Electrophilic cyclization of 1,5-enynes

Article abstract of DOI:10.1002/anie.201001113

Figure Presented Valuable six-membered carbocycles including highly substituted benzenes, 1,4-cyclohexadienes, and 4-fluorocyclohexenes can be prepared from simple 1,5-enynes by iodonium-induced cyclization. This cyclization strategy was applied to the total synthesis of cybrodol, a sesquiterpenoid with a pentasubstituted aromatic core (see scheme).

Full text of DOI:10.1002/anie.201001113

Angewandte
Chemie
DOI: 10.1002/anie.201001113
Carbocyclization
Electrophilic Cyclization of 1,5-Enynes**
Benedikt Crone, Stefan F. Kirsch,* and Klaus-Daniel Umland
Dedicated to Professor Horst Kessler on the occasion of his 70th birthday
The electrophilic cyclization of alkynes was studied in detail
by Barluenga, Larock, and others and has thus emerged as a
broadly applicable strategy for the efficient synthesis of small-
molecule targets.[1] For instance, the cyclization of heteroatom
nucleophiles with tethered alkynes is useful for the direct
formation of carbon–heteroatom bonds (Scheme 1a). Several
not been used as internal carbon nucleophiles in this way.[7]
Here we show that 1,5-enynes are powerful substrates in
iodonium-induced carbocyclizations yielding six-membered
carbocycles with great structural diversity (A–C; Scheme 1).
The reaction outcome is predictable over a broad range of
densely functionalized substrates.
The transition-metal-catalyzed cycloisomerization of 1,5-
enynes has evolved into a broadly useful transformation that
delivers various carbocyclic products of high complexity.[8,9]
As part of our ongoing studies on the use of enynes in domino
reactions,[10] we found that several transition-metal-catalyzed
processes can be run with traditional electrophiles such as I+
in an analogous way incorporating I rather than H in the final
product.[11] However, the electrophilic cyclization of simple
1,5-enynes is a particularly challenging chemical transforma-
tion given the fact that a range of background reactions can
arise from the positively charged intermediate I. Further-
more, reactive olefins may compete with the tethered alkyne
for the electrophile,[12] while olefins that lack the required
nucleophilicity do not undergo cyclization with the activated
alkyne.
Our initial attempts to react various 1,5-enynes with I2 in
CH2Cl2 at 238C resulted in the formation of complex
mixtures. We then discovered that potential side reactions
can be suppressed if an excess of N-iodosuccinimide (NIS) is
used at 508C. As shown in Table 1, treating a variety of 1,5-
enynes 1 with NIS (3 equiv) under aerobic conditions gave the
iodobenzenes 2 in high yields. Oxidative aromatization was
accomplished with R1 being aryl, alkyl and hydrogen sub-
stituents (Table 1, entries 1–10). Particularly notable is the
smooth reaction of bromoalkyne 1k to produce the 1-bromo-
2-iodobenzene derivative 2k (Table 1, entry 11).
Scheme 1. Iodonium-induced cyclizations of alkynes.
important syntheses of carbocycles and, in particular, hetero-
cycles have been developed using this approach,[2,3] most of
which employ iodine electrophiles to trigger the heterocyc-
lization. Despite an early report by Barluenga et al.,[4]
iodonium-induced carbocyclization has been demonstrated
only for the intramolecular arylation of alkynes (i.e., arene
nucleophiles; Scheme 1b)[5] and when malonates are used as
nucleophiles.[6] Remarkably, simple olefins have apparently
Awide range of 1,5-enynes with different substituents R2–
R5 were effectively converted into the corresponding ben-
zenes. Unfortunately, substrates lacking a substituent at C2
(i.e., R4 = H) turned out to be unreactive under the reaction
conditions. This result indicates that the substituent at C2 is
required to stabilize the positive charge in the cyclic
intermediate I. The reaction tolerates the presence of a
number of functional groups such as ester, ether, silyl ether,
nitro, and azide groups, although enynes containing a free
hydroxy or aldehyde group reacted in only poor yields. Most
remarkably, we found that even fully substituted benzenes
could be accessed in good yields (Table 1, entry 24).
[*] B. Crone, Dr. S. F. Kirsch, K.-D. Umland
Department Chemie
Technische Universitꢀt Mꢁnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-28913315
E-mail: stefan.kirsch@ch.tum.de
Several additional observations merit note. Transforma-
tions of enynes 1 into benzenes 2 most likely proceed through
dienes B as delineated in Scheme 1. For instance, if the
cyclization of enyne 1t is conducted under argon rather than
under aerobic conditions, aromatization is slowed dramati-
cally and accompanied by diene formation (argon: 8% yield
[**] This project was supported by the Deutsche Forschungsgemein-
schaft (DFG) and the Fonds der Chemischen Industrie (FCI). We
thank Prof. Thorsten Bach for helpful discussions and generous
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001113.
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Table 1: NIS-mediated formation of iodobenzenes.[a]
Entry
Product
R2 R3
Yield
[%][b]
R1
R4
R5
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
TIPSO(CH2)3 Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2 f
2g
2h
2i
96
93
76
59
70
95
86
75
81
88
90
82
derivative.[14] As outlined in Scheme 2, we began our synthesis
by transforming 3-oxo ester 7 into triisopropylsilyl (TIPS)
ether 8 using standard protocols. Addition of EtMgBr and
4-MeO(C6H4)
4-MeO2C(C6H4)
4-O2N(C6H4)
4-Cl(C6H4)
2-naphthyl
2-thienyl
H
Me
CH2Ph
Br
4-MeO(C6H4)
4-MeO(C6H4)
H
Ph
Ph
Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Me
Me
H
Me
Ph
Ph
Ph
Ph
Ph
Ph
Me
2j
2k
2l
2m 93
2n
2o
2p
78
92
92
nBu
MeO(CH2)3 Me
TBSO(CH2)3 Me
2q 44[c]
Ph
Ph
Ph
Ph
HO(CH2)3
OHC(CH2)2 Me
N3(CH2)3 Me
TIPSO(CH2)3 nBu
Me
2r
2s
2t
13[d]
19
76
74
71
2u
4-MeO(C6H4)
E-MeO2CMeC CH H Me
Me
Ph
Me 2v
Me 2w
=
TIPS-
O(CH2)2
Ph
62
24
4-MeO(C6H4) Me Me
Me 2x
71
Scheme 2. Total synthesis of cybrodol (12). Reagents and conditions:
a) LiAlH4, 08C, Et2O; b) TIPSCl, imidazole, 238C, DMF, 97% over two
steps; c) PCC, 238C, CH2Cl2, 73%; d) EtMgBr, 08C to 238C, THF,
81%; e) Martin’s sulfurane, NEt3, 238C, CH2Cl2, 98%; f) (E)-MeO2C-
[a] Conditions: Substrate (0.1m), NIS (3 equiv), 508C, CH2Cl2. [b] Yield
of pure product after column chromatography. [c] Cleavage of silyl ether
was observed (44% yield of 2q, and 54% yield of 2r). [d] A variety of by-
products were obtained, some of which arise from intramolecular
addition of the hydroxy group. TIPS=Si(iPr)3, TBS=SitBuMe2.
=
MeC CHBr, [PdCl2(PPh3)2] (2 mol%), CuI (4 mol%), NEt3, 508C,
quant.; g) DIBAL-H, ꢀ788C, CH2Cl2; h) TIPSCl, imidazole, 238C, DMF,
99% over two steps; i) NIS, 508C, CH2Cl2, 89%; j) 1) tBuLi, ꢀ788C,
THF, 2) MeOC(O)CN, ꢀ788C to 238C, 56%; k) DIBAL-H, ꢀ788C,
CH2Cl2; l) HF, 238C, MeCN, 80% over two steps. PCC=pyridinium
chlorochromate.
of 2t, 63% yield of 1,4-diene 13t, and unidentified com-
pounds after 2 h; air: 76% yield of 2t after 2 h). Iodonaph-
thalenes were produced from 1-alkenyl-2-alkynylbenzenes as
indicated by the conversion 3!4 in 58% yield [Eq. (1)].
subsequent regioselective elimination by exposure to Mar-
tinꢀs sulfurane ([PhC(CF3)2O]2SPh2)[15] produced 1,5-enyne 9
in 79% yield over two steps. Sonogashira coupling of 9 with
(E)-methyl 3-bromo-2-methylacrylate[16] installed the last side
chain, which was then elaborated to provide bis(silyl ether)
10. In the pivotal step, oxidative cyclization with NIS in
CH2Cl2 at 508C gave iodobenzene 11 in excellent 89% yield.
Addition of the lithium species generated from 11 to methyl
cyanoformate gave the fully constructed aromatic core, which
was finally subjected to reduction and desilylation to furnish
cybrodol in 45% yield over the three steps. The NMR data of
synthetic 12 was identical to that reported for the natural
substance.[17]
When aromatization is blocked through a further substituent
at C3, enynes (e.g., 5a) were found to give 1,3-diene products
as well [Eq. (2)].
The application of the new annulation strategy,[13] in which
the aromatic ring is assembled in a single step with all
substituents already in place, to the synthesis of the sesqui-
terpenoid cybrodol illustrates the utility of this methodology.
Isolated from the birdꢀs nest fungus Cyathus bulleri by Ayer
et al. in 1980, cybrodol (12) is a pentasubstituted benzene
We also found that 1,5-enynes bearing a substituent both
at C2 and C3 (R3, R4 ¼ H) can be smoothly transformed into
the corresponding cyclic 1,4-dienes 13 upon treatment with I2
and K3PO4 in CH2Cl2 at 08C (Scheme 3). A reaction time of
only 1–2 hours was required to reach completion. When the
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Scheme 3. Formation of 1-iodocyclohexa-1,4-dienes 13.
reactions were run for a longer period of time, significant
amounts of benzenes 2 arose from the subsequent aromatiza-
tion of 13.
Scheme 4. Formation of esters 15.
We next investigated the use of the bis(pyridine)iodonium
tetrafluoroborate reagent[18] (IPy2BF4) developed by the
Barluenga group in reactions of 1,5-enynes. To our surprise,
an equimolar mixture of IPy2BF4 and HBF4·Et2O at ꢀ788C in
CH2Cl2 rapidly (15 min) converted 1,5-enynes into 4-fluoro-1-
iodocyclohexenes 14 (Table 2). Under these optimized con-
cyclohexene derivative; as shown for the conversion of acid
1z, only the iodolactonization product 16 was obtained.
In conclusion, we have developed iodonium-induced
carbocyclizations of alkenes with appended alkynes. The
transition-metal-free processes are experimentally simple to
perform and convert 1,5-enynes into six-membered cyclic
products of high value including highly substituted benzenes,
1,4-cyclohexadienes, and 4-fluorocyclohexenes. We are con-
tinuing to explore the enormous potential of electrophilic
enyne cyclizations for the synthesis of diverse carbocyclic
scaffolds.
Table 2: Synthesis of 4-fluoro-1-iodocyclohexenes 14 by use of IPy2BF4.[a]
Received: February 23, 2010
Published online: May 17, 2010
Entry Substr.
Product
-(CH2)5-
HO(CH2)3
nBu
N3(CH2)3
Yield
[%][b]
Keywords: alkynes · arenes · carbocycles ·
electrophilic cyclization · iodine
.
R1
R3
R3’ R4
1
2
3
4
5
6
7
5a
5b
5c
1r
1o
1t
4-MeO(C6H4) Me
4-MeO(C6H4) Me
Me
Ph
Ph
Ph
Ph
Me Me 14a
Me Ph 14b
Me 14c
78
53
33
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H
H
H
H
Me 14d 59[c]
Me 14e 85[c]
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Me 14 f
80[c]
1y
MsO(CH2)3
Me 14g 73[c]
[a] Conditions: Substrate (0.05m), IPy2BF4 (1.3 equiv), HBF4·Et2O
(1.3 equiv), 15 min, ꢀ788C, CH2Cl2. [b] Yield of pure product after
column chromatography. [c] Obtained as a 1:1 mixture of diastereomers.
Ms=CH3SO2.
ditions, a range of substrates underwent fluorination in
moderate to excellent yields to give cyclohexene products.
Once again, only 1,5-enynes having a substituent at C2
reacted; enynes with R4 = H proved to be unreactive. This
cyclization–fluorination reaction of 1,5-enynes represents an
attractive and simple means of incorporating fluorine into
complex organic molecules.[19,20]
Having established the feasibility of incorporating a
nucleophile into the cyclohexene core, we sought to briefly
investigate the introduction of oxygen nucleophiles. To our
delight, when 1,5-enynes 5a and 5b were subjected to an
excess of NIS (3 equiv) and formic acid (10 equiv) at ꢀ208C
in CH2Cl2, the esters 15a and 15b were formed in 80% and
53% yield, respectively (Scheme 4). Of importance, an enyne
with a tethered carboxylic acid does not react to form a
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Communications
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1
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