Investigations on photochemistry of o-allyloxy-/crotyloxyacetophenones: Formation of unexpected intramolecular arene-olefin addition products on n-π* excitation of ketones

Article abstract of DOI:10.1016/S0040-4020(02)00868-2

Photochemistry of o-allyloxy-/crotyloxyacetophenones (6a,b) has been investigated under different conditions. Irradiation of 6a,b in dry benzene, under N₂ atmosphere (Pyrex filter), led to the isolation of (syn)-2-ethenyl/propenyl-3-hydroxy-3-m

Full text of DOI:10.1016/S0040-4020(02)00868-2

Tetrahedron 58 (2002) 7595–7606
Investigations on photochemistry of
o-allyloxy-/crotyloxyacetophenones: formation of unexpected
p
intramolecular arene–olefin addition products on n–p excitation
of ketones
*
Rajinder Singh and M. P. S. Ishar
Department of Pharmaceutical Sciences, Guru Nanak Dev University, Amritsar 143 005 Punjab, India
Received 3 May 2002; revised 24 June 2002; accepted 18 July 2002
Abstract—Photochemistry of o-allyloxy-/crotyloxyacetophenones (6a,b) has been investigated under different conditions. Irradiation of
6
2
a,b in dry benzene, under N atmosphere (Pyrex filter), led to the isolation of (syn)-2-ethenyl/propenyl-3-hydroxy-3-methyl-2,3-
dihydrobenzofurans (7a,b,10%) as the sole product. Irradiation of 6a,b in dry benzene in the presence of 0.1–0.4 mol equiv. of
triethylamine resulted in slightly increased formation of 7a,b, besides pinacols (9 and 10) and triethylamine addition products (11 and 12).
However, the formation of 7a,b is suppressed with increasing molar ratios of triethylamine, with increased formation of products (9–13). On
the other hand, irradiation of 6a,b in dry acetonitrile results in the formation of both syn-(7a,b) as well as anti-isomers of
benzodihydrofuranols (8a,b), besides some highly unexpected intramolecular arene-olefin addition products (14–16); formation of these
intramolecular arene–olefin addition products is quenched in the presence of 0.1 mol equiv. of triethylamine. With increasing molar ratios of
triethylamine, in acetonitrile solvent, the formation of 7, 8 is also suppressed with increased formation of products (9–13) derived from
photoreduction of carbonyl function through electron transfer from TEA. No product derived from intramolecular interaction of the ketone
derived anion radical/ketyl radical with an olefinic moiety has been detected. A plausible mechanistic rationalization of the results obtained is
presented. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
phenoxyacetates and related systems; however, yields are
4
low, particularly, in the case of aryl alkyl ketones. In the
Photochemical approaches to polycyclic molecules or their
precursors have become an important component of organic
latter case, the reaction is complicated by competing
reversible enolization, and formation of spirooxiranes (3),
the latter leading to dihydroisobenzofurans (4) and
diketones (5). These differences in the behavior of
benzophenones and aryl alkyl ketones have been attributed,
inter alia, to differences in the life times of the excited states/
1
synthetic methodology. Among the various photochemical
transformations utilized, intramolecular hydrogen abstrac-
tion, which is one of the primary reactions of a photoexcited
4
1
,2
carbonyl group, has proven to be quite valuable.
Normally, 1,4-biradical generation, followed by cyclization
1
leading to cyclobutanols, is favoured over 1,5-biradical
generation on account of the enthalpy and entropy of the
3
transition state for the H-abstraction step. However, if the
formation of 1,4-biradicals is blocked, i.e. if there are no
g-hydrogens, and reactive d-hydrogens are present as in
o-alkoxyaryl ketones (1), the 1,5-biradicals can be
generated which cyclise, e.g. in case of (1), to benzo-
dihydrofuranols (2, Scheme 1); the latter reaction has been
extensively investigated both mechanistically and for
4
synthetic applications. In general, the yields of benzo-
dihydrofuranols are high in the case of o-substituted-
benzophenones, o-benzyloxyarylglyoxalates, o-benzoyl-
Keywords: photochemistry; photocycloadditions; photoelectocyclization;
hydrogen abstraction; 1,5-biradical; electron-transfer; benzofuranols.
*
Corresponding author. Tel.: þ91-183-258808-09x3321; fax: þ91-183-
2
58819-20; e-mail: mpsishar@angelfire.com
Scheme 1.
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00868-2

7596
R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
Scheme 2.
rates of intersystem crossing, rotational freedom and
conformational preferences of diradicals involved (A,
2. Results and discussion
4
Scheme 1). The yield of 1,5-biradicals is also related to
conformational preferences as well the nature of the
Initially, o-allyloxy-/crotyloxy-acetophenones (6a,b) were
irradiated in anhydrous benzene with a 125-Watt medium
pressure Hg arc employing an immersion well type (Pyrex-
glass), water cooled photoreactor, under nitrogen
atmosphere for 100 h (50% conversion). Column chromato-
graphic resolution afforded a single identifiable product in
both cases (7a,b, ,10%), besides unreacted 6a,b and some
intractable material (Scheme 2); formation of any other
reaction product was rigorously excluded by H NMR
spectral scanning of all the chromatographic fractions. The
structures of compounds (7a,b) are based on spectroscopic
‡
data and its comparison with related systems. The
p
involved excited species (n–p or p–p ).
p 2,4,5
Investigations on this reaction both for synthetic applica-
4
l,6
tions
attract considerable attention. Recently this reaction has
and for mechanistic understanding continue to
7
4
l
been exploited to obtain a valuable precursor for
aflatoxin–M . However, it has been observed that the
1
2
yields of cyclized products, i.e. benzodihydrofuranols, are
very low, under identical irradiation conditions, from 2,6-
8
allyloxyacetophenone. Though not elaborated, it has been
attributed to possible role of an electron transfer component
in the cyclization process. A similar possible role of electron
transfer has also been invoked to explain the concomitant
formation of only syn-benzodihydrofuranols from
irradiation of some o-allyloxy/alkoxy-benzophenones
under comparable conditions has already been reported in
4
f–h,k
decarboxylation during similar photocyclization of
9
the literature.
(
o-acetylphenoxy)acetic acid.
Subsequently, irradiations of 6a,b were carried out in dry
benzene, in the presence of different molar equivalents of
TEA and chromatographic resolution of the photolysates
afforded various products (7–13), which have been
Another mode of the phototransformation in the case of
arenes bearing alkenyl substituents is the intramolecular
photocycloaddition. The mode, i.e. (2þ2), (2þ4) or meta-
addition, of these arene–alkene cycloadditions is reported
to depend, inter alia, on the electron-donor/acceptor
1
characterized by rigorous spectroscopic analysis (IR, H
1
3
and C NMR and Mass). The results are summarized in
Scheme 2 and Table 1.
1
0
properties of the arenes and alkenes. However, it has
been time and again emphasized that a minimum of three
carbon/hetero-atoms tether between the aromatic ring and
the olefinic moiety is a must for any intramolecular
The diastereomeric mixture of pinacols (9, 10) and
triethylamine addition products (11, 12) have been resolved
column chromatographically and characterized NMR
1
1
photocycloaddition.
§
spectroscopically. An important aspect brought out by
We have presently investigated the photochemistry of
o-allyloxy/crotyloxyacetophenones (6a,b) under varied
conditions of irradiation, including solvents such as
acetonitrile which is known to favor electron transfer, and
these investigations is that at lower molar ratios of
triethylamine, there is initially, observed an increase in the
formation of 7a,b, however, beyond 0.4 mol equiv. of
triethylamine, the formation of 7a,b starts decreasing with
the increasing formation of 9–13. The initial increase in
production of 7a,b in the presence of triethylamine may be
†
in the presence of triethylamine; the latter is well known to
quench the photoexcited carbonyl function by charge
1
2
transfer/electron transfer. A complete characterization of
the product profile, including, some unusual intramolecular
‡
p
The syn-stereochemistry in 7a,b was initially based on a comparison of
spectroscopic data with the data reported for syn-products obtained from
arene–olefin addition products obtained on n–p excitation
of the carbonyl function, has been attempted.
4
f,l
the irradiation of related systems and reaffirmed after isolation of the
corresponding anti-isomers (8a,b).
§
†
The phototransformations in presence of triethylamine were also of
The diastereomeric pinacols have been resolved chromatographically into
syn-(10) and anti-(9) isomers, and the NMR characterization has been
further authenticated by molecular modelling and finally by X-ray
crystallographic characterization of anti-diastereomer (10) (to be
published elsewhere).
interest because it has been reported that radical anions generated from
reduction of a photoexcited carbonyl function can be trapped
intramolecularly, by an olefinic moiety, in a synthetically useful
1d,13
manner.

R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
7597
Table 1. Reaction conditions and percentage yields of products obtained from irradiation of o-allyloxy/crotyloxyacetophenones (6a,b) in benzene
Entry
TEA (mol equiv.)
R
Irradiation time (h)
Conversion (%)
% Yield of products
7
9
10
11
12
13
1
2
3
4
5
6
Nil
0.1
0.4
0.5
1.0
2.5
H
CH
H
CH
H
CH
H
CH
H
CH
H
100
100
24
24
24
24
22
22
20
20
15
15
51
49
70
68
80
80
90
90
100
100
100
100
11
10
15
14
16
15
10
10
3
3
3
3
3
3
18
20
20
21
25
23
30
29
26
25
23
26
27
28
32
30
36
35
30
30
6
5
8
2
2
4
8
7
18
13
25
18
10
10
CH
4
l,17
due to the known biradical life-time enhancing effect of
Lewis bases through H-bonding, thereby preventing the
and in 8a it is cis to the C3-hydroxyl. As anticipated the
C3–Me resonance, which appeared at d 1.65 in 7a, was
1
4
13
quenching of biradicals by reverse-H transfer. Such
hydrogen bonding by Lewis bases is also reported to
diminish the stereoselectivity leading to benzodihydro-
furanols, having an anti-arrangement around the C2–C3
shifted upfield (d 1.45) in 8a. The C NMR assignments are
also in accordance with the assigned structures. In 7a, the
resonance for C2 appeared at d 101.41 and in 8a at d 100.60.
C3 appeared at d 92.10 in 7a, which is shifted downfield as
compared to C3 in 8a (d 94.21). The C3–Me appeared at d
24.63 in 7a and was shifted upfield in 8a (d 23.21).
Similarly, compound (8b) was obtained only as a mixture
with 7b and its structure has been assigned on the basis of
detailed NMR spectral analysis of the mixture.
1
4
bond; however, no such anti-product has been detected in
the present case. At present, there is no evidence available to
confirm the involvement of a reversible photoelectron
1
5
transfer catalysis as delineated in Scheme 3, though,
amines are reported to sensitize the benzophenone promoted
1
6
photoaddition of acetonitrile to olefins.
Two similar compounds isolated from irradiation of 6a,b
have been characterized, respectively, as polycyclic
unsaturated ketones (14a) and (14b). Though derived from
6a,b, the H NMR spectra of these molecules did not reveal
1
any resonance in the aromatic region. Instead, the H NMR
Subsequently, irradiations of 6a,b were carried out in dry
acetonitrile and in the presence of varying (increasing)
amounts of triethylamine. The products were resolved by
column and preparative layer chromatography, and charac-
terized spectroscopically. The results are summarized in
Scheme 4 and Table 2.
1
spectra of these molecules revealed an olefinic-H resonance
(broad singlet) at d 5.88 in 14a and d 5.74 in 14b. The
presence of unsaturated carbonyl moieties in 14a,b was
1
3
Compound (8a) was obtained only as a mixture with 7a. The
1
indicated by the IR spectra and corroborated by C NMR
spectra through a resonance at d 200.5 in the case of 14a,
and d 199.61 in the case of 14b. The presence of an intact
acetyl moiety was confirmed through the methyl resonance
at d 2.24 in 14a and d 2.15 in 14b. As the above-mentioned
IR and NMR spectral features suggested the presence of a
characteristic feature of its H NMR spectrum were a
doublet at d 4.86 (J¼6.99 Hz, C2–H), which is shifted
downfield as compared to the C2–H resonance in 7a (d
4
anticipated
.61, J¼6.54 Hz,); such a variation in chemical shifts is
because in 7a C2–H is cis to the C3-methyl
4
l,17
Scheme 3.

7598
R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
Scheme 4.
Table 2. Reaction conditions and percentage yields of products obtained from irradiation of o-allyloxy-/crotyloxyacetophenone in acetonitrile
Entry
TEA (mol equiv.)
R
Irradiation time (h)
Conversion (%)
Product yield (%)
7
7þ8
9
10
11
12
13
14
15
16
1
2
3
4
5
6
Nil
0.1
0.4
0.5
1.0
2.5
Nil
H
CH
H
CH
H
CH
H
CH
H
CH
H
CH
H
CH
100
100
24
24
23
23
22
22
19
19
15
15
30
30
60
59
80
80
90
88
100
100
100
100
100
100
90
17
17
20
18
18
21
14
13
6
5
7
6
6
5
4
4
7
8
4
6
3
3
3
3
3
3
3
15
16
22
22
25
24
28
27
20
20
22
23
30
29
32
31
36
34
26
27
10
8
3
4
6
7
7
14
11
15
13
20
14
25
18
10
10
20
20
a
7
24
23
7
7
15
16
7
12
90
a
Irradiation carried out by 400 W lamp; all other irradiations were carried out by 125 W lamp.
1
-acetylcyclobutene moiety (cf. Ref. 1m) and only one
these compounds derived from a meta-addition, was ruled
out because the hydrogens of the cyclopropane moiety in 17
olefinic-H resonance was present in each case, it was
inferred that the transformation leading to the products
1
1d,e,19
were anticipated to have low chemical shifts values
and no such upfield shifted resonances were present in the
(
14a,b) involved the addition of the side chain olefinic
moiety to the aromatic ring, through an intramolecular
2þ2) cycloaddition, which is followed by 4p-electro-
cyclization. Such 4p-photocyclizations are precedented in
1
H NMR spectra of 14a,b.
(
1
8
cyclohexadiene intermediates. The structures are also
supported by the presence of two resonances for oxygen
linked carbons in both compounds, i.e. d 69.41 (C1 in 14a),
d 78.8 (quat., C3 in 14a), d 69.06 (C1 in 14b), and d 78.40
(
quat., C3 in 14b). The assigned structures, stereochemistry
1
1
as well as H connectivities have been established by H
decoupling experiments and supported by comparison of the
NMR data with reported data for related systems.
Another product isolated in small amount (7%) from the
photolysis of o-allyloxyacetophenone, has been character-
ized as intramolecular cycloadduct (15a). The assigned
structure is again based on rigorous spectroscopic analysis.
1
h,18
An
1
important aspect of the H NMR spectra which needs
mention here is that the C3–Hs showed up as two distinct
resonances, i.e. as a doublet (1H, Jgem¼13.3 Hz) at d 4.52
and a double doublet at d 4.11 (1H, Jgem¼13.3 Hz and
1
Its H NMR spectrum did not reveal any resonance in the
aromatic region. However, it revealed two resonances in the
olefinic region i.e., a doublet at d 6.06 (1H, J¼2.84 Hz)
coupled to another broad doublet at d 5.88 (1H, J¼2.84 Hz).
The C3–Hs were located as a 1H double doublet at d 4.29
(J¼8.76, 4.73 Hz), and a 1H doublet at d 4.07 (J¼8.76 Hz).
A 1H broad doublet at d 3.88 (J¼3.06 Hz) has been assigned
to C7–H and a 2H multiplet at d 3.40 comprised the C4–H
and C10–H resonances. The upfield part of the spectrum
had a double doublet (1H) at d 2.08 (Jgem¼13.52 Hz and
J ¼5.43 Hz) in 14a, and in case of 14b as a doublet d 4.35
vic
(
J
1H, J¼13.35 Hz), and a double doublet at d 4.01 (1H,
gem vic
¼13.35 Hz and J ¼5.43 Hz). That only one of the
C3–Hs is coupled with C4–H is supported by molecular
modeling (DTMM, version 2.0) which indicated that one of
the C3–H has a dihedral angle of ,938 with C4–H. The
mass spectra also corroborated the assigned structures. It
may be mentioned here that an alternative structure (17) for

R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
7599
Scheme 5.
1
3
J ¼7.17 Hz, C5–H) and the other C5–H was located as a
overall C NMR assignments were in agreement with the
assigned structure. The assigned stereochemical disposi-
vic
multiplet at d 1.51. All these assignments were aided by
rigorous decoupling experiments to establish H con-
1
1
tions are based on J values and H connectivities worked out
by rigorous decoupling experiments.
1
3
nectivities. The C NMR spectrum revealed a carbonyl
carbon resonance at d 206.6. The oxygen-linked carbons
appeared at d 82.32 (C1) and d 71.24 (C3), and a quaternary
It may be mentioned here that some mixed fractions were
also obtained, which showed the presence of many other
related products as indicted by their H NMR spectra,
however, these could not be resolved sufficiently for any
structural assignments to be made. H NMR of one of the
fractions revealed resonances (multiplets) in the upfield part
of the spectrum, which were characteristic of a possible
1
3
carbon resonance at d 56.42 (C6); other C NMR spectral
assignments, which supported the assigned structure,
include resonances at d 54.24 (C7), d 48.72 (C10), d
1
1
4
and IR spectral data also supported the assigned structure.
0.80 (C5), d 30.72 (C4) and d 26.04 (COC H ). The mass
3
1
1d,e,19
Yet, another compound which was isolated in 12% yield
from the photolysis of o-allyloxyacetophenone has been
meta-addition product; the total amount of these
intramolecular addition products, including the unassigned
characterized as the intramolecular photoaddition product
(
fractions, accounted for ,50% of the photoproducts.
þ
two mass units during phototransformation. The presence of
16a). It revealed an M ion at 178 indicating the addition of
Subsequently, irradiations of 6a,b were carried out in dry
acetonitrile in the presence of different molar equivalents of
triethylamine and chromatographic resolution of the
photolysates afforded various products (Scheme 4, Table
2). Overall, the trend is similar to the one observed for
irradiation of 6a,b in dry benzene in the presence of
triethylamine. Initially, at lower molar ratios of triethyl-
amine, there was obtained an increase in the formation of
7a,b and with increase in the relative molar ratio of
triethylamine the formation of 7a,b decreased with the
concomitant increased formation of 9–13.
an intact CH CO-moiety was inferred from the IR spectrum
3
(
singlet at d 2.19), and C NMR (carbonyl carbon resonance
band at 1715 cm2 ) and confirmed by H NMR (3H,
1
1
1
3
1
at d 204.0). Its H NMR spectrum further revealed an –OH
1
resonance at d 8.02. The H NMR showed complete absence
of any resonance in the aromatic region of the spectrum, and
instead revealed two doublets in the olefinic region at d 6.00
and 5.79 (J¼2.70 Hz); as mentioned in the case of
compound (15a), this pattern was characteristic of a
1
k,m 1
cyclobutene moiety.
multiplet at d 4.20, which is assigned to a –OCH – moiety.
H NMR also revealed a 2H
2
The upfield part of the spectrum revealed two double
doublets at d 3.53 and 3.31 for C2–H and C5–H and two
multiplets at d 2.85 and 2.52 for C7–H and C6–H,
respectively. Further, a critical assignment made in its H
NMR spectrum was that of the C8–Hs, which appeared as
two distinct resonances, i.e. a double doublet at d 1.75
Mechanistically, the formation of intramolecular photo-
addition products is highly unexpected. As mentioned
earlier a minimum of a three-carbon/heteroatom tether has
been reported to be essential for any intramolecular arene-
1
1
1
alkene photocycloaddition. Accordingly, it has been
reported that no intramolecular photocycloaddition product
could be detected from benzyl vinyl ether and only
b-phenylpropanaldehyde was isolated. Similarly, allyl
phenyl ether is reported to afford only Claisen rearrange-
(J¼6.11, 12.93 Hz) and a multiplet at d 1.35, and implied
that only one of the C8–Hs is coupled with C7–H. That the
compound 16a has a free –CH OH group is also
2
13
21
corroborated by the C NMR spectrum which revealed a
single resonance for an oxygen linked carbon at d 65.21,
ment products on irradiation. 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
which was also shifted upfield as compared to the –OCH –
2
2
0
resonance in compound 14a (d 69.41) and 15a (d 71.24);

7600
R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
Scheme 6.
olefinic moiety can be considered as an electron donor. The
p
reaction is initiated by n–p excitation (Pyrex filter) and
group with the solvent molecule increases its bulkiness
leading to formation of the anti-adducts, albeit as minor
products.
4
f,g,23
involves intramolecular electron transfer as supported by
the observed solvent dependence of formation of these
products (14–16), i.e. they are formed in acetonitrile and
not in benzene, and their formation is inhibited by addition
of even 0.1 mol equiv. of triethylamine. The sequence of
events possibly leading to 14–16 is outlined in Scheme 5.
The conformational preferences play an overwhelming role
in controlling the product profile. Beyond 0.5 mol equiv. of
TEA, the whole of carbonyl moiety is reduced to anion
radicals and is partitioned into pinacols (9 and 10) and
triethylamine addition products (11 and 12), and no
intramolecular reactions of anion radicals or ketyl radicals,
It may be mentioned here that intramolecular (2þ2)
photoadditions followed by further rearrangement of the
photoadducts are precedented in the case of certain
1
3
as reported in certain cases, occurs (Scheme 6).
1
k,l
p-subsituted acetophenones
l,10b
and 2-subsituted-1-acetyl-
and among these, the former are
The role of conformational preference is also evident in not
obtaining any oxetane (Scheme 7) and intervention of rather
cumbersome transition states leading to photocycloadducts
(14–16).
1
naphthalenes,
postulated to involve donor–acceptor interactions. In
general, the donor–acceptor interactions are known to be
involved in ortho-photoaddition, which is particularly
favored if the addends differ substantially in their ionization
The above results have clearly established that the generally
accepted limiting condition of a three-atom tether between
an aryl ring and an alkene moiety for intramolecular photo-
cycloaddition is easily overcome by polar interactions.
Intramolecular H-abstraction by photo-excited carbonyl
followed by cyclization of 1,5-biradical is an inefficient
process in the case of o-alkenylmethoxyacetophenones,
which is slightly favored in presence of lower molar
equivalents of triethylamine and is quenched at higher
concentrations. A substantial amount of further investi-
gations will be necessary to completely unravel the
mechanistic details of these phototransformations, particu-
larly, the role of conformational preferences in determining
the overall photo-product profile from these molecules.
1
0a,22
potentials.
However, all the reported examples con-
form to the criterion of a minimum three methylenes/
heteroatom separation between arene and olefin. Therefore,
the reported addition, to the best of our knowledge, is the
first case wherein the above-mentioned limiting condition
for arene–olefin intramolecular photo-cycloaddition has
been surpassed. Exciplexes, which are generally believed to
be involved in such reactions between donor–acceptor
systems, have not been investigated in the present case. The
conversion of (15) to oxetane ring opened product (16) may
also be a photochemical transformation. The facile
4
number of cases.
p-photochemical electrocyclization is well known in a
8
1
In the case of a non-polar solvent, i.e. benzene, the preferred
formation of syn-adducts in which bulky groups are placed
3. Experimental
3.1. General
4
f,g
in trans-arrangement has been attributed
to the relative
sizes of the substitutents and their influence on the mode of
rotation of bonds during ring closure of 1,5-biradicals.
However, in polar solvents, hydrogen bonding of the –OH
A Bruker AC-200FT (200 MHz) NMR spectrometer was
Scheme 7.

R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
7601
1
13
used to record H and C NMR spectra. Chemical shifts are
reported in ppm as downfield displacements from tetra-
methylsilane used as internal standard and J values as Hertz.
IR spectra were recorded on Shimadzu DR 2001 FT-IR
spectrophotometer either as thin layer with few drops of
dissolved in thiophene-free dry benzene (250 mL) and taken
in an immersion well type Pyrex-glass, water cooled,
photoreactor. Solutions were purged with dry oxygen free
N for at least 15 min prior to irradiation. The irradiations
2
were carried out with a 125-Watt medium pressure Hg arc
placed coaxially inside the reactor and N was continuously
CHCl or as KBr pellets. Mass spectra were recorded (EI
3
2
method) on Shimadzu GCMS-QP-2000A spectrometer.
Microanalytical data were collected on Perkin–Elmer
bubbled through the solutions during irradiation. At the end
of reaction (Tlc), solvent from the photolysates was
distilled-off under reduced pressure using a rotary
evaporator and products were separated by column
chromatography over silica (60–120 mesh) using hexane–
chloroform (gradient) as eluent.
2
40C elemental analyzer and reported in percent atomic
abundance. All melting points and boiling points are
uncorrected; melting points are measured in open glass-
capillaries. 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).
3.2.1. Irradiation of o-allyloxyacetophenone (6a) in dry
benzene. Irradiation of a solution of o-allyloxyaceto-
phenone (6a, 500 mg) in thiophene-free dry benzene
(250 mL) for 100 h, afforded: unreacted 6a (245 mg).
(syn )-2-Ethenyl-3-hydroxy-3-methyl-2,3-dihydrobenzo-
furan (7a) as colorless viscous oil (55 mg), lmax (MeOH):
3
.1.1. o-Allyloxy/crotyloxy acetophenones (6a,b). These
were prepared by reacting o-hydroxyacetophenone (10 mL,
.3 mmol), respectively, with allyl bromide (10 mL,
8
,
,
1.4 mol equiv.) and crotyl bromide (12 mL,
1.4 mol equiv.) by stirring under reflux in dry alcohol
211, 237, 269, 272, 292 nm; nmax (CHCl ): 3488 (b), 1620
3
(w), 1600 (s), 1476 (s), 1465 (w), 1375 (m), 1280 (m), 1240
1
free acetone (100 mL), in presence of anhydrous K CO
(m), 1215 (m), 1094 (s)) cm2 ; dH (CDCl , 200 MHz):
7.33–7.20 (m, 2H, arom.-Hs), 6.97–6.85 (m, 2H, arom.-
2
3
3
(12.0 g). The reactions were monitored by TLC and after
completion of reactions (12 h), the K CO was filtered of,
0
Hs), 6.18–6.01 (m, 1H, C1 –H), 5.61 (dd, 1H, J¼15.9,
2
3
0
0
washed with little acetone and the solvent from the filtrates
was distilled off. Pure products (6a,b) were isolated by
vacuum distillation of the residual viscous oils and
characterized spectroscopically.
1.47 Hz, C2 –H), 5.44 (dd, 1H, J¼11.6, 1.3 Hz, C2 –H),
4.63 (bd, 1H, J¼6.54 Hz, C2–H), 1.65 (s, 3H, C3–Me); d
C
(CDCl , 50 MHz): 161.41 (C7a), 132.91 (C3a), 131.52
3
(CH), 130.27 (CH), 123.31 (CH), 121.11 (CH), 118.41
(H Cv), 110.64 (CH), 92.03 (C2), 77.80 (C3), 24.63 (C3–
Me); m/z(%): 176 (40, M ), 116 (100); (Found: C, 74.74, H,
2
þ
6.67. C H O requires C, 74.98, H, 6.86%).
3
.1.2. o-Allyloxyacetophenone (6a). A colorless oil (yield
2 g), bp 82–84 8C/10 mm; lmax (MeOH): 247, 305 nm;
nmax (CHCl ): 1680 (s), 1610 (s), 1595 (s), 1480 (s), 1450
1
11
12 2
3
(
(
s), 1420 (s), 1360 (s), 1290 (s), 1280 (s), 1160 (s), 1130
2
3.2.2. Irradiation of o-crotyloxyacetophenone (6b) in dry
benzene. Irradiation of a solution of (6b, 500 mg) in
thiophene-free dry benzene (250 mL) for 100 h afforded:
unreacted 6b (255 mg). (syn )-2-(1-Propenyl)-3-hydroxy-3-
methyl-benzodihydrofuran (7b) as colorless gummy
material (50 mg), lmax (MeOH): 218, 248, 278, 286 nm;
1
s)) cm ; dH (CDCl , 200 MHz): 7.72 (dd, 1H, J¼8.4,
3
1
arom.-Hs), 6.17–5.96 (m, 1H, C2 –H), 5.42 (dd, 1H,
.83 Hz, C6–H), 7.42 (m, 1H, arom.-H), 6.91 (m, 2H,
0
0
0
J¼14.0, 1.5 Hz, C3 –H), 5.31 (br d, 1H, J¼7.6 Hz, C3 –H),
0
4
5
1
1
.62 (d, 2H, J¼5.1 Hz, C1 –Hs), 2.63 (s, 3H); d (CDCl ,
C
3
0 MHz): 199.28 (CvO), 157.67 (C2), 133.27 (CH),
32.39 (CH), 130.04 (CH), 128.36 (C1), 120.3 (CH),
17.81 (CH v), 112.60 (CH), 69.16 (–OC H ), 31.73
nmax (CHCl ): 3480 (s), 1612 (m), 1599 (s), 1475 (m), 1455
3
(m), 1410 (w), 1381 (m), 1340 (m), 1315 (m), 1290 (w),
1275 (w), 1260 (w), 1240 (m), 1202 (m)) cm2 ; d (CDCl ,
1
2
2
H
3
þ
(
H, 6.99. C H O requires C, 74.98, H, 6.86%).
COCH ); Mass: m/z (%): 176 (M , 18); (Found: C, 74.79,
200 MHz): 7.24–7.11 (m, 2H, arom.-Hs), 6.88–6.74 (m,
0
3
2H, arom.-Hs), 5.98–5.84 (m, 1H, C1 –H), 5.74–5.62 (m,
0
1
1 12 2
1
H, C2 –H), 4.47 (bd, 1H, J¼7.56 Hz, C2–H), 1.79 (bd,
0
(CDCl , 50 MHz): 159.13 (C7a), 133.35 (C3a), 132.49
3
(
3
(
(
.1.3. o-Crotyloxyacetophenone (6b). A colorless oil
yield 11.9 g), bp 86–888C/10 mm; lmax (MeOH): 245,
05 nm; nmax (CHCl ): 1680 (s), 1600 (s), 1500 (s), 1460
3H, J¼6.30 Hz, C2 –Me), 1.53 (s, 3H, C3–Me); d
C
3
(CH), 130.24 (CH), 124.57 (CH), 123.39 (CH), 121.04
(CH), 110.70 (CH), 92.30 (C2), 77.92 (C3), 24.06 (C3–
3
s), 1380 (s), 1370 (s), 1300 (s), 1180 (s), 1160 (s), 1080
s)) cm2 ; dH (CDCl , 200 MHz): 7.72 (dd, 1H, J¼7.10,
1
Me), 18.23 (C2 –Me); 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);
(Found: C, 75.65, H, 7.50. C H O requires C, 75.76, H,
0
þ
3
1
.5 Hz, C6–H), 7.38 (dt, 1H, J¼8.24, 1.5 Hz, C4–H), 6.97
0
0
(
m, 2H, arom.-Hs), 5.85–5.74 (m, 2H, C2 –H and C3 –H),
0
4
3
(
(
6
(
.56 (br d, 2H, J¼5.52 Hz, C1 –H), 2.63 (s, 3H), 1.76 (d,
1
2 14 2
0
H, J¼4.6 Hz, C4 –Hs); d (CDCl , 50 MHz): 200.01
7.42%).
C
3
CvO), 158.01 (C2), 133.47 (CH), 130.74 (CH), 130.36
CH), 129.07 (C1), 125.56 (CH), 120.60 (CH), 112.91 (CH),
3.3. General procedure for irradiation of o-allyloxy-/
crotyloxyacetophenone (6a,b) in benzene in presence of
TEA
9.35 (–OCH ), 35.31 (COCH ), 17.81 (vCHC H ); m/z
2 3 3
þ
%): 190 (M , 27); (Found: C, 75.62, H, 7.61. C H O
12 14 2
requires C, 75.76, H, 7.42 %).
Irradiations of o-allyloxy-/crotyloxyacetophenones (6a,b,
500 mg) were carried out in thiophene-free dry benzene
with varying amounts of triethylamine (0.1–2.5 mol equiv.)
in an immersion well type Pyrex glass, water cooled
photoreactor with a 125-Watt medium pressure Hg arc.
3.2. General procedure for irradiation of o-allyloxy-/
crotyloxyacetophenones (6a,b) in dry benzene
o-Allyloxy-/crotyloxyacetophenones (6a,b, 500 mg) were

7602
R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
Solutions were purged with dry oxygen free N for at least
2
diastereomer) as pale yellow gummy material (30 mg), lmax
(MeOH): 220, 271, 280 nm; nmax (CDCl ): 3420 (b), 3017
1
5 min prior to irradiation and N was continuously bubbled
2
3
during irradiation. The progress of the reaction was
monitored by Tlc. At the end of the reaction, solvent was
distilled off from photolysates under reduced pressure and
the products were separated by column chromatography
over silica (60–120 mesh, column packed in hexane) using
hexane–chloroform (gradient) followed by chloroform, and
chloroform–acetone (gradient) as eluents.
(s), 2928 (s), 2857 (m), 1620 (m), 1598 (m), 1580 (m), 1560
(m), 1487 (m), 1458 (m), 1445 (m), 1355 (m), 1238 (m),
1227 (m), 1165 (m), 1040 (m), 1010 (m)) cm2 ; d (CDCl ,
1
H
3
200 MHz): 7.63 (dd, 1H, J¼7.0, 1.5 Hz, arom.-H), 7.16 (dt,
1H, J¼7.69, 1.5 Hz, arom.-H), 6.92 (t, 1H, J¼7.41 Hz,
arom.-H), 6.80 (bd, 1H, J¼8.05 Hz, arom-H), 6.15–5.98
(m, 1H, CHv), 5.44 (bs, 1H, OH), 5.36–5.27 (m, 2H,
CH v), 4.53 (bd, 2H, J¼5.20 Hz, –OCH ), 3.36 (q, 1H,
2
2
3
.3.1. Irradiation of o-allyloxyacetophenone in benzene
J¼6.85 Hz, N–CH–Me), 2.76–2.39 (m, (overlapping
quartets) 4H, 2£NCH Me), 1.75 (s, 3H, –CH ), 1.11 (t,
in presence of TEA. Irradiation of a solution of
o-allyloxyacetophenone 6a (500 mg) in dry benzene
2
3
6H, J¼7.02 Hz, 2£CH ), 0.92 (d, 3H, J¼7.06 Hz,
3
(
2
250 mL) and triethylamine (39.5 mL, 0.1 mol equiv.) for
4 h afforded: unreacted 6a (150 mg). (syn )-2-Ethenyl-3-
–CHCH ); d (CDCl , 50 MHz): 156.14 (quat.), 133.14
3
C
3
(quat.), 128.27 (CH), 127.96 (CH), 121.04 (CH), 118.19
(CH), 112.07 (CH), 109.0 (CH), 75.56 (quat.), 69.24
(OC H ), 64.09 (N–C H–CH ), 46.24 (NC H CH ), 26.96
hydroxy-3-methyl-2,3-dihydrobenzofuran (7a, 75 mg).
Pinacol (9a) as colorless crystals (90 mg), mp 83–848C
2
3
2
3
(
hexane–benzene 1:2), lmax (MeOH): 211, 237, 239 nm;
nmax (KBr): 3500 (s), 3000 (m), 1600 (s), 1575 (m), 1493
m), 1450 (m), 1425 (m), 1410 (w), 1400 (w), 1355 (m),
(vCHC H ), 12.22 (NCH C H ), 10.11 (N–CH–C H ); m/z
3 2 3 3
þ
þ
(%): 278 (M þ1, 9), 277 (M , 19), 261 (30), 260 (29), 252
(18), 250 (20), 232 (27), 230 (22), 220 (20), 219 (21), 218
(28), 207 (27), 205 (16), 204 (27), 203 (22), 202 (27), 100
(100); (Found: C, 73.39, H, 9.60, N, 5.29. C H NO
2
(
1
(
300 (m), 1233 (s), 1120 (w), 1065 (m), 1015 (m)) cm2 ; dH
1
CDCl , 200 MHz): 7.39 (bd, 2H, J¼7.7, 1.43 Hz, arom.-
3
17 27
Hs), 7.08 (dt, 2H, J¼7.46, 1.35 Hz, arom.-Hs), 6.90 (t, 2H,
J¼7.46 Hz, arom.-Hs), 6.60 (bd, 2H, J¼8.17 Hz, arom.-
Hs), 5.76–5.67 (m, 2H, 2£CHv), 5.43 (b s, 2H, exchanges
requires C, 73.61, H, 9.81, N, 5.05%).
Irradiation of a solution of 6a (500 mg) in dry benzene
(250 mL) in presence of triethylamine (197.6 mL,
0.5 mol equiv.) for 22 h, afforded: unreacted 6a (50 mg).
7a (50 mg). Pinacol (9a, 125 mg). Pinacol (10a, 160 mg).
Triethylamine addition product (11a, 40 mg).
with D O, 2£OH), 5.32 (dd, 2H, J¼17.84, 1.44 Hz, 2£trans
2
olefinic-H), 5.25 (dd, 2H, J¼10.94, 1.31 Hz, 2£cis-olefinic-
H), 4.15 (dd, 2H, J ¼11.88 Hz, J ¼5.97 Hz, –OCH ),
C
gem
vic
2
4
.05–3.88 (bm, 2H, –OCH ), 1.66 (s, 6H, 2£CH ); d
2
3
(
(
(
(
(
(
(
CDCl , 50 MHz): 156.26 (quat.), 133.07 (quat.), 132.62
3
CH), 130.42 (CH), 127.96 (CH), 120.89 (CH), 118.52
CH), 112.03 (CH), 81.74 (quat.), 69.49 (–OCH ), 24.74
Irradiation of a solution of 6a (500 mg) in dry benzene
(250 mL) in presence of triethylamine (395.2 mL,
1.0 mol equiv.) for 20 h afforded: pinacol (9a, 150 mg),
pinacol (10a, 180 mg), triethylamine addition product (11a,
90 mg).
2
þ
Me); m/z (%): 318 [M 236 (2£H O), 20], 303 (18), 295
2
37), 294 (23), 293 (62), 278 (19), 277 (30), 263 (24), 237
26), 236 (23), 235 (41), 223 (45), 221 (49), 211 (26), 203
22), 177 (100); (Found: C, 75.56, H, 7.49. C H O
2 26 4
2
requires C, 74.65, H, 7.39%). Pinacol (10a) as colorless
crystals (115 mg), mp 103–1048C (hexane–benzene 1:2);
lmax (MeOH): 211, 237, 240 nm; nmax: 3480 (s), 2995 (m),
Irradiation of a solution of 6a (500 mg) in dry benzene
(250 mL) in presence of triethylamine (987.9 mL,
2.5 mol equiv.) for 15 h afforded: pinacol (9a, 130 mg),
pinacol (10a, 150 mg), triethylamine addition product (11a,
125 mg), triethylamine dimer (13) as a pale viscous oil
1
1
1
7
599 (s), 1470 (m), 1491 (m), 1449 (m), 1426 (m), 1368 (s),
287 (m), 1227 (s), 1180 (w), 1150 (w), 1120 (m), 1105 (w),
2
1
085 (m), 1070 (m)) cm ; d (CDCl , 200 MHz): 7.15–
H
(50 mg), n
(CHCl ): 2966, 1600, 1455, 1396, 1242,
max 3
3
1170, 1160, 1060, 960, 908, 661) cm2 ; dH (CDCl3,
200 MHz): d 3.19–2.94 (m, 10H(q at d 3.14, J¼7.32 Hz,
partially overlapping another distorted q centered at d
1
.06 (dt, 2H, J¼7.68, 1.75 Hz, arom.-Hs), 6.99 (bd, 2H,
J¼7.24 Hz, arom.-Hs), 6.83–6.68 (m, 4H, arom.-Hs),
5
.97–5.80 (m, 2H, 2£CHvCH ), 5.46 (s, 2H, exchanges
2
þ
with D O, 2£OH), 5.28–5.19 (m, 4H, 2£H Cv), 4.34–
3.08)), 1.50–1.27 (m, 18H); m/z: 101 (15), 100 (100, M /2).
2
2
4
.12 (doublet of AB quartet, 4H, J¼15.61, 5.2 Hz, 2£–
2 3 C 3
OCH ), 1.62 (s, 6H, 2£CH ); d (CDCl , 50 MHz): 156.64
3.3.2. Irradiation of o-crotyloxyacetophenone (6b) in dry
benzene in presence of TEA. Irradiations of o-crotyl-
oxyacetophenone (6b, 500 mg) in dry benzene (250 mL) in
presence of triethylamine (36.6 mL, 0.1 mol eq.) under the
above conditions for 24 h afforded: unreacted 6b (160 mg).
7b (70 mg). Pinacol 9b as colorless crystals (100 mg), mp
95–968C (hexane–benzene 1:2); nmax (KBr): 3482 (s), 3010
(s), 1599 (s), 1580 (m-s), 1490 (s), 1449 (s), 1400 (s), 1375
(m), 1282 (s), 1240 (s), 1200 (m), 1001 (m-s)) cm2 ; dH
(
(
(
(
(
(
(
(
7
quat.), 132.97 (CH), 132.52 (quat.), 130.15 (CH), 128.06
CH), 120.53 (CH), 118.13 (CH), 112.83 (CH), 82.37
quat.), 69.67 (–OCH ), 24.61 (Me); m/z (%): 318 [M 236
þ
2
2£H O), 73], 303 (45), 295 (33), 293 (66), 278 (32), 277
2
53), 263 (39), 238 (33), 237 (38), 236 (37), 235 (66), 223
61), 221 (68), 219 (31), 211 (38), 207 (51), 205 (21), 203
26), 202 (25), 177 (99), 133 (26), 121 (100), 119 (96);
Found: C, 75.44, H, 7.57. C H O requires C, 74.65, H,
1
2
2 26 4
.39%).
(CDCl , 200 MHz): 7.40 (d, 2H, J¼6.77 Hz, arom.-Hs),
3
7
.06 (bt, 2H, J¼7.67 Hz, arom.-Hs), 6.87 (t, 2H, J¼7.37 Hz,
Irradiation of a solution of (6a, 500 mg) in dry benzene
250 mL) in presence of TEA (158.0 mL, 0.4 mol equiv.) for
4 h afforded: unreacted 6a (100 mg). 7a (80 mg). Pinacol
9a, 100 mg). Pinacol (10a, 135 mg). Triethylamine
addition product (11a, slightly contaminated with a minor
arom.-Hs), 6.58 (d, 2H, J¼8.14 Hz, arom.-Hs), 5.76–5.64
(
2
(
(m, 2H, 2£CHv), 5.52–5.41 (m, 4H, CH CHv£2 and
3
2£OH), 4.16–4.07 (m, 2H, –OCH ), 4.00–3.90 (m, 2H,
2
–OCH ), 1.75 (d, 6H, J¼6.26 Hz, 2£CH ), 1.66 (s, 6H,
2
3
2£CH ); d (CDCl , 50 MHz): 157.03 (quat.), 132.60
3
C
3

R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
7603
(
(
(
[
(
(
quat.), 130.64 (CH), 130.12 (CH), 128.03 (CH), 125.87
CH), 120.33 (CH), 112.72 (CH), 81.87 (quat.), 69.46
–OCH ), 24.86 (Me), 17.99 (vCHCH ); m/z (%): 346
2£NCH CH ), 1.70 (d, 3H, J¼5.70 Hz, vCHCH ), 1.60 (s,
3 2 3
2
3
3
3H, CH ), 0.97 (t, 6H, J¼7.05 Hz, 2£NCH CH ), 0.81 (d,
3H, J¼7.06 Hz, –CHCH ); d (CDCl , 50 MHz): 155.62
2
3
3
C
3
þ
M 236 (2£H O), 48), 331 (34), 309 (27), 292 (25), 291
(quat,), 135.98 (quat.), 129.82 (CH), 128.11 (CH), 127.73
(CH), 126.40 (CH), 120.70 (CH), 112.19 (CH), 77.23
(quat.), 68.80 (OCH ), 63.78 (N–C H–CH ), 45.13
2
28), 277 (34), 201 (29), 192 (43), 191 (95), 175 (80), 173
46), 159 (35), 137 (72), 121 (40), 56 (100); (Found: C,
2
3
7
5.18, H, 8.08. C H O requires C, 75.36, H, 7.91%).
2
(NC H CH ), 24.32 (vCHC H ), 18.97 (–CH ), 14.29
2 3 3 3
4
30
4
þ
Pinacol (10b) as colorless crystals (130 mg), mp 117–
188C (hexane–benzene 1:2); nmax (KBr): 3400 (b s), 3000
w-m), 1600 (s), 1578 (s), 1488 (m), 1448 (m), 1375 (m),
(NCH C H ), 9.88 (N–CH–C H ); m/z (%): 292 (M þ1,
2
3
þ
3
1
(
1
1
6
9), 291 (M , 23), 274 (29), 200 (48), 199 (35), 191 (27), 190
(59), 175 (45), 173 (36), 161 (29), 160 (20), 101 (19), 100
(100); (Found: C, 74.37, H, 10.19, N, 4.53. C H NO
280 (m), 1225 (s), 1180 (w), 1120 (w-m), 1082 (w-m),
1
1
8
29
2
2
057 (m), 1000 (m)) cm ; d (CDCl , 200 MHz): 7.15–
H
requires C, 74.18, H, 10.03, N, 4.81%).
3
.97 (dt, 2H, J¼7.68, 1.74 Hz, arom.-Hs), 6.97 (d, 2H,
J¼7.04 Hz, arom.-Hs), 6.81–6.68 (m, 4H, arom.-Hs),
Irradiation of a solution of 6b (500 mg) in dry benzene
(250 mL) in the presence of triethylamine (183.0 mL,
0.5 mol equiv.) for 22 h afforded: unreacted 6b (50 mg),
7b (50 mg), pinacol (9b, 115 mg), pinacol (10b, 150 mg),
triethylamine addition product (11b, 35 mg), triethylamine
addition product (12b, 10 mg).
5
.82–5.68 (m, 2H, 2£CHv), 5.64–5.53 (m, 4H, 2£OH
and 2£vCHCH ), 4.21–4.05 (doublet of AB quartet, 4H,
3
J¼11.28, 6.32 Hz, 2£–OCH ), 1.76 (dd, 6H, J¼6.05,
2
1
5
1
1
1
3
1
1
.02 Hz, 2£–CH ), 1.60 (s, 6H, 2£CH ); d (CDCl ,
3
3
C
3
0 MHz): 157.16 (quat.), 132.38 (quat.), 130.60 (CH),
30.00 (CH), 127.99 (CH), 125.74 (CH), 120.23 (CH),
12.60 (CH), 82.41 (quat.), 69.35 (–OCH ), 24.76 (Me),
Irradiation of a solution of 6b (500 mg) in dry benzene
(250 mL) in presence of triethylamine (366.1 mL,
1.0 mol equiv.) for 20 h afforded: pinacol (9b, 145 mg),
pinacol (10b, 175 mg), triethylamine addition product (11b,
65 mg), triethylamine addition product (12b, 20 mg).
2
þ
7.95 (vCHC H ); m/z (%): 346 [M 236 (2£H O), 44],
3
2
31 (36), 309 (26), 292 (27), 291 (26), 277 (30), 201 (30),
92 (45), 191 (97), 175 (82), 173 (42), 159 (36), 137 (73),
21 (43), 56 (100); (Found: C, 75.19, H, 8.02. C H O
4 30 4
2
requires C, 75.36, H, 7.91%).
Irradiation of a solution of 6b (500 mg) in dry benzene
(250 mL) in presence of triethylamine (915.3 mL
2.5 mol equiv.) for 15 h afforded: pinacol (9b, 125 mg),
pinacol (10b, 150 mg), triethylamine addition product (11b,
90 mg), triethylamine addition product (12b, 40 mg),
triethylamine dimer (13, 50 mg).
Irradiation of a solution of (6b, 500 mg) in dry benzene
250 mL) in the presence of triethylamine (146.4 mL,
(
0.4 mol equiv.) for 24 h afforded: unreacted 6b (105 mg).
b (75 mg). Pinacol (9b, 105 mg). Pinacol (10b, 140 mg).
7
Triethylamine addition product 11b as pale viscous oil
(
(
25 mg), lmax (MeOH): 215.8, 273, 279, 306 nm; nmax
CHCl ): 3425 (b), 3000 (s), 2980 (w), 1597 (s), 1581 (m),
3.4. General procedure for irradiation of o-allyloxy-/
crotyloxyacetophenone in dry acetonitrile
3
1
1
1
1
5
485 (s), 1448 (s), 1379 (s), 1290 (m), 1240 (s), 1165 (w),
2
1
074 (s), 1003 (m)) cm ; d (CDCl , 200 MHz): 7.50 (dd,
3
H
H, J¼7.72, 1.75 Hz, arom-H), 7.07 (dt, 1H, J¼7.77,
o-Allyloxy-/crotyloxyacetophenones (6a,b, 500 mg) were
dissolved in dry acetonitrile (250 mL) and taken in an
immersion well type Pyrex-glass, water cooled photo-
.70 Hz, arom.-H), 6.87–6.72 (m, 2H, arom.-Hs),
.74–5.65 (m, 2H, CHv and vCHCH ), 4.39 (d, 2H,
3
J¼4.64 Hz, OCH ), 4.20 (br s, OH), 3.05 (q, 1H,
reactor. Solutions were purged with dry oxygen free N for
2
2
J¼7.12 Hz, N–CH–CH ), 2.56–2.29 (m, 4H,
at least 15 min prior to irradiation. The irradiations were
carried out with a 125/400-Watt medium pressure Hg arc
placed coaxially inside the reactor and N was continuously
3
2
£NCH CH ), 1.70 (d, 3H, J¼5.25 Hz, vCHCH ), 1.47
3 2 3
2
3
3
(
(
s, 3H, –CH ), 0.95 (t, 6H, J¼7.22 Hz, 2£NCH CH ), 0.89
2
d, 3H, J¼6.97 Hz, –CHCH ); d (CDCl , 50 MHz):
bubbled during irradiation. At the end of reaction, solvent
was removed from the photolysate under reduced pressure
using a 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
3
C
3
1
1
7
55.99 (quat.), 136.52 (quat.), 129.85 (CH), 128.05 (CH),
27.79 (CH), 126.53 (CH), 120.77 (CH), 112.65 (CH),
5.02 (quat.), 69.17 (OCH ), 63.50 (N–C H–CH ), 44.98
2
3
(
NC H CH ), 24.37 (vCHC H ), 17.97 (–CH ), 13.48
2 3 3 3
þ
), 291 (M , 16), 274 (21), 200 (53), 199 (21), 191 (21), 190
(
NCH C H ), 10.28 (N–CH–C H ); m/z (%): 292 (M þ1,
2
3
þ
3
7
(
(
55), 175 (35), 173 (27), 161 (20), 160 (20), 121 (37), 101
20), 100 (100); (Found: C, 74.03, H, 10.31, N, 4.66.
plates (run in CHCl –benzene 1:1 and compounds extracted
3
with chloroform).
C H NO requires C, 74.18, H, 10.03, N, 4.81%).
1
8
29
2
Triethylamine addition product 12b, as a pale viscous oil
Irradiation of a solution of 6a (500 mg) in dry acetonitrile
(250 mL) for 50 h, with a 125-Watt medium pressure Hg arc
afforded: unreacted 6a (200 mg). 7a (85 mg). Mixture of 7a
and 8a as colorless viscous oil (1:1, 30 mg), nmax (CHCl3):
3488 (b), 1622 (m), 1601 (s), 1476 (s), 1468 (w-m), 1371
(m), 1282 (m), 1240 (m), 1219 (m), 1094 (s)) cm2 ; dH
(
(
10 mg), lmax (MeOH): 215.4, 272, 279, 306 nm; nmax
CHCl ): 3420 (b), 3010 (m), 2980 (w), 2800 (m), 1597 (m),
3
1
(
580 (w-m), 1485 (m), 1445 (s), 1380 (m), 1244 (m), 1074
m), 1060 (m)) cm2 ; d (CDCl , 200 MHz): 7.55 (dd, 1H,
1
H 3
1
J¼7.73, 1.39 Hz, arom.-H), 7.07 (bt, 1H, J¼7.70, 1.52 Hz,
arom.-H), 6.83 (t, 1H, J¼7.46 Hz, arom.-H), 6.71 (d, 1H,
J¼8.04 Hz, arom.-H), 5.75–5.66 (m, 2H, CHv and
vCHCH ), 4.37 (d, 2H, J¼4.76 Hz, OCH ), 3.17 (q, 1H,
(CDCl , 200 MHz): 7.33–7.20 (m, arom.-Hs), 6.97–6.82
3
0
(m, arom.-Hs), 6.18–6.00 (m, C1 –H in 7a), 5.92–5.75 (m,
0
7a), 5.44–5.29 (m, C2 –H in 7a and C2 –Hs in 8a), 4.86
0
1H, C1 –H in 8a), 5.61 (dd, 1H, J¼15.9, 1.47 Hz, C2 –H in
3
2
0
0
J¼7.0 Hz, N–CH–CH ), 2.54–2.09 (m, 4H, J¼7.34 Hz,
3

7604
R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
(
in 7a), 1.65 (s, C3–Me in 7a), 1.45 (s, C3–Me in 8a); d
bd, J¼6.99 Hz, C2–H in 8a), 4.63 (bd, J¼6.53 Hz, C2–H
photocycloadduct (15a, 35 mg), photocycloadduct (16a,
60 mg).
C
(
CDCl , 50 MHz): 161.41 (C7a), 133.72 (C3a in 8a), 132.91
3
(
1
1
C3a in 7a), 131.52 (CH), 131.48 (CH), 130.28 (CH),
3.4.1. Irradiation of o-crotyloxyacetophenone 6b in dry
acetonitrile. Irradiation of a solution of 6b (500 mg) in dry
acetonitrile (250 mL) for 50 h, with a 125-Watt medium
pressure Hg arc afforded: unreacted 6b (205 mg). 7b
30.24 (CH), 123.41 (CH), 123.31 (CH), 121.11 (CH),
21.09 (CH), 118.67 (H Cv), 118.43 (H Cv), 110.64
2
2
(
CH), 110.60 (CH), 92.81 (C2 in 8a), 92.03 (C2 in 7a),
7
2
7.80 (C3 in 7a), 77.30 (C3 in 8a), 24.62 (C3–Me in 7a),
þ
(85 mg). Mixture of 7b and 8b (,1:1, 25 mg), n
max
3.21 (C3–Me in 8a); m/z (%): 176 (53, M ), 161 (41), 160
(CHCl ): 3480 (s), 1610 (m), 1596 (s), 1473 (m), 1450
(m-s), 1410 (m), 1381 (m), 1340 (m), 1315 (m), 1290 (w),
3
(20), 133 (32), 119 (65), 116 (100), 77 (21), 56 (45); (Found:
C, 74.71, H, 7.05. C H O requires C, 74.98, H, 6.86%).
1278 (w-m), 1257 (m), 1240 (m), 1207 (m-s)) cm2 ; dH
(CDCl , 200 MHz): 7.27–7.12 (m, Ar-Hs), 6.90–6.74 (m,
1
1
1 12 2
Photocycloadduct (14a) as pale viscous oil (35 mg); nmax
CHCl ): 3011 (s), 2928 (s), 2338 (s), 1700 (s), 1603 (s),
3
0
(
Ar-Hs), 6.06–5.88 (m, 1H, C1 –H in 7b and 8b), 5.83–5.65
3
0
1
1
2
560 (m), 1540 (m), 1510 (w), 1410 (w), 1367 (s), 1238 (m),
2
(m, C2 –H in 7b and 8b), 4.77 (bd, J¼7.60 Hz, C2–H in
1
226 (m), 1215 (m), 1192 (m), 908 (s)) cm ; d (CDCl ,
3
8b), 4.49 (bd, J¼7.39 Hz, C2–H in 7b), 1.79 (bd, 3H,
H
0
0
00 MHz): 5.88 (bs, 1H, C8–H), 4.52 (d, 1H, J¼13.3 Hz,
J¼6.30 Hz, C2 –Me in 7b), 1.72 (d, 3H, J¼6.19 Hz, C2 –
Me in 8b), 1.53 (s, 3H, C3–Me in 7b), 1.38 (s, 3H, C3–Me
in 8b); d (CDCl , 50 MHz): 159.13 (C7a), 133.35 (C3a in
C3–H), 4.11 (dd, 1H, J¼13.3, 5.43 Hz, C3–H), 2.98–2.93
m, 1H, C7–H), 2.86–2.76 (m, 2H, C6–H and C10–H),
.70–2.56 (m, 1H, C5–H), 2.45–2.31 (m, 2H, C4–H and
C5–H), 2.24 (s, 3H, –COCH ); d (CDCl , 50 MHz): 200.5
(
2
C
3
7b), 132.61 (C3a in 8b), 132.49 (CH), 130.61 (CH), 130.24
(CH), 125.77 (CH), 124.57 (CH), 123.39 (CH), 121.04
(CH), 120.89 (CH), 112.18 (CH), 110.70 (CH), 93.42 (C2 in
8b), 92.30 (C2 in 7b), 77.92 (C3 in 7b), 77.25 (C3 in 8b),
3
c
3
(
3
(
1
CvO), 157.21, 147.50, 78.80 (C1), 69.41 (C3), 53.01 (C7),
6.30 (C10), 32.02 (C5), 30.99 (C4), 29.33 (C6), 28.88
Me); m/z (%): 177 (M þ1, 10), 176 (M , 25), 159 (24),
þ
58 (70), 147 (46), 121 (38), 105 (42), 91 (62), 84 (41), 83
þ
0
24.06 (C3–Me in 7b), 23.34 (C3–Me in 8b), 18.23 (C2 –
0
þ
Me in 7b), 17.8 2 (C2 –Me in 8b); m/z (%): 190 (M , 17),
173 (63), 172 (45), 147 (27), 145 (20), 137 (67), 131 (43),
121 (29), 119 (32), 107 (21), 91 (58), 77 (41), 56 (100);
(Found: C, 75.59, H, 7.57. C H O requires C, 75.76, H,
(
(
35), 81 (35), 79 (45), 78 (47), 71 (32), 69 (49), 67 (35), 65
38), 58 (56), 56 (100); (Found: C, 74.75, H, 7.12. C H O
1 12 2
1
requires C, 74.98, H, 6.86%). Photocycloadduct (15a) as
pale viscous oil (20 mg); nmax (CHCl ): 3023 (s), 2929 (s),
1
2 14 2
7.42%). Photocycloadduct (14b) as pale viscous oil
(40 mg): nmax (CHCl ): 3004 (s), 2919 (s), 2369 (s), 1699
3
1
1
2
718 (s), 1608 (s), 1520 (m), 1465 (m), 1362 (s), 1219 (m),
226 (m), 1218 (w-m), 1192 (m)) cm2 ; dH (CDCl3,
00 MHz): 6.06 (d, 1H, J¼2.84 Hz, C8–H), 5.88 (d, 1H,
3
1
(s), 1607 (m), 1461 (m), 1405 (m), 1367 (s), 1295 (m), 1252
(m), 1218 (m), 1166 (m), 1055 (m)) cm2 ; dH (CDCl3,
200 MHz): 5.74 (bs, 1H, C8–H), 4.35 (d, 1H, J¼13.35 Hz,
C3–H), 4.01 (dd, 1H, J¼13.35, 5.43 Hz, C3–H), 2.84 (m,
1H, C7–H), 2.76–2.63 (m, 2H, C6–H and C10–H), 2.56–
2.45 (m, 1H, C5–H), 2.21 (m, 1H, C4–H), 2.15 (s, 3H,
–COCH ), 1.07 (d, 3H, J¼7.2 Hz, C5–CH3); d (CDCl ,
1
J¼2.84 Hz, C9–H), 4.29 (dd, 1H, J¼8.76, 4.73 Hz, C3–H),
4
C7–H), 3.46–3.39 (m, 2H, C4–H and C10–H), 2.08 (dd,
.07 (d, 1H, J¼8.76 Hz, C3–H), 3.88 (d, 1H, J¼3.06 Hz,
1
1
1
H, J¼13.52, 7.17 Hz, C5–H), 1.52–1.49 (m, 1H, C5–H),
.31 (s, 3H, CH ); d (CDCl , 50 MHz): 206.6 (CvO),
C
3
3
3
C
3
31.69, 130.24, 82.32 (C1), 71.24 (C3), 56.42 (C6), 54.24
50 MHz): 199.61 (CvO), 157.34, 146.50, 78.40 (C1), 69.06
(C3), 49.51 (C7), 38.09 (C10), 36.32 (C5), 32.99 (C4),
29.74 (C6), 29.44 (COC H ), 21.17 (CH ); m/z (%): 191
(
C7), 48.72 (C10), 40.80 (C5), 30.72 (C4), 26.04. m/z (%):
þ
þ
177 (M þ1, 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); (Found: C, 74.69, H, 6.58. C H O
1 12 2
3
3
þ
þ
(M þ1, 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); (Found: C, 75.54, H, 7.60. C H O requires C,
1
requires C, 74.98, H, 6.86%). Photocycloadduct (16a) as
pale viscous oil (30 mg); nmax (CDCl ): 3345 (m), 2918 (s),
1
2 14 2
75.76, H, 7.42%).
3
2
854 (s), 1715 (CvO), 1611 (s), 1512 (m), 1462 (m), 1214
2
1
(
m), 1201 (m)) cm ; d (CDCl , 200 MHz): 8.02 (s, 1H,
H
A solution of 6b (500 mg) in dry acetonitrile (250 mL) for
30 h with a 400-Watt medium pressure Hg arc afforded:
unreacted 6b (50 mg), 7b (115 mg), mixture of 7b and 8b
(35 mg), photocycloadduct (14b, 80 mg).
3
–
OH), 6.00 (d, 1H, J¼2.70 Hz, C3–H), 5.79 (d, 1H,
J¼2.70 Hz, C4–H), 4.22–4.18 (m, 2H, –CH –OH), 3.53
2
(
dd, 1H, J¼7.4, 2.15 Hz, C2–H), 3.31 (dd, 1H, J¼7.4,
3
1
6
5
6
4
1
7
.21 Hz, C5–H), 2.90–2.79 (m, 1H, C7–H), 2.58–2.49 (m,
H, C6–H), 2.19 (s, 3H, –COCH ), 1.75 (dd, 1H, J¼12.93,
3.5. General procedure for irradiation of o-allyloxy-/
crotyloxyacetophenones (6a,b) in acetonitrile in the
presence of TEA
3
.11 Hz, C8–H), 1.35 (m, 1H, C8–H); dC (CDCl ,
3
0 MHz): 204 (CvO), 139.49 (CH), 134.86 (CH),
5.21 (CH –O–), 55.85 (C1), 54.65 (C2), 49.89 (C5),
2
7.00 (C6), 37.13 (C8), 29.78 (C7), 28.86 (CH ); m/z (%):
3
Irradiations of o-allyloxy-/crotyloxyacetophenone (6a,b,
500 mg) were carried out in presence of varying amounts
of triethylamine (0.1–2.5 mol equiv.) in an immersion well
type Pyrex-glass, water cooled photoreactor, with a 125-
Watt medium pressure Hg arc. Solutions were purged with
þ
1 (55), 70 (49), 69 (35), 58 (100), 57 (34), 56 (79);
þ
78 (M , 4), 177 (M 21, 9), 112 (20), 84 (17), 83 (31),
(
Found: C, 74.05, H, 8.11. C H O requires C, 74.13, H,
11 14 2
7
.92%).
dry oxygen free N for at least 15 min prior to irradiation
2
Irradiation of a solution of 6a (500 mg) in dry acetonitrile
250 mL) for 30 h, with a 400-Watt medium pressure Hg
arc afforded: unreacted 6a (50 mg), 7a (125 mg), mixture of
a and 8a (35 mg), photocycloadduct (14a, 75 mg),
and N2 was continuously bubbled during irradiation.
Progress of the reaction was monitored by Tlc. At the end
of the reaction, solvent was removed from the photolysate
under reduced pressure using a rotary evaporator and
(
7

R. Singh, M. P. S. Ishar / Tetrahedron 58 (2002) 7595–7606
7605
products were separated by column chromatography over
silica (60–120 mesh, column packed in hexane) using
hexane–chloroform (gradient) followed by chloroform, and
chloroform–acetone (gradient) as eluents.
19 h afforded: pinacol (9b, 135 mg), pinacol (10b, 170 mg),
triethylamine addition product (11b, 70 mg), triethylamine
addition product (12b, 35 mg), triethylamine dimer (13,
50 mg).
3
.5.1. Irradiation of o-allyloxyacetophenone in aceto-
Irradiation of a solution of 6b (500 mg) and triethylamine
(915.3 mL, 2.5 mol equiv.) in dry acetonitrile (250 mL) for
15 h afforded: pinacol (9b, 100 mg), pinacol (10b, 135 mg),
triethylamine addition product (11b, 90 mg), triethylamine
addition product (12b, 35 mg), triethylamine dimer (13,
100 mg).
nitrile in the presence of TEA. Irradiation of a solution of
o-allyloxyacetophenone 6a in dry acetonitrile (250 mL) in
presence of triethylamine (39.5 mL, 0.1 mol equiv.) for 24 h
afforded: unreacted 6a (100 mg), 7a (100 mg), mixture of
7
1
a and 8a (35 mg), pinacol (9a, 75 mg), pinacol (10a,
10 mg), triethylamine addition product (11a, 50 mg).
Irradiation of a solution of 6a (500 mg) in dry acetonitrile
250 mL) and triethylamine (158.0 mL, 0.4 mol equiv.) for
(
References
23 h afforded: unreacted 6a (50 mg), 7a (90 mg), mixture of
7a and 8a (30 mg), pinacol (9a, 110 mg), pinacol (10a,
150 mg), triethylamine addition product (11a, 70 mg).
1
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(
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Irradiation of a solution of 6a (500 mg) in dry acetonitrile
250 mL) in presence of triethylamine (197.6 mL,
(
0.5 mol equiv.) for 22 h afforded: 7a (70 mg, mixture of
7a and 8a (20 mg), pinacol (9a, 125 mg), pinacol (10a,
160 mg), triethylamine addition product (11a, 75 mg).
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87–495. (f) Oppolozer, W. Acc. Chem. Res. 1982, 15,
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250 mL) in presence of triethylamine (395.2 mL,
.0 mol equiv.) for 19 h afforded: pinacol (9a, 140 mg),
pinacol (10a, 180 mg), triethylamine addition product (11a,
00 mg), triethylamine dimer (13, 50 mg).
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(
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T.; Hiyama, T. J. Chem. Soc., Chem. Commun. 1999,
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Irradiation of a solution of 6a (500 mg) in dry acetonitrile
250 mL) in presence of triethylamine (987.9 mL,
.5 mol equiv.) for 15 h afforded: pinacol (9a, 100 mg),
pinacol (10a, 130 mg), triethylamine addition product (11a,
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Nahm, K. J. Am. Chem. Soc. 1987, 109, 6528–6530.
(
2
(
l) Wagner, P. J.; Sakamoto, M. J. Am. Chem. Soc. 1989,
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1
1
25 mg), triethylamine dimer (13, 100 mg).
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2
3
.5.2. Irradiation of o-crotyloxyacetophenone in dry
acetonitrile in the presence of TEA. Irradiation of a
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(
(
250 mL) for 24 h afforded: unreacted 6b (100 mg), 7b
100 mg), mixture of 7b and 8b (30 mg), pinacol (9b,
3
4
. Dorigo, A. E.; Houk, K. N. J. Am. Chem. Soc. 1987, 109,
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8
0 mg), pinacol (10b, 115 mg), triethylamine addition
product (11b, 40 mg), triethylamine addition product
12b, 15 mg).
2
(
(
Irradiation of a solution of 6b (500 mg) and triethylamine
146.4 mL, 0.4 mol equiv.) in dry acetonitrile (250 mL) for
(
(
1
23 h afforded: unreacted 6b (60 mg), 7b (90 mg), mixture of
7b and 8b (25 mg), pinacol (9b, 110 mg), pinacol (10b,
145 mg), triethylamine addition product (11b, 55 mg),
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Irradiation of a solution of 6b (500 mg) and triethylamine
(
183.0 mL, 0.5 mol equiv.) in dry acetonitrile (250 mL) for
2 h afforded: 7b (65 mg), mixture of 7b and 8b (20 mg),
2
pinacol (9b, 120 mg), pinacol (10b, 155 mg), triethylamine
addition product (11b, 65 mg), triethylamine addition
product (12b, 30 mg).
(
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(
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5
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995, 60, 1303–1308.
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