Chapter 5…

Investigations on Intramolecular Arene-olefine Photcycloadditions

CH3

O

O R

X

HH

H3C

O

H

O

H H

HH

H

H

O

H3C

H

R

OH

H

H

HHH

HH3C

O

HH

HHOH2C

HH

XO

HR

H

H3CO

H

O

HO

H

R

CH3

H

H

X X

O

H3C

H

RH

X

O

CH3

O

R

H

H

X1

H

H HH

H O

O

H3C

CH3

H

HHH

Cl

H

H

H

or

+ +

+ +

+

a X = H, R = H

b X = H, R = CH3

c X = Cl, R = H

d X = Cl, R = CH3

e X = Br, R = H

159

Chapter-5

Investigations on Intramolecular Arene-olefine Photocycloadditions

in Substituted-o-Alkenylmethoxy-Acetophenones

Introduction

New discoveries and inventions in organic chemistry and related fields are driven by the

ability to make molecules in a simple, safe, and practical fashion. Despite the relatively

long history of organic chemistry and noteworthy achievements, complex molecule

synthesis remains a formidable challenge.1,2

The efforts to address this challenge

inexorably rely on the development of complexity increasing strategy level reactions and

reaction cascades.1-5

Among the different organic transformations, including phototransformation,

intramolecular photocycloadditions of arene with alkene substituents provide easy access to

strained polycyclic molecules.6 Arene-alkene photo-cycloadditions are among the most

powerful and versatile strategy level reactions for complex molecule synthesisand these

remarkable reactions have attracted considerable mechanistic, and theoretical interest,

leading in recent years to spectacular applications in synthesis.6

The possible mode of addition, i.e., (2+2), (2+4) or meta-addition, is reported to

depend, inter alia, on the electron-donor/acceptor properties of the arenes and alkenes.7

Among these arene-olefine intramolecular photocycloadditions, the meta-cycloadditions are

more frequently reported in the literature7i

and the ortho-additions are also reported to

compete with meta cycloadditions.8

However, para- i.e., (4+2) photocycloaddition, is

reported to be the least efficient of the three modes of additions because of the poor orbital

overlap between an excited arene and a ground state alkene,9

but is a common pathway in

the photochemistry of extended aromatic systems.10

In this chapter emphasis is on the

development of the arene-alkene cycloaddition for the rational synthesis of complex

molecules of biological importance. Some of the examples of the arene-olefin cycloadditiosn

are:

Synthesis of the silphinene (4) could be easily achieved through arene-olefine meta-

photocycloaddition in 1 (Scheme 1).11

160

h

Li, MeNH2

+

( )-Silphinene

12 3

4

Scheme 1

Wander et. al. have reported the asymmetric synthesis of (-)-retigeranic acid (8, Scheme

2)12

through the meta-photocycloaddition of 5.

h H HH2NOC

HHOOC

H h

COh

(-)-Retigeranic Acid

5 6 7 8

Scheme 2

Wender et. al have further reported13

the application of the chiral ketals to regulate the

absolute stereochemistry of the cycloaddition process. Total synthesis of (±)-subergorgic

acid (11, Scheme 3) has been achieved through the arene alkene cycloaddition of a

benzylic ketal (9) and additionally features a free radical addition to a vinylcyclopropane

(10).

h

O

O

OO

H

HO2C

H

O

(±) -Subergorgic acid9 10 11

Scheme 3

Grayanotoxin II (14)14

has been synthesized through the irradiation of the 12, which lead to

the formation pentacyclic 13 with seven stereogenic centers through the meta-

161

cycloadditions; 13 through the series of steps furnishes Grayanotoxin II (14 Scheme 4).

This is an efficient approach in the synthesis of the seven membered rings through a

modification of cycloadduct cleavage procedure.

OAcOMe

TBOS

OAcOMe

TBOS HO

HO

OH

OH

OH

H

(±)-Grayanotoxin II

h

12 13 14

Scheme 4

Though (3 + 2) photocycloaddition has been studied the most,15

however, recent work has

shown that this cycloaddition competes with the [2 + 2] photocycloaddition;16

often the

products of the latter reaction are less stable. The silylated phenol derivative 15 reacts via

both a [3 + 2] and a [2 + 2] photocycloaddition to yield intermediates I and II, respectively

(Scheme 5).17

Intermediate I lead to the tetracyclic compound 18 by a radical cyclization

resulting in cyclopropanation. Since the reaction occurs in the singlet state, this

intermediate also possesses some zwitterionic character. Intermediate II resulting from [2

+ 2] addition generates III via a pericyclic reaction. Contraction of the cyclooctatriene

moiety leads to the cyclobutene intermediates 16 and 17. The final products of this reaction

are obtained by hydrolysis (20) or hydrolysis followed by dimerization (19).

OSi O Si O

Si

O Si

OSi

H

OSi

HH

OSi OH

H

SiO2

OH

h

+

15

16 17

18

19

I II

20

OSi

III

Scheme 5

Addition of electron-withdrawing substituents favors on aromatic ring [2 + 2]

photocycloaddition compared to the [3 + 2].18,19

For instance, the presence of a nitrile

group in the resorcinol derivative (21) favored a [2 + 2] cycloaddition in the -position

162

of this substituent (Scheme 6).20

A cyclohexane-1,3-dione derivative (23) was obtained by

spontaneous rearrangement of the intermediate 22; such products are likely to be used as

intermediates in the synthesis of a new generation of herbicides.

O

OH

NC

O

H

NC

OH

O

H

NC

O

h

21 22 23

Scheme 6

Primary adducts of [2 + 2] photocycloadditions with benzene derivatives are frequently

unstable. However, when the reaction is performed in an acidic medium, these reaction

adducts can be transformed into stable final products via acid-catalyzed reactions. The

resorcinol derivatives 24 yielded adducts IV and V (Scheme 7).21

These intermediates

rearranged to benzocyclobutenes 25 and 26 or, depending on the substitution, monocyclic

compounds such as 29. Benzocyclobutenes are interesting substrates in organic synthesis.

For instance, 25 has been transformed into the nitrogen-containing tricyclic compound 27,

which possesses an affinity for dopamine receptors.

O

OX

h O

HOX

+

OH

OX

HO

OX

HO

OX

NR

MeO

X = MeX= H, Me

OHOMeOMe OH

OMe

X= H, Me

+

+MeOH-H

24 25 26

272829

IV V

Scheme 7

Wagner et al.,22

have investigated the photochemistry of o-alkenoxy-acetophenones (30) and

have reported that intramolecular (2+2) photocycloaddition in these molecules occurs solely

at the bond between the two substituents (Scheme 8).

163

O

O

R1

R2

CH3OHO

R1

R2

O

O

O

R1

R2

O

O

R1

R2

hv hv

30

Scheme 8

On the other hand, in case of p-alkenoxyacetophenones (31), two possible modes of

cycloadditions (one towards and other away from the substituent) are possible and in general

the addition is highly regioselective i.e., syn to the substituent in all the cases with an

exception of meta-methoxy and -thiomethoxy substituent

(Scheme 9).22b

These

photocycloadditions are postulated to be intervened by polar intermediates/charge

separation.23

Y

X

O

O

Y

X

O

O

Y

X

O

O

O

O XY

O

O

Y

X

O O

O O

Y

X

Y

X

hvhv

hv

hvhv

31hv

Scheme 9

Chiral induction has also been studied for the [3 + 2] photocycloaddition. In compound 32,

the two reaction partners (phenol and vinyl ether) are linked together by a homochiral

tether (Scheme 10).24

Addition occurs stereospecifically, and intermediate VI is formed.

As the reaction takes place at the singlet state, the intermediate VI may be viewed as either

a biradical or a zwitterion. After regioselective formation of a cyclopropane ring, product

33 was isolated and then transformed into the enantiopure alcohol (-)-34.

164

O OO

O

OO

OH

O

O

h

32 VI 33 (-) 34

Scheme 10

In the above reported reactions, it was observed that minimum of the three carbon/hetero-

atom tether is necessary conditions between the aromatic ring and the olefinic moiety is a

must for any intramolecular photocycloaddition.25

However, Ishar et. al. have been recently

investigated the photochemistry o-allyloxy/-crotyloxyacetophenones26

(35a,b) under varied

conditions and it has been observed that the three carbon/hetero- atom tether between the

aromatic ring and the olefinic moiety is not a necessary condition for intramolecular

photocycloaddition (Scheme-11); various products have been characterized by NMR

spectroscopic analysis and it has been reported that many products have remained

uncharacterized.26

O

CH3

O R

O

HO CH3

HH

R

H

O R

O R

OHCH3OHCH3

H3CHC

H3CHC

N(CH2CH3)2

N(CH2CH3)2

HOCH3

N(CH2CH3)2

O R

O

HO CH3

HH

R

H

O R

O

O

CH3

HH

O

H3CO

OR

H3C O

R

OH

R

H

CH3

35

36

36 +

37( mixture of diastereomers)

+

39

+

38

(36

43

(A)

CH3

hv

CH3CN

hv

hv

PhH+TEAhv

CH3CN+TEA

Dry PhHDry

4041

42

+

++

(36+ 37 + 38 + 39)+

N2

N2

N2

,

,

,

,

N2

( mixture of diastereomers)

& 43)

(a) R=H

(b) CH3

Scheme 11

165

Thereby, in order to delineate the mechanistic transformation of the various products. It

was decided to synthesize various substituted o-allyloxy/-crotyloxyacetophenone and

characterize their product profile before and after irradiation.

Results and Discussion

Synthesis of the substituted -allyloxy-/crotoloxy acetophenone

Substituted o-allyloxy-/crotyloxy-acetophenones (45a-e) were prepared by reacting o-

hydroxyacetophenone (44a-e) with allyl bromide and crotyl bromide by stirring under

reflux in dry alcohol free acetone, in the presence of anhydrous K2CO3. After the

completion of reactions (Tlc), the K2CO3 was filtered off, washed with acetone and the

solvent from the filtrates was distilled off. Pure products (46a-e, Scheme 12) were isolated

by vacuum distillation of the residual viscous oils and characterized spectroscopically

(vide experimental).26

CH3

O

O R

O

CH3

OH

RBrX X

45a-e44 a X = H, R = H

b X = H, R = CH3

c X = Cl, R = H

d X = Cl, R = CH3

e X = Br, R = H

R = H, CH3

acetone, reflux

Scheme 12

Irradiation of substituted o-allyloxy-/crotyloxy-acetophenones

Initially, o-allyloxy-/crotyloxy-acetophenones (45ab) were irradiated in dry acetonitrile

with a 400 Watt medium pressure Hg arc employing an immersion well type (Pyrex-glass),

water cooled photoreactor, under nitrogen atmosphere for 30-50 h (90% conversion). The

obtained complex photolysates were resolved by column and preparative layer

chromatography to obtain compounds (Scheme 13, 46-51). An additional dehydrated

product 48a was isolated first time in the case of o-allyloxyacetophenones (45a). All the

purified compounds (46-51, Table 1) were characterized by various spectroscopic

techniques (IR, 1H and

13C NMR, mass) and by elemental analysis, and assigned by

comparison with the earlier reported data.26

After purification attempts to crystallize any of

the compounds 49-51 did not succeeded.

166

CH3

O

O R

HH

H3C

O

H

O

H H

HH

H

H

O

H3C

H

R

OH

H

H

H

HH

HH3C

O

HH

HHOH2C

HH

XO

HR

H

H3CO

H

O

HO

H

R

CH3

H

H

XX X

hv

dry CH3CN

46 (20-25%)

+

47 (5-8%)

+

51 (9-17%)49 (5-12%) 50 (6-9%)

1

23

4

5

67

81

2

3

4

5

6

78

9

101

23

4

5

6

7

8

9

10

45 a X = H, R = H

b X = H, R = CH3

O

H3C

H

R

H

X

+

48 (10-15%)

+

Scheme 13

Table 1: Reaction conditions and percentage yields of products obtained

from irradiation of o-allyloxy-/crotyloxyacetophenone in acetonitrile.

Subsequently, photochemistry of the o-allyloxy-/crotyloxy-acetophenones (45c-e) was

examined. Substituted o-allyloxy-/crotyloxy-acetophenones (45cd) were irradiated under

similar conditions for 20-40 h (95% conversion), leading to complex photolysates, which

were resolved by column chromatography to furnish compounds 52cd and 53cd along

with some uncharacterized components Scheme 14). All the purified compounds 52 and

53 were characterized by rigorous spectroscopic (IR, 1H and

13C NMR,

1H decoupling, 2D

1H -

13C H.et.-COSY NMR, mass) and by elemental analysis.

CH3

O

O R

X1hv

dry CH3CN

45 c X1= Cl, R = H

d X1= Cl, R = CH3

O

H

R

CH3

H

X1

52cd (40-45 %)

+

53cd (20-30%)

O

O

H3C

R

H

HHH

Cl

H

H

H1

23

4

5 6

7

8

99'

Scheme 14

% Yield of products Entry R X1 X2 Irradiation

Time (h) Conversion

% 46 47 48 49 50 51

1 H H H 30 95 24 5 10 8 8 9

2 CH3 H H 30 95 23 8 13 12 - -

167

Table 2: Reaction conditions and percentage yields of products obtained from irradiation of

o-allyloxy-/crotyloxyacetophenone in acetonitrile.

% Yield of products Entry R X1 X2 Irradiation

Time (h) Conversion

% 52 53

1 H Cl H 28 95 50 20

2 CH3 Cl H 38 95 55 30

However, it is pertinent to mention here no progress of reaction was observed in the case

of 4-bromo-o-allyloxy-/crotyloxy-acetophenones 45e even after prolonged irradiation 100h

(Tlc and NMR) under similar conditions (Scheme 15).

CH3

O

O R

X2

X1 h , 100 h

dry CH3CN

45e X1= Br, X2= H, R = H

No Reaction

Scheme 15

Characteristic features of 1H NMR spectrum of 52a were double doublets at 6.58 (J =

11.1 & 17.4 Hz) assigned to C1 H, 5.81 (J = 1.1 & 17.4 Hz) attributed to C2 H, besides

other resonances of proton NMR (Figure 1). 13

C NMR assignments made are also in

consonance with the assigned structures. In 52c, the resonance for C2 appeared at 112.65

(Figure 2) and resonances of oxygen linked carbons as observed ( 78 -93). In the case of

46 and 47 were absent in the spectrum of 52c. The mass spectra of 52c (ESI) revealed m/z

(M + K) +

peak at 232.

Figure 1: 1H NMR spectrum of 52c

O

H

H

CH3

H

Cl

168

Figure 2: 13C NMR spectrum of 52c

Another product was isolated from the photolysis of compound 45d, whose mass spectrum

[ESI m/z 247 (M +Na)+] indicted that there was no loss or gain of mass during

phototransformation. Though derived from 45d its 1H NMR did not reveal any resonance

in the aromatic region, which was indicative of the fact that product is an intramolecular

photocycloadduct. Based on the spectral data, in particular, the presence of only one 1H

proton resonance in the olefinic region of its proton NMR spectrum, following five

structures 53-57 were considered.

O

O

MeCH3H

HHH

H ClH

O

CH3

O

R

H

H

Cl

H

H HH

57

HO

O

H3C

CH3

H

HHH

Cl

H

H

H

54

ClH

H3C

O

H

O

H H

HH

H

H

55

ClHH

HH3C

O

HH

HHOH2C

H

53

56

Structures 55 and 56 were ruled out by comparison of spectroscopic data with the reported

for related structures26

and also because 13

C NMR revealed four carbons in the olefinic

O

H

H

CH3

H

Cl

169

region at 174.9(q), 132.0(q), 120.4(CH) and 101.9(q). Consequently, meta-addition

product 57 was also ruled out. Characteristic feature of the IR spectrum of the compound

were a conjugated carbonyl band at 1672 cm-1

. In the 13

C NMR the presence of an upfield

shifted quaternary olefinic carbon resonance ( 101.9) and a downfield shifted quaternary

olefinic carbon resonance ( 174.9) collates with push-pull substituted bond as present in

structures 53 and 54. 13

C NMR of the compound (Figure 4) showed carbonyl carbon at

204.2, an oxygen linked carbons at 71.2 (C2), besides other carbon resonances in the

aliphatic region. 1H NMR spectrum of the compound showed olefinic double doublet at

5.69 (J = 1.5 and 5.17 Hz); presence of 5.17 Hz splitting in the olefinic-H resonance does

not support structure 54 and favors structure 53. The compound 53 displayed max

(methanol) at 230 nm, which is also in keeping with the assigned structure. Other

assignments made are a double doublets at 4.05 (Jvic = 4.5 and Jgem = 9.0 Hz) attributed to

a C2H, doublet at 3.74 (Jgem = 9.0 Hz) attributed C2H, double doublet at 2.79 (J = 4.5

and 6.0 Hz) assigned to C3H (Figure 3). The assigned sterochemical dispositions are

based on the J values and 1H-connectivities worked out by

1H decoupling; the assignments

were aided by 2D 1H -

13C Het. COSY, NOESY NMR spectra.

Figure 3: 1H NMR spectrum of 53d

C7H

C2H

C3H

C9’H

C4H

-COCH3

CH3

O

O

H3C

CH3

H

HHH

Cl

H

H

H

53d

C5H

170

Figure 4: 13

C NMR spectrum of 53d

Plausible mechanisms

Mechanism for the formation of 46, 47 and 48

Mechanistically, it has been reported that the irradiation in non-polar solvent the preferred

formation of syn-adducts, , in which bulky groups are placed in trans-arrangement, through

triplet excited state, has been attributed27

to the relative sizes of the substituents and their

influence on the mode of rotation of bonds during ring closure of 1,5-biradicals. However,

hydrogen bonding of the –OH group with the solvent molecule increases its bulkiness

leading to formation of the anti-adducts (46), as minor products and syn as major product

(47). Further, dehydration of the photoproducts 46 or 47, leads to the formation of product

(48, Scheme 16).

53d

O

O

H3C

CH3

H

HHH

Cl

H

H

H174.9

120.4

101.9

71.2

47.4

17.7

132.0

171

ISC

O

H3C OH

HH

R

H

CH3CN

O

H3C OH

HH

R

H

O

H3CH

R

H

-H2O

-H2O

CH3

O

O R

CH3

O

O R

OH

CH3

RO C

HH

H

OH

H3C

R

H

O C

H

H

CH3

O

O R

H

OH

CH3

RO C

H

H

h

Route

a

Route b

47

46

45

Route

a

48

a

b

1 3

H-abs

ba Solvent

* *

Scheme 16

Proposed mechanism for the formation of 49, 50 and 51

Mechanistically, the formation of intramolecular photoaddition products is highly

unexpected. As mentioned earlier, a minimum of a three carbon/hetero-atom tether has

been reported to be essential for any intramolecular arene-alkene photocycloadditions. 28

Accordingly, it has been reported that no intramolecular photocycloaddition product could

be detected from benzyl vinyl ether and only -phenylpropanaldehyde was isolated.26

Similarly, allyl phenyl ether is reported to afford only Claisen rearrangement products on

irradiation.29

The formation of various intramolecular photoaddition products, therefore, is

a consequence of very special circumstances where the aryl ring is bearing an electron

withdrawing substituent and the olefinic moiety can be considered as a electron rich. The

reaction is initiated by n- * excitation (Pyrex-filter) and involves intramolecular electron

transfer as supported by the observed solvent dependence of formation of these products

(49-51). The sequence of events possibly leading to 49, 50 and 51 outlined in Scheme 17.

172

HH

H3C

O

H

O

H H

HH

H

H

CH3O

O

R

HHH

HH3C

O

HH

HHOH2C

H

RCH3

O

O

CH3

O

O R

CH3O

O

O

CH3

O

CH3

O

O R

H

O

HR

H

H3CO

H

*3

5149

50

1

2

3

4

5

67

812

3

4

5

6

78

9

10

1

23

4

5

6

7

8

ET

9

10

Scheme 17

It may be mentioned here that intramolecular (2+2) photoadditions followed by further

rearrangement of the photoadducts are precedented in the case of certain p-subsituted

acetophenones,22b,28b,30

and 2-subsituted-1-acetylnaphthalenes,22a

and among these the

former are postulated to involve donor-acceptor interactions. In general, the donor-acceptor

interactions are known to be involved in ortho-photoadditions, which are particularly

favored if the addends differ substantially in their ionization potentials,23a,31a,b

and it has

recently been reported that the regiochemistry of addition is mediated by orientation of

dipoles.31c

However, all the reported examples conform to the criterion of a minimum

three- carbon/heteroatom separation between arene and olefin. Therefore, the present

investigations on the phototransformations of o-alkenylmethoxyacetophenones, is the case

wherein the above-mentioned limiting condition for arene-olefin intramolecular photo-

cycloaddition has been surpassed.

Conclusions

The present investigations have led to the conclusion that irradiations of the substituted o-

allyloxy-/crotyloxy-acetophenones (46a-e) furnish different photo products (47-56). The

increased formation of benzofuran derivatives (52cd) in case of chloro-substituted ketone

(46cd) resulting from H-abstraction followed by cyclization is apparently a consequence of

higher triplet yield due to heavy atom effects, however, failure of reaction in case of

bromo-substituted ketone probably results from early deactivation of excited state. The

findings also suggest that intramolecular addition products may not be resulting from

173

triplet manifold. Isolation of the intramolecular photcycloaducts are suggestive of the fact

that the limiting condition of a three-atom tether between an aryl ring and an alkene moiety

is not necessary for the arene-olefine cycloadditions in solvents favoring intramolecular

charge/electron transfer.

Experimental

General Information: Starting materials, reagents and solvents were purchased from

commercial suppliers and purified/distilled/crystallized before use. Bruker AC-200 FT

(200 MHz) and JEOL AL-300FT (300 MHz) and Bruker AC-500 FT (500 MHz) and

spectrometers were used to record 1H NMR and

13C NMR (50, 75, 150 MHz) spectra.

Chemical shifts ( ) are reported as downfield displacements from TMS used as internal

standard and coupling constants (J) are reported in Hz. IR spectra were recorded on

Shimadzu DR 2001 FT-IR and Shimadzu FT-IR-8400S spectrophotometer either as thin

layer with few drops of CHCl3 or as KBr pellets. Mass spectra were recorded (EI and ESI-

method) on Shimadzu GCMS-QP-2000A and Bruker Daltonics Esquire 300 mass

spectrometer. Elemental Analyses were carried out on a Perkin-Elmer 240C elemental

analyzer and are reported in percent atomic abundance. All melting points are uncorrected

and measured in open glass-capillaries on a Veego (make) MP-D digital melting point

apparatus. Chromatographic separations have been carried out by either column

chromatography over silica gel (60-120 mesh) or preparative layer chromatography over

silica gel-G coated plates (2.0 mm thick layer).

o-Allyloxy/ crotyloxy acetophenones

These were prepared by reacting substituted-o-hydroxyacetophenone (10 ml, 8.3 mmol),

respectively, with allyl bromide (10 ml, ~1.4 molar equivalent) and crotyl bromide (12 ml,

~1.4 molar equivalent) by stirring under reflux in dry alcohol free acetone (100 ml), in

presence of anhydrous K2CO3 (12.0 g). The reactions were monitored by Tlc and after

completion of reactions (12 h), the K2CO3 was filtered of, washed with acetone and the

solvent from the filtrates was distilled off. Pure products (45a-e) were isolated by vacuum

distillation of the residual viscous oils and characterized spectroscopically.

o-Allyloxyacetophenone (45a)

Yield 85%; Colorless oil; IR (CHCl3): vmax 1680, 1610, 1595, 1480, 1450, 1420, 1360,

1290, 1280, 1160, 1130 cm-1

; 1H NMR (200 MHz, CDCl3): = 7.72(dd, 1H, J = 8.4, 1.83

174

Hz, C6H), 7.42(m, 1H, ArH), 6.91(m, 2H, ArH), 6.17-5.96(m, 1H, C2 H), 5.42(dd, 1H, J =

14.0, 1.5 Hz, C3 H), 5.31(br d, 1H, J = 7.6 Hz, C3 H), 4.62(d, 2H, J = 5.1 Hz, C1 H), 2.63(s,

3H); 13

C NMR (CDCl3, 50 MHz): = 199.28(C=O), 157.67(C2), 133.27(CH), 132.39(CH),

130.04(CH), 128.36(C1), 120.3(CH), 117.81(=CH2), 112.60(HC=), 69.16(-OCH2), 31.73

(COCH3); MS (EI) : m/z (%): 176 (M+, 18); Anal. calcd. For C11H12O2 : C, 74.92; H, 6.86,

Found: C, 74.66; H, 6.54 %.

o-Crotyloxyacetophenone (45b)

Yield 90%; Colorless oil; IR (CHCl3): vmax 1680, 1600, 1500, 1460, 1380, 1370, 1300,

1180, 1160, 1080 cm-1

; 1H NMR (CDCl3, 200 MHz): 7.72(dd, 1H, J = 7.10, 1.5 Hz,

C6H), 7.38(dt, 1H, J ~ 8.24, 1.5 Hz, C4H), 6.97(m, 2H, ArH), 5.85-5.74(m, 2H, C2 H,

C3 H), 4.56(brd, 2H, J = 5.52 Hz, C1 H], 2.63(s, 3H, CH3), 1.76(d, 3H, J = 4.6 Hz, C4 H);

13C NMR (CDCl3, 50 MHz): 200.01(C=O), 158.01(C2), 133.47(CH), 130.74(CH),

130.36(CH), 129.07(C1), 125.56(CH), 120.60(CH), 112.91(CH), 69.35(-OCH2),

35.31(COCH3), 17.81(=CHCH3); MS (EI): m/z (%): 190(M+, 27); Anal. calcd. For

C12H14O2 : C, 75.76; H, 7.42, Found: C, 75.44; H, 7.23 %.

1-(2-allyloxy-5-chloro-phenyl)-ethanone (45c)

Yield 92%; White solid, mp 112-115°C; IR (KBr): vmax 1684, 1615, 1565, 1473, 1442,

1363, 1290, 1284, 1164, 1125 cm-1

; 1H NMR (CDCl3, 500 MHz,): 7.70(d, 1H, J = 2.74

Hz, ArH), 7.36 (dd, 1H, J = 2.73 and 17.1 Hz, ArH), 6.89(d, 1H, J = 8.8 Hz, ArH), 6.08-

6.03 (m, 1H, C2 H), 5.40(dd, 1H, J = 1.4 & 17.2 Hz, C3 H), 5.34(dd, 1H, J = 1.5 & 21.0 Hz,

C3 H), 4.63(d, 2H, J = 5.3 Hz, C1 H), 2.62(s, 3H); 13

C NMR (500 MHz,CDCl3)

: 198.23(C=O), 156.4(C2), 133.05(CH), 132.19(CH), 130.01(CH), 129.47(C1),

126.0(CH), 118.58(=CH2), 114.36(HC=), 77.40 (-OCH2), 31.89 (COCH3); MS (EI): m/z

(%): 233 (M + Na). Anal. calcd. For C11H11O2Cl : C, 62.72; H, 5.26. Found: C, 60.02; H,

5.18 %.

1-(2-But-2-enyloxy-5-chloro-phenyl)-ethanone (45d)

Yield 90 %; White solid, mp 120-122°C; IR (KBr): vmax 1650, 1591, 1451, 1422, 1323,

1274, 1190, 1112 cm-1

; 1H NMR (CDCl3, 500 MHz,): 7.70(d, 1H, J = 2.74 Hz, ArH),

7.36 (dd, 1H, J = 2.73 and 17.1 Hz, ArH), 6.89(d, 1H, J = 8.8 Hz, ArH), 6.08-6.03 (m, 1H,

C2 H), 5.42(dd, 1H, J = 1.4 & 17.2 Hz, C3 H), 5.31(dd, 1H, J = 1.5 & 21.0 Hz, C3 H),

175

4.54(d, 1H, J = 6.0 Hz, C1 H), 2.62(s, 3H, CH3), 1.77(s, 3H, CH3); 13

C NMR (500

MHz,CDCl3) : 198.1(C=O), 152.1(C2), 133.2(CH), 132.4(CH), 130.5(CH), 128.1(C1),

127.1(CH), 124.2(=CH2), 120.1(HC=), 70.4(-OCH2), 36.2(COCH3), 17.6(=CHCH3); MS

(ESI): m/z: 247(M + Na); Anal. calcd. For C12H13O2Cl : C, 64.15; H, 5.83. Found: C,

63.99; H, 5.41 %.

1-(2-Allyloxy-5-bromo-phenyl)-ethanone (45g)

Yield 92 %; White solid, mp 115-120°C; IR (KBr): vmax 1672, 1593, 1458, 1419, 1321,

1271, 1181, 1108 cm-1

; 1H NMR (CDCl3, 500 MHz,): 8.05(s, 1H, ArH), 7.25-7.22(d, 1H,

J = 10, ArH), 6.13-6.00(m, 1H, C2 H), 5.45 (dd, 1H, J = 1.2 & 17.2 Hz, C3 H), 5.37(dd, 1H,

J = 1.0 & 8.2 Hz, C3 H), 4.70(d, 2H, J = 5.3 Hz, C1 H), 2.61(s, 3H, CH3); 13

C NMR (500

MHz,CDCl3) : 196.4(C=O), 161.3(C2), 133.5(CH), 131.7(CH), 131.6(CH), 130.1(C1),

127.9(CH), 119.0(=CH2), 112.7(HC=), 69.7(-OCH2), 31.9(COCH3); MS (ESI): m/z: 278

(M + Na); Anal. calcd. For C11H11O2Br : C, 51.79; H, 4.35. Found: C, 51.40; H, 4.22 %.

General procedure for Irradiation of substituted o-allyloxy-/crotyloxyacetophenone

(45a-e) in dry acetonitrile:

Substituted-o-Allyloxy-/crotyloxyacetophenones (45a-e, 500 mg) were dissolved in dry

acetonitrile (250 ml) and taken in an immersion well type Pyrex-glass, water cooled

photoreactor. Solutions were purged with dry oxygen free N2 for at least 15 minutes prior

to irradiation. The irradiations were carried out with a 400 Watt medium pressure Hg arc

placed coaxially inside the reactor and N2 was continuously bubbled during irradiation. At

the end of reaction, solvent was removed from the photolysate under reduced pressure

using Eyela-rotary evaporator and products were separated by column chromatography

over silica gel (60-120 mesh) using hexane-chloroform (gradient) as eluent. Some of the

column fractions were pooled and further resolved by preparative layer chromatography

over silica gel-G coated plates (run in CHCl3-Benzene 1:1 and compounds extracted with

chloroform).

(syn)-2-Ethenyl-3-hydroxy-3-methyl-2,3-dihydrobenzofuran (46a)

Yield 20 %; Colorless viscous oil; IR (CHCl3): vmax 3487, 1621, 1601, 1475, 1462, 1371,

1282, 1241, 1213, 1092 cm-1

; 1H NMR (CDCl3, 200 MHz): 7.31-7.21 (m, 2H, ArH),

6.97-6.85(m, 2H, ArH), 6.17-6.04(m, 1H, C1 H), 5.63(dd, 1H, J = 1.4 & 15.9 Hz, C2 H),

176

5.41(dd, 1H, J = 1.3 & 11.6, Hz, C2 H), 4.62(bd, 1H, J = 6.5 Hz, C2H), 1.62(s, 3H, C3-

CH3); 13

C NMR (CDCl3, 50 MHz): 161.2(C7a), 132.8(C3a), 131.4(CH), 130.4(CH),

123.2(CH), 121.5(CH), 118.2(=CH2), 110.2(HC=), 92.4(C2), 77.3(C3), 24.4(C3-CH3); MS

(EI): m/z: 176(40, M+), 116(100); Anal. Calcd. for C11H12O2: C, 74.98, H, 6.86; Found: C,

74.74, H, 6.67 %.

Mixture of 46a & 47a

Yiled 25%; colorless viscous oil; IR (CHCl3): vmax 3487, 1621, 1601, 1475, 1462, 1371,

1282, 1241, 1213, 1092 cm-1

; 1H NMR (CDCl3, 200 MHz): = 7.31-7.19(m, 4H, ArH),

6.94-6.80(m, 4H, ArH), 6.17-6.00(m, C1 H in 46a), 5.90-5.73(m, 1H, C1 H in 47a),

5.60(split d, 1H, J = 15.9, 1.4 Hz, C2 H in 46a), 5.41-5.27(m, C2 H in 46a & C2 H in 47a),

4.85(bd, J = 6.9 Hz, C2H in 47a), 4.62(bd, J = 6.5 Hz, C2H in 46a), 1.63(s, C3Me in 46a),

1.43(s, C3CH3 in 47a); 13

C NMR (CDCl3, 50 MHz): 161.2(C7a), 133.6(C3a in 47a),

132.8(C3a in 46a), 131.2(CH), 131.1(CH), 130.2(CH), 130.4(CH), 123.2(CH), 123.2(CH),

121.0(CH), 121.2(CH), 118.0(CH2=), 118.0(CH2=), 110.6(CH), 110.4(CH), 92.7(C2 in

47a), 92.0(C2 in 46a), 77.2(C3 in 46a), 77.1(C3 in 47a), 24.2(C3-CH3 in 46a), 23.0(C3-CH3

in 47a).

3-Methyl-2-vinyl-benzofuran (48a)

Yield 10 %; colorless viscous oil; IR (CHCl3): vmax 1604, 1452, 1479, 1374, 1261, 1231,

1219, 1074 cm-1

; 1H NMR (CDCl3, 300 MHz): = 7.48-7.37 (m, 2H, ArH), 7.26-7.14(m,

2H, ArH), 6.68(dd, 1H, J = 11.1 & 17.4 Hz, C1,H), 5.88(dd, 1H, J = 1.5 & 17.4 Hz, C2

,H),

5.32(dd, 1H, J = 0.9 & 11.1 Hz, C2,H), 2.23 (s, 3H, CH3);

l3C NMR (CDCl3, 75 MHz): =

154.0(C7a), 150.2(C3a), 139.2(CH), 130.3(CH), 124.7(CH), 123.2(CH), 122.2(=CH2),

119.3(HC=), 114.1(C2), 114.0(C3), 113.1(CH), 29.7(CH3); Mass (ESI) m/z : 181(M+ Na);

Anal. Calcd. for C1IH10O: C, 83.51, H, 6.37; Found: C, 83.00, H, 6.09 %.

Photocycloadduct (49a)

Yield 15%; Yellow viscous oil; IR (CHCl3): vmax 3012, 2923, 2332, 1710, 1611, 1563,

1542, 1511, 1409, 1362, 1237, 1198, 918 cm-1

; 1H NMR (CDCl3, 200 MHz): = 5.87(bs,

1H, C8H), 4.51(d, 1H, J = 13.3 Hz, C3H), 4.12(dd, 1H, J = 5.4 & 13.3, Hz, C3H), 2.97-

2.94(m, 1H, C7H), 2.85-2.75(m, 2H, C6H & C10H), 2.71-2.58(m, 1H, C5H), 2.40-2.32(m,

2H, C4H & C5H), 2.22(s, 3H, -COCH3); 13

C NMR (CDCl3, 50 MHz): = 200.2(C=O),

177

157.1, 147.2, 78.8(C1), 69.4(C3), 53.0(C7), 36.3(C10), 32.0(C5), 30.9(C4), 29.3(C6),

28.8(CH3); Mass (EI) m/z: 177(M++1, 10), 176(M

+, 25), 159(24), 158((70), 147(46),

121(38), 105(42), 91(62), 84(41), 83(35), 81(35), 79(45), 78(47), 71(32), 69(49), 67(35),

65(38), 58(56), 56(100); Anal. (%): Calcd. for C11H12O2: C, 74.98, H, 6.86; Found: C,

74.75, H, 7.12.

Photocycloadduct (50a)

Yield 7%; yellow viscous oil; IR (CHCl3): vmax 3024, 2928, 1717, 1601, 1522, 1464, 1360,

1218, 1191, 1015 cm-1

. 1H NMR (CDCl3, 200 MHz): = 6.05(d, 1H, J = 2.8 Hz, C8H),

5.83(d, 1H, J = 2.8 Hz, C9H), 4.30(dd, 1H, J = 8.7, 4.7 Hz, C3H), 4.06(d, 1H, J = 8.7 Hz,

C3H), 3.85(d, 1H, J = 3.1 Hz, C7H), 3.45-3.38(m, 2H, C4H & C10H), 2.05(dd, 1H, J = 13.5,

7.1 Hz, C5H), 1.51-1.49(m, 1H, C5H), 1.32(s, 3H, CH3); 13

C NMR (CDCl3, 50 MHz): =

206.1(C=O), 131.6, 130.2, 82.3(C1), 71.2(C3), 56.4(C6), 54.2(C7), 48.7(C10), 40.8(C5),

30.7(C4), 26.0; Mass (EI) m/z (%): 177(M++1, 9), 176(M

+, 18), 158((54), 147(42), 121(39),

105(43), 91(57), 84(45), 83(35), 81(33), 79(39), 78(41), 71(28), 56(100); Anal. Calcd. for

C11H12O2: C, 74.98, H, 6.86; Found: C, 74.69, H, 6.58 %.

Photocycloadduct (51a)

Yield 12%; Yellow viscous oil; IR (CHCl3): vmax 3342, 2912, 2856, 1712(C=O), 1610,

1518, 1469, 1211, 1209 cm-1

; 1H NMR (CDCl3, 200 MHz): = 8.01(s, 1H, -OH), 6.02(d,

1H, J = 2.7 Hz, C3H), 5.70(d, 1H, J = 2.7 Hz, C4H), 4.22-4.18(m, 2H, -CH2-OH), 3.53(dd,

1H, J = 2.1 & 7.4, Hz, C2H), 3.31(dd, 1H, J = 3.2 & 7.4, Hz, C5H), 2.91-2.78(m, 1H, C7H),

2.57-2.48(m, 1H, C6H), 2.18(s, 3H, -COCH3), 1.76(dd, 1H, J = 12.9, 6.1 Hz, C8H), 1.35(m,

1H, C8H); 13

C NMR (CDCl3, 50 MHz): = 204.0(C=O), 139.4(CH), 134.8(CH),

65.2(CH2-O-), 55.8(C1), 54.6(C2), 49.8(C5), 47.0(C6), 37.1(C8), 29.7(C7), 28.8(CH3); Mass

(EI) m/z (%): 178(M+, 4), 177(M

+-1, 9), 112(20), 84(17), 83(31), 71(55), 70(49), 69(35),

58(100), 57(34), 56(79); Anal. Calcd. for C11H14O2: C, 74.13, H, 7.92; Found: C, 74.05, H,

8.11 %.

Irradiation of o-crotyloxyacetophenone (45b) in dry acetonitrile

(syn)-2-(1-Propenyl)-3-hydroxy-3-methyl-benzodihydrofuran (46b)

Yield 23%; colorless gummy material; IR (CHCl3): vmax 3484, 1616, 1598, 1472, 1450,

1417, 1382, 1345, 1317, 1264, 1142, 1098 cm-1

; 1H NMR (CDCl3, 200 MHz): 7.23-

7.10(m, 2H, ArH), 6.89-6.73(m, 2H, ArH), 5.97-5.80(m, 1H, C1 H), 5.71-5.60(m, 1H,

178

C2 H), 4.48(bd, 1H, J = 7.5 Hz, C2H), 1.78(bd, 3H, J = 6.3 Hz, C2 -CH3), 1.56(s, 3H, C3-

CH3); 13

C NMR (CDCl3, 50 MHz): 159.1(C7a), 133.3(C3a), 132.4(CH), 130.2(CH),

124.5(CH), 123.3(CH), 121.0(CH), 110.7(CH), 92.30(C2), 77.9(C3), 24.0(C3-CH3),

18.2(C2 -CH3); Mass (EI) m/z (%): 190(M+, 8), 173(55), 172(31), 159(20), 147(20),

145(23), 137(48), 131(35), 121(44), 119(25), 107(23), 105(26), 91(44), 77(31), 56(100);

Anal. Calcd. for C12H14O2: C, 75.76, H, 7.42; Found: C, 75.65, H, 7.50 %.

Mixture of 46b & 47b

Yiled 20%; colorless viscous oil; (1:1, 25 mg); IR (CHCl3): vmax 3484, 1617, 1594, 1473,

1382, 1257, 1140, 1007 cm-1

; 1H NMR (CDCl3, 200 MHz): = 7.27-7.11(m, 2H, ArH),

6.92-6.73(m, 2H, ArH), 6.02-5.87(m, 1H, C1 H in 46b & 47b), 5.82-5.65(m, C2 H in 46b &

47b), 4.77(bd,, J = 7.6 Hz, C2H in 47b), 4.48(bd, J = 7.3 Hz, C2H in 46b), 1.78(bd, 3H, J =

6.3 Hz, C2 -CH3 in 46b), 1.70(d, 3H, J = 6.2 Hz, C2 -CH3 in 47b), 1.54(s, 3H, C3-CH3 in

46b), 1.32(s, 3H, C3-CH3 in 47b); 13

C NMR (CDCl3, 50 MHz): = 159.1(C7a), 133.3(C3a

in 46b), 132.6(C3a in 47b), 132.4(CH), 130.6(CH), 130.2(CH), 125.7(CH), 124.5(CH),

123.3(CH), 121.0(CH), 120.8(CH), 112.1(CH), 110.7(CH), 93.4(C2 in 47b), 92.3(C2 in

46b), 77.9(C3 in 46b), 77.2(C3 in 47b), 24.0(C3-CH3 in 46b), 23.3(C3-CH3 in 47b),

18.2(C2 -CH3 in 46b), 17.8(C2 -CH3 in 47b).

Photocycloadduct (49b)

Yield 30%; yellow viscous oil; IR (CHCl3): vmax 3011, 2918, 2360, 1691, 1627, 1468,

1425, 1363, 1293, 1156, 1025 cm-1

; 1H NMR (CDCl3, 200 MHz): = 5.73(bs, 1H, C8H),

4.33(d, 1H, J = 13.3 Hz, C3H), 4.08(dd, 1H, J = 13.3, 5.4 Hz, C3H), 2.82(m, 1H, C7H),

2.77-2.62(m, 2H, C6H & C10H), 2.55-2.42(m, 1H, C5H), 2.20(m, 1H, C4H), 2.14(s, 3H, -

COCH3), 1.06(d, 3H, J = 7.2 Hz, C5-CH3); 13

C NMR (CDCl3, 50 MHz): = 199.6(C=O),

157.3, 146.5, 78.4(C1), 69.0(C3), 49.5(C7), 38.1(C10), 36.3(C5), 32.9(C4), 29.74(C6),

29.4(COCH3), 21.1(CH3); Mass (EI) m/z (%): 191(M++1, 5), 190(M

+, 37), 147(8), 121(24),

91(35), 83(32), 79(23), 77(25), 71(47), 70(41), 69(45), 58(87), 56(100); Anal. Calcd. for

C12H14O2: C, 75.76, H, 7.42; Found: C, 75.54, H, 7.23 %.

179

Irradiation of 1-(2-allyloxy-5-chloro-phenyl)-ethanone (45c) in dry acetonitrile

5-Choloro-3-methyl-2-vinyl-benzofuran (52c)

Yield 50%; colorless viscous oil; IR (CHCl3): vmax 1608, 1458,1472, 1370, 1260, 1232,

1217, 1084 cm-1

; 1H NMR (CDCl3, 300 MHz): = 7.30(d, 1H, J = 3.3 Hz, ArH), 7.28-

7.09(m, 2H, ArH), 6.58(dd, 1H, J = 11.1 & 1.7 Hz, C1 H), 5.81(dd, 1H, J = 17.4 & 1.1Hz,

C2 H), 5.28(dd, 1H, J = 2.6 & 11.1 Hz, C2 -H), 2.11 (s, 3H, CH3); 13

C NMR (CDCl3, 50

MHz): = 152.4(C7a), 151.6(C3a), 131.7(CH), 128.0(CH), 124.8(CH), 123.0(CH),

119.4(=CH2), 115.3(HC=), 112.6(C2), 111.7(C3), 29.8(CH3); Mass (ESI) m/z: 232 (M +

K+); Anal. Calcd. for C11H9OCl: C, 68.58, H, 4.71; Found: C, 67.99, H, 4.66 %.

Photocycloadduct (53c)

Yield 20%; yellow viscous oil; IR (CHCl3): vmax 3018, 2929, 2399, 1716, 1683, 1521,

1508, 1423, 1363, 1217, 1099, 927 cm-1

; 1H NMR (CDCl3, 300 MHz): 5.74(split d, 1H, J =

1.4 and 5.7 Hz, allylic and homoallylic couplings C7H), 4.12(dd, 1H, J = 4.3 & 9.2 Hz,

C2Ha), 3.72 (d, 1H, J = 9.2, C2Hb), 2.76 (dd, 1H, J = 4.3 and 6.0 Hz, C3H), 2.62 (dd, 1H, J

= 4.3 & 7.5 Hz, C9’H ), 2.34(dd, 1H, J = 3.3 & 7.5 Hz, C5H), 2.25(s, 3H, COCH3), 2.05-

2.00(m, 2H, C4Hab); Mass (ESI) m/z (%): 212(M)-2

.

Irradiation of 1-(2-But-2-enyloxy-5-chloro-phenyl)-ethanone (45d) in dry acetonitrile

5-Choloro-3-methyl-2-propenyl-benzofuran (52d)

Yield 32 %; colorless viscous oil; IR (CHCl3): vmax 1604, 1442,1432, 1309, 1265, 1210,

1022, 962 cm-1

; 1H NMR (CDCl3, 300 MHz): = 7.36(d, 1H, J = 3.1 Hz, ArH), 7.31-

7.11(m, 2H, ArH), 6.53(dd, 1H, J = 11.5 & 17.0 Hz, C1 H), 5.78(dd, 1H, J = 0.9 & 16.8

Hz, C2 -H), 2.25 (s, 3H, CH3), 1.72(bd, 3H, J = 6.2 Hz, C2 -CH3); 13

C NMR (CDCl3, 50

MHz): = 152.1(C7a), 151.2(C3a), 131.8(CH), 128.5(CH), 124.1(CH), 123.9(CH),

118.9(=CH2), 115.0(HC=), 112.1(C2), 111.2(C3), 29.2(CH3); 17.9(C2 -CH3); Mass (ESI)

m/z: 229 (M + Na+); Anal. (%): Calcd. for C12H11ClO: C, 69.74, H, 5.36; Found: C, 69.42,

H, 5.05.

Photocycloadduct (53d)

Yield 20%, Yellow viscous oil ; IR (CHCl3): vmax 3016, 2962, 2929, 2434, 2399, 1672

(conjugated C=O), 1608, 1566, 1519, 1473, 1379, 1161, 1010, 927, 757 (C-Cl) cm-1

; 1H

NMR (CDCl3, 300 MHz): 5.69(split d, 1H, J = 1.5 and 5.7 Hz, allylic and homoallylic

180

couplings C7H), 4.05(dd, 1H, J = 4.5 & 9.0 Hz, C2Ha), 3.74 (d, 1H, J = 9.0, C2Hb), 2.79

(dd, 1H, J = 4.5 and 6.0 Hz, C3H), 2.60 (dd, 1H, J = 4.5 & 7.5 Hz, C9’H ), 2.36(dd, 1H, J =

3.3 & 7.5 Hz, C5H), 2.09(s, 3H, COCH3), 2.05(dd, 1H, J = 4.5 & 7.5 Hz, C4H), 1.21(s, 3H,

CH3); 13

C NMR (CDCl3, 75 MHz): 204.2(C=O), 174.9(C9), 132.0(C6), 120.4(C7), 101.9

(C8), 71.2(C2), 47.4(C3), 45.0(C9`), 37.3(C5), 26.6(C4); 21.3(COCH3), 17.7(CH3); MS

(ESI): m/z 247 (M + Na)+.

181

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