Novel one-pot synthesis of diverse γ,δ-unsaturated β-ketoesters by thermal cascade reactions of diazodicarbonyl compounds and enol ethers: Transformation into substituted 3,5-diketoesters

Article abstract of DOI:10.1039/c4ob00664j

Sequential Wolff rearrangement of α-diazo-β-ketoesters followed by trapping of the ketene intermediates with enol ethers generated a variety of γ,δ-unsaturated β-ketoesters. This method involves a novel thermal cascade reaction and allows the synthesis of γ,δ-unsaturated β-ketoesters with trans-stereochemistry under catalyst-free conditions. The synthesized compounds were further transformed into novel 3,5-diketoesters, which were used for the synthesis of naturally occurring 2-pyrone and 1-naphthoic acid ester. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00664j

Organic &
Biomolecular Chemistry
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Novel one-pot synthesis of diverse γ,δ-unsaturated
β-ketoesters by thermal cascade reactions of
diazodicarbonyl compounds and enol ethers:
transformation into substituted 3,5-diketoesters†
Cite this: Org. Biomol. Chem., 2014,
12, 4407
Rameshwar Prasad Pandit and Yong Rok Lee*
Sequential Wolff rearrangement of α-diazo-β-ketoesters followed by trapping of the ketene intermediates
with enol ethers generated a variety of γ,δ-unsaturated β-ketoesters. This method involves a novel thermal
cascade reaction and allows the synthesis of γ,δ-unsaturated β-ketoesters with trans-stereochemistry
under catalyst-free conditions. The synthesized compounds were further transformed into novel
3,5-diketoesters, which were used for the synthesis of naturally occurring 2-pyrone and 1-naphthoic
acid ester.
Received 29th March 2014,
Accepted 28th April 2014
DOI: 10.1039/c4ob00664j
www.rsc.org/obc
Ketenes have been used for the synthesis of a diverse selection
of carboxylic acids, esters, and amides via nucleophilic addition
Introduction
Carbon–carbon bond formation is of great importance and a on the sp-carbon of ketenes by water, alcohols, or amines
long-standing goal for synthetic chemists.1 As versatile organic (paths a–c).6 Thermal, photochemical, and Lewis-acid promoted
intermediates, ketenes provide an attractive and powerful [2 + 2] cycloaddition reactions between ketenes and alkenes or
means of chemical synthesis.2 Ketenes, though unstable and imines give rise to a diverse range of cyclobutanones (path d)
usually generated in situ for trapping,3 can serve as building and β-lactams (path e).7 Furthermore, cycloadditions between
blocks for the assembly of various organic materials.4 In ketenes and olefins have been used to produce various cyclo-
particular, the Wolff rearrangement is widely used for the hexenones through [4 + 2] cycloadditions (path f).8 However,
preparation of reactive ketene intermediates (Scheme 1).5 although numerous [2 + 2] and [4 + 2] cycloaddition reactions
between ketene intermediates and alkenes have been reported,
no previous report has described the direct generation of unsa-
turated enones by nucleophilic attack of enol ethers to ketenes
derived from diazodicarbonyl compounds.
γ,δ-Unsaturated β-ketoesters are widely used as important
intermediates and building blocks in the synthesis of organic
and natural products.9 Multifunctionality in unsaturated
ketoesters enables their consumption in the Robinson annula-
tion, in the synthesis of the corresponding enamines, pyrones,
and trans, trans-dienones, and in the preparation of substrates
for the Diels–Alder reactions.10
The reported general methods for the synthesis of γ,δ-un-
saturated β-ketoesters include the dianion reaction between
phosphine oxides and ketones,11 and coupling reaction
between the magnesium complex of ethyl hydrogen malonate
and acid chlorides.12 Although several approaches have been
reported for the synthesis of γ,δ-unsaturated β-ketoesters, they
Scheme 1 Reported reactions for the synthesis of various molecules
via the Wolff rearrangement.
have a limitation due to the harsh reaction conditions
required, low yields, and raw material availability.13 In parti-
School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749,
cular, there are a few reports on the synthesis of γ,δ-unsaturated
Republic of Korea. E-mail: yrlee@yu.ac.kr; Fax: +82 53-810-4631; Tel: +82 53-810-2529
†Electronic supplementary information (ESI) available. See DOI:
β-ketoesters bearing an (E)-alkoxy group.14 Accordingly, there
is a demand for a facile one-step synthetic method for the
10.1039/c4ob00664j
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Table 1 Synthesized acyclic diazodicarbonyl compounds 1a–1i
Scheme 2 Novel and efficient synthesis of γ,δ-unsaturated β-ketoesters
by the thermal cascade reaction of acyclic diazodicarbonyls and
enol ethers.
production of diverse γ,δ-unsaturated β-ketoesters under mild
thermal conditions.
Table 2 Thermal reactions of 1a with ethyl vinyl ether under several
solvents
We have developed several methods based on rhodium(II)-
catalyzed reactions between cyclic diazodicarbonyl compounds
and substrates, such as ketones, isocyanates, nitriles, and
halides, to generate a variety of 5-membered heterocycles15
and α-haloenones.16 In addition, we recently prepared novel
and diverse cyclic β-enaminoamides17 and indene derivatives18
via the thermal Wolff rearrangement of cyclic diazo
compounds.
Yielda (%)
Entry
Solvent
Condition
2a
3a
During our continued studies on the development of new
methodologies based on the Wolff rearrangement, we investi-
gated thermal reactions between diazodicarbonyls and several
enol ethers. Herein, we describe a novel and efficient one-pot
synthesis of a variety of γ,δ-unsaturated β-ketoesters bearing an
(E)-alkoxy group by thermal Wolff rearrangement of acyclic
diazodicarbonyl compounds followed by nucleophilic addition
of enol ethers to ketene intermediates (Scheme 2).
1
2
3
4
Benzene
Toluene
p-Xylene
DMF
Reflux, 24 h
Reflux, 24 h
140 °C, 6 h
120 °C, 12 h
32
5
0
21
51
60
b
—
—
a Isolated yield. b Unidentified complex mixtures.
Created bond.
Created bond.
and the coupling product 3a (21%) with the recovery of 35% of
1a (entry 1), whereas that in refluxing toluene for 24 h provided
products 2a and 3a in yields of 5 and 51%, respectively (entry 2).
When the temperature was raised to 140 °C using p-xylene
as a solvent for 6 h, the yield of compound 3a was increased to
60% and interestingly 2a was not observed (entry 3). However,
reaction of 1a in polar DMF at 120 °C for 12 h provided
complex mixtures (entry 4). The structures of 2a and 3a were
identified using spectral data. In the 1H NMR of the cyclo-
adduct 2a, a methine, a methylene, and a methyl proton on
the dihydrofuran ring showed peaks at δ 5.51 (1H, dd, J = 7.5,
3.0 Hz), 2.98 (1H, dd, J = 15.9, 7.5 Hz), 2.69 (1H, dd, J = 15.9,
3.0 Hz), and 2.20 (3H, s) ppm, respectively.19 In CDCl3 solu-
tion, compound 3a exclusively exists in a keto form. In the
1H NMR spectrum of 3a, a methine proton flanked by two carbo-
nyl groups showed a peak at δ 3.52 ppm as a quartet (J = 7.2
Hz) due to a keto form and there was no OH peak associated
with an enol form. Its trans-stereochemistry was determined by
analyzing coupling constants. The two vinylic peaks showed
absorptions at δ 7.60 (1H, J = 12.3 Hz) and 5.65 (1H, J =
12.3 Hz). Further confirmation of the structures of 2a and 3a
was obtained from 13C NMR spectra. For 2a, one carbonyl carbon
of a conjugated ester on the dihydrofuran ring produced a
peak at 166.1 ppm, whereas in 3a the two carbonyl carbons of
the enone and ester groups produced peaks at δ 194.5 and
171.6 ppm, respectively.
Results and discussion
We have recently disclosed a [3 + 2] cycloaddition reaction of
ruthenium-carbenoid generated from acyclic diazodicarbonyl
1a with olefin (Scheme 3).19 In this case, only dihydrofuran 2a
was produced in high yields and no other product associated
with the Wolff rearrangement was obtained.
To investigate thermal reactions between acyclic diazodicar-
bonyls and enol ethers, three types of acyclic diazodicarbonyl
compounds 1a–1i bearing methyl, aryl, and phenethyl groups
at C3 were prepared from the corresponding 1,3-dicarbonyl
compounds in 81–98% yield, as previously described
(Table 1).20
Thermal reactions between acyclic diazodicarbonyl com-
pound 1a and ethyl vinyl ether were first investigated at
different temperatures using different solvents. The results are
summarized in Table 2. Treatment of 1a with ethyl vinyl ether
in refluxing benzene for 24 h gave the cycloadduct 2a (32%)
The formation of the dihydrofuran 2a and the γ,δ-unsatu-
rated β-ketoester 3a is presumed to be initiated by the thermal
Scheme 3 Reported Ru(II)-catalyzed synthesis of dihydrofuran from
acyclic diazodicarbonyls.
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Scheme 4 Proposed mechanisms for formation of 2a and 3a from diazodicarbonyl compound 1a.
Table 3 Formation of γ,δ-unsaturated β-ketoesters by thermal
decomposition of acyclic diazodicarbonyl compounds with enol ethers
decomposition of methyl 2-diazo-3-oxobutanoate (1a) to the
corresponding carbene intermediate 4 via the initial loss of N2
(Scheme 4).21 Once formed, in refluxing benzene or toluene,
carbene 4 would competitively undergo cyclopropanation with
ethyl vinyl ether to give 5 or Wolff rearrangement to give
ketene 6. Ring opening of 5 followed by cyclization gives 2a,
whereas nucleophilic addition via attack of the vinyl group of
the enol ether to ketene 6 afford the zwitterionic adduct 7. The
intermediate 7 would undergo further deprotonation and sub-
sequent protonation to give 3a as an enol form. During this
step, the thermodynamically more stable trans-compound
would be produced and finally the enol form would adopt the
keto form due to a keto–enol tautomerism. Importantly, at
high temperature, compound 3a was only observed by a ketene
formation followed by nucleophilic attack of an enol ether.
Interestingly, it was reported by Wenkert that reaction of
acyclic diazomalonaldehyde in refluxing n-butyl vinyl ether
provides dihydro-4-pyrone in a good yield.22 However, in this
reaction, any possible adduct 8 via [4 + 2] cycloaddition was
not formed.
After optimizing the reaction conditions, we explored the
scope of thermal cascade reactions by employing three types of
acyclic diazodicarbonyl compounds and enol ethers (Table 3).
Several cascade reactions through the migration of methyl
groups of diazodicarbonyl compounds 1a–1c were first
attempted. Reactions between methyl 2-diazo-3-oxobutanoate
(1a) and n-propyl vinyl ether or n-butyl vinyl ether in refluxing 66–70%. Similarly, reaction between ethyl 2-diazo-3-oxo-
p-xylene for 6 h provided the desired products 3b and 3c in 3-phenylpropanoate (1g) and 2-methoxypropene produced com-
yields of 56 and 45%, respectively. Treatment of 1a with pound 3k in 72% yield. To extend the utility of these cascade
2-methoxypropene in refluxing xylene for 6 h gave 3d in 66% reactions, we performed further reactions using diazodicarbo-
yield. Similarly, thermal reaction between ethyl 2-diazo-3-oxo- nyl compounds 1h–1i bearing the phenethyl or 4-methoxy-
butanoate (1b) or allyl 2-diazo-3-oxobutanoate (1c) furnished phenethyl group. Reactions between methyl 2-diazo-3-oxo-
the desired products 3e–3g in yields of 59–76%. To investigate 5-phenylpentanoate (1h) or methyl 2-diazo-5(4-methoxyphenyl)-
the migratory aptitude of aryl groups, additional reactions of 3-oxopentanoate (1i) and 2-methoxypropene afforded 3l (68%)
diazodicarbonyl compounds 1d–1g were next carried out. Reac- and 3m (61%) through the migration of the phenethyl or
tions between methyl 2-diazo-3-oxo-3-phenylpropanoate (1d) 4-methoxyphenethyl group. Interestingly, additional reactions
or methyl 2-diazo-3-oxo-3-arylpropanoates 1e–1f bearing an between 1a or 1g and 2,3-dihydropyran produced 3n and 3o in
electron donating group on the benzene ring and 2-methoxy- yields of 58 and 61%, respectively. These results show that the
propene provided the desired products 3h–3j in yields of reactions of various diazodicarbonyls bearing methyl, aryl,
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and phenethyl groups provided the desired products 3b–3o in
moderate yields through the migration of methyl, aryl, and
phenethyl groups followed by the addition of vinyl ethers. All
products were characterized by spectral analysis and 3b–3g, 3k,
and 3l–3o exist exclusively as keto tautomers (in CDCl3
solution).
To examine the potential use of the γ,δ-unsaturated β-keto-
esters synthesized by the nucleophilic addition of enol ethers
to ketene intermediates, we envisioned that the synthesized
enones could be utilized to produce 3,5-diketoesters. Mole-
cules bearing 3,5-diketoesters are found in a variety of biologi-
cally active natural polyketides23 and are important starting
materials and building blocks for the synthesis of polyols.24
Representative synthetic approaches for 3,5-diketoesters
include the condensations of 1,3-dicarbonyl dianions with
esters, Weinreb amides, chloroacetates, or other acylating
agents.25 In addition, 3,5-diketoesters have also been prepared
by the condensation of 1,3-bis(silyl enol ethers) with acid
chlorides26 and by reaction between 1-methoxy-1,3-bis(tri-
methylsiloxy)-1,3-butadiene and acetyl chloride.27 Based on
these precedents, conversion of the synthesized γ,δ-unsatu-
rated β-ketoesters to 2-substituted 3,5-diketoesters was
attempted using p-TsOH as a Brønsted acid. Results are sum-
marized in Table 4. Methanol, ethanol, and allyl alcohol were
used as solvents to avoid transesterification. Reaction of 3d in
the presence of 1.0 equivalent of p-TsOH in refluxing methanol
for 10 h afforded 9a (71%), whereas reactions of 3f and 3g in
refluxing ethanol or allyl alcohol resulted in 9b and 9c at
yields of 73 and 60%, respectively. Similarly, when γ,δ-unsatu-
rated β-ketoesters 3i–3k bearing phenyl or aryl groups were
used, compounds 9d–9f were obtained at yields of 50–54%,
whereas when 3l–3m bearing arylethyl groups at the C2-posi-
tion were used, the desired products 9g–9h were isolated at
yields of 62 and 53%, respectively. The synthesized com-
pounds 9a–9h were characterized by spectral analysis and were
found to exist predominantly as the enol tautomers. For
example, in the 1H NMR spectrum of 9a, a methine proton
Scheme 5
flanked by 1,3-dicarbonyl groups showed a peak at δ 3.34 ppm
as a quartet (J = 7.2 Hz) and a vinyl proton on C4 due to an
enol form showed a peak at 5.46 ppm as a singlet. The ratio of
a keto/enol (= 14 : 86) tautomer was calculated by the inte-
gration of a methyl peak on C5. However, reaction of 3a in the
presence of 1.0 equivalent of p-TsOH in refluxing methanol for
12 h provided 9i (55%) through acetal formation from reactive
aldehyde.
3,5-Diketoesters were also used to synthesize a known
natural pyrone and a novel β-naphthol derivative, as shown in
Scheme 5. Reaction of 9a with DBU in refluxing benzene for
1 h provided the naturally occurring methyltriacetic lactone 10
(or USF 2550A; 81%), which was isolated from Penicillium stipi-
tatum,28 via condensation and an enolization, and treatment
of 9f with 1.0 equivalent of TFA in refluxing toluene for 12 h
gave functionalized 1-naphthoic acid ester 11 via an intramole-
cular Friedel–Crafts reaction at a yield of 45%.
Conclusions
In summary, we describe a novel one-pot approach for the syn-
thesis of γ,δ-unsaturated β-ketoesters based on thermal
cascade reactions between α-diazo-β-ketoesters and enol
ethers. This catalyst-free reaction allows the synthesis of
various trans-γ,δ-unsaturated β-ketoesters and could be used to
synthesize natural products and pharmaceuticals. Further
expansion of the scope of this reaction using other nucleo-
philic components in the presence of Lewis acid catalysts is in
progress.
Table 4 Synthesis of 3,5-diketoesters from the corresponding
γ,δ-unsaturated β-ketoesters
Acknowledgements
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2012R1A1A4A01009620).
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