Additional nucleophile-free FeCl3-catalyzed green deprotection of 2,4-dimethoxyphenylmethyl-protected alcohols and carboxylic acids

Article abstract of DOI:10.1248/cpb.c16-00161

The deprotection of the methoxyphenylmethyl (MPM) ether and ester derivatives can be generally achieved by the combinatorial use of a catalytic Lewis acid and stoichiometric nucleophile. The deprotections of 2,4-dimethoxyphenylmethyl (DMPM)-protected alcohols and carboxylic acids were found to be effectively catalyzed by iron(III) chloride without any additional nucleophile to form the deprotected mother alcohols and carboxylic acids in excellent yields. Since the present deprotection proceeds via the self-assembling mechanism of the 2,4-DMPM protective group itself to give the hardly-soluble resorcinarene derivative as a precipitate, the rigorous purification process by silica-gel column chromatography was unnecessary and the sufficiently-pure alcohols and carboxylic acids were easily obtained in satisfactory yields after simple filtration.

Full text of DOI:10.1248/cpb.c16-00161

778
Chem. Pharm. Bull. 64, 778–784 (2016)
Vol. 64, No. 7
Special Collection of Papers
This article is dedicated to Professor Satoshi Ōmura in celebration of his 2015 Nobel Prize.
Regular Article
Additional Nucleophile-Free FeCl3-Catalyzed Green Deprotection of
2,4-Dimethoxyphenylmethyl-Protected Alcohols and Carboxylic Acids
Yoshinari Sawama,* Masahiro Masuda, Akie Honda, Hiroki Yokoyama, Kwihwan Park,
Naoki Yasukawa, Yasunari Monguchi, and Hironao Sajiki*
Gifu Pharmaceutical University; 1–25–4 Daigaku-nishi, Gifu 501–1196, Japan.
Received February 22, 2016; accepted March 10, 2016
The deprotection of the methoxyphenylmethyl (MPM) ether and ester derivatives can be generally
achieved by the combinatorial use of a catalytic Lewis acid and stoichiometric nucleophile. The deprotections
of 2,4-dimethoxyphenylmethyl (DMPM)-protected alcohols and carboxylic acids were found to be effectively
catalyzed by iron(III) chloride without any additional nucleophile to form the deprotected mother alcohols
and carboxylic acids in excellent yields. Since the present deprotection proceeds via the self-assembling
mechanism of the 2,4-DMPM protective group itself to give the hardly-soluble resorcinarene derivative as a
precipitate, the rigorous purification process by silica-gel column chromatography was unnecessary and the
sufficiently-pure alcohols and carboxylic acids were easily obtained in satisfactory yields after simple filtra-
tion.
Key words deprotection; alcohol; carboxylic acid; 2,4-dimethoxyphenylmethyl (DMPM) protecting-group;
iron catalyst; self-cleaving mechanism
The protection/deprotection method is important and the ethers without the addition of nucleophiles30) [Chart 1, (e)].
deprotection under mild conditions associated with perfect We now report the extensive results of the FeCl3-catalyzed
tolerance of the non-target functional groups can be a power- deprotection method of 2,4-dimethylphenylmethyl (DMPM)-
ful tool to effectively construct the target molecules in organic protected alcohols without purification using silica-gel column
reactions.1) The 4-methoxyphenylmethyl (MPM) group and chromatography together with a novel application for the de-
its derivatives are widely utilized as the protecting group of protection of 2,4-DMPM-protected carboxylic acids. Because
alcohols, and the various deprotection methods have been 2,4-DMPM-protected alcohols were found to be preferentially
developed.1–21) 4-MPM ethers are generally deprotected by the deprotected in comparison with 4-MPM-protected alcohols in
use of a stoichiometric amount of oxidants [2,3-dichloro-5,6- the reported methods (DDQ,1,31) the combination of CeCl3 and
dicyano-p-benzoquinone (DDQ)2,3) and ceric ammonium NaI10) or Ph3C·BF419)), the synthetic route via the 2,4-DMPM-
nitrate (CAN)4,5)], Pd/C-catalyzed hydrogenation under atmo- protected alcohol could be adapted in the efficient synthesis
spheric hydrogen gas1) or the combination of a stoichiometric of target molecules (e.g., total syntheses of natural prod-
amount of nucleophiles and the catalytic or stoichiometric ucts).19,31,32) However, the problems of side-products derived
Lewis acids6–17) [Chart 1, (a)–(d)]. However, the stoichiomet- from 2,4-DMPM group, which should be separated by silica-
ric byproducts derived from 4-MPM protective groups (e.g., gel column chromatography, have been unsolved.
anisaldehydes, and toluene derivatives) and the acidic residues
The 4-MPM-protected 1-decanol (1a) effectively underwent
derived from reagents when using DDQ and CAN should be deprotection in the presence of 5mol% of FeCl3 without any
removed from the reaction mixture by the purification pro- additional nucleophile in CH2Cl2 at room temperature to pro-
cess using silica-gel column chromatography. In addition, a duce the mother alcohol (2a) and many side-products derived
stoichiometric amount of Ph3C·BF418,19) and the chlorosulfonyl from the 4-MPM moiety, which should be separated by silica-
isocyanate (CSI)–NaOH combination20) has also been used gel column chromatography (Eq. 1). Since benzyl (Bn) ether
for the deprotection of the 4-MPM ether. Although a catalytic was tolerant under the FeCl3-catalyzed conditions, the che-
method using ZrCl4 in CH3CN has been developed for the de- moselective deprotection of the 4-MPM protective group of 1b
protection of the 4-MPM ether, silica-gel column chromatog- could be accomplished (Eq. 2). Meanwhile, the deprotection of
raphy purification is necessary to remove the undefined side- 3,4-DMPM also efficiently proceeded to form the correspond-
products.21) We have found that the widely used iron(III) chlo- ing deprotected alcohol (2a) and an inseparable mixture of the
ride (FeCl3) could effectively activate the benzylic position corresponding cyclic trimer, tetramer, pentamer and hexamer
and achieved various reactions accompanied by the cleavage (m/z; 478.1934, 623.2623, 773.3293, 923.3951 [M+Na]+)33–35)
of the benzylic C–O bond to construct unique skeletons.22–29) of the 3,4-DMPM protective group, which were detected and
Furthermore, we have recently developed the FeCl3-catalyzed determined by an electrospray ionization (ESI)/MS spectral
self-cleaving deprotection method of various types of MPM analysis (Eq. 3, the chart is in the Supplementary Materials).
*To whom correspondence should be addressed. e-mail: sawama@gifu-pu.ac.jp; sajiki@gifu-pu.ac.jp
© 2016 The Pharmaceutical Society of Japan

Vol. 64, No. 7 (2016)
Chem. Pharm. Bull.
779
Chart 1. General and Novel Deprotection Methods of MPM Type-Protected Alcohols
the sufficiently-pure 1-decanol (2a). In a similar fashion, the
FeCl3-catalyzed deprotection of 2,4-dibutoxyphenylmethyl
It is noteworthy that the FeCl3-catalyzed deprotection of the ether (1e) also efficiently proceeded to produce lipophilic
2,4-DMPM ether (1d) was completed within 5min and only resorcin[4]arene octabutyl ether (3e: m/z; 959.6376 [M+Na]+
the sufficiently-pure 1-decanol (2a) was isolated without the see Supplementary Materials) together with the deprotected
purification process using silica-gel column chromatography 2a (Eq. 5), which strongly supported that the hardly-soluble
1
(Eq. 4, Chart 2, middle H-NMR spectrum). Peaks derived tetramer of 2,4-DMPM was obtained as nearly the only side-
from the 2,4-DMPM group of the substrate (1d, Chart 2, top) product from the deprotection of 1d (Eq. 4).
were never observed after the filtration and subsequent extrac-
The present deprotection of 2,4-DMPM ether (1d) should
tion of the reaction mixture of the deprotection reaction of 1d proceed via the FeCl3-catalyzed self-assembling mechanism
(Chart 2, middle 1H-NMR spectrum), and sufficiently-pure (Chart 3). The electron-rich aromatic ring of 1d attacked the
1-decanol (2a) could be obtained in comparison to the spec- benzylic position of another substrate (1d) activated by FeCl3
trum of the commercially available 1-decanol as a standard to give the deprotected alcohol (2a) and a corresponding
sample (2a, Chart 2, bottom). During the deprotection of 1d, dimer (4). The repeated similar intermolecular nucleophilic
the resorcinarene derivative (3d) was produced as a hardly- attacks continue until the formation of the tetramer (5), which
soluble precipitate, which was slightly soluble in CH2Cl2, undergoes the intramolecular annulation to give the hardly-
but hardly dissolved in Et2O. Therefore, the residual mixture soluble 3d accompanied by the complete deprotection. Alter-
containing the deprotected 2a and 3d was treated with Et2O natively, the homo-coupling of the dimer (4) can also directly
after removal of CH2Cl2 as the solvent under reduced pres- produce the tetramer (5).
sure, and the insoluble 3d was simply removed by filtration.
The green and self-cleaving deprotection method of the
The subsequent extraction (Et2O washing) of the filtrate gave 2,4-DMPM ether (1d) without the silica-gel column chro-

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Chem. Pharm. Bull.
Vol. 64, No. 7 (2016)
(Top) substrate 1d; (Middle) deprotection product; (Bottom) standard sample 2a.
1
Chart 2. Comparison of H-NMR Spectra of 1d and 2a
Chart 3. Proposed Reaction Mechanism of Deprotection of 2,4-DMPM Ether
matography purification could be accomplished using other
Lewis acids (e.g., AuCl3,36–38) trimethylsilyl trifluoromethane-
sulfonate (TMSOTf), BF3·Et2O) instead of FeCl3 in CH2Cl2
to produce the deprotected 1-decanol (2a) in high yields
within 5min (Table 1, entries 1–4). On the other hand, Brøn-
sted acids, such as trifluoroacetic acid (TFA) and AcOH, were
insufficient and most of the starting material (1d) remained
unchanged. CH2Cl2 or CH3CN was found to be an adequate
solvent (Table 1, entries 1, 10), while the use of n-hexane, 2,4-DMPM-protected palmitic acid (6a) was also effectively
THF or toluene gave the desired deprotected alcohol (2a) in a deprotected using FeCl3 and AuCl3 in a short time to give the
relatively low yield (entries 7–9).
corresponding deprotected carboxylic acid (7a) in excellent
Furthermore, the deprotection reactions using 2,4-dibutoxy- yields (Table 2, entries 1, 2). Although TMSOTf and BF3·Et2O
phenylmethyl ether (1e) as a protected alcohol produced the were also effective (entries 3, 4), Brønsted acids, such as TFA
resorcin[4]arene octabutyl ether (3e) as nearly the only tetra- and AcOH, were insufficient (entries 5, 6). While the FeCl3-
mer side-product even when using AuCl3, FeBr3 and ZnCl2 as catalyzed deprotection could smoothly proceed in CH2Cl2,
the Lewis acid catalysts instead of FeCl3 (Eq. 6).
toluene and CH3CN as the solvent, n-hexane and THF were
MPM-type protecting groups are also utilized for the found to be inadequate solvents (entries 1, 9, 10 vs. 7, 8). From
protection of carboxylic acids.1,39,40) Similarly, DDQ or the the viewpoint of cost performance and reaction efficiency, the
nucleophile in the presence of a catalytic Lewis acid is used use of a catalytic amount (5mol%) of FeCl3 in CH2Cl2 was
for the deprotection of MPM-protected carboxylic acids. The chosen as the optimum reaction conditions for the deprotec-

Vol. 64, No. 7 (2016)
Chem. Pharm. Bull.
781
tion of the 2,4-DMPM-protected carboxylic acids.
could also produce the highly pure carboxylic acid (7a). Since
For the FeCl3-catalyzed deprotection of 2,4-DMPM- resorcin[4]arene octabutyl ether (3e) was also obtained in 95%
protected carboxylic acid, the rigorous purification process yield as a tetramer side-product by the FeCl3-catalyzed depro-
to remove the side-products derived from the 2,4-DMPM tection of the 2,4-dibutoxyphenylmethyl ester (6b) (Eq. 7), the
protective group was also unnecessary (Chart 4). After the formation and structure of the hardly-soluble 3d was equally
filtration of the resulting precipitate derived from 2,4-DMPM probable.
1
using a small amount of a silica-gel pad, the H-NMR peaks
The versatility of the substrate applicability was next
resulting from 2,4-DMPM moiety were hardly observed (top investigated regarding the FeCl3-catalyzed deprotection of
vs. middle) and the sufficiently-pure palmitic acid (7a) was 2,4-DMPM-protected alcohols and carboxylic acids (Table 3).
obtained (middle vs. bottom). Due to the highly polar property The 2,4-DMPM ethers prepared from primary and secondary
of the deprotective carboxylic acid (7), the use of water should
be avoided during work-up to avoid any undesired loss. There-
fore, the reaction mixtures including the deprotected carbox-
ylic acid (7) and the hardly-soluble resorcinarene derivative
(3d) were directly filtered with a small amount of a siliga-gel
pad. Alternatively, the use of the celite pad for the filtration
Table 1. Catalyst and Solvent Efficiencies of Deprotection of 2,4-DMPM-
Table 2. Catalyst Efficiencies of Deprotection of 2,4-DMPM-Protected
Protected Alcohol (1d)
Carboxylic Acid (6a)
Entry
Catalyst
Solvent
Time
Yield (%)
Entry
Catalyst
Solvent
Time
Yield (%)
1
2
FeCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
n-Hexane
THF
5min
5min
5min
5min
3h
99
1
2
FeCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
n-Hexane
THF
5min
20min
5min
5min
3h
99
AuCl3
TMSOTf
BF3·Et2O
TFA
99
AuCl3
TMSOTf
BF3·Et2O
TFA
96
3
99
3
88
4
97
4
86
5
11 (70)a)
0 (100)a)
38 (59)a)
25 (55)a)
75
5
34 (46)a)
0 (99)a)
75 (25)a)
46 (50)a)
98
6
AcOH
FeCl3
3h
6
AcOH
FeCl3
3h
7
3h
7
3h
8
FeCl3
3h
8
FeCl3
3h
9
FeCl3
Toluene
CH3CN
30min
5min
9
FeCl3
Toluene
CH3CN
45min
5min
10
FeCl3
92
10
FeCl3
96
a) The yield in parenthesis indicates the recovered starting material (1d).
a) The yield in parenthesis indicates the recovered starting material (6a).
(Top) substrate 1d; (middle) deprotection product; (Bottom) standard sample 2a.
1
Chart 4. Comparison of H-NMR Spectra of 6a and 7a

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Vol. 64, No. 7 (2016)
Table 3. Scope and Limitationa)
Entry
1
Substrate
Product
Time
5min
Yield (%)
99
2
3
5min
5min
99
92
4
5
6
7
5min
5min
5min
5min
99
Quant
Quant
69
31
8
9
15min
5min
99
80b)
10
5min
96
11
12
13
5min
6h
78c)
No reaction
96
5min
14
5min
99
15
16
5min
5min
94
96
17
18
5min
5min
75
84
19
20
5min
5min
99
87
21
5min
86
a) The reactions were carried out according to the general procedures shown in Experimental unless otherwise noted. Spectral data without the rigorous purification process
are shown in the Supplementary Materials. b) The product was purified by silica-gel column chromatography due to the contamination of unidentified byproducts. c) The yield
after the purification using silica-gel column chromatography.

Vol. 64, No. 7 (2016)
Chem. Pharm. Bull.
783
alcohols were effectively deprotected into the corresponding small amount of a silica-gel pad and washed with Et2O. The
mother alcohols within 5min in nearly quantitative yields filtrate was concentrated in vacuo to give the sufficiently-pure
without purification by the silica-gel column chromatogra- carboxylic acid. When some peaks derived from undeter-
1
phy (entries 1, 2). Because benzyl ether was stable under the mined side-products were observed in the H-NMR spectra,
FeCl3-catalyzed conditions, benzyl 2,4-DMPM ether was per- the analytically-pure deprotected carboxylic acid was obtained
fectly converted into the corresponding benzyl alcohol by the after the normal silica-gel column chromatography (Table 3).
chemoselective cleavage at the electron-rich benzylic position
(entry 3) and the 2,4-DMPM ether coexisting benzyl ether
Acknowledgment This work was supported by Grant-in-
within the same molecule was selectively deprotected without Aid for Young Scientists (B) from the Japan Society for the
the deprotection of the benzyl ether moiety (entry 4). The tert- Promotion of Science (JSPS).
butyldimethylsilyl (TBS) ether and acetoxy (AcO) group could
be tolerant under the present deprotection conditions (entries
Conflict of Interest The authors declare no conflict of
5, 6), while the ketal as a protective group of the ketone was interest.
partially deprotected along with the complete deprotection of
the 2,4-DMPM ether protective group to form a mixture of the
Supplementary Materials The online version of this ar-
ketal and ketone (entry 7). The acidic proton on the hydroxyl ticle contains supplementary materials.
group never influenced the deprotection (entry 8) and the
2,4-DMPM-protected phenol was also efficiently deprotected
into mother phenol (entry 9).
References and Notes
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deprotection mixture through a small amount of a silica-gel
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solution of the 2,4-DMPM ether (0.2mmol) in CH2Cl2 (1mL)
was added FeCl3 (0.01mmol), then stirred at room temperature
under argon. After being stirred until the reaction was com-
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and the organic layer (CH2Cl2) was evaporated in vacuo. Et2O
was added to the aqueous residue and the resulting precipitate
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