Chapter 5…
Investigations on Intramolecular Arene-olefine Photcycloadditions
CH3
O
O R
X
HH
H3C
O
H
O
H H
HH
H
H
O
H3C
H
R
OH
H
H
HHH
HH3C
O
HH
HHOH2C
HH
XO
HR
H
H3CO
H
O
HO
H
R
CH3
H
H
X X
O
H3C
H
RH
X
O
CH3
O
R
H
H
X1
H
H HH
H O
O
H3C
CH3
H
HHH
Cl
H
H
H
or
+ +
+ +
+
a X = H, R = H
b X = H, R = CH3
c X = Cl, R = H
d X = Cl, R = CH3
e X = Br, R = H
159
Chapter-5
Investigations on Intramolecular Arene-olefine Photocycloadditions
in Substituted-o-Alkenylmethoxy-Acetophenones
Introduction
New discoveries and inventions in organic chemistry and related fields are driven by the
ability to make molecules in a simple, safe, and practical fashion. Despite the relatively
long history of organic chemistry and noteworthy achievements, complex molecule
synthesis remains a formidable challenge.1,2
The efforts to address this challenge
inexorably rely on the development of complexity increasing strategy level reactions and
reaction cascades.1-5
Among the different organic transformations, including phototransformation,
intramolecular photocycloadditions of arene with alkene substituents provide easy access to
strained polycyclic molecules.6 Arene-alkene photo-cycloadditions are among the most
powerful and versatile strategy level reactions for complex molecule synthesisand these
remarkable reactions have attracted considerable mechanistic, and theoretical interest,
leading in recent years to spectacular applications in synthesis.6
The possible mode of addition, i.e., (2+2), (2+4) or meta-addition, is reported to
depend, inter alia, on the electron-donor/acceptor properties of the arenes and alkenes.7
Among these arene-olefine intramolecular photocycloadditions, the meta-cycloadditions are
more frequently reported in the literature7i
and the ortho-additions are also reported to
compete with meta cycloadditions.8
However, para- i.e., (4+2) photocycloaddition, is
reported to be the least efficient of the three modes of additions because of the poor orbital
overlap between an excited arene and a ground state alkene,9
but is a common pathway in
the photochemistry of extended aromatic systems.10
In this chapter emphasis is on the
development of the arene-alkene cycloaddition for the rational synthesis of complex
molecules of biological importance. Some of the examples of the arene-olefin cycloadditiosn
are:
Synthesis of the silphinene (4) could be easily achieved through arene-olefine meta-
photocycloaddition in 1 (Scheme 1).11
160
h
Li, MeNH2
+
( )-Silphinene
12 3
4
Scheme 1
Wander et. al. have reported the asymmetric synthesis of (-)-retigeranic acid (8, Scheme
2)12
through the meta-photocycloaddition of 5.
h H HH2NOC
HHOOC
H h
COh
(-)-Retigeranic Acid
5 6 7 8
Scheme 2
Wender et. al have further reported13
the application of the chiral ketals to regulate the
absolute stereochemistry of the cycloaddition process. Total synthesis of (±)-subergorgic
acid (11, Scheme 3) has been achieved through the arene alkene cycloaddition of a
benzylic ketal (9) and additionally features a free radical addition to a vinylcyclopropane
(10).
h
O
O
OO
H
HO2C
H
O
(±) -Subergorgic acid9 10 11
Scheme 3
Grayanotoxin II (14)14
has been synthesized through the irradiation of the 12, which lead to
the formation pentacyclic 13 with seven stereogenic centers through the meta-
161
cycloadditions; 13 through the series of steps furnishes Grayanotoxin II (14 Scheme 4).
This is an efficient approach in the synthesis of the seven membered rings through a
modification of cycloadduct cleavage procedure.
OAcOMe
TBOS
OAcOMe
TBOS HO
HO
OH
OH
OH
H
(±)-Grayanotoxin II
h
12 13 14
Scheme 4
Though (3 + 2) photocycloaddition has been studied the most,15
however, recent work has
shown that this cycloaddition competes with the [2 + 2] photocycloaddition;16
often the
products of the latter reaction are less stable. The silylated phenol derivative 15 reacts via
both a [3 + 2] and a [2 + 2] photocycloaddition to yield intermediates I and II, respectively
(Scheme 5).17
Intermediate I lead to the tetracyclic compound 18 by a radical cyclization
resulting in cyclopropanation. Since the reaction occurs in the singlet state, this
intermediate also possesses some zwitterionic character. Intermediate II resulting from [2
+ 2] addition generates III via a pericyclic reaction. Contraction of the cyclooctatriene
moiety leads to the cyclobutene intermediates 16 and 17. The final products of this reaction
are obtained by hydrolysis (20) or hydrolysis followed by dimerization (19).
OSi O Si O
Si
O Si
OSi
H
OSi
HH
OSi OH
H
SiO2
OH
h
+
15
16 17
18
19
I II
20
OSi
III
Scheme 5
Addition of electron-withdrawing substituents favors on aromatic ring [2 + 2]
photocycloaddition compared to the [3 + 2].18,19
For instance, the presence of a nitrile
group in the resorcinol derivative (21) favored a [2 + 2] cycloaddition in the -position
162
of this substituent (Scheme 6).20
A cyclohexane-1,3-dione derivative (23) was obtained by
spontaneous rearrangement of the intermediate 22; such products are likely to be used as
intermediates in the synthesis of a new generation of herbicides.
O
OH
NC
O
H
NC
OH
O
H
NC
O
h
21 22 23
Scheme 6
Primary adducts of [2 + 2] photocycloadditions with benzene derivatives are frequently
unstable. However, when the reaction is performed in an acidic medium, these reaction
adducts can be transformed into stable final products via acid-catalyzed reactions. The
resorcinol derivatives 24 yielded adducts IV and V (Scheme 7).21
These intermediates
rearranged to benzocyclobutenes 25 and 26 or, depending on the substitution, monocyclic
compounds such as 29. Benzocyclobutenes are interesting substrates in organic synthesis.
For instance, 25 has been transformed into the nitrogen-containing tricyclic compound 27,
which possesses an affinity for dopamine receptors.
O
OX
h O
HOX
+
OH
OX
HO
OX
HO
OX
NR
MeO
X = MeX= H, Me
OHOMeOMe OH
OMe
X= H, Me
+
+MeOH-H
24 25 26
272829
IV V
Scheme 7
Wagner et al.,22
have investigated the photochemistry of o-alkenoxy-acetophenones (30) and
have reported that intramolecular (2+2) photocycloaddition in these molecules occurs solely
at the bond between the two substituents (Scheme 8).
163
O
O
R1
R2
CH3OHO
R1
R2
O
O
O
R1
R2
O
O
R1
R2
hv hv
30
Scheme 8
On the other hand, in case of p-alkenoxyacetophenones (31), two possible modes of
cycloadditions (one towards and other away from the substituent) are possible and in general
the addition is highly regioselective i.e., syn to the substituent in all the cases with an
exception of meta-methoxy and -thiomethoxy substituent
(Scheme 9).22b
These
photocycloadditions are postulated to be intervened by polar intermediates/charge
separation.23
Y
X
O
O
Y
X
O
O
Y
X
O
O
O
O XY
O
O
Y
X
O O
O O
Y
X
Y
X
hvhv
hv
hvhv
31hv
Scheme 9
Chiral induction has also been studied for the [3 + 2] photocycloaddition. In compound 32,
the two reaction partners (phenol and vinyl ether) are linked together by a homochiral
tether (Scheme 10).24
Addition occurs stereospecifically, and intermediate VI is formed.
As the reaction takes place at the singlet state, the intermediate VI may be viewed as either
a biradical or a zwitterion. After regioselective formation of a cyclopropane ring, product
33 was isolated and then transformed into the enantiopure alcohol (-)-34.
164
O OO
O
OO
OH
O
O
h
32 VI 33 (-) 34
Scheme 10
In the above reported reactions, it was observed that minimum of the three carbon/hetero-
atom tether is necessary conditions between the aromatic ring and the olefinic moiety is a
must for any intramolecular photocycloaddition.25
However, Ishar et. al. have been recently
investigated the photochemistry o-allyloxy/-crotyloxyacetophenones26
(35a,b) under varied
conditions and it has been observed that the three carbon/hetero- atom tether between the
aromatic ring and the olefinic moiety is not a necessary condition for intramolecular
photocycloaddition (Scheme-11); various products have been characterized by NMR
spectroscopic analysis and it has been reported that many products have remained
uncharacterized.26
O
CH3
O R
O
HO CH3
HH
R
H
O R
O R
OHCH3OHCH3
H3CHC
H3CHC
N(CH2CH3)2
N(CH2CH3)2
HOCH3
N(CH2CH3)2
O R
O
HO CH3
HH
R
H
O R
O
O
CH3
HH
O
H3CO
OR
H3C O
R
OH
R
H
CH3
35
36
36 +
37( mixture of diastereomers)
+
39
+
38
(36
43
(A)
CH3
hv
CH3CN
hv
hv
PhH+TEAhv
CH3CN+TEA
Dry PhHDry
4041
42
+
++
(36+ 37 + 38 + 39)+
N2
N2
N2
,
,
,
,
N2
( mixture of diastereomers)
& 43)
(a) R=H
(b) CH3
Scheme 11
165
Thereby, in order to delineate the mechanistic transformation of the various products. It
was decided to synthesize various substituted o-allyloxy/-crotyloxyacetophenone and
characterize their product profile before and after irradiation.
Results and Discussion
Synthesis of the substituted -allyloxy-/crotoloxy acetophenone
Substituted o-allyloxy-/crotyloxy-acetophenones (45a-e) were prepared by reacting o-
hydroxyacetophenone (44a-e) with allyl bromide and crotyl bromide by stirring under
reflux in dry alcohol free acetone, in the presence of anhydrous K2CO3. After the
completion of reactions (Tlc), the K2CO3 was filtered off, washed with acetone and the
solvent from the filtrates was distilled off. Pure products (46a-e, Scheme 12) were isolated
by vacuum distillation of the residual viscous oils and characterized spectroscopically
(vide experimental).26
CH3
O
O R
O
CH3
OH
RBrX X
45a-e44 a X = H, R = H
b X = H, R = CH3
c X = Cl, R = H
d X = Cl, R = CH3
e X = Br, R = H
R = H, CH3
acetone, reflux
Scheme 12
Irradiation of substituted o-allyloxy-/crotyloxy-acetophenones
Initially, o-allyloxy-/crotyloxy-acetophenones (45ab) were irradiated in dry acetonitrile
with a 400 Watt medium pressure Hg arc employing an immersion well type (Pyrex-glass),
water cooled photoreactor, under nitrogen atmosphere for 30-50 h (90% conversion). The
obtained complex photolysates were resolved by column and preparative layer
chromatography to obtain compounds (Scheme 13, 46-51). An additional dehydrated
product 48a was isolated first time in the case of o-allyloxyacetophenones (45a). All the
purified compounds (46-51, Table 1) were characterized by various spectroscopic
techniques (IR, 1H and
13C NMR, mass) and by elemental analysis, and assigned by
comparison with the earlier reported data.26
After purification attempts to crystallize any of
the compounds 49-51 did not succeeded.
166
CH3
O
O R
HH
H3C
O
H
O
H H
HH
H
H
O
H3C
H
R
OH
H
H
H
HH
HH3C
O
HH
HHOH2C
HH
XO
HR
H
H3CO
H
O
HO
H
R
CH3
H
H
XX X
hv
dry CH3CN
46 (20-25%)
+
47 (5-8%)
+
51 (9-17%)49 (5-12%) 50 (6-9%)
1
23
4
5
67
81
2
3
4
5
6
78
9
101
23
4
5
6
7
8
9
10
45 a X = H, R = H
b X = H, R = CH3
O
H3C
H
R
H
X
+
48 (10-15%)
+
Scheme 13
Table 1: Reaction conditions and percentage yields of products obtained
from irradiation of o-allyloxy-/crotyloxyacetophenone in acetonitrile.
Subsequently, photochemistry of the o-allyloxy-/crotyloxy-acetophenones (45c-e) was
examined. Substituted o-allyloxy-/crotyloxy-acetophenones (45cd) were irradiated under
similar conditions for 20-40 h (95% conversion), leading to complex photolysates, which
were resolved by column chromatography to furnish compounds 52cd and 53cd along
with some uncharacterized components Scheme 14). All the purified compounds 52 and
53 were characterized by rigorous spectroscopic (IR, 1H and
13C NMR,
1H decoupling, 2D
1H -
13C H.et.-COSY NMR, mass) and by elemental analysis.
CH3
O
O R
X1hv
dry CH3CN
45 c X1= Cl, R = H
d X1= Cl, R = CH3
O
H
R
CH3
H
X1
52cd (40-45 %)
+
53cd (20-30%)
O
O
H3C
R
H
HHH
Cl
H
H
H1
23
4
5 6
7
8
99'
Scheme 14
% Yield of products Entry R X1 X2 Irradiation
Time (h) Conversion
% 46 47 48 49 50 51
1 H H H 30 95 24 5 10 8 8 9
2 CH3 H H 30 95 23 8 13 12 - -
167
Table 2: Reaction conditions and percentage yields of products obtained from irradiation of
o-allyloxy-/crotyloxyacetophenone in acetonitrile.
% Yield of products Entry R X1 X2 Irradiation
Time (h) Conversion
% 52 53
1 H Cl H 28 95 50 20
2 CH3 Cl H 38 95 55 30
However, it is pertinent to mention here no progress of reaction was observed in the case
of 4-bromo-o-allyloxy-/crotyloxy-acetophenones 45e even after prolonged irradiation 100h
(Tlc and NMR) under similar conditions (Scheme 15).
CH3
O
O R
X2
X1 h , 100 h
dry CH3CN
45e X1= Br, X2= H, R = H
No Reaction
Scheme 15
Characteristic features of 1H NMR spectrum of 52a were double doublets at 6.58 (J =
11.1 & 17.4 Hz) assigned to C1 H, 5.81 (J = 1.1 & 17.4 Hz) attributed to C2 H, besides
other resonances of proton NMR (Figure 1). 13
C NMR assignments made are also in
consonance with the assigned structures. In 52c, the resonance for C2 appeared at 112.65
(Figure 2) and resonances of oxygen linked carbons as observed ( 78 -93). In the case of
46 and 47 were absent in the spectrum of 52c. The mass spectra of 52c (ESI) revealed m/z
(M + K) +
peak at 232.
Figure 1: 1H NMR spectrum of 52c
O
H
H
CH3
H
Cl
168
Figure 2: 13C NMR spectrum of 52c
Another product was isolated from the photolysis of compound 45d, whose mass spectrum
[ESI m/z 247 (M +Na)+] indicted that there was no loss or gain of mass during
phototransformation. Though derived from 45d its 1H NMR did not reveal any resonance
in the aromatic region, which was indicative of the fact that product is an intramolecular
photocycloadduct. Based on the spectral data, in particular, the presence of only one 1H
proton resonance in the olefinic region of its proton NMR spectrum, following five
structures 53-57 were considered.
O
O
MeCH3H
HHH
H ClH
O
CH3
O
R
H
H
Cl
H
H HH
57
HO
O
H3C
CH3
H
HHH
Cl
H
H
H
54
ClH
H3C
O
H
O
H H
HH
H
H
55
ClHH
HH3C
O
HH
HHOH2C
H
53
56
Structures 55 and 56 were ruled out by comparison of spectroscopic data with the reported
for related structures26
and also because 13
C NMR revealed four carbons in the olefinic
O
H
H
CH3
H
Cl
169
region at 174.9(q), 132.0(q), 120.4(CH) and 101.9(q). Consequently, meta-addition
product 57 was also ruled out. Characteristic feature of the IR spectrum of the compound
were a conjugated carbonyl band at 1672 cm-1
. In the 13
C NMR the presence of an upfield
shifted quaternary olefinic carbon resonance ( 101.9) and a downfield shifted quaternary
olefinic carbon resonance ( 174.9) collates with push-pull substituted bond as present in
structures 53 and 54. 13
C NMR of the compound (Figure 4) showed carbonyl carbon at
204.2, an oxygen linked carbons at 71.2 (C2), besides other carbon resonances in the
aliphatic region. 1H NMR spectrum of the compound showed olefinic double doublet at
5.69 (J = 1.5 and 5.17 Hz); presence of 5.17 Hz splitting in the olefinic-H resonance does
not support structure 54 and favors structure 53. The compound 53 displayed max
(methanol) at 230 nm, which is also in keeping with the assigned structure. Other
assignments made are a double doublets at 4.05 (Jvic = 4.5 and Jgem = 9.0 Hz) attributed to
a C2H, doublet at 3.74 (Jgem = 9.0 Hz) attributed C2H, double doublet at 2.79 (J = 4.5
and 6.0 Hz) assigned to C3H (Figure 3). The assigned sterochemical dispositions are
based on the J values and 1H-connectivities worked out by
1H decoupling; the assignments
were aided by 2D 1H -
13C Het. COSY, NOESY NMR spectra.
Figure 3: 1H NMR spectrum of 53d
C7H
C2H
C3H
C9’H
C4H
-COCH3
CH3
O
O
H3C
CH3
H
HHH
Cl
H
H
H
53d
C5H
170
Figure 4: 13
C NMR spectrum of 53d
Plausible mechanisms
Mechanism for the formation of 46, 47 and 48
Mechanistically, it has been reported that the irradiation in non-polar solvent the preferred
formation of syn-adducts, , in which bulky groups are placed in trans-arrangement, through
triplet excited state, has been attributed27
to the relative sizes of the substituents and their
influence on the mode of rotation of bonds during ring closure of 1,5-biradicals. However,
hydrogen bonding of the –OH group with the solvent molecule increases its bulkiness
leading to formation of the anti-adducts (46), as minor products and syn as major product
(47). Further, dehydration of the photoproducts 46 or 47, leads to the formation of product
(48, Scheme 16).
53d
O
O
H3C
CH3
H
HHH
Cl
H
H
H174.9
120.4
101.9
71.2
47.4
17.7
132.0
171
ISC
O
H3C OH
HH
R
H
CH3CN
O
H3C OH
HH
R
H
O
H3CH
R
H
-H2O
-H2O
CH3
O
O R
CH3
O
O R
OH
CH3
RO C
HH
H
OH
H3C
R
H
O C
H
H
CH3
O
O R
H
OH
CH3
RO C
H
H
h
Route
a
Route b
47
46
45
Route
a
48
a
b
1 3
H-abs
ba Solvent
* *
Scheme 16
Proposed mechanism for the formation of 49, 50 and 51
Mechanistically, the formation of intramolecular photoaddition products is highly
unexpected. As mentioned earlier, a minimum of a three carbon/hetero-atom tether has
been reported to be essential for any intramolecular arene-alkene photocycloadditions. 28
Accordingly, it has been reported that no intramolecular photocycloaddition product could
be detected from benzyl vinyl ether and only -phenylpropanaldehyde was isolated.26
Similarly, allyl phenyl ether is reported to afford only Claisen rearrangement products on
irradiation.29
The formation of various intramolecular photoaddition products, therefore, is
a consequence of very special circumstances where the aryl ring is bearing an electron
withdrawing substituent and the olefinic moiety can be considered as a electron rich. The
reaction is initiated by n- * excitation (Pyrex-filter) and involves intramolecular electron
transfer as supported by the observed solvent dependence of formation of these products
(49-51). The sequence of events possibly leading to 49, 50 and 51 outlined in Scheme 17.
172
HH
H3C
O
H
O
H H
HH
H
H
CH3O
O
R
HHH
HH3C
O
HH
HHOH2C
H
RCH3
O
O
CH3
O
O R
CH3O
O
O
CH3
O
CH3
O
O R
H
O
HR
H
H3CO
H
*3
5149
50
1
2
3
4
5
67
812
3
4
5
6
78
9
10
1
23
4
5
6
7
8
ET
9
10
Scheme 17
It may be mentioned here that intramolecular (2+2) photoadditions followed by further
rearrangement of the photoadducts are precedented in the case of certain p-subsituted
acetophenones,22b,28b,30
and 2-subsituted-1-acetylnaphthalenes,22a
and among these the
former are postulated to involve donor-acceptor interactions. In general, the donor-acceptor
interactions are known to be involved in ortho-photoadditions, which are particularly
favored if the addends differ substantially in their ionization potentials,23a,31a,b
and it has
recently been reported that the regiochemistry of addition is mediated by orientation of
dipoles.31c
However, all the reported examples conform to the criterion of a minimum
three- carbon/heteroatom separation between arene and olefin. Therefore, the present
investigations on the phototransformations of o-alkenylmethoxyacetophenones, is the case
wherein the above-mentioned limiting condition for arene-olefin intramolecular photo-
cycloaddition has been surpassed.
Conclusions
The present investigations have led to the conclusion that irradiations of the substituted o-
allyloxy-/crotyloxy-acetophenones (46a-e) furnish different photo products (47-56). The
increased formation of benzofuran derivatives (52cd) in case of chloro-substituted ketone
(46cd) resulting from H-abstraction followed by cyclization is apparently a consequence of
higher triplet yield due to heavy atom effects, however, failure of reaction in case of
bromo-substituted ketone probably results from early deactivation of excited state. The
findings also suggest that intramolecular addition products may not be resulting from
173
triplet manifold. Isolation of the intramolecular photcycloaducts are suggestive of the fact
that the limiting condition of a three-atom tether between an aryl ring and an alkene moiety
is not necessary for the arene-olefine cycloadditions in solvents favoring intramolecular
charge/electron transfer.
Experimental
General Information: Starting materials, reagents and solvents were purchased from
commercial suppliers and purified/distilled/crystallized before use. Bruker AC-200 FT
(200 MHz) and JEOL AL-300FT (300 MHz) and Bruker AC-500 FT (500 MHz) and
spectrometers were used to record 1H NMR and
13C NMR (50, 75, 150 MHz) spectra.
Chemical shifts ( ) are reported as downfield displacements from TMS used as internal
standard and coupling constants (J) are reported in Hz. IR spectra were recorded on
Shimadzu DR 2001 FT-IR and Shimadzu FT-IR-8400S spectrophotometer either as thin
layer with few drops of CHCl3 or as KBr pellets. Mass spectra were recorded (EI and ESI-
method) on Shimadzu GCMS-QP-2000A and Bruker Daltonics Esquire 300 mass
spectrometer. Elemental Analyses were carried out on a Perkin-Elmer 240C elemental
analyzer and are reported in percent atomic abundance. All melting points are uncorrected
and measured in open glass-capillaries on a Veego (make) MP-D digital melting point
apparatus. Chromatographic separations have been carried out by either column
chromatography over silica gel (60-120 mesh) or preparative layer chromatography over
silica gel-G coated plates (2.0 mm thick layer).
o-Allyloxy/ crotyloxy acetophenones
These were prepared by reacting substituted-o-hydroxyacetophenone (10 ml, 8.3 mmol),
respectively, with allyl bromide (10 ml, ~1.4 molar equivalent) and crotyl bromide (12 ml,
~1.4 molar equivalent) by stirring under reflux in dry alcohol free acetone (100 ml), in
presence of anhydrous K2CO3 (12.0 g). The reactions were monitored by Tlc and after
completion of reactions (12 h), the K2CO3 was filtered of, washed with acetone and the
solvent from the filtrates was distilled off. Pure products (45a-e) were isolated by vacuum
distillation of the residual viscous oils and characterized spectroscopically.
o-Allyloxyacetophenone (45a)
Yield 85%; Colorless oil; IR (CHCl3): vmax 1680, 1610, 1595, 1480, 1450, 1420, 1360,
1290, 1280, 1160, 1130 cm-1
; 1H NMR (200 MHz, CDCl3): = 7.72(dd, 1H, J = 8.4, 1.83
174
Hz, C6H), 7.42(m, 1H, ArH), 6.91(m, 2H, ArH), 6.17-5.96(m, 1H, C2 H), 5.42(dd, 1H, J =
14.0, 1.5 Hz, C3 H), 5.31(br d, 1H, J = 7.6 Hz, C3 H), 4.62(d, 2H, J = 5.1 Hz, C1 H), 2.63(s,
3H); 13
C NMR (CDCl3, 50 MHz): = 199.28(C=O), 157.67(C2), 133.27(CH), 132.39(CH),
130.04(CH), 128.36(C1), 120.3(CH), 117.81(=CH2), 112.60(HC=), 69.16(-OCH2), 31.73
(COCH3); MS (EI) : m/z (%): 176 (M+, 18); Anal. calcd. For C11H12O2 : C, 74.92; H, 6.86,
Found: C, 74.66; H, 6.54 %.
o-Crotyloxyacetophenone (45b)
Yield 90%; Colorless oil; IR (CHCl3): vmax 1680, 1600, 1500, 1460, 1380, 1370, 1300,
1180, 1160, 1080 cm-1
; 1H NMR (CDCl3, 200 MHz): 7.72(dd, 1H, J = 7.10, 1.5 Hz,
C6H), 7.38(dt, 1H, J ~ 8.24, 1.5 Hz, C4H), 6.97(m, 2H, ArH), 5.85-5.74(m, 2H, C2 H,
C3 H), 4.56(brd, 2H, J = 5.52 Hz, C1 H], 2.63(s, 3H, CH3), 1.76(d, 3H, J = 4.6 Hz, C4 H);
13C NMR (CDCl3, 50 MHz): 200.01(C=O), 158.01(C2), 133.47(CH), 130.74(CH),
130.36(CH), 129.07(C1), 125.56(CH), 120.60(CH), 112.91(CH), 69.35(-OCH2),
35.31(COCH3), 17.81(=CHCH3); MS (EI): m/z (%): 190(M+, 27); Anal. calcd. For
C12H14O2 : C, 75.76; H, 7.42, Found: C, 75.44; H, 7.23 %.
1-(2-allyloxy-5-chloro-phenyl)-ethanone (45c)
Yield 92%; White solid, mp 112-115°C; IR (KBr): vmax 1684, 1615, 1565, 1473, 1442,
1363, 1290, 1284, 1164, 1125 cm-1
; 1H NMR (CDCl3, 500 MHz,): 7.70(d, 1H, J = 2.74
Hz, ArH), 7.36 (dd, 1H, J = 2.73 and 17.1 Hz, ArH), 6.89(d, 1H, J = 8.8 Hz, ArH), 6.08-
6.03 (m, 1H, C2 H), 5.40(dd, 1H, J = 1.4 & 17.2 Hz, C3 H), 5.34(dd, 1H, J = 1.5 & 21.0 Hz,
C3 H), 4.63(d, 2H, J = 5.3 Hz, C1 H), 2.62(s, 3H); 13
C NMR (500 MHz,CDCl3)
: 198.23(C=O), 156.4(C2), 133.05(CH), 132.19(CH), 130.01(CH), 129.47(C1),
126.0(CH), 118.58(=CH2), 114.36(HC=), 77.40 (-OCH2), 31.89 (COCH3); MS (EI): m/z
(%): 233 (M + Na). Anal. calcd. For C11H11O2Cl : C, 62.72; H, 5.26. Found: C, 60.02; H,
5.18 %.
1-(2-But-2-enyloxy-5-chloro-phenyl)-ethanone (45d)
Yield 90 %; White solid, mp 120-122°C; IR (KBr): vmax 1650, 1591, 1451, 1422, 1323,
1274, 1190, 1112 cm-1
; 1H NMR (CDCl3, 500 MHz,): 7.70(d, 1H, J = 2.74 Hz, ArH),
7.36 (dd, 1H, J = 2.73 and 17.1 Hz, ArH), 6.89(d, 1H, J = 8.8 Hz, ArH), 6.08-6.03 (m, 1H,
C2 H), 5.42(dd, 1H, J = 1.4 & 17.2 Hz, C3 H), 5.31(dd, 1H, J = 1.5 & 21.0 Hz, C3 H),
175
4.54(d, 1H, J = 6.0 Hz, C1 H), 2.62(s, 3H, CH3), 1.77(s, 3H, CH3); 13
C NMR (500
MHz,CDCl3) : 198.1(C=O), 152.1(C2), 133.2(CH), 132.4(CH), 130.5(CH), 128.1(C1),
127.1(CH), 124.2(=CH2), 120.1(HC=), 70.4(-OCH2), 36.2(COCH3), 17.6(=CHCH3); MS
(ESI): m/z: 247(M + Na); Anal. calcd. For C12H13O2Cl : C, 64.15; H, 5.83. Found: C,
63.99; H, 5.41 %.
1-(2-Allyloxy-5-bromo-phenyl)-ethanone (45g)
Yield 92 %; White solid, mp 115-120°C; IR (KBr): vmax 1672, 1593, 1458, 1419, 1321,
1271, 1181, 1108 cm-1
; 1H NMR (CDCl3, 500 MHz,): 8.05(s, 1H, ArH), 7.25-7.22(d, 1H,
J = 10, ArH), 6.13-6.00(m, 1H, C2 H), 5.45 (dd, 1H, J = 1.2 & 17.2 Hz, C3 H), 5.37(dd, 1H,
J = 1.0 & 8.2 Hz, C3 H), 4.70(d, 2H, J = 5.3 Hz, C1 H), 2.61(s, 3H, CH3); 13
C NMR (500
MHz,CDCl3) : 196.4(C=O), 161.3(C2), 133.5(CH), 131.7(CH), 131.6(CH), 130.1(C1),
127.9(CH), 119.0(=CH2), 112.7(HC=), 69.7(-OCH2), 31.9(COCH3); MS (ESI): m/z: 278
(M + Na); Anal. calcd. For C11H11O2Br : C, 51.79; H, 4.35. Found: C, 51.40; H, 4.22 %.
General procedure for Irradiation of substituted o-allyloxy-/crotyloxyacetophenone
(45a-e) in dry acetonitrile:
Substituted-o-Allyloxy-/crotyloxyacetophenones (45a-e, 500 mg) were dissolved in dry
acetonitrile (250 ml) and taken in an immersion well type Pyrex-glass, water cooled
photoreactor. Solutions were purged with dry oxygen free N2 for at least 15 minutes prior
to irradiation. The irradiations were carried out with a 400 Watt medium pressure Hg arc
placed coaxially inside the reactor and N2 was continuously bubbled during irradiation. At
the end of reaction, solvent was removed from the photolysate under reduced pressure
using Eyela-rotary evaporator and products were separated by column chromatography
over silica gel (60-120 mesh) using hexane-chloroform (gradient) as eluent. Some of the
column fractions were pooled and further resolved by preparative layer chromatography
over silica gel-G coated plates (run in CHCl3-Benzene 1:1 and compounds extracted with
chloroform).
(syn)-2-Ethenyl-3-hydroxy-3-methyl-2,3-dihydrobenzofuran (46a)
Yield 20 %; Colorless viscous oil; IR (CHCl3): vmax 3487, 1621, 1601, 1475, 1462, 1371,
1282, 1241, 1213, 1092 cm-1
; 1H NMR (CDCl3, 200 MHz): 7.31-7.21 (m, 2H, ArH),
6.97-6.85(m, 2H, ArH), 6.17-6.04(m, 1H, C1 H), 5.63(dd, 1H, J = 1.4 & 15.9 Hz, C2 H),
176
5.41(dd, 1H, J = 1.3 & 11.6, Hz, C2 H), 4.62(bd, 1H, J = 6.5 Hz, C2H), 1.62(s, 3H, C3-
CH3); 13
C NMR (CDCl3, 50 MHz): 161.2(C7a), 132.8(C3a), 131.4(CH), 130.4(CH),
123.2(CH), 121.5(CH), 118.2(=CH2), 110.2(HC=), 92.4(C2), 77.3(C3), 24.4(C3-CH3); MS
(EI): m/z: 176(40, M+), 116(100); Anal. Calcd. for C11H12O2: C, 74.98, H, 6.86; Found: C,
74.74, H, 6.67 %.
Mixture of 46a & 47a
Yiled 25%; colorless viscous oil; IR (CHCl3): vmax 3487, 1621, 1601, 1475, 1462, 1371,
1282, 1241, 1213, 1092 cm-1
; 1H NMR (CDCl3, 200 MHz): = 7.31-7.19(m, 4H, ArH),
6.94-6.80(m, 4H, ArH), 6.17-6.00(m, C1 H in 46a), 5.90-5.73(m, 1H, C1 H in 47a),
5.60(split d, 1H, J = 15.9, 1.4 Hz, C2 H in 46a), 5.41-5.27(m, C2 H in 46a & C2 H in 47a),
4.85(bd, J = 6.9 Hz, C2H in 47a), 4.62(bd, J = 6.5 Hz, C2H in 46a), 1.63(s, C3Me in 46a),
1.43(s, C3CH3 in 47a); 13
C NMR (CDCl3, 50 MHz): 161.2(C7a), 133.6(C3a in 47a),
132.8(C3a in 46a), 131.2(CH), 131.1(CH), 130.2(CH), 130.4(CH), 123.2(CH), 123.2(CH),
121.0(CH), 121.2(CH), 118.0(CH2=), 118.0(CH2=), 110.6(CH), 110.4(CH), 92.7(C2 in
47a), 92.0(C2 in 46a), 77.2(C3 in 46a), 77.1(C3 in 47a), 24.2(C3-CH3 in 46a), 23.0(C3-CH3
in 47a).
3-Methyl-2-vinyl-benzofuran (48a)
Yield 10 %; colorless viscous oil; IR (CHCl3): vmax 1604, 1452, 1479, 1374, 1261, 1231,
1219, 1074 cm-1
; 1H NMR (CDCl3, 300 MHz): = 7.48-7.37 (m, 2H, ArH), 7.26-7.14(m,
2H, ArH), 6.68(dd, 1H, J = 11.1 & 17.4 Hz, C1,H), 5.88(dd, 1H, J = 1.5 & 17.4 Hz, C2
,H),
5.32(dd, 1H, J = 0.9 & 11.1 Hz, C2,H), 2.23 (s, 3H, CH3);
l3C NMR (CDCl3, 75 MHz): =
154.0(C7a), 150.2(C3a), 139.2(CH), 130.3(CH), 124.7(CH), 123.2(CH), 122.2(=CH2),
119.3(HC=), 114.1(C2), 114.0(C3), 113.1(CH), 29.7(CH3); Mass (ESI) m/z : 181(M+ Na);
Anal. Calcd. for C1IH10O: C, 83.51, H, 6.37; Found: C, 83.00, H, 6.09 %.
Photocycloadduct (49a)
Yield 15%; Yellow viscous oil; IR (CHCl3): vmax 3012, 2923, 2332, 1710, 1611, 1563,
1542, 1511, 1409, 1362, 1237, 1198, 918 cm-1
; 1H NMR (CDCl3, 200 MHz): = 5.87(bs,
1H, C8H), 4.51(d, 1H, J = 13.3 Hz, C3H), 4.12(dd, 1H, J = 5.4 & 13.3, Hz, C3H), 2.97-
2.94(m, 1H, C7H), 2.85-2.75(m, 2H, C6H & C10H), 2.71-2.58(m, 1H, C5H), 2.40-2.32(m,
2H, C4H & C5H), 2.22(s, 3H, -COCH3); 13
C NMR (CDCl3, 50 MHz): = 200.2(C=O),
177
157.1, 147.2, 78.8(C1), 69.4(C3), 53.0(C7), 36.3(C10), 32.0(C5), 30.9(C4), 29.3(C6),
28.8(CH3); Mass (EI) m/z: 177(M++1, 10), 176(M
+, 25), 159(24), 158((70), 147(46),
121(38), 105(42), 91(62), 84(41), 83(35), 81(35), 79(45), 78(47), 71(32), 69(49), 67(35),
65(38), 58(56), 56(100); Anal. (%): Calcd. for C11H12O2: C, 74.98, H, 6.86; Found: C,
74.75, H, 7.12.
Photocycloadduct (50a)
Yield 7%; yellow viscous oil; IR (CHCl3): vmax 3024, 2928, 1717, 1601, 1522, 1464, 1360,
1218, 1191, 1015 cm-1
. 1H NMR (CDCl3, 200 MHz): = 6.05(d, 1H, J = 2.8 Hz, C8H),
5.83(d, 1H, J = 2.8 Hz, C9H), 4.30(dd, 1H, J = 8.7, 4.7 Hz, C3H), 4.06(d, 1H, J = 8.7 Hz,
C3H), 3.85(d, 1H, J = 3.1 Hz, C7H), 3.45-3.38(m, 2H, C4H & C10H), 2.05(dd, 1H, J = 13.5,
7.1 Hz, C5H), 1.51-1.49(m, 1H, C5H), 1.32(s, 3H, CH3); 13
C NMR (CDCl3, 50 MHz): =
206.1(C=O), 131.6, 130.2, 82.3(C1), 71.2(C3), 56.4(C6), 54.2(C7), 48.7(C10), 40.8(C5),
30.7(C4), 26.0; Mass (EI) m/z (%): 177(M++1, 9), 176(M
+, 18), 158((54), 147(42), 121(39),
105(43), 91(57), 84(45), 83(35), 81(33), 79(39), 78(41), 71(28), 56(100); Anal. Calcd. for
C11H12O2: C, 74.98, H, 6.86; Found: C, 74.69, H, 6.58 %.
Photocycloadduct (51a)
Yield 12%; Yellow viscous oil; IR (CHCl3): vmax 3342, 2912, 2856, 1712(C=O), 1610,
1518, 1469, 1211, 1209 cm-1
; 1H NMR (CDCl3, 200 MHz): = 8.01(s, 1H, -OH), 6.02(d,
1H, J = 2.7 Hz, C3H), 5.70(d, 1H, J = 2.7 Hz, C4H), 4.22-4.18(m, 2H, -CH2-OH), 3.53(dd,
1H, J = 2.1 & 7.4, Hz, C2H), 3.31(dd, 1H, J = 3.2 & 7.4, Hz, C5H), 2.91-2.78(m, 1H, C7H),
2.57-2.48(m, 1H, C6H), 2.18(s, 3H, -COCH3), 1.76(dd, 1H, J = 12.9, 6.1 Hz, C8H), 1.35(m,
1H, C8H); 13
C NMR (CDCl3, 50 MHz): = 204.0(C=O), 139.4(CH), 134.8(CH),
65.2(CH2-O-), 55.8(C1), 54.6(C2), 49.8(C5), 47.0(C6), 37.1(C8), 29.7(C7), 28.8(CH3); Mass
(EI) m/z (%): 178(M+, 4), 177(M
+-1, 9), 112(20), 84(17), 83(31), 71(55), 70(49), 69(35),
58(100), 57(34), 56(79); Anal. Calcd. for C11H14O2: C, 74.13, H, 7.92; Found: C, 74.05, H,
8.11 %.
Irradiation of o-crotyloxyacetophenone (45b) in dry acetonitrile
(syn)-2-(1-Propenyl)-3-hydroxy-3-methyl-benzodihydrofuran (46b)
Yield 23%; colorless gummy material; IR (CHCl3): vmax 3484, 1616, 1598, 1472, 1450,
1417, 1382, 1345, 1317, 1264, 1142, 1098 cm-1
; 1H NMR (CDCl3, 200 MHz): 7.23-
7.10(m, 2H, ArH), 6.89-6.73(m, 2H, ArH), 5.97-5.80(m, 1H, C1 H), 5.71-5.60(m, 1H,
178
C2 H), 4.48(bd, 1H, J = 7.5 Hz, C2H), 1.78(bd, 3H, J = 6.3 Hz, C2 -CH3), 1.56(s, 3H, C3-
CH3); 13
C NMR (CDCl3, 50 MHz): 159.1(C7a), 133.3(C3a), 132.4(CH), 130.2(CH),
124.5(CH), 123.3(CH), 121.0(CH), 110.7(CH), 92.30(C2), 77.9(C3), 24.0(C3-CH3),
18.2(C2 -CH3); Mass (EI) m/z (%): 190(M+, 8), 173(55), 172(31), 159(20), 147(20),
145(23), 137(48), 131(35), 121(44), 119(25), 107(23), 105(26), 91(44), 77(31), 56(100);
Anal. Calcd. for C12H14O2: C, 75.76, H, 7.42; Found: C, 75.65, H, 7.50 %.
Mixture of 46b & 47b
Yiled 20%; colorless viscous oil; (1:1, 25 mg); IR (CHCl3): vmax 3484, 1617, 1594, 1473,
1382, 1257, 1140, 1007 cm-1
; 1H NMR (CDCl3, 200 MHz): = 7.27-7.11(m, 2H, ArH),
6.92-6.73(m, 2H, ArH), 6.02-5.87(m, 1H, C1 H in 46b & 47b), 5.82-5.65(m, C2 H in 46b &
47b), 4.77(bd,, J = 7.6 Hz, C2H in 47b), 4.48(bd, J = 7.3 Hz, C2H in 46b), 1.78(bd, 3H, J =
6.3 Hz, C2 -CH3 in 46b), 1.70(d, 3H, J = 6.2 Hz, C2 -CH3 in 47b), 1.54(s, 3H, C3-CH3 in
46b), 1.32(s, 3H, C3-CH3 in 47b); 13
C NMR (CDCl3, 50 MHz): = 159.1(C7a), 133.3(C3a
in 46b), 132.6(C3a in 47b), 132.4(CH), 130.6(CH), 130.2(CH), 125.7(CH), 124.5(CH),
123.3(CH), 121.0(CH), 120.8(CH), 112.1(CH), 110.7(CH), 93.4(C2 in 47b), 92.3(C2 in
46b), 77.9(C3 in 46b), 77.2(C3 in 47b), 24.0(C3-CH3 in 46b), 23.3(C3-CH3 in 47b),
18.2(C2 -CH3 in 46b), 17.8(C2 -CH3 in 47b).
Photocycloadduct (49b)
Yield 30%; yellow viscous oil; IR (CHCl3): vmax 3011, 2918, 2360, 1691, 1627, 1468,
1425, 1363, 1293, 1156, 1025 cm-1
; 1H NMR (CDCl3, 200 MHz): = 5.73(bs, 1H, C8H),
4.33(d, 1H, J = 13.3 Hz, C3H), 4.08(dd, 1H, J = 13.3, 5.4 Hz, C3H), 2.82(m, 1H, C7H),
2.77-2.62(m, 2H, C6H & C10H), 2.55-2.42(m, 1H, C5H), 2.20(m, 1H, C4H), 2.14(s, 3H, -
COCH3), 1.06(d, 3H, J = 7.2 Hz, C5-CH3); 13
C NMR (CDCl3, 50 MHz): = 199.6(C=O),
157.3, 146.5, 78.4(C1), 69.0(C3), 49.5(C7), 38.1(C10), 36.3(C5), 32.9(C4), 29.74(C6),
29.4(COCH3), 21.1(CH3); Mass (EI) m/z (%): 191(M++1, 5), 190(M
+, 37), 147(8), 121(24),
91(35), 83(32), 79(23), 77(25), 71(47), 70(41), 69(45), 58(87), 56(100); Anal. Calcd. for
C12H14O2: C, 75.76, H, 7.42; Found: C, 75.54, H, 7.23 %.
179
Irradiation of 1-(2-allyloxy-5-chloro-phenyl)-ethanone (45c) in dry acetonitrile
5-Choloro-3-methyl-2-vinyl-benzofuran (52c)
Yield 50%; colorless viscous oil; IR (CHCl3): vmax 1608, 1458,1472, 1370, 1260, 1232,
1217, 1084 cm-1
; 1H NMR (CDCl3, 300 MHz): = 7.30(d, 1H, J = 3.3 Hz, ArH), 7.28-
7.09(m, 2H, ArH), 6.58(dd, 1H, J = 11.1 & 1.7 Hz, C1 H), 5.81(dd, 1H, J = 17.4 & 1.1Hz,
C2 H), 5.28(dd, 1H, J = 2.6 & 11.1 Hz, C2 -H), 2.11 (s, 3H, CH3); 13
C NMR (CDCl3, 50
MHz): = 152.4(C7a), 151.6(C3a), 131.7(CH), 128.0(CH), 124.8(CH), 123.0(CH),
119.4(=CH2), 115.3(HC=), 112.6(C2), 111.7(C3), 29.8(CH3); Mass (ESI) m/z: 232 (M +
K+); Anal. Calcd. for C11H9OCl: C, 68.58, H, 4.71; Found: C, 67.99, H, 4.66 %.
Photocycloadduct (53c)
Yield 20%; yellow viscous oil; IR (CHCl3): vmax 3018, 2929, 2399, 1716, 1683, 1521,
1508, 1423, 1363, 1217, 1099, 927 cm-1
; 1H NMR (CDCl3, 300 MHz): 5.74(split d, 1H, J =
1.4 and 5.7 Hz, allylic and homoallylic couplings C7H), 4.12(dd, 1H, J = 4.3 & 9.2 Hz,
C2Ha), 3.72 (d, 1H, J = 9.2, C2Hb), 2.76 (dd, 1H, J = 4.3 and 6.0 Hz, C3H), 2.62 (dd, 1H, J
= 4.3 & 7.5 Hz, C9’H ), 2.34(dd, 1H, J = 3.3 & 7.5 Hz, C5H), 2.25(s, 3H, COCH3), 2.05-
2.00(m, 2H, C4Hab); Mass (ESI) m/z (%): 212(M)-2
.
Irradiation of 1-(2-But-2-enyloxy-5-chloro-phenyl)-ethanone (45d) in dry acetonitrile
5-Choloro-3-methyl-2-propenyl-benzofuran (52d)
Yield 32 %; colorless viscous oil; IR (CHCl3): vmax 1604, 1442,1432, 1309, 1265, 1210,
1022, 962 cm-1
; 1H NMR (CDCl3, 300 MHz): = 7.36(d, 1H, J = 3.1 Hz, ArH), 7.31-
7.11(m, 2H, ArH), 6.53(dd, 1H, J = 11.5 & 17.0 Hz, C1 H), 5.78(dd, 1H, J = 0.9 & 16.8
Hz, C2 -H), 2.25 (s, 3H, CH3), 1.72(bd, 3H, J = 6.2 Hz, C2 -CH3); 13
C NMR (CDCl3, 50
MHz): = 152.1(C7a), 151.2(C3a), 131.8(CH), 128.5(CH), 124.1(CH), 123.9(CH),
118.9(=CH2), 115.0(HC=), 112.1(C2), 111.2(C3), 29.2(CH3); 17.9(C2 -CH3); Mass (ESI)
m/z: 229 (M + Na+); Anal. (%): Calcd. for C12H11ClO: C, 69.74, H, 5.36; Found: C, 69.42,
H, 5.05.
Photocycloadduct (53d)
Yield 20%, Yellow viscous oil ; IR (CHCl3): vmax 3016, 2962, 2929, 2434, 2399, 1672
(conjugated C=O), 1608, 1566, 1519, 1473, 1379, 1161, 1010, 927, 757 (C-Cl) cm-1
; 1H
NMR (CDCl3, 300 MHz): 5.69(split d, 1H, J = 1.5 and 5.7 Hz, allylic and homoallylic
180
couplings C7H), 4.05(dd, 1H, J = 4.5 & 9.0 Hz, C2Ha), 3.74 (d, 1H, J = 9.0, C2Hb), 2.79
(dd, 1H, J = 4.5 and 6.0 Hz, C3H), 2.60 (dd, 1H, J = 4.5 & 7.5 Hz, C9’H ), 2.36(dd, 1H, J =
3.3 & 7.5 Hz, C5H), 2.09(s, 3H, COCH3), 2.05(dd, 1H, J = 4.5 & 7.5 Hz, C4H), 1.21(s, 3H,
CH3); 13
C NMR (CDCl3, 75 MHz): 204.2(C=O), 174.9(C9), 132.0(C6), 120.4(C7), 101.9
(C8), 71.2(C2), 47.4(C3), 45.0(C9`), 37.3(C5), 26.6(C4); 21.3(COCH3), 17.7(CH3); MS
(ESI): m/z 247 (M + Na)+.
181
References
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