Photochemical approach to new polycyclic substrates suitable for further photocatalytic functionalization

Article abstract of DOI:10.5562/cca2454

New polycyclic compounds are synthesized by photocycloaddition reactions of methoxy, methyl or phenyl substituted butadiene derivatives 11-14. Mono- and dimethoxy butadiene derivatives 11 and 12 undergo intramolecular [2+2] photocycloaddition giving benzobicyclo[3.2.1]octadiene structures (endo-15, endo,trans-17) as main products. As minor photoproducts tricyclic compounds endo-16 and endo,trans-18 are isolated, respectively, formed by [4+2] photoinduced cycloaddition of the starting molecules. The reaction of compounds 13 and 14 is more selective and only benzobicyclo[3.2.1]octadienes endo,endo-19 and endo,endo-20 are formed, respectively. New bicyclo[3.2.1]octadienes with isolated double bond can be suitable substrates for further efficient photocatalytic oxygenations in course of new functionalized polycycles, potentially new biologically active compounds.

Full text of DOI:10.5562/cca2454

CROATICA CHEMICA ACTA
CCACAA, ISSN 0011-1643, e-ISSN 1334-417X
■
hkd
ATICA
CT
| H HEMICA
Croat. Chem. Acta 87 (4) (2014) 465A73.
1
9 2 6 H
http://dx.doi.org/!0.5562/cca2454
Original ScientificArticle
Photochemical Approach to New Polycyclic Substrates
Suitable for Further Photocatalytic Functionalization*
Dragana Vuk,aŽeljko Marinić,band Irena Škorića*
aDepartment ofOrganic Chemistry, Faculty of Chemical Engineering and Technology,
University ofZagreb, Marulićev trg 19, 10000 Zagreb, Croatia
bCenterfor NMR, Rudjer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
RECEIVED APRIL 3,2014; REVISED OCTOBER 9, 2014; ACCEPTED NOVEMBER 6, 2014
Abstract. New polycyclic compounds are synthesized by photocycloaddition reactions of methoxy, methyl
or phenyl substituted butadiene derivatives 11-14.Mono- and dimethoxy butadiene derivatives 11 and 12
undergo intramolecular [2+2] photocycloaddition giving benzobicyclo[3.2.1]octadiene structures (endo-
1
5,endo,trans-\l) as main products. As minor photoproducts tricyclic compounds endo-16 and en-
do,trans-\H are isolated, respectively, formed by [4+2] photoinduced cycloaddition of the starting mole-
cules. The reaction of compounds 13 and 14 is more selective and only benzobicyclo[3.2.1]octadienes en-
do,endo-19 and endo,endo-19 are formed, respectively. New bicyclo[3.2.1]octadienes with isolated dou-
ble bond can be suitable substrates for further efficient photocatalytic oxygenations in course of new func-
tionalized polycycles, potentially new biologically active compounds.
Keywords: cycloaddition, photochemistry, butadienes, benzobicyclo[3.2.1.]octadiene, polycycles
INTRODUCTION
mations on the isolated double bond, easily derivatized
to new compounds with various functionalities.21-27
Photochemistry of mono- and dibutadienyl derivatives
la-2d (Figure 1) has been studied in detail.1-3 Unsubsti-
tuted starting compounds la and 2a, upon irradiation at
As a part of our increasing interest in the photo-
chemistry of conjugated butadiene systems and to ex-
plore the effect of substituents on the butadiene units on
the photochemical reactions, studies are expanded to
methyl- and chloro-butadiene derivatives lb d and
2b-d.2,3 Methyl group effects sterically to photochemi-
cal reactions, causing diverse photoinduced behaviour
and formation of completely different photoproducts.2
Monosubstituted methyl derivative lb upon irradiation
gives dihydronaphtalene derivative 5 (Figure 2), while
disubstituted methyl derivative 2b shows only geomet-
ric isomerization. Introduction of the chlorine atom to
butadiene unit can lead the reaction course to various
direction, under its steric and electronic effects. Upon
irradiation, mono- and di-a-chloro derivatives lc and 2c
photocyclize to give six-membered ring products 6 and
3
50 nm, give as the main products of intramolecular
cycloaddition benzobicyclo[3.2.1]octadiene structures
endo-3 and endo,trans-4, respectively.1,2 The bicy-
clo[3.2.1]octane skeleton, saturated analogue of the
bicyclo[3.2.1]octadiene, is found in numerous biologi-
cally important active natural products.4-20 Moreover,
benzobicyclo[3.2.1]octadiene skeleton is also an im-
portant fragment in biologically active compounds or it
can be used as a suitable substrate for further transfor-
7.3 In continuation of our studies on photochemical be-
haviour of butadiene derivatives we extended research to
^
-substituted chloro derivatives Id and 2d.3 The p-
substitution increases molecular planarity, relative to a-
substitution and shift conformer equilibrium, affecting the
reaction pathways and yields. As the main products upon
irradiation of p-chloro-butadiene derivatives Id and 2d
new benzobicyclo[3.2.1]octa-diene structures endo-8 and
la : R = H,
b: R = CH3 R' = H
c: R = Cl, R’= H
R' = H
b: R = CH3 R’ = H
c: R = Cl, ’R’= H
d: R =H,
R' = Cl
d: R =H,
R’= Cl
Figure 1. Structures of the known butadiene derivatives.
t
*
Dedicated to Dr. Mirjana Eckert-Maksić on the occasion of her 70th birthday.
Author to whom correspondence should be addressed. (E-mail: irena.skoric@fkit.hr)

4
66
D. Vuk et al, Photochemical Approach to New Polycyclic Substrates
Figure 2. Structures of the isolated photoproducts.
endo,trans-9 are formed in very good yields, with smaller
amounts (10 %) of dihydronaphtalene derivative endo-10
in case of 2d(Figure 2).
Starting compounds la-2d(Figure 1) possess very
similar structures. Under steric and electronic effect of
substituents, different products are formed. Therefore,
understanding the influence and nature of substituents in
the molecule is crucial for understanding the preffered
reaction path and for prediction the behaviour of similar
unresearched related compounds.
tained using microscope equipped apparatus and are
uncorrected. HRMS analysis were carried out on a mass
spectrometer (MALDI TOF/TOF analyzer) equipped
with Nd:YAG laser operating at 355 nm with firing rate
200 Hz in the positive (H+) or negative (H­) ion reflec­
tor mode. Silica gel (0.063­0.2 mm) was used for
chromatographic purifications. Thin­layer chromatog­
raphy (TLC) was performed silica gel 60 F254 plates.
Solvents were purified by distillation.
Benzaldehyde and p­methoxy­cinnamaldehyde
In the continuation of our research on photochem­
ical behaviour of butadiene derivatives, we inserted one
or two methoxy groups (as strong electron donating
groups) to the p­position(s) of the aromatic ring(s) of
the investigated compounds 11and 12(Figure 3). To
explore the effect of methyl or phenyl group in the [i-
position of the o­vinyl group of la(Figure 1) on photo­
chemical behaviour, butadiene derivatives 13and 14
were obtained from a commercial source, pfi-o-
xylyl(ditriphenylphosphonium) dibromide was prepared
from o­xylyldibromide and triphenylphosphine in dime­
thylformamide.
The starting compounds 1­(p­methoxyphenyl)­4­(o­
styryl)­l,3­butadiene (11), 1­{o­[­4­(p­methoxyphe­nyl)­
1,3­butadienyljphenyl}­4­(/>methoxyphenyl)­1,3­
butadiene (12) and (1E,3E)-1­(o­[(£)­1­propenyljphe­
nyl)­4­phenyl­1,3­butadiene {trans,trans-13) are de­
scribed.3The data ofthe new compounds are given below.
(
Figure 3) are prepared as related compounds to the
molecule of la(Figure 1), also in course of the investi­
gation the effect of substitution.
(
(
lZ,3E)-l-[(Z)-2-Stilbenyl]-4-phenyl-l,3-butadiene
cis,cis-14)
EXPERIMENTAL
Starting compound 14was prepared by Wittig reaction
from o­xylylbis(triphenylphosphonium bromide) and
General Experimental Information
The 'H spectra were recorded on a spectrometer at 600
MFIz. The 13C NMR spectra were registered at 150
MHz, respectively. All NMR spectra were measured in
CDCb using tetramethylsilane as reference. The as­
signment of the signals is based on 2D­CH correlation
and 2D­HH­COSY experiments. UV spectra were
measured on a UV/VIS Cary 50 spectrophotometer. IR
spectra were recorded on a FTIR­ATR (film). Irradia­
tion experiments were preformed in a Quartz vessel in
toluene solution in a photochemical reactor equipped
with 3500 A lamps. All irradiation experiments were
carried out in deaerated solutions by bubbling a stream
of argon prior to irradiation. Melting points were ob­
Figure 3. Structures of the starting butadiene derivatives
11-14.
Croat. Chem. Acta 87 (2014) 465.

D. Vuk et al, Photochemical Approach to New Polycyclic Substrates
467
endo,endo-19
endo,endo-20
Chart 1.
cinnamaldehyde. To a stirred solution of the tri-
phenylphosphonium salt (0.001 mol) and the cinnamal-
dehyde (0.011 mol) in absolute ethanol (200 mL), a
solution of sodium ethoxide (0.253 g, 0.011 mol in 15
mL of absolute ethanol) was added dropwise. Stirring
was continued under a stream of nitrogen for one hour
at RT, when was 1.1 eq of benzaldehyde (0.011 mol)
introduced and the next quantity of sodium ethoxide
126.6 (2d), 125.5 (d); HRMS (TOF ES+) m/z for C24
M caicd308.1559; M found308.1559.
H :
20
Irradiation Experiments
A mixture of isomers of 11-14 in toluene (3.0-10 3 M)
was purged with argon for 20 min and irradiated at 350
nm in a Rayonet reactor (16 lamps) in a quartz vessel
for 16 h (11), 96 h (12), 4 h (13) and 8 h (14). After
irradiation the solvent was removed in vacuo and the
oily residue chromatographed on a silica gel column
using petroleum ether as the eluent. The photoproducts
endo-15, endo-16 (obtained from 11), 17,18 (from 12),
endo.endo-19 (from 13) and endo,endo-20 (from 14)
(0.253 g, 0.011 mol in 15 mL of absolute ethanol) was
added dropwise. The reaction was completed within 3-4
h (usually was left to stand overnight). After removal of
the solvent, the residue was worked up with water and
toluene. The toluene extracts were dried (anhydrous
MgSCL), concentrated and the crude reaction mixture
was purified. The reaction mixture contained cis,cis-,
cis,trans-, trans,cis- and trans,trans-isomers in the ratio
(
Chart 1) were isolated from the enriched first chroma-
tographic fractions followed by a mixture of several
unidentified products (< 2 %). High-molecular-weight
products remained on the column.
3
:2:2:3. After repeated column chromatography on
silica gel using petroleum ether as the eluent only the
cis,cis-14 was isolated and completely characterized.
cis,cis-14: yield 30 %; Rf 0.88 (petroleum
ether/dichloromethane = 1:1); colourless oil; UV (96 %
EtOH) /Imax / nm: 284 and 254 (log e l dm3moL1cm”':
(11S)-1 l-(4-methoxyphenyl)tricyclo[6.3.1.02, 7] dodeca-
2,4,6,9-tetraene (endo-75):
Yield 55 %; Rf 0.27 (petroleum ether/dichloromethane =
8:2); colourless crystals; m.p. 45-47 °C; UV (96 %
EtOH) Amax/nm: 284, 276, 269 and 226 (log e / dm3
mob1 cnT1: 3.55, 3.67, 3.65 and 4.16); IR iw ./cnT 1
(evaporated from CH2CI2): 2952, 1605 (C=C, ar), 1510
(C=C), 1249 (Car-0-CH3), 1174, 1035, 771; 'H NMR
(CDCb, 600 MHz) <5/ppm: 7.11 (d, IH, J = 7.3 Hz),
7.03 (dt, 1H, J= 7.3 Hz; 1.0 Hz), 6.82 (dt, 1H, J= 7.3
Hz; 1.0 Hz), 6.72 (d, 2H, J= 8.6 Hz), 6.64 (d, 2H, J =
8.6 Hz), 6.32-6.36 (m, 1H, HA), 6.25 (d, IH, J = 7.3
Hz), 5.28 (dt, 1H, Hb, Jab = 9.6 Hz; JBC= 2.6 Hz),
3.91-3.94 (m, 1H, Hc), 3.77 (s, 3H, -OCH3), 3.35 (t,
1H, Jdc = J df = 4.5 Hz, Hd), 3.27 (dd, 1H, J ea = 6.3 Hz;
3
.78 (sh) and 3.93); IR vmax./cm_1 (evaporated from
CH2C12): 3016, 2923, 1492, 1446, 1218, 958, 774, 692;
HNMR (CDCb; 300 MHz) 81ppm: 7.41 (d, 1H, J= 7.6
Hz), 7.33 (d, 2H, J = 7.5 Hz), 7.24-7.29 (m, 4H), 7.20
t, 1H, J= 7.6 Hz), 7.07-7.18 (m, 7H), 6.65 (d, 1H, J =
>
(
1
1
1
1
1
1
5.6 Hz), 6.63 (d, 1H, J = 12.2 Hz), 6.61 (d, 1H, J =
2.2 Hz), 6.56 (d, IH, J = 11.3 Hz), 6.40 (t, 1H, J =
1.3); 13C NMR (CDC13; 75 MHz) č/ppm: 137.4 (s),
36.9 (2s), 136.3 (s), 134.4 (d), 130.9 (d), 130.8 (d),
30.0 (d), 129.4 (d), 129.4 (d), 129.2 (d), 129.0 (2d),
28.6 (2d), 128.1 (2d), 127.6 (d), 127.1 (2d), 127.0 (d),
Croat. Chem. Acta 87 (2014) 465.

4
68
D. Vuk et al., Photochemical Approach to New Polycyclic Substrates
Jef = 4.7 Hz, He), 2.49-2.53 (m, 1H, HF), 2.37 (d, 1H,
Jg f = 9.9 Hz, Hg); l3C NMR (CDC13, 75 MHz) <S/ppm:
and 230 (log e / dm3 mol ' e m 1: 4.24 (sh), 4.33 and
4.14); IR Vmax/cm-1 (evaporated from CH2CI2): 2922,
1606 (C=C, ar), 1510 (C=C), 1247 (Car-O-CHj), 1076,
1033, 964; 'H NMR (CDCb, 600 MHz) SIppm: 7.28 (d,
1H, J = 7.6 Hz), 7.06 (dt, 1H,J=7.6; 1.0 Hz), 7.00 (d,
2H, J = 8.7 Hz), 6.88 (dt, 1H, J = 7.6; 1.0 Hz), 6.80 (d,
1
57.6 (s), 152.0 (s), 141.7 (s), 134.2 (s), 134.1 (d), 128.8
(2d), 126.3 (d), 125.8 (d), 125.5 (d), 124.5 (d), 119.6
(d), 112.6 (2d), 43.7 (t); HRMS (TOF ES+) mlz for
C,9H,80: Mealed 262.1352; M+found262.1351.
2
=
1
H, J = 8.7 Hz), 6.71 (d, 2H, J=8. 7 Hz), 6.61 (d, IH, J
7.6 Hz), 6.56 (d, 2H, J = 8.7 Hz), 6.32 (d, IH, J =
5.7 Hz), 5.25 (dd, 1H, J = 15.7; 8.2 Hz), 3.73 (s, 3H, -
(
1OS)-10-(4-methoxyphenyl)tetracy-
clo[7.2.1.02,n.03, 8] dodeca-3,5,7-triene (endo-16)
Yield 13 %; R{ 0.21 (petroleum ether/dichloromethane =
8
EtOH) /Imax/nm: 285, 277, 270 and 227 (log e / dm3
mol’1 cm'1: 4.10, 3.27, 3.19 and 4.18); !H NMR
OCH3), 3.66 (s, 3H, -OCH3), 3.51-3.54 (m, 1H, HA),
3
2
:2); colourless crystals; m.p. 91-93 °C; UV (96 %
.20 (t, 1H, J = 4.6 Hz, HB), 3.11-3.16 (m, 1H, HG),
.40 (t, 1H, J = 7.1 Hz, Hc), 1.95 (dt, 1H, J = 6.4; 1.8
Hz, HE), 1.81 (dt, 1H, J = 6.4; 1.8 Hz, HF); 13C NMR
CDCb, 75 MHz) <S/ppm: 158.4 (s), 157.6 (s), 136.4 (s),
33.3 (s), 132.2 (s), 130.6 (d), 129.8 (d), 129.0 (d),
(
7
CDC13, 600 MHz) <5/ppm: 7.26 (d, IH, J = 7.4 Hz),
.06 (dt, 1H, J = 1A Hz; 0.8 Hz), 6.89 (dt, 1H, J - 7.4
Hz; 0.8 Hz), 6.79 (d, 2H, J = 8.6 Hz), 6.66 (d, 1H, J =
(
1
1
1
(
28.6 (2d), 127.1 (d), 126.8 (d), 126.0 (d), 125.2 (d),
24.5 (d), 113.7 (d), 113.7 (d), 112.9 (2d), 55.3 (d), 55.0
d), 48.3 (d), 43.8 (d), 38.8 (d), 22.7 (d), 21.5 (d), 17.9
7
3
1
.4 Hz), 6.55 (d, 2H, J = 8.6 Hz), 3.65 (s, 3H, -OCH3),
.37 (dd, 1H, Jab = 4.6 Hz; JAE= 2.8 Hz, HA), 3.16 (t,
H, Jab = Jbd = 4.6 Hz, HB), 2.33 (t, 1H, JCE= Jc f = 7.2
(d); HRMS (TOF ES+) mlz for CzsHzeOz: M+ Ca icd
Hz, Hc), 2.12 (ddd, 1H, JDG=11.7 Hz; JBD= 4.6 Hz;
Jdf = 2.8 Hz, Hd), 1.88 (dt, 1H, JCE = Jef = 7.2 Hz; JAE
3
94.1927; M+found394.1920.
=
2
(
1
2.8 Hz, He), 1.77 (dt, 1H, JCE = Jef = 7.2 Hz; JAE=
.8 Hz, Hf), 1.19 (d, IH, JDG= 11.7 Hz, HG); I3C NMR
CDCb, 75 MHz) <5/ppm: 157.0 (s), 136.3 (s), 135.2 (s),
32.3 (s), 128.0 (2d), 125.3 (d), 124.9 (d), 124.5 (d),
23.8(d), 112.5 (2d), 29.5 (t).
(
2
11S)-12-methyl-l1-phenyltricyclof6.3.1.02, 7] dodeca-
,4,6,9-tetraene (endo,endo-/9)
Yield 77 %; Rf 0.88 (petroleum ether/dichloromethane =
:1); colourless oil; UV (96 % EtOH) Jmax/nm: 324,
75, 268 and 259 (log e l dm3 mol-1 cm-1: 2.94, 3.00,
.02 and 2.96 (sh)); IR iw./cm ”1 (evaporated from
1
2
3
1
(
11S)-1l-(4-methoxyphenyl)-12-[(E)-2-(4-methoxyphe-
CHzCb): 3025, 1601, 1495, 1450, 1218, 1032,755,698;
H NMR (CDCb, 600 MHz) d/ppm: 7.14-7.18 (m, 3H),
nyl)ethenyl]tricyclo[6.3.1.02, 7] dodeca-2,4,6,9-tetraene
endo,trans-/ 7)
Yield 24 %; Rf 0.22 (petroleum ether/dichloromethane =
:2); colourless oil; UV (96 % EtOH) Xm-Jnnr. 260 and
29 (log £ / dm3 mol-1 cm"': 4.28 and 4.22); IR
'
(
7
6
6
.10 (d, 1H, J = 7.4 Hz), 7.03 (dt, 1H, J = 7.4; 0.8 Hz),
.80 (dt, 1H, J= 7.4; 0.8, Hz), 6.70-6.74 (m, 2H), 6.36-
.42 (m, 1H, Ha), 6.16 (d, 1H, J = 7.4 Hz), 5.30-5.32
8
2
(
m, 1H, Hb), 3.96-4.00 (m, 1H, Hc), 3.04 (d, 1H, J =
Vmax./cnT1(evaporated from CH2CI2): 3021, 1607 (C=C,
ar), 1510 (C=C), 1247 (Car-0-CH3), 1034, 833, 737; 'H
NMR (CDCb, 600 MHz) <S/ppm: 7.32 (d, 2H, J = 8.5
Hz), 7.14 (d, 1H, J = 7.3 Hz), 7.07 (dt, 1H, J = 7.3; 0.9
Hz), 6.95 (d, 2H, J = 8.5 Hz), 6.83-6.88 (m, 2H), 6.72
4.8 Hz, Hd), 2.96 (d, 1H, J = 6.6 Hz, HE), 2.75 (dd, 1H,
J = 6.6, 13.1 Hz, Hf), 0.93 (d, 3H, J = 6.6 Hz, -CH3);
13C NMR (CDCb, 150 MHz) <5/ppm: 150.5 (s), 141.9
(s), 139.8 (s), 135.1 (d), 127.9 (2d), 127.1 (2d), 126.9
(d), 125.8 (d), 125.7 (d), 125.6 (d), 124.6 (d), 120.9 (d),
55.0 (d), 49.1 (d), 47.1 (d), 46.7 (d), 17.8 (q); HRMS
(TOF ES+) mlz for Ci9Hi8: [M-H]”caicd 245.1325;
[M-H]-found 245.1329.
(
1
(
3
d, 2H, J = 8.7 Hz), 6.65 (d, 2H, J = 8.7 Hz), 6.41 (ddd,
H, J = 8.3; 3.2; 2.5 Hz), 6.25-6.29 (m, 2H), 5.32-5.33
m, 1H), 3.97-4.00 (m, 2H, Hc), 3.85 (s, 3H, -OCH3),
.77 (s, 3H, -OCH3), 3.75-3.79 (m, 1H, HD), 3.24 (d,
H, J = 4.7 Hz, He / HF), 3.20 (d, 1H, J = 6.6 Hz, HE/
Hf); l3C NMR (CDCb, 75 MHz) <S/ppm: 158.0 (s),
57.7 (s), 150.5 (s), 150.2 (s), 140.0 (s), 139.7 (s), 134.1
1
(
llS )-ll, 12-diphenyltricyclo[6.3.1.02, 7] dodeca-2,4,6,9-
tetraene (endo,endo-20)
Yield 45 %; R f 0.88 (petroleum ether/dichloromethane =
1
dm3mol”1cm”1: 3.55 (sh) and 3.71; IR iw /cm ”1(evap-
orated from CHzCb): 3022, 2956, 1452, 1375, 1016,
7
7
7
1
(
(
d), 132.7 (d), 129.5 (2d), 128.9 (2d), 128.2 (d), 126.8
d), 126.7 (d), 126.7 (d), 126.5 (d), 125.9 (d), 125.9 (d),
:1); UV (96 % EtOH) /W nm : 278 and 253 (log e!
124.9 (d), 124.8(d), 120.7 (d), 113.3 (q), 112.7 (q), 54.7
(d), 53.7 (d), 46.4 (d), 45.8 (d); HRMS (TOF ES+) mlz
46, 701; ‘H NMR (CDCb, 600 MHz) <S/ppm: 7.16-
.20 (m, 3H), 7.12 (d, 2H, J = 7.6 Hz), 7.08 (d, 2H, J =
.6 Hz), 7.03 (d, 2H, J = 7.6 Hz), 7.00-7.03 (m, 1H),
for CzsHseOj: [M+K]+ Ca icd 433.1564; [M+K]+f0Und
33.1581.
4
(
10S)-10-(4-methoxyphenyl)-12-[(E)-2-(4-methoxyphe-
6.80 (dt, 1H, J = 7.7; 0.9 Hz), 6.75-6.78 ( m, 2H), 6.50-
6.55 (m, 1H, Ha), 6.15 (d, 1H, J = 7.3 Hz), 5.42 (ddd,
1H, J = 9.7; 3.5; 1.7 Hz, HB), 4.16-4.20 (m, 1H, Hc),
3.88 (s, 1H, Hf), 3.57 (d, 1H, J = 4.7 Hz, HD), 3.48 (d,
1H, J = 6.7 Hz, He); 13C NMR (CDCb, 150 MHz)
nyl)ethenyl]tetracyclo
triene (endo,trans-/#/
[7.2.1.02,u.03 8] dodeca-3,5,7-
Yield 7 %; Rf 0.20 (petroleum ether/dichloromethane =
:2); colourless oil; UV (96 % EtOH) /Lax/nm: 274, 264
8
Croat. Chem. Acta 87 (2014) 465.

D. Vuk et al., Photochemical Approach to New Polycyclic Substrates
469
Scheme 1.
<
S/ppm: 150.5 (s), 143.6 (s), 141.8 (s), 139.8 (s), 135.2
derivative 16, while the high­molecular­weight products
remained on the column (Scheme 1). The proposed
mechanism of the formation of benzobicyclo[3.2.1]
structure 15 involves intramolecular [2+2] cycloaddition
(
d), 128.0 (2d), 127.6 (d), 127.3 (2d), 126.9 (d), 126.3
d), 126.24 (d), 126.22 (d), 126.1 (2d), 125.9 (d), 125.5
d), 124.9 (d), 120.4 (d), 58.8 (d), 54.2 (d), 47.6 (d),
(
(
4
3
6.6 (d). HRMS (TOF ES+) mlz for C24H2o: [M-H]-caicd via biradical intermediate 15’ followed by preffered 1,6
07.1481; [M-H]“found307.1493.
ring closure. Photoproduct 16 can be formed by 67
c
electrocyclization process followed by [4+2] cycloaddi­
tion of the intermediate 16’ (Scheme 2).
RESULTS AND DISCUSSION
Inserting a second styryl substituent at the [i-
position of the vinyl group of la (Figure 1), we pre­
pared dimethoxy derivative 12 and obtained the system
with extended conjugation, which can influence on the
reaction course. Irradiation of compound 12 under the
same conditions gave, after chromatograpfic separation
on silica gel, very similar results (Scheme 2). Benzobi­
cyclo[3.2.1]octadiene 17 and tricyclic derivative 18
were isolated. Their formation can be explained by the
same mechanism as in the previous case of mono­
methoxy derivative 11. The difference in photochemical
behaviour between mono­ and dimethoxy­derivative is
the prolonged time of irradiation to the full conversion
in case of 12. This may be caused by insertion of the
second styryl group, which might have influence on the
additional stability of the starting molecule 12 in com­
parison to 11.
Starting compounds 11-14 are prepared by Wittig reac­
tion according to the procedure described for butadienes
in previous papers1-3 from o­xylylbis(triphenylphospho­
nium bromide) and corresponding aldehydes. The prod­
ucts are obtained as mixture of two (in case of 11), three
(in case of 12) or four (in case of 13 and 14) isomers
and subjected to irradiation. Irradiation experiments are
performed in Rayonet reactor at 350 nm in toluene solu­
tions under anaerobic conditions at low concentrations.
Obtained photoproducts are isolated and completely
characterized by spectroscopic methods.
Mono­ and di­methoxy derivatives 11 and 12 un­
dergo photochemical reaction to form new polycyclic
structures 15-18 (Scheme 1). On irradiation of mono­
methoxy derivative 11, bicyclic derivative 15 was iso­
lated as main product (55 %), with 13 % of tricyclic
Croat. Chem. Acta 87 (2014) 465.

470
D. Vuk et cd., Photochemical Approach to New Polycyclic Substrates
OCH,
Scheme 2.
Irradiation of starting compounds 13 and 14 gave
different results. Under irradiation starting compounds
3 and 14 react giving new bicyclic cycloaddition prod­
ucts, interesting for further functionalization as it was
shown for previously synthesized hetero­bicyclic pho­
toproducts.21­23 After chromatographic purification of
crude reaction mixture, only bicyclo derivatives 19 and
20 were isolated, respectively (Scheme 3.) The mecha­
1
Scheme 3.
Croat. Chem. Acta 87 (2014) 465.

D. Vuk et al., PhotochemicalApproach toNewPolycyclic Substrates
471
nism of their formation, as seen previously, involves
intramolecular [2+2] cycloaddition and 1,6 ring closure
of intermediate 19’ or 20’. Benzobicyclo[2.1.1] hexene
derivative 21 or 22, which could be formed by 1,4-ring
closure of 13 or 14, was not detected.
of Hd proton in endo,trans-18 due to the entry of the
second styryl or benzyl group to the molecule. For the
same reason proton Hg is strongly shifted to lower field
from 1.2 ppm in endo-16 to 3.1 ppm in endo,trans-18.
Methoxy group might have electronic and/or steric
The structures of new photoproducts have been
elucidated from their spectral data, *HNMR being most
informative. Figure 4 shows comparison of aliphatic
region of isolated bicyclic derivatives endo-15, en-
do,trans-Yl, endo,endo-19 and endo,endo-20. The rec-
ognizable pattern of 'H NMR spectra of photoproducts
in the region between 2.4 and 4.2 ppm indicates the
same bicyclo[3.2.1]octadiene structure. Depending on
substitution at the //-position of the vinyl group, the
aliphatic protons are shifted to higher or lower field.
Protons of compounds endo,trans-\l and endo,endo-19,
which possess the second phenyl group are deshielded
and shifted to lower field in comparison to protons of
endo-15 and endo,endo-20. The accent is on Hf proton,
which is shifted from 2.5 ppm (in endo-15) to 3.2 ppm
effects on the photochemical behaviour of the analyzed
compound. But the results obtained under irradiation of
mono-methoxy derivative 11 in comparison with previ-
ously reported unsubstituted la and chloro-derivative
Id are very similar. In all cases benzobicy-
clo[3.2.1]octadiene structures endo-3, endo-8 and endo-
15 are isolated, respectively, as main product. Besides,
methoxy derivative 11 gives tricyclic derivative endo-
16. The yield on the isolated bicyclic derivative is the
largest by use of the unsubstituted compound (90 %)
where the steric and electronic effects are reduced. In
case of chloro derivative the yield of the isolated prod-
uct (77 %) is slightly higher compared to the methoxy
compound endo-15 (55 %), having potentially increased
steric and electronic influence relative to a chlorine
atom. Electronic effects may have influence on the
reaction course, as in the case of methoxy derivative the
reaction was less selective giving two photoproducts, in
contrary to the previously described derivatives, where
only the bicyclo derivative is isolated. In case of dibuta-
diene derivatives, unsubstituted compound 2a under
{
endo,trans-17) and from 2.8 ppm {endo,endo-19) to 3.9
ppm {endo,endo-20). Tricyclic derivatives endo-16 and
endo,trans-18 has a completely different pattern in 'H
NMR spectrum (Figure 5) compared to bicyclic photo-
products. Patterns of aliphatic protons of both deriva-
tives are very similar. The difference is disappearance
Croat. Chem. Acta 87 (2014)465.

472
D. Vuk et al., Photochemical Approach to New Polycyclic Substrates
irradiation gives only benzobicyclo[3.2.1]octadiene
structure (endo,trans-4). Chloro and methoxy deriva­
tives, due to their different steric and electronic effect
give the same type of products, benzobicy­
clo[3.2.1]octadiene structure (endo,trans-9, endo,trans-
XT)and tricyclic derivative (endo-10, endo,trans-18).
On the other hand compounds 13 and 14 under irradia­
tion behave as the unsubstituted derivative la giving
selectively only bicyclic structures endo,endo-19 and
endo,endo-20, respectively, but in lower yields.
selectivity, stereoselectivity and the reaction course
leading to bicyclic photoproducts in moderate to good
yields (24­77 %). All new prepared and fully character­
ized bicyclic molecules are suitable substrates for fur­
ther efficient photocatalytic oxygenation reactions in
course of getting new functionalized polycycles, poten­
tially biologically more active compounds with greater
similarity to the structure of some naturally occurred
terpenes.
Supplementary Materials. ­ Supporting informations to the
paper are enclosed to the electronic version of the article.
These data can be found on the website of Croatica Chemica
Acta (http://public.camet.hr/ccacaa).
In cases of all bicyclic photoproducts the ring clo­
sure predominantly gives
e«t/o­isomers.
The
stereoselectivity of the cycloaddition reaction and pref­
erable ring closure to endo-isomer can be ascribed to the
stabilization of the transition state in enifo­orientation
by the strong attractive intramolecular 71-71 interactions
of the benzo­phenyl.30”32
Acknowledgements. This research was founded by grants from
the Croatian Ministry of Science, Education and Sports (125­
0982933­2926 and 098­0982929­2917) and University of
Zagreb short term scientific support under the title “Function­
alization of the benzobicyclo[3.2.1]octadiene skeleton using
photocatalytic oxygenation reactions”.
CONCLUSION
Different substituted butadiene derivatives under irradi­
ation show not so diverse mechanism of photochemical
behavior but with different selectivity and yields. Con­
sidering the nature and the position of the substituents,
new polycyclic structures are formed. Starting materials
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