1. LETTER ▌2649
letterUnprecedented C-Methylation at the 2-Position of 2-Carboxy-4-Chromanones
– A Case Study with the Corey–Chaykovsky Reagent
2-Methylation of 4-Chromanone-2-carboxylates by Sulfur Ylide
Subrata Ghosh,a
Nellore Bhanu Chandar,b,e
Debayan Sarkar,c
Manoj Kumar Ghosh,c
Bishwajit Ganguly,*b,e
Indrajit Chakraborty*d
a
Nano Characterization Unit, Advanced Key Technologies Division, National Institute of Material Science, Ibaraki 3050047, Japan
b
Computation and Simulation Unit (Analytical Discipline & Centralized Instrument Facility), Central Salt and Marine Chemicals Research
Institute, G.B. Marg, Bhavnagar, Gujarat, 3624002 , India
Fax +91(278)2567562; E-mail: ganguly@csmcri.org
c
Department of Chemistry, National Institute of Technology, Rourkela, Odissa, 769008, India
d
Department of Chemistry, KIIT University, Bhubaneswar, Odissa, 751024, India
Fax +91(674)2725113; E-mail: indraji2001@gmail.com
e
Academy of Scientific and Innovative Research, CSIR-CSMCRI, Bhavnagar, Gujarat, 364002, India
Received: 25.08.2014; Accepted after revision: 01.09.2014
Abstract: An unprecedented C-methylation at the 2-position of 4-
chromanone-2-carboxylates was achieved in good yield on treat-
ment with dimethylsulfoxonium methylide. The reaction was per-
formed with excellent chemo- as well as regioselectivity. It is the
first synthetic report of alkylation at the 2-position of the chroma-
none framework through a very mild and simple approach. Such an
uncommon behavioral pattern of the Corey–Chaykovsky reagent is
justified by theoretical potential energy surface calculations.
Key words: 4-chromanone-2-carboxylate, C-methylation, regiose-
lectivity, dimethylsulfoxonium methylide, theoretical calculations
The widespread versatility of ylide chemistry in function-
al-group transformations has generated considerable in-
terest over more than 60 years.1
The successful extension
of this chemistry to sulfonium ylides as well as their oxi-
dized analogues, the sulfoxonium ylides, was due to the
pioneering work of Corey and Frazen.2
Since then, sulfur
ylide mediated chemistry has been exploited extensively
in organic synthesis.3
Dimethylsulfoxonium methylide
(DIMSOY), popularly known as the Corey–Chaykovsky
reagent, is one of the most versatile sulfur ylides for pre-
parative organic chemistry.4
In organic chemistry, the nucleophilic reactivity of sulfur
ylides is well recognized through three-membered ring
(epoxide, cyclopropane, or aziridine) formation,3a
al-
though their enhanced stability is attributed to additional
d-orbital participation.5
The zwitterionic character of the
ylides contributes significantly to their reactivity, confer-
ring high stereo- and regiocontrol.6
Another type of reac-
tion exemplified by these reagents, although reportedly a
more limited one, is methylation. Thus a number of C-
methylation, N-methylation, O-methylation, and S-meth-
ylation reactions has been reported with DIMSOY.4,7
Re-
gioselective C-methylation of various nitrobenzenes by
DIMSOY was first reported by Traynelis et al.7a
The ten-
tative mechanistic explanations, however, could not fur-
nish the reasons behind such a regioselective methylation
process. Herein we wish to report DIMSOY-mediated
one-step methylation at the 2-position of 4-chromanone-
2-carboxylates in good yield and excellent regio- and che-
moselectivity. We also present supporting evidence for
such selectivity through computational calculations.
The chromanone framework abounds in a wide spectrum
of natural products with pharmacological properties such
as antioxidant, antitumor, and antibacterial activity.8
This
key motif for synthesizing chromone derivatives with bi-
ological activities9
makes it a privileged structure. To syn-
thesize chromone derivatives, a significant amount of
work has been carried out over the last four decades.10
While much effort has been expended on synthesizing 2-
aryl-substituted chromanones (flavones),11
only a limited
number of reports are available for installing an alkyl
group at that same position.12
However, the alkylation of
chroman-2-carboxylates is documented.13
The prepara-
tion of 2-alkyl-4-chromanone-2-carboxylates has been re-
ported by three different groups,14
although unlike the
examples presented herein, none of them was based on a
direct alkylation approach at the 2-position of 4-chroma-
none-2-carboxylates. Furthermore these earlier approach-
es failed to produce 2-alkylated products in good yields.
As with other conjugated enones, 4-chromone-2-carbox-
ylates may also be cyclopropanated in fair yield when
treated with DIMSOY.15
Subsequently changing the basic
framework from chromone to chromanone, we envisaged
the formation of epoxide 2 from the corresponding 4-
chromanone-2-carboxylates 1a–e on treatment with DIM-
SOY. However, in the event, no such epoxide was
formed, but rather the 2-alkylated products 3a–e were ob-
tained exclusively. Careful monitoring of the reaction by
TLC also indicated no starting material remaining
(Scheme 1). The best yield for this alkylation reaction was
obtained using DMSO as solvent.
SYNLETT 2014, 25, 2649–2653
Advanced online publication: 07.10.20140936-52141437-2096
DOI: 10.1055/s-0034-1379203; Art ID: st-2014-d0136-l
© Georg Thieme Verlag Stuttgart · New York
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2. 2650 S. Ghosh et al. LETTER
Synlett 2014, 25, 2649–2653 © Georg Thieme Verlag Stuttgart · New York
Computational calculations by Aggarwal et al. indicate
good leaving-group ability for the sulfonium group.16
This
ability becomes all the more manifest for the sulfoxonium
group, thus implying the role of trimethylsulfoxonium
cation (TMSO+
) as an active intermediate during methyl-
ation by DIMSOY. In our system, formation of TMSO+
is
achievable through the uptake of an acidic proton by the
ylidic carbon atom of DIMSOY (Scheme 2). In fact,
N-methylation of pyrimidines and the corresponding nu-
cleosides has been reported using trimethylsulfoxonium
hydroxide.17
Although the ylide carbon of the sulfur ylide is nucleop-
hilic in nature, various examples of methylation by DIM-
SOY4,7
as well as Aggarwal’s recent work on sulfonium
ylide mediated cyclopropanation18
clearly demonstrate
that it can also act as a base. However, to the best of our
knowledge no previous report is available in literature
where the basic nature of DIMSOY prevailed over its nu-
cleophilic nature in the presence of a keto functionality by
performing selective alkylation at a less reactive sp3
car-
bon atom C(2). Reportedly, 4-chromanones are much
more prone to undergo enolization, in the presence of a
base, followed by alkylation at C(3).19
From that point of
view methylation at the 2-position of 4-chromanone-2-
carboxylates by DIMSOY was totally unexpected. To ex-
plore this uncommon behavioral pattern of Corey–
Chaykovsky reagent, a computational study was under-
taken.
We first examined the formation of the epoxide of methyl
4-oxochroman-2-carboxylate 4 in the place of substituted
ethyl 4-oxochroman-2-carboxylates 1a–c for computa-
tional simplicity. The Corey–Chaykovsky reaction mech-
anism was computationally investigated by Aggarwal et
al.20
The mechanistic pathways seem to suggest that C–C
bond formation occurs via cisoid or transoid (see Support-
ing Information) addition of DIMSOY to the carbonyl
compounds, which leads towards the formation of the be-
taine. In the subsequent step, the formation of the epoxide
occurs with the elimination of DMSO. The elimination of
DMSO can occur either via a syn or anti fashion in the
transition state. The formation of epoxide of chromanone
4 with DIMSOY has been calculated employing M05-
2X/6-31+G* in DMSO. The potential-energy profile ob-
tained for the formation of epoxide suggests that only
Scheme 1 Regio- and chemoselective methylation of chromone-2-carboxylates 1 by DIMSOY
R2
R3
R1
O
O
CO2Et
i) Me3SOI, NaH, DMSO
ii) addition of compound
to ylide solution
iii) stirring, r.t., 4–12 h
R2
R3
R1
O
O
CO2Et
R2
R3
R1
O
O
CO2Et
2
1a: R1
= H, R2
= H, R3
= H, R4
= H
1b: R1 = H, R2 = Me, R3 = H, R4 = H
1c: R1
= H, R2
= H, R3
= H, R4
= Me
1d: R1 = H, R2 = OMe, R3 = Me, R4 = H
1e: R1 = Me, R2 = H, R3 = OMe, R4 = H
1f: R1 = H, R2 = H, R3 = Me, R4 = H
3a, 47%
3b, 86%
3c, 75%
3d, 80%
3e, 82%
3f, 83%
+ –
R4
R4
R4
Scheme 2 A Simple mechanistic rationale for chromanone-2-carboxylate 2-methylation by DIMSOY
O
O
H
CO2Me
S O
Me
Me
H2C
O
O
OMe
O
S
O
Me Me
Me
O
O
CO2Me
S
O
Me Me
Me
δ–
δ+
O
O
Me
CO2Me
DMSO
ester enolate
complex (A)
C(2)-methyl transfer TS (B)
C(2)-methylated product (5)
4 DIMSOY
TMSO+
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3. LETTER 2-Methylation of 4-Chromanone-2-carboxylates by Sulfur Ylide 2651
© Georg Thieme Verlag Stuttgart · New York Synlett 2014, 25, 2649–2653
cisoid-type addition is possible in this case (Figure 1). The
attack of DIMSOY as a nucleophile to the carbonyl group
of chromanone 4 leads to the formation of betaine, where
the O–C–C–S dihedral angle is 40° (Figure 1).
The geometric constraints and steric effects do not allow
the DIMSOY to approach the carbonyl group of chroma-
none 4 in a transoid fashion. Our efforts failed to locate
the transoid transition states in this case. Furthermore, an
alternative approach to arrive at the transiod betaine via
the cisoid pathway also failed as the rotational transition
state from cisoid to transoid form was also not accessible.
The steric crowding in the cisoid betaine prevents the ro-
tational transition state in this case. The cisoid betaine was
found to be ca. 8.0 kcal·mol–1
energetically more stable
compared to the separate reactants. A second transition-
state geometry was identified for the ring closure to form
the epoxide via the elimination of DMSO (Figure 1).The
O–C–C–S dihedral angle changes from 40° to 31° in the
elimination transition state which supports the cisoid ori-
entation of the molecule. The calculated activation barrier
to form the epoxide is ca. 34.5 kcal·mol–1
compared to the
cisoid betaine at M05-2X/6-31+G* level in DMSO.
The possibility for methyl 4-oxochroman-2-carboxylate
(4) to undergo methylation at C(2) and/or C(3) positions
with DIMSOY due to the availability of acidic protons
was also examined (Figure 3 and Figure 4). To examine
the methylation processes, the potential energy surface
was generated with M05-2X/6-31+G* in DMSO. The
methylation of methyl 4-oxochroman-2-carboxylate with
DIMSOY is a two-step process; the first step being the
deprotonation at C(2) or C(3) by DIMSOY followed by
methyl transfer to the respective carbons from DIMSOY.
Both steps are concerted in nature (Scheme 2 and Scheme
3, Figures 3 and 4).
Scheme 3 Methylation of chromanone 4 by DIMSOY at 3-position
The enolate complex formation after deprotonation at
C(3) is energetically more stable compared to the separat-
ed reactants by ca. 7.0 kcal·mol–1
; whereas, the enolate
complex formed via C(2) deprotonation is ca. 3.0
kcal·mol–1
less stable than the reactant molecules (Figures
2–4). In both cases, the methyl transfer process is the rate-
determining step (Figure 2). The activation energy barrier
calculated with M05-2X/6-31+G* for methyl transfer to
the C(2) position of chromanone 4 is 18.0 kcal·mol–1
.
However, the activation barrier calculated with respect to
the stable enolate complex is relatively higher for the cor-
Figure 1 Potential energy surface for epoxide formation of methyl 4-oxochroman-2-carboxylate with DIMSOY at M05-2X/6-31+G*(DMSO)
level of theory. Selected bond distances are given in Å and the relative energies are given in kcal·mol–1
.
–8.1
26.4
–26.8
0.0
cisoid TS
ring closure TS
PRODUCTS
REACTANTS
cisoid betaine
3.4
methyl
4-oxochroman-2-
carboxylate
2.20
epoxide DMSO
DIMSOY
O
O
H
CO2Me
S
O
MeMe
C
H2
O
O
H
CO2Me
S
O
MeMe
CH2
O
O
H
CO2Me
S
O
Me
Me
CH2
2.47
O
O
H
CO2Me
S
O
Me Me
CH2
O
O
H
CO2Me
S
O
MeMe
H2C
+
+
O
O
CO2Me
S O
Me
Me
H2C
O
O
S
O
Me Me
Me
O
O
CO2Me
S
O
Me Me
Me
δ–
δ+
O
O
CO2Me
DMSO
keto enolate
complex (B)
C(3)-methyl transfer TS
4 DIMSOY
H
CO2Me
TMSO+
C(3)-methylated product (5)
(trans)
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4. 2652 S. Ghosh et al. LETTER
Synlett 2014, 25, 2649–2653 © Georg Thieme Verlag Stuttgart · New York
responding process at C(3). The calculated barrier was
found to be 19.9 kcal·mol–1
(Figure 2). These calculated
results suggest that methyl 4-oxochroman-2-carboxylate
can preferentially undergo C(2) methylation. Further-
more, the formation of the epoxide of chromanone 4 with
DIMSOY is energetically unfavorable compared to the
methylation processes (Figures 1 and 2). The calculated
transition states and corresponding enolate complexes for
C(2) and C(3) methylation processes are given in Figures
3 and 4, respectively.
Figure 2 The potential energy surface for C(2)-methylation and
C(3)-methylation of methyl 4-oxochroman-2-carboxylate with DIM-
SOY at M05-2X/6-31+G* level of theory in DMSO
Figure 3 Optimized geometries and selected bond distances (Å) for
species involved in the C(2)-methylation of methyl 4-oxochroman-2-
carboxylate with DIMSOY at M05-2X/6-31+G* level in DMSO (red
= oxygen; white = hydrogen; yellow = sulfur; gray = carbon)
In summary, we have reported the methylation at the 2-
position of 4-chromanone-2-carboxylic acids with di-
methylsulfoxonium methylide (DIMSOY). This is the
first example of DIMSOY-mediated methylation at C(2)
instead of C(3) in these systems. DFT calculations re-
vealed that the epoxide formation with DIMSOY via
Corey–Chaykovsky reaction mechanism is unfavored in
this case, and the methylation at C(2) of the 4-chroma-
none-2-carboxylic acids can occur under such conditions.
Computational Section
All geometries were fully optimized with M05-2X/6-31+G* level
in DMSO with the polarizable continuum solvation model (PCM).21
All calculations were performed with the Gaussian 09 suite pro-
gram.22
The stationary points were characterized by frequency cal-
culations in order to verify that the transition structures had one, and
only one, imaginary frequency. To verify that each saddle point
connects two minima, intrinsic reaction coordinate (IRC) calcula-
tions of transition states were performed in both directions; that is,
by following the eigenvectors associated to the unique negative ei-
genvalue of the Hessian matrix, using the González and Schlegel in-
tegration method.23
The calculated M05-2X/6-31+G* electronic
energies have been reported here.
Typical Procedure for the 2-Methylation of 4-Chromanone-2-
carboxylates 1 by DIMSOY
Freshly prepared trimethylsulfoxonium iodide (1.03 mmol) was
weighed and dissolved in dry DMSO (2 mL). It was then added
slowly to a stirred suspension of oil-free NaH (1.12 mmol) in dry
DMSO (2 mL) under an inert atmosphere with cooling in an ice
bath. After 30 min, a solution of ester 1 (1 mmol) in DMSO (2 mL)
was slowly added in a dropwise manner and the reaction mixture
stirred for 20 min at ice-bath temperature. Subsequently, it was al-
lowed to warm to r.t. and stirred till completion of the reaction (ver-
ified by TLC; hexane–EtOAc, 10:1). The reaction mixture was then
poured into ice water and extracted with Et2O. The solvent was re-
moved under reduced pressure to obtain a crude mass which was
purified by column chromatography on silica gel (hexane–EtOAc)
to afford the pure compound 3.
Acknowledgment
I.C. is grateful to KIIT University, Bhubaneswar and BG is grateful
to the Central Salt and Marine Chemicals Research Institute
(CSIR), Gujarat, for providing research facilities during this pro-
gram. S.G. thanks Dr. A. Bandyopadhyay (NIMS) & MEXT for full
support of laboratory facilities. We are also grateful to Dr. T. K. Pai-
ne and Dr. S. Ghosh of IACS, Kolkata for providing us with specific
laboratory facilities. N.B.C. is grateful to the CSIR, New Delhi, for
O CO2Me
O
2
3
45
6
7
8
9
10
C(2)-methylation
C(3)-methylation
13.1
11.0
2.9
–7.2
18.0
12.7
–53.4
–57.1
0.0
TS1
PRODUCTS
REACTANTS
INT
methyl 4-oxochromane-2-carboxylate
1
TS2
Figure 4 Optimized geometries and selected bond distances (Å) for
species involved in the C(3)-methylation of methyl 4-oxochroman-2-
carboxylate) with DIMSOY at M05-2X/6-31+G* level in DMSO (red
= oxygen; white = hydrogen; yellow = sulfur; gray = carbon)
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5. LETTER 2-Methylation of 4-Chromanone-2-carboxylates by Sulfur Ylide 2653
© Georg Thieme Verlag Stuttgart · New York Synlett 2014, 25, 2649–2653
the award of a Fellowship and also thankful to AcSIR for enrolment
into a PhD program. We thank the anonymous reviewers for their
valuable comments/suggestions that have helped us to improve this
manuscript.
Supporting Information for this article is available online
at http://www.thieme-connect.com/products/ejournals/journal/
10.1055/s-00000083.SupportingInformationSupportingInformation
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