Photochemistry of Structural Analogues of Previtamin D3: Generality of the Wavelength-Dependent Triene Photocyclization

Article abstract of DOI:10.1021/jo9622183

Two structural previtamin D₃ analogues (R = H, CH₃) cyclized photochemically with a 1.4-1.8-fold increase in quantum yields over a 3-nm change in excitation wavelength. The sudden increase in quantum yields is due to the participation and mixing of both 2A and 1B excited states. At λ ≤ 306 nm, the 1B state is initially excited and then (a) partitions between isomerization to the corresponding trans triene isomers, (b) decays to the 2A state to give the corresponding cyclohexadienes, and (c) decays to the ground state. At λ ≥ 309 nm, the 2A state is directly excited to give the corresponding cyclohexadienes and the relaxation path from 1B state to the ground state or isomerization in the 1B state is diminished.

Full text of DOI:10.1021/jo9622183

J . Org. Chem. 1997, 62, 9005-9008
9005
P h otoch em istr y of Str u ctu r a l An a logu es of P r evita m in D3:
Gen er a lity of th e Wa velen gth -Dep en d en t Tr ien e P h otocycliza tion
William G. Dauben, Boli Zhou, and J oe Y. L. Lam*,†
Department of Chemistry, University of California, Berkeley, California 94720
Received November 26, 1996X
Two structural previtamin D3 analogues (R ) H, CH3) cyclized photochemically with a 1.4-1.8-
fold increase in quantum yields over a 3-nm change in excitation wavelength. The sudden increase
in quantum yields is due to the participation and mixing of both 2A and 1B excited states. At λ e
306 nm, the 1B state is initially excited and then (a) partitions between isomerization to the
corresponding trans triene isomers, (b) decays to the 2A state to give the corresponding
cyclohexadienes, and (c) decays to the ground state. At λ g 309 nm, the 2A state is directly excited
to give the corresponding cyclohexadienes and the relaxation path from 1B state to the ground
state or isomerization in the 1B state is diminished.
In tr od u ction
state surfaces of pre D3 models.4 However, few chemical
experimental studies have been performed to test the
utility of the calculated surfaces and their interactions.
With the foregoing information at hand, it is of interest
to search for wavelength effects in other conjugated triene
systems. We report in this manuscript a photochemical
study of simple conjugated triene systems mimicking the
pre D3 system.
The photochemistry of previtamin D3 (pre D3) has been
intensively studied (Scheme 1).1 The quantum yields of
the photoproducts from previtamin D3 (pre D3) have been
measured at different wavelengths with monochromatic
light between 285 and 325 nm.2 While the quantum
yields for the cis-trans isomerization decrease with
increasing wavelength in the 285-325 nm range, the
quantum yields for formation of ring-closed products
increase, dramatically, in the same wavelength range.3a
This increasing photocyclization yield with decreasing
photoenergy has been attributed to the involvement of
both the 1B and 2A excited states of pre D3. This
hypothesis is supported by measurements of the pre D3
fluorescence specturm, fluorescence lifetime, wavelength
dependence of the fluorescence quantum yield, and
temperature dependence of fluorescence intensity. All
these data have been integrated into a potential energy
surface diagram that is consistent with both the photo-
chemical and spectroscopic behavior.3
Resu lts a n d Discu ssion
Syn th esis of P r e D3 Str u ctu r a l An a logu es 1 a n d
2. Trienes 1 and 2 were obtained by convergent synthe-
sis. A mixture of cis- and trans-1,2-di(1-cyclohexenyl)-
ethene were obtained in the reaction of 1-cyclohexenyl-
carbox-aldehyde5a (3a ) with triphenyl(1-cyclohexenyl-
methyl)phosphonium bromide6 in the presence of nBuLi.
The pure cis isomer 1a was obtained either by column
chromatography of the mixture of cis and trans isomers
or by photochemical sensitization7 of trans isomer 1b
with 9-fluorenone. Similarly, 2a was also obtained from
the reaction of 2-methyl-1-cyclohexenecarboxaldehyde5,6
(3b) with 4.
Since the postulation of the above-mentioned potential
energy surface diagram, much effort has been directed
toward obtaining detailed knowledge, by calculation, of
the potential energy surfaces of both excited- and ground-
* Current address: PE Applied Biosystems, 850 Lincoln Centre Dr.,
Foster City, CA 94404.
† Dedicated with love and appreciation to Professor William G.
Dauben, deceased.
X Abstract published in Advance ACS Abstracts, November 15, 1997.
(1) General reviews of the photochemistry related to the previtamin
D3 system: (a) Dauben, W. G.; McInnis, D. M. In Rearrangements in
Ground and Excited States; de Mayo, P., Ed.; Academic Press: New
York, 1980; Vol. 3, p 81. (b) J acobs, H. J . C.; Havinga, E. In Advances
in Photochemistry; Pitts, J . N., J r., Hammond, G. S., Gollnick, K., Eds.;
Interscience Publishers: New York, 1979; Vol. 11, p 305. (c) Laarhoven,
W. H. In Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker: New
York, 1987; Vol. 9, p 129. (d) Norman, A. W. Vitamin D; Academic
Press: New York, 1979. (e) Dauben, W. G.; Phillips, R. B. J . Am. Chem.
Soc. 1982, 104, 355. (f) Kobayashi, T.; Yasumura, M. J . Nutr. Sci.
Vitaminol. 1973, 19, 123. (g) Sato, T.; Yamachuchi, H,; Ogata, Y.; Kunii,
T.; Kagei, K.; Katsui, G.; Toyoshima, S.; Yasumura, M.; Kobayashi, T.
J . Nutr. Sci. Vitaminol. 1980, 26, 545. (h) Barton, D. H. R.; Hesse, R.
H.; Pechet, M. M.; Rizzardo, E. J . Am. Chem. Soc. 1973, 95, 2748. (i)
Pfoertner, K.; Weber, J . P. Helv. Chim. Acta 1972, 55, 921. (j) Pfoertner,
K. Helv. Chim. Acta 1972, 55, 937. (k) J acobs, H. J . C.; Gielen, J . W.
H.; Havinga, E. Tetrahedron Lett. 1980, 21, 4013. (l) Andrews, J . R.;
Hudson, B. S. Chem. Phys. Lett. 1979, 60, 380. (m) Pierce, B. M.;
Bennett, J . A.; Birge, R. R. J . Chem. Phys. 1982, 77, 6343.
(2) Dauben, W. G.; Phillips, R. B. J . Am. Chem. Soc. 1982, 104, 5781.
(3) (a) Dauben, W. G.; Disanayaka, B.; Funhoff, D. J . H.; Kohler, B.
E.; Schilke, D. E.; Zhou, B. J . Am. Chem. Soc. 1991, 113, 8367. (b)
Dauben, W. G.; Share, P. E.; Ollmann, R. R., J r. J . Am. Chem. Soc.
1988, 110, 2548.
P h otoch em ica l Rea ction s of Con ju ga ted Tr ien e
Ch r om p h or e. The cis triene at a concentration of 5 ×
10-3 to 1 × 10-2 M in THF was purged with nitrogen in
a quartz tube sealed with a septum and irradiated with
a Rayonet apparatus at 300 nm. The same photochemi-
cal reactions as those found for pro D3 have been observed
with model trienes 1a and 2a upon irradiation, cis-trans
isomerization around the central double bond, and the
(4) (a) Bernardi, F.; Olivucci, M.; Ragazos, I. N.; Robb, M. A. J . Am.
Chem. Soc. 1992, 114, 8211. (b) Celani, P.; Ottani, S.; Olivucci, M.;
Bernardi, F.; Robb, M. A. J . Am. Chem. Soc. 1994, 116, 10141.
(5) (a) Courtin, J . M. L.; Verhagen, L.; Biesheuvel, P. L.; Lugtenburg,
J .; van der Bend, R. L.; van Dam, K. Recl. Trav. Chim. Pay-Bas 1987,
106, 112. (b) Harding, K. E.; Ligon, R. C. Synth. Commun. 1974, 4(5),
297.
(6) Inhoffen, H. H.; Irmscher, K. Chem. Ber. 1956, 89, 1833.
(7) Snoeren, A. E. C.; Doha, M. R.; Lugtenburg, J .; Havinga, E. Recl.
Trav. Chim. Pays-Bas 1970, 89, 261.
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9006 J . Org. Chem., Vol. 62, No. 26, 1997
Dauben et al.
Sch em e 1
F igu r e 2. Wavelength-Φ profiles for the formation of 1c from
1a (squares) and 2c from 2a (circles) in ether.
F igu r e 1. Plot of relative percent composition of cis-triene
1a (circles), trans-triene 1b (triangles), and the diene 1c
(squares) versus time upon irradiation of 1a at 300 nm in THF.
Wa velen gth Dep en d en ce of P h otoch em ica l Qu a n -
tu m Yield s. The quantum yields were measured by an
electronic quantacount apparatus and a pulsed UV dye
laser, which provide a narrow-band tunable excitation
source with reasonable intensity.3 The quantum yields
of ring closure (1a f b1c and 2a f 2c) and cis-trans
isomerization (1a f 1b and 2a f 2b) were measured in
both diethyl ether and hexane. The numerical results
of Φ(1a f 1c), Φ(2a f 2c),Φ(1a f 1b) and Φ(2a f 2b)
are reported in the Supporting Information, and a typical
plot of quantum yield versus wavelength of 1a and 2a in
diethyl ether is shown in Figure 2.
The quantum yields over a wavelength of 296-312 nm
for the cis-trans isomerization, i.e., 1a f 1b and 2a f
2b, are more efficient than the ring closure reaction, i.e.,
1a f 1c and 2a f 2c. The quantum yield ranges from
Φ 0.42 to 0.51 for 1a and Φ 0.35 to 0.46 for 2a in cis-
trans isomerization about the central double bond, and
the quantum yield for ring-closure ranges from Φ 0.05
to 0.2 for 1a and Φ 0.10 to 0.44 for 2a in ether as the
solvent. The data show that, within experimental error
((10%), the quantum yields for cis-trans isomerization
about the central double bond is independent of wave-
lengths (296-312 nm). The ring-closure quantum yields
for 1a and 2a increase slowly from 296 to 306 nm and
then jump sharply from 306 to 309 nm with a 1.4-fold
to1.8-fold increase.
Sch em e 2
photocyclization via Woodward-Hoffmann rules (Scheme
2).8 During the irradiation, the photoproducts experience
a buildup up period and then reach a quasiphotostation-
ary state with the reactants, which is characteristic and
indicative of reversible photochemical processes.1d,9 At
the quasiphotostationary state, the ratio of 1b:1a :1c is
3:2:1 and the ratio of 2b:2a :2c is 2:5:1. The progress of
the reaction is illustrated in Figure 1 for compound 1a
with the relative percent composition of 1a , 1b, and 1c
versus time upon irradiation. It is interesting to note
that the reversible photochemical transformations of
these simple structural analogues very closely resemble
the reversible photochemical interconversions of pre D3
series.
(8) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl.
1969, 8, 814.
Mech a n ism of Rin g Closu r e. The quantum yields
for ring closure from pre D3 to pro D3 increase slowly from
(9) Rappoldt, M. P.; Havinga, E. Recl. Trav. Chim. Pays-Bas 1960,
79, 369.

Structural Analogs of Previtamin D3
J . Org. Chem., Vol. 62, No. 26, 1997 9007
285 to 325 nm (Φ 0.01 to 0.08) with a sudden 2-fold
increase (Φ 0.02 to 0.04) from 302 to 306 nm. With
evidence from the previtamin D3 fluorescence spectrum,
fluorescence lifetime, wavelength dependence of the
fluorescence quantum yield, and temperature dependence
of fluorescence intensity studies, the 2-fold increase in
quantum yields from pre D3 to pro D3 has been inter-
preted as the participation of both 2A (S1) and 1B (S2)
excited states in the phototransformation.3,10 The two
structural analogues 1a and 2a bearing the same triene
chromophore display a similar wavelength-quantum
yield profile for the photochemical ring closure (see
Figure 2) as the pre D3 to pro D3, including a 1.4-1.8-
fold increase in the quantum yields for the conversion of
1a f 1c and 2a f 2c between the excitation wavelengths
of 306 and 309 nm. This finding points to the fact that
in the ring-closure process 1a and 2a also react photo-
chemically by the participation of both the low-lying 2A
(S1) and 1B (S2) excited states. Schematically, the
potential surfaces of 2A and 1B states along the conro-
tatory coordinate of compounds 1a and 2a are similar to
that of the pre D3 potential surface model.3 With the
wavelength shorter than 306 nm, excitation occurs at the
higher energy 1B (S2) allowed state11e and eventually
partitions between (a) cis-trans isomerization such as
(1a f 1b) or (2a f 2b), (b) decay to the 2A surface giving
ring-closure products, or (c) decay to the starting ground
state. Above 306 nm, the mixing3,11 of 2A and 1B states
intensifies the extinction coefficient for absorption to the
2A state and allows the molecules, preferentially, to be
excited to the 2A surface at the red edge of the absorption
band and so the quantum yield for ring-closure slowly
increases from 296 to 306 nm. At wavelengths longer
than 309 nm, the 2A state is directly exicited to give ring
closure product 1c and 2c in high yield, and the relax-
ation path from the 1B (S2) state to the ground state or
isomerization to 1b or 2b in the 1B (S2) state is dimin-
ished.
1a and 2a , over a 3-nm change in excitation wavelength
is due to the participation and mixing of both 2A and 1B
excited states. At λ e 306 nm, the 1B state is initially
excited and then (a) partitions between isomerization to
1b and 2b, (b) decays to the 2A state to give 1c and 2c,
and (c) decays to the ground state. At λ g 309 nm, the
2A state is directly excited to give 1c and 2c, and the
relaxation path from 1B state to the ground state or
isomerization in the 1B state is diminished.
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR (400 MHz) and 13C NMR
(100.6 MHz) spectra were measured in CDCl3 solutions
calibrated with MeSi4. Et2O and THF were distilled from
sodium and benzophenone. UV spectra were run in hexane
unless otherwise stated. Solvents used in irradiation were
spectroscopic grade. Unless otherwise stated, other solvents
and reagents were reagent-grade commercial materials used
as supplied. Flash chromatography was done on 230-400
mesh silica gel. Irradiation mixtures were separated by using
preparative TLC silica plates (20 × 20, 1000 microns). Ana-
lytical GLC was performed on a carbowax capillary column
(30 m × 0.25 mm). Solvent evaporation after extraction was
carried out under reduced pressure using a Rotovap apparatus.
tr a n s-1,2-Di(1-cycloh exen yl)et h en e (1b ). 1-Cyclohex-
enylcarboxaldehyde5a (3a ) (3.0 g, 27 mmol) was added to a
solution of triphenyl(1-cyclohexenylmethyl)phosphonium bro-
mide6 (4) (9.0 g, 21 mmol), n-butyllithium (11 mL, 2.3 M in
hexane, 25 mmol), and THF (40 mL) with stirring at 0 °C. The
reaction mixture was refluxed for 3 h, and then water was
added. The reaction mixture was extracted with petroleum
ether. The organic portion was separated and dried (MgSO4).
The solvent was evaporated, and the crude residue was then
chromatographed with petroleum ether to give a pale yellow
oil consisting of trans-1,2-di(1-cyclohexenyl)ethene (2.0 g, 52%)
and cis-1,2-di(1-cyclohexenyl)ethene (0.2 g, 5%). Trans iso-
mer: IR νmax 2950, 1644, 1459, 1448, 1141, 962 cm-1; 1H NMR
δ 1.62 (m, 8H), 2.14 (m, 8H), 5.73 (m, 2H), 6.13 (s, 2H); 13C
NMR δ 22.6, 22.7, 24.6, 26.0, 128.6, 128.8, 136.0, MS m/z 188,
145 (base), 117,105, 91, 77; UV λmax (ꢀ) 259 (20 570), 268
(24 900), 280 (20 000).
When 1b and 2b were irradiated in the presence of
9-fluorenone as a triplet sensitizer, a mixture of cis and
trans isomers was obtained and there was no sign of any
ring-closure product being observed. This points to the
fact that the triplet state is only limited to cis/trans
isomerization and the photochemical ring closure reaction
for 1a f 1c and 2a f 2c occurs in the singlet excited
state.1b,c The sudden increase in quantum yields in the
ring-closure closure reaction within 306-309 nm could
not be due to the ground-state conformational effect
because the molar extinction coefficient of 1a and 2a
decreased by 36% for 1a and by 29% for 2a within that
narrow range.3a
cis-1,2-Di(1-cycloh exen yl)eth en e (1a ). A solution of
trans-1,2-di(1-cyclohexenyl)ethene (0.2 g, 1.1 mmol), 9-fluo-
renone (0.58 g, 3.2 mmol), hexane (150 mL), and benzene (25
mL) contained in a Pyrex tube was sealed with a rubber
septum at 0 °C and purged with N2. The solution was
irradiated in a Rayonet apparatus equipped with 350 nm
lamps through a Uranium glass filter for 3 h (N2 purging).
The solvent was evaporated, and the crude residue was
chromatographed with petroleum ether to give a pale yellow
oil of cis-1,2-di(1-cyclohexenyl)ethene (0.08 g, 40%) and trans-
1,2-di(1-cyclohexenyl)ethene (0.07 g, 35%). The spectral data
for 1b were same as above. Cis isomer (1a ): IR νmax 2952,
1
1643, 1459, 1447, 1138, 922 cm-1; H NMR δ 1.57, (m, 8H),
2.09 (m, 8H), 5.60 (m, 2H), 5.70 (s, 2H); 13C NMR δ 22.2, 22.9,
25.6, 28.4, 127.6, 130.7, 136.4; MS m/z 188, 145 (base), 131,
117, 105, 91, 77; UV λmax (ꢀ) 246 (12 840).
Con clu sion s
tr a n s-2-(1-Cycloh exen yl)-1-(2-m eth yl-1-cycloh exen yl)-
eth en e (2b).6 The reaction procedure was as described above.
2-Methyl-1-cyclohexene carboxaldehyde5b (3b) (0.9 g, 7.3 mmol)
was allowed to react with (1-cyclohexenylmethylidene)triph-
enylphosphorane to give trans-2-(1-cyclohexenyl)-1-(2-methyl-
1-cyclohexenyl)ethene (0.8 g, 55%): IR νmax 2950, 1643, 1468,
The 1.4-1.8-fold increase in quantum yields of photo-
chemical cyclization of two structural pre D3 analogues,
(10) Alternatively, Fuss, Olivucci, and Robb (Fuss, W.; Lochbrunner,
S. J . Photochem., submitted) have studied the reaction route that
occurs once the excited reactant is on the lower lying excited state (2A,
S1 state) and have suggested the conical intersection mechanism to
explain the wavelength dependence of the quantum yields of pre D3.
The excited pre D3 enters two different conical intersection points
where ring closure to pro D3 competed with the double bond isomer-
ization to Tachy3 that are of different barriers (Bernardi, F.; Olivucci,
M.; Robb, M. Isr. J . Chem. 1993, 33, 265).
(11) (a) Granville, M. F.; Kohler, B. E.; Snow, J . B. J . Chem. Phys.
1981, 75, 3765. (b) Ackerman, J . R.; Kohler, B. E. J . Chem. Phys. 1982,
77, 3967. (c) Kohler, B. E. J . Chem. Phys. 1990, 93, 5838. (d) Buma,
W. J .; Kohler, B. E.; Song, K. J . Chem. Phys. 1991, 94, 4691. (e) Kohler,
B. E. Chem. Rev. 1993, 93, 41.
1
1459, 1144, 961 cm-1; H NMR δ 1.63 (m, 8H), 1.77 (s, 3H),
2.14 (m, 8H), 5.76 (m, 1H), 6.19, 6.61 (AB quartet, 2H, J ) 16
Hz); 13C NMR δ 19.4, 22.6, 22.7, 22.9, 23.0, 24.6, 25.5, 26.1,
33.1, 123.6, 127.9, 128.6, 129.0, 132.7, 136.4; MS m/z 202, 187,
159, 145 (base), 131, 117, 91, 77; UV λmax (ꢀ) 268 (34 310), 278
(42 100), 289 (31 750).
cis-2-(1-Cycloh exen yl)-1-(2-m et h yl-1-cycloh exen yl)-
eth en e (2a ). The sensitized photochemical isomerization
procedure was as described above. trans-2-(1-cyclohexenyl)-
1-(2-methyl-1-cyclohexenyl)ethene (2b) (0.25 g, 1.2 mmol) was

9008 J . Org. Chem., Vol. 62, No. 26, 1997
Dauben et al.
irradiated in a solution of 9-fluorenone (0.6 g, 3.3 mmol),
hexane (150 mL), and benzene (25 mL) to give cis-2-(1-cyclo-
hexenyl)-1-(2-methyl-1-cyclohexenyl)ethene (0.15 g, 55%): IR
(m, 2H); 13C NMR δ 16.1, 22.6, 22.7, 23.9, 25.0, 25.4, 26.7, 32.7,
37.6, 40.5, 48.0, 117.4, 117.5, 128.4, 139.2; GC-MS m/z 202,
187, 159, 145 (base), 131, 117, 91, 77 (1H NMR and 13C NMR
corrected for the presence the triene).
1
νmax 2950, 1642, 1457, 1445, 1143, 855 cm-1; H NMR δ 1.55
(m, 11H), 2.02 (m, 8H), 5.68 (m, 2H), 5.80 (AB quartet, 1H, J
) 12 Hz); 13C NMR δ 20.6, 22.3, 23.0, 23.11, 23.13, 25.9, 26.8,
29.8, 31.3, 128.47, 128.52, 128.7, 129.6, 132.1, 136.8; MS m/z
202, 187, 159, 145 (base), 131, 117, 91, 77; UV λmax (ꢀ) 238
(14 980).
Kin etic Ir r a d ia tion . The relative concentration vs time
plot was obtained by irradiating a solution of the corresponding
trienes (5 × 10-3-1 × 10-2 M), hexadecane (internal standard;
1 × 10-3 M), and THF in a Rayonet at 300 nm in a quartz
tube sealed with septum. Aliquots of the samples were taken
out with syringes and analyzed by GC, and the relative
percentages of cis and trans triene and cyclohexadiene were
ploted vs time. The quantum yields were measured by an
electronic quantacount apparatus equipped with either a 1000
W HP lamp or a pulsed UV dye laser and were calibrated by
potassium ferrioxalate actinometry.3
P h otoch em ica l Rea ction of cis-1,2-Di(1-cycloh exen yl)-
eth en e (1a ). A solution of 1a (0.2 g, 1.1 mmol) and hexane
(120 mL) in a Pyrex tube purged with nitrogen was irradiated
at 0 °C in a Rayonet apparatus with 300 nm lamps. The
solvent was evaporated, and the crude residue was purified
by using preparative TLC with petroleum ether to give an oil
(0.12 g, 60%). The 1H NMR spectrum showed a mixture of
1a , 1b, and 1c in a ratio of 28:10:15. The spectral data of 1a
and 1b were the same as above. 1,2,3,4,4a,4b,5,6,7,8-Decahy-
drophenanthrene (1c): 1H NMR δ 1.33-2.18 (m, 18H), 5.38
(s, 2H); 13C NMR δ 22.6, 28.7, 34.8, 37.2, 44.1, 115.2, 139.2;
GC-MS m/z 188, 145 (base), 131, 117, 105, 91, 77 (1H NMR
and 13C NMR corrected for the presence the triene).
Ack n ow led gm en t. This research was supported by
National institute of Diabetes and Digestive and Kidney
Disease, PHS Grant 00709.
Su p p or tin g In for m a tion Ava ila ble: UV spectra of com-
pounds 1a and 2a and a table of quantum yields (Φ) of 1b
and 1c from 1a and 2b and 2c from 2a in hexane and ether
are available (3 pages). This material is contained in libraries
on microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
P h otoch em ica l Rea ction of cis-2-(1-Cycloh exen yl)-1-
(2-m eth yl-1-cycloh exen yl)eth en e (2a ). A solution of 2a (0.2
g, 1.0 mmol) and hexane (120 mL) was irradiated at 300 nm
in a Rayonet apparatus as was described above to give an oil
1
(0.13 g, 65%). The H NMR spectrum showed a mixture of 2a
and 2c in a ratio of 1:1. The spectral data of 2a were the same
as above. 1,2,3,4,4b,5,6,7,8-Nonahydro-4a-methylphenan-
threne (2c): 1H NMR δ 0.86 (s, 3H), 1.24-2.35 (m, 17H), 5.46
J O9622183