Formation of 2,2-dimethylchroman-4-ones during the photoinduced rearrangement of some aryl 3-methyl-2-butenoate esters. A mechanistic insight

Article abstract of DOI:10.1016/j.tet.2016.02.039

Several aryl 3-methyl-2-butenoate esters upon irradiation lead to the formation of [1,3]-migrated photoproducts, phenol and, surprisingly 2,2-dimethylchroman-4-one derivatives. The starting photochemical reaction takes place from the singlet excited state of the ester and as a total mechanism two consecutive reaction pathways are proposed. The former involves the photo-Fries rearrangement of the esters in all the solvents studied and, depending on the proticity of the solvent, the latter involves an ESIPT process followed by thermal 6π-electrocyclic reaction and/or thermal (intramolecular oxa-Michael addition) cyclization of the ortho regioisomers photoformed. This second pathway is responsible of the formation of the 2,2-dimethylchroman-4-one derivatives.

Full text of DOI:10.1016/j.tet.2016.02.039

Accepted Manuscript
Formation of 2,2-dimethylchroman-4-ones during the photoinduced rearrangement of
some aryl 3-methyl-2-butenoate esters. A mechanistic insight
Daniela Iguchi, Rosa Erra-Balsells, Sergio M. Bonesi
PII:
S0040-4020(16)30101-6
10.1016/j.tet.2016.02.039
TET 27509
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 December 2015
Revised Date: 16 February 2016
Accepted Date: 18 February 2016
Please cite this article as: Iguchi D, Erra-Balsells R, Bonesi SM, Formation of 2,2-dimethylchroman-4-
ones during the photoinduced rearrangement of some aryl 3-methyl-2-butenoate esters. A mechanistic
insight, Tetrahedron (2016), doi: 10.1016/j.tet.2016.02.039.
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1
Graphical Abstract.
Photo-Fries rearrangement
Main photoproduct
OH
O
OH
OH
O
h
Solvent
X
O
+
+
25ºC, Ar
X
X
X
O
ESIPT
Intramolecular
oxa-Michael addition
MeOH
6-Electrocyclization
Cyclohexane or
MeCN
H
O
O
O
O
OH
O
X
X
normal form
trans - keto form
A series of (hetero)aryl 3-methy-2-butenoate esters upon irradiation undergo a photo-Fries
rearrangement leading to the expected ortho- and para-regioisomers and phenols.
Consecutively, the ortho-regioisomers under photochemical conditions followed by a
thermal 6-electrocyclization and/or thermal (intramolecular oxa-Michael addition)
conditions cyclizise to give the corresponding 2,2-dimethylchroman-4-one derivatives
depending on the proticity of the solvent. The photo-Fries reaction occurs from the singlet
excited state and provides the preparation in situ of o-hydroxyphenone synthons that are
key intermediates in the synthesis of chroman-4-ones.

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Formation of 2,2-dimethylchroman-4-ones during the photoinduced rearrangement of
some aryl 3-methyl-2-butenoate esters. A mechanistic insight.
Daniela Iguchi, Rosa Erra-Balsells and Sergio M. Bonesi*
CIHIDECAR—CONICET, Departamento de Química Orgánica, FCEyN, University of
Buenos Aires, Pabellón 2, 3er Piso, Ciudad Universitaria, 1428, Buenos Aires, Argentina.
Fax: +54-11-4576-3346; Tel: +54-11-4576-3346; E-mail: smbonesi@qo.fcen.uba.ar
Abstract. Several aryl 3-methyl-2-butenoate esters upon irradiation lead to the formation of
[1,3]-migrated photoproducts, phenol and, surprisingly 2,2-dimethylchroman-4-one
derivatives. The starting photochemical reaction takes place from the singlet excited state
of the ester and as a total mechanism two consecutive reaction pathways are proposed. The
former involves the photo – Fries rearrangement of the esters in all the solvents studied and,
depending on the proticity of the solvent, the latter involves an ESIPT process followed by
thermal 6-electrocyclic reaction and/or thermal (intramolecular oxa-Michael addition)
cyclization of the ortho – regioisomers photoformed. This second pathway is responsible of
the formation of the 2,2-dimethylchroman-4-one derivatives.
Keywords: 2,2-dimethylchroman-4-ones; Photo-Fries rearrangement; aryl 3-methyl-2-
butenoate esters; thermal 6-electrocyclic reaction; oxa-Michael cyclization

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1. Introduction.
The photo Fries-rearrangement has received considerable attention since its discovery in
1960.1 Upon irradiation aryl esters, amides, thioesters and sulfonates undergo the photo
Fries-rearrangement and these compounds have been used mainly to elucidate the
underlying reaction mechanism.2 However, this photochemical reaction has been
successfully used in the synthesis of natural products such as griseofulvin,3 daunomycine4
and flavonoids.5 The photo-Fries rearrangement is a mild and clean reaction where no
hazardous solvents and Lewis acid catalysts are needed to accomplish the photoreaction.
The photoproducts are formed in good chemical yield with a predictable regioselectivity.
The radical mechanism of the photo-Fries rearrangement is well established and it is known
that this rearrangement occurs mainly through the excited singlet state.6
Irradiation of aryl esters provides o-acylphenols which are useful organic
compounds and versatile intermediates in the synthesis of biologically active
naphthoquinones, pesticides, photographic agents and UV absorbents.7 Indeed, substituted
o-hydroxyphenones are also valuable intermediates in the preparation of substituted 2,2-
dimethylchroman-4-one moieties that show biological activity.8-12 The synthesis, chemistry,
occurrence in nature and biological activity of chromanones has been the subject of several
comprehensive reviews.13 On the other hand, ortho-hydroxyphenones undergo
intramolecular proton transfer in the excited state, which provides an efficient energy
wasting channel, responsible for their ability to act as UV-stabilizers.14a However, ortho-
hydroxyaryl alkenyl ketones, e.g. 2’-hydroxychalcones, show a different behavior and a 6-
exo-trig intramolecular cyclization of the cis-isomer takes place efficiently to give 4-
flavanones. In fact, a well-studied example was the photocyclization of 2’-
hydroxychalcones to 4-flavanones, which is enzymatically catalyzed in plants and appear to

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be involved in the biosynthesis of flavonoids.14b-d Matsushima et al. have studied the
photocyclization of substituted 2’-hydroxychalcones to 4-flavanones and concluded that the
cyclization proceeded via a thermal 6-electrocyclic process.15a,b In this regard, Miranda et
al. have also studied the cyclization of different cis- and trans-o-hydroxyaryl alkenyl
ketones from a mechanistic point of view under basic conditions, e.g. sodium acetate in
DMSO (other bases likewise used are sodium hydroxide, tetraethyl amonium hydroxide
and potassium carbonate) and, under preparative conditions, a biphasic system was used,
viz. hexane and aqueous sodium hydroxide (10%). In both cases, 2-methyl chroman-4-one
derivatives were obtained quantitatively.15c
Previous studies on the product distribution of the photo-Fries rearrangement
reaction of (hetero)aryl esters in isotropic media afforded ortho and para rearranged
photoproducts as well as the corresponding phenol.16 However, irradiation of aryl 3-
methyl-2-butenoate esters in different organic solvents (cyclohexane, methanol and
acetonitrile) undergoes the photo-Fries rearrangement yielding the expected photoproducts
such as the rearranged photoproducts, phenol, 3-methyl-2-butenoic acid and, surprisingly,
the substituted 2,2-dimethylchroman-4-ones as well (Scheme 1). These interesting results
encouraged us to study the mechanism of formation of the 2,2-dimethylchroman-4-one
derivatives since these heterocyclic compounds are not an expectable primary photoproduct
from a photo-Fries rearrangement reaction.

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5
OH
OH
+
R
R
O
h
O
O
O
O
O
Solvent
OH
Ar; 25 °C
R
+
R: electron withdrawing and
electron donor groups
R
R
Substituted 2,2-dimethylchroman-4-ones
Scheme 1. Product distribution of the irradiation of substituted aryl 3-methyl-2-butenoate
esters.
In view of this background and in our interest on photo-Fries reaction,17 we have
selected representative (hetero)aryl 3-methyl-2-butenoate esters 1 – 9 (Scheme 2) which
have been synthetized previously in our laboratory.18,19 Then, a systematic study of their
photochemical behavior has been performed in solution under UV irradiation. Noteworthy,
Miranda et al. studied the photochemistry of ester 6 in hexane and in hexane containing
anhydrous potassium carbonate.17b,20
O
O
O
O
O
O
O
O
R
7
6
O
O
1; R = H
2; R = CH3
3; R = OPh
4; R = Cl
O
O
N
5; R = NO2
8
9
Scheme 2. Aryl 3-methyl-2-butenoate esters studied.
Herein, the photo-Fries rearrangement of the esters will be conducted without any
acid- or base-mediated catalysis with special attention to the preparative and mechanistic

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aspects. Also, we attempt to demonstrate that the formation of 2,2-dimethylchroman-4-one
derivatives occurs through a thermal 6-electrocyclization reaction when 2’-hydroxyphenyl
2-methylpropenyl ketone is irradiated in polar and non-polar aprotic solvents. However,
when polar protic solvents are used, i.e. methanol, a competitive photochemical and
thermal cyclization processes occur providing the 2,2-dimethylchroman-4-one derivative in
fairly good yields.
2. Results and discussion.
2.1. Spectrosocpic properties of esters 1 – 9.
The absorption spectra of compounds 1 – 9 were recorded in methanol at 298 K and the

max of absorption of the band assigned to the lowest-lying excited electronic state (1Lb(S1 –
S0) electronic transition) are shown in Table 1. In general, on changing the solvent polarity
from cyclohexane to acetonitrile, no shift of the max was observed.
Excitation at 265 nm of phenyl ester (1) induced a very weak fluorescence emission
at 281 nm. Excitation at 255 – 285 nm for compounds 2 – 9 induced a weak fluorescence
emission located in the 270 – 346 nm regions. The high-energy edge of the broad emission
spectrum overlaps with the 0 – 0 band. The nearly negligible Stoke’s shift reflects a high
structural similarity between the ground and excited states. Table 1 includes the max of
fluorescence obtained for compounds 1 – 9 and Fig. 1 shows the UV-visible absorption
spectrum and the fluorescence emission spectrum of compound 7 recorded in cyclohexane
at 298 K. Also, the fluorescence quantum yields have been measured at 298 K using
naphthalene as a fluorescence reference (f = 0.0421) and the values are collected in Table

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1. Compound 5 did not show fluorescence emission due to the presence of a nitro group
attached to the phenyl moiety which promotes the intersystem crossing process populating
efficiently the triplet excited state.22, 23
Table 1. Spectroscopic data for esters 1 – 9 in methanol solution.
Absorption Fluorescence
Esters

max/nm
265
254
255
254
260
287
285
285
280

max/nm fa
281
281
280
281
NDb
343
346
346
342
0.01
0.02
0.02
0.02
NDb
0.02
0.03
0.07
0.01
1
2
3
4
5
6
7
8
9
a
Fluorescence reference: naphthalene (exc: 285 nm; max(fluo): 318 nm; f: 0.04)21; Error
of ± 10%. b ND: not detected.
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
250
300
350
400
450
 / nm
Figure 1. UV-visible absorption spectrum (black line) and fluorescence emission spectrum
(blue line) of ester 7 (1.04 x 10-5 mol.dm-3) recorded in cyclohexane at 298 K.

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2.2. Preparative photochemistry of aryl 3-methyl-2-butenoate esters 1 – 9 in solution.
We have irradiated systematically solutions of esters 1 – 9 (see Scheme 2) in preparative
scale in order to isolate and characterize the photoproducts. The experiments were
performed in different organic media (cyclohexane, acetonitrile and methanol) with
excitation wavelengths of 254 and 313 nm under inert atmosphere (Ar) and at room
temperature. After irradiation in preparative scale, the photolyzed solutions were
evaporated under vacuum and the photoproducts were separated by column
chromatography over silica gel from the solid residue and were identified and characterized
1
by means of physical and spectroscopical methods (m.p., H-NMR and 13C-NMR). The
photoreactions were followed spectrophotometrically and by GLC and HPLC as cross-
checking. Similar photoproducts and yields were obtained by both chromatographic
methods. This means that the phenols and the 2,2-dimethylchroman-4-ones, compounds
that can sublimate, were not lost during chromatographic analysis.
Phenyl 3-methyl-2-butenoate ester (1) was irradiated in three different solvents at
254 nm affording the migrated products 1-(2-hydroxyphenyl)-3-methyl-2-buten-1-one (1b)
and 1-(4-hydroxyphenyl)-3-methyl-2-buten-1-one (1d), together with 2,2-dimethyl
chroman-4-one (1a), phenol (1c) and 3-methyl-2-butenoic acid (see Scheme 3 and Table 2).
No secondary photoproducts were detected in the photolyzed solutions under our
experimental conditions, viz. aryl aryl radical coupling, aryl alkyl radical coupling and
extrusion of carbon monoxide. The molar ratio of 1a and 1b is ca. 1:1 in apolar
(cyclohexane) and polar aprotic (MeCN) solvents while in MeOH compound 1b is formed
in amounts higher than compound 1a. It is interesting to point out that thermal Fries
rearrangement studies on compound 1 were carried out to produce the ortho regioisomer
and the expected 2,2-dimethylchroman-4-one 1a and they were obtained in low chemical

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yield from complicated reaction mixtures.24 However, the photochemical reaction allows
the formation of 1b as well as 1a under mild conditions and at room temperature. This
photoreaction is advantageous over the thermal Fries rearrangement where higher
temperature (80 – 120°C) and Lewis acid catalysts (AlCl3 or FeCl3) are needed to
accomplish the reaction.25
Table 2. Yields of photoproducts obtained and quantum yields () of irradiation of esters 1
– 5 in different organic media at exc = 254 nma.
Yields of photoproducts (%)b
c
Compound Solvent
1a
14
17
14
2a
23
20
5
1b
11
15
25
2b
23
27
40
3b
21
27
36
4b
25
21
28
5b
5
1c
21
22
19
2c
12
15
31
3c
23
25
21
4c
22
26
25
5c
23
38
54
1d
22
20
20
Cyclohexane
0.13
0.09
0.08
1
MeCN
MeOH
2
Cyclohexane
MeCN
0.10
0.11
0.07
MeOH
3
4
5
3a
20
23
11
4a
27
25
16
5a
---
---
---
Cyclohexane
MeCN
0.12
0.11
0.06
MeOH
Cyclohexane
MeCN
0.14
0.17
0.07
MeOH
Cyclohexane
MeCN
0.04
0.02
0.01
---
---
MeOH

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10
a
c
Concentration: 1.06x10-3 mol.dm-3; T: 298 K; Atmosphere: Ar.b Calculated by GC.
Quantum yields of disappearance of esters 1 – 5 measured at low conversions (ca. 10%)
under argon atmosphere; actinometer: potassium ferrioxalate; exc = 254 nm; Error: ± 0.02.
Examination of the photochemical reaction of several 4-substituted esters 2 – 5
bearing electron – withdrawing and electron – donating groups was conducted. Upon
irradiation with wavelength of 254 nm these esters gave the ortho regioisomers (2b – 4b),
phenols (2c – 4c), and the 2,2-dimethylchroman-4-one derivatives (2a – 4a), with the
exception of compound 5 (see Table 2). Similar solvent effect on the chemical yield of
esters 2 – 4 was observed which is comparable to the yields obtained for ester 1 in the same
solvents.
A different photochemical behavior was observed for ester 5. No photoproducts 5a
and 5b were formed during the irradiation of ester 5 with the exception in cyclohexane
where a very low amount (ca. 5%) of 5b was observed. Instead, the phenol 5c was obtained
independently of the medium. These results suggested that a different reactive excited state
is involved in the photoreaction of ester 5. We attributed this behavior to the nitro group
attached to the phenyl moiety that populates efficiently the triplet excited state (T1) of ester
5 rendering 5c as the sole photoproduct. It is known that the photochemical reactivity of
nitro arenes takes place from T1 since they show isc in the order or higher than 0.50.22, 23

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OH
OH
O
O
OH
O
h
O
+
+
+
O
Solvent
Ar, 25 ºC
R
R
R
R
1c
O
1a
1b
1, R = H
2a
3a
4a
5a
2b
3b
4b
5b
2c
3c
4c
5c
2, R = CH3
3, R = OPh
4, R = Cl
5, R = NO2
1d
OH
OH
O
O
OH
O
O
O
O
h
O
O
O
O
O
+
+
Solvent
Ar, 25 ºC
O
O
6a
6c
6b
6
O
OH
O
OH
O
O
h
+
+
+
O
+
Solvent
Ar, 25 ºC
O
7
7a
7c
7b
7d
O
O
O
O
OH
OH
h
O
+
Solvent
Ar, 25 ºC
8
9
8a
8c
8b
O
OH
OH
O
O
O
N
N
h
N
N
+
+
O
Solvent
Ar, 25 ºC
9a
9c
9b
Scheme 3. The photo-Fries rearrangement of esters 1 – 9. Product distribution.
The photochemical behavior of esters 6 – 9, which are examples of polycyclic and
heterocyclic systems, was also studied (Scheme 3). Irradiation at two different excitation
wavelength (254 and 313 nm) of esters 6 – 9 in cyclohexane and under Argon atmosphere,
gave the ortho regioisomers (6b – 9b), the corresponding phenols (6c – 9c) and the 2,2-
dimethylchroman-4-one derivatives (6a – 9a) (Scheme 3 and Table 3). The ortho
regioisomers (6b – 9b) are always the main photoproducts. On the other hand, Miranda et

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12
al.21 studied the photochemistry of ester 6 and after 20 hours of irradiation at exc = 254 nm
in hexane no photochemical reaction took place. Otherwise, when the photoreaction of 6
was carried out in the same experimental conditions but in the presence of anhydrous
K2CO3 compound 6b was formed in only 22 % yield. However, in our experimental
conditions, ester 6 gives the expected photo – Fries products as well as the 2,2-
dimethylchroman-4-one 6a at irradiation time lower than 120 minutes without any basic
catalysis (see Schme 3).
Table 3. Yields of photoproducts obtained and quantum yields () of irradiation of esters 6
– 9 in cyclohexane at different excitation wavelength.a
Yield of Photoproducts (%)b
Compounds
6a 6b
24 30
25 27
7a 7b
15 48
17 47
8a 8b
17 41
19 39
9a 9b
18 40
19 41
6c
20
21
7c
10
11
8c
19
21
9c
19
20

254
313
exc (nm)
c
0.10
0.11
6
7d
9
10
254
313
0.12
0.12
7
8
9
254
313
0.14
0.13
254
313
0.14
0.14
a
b
c
Concentration: 1.06x10-3 mol.dm-3; T: 298 K; Atmosphere: Ar. Calculated by GC.
Quantum yields of disappearance of esters 6 – 9 measured at low conversions (ca. 10%)
under argon atmosphere; actinometer: potassium ferrioxalate; exc = 254 nm; Error: ± 0.02.
Noteworthy, the data collected in Table 3 also show the same photoproduct
distribution and no significant differences in the chemical yield are observed as a function

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13
of excitation wavelength. This behavior implies that the lowest excited state of the esters is
the photo reactive state responsible of the formation of the photoproducts.
The quantum yields () of disappearance of esters 1 – 9 were determined in
different organic media using potassium ferrioxalate as actinometer and are collected in
Tables 2 and 3.26–28 The conversion of the esters was monitored by GC and HPLC analysis
and was not higher than 10%. The  values for esters 1 – 5 in all the solvents studied are
almost the same within the experimental error (± 0.02) which accounts for a similar degree
of stabilization of the reaction intermediates in polar and non-polar solvents. In the case of
esters 6 – 9 the  values do not depend on the excitation wavelength which accounts for the
same photo reactive excited state involved in the photoreaction.
2.3. Formation of chroman-4-ones during the photochemical reaction.
During the photochemical rearrangement of esters 1 – 9, it was observed that the 2,2-
dimethylchroman-4-one derivatives were formed. Thus, we believe that the whole
photochemical reaction takes place really in two consecutive steps: i) the photochemical
Fries rearrangement of the esters that provides the rearranged ortho- and para-regioisomers
and the phenols (path a; Scheme 4); ii) the intramolecular photoinduced cyclization of the
ortho regioisomer to the corresponding 2,2-dimethylchroman-4-ones through a thermal 6-
electrocyclization reaction (path b; Scheme 4) in apolar and polar aprotic solvents.
However, when the solvent is a polar protic one, e.g. methanol, an intramolecular oxa –
Michael addition reaction takes place (path c; Scheme 4) even though a low photochemical
contribution to the cyclization process can not be rule out.

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14
H
H
Me
Me
O
H
O
O
O
O
O
H
O
O
X
X
X
Path (c)
(Thermal)
Intramolecular oxa - Michael reaction
MeOH
O
X
OH
H
OH
O
O
O
h
+
+
X
Path (a)
Photo-Fries rearrangement
X
X
O
Path (b)
h
Cyclohexane
or MeCN
ESIPT process
H
*
O
O
O
O

H
O
O
X
X
X
cis-keto form
trans-keto form
6-Electrocylization reaction
Scheme 4. Proposed reaction mechanism for the photochemical reaction of esters 1 – 9.
The success of the electrocyclization reaction can be ascribed to the formation of
trans-keto tautomer in the ground state which is the precursor for the production of the
cyclization product. This tautomer was formed by the intramolecular proton transfer
process (ESIPT)29 of the ortho regioisomer to produce cis-keto form in the excited state
followed by deactivation with twisting of the newly produced C=C double bond of the
tautomer before undergoing intersystem crossing to triplet excited state. The rapid
radiationless deactivation of the cis-keto form in the singlet excited state gives the trans
tautomer in its ground state which has the required geometry to proceed effectively via a
thermal 6-electrocyclic process recovering the aromaticity and furnishing the 2,2-

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15
dimethylchroman-4-one derivatives. This behavior is usually observed with -hydroxy
carbonyl compounds such as 2’-hydroxychalcones where, in * excited states, phenolic
groups usually become more acidic and carbonyl groups more basic, enhancing the proton
transfer and, therefore, favoring the one way cis – trans isomerization of 2’-
hydroxychalcones.30 Curiously, this behavior depends on the nature of the solvent used.
Table 4 depicts the results obtained under photoinduced and thermal conditions for
compound 2b. When the solvents are polar aprotic (MeCN) or non polar (cyclohexane), the
6-electrocyclization reaction takes place efficiently due to a favored intramolecular
hydrogen bonding providing 2,2-dimethylchroman-4-one 2a in good yields. Noteworthy,
irradiation of 2b at different wavelength (254 nm and 366 nm) provides compound 2a with
similar chemical and quantum yields. This behavior accounts for the same photo reactive
excited state involved in the cyclization photoreaction.
Table 4. Yields of 6-methylchroman-4-one (2a) in different experimental conditions.
O
OH
h or 60ºC
Solvent, Ar
H3C
H3C
O
O
2b
2a

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16
Solvent
MeOH
T / ºC
25
Yields of 2a (%)a

exc / nm
254
366
---
c
0.02
0.01
---
23
16
32
79
48
0
25
60b
25
Cyclohexane
MeCN
254
366
---
0.05
0.04
---
25
60b
25
254
366
---
74
44
0
0.05
0.06
---
25
60b
b
aIsolated yields; t = 120 min. Under thermal conditions (bath temperature: 60 ºC); t =
c
300 min; Atmosphere: Ar. Quantum yields of disappearance of 2b measured at low
conversions (ca. 10%); actinometer: potassium ferrioxalate.
Figure 2 shows that irradiation of compound 2b undergoes a cyclization reaction to
compound 2a as suggests from the spectral profile of the UV-visible absorption spectra run
in cyclohexane during 120 min.
Abs.
0.3
O
O
O
0.2
0.1
0.0
H3C
H3C
OH
2a
2b
0 min
120 min
250
300
350
400
450
 / nm
Figure 2. Change of the UV-visible absorption spectrum of compound 2b (1.10x10-5
mol.dm-3) upon irradiation at 254 nm in cyclohexane. Total irradiation time 120 min.
On the other hand, in polar protic solvents (methanol), the 6-electrocyclization also

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17
undergoes to produce the desired 2,2-dimethylchroman-4-one 2a but in lower chemical and
quantum yields (Scheme 4). In this case, methanol breaks the intramolecular hydrogen
bond by forming an intermolecular hydrogen bonds with compound 2b (see Path (c),
Scheme 4), as it was observed for different 2’-hydroxychalcones.30c Additionally, we have
measured the UV-visible spectra of compound 2b in different solvents and a hypsochromic
shift of the lower energy band was observed (compare these data: max = 357 nm in
cyclohexane, max = 356 nm in acetonitrile and max = 349 nm in methanol). These facts
indicate that the hypsochromic shift of the absorption band of 2b in methanol should be
related to an intermolecular hydrogen bonding interactions.31 However, the thermal
cyclization reaction of 2b to 2a undergoes in fairly good yield when the reaction is
conducted in methanol and at 60 ºC (Table 4). Likewise, the cyclization process of 2b also
takes place at room temperature (25 ºC) but requiring long period of reaction time.
Noteworthy, the thermal cyclization reaction of 2b does not take place in acetonitrile and in
cyclohexane at 60 ºC (Table 4). These results demonstrate that under polar aprotic and non-
polar solvents the intramolecular hydrogen bond is so strong that no thermal reaction
occurs while upon irradiation 2,2-dimethylchroman-4-one 2a is produced efficiently.
On the other hand, both photochemical and thermal cyclization reactions of 2b in
methanol occur, being more efficient at 60 ºC. Therefore, we suggest that there must be a
compromise of the intramolecular and intermolecular hydrogen bonding in methanol
solution. Thus, those molecules of 2b bearing intramolecular hydrogen bonds may undergo
photoinduced hydrogen atom transfer in the excited state, as depicted in Path (b) (Scheme
4), giving the cyclization product 2a. Instead, those molecules showing intermolecular
hydrogen bonds with methanol may undergo through an intramolecular cyclization oxa –

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Michael addition reaction in the ground state of compound 2b (thermal condition; Path (c)
in Scheme 4), since 2,2-dimethylchroman-4-one 2a is formed in fairly good yields when
the reaction is carried out at 60 ºC (Table 4). The success of the oxa-Michael addition
reaction in the ground state can be ascribed to the polarization of the double bond, due to
conjugation with the ketone group that determines the occurrence of the nucleophilic attack
of the hydroxyl group exclusively at C(3) of the 3-methyl acrylic moiety (reactivity of -
ethylenic carbonyl compounds).14 Therefore, our results show that the nucleophilicity of the
hydroxy group in synthon 2b is enough for promoting the nucleophilic attack exclusively at
C(3) to achieve the intramolecular cyclization process under thermal condition.
In this regard, we selected and irradiated esters 3 and 4 in cyclohexane, acetonitrile
and methanol with the aim to show that the cyclization reaction of the ortho-regioisomers
to the corresponding 2,2-dimethylchroman-4-ones takes place during the irradiation of the
esters. Since the ortho-regioisomers 3b and 4b show a characteristic absorption band
around 350 nm, the irradiation of the esters were followed by UV-visible absorption
spectroscopy. Fig. 3 shows nicely the rapid formation of the ortho-regioisomers and their
consumption to the formation of the corresponding 2,2-dimethylchroman-4-one derivatives
(3a and 4a) during the irradiation of the esters when the solvent is cyclohexane and
acetonitrile. In the same figure the formation of the 2,2-dimethylchroman-4-one derivatives
are also shown. Instead, when the solvent is methanol, the formation of the ortho-
regioisomers are also fast but their consumption are noticeable slower and consequently, a
slow formation of the 2,2-dimethylchroman-4-one derivatives are observed.

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1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
0
50
100
150
200
250
300
350
t / min
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
0
50
100
150
200
250
300
t / min
Figure 3. Relative absorbance (Arel) profiles against time of compounds (a) 3a and 3b and
(b) 4a and 4b upon irradiation of esters 3 and 4 in different solvents. Solvents: ○
cyclohexane; ∆ methanol; □ acetonitrile. Unfilled black symbols belong to the ortho-
regioisomers and dotted blue symbols belong to 2,2-methylchroman-4-one derivatives.
The profiles shown in Fig. 3 are in agreement with the proposed reaction mechanim
described in Scheme 4. The formation of the 2,2-dimethylchroman-4-ones from the ortho-
regioisomers in cyclohexane and acetonitrile takes place through an ESIPT process

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followed by a thermal 6-electrocyclization reaction (Path (b); Scheme 4) while the ortho-
regioisomers are formed by the photo-Fries rearrangement of the corresponding esters
(Path (a); Scheme 4). On the other hand, the slow consumption profile of the ortho-
regioisomers (3b and 4b) during the irradiation in methanol of esteres 3 and 4, respectively,
implies how significative is the disruption of the intramolecular hydrogen bonding in the
ortho-regioisomers disfavoring the 6-electrocyclization reaction (Path (b); Scheme 4) and
favoring the intramolecular oxa-Michael addition reaction (Path (c); Scheme 4).
3. Conclusions
Esters 1 – 4 undergo the photo-Fries rearrangement reaction upon irradiation with 254 nm
wavelength providing the ortho and para regioisomers, the corresponding phenols and the
2,2-dimethylchroman-4-one when the photoreaction is carried out in different solvents at
room temperature. Polycyclic aryl and heteroaryl esters (6 – 9) show a similar
photochemical behavior when are irradiated with 254 nm and 313 nm wavelength rendering
similar photoproduct distribution. These photoreactions take place from the lowest singlet
excited state with quantum yields ranging from 0.01 to 0.15. In the case of ester 5 the
lowest triplet excited state is efficiently populated and the photochemical behavior turned
out quite different providing the p-nitrophenol as the solely photoproduct in all the solvents
used.
Also, we were able to demonstrate that the ortho regioisomers are converted to the
corresponding 2,2-dimethylchroman-4-one derivatives under photochemical or thermal
conditions depending on the nature of the reaction medium. Thus, a 6-electrocyclization
process from the trans-keto form of the ortho regioisomer after an excited state
intramolecular proton transfer (ESIPT) process is involed in the formation of the 2,2-

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dimethylchroman-4-one derivatives when the solvents are cyclohexane and acetonitrile
(Path (b); Scheme 4). On the other hand, when the solvent is methanol, production of 2,2-
dimethylchroman-4-one derivatives can be achieved mainly through an intramolecular oxa-
Michael addition reaction, a thermal process, that takes place during the photoreaction
(Path (c); Scheme 4). However, a photoinduced trans-keto form formation followed by a
thermal 6-electrocyclization process can participate with sluggish quantum efficiency.
Further experiments related to the photochemical behavior of a series of substituted
(hetero)aryl 3-methyl-2-butenoate esters in homogeneous and heterogeneous media are in
course in our laboratory.
4. Experimental
4.1. General Information
3,3-Dimethylacryloyl chloride, phenol, 4-methyl-, 4-phenoxy-, 4-chloro-, 4-nitrophenol,
oxine and 3,4-methylenedioxyphenol were purchased from Aldrich and were used without
further purification. -Naphthol and -naphthol were purchased from Aldrich and were
purified by column chromatography. Spectrograde solvents were obtained from J. T. Baker
and were used as received. The esters 1 – 9 were synthesized in our laboratory according to
the procedure described in the literature.18,19 Melting Points were determined on a Fisher
Jones apparatus and are not corrected. 1H and 13C NMR spectra were registered on a Bruker
AC-200 spectrometer; chemical shifts () are reported in part per million (ppm), relative to
internal tetramethylsilane. Coupling constant (J) values are given in Hz. The measurements
were carried out using the standard pulse sequences. GC analysis was carried out on a
Hewlett Packard 5890 gas chromatograph using an Ultra 2 capillary chromatographic
column. For esters 1 – 6, the chromatograms were recorded with the following program:

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initial temperature: 100 ºC, 2 minutes; rate: 10 ºC.min-1; final temperature: 250 ºC, 10
minutes. For esteres 7 – 9, the chromatograms were recorded as follows: initial
temperature: 150 ºC, 2 minutes; rate: 10 ºC.min-1; final temperature: 250 ºC, 15 minutes.
Reverse phase HPLC analysis was carried out on a JASCO PU 1580 with a UV detector
PU 1575 using a RP – 18 reverse chromatographic column (Supelco; eluent: MeCN – H2O
mixtures; flow: 1 mL.min-1; detector: UV,  230 and 260 nm). The chromatrographic runs
were conducted in the presence of biphenyl as internal standard. UV absorption spectra
were obtained using a Shimadzu UV-1203 UV-visible spectrophotometer. All the
measurements were made with 1 cm stopped quartz cells at 298 K. Fluorescence
measurements were performed using a Hitachi F500 spectrofluorimeter. Corrected
fluorescence spectra, measured at room temperature, were recorded in a 1 cm path length
(in both the excitation and emission directions) quartz cell. The fluorescence quantum
yields (f) at room temperature were determined from the corrected fluorescence spectra,
integrated over the entire emission profile, using naphthalene as reference (exc: 285 nm;

max(fluo): 318 nm; f: 0.040).21 The error of the measurements was ± 10%. To avoid inner
filter effects the absorbance of the solutions, at the excitation wavelength, was kept below
0.10.
4.2. Quantum yields
Quantum yields were determined using potassium ferrioxalate as an actinometer.26
A
6.0x10-3 M solution of K3Fe(C2O4)3.3H2O was prepared for measurements at 254 and 313
nm. The aryl 3-methyl-2-butenoate esters (1.0x10-4 M) was dissolved in the selected
organic solvent and irradiated simultaneously with the actinometer using a medium
pressure Hg lamp (Hereaus TQ150-Z3) and an interference filter (Schott, band path 5 nm)

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which gives a nearly parallel beam at 313 nm for compounds 6 – 9 or using four 18 W
lamps (Philips) which gives a nearly parallel beam at 254 nm for compounds 1 - 5. The
conversions of the esters were monitored by GC and were lower than 10%.
4.3. Photoirradiations
General procedure for aryl 3-methyl-2-butenoate esters. Solutions of esters (1 – 9; 0.106
mmol) were prepared in different organic media (100 mL). Photoirradiations of the esters
were performed as follow: i) analytical scale: a 2 ml aliquot of solution was placed in a
stoppered 3 ml quartz cell and degassed with argon for 20 min; ii) preparative scale: a 65
ml aliquot was placed in a stoppered 100 ml Erlenmeyer quartz flask and degassed with
argon for 30 min. The quartz cell as well as the Erlenmeyer quartz flask were placed in a
home made optical bench provided with the possibility to use two or four lamps. The
solutions of the esters were stirred during the irradiation process. Irradiations with λexc
=
313 nm were carried out with four phosphorous – coated lamps (HelioQuartz, each of 18
Watts) that give a nearly parallel beam at 313 nm. Irradiations with λexc = 254 nm were
carried with four germicide lamps (Philips, each of 20 Watts).
The progress of the reaction was monitored by four different methods: (i) UV spectroscopy;
(ii) TLC [eluent: hexane–ethyl acetate (8 : 2 v/v); spots were visualized with UV light (254
and 366 nm) and with I2]; (iii) GC analysis (Ultra 2 capillary column); (iv) HPLC analysis
(RP-18 column).
In order to isolate, purify and characterize the photoproducts formed, preparative photolysis
(preparative scale) was conducted according to the following procedure. A solution of ester
(0.106 mol) in cyclohexane (65 mL) was placed in a stoppered Erlenmeyer quartz flask and
was irradiated with stirring under an Ar atmosphere employing the optical bench above

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described. The irradiation time varied depending on the ester studied and the progress of
the reaction was monitored by TLC and GC as well as HPLC. When the conversion of the
starting material reached the appropriate level, the photolyzed solution was evaporated
carefully to dryness under reduced pressure. The yellowish solid residue obtained was
worked up by silica gel column chromatography (eluent: hexane 100% followed by
hexane–ethyl acetate mixtures). From the eluted fractions, the photoproducts were isolated
and characterized by mean of physical and spectroscopic methods.
Photoproducts 1a – 1d18, 2a14b, 2b14b, 3a19, 4a19, 6a18, 6b20, 7a18, 7b18, 7d32, 8a18 and 9a18
as well as the esters 118, 224b, 319, 424b and 519 show similar physical and spectroscopic
properties as those reported in the literature.
1-(2-Hydroxy-5-phenoxyphenyl) isobutenyl ketone (3b): colorless oil; H(200 MHz; CDCl3;
Me4Si)  (sArd, J3,5 = 2.8 Hz, 1 H, H-3), 7.37 – 7.26 (m, 1 H, H-4’),
7.19 (dd, J3,5 = 2.6 and J5,6 = 8.8 Hz, 1 H, H-5), 7.09 (d, J5,6 = 8.8 Hz, 1 H, H-6), 7.02 –
6.92 (m, 2 H, H-2’ and H-3’), 6.68 (s, 1 H, H-C=C), 2.27 (s, 3 H, CH3), 2.02 (s, 3 H, CH3).
C(50 MHz, CDCl3, Me4Si) 195.5 (C=O), 159.8 (C – 1’), 159.0 (C – 1), 158.5 (C – 4),
147.4 (C=C-H), 129.8 (C – 3’), 128.6 (C – 5), 122.5 (C- 4’), 120.5 (C – 2), 120.7 (C – 3),
119.7 (C – 2’), 119.6 (C – 6), 117.9 (C=C-H), 28.3 (CH3), 21.5 (CH3). Anal. Found: C,
76.03; H, 5.97. C17H16O3 requires C, 76.10; H, 6.01%.
1-(2-Hydroxy-5-chlorophenyl) isobutenyl ketone (4b): colorless oil33; H(200 MHz; CDCl3;
Me4Si) 12.70 (1 H, Ar-OH), 7.35 (1 H, dd, J5,6 9.0 and J3,5 2.2, 5-H), 7.32 (1 H, d, J3,5 2.2,
3-H), 7.05 (1 H, d, J5,6 9.0, 6-H), 5.91 (1 H, s, C=C-H), 2.24 (3 H, s, Me), 2.00 (3 H, s, Me).
C(50 MHz, CDCl3, Me4Si) 195.0 (C=O), 160.6 (1-C), 149.2 (9-C), 135.5 (4-C), 129.3 (3-
C), 123.2 (5-C), 120.1 (2-C), 119.4 (8-C), 114.9 (6-C), 27.7 (Me), 20.5 (Me).

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1-(2-hydroxy-5-nitrophenyl) isobutenyl ketone (5b) yellowish oil; H(200 MHz; CDCl3;
Me4Si) 13.10 (1 H, Ar-OH), 8.27 (1 H, d, J5,6 8.3, 6-H), 8.10 (1 H, d, J3,5 2.1, 3-H), 6.91 (1
H, dd, J3,5 8.3 and J5,6 2.1, 5-H), 5.97 (1 H, s, C=C-H), 2.17 (3 H, s, Me), 1.97 (3 H, s, Me).
C(50 MHz, CDCl3, Me4Si) 195.0 (C=O), 166.1 (1-C), 149.6 (9-C), 140.1 (4-C), 132.4 (3-
C), 124.8 (5-C), 118.0 (2-C), 117.3 (8-C), 117.4 (6-C), 27.9 (Me), 21.4 (Me). Anal. Found:
C, 59.81; H, 5.05; N, 6.28. C11H11NO4 requires C, 59.73; H, 5.01; N, 6.33%.
1-(2-Hydroxynaphthyl) isobutenyl ketone (8b): mp 110 ºC; H (200 MHz; CDCl3; Me4Si)
15.20 (1 H, bs, OH), 8.43 (1 H, m, 8-H), 7.80–7.12 (5 H, m, Ar-H), 6.80 (1 H, m, C=C-H),
2.20 (3 H, s, Me), 2.00 (s, 3 H, Me). C(50 MHz, CDCl3, Me4Si) 182 (C=O), 166.9 (2-C),
149.7 (3’-C), 139.3 (4-C), 134.3 (8a-C), 131.9 (5-C), 129.7 (7-C), 127.8 (4a-C), 125.4 (8-
C), 125.1 (6-C), 121.4 (3-C), 117.2 (2’-C), 113.5 (1-C), 27.9 (Me), 23.1 (Me). Anal. Found:
C, 79.61; H, 6.21. C15H14O2 requires C, 79.62; H, 6.24%.
1-(8-Hydroxyquinolyl) isobutenyl ketone (9b): mp 143 ºC. H(200 MHz; CDCl3; Me4Si)
11.8 (1 H, bs, OH), 8.71 (1 H, dd, J2,3 5 and J2,4 2, 2-H), 7.97 (1 H, d, J6,5 8.5, 6-H), 7.89 (1
H, dd, J4,3 5 and J4,2 2, 4-H), 7.54 (1 H, d, J5,6 8.5, 5-H), 7.32 (1 H, q, J3,2 5, 3-H), 6.37 (1
H, s, C=C-H), 2.17 (3 H, s, Me), 1.97 (3 H, s, Me). C(50 MHz; CDCl3; Me4Si) 178.1
(C=O), 162.6 (8-C), 150.9 (1-C), 150.0 (3’-C), 141.8 (4a-C), 139.4 (3-C), 138.3 (8a-C),
126.9 (6-C), 124.6 (5-C), 122.1 (2-C), 121.2 (7-C), 117.3 (2’-C), 28.0 (Me), 23.1 (Me).
Anal. Found: C, 73.97; H, 5.79; N, 6.14. C14H13NO2 requires C, 73.99; H, 5.77; N, 6.16.
General procedure for the photochemical cyclization of 1-(2-hydroxy-5-methylphenyl)
isobutenyl ketone (2b). Solutions of ketone 2b (0.10 mmol) were prepared in different
organic media (100 mL). Photoirradiations were performed in analytical scale: a 2 ml
aliquot of solution was placed in a stoppered 3 ml quartz cell and degassed with argon for

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20 min. The quartz cell was placed in a home made optical bench provided with four
germicide lamps (Philips, each of 20 Watts) that give a nearly parallel beam at 254 nm and
the solutions were stirred during the irradiation process. The progress of the reaction was
monitored by UV spectroscopy, TLC [eluent: hexane–ethyl acetate (8 : 2 v/v); spots were
visualized with UV light (254 and 366 nm) and with I2] and GC analysis (Ultra 2 capillary
column).
General procedure for the thermal cyclization of 1-(2-hydroxy-5-methylphenyl) isobutenyl
ketone (2b). Solutions of ketone 2b (0.10 mmol) were prepared in different organic media
(100 mL) and a 2 mL aliquot of solution was placed in a stoppered 3 mL quartz cell and
degassed with argon for 20 min. The quartz cell was placed in a thermal bath at 60ºC. The
progress of the reaction was monitored by UV spectroscopy, TLC [eluent: hexane–ethyl
acetate (8 : 2 v/v); spots were visualized with UV light (254 and 366 nm) and with I2] and
GC analysis (Ultra 2 capillary column).
Acknowledgements
The authors thank Universidad de Buenos Aires (X 0055BA), CONICET (PIP0072CO and
PIP0155) and ANPCyT (PICT 2012-0888) for financial support. R.E.B. and S.M.B. are
research members of CONICET. D. Iguchi was recipendant of a scholarship from
CONICET (Doctorate Program).
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