Facile synthesis of diphenylmethyl esters from 2-diphenylmethoxypyridine using catalytic boron trifluoride·diethyl etherate

Article abstract of DOI:10.1016/j.tetlet.2018.04.002

A practical method for the direct preparation of diphenylmethyl (DPM) esters from 2-diphenylmethoxypyridine is described. The reaction was readily performed in the presence of a catalytic amount of boron trifluoride-diethyl etherate at room temperature. Using this reaction protocol, various carboxylic acids were converted to DPM esters with high yields. This method is highly effective for the protection of carboxylic acids and the synthesis of DPM esters, and offers a promising approach for facile esterification of a variety of carboxylic acids.

Full text of DOI:10.1016/j.tetlet.2018.04.002

Accepted Manuscript
Facile Synthesis of Diphenylmethyl Esters from 2-Diphenylmethoxypyridine
using Catalytic Boron Trifluoride·Diethyl Etherate
Minh Thanh La, Hee-Kwon Kim
PII:
S0040-4039(18)30433-7
https://doi.org/10.1016/j.tetlet.2018.04.002
TETL 49865
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 February 2018
30 March 2018
2 April 2018
Please cite this article as: La, M.T., Kim, H-K., Facile Synthesis of Diphenylmethyl Esters from 2-
Diphenylmethoxypyridine using Catalytic Boron Trifluoride·Diethyl Etherate, Tetrahedron Letters (2018), doi:
https://doi.org/10.1016/j.tetlet.2018.04.002
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Facile Synthesis of Diphenylmethyl Esters
from 2-Diphenylmethoxypyridine using
Catalytic Boron Trifluoride·Diethyl Etherate
Minh Thanh Laa,b, Hee-Kwon Kim a,b *
Leave this area blank for abstract info.
O
BF3 OEt2 (20 mol%)
O
+
O
R
OH
R
DCE
rt, 6 h
O
N
24 examples

1
Tetrahedron Letters
Facile Synthesis of Diphenylmethyl Esters from 2-Diphenylmethoxypyridine using
Catalytic Boron Trifluoride·Diethyl Etherate
Minh Thanh Laa,b , Hee-Kwon Kim a,b 
a Department of Nuclear Medicine, Molecular Imaging & Therapeutic Medicine Research Center, Chonbuk National University Medical School and Hospital,
Jeonju, 54907, Republic of Korea
b Research Institute of Clinical Medicine of Chonbuk National University-Biomedical Research Institute of Chonbuk National University Hospital, Jeonju,
54907, Republic of Korea
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A practical method for the direct preparation of diphenylmethyl (DPM) esters from 2-
diphenylmethoxypyridine is described. The reaction was readily performed in the presence of a
catalytic amount of boron trifluoride-diethyl etherate at room temperature. Using this reaction
protocol, various carboxylic acids were converted to DPM esters with high yields. This method
is highly effective for the protection of carboxylic acids and the synthesis of DPM esters, and
offers a promising approach for facile esterification of a variety of carboxylic acids.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Esterification
Diphenylmethyl ester
Carboxylic acid
Protecting groups
1. Introduction
Thus, the discovery of a novel efficient synthetic method
using inexpensive starting materials under mild conditions for
Protection and deprotection of carboxylic acids is one of the
most popular methodologies in organic synthesis. The
diphenylmethyl (DPM) group is one of the commonly used
protecting groups for carboxylic acids and hydroxyl groups. The
cleavage of DPM ester is known to be readily achieved under
acidic condition using acetic acid, trifluoroacetic acid, and
hydrofluoric acid, in the presence of hydrazine, or AlCl3 or by
hydrogenation with Pd.1-7 These various options for removal of a
DPM group provide an advantage in multistep synthesis. Thus,
the DPM group has been used in a wide range of organic
reactions and the syntheses of various compounds including
enzyme inhibitors, antibacterial agents, leukotriene receptor
antagonists, and amphiphilic chelators.8-11
DMP ester formation is still a challenge. Dudley and co-workers
reported a reaction protocol using 2-benzyloxy-1-methylpyridine
to prepare benzyl esters.17 In this method, 2-benzyloxy-1-
methylpyridine
was
converted
to
2-benzyloxy-1-
methylpyridinium triflate salt by MeOTf. In the reaction
mechanism, the oxypyridinium salt serves as a key intermediate
to produce benzyl esters. However, this method required two
steps and reaction temperatures as high as 83 °C. Moreover it
took one day to complete the reaction. However, from the
reactions using 2-benzyloxy-1-methylpyridine, we hypothesized
that 2-diphenyloxy-1-methylpyridine, which contains groups
more bulky than the benzyl group, can be used as a starting
material for the preparation of DPM esters with the employment
of novel catalytic reagents. To the best of our knowledge, a
synthetic method for the direct preparation of DPM ester from 2-
diphenylmethoxypyridine has not been reported. We predicted
that a Lewis acid reagent could serve as a key reagent for DPM
esterification because of the versatility of Lewis acids as catalysts
in several organic reaction.18-20 Here, we report the novel facile
synthesis of DPM esters from 2-diphenylmethoxypyridine and
various carboxylic acids using a catalytic amount of Lewis acid.
Several synthetic methods for DPM ester have been reported.
The preparation of DPM esters can be performed by the
treatment of benzophenone hydrazine in the presence of iodine
and acetic acid, in the presence of PhI(OAc)2,12 or in the presence
of peracetic acid.13 DPM esters was also produced from
diazodiphenylmethane in acetone.14 The employment of
(Ph2CHO)3PO and trifluoroacetic acid has also been shown to
protect esters with DPM groups.15 However, these conditions use
expensive reagents and require strong acidic conditions which
can restrict the scope of the esterification. In addition, metal
2. Result and Discussion
catalyst-mediated methods using
5
mol% MoO2Cl2
The overall equation for the preparation of DPM ester from
diphenylmethyl alcohol is shown in Scheme 1. In the first step, 2-
diphenylmethoxypyridine was prepared using a modification of
the previously reported method.21 The reaction of diphenylmethyl
(molybdenum(VI) dichloride dioxide) in the presence of benzoic
anhydride have been reported for synthesis of DMP ester, but the
reaction took more than one day to complete.16
———
 Corresponding author. Tel.: Tel: +82 63 250 2768; Fax: +82 63 255 1172; e-mail: hkkim717@jbnu.ac.kr (H. Kim)

2
Table 1. Screening of catalysts for DMP esterificationa
alcohol with 2-chloropyridine in the presence of potassium
hydroxide and 18-crown-6 produced 2-diphenylmethoxypyridine
(compound 2) with 93% yield. In the initial study, we used the
previously reported synthetic method using oxypyridinium salt to
prepare DPM ester. Compound 2 was treated with MeOTf to
Base
O
or
O
Lewis acid
OH
+
O
O
N
DCE, rt, 6h
induce
alkylation
yielding
2-diphenylmethoxy-1-
2
5
4
methylpyridinium triflate, which can be used in situ for reaction
with carboxylic acids. Several reagents including K2CO3, and
Et3N were employed for the reaction of 2-diphenylmethoxy-1-
methylpyridinium triflate, according to the previous reported
procedure.22 However, no advantage was observed during the
reaction (17 % for K2CO3 and 18% for Et3N).
Entry
Base or Lewis acid
K2CO3
time
6
Temp.
r.t.
Yieldb (%)
1
2
3
4
5
6
7
8
9
4
3
NRc
NRc
8
Triethylamine
NaHCO3
DBU
6
r.t.
6
r.t.
6
r.t.
TiCl4
6
r.t.
O
N
AlCl3
6
r.t.
7
MeOTf
OTf
3
SnCl4
6
r.t.
9
Cl
N
O
KOH, 18-C-6
110 oC, 15h
O
OH
OH
BF3∙OEt2
None
6
r.t.
95
NRc
R
O
N
4
base
OH
R
4
1
2
6
r.t.
catalyst
a Reaction conditions: compound 2 (1.5 mmol), carboxylic acid
O
O
(1.0 mmol), Bases or Lewis acids (0.2 mmol), DCE (4 mL), 6 h
b
Isolated yield after purification of flash column
5
chromatography.
c No reaction.
Scheme 1. Synthetic route to diphenylmethyl esters.
Table 2. Screening of solvent for in situ esterification a
Thus, a novel reaction protocol for the treatment of carboxylic
acids with 2-diphenylmethoxypyridine was explored to achieve
better direct transformation of carboxylic acids to DPM esters
without the formation of oxypyridinium salt. Benzoic acid was
used as the substrate to select the initial optimal reaction
condition for the synthesis of DPM ester. The screening of
reaction condition was carried out with a series of organic
reagents, including bases and Lewis acids to find a better
catalytic reagent for DPM ester formation. In this study, the
esterification reaction of benzoic acid was performed in the
presence of 1.0 equiv. of compound 2, 1.0 equiv. of benzoic acid
and 0.2 equiv. of either Lewis acid or base in dichloroethane
(DCE). After reaction for 6 h, the effect of each reagent on
esterification was measured. Our initial screening results suggest
that the catalytic activities of the organic reagents varied
significantly according to catalyst type, as shown in Table 1. The
catalytic activities of the bases K2CO3, Et3N, NaHCO3, and DBU
were very low (0-5% yield). Addition of Lewis acids such as
TiCl4, AlCl3, and SnCl4, resulted in a small increase in DPM
esterification, but yield remained less than 10%. Finally, the
screening reaction experiment with BF3∙OEt2 was investigated.
Unexpectedly, the conversion yield for the desired product was
enhanced to 95%, suggesting that direct in situ reaction with
BF3∙OEt2 is the most effective for DPM esterification.
O
O
BF3 OEt2
OH
+
O
O
N
Solvent, rt, 6h
2
4
5
Entry
Lewis acids
(equiv)
Solvent
Temp.
Yieldb
(%)
21
1
2
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.2)
BF3∙OEt2 (0.05)
BF3∙OEt2 (0.1)
BF3∙OEt2 (1.0)
THF
MeCN
1,4-dioxane
Toluene
CH2Cl2
DCE
r.t.
r.t
27
NRc
3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
4
58
5
92
6
95
7d
8d
9e
DCE
41
DCE
95
DCE
78
a Reaction conditions: compound 2 (1.5 mmol), carboxylic acid
Next, the effect of solvent choice on the reaction was
explored. Reactions performed with BF3∙OEt2 in solution with
THF, MeCN, 1,4-dioxane, and toluene led to low yield synthesis
of DPM ester. When the reaction was carried out in toluene, the
conversion yield for the desired product increased to 58%, but
still unsatisfactory. However, when chlorinated solvents such as
dichloromethane (DCM) and dichloroethane (DCE) were
employed in the reaction, the yield of the desired DMP ester
increase significantly (92% for dichloromethane, and 95% for
dichloroethane; Table 2). The solvent screening results suggest
that DCE is the most suitable solvent for the reaction.
(1.0 mmol), Lewis acids (0.2 mmol), solvent (4 mL), 6 h
b
Isolated yield after purification of flash column
chromatography.
c No reaction.
d Reaction conducted for 24 h.
e Reaction conducted for 30 min.
equiv) in DPM esterification. Reactions performed using 1.0
equiv of BF3∙OEt2 resulted in synthesis of the desired DPM ester
with 95% yield in 30 min, while 0.05 equiv and 0.1 equiv of
BF3∙OEt2 resulted in synthetic yields of 41% and 78%,
respectively, after performing the reaction for 24 h (Table 2,
entries 7, 8 and 9). These results suggest that the loading amount
of BF3∙OEt2 significantly influences the synthetic yield of the
DPM ester compounds. Based on these conversion yields, a
Based on our primary results, we further tested the use of
BF3∙OEt2 in different amounts (0.05 equiv, 0.1 equiv and 1.0

3
catalytic amount of BF3∙OEt2 of 0.2 equiv (20 mol%) was
selected for the subsequent studies of the DPM ester synthesis,
although reactions containing 1.0 equiv of BF3∙OEt2 resulted in a
sufficient yield of the desired product in short time.
performed, and DPM ester with pyridine ring was obtained in
high yield (Table 3, entry 21).
Table 3. Scope of DPM esterification using 2-
diphenylmethoxypyridine a
The suitability and scope of the optimized reaction conditions
for the preparation of various other DPM esters were examined
(Table 3). These reactions were performed by mixing 1.0 equiv.
of carboxylic acid substrates with 1.5 equiv. of 2-
diphenylmethoxypyridine in the presence of 20 mol% of
BF3∙OEt2 in anhydrous DCE at room temperature. All DPM
esterification yields of benzoic acids were evaluated for 6 h.
First, benzoic acids containing electron-donating and electron-
withdrawing groups were used for the preparation of DPM esters.
Reactions of benzoic acids with electron-donating group
generated the corresponding DPM ester in high yield ranging
from 94% to 96% (Table 3, entries 2 and 3). Similarly, benzoic
acids with electron-withdrawing group were esterified with good
yield under the same conditions (Table 3, entries 4-8). This result
indicates that the reactions using catalytic amounts of BF3∙OEt2
work well with aromatic carboxylic acids to produce the
corresponding DPM ester with good yield.
Ph
Ph
O
N
2
Ph
O
O
BF3 OEt2 (20 mol%)
DCE, rt, 6h
O
Ph
R
OH
R
4
5
Entry Carboxylic acid
Product
Yieldb (%)
O
O
ODPM
95
OH
1
5a
4a
O
O
OH
ODPM
O
96
94
5b
5c
2
4b
4c
H3CO
H3CO
O
ODPM
OH
3
O
O
O
Next esterification of alkyl-substituted carboxylic acids was
examined. The reactions of alkyl-substituted carboxylic acids
with 2-diphenylmethoxypyridine in the presence of catalytic
BF3∙OEt2 produced the corresponding DPM ester in excellent
yields (Table 3, entries 9-13). Cyclohexanecarboxylic acid was
also employed as an alicyclic substrate to test the possibility of
this application. The desired product was easily obtained with
94% yield from the reaction of cyclohexanecarboxylic acid with
2-diphenylmethoxypyridine (Table 3, entry 14). In additions,
both alkenyl and alkynyl-substituted carboxylic acids were used
to assess the scope of the novel synthetic method for
esterification of unsaturated carboxylic acids. The reaction
protocol was proven useful for the synthesis of both alkene-
substituted and alkyne-substituted esters. These BF3∙OEt2
catalyzed reactions produced DPM esters in high yields with the
same method (96% for compound 5o and 94% for compound
5p). Interestingly, the result showed that the yields of reactions of
alkyl-substituted carboxylic acids were generally higher than
those of benzoic acids.
OH
OH
ODPM
ODPM
5d
5e
4
5
4d
4e
82
92
Cl
Cl
O
F
F
O
O
Br
Br
ODPM
ODPM
OH
5f
94
91
6
4f
O
O
OH
OH
5g
7
8
4g
O2N
O
O2N
O
O
ODPM
5h
5i
82
4h
O
O
O
9
10
11
ODPM
O
93
81
OH
4i
4j
O
ODPM 5j
OH
O
O
It is known that the steric effect of bulky groups usually
influences the reaction yield. Thus, carboxylic acids with more
bulky groups were used to explore the application of the new
catalytic protocol. When adamantine carboxylic acid containing a
hindered bulky group was used, the corresponding ester was
formed with 90% yield under the same conditions after 6 h
(Table 3, entry 17). The reaction of 3,5-dimethylbenzoic acid
also produced the desired product with 93% yield (Table 3, entry
18). Diphenylacetic acid, which contains two bulky benzene
groups, was also utilized for DPM esterification, and the reaction
yield was not significantly different from those of the other
phenyl carboxylic acids (Table 3, entry 19). These results clearly
demonstrate the importance of BF3∙OEt2 as an efficient catalyst
for the conversion of bulky carboxylic acids to DPM esters under
mild reaction conditions using 2-diphenylmethoxypyridine.
92
92
5k
ODPM
4k
4l
OH
O
O
5l
ODPM
12
OH
O
O
OH
ODPM
4m
5m
93
13
14
15
O
O
ODPM
O
OH
O
4n
4o
5n
5o
5p
94
96
ODPM
OH
OH
O
O
ODPM
4p
4q
16
17
18
94
O
O
The scope of utilization of catalytic BF3∙OEt2 was extended to
a heterocyclic acid compound. For example, the treatment of 2-
furoic acid containing oxygen with 2-diphenylmethoxypyridine
readily resulted in the successful synthesis of the corresponding
DPM ester with 92% yield in 6 h (Table 3, entry 20),
demonstrating that heterocyclic DPM compounds can be
obtained in high yields in the presence of BF3∙OEt2 under mild
conditions. In additions, reaction of picolinic acid was
ODPM
ODPM
5q
5r
90
93
OH
OH
O
O
4r
O
O
ODPM
OH
91
5s
4s
19

4
Scheme 2. Plausible reaction mechanism.
Table 3. (Contd.)
3. Conclusion
Entry Carboxylic acid
Product
Yieldb (%)
In summary,
a
novel facile BF3∙OEt2–catalyzed DPM
O
O
O
O
esterification of various carboxylic acids was developed.
Developments of novel reaction methods to generate DPM esters
are of particular importance due to their popular use as a
protecting group for carboxylic acids. In the present study,
BF3∙OEt2 was used as a catalyst for the reaction of carboxylic
acids with 2-diphenylmethoxypyridine for the production of the
desired DPM ester compounds with high yield. This method is
noteworthy because the formation of DPM esters from carboxylic
acid substrates was achieved under mild reaction conditions. Our
results suggest that the novel protocol using catalytic BF3∙OEt2 is
efficient and applicable to the protection of a variety of
carboxylic acids in a variety of organic syntheses.
20
21
4t
5t
ODPM
92
OH
OH
O
O
O
O
N
N
73
4u
5u
ODPM
OH
OH
88
22
23
4v
5v
ODPM
O
OH
O
ODPM
40
71
OH 4w
5w
NHBoc
NHBoc
HO
HO
O
H3C
O
H3C
ODPM
OH
Acknowledgments
H
O
H
O
CH3 CH3
CH3
CH3
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (2014R1A1A2057943).
24
CH3 CH3
CH3
CH3
H
H
H
H
HO
5x
4x
HO
a Reaction conditions: Carboxylic acid (1.0 mmol), compound 2
(1.5 mmol), BF3∙OEt2 (0.2 mmol), DCE (4 mL), rt for 6 h,
Isolated yield after purification of flash column
chromatography.
References and notes
1. Haslam, R. D.; Haworth, R. D.; Makinson. G. K. J. Chem.
Soc. 1961, 5153-5156.
b
2. Hiskey, R. G.; Smithwick, E. L. J. Am. Chem. Soc. 1967, 89,
Salicylic acid bearing OH group was employed in our reaction
method for the preparation of DPM esters. Reaction of salicylic
acid produced desired DPM ester in yield of 88% (Table 3, entry
22). However, reaction of 4-aminobenzoic acid bearing NH2
group gave DPM protected amine compound instead of DPM
ester. The result suggested that the carboxylic group was more
reactive to prepare DPM esters than the hydroxyl group, and the
amine showed to be the most reactive group which reacted with
2-diphenylmethoxypyridine more rapidly than carboxylic acid.
Besides, Boc-protected tyrosine was utilized for DPM
esterification, and the corresponding ester was prepared under the
same condition (Table 3, entry 23).
437-441.
3. Stelakatos, G. C.; Paganou, A.; Zervas, L. J. Chem. Soc. C.
1966, 1191-1199.
4. Hills, L. R.; Ronald, R. C. J. Org. Chem. 1985, 50, 470−473.
5. Hiskey, R. G.; Adams, J. B. J. Am. Chem. Soc. 1965, 87, 3967-
3973.
6. Ohtani, M.; Watanabe, F.; Narisada, M. J. Org. Chem. 1984,
49, 5271−5272.
7. De Bernardo, S.; Tengi, J. P.; Sasso, G. J.; Weigele, M. J.
Org. Chem. 1985, 50, 3457−3462.
Finally, 18β-glycyrrhetinic acid, which is more complex
biological carboxylic acid, was employed for DPM esterification
and the desired product was obtained (Table 3, entry 24).
8. Aoyama, Y.; Uenaka, M.; Kii, M.; Tanaka, M.; Konoike, T.;
Hayasaki-Kajiwara, Y.; Naya, N.; Nakajima, M. Bioorg. Med.
Chem. 2001, 9, 3065–3075.
Based on the present result, a plausible mechanism for this
reaction could be proposed, as shown in Scheme 2. In the first
step, boron trifluoride diethyl etherate (BF3∙OEt2) makes a
complex with 2-diphenylmethoxypyridine which can easily
undergo nucleophilic attack by carboxylic acid to yield the DPM
ester product. Then, 2-pyridone was separated from 2-pyridone-
BF3 complex and BF3∙OEt2 was given back to the reaction and
continuously catalyze DPM ester reaction.
9. Yamawaji, K.; Nomura, T.; Yasukata, T.; Uotani, K.; Miwa,
H.; Takeda, K.; Nishitani, Y. Bioorg. Med. Chem. 2007, 15,
6716–6732.
10. Marshall, W. S.; Goodson, T.; Cullinan, G. J.; Swanson-
Bean, D.; Haisch, K. D.; Rinkema, L. E.; Fleisch, J. H. J. Med.
Chem. 1987, 30, 682−689.
11. De Sa, A.; Prata, M. I. M. P.; Geraldes, C. F.G.C.; André, J.
P.. J. Inorg. Biochem. 2010, 104, 1051–1062.
O
12. Bywood, R.; Gallagher, G.; Sharma, G. K.; Walker, D. J.
Chem. Soc.Perkin Trans. 1. 1975, 2019-2022.
N
O
13. Micetich, R. G.; Malti, S. M.; Spevak, P.; Tanaka, M.;,
Yamazaki, T.; Ogawa, K. Synthesis. 1986, 4, 292-296.
O
B
F
N
O
B
F
F
F
F
F
14. Kametani, H.; Sekine, H.; Hondo, T. Chem. Pharm. Bull.
1982, 30, 4545-4547.
O
N
O
H
R
OH
15. Lapatsanis, L. Tetrahedron Lett. 1978, 19, 4697–4698.
N
O
B
F
O
H
O
16. Chen, C.-T.; Kuo, J.-H.; Ku, C.-H.; Weng, S,-S,; Liu, C.-Y. J.
Org. Chem. 2005, 70, 1328−1339.
F
F
R
O

5
17. Tummatorn, J.; Albiniak, P. A.; Dudley, G. B. J. Org. Chem.
2007, 72, 8962−8964.
22. General procedure for the preparation of DPM esters (5a-
5x): To a solution of benzoic acid (0.12 g. 1.00 mmol) and 2-
diphenylmethoxypyridine (0.39 g, 1.50 mmol) in DCE (4 mL)
BF3∙OEt2 (0.028 g. 0.20 mmol) was added. The mixture was
stirred at room temperature for 6h. The reaction mixture was
extracted with ethyl acetate (2 x 10 mL), and then washed with
water (10 mL), followed by brine (10 mL). The organic layer was
dried over anhydrous sodium sulfate and concentrated under
reduced pressure. The resulting residue was then purified by flash
column chromatography on silica gel with hexane-CH2Cl2 as
eluent to afford the desired product 5a as a white solid (0.175 g,
95%).
18. Li, Y.; Xiong, Y.; Li, X. M.; Ling, X. G.; Huang, R. F.;
Zhang, X. H.; Yang, J. C. Green Chem. 2014, 16, 2976–2981.
19. Huang, R. F.; Zhang, X. H.; Pan, J.; Li, J. Q.; Shen, H.; Ling,
X. G.; Xiong, Y. Tetrahedron 2015, 71, 1540–1546.
20. Sheng, H.; Li, J. Q.; Liu, Q.; Pan, J.; Huang, R. F.; Xiong, Y.
J. Org. Chem. 2015, 80, 7212–7218.
21. Poon, K. W. C.; Dudley, G. B. J. Org. Chem. 2006, 71,
3923−3927.

6
Highlights
- Direct DPM esterification from carboxylic acids.
- Catalytic boron trifluoride-diethyl etherate was
used.
- DPM esterification at room temperature.
- Easy to perform in one step.