C-H bond activation of hydrocarbon segments in complex organic molecules: Total synthesis of the antimitotic rhazinilam [18]

Full text of DOI:10.1021/ja0003223

J. Am. Chem. Soc. 2000, 122, 6321-6322
6321
C-H Bond Activation of Hydrocarbon Segments in
Complex Organic Molecules: Total Synthesis of the
Antimitotic Rhazinilam
James A. Johnson and Dalibor Sames*
Department of Chemistry, Columbia UniVersity
New York, New York 10027
ReceiVed January 31, 2000
Figure 1. Selective C-H functionalization of diethyl alkane segment
C-H bond activation represents a fundamental chemical
process of broad synthetic consequence.1 To harness the full
potential of these intriguing processes, the C-H activation step
must be accompanied by a functionalization event, such as
â-hydride elimination (dehydrogenation)2 or atom group transfer
(oxygen,3 carbene,4 and boron5). The possibility of programmable
site selectivity of these transformations would provide entirely
new opportunities to the synthetic community.6 Despite significant
advances in this area, most transition metal complexes capable
of C-H bond activation are intolerant to functional groups and
have a strong preference for the activation of aryl C-H bonds.7
By assuming the possibility of overcoming such boundaries, novel
and unique strategies for the assembly of complex organic
molecules can be envisioned. We wish to communicate our
investigations where such constraints were overcome and selective
C-H functionalization (dehydrogenation) was achieved as the
key step in the complete synthesis of the natural product
rhazinilam (Figure 1).
After a stimulating discussion with Professor Erik J. Sorensen,
the antitumor agent rhazinilam, a member of the Aspidosperma
class of alkaloids, was selected as our first target.8 The assembly
of rhazinilam would be drastically simplified by selective activa-
tion of diethyl intermediate 1 (Figure 1). The quaternary stereo-
genic center, a central element of the molecule, would then be
constructed via selective functionalization of the prochiral ethyl
groups. This proposal posed a daunting challenge, considering
the multiple functionalities present in 1.
defines strategy for rhazinilam assembly.
Scheme 1a
a Conditions: (a) DMF, 100 °C, 90%; (b) Ag2CO3 (2 equiv), toluene,
reflux, 70%; (c) CCl3COCl; (d) NaOMe, MeOH; (e) H2 (l atm), Pd/C,
88% for steps c-e.
formation of a Schiff base linkage, followed by a metal com-
plexation. Molecular model analysis showed that a carefully
designed system was needed for the selective delivery of an
activated metal complex to the ethyl group in question.
In the first phase of the investigation, intermediate 1 was
synthesized in an efficient sequence as depicted in Scheme 1.
Iminium salt 4 was generated from readily available imine 210
and o-nitrocinnamyl bromide 3. Heating of 4 in the presence of
silver carbonate accomplished both cyclization and aromatization
yielding pyrrole intermediate 5 in 70% yield.11 The methyl
carboxylate group was then installed as a temporary protection
to stabilize the electrophile-sensitive pyrrole ring, followed by
reduction of the nitro group to furnish amine 1.
The initial exploratory stage, involving a variety of metals and
ligands, directed our focus toward cationic platinum(II) complexes
which have previously been shown to activate methane.12 Also,
sp2-hybridized nitrogen atoms are suitable ligands for stable and
active platinum complexes in the contex of C-H bond activa-
tion.13 Thus, dimethyl platinum complex 7 (Figure 2) was
constructed via Schiff base preparation, followed by treatment
with [Me2Pt(µ-SMe2)]214 (Supporting Information). The generation
of a platinum cation was then to be achieved via the action of a
weakly coordinating acid. A detailed correlation between the
structure (X-ray crystallography) and reactivity of the following
systems (see below) provided the critical insights which guided
our explorations.
Our approach builds on the opportunity afforded by the
proximity of the amino group to the ethyl groups, a favorable
scenario for directed C-H activation.9 According to this outline,
selected ligands would be attached to intermediate 1 via the
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10.1021/ja0003223 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/16/2000

6322 J. Am. Chem. Soc., Vol. 122, No. 26, 2000
Communications to the Editor
Figure 2. (A) Selective C-H bond activation of highly functionalized intermediate 1 via strategically placed platinum complex. (B) X-ray structure
of complex 8 (triflate anion removed for clarity). Selected bond lengths: Pt(2)-C(138) 2.165(8) Å; Pt(2)-N(5) 2.007(7) Å; Pt(2)-N(5) 2.033(7) Å.
Scheme 2a
Addition of triflic acid to complex 7 led to the rapid formation
of a new intermediate with concomitant loss of methane.
Interestingly, complex 8, possessing unusual coordination bonding
between the platinum metal and the pyrrole moiety was formed
(Figure 2B).15 Note that bond “a” is bent out of the plane of the
pyrrole ring, suggesting sp3-like character of the pyrrole carbon.
Heating of complex 8 in trifluoroethanol led to decomposition
providing intractable mixtures. To liberate the platinum cation
from the pyrrole ring and to facilitate its rotation toward the ethyl
group, the stability or formation of intermediate 8 had to be
a (a) KCN (0.5 M), CH2Cl2, H2O; NH2OH, MeOH, 60% for four steps
undermined. It had previously been demonstrated that cationic
from 9; (b) Boc2O, DMAP, 76%; (c) OsO4, NaIO4; (d) Ph3PdCHCO2tBu;
(e) H2, Pd/C, 70% for steps c-e; (f) TFA, CH2Cl2, 75%; (g) PyBOP,
HOBT, iPr2NEt; (h) NaOH (aq), MeOH then HCl (aq), 80% for g and h.
platinum complexes require a weakly coordinated ligand (e.g.,
H2O, pentafluoropyridine) which can readily depart and vacate a
coordination site for an alkane.16 Mindful of such prerequisites,
the installation of a bulky R group was proposed to disfavor
coplanarity of phenyl ring A and platinum complex B due to steric
congestion and, in turn, weaken pyrrole complexation to the
platinum metal in complex 8 (Figure 2). Thus, phenyl-substituted
complex 9 was synthesized from readily available phenylpyridyl
ketone and exposed to triflic acid (Supporting Information).
Although analogous complex 10 was still formed, as established
by an X-ray structure analysis,17 its reactivity profile proved to
be profoundly different. Remarkably, thermolysis of complex 10
proVided platinum hydride 11 as a single product in excellent
yield (>90%) as determined by proton NMR spectroscopy. Indeed,
actiVation of the desired ethyl group took place with concomitant
loss of methane, followed by â-H-elimination affording alkene-
hydride platinum(II) complex 11.
extension of the vinyl group and the subsequent macrocycle
closure was then carried out in a standard fashion. Namely,
transformation of the alkene double bond of 12 to an aldehyde
was followed by Horner-Emmons reaction, catalytic hydrogena-
tion, tert-butyl ester and Boc deprotection, and finally a macro-
lactam formation.19
In summary, the total synthesis of the antitumor agent rhazi-
nilam was achieved through a novel strategy centered on selective
C-H bond activation. Dehydrogenation of the ethyl group,
mediated by a platinum complex, was accomplished in the
presence of a variety of functional groups including an ester,
pyrrole and arene rings. Introduction of new C-H activation metal
catalysts to the realm of functionalized substrates opens an
exciting frontier in the assembly of complex organic molecules.
The platinum metal was subsequently removed via treatment
with aqueous potassium cyanide,18 followed by hydrolysis of the
resulting Schiff base in the presence of hydroxylamine. Isolation
and characterization of alkene 12 provided solid evidence for this
sequence (Scheme 2, 60% yield for a four-step sequence 9f12).
To complete the total synthesis of rhazinilam, a one-carbon
Acknowledgment. We thank Professor E. J. Sorensen, Dr. L. J.
Williams and Vitas Votier Chmelar for intellectual contributions.
Columbia University and The Camille and Henry Dreyfus Foundation
are acknowledged for financial support of this work. We gratefully
acknowledge Dr. Brian M. Bridgewater, David Churchill, and Professor
Ged Parkin (X-ray work), Dr. Y. Itagaki (MS), and Dr. J. B. Schwarz
(manuscript preparation).
(15) Figure 2 shows only one of two independent molecules found in the
ORTEP diagram. The second molecule is structurally similar to that shown
in Figure 2, with the exception that the platinum interaction with the pyrrole
possesses more η2 character (Supporting Information). See also: (a) Brunkan,
N. M.; White, P. S.; Gagne, M. R. J. Am. Chem. Soc. 1998, 120, 11002. (b)
Kocovsky, P.; Malkov, A. V.; Vyskocil, S.; Lloyd-Jones, G. C. Pure Appl.
Chem. 1999, 71, 1425-1433. (c) Meyers, W. H.; Sabat, M.; Harman, W. D.
J. Am. Chem. Soc. 1991, 113, 6682.
Supporting Information Available: Experimental details for 9-12.
Crystallographic data for 8. This material is available free of charge via
the Internet at http://pubs.acs.org.
JA0003223
(16) See ref 12.
(17) The X-ray structure of 10 will be disclosed elsewhere.
(18) Albrecht, M.; Gossage, R. A.; Spek, A. L.; van Koten, G. J. Am. Chem.
Soc. 1999, 121, 11898.
(19) Ratcliffe, A. H.; Smith, G. F.; Smith, G. N. Tetrahedron Lett. 1973,
5179.