Photolysis of phenacyl esters in a two-phase system

Article abstract of DOI:10.1081/SCC-120005943

Phenacyl esters are useful photoremovable protecting groups for carboxylic acids in organic synthesis and biochemistry. In this work, simple one-pot arrangements of the phenacyl and 2,5-dimethylphenacyl ester photolysis are proposed. The reactions were performed in both the benzene/water two-phase system and in water. Cetyltrimethylammonium bromide was found to increase substantially the efficiency of the deprotection as well as the purity of the products by lowering the interfacial tension between the phases. Utilizing water as a medium significantly reduced the necessity to use environmentally malign organic solvents. The overall yields varied from 72 to 98% depending on the reaction conditions.

Full text of DOI:10.1081/SCC-120005943

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PHOTOLYSIS OF PHENACYL ESTERS IN A TWO-PHASE
SYSTEM
Radovan Ruzicka a , Miroslav Zabadal a & Petr Klán b
a Department of Organic Chemistry, Faculty of Science, Masaryk University, Kotlarska 2,
Brno, CZ-611 37, Czech Republic
b Department of Organic Chemistry, Faculty of Science, Masaryk University, Kotlarska 2,
Brno, CZ-611 37, Czech Republic
Published online: 16 Aug 2006.
To cite this article: Radovan Ruzicka , Miroslav Zabadal & Petr Kln (2002) PHOTOLYSIS OF PHENACYL ESTERS IN A TWO-PHASE
SYSTEM, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:16,
2581-2590, DOI: 10.1081/SCC-120005943
To link to this article: http://dx.doi.org/10.1081/SCC-120005943
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 16, pp. 2581–2590, 2002
PHOTOLYSIS OF PHENACYL
ESTERS IN A TWO-PHASE SYSTEM
Radovan Ruzicka, Miroslav Zabadal,y and Petr Klanz
*
Department of Organic Chemistry, Faculty of Science,
Masaryk University, Kotlarska 2, CZ-611 37 Brno,
Czech Republic
ABSTRACT
Phenacyl esters are useful photoremovable protecting groups
for carboxylic acids in organic synthesis and biochemistry.
In this work, simple one-pot arrangements of the phenacyl
and 2,5-dimethylphenacyl ester photolysis are proposed.
The reactions were performed in both the benzene/water
two-phase system and in water. Cetyltrimethylammonium
bromide was found to increase substantially the efficiency of
the deprotection as well as the purity of the products by low-
ering the interfacial tension between the phases. Utilizing
water as a medium significantly reduced the necessity to use
environmentally malign organic solvents. The overall yields
varied from 72 to 98% depending on the reaction conditions.
Key Words: Phenacyl; Two-phase system; Photochemistry;
Protecting group
*E-mail: jeanpaul@mail.muni.cz
yE-mail: mzabadal@chemi.muni.cz
zCorresponding author. E-mail: klan@sci.muni.cz
2581
DOI: 10.1081/SCC-120005943
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2582
RUZICKA, ZABADAL, AND KLAN
Photochemically removable protecting (PRP) groups are recognized
today as a class of groups of great practical significance. They found their
applications in organic synthesis as well as in biochemistry.[1–4] Phenacyl
ester derivatives certainly belong among the widely used PRP groups
for carboxylic acids and phosphates. The p-hydroxyphenacyl group received
a considerable attention as quite a versatile moiety by Givens and other
authors, especially in terms of oligopeptide synthesis.[5–10] Phenacyl and p-
methoxyphenacyl groups have been reported as efficient PRP groups for
photochemical release of carboxylic acids, alcohols, and phosphates.[11–17]
Recently we proposed the 2,5-dimethylphenacyl chromophore as a new
photoremovable protecting group for carboxylic acids.[18,19]
There has been long-standing interest to introduce water to the
chemical processes and consequently replace environmentally malign
solvents. By reducing or eliminating the application of those substances
associated with a particular synthesis, chemists can greatly reduce risk to
human health and the environment. Such efforts belong to a new program
called green chemistry.[20] Now we wish to report on the photoreactions
of phenacyl and 2,5-dimethylphenacyl esters in aqueous two-phase systems.
RESULTS AND DISCUSSION
The protection reactions of carboxylic acids with the phenacyl group
is well-described in the literature.[11–17] The carboxylic acid usually reacts
with phenacyl halide in the presence of a base to obtain the corresponding
ester in a high yield. The photorelease of the acid could be affected through
photosensitization using excited-state electron donors[14–16,21] or efficient
hydrogen donors.[17] Falvey and Banerjee proposed a mechanism of the
sensitized deprotection of the phenacyl esters which results in C–O bond
scission and consequently the formation of the corresponding carboxylic
acid and acetophenone (Scheme 1).[15]
Scheme 1.

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PHOTOLYSIS OF PHENACYL ESTERS
2583
Scheme 2.
It is known that aryl ketones with o-alkyl groups undergo an efficient
photoinduced intramolecular photoenolization.[22–24] 2,5-Dimethylphenacyl
(DMP) group is released in the reaction starting with triplet excitation of the
ketone and followed by intramolecular hydrogen abstraction (Scheme 2).
Chloride[25,26] and carboxylates[18,19] are the only leaving groups (X) so far
described.
In this work, the esters 1–5 were synthesized from the phenacyl halide
derivatives and the sodium salt of the corresponding acid in high yields
(67–95%).
Phenacyl esters were irradiated in the presence of electron or hydrogen
donors; DMP esters released the carboxylic acid upon a direct irradiation.
The starting compound was photolyzed either in
a
two-phase
(benzene/aqueous Na2CO3 solution) system or in the aqueous Na2CO3 sol-
ution only. Since phenacyl and DMP esters are insoluble in the latter system
they remained in the crystalline state during the irradiation until they
reacted off. We have not found any significant solid-state photoreactivity
of any ester derivative, thus photoreactions had to occur in the liquid phase.
Cetyltrimethylammonium bromide (CTAB) as a surfactant was added in
some experiments to increase the reaction efficiency. The experiments
were performed in a sealed flask equipped with a stir bar at 20ꢀC.
The wavelength used (>280 nm) guaranteed an efficient photochemical

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2584
RUZICKA, ZABADAL, AND KLAN
transformation. The reaction mixtures were vigorously stirred and
irradiated until the ester was completely consumed. The acids were
obtained as the corresponding sodium salt solutions in water. The following
acidification and purification gave the desired products.
The following tables compare the results from the experiments in
two-phase systems: Table 1 shows the photolysis with the presence and
Table 2 without the presence of a surfactant (CTAB).
The phenacyl ester in the water/benzene system was dissolved in
the organic layer while the resulting acid was transferred into an alkaline
aqueous solution. When the phenacyl esters were used diethylamine (DEA)
and triethylamine (TEA) served as the electron donors to afford the
reaction, benzylalcohol (BzOH) was used as a hydrogen donor. While
DEA is almost completely miscible with water but partially soluble in
benzene, TEA and BzOH are poorly soluble in water but well soluble in
the organic layer.
Photochemistry occurred with or without the presence of a surfactant,
however, the chemical yields and the purity of the acids, as well as the
photochemical reaction efficiencies, were considerably higher in the former
experiments. Time necessary for the reactions without CTAB was
approximately 2-times longer. Furthermore, the products were contaminated
Table 1. Photolysis of Phenacyl Esters in a Two-Phase System with CTABa
Hydrogen or
Electron
Donor
Photolysis Yield
Time [h] [%]b
Reaction Mixture
1 Phenacyl benzoate (water)
2 Phenacyl benzoate (water/benzene)
3 Phenacyl phenylacetate (water)
4 Phenacyl phenylacetate (water/benzene)
5 Phenacyl phenylacetate (water)
6 Phenacyl phenylacetate (water/benzene)
7 Phenacyl octanoate (water)
8 Phenacyl octanoate (water/benzene)
9 2,5-dimethylphenacyl benzoate (water)
10 2,5-dimethylphenacyl benzoate (water/benzene)
11 2,5-dimethylphenacyl palmitate (water/benzene)
BzOH
BzOH
TEA
TEA
DEA
DEA
TEA
TEA
–
16
16
12
14
24
24
19
19
12
14
12
98
94
93
93
88c
92c
86
82
0d
–
–
82
86
aIrradiated at >280 nm at 20ꢀC. No ground state deprotection reaction was
observed at given reaction conditions; bIsolated chemical yields of the crude acids
(>95% purity); cThe GC purity ꢁ 88%; dThe ester remained in the solid state during
the experiment.

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PHOTOLYSIS OF PHENACYL ESTERS
2585
Table 2. Photolysis of Phenacyl Esters in a Two-Phase System without CTABa
Hydrogen or
Electron
Donor
Yield
Photolysis (Purity)
Reaction Mixture
Time [h]
[%]
1
2
3
4
5
6
Phenacyl benzoate (water)
BzOH
BzOH
TEA
TEA
DEA
DEA
24
24
24
24
24
24
72 (74)
64 (88)
83 (91)
81 (95)
72 (86)
76 (82)
Phenacyl benzoate (water/benzene)
Phenacyl phenylacetate (water)
Phenacyl phenylacetate (water/benzene)
Phenacyl phenylacetate (water)
Phenacyl phenylacetate (water/benzene)
aIrradiated at >280 nm at 20ꢀC. No ground state deprotection reaction was
observed at given reaction conditions.
by additional unwanted byproducts, thus photoproduct purification was
more difficult (the overall loss was almost 50% in comparison to the
experiments from Table 1). The surfactant apparently lowered the inter-
facial tension, thus increased the emulsification of the reaction mixture.
This facilitated the transport of a water soluble material out of the organic
phase.
The reactions in the aqueous Na2CO3 solution can be classified as a
solid/liquid phase system because the esters are practically insoluble in
water. Surprisingly, the presence of the surfactant was not obligatory for
the reaction to take place, however, the yields as well as the purity was
considerably higher when CTAB was added. The reaction between the
ester and an electron/hydrogen donor probably occurred in a quasi-liquid
layer at the surface of the crystalline material (the organic donor molecules
dissolve the esters). This procedure requires removing the unwanted reaction
material from the resulting acid salt solution by washing with an organic
solvent (dichloromethane, petroleum ether, ethyl acetate etc.). Moreover,
the sodium salt is a practical form of the acid for a further purification.
Photolysis of DMP esters was successful only in the benzene/water system.
No reaction was observed in the aqueous environment where the ester
remains in the solid state.
In conclusion, we propose simple reaction conditions for phenacyl
and 2,5-dimethylphenacyl ester photolysis with the emphasis to environmen-
tally friendly reactions in water. Application of a surfactant increased
the overall reaction efficiency—the products were obtained in very high
chemical yields (82–98%) as well as purity. Moreover, all reactions have
an ‘‘one-pot’’ arrangement which might be beneficial in organic synthesis
when the phenacyl is used as a protecting group.

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2586
RUZICKA, ZABADAL, AND KLAN
EXPERIMENTAL
Materials and Methods
Benzene and methanol were purified by distillation through a vacuum-
sealed column (70 cm) packed with glass detritus. Phenacyl bromide
and a-chloro-20,50-dimethylacetophenone were prepared according to the
standard procedure described elsewhere.[15,25,26]
NMR spectra were recorded on a Bruker 300 MHz spectrometer.
1H and 13C NMR data were measured in the indicated solvent with tetra-
methylsilane as an internal standard. Mass spectra were obtained using EI
(30 eV). Gas chromatography was performed on a Shimadzu GC 17A gas
chromatograph equipped with a DB1 column. Melting points were
determined on a Kofler hot-stage Rapido 79–2106 and were not corrected.
Preparation of Phenacyl Esters: A Typical Procedure
a-Chloro-20,50-dimethylacetophenone (32 mmol) in acetone (25 mL)
was added to a stirred mixture of the acid (1.3–2 equiv.) or its sodium salt
(1.2 equiv.), sodium iodide (1.2 equiv.) and triethylamine (1.1 equiv.) in
250 mL of acetone. The mixture was stirred and heated under reflux for
ꢁ 8 h. The product formation was monitored by TLC. The solids were
removed, the solvent was distilled off, and the residue was extracted with
benzene. The benzene layer was washed three times with 10% HCl (50 mL),
1 M aqueous sodium hydroxide (50 mL), and water. The organic layer
was dried over MgSO4 and the solvent was removed to obtain the crude
product. The crude products were purified by crystallization or by flash
chromatography (petroleum ether–diethyl ether mixture, 4 : 1).
2-Oxo-2-phenyl-ethyl Ester of Benzoic Acid (Phenacyl Benzoate, 1):
Synthesized from phenacyl bromide with 71% yield; white solid; m.p.
123–125ꢀC (from diethyl ether); IR (KBr) 3058, 2944, 1721, 1699, 1597,
1449, 1283, 1229, 1122, 959 cmÀ1 1H NMR (300 MHz, CDCl3) ꢀ 5.58
;
(s, 2H), 7.46–7.65 (m, 6H), 7.98 (d, J ¼ 7.2 Hz, 2H), 8.16 (d, J ¼ 7.2 Hz,
2H); 13C NMR (75.5 MHz, CDCl3) ꢀ 66.7, 128.1, 128.7, 129.1, 130.2,
133.5, 134.1, 134.6, 166.3, 192.3.
2-Oxo-2-phenyl-ethyl Ester of Phenylacetic Acid (Phenacyl
Phenylacetate, 2): Synthesized from phenacyl bromide with 93% yield;
yellowish solid; m.p. 48–50ꢀC (from ethanol); IR (KBr) 3060, 2939, 1731,
1702, 1596, 1450, 1282, 1225, 1142, 971 cmÀ1; 1H NMR (300 MHz, CDCl3) ꢀ
3.84 (s, 2 H), 5.37 (s, 2H), 7.37 (d, J ¼ 4.3 Hz, 5H), 7.49 (t, J ¼ 7.2 Hz, 2H),
7.62 (d, J ¼ 7.2 Hz, 1H), 7.91 (d, J ¼ 7.9 Hz, 2H); 13C NMR (75.5 MHz,

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PHOTOLYSIS OF PHENACYL ESTERS
2587
CDCl3) ꢀ 41.0, 66.4, 127.3, 127.9, 128.7, 128.9, 129.5, 133.7, 133.9, 134.4,
171.1, 192.1.
2-Oxo-2-phenyl-ethyl Ester of Octanoic Acid (Phenacyl Octanoate, 3):
Synthesized from phenacyl bromide with 71% yield; yellowish liquid; IR
(NaCl) 2953, 2928, 2856, 1746, 1705, 1597, 1450, 1375, 1222, 1161,
1
967 cmÀ1; H NMR (300 MHz, CDCl3) ꢀ 0.81 (s, 3H), 1.21 (bs, 8H), 1.63
(d, J ¼ 6.6 Hz, 2H), 2.40 (t, J ¼ 7.3 Hz, 2H), 5.24 (s, 2H), 7.39 (t, J ¼ 7.0 Hz,
2H), 7.50 (t, J ¼ 7.0 Hz, 1H), 7.82 (d, J ¼ 7.0 Hz, 2 H); 13C NMR (75.5 MHz,
CDCl3) ꢀ 14.2, 22.7, 25.1, 29.1, 29.2, 31.8, 34.1, 66.0, 127.9, 129.0, 133.9,
134.6, 173.3, 192.5.
2-(2,5-Dimethyl-phenyl)-2-oxo-ethyl Ester of Benzoic Acid (2,5-
Dimethylphenacyl Benzoate, 4): Synthesized from a-chloro-20,50-dimethyl-
acetophenone with 95% yield; white solid; mp. 90–91ꢀC (from diethyl
ether); IR (KBr) 2944, 1720, 1700, 1597, 1450, 1372, 1283, 1229, 1122,
1
1019, 958 cmÀ1; H NMR (300 MHz, CDCl3) ꢀ 2.37 (s, 3H), 2.48 (s, 3H),
5.40 (s, 2H), 7.15–7.25 (m, 2H), 7.43–7.48 (m, 3H), 7.58 (t, J ¼ 7.6 Hz, 1H),
8.11–8.14 (m, 2H); 13C NMR (75.5 MHz, CDCl3) ꢀ 20.8, 21.1, 67.9, 128.6,
128.9, 129.7, 130.1, 132.3, 133.1, 133.5, 134.7, 135.6, 136.1, 166.3, 196.0; MS
(EI) m/z 268 (Mþ), 147, 133, 105, 77. Anal. calcd. for C17H16O3: C, 76.10;
H, 6.01. Found: C, 76.53; H, 6.11.
2-(2,5-Dimethyl-phenyl)-2-oxo-ethyl Ester of Palmitoic Acid (2,5-
Dimethylphenacyl Palmitate, 5): Synthesized from a-chloro-20,50-dimethyla-
cetophenone with 67% yield; white solid; mp. 38–39ꢀC (from aqueous
ethanol); IR (KBr) 2916, 2851, 1746, 1701, 1572, 1467, 1420, 1282, 967,
818 cmÀ1
;
1H NMR (300 MHz, CDCl3) ꢀ 0.88 (t, J ¼ 6.2 Hz, 3H), 1.26
(s, 26H), 1.67 (k, J ¼ 7.3 Hz, 2H), 2.36 (s, 3H), 2.45 (s, 3H), 5.14 (s, 2H),
7.15 (d, J ¼ 7.8 Hz, 1H), 7.21 (d, J ¼ 7.8 Hz, 1H), 7.39 (s, 1H); 13C NMR
(75.5 MHz, CDCl3) ꢀ 14.3, 20.7, 21.1, 22.9, 25.1, 29.3, 29.5, 29.6, 29.7, 29.8,
29.9, 32.1, 34.1, 67.4, 128.8, 132.2, 132.9, 134.9, 135.5, 135.9, 173.5, 196.5;
MS (EI) m/z 402 (Mþ), 239, 133, 105, 77. Anal. calcd. for C26H42O3:
C, 77.56; H, 10.51. Found: C, 77.22; H, 10.76.
Photolysis of Phenacyl Esters: The reaction mixture consisted either of
ꢁ 0.2-M ester solution in benzene (5 mL) mixed with a 2-M Na2CO3 aque-
ous solution (5 mL) or the ester (1 mmol) added to a 2-M Na2CO3 aqueous
solution (5 mL). A hydrogen donor (benzylalcohol) or an electron donor
(triethylamine or diethylamine) (10 equiv.), respectively, were added in case
of phenacyl esters. In some experiments, a small amount (1–3%, w/w) of
cetyltrimethylammonium bromide was added. The reaction mixtures, placed
into a Simax vessel sealed with a septum and equipped with a stir bar, were
degassed by bubbling the solution with argon for 5 min. The Simax (Pyrex
equivalent) glass was used in all cases to filter off UV radiation below 280 nm.
The well-stirred mixture was irradiated using a 125-W medium-pressure

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2588
RUZICKA, ZABADAL, AND KLAN
mercury lamp (Teslamp) at the ambient temperature for 12–24 h. The reac-
tion conversion was monitored by GC. The main by-product from the
phenacyl ester photolysis was identified as acetophenone which confirms
that the reaction is a photochemical process as described elsewere.[11–17]
No deprotection reaction was observed in dark at given reaction conditions.
The benzene/water system. When the photolysis reached a maximal
concentration of the photoproduct (the carboxylic acid) the aqueous
layer was separated, acidified with HCl, and the acid was extracted into
dichloromethane. The organic layer was dried over MgSO4 and the solvent
was evaporated to yield the crude product. The isolated acid was analyzed
by 1H NMR and GC methods and compared to the authentic samples
obtained from commercial sources.
The water/solid ester system. The water layer was washed with a small
amount of an organic solvent (e.g., dichloromethane), acidified with HCl,
and the acid was extracted into dichloromethane. The organic layer was
worked up in the same way as described above.
ACKNOWLEDGMENT
We thank the Czech Ministry of Education, Youth and Sport for the
support of this research (FRVS 0587/2001).
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Received in the UK June 15, 2001