Aqueous Titanium Trichloride Promoted Reductive Cyclization of o-Nitrostyrenes to Indoles: Development and Application to the Synthesis of Rizatriptan and Aspidospermidine

Article abstract of DOI:10.1002/anie.201505713

Treatment of o-nitrostyrenes with aqueous TiCl₃ solution at room temperature afforded indoles through a formal reductive C(sp²)-H amination process. A range of functions such as halides (Cl, Br), carbonyl (ester, carbamate), cyano, hydroxy, and amino groups were tolerated. From β,β-disubstituted o-nitrostyrenes, 2,3-disubstituted indoles were formed by a domino reduction/cyclization/migration process. Mild conditions, simple experimental procedure, ready accessibility of the starting materials and good to excellent yields characterize the present transformation. The methodology was used as a key step in a concise synthesis of rizatriptan and a formal total synthesis of aspidospermidine. Mild and efficient treatment of o-nitrostyrenes with aqueous TiCl₃ solution at room temperature afforded indoles through a formal reductive C(sp²)-Hamination process. A concise synthesis of a marketed drug (rizatriptan) and a formal total synthesis of aspidospermidine featuring this novel N-heterocyclization process are reported.

Full text of DOI:10.1002/anie.201505713

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505713
German Edition: DOI: 10.1002/ange.201505713
Synthetic Methods
Hot Paper
Aqueous Titanium Trichloride Promoted Reductive Cyclization of
o-Nitrostyrenes to Indoles: Development and Application to the
Synthesis of Rizatriptan and Aspidospermidine
Shuo Tong, Zhengren Xu, Mathias Mamboury, Qian Wang, and Jieping Zhu*
Abstract: Treatment of o-nitrostyrenes with aqueous TiCl3
solution at room temperature afforded indoles through
a formal reductive C(sp2)–H amination process. A range of
functions such as halides (Cl, Br), carbonyl (ester, carbamate),
cyano, hydroxy, and amino groups were tolerated. From b,b-
disubstituted o-nitrostyrenes, 2,3-disubstituted indoles were
formed by a domino reduction/cyclization/migration process.
Mild conditions, simple experimental procedure, ready acces-
sibility of the starting materials and good to excellent yields
characterize the present transformation. The methodology was
used as a key step in a concise synthesis of rizatriptan and
a formal total synthesis of aspidospermidine.
procedures for this reductive cyclization have been developed
using high-pressure carbon monoxide as a stoichiometric
reductant in the presence of a transition-metal catalyst.[12]
Recently, Driver and co-workers reported an efficient syn-
thesis of 3H-indoles by reductive cyclization of nitrostyrenes
in the presence of Mo(CO)6 and a catalytic amount of
Pd(OAc)2.[13] While o-nitrostyrenes have been recognized to
be ideal precursors for indoles, the potential of this trans-
formation has not been fully exploited because of the
moderate synthetic efficiency and harsh conditions often
associated with the established protocols. In connection with
our ongoing research program, we serendipitously discovered
that aqueous titanous chloride was able to promote the
reductive cyclization of o-nitrostyrenes to indoles in good to
excellent yields.[14] Literature search after our own observa-
tion revealed a single isolated example from the group of
Banwell.[15] We report herein the development and applica-
tions of this novel reductive C(sp2)–H amination process for
the synthesis of indoles. The characteristic features of our
reaction included mild conditions, good to excellent yields,
a wide application scope, and tolerance of functional groups
including the reducible ones.
T
he indole ring system, one of the most important hetero-
cycles, appears as a motif in a large number of bioactive
natural products, pharmaceuticals, and agrochemicals.[1] The
synthesis and functionalization of indoles have attracted
chemists for over a century and remain an active research
area.[2,3] Nitroarenes, due to their widespread availability and
their ease of synthesis,[4] are attractive starting materials for
the preparation of indoles as witnessed by the existence of
a number of named transformations such as Reissert indole
synthesis,[5] Leimgruber–Batcho reaction,[6] Bartoli indole
synthesis,[7] and Cadogan–Sundberg reaction (Scheme 1).[8]
The classic Cadogan–Sundberg conditions involved heat-
ing to reflux a solution of o-nitrostyrenes in neat triethyl
phosphite (b.p. 1568C).[9,10] The yields of the corresponding
indoles are typically moderate because of the concurrent
formation of N-hydroxy-, and N-ethoxyindoles.[11] Alternative
Conditions were surveyed using 1-(2’-nitrophenyl)-cyclo-
hexene (1a) as a test substrate (Table 1). Six equivalents of
titanous chloride were initially employed since this is the
theoretical amount of TiCl3 needed to reduce nitroarenes to
anilines. Performing the reaction in acetone in an ammonium
acetate-buffered solution afforded the tetrahydrocarbazole
2a in 58% yield. Further increasing the amount of TiCl3
Table 1: TiCl3-promoted reductive cyclization of o-nitrostyrenes: optimi-
zation of reaction conditions.[a]
NH4OAc [equiv][b]
2a [%][c]
Scheme 1. Reductive cyclization of o-nitrostyrenes.
Entry
TiCl3 [equiv]
Solvent
1
2
3
4
5
6
6.0
8.0
10.0
4.0
8.0
8.0
8.0
acetone
acetone
acetone
acetone
acetone
acetone
MeCN
33
33
33
33
25
0
58
63
65
32
67
90
90
73
[*] Dr. S. Tong,[+] Dr. Z. Xu,[+] M. Mamboury, Dr. Q. Wang, Prof. Dr. J. Zhu
Laboratory of Synthesis and Natural Products
Institute of Chemical Sciences and Engineering
Ecole Polytechnique FØdØrale de Lausanne
EPFL-SB-ISIC-LSPN, BCH 5304, 1015 Lausanne (Switzerland)
E-mail: jieping.zhu@epfl.ch
7[d]
8
0
0
8.0
MeOH
Homepage: http://lspn.epfl.ch
[+] These authors contributed equally to this work.
[a] Reaction conditions: 1a (0.2 mmol), TiCl3 (20% solution in 2.0n
HCl), solvent (c=0.2m), RT, 8 h. [b] NH4OAc (2.5n) was used as
a buffer solution. [c] Isolated yield. [d] c=0.4m.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505713.
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augmented slightly the yield of 2a, whereas reducing the
stoichiometry of TiCl3 (4.0 equiv, entry 4) diminished signifi-
cantly the efficiency of the reaction. Gratefully, compound 2a
was isolated in 90% yield when the reaction was performed
without adding the ammonium acetate buffer (entry 6). A
similar yield of 2a was isolated when MeCN was used as co-
solvent, although the reaction mixture was biphasic in the
latter case (entry 7). The reaction proceeded less efficiently
using MeOH as co-solvent (entry 8).
Treatment of 1t and 1u with TiCl3 under standard
conditions provided 3,4-bridged indoles 2t and 2u in yields
of 79% and 82%, respectively (Scheme 3).[17] Compound 2u,
the identity of which was confirmed by X-ray crystallographic
analysis,[18] is the core structural unit of aurantioclavine (3)
and other related natural products.[19]
With the optimized conditions in hand, the generality of
this novel reductive cyclization method was examined
(Scheme 2). The presence of reducible functional groups
Scheme 3. Synthesis of 3,4-bridged indoles.
The reductive cyclization of b,b-disubstituted o-nitrostyr-
enes was next investigated as this could provide not only an
entry to 2,3-disubstituted indoles, but also provide us with
useful mechanistic information. Gratefully, treatment of 1v
under standard conditions afforded 2-methyl-3-phenylindole
(2v) by a domino sequence involving reductive cyclization
and migration of the phenyl group (Scheme 4). Of mecha-
nistic importance, we observed a) the process worked better
with substrates having an electron-rich aromatic ring (2x–2z)
than an electron-poor one (2w). This result is indicative of
a cationic rather than a radical rearrangement process; b) the
high migratory aptitude of the electron-rich aryl group is
independent of the geometry of the olefin as both Z-1aa and
E-1aa were converted to indole 2aa with similar efficiency.
This result supported the notion that the reaction went
through a stepwise process and that nitrene was not an
intermediate of this indolization process.
Scheme 2. General conditions: 1 (0.2 mmol), TiCl3 (20% solution in
2.0n HCl, 1.6 mmol), solvent (c=0.2m), RT, 8 h. [a] Acetone as co-
solvent. [b] Acetonitrile as co-solvent (c=0.4m).
(Cl, Br, NCbz, CN, COOMe) and polar functions (OH,
NMe2) are well tolerated and indoles fused to 6- (2a–2d), 7-
(2e), 8-membered (2 f) rings as well as to the steric hindered
bridged bicyclic motif (2g) are readily accessible. However,
when 1-(2’-nitrophenyl)-cyclopentene was submitted to our
standard conditions, only the corresponding aniline was
produced. An acyclic trisubstituted olefin was converted to
2,3-disubstituted indole 2h in 87% yield. The 3-substituted
indoles were also prepared in good to excellent yields from
the corresponding 1-substituted-1-(2’-nitrophenyl)-ethylenes,
providing therefore an easy access to medicinally important
tryptamine (2o) and tryptophol derivatives (2q–2s). In
general, both acetonitrile and acetone are appropriate co-
solvents for the synthesis of 2,3-disubstituted indoles (2a–2h).
However, the former turned out to be the solvent of choice
for 3-substituted ones (2i–2s). Most of the a-substituted o-
nitrostyrenes 1i–1s were synthesized by a regioselective Heck
reaction according to Zhouꢀs protocol.[16]
Scheme 4. Domino process involving N-cyclization and aryl migration.
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Applying the standard conditions to methyl o-nitrocinna-
mate (1ab), o-nitrocinnamyl alcohol (1ac) and o-nitrostil-
bene (1ad, Figure 1) provided only the corresponding ani-
Scheme 6. Formal synthesis of aspidospermidine.
Figure 1. Substrates that failed to undergo the indolization.
lines. Therefore, the present method is not applicable to the
synthesis of 2-substituted indoles. When 1-(2’-aminophenyl)-
cyclohexene (4, Figure 1) was submitted to our standard
conditions, indole 2a was not formed indicating that aniline 4
was not an intermediate on the way to indole. This result is
also in line with the conversion of 1m to indole 2m. Should
the aniline be an intermediate, an oxindole resulting from the
5-exo-trig cyclization would be produced.[20]
A possible reaction pathway for this reductive cyclization
is presented in Scheme 5. Reduction of the nitro group to
Scheme 7. Synthesis of rizatriptan. [a] Pd2(dba)3 (0.05 equiv), dppf
(0.12 equiv), urotropine (2.0 equiv), THF, 808C, 48 h, 91%. [b] CBr4,
PPh3, CH2Cl2, 08C, 2 h, 83%. [c] Sodium salt of 1H-1,2,4-triazole, DMF,
À208C, 2 h, 64%. [d] TiCl3 (8.0 equiv), MeCN, RT, 4 h, 51%. [e] Dime-
thylamine (excess), MeOH, 508C, 48 h, 98%. dba=dibenzylidenace-
tone, dppf=1,1’-bis(diphenylphosphanyl)ferrocene, THF=tetrahydro-
furan, and DMF=N,N-dimethylformaide.
(Scheme 6). Tricycle 11 has been converted to aspidospermi-
dine 12 and other aspidosperma-type alkaloids.[24]
Rizatriptan 13, one of the tryptamine-based triptan drugs
used in the treatment of migraines and cluster headaches, was
synthesized as shown in Scheme 7.[25] Regioselective Heck
reaction between aryl triflate 14 and but-3-en-1-ol (15)
according to Zhou provided terminal olefin 16 in 91%
yield.[16] Bromination of diol followed by chemoselective
displacement of the benzyl bromide by sodium salt of 1H-
1,2,4-triazole provided 17. TiCl3-promoted reductive cycliza-
tion of 17 afforded indole 18 (51%) that was converted to
rizatriptan (13) under standard conditions. We note that the
classic Cadogan-Sundberg conditions wonꢀt be applicable to
17 because of the presence of an alkyl bromide function.
In summary, we developed a TiCl3-promoted reductive
cyclization of o-nitrostyrenes for the synthesis of functional-
ized indoles. Mild conditions, simple experimental procedure,
ready accessibility of the starting materials, and good to
excellent yields characterized the present transformation. We
anticipate that the reaction will find applications in the
synthesis of natural products and pharmaceuticals.
Scheme 5. Reaction pathways for the reductive cyclization of o-nitro-
styrenes.
a nitroso[21] group followed by a 6e electrocyclization would
give zwitterion 6.[22] In the case of R3 = H, a rapid re-
aromatization would occur leading to N-hydroxy indole 7,
that would be further reduced by TiCl3 to afford indoles 2a–
2u. In case of R1 = H, the group with high migratory aptitude,
irrespective of the initial olefin geometry, will shift from C2 to
C3 position which, upon deprotonation, would provide N-
hydroxy indole 9. Reduction of the latter would then furnish
the 2,3-disubstituted indoles 2v–2aa.
Acknowledgements
The application of our protocol to the synthesis of natural
products and drugs was pursued. Compound 10 was a key
intermediate in our total synthesis of mersicarpine and
leuconoxines.[23] We were pleased to find that simply stirring
a solution of 10 in acetone-NH4OAc in the presence of TiCl3
at RT afforded the tetrahydrocarbazolone 11 in 70% yield
Financial supports from EPFL (Switzerland), Swiss State
Secretariat for Education and Research (SER) and the Swiss
National Science Foundation (SNSF) are gratefully acknowl-
edged. We thank Dr. R. Scopelliti for X-ray crystallographic
analysis of 2u.
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Keywords: C H amination · indoles · nitroarenes ·
reductive cyclization · total synthesis
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11809–11812
Angew. Chem. 2015, 127, 11975–11978
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Received: June 21, 2015
Revised: July 12, 2015
Published online: August 18, 2015
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