Samarium(II) dibromide-promoted selective deprotection of a benzoyl protective group

Article abstract of DOI:10.1080/00397911.2018.1429634

The selective deprotection of a benzoyl group was very important methodology in the field of organic synthesis. Various methods for debenzoylation were investigated and developed in the past six decades, but more useful and selective strategies are now being strongly desired. In response to this strong demand, we developed the novel and selective deprotection of a benzoyl group by use of samarium(II) dibromide and a proton source. This deprotective reaction proceeded smoothly and the desired compound was obtained in good to excellent yields. In this paper, we will report the details of this deprotective reaction.

Full text of DOI:10.1080/00397911.2018.1429634

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Samarium(II) dibromide-promoted selective
deprotection of a benzoyl protective group
Yasuo Yokoyama, Shun Oyamada, Junya Suzuki, Shou Maruyama, Takahiro
Sakusabe & Shoko Suzuki
To cite this article: Yasuo Yokoyama, Shun Oyamada, Junya Suzuki, Shou Maruyama, Takahiro
Sakusabe & Shoko Suzuki (2018): Samarium(II) dibromide-promoted selective deprotection of a
benzoyl protective group, Synthetic Communications, DOI: 10.1080/00397911.2018.1429634
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https://doi.org/10.1080/00397911.2018.1429634
Samarium(II) dibromide-promoted selective deprotection of a
benzoyl protective group
Yasuo Yokoyamaa, Shun Oyamadab, Junya Suzukib, Shou Maruyamab,
Takahiro Sakusabeb, and Shoko Suzukic
aDepartment of Chemical and Biological Engineering, National Institute of Technology, Akita, Japan;
c
bEnvironment Systems Engineering Course, National Institute of Technology, Akita, Japan; Faculty of
Education, Kyoto University of Education, Kyoto, Japan
ABSTRACT
ARTICLE HISTORY
Received 2 December 2017
The selective deprotection of a benzoyl group was very important
methodology in the field of organic synthesis. Various methods for
debenzoylation were investigated and developed in the past six
decades, but more useful and selective strategies are now being
strongly desired. In response to this strong demand, we developed
the novel and selective deprotection of a benzoyl group by use of
KEYWORDS
Benzoyl group; reduction;
samarium(II) dibromide;
selective deprotection
samarium(II) dibromide and
a proton source. This deprotective
reaction proceeded smoothly and the desired compound was
obtained in good to excellent yields. In this paper, we will report
the details of this deprotective reaction.
GRAPHICAL ABSTRACT
CONTACT Yasuo Yokoyama
yokoyama@akita-nct.ac.jp
Department of Chemical and Biological Engineering,
National Institute of Technology, Akita College, Iijimabunkyo-cho 1-1, Akita 011-8511, Japan.
Supplemental material for this articl can be accessed on the publisher’s website.
© 2018 Taylor & Francis

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Y. YOKOYAMA ET AL.
Scheme 1. Deprotection using samarium(II) dibromide.
Introduction
Recently, synthetic studies of natural products have come to be paid to attention again as
the application of it to a medicine have become active. The practical use degree of the
present study is extremely high, because it becomes possible to supply reasonable amount
of a natural product to research of its bioactivity. Thus, the development of various organic
synthetic reactions is very important. One of the important techniques in organic synthesis
of a natural product includes protection and deprotection of a functional group.[1] This
method has been developed based on inactivation of an active substituent. An active
substituent of a starting material is converted into an inactive organic structure beforehand,
and then the desired reaction is performed using this compound. After that, the original
active substituent is regenerated. This method was often applied to a hydroxy group
especially. The mutual conversion of a hydroxy group and a benzoyloxy group was well
known as a protection–deprotection method.[2] Actually, this method was used for total
synthesis of isoajmaline, leukotriene B4, 5S,12S-dihydroxy-6,10-E,8,14-Z-eicosatetraenoic
acid, 5-desoxyleukotriene D4, and so on.[2] In this strategy, a strong base was used for this
purpose, but some kinds of structures were base-sensitive and were decomposed or
mutated. Another type deprotection methods, such as using KCN as a nucleophile,[3]
hydrazine method,[4] Et3N-MeOH,[5] and BF3 · Et2O/Me2S combination[6] were well
known, but these methods had some serious disadvantages. For example, in cases of
deprotection using KCN or hydrazine, these reagents were highly toxic, and sometimes,
there was a risk of causing an explosion. Deprotection methods using Et3N-MeOH or
BF3 · Et2O/Me2S decomposed other ester type protective groups. Therefore, the develop-
ment of the efficient deprotection of a benzoyl group under mild reaction conditions
was strongly desired. Concerning this problem, we investigated the selective debenzoyla-
tion using samarium(II) dibromide (SmBr2).[7,8] This samarium salt had an appropriate
reduction power for organic synthesis, was handle easily, and was stable in inert gas. If
a benzoyloxy group was able to convert to a hydroxy group using samarium(II) dibromide,
it was thought that this deprotection method was very useful. We will report the result of
applying the reductive reaction using samarium(II) dibromide to removing of a benzoyl
protective group (Scheme 1).
Results and discussion
At first, hexadecyl benzoate (C16H33OCOPh) was used as a model compound, and we
examined efficacy of samarium(II) dibromide for removal of a benzoyl group. Some results
of this reductive deprotection of a benzoyl group were shown in Table 1.
When this reaction was performed for 24 h in the presence of methyl alcohol, hexadecyl
alcohol was obtained in 51% (Entry 1). However, when hexadecyl benzoate was treated
with samarium(II) dibromide for 48 h, the yield of the corresponding alcohol was
improved to 98% (Entry 2). In this reaction, when samarium(II) diiodide[9] was used

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Table 1. Reductive removal of a benzoyl group using SmBr2.
Entry
Reaction time (h)
Eq. of SmBr2
Proton source
Yielda (%)
1
2b
3
24
48
48
48
48
72
120
48
48
48
48
6.0
6.0
1.0
2.0
4.0
6.0
6.0
6.0
6.0
6.0
6.0
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
51
98
34
45
78
96
99
92
87
93
75
4
5
6
7
8
9
i-PrOH
H2O
AcOH
10
11
aIsolated yield.
bWhen samarium(II) diiodide was used instead of samarium(II) dibromide, hexadecyl benzoate was recovered perfectly.
instead of samarium(II) dibromide, hexadecyl benzoate was recovered perfectly. These
facts indicated that a reduction power of samarium(II) dibromide was suitable and that
of samarium(II) diiodide was too weak for this debenzoylation. The first step of this
debenzoylation was probably reduction of carbonyl substituent to form the corresponding
ketyl radical by this samarium salt. We presumed that reduction of carbonyl substituent
did not occur by samarium(II) diiodide having a low reduction power. Unfortunately,
when 1.0, 2.0, and 4.0 equivalent amounts of samarium(II) dibromide were used in this
reaction, the corresponding compound was obtained in low yields (Entries 3–5). The
tendency for hexadecyl alcohol to be obtained in high yield was observed, when this reac-
tion was performed for 72 h or 120 h (Entries 6 and 7). These facts suggested that this
deprotection proceeded gently, and the desired compound was stable in this reaction con-
ditions. Moreover, some alcohols and other compounds having active proton could be used
as proton sources in this method. Ethyl alcohol and isopropyl alcohol were efficient proton
[8h]
sources for this reaction (Entries 8 and 9). Fortunately, water
could also be used as an
effective proton source of this reductive deprotection method (Entry 10). This fact indi-
cated that the cost concerning performing of this deprotection method was reasonable.
Strangely, acetic acid was undesirable as a proton source. The target compound was
obtained in moderate yield in the presence of acetic acid (Entry 11). Now, we think that
further investigation is needed to elucidate this phenomenon. It was very interesting that
the acetyl group of hexadecyl acetate could not be deprotected at all by this reaction,
although a benzoyl group and an acetyl group were structurally similar each other. In this
reaction system, only the benzoyl group was deprotected.
At next investigation, we tried to apply this method for debenzoylation of the
derivative of a natural product. In this study, we selected cholesterol as a natural product.
Commercially available cholesteryl benzoate was used as a starting compound and was
treated with samarium(II) dibromide. Some results were summarized in Table 2. In these
reactions, it was decided to use methyl alcohol and 10 equivalent amounts of samarium(II)
dibromide from the results of examining the type of a proton source and the amount of this
samarium salt.

4
Y. YOKOYAMA ET AL.
Table 2. Reaction of cholesteryl benzoate.
Entry
Temp. (°C)
Yielda (%)
1
10
20
30
40
50
50
–
–
9.3
90
99
88
2
3
4
5
6b
aIsolated yield.
bIn total, 10% (v/v) of hexamethylphosphoramide (HMPA) was used as an additive.
When cholesteryl benzoate was treated with samarium(II) dibromide at low
temperature, deprotection did not proceed at all (Entries 1 and 2). This deprotective
reaction was performed at 30 °C to give cholesterol in moderate yield (Entry 3). These
results suggested that this reaction rate was slow at low temperature, so we decided to
raise reaction temperature. It was found that the yield of the target compound
was improved as the reaction temperature was raised to 40 and 50 °C (Entries 4 and 5).
Particularly, cholesterol was obtained perfectly at 50 °C. Interestingly, when
hexamethylphosphoramide[8c,10] (HMPA) was added to this reaction system, many
unknown compounds were formed and cholesterol was obtained in moderate yield
(Entry 6). The reason of this result is now unclear. We will further investigate in future.
Next, we tried selective deprotection of a benzoyl group of other derivative of a natural
product. In this study, we selected 17β-estradiol as a natural product, because this
compound had two types of hydroxy groups. One was the phenol type hydroxy group,
and the other was the secondary hydroxy group. We synthesized benzoate of 17β-
acetylestradiol to use as a starting compound, and investigated about effect of reaction
temperature and reaction time to a yield of 17β-acetylestradiol. Some results were shown
in Table 3. We decided to use water and 8.0 equivalent amounts of samarium(II) dibro-
mide in these reactions, as a result of examining the type of a proton source and the
amount of this samarium salt.
This reductive deprotection of a benzoyl group proceeded smoothly at 0 °C to give the
corresponding phenol derivative in 80% yield (Entry 1). We could not find 17β-estradiol at
[11]
all. However, this compound was treated with samarium(II) diiodide/Et3N/water
to
give many products including 17β-estradiol. These facts indicated that samarium(II)

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Table 3. Reductive deprotection of a benzoyl group of 17β-acetylestradiol derivative.
Entry
Temp. (°C)
Reaction time (h)
Yielda (%)
1
2
3
4
5
6
7
8
0
17
17
17
17
1
80
99
93
97
72
88
91
98
rt
50
60
60
60
60
60
2
4
8
aIsolated yield.
dibromide-promoted reductive method affected to the benzoyl group only, and an acetyl
group was not removed from a starting material at all. When this reaction was performed
at room temperature for 17 h, the desired compound was obtained, quantitatively (Entry 2).
In this case, decomposition of an acetyloxy group was not also observed. This phenomenon
could be found at higher temperature. This method was performed at 50 or 60 °C to give
the corresponding compound in excellent yields (Entries 3 and 4). Results of entries 1, 2, 3,
and 4 suggested that this reductive deprotection of a benzoyl group was very useful,
because only the benzoyl protective group was removed selectively. Moreover, it was
revealed that this reaction proceeded slowly, under high temperature. When this method
was performed for 1 h at 60 °C, the corresponding compound was obtained in 72% yield
(Entry 5). When this deprotection was performed for 2 or 4 h, this reaction was not
finished (Entries 6 and 7). However, this deprotection proceeded perfectly for 8 hours to
give the desired alcohol in excellent yield (Entry 8). These facts suggested that
samarium(II) dibromide-promoted reaction was effective for the selective deprotection
of a benzoyl group.
We found that the selective deprotection of a benzoyl group as mention above, thus we
examined this reaction using 1,10-decanediol derivative as a substrate based on this knowl-
edge (Scheme 2). When 1-acetyloxy-10-benzoyloxydecane was treated with samarium(II)
dibromide and water in THF, 10-acetyloxydecan-1-ol was obtained in excellent yield.[12]
In this reaction, 1,10-decanediol to be produced by over-reduction was not observed at
all. Then it was revealed that this deprotection method was useful, because only the benzoyl
group was decomposed selectively.[13] These results indicated that this method was

6
Y. YOKOYAMA ET AL.
Scheme 2. Selective deprotection of a benzoyl group.
powerful because of effective removal of a benzoyl group from both an aromatic benzoate
and an aliphatic benzoate.
Conclusion
In this paper, we presented the selective deprotection method of a benzoyl group by use of
samarium(II) dibromide. This deprotection proceeded under mild conditions, and the
corresponding alcohol or phenol were obtained in good to excellent yields. When 17β-estradiol
derivative or 1,10-decanediol were used as substrates, the corresponding acetylated compounds
were given in excellent yield without decomposition of an acetyloxy group. The mechanism of
this deprotection is unclear at present. Furthermore, investigation is now in progress.
Experimental
Standard bench top techniques were used for handling air-sensitive reagent. THF was
distilled from sodium benzophenoneketyl before use. Ethyl alcohol was distilled from
magnesium cake before use. Methyl alcohol and isopropyl alcohol were distilled from mol-
ecular sieves (4 Å) before use. HMPA was distilled from calcium hydride before use. Acetic
acid and water were distilled under nitrogen atmosphere. All reactions were performed
under nitrogen atmospheres. Reaction temperatures referred to bath temperatures. Melting
1
point was uncorrected. All yields of target compounds were isolated yields. H NMR and
13C NMR spectra were measured on Bruker BioSpin AVANCE III 400 Nanobay
spectrometer at 400.1 and 100.6 MHz, respectively. Chemical shifts were given in ppm
relative to TMS. IR spectra were recorded on an FT/IR-610 (JASCO) spectrophotometer.
Elemental analyses were determined on a Perkin-Elmer PE2400 Series II CHNS/O
analyzer. MS were recorded on a Shimadzu PARVUM2 spectrometer.
Preparation of a THF suspension of samarium(II) dibromide
Fine powder of samarium metal (4.96 g, 33 mmol) was suspended in THF (300 mL) at 0 °C
in ice-water bath. Bromine (4.79 g, 30 mmol) was added dropwise by glass syringe under
nitrogen atmosphere. After stirring for 3 days at room temperature, the color of this sus-
pension turned deep-violet. This well-stirred THF suspension of samarium(II) dibromide

SYNTHETIC COMMUNICATIONS®
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could be used as a reducing agent in this reductive deprotection as 0.1 mol/L suspension.
Samarium(II) dibromide was dissolved perfectly, when HMPA was added.
Reductive deprotection of a benzoyl group from hexadecyl benzoate by use of
samarium(II) dibromide
To a stirring solution of hexadecyl benzoate (34.6 mg, 0.1 mmol) and proton source
(0.2 mmol) in dry THF (0.5 mL), was added dropwise a THF suspension of samarium(II)
dibromide (6.0 mL, 0.1 mol/L). After staring at ambient temperature for several hours, a
silica gel (0.5 g) and n-hexane (1.0 mL) was added to the mother liquor and this mixture
was passed through a short column of silica gel (2.0 g, n-hexane) and eluted with Et2O
(100 mL). The crude mixture was purified by silica gel chromatography (n-hexane/Et2O ¼
4/1) to yield the product as white solid. 1H NMR, 13C NMR, MS, and other physical data of
1
this product were consisted with those of 1-hexadecanol.[14]; H NMR (CDCl3) δ (ppm):
0.88 (t, J ¼ 7.1 Hz, 3H), 1.26–1.36 (m, 26H), 1.53–1.60 (m, 3H), 3.64 (t, J ¼ 4.3 Hz, 2H);
13C NMR (CDCl3) δ (ppm): 14.15, 22.72, 25.76, 29.40, 29.47, 29.64 (3C), 29.69 (2C),
29.72 (3C), 31.95, 32.81, 63.06; MS (EI, 70 eV) m/z (%): 69 (100), 83 (92), 97 (78), 111
(46), 125 (17), 140 (4), 154 (1), 168 (1), 182 (1), 196 (4), 222 (1), 242 (2) [Mþ].
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