A metal-free oxidation of benzo[c]chromen to benzo[c]chromen-6-ones by t-butyl hydroperoxide in the presence of potassium iodide

Article abstract of DOI:10.3987/COM-13-12760

An effective and eco-friendly oxidation method for the synthesis of benzo[c]chromen-6-ones is described. The condition constitutes 6 molar ratio TBHP, catalytic KI and pyridine in acetonitrile as the solvent. Altogether, 16 structurally diverse substituted benzo[c]chromen-6-ones were prepared through this protocol from corresponding benzo[c]chromenes in excellent yields. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-13-12760

HETEROCYCLES, Vol. 87, No. 9, 2013
1889
HETEROCYCLES, Vol. 87, No. 9, 2013, pp. 1889 - 1896. © The Japan Institute of Heterocyclic Chemistry
Received, 24th June, 2013, Accepted, 29th July, 2013, Published online, 7th August, 2013
DOI: 10.3987/COM-13-12760
A
METAL-FREE OXIDATION OF BENZO[C]CHROMEN TO
BENZO[C]CHROMEN-6-ONES BY t-BUTYL HYDROPEROXIDE IN THE
PRESENCE OF POTASSIUM IODIDE
Jing Zhou,1 Pan Han,1 Ying-Meng Xu,1 Tao Zhang,1 and Zhen-Ting Du1,2*
1College of Science, Northwest A&F University, Yangling 712100, Shaanxi
Province, P. R. China
2Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 20032, P. R.
China
Abstract – An effective and eco-friendly oxidation method for the synthesis of
benzo[c]chromen-6-ones is described. The condition constitutes 6 molar ratio
TBHP, catalytic KI and pyridine in acetonitrile as the solvent. Altogether, 16
structurally diverse substituted benzo[c]chromen-6-ones were prepared through
this protocol from corresponding benzo[c]chromenes in excellent yields.
Selective oxidation of C–H bonds to give oxygen-containing compounds is an extremely important
reaction in synthetic chemistry. In this field, benzylic oxidation is one of the most valuable
transformations in not only manufacture of pharmaceuticals and agrochemicals, but also in synthesis of
natural products.1 However, the traditional oxidation methods based heavy mental have provoked
environmental issue in recent years.2 Therefore the use of eco-friendly oxidants to replace metals is
highly desirable in terms of sustainable chemical production.3
Benzo[c]chromen-6-one is a privileged scaffold in a number of pharmacologically relevant natural
products such as graphislactones,4 autumnariol,5 autumnariniol,6 alternariol7 and altenuisol.8 It has also
been demonstrated that benzo[c]chromen-6-one can be used as synthetic intermediates in the synthesis of
several pharmaceutically interesting compounds including progesterone, glucocorticoids modulators,9 and
endothelial cell proliferation inhibitors.10 Furthermore, benzo[c]chromen-6-ones occur naturally in many
food sources including citrus fruits, herbs and vegetables.11
The most popular synthesis route for benzo[c]chromen-6-one is Suzuki-Miyaura cross-coupling followed
by intramolecular cyclization process (Figure 1, eq 1). Indeed, a plethora of papers have been published
on modifications of this protocol.12 Despite these advancements, this methodology suffers from some
drawbacks from a practical point of view, such as expensive organoborane compounds and catalyst. Other

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HETEROCYCLES, Vol. 87, No. 9, 2013
synthetic approaches involving Diels-Alder cycloaddition to construct benzene ring,13 Pd-catalyzed
decarboxylative cross-coupling and lactonization14 can also be found in the literature. More recently, an
H2O2 oxidation of benzo[c]chromenes under microwave irradiation was reported.15 However, the
practicality of these methodologies are confined, owing to toxic residues, more steps, additional apparatus,
low practicability and poor yields to some extent.
Main synthetic routes
Cross coupling/cyclization
cross
coupling
B(OH)2
OMe
MeOOC
cyclization
+
(1)
X
O
O
This work:
Eco-f riendly oxidation
TBHP
(2)
selective oxidation
O
O
O
Figure 1
Our research group is searching for new heterocyclic structures to serve as potential bioactive compounds
in agriculture.16 Recently, we have reported the efficient and convenient palladium-catalyzed formation of
6H-benzo[c]chromen from inexpensive diazonium salts.17 We envisaged an eco-friendly oxidation of
benzo[c]chromen without any heavy metal in order to obtain benzo[c]chromenes. Herein, we would like
to report an efficient synthesis of benzo[c]chromen-6-one from benzo[c]chromen by homogeneous TBHP
oxidation with acetonitrile as the solvent in excellent yields.
Initially, the oxidation of simple 6H-benzo[c]chromene was set as a model reaction and we were strongly
interested in a general oxidizer such as potassium persulfate. Regarding potassium persulfate, the
oxidation reaction did not occur, and those conditions proved to be inappropriate in a non-polar solvent
(Table 1, entries 1, 2). Then, sodium bromide was added as a co-oxidizer, but the oxidation reaction did
not occur in nitromethane, either (Table 1, entry 3). Our studies subsequently showed that the nature of
the reaction solvent was very important for this reaction. When acetonitrile was chosen as the solvent,
benzo[c]chromen-6-ones together with an unknown byproduct could be obtained in 55% yield. Then, the
oxidizer t-butyl hydroperoxide (TBHP) (in aqueous solution) was tested in acetonitrile without any
additive. The transformation was sluggish, and a 50% yield of the product as obtained at 80 C after 48 h
(Table 1, entry 5). Then, a simple 10% molar ratio of KI (48 h) or a combination of a 10% molar ratio of
KI and a 10% molar ratio N-methylmophine were used as additives; 60% yields (48 h) were obtained, but
the reaction is still sluggish (Table 1, entries 5-7). After the combination of a 10% molar ratio of KI and a
10% molar ratio pyridine as the additive was employed, the reaction was markedly more effective (Table

HETEROCYCLES, Vol. 87, No. 9, 2013
1891
1, entry 8). Then, the use of TBHP was investigated for the model reaction. When 3 equiv. of TBHP was
used, 90% yield was obtained and when 6 equiv. TBHP was used, 95% yield was obtained (Table 1,
entries 8-9). Indeed, a large excess of TBHP was found to be needless because of the inconvenience in the
work-up (Table 1, entry 10). So, the optimum conditions included 3 equiv. 70% aqueous TBHP as the
oxidizer, acetonitrile as the solvent, a 10% molar ratio of KI21 and a 10% molar ratio pyridine as additives
and the temperature as 80 C.
Table 1. Screening conditions of oxidation
O
O
O
oxidation
2a
1a
entry
1
2
oxidizer
K2S2O8 (3 eq)
K2S2O8 (3 eq)
solvent
CH2Cl2
CH2Cl2/H2O(1:1)
MeNO2/H2O(10:1)
MeCN/H2O(10:1)
MeCN
additive
temperature
yielda
-
-
40 oC
40 oC
80 oC
40 oC
80 oC
0
0
3
4
5
K2S2O8 (3 eq)
K2S2O8 (3 eq)
TBHP (3 eq)
NaBr(0.5 eq)
NaBr(0.5 eq)
-
0
55%b
50%c
6
7
TBHP (3 eq)
TBHP (3 eq)
MeCN
MeCN
KI (10%)
80 oC
60%c
KI(10%)/
NMO (10%)
KI (10%)
Pyr (10%)
KI (10%),
Pyr (10%)
KI (10%),
Pyr (10%)
80 oC
60%c
8
9
TBHP (3 eq)
TBHP (6 eq)
TBHP (8 eq)
MeCN
MeCN
MeCN
80 oC
80 oC
80 oC
90%
95%
95%
10
With the optimized reaction conditions, we next examined the scope of TBHP selective oxidation for the
synthesis of substituted benzo[c]chromen-6-ones. The results are summarized in Table 2. A wide range of
structurally diverse benzo[c]chromens (Table 2), including chloro (Table 2, entries 9-12), bromo (Table 2,
entries 13-16), fluoro (Table 2, entry 8), and ester (entry 3) groups were subjected to this protocol to
provide the corresponding benzo[c]chromen-6-ones in excellent yields. As for the variations of R2,
electron-neutral (Table 2, entries 3-6) and electron-deficient (Table 2, entry 7) groups were good
substrates for this reaction. It was found that R3 did not show a great influence (Table 2, entries 9-16) to
this protocol. The bromo moieties could be easily functionalized to boric acid substituents, which make it
possible to obtain more complicated substituted benzo[c]chromen-6-ones by the subsequent coupling
reaction.

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Table 2. TBHP Oxidation of benzo[c]chromens to benzo[c]chromen-6-ones
R1
R1
O
O
O
6 equiv. TBHP
0.1 equiv. KI
R4
R4
R2
R2
1 equiv. Py
80 oC
2
1
R3
R3
Entry
Compound 1
Product 2
2a
Yield (%)a
95
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
R1=R2=R3=R4=H
R1=R2=R4=H, R3=Me
R1=R3=R4=H, R2=Me,
R1=R3=H, R2=t-Bu
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
88
86
95
92
86
98
96
96
95
90
92
86
95
95
92
R1=R4=H, R2=t-Bu, R3=Me
R1=R4=H, R2=R3=Me
R1=R3=R4=H, R2=COOEt
R1=R2=R4=H, R3=F
R1=R2=R4=H, R3=Cl
R1=R2=R3=H, R4=Cl
R1=R4=H, R2=Me, R3=Cl
R1=R4=H, R2=t-Bu, R3=Cl
R1=R4=H, R2=Me, R3=Br
R1=R2=R4=H, R3=Br
R1=R4=H, R2=t-Bu, R3=Br
R1=R3=R4=H, R2=Br
2p
In conclusion, an efficient and convenient oxidation method was developed for the formation of
benzo[c]chromen-6-ones from benzo[c]chromens in the presence of 70% aqueous TBHP, a catalytic
amount of KI and pyridine. This method is bestowed with several unique merits, such as high conversions
and yields, simplicity in operation, cost-effectiveness and functional group tolerance. Thus, we believe
that this novel methodology will be a practical alternative to the existing procedures and cater to the need
of academia as well as industry.
EXPERIMENTAL
To a solution of the 6H-benzo[c]chromene (0.5 mmol) in MeCN (3 mL), tert-butylhydroperoxide (TBHP)
(70% aqueous solution, 6 eq.), KI (0.1 eq.) and pyridine (0.1 eq.) were added in turn at room temperature.
The solution was stirred at 80 °C for 48 h. The reaction mixture was concentrated under reduced pressure.
The mixture was dissolved in EtOAc and then washed by saturated aqueous CuSO4 solution and saturated
aqueous Na2SO3 solution. The organic phase was concentrated under reduced pressure. The product was
1
isolated by column chromatography on silica gel (Table 2). The H-NMR and 13C-NMR data were
recorded in deuterated chloroform solution with Bruker-NMR spectrometers (DRX 500, AM 400) if not
noted otherwise.

HETEROCYCLES, Vol. 87, No. 9, 2013
1893
6H-Benzo[c]chromen-6-one (2a)
Mp 91-92 C (lit.,12h 91-92 C). 1H NMR (500 MHz, CDCl3): δ 8.39 (d, 1H, J = 7.5 Hz), 8.10 (d, 1H, J =
8.0 Hz), 8.04 (d, 1H, J = 8.0 Hz), 7.82 (t, 1H, J = 7.5 Hz), 7.57 (t, 1H, J = 8.0 Hz), 7.47 (t, 1H, J = 7.5
Hz), 7.36-7.31 (m, 2H) ppm. 13C NMR (125 MHz, CDCl3): δ 161.1, 151.3, 134.8, 134.7, 130.6, 130.4,
130.5, 128.9, 124.5, 122.8, 121.7, 121.3, 118.0, 117.8 ppm. IR (cm-1): 1730, 1605, 1236, 1077.
9-Methyl-6H-benzo[c]chromen-6-one (2b)
1
Mp 100-102 C (lit.,18 102.5-103 C). H NMR (500 MHz, CDCl3): δ 8.26 (d, 1H, J = 8.0 Hz), 8.02 (d,
1H, J = 8.0 Hz), 7.87 (s, 1H), 7.46 (d, 1H, J = 7.5 Hz), 7.38-7.30 (m, 3H), 2.55 (s, 3H) ppm. 13C NMR
(125 MHz, CDCl3): δ 161.2, 151.5, 145.9, 134.7, 130.5, 130.3, 130.2, 124.4, 122.7, 121.8, 118.8,
118.1,117.7, 22.3 ppm. IR (cm-1): 1723, 1615, 1302, 1268, 1101.
2-Methyl-6H-benzo[c]chromen-6-one (2c)
Mp 126-128 C (lit.,19 127-129 C). 1H NMR (400 MHz, CDCl3): δ 8.22 (d, 1H, J = 8.1 Hz), 7.98 (d, 1H,
J = 7.6 Hz), 7.82 (s, 1H), 7.44 (t, 1H, J = 7.3 Hz), 7.35-7.27 (m, 3H), 2.53 (s, 3H) ppm. 13C NMR (100
MHz, CDCl3): δ 161.2, 151.4, 145.9, 134.6, 130.5, 130.2, 130.1, 124.4, 122.7, 121.8, 118.7, 118.0, 117.7,
22.3 ppm. IR (cm-1): 1715, 1604, 1296, 1234, 1096.
2-tert-Butyl-6H-benzo[c]chromen-6-one (2d)
1
Mp 100-102 C (lit.,20 106.9-107.4 C). H NMR (400 MHz, CDCl3): δ 8.40 (dd, 1H, J = 0.8, 8.0 Hz),
8.17 (d, 1H, J = 8.1 Hz), 8.05 (d, 1H, J = 2.3 Hz), 7.82 (td, 1H, J = 1.2, 10.3 Hz), 7.59-7.52 (m, 2H), 7.30
(d, 1H, J = 8.7 Hz), 1.41 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ 156.7, 144.5, 142.8, 130.4, 130.0,
125.9, 123.9, 123.3, 116.8, 116.5, 114.2, 112.6, 112.4, 30.0, 26.7 ppm. IR (cm-1): 1729, 1609, 1363, 1296,
1072.
2-tert-Butyl-9-methyl-6H-benzo[c]chromene (2e)
1
Mp 117-119 C. H NMR (400 MHz, CDCl3): δ 8.27 (d, 1H, J = 8.1 Hz), 8.03 (d, 1H, J = 2.2 Hz), 7.92
(td, 1H, J = 2.1, 10.3 Hz), 7.52 (dd, 1H, J = 2.2, 8.7 Hz), 7.37 (d, 1H, J = 7.6 Hz), 7.29 (d, 1H, J = 8.7
Hz), 2.58 (s, 3H), 1.42 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ 161.6, 149.4, 147.3, 145.8, 137.9,
135.1, 130.6, 130.0, 127.9, 121.6, 118.9, 117.3, 117.2, 34.8, 31.5, 22.3 ppm. IR (cm-1): 1728, 1616, 1273,
1072.
2,9-Dimethyl-6H-benzo[c]chromen-6-one (2f)
Mp 180-182 C. 1H NMR (500 MHz, CDCl3): δ 8.25 (d, 1H, J = 8.0 Hz), 7.85 (s, 1H), 7.79 (s, 1H), 7.35
(d, 1H, J = 8.0 Hz), 7.24-7.21 (m, 2H), 2.54 (s, 3H), 2.45 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3): δ
161.4, 149.6, 145.7, 134.8, 133.9, 131.2, 130.5, 130.0, 122.7, 121.7, 118.9, 117.7, 117.4, 22.2, 21.1 ppm.
IR (cm-1): 1722, 1616, 1275, 1219, 1072, 1043.
Ethyl 6-oxo-6H-benzo[c]chromene-2-carboxylate (2g)
Mp 145-146 C. 1H NMR (400 MHz, CDCl3): δ 8.75 (d, 1H, J = 1.9 Hz), 8.39 (dd, 1H, J = 0.9, 8.0 Hz),
8.21 (d, 1H, J = 8.0 Hz), 8.13 (dd, 1H, J = 1.9, 8.6 Hz), 7.87 (td, 1H, J = 1.3, 7.9 Hz), 7.63 (td, 1H, J =

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HETEROCYCLES, Vol. 87, No. 9, 2013
0.8, 7.7 Hz), 7.39 (d, 1H, J = 8.6 Hz), 4.45 (q, 2H, J = 7.2 Hz), 1.45 (t, 3H, J = 7.2 Hz) ppm. 13C NMR
(100 MHz, CDCl3): δ 165.6, 160.5, 154.1,135.2, 134.1, 131.4, 129.5, 126.9, 125.0, 122.0, 121.1, 118.0,
117.9, 61.46, 14.4 ppm. IR (cm-1): 1755, 1720, 1612, 1317, 1263, 1108, 1093, 1070, 1033.
9-Fluoro-6H-benzo[c]chromen-6-one (2h)
Mp 157-159 C. 1H NMR (500 MHz, CDCl3): δ 8.41(d, 1H, J = 8.0 Hz), 8.03 (d, 1H, J = 8.0 Hz), 7.85 (t,
1H, J = 7.5 Hz), 7.71 (dd, 1H, J = 2.6, 9.0 Hz), 7.63 (t, 1H, J = 7.6 Hz), 7.35 (m, 1H), 7.20 (td, 1H, J =
2.6, 8.6 Hz) ppm. 13C NMR (125 MHz, CDCl3): δ 160.5 (d, 1C, J = 63.8 Hz), 158.3, 147.4, 135.0, 130.7,
129.6, 121.9, 121.3, 119.4,119.3, 117.7 (d, 1C, J = 23.8 Hz), 108.8 (d, 1C, J = 25.0 Hz) ppm. IR (cm-1):
1721, 1487, 1299, 1273, 1165, 1076.
9-Chloro-6H-benzo[c]chromen-6-one (2i)
1
Mp 182-184 C. H NMR (500 MHz, CDCl3): δ 8.33 (d, 1H, J = 8.5 Hz), 8.07 (s, 1H), 7.98 (d, 1H, J =
8.0 Hz), 7.52 (t, 2H, J = 9.0 Hz), 7.38-7.34 (m, 2H) ppm. 13C NMR (125 MHz, CDCl3): δ 160.4, 151.7,
141.9, 136.3, 132.2, 131.2, 129.3, 124.8, 122.9, 121.8, 119.6, 117.9, 117.0 ppm. IR (cm-1): 1738, 1603,
1265, 1232, 1097, 747.
7-Chloro-6H-benzo[c]chromen-6-one (2j)
Mp 187-189 C. 1H NMR (500 MHz, CDCl3): δ 8.40 (d, 1H, J = 7.8 Hz), 8.10 (d, 1H, J = 8.0 Hz), 7.96 (d,
1H, J = 8.0 Hz), 7.85 (t, 1H, J = 7.6 Hz), 7.62 (t, 1H, J = 7.6 Hz), 7.53 (d, 1H, J = 7. 8 Hz), 7.26 (t, 1H, J
= 8.2 Hz) ppm. 13C NMR (125 MHz, CDCl3): δ 160.0, 147.1, 135.1, 134.2, 130.9, 130.7, 129.5, 124.5,
122.7, 122.0, 121.2, 121.1, 119.6 ppm. IR (cm-1): 1745, 1604, 1272, 1224, 1038, 748.
9-Chloro-2-methyl-6H-benzo[c]chromen-6-one (2k)
Mp 188-190 C. 1H NMR (500 MHz, CDCl3): δ 8.28 (s, 1H), 7.98 (s, 1H), 7.69 (s, 1H), 7.49 (s, 1H), 7.27
(s, 1H), 7.21 (s, 1H), 2.44 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3): δ 160.5, 149.6, 141.7, 136.3, 134.4,
132.2, 132.1, 129.1, 122.8, 121.7, 119.6, 117.6, 21.1 ppm. IR (cm-1): 1724, 1603, 1412, 1186, 1068, 809,
774.
2-tert-Butyl-9-chloro-6H-benzo[c]chromen-6-one (2l)
Mp 149-150 C. 1H NMR (500 MHz, CDCl3): δ 8.33 (d, 1H, J = 8.5 Hz), 8.10 (s, 1H), 7.96 (s, 1H), 7.57
(dd, 1H, J = 1.0, 8.5 Hz), 7.52 (d, 1H, J = 8.5 Hz), 7.31 (d, 1H, J = 8.5 Hz), 1.42 (s, 9H) ppm. 13C NMR
(125 MHz, CDCl3): δ160.7, 149.6, 147.8, 141.7, 136.7, 132.3, 129.1, 128.9, 121.6, 119.6, 119.0, 117.5,
116.2, 34.8, 31.4 ppm. IR (cm-1): 1730, 1604, 1258, 1213, 1096, 1038, 680.
9-Bromo-2-methyl-6H-benzo[c]chromen-6-one (2m)
Mp 180-183 C. 1H NMR (500 Hz, CDCl3): δ 8.23 (d, 2H, J = 8.0 Hz), 7.55 (s, 1H), 7.67 (d, 1H, J = 8.5
Hz), 7.31 (d, 1H, J = 8.5 Hz), 7.25 (d, 1H, J = 8.5 Hz), 2.46 (s, 3H) ppm. 13C NMR (125 MHz, CDCl3): δ
160.7, 149.7, 136.4, 134.4, 132.2, 132.1, 132.0, 130.5, 124.8, 122.8, 120.0, 117.6, 116.5, 21.1 ppm. IR
(cm-1): 1725, 1599, 1294, 1214, 1086, 1066, 774, 676.
9-Bromo-6H-benzo[c]chromen-6-one (2n)

HETEROCYCLES, Vol. 87, No. 9, 2013
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Mp 181-183 C. 1H NMR (500 MHz, CDCl3): δ 8.24-8.23 (m, 2H), 7.98 (d, 1H, J = 7.8 Hz), 7.69 (dd, 1H,
J = 1.2, 8.5 Hz), 7.52 (t, 1H, J = 7.2 Hz), 7.37-7.35 (m, 2H) ppm. 13C NMR (125 MHz, CDCl3): δ 160.5,
151.6, 136.3, 132.2, 132.19, 131.2, 130.6, 124.9, 124.8, 122.9, 120.0, 118.0, 116.9 ppm. IR (cm-1): 1738,
1597, 1261, 1231, 1090, 1014, 817.
9-Bromo-2-tert-butyl-6H-benzo[c]chromen-6-one (2o)
Mp 156-157 C. 1H NMR (500 MHz, CDCl3): δ 8.35 (d, 1H, J = 9.0 Hz), 8.12 (s, 1H),7.98 (d, 1H, J = 2.0
Hz), 7.59 (dd, 1H, J = 2.0, 8.6 Hz), 7.54 (dd, 1H, J = 1.2, 8.4 Hz), 7.33 (d, 1H, J = 8.6 Hz), 1.45 (s, 9H)
ppm. 13C NMR (125 MHz, CDCl3): δ 160.6, 149.6, 147.8,141.7, 136.7, 132.3, 129.0, 128.9, 121.6, 119.6,
119.0, 117.5, 116.1, 34.8, 31.5 ppm. IR (cm-1): 1730, 1604, 1364, 1258, 1096, 1038, 821, 776, 680.
2-Bromo-6H-benzo[c]chromen-6-one (2p)
Mp 183-185 C. 1H NMR (500 MHz, CDCl3): δ 8.41(d, 1H, J = 8.0 Hz), 8.12 (d, 1H, J = 2.0 Hz), 8.06 (d,
1H, J = 8.0 Hz), 7.85 (td, 1H, J = 0.5, 8.0 Hz), 7.63 (t, 1H, J = 7.5 Hz), 7.57 (dd, 1H, J = 2.5, 8.5 Hz),
7.26 (d, 1H, J = 8.5 Hz) ppm. 13C NMR (125 MHz, CDCl3): δ 160.5, 150.2, 135.1, 133.5, 133.2, 130.8,
129.6, 125.7, 121.8, 121.3, 119.9, 119.5, 117.5 ppm. IR (cm-1): 1739, 1602, 1263.5, 1213.7, 1035, 1018,
810.
ACKNOWLEDGEMENTS
Financial support from the open funding of Key Laboratory of Synthetic Chemistry of Natural Substances
of SIOC as well as the National Natural Science Foundation of China (31270388) is greatly appreciated.
We are grateful to Special Fund for Forestry Scientific Research in Public Interest (No. 200904004) for
partial support of this work.
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protocol.
22. The first two authors contributed equally to this work.