Synthesis of pyrano[2,3-: F] chromen-2-ones vs. pyrano[3,2-g] chromen-2-ones through site controlled gold-catalyzed annulations

Article abstract of DOI:10.1039/c9ob01695c

Regioselective access to 10-substituted-2H,8H-pyrano[2,3-f]chromen-2-ones through the gold-catalyzed intramolecular hydroarylation of readily available 7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivatives at their C-8 congested position was investigated by tuning the electronic and steric properties of the ligand on the gold complex. On the other hand, the combination of the JohnPhosAu(MeCN)SbF₆ catalyzed intramolecular hydroarylation of 8-iodo-7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivatives followed by selective palladium/formate C-I reduction allows for the exclusive formation of 2H,8H-pyrano[3,2-g]chromen-2-one regioisomers. The development of these two protocols provides versatile synthetic tools required for exploring the biological activities of these new pyranocoumarin derivatives.

Full text of DOI:10.1039/c9ob01695c

Organic &
Biomolecular Chemistry
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Synthesis of pyrano[2,3-f]chromen-2-ones
vs. pyrano[3,2-g]chromen-2-ones through
site controlled gold-catalyzed annulations†
Cite this: Org. Biomol. Chem., 2019,
17, 10065
b
Antonio Arcadi, a Alessia Ciogli, b Giancarlo Fabrizi, b Andrea Fochetti,
Roberta Franzini, b Francesca Ghirga, c Antonella Goggiamani *b and
b
Antonia Iazzetti
*
Regioselective access to 10-substituted-2H,8H-pyrano[2,3-f]chromen-2-ones through the gold-cata-
lyzed intramolecular hydroarylation of readily available 7-(prop-2-yn-1-yloxy)-2H-chromen-2-one
derivatives at their C-8 congested position was investigated by tuning the electronic and steric properties
of the ligand on the gold complex. On the other hand, the combination of the JohnPhosAu(MeCN)SbF6
catalyzed intramolecular hydroarylation of 8-iodo-7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivatives
followed by selective palladium/formate C–I reduction allows for the exclusive formation of 2H,8H-
pyrano[3,2-g]chromen-2-one regioisomers. The development of these two protocols provides versatile
synthetic tools required for exploring the biological activities of these new pyranocoumarin derivatives.
Received 31st July 2019,
Accepted 8th November 2019
DOI: 10.1039/c9ob01695c
rsc.li/obc
Umbelliferone (7-hyroxycoumarine), a phenolic metabolite regio- and stereoselectivity depending on the reaction partners,
found in many familiar plants, and its derivatives exhibit the catalyst, and the reaction conditions.20–22 Gold-catalyzed
various
pharmacological
properties
including
anti- intramolecular hydroarylation (IMHA) of alkynes provides
inflammatory,1,2 antioxidant,3 anti-metastatic,4 proapoptotic,5 a powerful tool for the construction of polyheterocycles.23–29
and neuroprotective activities.6 Umbelliferone glycosides show As part of our ongoing interest in the development of new
antidiabetic and antihyperlipidemic properties7 and have been approaches for the synthesis of heterocycles employing
found to prevent serum deprivation-induced PC12 cell efficient and atom-economical routes, we previously investi-
damage.8 Furthermore, 7-substituted umbelliferone derivatives gated the IMHA of reactions of 3-[(3-arylprop-2-ynyl)oxy]
resulted in androgen receptor antagonists for the potential benzene derivatives and showed that gold catalysis exhibits a
treatment of prostate and breast cancer9 and showed antifun- unique blend of reactivity and selectivity making it the only
gal and antibacterial activities.10 Structure–activity relationship choice for the IMHA of substrates bearing electron-deficient
(SAR) studies drove forward suitable umbelliberone modifi- arenes (Scheme 1a).30 Subsequently, we envisaged to investi-
cations for the identification of biologically active gate the intramolecular gold-catalyzed hydroarylation of the
compounds.11–13 Also, researchers are extensively exploring the readily
synthesis of various umbelliferone derivatives as a source of
potential candidates in drug development.14–19 The insertion
of a C–C triple bond directly into a C–H bond of aromatic com-
pounds represents an attractive alternative to the traditionally
adopted Heck and cross coupling reactions, and can be per-
formed in an inter- or intramolecular fashion with variable
available
7-(prop-2-yn-1-yloxy)-2H-chromen-2-one
aDipartimento di Scienze Fisiche e Chimiche Università degli Studi di L’Aquila,
Via Vetoio, 67100 Coppito (AQ), Italy
bDipartimento di Chimica e Tecnologie del Farmaco, Sapienza, Università di Roma,
P.le A. Moro 5, 00185 Rome, Italy. E-mail: antonia.iazzetti@uniroma1.it
cCenter for Life Nano Science@Sapienza, Istituto Italiano di Tecnologia,
Viale Regina Elena 291, 00161 Rome, Italy
†Electronic supplementary information (ESI) available: Experimental details,
characterization data of all compounds, and copies of 1H and 13C NMR spectra
(PDF). See DOI: 10.1039/c9ob01695c
Scheme 1 (a) IMHA of 3-[(3-arylprop-2-ynyl)oxy]benzene derivatives;
(b) work hypothesis.
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derivatives 1 to selectively afford the corresponding 10-substi- feasibility of the reaction. On the basis of our previous work,30
tuted-2H,8H-pyrano[2,3-f]chromen-2-ones 2 or the 6-substi- we carried out some preliminary experiments using the com-
tuted-2H,8H-pyrano[3,2-g]chromen-2-ones
Pyranocoumarin derivatives show variety of biological evidence of reaction products was obtained by performing
activities31,32 and this aspect may justify our efforts to develop the reaction in DMSO and in DMF at 60 °C (Table 1 entries 1
3
(Scheme 1b). mercially available gold catalyst JohnPhosAu(CH3CN)SbF6. No
a
a new and efficient protocol for their preparation.
and 2), whereas the use of CH2Cl2 as a solvent gave, in about
quantitative yield, a mixture of regioisomers 2a and 3a in a
relative product ratio 2a/3a of 60/40 (Table 1, entry 4). This
result is similar to a previous report in which the Ph3PAuNTf2
catalyzed IMHA of 7-((2-methylbut-3-yn-2-yl)oxy)-2H-chromen-
2-one led to the formation of a 60 : 40 mixture of the corres-
ponding pyranocoumarins seselin and xanthyletin.33
Switching to the less polar solvents such as CHCl3 and 1,4-
Hereafter we report the results of our investigation.
Results and discussion
We started our study using the reaction of 7-((3-phenylprop-2-
ynil)oxy)-2H-chromen-2-one 1a as a probe for evaluating the
Table 1 Screening optimal conditions for the IMHA of 7-((3-phenylprop-2-ynil)oxy)-2H-chromen-2-one 1aa
Entry
Catalyst (mol %)
Solvent
T (°C)
Time (h)
Yieldb (%)
Ratioc 2a/3a
d
1
2
3
4
5
6
7
8
JohnPhosAu(MeCN)SbF6
JohnPhosAu(MeCN)SbF6
JohnPhosAu(MeCN)SbF6
JohnPhosAu(MeCN)SbF6
JohnPhosAu(MeCN)SbF6
JohnPhosAu(MeCN)SbF6
CyJohnPhosAuCl/AgSbF6
CyJohnPhosAuCl/AgSbF6
XPhosAuCl/AgSbF6
DMSO
DMF
60
60
r.t.
60
60
60
r.t
60
r.t.
r.t.
r.t.
r.t.
24
24
8
2
4
—
—
—
—
d
—
CH2Cl2
CH2Cl2
CHCl3
1,4-Dioxane
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
-
99
99
99
60/40
62/38
76/24
—
72/28
67/33
70/30
70/30
93/7
3
d
3.0
2.0
2.0
3.0
3.0
1
—
99
99
99
99
98
9
10
11
12
SPhosAuCl/AgSbF6
RuPhosAuCl/AgSbF6
13
CH2Cl2
r.t.
24
—
—
d
14
15
16
17
18
19
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
r.t.
r.t.
r.t.
60
24
14
24
1.5
6
—
—
96
32
87
96
93
95/5
90/10
90/10
95/5
92/8
r.t.
r.t
24
20
CH2Cl2
r.t
1.5
96
90/10
21
22
CH2Cl2
CH2Cl2
r.t.
r.t.
24
24
57e
91/9
d
—
—
d
23
24
25
26
AgSbF6
CH2Cl2
CH2Cl2
MeCN
60
60
80
120
4
4
24
48
—
—
—
—
—
d
AgSbF6/PPh3
PdCl2(MeCN)2
NaAuCl4 H2O
—
d
—
d
BMIMBF4
—
a Unless otherwise stated, reactions were carried out on a 0.3 mmol scale using 0.02 equiv. of catalyst in 2.0 mL of solvent. b Overall yield values
are related to a mixture of the two isomers obtained after filtration on a SiO2 pad. c Isomeric ratios were calculated from the 1H NMR analyses.
d Starting alkyne recovered in almost quantitative yield. e The starting alkyne was recovered in 43% yield.
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Table 2 Au(I)-Catalyzed intramolecular hydroarylation of 7-(prop-2-yn-1-yloxy)-2H-chromen-2-one derivativesa
Entry
1
Starting 1
Catalyst
T (°C)
Time (h)
3
Yieldb (%)
99
Ratioc 2/3
A
60
60/40
2
3
1a
B
A
r.t.
60
3
1
96
99
95/5
15/85
4
5
1b
1c
B
A
r.t.
60
5
1
99
99
77/23
80/20
6
7
B
A
r.t.
60
8
24
90
99
92/8
43/57
8
9
1d
1e
B
A
r.t.
60
2
48
93
86/14
85/15
26d
10
11
B
A
r.t.
60
3
3
99
92
84/16
85/15
12a
12be
13
1f
1f
B
B
A
r.t.
r.t.
60
5
12
1
94
70
99
85/15
75/25
80/20
14
15
1g
B
A
r.t.
60
5
24
95
99
88/12
77/23
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Table 2 (Contd.)
Entry
Starting 1
Catalyst
T (°C)
Time (h)
Yieldb (%)
Ratioc 2/3
16
17
1h
B
A
r.t
60
2
48
93
94/6
65/35
67f
18
19
1i
1j
B
A
r.t.
60
3
48
99
92
90/10
85/15
20
21
B
A
r.t.
60
24
3
15g
97
85/15
84/16
22
23
1k
1l
B
A
r.t.
60
3
1
99
99
84/16
63/37
24
B
r.t.
24
65h
65/35
a Unless otherwise stated, reactions were carried out on a 0.3 mmol scale using 0.02 equiv. of a catalyst in 2.0 mL of CH2Cl2. b Overall yield values
are related to a mixture of the two isomers obtained after filtration on a SiO2 pad. c Isomeric ratios were calculated from the 1H NMR analyses.
d The starting alkyne was recovered in 73% yield. e Reaction carried out in 2 mL of CHCl3. f The starting alkyne was recovered in 30% yield. g The
starting alkyne was recovered in 90% yield. h The starting alkyne was recovered in 10% yield.
dioxane a slight increase in the ratio 2a/3a was observed tely failed both with tris-(pentafluorophenyl)phosphine gold(I)
(Table 1, entries 5 and 6).
In order to enhance the regioselective control, we decided phosphine gold chloride/AgSbF6 (Table 1, entry 14). By con-
to explore various cationic Au(I) catalysts.25,34
trast, the formation of 10-phenyl-2H,8H-pyrano[2,3-f]-2-one 2a
chloride/AgSbF6 (Table 1, entry 13) and with tri(furan-2-yl)
A variety of Au(I)complexes have been screened generating was observed in high yield and with good regioselectivity with
in situ the active catalysts by the combined use of LAuCl (L = the gold(I) catalyst bearing tris-(4-chlorophenyl)-phosphine, tri-
dialkylbiaryl phosphine ligands or alkyldiaryl phosphine phenylphosphine, and methyldiphenylphosphine (Table 1,
ligands or triarylphosphine ligands) and AgSbF6. In the pres- entries 15–20). Particularly, the Ph3PAuCl/AgSbF6 catalytic
ence of different dialkylbiaryl phosphine Au(I) complexes, system revealed remarkable applicability for the regioselective
almost similar results were reported (Table 1, entries 7–11). formation of complex coumarins useful as neuroimaging
Switching to the electron-deficient tris(2,4-di-tert-butylphenyl) agents.36 Moreover, triphenylphosphine-gold(I) catalysis pro-
phosphite ligand led to selective cyclization at the C-8 position vided better results in terms of yield and selectivity compared
of the starting umbelliferone derivative (Table 1, entry 12) to the platinum-catalyzed IMHA of functionalised propargyl
giving 98% yield. This result suggested that the electron- ethers.37
deficient ligand might favor IMHA in accordance with a
Further optimization studies revealed that worse results are
Friedel–Crafts type reactivity by enhancing the electrophilic obtained by replacing AgSbF6 with AgNTf2, as the activator of
nature of the gold center.35 Surprisingly, the reaction comple- the gold precatalyst (Table 1, entry 21). Control experiments
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with either the gold catalyst alone or the silver salt showed no according to its natural reactivity being more nucleophilic at
conversions (Table 1, entries 22–24).
C8, as for example observed in the regioselective C8 umbelli-
IMHA failed in the presence of NaAuCl4 2H2O using [bmin] ferone iodination.47 Furthermore, this view is supported by the
BF4 as the reaction medium (Table 1, entries 26).38 Also the 13C NMR spectra of compounds 1a–l in which the signal of C8
activity of other metal catalysts was evaluated. The reaction appears more shielded than the C6 signal.48
conducted in the presence of a Pd(II) catalyst did not provide
any product, and the starting materials were quantitatively to force the formation of regioisomer 3, we prepared 8-iodo-
recovered after 4 h in MeCN at 80 °C (Table 1, entry 16).39,40
2H-chromen-2-one derivatives 4a–e. Their IMHA with catalyst A
To examine the limits of this hydroarylative cyclization and
We next compared the effect of JohnPhosAu(MeCN)SbF6 (A) under the optimized conditions led to the corresponding
and Ph3PAuCl/AgSbF6 (B) catalytic systems on the control of 10-iodo-2H,8H-pyrano[3,2-g] chromen-2-one derivatives 5a–e in
the regioselective outcome of the IMHA reaction of a variety of high yields, (Table 3). To evaluate the possible use of the
7-((3-arylprop-2-yn-1-yl)oxy)-2H-chromen-2-ones 1 in terms of cheaper PPh3 ligand, we compared the efficiency of both cata-
aryl ring substitution (Table 2). In agreement with the gold(I)- lysts A and B in the annulation reaction of 4a. The formation
catalyzed IMHA of [(3-arylprop-2-ynyl)oxy]-benzene deriva-
tives,32 reactions were carried out under air, and rigorous
exclusion of moisture was unnecessary. It is worth noting that
Table 3 IMHA of 8-iodo-7-(prop-2-yn-1-yloxy)-2H-chromen-2-one
derivativesa
only products derived from the 6-endo cyclization of substrate
1 were observed: in fact, no benzofuran by-products41 and
dimeric derivatives42 were detected during the cycloisomeriza-
tion reactions. By using catalyst A (2 mol%), the cyclization of
derivatives 1a–l was quite general and proceeded smoothly at
60 °C in excellent yields, while the presence of ortho-methyl
group was detrimental to furnishing the IMHA products in
25% overall yield (Table 2, entry 9). Only substrate 1b bearing
an Ar substituted with the –OMe group at the para position
showed the prevalent formation of 6-substituted-2H,8H-pyrano
[3,2-g]chromen-2-ones 3b (Table 2, entry 3). Conversely, regio-
isomer 2b was isolated with good regioselectivity in the pres-
ence of catalytic system B (Table 2, entry 4). It was previously
shown that ion pairing to a small noncoordinating counter ion
Entry
1
Starting 4
Time (h)
1
Yieldb (%)
90
2
3
4a
6
3.5
70c
98
−
such as SbF6 can influence the regioselective outcome of the
gold-catalyzed IMHA reactions.43 Considering that Au(I) com-
plexes were reported to form tight ion pairs with their counter-
ions in solvents with ε ≤ 5,44,45 we carried out the catalytic
IMHA with catalyst B of 1c also in CHCl3 with surprisingly dis-
appointing results (Table 1, entry 5 and Table 2, entry 12b).
For the purpose of exploring the scope of the gold–silver
system B, we employed milder room temperature conditions
with a 2 mol% loading. The catalytic IMHA of substrates 1a–k
showed excellent tolerance for both the functional groups and
steric hindrance. The cyclization of substrate 1 proceeded gen-
erally with good regioselectivity affording the more sterically
congested product 2.46 Better selectivity was observed in the
presence of electron-withdrawing substituents in the aryl
moiety, although a low yield occurred with the p-CN substitu-
ent (Table 2, entry 20).
4
5
3
5
99
96
The regioselective outcome of substrates containing ortho-
substituted aryl rings, as well as of the unsubstituted pro-
pargylic aryl ether 1l, was comparable to both the catalytic
systems A and B (Table 2, entries 9–10 and 23–24). The for-
mation of isomer 2 increases with the increased electron-with-
drawing effect of the substituents on the aryl moiety (Table 2
entries 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23). By using the
less hindered PPh3 ligand, isomer 2 formation significantly
prevails on isomer 3 in all the cases. These data seem to indi-
cate that under these conditions, the coumarin ring reacts
6
6
98
a Unless otherwise stated, reactions were carried out on a 0.3 mmol
scale using 0.02 equiv. of JohnPhosAu(MeCN)SbF6 in 2.0 mL of CH2Cl2
at 60 °C. b Yields refer to the isolated products. c Reaction was carried
out using Ph3PAuCl/AgSbF6 as a catalyst (catalyst B); the starting 4a
was recovered in 28% yield.
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amine (7 mL) and N,N-dimethylformamide (5 mL). The resul-
tant solution was stirred under nitrogen at room temperature
for 10 minutes before adding iodobenzene (714 mg, 3.5 mmol,
1.0 equiv.) in diisopropylamine (3 mL) and 7-(prop-2-ynyloxy)-
2H-chromen-2-one (841 mg, 4.2 mmol, 1.2 equiv.) and stirred
for 3 hours at room temperature. After this, the reaction
mixture was diluted with Et2O and washed with a saturated
NH4Cl solution and with brine. The organic layer was dried
over Na2SO4, filtered and concentrated under reduced
pressure. The residue was purified by chromatography on SiO2
(25–40 μm), eluting with an 80/20 (v/v) n-hexane/AcOEt mixture
(Rf = 0.24) to obtain 870 mg (90% yield) of 7-(3-phenylprop-2-
ynyloxy)-2H-chromen-2-one 1a.
Scheme 2 Pd-Catalyzed reduction of the C–I bond of 5a and 5e.
1a 50: white solid; mp: 140–142 °C; IR (KBr): 2963, 2219,
1711, 1615, 1506, 1488, 1209 cm−1 1H NMR (400.13 MHz)
;
(CDCl3): δ = 7.67 (d, J = 9.5 Hz, 1 H), 7.47–7.28 (m, 6 H), 7.03 (d,
J = 2.4 Hz, 1 H), 6.98 (dd, J1 = 8.4 Hz, J2 = 2.4 Hz, 1 H), 6.29 (d,
J = 9.5 Hz, 1 H), 5.00 (s, 2 H); 13C NMR (100.6 MHz) (CDCl3): δ =
161.1, 160.9, 155.7, 143.3, 131.9, 129.0, 128.8, 128.4, 121.8,
113.5, 113.2, 113.1, 102.2, 88.2, 82.6, 57.1; MS (EI ion source):
m/z (%) = 276 (100, [M+]), 247 (32), 199(15), 63.00 (43).
of 5a occurred in a lower yield when catalyst B was used instead
of A at 60 °C, while no interconversion of 4a was observed with
this latter catalyst at r.t. (Table 3, entries 1 and 2).
The following palladium-catalyzed selective reduction of
the C–I bond by ammonium formate, carried out on 5a and 5e
derivatives, led to the desired 2H,8H-pyrano[3,2-g] chromen-2-
one derivatives in good yields (Scheme 2).49
Typical procedure for the preparation of 2a/3a
A flask equipped with a magnetic stirring bar was charged
with 7-(3-phenylprop-2-ynyloxy)-2H-chromen-2-one (82.9 mg,
0.3 mmol, 1 equiv.) and CH2Cl2 (2 mL) before adding catalyst
A or catalyst B (catalyst A: JohnPhosAu(MeCN)SbF6 4.6 mg,
0.006 mmol, 0.02 equiv.; catalyst B: Ph3PAuCl 3.0 mg,
0.006 mmol, 0.02 equiv. and AgSbF6 2.1 mg, 0.006 mmol,
0.02 equiv.). The resulting mixture was stirred for 3 hours,
then it was concentrated under reduced pressure and the
residue was filtered on a pad of SiO2 to afford 81.8 mg of 2a +
3a (99% overall yield) using catalyst A or 79.5 mg of 2a + 3a
(96% overall yield) using catalyst B. The 2a/3a ratio was calcu-
Conclusions
In conclusion, we have developed a regioselective intra-
molecular hydroarylation of 7-(prop-2-yn-1-yloxy)-2H-chromen-
2-one derivatives to afford 10-substituted-2H,8H-pyrano[2,3-f]
chromen-2-ones by means of Au(I) catalysis. The regioselective
outcome of the cyclization is established through fine-tuning
the electronic and steric effects of the ligands of gold com-
plexes. The Ph3PAuCl/AgSbF6 catalytic system accomplished
the formation of the 10-substituted-2H,8H-pyrano[2,3-f]
chromen-2-ones in general in good to excellent yields under
mild conditions with compatibility with many functional
groups, although the procedure was found to be sensitive to
some extent to the electronic and steric effects of the arene
substituents. The exclusive formation of the 2H,8H-pyrano[3,2-
g]chromen-2-one regioisomers was allowed by combining the
JohnPhosAu(MeCN)SbF6 catalyzed intramolecular hydroaryla-
tion of 8-iodo-7-(prop-2-yn-1-yloxy)-2H-chromen-2-one deriva-
tives with selective C–I reduction by the palladium/formate
system.
1
lated from the H NMR analyses. Afterwards, the two isomers
were separated by semi-preparative HPLC to obtain suitable
NMR spectra of each compound.
Overall yield (catalyst A): 99% (81.8 mg); 2a/3a = 60/40
Overall yield (catalyst B): 96% (79.5 mg); 2a/3a = 95/5
2a: white solid; mp: 170–172 °C; IR (KBr): 3060, 2922, 1702,
1593, 1480, 1402, 1343, 1232 cm−1 1H NMR (400.13 MHz)
;
(CDCl3): δ = 7.59 (d, J = 9.5 Hz, 1 H), 7.40–7.39 (m, 3 H), 7.34
(d, J = 8.4 Hz, 1 H), 7.28–7.26 (m, 2 H), 6.96 (d, J = 8.4 Hz, 1 H),
6.15 (d, J = 9.5 Hz, 1 H), 5.96 (t, J = 4.5 Hz, 1 H), 4.81 (d, J =
4.5 Hz, 2 H); 13C NMR (100.6 MHz) (CDCl3): δ = 159.6, 159.2,
150.9, 143.6, 139.2, 135.1, 128.6, 128.1, 127.7, 127.2, 121.9,
113.9, 113.5, 113.2, 112.3, 65.3; MS (EI ion source): m/z (%) =
276 (21 [M+]), 207 (16), 135 (11), 73 (100); HRMS: m/z [M + H]+
calcd for C13H10N: 180.0808; found: 180.0808.
3a: solid; mp: 163–164 °C; IR (KBr): 3076, 2818, 1734, 1620,
1487, 1401 cm−1; 1H NMR (400.13 MHz) (CDCl3): δ = 7.51–7.43
(m, 4 H), 7.36–7.34 (m, 2 H), 7.06 (s, 1 H), 6.83 (s, 1 H), 6.22 (d,
J = 9.5 Hz, 1 H), 5.83 (t, J = 3.8 Hz, 1 H), 5.01 (d, J = 3.8 Hz,
Experimental
A list of chemicals and instrumentation is provided in the
ESI.†
Typical procedure for the preparation of 7-(3-phenylprop-
2-ynyloxy)-2H-chromen-2-one 1a
A flask equipped with a magnetic stirring bar was charged 2 H); 13C NMR (100.6 MHz) (CDCl3): δ = 161.0, 158.3, 155.3,
with PdCl2(PPh3)2 (49 mg, 0.07 mmol, 0.02 equiv.) and CuI 143.5, 137.6, 135.6, 128.7, 128.5, 128.3, 124.6, 120.7, 120.3,
(26.5 mg, 0.14 mmol, 0.04 equiv.) dissolved in diisopropyl- 113.4, 113.0, 104.4, 66.1; MS (EI ion source): m/z (%) = 276
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L. Argyri, E. Pontiki, A. Chroni and D. Hadjipavlou-Litina,
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18 A. Nagarsenkar, L. Guntuku, S. K. Prajapti, S. D. Guggilapu,
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge the University “La Sapienza”,
Rome, and the University of L’Aquila for financial support.
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Organic & Biomolecular Chemistry
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10072 | Org. Biomol. Chem., 2019, 17, 10065–10072
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