1996, 37, 7695-7698. (d) Ohmori, K.; Nishiyama, S.;
Yamamura, S. Tetrahedron Lett. 1995, 36, 6519-6522. (e) For a review
covering bryostatin synthesis through the end of 1994, including work from
the groups of Evans, Hale, Roy, Vandewalle, and Yamamura and Masamune’s
total synthesis of bryostatin 7, see: Norcross, R. D.; Paterson, I. Chem. ReV.
1995, 95, 2041-2114.
S0002-7863(97)02763-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/23/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 18, 1998 4535
Scheme 2a
dehydration gave pyranones 6a,b (1:1). The â-isomer (6a) was
easily separated and reduced under Luche conditions,11 and the
resulting glycal was epoxidized with m-CPBA in the presence of
MeOH to afford a C19-methoxylated C20,C21-diol (71% from
6a). Selective benzoylation of the C21 equatorial alcohol
followed by oxidation of the remaining C20 hydroxyl group with
Dess-Martin periodinane12 (DMP) provided benzoate 7 (90%,
two steps). Treatment of 7 with SmI2 (2 equiv) selectively
deoxygenated C21 to give ketone 8 (95%). From 8, a straight-
forward aldol condensation/elimination sequence with OHCCO2-
Me13 was sufficient to install the desired E-exocyclic unsaturated
ester at C21, providing enone 9. Luche reduction11 of 9 gave
exclusively the C20 axial alcohol which was esterified14 with
octanoic acid to afford compound 10 (93%, two steps). Comple-
tion of the target fragment, requiring a demanding two carbon
homologation at C17, proceeded with removal of the TBS group
with HF/pyridine and oxidation of the resulting alcohol with
DMP12 to give aldehyde 12 (86%, two steps). After much
experimentation, this hindered aldehyde was found to react with
allyl-BEt2 followed by Ac2O to generate an inconsequential
mixture of acetates in high yield (95%, two steps). Dihydroxyl-
ation of the terminal olefin with catalytic OsO4 (NMO co-oxidant)
and Pb(OAc)4-mediated glycol cleavage in the presence of Et3N
and DBU afforded enal 13 (80%, two steps). Exposure of 13 to
DDQ selectively liberated the C25 alcohol to give 14 (79%),
which, when treated with aqueous HF, gave the target hemiketal
15 in >95% yield.
With an efficient route to the C15-C27 segment of the
bryostatins established, attention turned toward installing our first
series of C1-C14 inserts. Molecular modeling indicated that
B-ring acetal/A-ring pyrans of type 24a (Scheme 2) would closely
mimic the conformation of bryostatin, allowing for the proper
display of putative PKC recognition elements.6a Accordingly,
menthone-derived spacer segments 16 and 1715 were prepared
independently and coupled by Yamaguchi esterification14 with
alcohol 15 to provide ester adducts 18 (81%) and 19 (81%),
respectively. For 19, the C3 TES group was removed with HF/
pyridine (81%). At this point, a remarkable macrotransacetal-
ization was initiated by stirring 18 and 20 independently in a dilute
(0.004 M) solution of Amberlyst-15 acidic resin with 4-Å
molecular sieves in CH2Cl2. This pivotal reaction served to close
the 20-membered macrocycle via acetal formation.16 In each case,
a single isomer of cyclized product was detected in the crude
macrocyclization mixture; a result consistent with a thermody-
namically controlled acetalization establishing the equatorial
configuration at C15.17 The cyclized products were independently
hydrogenated over Pd(OH)2 to afford bryostatin analogues 23
(56% from 18) and 24a (88% from 20).
a (a) 2,4,6-Trichlorobenzoyl chloride, Et3N, toluene, rt, then 15,
DMAP: 18, 81% from 16; 19, 81% from 17. (b) 1:1 HF:pyridine, THF,
rt, 81%. (c) Amberlyst-15, 4-Å molecular sieves, CH2Cl2, rt. (d) cat.
Pd(OH)2/C, H2 (1 atm), EtOAc, rt: 23, 56% from 18; 24a, 88% from
20. (e) Ac2O, DMAP, CH2Cl2, rt, 85%.
Acetals 23 and 24a bind rat brain PKC isozymes with Ki )
297 and 3.4 nM, respectively. As previously found for bryosta-
tin,6 acylation of the C26 hydroxyl group of 24a produced
compound 24b with substantially reduced affinity for PKC (>10
µM). Most importantly, analogue 24a shows significant leVels
(1.8-170 ng/mL) of in Vitro growth inhibitory actiVity against
seVeral human tumor cell lines.6a
In summary, this study establishes a novel and effective route
to the first generation of simplified biologically active analogues
of bryostatin. An esterification-macrotransacetalization strategy
allows for a modified bryostatin ring system to be convergently
assembled from readily prepared subunits with high efficiency.
Of the first analogues tested, acetal 24a exhibits potent affinity
for PKC and exceptional growth inhibitory activity. Efforts to
further simplify these new leads, to elucidate the molecular basis
for bryostatin’s activity, and to develop improved clinical
candidates of bryostatin are in progress.
(9) Prepared in four steps from commercially available methyl isopropyl
ketone (see the Supporting Information).
(10) Synthesized in five steps from (R)-(+)-methyl lactate according to
minor modifications of a closely related sequence. See ref 8a.
(11) Luche, J. L.; Rodriguez-Hahn, L.; Crabbe´, P. J. Chem. Soc., Chem.
Commun. 1978, 601-602.
Acknowledgment. Support of this work through a grant (CA31845)
provided by the National Institutes of Health is gratefully acknowledged.
HRMS analyses were performed at the UC San Francisco and the UC
Riverside Mass Spectrometry Facilities. Fellowship support from the
following sources is also gratefully recognized: Fulbright-Hays/NATO
(J.D.B.), NIH (P.G.H.), Fulbright/Spanish Ministry of Education and
Science (J.-M.J.), Eli Lilly (B.L., M.F.T.K.), the Korean Science and
Engineering Foundation (C.-M.P.), and Japan Tobacco (M.S.).
(12) (a) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277-
7287.
(13) Kelly, T. R.; Schmidt, T. E.; Haggerty, J. G. Synthesis 1972, 544-
545.
(14) (a) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989-1993.
(15) Prepared in seven (16) and eleven (17) steps from the 1,3-menthone
acetal of 1,3,5-pentanetriol (see the Supporting Information).
(16) To our knowledge, there is only a single report of a related
transformation: Li, G.; Still, W. C. J. Am. Chem. Soc. 1993, 115, 3804-
3805.
Supporting Information Available: Spectroscopic data and experi-
mental procedures for reported compounds (24 pages, print/PDF). See
any current masthead page for ordering information and Web access
instructions.
(17) The all-equatorial configuration of 24a was confirmed by 2D-NOESY
1H-NMR analysis. See ref 6a.
JA9727631