Hypervalent iodine promoted transformation of 4H-chromenes into dihydrofurans and 2H-chromenes

Article abstract of DOI:10.1080/00397911.2020.1799013

A novel protocol to synthetic transformation of 4H-chromenes into dihydrofurans (3) and 2H-chromenes (11) promoted by hypervalent iodine has been accomplished in a one-pot reaction at ambient temperature. Dihydrofurans are exclusively formed under basic reaction conditions, whereas, 2H-chromenes are the products under acidic condition. Simple reaction conditions with a broad substrate scope inclusive of various heterocycles and high yields are the attractive features of this protocol.

Full text of DOI:10.1080/00397911.2020.1799013

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Hypervalent iodine promoted transformation
of 4H-chromenes into dihydrofurans and 2H-
chromenes
Ashok Kale , Nagaraju Medishetti , Jagadeesh Babu Nanubolu & Krishnaiah
Atmakur
To cite this article: Ashok Kale , Nagaraju Medishetti , Jagadeesh Babu Nanubolu & Krishnaiah
Atmakur (2020): Hypervalent iodine promoted transformation of 4H-chromenes into dihydrofurans
and 2H-chromenes, Synthetic Communications, DOI: 10.1080/00397911.2020.1799013
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https://doi.org/10.1080/00397911.2020.1799013
Hypervalent iodine promoted transformation of 4H-
chromenes into dihydrofurans and 2H-chromenes
Ashok Kalea,b, Nagaraju Medishettia,b, Jagadeesh Babu Nanubolub,c, and
Krishnaiah Atmakura,b
aFluoro & Agro Chemicals Department, CSIR-Indian Institute of Chemical Technology, Tarnaka,
b
Hyderabad, India; Academy of Scientific and Innovative Research, CSIR- Indian Institute of Chemical
c
Technology, Tarnaka, Hyderabad, India; Laboratory of X-ray Crystallography, CSIR-Indian Institute of
Chemical Technology, Tarnaka, Hyderabad, India
ABSTRACT
ARTICLE HISTORY
Received 13 May 2020
A novel protocol to synthetic transformation of 4H-chromenes into
dihydrofurans (3) and 2H-chromenes (11) promoted by hypervalent
iodine has been accomplished in a one-pot reaction at ambient tem-
perature. Dihydrofurans are exclusively formed under basic reaction
conditions, whereas, 2H-chromenes are the products under acidic
condition. Simple reaction conditions with a broad substrate scope
inclusive of various heterocycles and high yields are the attractive
features of this protocol.
KEYWORDS
1,3-Diones; hypervalent
iodine; 2H-chromenes; 4H-
chromenes; dihydrofurans
GRAPHICAL ABSTRACT
Introduction
In the recent past, hypervalent iodine reagents have been widely used in synthetic trans-
formations, especially in the field of C–C, C–N, C–S and C–O bond formation and
CONTACT Krishnaiah Atmakur
krishnu@iict.res.in
Fluoro & Agro Chemicals Department, CSIR-Indian Institute of
Chemical Technology, Tarnaka, Hyderabad, India
ß 2020 Taylor & Francis Group, LLC

2
A. KALE ET AL.
cyclization reactions[1] and have become an alternate to transition metal-catalyzed reac-
tions[2]. Further, hypervalent iodine mediated reactions most of the time proceed
straightforward, finds compatibility and meets the environmental standards for its
greater importance toward green chemistry.[3] Some of the chiral hypervalent iodine
reagents are in the glare of asymmetric synthesis[4] and several books[5] as well as
[6]
reviews
have been published based on the synthetic applications of these reagents.
Wide applicability of these reagents in synthetic organic chemistry are mainly based on
extremely high leaving ability of iodobenzene and high electrophilicity at the iodine
atom.[6d]On the other hand, furan scaffolds are the key structural units in many bio-
active compounds and gained prominence in modern heterocyclic chemistry. For
example, Moracins (A–Z),[7] furodysinin,[8] pinguisone,[9] methyl vouacapenate[10] con-
stitutes a characteristic structural features in a number of bioactive natural products.
Further, these compounds are valuable synthons in synthesis of several heterocyclic
compounds.[11]
Similarly, chromenes are considered as one of the important molecular structures in
chemistry, biology and medicine.[12] Among them, 2H-chromene motifs are present in a
large number of natural products that are biologically active. For example, polyphenols
can be found in tea leaves, vegetables and fruits.[13] Further, 2H-chromene scaffolds
exhibit a wide range of biological activities such as anti-HIV, anti-diabetic, anti-inflam-
matory, anti-tumour, anti-oxidant, anti-viral, anti-microbial, anti-tubercular, diuretic
activity including activators of K-channels, sex pheromones or phosphodiesterase IV
and dihydrofolate reductase inhibitors.[14] Therefore, development of novel methodolo-
gies for synthesis and transforming them into various structural motifs is a continuous
urge of organic chemists. With this background coupled with our recent work on the
transformation of 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chro-
mene-3-carbonitrile (1) into several new chemical entities through novel protocols,[15]
herein, we report the transformation of 1 into products 3 and 11 promoted by bis ace-
toxy iodobenzene (BAIB) under simple reaction conditions with good compatibility and
high yields.
Results and discussion
Our studies have commenced by treating compound 1 (1.0 equiv) with BAIB (1.0 equiv)
in trifluoroethanol at ambient temperature anticipating the formation of compound 2 as
per the previous reports (Scheme 1). However, formation of product 3 as major and
4[16a] in minor quantities were observed (Scheme 2). Structure of 3 was confirmed by
the spectral data and a representative compound 3k (Fig. 1) was analyzed by X-ray
crystallographic studies (CCDC 1921313 contains the supplementary crystallographic
data for this paper). Based on the X-ray studies the relative stereochemistry of com-
pound 3k assigned as cis since both the phenyl and imidate groups are on the
same plane.
Amazed over the formation of 3 and 4, studies were focus on preparing compound 3
exclusively. Accordingly, an experiment was conducted by reacting 1 with BAIB in tri-
fluoroethanol employing triethylamine (1.0 equiv) at ambient temperature. Interestingly,
a conspicuous increase in the yields of 3 was observed (Table 1, entry 2). Next, the

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SYNTHETIC COMMUNICATIONSV
3
Scheme 1. Synthesis of compound 3.
Scheme 2. Previous reports[16] and our work.
dosage of BAIB, TEA and the impact of other bases were examined. The outcome of
the study (Table 1, entries 3–10) showed that one equiv of BAIB and two equiv of base
is optimum for exclusive formation of 3 in 89% yield.
Further, studies were also conducted by replacing trifluoroethanol with ethanol,
methanol and isopropanol. Of these, ethanol and methanol could produce the desired
products in moderate yields and no reaction was observed in isopropanol (Scheme 3).
Next, studies were conducted by employing other hypervalent iodine reagents such as
PIFA, HTIB, PhIO (Table 1, entries 11–13). However, BAIB was proved to be the most
suitable. With the set reaction parameters on hand for the formation of 3, the scope of

4
A. KALE ET AL.
Table 1. Optimization of reaction conditions for the compound 3a.
reagent
(eq)
base
(eq)
solvent
(mL)
temp
(ꢀC)
time
(min)
yield 3
(%)
yield 4
(%)
entry
1
2
3
4
5
6
7
8
BAIB (1)
BAIB (1)
BAIB (1)
BAIB (1)
BAIB(0.4)
BAIB (0.8)
BAIB (1)
BAIB (1)
BAIB (1)
BAIB (1)
PIFA
–
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
10
10
10
10
10
64
71
79
89
36
77
64
59
79
81
–
19
10
trace
-
–
–
19
19
trace
trace
–
TEA (1)
TEA (1.5)
TEA (2)
TEA (2)
TEA (2)
10
K2CO3 (2)
NaOH (2)
Piperidine (2)
DBU (2)
TEA (2)
TEA (2)
TEA (2)
120
120
120
120
120
120
120
9
10
11
12
13
HTIB (1)
PhIO
51
34
–
–
eq ¼ equivalents; (-) ¼ no product; temp ¼ temperature (ꢀC);
TFE ¼ trifluoroethanol: time in minutes.
Bold indicates the optimum parameter.
Scheme 3. Scope of the protocol.b
aReaction Conditions: 1, (0.11 mmol), BAIB (0.11 mmol), triethylamine (0.22 mmol), TFE (5 mL), rt.
bIsolated yield and reaction time.

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5
Scheme 4. Functionalization of compound 3.
a) EtOH KOH, reflux, 2 h; b) 1N-HCl, reflux, 2 h; c) Conc HCl, reflux, 2 h.
this protocol was extended to various electron donating, neutral and electron withdraw-
ing substituted 4H chromene derivatives. To our satisfaction, this protocol is compatible
to be able to produce the corresponding products (3–3l) in very good yields
(Scheme 3).
Later, compounds 3 and 3j were subjected to hydrolysis employing acid and base
independently. Initially, base mediated hydrolysis of 3 was conducted employing etha-
nolic KOH at reflux temperature for 2 h. Interestingly, ethyl 6,6-dimethyl-4-oxo-3-phe-
nyl-4,5,6,7-tetrahydrobenzofuran-2-carbimidate (6) was obtained in (78%) yields.
Subsequent acid hydrolysis with 1 N HCl and conc HCl separately lead to compounds 7
with 94% and 8 with 88% yields. Surprisingly, no reaction was observed when replaced
ethanolic KOH with trifluoroethanolic KOH. However, in the case of compound 3j
under (acidic/basic) reaction conditions formation of compounds 6, 8 and 9[15d] was
observed (Scheme 4).
Additionally, reactivity of compound 1 with BAIB was also investigated in trifluoro-
acetic acid presuming the formation of product 10 if any. To our surprise, formation of
11 observed in 84% yield (Scheme 5). Structure of 11 was established by the spec-
tral data.[16b]
Excited over the formation of unexpected product 11, reactivity of 1 was examined
with other halogen electrophiles such as NCS, NBS, NIS, HTIB, PIFA and PhIO inde-
pendently under similar reaction conditions. Of these, only PhIO could produce 11 in
similar yields and HTIB 46% and PIFA 20% respectively (Table 2). Having obtained the
product 11 in excellent yields, this protocol was verified by reacting various 4H chro-
mene compounds having electron donating, neutral and electron withdrawing groups

6
A. KALE ET AL.
Scheme 5. Synthesis of 2H-chromenes.
Table 2. Optimization of reaction conditions for the compound 11a.
reagent
(eq)
solvent
(mL)
temp
(ꢀC)
time
(min)
yield 10
(%)
yield 11
(%)
entry
1
2
3
4
5
6
7
8
BAIB (0.1)
BAIB (0.2)
BAIB (0.4)
BAIB (0.6)
BAIB(0.8)
BAIB (1)
BAIB (1.2)
BAIB (1.5)
NCS
NBS
NIS
HTIB
PIFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
30
30
30
30
30
30
30
30
30
30
30
30
30
30
–
–
–
–
–
-
–
–
–
–
–
–
–
–
trace
15
48
66
71
84
84
84
-
-
-
46
20
81
9
10
11
12
13
14
PhIO
eq ¼ equivalents; (-) ¼ no product; temp ¼ temperature (ꢀC); TFA ¼ trifluoroacetic acid: time in minutes.
Bold indicates the optimum parameter.
on C-4 phenyl ring and also other enones at C-5 and C-6 position with BAIB under
similar reaction conditions (Scheme 6). Of these, compounds having electron withdraw-
ing groups gave high yields when compared to electron donating group substituted
4H-chromenes. Further, 4H-chromenes with hetero atoms (O, N, and S) on phenyl ring
and other hetero cyclic compounds could not produce the desired product. This is
probably due to the decomposition of 4H-chromenes in a strong acidic reac-
tion medium.
Plausible mechanism
Based on the previous reports,[15d, 17] a plausible mechanism has been proposed, where
the initial reaction between compound 1 and BAIB in presence of base lead to
the formation of intermediate I with the liberation of Et3NHOAc, which on
C–O bond dissociation actuated by the alkoxy group leads to II. Finally, the attack

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7
Scheme 6. Substrate scope for the 2H-chromenes.b aReaction Conditions: 1, (0.11 mmol), BAIB
b
(0.11 mmol), TFA (5 mL), rt. Isolated yield and reaction time.
Plausible mechanism
of hydroxy functional at the C-3 carbon leads to the ring contraction product
3 with the elimination of iodobenzene and one more equivalent of Et3NHOAc
(Scheme 7).
Whereas in the case of formation of compound 11, treatment of compound 1 with
BAIB in the presence of trifluoroacetic acid lead to I with the elimination of AcOH.
Next, formation of intermediate III possible via simultaneous liberation of PhI and
AcOH with the removal of benzylic proton and cleavage of C-I bond. Finally, hydrolysis
of imine results in product 11 (Scheme 8).

8
A. KALE ET AL.
Scheme 7. Mechanism for compound 3.
Scheme 8. Mechanism for compound 11.
Conclusions
In summary, we have come out with a novel transformation of 4H-chromenes into
dihydrofuran and 2H-chromene scaffolds promoted by BAIB. Simple reaction condi-
tions, high compatibility with good yields are the advantages of this protocol which
may be useful to accomplish more such new chemical entities.

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SYNTHETIC COMMUNICATIONSV
9
Figure 1. Crystal structure of the compound 3k (ORTEP diagram of compound 3k).
Experimental
General
Melting points were measured by CINTEX programable melting point apparatus and
1
are uncorrected. H and 13C NMR spectra of samples in CDCl3 recorded on AVANCE-
300 MHz, 400 MHz, 500 MHz and 700 MHz spectrometers. Chemical shifts (d) are
reported relative to TMS (d ¼ 0.0) as the internal standard. Mass spectra were recorded
in ESI spectrometers. All high-resolution mass spectra were recorded on QSTAR XL
hybrid ms/ms system (Applied Bio Systems/MDS Sciex, Foster City, USA), equipped
with an ESI source (IICT, Hyderabad). TLC was performed on Merck 60F-254 silica gel
plates. The chemicals used in this work were obtained from commercial channels and
were used without purification.
General procedure for the preparation of 2,2,2-trifluoroethyl 2-cyano-6,6-dimethyl-4-
oxo-3-phenyl-2,3,4,5,6,7-hexahydrobenzofuran-2-carbimidate (3)
To a 100 mL round-bottomed flask was added 2-amino-7,7-dimethyl-5-oxo-4-phenyl-
5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (1) (1.0 equiv), BAIB (1.0 equiv) and
triethylamine (2.0 equiv) in trifluoroethanol (5 mL) and stirred the reaction mixture at
room temperature. After completion of the reaction in 10 min as indicated by TLC,
10 mL water was added to the reaction mass and extracted with ethyl acetate. Organic
layer dried over anhydrous sodium sulfate and evaporated under reduced pressure. Left
over mass passed through a column of silica gel using ethyl acetate and hexane (25:75)
mixture as eluent to give pure compound (3) 89% yield.
2,2,2-Trifluoroethyl 2-cyano-6,6-dimethyl-4-oxo-3-phenyl-2,3,4, 5,6,7 hexahydroben-
zofuran-2-carbimidate (3)
Isolated as color less solid; Yield 89%; m.p. 140–142 ꢀC, IR (KBr): 2925, 2228, 1665,
1
1542, 1320, 1059, 925, 860, 775 cmꢁ1, H NMR (400 MHz, CDCl3): d 8.16 (br, 1H),
7.33–7.22 (m, 3H), 7.07–6.97 (m, 2H), 4.85 (s, 1H), 4.36–4.25 (m, 1H), 3.63–3.52
(m, 1H), 2.68–2.54 (m, 2H), 2.43–2.22 (m, 2H), 1.23 (s, 6H) ppm, 13C NMR (100 MHz,

10
A. KALE ET AL.
CDCl3): d 192.7, 172.7, 158.7, 133.6, 128.9, 128.6, 128.2, 122.4 (q, J ¼ 276.7 Hz), 115.3,
83.6, 62.7 (q, J ¼ 37.4 Hz), 56.3, 51.0, 37.3, 34.3, 29.0, 28.3 ppm, ESI-MS: m/z 393
[M þ H]þ, HRMS (ESI) Anal. calcd. for C20H20F3N2O3 m/z 393. 1417 [M þ H]þ,
found 393.1426.
Experimental procedures, characterization and Spectral data for all the synthesized
compounds and X-ray diffraction crystallographic data for the compound 3k which has
been deposited on CCDC with deposition number 1921313 can be available free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Center (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax:
þ44(0) 1223 336 033].
Acknowledgements
We gratefully acknowledge the help received from Centre for NMR & Structural Chemistry and
Analytical Chemistry & Mass Spectrometry, CSIR–IICT.
Funding
We are grateful to SERB-DST, New Delhi, for financial support (grant no. EEQ/20l6/000066) and
we thank director CSIR-IICT for the support (IICT/pubs./2019/333).
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