A One-Pot Synthesis of Pyranocoumarins Through Microwave-Promoted Propargyl Claisen Rearrangement/Wittig Olefination

Article abstract of DOI:10.1002/ejoc.201701684

The reaction between propargyl ethers of hydroxybenzaldehydes and the ylide ethyl (triphenylphosphoranylidene)acetate was carried out under microwave irradiation to regioselectively afford angular pyranocoumarins. The chromene and coumarin heterocyclic scaffolds were simultaneously formed in the same synthetic step without changing the reaction conditions. The natural products seselin, braylin, and dipetalolactone were among the products synthesized by this method.

Full text of DOI:10.1002/ejoc.201701684

DOI: 10.1002/ejoc.201701684
Full Paper
Microwave-Promoted Synthesis
A One-Pot Synthesis of Pyranocoumarins Through Microwave-
Promoted Propargyl Claisen Rearrangement/Wittig Olefination
Bernd Schmidt*[a] and Christiane Schultze[a]
Abstract: The reaction between propargyl ethers of hydroxy- formed in the same synthetic step without changing the reac-
benzaldehydes and the ylide ethyl (triphenylphosphoranyl-
idene)acetate was carried out under microwave irradiation to
regioselectively afford angular pyranocoumarins. The chromene
and coumarin heterocyclic scaffolds were simultaneously
tion conditions. The natural products seselin, braylin, and
dipetalolactone were among the products synthesized by this
method.
Introduction
ples include a Suzuki coupling/nitro reduction sequence[8] and
a tandem imination/cycloisomerization reaction for the synthe-
sis of 3-benzylisoquinolines.[9] In contrast, the carbonyl alkynyl-
ation/Sonogashira coupling/cyclization sequence for the syn-
thesis of indoles — although synthetically attractive — requires
the interruption of the microwave irradiation and addition of
further reagents and catalysts.[10]
Over the past few years, we have contributed to the field of
microwave-promoted tandem reactions with the development
of a Claisen rearrangement/oxa-Michael cyclization reaction se-
quence for the synthesis of substituted chromones and chro-
man-4-ones.[11] Various 8-allyl- and prenyl-substituted coumar-
ins have been obtained by us[12] and others[13] through a tan-
dem sequence that involves a Claisen rearrangement of allyl
phenyl ethers, a Wittig olefination, E/Z isomerization, and cycli-
zation reactions. During a latter study, we also investigated an
example of a tandem propargyl Claisen rearrangement/Wittig
olefination reaction sequence (Scheme 1). Propargyl ether 1
was converted into a mixture of cinnamates 4 and 5 when irra-
diated in the presence of stable ylide 2. The formation of 5 as
the major product is remarkable, because the primary products
of thermal propargyl Claisen rearrangements, that is, the ortho-
Microwave irradiation as an alternative energy supply for or-
ganic reactions was first reported in the 1980s with the use of
the domestic microwave oven.[1] Since then, the number of re-
ports of organic transformations carried out under microwave
irradiation — now exclusively performed in dedicated micro-
wave reactors — has increased dramatically to more than 1000
per year. Compared with conventional heating, microwave irra-
diation has the potential to reduce reaction times by heating
mixtures in closed systems at temperatures greater than the
boiling point of the solvent (superheating), and shorter times
may result in fewer side products. Thus, microwave irradiation
may be the chosen method if reactions that use conventional
heating are excessively slow.[2]
One-pot processes and tandem reactions[3,4] often achieve
their high selectivity by the successive addition of reagents or
catalysts in due course.[5] Such reaction protocols are, however,
not practical in a microwave reactor under closed conditions,
as the addition of catalysts, reagents, or solvents over the
course of a synthesis implies that microwave irradiation is inter-
rupted and the reaction vessel is cooled and opened. Alterna-
tively, special pumps can be used to inject reagents into sealed
systems without interrupting the microwave irradiation or the
internal pressure. Unfortunately, microwave-assisted reactions
that are conducted in an open vessel do not provide the posi-
tive effects normally expected of microwave heating, as super-
heating the reaction mixture is not possible[6] and nonthermal
microwave effects are no longer believed to play a substantial
role in these transformations.[7] For these reasons, microwave-
promoted one-pot and tandem reactions are more conve-
niently performed if the reaction conditions tolerate the pres-
ence of all reagents and catalysts at the outset. Recent exam-
[a] Institut für Chemie, Universität Potsdam,
Karl-Liebknecht-Strasse 24-25, 14476 Potsdam-Golm, Germany
E-mail: bernd.schmidt@uni-potsdam.de
http://www.chem.uni-potsdam.de/groups/schmidt/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701684.
Scheme 1. Previous investigation of a propargyl Claisen rearrangement/Wittig
olefination sequence by our group (MW = microwave)..[12]
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allenyl phenols, normally cyclize selectively to give chrom- ular cyclizative condensation of a hydroxycoumarin and a C3
enes.[14,15] Chromenes have also been obtained from propargyl building block were investigated under microwave irradia-
ethers under microwave irradiation[16] and in the presence of tion.[38]
gold catalysts.[17] The alternative 5-endo cyclization pathway
In continuation of our investigations into the microwave-pro-
moted synthesis of 8-prenyl coumarins,[12] we have pursued a
related synthetic approach to pyranocoumarins that relies on
the combination of a propargyl Claisen rearrangement and in-
termolecular Wittig olefination reaction. We are aware of one
report by Mali et al. that describes such a one-pot sequence
has only been reported in unusual cases (e.g., in the presence
of Ag salts[18] or fluoride bases[19]). Notably, no coumarins such
as 6 or its isomers were detected in our case, which suggests
that the cyclization proceeding through the allenyl substituent
is much faster than E/Z isomerization and lactonization.
In general, coumarins are widespread secondary plant under thermal conditions. These authors report that heating
metabolites.[20] In plants, they act as fungal pathogen inhibitors, monopropargyl dihydroxybenzaldehydes in the presence of yl-
insect repellants, growth regulators, and UV light absorbers.[21] ide 2 at 180–190 °C for prolonged periods of time (3 to 10 h)
In mammals, numerous bio- and pharmacological activities, yielded angular pyranocoumarins, such as seselin (8) in 55 %
such as anti-inflammatory,[22] anticoagulant, antibacterial, anti- yield.[39,40] The objective herein is to identify and evaluate suit-
viral, antifungal, anticancer,[23] and neuroprotective[24] proper- able microwave conditions for a one-pot propargyl Claisen rear-
ties, have been reported for natural and synthetic coumarins.[25] rangement/Wittig-olefination/cyclization reaction sequence
A classification scheme based on structural scaffolds has also and determine whether the high selectivity of the rearrange-
been proposed.[25] Pyranocoumarins, in which a pyran ring ment step towards angular pyranocoumarins is maintained in a
(most commonly a 2,2-dimethyl-substituted pyran) is annulated closed vessel under superheating microwave conditions. Some
to the benzene ring of a coumarin, is an important class of reported observations in the literature regarding the syntheses
this scheme. Other classes include simple coumarins (that have of microwave-promoted pyranocoumarin under these condi-
aliphatic or hydroxy groups on the benzene ring), furano- tions point to the predominant formation of linear byprod-
coumarins, and coumarins with substituents on the pyrone ring. ucts.[38,41]
The pyranocoumarins are further subdivided into linear [e.g.,
xanthyletin (7)] and angular representatives [e.g., seselin (8),[26]
braylin (9),[27] dipetalolactone (10),[28] Figure 1].
Results and Discussion
The results for the preparation of the 1,1-dimethylpropargyl aryl
ethers 12 are summarized in Table 1. We used an adaptation of
a procedure that was previously published by Godrey et al.[42]
Table 1. Synthesis of aryl propargyl ethers 12.
Figure 1. Representative examples of linear and angular pyranocoumarins.
Recent syntheses of pyranocoumarins, inter alia seselin (8),
involve catalyst-free multicomponent[29] or two-step condensa-
tion reactions[28] that start from phloroglucinol, ring-closing
metathesis reactions at preformed coumarin scaffolds,[30]
cobalt-catalyzed cyclizative carbonylations of vinylphenols,[31]
cyclizative Wittig olefinations of 6-formyl chromenes,[32] or oxid-
ative selenium-mediated cyclizations of prenylated hydroxy-
coumarins.[33] Most syntheses of seselin and structurally related
angular pyranocoumarins are prepared from preformed
coumarins, either by thermal propargyl Claisen rearrangements
of the corresponding coumarin 2,2-dimethylpropargyl
ethers[34,35] or by acid-catalyzed condensations of hydroxy-
coumarins and acetals of senecialdehyde.[36] Microwave condi-
tions have been used only in a limited number of pyrano-
coumarin syntheses. In these cases, either a propargyl Claisen
rearrangement of a preformed coumarin[37] or an intermolec-
Entry 11
R1
R2
R3
T [°C]
12
Yield [%]
1[a]
2
3
11a
CH3
CH3
H
Ph
H
H
H
H
H
H
H
H
H
H
H
H
OCH3
OCH3
OCH3
65
20
20
20
20
0
12a
12a
12b
12c
12d
12d
12d
29
83
93
80
<5
14
41
11a
11b
11c
11d
11d
11d
4
5[b]
6[c]
7[d]
H
H
0
[a] Compound 14a (17 %) was also formed as a product. [b] Compounds 13d
(36 %) and 14d (10 %) were formed as additional products. [c] Compound
13d (58 %) was formed as well. [d] Dimethylpropargyl chloride was slowly
added over 1 h. Compound 13d (35 %) was also produced.
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and Mann and co-workers.[43] These authors reported that 1,1- diation, compound 12a cleanly afforded pyranocoumarin 15.
dialkylpropargyl ethers were only obtained in synthetically use- After 10 min of reaction time, the starting material was fully
ful yields if catalytic amounts of CuI salts were present. The Cu- consumed, and product 15 was isolated in 71 % yield (Table 2,
catalyzed propargylation of phenols, which presumably pro- Entry 1). Longer reaction times of 30 min (Table 2, Entry 2) and
ceeds through a cross-coupling mechanism,[42] was later used 1 h (Table 2, Entry 3) gave comparable yields, which verify the
by Winssinger and co-workers in the synthesis of propargyl stability of the pyranocoumarin at the reaction temperature.
ether 12b[16] and by Stratakis and co-workers for the prepara- Next, we examined whether microwave irradiation was indeed
tion of starting materials for Au-catalyzed cycloisomerization better than conventional heating. To this end, a solution of 12a
reactions.[17]
was heated to the same reaction temperature in a silicon oil
bath for 1 h and 12 h, respectively. After 1 h, TLC analysis indi-
cated virtually no conversion and the presence of unreacted
starting material (Table 2, Entry 4). After 12 h, some formation
of product 15 was observed, but substantial amounts of start-
ing material were still present. Upon workup, compound 15
was isolated in a low yield of 28 % (Table 2, Entry 5). We were
confident that our preferred reaction temperature of 250 °C
could not be substantially lowered, because both our research
group[12] and that of Pospíšil[13] recently found that E/Z isomeri-
zation of the intermediate cinnamates, a prerequesite for the
cyclization into coumarin, does not efficiently proceed at tem-
peratures lower than 200 °C. There is literature precedent for
the conversion of preformed cinnamates into coumarins
through an E/Z isomerization/cyclization sequence at tempera-
tures below 100 °C, but this method requires the presence of
air-sensitive trialkylphosphines as catalysts.[44] Nevertheless, we
submitted 12a under otherwise identical conditions to micro-
wave irradiation at 200 °C for 1 h (Table 2, Entry 6). This resulted
in complete consumption of the starting material but, not sur-
prisingly, with very poor selectivity, as a complex mixture of
Under the original conditions by Mann and co-workers, the
desired product 12a was obtained in only 29 % yield along with
considerable amounts of chromene 14a, a product that resulted
from a propargyl Claisen rearrangement (Table 1, Entry 1). The
formation of this rearrangement product was fully suppressed,
and the yield of 12a increased to 83 % by lowering the reaction
temperature to 20 °C (Table 1, Entry 2). These modified condi-
tions were then applied to 11b and 11c, which resulted in the
regioselective formation of monopropargyl ethers 12b and 12c,
respectively, in good yields (Table 1, Entries 3 and 4). Unfortu-
nately, the more electron-rich methoxy-substituted phenol 11d
did not undergo a reaction at ambient temperature to give the
desired 12d but only gave chromene 14d and double propar-
gylated product 13d (Table 1, Entry 5). Lowering the reaction
temperature to 0 °C completely suppressed the propargyl Clai-
sen rearrangement, but the isolated yield of 12d was still unsat-
isfactory (Table 1, Entry 6). The slow addition of the alkylating
agent over 1 h, however, led to an increased yield of 12d along
with the formation of dipropargyl ether 13d (Table 1, Entry 7).
The synthesis of dipetalolactone precursor 12e was trouble-
some because of the occurrence of a double propargyl Claisen
rearrangement to give pyranochromene 14e and threefold
propargylation to provide 13e. The separation of the three
products was possible and allowed us to isolate sufficient quan-
tities of 12e to investigate the envisaged microwave-promoted
one-pot conversion into the dipetalolactone (Scheme 2).
Table 2. Microwave-promoted one-pot propargyl Claisen rearrangement/
Wittig olefination/cyclization reaction sequence.
Entry
12
Product R1
R2
R3
t [min] % Yield[a]
1
2
3
12a
12a
12a
12a
12a
12a
12b
12b
12c
12c
12d
12d
12e
12e
15
15
15
15
15
15
8
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
OCH3
OCH3
10
30
60
60
720
60
10
60
10
60
10
60
10
60
71
73
78
CH3
CH3
CH3
CH3
CH3
H
4[b]
5[b]
6[e]
7
–
[c]
28[d]
[f]
–
66
67
54
56
75
74
45
43
8
9
8
H
16
16
9
9
10
10
Ph
Ph
H
H
H
10
11
12
13
14
OCMe2CH=CH
OCMe2CH=CH
Scheme 2. Synthesis of dipetalolactone precursors (DMF = N,N-dimethyl-
H
formamide).
[a] Yields of isolated products are reported. Unless otherwise stated, full con-
version was observed by TLC analysis. [b] Reaction mixture was heated to
250 °C with conventional heating (oil bath). [c] No conversion was observed.
[d] Incomplete conversion (by TLC analysis) was observed. [e] Reaction was
carried out under microwave irradiation at 200 °C. [f] Complex mixture of
products resulted.
The microwave-promoted one-pot sequence to give the
pyranocoumarins was first investigated by starting from precur-
sor 12a. In the presence of 1.5 equiv. of ylide 2 and N,N-
diethylaniline in a closed vessel at 250 °C under microwave irra-
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products was observed. We assume that the high reaction tem- Conclusions
perature of 250 °C ensures the rapid conversion into the stable
In summary, we described a microwave-promoted one-pot syn-
thesis for angular pyranocoumarins by starting from propargyl
ethers. The sequence proceeds through a single or double
propargyl Claisen rearrangement, a Wittig olefination, an E/Z
isomerization, and eventually a cyclization to give the coumarin
scaffold. The utility of this synthetic method was demonstrated
for the seselin, braylin, and dipetalolactone natural products.
pyranocoumarin 15 (see Table 2, Entry 3 and the discussion
referring to this) through a defined order of steps, whereas at
lower temperatures intermediates are formed that are prone to
undefined side reactions.
We next applied the optimized conditions from Table 2, En-
try 1 (i.e., 250 °C, 10 min of reaction time) to precursors 12b–
12e, and for a comparison, we also employed the conditions
from Table 2, Entry 3 (250 °C, 60 min of reaction time). As a
result, four other pyranocoumarins, including the natural prod-
ucts seselin (8, Table 2, Entries 7 and 8), braylin (9, Table 2,
Entries 11 and 12), and dipetalolactone (10, Table 2, Entries 13
and 14) were formed. The latter product was obtained from 12e
proceeding through a double propargyl Claisen rearrangement/
Wittig olefination/cyclization sequence. We also demonstrated
that 4-aryl pyranocoumarins are also accessible through this
sequence by the formation of product 16 (Table 2, Entries 9
and 10). Such phenyl coumarins represent another important
class of naturally occurring coumarins. Although 4-aryl substitu-
ents and pyranoannulated systems are not found concurrently
in natural products, 4-arylpyranocoumarin hybrids have been
synthesized and biologically evaluated.[45–47] For all of the prod-
ucts listed in Table 2, Entries 7–14, full conversion into the cor-
responding pyranocoumarin was observed after 10 min. Irradia-
tion for 1 h did not result in any notable decomposition of the
products, which were obtained in yields similar to those result-
ing after an irradiation time of 10 min.
The isolation of chromenes 14 as side products during the
Cu-catalyzed propargylation (Table 1 and Scheme 2) provided
an opportunity to investigate the Wittig olefination and cycliza-
tion steps independently from the propargyl Claisen rearrange-
ment (Scheme 3). To this end, compounds 14d and 14e were
treated with the Wittig reagent under the standard microwave
conditions as shown in Table 2, Entry 3. Interestingly, braylin (9)
and dipetalolactone (10) were isolated by the two-step se-
quence in yields very similar to those obtained from the three-
step sequence (Table 2, Entries 11–14). This observation sug-
gests that the propargyl Claisen rearrangement proceeds with
high selectivity and conversion, but that either the Wittig olefin-
ation or cyclization, which follow, is the rate-limiting step.
Experimental Section
General Methods: All experiments were conducted in dry reaction
vessels under dry nitrogen. Solvents were purified by standard pro-
cedures. The 1H NMR spectroscopic data were obtained at 300 MHz
in CDCl3 with CHCl3 (δ = 7.26 ppm) as an internal standard. Cou-
pling constants are reported in Hz. The 13C NMR spectroscopic data
were recorded at 75 MHz in CDCl3 with CDCl3 (δ = 77.0 ppm) as
an internal standard. IR spectra were recorded as ATR-FTIR (ATR =
attenuated total reflectance) spectra. Wavenumbers (ν) are reported
˜
in cm–1. The intensities of the bands are defined as strong (s), me-
dium (m), or weak (w). Low and high resolution mass spectral data
were obtained by EI/TOF or ESI-TOF analysis. Microwave reactions
were carried out in an Anton-Paar-monowave-300 reactor (mono-
wave, maximum power 850 W, temperature control by IR sensor, vial
volume: 20 mL). Starting material 11d was synthesized according
to a reported procedure.[48]
General Procedure for the Microwave-Promoted One-Pot Reac-
tion Sequence: The appropriate propargyl ether 12 (1.00 mmol)
was dissolved in N,N-diethylaniline (10 mL) in a vessel that was
suited for microwave irradiation. Ylide 2 (522 mg, 1.50 mmol) was
added, and the vessel was sealed. The reaction mixture was irradi-
ated at 250 °C for 10 min in the dedicated microwave reactor. After
cooling to ambient temperature, the mixture was diluted with ethyl
acetate (50 mL), and the resulting mixture was washed with HCl
(2
M aqueous solution, 3 × 30 mL). The organic layer was separated,
dried with MgSO4, and filtered. The filtrate was concentrated, and
the crude residue was purified by column chromatography on silica
gel [hexanes/methyl tert-butyl ether (MTBE) mixtures of increasing
polarity].
4,8,8-Trimethyl-2H,8H-pyrano[2,3-f]chromen-2-one (15):[46] By
following the general procedure, 12a (218 mg, 1.00 mmol) provided
15 (172 mg, 0.71 mmol, 71 %) as a colorless solid; m.p. 119–121 °C.
1H NMR (300 MHz, CDCl3): δ = 7.33 (d, J = 8.7 Hz, 1 H), 6.88 (d, J =
10.1 Hz, 1 H), 6.73 (d, J = 8.7 Hz, 1 H), 6.10 (q, J = 1.1 Hz, 1 H), 5.71
(d, J = 10.1 Hz, 1 H), 2.36 (d, J = 1.1 Hz, 3 H), 1.46 (s, 6 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 161.2, 156.2, 153.0, 149.6, 130.8, 124.6,
115.5, 113.7, 113.2, 111.7, 109.4, 77.6, 28.2, 18.9 ppm. IR (ATR): ν =
˜
2975 (w), 1723 (s), 1624 (m), 1589 (s), 1435 (m), 1384 (s), 1373 (s),
1288 (s), 1212 (m), 1171 (s), 1112 (s), 1072 (s) cm–1. HRMS (EI): calcd.
for C15H14O3 [M]+ 242.0943; found 242.0949.
Seselin (8):[31] By following the general procedure, 12b (204 mg,
1.00 mmol) provided 8 (151 mg, 0.66 mmol, 66 %) as a yellowish
1
oil. H NMR (300 MHz, CDCl3): δ = 7.58 (d, J = 9.5 Hz, 1 H), 7.18 (d,
J = 8.4 Hz, 1 H), 6.85 (d, J = 10.1 Hz, 1 H), 6.69 (d, J = 8.5 Hz, 1 H),
6.20 (d, J = 9.5 Hz, 1 H), 5.71 (d, J = 10.1 Hz, 1 H), 1.45 (s, 6 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 161.1, 156.4, 150.2, 144.0, 130.9, 127.9,
115.1, 113.6, 112.7, 112.7, 109.4, 77.7, 28.2 ppm. IR (ATR): ν = 2975
˜
(w), 2930 (w), 1720 (s), 1636 (m), 1593 (s), 1484 (m), 1403 (m), 1290
(m), 1258 (m), 1153 (m), 1109 (s), 1073 (s), 1007 (s) cm–1. HRMS (EI):
calcd. for C14H12O3 [M]+ 228.0786; found 228.0789.
Scheme 3. Microwave-promoted Wittig olefination/cyclization sequence from
preformed chromenes.
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8,8-Dimethyl-4-phenyl-2H,8H-pyrano[2,3-f]chromene-2-one
(16):[46] By following the general procedure, 12c (280 mg,
1.00 mmol) provided 16 (164 mg, 0.54 mmol, 54 %) as an orange
1
oil. H NMR (300 MHz, CDCl3): δ = 7.53–7.47 (3 H), 7.46–7.39 (2 H),
7.21 (d, J = 8.8 Hz, 1 H), 6.96 (d, J = 10.1 Hz, 1 H), 6.67 (d, J = 8.8 Hz,
1 H), 6.19 (s, 1 H), 5.74 (d, J = 10.1 Hz, 1 H), 1.48 (s, 6 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 161.2, 156.4, 156.4, 150.3, 135.8, 130.8,
129.6, 128.9, 128.5, 127.2, 115.5, 113.3, 112.7, 111.6, 109.7, 77.7,
28.3 ppm. IR (ATR): ν = 3060 (w), 2974 (w), 1727 (s), 1614 (m), 1586
˜
(s), 1445 (m), 1372 (s), 1289 (s), 1210 (m), 1154 (m), 1113 (s), 1081
(s) cm–1. HRMS (EI): calcd. for C20H16O3 [M]+ 304.1099; found
304.1092.
Braylin (9):[46] By following the general procedure, 12d (234 mg,
1.00 mmol) provided 9 (194 mg, 0.75 mmol, 75 %). Alternatively,
14d (234 mg, 1.00 mmol) also afforded 9 (183 mg, 0.71 mmol, 71 %)
as an orange solid, m.p. 139–141 °C. 1H NMR (300 MHz, CDCl3): δ =
7.57 (d, J = 9.5 Hz, 1 H), 6.87 (d, J = 10.0 Hz, 1 H), 6.76 (s, 1 H), 6.24
(d, J = 9.4 Hz, 1 H), 5.74 (d, J = 10.1 Hz, 1 H), 3.88 (s, 3 H), 1.51 (s, 6
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 161.3, 146.1, 145.8, 145.1,
143.8, 131.0, 115.3, 113.3, 111.6, 110.4, 108.8, 78.1, 56.7, 28.1 ppm.
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IR (ATR): ν = 2972 (w), 2925 (w), 1713 (s), 1599 (m), 1566 (s), 1463
˜
(m), 1480 (m), 1408 (s), 1292 (s), 1147 (s), 1128 (s), 1078 (m), 1015
(s) cm–1. HRMS (EI): calcd. for C15H14O4 [M]+ 258.0892; 258.0906.
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1
0.40 mmol, 40 %) as a yellowish oil. H NMR (300 MHz, CDCl3): δ =
7.93 (d, J = 9.6 Hz, 1 H), 6.79 (d, J = 10.0 Hz, 1 H), 6.62 (d, J =
10.0 Hz, 1 H), 6.11 (d, J = 9.6 Hz, 1 H), 5.58 (d, J = 10.1 Hz, 1 H), 5.54
(d, J = 9.5 Hz, 1 H), 1.46 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
161.4, 152.1, 150.3, 138.9, 127.8, 127.7, 116.1, 115.4, 110.8, 106.1,
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103.3, 102.4, 78.2, 78.1, 77.4, 28.4, 28.2 ppm. IR (ATR): ν = 2975 (w),
˜
1731 (s), 1640 (m), 1592 (s), 1442 (m), 1364 (m), 1134 (s), 1018
(m) cm–1. HRMS (EI): calcd. for C19H18O4 [M]+ 310.1205; found
310.1209.
Acknowledgments
We thank Evonik for the generous donations of solvents.
Keywords: Domino reactions · Alkynes · Arenes · Oxygen
heterocycles · Microwave chemistry · Rearrangement
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Received: December 4, 2017
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