An efficient photoinduced deprotection of aromatic acetals and ketals

Article abstract of DOI:10.1002/hlca.201000333

A novel type of photodeprotection reaction of simple aromatic acetals and ketals is described. The deprotection is highly efficient under optimized conditions. The aromatic ring confers the photoreactivity to the compounds. The efficiency of the process depends on the structure of the acetal moiety. The common minor side reaction, the photooxidation, becomes the major reaction pathway in the cases of cyclic acetals. The use of photon as only reagent makes this procedure especially attractive for acetal deprotection.

Full text of DOI:10.1002/hlca.201000333

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An Efficient Photoinduced Deprotection of Aromatic Acetals and Ketals
by Damien Thevenet and Reinhard Neier*
ˆ
ˆ
Department of Chemistry, University of Neuchatel, Avenue de Bellevaux 51, CH-2009 Neuchatel
(phone: þ 41-32-7182428; fax: þ 41-32-718 25 11; e-mail: reinhard.neier@unine.ch)
A novel type of photodeprotection reaction of simple aromatic acetals and ketals is described. The
deprotection is highly efficient under optimized conditions. The aromatic ring confers the photoreactivity
to the compounds. The efficiency of the process depends on the structure of the acetal moiety. The
common minor side reaction, the photooxidation, becomes the major reaction pathway in the cases of
cyclic acetals. The use of photon as only reagent makes this procedure especially attractive for acetal
deprotection.
Introduction. – Using laser beams to trigger photochemical reactions allows
controlling the timing and the localization of the photochemically induced trans-
formations with high precision. This is a major asset when photochemical reactions are
applied in life sciences and material sciences. Broadening the scope of a photochemical
deprotection reaction widens the toolbox developed by chemists for those two fields.
We have discovered a novel and efficient photochemical deprotection of aromatic
dialkyl acetals and ketals. This reaction can be applied to normal dialkyl acetals without
having to introduce a specially designed chromophore.
Photochemical transformations have attracted the attention of chemists from the
beginning of systematic organic synthesis [1]. The scientific interest concentrated on
the challenging mechanisms, the complex and synthetically attractive product
structures, and the selectivity of the processes as a function of the chromophore
involved. In recent years, the emphasis of the research in photochemistry focused on
applications in live sciences and in material sciences. The in vitro detection of biological
processes in living cells, even in whole living organisms, has given new insights into vital
biochemical processes [2]. Caged compounds have become valuable tools of chemical
biology [3]. Significant applications of caged compounds in neurochemistry have been
reported [4]. The recent growing understanding of the photophysics and photo-
chemistry of polymeric materials [5] have led to the creation of micro- and nano-
patterned structures on surfaces and functional assemblies with unprecedented
precision. Photodeprotection reactions have gained in importance as the chemical,
biological, and material properties of a compound can be changed on purpose [6].
Protection and deprotection reactions are an important, often inevitable part of
modern natural-product syntheses. One of the key issues in protecting-group
procedures is orthogonality [7]. Orthogonal protecting groups allow the selective
removal of one of them without affecting the others. Photochemical liberation of
functional groups occurs without adding a chemical reagent, and the deprotection is
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

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often fast and neat. The cleavage of a photolabile protecting group (PPG) is, therefore,
most of the times orthogonal to the conditions needed for the thermal deprotections;
the inverse is often not true. Acetals are widely used protecting groups in organic
synthesis [8]. The use of PPGs for carbonyl compounds has attracted considerable
attention recently [9].
Different classes of photochemical deprotection reactions for the release of
carbonyl and dihydroxy groups have been described. A common feature of all these
photochemical reactions is the need of a specific chromophore or of a allocated
functional group to generate the activated intermediate during the photochemical
process. The position of the chromophore can be used as criterium to classify PPGs.
Photolabile acetals can be employed for protecting either carbonyl or dihydroxy
groups, depending on the position of the photoreactive moiety.
Several different photoremovable protecting groups for aldehydes and ketones
have been reported. The 2-nitrobenzyl function has been used since its discovery in
1901 [10] to protect carbonyl functions [11][12]. Carbonyl groups protected with 3,5-
dimethoxy-a,a-diphenylsalicylic alcohol (¼ 3,5-dimethoxy-a,a-diphenylbenzeneme-
thanol) derivatives can be released upon UV irradiation [13]. The so-called ꢂmetaꢃ
effect [14][15] has been invoked to rationalize this photodeprotection reaction. Acetals
derived from (2,5-dihydroxyphenyl)ethylene glycol take advantage of a photochemi-
cally induced hydrogen shift [16]. Caged carbonyl groups derived from 6-bromo-4-(1,2-
dihydroxyethyl)-7-hydroxycoumarin (Bhc-diol ¼ 6-bromo-4-(1,2-dihydroxyethyl)-7-
hydroxy-2H-1-benzopyran-2-one) [17] and 2-anthraquinonylethyl-1’,2’-diols (Aqe-
diol ¼ 2-(1,2-dihydroethyl)anthracene-9,10-dione) [18] have been successfully used as
PPGs.
Alternatively, the chromophore can be attached to the carbonyl part and will,
thereby, release 1,2- or 1,3-diols after the photodeprotection. The same type of
chromophores are used than in the cases described above. The heterolysis of the
benzylic CꢀO bond can be triggered, e.g., by the photoexcitation of (6-bromo-7-
hydroxycoumarin-4-yl)methylene acetals (¼6-bromo-7-hydroxy-4-(1,3-dioxolan-2-yl)-
2H-1-benzopyran-2-ones) [19]. The change of the electronic distribution in the excited
state of the acetals leads to the cleavage of the benzylic CꢀO bond followed by
chemical hydrolysis of the intermediate created by the photoexcitation (Scheme 1).
The acetals of 5-methoxy- and 5-hydroxysalicylaldehyde (¼ 5-methoxy-2-hydroxy- and
2,5-dihydroxybenzaldehyde) photodecompose with good to excellent yields [20]. The
release of the protecting group is induced by a photochemical proton shift. The
photorelease of 1,2 and 1,3-diols from 2-nitrobenzylidene acetals has been described as
well [21].
In our research program aiming at the synthesis of triphenylene-based dimers for
the application in material sciences, we synthesized the triphenylene-biased acetal 3.
During UV irradiation (TLC lamp, 366 nm) of 3 dissolved in CH2Cl2, the unexpected
photochemical deprotection of acetal 3 occurred with high efficiency. The light-induced
process was identified by means of a color change in the fluorescence emission (from
blue to yellow).
Based on this observation, we decided to study the scope of this transformation. We
could demonstrate that the photodeprotection is not limited to the 2-(prop-1-yn-1-
yl)triphenylene chromophore of compound 3. We describe a new and efficient

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Scheme 1. Plausible Mechanism of Light-Mediated Uncaging of Coumarin-Derived Acetals Proposed by
Lin and Lawrence [19]
procedure to deprotect simple benzene- and naphthalene-based acetals and ketals
photochemically. We report our studies of the reaction conditions of the photolysis and
of the identification of the by-products formed in some cases.
2. Results. – 2.1. Synthesis and Photodeprotection of Triphenylene-Based Acetal 31).
For the synthesis of the triphenylene-based acetal 3 (Scheme 2), we started from
2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (1), which was synthesized by oxidative
coupling of 1,2-bis(hexyloxy)benzene with MoCl5, following a procedure described by
Kumar and Manickam. [22]. Selective ether cleavage [23] was achieved with ꢂB-
bromocatecholboraneꢃ (¼2-bromo-1,3,2-benzodioxaborole) in CH2Cl2 to give the
pentakis(hexyloxy)triphenylen-monool in a complex mixture of tetrakis(hexyloxy)-
triphenylene-diol and tris(hexyloxy)triphenylene-triol regioisomers which could not be
Scheme 2
a) ꢂB-Bromocatecholboraneꢃ, CH2Cl2, r.t., 24 h. b) Trifluoromethanesulfonic anhydride, pyridine,
CH2Cl2, r.t., 1 h; 40% (2 steps). c) 3,3-Diethoxyprop-1-yne, [Pd(Ph3P)4], Ph3P, CuI, (i-Pr)2NH, toluene,
608, 24 h; 79%.
Arbitrary atom numbering (see 13C-NMR spectra in the Exper. Part).
1
)

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Helvetica Chimica Acta – Vol. 94 (2011)
separated efficiently. The crude mixture was filtered through a pad of silica gel to
remove the catechol (¼ benzene-1,2-diol) generated after quenching with cold H2O.
The mixture of triphenylene-monool, -diol, and -triol was directly treated with triflic
anhydride (¼trifluoromethanesulfonic anhydride) and pyridine in CH2Cl2 to give the
corresponding mono-, bis-, and tris-trifluoromethanesulfonate derivatives. In contrast
to the partially deprotected triphenylenols, compound 2 could be isolated by
chromatography on silica gel. The crude 2 was purified by recrystallization in EtOH
to give a moderate yield of 40% after two steps. The introduction of the
propargylaldehyde diethyl acetal (¼ 3,3-diethoxyprop-1-yn-1-yl) moiety at the 2-
position of 2 was achieved under Sonogashira coupling conditions [24], i.e., 3,3-
diethoxyprop-1-yne, [Pd(Ph3P)4], and CuI were used in catalytic amount in toluene
containing 5% of (i-Pr)2NH. A higher amount of base in toluene led to degradation of
the triflate function of 2. The 2-(3,3-diethoxyprop-1-yn-1-yl)-3,6,7,10,11-pentakis(hex-
yloxy)triphenylene (3) could be chromatographed on silica gel and was isolated pure in
79% yield.
Following the unexpected fast color change of fluorescence emission of tripheny-
lene-based acetal 3, we identified the product formed as the 3-(triphenylen-2-yl)prop-
2-ynal 4. Fluorescence measurement for 3 and 4 (3.0 · 10ꢀ6 m in CHCl3) with an
excitation wavelength of 366 nm, showed emission bands at 413 nm for 3 and a broader
band at 531 nm for 4, respectively (Fig.).
Figure. Absorption and fluorescence spectra of 3 and 4. The fluorescence spectra are normalized to the
same maximum intensity.
We tested the photodeprotection of 3 first, using a 125 W medium-pressure
mercury lamp equipped with a Pyrex filter. Under these conditions, the substrate was
essentially irradiated with the 366 nm Hg-line. Under optimized conditions, a 0.005m
solution of 3 in CD3CN/CDCl3/H2O 70 :25 :5, without degassing the solvents, was
irradiated at room temp. for 20 min converting thus acetal 3 quantitatively into
aldehyde 4. No traces of the starting material 3 could be detected by 1H-NMR analysis
of the reaction mixture (Scheme 3).

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335
Scheme 3
The ease and efficiency of the photodeprotection of acetal 3 containing none of the
functional groups known to facilitate the photoactivation, came as a surprise. We asked
therefore the question if the success of this photoreaction depends on the p-extended
structure of the (alkyloxy)-substituted 2-(prop-1-yn-1-yl)triphenylene chromophore.
We decided to study the scope of this photodeprotection with simpler aromatic
compounds lacking the propynyl unit, such as benzene- and naphthalene-based acetals
and ketals.
2.2. Synthesis and Photodeprotection of Simple Aromatic Ketals and Acetals 5 – 16.
For the studies of the photochemical deprotection reaction, we needed simpler
aromatic acetals and ketals as substrates. We chose to replace the 2-(prop-1-yn-1-
yl)triphenylene aromatic part by the simpler benzene and naphthalene moieties. These
simpler substrates lacking the triple bond should facilitate the study of their
photohydrolysis. These simpler chromophores could be excited with medium-pressure
Hg-lamps, equipped with a Pyrex filter in the case of naphthalene derivatives. The
aromatic rings we used were unsubstituted, so any special effect of the substituent could
be excluded. If photodeprotection was observed with these simple model compounds,
this could confidently be attributed to the change of electron distribution in the excited
state, and no other effect would have to be invoked.
Trialkyl orthoformates (¼tris(alkyloxy)methanes) in the presence of acid catalysts
have been reported to convert carbonyl groups efficiently into their corresponding
acetals [25]. Many of the reported procedures require a large excess of reagents, long
reaction times or harsh reaction conditions, and moisture-sensitive and expensive
reagents. Some of these acetal-forming reaction conditions were not giving satisfactory
results for the acetalization of aromatic ketones.
The starting materials 5 – 16 needed for our studies were synthesized adapting the
procedure described by Patel and co-workers [26] (Scheme 4 and Table 1). At room
temperature, tetrabutylammonium tribromide (0.04 equiv.) functioned as promoter in
the presence of trimethyl orthoformate (2.2 equiv.) and the corresponding alcohol (8.0
equiv.) in an adequate solvent. The in situ generation of HBr from tetrabutylammo-
nium tribromide [27] is supposed to catalyze the reaction. The acetalization of
naphthalene-2-carboxaldehyde gave 80% of acetal 8 in dry MeOH and 96% in dry
MeNO2. In our hands, MeCN was the best solvent for the synthesis of benzene-based
ketals and acetals 13 – 16 (Table 2, Entries 9 and 10). MeNO2 led to the best yield in the

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Scheme 4a)
a) Trimethyl orthoformate, tetrabutylammonium tribromide (Bu4NBr3), solvent (Method A: MeCN;
Method B: MeNO2).
a) For Ar, R, and R’, see Table 1.
Table 1. Starting Ketals and Acetals 5 – 16
Ar
R’
RO or ORꢀRO
Ar
R’
RO or ORꢀRO
5
6
7
8
9
10
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
Me
Me
Me
H
H
H
MeO
11
12
13
14
15
16
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
H
H
H
MeO
OCH2CH2O
OCH2CH2CH2O
MeO
OCH2CH2O
OCH2CH2CH2O
OCH2CH2O
OCH2CH2CH2O
MeO
OCH2CH2O
OCH2CH2CH2O
Table 2. Synthesis of Starting Ketals and Acetals 5 – 13 and 16
Entry
Ar
R’
ROH or HORꢀROH
Method
Product (yield [%])
1
2
3
4
5
6
7
8
9
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
naphthalen-2-yl
Ph
Ph
Ph
Ph
Me
Me
Me
H
H
H
Me
Me
Me
H
MeOH
B
B
B
B
B
B
A
A
A
A
5 (90)
6 (81)
7 (75)
8 (96)
9 (79)
10 (85)
11 (95)
12 (80)
13 (74)
16 (90)
HO(CH2)2OH
HO(CH2)3OH
MeOH
HO(CH2)2OH
HO(CH2)3OH
MeOH
HO(CH2)2OH
HO(CH2)3OH
HO(CH2)3OH
10
naphthalene series 5 – 10 (Entries 1 – 6). The acetals and ketals could be isolated under
these conditions in 74 – 96% yield (Table 2).
Our preliminary photodeprotection tests were conducted with 2-(dimethoxyme-
thyl)naphthalene (8) in a quartz NMR tube in CDCl3 as solvent and were monitored by
1H-NMR measurements. To avoid photochemical heating of the solutions and thereby
creating a thermal activation, we used a water-cooling system to maintain the
temperature at 268. Thus, 75% of 8 was consumed after 60 min of irradiation to form
65% of aldehyde 17 and 10% of the photooxidation product 18 (Scheme 5).
Simultaneously, a control experiment without UV irradiation showed no sign of
hydrolysis. A purely thermal deprotection of the acetal could be excluded by this result.

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It is known that treating CHCl3 solutions by intensive UV irradiation leads to the
formation of HCl [28]. The photochemically induced formation of small amounts of
HCl in CHCl3 could be possibly explain the hydrolysis of acetal 8. We, therefore,
replaced CHCl3 by MeCN. MeCN possesses no end absorption extending towards the
emission lines of a medium-pressure Hg-lamp. Thus, the solvent is not activated directly
upon UV irradiation, and we could find no report showing the formation of protons
under UV irradiation with the Hg-lines 254 or 366 nm for short times.
Scheme 5
To compare the photolability of the various benzene- and naphthalene-based
acetals and ketals, we developed standardized conditions for the photolysis experi-
ments. The reactions were conducted on a 10 mg scale (0.50 mmol) of the acetal or
ketal. The latter was dissolved in CD3CN (0.6 ml), and H2O (32 ml) was added, to give a
0.08m solution of CD3CN/H2O 95 :5. The solution was transferred to an NMR quartz
tube (diameter 5 mm) and irradiated with a 125 W Hg-lamp by using the 254 nm and/or
the 365 nm bands depending on the filter used (quartz or Pyrex). The reaction was
monitored by 1H-NMR measurements observing the decreasing starting-material
concentration and increasing formation of products. At the end of the reaction, an
aliquot (150 ml) of the sample was analyzed by GC (see Exper. Part) identifying the
peaks by comparison with samples of known structure.
We tested the influence of the presence of H2O in the reaction media on the
photodeprotection of 8 by using a Pyrex filter (Table 3). At least 5% of H2O were
needed to obtain a good conversion. Increasing the amount of H2O from 5 to 50% did
not significantly change the outcome of the reaction. The limit of solubility of 8 at a
concentration of 10ꢀ2 m was reached in 65% of H2O/MeCN.
Table 3. Influence of Solvent and Time on the Photodeprotection (Pyrex filter) of Acetal 8 (Scheme 5)
Solvent
Time [min]
Yield [%]
8
17
18
CD3CN
200
20
20
20
20
84
38
n.d.a)
n.d.a)
n.d.a)
n.d.a)
56
97
100
100
16
6
3
n.d.a)
n.d.a)
CD3CN/H2O 99.5 : 0.5
CD3CN/H2O 95 : 5
CD3CN/H2O 50 : 50
CD3CN/H2O 36 : 64
a) n.d. ¼ not detected.
Irradiating the naphthalene-based ketals 5 – 7, dissolved in CD3CN/H2O 95 :5, in a
quartz vessel, total transformation to 2-acetonaphthone (¼1-(naphthalen-2-yl)etha-
none; 19) could be achieved within 20 min (Scheme 6 and Table 4, Entries 1, 3, and 5).

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Helvetica Chimica Acta – Vol. 94 (2011)
Irradiating selectively at 365 nm by using a Pyrex filter led to very good conversions (93
to 100%) to the ketone 19 within 30 to 60 min (Table 4, Entries 2, 4, and 6). Longer
irradiation time did not lead to higher conversions. Despite the low molar extinction of
compounds 5 – 10 at 366 nm, their photodeprotection was still very efficient when a
Pyrex filter was used (irradiation at 365 nm, see Entries 2, 4, and 6 in Table 4). The
photodeprotection could be successfully done on a 1-g scale with 5 as substrate. A 4
times higher concentration of the starting material 5 than that described above still
allowed an efficient photodeprotection. After 20 min of irradiation, evaporation of the
solvent, followed by a recrystallization in MeOH, the product 19 could be isolated in
96% yield with high purity.
Scheme 6
Table 4. Photodeprotection of Naphthalene-Based Ketals 5 – 7 under Different Conditions (Scheme 6)
Entry Ketal lirradiation (filter) [nm] Time [min] Yield of 19 [%] ROH or HORꢀROH (product)
1
2
3
4
5
6
5
5
6
6
7
7
254, 365 (quartz)
365 (Pyrex)
254, 365 (quartz)
365 (Pyrex)
254, 365 (quartz)
365 (Pyrex)
20
30
20
40
20
60
100
100
100
93
100
94
MeOH
MeOH
HO(CH2)2OH
HO(CH2)2OH
HO(CH2)3OH
HO(CH2)3OH
Irradiating the naphthalene-based acetals 8 – 10 dissolved in CD3CN/H2O 95 :5
through a Pyrex filter, slightly different results were obtained compared to experiments
reported for the naphthalene-based ketals 5 – 7 (Scheme 7 and Table 5). The acyclic
acetal 8 was completely deprotected to aldehyde 17 after 20 min irradiation with the
formation of 3% of by-product 18 formed by photooxidation (Table 5, Entry 1);
whereas the cyclic acetals 9 and 10 formed no or a very low amount (4%) of aldehyde
17, even when we irradiated for 200 min (Entries 2 and 3). The major products in those
cases were the corresponding ring-opened esters 20 and 21 formed in 20 and 9% yield,
respectively. Irradiation of 8 – 10 in a quartz vessel led to a similar product distribution.
Degassing the solvent by bubbling N2 for 20 min prior to irradiation did not diminish
the importance of the ester formation significantly. A control experiment with the
naphthalene-2-carboxaldehyde (17) under the photodeprotection conditions showed
that the product is photostable even after 200 min of irradiation. The photodepro-
tection could be applied successfully on a synthetic scale to 1 g of 8. Under the
conditions described above but with a 4 times higher concentration of the starting
material 8, 17 could be isolated in pure form in a 98% yield after evaporation of the
solvent and recrystallization in MeOH.

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339
Scheme 7
Table 5. Photodecomposition (Pyrex filter) of Naphthalene-Based Acetals 8 – 10 (Scheme 7)
Entry
Acetal
Time [min]
Yield [%]
acetal
ROH or HORꢀROH (product)
17
ester
1
2
3
8
9
10
20
200
200
n.d.a) (8)
76 (9)
91 (10)
97
3 (18)
20 (20)
9 (21)
MeOH
HO(CH2)2OH
HO(CH2)3OH
4
n.d.a)
a) n.d. ¼ not detected.
The benzene-based ketals 11 – 13 absorbed exclusively below 300 nm. Therefore,
we irradiated all the samples in a quartz vessel. Photodeprotection of the ketals to
acetophenone (¼1-phenylethanone; 22) occurred but the desired products were
accompanied by the products of detrimental side reactions (Scheme 8 and Table 6). It is
well documented that H-abstraction may occur from the triplet excited state of
acetophenone [29]. Several reactions starting from the ketyl radical have been reported
in the literature. The generated transient ketyl radical may dimerize to give pinacols, it
may undergo further H-abstraction or it may combine with the donor radical yielding
Scheme 8
Table 6. Photodecomposition (Quartz vessel) of Benzene-Based Ketals 11 – 13 (Scheme 8)
Ketal
Time [min]
Yield [%]
ketal
22
by-products
11
12
13
20
20
20
8 (11)
69
97
73
23
3
22
n.d.a) (12)
1 (13)
a) n.d. ¼ not detected.

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adducts. In the cases of the photodeprotection of 11 – 13, we identified by GC/MS
analysis three different by-products. The pinacol adduct 23 was the only by-product
characterized. In a control experiment, acetophenone (22) was irradiated under the
same conditions for 20 min and formed the same mixture of three products with a
conversion of 65%.
Irradiating the benzene-based acetals 14 – 16 dissolved in CD3CN/H2O 95 :5 in a
quartz vessel did not show any formation of benzaldehyde 24 even when we irradiated
for 200 min (Scheme 9 and Table 7). The only products formed were the corresponding
esters 25 – 27 in 8, 11, and 7%, respectively. Degassing the solvent by bubbling N2 before
irradiation did not lead to significant changes in the amount of ester formed.
Scheme 9
Table 7. Photodecomposition (Quartz vessel) of Benzene-Based Acetals 14 – 16 (Scheme 9)
Acetal
Time [min]
Yield [%]
acetal
24
ester
14
15
16
50
200
200
92 (14)
89 (15)
93 (16)
n.d.a)
n.d.a)
n.d.a)
8 (25)
11 (26)
7 (27)
a) n.d. ¼ not detected.
3. Discussion. – The photohydrolysis can be successfully applied to a series of simple
aromatic acetals and ketals. Adding at least 5% of H2O to the polar solvent MeCN is
necessary to perform the reaction efficiently. The acyclic and cyclic naphthalene-based
ketals 5 – 7 could be deprotected cleanly and efficiently by UV irradiation, whereas the
cyclic naphthalene-based acetals 9 and 10 could not be transformed into the
corresponding aldehyde. Comparing the results from the six different ketals 5 – 7 and
11 – 13 showed that the photodeprotection can be achieved efficiently. The reactions
with the benzene-based ketals 11 – 13 as starting material suffered from a subsequent
photodegradation of the released acetophenone (22). In the case of the cyclic acetals 9,
10, 15, and 16, a photooxidation process became the major pathway forming the
corresponding ring-opened ester 20, 21, 26, and 27 in moderate yields.
The mechanism of this novel photoinduced hydrolysis of aromatic acetals and ketals
remains to be studied in depth. A hypothetical mechanism (Scheme 10) can be
proposed, based on the mechanism suggested by Lin and Lawrence [19] for the
photouncaging of coumarin acetal derivatives. Excitation of the chromophore should
induce a heterolytic-bond photodissociation [30] to generate a resonance-stabilized

Helvetica Chimica Acta – Vol. 94 (2011)
341
Scheme 10
carbocation as transient species. The intermediate created after the heterolytic-bond-
breaking process is supposed to be captured by H2O. The hemiacetal obtained by this
process is then thermally transformed into the carbonyl compound. At this point of our
studies, it cannot be excluded that the reaction might proceed via a radical-cation
species. Miranda and co-workers [31] have reported the triphenylpyrylium tetrafluoro-
borate sensitized photolysis of acetals occurring via radical-cation species.
Summarizing the experimental evidence collected, the difference of reactivity
observed in the photodeprotection between the naphthalene-based ketals 5 – 7 and the
acetals 8 – 10 is consistent with the difference in reactivity in the thermal hydrolysis. The
2-methyl-2-(naphthalen-2-yl)-1,3-dioxane derivatives have been shown to hydrolyze
ca. 7 – 8 times faster than their corresponding 2-(naphthalen-2-yl)-1,3-dioxane [32].
The authors proposed that the presence of the axial Me group in cyclic ketals may
accelerate the hydrolysis. The release of strain when ring opening occurs is cited as
reason for the rate difference.
In the cases of benzene- and naphthalene-based acetals, the major side reaction is a
competing photooxydation to the ester. The side reaction is probably due to the
formation and the reaction of singlet oxygen or of a photocreated reactive species with
triplet oxygen. Direct oxidation of acetals to the corresponding esters involving
molecular oxygen and a radical generator has been previously described [33].
4. Conclusion. – We detected a novel, efficient photodeprotection reaction
independent on the presence of specific substituents at the aromatic ring. We showed
that naphthalene-based ketals 5 – 7 and 2-(prop-1-yn-1-yl)triphenylene-based acetal 3
cleanly release simple alcohols upon exposure to UV light in an excellent yield. The 2-
acetonaphthone moiety (see 19) represents an interesting platform for further
application as a PPGs of 1,2- and 1,3-diols derivatives such as carbohydrates.
We thank Prof. Christian Bochet for the helpful discussion. We thank Dr. Armelle Vallat-Michel
(SAF UniNE) for assistance with mass spectrometry and Dr. Julien Furrer (SAF UniNE) for NMR
spectroscopic assistance. This work was financially supported by the Swiss National Science Foundation
ˆ
(grants 200021_121846 and 200020_124696) and the University of Neuchatel.

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Helvetica Chimica Acta – Vol. 94 (2011)
Experimental Part
General. Purchased chemicals were used without further purification. The solvents CH2Cl2, toluene,
and MeCN were purchased from SigmaꢀAldrich with the grade puriss., absolute, on molecular sieve
(H2O ꢁ 0.005%), ꢂ 99.5% (GC). Compounds 14 (benzaldehyde dimethyl acetal) was purchased from
Acros in puriss. grade. Compounds 15 (2-phenyl-1,3-dioxolane), 17 (2-naphthaldehyde), 19 (2-
acetonaphthone) were bought from SigmaꢀAldrich in puriss. grade. All photolyses were conducted in
a quartz NMR tube or a quartz vessel at 208 with water circulation in a photochemical reactor equipped
with a medium-pressure Hg-lamp (HPK125 Philips; 125 W; emission lines: 253.7, 296.7, 365.4, 404.7, and
435.8 nm) and a Pyrex (>280 nm) or quartz filter (>190 nm). TLC: Merck 60 F254 acidic silica-gel-coated
(SiO2) or neutral Al2O3-coated Al plates, 0.2 mm; detection under UV light (254 nm) or with KMnO4
soln. Column chromatography (CC): SiO2 60 ꢄ, 32 – 63 mm (Brunschwig). M.p.: Gallenkamp melting-
point apparatus; uncorrected. GC: Agilent-6850A (column HP-1, length 3 m, i.d. 0.32 mm, film 0.25 mm);
temp. gradient: initial temp. 1008 for 3 min, then heating 158/min up to 2808, further 8 min at 2808. UV/
VIS: Varian-Cary300-1E UV/VIS spectrophotometer; lmax (e[l molꢀ1 cmꢀ1]) in nm. Fluorescence
spectra: Perkin-Elmer-LS-50-B luminescence spectrometer. 1H- and 13C-NMR Spectra: Bruker-Avance-
400 instrument; at 298 K; d in ppm rel. to the used solvent, J in Hz; NMR solvents from Cambridge
ˆ
Isotope Laboratories, Inc. ESI-MS: Finnigan-LCQ mass spectrometer at the University of Neuchatel; in
m/z. HR-MS: Bruker-BioAPEX-II-Daltonics apparatus at the University of Fribourg; performed by Mr.
F. Nydegger; in m/z.
1. 3,6,7,10,11-Pentakis(hexyloxy)triphenylen-2-yl 1,1,1-Trifluoromethanesulfonate (2) [34]. To a soln.
of 1 (1.05 g, 1.27 mmol) in anh. CH2Cl2 (15 ml) at 08, a soln. of ꢂB-bromocatecholboraneꢃ (3.18 mmol,
0.3m in CH2Cl2) was added. Then the mixture was stirred at r.t. for 24 h. The mixture was poured into ice-
water and extracted with CH2Cl2, the combined org. layer dried (Na2SO4) and concentrated to 2 ml, and
the concentrate filtered through a pad of SiO2 with CH2Cl2 (to remove the catechol). The CH2Cl2 soln.
was concentrated to 5 ml, and pyridine (315 ml, 3.91 mmol) was added. Then 1,1,1-trifluoromethane-
sulfonic anhydride (255 ml, 1.52 mmol) was added slowly, and the mixture was stirred at r.t. for 1 h under
Ar. The reaction was quenched with H2O (5 ml), the mixture extracted with CH2Cl2 (3 ꢃ 15 ml), the
combined org. layer washed with 1m HCl (20 ml) and brine (20 ml), dried (Na2SO4), and concentrated,
the residue purified by CC (SiO2, CH2Cl2/petroleum ether 1:1), and then the product recrystallized from
EtOH: 2 (444 mg, 40% over 2 steps). Purple solid. Rf (CH2Cl2/petroleum ether 1:1) 0.55. M.p. 172 – 1758.
UV/VIS (CHCl3): 280 (134000). 1H-NMR (CDCl3, 400 MHz): 8.21 (s, HꢀC(1)); 7.89 (s, HꢀC(4)); 7.83 (s,
HꢀC(5)); 7.81 (s, HꢀC(8), HꢀC(9)); 7.73 (s, HꢀC(12)); 4.28 – 4.22 (m, 5 CH2O); 2.01 – 1.89 (m,
5 CH2CH2O); 1.67 – 1.53 (m, 5 CH2CH2)2O); 1.49 – 1.35 (m, 5 CH2CH2(CH2)3O); 1.01 – 0.89 (m,
5 Me(CH2)5O). 13C-NMR (CDCl3, 100 MHz)1): 150.6 (C(7)); 149.8, 149.7 (C(10,11)); 149.2 (C(3,6));
138.6 (C(2)); 129.8 (C(15)); 125.4 (C(17)); 123.7 (C(18)); 123.1 (C(13,14)); 122.5 (C(16)); 119.1 (q,
1J(C,F) ¼ 320, CF3); 116.9 (C(1)); 108.3 (C(4)); 107.5 (C(9)); 107.0, 106.8, 106.7 (C(5,8,12)); 70.2, 70.0,
69.70, 69.66, 69.6 (5 CH2O); 31.9 (CH2(CH2)3O); 29.7 (CH2CH2O); 26.1 (CH2(CH2)2O); 22.9
(CH2(CH2)3O); 14.3 (Me(CH2)5O). HR-MS: 899.4712 ([M þ Na]þ, C49H71F3NaO8Sþ; calc. 899.4719).
2. 2-(3,3-Diethoxyprop-1-yn-1-yl)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (3). To a soln. of 2
(220 mg, 0.250 mmol), 3,3-diethoxyprop-1-yne (55 ml, 0.37 mmol), CuI (10 mg, 0.050 mmol), Ph3P
(20 mg, 0.075 mmol), and (i-Pr)2NH (0.15 ml) in degassed toluene (1.0 ml) was added [Pd(Ph3P)4]
(29 mg, 0.025 mmol dissolved in 1.0 ml of toluene) under Ar. The homogeneous mixture was stirred at
608 for 20 h (TLC monitoring). After completion of the reaction, the mixture was filtered through Celite,
H2O (20 ml) was added, and the mixture was extracted with Et2O (3 ꢃ 20 ml). The org. layer was washed
with brine (30 ml), dried (anh. Na2SO4), and concentrated. The yellow solid was purified by CC (SiO2)
followed by recrystallization from dioxane/MeOH: 3 (161 mg, 79%). Yellow solid. Rf (CH2Cl2/petroleum
1
ether 2 :3) 0.20. M.p. 133 – 1368. UV/VIS (MeCN): 284 (124000). H-NMR (CDCl3, 400 MHz): 8.56 (s,
HꢀC(1)); 7.86 (s, HꢀC(5), HꢀC(12)); 7.81 (s, HꢀC(8), HꢀC(9)); 7.75 (s, HꢀC(4)); 5.63 (s, CH(OEt)2);
4.24 (m, 5 CH2O); 3.94 (dq, J ¼ 9.5, 7.1, A of ABX3, 2 MeCH2); 3.75 (dq, J ¼ 9.5, 7.1, B of ABX3,
2 MeCH2); 1.94 (m, 5 CH2CH2O); 1.57 (m, 5 CH2(CH2)2O); 1.41 (m, 5 CH2CH2(CH2)3O); 1.33 (t, J ¼ 7.1,
X of ABX3, 2 MeCH2); 1.01 – 0.89 (m, 5 Me(CH2)5O). NOESY (CDCl3): irrad. HꢀC(1) (8.56) ! enh.
HꢀC(12) (7.86); irrad. HꢀC(4) (7.75) ! enh. HꢀC(5) (7.86) and CH2O (4.24); irrad. HꢀC(5) þ HꢀC(12)

Helvetica Chimica Acta – Vol. 94 (2011)
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(7.86) ! enh. HꢀC(1) (8.56), HꢀC(4) (7.75), and CH2O (4.24); irrad. HꢀC(8) þ HꢀC(9) (7.81) ! enh.
CH2O (4.24). 13C-NMR (CDCl3, 100 MHz)1): 158.0 (C(3)); 150.4 (C(6)); 149.7 (C(10)); 149.1 (C(11));
149.0 (C(7)); 130.9 (C(15)); 129.2 (C(1)); 125.6 (C(17)); 123.6 (C(13)); 123.1 (C(14)); 122.9 (C(18));
122.7 (C(16)); 111.2 (C(2)); 108.3 (C(5)); 107.7 (C(9)); 107.0 (C(8)); 106.4 (C(12)); 104.4 (C(4)); 92.3
(CH(OEt)2); 88.5 (CꢄCCH(OEt)2); 82.6 (CꢄCCH(OEt)2); 70.14, 70.09, 69.6, 69.5, 69.1 (5 CH2O); 61.2
(MeCH2O); 31.84, 31.83 (CH2)(CH2)3O; 29.6, 29.5 (CH2CH2O); 26.00, 25.98 (CH2(CH2)2O); 22.8
(CH2(CH2)4O); 15.4 (MeCH2O); 14.2 (Me(CH2)5O). HR-MS: 877.5963 ([M þ Na]þ, C55H82NaO7þ ; calc.
877.5958).
3. 3-[3,6,7,10,11-Pentakis(hexyloxy)triphenylen-2-yl]prop-2-ynal (4). A soln. of 3 (3.3 mg,
0.004 mmol) in CD3CN/CDCl3/H2O 70 :25 :5 (0.832 ml) was irradiated for 20 min in an NMR quartz
tube. The solvent was evaporated, and the residue analyzed by 1H-NMR: 4 (3.0 mg, quant.) Yellow solid.
UV/VIS (MeCN): 278 (13100). 1H-NMR (CDCl3, 400 MHz): 9.53 (s, CꢄCCHO); 8.64 (s, HꢀC(1));
7.83 – 7.77 (m, HꢀC(5), HꢀC(8), HꢀC(9), HꢀC(12)); 7.75 (s, HꢀC(4)); 4.30 – 4.21 (m, 5 CH2O); 1.95 (m,
5 CH2CH2O); 1.65 – 1.56 (m, 5 CH2(CH2)2O); 1.41 (m, 5 CH2CH2(CH2)2O); 1.05 – 0.90 (m, 5 Me-
(CH2)5O). 13C-NMR (CDCl3, 100 MHz)1): 177.0 (CꢄCCHO); 158.7 (C(3)); 151.1, 149.9, 149.1, 149.0
(C(6,7,10,11)); 133.2 (C(2)); 131.4 (C(1)); 126.3, 123.8, 123.2, 123.1, 122.9, 122.4 (C(13,14,15,16,17,18));
108.3, 107.6, 106.7, 106.0 (C(5,8,9,12)); 104.6 (C(4)); 94.0 (CꢄCCHO); 93.1 (CꢄCCHO); 70.1, 69.9, 69.5,
69.4, 69.3 (CH2O); 31.8 (CH2(CH2)3O); 29.6 (CH2CH2O); 26.0 (CH2(CH2)2O); 22.8 (CH2(CH2)4O); 14.2
(Me(CH2)5O). ESI-MS: 803.52 ([M þ Na]þ).
4. Aromatic Acetals and Ketals 5 – 13 and 16: General Procedure [26]. To a soln. of carbonyl
compound (0.640 mmol), trimethyl orthoformate (155 ml, 1.410 mmol), and the corresponding alcohol
(5.120 mmol) in dry MeCN (2.0 ml, Method A) or dry MeNO2 (2.0 ml, Method B) was added
tetrabutylammonium tribromide (12 mg, 0.025 mmol). The homogeneous mixture was stirred at r.t.
(TLC and GC monitoring). After completion of the reaction, the mixture was poured into NaHCO3 soln.
(10 ml) and extracted with Et2O (3 ꢃ 10 ml), and the org. layer dried (anh. Na2SO4) and concentrated.
The crystalline compounds 6, 7, 8, 10, 12, and 13 were purified by recrystallization from MeOH.
5. 2-(1,1-Dimethoxyethyl)naphthalene (5). According to Exper. 4 (Method B): 125 mg (90%). Clear
oil. Rf (CH2Cl2/petroleum ether 1:1) 0.30. UV/VIS (MeCN): 224 (108000), 254 (3500), 366 (<200).
1H-NMR (CD3CN, 400 MHz): 8.01 (s, HꢀC(1)); 7.93 – 7.85 (m, 3 arom. H); 7.58 (dd, J ¼ 8.6, 1.8, 1 arom.
H); 7.52 – 7.49 (m, 2 arom. H); 3.18 (s, 2 MeO); 1.57 (s, Me). 13C-NMR (CD3CN, 100 MHz): 142.1 (C(2)
(naph)); 134.4, 134.2 (q, arom. C); 129.6, 129.1, 128.8, 127.5, 127.4, 126.6, 125.8 (arom. CH); 102.9 (q, ketal
C); 49.8 (MeO); 26.8 (Me). ESI-MS: 239.1 ([M þ Na]þ).
6. 2-Methyl-2-(naphthalen-2-yl)-1,3-dioxolane (6) [35]. According to Exper. 4 (Method B): 111 mg
(81%). White solid. M.p. 58.5 – 60.28. Rf (CH2Cl2/petroleum ether 1:1) 0.20. UV/VIS (MeCN): 224
1
(93000), 254 (3500), 366 (<100). H-NMR (CD3CN, 400 MHz): 7.98 (s, 1 arom. H); 7.91 – 7.87 (m, 3
arom. H); 7.60 (m, 1 arom. H); 7.52 (m, 2 arom. H); 4.04 (m, A of AA’BB’, CH2O); 3.78 (m, B of AA’BB’,
CH2O); 1.68 (s, Me). 13C-NMR (CD3CN, 100 MHz): 140.6 (C(2) (naph)); 133.0, 128.2, 128.1, 127.6, 126.4,
126.1, 126.0, 124.0, 123.7 (q, arom. C); 109.0 (q, ketal C); 64.5 (OCH2CH2O); 27.6 (Me). ESI-MS: 214.1
([M þ H]þ).
7. 2-Methyl-2-(naphthalen-2-yl)-1,3-dioxane (7). According to Exper. 4 (Method B): 124 mg (85%).
White solid. M.p. 90.5 – 93.88. Rf (CH2Cl2/petroleum ether 1:1) 0.20. UV/VIS (MeCN): 224 (92000), 254
(3500), 366 (1200). 1H-NMR (CD3CN, 400 MHz): 7.94 – 7.89 (m, 4 arom. H); 7.55 – 7.51 (m, 3 arom. H);
3.89 – 3.85 (m, 2 Heq of CH2CH2); 3.82 – 3.75 (m, 2 Hax of CH2CH2O); 2.06 – 1.92 (m, Hax of CH2CH2O);
1.49 (s, Me); 1.25 (dtt, J ¼ 13.5, 2.7, 1.5, Heq of CH2CH2O). 13C-NMR (CD3CN, 100 MHz): 140.2 (C(2)
(naph)); 134.4, 133.8 (q, arom. C); 129.3, 129.0, 128.5, 127.2, 127.1, 126.9, 125.7 (arom. CH); 101.2 (q, ketal
C); 62.0 (CH2CH2O); 32.6 (CH2CH2O); 26.3 (Me). ESI-MS: 229.1 ([M þ H]þ).
8. 2-(Dimethoxymethyl)naphthalene (8) [36]. According to Exper. 4 (Method B): 124 mg (96%).
Clear oil. Rf (CH2Cl2/petroleum ether 1:1) 0.32. UV/VIS (MeCN): 224 (158000), 254 (5500), 366
(< 300). 1H-NMR (CD3CN, 400 MHz): 7.91 (m, 4 arom. H); 7.53 (m, 3 arom. H); 5.51 (s, CH(OMe)2);
3.34 (s, 2 MeO). 13C-NMR (CD3CN, 100 MHz): 137.2 (C(2) (naph)); 134.4, 133.9 (arom. C); 129.2, 128.9,
128.6, 127.3, 127.2, 126.7, 125.4 (arom. CH); 104.4 (acetal CH); 53.5 (MeO). HR-MS: 225.0888 ([M þ
Na]þ, C13H14NaOþ2 ; calc. 225.0891).

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9. 2-(Naphthalen-2-yl)-1,3-dioxolane (9) [37]. According to Exper. 4 (Method B): 101 mg (79%).
White solid. Rf (CH2Cl2/petroleum ether 1:1) 0.20. UV/VIS (MeCN): 224 (82000), 254 (4100), 366
(<100). 1H-NMR (CD3CN, 400 MHz): 7.93 (m, 4 arom. H); 7.56 (m, 3 arom. H); 5.90 (s, HꢀC(2) (diox));
4.15 (m, A of AA’BB’, CH2O); 4.04 (m, B of AA’BB’, CH2O). 13C-NMR (CD3CN, 100 MHz): 135.3 (C(2)
(naph)); 133.9, 133.0 (q, arom. C); 128.4, 128.3, 127.8 (arom. CH(4,5,8)); 126.4, 126.2 (arom. CH(6,7));
126.1 (arom. CH(1)); 123.8 (arom. CH(3)); 104.0 (acetal CH); 65.4 (OCH2CH2O). HR-MS: 223.0730
([M þ Na]þ; C13H12NaOþ2 ; calc. 223.0735).
10. 2-(Naphthalen-2-yl)-1,3-dioxane (10) [38]. According to Exper. 4 (Method B): 103 mg (75%).
White solid. Rf (CH2Cl2/petroleum ether 1:1) 0.18. M.p. 82.1 – 83.98. UV/VIS (MeCN): 222 (124000), 254
(4800), 366 (< 300). 1H-NMR (CD3CN, 400 MHz): 7.97 (br. s, arom. HꢀC(1)); 7.85 (m, arom. HꢀC(4),
HꢀC(5), HꢀC(8)); 7.60 (dd, J ¼ 8.5, 1.7, arom. HꢀC(3)); 7.48 (m, arom. HꢀC(6), HꢀC(7)); 5.68 (s,
HꢀC(2) (dioxane)); 4.33 (ddd, J ¼ 13.0, 5.0, 2.7, 2 Heq of CH2CH2O); 4.06 (ddd, J ¼ 13.0, 12.5, 1.5, 2 Hax of
CH2CH2O); 2.29 (dtt, J ¼ 13.5, 12.4, 5.0, Hax of CH2CH2O); 1.50 (dtt, J ¼ 13.5, 2.7, 1.5, Heq of CH2CH2O).
13C-NMR (CD3CN, 100 MHz): 136.1 (arom. C(2)); 133.6, 133.1 (q, arom. C); 128.4, 128.1, 127.7 (arom.
CH(4,5,8)); 126.2, 126.0 (arom. CH(6,7)); 125.3 (arom. CH(1)); 123.8 (arom. CH(3)); 101.8 (q, acetal
CH); 67.5 (CH2CH2O); 32.6 (CH2CH2O); 25.9 (Me). HR-MS: 237.0886 ([M þ Na]þ; C14H14NaO2þ ; calc.
237.0891).
11. (1,1-Dimethoxyethyl)benzene (11) [26]. According to Exper. 4 (Method A): 101 mg (95%).
Yellow oil. Rf (CH2Cl2/petroleum ether 1:2) 0.85. 1H-NMR (CD3CN, 400 MHz): 7.46 (m, 2 arom. H); 7.37
(m, 2 arom. H); 7.30 (tt, J ¼ 6.3, 1.4, HꢀC(4)); 3.12 (s, 2 MeO); 1.48 (s, Me). 13C-NMR (CD3CN,
100 MHz): 142.9 (arom. C(1)); 128.1, 127.5, 126.0 (arom. CH); 101.6 (q, ketal C); 49.0 (MeO); 26.0 (Me).
ESI-MS: 189.2 ([M þ Na]þ).
12. 2-Methyl-2-phenyl-1,3-dioxolane (12) [35]. According to Exper. 4 (Method A): 93 mg (88%).
White solid. Rf (CH2Cl2/petroleum ether 1:2) 0.80. 1H-NMR (CD3CN, 400 MHz): 7.47 (m, 2 arom. H);
7.35 (m, 3 arom. H); 3.99 (m, A of AA’BB’, CH2O); 3.73 (m, B of AA’BB’, CH2O); 1.58 (s, Me).
13C-NMR (CD3CN, 100 MHz): 143.8 (arom. C(1)); 128.7, 128.2, 125.7 (arom. CH); 109.2 (q, ketal C); 64.8
(OCH2CH2O); 26.0 (Me). ESI-MS: 187.2 ([M þ Na]þ).
13. 2-Methyl-2-phenyl-1,3-dioxane (13) [39]. According to Exper. 4 (Method A): 84 mg (74%).
White solid. Rf (CH2Cl2/petroleum ether 1:2) 0.80. 1H-NMR (CD3CN, 400 MHz): 7.40 (m, 4 arom. H);
7.32 (m, 1 arom. H); 3.81 (ddd, J ¼ 13.0, 5.0, 2.7, 2 Heq of CH2CH2); 3.70 (ddd, J ¼ 13.0, 12.5, 1.5, 2 Hax of
CH2CH2); 1.98 (dtt, J ¼ 13.5, 12.4, 5.0, Hax of CH2CH2O); 1.40 (s, Me); 1.27 (dtt, J ¼ 13.5, 2.7, 1.5, Heq of
CH2CH2O). 13C-NMR (CD3CN, 100 MHz): 141.7 (arom. C(1)); 129.1, 129.0, 128.2 (arom. CH); 100.9 (q,
ketal C); 61.6 (CH2CH2O); 32.8 (CH2CH2O); 26.0 (Me). ESI-MS: 201.1 ([M þ Na]þ).
14. 2-Phenyl-1,3-dioxane (16) [40][41]. According to Exper. 4 (Method A): 94 mg (90%). Clear oil.
Rf (CH2Cl2/petroleum ether 1:4) 0.60. 1H-NMR (CD3CN, 400 MHz): 7.45 – 7.35 (m, 5 arom. H); 5.51 (s,
HꢀC(2) (dioxane)); 4.18 (ddd, J ¼ 13.0, 5.0, 2.7, 2 Heq of CH2CH2O); 3.98 (ddd, J ¼ 13.0, 12.5, 1.5, 2 Hax of
CH2CH2O); 2.09 (dtt, J ¼ 13.5, 12.4, 5.0, Hax of CH2CH2O); 1.45 (dtt, J ¼ 13.5, 2.7, 1.5, Heq of CH2CH2O).
13C-NMR (CD3CN, 100 MHz): 138.5 (arom. C(1); 128.1, 127.6, 125.6 (arom. CH); 100.9 (acetal CH); 66.7
(CH2O); 25.2 (CH2CH2O). ESI-MS: 187.2 ([M þ Na]þ).
15. Photoinduced Hydrolysis of Ketal 5 and 8 on a 1-g Scale in a Quartz Vessel: Procedure 1. A soln. of
ketal or acetal (5 mmol) in MeCN/H2O 95 :5 (15 ml) was irradiated for 20 min in a quartz vessel. The
solvent was evaporated and the residue recrystallized from MeOH: 5 (1.081 g) gave 19 (817 mg, 96%),
and 8 (1.011 g) gave 17 (765 mg, 98%).
16. Photoinduced Hydrolysis of Ketals and Acetals 5 – 16 on a 10-mg Scale in an NMR Quartz Tube:
Procedure 2. A soln. of ketal or acetal (0.050 mmol) in CD3CN/H2O 95 :5 (0.632 ml) was irradiated for
1
20 min in an NMR quartz tube, and the mixture was analyzed by GC and H-NMR.
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Received September 1, 2010