Photosensitized tetrahydropyran transfer

Article abstract of DOI:10.1021/jo800519h

(Chemical Equation Presented) THP ethers were formed cleanly during photolysis of 3,4-dihydro-2H-pyran, an alcohol, and catalytic 1,5-dichloro9,10-anthraquinone with use of visible light. The reaction could be conducted under ambient fluorescent lighting or with sunlight as well as in a Rayonet reactor. The scope and mechanism are discussed.

Full text of DOI:10.1021/jo800519h

Photochemical methods for the protection and deprotection
of alcohols are well-known.5 Methods for similar transforma-
tions involving carbonyls and acetals are less widely used.
However, photoinitiated release of carbonyl compounds has
recently received much attention.6 Photosensitized conversion
of carbonyls to their corresponding dimethyl acetals by pho-
tolysis in methanol with various quinones as sensitizer was
recently described.7 This reaction was apparently due to a
photochemically generated acid. During the course of our
investigation of anthraquinone photochemistry, we have dis-
covered a similar reaction that provides a quick and easy method
for installing the tetrahydropyran (THP) protecting group on
an alcohol.
Photosensitized Tetrahydropyran Transfer
R. P. Oates and Paul B. Jones*
Department of Chemistry, Wake Forest UniVersity,
Winston-Salem, North Carolina 27109
jonespb@wfu.edu
ReceiVed March 6, 2008
We observed that photolysis of a mixture of THP-protected
R-terpineol (1),8 1-pentanol (2), and 1,5-dichloro-9,10-an-
thraquinone (DCQ) in dry dichloromethane using 419 nm
Rayonet lamps resulted in transfer of the THP group from 1 to
2. This proved to be a general reaction: the THP group of 1
was efficiently transferred to other alcohols under the photolysis
conditions. The reaction was also observed under standard
fluorescent lights or in sunlight. THP transfer was also observed
when other THP ethers were photolyzed in the presence of
alcohols and DCQ. Apparently, this photolysis produced an
equilibrium mixture of the THP ethers. When a 3° THP ether
was photolyzed in the presence of either a 2° or 1° alcohol, the
THP was completely transferred to the less hindered hydroxyl
while photolysis of a 1° THP ether in the presence of a 1°
alcohol resulted in a mixture of the possible THP ethers.
THP ethers were formed cleanly during photolysis of 3,4-
dihydro-2H-pyran, an alcohol, and catalytic 1,5-dichloro-
9,10-anthraquinone with use of visible light. The reaction
could be conducted under ambient fluorescent lighting or
with sunlight as well as in a Rayonet reactor. The scope and
mechanism are discussed.
Photochemistry often provides methods for organic transfor-
mations that complement ground state reactions. The utility of
photolabile protecting groups is well-documented and further
development of such groups is of much current interest.1 In
certain cases, photochemistry can give selectivity based on
absorbance, excited state energies, and conformation, potentially
reversing selectivities seen in thermal reactions.2 Additionally,
photons are a cheap reagent and generate no waste. The redox
and photochemistry of quinones has been studied extensively;
this chemistry has been recently applied to photolabile protecting
groups in our laboratory and used in synthetic applications
elsewhere.3,4
The reaction could also be sensitized by p-chloroanil (CA).
The efficacy of CA was similar to that found by de Lijser in
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10.1021/jo800519h CCC: $40.75
Published on Web 05/28/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4743–4745 4743

SCHEME 1. Photosensitized Formation of THP Ethers
TABLE 1. Photosensitized THP Protection of Alcohols
CHART 1. Possible Photogenerated Acids
solvent
yieldcor
entry
ROH (THP ether)
1-C5H9OH (3)
(light)a, b
(conv)d
1
CH2Cl2
99
Unsatisfactory yields or conversions were observed in three
cases. None of the desired THP ether was observed when N,N-
dimethylaminopropanol (entry 11) was used. This was due either
to quenching of DCQ by the amine3a,9 or to the presence of a
base, which may neutralize any photogenerated acid. Although
both hindered alcohols and potential electron donors were
tolerated in some substrates (entries 5, 6, 9, 10, and 12), neither
R-terpineol nor 6-methyl-5-hepten-2-ol (entries 16 and 17) gave
good yields. In both cases, an alkene located five atoms from
the alcohol may allow interaction of acid, alcohol, and alkene.
However, 5-methyl-5-hexen-1-ol did give satisfactory conver-
sion (entry 18). The reason for the poor yields in entries 16
and 17 is still under investigation.
An attempt to use DCQ to sensitize the hydrolysis of a THP
ether was made by using 5 in 95:5 CH3CN:H2O. After 1 h, no
reaction had occurred. A few drops of CH3OH was added and
benzyl alcohol was cleanly produced. Although the reaction was
slow, complete conversion was achieved. Thus, it appears that
THP ethers may either be added or removed with this method.
This experiment also demonstrated the importance of including
a reducing agent in the reaction mixture.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2-C5H9OH (4)
PhCH2OH (5)
1-C9H19OH (6)
cholesterol (7)
CH3S(CH2)3OH (8)
BOCNH(CH2)3OH (9)
allyl alcohol (10)
CH2dCHC(CH3)2OH (11)
(CH3)2CdCHCH2OH (12)
(CH3)2N(CH2)2OH (13)
methyl p-hydroxybenzoate (14)
TBDMSOCH2CH2OH (15)
PhCH2OH (5)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
91
89
96
94
90
94
98
99
86
(0)
90
48
98
(100)
(20)
(0)
(100)
(62)
(22)
e f
,
g f
,
sunlighth
1-C5H9OH (3)
R-terpineol (1)
6-methyl-5-hepten-2-ol (16)
5-methyl-5-hexen-1-ol (17)
PhCH2OH (5)
CH3CN
CH3CNi
CH3CNi,f
CH3CNi,f
CH3CN (Ar)j
CH3CN (O2)j
PhCH2OH (5)
a 30 min irradiation with 8 419 nm lamps in a Rayonet reactor unless
otherwise indicated. b 5 mol
% DCQ unless otherwise indicated.
c Isolated yield. d Conversion measured by GC analysis. e 50 min. f 50
mol %. g 3 h. h Sunny day, 1-3 pm local time, late January at 35 °N.
i 90 min. j Reaction mixture purged 20 min with the indicated gas.
The data strongly suggest that the photosensitized THP
transfer is due to a photochemically generated acid. Addition
of base slows or prevents the reaction. Under the reaction
conditions, equilibrium is established between the various
possible THP ethers and DHP.
the formation of dimethyl acetals.7 This similarity led to the
hypothesis that a photochemically produced acid was at work
in the DCQ-sensitized transfer of THP groups. To test this
hypothesis, the reaction was carried out in the presence of base.
When solid NaHCO3 was added to the reaction, the rate slowed
considerably, but did not completely cease. After 3 h, 90%
conversion was achieved. However, in the presence of 5 equiv
of 2,6-lutidine, no reaction was observed after 6 h of irradiation.
Installation of a THP group on free hydroxyl groups was also
accomplished with use of 3,4-dihydropyran (DHP) (Scheme 1).
This proved to be a very clean, efficient method for THP
protection of alcohols. Addition of the THP group to 1° alcohols
was generally complete in a few minutes, using only 5 mol %
DCQ. Results are summarized in Table 1.
Table 1 shows the result of tetrahydropyranylation of alcohols
with DHP, DCQ, and visible light. A variety of functional
groups were tolerated and even 3° alcohols give good yields.
Acid-sensitive groups such as BOC and TBDMS were tolerated.
The low yield in the formation of 15 appeared to be a problem
in workup and purification. When mono-TBDMS ethylene
glycol was mixed with DCQ in CH2Cl2 and then photolyzed,
no cleavage of the TBDMS group was observed. The addition
of water to this photolysis did not change the outcome.
The reaction was relatively insensitive to the presence of
oxygen (see below) and was simple to carry out. Typically, DCQ
was added to a solution of DHP and alcohol in CH2Cl2 and
then irradiated. Once the reaction was found to be complete by
TLC or GC, the solvent was removed. At that point, the desired
THP ether could be isolated either by chromatography or
distillation. In many cases, it was possible to dissolve the crude
residue in a small amount of cold ether and decant or filter the
liquid to separate the THP ether from DCQ. The THP ether
thus obtained was better than 95% pure as judged by 1H NMR.
Several possibilities exist for the identity of the photogener-
ated acid. HCl, potentially formed from either DCQ or CH2Cl2,
was ruled out. In contrast to one report that claimed photolysis
of DCQ with visible light resulted in dechlorination to give AQ
and HCl,10 we found that extended photolysis (24 h) of DCQ
in CH3OH/CH3CN at 419 nm did not result in an increase in
acidity and DCQ was obtained unchanged (after reoxidation-pho-
toreduction was evident during the photolysis). Anthraquinone
was also capable of sensitizing the reaction. Clean formation
of THP ether 5 occurred when AQ was used as sensitizer,
although at a slower rate, due to lower absorbance (at 419 nm)
by AQ relative to DCQ. Similarly, THP transfer was observed
when the reaction was carried out in CH3CN (entry 13), which
ruled out HCl from CH2Cl2 as the source of the acid. However,
photoreduction of DCQ in CH2Cl2 with a small amount of
CH3OH did lead to an increase in acidity.11 No hydrolysis of
the BOC group (entry 7) in the formation of 9 was observed,
which also argues against HCl as the acid.
This leaves five possible sources of acid (see Chart 1):
DCQH2 (reduced DCQ), DCQH2 radical cation, DCQH radical,
DCQH2*, and peroxyl radical (HOO•). All may form during a
photoreduction of DCQ.3a,9 Given our failure to hydrolyze 5 in
the absence of a photoreductant, a photoreduction must initiate
(9) Kausche, T.; Sauberlich, J.; Trobitzsch, E.; Beckert, D.; Dinse, K. P.
Chem. Phys. 1996, 208, 375.
(10) Hamanoue, K.; Sawada, K.; Yokoyama, K.; Nakayama, T.; Hirase, S.;
Teranishi, H. J. Photochem. 1986, 33, 99.
(11) Extraction of the organic solution with neutral water produced an aqueous
layer with a pH of approximately 3.
4744 J. Org. Chem. Vol. 73, No. 12, 2008

the reaction. The pKa of DCQH2 is not known, but can reliably
be estimated as approximately 10, a value too high to catalyze
THP transfer.12 Likewise, the pKa of DCQH2* is not known,
but true photoacids (that is, compounds that are much more
acidic in the excited state) tend to have lifetimes too brief to
transfer a proton to anything other than a solvent molecule;13
in CH2Cl2, this seems unlikely, though it cannot be ruled out.
Peroxyl, which can be formed during aerobic photolysis of
anthraquinones, has a pKa of 4.8.14 The pKa of the AQH radical
is approximately 5.3.15 Therefore, it is unlikely that the DCQH
radical has a pKa lower than 4. When AcOH (pKa 4.75) was
used to catalyze the reaction (10 mol % AcOH), no conversion
was observed after 20 min, while the same reaction sensitized
by DCQ (10 mol %) had undergone 80% conversion in the same
time. The radical cation of DCQH2 is most likely a much
stronger acid than peroxyl and is the conjugate acid of the
DCQH radical and was present in approximately the same
concentration as these species.16 Thus, we favored the DCQH2
radical cation as the likely catalytic species.
Unfortunately, distinguishing between the DCQH radical and
the DCQH2 radical cation with reaction chemistry was difficult.
Both reacted with oxygen and could, therefore, be distinguished
from peroxyl. If increasing the O2 concentration decreased the
reaction rate, then peroxyl was not the acid. When carried out
under identical conditions, THP transfer in O2-saturated solution
(entry 20) occurred at about one-third the rate as in Ar-saturated
solution (entry 19). This result showed that peroxyl was not
the acid responsible for THP. Combined with the lack of reaction
with AcOH as catalyst, this supports the DCQH2 radical cation
as the acid responsible for catalysis in the sensitized reactions.
Our investigation of this chemistry continues.
Experimental Section
Representative Procedure for Photochemical THP Ether
Formation. 3-Thiomethyl-1-propanol THP Ether (8). To a
solution of 3-(methylthio)-1-propanol (128 mg, 1.21 mmol) in dry
CH2Cl2 was added 3,4-dihydro-2H-pyran (111 mg, 1.32 mmol, 1.1
equiv). 1,5-Dichloroanthraquinone (16 mg, 0.06 mmol) was then
added to the reaction mixture. The solution was irradiated at 419
nm for 1 h until the 3-(methylthio)-1-propanol was shown to be
consumed by GC analysis. The reaction was neutralized with TEA
and concentrated in vacuo to afford a yellow oil. The crude product
was then dissolved in cold Et2O and insoluble material removed
by gravity filtration. 1H NMR indicated that the crude product thus
obtained was quite pure (see the Supporting Information). The oil
could be further purified by flash column chromatography (7/1
petroleum ether/ethyl acetate f 6/1 petroleum ether/ethyl acetate)
to afford a yellow oil (220 mg, 1.17 mmol, 97%). An analytically
pure sample was obtained by vacuum distillation over K2CO3 (head
temperature of 110 °C (0.5 Torr)). 1H NMR (500 MHz, CDCl3) δ
4.56 (m, 1H), 3.81 (m, 2H), 3.47 (m, 2H), 2.59 (m, 2H), 2.09 (s,
3H), 1.86 (m, 3H), 1.57 (m, 1H), 1.50 (m, 4H). 13C NMR (125
MHz, CDCl3) δ 99.2, 66.3, 62.7, 31.3, 31.0, 29.7, 25.8, 19.9, 15.8.
HRMS (ESI+) calcd for C9H18O2SNa+ 213.0919, found 213.0910.
Anal. Calcd for C9H18O2S: C, 56.80; H, 9.53; S, 16.85. Found: C,
56.83; H, 9.53; S, 16.58.
N-tert-Butoxycarbonyl-3-amino-1-propanol THP Ether (9).
The crude oil, obtained as described for 7, could be further purified
by flash column chromatography (3/1 petroleum ether/ethyl acetate
f 2/1 petroleum ether/ethyl acetate) to yield a yellow oil (200 mg,
0.76 mmol, 94%). An analytically pure sample was obtained by
vacuum distillation over K2CO3 (head temperature of 130 °C (0.5
Torr)). 1H NMR (500 MHz, CDCl3) δ 4.91 (s, 1H), 4.57 (m, 1H),
3.62 (m, 2H), 3.47 (m, 2H), 3.24 (m, 2H), 1.71 (m, 4H), 1.55 (m,
4H), 1.41 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 156.2, 99.0,
78.6, 65.9, 62.0, 39.2, 31.3, 30.4, 28.9, 26.2, 20.0. HRMS (ESI+)
calcd for C13H25NO4Na+ 282.1676, found 282.1666. Anal. Calcd
for C13H25NO4: C, 60.21; H, 9.72; N, 5.40. Found: C, 60.24; H,
9.77; N, 5.20.
In conclusion, we report a quick and clean photochemical
method for introducing a THP group to a hydroxyl group. The
method tolerates a variety of functional groups, although strong
bases are precluded. The same reaction can also sensitize the
hydrolysis of a THP ether. The reaction can be carried out by
using photochemical reactors, ambient fluorescent lighting, or
sunlight and is only slightly sensitive to the presence of oxygen.
This makes the reaction convenient in a typical laboratory
setting. The mechanism of the reaction appears to involve a
photochemically generated acid that is most likely a reactive
anthraquinone intermediate.
2-(tert-Butyldimethylsiloxy)ethanol THP Ether (15). The crude
oil, obtained as described for 7, could be further purified by flash
column chromatography (7/1 petroleum ether/ethyl acetate) to give
a yellow oil (80 mg, 0.31 mmol, 48%). An analytically pure sample
was obtained by vacuum distillation over K2CO3 (head temperature
of 82 °C (0.5 Torr)). 1H NMR (500 MHz, CDCl3) δ 4.64 (m, 1H),
3.87 (m, 1H), 3.75 (m, 3H), 3.49 (m, 2H), 1.70 (m, 1H), 1.60 (m
1H), 1.52 (m, 4H), 0.87 (s, 9 H), 0.08 (s, 6H). 13C NMR (75 MHz,
CDCl3) δ 98.9, 68.7, 62.7, 62.0, 30.6, 25.9, 19.4, 18.4, -5.22,
-5.25. HRMS (ESI+) calcd for C13H28O3SiNa+ 283.1699, found
283.1691. Anal. Calcd for C13H28O3Si: C, 59.95; H, 10.84. Found:
C, 60.16; H, 10.98.
(12) Da Silva, R. R.; Ramalho, T. C.; Santos, J. M.; Figueroa-Villar, J. D. J.
Phys. Chem. A 2006, 110, 1031. The pKa of p-hydroquinone is given here as
9.96. The pKa of 9,10-dihydroxyanthracene is reasonably expected to be similar;
the presence of two Cl substituents should lower the pKa somewhat. This allows
a rough estimate of 9 for the pKa of DCQH2.
(13) (a) Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc. 1994, 116, 10593.
(b) Solntsev, K. M.; Huppert, D.; Agmon, N.; Tolbert, L. M. J. Phys. Chem. A
2000, 104, 4658. (c) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35,
19. (d) Clower, C.; Solntsev, K. M.; Kowalik, J.; Tolbert, L. M.; Huppert, D. J.
Phys. Chem. A 2002, 106, 3114. (e) Solntsev, K. M.; Huppert, D.; Agmon, N.
J. Phys. Chem. A 1999, 103, 6984.
Acknowledgement. This work was supported in part by the
NIH (R15GM070468-01) and NSF (CHE-0514576).
Supporting Information Available: Spectroscopic data of
compounds 1, 3-9, 11, 14, and 15 and crude products for 3
and 5, as well as general experimental methods. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) (a) Rabani, J.; Nielsen, S. O. J. Phys. Chem. 1969, 73, 3736. (b) Behar,
D.; Czapski, G.; Rabani, J.; Dorfman, L.; Schwarz, H. A. J. Phys. Chem. 1970,
74, 3209.
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(16) Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991, 113, 1736. In
this paper, the pKa of the structurally related p-hydroquinone radical cation is
estimated to be-5.5.
JO800519H
J. Org. Chem. Vol. 73, No. 12, 2008 4745