Photochemical reductive desulfonylation of β-ketosulfones by ascorbic acid

Article abstract of DOI:10.1016/j.tetlet.2006.01.023

Photoinduced electron-transfer reaction between excited β-ketosulfones and ascorbic acid provides an efficient and green approach for the desulfonylation of β-ketosulfones.

Full text of DOI:10.1016/j.tetlet.2006.01.023

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Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1805–1807
Photochemical reductive desulfonylation of b-ketosulfones
by ascorbic acid
Qiang Liu, Bing Han, Zhengang Liu, Li Yang, Zhong-Li Liu* and Wei Yu*
National Laboratory of Applied Organic Chemistry, Lanzhou University Lanzhou, Gansu 730000, PR China
Received 18 November 2005; revised 28 December 2005; accepted 6 January 2006
Available online 25 January 2006
Abstract—Photoinduced electron-transfer reaction between excited b-ketosulfones and ascorbic acid provides an efficient and green
approach for the desulfonylation of b-ketosulfones.
Ó 2006 Elsevier Ltd. All rights reserved.
b-Ketosulfones are useful intermediates in organic syn-
thesis.1 The sulfonyl group can stabilize the adjacent
carbanions, thus facilitating the carbon–carbon bond
formation via alkylation or condensation reaction.
However, the usefulness of the sulfonyl group is depen-
dent, in most cases, on the ease of its subsequent re-
moval from the molecule.2 Several reducing agents,
such as Al(Hg),3 Zn/HOAc,4 Bu3SnH,5 Na2S2O4,6 Mg/
MeOH,7 SmI2,8 and TiCl4–Zn,9 have been used to
accomplish this desulfonylation. Desulfonylation of b-
ketosulfones could also be realized under photochemical
conditions with Hantzsch ester in pyridine in the pres-
ence of Ru(II).10
O
O
R2
H2A, CH3CN-H2O (5:1)
> 300 nm
SO2Ph
Ph
Ph
h
ν
,
λ
R1 R2
R1
2
1
Scheme 1.
diated in the presence of ascorbic acid (H2A), the reduc-
tive desulfonylation could take place effectively (Scheme
1).
In a typical procedure, a mixture of a-phenylsulfonyl
acetophenone (1a, 1 mmol) and H2A (3 mmol) was irra-
diated in CH3CN–H2O (5:1, 20 mL) with a Pyrex-
filtered high-pressure mercury lamp (k > 300 nm) under
an argon atmosphere at ambient temperature. After
completion of the reaction as monitored by TLC, the
solvent was removed under reduced pressure and the
product 2a was isolated from the residue by chromato-
graphy over silica gel in 88% yield (Table 1, entry 1).
No reaction took place in the absence of H2A under
the otherwise same conditions. This method worked well
with a variety of a-substituted substrates (1b–g), and the
double bond or triple bond incorporated in the sub-
strates (1c–e) was left intact under the photoreductive
conditions (Table 1, entries 3, 4 and 5). Desulfonylation
of a-disubstituted 1g was also achieved in high yield
(entry 7).
Ascorbic acid (vitamin C) is a well-known natural anti-
oxidant, which has been widely used in people’s daily life
for treatment and prevention of various diseases. The
structure of ascorbic acid, which features a 2,3-enediol
group on a five-membered lactone ring, renders it an
effective electron donor in single electron-transfer reac-
tions. However, only limited reports have appeared in
the literature dealing with the application of this com-
pound in organic synthesis.11,12 Pandey et al.11 and oth-
ers12 have successfully used ascorbic acid as a primary
and/or secondary and sacrificial electron donor in syn-
thetically useful photoinduced electron-transfer (PET)
reactions. In our ongoing research program on the syn-
thetic potentials of PET reactions,13 we found that when
a-phenylsulfonyl acetophenone derivatives (1) were irra-
It is believed that the reaction proceeded via a photo-
induced electron-transfer mechanism as illustrated in
Scheme 2. In the scheme the electron transfer from
ascorbate to the excited b-ketosulfone (1) produces
Keywords: b-Ketosulfones; Ascorbic acid; Desulfonylation; Photo-
induced electron transfer.
*
Corresponding authors. Fax: +86 931 8625657 (W.Y.); e-mail:
yuwei@lzu.edu.cn
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.023

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1806
Q. Liu et al. / Tetrahedron Letters 47 (2006) 1805–1807
Table 1. Photoinduced desulfonylation of b-ketosulfones 1 by ascorbic
O
O
O
acida
H2A, CH3CN-H2O (5:1)
SO2Ph
Entry
Substrate 1
R1 R2
Time Product Yield
h
ν, > 300 nm
λ
R
(%)b
R
(h)
2
O
5
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
H
H
H
H
H
H
H
Me
4
10
15
15
2a
2b
2c
2d
2e
2f
88
96
94
90
75c
98
89
4
Scheme 3.
H2C@CHCH2
HC„CCH2
(CH3)2C@CHCH2 24
PhCH2
15
12
Table 2. Photochemical desulfonylation of 3-phenylsulfonyl-4-chro-
manones 4 by ascorbic acida
1g Me Me
2g
a The reaction conditions see text.
Entry Substrate
R
Time Product Yield
(h)
b Isolated yields. All products are known compounds and were char-
acterized by EI-MS, 1H and 13C NMR.
(%)b
1
2
3
4
5
4a
4b
4c
4d
4e
H
Me
6
8
5a
5b
5c
5d
5e
86
90
81
85
84
c 15% 1e was recovered after irradiation.
H2C@CHCH2 10
HC„CCH2
PhCH2
10
12
h
ν
HA + H+
a The reaction conditions were the same as those for desulfonylation of
1, see text.
H2A
1*
1
b Isolated yields. All products are known compounds and were char-
ET
acterized by EI-MS, H and 13C NMR.
1
1/2 HA + 1/2 A + 1/2 H+
HA
1
SO2Ph
Ph
H2A
R2
is advantageous in terms of nontoxicity and mild condi-
tions. It is expected that more applications could be
found for using ascorbic acid as a reducing agent in
organic synthesis.
O
2
R1
3
Scheme 2.
Acknowledgement
We thank the National Natural Science Foundation of
China (grant No. 20372030) for financial support.
ascorbic acid radical HAÅ and the radical anion of 1. It is
well known that the formation of radical ions would
remarkably weaken the chemical bond in the species,14
hence the mesolystic cleavage15 of the radical anion of
1 facilitates the desulfonylation. The a-ketone radical
(3) thus formed abstracts a hydrogen atom from ascor-
bic acid, producing the corresponding ketone 2. HAÅ
would disproportionate to dehydroascorbic acid (A) as
reported in the literatures.11a,b,16
References and notes
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The present desulfonylation method can also be applied
for the synthesis of 3-substituted chromanones. It has
been reported that the alkylation of 3-phenylsulfonyl-
4-chromanone at the 3-position provides a facile ap-
proach for the synthesis of 3-substituted chromanones,
which constitute an important class of biologically
active compounds.17 The necessity of using 3-phenyl-
sulfonyl substituent to activate the adjacent 3-carbon
for alkylation is due to the fact that the direct alkylation
of 4-chromanone gave quite unsatisfactory result. For
this purpose, several 3-substituted-3-phenylsulfonyl-4-
chromanones (4) were prepared according to the litera-
ture procedure,17a then the phenylsulfonyl group was
removed using the protocol described above, giving
corresponding 3-substituted chromanones (5) in high
yield (Scheme 3 and Table 2).
In summary, a simple photochemical method employing
ascorbic acid as the reducing agent has been developed
for the desulfonylation of b-ketosulfones. The reaction

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