Iron(III) Chloride/Phenylsilane-Mediated Cascade Reaction of Allyl Alcohols with Maleimides: Synthesis of Poly-Substituted γ-Butyrolactones

Article abstract of DOI:10.1002/adsc.201900843

A iron-catalyzed free radical cascade reaction of allyl alcohols with N-substituted maleimides for accessing poly-substituted γ-butyrolactones has been developed. In this protocol, various allyl alcohols can open N-substituted maleimide rings to form allyl ester intermediates, and the allyl ester intermediates can be converted into an allyl ester alkyl radicals and undergo intramolecular free radical addition cyclization to form a polysubstituted γ-butyrolactones. In this protocol, spiro γ-butyrolactone compounds can be also synthesized. Meanwhile, the strategy could be applied to further construct a fully substituted tetrahydrofuran. The reaction is not sensitive to oxygen or moisture and has been performed on gram-scale. (Figure presented.).

Full text of DOI:10.1002/adsc.201900843

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DOI: 10.1002/adsc.201900843
Iron(III) Chloride/Phenylsilane-Mediated Cascade Reaction of
Allyl Alcohols with Maleimides: Synthesis of Poly-Substituted
γ-Butyrolactones
Hua Zhang,a, b Xiao-Yu Zhan,a, b Xu-Ling Chen,a, b Lei Tang,c Shuai He,d
Zhi-Chuan Shi,d Yu Wang,e and Ji-Yu Wanga,*
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
E-mail: Jiyuwang@cioc.ac.cn
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
Laboratory of Anaesthesia & Critical Care Medicine, Translational Neuroscience Center and Department of Anaesthesiology,
b
c
West China Hospital, Sichuan University, Chengdu 610041, People’s Republic of China
Southwest Minzu University, Chengdu 610041, People’s Republic of China
School of Chemistry and Materials Sciences, Guizhou Education University, Guiyang, People’s Republic of China
d
e
Manuscript received: July 19, 2019; Revised manuscript received: August 26, 2019;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201900843
tone by free radical addition reaction of α-halocarbox-
Abstract: A iron-catalyzed free radical cascade
ylate with olefin.[4] Despite these methods, it is still an
reaction of allyl alcohols with N-substituted malei-
important challenge to find a more convenient and
mides for accessing poly-substituted γ-butyrolac-
efficient method for synthesizing fully substituted γ-
tones has been developed. In this protocol, various
butyrolactones from simple starting materials.
allyl alcohols can open N-substituted maleimide
rings to form allyl ester intermediates, and the allyl
How to construct CÀ C bonds quickly and easily has
always been the direction of organic chemists. In
ester intermediates can be converted into an allyl
recent years, the transition-metal-catalyzed radical
ester alkyl radicals and undergo intramolecular free
olefin hydrofunctionalization has been shown as an
radical addition cyclization to form a polysubsti-
effective and distinct approach for CÀ C and CÀ X
tuted γ-butyrolactones. In this protocol, spiro γ-
bonds formation.[5] Currently, iron has been widely
butyrolactone compounds can be also synthesized.
used as a catalyst in organic chemistry for quite a few
Meanwhile, the strategy could be applied to further
reasons. Iron is the most abundant metal element after
construct a fully substituted tetrahydrofuran. The
aluminum in earth’s crust. Iron catalysts are partic-
reaction is not sensitive to oxygen or moisture and
ularly attractive because of their low cost, environ-
has been performed on gram-scale.
mental friendliness and low toxicity.[6] Therefore, the
use of ferric chloride as the most abundant iron catalyst
Keywords: Ferric chloride; poly-substituted γ-butyr-
in organic chemistry has been reviewed by Martín.[7]
olactones; free radical; fully substituted tetrahydrofur-
At present, iron-catalyzed non-activated olefin reduc-
an; free radical cascade reaction
tion cross-coupling reaction has shown its extraordi-
nary significance in organic synthesis as a powerful
γ-Lactone skeletons are crucial structures in organic
chemistry, which can be found in various spices,
natural products and drug molecules (Figure 1).[1]
Therefore, scientists have established a few meth-
ods for synthesizing γ-lactones.[2] Meanwhile, the
synthesis of γ-butyrolactones by N-substituted malei-
mide has also been reported.[3] Additionally, there are
only several reports on the synthesis of fully sub-
Figure 1. Biological compounds containing fully substituted γ-
butyrolactone skeleton.
stituted γ-butyrolactones, such as Metzger and Liu
succeeded in accessing fully substituted γ-butyrolac-
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means of constructing CÀ C bond, CÀ N bond, CÀ X butyrolactone. Herein, we report a less expensive Fe
(X=F, Cl, S) bond. For example, Boger reported the (III)-catalyzed radical cascade reaction to construct
hydrofluorination of non-activated olefins and the poly-substituted γ-butyrolactones via ring-opening/rad-
synthesis of vincristine analogs.[8] Baran developed the ical addition.
free radical reduction cross-coupling of non-activated
We commenced our investiongations by examining
olefins using various electron-deficient olefins as the cascade reaction between N-phenylmaleimide (1a)
acceptors, which can easily and quickly prepare and 2-methylallyl alcohol (2a) under various reaction
various natural products that are difficult to synthesize, conditions (Table 1). Initially we carried out the
and he described the reaction mechanism in detail.[9] reaction using Baran’s conditions[9] at 75 C. Gratify-
°
The Cui team continued to study the functionalization ingly, a γ-butyrolactone product 3a was observed and
of non-activated olefins.[10] Our group reported the isolated in 68% yield (entry 1). When we carried out
reduction cross-coupling reaction using chromone as the reaction using copper sulfate as a catalyst, the ethyl
the acceptor olefin.[11] Although these studies have esterified product 4a (entry 2) was obtained only in a
exhibited various olefinic acceptors, there is currently moderate yield. This phenomenon indicated that
no reductive coupling reaction in which maleimide is copper sulfate was a good catalyst for the esterifica-
involved as an olefin acceptor.
tion, but was not a good catalyst for radical addition
At present, it has been reported that N-substituted cyclization. Cobalt and nickel catalysts had a small
maleimide can form an ester with an alcohol ring- amount of product formed (entries 3–4). When iron
opening under the catalysis of Lewis acid.[12] We tribromide was used as a catalyst, both γ-butyrolactone
suspected that the N-substituted maleimide could be and ring-opening products were formed (entry 5). In
opened with allyl alcohol under Lewis acid catalysis, particular, Boger’s catalyst[8] iron oxalate did not give
and the formed allyl ester could undergo an intra- γ-butyrolactone (entry 6). To our surprise, when the
molecular free radical addition reaction to obtain a γ- reaction was conducted in FeCl3 as a catalyst, we were
Table 1. Optimization of the Reaction Conditions.[a,b]
entry
catalyst
reductant
sovent
T
Yield (%)
3a
°
( C)
4a
1
2
3
4
5
6
7
8
Fe(acac)3
CuSO4 ·5H2O
Co(acac)2
Ni(acac)2
FeBr3
Fe(ox)3 ·6H2O
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
HSiCl3
Et3SiH
(EtO)3SiH
NaBH4
NaCNBH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
cyclohexane
MeOH
THF
75
68
0
0
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
75
30
trace
trace
36
trace
trace
0
–
20
–
14
0
trace
0
0
0
trace
16.2
26
trace
82
0
0
51
0
9
10
11
12
13
14
15
16
17b
18
19
20[c]
20
0
41
trace
51
22
59
68
32
2a
EtOH
EtOH
EtOH
EtOH
60
100 (seal)
75
0
0
tance
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst (30 mol%), reductant (2 equiv.), solvent (1 mL), 4 h; yield refers to
isolated product.
[b] FeCl3 (10 mol%)was used.
[c] Argon atmosphere reaction.
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pleased to find that the yield of 3a could be
dramatically improved to 82% (entry 7). What’s more,
when other reductants like HSiCl3, Et3SiH, (EtO)3SiH
were used, only (EtO)3SiH was able to obtain the target
product in a moderate yield (entries 8–10). When
NaBH4 and NaCNBH3 were used as a reducing agent,
NaCNBH3 could obtain γ-butyrolactone in a lower
yield (entries 11–12). The survey of solvents showed
that cyclohexane did not give the target product
(entry 13), while THF and MeOH were inferior to give
the product in lower yield (entries 14–15). To our
surprise, when the reaction was conducted in 2a as a
solvent, the γ-butyrolactone product could also be
formed, albeit in moderate yield (entry 16). We also
tried to decrease the amount of FeCl3 to 10 mol% and
found the yield was lower (entry 17). In addition,
°
°
changing the temperature to 60 C and 100 C (seal)
also resulted in a lower yield (entries 18–19). The
argon atmosphere yield is only 32% under standard
conditions (entry 20).
With the optimized reaction conditions in hand, we
next set out to explore the universality of this method
(Scheme 1). Therefore, a series of allyl alcohols with
significant structural diversity, including chain and
cyclic allyl alcohols, were subjected to this process. In
addition, the single-crystal structure of 3a, 3t, 3u
further confirmed that the product of the cascade
reaction was γ-butyrolactone.[13] When we carried out
the reaction with 1-phenyl-2-methyl-allyl alcohol, we
obtained 4-phenyl substituted γ-butyrolactone in mod-
erate yield (3b). Interestingly, the phenyl para-elec-
tron-withdrawinggroup substituted 2-methyl-1-substi-
tuted allyl alcohols, such as fluoro, chloro, bromo,
trifluoromethyl and cyano, were also amenable to this
protocol to generate the corresponding γ-butyrolac-
tones in good to excellent yield (3c–3g). When the
phenyl ortho and meta substituted 2-methyl-1-substi-
tuted allyl alcohols were used, the γ-butyrolactone
products could also be formed, in moderate yield (3h–
3i). The 2-methyl-1-disubstituted phenyl allyl alcohol
could engage in this process to afford the product in
excellent yield (3j, 94%). Moreover, phenyl para-
electron-donating -group substituted 2-methyl-1-substi-
tuted allyl alcohols were also applicable in this process
to furnish the product in moderate yield (3k–3m). It is
Scheme 1. Substrate scope of allyl alcohol.a,b,c
possible that the electron withdrawing group enhances ylmethanol, could engage in this process to afford the
the electron withdrawing power of the ester group in spiro γ-butyrolactone products in moderate to good
the allyl ester intermediate to the acceptor olefin, and yields (3s–3t). The methyl 2-(hydroxy(phenyl)methyl)
the acceptor olefin is more polarized to make the alkyl acrylate was also applicable in this process to furnish
radical easier to attack the acceptor olefin. The 2- the product in moderate yield (3u, 55%). Furthermore,
methyl-1-(naphthalen-2-yl)prop-2-en-1-ol were well the 3-methylbut-2-en-1-ol was used, and the δ-valer-
amenable to this protocol to furnish the product in olactone product could also be formed, albeit in
moderate yield (3n). When the 2-methyl-1-alkyl- moderate yield (3v, 53%).
substituted allyl alcohols were used, the γ-butyrolac-
We next focused our attention to the scope of
tone products could also be formed, albeit in moderate maleimides (Scheme 2). Gratifyingly, various malei-
yield (3o–3r). In addition, cyclic allyl alcohols, such mides were well applicable in this free radical
as cyclopent-1-en-1-ylmethanol and cyclohex-1-en-1- reduction cascade reaction process. For example, the
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substituted tetrahydrofuran.[15] However the rapid and
simple synthesis of fully substituted tetrahydrofuran
still faces challenges. Therefore, the derivatization of
the fully substituted γ-butyrolactone demonstrated the
comprehensive utility of the method (Scheme 3).
Initially, we used benzyl magnesium chloride to carry
out the addition reaction with 3j, and simultaneously
carried out dehydration reaction to obtain intermediate
6a.[16] We found the carbon-carbon double bond could
be successively reduced by H2,[17] and the fully
substituted tetrahydrofuran products 7a could be
afforded in moderate yield.
To gain insight into the reaction mechanism, some
control experiments were carried out (Scheme 4).
Omission of FeCl3 resulted in obtaining trace carbon-
carbon double bond-reduced product (5a) and recovery
of the starting material, demonstrating that the Fe(III)
catalyst was necessary for the free radical reduction
Scheme 2. Substrate scope of allyl maleimide.a,b
N-4-substituted phenyl maleimides, regardless of the cascade reaction. Interestingly, when the reaction was
electron-donating or electron-withdrawing substitution, performed using 2a as the solvent in the absence of
could proceed smoothly in this process to deliver
corresponding γ-butyrolactones in moderate to good
yields (3x–3aa), with the valuable functional group
such as methyl, benzyloxy, chloro and bromo. When
the N-3-chlorophenyl maleimide was used, the γ-
butyrolactone product could also be formed, albeit in
moderate yield (3ab, 55%). Interestingly, the N-
disubstituted phenyl and heterocyclic substituted mal-
eimide, like pyridine, were also amenable to this
protocol to generate the corresponding γ-butyrolac-
tones in moderate to good yield (3ac–3ad).
In order to reveal the potential application of this
protocol, an amplification scale reaction was imple-
mented. A 1.73 g scale reaction was carried out for
allyl alcohol 2a and N-phenylmaleimide 1a, and the γ-
butyrolactone product 3a was isolated in 71% yield
(Scheme 3). Fully substituted tetrahydrofuran is a very
important structural model widely found in various
natural products.[14] At present, there are only several
reports about the synthesis of some special fully
Scheme 3. Scale-up reaction and synthetic application.
Scheme 4. Control experiments.
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PhSiH3, intermediate 8a was obtained in 12% yield. form radical adduct D. Then the single-electron trans-
When a radical scavenger such as TEMPO was added, fer between FeIICln and D delivers E, which was
the γ-butyrolactone product was not observed and ring- protonated to lactone.
opened product (4a) was observed, when intermediate
In summary, an iron-catalyzed free radical reduc-
8a is added to the TEMPO reaction under standard tion cascade reaction of allyl alcohols with N-
conditions, no γ-butyrolactone is formed, indicating substituted maleimides has been reported for the
that the transformation proceeded via a radical process. preparation of polysubstituted γ-butyrolactones. This
Moreover, intermediate 8a could form γ-butyrolactone protocol is not only featured with mild conditions and
under standard conditions in moderate yield, while broad scope, but also reveals a free radical cascade
29% of the ethyl ester product 4a was formed. This reaction of N-substituted maleimides, which would
result was consistent with the reaction of one equiv- offer a new insight for synthesis of γ-butyrolactones
alent of allyl alcohol, demonstrating that intermediate and spiro γ-butyrolactones. Meanwhile, the fully
8a was not stable. There was a transesterification substituted γ-butyrolactone can be further derivatived
equilibrium in the reaction. Unfortunately, when 2- as a fully substituted tetrahydrofuran. In the future, we
methylpropene (2w) was reacted with N-phenylmalei- will examine the biological activities of this series of
mide under standard conditions, no free radical compounds to explore the application value of these
addition product (3w) was obtained, and the ring- compounds.
opening product (4a) was obtained in only 21% yield.
Regrettably, we did not obtain the γ-butyrolactone
product when ((2-methylallyl)oxy)benzene (2x) and
Experimental Section
N-phenylmaleimide reacted under standard conditions. General Procedure for Polysubstituted γ-Butyrolac-
It is indicated that N-phenylmaleimide needed to be tone (3a–3x, 3aa–3ad)
opened before free radical addition. In addition, when
the intermediate 8a reacted with 2w or 2x under
standard conditions and no coupling product was
produced, the γ-butyrolactone product was obtained
only in a moderate yield, indicating that the intra-
molecular cyclization reaction was advantageous.
Add FeCl3 (14.6 mg, 0.09 mmol) to the reaction tube and
dissolve immediately with ethanol (1.5 ml). Afterwards, allyl
alcohol 2 (0.9 mmol), and PhSiH3 (64.9 mg, 0.6 mmol) were
°
added via syringe. The solution was heated to 75 C. After-
wards, N-substituted maleimide 1 (0.3 mmol) added to the
°
solution. The solution was kept at 75 C for 4 h. Then the
Based on these results and others previously solution was diluted with H2O. Ethyl acetate extracted and
reported in the literature,,[9][10] a plausible mechanism
transferred to a round bottom flask. Silica gel was added to the
flask and volatiles were evaporated under vacuum. The
was proposed in Scheme 5. Initially, the Fe(III)-
purification was performed by column chromatography on silica
gel using ethyl acetate/petroleum ether (v/v, 1:3) as eluent to
give polysubstituted γ-butyrolactone 3.
General Procedure for Fully Substituted Tetrahy-
drofuran (7a)
2-(5-(3,4-dichlorophenyl)-4,4-dimethyl-2-oxotetrahydrofuran-3-
yl)-N-phenylacetamide 3j (118 mg, 0.3 mmol) was added to the
reaction tube which was dried under an argon atmosphere and
dissolved in dry THF (1.5 ml). Benzyl magnesium chloride
°
(1 M, 1.8 ml) was added dropwise at À 20 C and kept for one
Scheme 5. Proposed mechanism.
hour. Then the solution was diluted with 1 N HCl. Ethyl acetate
extracted and transferred to a round bottom flask. The volatiles
were evaporated to dryness and dissolved in dichloromethane
catalyst was converted to Fe hydride species A in the (1 ml) to be transferred to a reaction tube. Et3SiH (105 mg,
0.9 mmol), BF3.Et2O (64 mg, 0.45 mmol) were added in
presence of phenylsilane and ethanol. Meanwhile, N-
substituted maleimide and alcohol were ring-opened
under Lewis acid catalysis to form intermediate 4 and
B.[12] Intermediate 4 had a transesterification equili-
brium reaction with intermediate B, but with the
consumption of intermediate B, intermediate 4 was
continuously converted to intermediate B. The follow-
ing step involved the transfer of a hydrogen atom from
the transient Fe hydride (A) to the donor olefin B,
giving the intermediate alkyl radical C, which could be
able to carry out intramolecular free radical addition to
°
sequence, and kept at 0–5 C for 6 hours. Then the solution was
diluted with H2O. Ethyl acetate extracted and transferred to a
round bottom flask. Silica gel was added to the flask and
volatiles were evaporated under vacuum. The purification was
performed by column chromatography on silica gel using ethyl
acetate/petroleum ether (v/v, 1:7) as eluent to give 6a (2-(2-
benzylidene-5-(3,4-dichlorophenyl)-4,4-dimeth-
yltetrahydrofuran-3-yl)-N-phenylacetamide) as a white solid
(71 mg, 51% yield).
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To a solution of 6a (70 mg, 0.15 mmol) in MeOH (5 ml) under
an inert atmosphere, was added 10 percent Pd/C (16 mg). H2
was bubbled through the resulting suspension for 30 min. The
reaction mixture was then stirred under 1 atmosphere of H2 for
12 h. Pd/C was filtered off and silica gel was added to the flask
and volatiles were evaporated under vacuum. The purification
was performed by column chromatography on silica gel using
ethyl acetate/petroleum ether (v/v, 1:15) as eluent to give 7a (2-
(2-benzyl-5-(3,4-dichlorophenyl)-4,4-dimethyltetrahydrofuran-
3-yl)-N-phenylacetamide) as a white solid (46 mg, 65% yield).
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Acknowledgements
We are grateful for the financial support from CAS “Light of
West China” Program, Technological Innovation Program of
Chengdu, Sichuan province, China (Nos. 2018-YF05-00244-SN)
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Iron(III) Chloride/Phenylsilane-Mediated Cascade Reaction
of Allyl Alcohols with Maleimides: Synthesis of Poly-Substi-
tuted γ-Butyrolactones
Adv. Synth. Catal. 2019, 361, 1–8
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Y. Wang, J.-Y. Wang*