Photoisomerization of alfa calcidol by a sensitized quantum chain reaction

Article abstract of DOI:10.1111/j.1751-1097.2011.01054.x

The production of vitamin D3 is a pharmaceutically relevant process, producing high added-value products. Precursors are extracts from vegetal origin but bearing mainly an E geometry in the 5,6 double bond. The synthesis of vitamin D3 (5-E-α-calcidol) wit

Full text of DOI:10.1111/j.1751-1097.2011.01054.x

Photochemistry and Photobiology, 2012, 88: 769–773
Photoisomerization of Alfa Calcidol by a Sensitized Quantum
Chain Reaction†
´
Gaston A. Estruch and Pedro F. Aramendıa*
´
´
´
INQUIMAE and Departamento de Quımica Inorganica, Analıtica y Quımica Fısica, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria,
´
´
´
´
1428 Ciudad de Buenos Aires, Argentina
Received 1 October 2011, accepted 22 November 2011, DOI: 10.1111/j.1751-1097.2011.01054.x
pharmaceutically relevant process, producing high added-
value products.
ABSTRACT
The production of vitamin D3 is a pharmaceutically relevant
process, producing high added-value products. Precursors are
extracts from vegetal origin but bearing mainly an E geometry in
the 5,6 double bond. The synthesis of vitamin D3 (5-E-a-calcidol)
with the correct Z stereochemistry in the 5,6 double bond from
the E isomer using anthracene and triethylamine (TEA) as the
sensitizer system was studied from the kinetic and mechanistic
point of view. The sensitized isomerization of E-calcidol by
irradiation of anthracene takes place only in deoxygenated
solution and yields the Z isomer in ca 5% yield in the
photostationary state. When TEA is added to the system, the
E–Z reaction is not inhibited by oxygen any more, the quantum
yield of photoisomerization to the Z isomer grows linearly with
the concentration of E-calcidol, while conversions higher than
95% to the Z isomer are reached in the photostationary state
and E–Z quantum yields as high as 45 at [E-calcidol] = 25 mM
are reached. If TEA is replaced by 1,4-diazabicyclo[2.2.2]octane,
the reaction rate drops to one-third at the same amine
concentration. The observations can be explained by a quantum
chain reaction mechanism. The high conversion achieved elim-
inates the need of isomer separation.
Industrially, the photosensitized irradiation to produce
Vitamin D photoisomerization has two purposes: on one side,
it produces an increase in the total yield of product by
adequately choosing the triplet photosensitizer to increase the
steady state concentration of the isomer with the correct
stereochemistry. On the other side, it allows the use of UVA
light from mercury sources.
In 1987, Calverley reported the synthesis of Vitamin D3
with the correct Z stereochemistry in the 5,6 double bond from
the E isomer (Scheme 1) using anthracene and triethylamine
(TEA) as sensitizer system and irradiating the mixture at
365 nm (8). Direct excitation of vitamin D implies the use of
254 nm low pressure mercury sources and quartz vessels,
whereas the photosensitized reaction can be performed with
normal mercury lamps and using Pyrex reactors. Being
photons are normally an expensive reactant, chemical ampli-
fication of its effect is a desirable process, this being achieved
by a chain reaction mechanism.
Photoisomerization reactions by a chain mechanism were
reported by Hammond et al. in 1969 for the isomerization of
2,4-hexadiene (9). Subsequent work by Saltiel and Liu et al.
established the mechanism of this reaction in dienes and
polyenes (10–12). A triplet state is the relay responsible for the
propagation in these cases.
INTRODUCTION
Early work on the isomerization of stilbene under pulse
radiolysis conditions established a chain mechanism (13,14).
The radical anion of trans-stilbene (Strans)Æ) is the reactive
species in this case, propagating the chain by selectively
producing the trans isomer (Strans) upon reaction with a cis-
stilbene (Scis) molecule (Eq. 1).
The vitamin D family of compounds is crucial for calcium
fixation and metabolism. Low vitamin D levels are linked to
rickets, bone mass loss, multiple sclerosis, hypertension, breast
cancer, prostate cancer, colorectal cancer, insulin dependent
diabetes and schizophrenia (1,2). For these reasons, vitamin D
and its precursors are added to different kinds of foods.
Biologically and in industry, vitamin D and its derivatives
are produced photochemically (3–7) by pericyclic and photo-
induced isomerization reactions. The first reaction involves the
ring aperture in the C9–C10 bond of the 7-dehydrocholesterol,
present in animal skin. This ring aperture is followed by
photosensitized E–Z isomerization to produce the biologically
active compound for human use. The production of
hydroxylated vitamin D derivatives, such as Vitamin D3, is a
ꢀ
Strans þ Scis ꢀ! Strans þ Stꢀrans
ð1Þ
Æ
Æ
This mechanism leads to a stationary state with practically
complete conversion of the cis to the trans isomer. This fact is
in contrast with triplet sensitized reactions of stilbenes. In this
latter case, the proportion of the two isomers under photosta-
tionary state approaches more balanced situations (ca 50% of
each compound), unless there is a great energy difference
between the triplet states of the cis and trans isomers and a
sensitizer is chosen that can sensitize preferentially only the
lower energy triplet.
†This paper is part of the Special Issue on the 21th Conference of the IAPS.
*Corresponding author email: pedro@qi.fcen.uba.ar (Pedro F. Aramendıa)
ꢀ 2011 Wiley Periodicals, Inc.
Photochemistry and Photobiology ꢀ 2011 The American Society of Photobiology 0031-8655/12
769

´
770 Gaston A. Estruch and Pedro F. Aramendıa
´
regulated by water circulation from a thermostat. Anthracene concen-
tration was adjusted in the millimolar range to provide an absorbance
of ca 2 in 1 cm at 365 nm to assure complete light absorption
(to obtain an estimate of the anthracene concentration, we assume that
the absorption coefficient at 365 nm in toluene is equal to the value in
hν
)1
cyclohexane eAnthr [365 nm, cyclohexane] = 2400 M cm)1) (http: ⁄ ⁄
6
6
omlc.ogi.edu ⁄ spectra ⁄ PhotochemCAD ⁄ html ⁄ index.html). Irradiation
was carried out in vigorously stirred samples.
5
5
Flash photolysis. Deactivation of the triplet state of anthracene by
TEA and E was monitored at 420 nm after 354 nm excitation of Ar-
bubbled anthracene solutions containing variable amounts of quencher
using an apparatus described earlier (20).
OH
OH
OH
OH
5Z-α calcidol
5E-α calcidol
Scheme 1. E-Z photoisomerization of 5E-a-calcidol.
RESULTS
Mizuno and coworkers studied the cis–trans reactions of
diarylcyclopropanes photosensitized by 9,10-dicyanoanthra-
cene (15,16). The reaction occurs by a chain mechanism
initiated by electron transfer from the sensitizer to the
cyclopropane, in the presence of additives, such as salts and
aromatic hydrocarbons. The radical cation of the cyclopro-
pane is the chain carrier. Also in the isomerization of
stilbazolium salts, the chain carrier is a reduced radical
resulting from electron transfer from Ru(II)(tris-bipyridyl)
(17). Some olefins, typically but not exclusively 2-styrylanthra-
cene, upon direct excitation or upon photosensitization, react
from the cis to the trans isomer by a quantum chain mechanism
where the triplet of the trans isomer is the chain carrier (18,19).
High quantum yields for photoisomerization, increasing
with the concentration of the substrate, and almost a complete
conversion to one of the photoisomers in the photostationary
state are facts indicative of a one-way photoisomerization via a
quantum chain reaction. These reactions can proceed with ion-
radicals or with triplets as the chain-propagating species.
In this work, we analyze the sensitized photoisomerization
of 5E-a-calcidol, as proposed by Calverely (8). We find that the
reaction proceeds through a charge transfer chain mechanism.
With quantum yield higher than 40 for the build up of the 5Z
isomer, a conversion higher than 95% to this isomer can be
achieved by irradiation at 365 nm. The high conversion
attained eliminates the need of isomer separation.
The photosensitized E fi Z isomerization of the C5–C6
double bond of a-calcidol in the presence of anthracene (An)
takes place upon steady-state 365 nm irradiation only in Ar-
bubbled samples. The system reaches a photostationary state
with a 5% conversion to the Z isomer under these conditions
(Fig. 1). Triplet anthracene is the photochemically active
excited state in this case.
When TEA is added to the previous mixture, the reaction
proceeds at the same speed in aerated or in Ar-bubbled
samples and conversion to the Z isomer as high as 95% can be
achieved. Figure 2 displays the kinetics of the early stages of
the reaction. Under these conditions, the isomerization is
triggered by the product of the interaction of excited singlet
anthracene and TEA.
The deactivation of the first excited singlet state of anthra-
cene (1An) by TEA and by E takes place with Stern-Volmer
constants of 7.6 and 64 M)1, respectively (Fig. 3). Considering
the measured singlet state lifetime of 3.3 ns for anthracene,
1
quenching rate constants for deactivation of An by TEA of
)1
)1
(2.8
0.2) 109
M
s)1, and by E of (2.3
0.2) 1010
M
s)1
can be obtained. These rate constants correspond to quenching
rate constants kQ1 and kQ2 of Scheme 2 (see next).
On the other hand, the anthracene triplet (3An) is not
quenched by TEA at the concentrations at which 1An quench-
ing takes place by this compound, whereas 3An is quenched by
)1
E at a rate of (5
1) 109
M
s)1 (results not shown).
MATERIALS AND METHODS
Chemicals. 5E-a-calcidol (E) was prepared as suggested by Calverley
(8), >98% checked by HPLC. Toluene (99.95% for HPLC; Backer),
TEA (HPLC, ‡99.5% GC; Fluka), DABCO (Aldrich), and anthracene
(99.9%; Aldrich) were used as received.
Reaction progress. The reaction progress was followed by HPLC
analysis, performed in a Varian equipment using Kromasil C18, 100-5,
4.6 · 250 mm column, acetonitrile-water (80:20) 2 mL min)1, and
detection at 254 nm. Elution times were 16 and 18 min for 5Z and 5E
a-calcidol, respectively.
Spectroscopy. Absorption spectra were recorded on a Shimadzu
3101 PC or on a Shimadzu 160 UV spectrophotometer. Steady-state
fluorescence emission and fluorescence quenching was measured on a
PTI Quantamaster spectrofluorimeter. Time-resolved emission decays
were recorded on a PTI Timemaster apparatus.
Steady-state irradiation. A UVLED 365-10 from Roithner (365 nm
maximum emission wavelength, 10 nm FWHM emission) was used as
light source for steady state irradiation. Photon flux was measured
with a calibrated photodiode and a Labmaster Ultima detector
(Coherent). Reaction progress was determined by HPLC analysis of
E and Z isomers from an extracted aliquot of the sample, as described
in the Reaction progress section. The sample was contained either in a
10 · 10 mm quartz standard absorption cuvette or in a quartz cuvette
of similar size provided with a septum when gas bubbling was
necessary. When required, the cuvette holder temperature was
Figure 1. Fractional build up of Z isomer as a function of time upon
E fi Z isomerization of 5E-a-calcidol (7 mM) sensitized by anthra-
cene in Ar-bubbled toluene solution by irradiation at 365 nm. The line
is to guide the eye.

Photochemistry and Photobiology, 2012, 88 771
The reaction proceeds in the same way if TEA is replaced by
DABCO, even though the efficiency drops to one-third under
otherwise identical conditions (Fig. 4).
Figure 5 shows the dependence of the quantum yield of the
E fi Z isomerization under irradiation with a radiant power
of 0.78 mW at 365 nm (equivalent to a photon absorbed rate
of 0.79 lM s)1) of solutions of E containing anthracene and
Ar
8
6
4
2
0
a
Air
b
10 mM TEA. The photoisomerization quantum yield (/E fi Z
)
increases almost linearly with the concentration of E and
values as high as 50 can be reached at [E] > 25 mM. This fact
points undoubtedly to a chain reaction.
c
DISCUSSION
The chain reaction responsible for the photosensitized
E fi Z isomerization of 5E-a-calcidol is initiated by the
interaction of An and the amine (TEA in the majority of our
0
2
4
6
8
10 12 14 16 18
Time (min)
1
Figure 2. Build up of the Z isomer as a function of time upon 365 nm
irradiation in toluene of samples containing 10 mM 5E-a-calcidol,
10 mM TEA and anthracene. Full symbols and continuous line: Ar-
bubbled samples; open circles and dashed line: air saturated samples.
The lines are drawn to guide the eye. Chromatograms of E and Z
isomers for a Z conversion proportion of <1% (a), 54% (b) and 98%
(c), respectively.
experiments). The product of the appointed reaction is the
oxidized TEA radical and the reduced anthracene radical
(reaction 3 of Scheme 2). This reaction is exergonic for 1An
and endergonic for 3An. The reaction initiation from the
singlet state explains the absence of the molecular oxygen
inhibiting effect. Furthermore, two facts point to a quantum
chain one way isomerization reaction: the high values of the
/E fi Z and the high (almost complete) conversions attained in
the photostationary state. The active chain carrier in this case
is postulated to be a twisted cation radical of the polyene
(Tw+·) produced by the reaction of TEA+· and E (reaction 5
of Scheme 2). Upon reaction of Tw+· with an E isomer,
electron transfer takes place from this species to the cation,
regenerating the chain carrier and yielding also a Z isomer
(reaction 6 of Scheme 2). This reaction is almost energetically
neutral taking into account that E and Z are almost symmetric
around the C5–C6 double bond.
Scheme 2 summarizes the mechanism. Singlet anthracene is
efficiently quenched by 5E-a-calcidol, in fact this quenching is
more efficient than the quenching of An by TEA. Neverthe-
1
less, if the quenching of singlet anthracene by E-a-calcidol
would lead to the build up of the cation radical of the polyene
in an appreciable amount, the determinant role of the amine in
the reaction cannot be explained. Neither is it expected that
anthracene could have a catalytic effect mediating the energy
transfer between the excited states of E and Z isomers as in
1
1) An + hυ ⎯⎯→ An
I
a
1
1
2 ) An ⎯⎯→ decay
(1/τ ).[ An]
f0
1
−•
+•
1
3) An + TEA⎯⎯→ An + TEA
k
k
[TEA][ An]
Q1
Q2
1
1
4) An + E ⎯⎯→ An + E
[E][ An]
+•
+•
+•
5) E + TEA ⎯⎯→Tw + TEA
k [E][TEA
]
tr
+•
+•
+•
6) Tw + E ⎯⎯→ Z + Tw
k [Tw ][E]
r
+•
-•
+•
-•
Figure 3. Stern-Volmer plots for the deactivation of 1An (A) by E and
(B) by TEA in toluene. Insets show the emission spectrum of
anthracene alone (full line) and in the presence of 10 mM of the
respective quencher (dashed line).
7) Tw + An ⎯⎯→ An + E or Z k [Tw ][An ]
t
Scheme 2. Reaction mechanism proposed for the photosensitized
quantum chain reaction.

´
772 Gaston A. Estruch and Pedro F. Aramendıa
´
Figure 6. Representation of the data of Fig. 5 according to the kinetic
relation derived from the mechanism of Scheme 2 and represented in
Eq. (3).
Figure 4. Build up of the Z isomer upon 365 nm irradiation in aerated
toluene solutions of 5E-a-calcidol 10 mM, with 10 mM amine (full
symbols: TEA; open symbols: DABCO) in the presence of anthracene.
ꢀ
ꢁꢀ1=2
kQ1½TEAꢁ
fðIa; TEA; EÞ ¼ /E!Z
1=sf0 þ kQ1½TEAꢁ þ kQ2½Eꢁ
ð3Þ
kr
¼
ƽEꢁ:
I1a=2Æk1t =2
The good linear correlation obtained: f(Ia, TEA, E) =
()30 140) + (2.8
1.0) · 104 )1.[E], with zero intercept
M
under the experimental uncertainty, demonstrates that the
mechanism accounts satisfactorily for the kinetics observed.
The competition of the E isomer and TEA lowers the chain
initiation efficiency (see next). Scheme 2 assumes that all the
quenching events of 1An by TEA (reaction 3) lead to free
diffusing ions. In toluene, this is certainly not the case, which
adds a factor to the rate of reaction (3) to account for the
fraction of quenching events that lead to separated ions. This
fact further contributes to lower the chain initiation efficiency
but does not alter the rate equation (Eqs 2 and 3). In the
reaction mechanism, the low efficiency of production of chain
carriers is compensated by the favorable competition of
reaction (6), the chain propagation, with reaction (7), the
chain termination. The low efficiency of production of the
chain carriers is given by the high quenching rate of 1An by 5E-
Figure 5. Quantum yield for the E fi Z isomerization of 5E-a
calcidol (/E fi Z) as a function of the reactant concentration in the
presence of anthracene and 10 mM TEA. Irradiation at 365 nm
(0.78 mW) in aerated toluene solution.
a-calcidol. In fact, at [E-a-calcidol] = 25 mM, and
M
previously reported examples (21). So considering the deter-
minant role of the amine, it is postulated that TEA reacts with
1An by electron transfer (reaction 3) and that the cation radical
of the amine reacts readily with the polyene to produce the
cation radical of this later species (Tw+· in Scheme 2). This
species is the chain carrier (17) (reaction 6). Reaction (5)
accounts for the very low concentration of TEA+·. Chain
termination takes place via reaction of the most abundant
anion and cation species.
)1
[TEA] = 10 mM, considering the values of 2.8 · 109
M
s)1
,
)1
and 2.3 · 1010
s)1 for kQ1 and kQ2, respectively, and
3.0 · 108s)1 for 1 ⁄ sf0, we can calculate that only 3% of 1An are
quenched by TEA. Under these conditions, /E fi Z = 43
(Fig. 5), so the chain length (defined as the rate of step 6
divided by the rate of step 7 in Scheme 2) is ca 1500 (assuming
that all quenching events by TEA lead to separated ions;
otherwise, it is higher). At [E-a-calcidol] = 1 mM, /E fi Z = 6
and the chain length is 73.
Under steady-state conditions, the mechanism of Scheme 2
leads to the following expression for /E fi Z Eq. (2).
Finally, the reaction is possible in such a long chain length
because the two isomers do not differ much in energy, due to
the almost symmetric geometry with respect to the C5–C6
double bond.
The system appears as an excellent candidate for industrial
preparation of vitamin D derivatives. The reaction can be
carried out at room temperature under aerated conditions. A
ꢀ
ꢁ
1=2
kr ½Eꢁ
kQ1½TEAꢁ
/
¼
Æ
Æ
:
ð2Þ
E!Z
k1t =2 I1a=2
1=sf0 þ kQ1½TEAꢁ þ kQ2½Eꢁ
This can be rearranged to Eq. (3), which is represented in
Fig. 6 for the data of Fig. 5.

Photochemistry and Photobiology, 2012, 88 773
10. Saltiel, J., D. E. Townsend and A. Sykes (1973) The quantum
chain process in the sensitized cis–trans photoisomerization of
1,3-dienes. J. Am. Chem. Soc. 95, 5968–5973.
good conversion is achieved for the biologically active isomer
with no need of expensive isomer separation.
11. Ganapathy, S., A. Trehan and R. S. H. Liu (1990) Photoisomer-
ization of 7-cis-retinal. The concentration effect. J. Chem. Soc.
Chem. Commun. 199–200.
12. Ganapathy, S. and R. S. H. Liu (1992) Photoisomerization of
polyenes. 30. Quantum chain processes in photoisomerization of
the all-trans, 7-cis, and 11-cis isomers of retinal. J. Am. Chem. Soc.
114, 3459–3464.
13. Hentz, R. R., K. Shima and M. Burton (1967) High-energy
induced isomerization of stilbene in benzene solution. J. Phys.
Chem. 71, 461–462.
14. Dainton, F. S., C. T. Peng and G. A. Salmon (1968) Transient
species in the radiolysis of solutions of stilbene. J. Phys. Chem. 72,
3801–3807.
Acknowledgements—P.F.A. is
a member of CONICET (Consejo
Nacional de Investigaciones Cientıficas y Tecnicas, Argentina). We
thank Delta Biotech (Argentina) for providing 5E-a-calcidol and
facilities for HPLC analysis. The work was performed under financial
support from UBA (X006), CONICET (PIP5470) and ANPCyT
(PICT33973).
REFERENCES
1. Kimlin, M. G. (2008) Geographic location and vitamin D syn-
thesis. Mol. Asp. Med. 29, 453–461.
2. Moon, S. J., A. A. Fryer and R. C. Strange (2005) Ultraviolet
radiation, Vitamin D and risk of prostate cancer and other
diseases. Photochem. Photobiol. 81, 1252–1260.
3. Lehmann, B. (2005) The Vitamin D3 pathway in human skin and
its role for regulation of biological processes. Photochem. Photo-
biol. 81, 1246–1251.
4. Holick, M. F., J. A. MacLaughlin and M. B. Clark (1980) Pho-
tosynthesis of previtamin D3 in human skin and the physiologic
consequences. Science 210, 203–205.
5. Braun, A. M., M.-T. Maurettte and E. Oliveros (1986) Technol-
ogie Photochimique, pp 483–505. Presses Polytechniques Roman-
des, Lausanne, Switzerland, Chapter 12.
15. Mizuno, K., N. Ichinose and Y. Otsuji (1992) Photochemistry of
9,10-dicyanoanthracene-1,2-diarylcyclopropane systems. Photo-
cycloaddition and photoisomerization. J. Org. Chem. 57, 1855–
1860.
16. Mizuno, K., K. Nire, H. Sugita and H. Maeda (2001) A novelcis–
trans photoisomerization of vinylidenecyclopropanes via an elec-
tron-transfer chain process. Tetrahedron Lett. 42, 2689–2692.
17. Takagi, K., K. Aoshima, Y. Sawaki and H. Iwamura (1985)
Electron-relay chain mechanism in the sensitized photoisomer-
ization of stilbazole salts in aqueous anionic micelles. J. Am.
Chem. Soc. 107, 47–52.
18. Arai, T. and K. Tokumaru (1993) Photochemical one-way adia-
batic isomerization of aromatic olefins. Chem. Rev. 93, 23–39.
19. Arai, T.. (1999) Photochemical cis–trans isomerization in the
triplet state. In Molecular and Supramolecular Photochemistry.
Vol. 3. Organic Molecular Photochemistry (Edited by V. Rama-
murthy and K. S. Schanze), pp 131–167. Marcel Dekker, New
York.
20. Daraio, M. E., A. Volker, P. F. Aramendıa and E. San Roman
(1996) Reaction of zinc phtalocyanine excited states with amines in
cationic micelles. Langmuir 12, 2932–2938.
6. Kubodera, N. (2010) Pharmaceutical studies on vitamin
D
derivatives and practical syntheses of six commercially available
vitamin D derivatives that contribute to current clinical practice.
Heterocycles 80, 83–98.
7. Moriarty, R. M. and D. Albinescu (2005) Synthesis of 1a-Hy-
droxyvitamin D5 using a modified two wavelength photolysis for
vitamin D formation. J. Org. Chem. 70, 7624–7628.
8. Calverley, M. J. (1987) Synthesis of mc 903, a biologically active
vitamin D metabolite analogue. Tetrahedron 43, 4609–4619.
9. Hyndman, H. L., B. M. Monroe and G. S. Hammond (1969)
Mechanisms of photochemical reactions in solution. LX. Photo-
chemical isomerization of 2,4-hexadiene via a quantum-chain
mechanism. J. Am. Chem. Soc. 91, 2852–2859.
21. Brink, M., H. Jonson and M. Sundahl (1998) Catalysis of
triplet state cis–trans isomerizations making a quantum chain
process more efficient. J. Photochem. Photobiol. A: Chem. 112,
149–153.