Photochemical transformations of 7-dehydrocholesterol

Article abstract of DOI:10.1134/S1070427209030306

Photochemical transformations of 7-dehydrocholesterol were performed with a two-lamp experimental circulation installation equipped with a ferrioxalate actinometer and were monitored by UV spectroscopy and high-performance liquid chromatography. The quant

Full text of DOI:10.1134/S1070427209030306

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 3, pp. 509–511. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © Z.L. Grigoryan, A.I. Martiryan, L.A. Nersesyan, Sh.A. Markaryan, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82,
No. 3, pp. 515–518.
BRIEF
COMMUNICATIONS
Photochemical Transformations of 7-Dehydrocholesterol
Z. L. Grigoryan, A. I. Martiryan, L. A. Nersesyan, and Sh. A. Markaryan
Yerevan State University, Yerevan, Armenia
Received February 21, 2008
Abstract―Photochemical transformations of 7-dehydrocholesterol were performed with a two-lamp
experimental circulation installation equipped with a ferrioxalate actinometer and were monitored by UV
spectroscopy and high-performance liquid chromatography. The quantum yields of the overall conversion of 7-
dehydrocholesterol at wavelengths of 254, 313, and 365 nm were determined.
DOI: 10.1134/S1070427209030306
Photochemical transformations of 7-dehydrocho-
Photolysis o 7-DHC (I) yields a tarry photolyzate
lesterol (provitamin D , 7-DHC) is the principal
consisting mainly of provitamin D (II), lumisterol III,
3
3
precursor for commercial production of cholecalciferol
and also small amounts of tachysterol (V) and
(
vitamin D ) [1–3]. Photolysis of 7-DHC follows
toxisterols (not shown in the scheme). Vitamin D (IV)
3
3
Scheme 1.
is produced by the subsequent thermal isomerization of
Scheme 1.
R
R
R
H3C
H
H3
H3C
H
C
hν
hν
hν
hν
H C
H C
3
3
H
H
HO
HO
H C
3
I
II
Δt
hν
OH
R
Δt
hν
III
H
3C
R
H3C
H C
3
H
H
H
CH2
HO
OH
IV
V
CH3
H C
3
CH3
.
R =
CH3
5
CH3
09

5
10
GRIGORYAN et al.
second step, tachysterol transforms into provitamin D3
6, 7]. To accelerate the transformation of tachysterol
[
into provitamin D [5], such sensitizers are used in the
3
second step as anthaquinone, benzpyrene, or an-
thracene. The mechanism of photolysis of 7-DHC
upon laser irradiation was examined in [7–9].
Despite numerous studies on photochemical
transformations of 7-DHC, available data on the
kinetic relationships of the 7-DHC photolysis and
quantum yield of the process are scarce.
The goal of this study was determination of the
quantum yields of the process at various wavelengths
and elucidation of kinetic relationships of the
photochemical transformations of 7-DHC.
Fig. 1. Two-lamp experimental circulation installation:
1) ultraviolet irradiation lamps, (2) temperature-control
system, (3) reaction zone, (4) circulation pump, (5) vessel
with the reaction mixture, (6) power supply to lamps, and
(
(
7) thermostat.
EXPERIMENTAL
The quantum yields of photochemical trans-
formations of 7-DCH at various wavelengths were de-
termined with a ferrioxalate actinometer [3]. The UV
spectra were recorded with a Specord 50 PC spec-
trometer.
provitamin D . Photolysis is usually performed with
3
2
0–30-W low-pressure mercury lamps. It was found
[
2] that, to ensure high selectivity with respect to
provitamin D , it is appropriate to use luminophores
3
with which the major fraction of the emission power of
the mercury lamp would correspond to a wavelength λ =
Kinetic experiments were performed with a two-
lamp experimental circulation installation schemat-
ically shown in Fig. 1.
3
00–310 nm.
In some studies, with the aim to increase the yield
of provitamin D , the photochemical process is
Irradiation was performed with 30-W low-pressure
mercury lamps made of Uviol glass. Walls of one of
the lamps were coated with a luminophore having the
emission maximum at 310–313 nm, and the second
lamp had no luminophore coating, and its emission
maximum was at 254 nm. The circulation rate of 7-
(b)
3
performed in two steps: solutions of 7-DCH in various
solvents (ethanol, diethyl ether, etc.) are irradiated with
a laser first at λ = 248–254 and then at λ = 350–
4
00 nm [4–7]. In the first step, the product of the
photochemical reaction is tachysterol, and in the
(
a)
τ, min
τ, min
Fig. 2. Irradiation of 7-dehydrocholesterol with a lamp having the emission maximum at (a) 254 and (b) 313 nm. (c) Concentration
and (τ) irradiation time; the same for Fig. 3. (1) Unchanged 7-DHC, (2) provitamin D , and (3) vitamin D ; the same for Fig. 3.
3
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 3 2009

PHOTOCHEMICAL TRANSFORMATIONS OF 7-DEHYDROCHOLESTEROL
511
successively. In this case, 7-DHC is completely
converted in 40 min. Upon 7-DHC irradiation at λ =
3
13 nm, only provitamin D was found in the reaction
3
products, with no vitamin D detected, and the 7-DHC
3
conversion after irradiation for 80 min was 75%. It
should be noted that vitamin D was detected in 7-
3
DHC irradiation products for the first time. It was
believed previously that the products of 7-DHC
irradiation are provitamin D , lumisterol, tachysterol,
3
and toxisterols. Detection of vitamin D indicates that
3
the generally accepted mechanism of 7-DHC photo-
lysis should be revised.
τ, min
Fig. 3. Successive irradiation of 7-dehydrocholesterol at
λ = 254 and 313 nm.
CONCLUSIONS
–
1
DHC solutions was 14–15 ml s . Analysis of the
initial substances and reaction products was performed
on a Waters 486 high-performance liquid chromato-
graph equipped with a tunable absorbance detector, a
Waters 6005 controller, and a Waters 626 pump. The
UV detector operated at λ = 265 nm. The column (150 ×
(1) The optimal quantum yield of the overall
conversion of 7-dehydrocholesterol is observed at the
irradiation wavelength of 313 nm.
(
2) Along with provitamin D , the products of
3
photochemical transformations of 7-dehydrocholesterol
contain vitamin D3.
4
.6 mm) was packed with C adsorbent. The eluent was
acetonitrile–methanol (75 : 25), elution rate 1 ml min .
18
–1
REFERENCES
7
-DHC (main substance content 98%) was
1
. CRC Handbook of Organic Photochemistry and Photo-
biology, Horspool, W.M., Ed., New York: CRC, 1995,
p. 155.
purchased from Aldrich–Sigma. Ethanol, methanol,
and acetonitrile were purified by published procedures
10].
[
2
3
. Yakhimovich, R.I., Khimiya vitaminov D (Chemistry of
Vitamins D), Kiev: Naukova Dumka, 1978.
. Vvedenie v fotokhimiyu organicheskikh soedinenii
(Introduction to Photochemistry of Organic Com-
pounds), Bekker, G.O. and El’tsov, A.V., Eds.,
Leningrad: Khimiya, 1976.
The quantum yields of the overall conversion of 7-
DHC were determined at λ = 254, 313, and 365 nm.
From the results obtained (φ254 = 0.35, φ313 = 0.42, and
φ365 = 0.05, respectively), it follows that the wave-
length of 313 nm is optimal.
4
. Dauben, W.G. and Funhoff, D.J.H., J. Org. Chem.,
The kinetics of photochemical transformations of 7-
DHC were studied by taking samples from the flask at
1
988, vol. 53, p. 5070.
5
6
. US Patent 4686023
. Malatesta, V., Wllis, C., and Hacket, P.A., J. Am. Chem.
Soc., 1981, vol. 103, p. 6781.
5
-min intervals for chromatographic analysis to
determine the content of the unchanged 7-DHC and
reaction products, provitamin and vitamin D . Figures 2
3
7
8
. Terenetskaya, I.P., Gundorov, S.I., Kravchenko, V.I.,
and Berik, E.B., Quant. Electron., 1988, vol. 18, no. 18,
p. 1323.
and 3 show the kinetic curves of variation of the 7-
DHC and reaction product concentrations in the course
of irradiation at λ = 254 and 313 nm and successive
irradiation first at λ = 254 and then at λ = 313 nm.
. Gundorov, S.I., Davidenko, V.A., Terenetskaya, I.P.,
and Yuschuk, O.I., Quant. Electron., 1991, vol. 21, no. 3,
p. 339.
The results obtained show that, upon irradiation of
a 7-DHC solution with a lamp having an emission
maximum at 254 nm, the reaction products contain
vitamin D along with provitamin D , and 7-DHC is
9. Repeev, Yu.A. and Terenetskaya, I.P., Quant. Electron.,
1996, vol. 26, no. 8, p. 746.
3
3
10. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion. A Handbook of Practical Data, Techniques, and
References, New York: Wiley, 1972.
completely consumed in 60 min. A similar pattern is
observed when 7-DHC is irradiated at λ = 254 and 313 nm
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