References and Notes
<A NAME="RU04008ST-1A">1a</A>
Kocienski PJ.
Protecting
Groups
Thieme;
New York:
1994.
<A NAME="RU04008ST-1B">1b</A>
Wuts PGM.
Greene TW.
Protective Groups in Organic Synthesis
4th
ed.:
Wiley-Interscience;
New York:
2006.
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Okano K.
Tokuyama H.
Fukuyama T.
J. Am. Chem. Soc.
2006,
128:
7136
<A NAME="RU04008ST-2B">2b</A>
Okano K.
Tokuyama H.
Fukuyama T.
Chem.
Asian J.
2008,
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296
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Chong R.
Gray RW.
King RR.
Whalley WB.
J. Chem.
Soc. D
1970,
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Kiso Y.
Isawa H.
Kitagawa K.
Akita T.
Chem. Pharm.
Bull.
1978,
26:
2562 ;
treatment of O-benzyltyrosine with TFA
yielded tyrosine and undesired 3-benzyltyrosine in the ratio of
57:43
<A NAME="RU04008ST-4B">4b</A>
Kiso Y.
Ukawa K.
Nakamura S.
Ito K.
Akita T.
Chem.
Pharm. Bull.
1980,
28:
673 ;
a combination of thioanisole with TFA was effective to debenzylation
of O-benzyltyrosine without formation
of 3-benzyltyrosine
<A NAME="RU04008ST-5A">5a</A> Various
cation scavengers on debenzylation of O-benzyltyrosine
in TFA were investigated. It was reported that pentamethylbenzene
increased the rate of deprotection most efficiently among other
scavengers such as thioanisole, anisole, 1,3-dimethoxybenzene, 1,2,3-dimethoxybenzene,
m-xylene, 1,2,3-trimethylbenzene, and
1,2,3,4-tetramethylbenzene. Isolation and characterization of benzylpentamethylbenzene
generated by trapping of benzyl cation were also reported. See:
Yoshino H.
Tsuchiya Y.
Saito I.
Tsujii M.
Chem.
Pharm. Bull.
1987,
35:
3438
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Yoshino H.
Tsujii M.
Kodama M.
Komeda K.
Niikawa N.
Tanase T.
Asakawa N.
Nose K.
Yamatsu K.
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Khorphueng P.
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Physical and Spectroscopic
Data for 2-Benzyl-4,5-dimethoxyphenol (5)
Colorless
oil; R
f
= 0.43
(hexanes-EtOAc, 1:1). ¹H NMR (600 MHz,
CDCl3): δ = 7.32-7.26 (m,
2 H), 7.25-7.17 (m, 3 H), 6.66 (s, 1 H), 6.45 (s, 1 H),
4.37 (s, 1 H), 3.93 (s, 2 H), 3.83 (s, 3 H), 3.80 (s, 3 H). ¹³C
NMR (151 MHz, CDCl3):
δ = 148.5,
147.7, 143.1, 140.0, 128.7, 128.4, 126.4, 117.6, 114.5, 101.3, 56.6,
55.9, 36.0. IR (neat): 3445, 2932, 1614, 1520, 1452, 1416, 1205,
1109, 997, 669 cm-¹. HRMS (EI+): m/z calcd for C15H16O3 [M+]:
244.1099; found: 244.1089.
<A NAME="RU04008ST-8">8</A>
Boron trichloride (1.0 M in CH2Cl2)
was purchased from Aldrich Chemical Company and was used as supplied. Pentamethylbenzene
was purchased from Tokyo Chemical Industry Co., Ltd. and was used
as supplied.
Typical Procedure (Table
3, Entry 20)
To a stirred solution of N-[3-(benzyloxy)phenyl]-2,2,2-trifluoroethanamide
(1.61 g, 5.44 mmol, 1.0 equiv) and pentamethylbenzene (2.42 g, 16.3
mmol, 3.0 equiv) in dry CH2Cl2 (27.0 mL) was
added BCl3 (1.0 M in CH2Cl2, 10.9 mL,
2.0 equiv) dropwise over 10 min via syringe at -78 ˚C. After
15 min, TLC indicated complete consumption of the starting material.
The reaction was quenched with CHCl3-MeOH (10:1,
20 mL) at -78 ˚C, and the resulting mixture was
warmed to r.t. The excess organic solvents were removed under reduced
pressure. The residue was purified by column chromatography on SiO2 (eluent:
hexanes to hexanes-EtOAc = 1:1,
gradient) to provide 2,2,2-trifluoro-N-(3-hydroxyphenyl)ethanamide
(1.10 g, 5.36 mmol, 99%).
Physical
and Spectroscopic Data for 2,2,2-trifluoro-
N
-(3-hydroxyphenyl)ethanamide
Colorless
crystals; R
f
= 0.54
(hexanes-EtOAc, 1:1); mp 128.4-129.6 ˚C
(hexanes-EtOAc). ¹H NMR (600 MHz, CDCl3): δ = 7.81
(br s, 1 H), 7.37-7.34 (m, 1 H), 7.25 (dd, 1 H, J = 7.8, 7.8
Hz), 6.95 (dd, 1 H, J = 7.8,
1.2 Hz), 6.74 (dd, 1 H, J = 7.8,
2.4 Hz), 5.22 (s, 1 H). ¹³C NMR (100
MHz, CD3OD): δ = 158.9, 156.5 (q, ²
J
C-F = 37
Hz), 138.5, 130.6, 117.3 (q, ¹
J
C-F = 285
Hz), 113.8, 113.1, 109.2. IR (neat): 3312, 1715, 1614, 1566, 1495,
1456, 1188 cm-¹. HRMS (FAB+): m/z calcd for C8H7NO2F3 [M + H+]:
206.0429; found: 206.0427.
