Bis(4,5-dimethoxy-2-nitrophenyl)ethylene glycol: A new and efficient photolabile protecting group for aldehydes and ketones

Article abstract of DOI:10.1016/j.tet.2005.04.007

Synthesis of a new photolabile protecting group, bis(4,5-dimethoxy-2- nitrophenyl)ethylene glycol (4) from 4,5-dimethoxy-2-nitrobenzyl alcohol in three steps in good yields is described. The acetals and ketals of 4 are stable against acidic and basic reaction conditions and are cleaved smoothly on irradiation at 350 and 400 nm with regeneration of carbonyl compounds in high yields and efficiency.

Full text of DOI:10.1016/j.tet.2005.04.007

Tetrahedron 61 (2005) 5849–5854
Bis(4,5-dimethoxy-2-nitrophenyl)ethylene glycol: a new and
efficient photolabile protecting group for aldehydes and ketones
*
Srinivas Kantevari, Ch. Venkata Narasimhaji and Hari Babu Mereyala
Organic Chemistry Division-II, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 31 January 2005; revised 22 March 2005; accepted 7 April 2005
Available online 10 May 2005
Abstract—Synthesis of a new photolabile protecting group, bis(4,5-dimethoxy-2-nitrophenyl)ethylene glycol (4) from 4,5-dimethoxy-2-
nitrobenzyl alcohol in three steps in good yields is described. The acetals and ketals of 4 are stable against acidic and basic reaction
conditions and are cleaved smoothly on irradiation at 350 and 400 nm with regeneration of carbonyl compounds in high yields and efficiency.
q 2005 Published by Elsevier Ltd.
1. Introduction
wavelength (350 nm). We therefore, undertook the design
of an efficient protecting group in terms of stability and ease
of deprotection. We chose the 4,5-dimethoxy-2-nitrobenzyl
group as it exhibited absorbance at longer wavelength than
the 2-nitrobenzyl group. We describe herein, the synthesis
of the new photolabile protecting group bis(4,5-dimethoxy-
2-nitrophenyl)ethylene glycol 4 from 4,5-dimethoxy-2-
nitrobenzyl alcohol in three steps in good yields. The
acetals and ketals of 4 have been found to be stable under all
normal reaction conditions and were efficiently deprotected
at 350 and 400 nm with liberation of carbonyl compounds in
high yields.
The use of photolabile molecules as protecting groups has
received considerable attention in recent years, as it offers a
mild method to deprotect without the requirement of any
reagent.1 It also provides a greater specificity in the presence
of other protecting groups and can be smoothly handled in
variable acidic, basic and other very sensitive reaction
conditions.2 Although, many photo-removable protecting
groups are known for carboxylic acids,3 amines,4 amides,5
carbamates,6 phenols,7 alcohols8 and phosphates,9,10
surprisingly, less attention has been paid to develop
photo-removable groups for aldehydes and ketones, though,
they are most commonly used in organic synthesis.1a Earlier
6-bromo-4-(1,2-dihydroxyethyl)-7-hydroxy coumarin 1
(Bhc-diol),11 2-nitrophenyl ethylene glycol 212 and bis(2-
nitrophenyl)ethane diol 313 were developed as photolabile
protecting molecules for carbonyl compounds and found to
have certain drawbacks.
2. Results and discussion
4,5-Dimethoxy-2-nitrobenzyl alcohol (5),17 was converted
to 4,5-dimethoxy-2-nitro benzyl chloride (6) by reaction
with PCl5 and treated further, with KOH in DMSO–ethanol
for 45 h to afford stilbene derivative 7 as pale yellow
crystals. Stillbene 7 was dihydroxylated with a catalytic
amount of OsO4/NMO to obtain the diol 4 as a crystalline
pale yellow solid, mp 154–157 8C (Scheme 1).
In order to study the efficiency of the new photosensitive
protecting group, 4 was reacted with various aldehydes
8a–b and ketones 8c–f (entries i–vi, Table 1) by refluxing in
benzene containing a catalytic amount of pyridinium
p-toluenesulphonate (PPTS) to obtain the corresponding
acetals 9a–b and ketals 9c–f, respectively, (Scheme 1).
The products 9a–f were purified by silica gel column
chromatography and characterized by IR, NMR and mass
spectra. It is worth mentioning the absence of double bond
isomerization in enone substrates (entries iii, vi and viii).
Bhc-diol is not a suitable substrate due to the more sensitive
coumarin moiety and diol 2 takes several hours for
photochemical deprotection under UV-light at longer
Keywords: Protecting groups; Cage compounds; Photolysis; Radical ions.
*
Corresponding author. Tel.: C91 40 270 16329; fax: C91 40 271 60123;
e-mail: kantevari@yahoo.com
0040–4020/$ - see front matter q 2005 Published by Elsevier Ltd.
doi:10.1016/j.tet.2005.04.007

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S. Kantevari et al. / Tetrahedron 61 (2005) 5849–5854
Scheme 1. (a) PCl5/CHCl3; (b) KOH/DMSO–ethanol; (c) OsO4/NMO; (d) PPTS, benzene.
˚
The use of 4 A molecular sieves was essential to avoid
deconjucation during the formation of the ketal.
showed very good absorbance at 400 nm (Fig. 1). Accord-
ingly, on irradiation of 9e at 400 nm for 10 h in a Pyrex
vessel, 75% of cyclohexanone 8e was regenerated (Table 1).
The acetals 9a–b and ketals 9c–f were exposed to UV-light
at 350 and 400 nm for 2–10 h to regenerate the carbonyl
compounds 8a–f, respectively, in good yields (Table 1). In a
typical deprotection experiment 0.1 mmol solution of 9e in
acetonitrile in a Pyrex vessel was irradiated at 350 nm using
Rayonet apparatus until TLC showed the disappearance of
the starting material. Chromatography of the residue on
silica gel afforded cyclohexanone 8e in 92% yield.
Examination of the UV–visible spectra revealed that
dimethoxy diol 4 and its acetals 9a–b and ketals 9c–f
The stability of the compounds was tested in a broad variety
of chemical conditions and for commonly used reagents to
assess the general applicability of the new protecting group
(Table 2). Here, it is worth mentioning that the acetals 9a–b
and ketals 9c–f were found to be highly stable in acidic and
basic reaction media. They did not show any significant
decomposition in polar solvents such as DMSO and
alcohols. We noticed that 9e did not undergo reduction
with LiAlH4 (5 equiv) even after 48 h at rt in solvents such
Table 1. Protection and deprotection of aldehydes and ketones with diols 3 and 4
Entry
Diol
Substrate (8)
Ketal
Yield (%)
Photolysis
350 nm
80
400 nm
58
(a)
i
4
4
9a
9b
77
95
(b)
ii
82
68
62
—
(c)
iii
iv
4
4
9c
55
65
(d)
9d
74
—
(e)
v
4
4
3
3
9e
9f
90
73
90
80
92
81
89
78
75
—
50
—
(f)
(e)
(f)
vi
vii
viii
10a
10b

S. Kantevari et al. / Tetrahedron 61 (2005) 5849–5854
5851
Figure 1. UV–visible spectra of diols 3, 4; acetals 9a, 9b and ketals 9e, 9f, 10a and 10b.
Table 2. Stability of ketal 8c in acidic, basic, oxidation and reduction conditions
S. No
Solvent
Reagent
Reaction conditionsa
Ketal 8cb (%)
Cyclohexanone
Temp (8C)
Time (h)
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
THF
THF
Dioxane
Dioxane
THF
DMSO
DMSO
CH3CN
Ether
Ether
THF
DME
t-Butanol
Methanol
CH3CN
THF
CH3CN
TFA
aq HCl (5%)
aq H2SO4 (10%)
2 N NaOH
2 N NaOH
NaH
rt
rt
rt
48
78
24
24
24
24
10
20
24
48
48
24
24
24
18
20
24
1
100
99
100
89
98
95
65
100
100
98
96
100
80
100
85
95
85
58
98
0
0
Trace
0
8
Trace
Trace
Trace
0
Reflux
rt
rt
100
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
NaH
NaH
NaH
NaH
LiAlH4
LiAlH4
NaH
0
Trace
Trace
0
Trace
0
Trace
Trace
Trace
40
Trace
0
Potassium t-butoxide
NaBH4
DDQ
TBAF
CAN
TFA
NaBH4CCeCl3$7H2O
Pd/C, H2
s
t
Methanol
Methanol
24
24
a 0.04 mmol of ketal in appropriate solvent was stirred with 5 equiv of reagent in the dark.
b Yields are determined by 1H NMR analysis of crude reaction mixture after workup.

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S. Kantevari et al. / Tetrahedron 61 (2005) 5849–5854
Scheme 2. Proposed mechanism for the photodeprotection of ketal 9a–f.
as ethyl ether and THF (entries j and k, Table 2).
Hydrogenation of 9e with Pd/C/H2 resulted in the formation
of the corresponding aniline and deprotection of acetal was
not observed. Under aqueous acidic conditions, ketal 9e
showed high stability, whereas in neat TFA at rt, the
respective carbonyl compound was isolated in 1 h (entry r,
Table 2).
3. Experimental
3.1. General
Melting points were recorded on Buchi 535 melting point
apparatus and are uncorrected. All the reactions were
monitored by thin layer chromatography performed on pre-
coated silica gel 60F254 plates (Merck). Compounds were
visualized with UV-light at 254 and 365 nm, iodine and
heating plates after dipping in 2% phosphomolybdic acid in
15% aq H2SO4 solution. Column chromatography was
carried out using silica gel 60–120 mesh purchased from
ACME Chemical Company, Bombay. All solvents used
were purified and dried according to the standard pro-
cedures. IR spectra were recorded on Perkin-Elmer 683 or
1310 FT-IR spectrometer with KBr pellets. NMR spectra
were recorded on Varian Unity-400 MHz and BRUKER
AMX 300 MHz spectrometers using tetramethylsilane as an
internal standard. 13C NMR was recorded on Varian Unity
100 MHz using CDCl3 as internal standard. Mass spectra
were recorded on a VG Micromass 7070H and Finnigan Mat
1020B mass spectrometers operating at 70 eV. UV-spectra
were recorded on a Perkin-Elmer UV/Vis spectro-
photometer. Benzaldehyde, p-methoxybenzaldehyde,
cyclohexanone, 2-cyclohexenone, 3,5,5-trimethyl-2-cyclo-
hexenone were purchased from SD-Fine chemicals,
Although, the exact mechanism of the photochemical
deprotection is not yet fully understood, it is probably
analogous to that described for uncaging of caged
calcium,14 caged neurotransmitters15 caged peptides16 and
protected diols12 (Scheme 2). The key step involves
intramolecular abstraction of benzylic hydrogen in a singlet
or a triplet state to give 11a or 11b, its rearrangement to
hemiacetal 12 and collapse of the latter afford the free
carbonyl compound 8 and nitroso compound 13. The
sensitivity of 13 to irradiated light prevented its isolation
and it could not therefore, be characterized.
In conclusion, we have synthesized a new photolabile
protecting group bis(4,5-dimethoxy-2-nitrophenyl)ethylene
glycol 4 for aldehydes and ketones and demonstrated the
stability of its ketals in various reaction conditions. The
ketals were efficiently deprotected both at 350 and 400 nm
to regenerate carbonyl compounds in good yields.

S. Kantevari et al. / Tetrahedron 61 (2005) 5849–5854
5853
Bombay. Bis(2-nitrophenyl)ethanediol 3 and its cyclo-
hexanone ketal 10a and cyclohexanone ketal 10b were
synthesized according to the literature procedure.13
3.2. General procedure for preparation of acetals 9a–b
and ketals 9c–f
A solution of bis(4,5-dimethoxy-2-nitrophenyl)ethylene
glycol
4
(1.0 mmol), carbonyl compounds 8a–f
3.1.1. 4,5-Dimethoxy-2-nitrobenzyl chloride 6. 4,5-
Dimethoxy-2-nitrobenzyl alcohol 5 (3.0 g, 14.0 mmol) in
chloroform (80 ml) was treated with PCl5 (3.2 g,
15.4 mmol) at rt for 30 min. The reaction was quenched
with addition of water (80 ml), organic layer was separated,
dried over sodium sulfate and evaporated under reduced
pressure to give a solid residue, which was passed over
silica gel column (eluted with ethyl acetate/hexane 1:9) to
isolate the titled compound 6 (2.72 g, 85%) as a pale yellow
solid, mp 65–66 8C. IR (nmax, cmK1) 3030, 2985, 1425,
(1.0 mmol), pyridinium p-toluenesulfonate (0.1 mmol) in
dry benzene (10 ml) was taken in a flask equipped with
Dean-Stark water separator (protected from day light) and
was heated to reflux. Progress of the reaction was monitored
by TLC After completion of reaction, benzene was
evaporated under vacuum to obtain residue, which
was dissolved in ethyl acetate (10 ml). The organic phase
was washed with saturated NaHCO3, brine, dried over
sodium sulfate, filtered and evaporated to obtain a residue,
which was purified by silica gel column chromatography
(eluted with hexane:ethyl acetate) to isolate the acetals and
ketals as crystalline solids.
1
1505, 1425, 1285, 1100, 675. H NMR (400 MHz, CDCl3)
d: 3.94 (s, 3H), 4.04 (s, 3H), 5.00 (s, 2H), 7.14 (s, 1H), 7.55
(s, 1H). MS (m/z, %) 231 (MC, 10), 183 (100), 153 (8), 79
(28). Analysis found: C, 46.84; H, 4.44; Cl, 15.22; N, 6.12.
Calcd for C9H10ClNO4: C, 46.67; H, 4.35; Cl, 15.35; N,
6.05.
3.2.1. Benzaldehyde acetal 9a. Yield: 77%, pale yellow
solid, mp 117 8C. IR (nmax, cmK1): 2923, 2852, 1582, 1516,
1459, 1269, 1064, 869, 752. 1H NMR (300 MHz, CDCl3) d:
3.70 (s, 3H), 3.92 (s, 3H), 3.98 (s, 3H), 4.98 (s, 3H), 5.90 (d,
JZ5 Hz, 1H), 6.00 (d, JZ5 Hz, 1H), 6.39 (s, 1H), 7.10 (s,
1H), 7.35 (s, 1H), 7.44 (m, 3H), 7.51 (s, 1H), 7.54 (s, 1H),
7.60 (d, JZ7.6 Hz, 2H). 13C NMR (100 MHz, CDCl3) d:
153.7, 153.5, 148.7, 148.6, 141.3, 140.6, 137.7, 129.5,
128.6, 110.1, 109.4, 108.3, 107.7, 104.5, 81.0, 80.5, 56.5,
56.4, 56.3, 56.1. MS (ESIKve, CHCl3): 547 (MC ClK),
1059 (2MCClK). Analysis found: C, 58.32; H, 4.68; N,
5.48. Calcd for C25H24N2O10: C, 58.59; H, 4.72; N, 5.47.
3.1.2. trans-2,20-Dinitro-3,30,4,40-tetramethoxystilbene 7.
To a solution of 4,5-dimethoxy-2-nitrobenzyl chloride 6
(2.0 g, 8.6 mmol) in DMSO (1 ml) and ethanol (3 ml) was
added KOH (1.54 g, 26.8 mmol) in ethanol (14 ml) drop-
wise slowly and stirred for 45 h at rt. The solid precipitate
was filtered and washed with ethanol (10 ml) and
redissolved in hot ethyl acetate. The insoluble portion was
filtered and filtrate was cooled to give the title compound 7
(0.300 g, 19%) as yellow solid, mp 245–248 8C. IR (nmax
,
cmK1) 2950, 1650, 1500, 1250, 1200, 820, 740. H NMR
(300 MHz, CDCl3) d: 3.95 (s, 6H), 4.60 (s, 6H), 7.14 (s, 2H),
7.59 (s, 2H), 7.65 (s, 2H). 13C NMR (100 MHz, CDCl3) d:
153.5, 148.8, 145.2, 128.8, 128.1, 110.0, 107.9, 56.4. MS
(EI, m/z, %): 390 (MC, 28), 358 (10), 211 (18), 194 (18),
179 (24), 164 (70), 152 (38), 136 (100), 125 (14), 108 (24),
79 (20). Analysis found: C, 55.12; H, 4.58; N, 7.19. Calcd
for C18H18N2O8: C, 55.39; H, 4.65; N, 7.18.
1
3.2.2. p-Anisaldehyde acetal 9b. Yield: 95%; yellow solid,
mp 167 8C. IR (nmax, cmK1): 2933, 2880, 1600, 1520, 1456,
1380, 1200, 1140, 1115, 799. H NMR (300 MHz, CDCl3)
1
d: 3.76 (s, 3H), 3.85 (s, 3H), 3.95 (s, 3H), 3.98 (s, 3H), 4.09
(s, 3H), 5.90 (d, JZ7.8 Hz, 1H), 5.98 (d, JZ7.8 Hz, 1H),
6.30 (s, 1H), 6.98 (d, JZ10.5 Hz, 2H), 7.18 (s, 1H), 7.35 (s,
1H), 7.52–7.54 (d, JZ10.5 Hz, 2H; s, 2H). 13C NMR
(100 MHz, CDCl3) d: 160.6, 153.6, 153.4, 148.5, 148.4,
141.0, 140.5, 129.5, 129.4, 128.9, 127.7, 114.0, 109.9,
109.2, 108.1, 107.5, 104.5, 80.8, 80.4, 56.4, 56.2, 56.1, 55.3.
MS (ESIKve, CHCl3): 577 (MCClK), 1119 (2MCClK).
Analysis found: C, 57.78; H, 4.78; N, 5.12. Calcd for
C26H26N2O11: C, 57.56; H, 4.83; N, 5.16.
3.1.3. Bis(4,5-dimethoxy-2-nitrophenyl)ethylene glycol 4.
To a mixture of trans-2,20dinitro-3,30,4,40-tetramethoxy-
stilbene 7 (150 mg, 0.38 mmol) in dichloromethane (4 ml)
and water (1 ml) and N-methyl morpholine oxide (NMO)
(50 mg, 0.5 mmol) in water (0.5 ml), OsO4 (0.1 ml,
0.02 mmol, 4% in water) was added. The reaction mixture
was vigorously stirred at rt. After 48 h, reaction mixture was
quenched with Na2S2O4 (0.45 g in 4.5 ml water) and stirred
for an additional 24 h. The dichloromethane layer was
separated and the aqueous phase was extracted with ethyl
acetate (2!5 ml). The combined organic layers were dried
over sodium sulfate and evaporated under vacuum. The
residue obtained was chromatographed over silica gel
(eluted with hexane/ethyl acetate, 1:1) to isolate diol 4
(50 mg, 46%) as a pale yellow solid, mp 154–157 8C. IR
(nmax, cmK1): 3320, 2985, 2895, 1420, 1340, 1280, 100,
1080, 810, 780. 1H NMR (300 MHz, CDCl3) d: 3.99 (s, 6H),
4.00 (s, 6H), 5.69 (s, 2H), 7.19 (s, 2H), 7.31 (s, 2H). 13C
NMR (100 MHz, CDCl3) d: 153.4, 148.7, 141.3, 129.8,
111.4, 108.4, 72.3 and 56.4. MS (ESIKve, CHCl3): 459
(MCClK), 884 (2MCClK). Analysis found: C, 50.88; H,
4.62; N, 6.71. Calcd for C18H20N2O10: C, 50.95; H, 4.75; N,
6.60.
3.2.3. 3,5,5-Trimethyl-2-cyclohexenone ketal 9c. Yield:
55%; yellow solid, mp 152–155 8C. IR (nmax, cmK1): 2925,
2858, 2359, 1667, 1585, 1458, 1274, 1171, 1080, 798, 760.
1H NMR (300 MHz, CDCl3) d: 1.09 (s, 6H), 1.29 (s, 3H),
1.80 (s, 2H), 2.00 (s, 2H), 3.91 (s, 6H), 4.09 (s, 6H), 5.51–
5.60 (m, 3H), 7.32 (s, 1H), 7.34 (s, 1H), 7.38 (s, 2H). MS
(ESIKve, CHCl3): 579 (MCC1K), 1123 (2MCC1K).
Analysis found: C, 59.68; H, 5.88; N, 5.12. Calcd for
C27H32N2O10: C, 59.55; H, 5.92; N, 5.14.
3.2.4. Ketal 9d. Yield: 65%, 61–64 8C. IR (nmax, cmK1):
2924, 2855, 2360, 1514, 1458, 1266, 1215, 1083, 868, 802.
1H NMR (300 MHz, CDCl3) d: 0.95 (d, JZ6.8 Hz, 3H),
1.08 (d, JZ6.8 Hz, 3H), 2.20–2.40 (m, 3H), 2.75 (m, 1H),
3.58 (t, JZ3.4 Hz, 1H), 3.65 (d, JZ3.6 Hz, 1H), 4.31 (m,
1H), 3.71 (s, 3H), 3.96 (s, 3H), 4.10 (s, 3H), 4.55 (ABq, JZ
10.8 Hz, 2H), 5.82 (d, JZ8.7 Hz, 1H), 6.01 (d, JZ8.7 Hz,

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S. Kantevari et al. / Tetrahedron 61 (2005) 5849–5854
Organic Synthesis, 3rd ed.; Wiley: New York, 1999. (b) Pillai,
V. N. R. Synthesis 1980, 1–26. (c) Pillai, V. N. R. Org.
Photochem. 1987, 9, 225–323. (d) Dorman, G.; Prestwich,
G. D. Trends Biotechnol. 2000, 18, 64–72.
1H), 7.12–7.62 (m, 9H). Analysis found: C, 61.51; H, 5.78;
N, 4.26. Calcd for C24H28N2O10: C, 61.25; H, 5.75; N, 4.20.
3.2.5. Cyclohexanone ketal 9e. Yield: 90%; pale yellow
solid, mp 197–200 8C. IR (nma1x, cmK1): 2930, 1520, 1350,
1290, 1210, 1130, 1090, 790. H NMR (300 MHz, CDCl3)
d: 1.50 (m, 2H), 1.75 (m, 4H), 1.92 (m, 4H), 3.90 (s, 6H),
4.02 (s, 6H), 5.60 (s, 2H), 7.39 (s, 2H), 7.40 (s, 2H). 13C
NMR (100 MHz, CDCl3) d: 153.5, 148.5, 141.3, 127.8,
109.8, 107.6, 79.3, 56.3, 56.2, 36.8, 25.5, 23.7. MS (ESIK
ve, CHCl3): 539 (MCClK), 1043 (2MCClK). Analysis
found: C, 57.39; H, 5.58; N, 5.62. Calcd for C24H28N2O10:
C, 57.14; H, 5.59; N, 5.55.
2. (a) Bochet, C. G. Synlett 2002, 2268–2274. J. Chem. Soc.,
Perkin Trans. 1 2002, 125–142. (b) Blank, A.; Bochet, C. G.
J. Org. Chem. 2002, 67, 5567–5577. (c) Kessler, M.; Glatthar,
R.; Giese, B.; Bochet, C. G. Org. Lett. 2003, 5, 1179–1181.
3. (a) Bochet, C. G. Tetrahedron Lett. 2000, 41, 6341. (b)
Schaper, K.; Mobarekah, S. A. M.; Grewer, C. Eur. J. Org.
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4. Peng, L.; Wirz, J.; Goeldner, M. Angew. Chem., Int. Ed. 1997,
36, 398–400. Tetrahedron Lett. 1997, 38, 2961–2964.
5. (a) Salerrno, C. P.; Resat, M.; Magde, D.; Kraut, J. J. Am.
Chem. Soc. 1997, 119, 3403–3404. (b) Cohen, B. E.; Stoddard,
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2483–2486. (b) Milburn, T.; Matsubara, N.; Billington, A. P.;
Udgaonkar, J. B.; Walker, J. W.; Carpenter, B. K.; Webb,
W. W.; Marque, J.; Denk, W.; McCray, J. A.; Hess, G. P.
Biochemistry 1989, 28, 49–55.
3.2.6. Cyclohexenone ketal 9f. Yield: 73%; yellow solid,
mp 140–142 8C. IR (nmax, cmK1): 2924, 2855, 1584, 1515,
1464, 1386, 1332, 1216, 1117, 1071, 1026, 873. H NMR
1
(300 MHz, CDCl3) d: 1.80–1.90 (m, 2H), 2.19 (m, 4H), 3.90
(s, 6H), 4.08 (s, 6H), 5.61 (d, JZ7.5 Hz, 1H), 5.72 (d, JZ
7.5 Hz, 1H), 5.89 (d, JZ9.3 Hz, 1H), 6.11 (d, JZ9.2 Hz,
1H), 7.39 (s, 2H), 7.40 (s, 2H). MS (ESIKve, CHCl3): 537
(MCClK), 1040 (2MCClK). Analysis found: C, 57.56; H,
5.08; N, 5.11. Calcd for C24H26N2O10: C, 57.37; H, 5.22; N,
5.01.
7. Walker, J. W.; Martin, H.; Schmitt, F. R.; Barsotti, R. J.
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7957–7959. (b) Corrie, J. E. T. J. Chem. Soc., Perkin Trans. 1
1993, 2161–2166.
3.3. General procedure for photolytic deprotection of
acetals and ketals 9a–f
9. (a) Dinkel, C.; Wichmann, O.; Schultz, C. Tetrahedron Lett.
2003, 44, 1153–1155. (b) Dinkel, C.; Schultz, C. Tetrahedron
Lett. 2003, 44, 1157–1159. (c) Baldwin, J. E.; McConnanghie,
A. W.; Moloney, M. G.; Pratt, A. J.; Shin, S. B. Tetrahedron
1990, 46, 6879–6884.
A solution of acetals 9a–b or ketals 9c–f (0.1 mmol) in
acetonitrile (10 ml) in a Pyrex vessel was degassed by
bubbling dry nitrogen gas and irradiated at 350 and 450 nm
separately with stirring in Rayonet apparatus equipped with
monochromatic lamps. Progress of deprotection was
monitored by thin layer chromatography (hexane/ethyl
acetate 4:1). Samples were drawn intermittently and yields
were determined by GC and proton NMR spectroscopy
(Table 1). The reddish brown solution so obtained was
evaporated to a residue and purified by flash column
chromatography over silica gel to recover carbonyl
compounds 8a–e. The irradiation at 400 nm was carried
out for 10 h and analogously analyzed by thin layer
chromatography and NMR spectroscopy. The carbonyl
compounds 8a–f recovered were characterized by compari-
son with the authentic samples.
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Acknowledgements
Authors thank Dr. J. S. Yadav, Director, IICT for his
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