Simultaneous synthesis of both rings of chromenes via a benzannulation/ o -quinone methide formation/electrocyclization cascade

Article abstract of DOI:10.1021/ja210655g

A new route to the chromene ring system has been developed which involves the reaction of an α,β-unsaturated Fischer carbene complex of chromium with a propargyl ether bearing an alkenyl group on the propargylic carbon. This transformation involves a cascade of reactions that begins with a benzannulation reaction and is followed by the formation of an o-quinone methide, and finally results in the emergence of a chromene upon an electrocyclization. This reaction was extended to provide access by employing an aryl carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a single step and is highlighted in the synthesis of lapachenole and vitamin E.

Full text of DOI:10.1021/ja210655g

Article
pubs.acs.org/JACS
Simultaneous Synthesis of Both Rings of Chromenes via a
Benzannulation/o-Quinone Methide Formation/Electrocyclization
Cascade
Nilanjana Majumdar, Keith A. Korthals, and William D. Wulff*
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
S
* Supporting Information
ABSTRACT: A new route to the chromene ring system has
been developed which involves the reaction of an α,β-
unsaturated Fischer carbene complex of chromium with a
propargyl ether bearing an alkenyl group on the propargylic
carbon. This transformation involves a cascade of reactions
that begins with a benzannulation reaction and is followed by
the formation of an o-quinone methide, and finally results in
the emergence of a chromene upon an electrocyclization. This
reaction was extended to provide access by employing an aryl
carbene complex. This constitutes the first synthesis of chromenes in which both rings of the chromene system are generated in a
single step and is highlighted in the synthesis of lapachenole and vitamin E.
one in which both rings of the chromene core are constructed
at the same time. We report here an efficient method for the
preparation of chromenes in which both rings of the chromene
unit are generated from the reaction of the carbene complex 3
and the alkoxyenyne 4 in a benzannulation/o-quinone methide
formation/electro-cyclization cascade.
1. INTRODUCTION
As a consequence of its appearance in compounds with a broad
spectrum of biological activity, the chromene ring system (1 in
Scheme 1) has been identified as one of the privileged scaffolds
Scheme 1
The anticipated events in the transformation of carbene
complex 3 and alkoxyenyne 4 into a chromene are the
benzannulation to give the phenol complex 5,20 the elimination
of an alcohol to generate the o-quinone methide complex 6,21,22
and finally an electrocyclization to give the chromene
chromium tricarbonyl complex 723 (Scheme 2). The chromene
oxygen in 7 has as its origin one of the carbon monoxide
ligands of the carbene complex, and the overall integration of
the pieces is indicated diagrammatically in the assembly 8.
In previous studies we had shown that the reaction of alkenyl
carbene complexes would react with propargyl ethers of the
type 10 with a tethered alkene unit to generate hexahydrodi-
benzopyrans of the type 11.21 Significant yields of 11 were only
observed if the reaction was performed in the presence of
for drug discovery1 and thus for combinatorial libraries.2 Nearly
all of the methods for the synthesis of chromenes involve the
closure of the pyran ring on a substrate containing a preformed
phenol unit of the type 2. Recent representative methods
include Claisen rearrangement3 or electrophile-induced cycliza-
tion of aryl propargyl ethers,4 palladium-catalyzed5 and selenium-
mediated2a cyclization of 2-butenylphenols, ring closing meta-
thesis,6 cyclization of salicylaldehydes with enamines,7 Wittig
olefination,8 Petasis reaction of salicylaldehydes9 and the
reaction of salicylaldehydes with vinyltrifluoroborates,10 palladium-
catalyzed oxidative cyclization of aryl-3-butenyl ethers,11 ene,12
and Baylis−Hillman13 reactions of salicylaldehydes, ylide-
induced annulation,14 enolization of vinylquinones,15 cycliza-
tion onto 3,4-epoxy alcohols,16 the reactions of phenols with
α,β-unsaturated aldehydes,17 the oxa-Michael-aldol/Henry
reaction of salicylaldehydes,18 and the dehydrative cyclizations
of 2-(1-hydroxy-2-propenyl)phenols.19 Of all the methods of
the myriad, to the best of our knowledge, there is not a single
Hunig’s base that presumably aided in the elimination of the
̈
propargyl oxygen unit from the benzannulated product 12 to
generate the o-quinone methide complex 13. The intra-
molecular Diels−Alder reaction that concludes the cascade
must have occurred via the intermediate 13 with an E-alkene
such that the trans-stereochemistry of 11 is established. This in
turn requires that during o-quinone methide formation the alkyl
group moves away from the phenol unit in 12 to establish the
E-stereochemistry in the o-quinone methide. This was a source of
concern in the original planning of the chromene synthesis in
Received: November 12, 2011
Published: December 16, 2011
© 2011 American Chemical Society
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Scheme 2
Table 1. Optimization of Chromene Formation from
a
Chromium Carbene Complex 14
b
entry
solvent
oxidative workup
% yield 17
cd
,
1
toluene
toluene
benzene
hexane
THF
FeCl3·DMF
FeCl3·DMF
FeCl3·DMF
FeCl3·DMF
FeCl3·DMF
none
35
62
74
70
65
95
76
c
2
3
4
5
6
7
8
9
Scheme 2, since the ultimate electrocyclic ring closure would
require the Z-configuration of the alkene in the o-quinone methide
unit.
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
FeCl3·DMF
CAN
e
nd
f
none
nd
a
2. CHROMENES FROM ALKENYL CARBENE
COMPLEXES
In our initial foray into exploring the cascade process in Scheme 2,
we chose to examine the reaction of the trans-styrenyl
carbene complex 14 and the siloxyenyne 15. The first reaction
Unless otherwise specified, all reactions were run at 0.03 M in 14
with 1.2 equiv of 15 at 60 °C for 24 h and worked up with 7.5 equiv of
the indicated oxidant (if employed). Isolated yield after chromatog-
raphy on silica gel. All yields are the average of two runs except for
b
c
entry 7; nd = not determined. Reaction performed at 80 °C for 24 h.
d
e
Reaction performed with 5 equiv of (i-Pr)2EtN. TLC indicated the
was carried out in toluene with 5 equiv of Hunig’s base, and the
̈
absence of 16 and 17 and the presence of compounds more polar than
either. A mixture of 16 and 17 from which 16 could not be separated
f
reaction mixture was subjected to oxidation with ferric chloride
DMF complex to remove the metal from the product and
to simplify purification. This reaction gave a 35% yield of
chromene 17, but the yield could be improved to 62% yield if
the base was excluded (Table 1, entries 1 vs 2). This was a
surprise since we had previously found that o-quinone methide
formation was facilitated by the presence of a base (Scheme 3).21
The more facile elimination in the present case may be related
to the fact that a chromium tricarbonyl unit in an arene
complex can stabilize a benzylic cation (19 in Table 1).24
Substitution reactions on the chromium tricarbonyl complex of
benzyl chloride are 105 times faster than on benzyl chloride
itself.24a The benzylic cation 19 (R = alkenyl) derived from
alkoxyenyne 15 would be both benzylic and allylic rather
than just benzylic (R = alkyl) as in previous reactions, where
the base was employed to effect o-quinone methide
formation which was presumably initiated via base-induced
deprotonation in phenol complexes of the type 12 (Scheme
3).21 The fact that base is not needed in the present case
could be explained if o-quinone methide formation could be
initiated by a chromium-induced loss of an alkoxide in the
complex 5 (Scheme 2).
A survey of different solvents for this reaction shown in
Table 1 found that acetonitrile was the optimum for this
particular reaction and had the additional advantage that an
oxidative workup was not needed since the solvent was capable
of completely displacing the metal from the product under the
reaction conditions. Ceric ammonium nitrate was too strong an
oxidant since neither the chromene complex 16 nor the
chromene 17 could be detected in the crude reaction mixture.
In the absence of an oxidative workup, a mixture of 16 and 17
was obtained from which 16 could not be purified since it
slowly oxidizes to 17 in air. The chromium tricarbonyl arene
complex 21 derived from the trans-2-butenyl carbene complex
to purity due to its slow and continuous decomposition to 17.
Scheme 3
20 was much more stable to loss of the metal and could be
isolated in 61% yield after purification by chromatography on
silica gel (Scheme 4). The corresponding chromene 22 could
Scheme 4
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be obtained free of the metal in 88% yield if an oxidative
workup with FeCl3·DMF complex was employed.
The formation of chromenes from α,β-unsaturated carbene
complexes and alkoxyenynes is efficient for a number of
substituent patterns and with substituents of varying sizes
(Table 2). It should be noted that alkoxyenynes with an
that the yield of 24 is lower than 17 and that the yield of 26 is
lower than 22 suggests the substitution pattern in cation 19
may play some role in this reaction. While Hunig’s base was
̈
detrimental (Table 1), it was found that the yield of 28 could
be slightly improved if the reaction was performed in the
presence of 10 equiv of aniline, but the generality of this effect
was not examined.
In many of the reactions, the solvents CH2Cl2 and CH3CN
were found to give similar yields of the chromenes. However,
for carbene complexes that only had a substituent in the α-
position (R2), CH2Cl2 was superior to CH3CN as solvent (29,
30, 33, and 34). In the case of the chromenes 33 and 34, which
are obtained from the carbene complex with R2 = methyl, the
best solvent was found to be hexane. While the source of these
differences is not fully appreciated, α,β-unsaturated carbene
complexes that do not have a substituent at the β-carbon tend
to be less stable as a result of a tendency to undergo poly-
merization.25a While the reactions in CH3CN do not need an
oxidative workup to remove the chromium tricarbonyl group
from the chromene, these reactions produce (CO)5Cr(CH3CN)
which slowly air oxidizes and which can coelute with the
chromene product. Therefore, many of the reactions in
CH3CN also utilized the FeCl3·DMF complex in an oxidative
workup to prevent (CO)5Cr(CH3CN) from complicating
product isolation.25b
Table 2. Chromene Synthesis via Carbene Complexes and
Alkoxyenynes
a
A side-product was isolated from the reaction of the trans-t-
butyl alkenyl carbene complex 37 with the enyne 38. The
phenol 39 is the major product for the reaction in acetonitrile
formed in a 3:2 ratio over the chromene 28. Clearly, this
product is the result of the failure of the o-quinone methide to
form. Nonetheless, phenol 39 can be quantitatively converted
to the chromene 28 by treatement with triflic acid. The yield
given for chromene 28 in acetonitrile that is indicated in Table
2 involved treatment of the crude reaction mixture with triflic
acid before purification. A possible explanation for the partial
failure of o-quinone methide formation in these reactions is that
the H-bonding indicated in structure 40 prevents the proper
anti-orientation of the benzylic oxygen with respect to the
chromium for the assisted elimination to generate a benzylic
cation of the type 19 (Table 1). The role of t-butyl group in
this process may be related in some fashion to the orientation
of the chromium tricarbonyl group relative to the arene
carbons.26 As a test, this reaction was repeated in the presence
of increasing amounts of isopropanol which was added to
dissrupt the H-bonding, and indeed, at 100 equiv no detectable
amount of 39 was observed. Acetonitrile would not be expected
to disrupt this H-bonding since the pKa of protonated aceto-
nitrile has been reported to be −10.27 Instead, the role of the
acetonitrile in favoring the phenol product 39 is suspected of
being related to its ability to displace the chromium tricarbonyl
from the benzannulated product before it has the chance to
completely ionize the benzylic oxygen.
a
Unless otherwise specified, all reactions were run at 0.03 M in
carbene complex in the indicated solvent with 1.2 equiv of the enyne at
60 °C for 24 h. If oxidation was used, it was carried out with 7.5 equiv
of FeCI3-DMF complex. All yields are isolated yields after
chromatography on silica gel. Reaction performed with 10 equiv of
aniline. Oxidative workup not used. Isolation after treatment of
crude reaction mixture with trifluoromethane sulfonic acid.
b
c
d
internal alkyne function (R4 ≠ H) gave a single regioisomer of
the chromene, specifically, that in which the R4 substituent is
ortho to the methoxy group in complex 5 in Scheme 2.20 Most
of the cases examined involved chromenes in which the sp3
ethereal carbon of the chromene bears two substituents, but we
were particularly pleased to note that chromenes with either
one (26) or no substituents (24) could also be generated. The
concern was that alkoxyenynes with either one or both of the
substituents R7 and R8 as hydrogen would fail since the cation
19 would be less stable. While this was not the case, the fact
3. NAPHTHOPYRANS FROM ARYL CARBENE
COMPLEXES
The benzannulation/o-quinone methide formation/electro-
cyclization cascade of an aryl carbene complex with a propargyl
enyne has the potential for providing access to 2H-benzo[h]-
chromenes (naphthopyrans) 43 in a single step (Scheme 6).
The 2H-benzo[h]chromene core 45 is quite common and
occurs in a large number of natural and unnatural products.28
One of the simplest members is the natural product
lapachenole 46 which has been isolated from different sources,
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The key reaction for the synthesis of lapachenole via the
benzannulation/o-quinone methide formation/electrocycliza-
tion cascade is that of carbene complex 53 and enyne 15
(Scheme 7). This reaction only produced the natural product in
Scheme 5
Scheme 7
Scheme 6
37% yield, but it was found that the yield could be improved
slightly to 48% if the reaction was performed in the presence of
10 equiv of aniline. Despite the moderate yield, it represents a very
short synthesis of lapachenole: Two steps from bromobenzene or
two to three steps from the commercially available prenal.34 No
significant by-products were observed to form along with the
desired product lapachenole. Collection of other fractions from the
silica gel column yielded a complexed mixture of compounds,
none of which were predominate or separable. This is suggestive
of incorporation of multiple units of the enyne, and this has been
observed in other reactions to produce phenols, trisubstituted
benzenes or oligomers.35 Previous experience suggests that if this
was the case, improved yields could be achieved by contolling
concentration. However, in the present case, there was not a great
response in the yield to changes in the concentration. The yield
was 37% at 0.035 M and, while this fell as expected when the
concentration was increased (26% at 0.1 M), it also fell when the
concentration was reduced (31% at 0.005 M). If multiple insertions
of alkyne 15 containing a terminal alkyne function are responsible
for the moderate yields of lapachenole, then increased yields would
be expected for similar reactions with internal alkynes. Indeed, the
synthesis of 5-methyllapachenole 54 was possible with a much
higher yield (85%) than lapachenole.
An alternative approach to lapachenole 46 that has the potential
to be more efficient is the reaction of the carbene complex 53 with
the enyne 55 bearing an internal alkyne as the trimethylsilylated
analog of enyne 15 (Scheme 8). In analogy with enyne 38 bearing
an internal alkyne, the product from the reaction of enyne 55
would be expected to be the naphthopyran 56 from which the
trimethylsilyl group could be removed by protonolysis to give
lapachenole 46. However, it was found that the reaction of the
silylated enyne 55 gave the indene product 57 rather than the
expected naphthopyran 56. The indene 57 was isolated in 65%
yield as a 1.14:1.0 mixture of diastereomers and was the only
product that was observed that was mobile on TLC. This type of
five-membered ring cyclized product is perhaps the most common
of the many side products that have been observed in the
benzannulation reaction.20 There is a tendency to see increased
amounts of five-membered ring products with increased steric
bulk of the two acetylene substituents. Mechanistically, this
reaction should occur by initial insertion of the alkyne function
including Avicennia rumphiana.29 This compound has been
used as a fluorescent photoaffinity label30 and has been shown
to have cancer chemopreventitive activity.29 It occurs in
Tabebuia heptaphylla which is the source of the Paraguayan
̈
́
traditional medicine “tayi pyta” used in the treatment of
wounds, cancer, and inflammations.31 Both 2H-benzo[h]-
chromenes 4530 and 3H-benzo[f ]chromenes 4432 are of
interest for their photochromic properties which are
associated with photoinduced electrocyclic ring opening to
o-quinone methides. We have previously reported an
approach to 3H-benzo[ f ]chromenes via the simple benzan-
nulation reaction of a chromene carbene complex and an
alkyne.33 This approach required the preparation of the
chromene carbene complex 49 which was accomplished in
six steps from o-methoxybenzaldehyde. The proposed route
to 2H-benzo[h]chromenes would potentially be much more
efficient since aryl carbene complex of the type 41 can be
prepared in one step from the corresponding aryl bromide or
iodide in good to excellent yields.
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alcohol as a TBS ether to give the key enyne 62. The chromene 63
is generated in 85% yield directly from the reaction of carbene
complex 20 and the internal alkyne 62. Reduction of the double
bond is quantitative, and cleavage of the methyl ether with
BF3·SMe2 complex and aluminum chloride gives vitamin E in 73%
overall yield from the carbene complex 20.
Scheme 8
5. CONCLUSIONS
The formation of chromenes from the reaction of a chromium
carbene complex with a 3-siloxypent-4-en-1-yne has been
shown to proceed via an initial benzannulation reaction that
produces a chromium tricarbonyl complexed phenol. Sponta-
neous loss of a silanol generates an o-quinone methide that
undergoes a six-electron electrocyclic ring closure to the
chromene. The reaction is general giving good to high yields of
a variety of 4-alkoxychromenes with mono- and di-substituted
alkenyl carbene complexes and mono-, di-, and trisubstituted
3-silyloxypent-4-en-1-ynes. The primary product of the reaction
is a chromium tricarbonyl complexed chromene that can be
isolated but is typically stripped of the metal either by ligand
exchange with acetonitrile as solvent or by workup with an
oxidizing agent. The reaction can also be extended to aryl
carbene complexes for the synthesis of 2-H-benzo[h]-
chromenes, and this process was employed in the synthesis
of lapachenole and 5-methyllapachenole. The benzannulation/
o-quinone methide formation/electrocyclization cascade was
featured in a synthesis of vitamin E in which both rings of
vitamin E were generated in a single step.
of 55 into the metal-carbene bond in carbene complex 53 to
give the vinyl carbene complexed intermediate 58. The
subsequent events normally would be migratory insertion of a
CO ligand to give the chromium complexed vinyl ketene 59
and then electrocyclic ring closure to give the phenol chromium
tricarbonyl complex 60. Apparently, in the case of the vinyl
carbene complexed intermediate 58, there is a preference for
direct cyclization to 57 rather than CO insertion to give 59.
The reasons for this are not clear, but one might expect that it
may be related to the increase in bond angles for the sp2
carbons of 58, as cyclization occurs to give a five-membered
ring and the associated decrease in strain energy as the large
substituents move further apart.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and spectral data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
4. SYNTHESIS OF VITAMIN E VIA THE
BENZANNULATION/O-QUINONE METHIDE
FORMATION/ELECTROCYCLIZATION CASCADE
As an illustration of the utility of the benzannulation induced
cascade in the synthesis of chromenes, we undertook the
synthesis of vitamin E outlined in Scheme 9.36,37 The most
AUTHOR INFORMATION
■
Corresponding Author
wulff@chemistry.msu.edu
Scheme 9
ACKNOWLEDGMENTS
■
This work was supported by NSF grant CHE-0750319 and the
National Institute of General Medical Sciences (GM094478).
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