Ti(III) chloride: A novel reagent for the chemoselective deprotection of tetrahydropyranyl ethers

Article abstract of DOI:10.1055/s-2004-834949

Ti(III) chloride was found to be an effective catalyst for the deprotection of tetrahydropyranyl ethers of alcohols and phenols at an ambient temperature. Allyl ether, benzyl ether, tert-butyl-diphenylsilyl (TBDPS) ether, p-toluenesulfonate ester and isomerizable double bond were found to be compatible under the reaction conditions.

Full text of DOI:10.1055/s-2004-834949

PAPER
71
Ti(III) Chloride: A Novel Reagent for the Chemoselective Deprotection of
Tetrahydropyranyl Ethers
Ti(III) Chloride:
N
b
ovel
R
eagen
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t
for Chemos
a
e
lective
D
eprotec
S
tion of Tetrah
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ydropyrany
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l
Ethers wal, Sandip K. Nayak*
Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai–400085, India
Fax +91(22)25505151; E-mail: sknayak@magnum.barc.ernet.in
Received 25 November 2003; revised 5 October 2004
ry for the synthesis of alkenes/1,2-diols from carbonyl
compounds.7 We have also demonstrated single electron
transfer mediated deprotection of propargyl and allyl
ethers and tosylate esters with LVT reagents.8 Amongst
Abstract: Ti(III) chloride was found to be an effective catalyst for
the deprotection of tetrahydropyranyl ethers of alcohols and phe-
nols at an ambient temperature. Allyl ether, benzyl ether, tert-butyl-
diphenylsilyl (TBDPS) ether, p-toluenesulfonate ester and
isomerizable double bond were found to be compatible under the re- the high valent titanium reagents [Ti(III) and Ti(IV)], Mu-
action conditions.
kaiyama et al. have extensively used TiCl4 as a Lewis acid
in directed Aldol condensation.9 Similarly, aqueous TiCl3
Key words: allyl ether, benzyl ether, chemoselectivity, depyranyl-
ation, titanium(III) chloride, TBDPS ether, p-toluenesulfonate ester
solution has been used for the reductive hydrodimeriza-
tion of aromatic carbonyl compounds.10 However, use of
anhydrous TiCl3 as a Lewis acid is rather limited.11,12
A
recent report on the hydrolysis of isopropylidene acetals
with ytterbium triflate13 prompted us to investigate the
role of anhydrous TiCl3 as a Lewis acid in the deprotec-
tion of tetrahydropyranyl ethers.
Protection and deprotection of alcohols and phenols play
a key role in the synthesis of polyfunctional organic mol-
ecules, and a variety of protecting groups have been de-
veloped for this purpose.1 Tetrahydropyranyl (THP)
ethers are widely used for the protection of alcohols be-
cause of their easy preparation and stability under a vari-
ety of reaction conditions involving alkaline media,
Grignard reagents, organolithiums, metal hydrides and
oxidative reagents.2 The most common methods for the
deprotection of the THP ethers are acid catalyzed hydro-
lysis in protic solvents.3 However, these methodologies
are inappropriate in the case of acid sensitive molecules.
Many other methods have been reported for the deprotec-
In a model reaction, the THP ether 1 of 1-octanol prepared
by following the standard procedure [dihydropyran
(DHP) and pyridinium p-toluenesulfonate (PPTS) in
CH2Cl2], was exposed to Ti(III) chloride in MeCN at
room temperature. The reaction proceeded smoothly (2 h)
to afford 1-octanol in 97% yield (Table 1, entry 1). In the
above reaction, 20 mol% of TiCl3 was required for com-
plete conversion of the substrate. Similarly, the THP
ethers of 2-octanol and 1-hexadecanol (2 and 3, respec-
tively) yielded the corresponding alcohols in good yields
(Table 1, entries 2 and 3). The THP ethers 4–8 of phenols
also underwent facile deprotection in good yields at room
temperature (Table 1, entries 4–8). From the results in
Table 1, it is clear that with protected phenolic substrates,
deprotection was more facile when electron-withdrawing
substituents are attached to the aromatic ring. Further, var-
ious functional groups such as ketone, ester, nitro, and
methylenedioxy were found to be compatible under the
reaction conditions.
5
tion of the THP ethers.4 More recently, In(OTf)3 and
6
AlCl3 have been described as efficient reagents for the
deprotection of the THP ethers. Many of these procedures
have several drawbacks including elevated temperatures,
longer reaction times and use of expensive and toxic re-
agents. Therefore, the developments of new reagents that
are more efficient and better yielding are desirable. In this
communication, we report an efficient method for the se-
lective removal of the THP ethers using catalytic amount
of Ti(III) chloride at an ambient temperature (Scheme 1).
Earlier, several variety of low-valent titanium reagents
[Ti(0) and Ti(II), prepared by the reduction of TiCl3/TiCl4
with different reducing metals (Li, K, Mg, Zn) and sol-
vents (THF, DME)] have been developed in our laborato-
Investigations were also carried out with the THP ethers
of allylic alcohols, which are vulnerable to isomerization.
To our satisfaction, the THP ethers 9 and 10 of the Baylis–
Hillman adducts also underwent smooth deprotection to
yield the corresponding allylic alcohols in moderate
yields (Table 1, entries 9 and 10) without any isomeriza-
tion.
TiCl3 (20 mol%)
R1OTHP/R2O(CH2)nOTHP
R1 = alkyl, aryl
R1OH/R2O(CH2)nOH
CH3CN, r.t.
In presence of other hydroxyl protecting functionalities,
chemoselectivity for the depyranylation reaction using
anhydrous TiCl3 was also investigated. It was observed
that the reaction proceeded smoothly even in the presence
of sensitive functional groups such as TBDPS and tosyl
(Table 1, entries 11 and 12). The O-allyl and O-benzyl
groups, which get cleaved with the LVT reagents, were
R
2 = allyl, benzyl, TBDPS, THP, tosyl
Scheme 1
SYNTHESIS 2005, No. 1, pp 0071–0074
x
x
.
x
x
.2
0
0
5
Advanced online publication: 03.12.2004
DOI: 10.1055/s-2004-834949; Art ID: Z16603SS
© Georg Thieme Verlag Stuttgart · New York

72
A. Semwal, S. K. Nayak
PAPER
Table 1 Ti(III) Chloride-Induced Deprotection of Tetrahydropyranyl Ether of Alcohols/Phenols
Entry
Substrate
Product
Time (h)
Yield (%)
1
2
3
4
CH3(CH2)7OTHP
CH3(CH2)5CH(OTHP)CH3
CH3(CH2)15OTHP
(1)
(2)
(3)
(4)
CH3(CH2)7OH
CH3(CH2)5CH(OH)CH3
CH3(CH2)15OH
2
3
97
96
85
82
16
16
5
6
7
8
(5)
(6)
(7)
(8)
3
2.5
1
72
88
69
81
24
9
(9)
6
7
46
66
10
(10)
11
12
13
14
15
CH3CH(OTBDPS)(CH2)9OTHP
TsO(CH2)8OTHP
(11)
(13)
(14)
(15)
(16)
CH3CH(OTBDPS)(CH2)9OH (12)
TsO(CH2)8OH
6
6
4
6
5
63
80
91
84
91
AllylO(CH2)8OTHP
AllylO(CH2)8OH
BnO(CH2)8OTHP
BnO(CH2)8OH
CH3CH(OTHP)(CH2)9OTHP
CH3CH(OH)(CH2)9OH (17)
also unaffected by TiCl3 (Table 1, entries 13 and 14).
However, the reagent did not show any selectivity be-
tween the THP protected primary and secondary carbinol
functionalities. For example, the substrate 16 furnished
the diol 17 in excellent yield (Table 1, entry 15).
Preparation of the THP Ethers; General Procedure
A solution of the alcohol/phenol (30 mmol), PPTS (1.51 g, 20
mol%) and DHP (2.945 g, 35 mmol) in anhyd CH2Cl2 (100 mL) was
stirred at r.t. till the reaction was complete (monitored by TLC).
Then the reaction mixture was washed with aq 10% NaHCO3, the
organic layer separated and washed with water and brine, and dried
(Na2SO4). Removal of solvent followed by column chromatography
(silica gel) of the crude product using EtOAc–petroleum ether (bp
60–80 °C) as the eluent yielded the respective THP ethers in 54–
84% yields. The THP ethers 1–9, 13 and 15 were characterized from
their known spectral data.
In conclusion, Ti(III) chloride-mediated depyranylation
of alcohols and phenols has been demonstrated for the
first time. Chemoselective deprotection of the THP ethers
even in the presence of TBDPS ether, p-toluenesulfonate
ester, allyl ether and benzyl ether, was also observed.
Thus, the present protocol offers several advantages in-
cluding compatibility with various sensitive and hydroxyl
protecting functional groups, use of catalytic amount of
the reagent, high yields and experimental simplicity,
which makes it a useful addition to the existing synthetic
methodologies.
1-(2-Tetrahydropyranyloxy)octane (1)15a
Yield: 74%; colorless viscous oil.
IR (neat): 2927, 2870, 1466, 869, 815 cm–1.
1H NMR (CDCl3): d = 0.86 (t, J = 6.6 Hz, 3 H), 1.09–1.97 (m, 18
H), 3.25–3.97 (m, 4 H), 4.56 (br s, 1 H).
2-(2-Tetrahydropyranyloxy)octane (2)15a
Yield: 75%; colorless viscous oil.
IR (neat): 2930, 2857, 1466, 870, 814 cm–1.
The FT-IR spectra were scanned with a Nicolet FT-IR spectropho-
tometer (model 410). The H NMR spectra were recorded with a
1
Bruker AC 200 (200 MHz) spectrometer. Mass spectra were record-
ed with a Fisons MD 800 mass spectrometer. Microanalyses were
performed with a Carlo Erba elemental analyzer (model 1110). All
reactions were carried out under an Ar atmosphere. MeCN and
CH2Cl2 were distilled over P2O5. Ti(III) chloride and 3,4-dihydro-
2H-pyran (DHP) were purchased from Aldrich Chemical Co. and
used as such. The Baylis–Hillman adducts were prepared following
the literature procedure.14
1H NMR (CDCl3): d = 0.85 (t, J = 6.6 Hz, 3 H), 1.08–1.98 (m, 18
H), 3.25–3.93 (m, 4 H), 4.55 (br s, 1 H).
1-(2-Tetrahydropyranyloxy)hexadecane (3)15b
Yield: 80%; colorless viscous oil.
IR (neat): 2925, 2853, 1466, 869, 816 cm–1.
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Ti(III) Chloride: Novel Reagent for Chemoselective Deprotection of Tetrahydropyranyl Ethers
73
1H NMR (CDCl3): d = 0.87 (t, J = 6.6 Hz, 3 H), 1.24 (br s, 28 H),
1.44–1.94 (m, 6 H), 3.30–3.56 (m, 2 H), 3.62–3.94 (m, 2 H), 4.57
(br s, 1 H).
10-[(1,1-Dimethylethyl)diphenylsilyloxy]-1-(2-tetrahydropyra-
nyloxy)undecane (11)
Yield: 84%; colorless viscous oil.
IR (neat): 3070, 2930, 2856, 1472, 1430, 1114, 1035 cm–1.
4-(2-Tetrahydropyranyloxy)toluene (4)15c
Yield: 79%; colorless viscous oil.
IR (neat): 2944, 2873, 970, 921, 872, 818 cm–1.
1H NMR (CDCl3): d = 1.54–2.16 (m, 6 H), 2.34 (s, 3 H), 3.52–3.70
(m, 1 H), 3.88–4.08 (m, 1 H), 5.43 (br s, 1 H), 6.80–7.18 (m, 4 H).
1H NMR (CDCl3): d = 0.99–1.08 (m, 12 H), 1.12–1.75 (m, 22 H),
3.25–3.54 (m, 2 H), 3.62–3.90 (m, 3 H), 4.57 (m, 1 H), 7.35–7.41
(m, 6 H), 7.65–7.69 (m, 4 H).
13C NMR (CDCl3): d = 19.1, 19.6, 23.1, 25.1, 25.4, 26.2, 26.9, 29.4,
29.7, 30.7, 39.3, 62.1, 67.5, 69.5, 98.6, 127.3, 129.3, 134.5, 134.8,
135.8.
4-(2-Tetrahydropyranyloxy)acetophenone (5)15d
Yield: 81%; low-melting solid.
EI–MS: 510 [M+].
IR (CHCl3): 2948, 2871, 1714, 916, 841 cm–1.
Anal. Calcd for C32H50O3Si: C, 75.22; H, 9.87. Found: C, 75.09; H,
9.85.
1H NMR (CDCl3): d = 1.46–2.07 (m, 6 H), 2.55 (s, 3 H), 3.50–3.68
(m, 1 H), 3.75–3.91 (m, 1 H), 5.51 (br s, 1 H), 7.07 (d, J = 8.8 Hz,
2 H), 7.91 (d, J = 8.8 Hz, 2 H).
1-(4-Methylphenylsulfonyloxy)-8-(2-tetrahydropyranyloxy)oc-
tane (13)8c
Yield: 78%; colorless viscous oil.
IR (neat): 2938, 2858, 1363, 1177, 1033, 966, 815 cm–1.
Ethyl 4-(2-Tetrahydropyranyloxy)benzoate (6)15e
Yield: 80%; colorless viscous oil.
1H NMR (CDCl3): d = 1.12–1.33 (m, 8 H), 1.41–1.92 (m, 10 H),
2.42 (s, 3 H), 3.23–3.53 (m, 2 H), 3.65–3.89 (m, 2 H), 3.98 (t, J =
6.5 Hz, 2 H), 4.54 (br s, 0.8 H), 4.93 (br s, 0.2 H), 7.32 (d, J = 8.1
Hz, 2 H), 7.76 (d, J = 8.3 Hz, 2 H).
IR (CHCl3): 2950, 1694, 1608, 852, 773 cm–1.
1H NMR (CDCl3): d = 1.37 (t, J = 7.1 Hz, 3 H), 1.50–2.11 (m, 6 H),
3.25–3.68 (m, 1 H), 3.75–3.95 (m, 1 H), 4.34 (q, J = 7.1 Hz, 2 H),
5.49 (br s, 1 H), 7.06 (d, J = 8.8 Hz, 2 H), 7.98 (d, J = 8.8 Hz, 2 H).
1-Allyloxy-8-(2-tetrahydropyranyloxy)octane (14)
Yield: 81%; colorless viscous oil.
IR (neat): 2939, 2867, 1441, 1125, 1075, 1035, 967 cm–1.
1H NMR (CDCl3): d = 1.17–1.92 (m, 18 H), 3.25–3.58 (m, 4 H),
3.65–4.01 (m, 4 H), 4.55 (br s, 0.2 H), 4.93 (br s, 0.8 H), 5.08–5.33
(m, 2 H), 5.75–6.01 (m, 1 H).
4-(2-Tetrahydropyranyloxy)nitrobenzene (7)15a
Yield: 65%; low melting solid.
IR (CHCl3): 2948, 1495, 1173, 849, 823 cm–1.
1H NMR (CDCl3): d = 1.42–2.12 (m, 6 H), 3.44–3.94 (m, 2 H), 5.52
(br s, 1 H), 7.11 (d, J = 9.3 Hz, 2 H), 8.17 (d, J = 9.3 Hz, 2 H).
13C NMR (CDCl3): d = 19.2, 25.1, 25.8, 29.0, 29.3, 30.2, 61.6, 62.2,
3,4-Methylenedioxy-1-(2-tetrahydropyranyloxy)benzene (8)15f
Yield: 60%; colorless viscous oil.
IR (CHCl3): 2945, 2875, 967, 931, 872, 819 cm–1.
67.1, 69.9, 71.3, 93.9, 98.2, 116.0, 134.6.
EI–MS: 270 [M+].
1H NMR (CDCl3): d = 1.44–2.10 (m, 6 H), 3.46–3.70 (m, 1 H),
3.78–4.04 (m, 1 H), 5.25 (br s, 1 H), 5.99 (s, 2 H), 6.41–6.78 (m, 3
H).
Anal. Calcd for C16H30O3: C, 71.05; H, 11.19. Found: C, 70.92; H,
11.17.
1-Benzyloxy-8-(2-tetrahydropyranyloxy)octane (15)15h
Yield: 76%; colorless viscous oil.
IR (neat): 2937, 2855, 1454, 1123, 1034, 968 cm–1.
1H NMR (CDCl3): d = 1.08–1.30 (m, 8 H), 1.32–1.77 (m, 10 H),
3.26–3.85 (m, 6 H), 4.47 (s, 2 H), 4.55 (br s, 0.7 H), 4.94 (br s, 0.3
H), 7.19–7.36 (m, 5 H).
Methyl 2-Methylene-3-phenyl-3-(2-tetrahydropyranyloxy)pro-
panoate (9)15g
Yield: 75%; colorless viscous oil.
IR (neat): 2949, 2871, 1727, 1352, 911, 870, 817 cm–1.
1H NMR (CDCl3): d = 1.37–1.97 (m, 6 H), 3.34–3.94 (m, 5 H), 4.51
(m, 0.5 H), 4.77 (m, 0.5 H), 5.63 (m, 1 H), 5.97 (br s, 0.5 H), 6.09
(br s, 0.5 H), 6.32 (s, 0.5 H), 6.39 (s, 0.5 H), 7.16–7.50 (m, 5 H).
1,10-Bis(2-tetrahydropyranyloxy)undecane (16)
Yield: 75%; colorless viscous oil.
IR (neat): 2934, 2854, 1441, 1354, 1200, 1123, 1035 cm–1.
1H NMR (CDCl3): d = 1.01–1.92 (m, 31 H), 3.29–3.56 (m, 3 H),
3.62–3.92 (m, 4 H), 4.52–4.73 (m, 1.4 H), 4.95 (br s, 0.6 H).
13C NMR (CDCl3): d = 18.6, 19.3, 19.7, 21.2, 25.2, 25.9, 29.2, 30.4,
30.9, 36.2, 37.2, 61.7, 61.9, 62.3, 67.2, 70.6, 73.5, 94.0, 95.1, 98.2,
98.3.
Methyl 2-Methylene-3-(3,4-methylenedioxyphenyl)-3-(2-tet-
rahydropyranyloxy)propanoate (10)
Yield: 54%; colorless viscous oil.
IR (neat): 2948, 2872, 1725, 1641, 996, 938, 896, 822 cm–1.
1H NMR (CDCl3): d = 1.34–1.96 (m, 6 H), 3.39–3.92 (m, 5 H), 4.52
(m, 0.5 H), 4.71 (m, 0.5 H), 5.53 (m, 1 H), 5.92 (m, 2.5 H), 6.09 (br
s, 0.5 H), 6.30 (s, 0.5 H), 6.37 (s, 0.5 H), 6.64–6.96 (m, 3 H).
13C NMR (CDCl3): d = 18.4, 24.6, 29.7, 50.9, 61.2, 62.1, 72.8, 74.4,
93.7, 96.2, 100.3, 107.2, 121.4, 123.5, 124.1, 132.3, 133.5, 140.2,
140.9, 146.3, 146.6, 146.8, 146.9, 165.4, 165.5.
EI–MS: 356 [M+].
Anal. Calcd for C21H40O4: C, 70.72; H, 11.32. Found: C, 70.61; H,
11.31.
EI–MS: 320 [M+].
Depyranylation; General Procedure
To a solution of the THP ethers (2 mmol) in anhyd MeCN (10 mL),
anhyd TiCl3 (0.062 g, 20 mol%) was added and the mixture stirred
at r.t. for the time specified in Table 1. Then the reaction mixture
was concentrated in vacuo, taken in Et2O and the extract was suc-
cessively washed with 10% aq NaHCO3, water and brine, and dried
Anal. Calcd for C17H20O6: C, 63.72; H, 6.30. Found: C, 63.54; H,
6.28.
Synthesis 2005, No. 1, 71–74 © Thieme Stuttgart · New York

74
A. Semwal, S. K. Nayak
PAPER
(Na2SO4). Removal of solvent followed by column chromatography
(silica gel) of the crude product using EtOAc–petroleum ether (bp
60–80 °C) as the eluent afforded the respective alcohols/phenols in
46–97% yields. The products obtained from the substrates 1–10 and
13–15 were characterized from their known spectral data.
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10-[(1,1-Dimethylethyl)diphenylsilyloxy]undecan-1-ol (12)
Yield: 63%; colorless viscous oil.
IR (neat): 3365, 3070, 2929, 2855, 1463, 1427, 1111, 1059 cm–1.
1H NMR (CDCl3): d = 1.01–1.07 (m, 12 H), 1.12–1.75 (m, 16 H),
1.94 (br s, 1 H), 3.63 (t, J = 6.5 Hz, 2 H), 3.75–3.90 (m, 1 H), 7.26–
7.49 (m, 6 H), 7.65–7.78 (m, 4 H).
13C NMR (CDCl3): d = 19.2, 23.3, 25.1, 25.7, 26.9, 27.1, 29.5, 32.7,
39.4, 62.8, 69.5, 127.3, 129.3, 134.5, 134.9, 135.9.
EI–MS: 426 [M+].
Anal. Calcd for C27H42O2Si: C, 75.98; H, 9.93. Found: C, 75.75; H,
9.90.
1,10-Dihydroxyundecane (17)
Yield: 91%; colorless viscous oil.
IR (neat): 3364, 2927, 2854, 1455, 1075 cm–1.
1H NMR (CDCl3): d = 1.01–1.82 (m, 19 H), 2.74 (br s, 1 H), 3.53 (t,
J = 6.5 Hz, 2 H), 3.59–3.77 (m, 2 H).
13C NMR (CDCl3): d = 23.1, 23.3, 25.5, 29.3, 32.5, 39.1, 62.6, 67.9.
EI–MS: 188 [M+].
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12.83.
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Synthesis 2005, No. 1, 71–74 © Thieme Stuttgart · New York