Novel and versatile photosensitized oxygenation reaction of α-cedrene

Article abstract of DOI:10.2174/1570178611310030015

Three types of photosensitized oxygenations of α-cedrene (CED) were investigated, including: rose bengal(RB)-sensitized photooxygenation of α-cedrene in acetonitrile produced one major stereospecific product (cedr-8-exoen-9α-ol), in which the double bond migrated to an adjacent position, this result demonstrated the singlet oxygen process; 9,10-dicyanoanthrathene (DCA) sensitized photooxygenation led to the formation of cedr-8-en-10β-ol; benzyl (BZ)-sensitized photoepoxidation furnished a stereospecific 8α,9α-cedrene epoxide, this reaction includes radical reaction mechanism.

Full text of DOI:10.2174/1570178611310030015

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Letters in Organic Chemistry, 2013, 10, 223-227 223
Novel and Versatile Photosensitized Oxygenation Reaction of ꢀ-Cedrene
Weiping Wu, Feng An, Zhengsong Geng, Ronghua Zhang* and Zhiqin Jiang
Department of Chemistry, Tongji University, Shanghai 200092, China
Received November 08, 2012: Revised January 21, 2013: Accepted February 25, 2013
Abstract: Three types of photosensitized oxygenations of ꢀ-cedrene (CED) were investigated, including: rose ben-
gal(RB)-sensitized photooxygenation of ꢀ-cedrene in acetonitrile produced one major stereospecific product (cedr-8-
exoen-9ꢀ-ol), in which the double bond migrated to an adjacent position, this result demonstrated the singlet oxygen
process; 9,10-dicyanoanthrathene (DCA) sensitized photooxygenation led to the formation of cedr-8-en-10ꢀ-ol; benzyl
(BZ)-sensitized photoepoxidation furnished a stereospecific 8ꢀ,9ꢀ-cedrene epoxide, this reaction includes radical reaction
mechanism.
Keywords: Sensitized photooxygenation, ꢀ-cedrene, singlet oxygen, electron transfer, regioselectivity.
1. INTRODUCTION
Each of these prior art, cedrenol and cedrenone produc-
tion processes, has disadvantages, where it requires not only
a prolonged period of reaction time but also complicated by-
products that require extensive separation processes for the
resulting cedrenol and cedrenone. There is no effective
chemical method to synthesize the cedrenol and the ce-
drenone.
It is established that the photochemical synthesis of ter-
penoids and natural products etc. has become a useful plat-
form for methodological and synthetic studies [1]. Particu-
larly, application of the photooxygenation for a variety of
organic compounds, including olefins, heterocycles and
natural products etc., has been extensively studied in recent
years [2]. The photooxygenation provides an efficient and
convenient method in organic synthesis, in which versatile
oxygen-containing functional groups were introduced into
the target molecules and the reactions frequently proceed in
stereocontrolled manner to furnish the stereospecific selec-
tivity. The photooxygenations also make important contribu-
tion to the green chemistry [3]. On the other hand, biologi-
cally active molecules such as amino acid, protein, nucleic
acid and hormone etc. were found to undergo the photo-
chemical damage through photooxygenation [4]. Therefore,
both theoretical and applied studies on the photooxygenation
are of significance.
In this paper, three types of sensitized photooxygenations
of ꢀ-cedrene involving the ene reaction by singlet oxygen
(1O2)[9], electron transfer (ET) and free radical reaction were
explored. The results showed that these syntheses are effec-
tive and convenient to furnish stereospecific products, cedre-
nol, cedrenone and cedrene epoxide in high and moderate
yields.
2. RESULTS AND DISCUSSION
RB Sensitized Photooygenation of ꢀ-cedrene
The rose bengal (RB) sensitized photooxygenation of ꢀ-
cedrene (CDE) 1 was carried out in acetonitrile under con-
tinuous bubbling of O2. The reaction gave rise to a single
stereospecific enol, Cedr-8-exoen-9ꢀ-ol 3[10], in high yield
(88.8 %) after the reduction of the unstable 9-allylic hydrop-
eroxide 2 by PPh3. Obviously, the reaction proceeded via the
ene reaction of singlet oxygen (1O2) according to the C=C
shift manner and formation of the allylic hydroxyl group. RB
is well known as a typical sensitizer for the formation of1O2
Cedrene and its oxygenated derivatives such as ses-
quiterpenoids are major components of perfume essentials,
flavors and some pharmaceuticals [5]. They are also attrac-
tive topics of total synthesis [6]. Abraham and coworkers [7]
reported that Corynespora cassicola DSM 62474 and
Rhodococcus rhodochrous ATCC 999 converted ꢀ-cedrene
(Scheme 1) to a number of oxygenated products, such as 3-
hydroxy- and 12-hydroxy-ꢀ-cedrenes and cedrenone and 10-
methoxy-ꢀ-cedrene, but the biotransformations of ꢀ-cedrene
require a long cultivation time and the product yields were
extremely low. The epoxidation of the acid sensitive (+)-
(ꢁ = 0.76) [11]. Upon excitation, the triplet RB can convert
1
oxygen to O2 efficiently. The branched 8-methyl group in ꢂ-
cedrene led to the migration of C=C to the adjacent 8-exo
1
ꢁ
position when the mild electrophilic O2 attacked the more
8(15)-cedren-9-ol with sulfonyl imidazolides / H2O2 / OH
nucleophilic 9-position of 8-C=C bond from the ꢂ-
orientation and the formation of 9ꢀ-allylic hydroperoxide 2
via the suprafacial H shift.
gave the corresponding cedrene epoxide as a single di-
astereoisomer [8].
This stereospecific phenomenon known as the ‘cis effect’
was first recognized in enol ethers by Conia [12] and Foote
[13]. The cyclohexene and its derivatives show no cis effect,
only gave the anti ene addition.
*Address correspondence to this author at the Department of Chemistry,
Tongji University, Shanghai 200092, China; Tel: 00862165988570-8542;
Fax: 00862165981097; E-mail: rhzhang@tongji.edu.cn
1875-6255/13 $58.00+.00
© 2013 Bentham Science Publishers

224 Letters in Organic Chemistry, 2013, Vol. 10, No. 3
Wu et al.
OH
OOH
PPh3
hv, RB
O2
1
2
3
Scheme 1. RB-sensitized photooxygenation of ꢀ-cedrene.
OOH
OOH
hv, DCA
+
O2
2
4
1
OH
OH
3
5
Scheme 2. DCA sensitized photooxygenation of ꢀ-cedrene.
OH
HO
hv, DCA/BP
+
O2
1
5
6
Scheme 3. DCA/BP co-sensitized photooxygenation of ꢀ-cedrene.
This mechanism is also verified by our experiments. The
attack of 1O2 at the C(8)=C(9) bond in ꢀ-cedrene yields only
9ꢀ-allylic hydroperoxide 2.
a electron-deficient sensitizer. Upon excitation, the singlet
state of DCA will be obtained.
To demonstrate above DCA-sensitized ET reaction pos-
tulate, the DCA/biphenyl (BP) cosensitized photooxygena-
tion of CDE was further carried out. The result showed that
the reaction led to the formation of two cedr-8-en-10-ol epi-
mers, cedr-8-en-10ꢀ-ol 5 and cedr-8-en-10ꢀ-ol 6. This re-
gioselectivity is consistent with the major product of cedr-8-
en-10ꢀ-ol 5 for the DCA sensitized photooxygenation of
CDE and no formation of C=C-shifted cedr-8-exoen-9ꢀ-ol 3
was observed. In addition, the reaction was greatly acceler-
ated and completed in 14 min, shortened about 20 folds as
compared with that for DCA-sensitized reaction.
DCA Sensitized and DCA/BP Co-sensitized Photooxy-
genation of ꢀ-cedrene
It was found that 9, 10-dicyanoanthrathene (DCA) sensi-
tized photooxygenation of ꢀ-cedrene showed different prod-
uct distribution pattern as compared with that in Rose ben-
gal-sensitized photooxygenation of ꢀ-cedrene (see Scheme
1). As shown in Scheme 2, the reaction initially led to the
formation of the major product, cedr-8-en-10ꢀ-
hydroperoxide 4, and minor product, cedr-8-exoen-9ꢀ-
hydroperoxide 2. Similarly, the reduction by PPh3 was fol-
lowed to give the corresponding enols, cedr-8-en-10ꢀ-ol 5, in
45 % yield, and cedr-8-exoen-9ꢀ-ol 3, in 30 % yield. DCA is
It was well established that DCA/BP co-sensitization
could efficiently enhance the ET reaction of less reactive
substrates (S) via the secondary (or indirect) ET process

Novel and Versatile Photosensitized Oxygenation Reaction
Letters in Organic Chemistry, 2013, Vol. 10, No. 3 225
ꢀ
+·
+·
[14], i.e. BP ꢀꢁꢀSꢂꢀBPꢀꢁꢀS . In our case, therefore,
CDE can be regarded as such substrate and suggested to un-
dergo the following ET photooxygenation:
exoen-9ꢀ-ol 3 (0.95 g) was obtained as a white, crystalline
solid, m.p. 123-125 °C; [ꢀ]20 +8.5, (c 1, CHCl3) [16]. The
589
yield was 88.8 %. 1H NMR(400Mz, CDCl3):ꢁ4.99 (t, 1H, J =
2.4 Hz), 4.75 (t, 1H, J = 2.4 Hz), 4.34 (m, 1H), 2.35 (d, 1H, J
= 4.2 Hz), 1.72-1.92 (m, 3H), 1.51-1.61 (m, 2H), 1.16-1.47
(m, 4H), 0.97 (s, 3H), 0.94 (s, 3H), 0.86 (d, 3H, J = 6.9 Hz);
13C NMR (100Mz, CDCl3):ꢁ154.0, 106.6, 70.0, 60.4, 57.0,
54.9, 45.0, 44.9, 42.3, 41.6, 36.7, 26.7, 26.1, 25.8, 15.5; MS
(m/z, %): 220 (24), 205 (26), 187 (23), 177 (47), 162 (60),
159 (59), 136 (56), 135 (66), 118 (54), 109 (54), 91 (59), 69
(100), 41 (53).
.
.
+
BP
BP
+
DCA
BP
+
+
DCA
.
+
.
+
BP
CDE
+ CDE
.
.
O2
DCA
CDE
O2
O2
+
+
+
+
DCA
.
+
.
5
6
3.2. DCA Sensitized Photooxygenation of ꢀ-cedrene
BZ Sensitized Photoepoxidation of ꢀ-cedrene
89 mg of ꢀ-cedrene and 7.8 mg of DCA were added in
80 mL of acetonitrile. The solution was irradiated under con-
tinuous bubbling of O2 with medium pressure mercury lamp
(500 W) for 4 h and the reaction was followed by TLC. 130
mg of triphenylphosphine (PPh3) was then added to the solu-
tion with stirring for 30 min. After removal of solvent, the
mixture was isolated by column chromatography on silica
gel with eluent of petroleum/ ethyl acetate (8 : 1). Two com-
ponents were obtained: 43.4 mg of the major product, Cedr-
8-en-10ꢀ-ol 5, yield 45%, m.p. 88-90 °C and 28.5 mg of
minor product, Cedr-8-exoen-9ꢀ-ol 3, yield 30 %, m.p. 123-
125 °C. Spectral data for 5, IR (cm-1) 3246, 2946, 2932,
2843, 1457, 1056, 998, 892. 1H NMR(400Mz, CDCl ): 5.37
(1H, dd, J = 4.2 Hz 10ꢀH), 3.86 (1H, d, J = 4.0 Hz 9-H),
1.52-1.93 (m, 9H), 1.13-1.49 (m, 4H), 0.98 (s, 3H), 0.92 (s,
3H), 0.80 (d, 3H, J = 7.0 Hz); 13C NMR (100Mz,
CDCl3):ꢁ144.3, 124.2, 72.3, 60.2, 54.5, 51.9, 45.7, 38.6,
35.2, 33.9, 28.5, 25.7, 25.0, 23.8, 14.9; MS (m/z, %): 220
(11), 202 (12), 187(27), 177(51), 159 (84), 136 (51), 118
(68), 105 (2), 91 (61), 69 (100).Spectral data for 3 were the
same as that in above rose bengal-sensitized photooxygena-
tion of ꢀ-cedrene.
When the photosensitized oxygenation reaction of ꢀ-
cedrene was performed under the photosensitization of ben-
zil (BZ), cedrene epoxide was obtained (Scheme 4) in total
yield of 60.6 %, which was further isolated to give two
stereoisomers, minor 8ꢀ,9ꢀ- cedrene epoxide (7) and major
8ꢀ, 9ꢀ-cedrene epoxide 8 in the ratio of 0.43 : 1. This reac-
tion could be illustrated by the Bartlett reaction [15], in
which the photoxidation of benzil produces benzoylperoxy
radicals which can transfer an oxygen atom effectively to
olefins producing the stereospecific ꢀ-epoxides. Further-
more, 8, 9-epoxycedrane can be easily transformed into ce-
ꢁ
3
dran-9-one
9 through the isomerization with boron
trifluoride ether complex (BF3·OEt2).
3. EXPERIMENTAL PROCEDURES
3.1. Rose Bengal-sensitized Photooxygenation of ꢀ-
cedrene
ꢀ-cedrene (1.05 g, 5.0 mmol) and Rose Bengal (0.11 g)
were dissolved in 80 ml of acetonitrile. The solution was
irradiated under continuous bubbling of O2 with medium
pressure mercury lamp (500 w) for 4 h and the reaction was
followed by TLC until the reaction was completed. 1.51 g of
triphenylphosphine (PPh3) was then added to the solution
with vigorous stirring for 1 h. After removal of solvent, the
mixture was isolated by column chromatography on silica
gel with eluant of petroleum: ethyl acetate (26 : 1). Cedr-8-
3.3. DCA-BP Co-sensitized Photooxygenation of ꢀ-
cedrene
99 mg of ꢀ-cedrene, 6.3 mg of DCA and 523 mg of
biphenyl were dissolved in 80 ml of acetonitrile. The solu-
O
O
hv, BZ
BF3ꢀOEt2
O2
1
9
O
O
+
8
7
Scheme 4. BZ-sensitized photoepoxynation of ꢀ-cedrene.

226 Letters in Organic Chemistry, 2013, Vol. 10, No. 3
Wu et al.
tion was irradiated under continuous bubbling of O2 with
medium pressure mercury lamp (500 W) for 20 h and the
reaction was followed by TLC until the reaction was com-
pleted. After removal of solvent, the mixture was isolated by
column chromatography on silica gel with eluent of petro-
leum/ ethyl acetate (8 : 1). 32.5 mg of the component was
obtained as Cedr-8-en-10-ol. HPLC analysis indicated the
component was consist of equal amount of two epimers, Fur-
ther separation by column chromatography on silica gel with
eluent of petroleum/ ethyl acetate (8 : 1) to give two epimers,
16.2 mg Cedr-8-en-10ꢀ-ol 6(15.2%) and 16.2 mg Cedr-8-en-
10ꢀ-ol 5(15.2%). IR (cm-1) 3246, 2946, 2932, 2843, 1457,
4.2 Hz 10ꢀH), 3.86 (1H, d, J = 4.0 Hz39-H), 1.52-1.93 (m,
9H), 1.13-1.49 (m, 4H), 0.98 (s, 3H), 0.92 (s, 3H), 0.80 (d,
3H, J = 7.0 Hz); 13C NMR (100Mz, CDCl3):ꢁ144.3, 124.2,
72.3, 60.2, 54.5, 51.9, 45.7, 38.6, 35.2, 33.9, 28.5, 25.7, 25.0,
23.8, 14.9; MS (m/z, %): 220 (11), 202 (12), 187(27),
177(51), 159 (84), 136 (51), 118 (68), 105 (2), 91 (61), 69
(100).
ACKNOWLEDGEMENTS
This research was supported by the National Science
Foundation of China (Project No. 20972113/B020502).
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CONFLICT OF INTEREST
The author(s) confirm that this article content has no con-
flict of interest.

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