Autoxidation of isotachysterol

Article abstract of DOI:10.1016/j.tet.2004.01.075

Isotachysterol, the acid-catalyzed isomerization product of vitamin D ₃, produces seven previously unknown oxygenation products in a self-initiated autoxidation reaction under atmospheric oxygen in the dark at ambient temperature. They are (5R)

Full text of DOI:10.1016/j.tet.2004.01.075

Tetrahedron 60 (2004) 2881–2888
Autoxidation of isotachysterol
a
Xiaoling Jin, Xinping Yang, Li Yang, Zhong-Li Liu and Fa Zhang
a
a,*
a,*
b
a
National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China
Johnson and Johnson, Skillman, NJ 08558-9418, USA
b
Received 22 September 2003; revised 14 January 2004; accepted 15 January 2004
Abstract—Isotachysterol, the acid-catalyzed isomerization product of vitamin D
products in a self-initiated autoxidation reaction under atmospheric oxygen in the dark at ambient temperature. They are (5R)-5,10-epoxy-
,10-secocholesta-6,8(14)-dien-3b-ol (6a), (5S)-5,10-epoxy-9,10-secocholesta-6,8(14)-dien-3b-ol (6b), (10R)-9,10-secocholesta-5,7,14-
trien-3b,10-diol (7a), (10S)-9,10-secocholesta-5,7,14-trien-3b,10-diol (7b), (7R,10R)-7,10-epoxy-9,10-secocholesta-5,8(14)-dien-3b-ol (8),
,10-epidioxyisotachysterol (9) and 3,10-epoxy-5-oxo-5,10-seco-9,10-secocholesta-6,8(14)-dien-10-ol (10). The formation of these products
3
, produces seven previously unknown oxygenation
9
5
is explained in terms of free radical peroxidation chemistry.
q 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
and identification of the principal autoxidation products
of isotachysterol, including (5R)-5,10-epoxy-9,10-seco-
cholesta-6,8(14)-dien-3b-ol (6a), (5S)-5,10-epoxy-9,10-
secocholesta-6,8(14)-dien-3b-ol (6b), (10R)-9,10-seco-
cholesta-5,7,14-trien-3b,10-diol (7a), (10S)-9,10-seco-
cholesta-5,7,14-trien-3b,10-diol (7b), (7R,10R)-7,10-
epoxy-9,10-secocholesta-5,8(14)-dien-3b-ol (8), 5,10-epi-
dioxy-isotachysterol (9) and 3,10-epoxy-5-oxo-5,10-seco-
9,10-secocholesta-6,8(14)-dien-10-ol (10). The formation
of these products is discussed in terms of free radical
peroxidation chemistry.
The chemistry and biochemistry of cholecalciferol (vitamin
D , 1) have been extensively studied for over half a century
due to the great diversity of its chemistry and, especially, its
important roles in calcium regulation, immunological
3
1
regulation and inducing cancer cell differentiation. Over
3
0 natural metabolites of vitamin D have been identified
3
2
from human beings and animals and much more synthetic
analogues, especially those of 1,25-dihydroxyvitamin D3
(1,25(OH) D ), have been made to explore their anticancer
2 3
potentials and other biological activities. Structural
3
alterations of vitamin D by metabolism mostly occur at
3
2
the 1a-position and the side chain, while the oxidation of
–6
4
2. Results
the conjugated triene part has scarcely been reported.
The unique epoxide found in natural metabolites of vitamin
4
Isotachysterol (5) was prepared by HCl-catalyzed isomer-
8
D is 7,8-epoxy-25-hydroxy-19-nor-10-oxovitamin D (2).
3
3
5
ization of vitamin D
3
(1) in methanol. The pale yellow oil
Takayama and co-workers found that 1 could be regio- and
stereoselectively oxidized by m-chlorobenzoic acid and tert-
butyl hydroperoxide catalyzed by VO(acac) , giving (7R)-
of 5 was laid in a small beaker at ambient temperature in the
dark. 5 was found oxidized rapidly to a very complex
mixture as monitored by TLC and after 1–2 days only a
little 5 was left. Oxidation by bubbling oxygen to a benzene
solution of 5 at ambient temperature gave the similar result
together with some polymeric/oligomeric materials which
deposited out of the solution, particularly in the later stage
2
7
respectively. Photosensitized oxidation of vitamin D by
,8-epoxyvitamin D (3) and (5S)-5,6-epoxyvitamin D (4)
3 3
3
6
singlet oxygen has also been reported. However, autoxida-
tion of vitamin D and its isomers has not been reported
3
previously. It is well-known that vitamin D is relatively
3
0
6
a
of the oxidation. Addition of 2,2 -azobisisobutyronitrile
(AIBN) to the benzene solution of 5 significantly acceler-
stable in the air at ambient temperature, while its acid-
catalyzed isomerization product, isotachysterol (5), is very
7
,8
ated the reaction, suggesting that the reaction proceeded by
a free radical chain mechanism. The soluble materials were
separated by reverse phase HPLC. Figure 1 shows the
chromatogram of the reaction mixture obtained at the early
stage (8 h) of the autoxidation of 5 at room temperature in
benzene solution, corresponding to ca. 50% conversion of 5.
UV–Vis spectra in the range of 190–400 nm were obtained
for all major products. The mixture was also examined at
labile in the air even in the dark. However, no effort has
been made previously to identify the complex autoxidation
products of isotachysterol. We report herein the isolation
Keywords: Vitamin D
*
3
; Isotachysterol; Autoxidation; Epoxide; Dioxetane.
Corresponding authors. Tel.: þ86-931-8911186; fax: þ86-931-
625657; e-mail address: liuzl@lzu.edu.cn
8
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.01.075

2882
X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
chemical shifts for 19-Me, and to a less extent, for 4-C. The
coupling constants of 3-H are 8.0, 8.0, 4.5 and 4.5 Hz for 6a,
and 9.6, 9.6, 4.7 and 4.7 Hz for 6b, respectively,
demonstrating that the 3-H is axial in both 6a and 6b. The
NOESY spectrum of 6a showed clear cross peaks
between 1a-H, 3a-H and 19-CH and between 6-H, 1a-H
3
and 19-CH (Fig. 2), indicating that the epoxy ring and the
3
3
-hydroxyl locate at the same side of the molecule. On the
other hand, clear NOESY correlations between 6-, 4b-, 2b-,
b-Hs and 19-CH of 6b (Fig. 2) demonstrates that the
1
3
Figure 1. 3D HPLC diagram recorded from the reaction mixture of the
autoxidation of isotachysterol in benzene at room temperature for 8 h on a
epoxy ring and the 3-hydroxyl are at the opposite sides of
the molecule. In addition, epoxidation of isotachysterol (5)
with anhydrous tert-butyl hydroperoxide (TBHP) in
benzene in the presence of VO(acac) (0.01 equiv.) at 0 8C
2
gave 6a as the sole epoxy product (yield 45%). It is well
known that epoxidation of homoallylic alcohols with
Phenomenex Nucleosil C18 column (5 mm) eluted with MeOH–H
2
O
(88:12 v/v) at a flow rate of 1 ml/min. x-axis: retention time (min), y-axis:
UV absorption; z-axis: intensity of the UV absorption. The peak numbers
correspond to the numbers of compounds.
various stages of oxidation by coupled LC–MS using the
same column and solvent system. The total ion current
TBHP/VO(acac) produces stereospecifically syn-epoxy
2
1
1
alcohols. Therefore, 6a and 6b are assigned as (5R)-
5,10-epoxy-9,10-secocholesta-6,8(14)-dien-3b-ol (5b,10-
epoxy-isotachysterol) and (5S)-5,10-epoxy-9,10-seco-
cholesta-6,8(14)-dien-3b-ol (5a,10-epoxy-isotachysterol)
respectively.
(TIC) chromatograms were similar to those obtained in the
analytical HPLC, except of different peak intensities.
The peak 5 with the longest retention time (R ) of 29.9 min
t
and the molecular ion peak of 385.3463 (C H OþH
2
7 44
requires 385.3470) was identified as unreacted isotachy-
sterol (5) by comparing its R and UV spectrum with that of
The peaks 7a and 7b with R of 10.6 and 7.1 min
t
respectively, gave molecular ion peak of 401.3416 and
401.3411 respectively, corresponding to a same molecular
t
the authentic sample. The peaks 6a and 6b with R of 9.0 and
t
6
and 401.3422 respectively, corresponding to a same
.8 min respectively, gave molecular ion peaks of 401.3413
formula with one more oxygen than 5 (C27
H
44
O
2
þH
requires 401.3420). Both of them exhibited an UV
absorption maximum at 278 nm, suggesting the existence
of a conjugated triene chromophore. The comparison of
molecular formula with one more oxygen than 5
C H O þH requires 401.3420). The UV spectra of 6a
(
and 6b were almost identical, showing a band at 248 nm
27 44 2
1
13
their H and C NMR spectra (Table 1) with those of
1
0
13
which is characteristic of conjugated double bonds. The
1
isotachysterol (5) showed remarkable differences on
C
13
comparison of their H and C NMR spectra (Table 1) with
9
chemical shifts of 5-, 10- and 15-Cs, and to a less extent, on
16 and 19-Cs, which suggests that the 5(10), 6, 8(14)-triene
structure in 5 might change to a 5,7,14-triene system in both
7a and 7b. The A-ring structure of 7a and 7b was confirmed
by their gCOSY spectra which exhibited spin coupling
network between the hydroxymethine proton 3-H and 4a-,
those of vitamin D and its metabolites and with that of
3
1
0
isotachysterol clearly demonstrates that 6a and 6b are
1
3
5
chemical shifts are only observable for 5- and 10-Cs (from
,10-epoxides of 5 since the remarkable changes on
C
1
3
1
double bond carbons to epoxy carbons) and on C and H

X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
2883
1
13
a
6
Table 1. H and C NMR chemical shifts of compounds 5–10 (acetone-d )
Carbon
5
6a
6b
7a
7b
8
10
Proton
5
6a
6b
7a
7b
8
10
1
2
32.3
32.1
33.7
30.4
35.4
32.7
38.7
31.5
38.8
32.1
35.0
31.2
39.0
31.7
1a
1.82
2.17
1.86
1.48
3.81
2.53
2.04
1.42
1.86
1.73
1.64
3.81
1.76
1.88
1.87
1.46
1.82
1.38
3.97
1.95
1.60
1.42
1.89
1.86
1.74
3.62
2.65
2.55
1.51
1.79
1.90
1.51
3.52
3.01
2.04
2.13
1.53
1.62
1.80
4.06
2.28
2.51
1.68
1.98
2.08
1.75
4.48
2.95
2.69
1b
2a
2b
b
b
b
b
b
3
4
67.3
35.5
67.3
32.5
66.7
43.3
69.7
35.2
69.7
35.4
66.5
34.4
77.5
48.9
3a
4a
4b
5
6
7
8
9
127.1
124.6
125.9
125.4
26.3
77.6
129.5
128.6
124.5
26.1
77.1
131.2
128.0
124.5
26.1
144.2
118.0
120.4
136.5
28.2
143.8
116.4
119.3
135.1
27.7
142.9
122.3
83.2
122.3
25.3
198.7
124.4
142.2
124.5
26.2
6
7
6.53
6.36
5.73
6.61
5.82
6.45
6.41
6.39
6.59
6.38
5.05
5.48
6.12
7.51
9a
9
2.38
2.47
2.36
2.47
2.35
2.47
1.80
2.80
1.83
2.81
2.21
2.42
2.57
2.62
b
1
1
0
1
131.6
27.6
73.1
27.6
73.8
27.6
71.6
22.8
71.7
21.9
88.1
28.1
108.2
27.3
11a
1b
12a
2b
1.92
1.46
1.18
2.01
1.89
1.47
1.19
2.01
1.90
1.46
1.18
1.98
1.77
1.64
1.64
2.02
1.82
1.68
1.45
2.00
1.92
1.45
1.20
1.98
1.97
1.53
1.23
2.07
1
12
38.6
38.0
38.0
41.6
42.8
38.3
37.3
1
1
1
1
3
4
5
44.6
149.3
24.8
44.4
148.1
25.0
44.4
148.8
25.0
47.9
153.9
119.9
47.0
153.1
118.9
44.9
144.4
23.2
45.5
161.4
24.4
15a
5b
16a
6b
17
18
19
20
21
22
2.04
2.24
1.90
1.74
1.18
0.90
1.75
1.50
0.97
1.10
1.98
2.12
2.01
1.75
1.19
0.90
1.08
1.48
0.97
1.43
1.93
2.15
2.01
1.73
1.20
0.90
1.18
1.51
0.97
1.43
5.55
5.55
1.98
2.09
1.96
1.65
1.17
0.86
1.25
1.50
0.97
1.14
1.44
1.10
2.20
1.99
1.80
2.03
1.25
0.95
1.33
1.58
0.98
1.42
1
16
19.6
19.6
19.6
36.5
35.6
20.0
19.2
2.16
1.90
1.65
0.90
1.33
1.50
1.04
1.41
2.20
1.97
1.64
0.88
1.33
1.48
0.94
1.42
1
1
1
1
2
2
2
7
8
9
0
1
2
57.2
18.4
18.9
35.3
19.4
36.6
57.1
18.4
23.9
35.3
19.3
36.6
57.2
18.4
23.8
35.3
19.4
36.6
59.5
17.6
27.3
39.7
21.4
36.6
58.8
16.7
27.3
38.8
20.4
36.6
56.7
18.8
23.4
35.4
19.4
36.7
56.8
18.3
22.1
35.2
19.3
36.5
c
c
c
c
c
c
1
.36
1.10
.43
c
c
2
3
24.4
24.3
24.3
24.3
24.3
24.4
24.4
23
1.36
1.10
1.36
1.11
1.52
0.86
0.86
1.08
1.10
1.12
1
2
2
2
2
4
5
6
7
40.1
28.6
22.8
23.0
40.2
28.6
23.0
22.8
40.1
28.6
23.0
22.8
43.6
28.6
23.2
22.8
42.8
28.6
23.0
23.0
40.2
28.8
23.0
22.8
40.2
28.6
23.0
22.8
24
25
26
27
1.17
1.50
0.86
0.86
1.15
1.48
0.87
0.87
1.16
1.50
0.87
0.87
1.17
1.52
0.87
0.87
1.18
1.53
0.87
0.87
1.19
1.54
0.86
0.86
a
b
c
Data for compound 9 not included, see text.
J values see text.
a or b protons.
4b-, 2a-, 2-b, 1a- and 1b-Hs, and by their HMBC spectra
which showed correlations between 19-CH and 1-, 5- and
NOESY spectra which showed clear correlations between
3a-H and 4a-, 2a- and 1a-Hs, and between 19-CH and 6-H
3
3
1
0-Cs. The structure of the C ring was confirmed by their
in 7a, while no such correlations occurred in 7b. Instead,
clear NOESY correlations were observed between 19-CH3
and 4b-, 2b- and 1b-Hs in 7b (Figure 2). This demonstrates
gCOSY spectra which showed correlations between the
allylic 9b-H and 9a-, 11a-, 11b-, 12a- and 12b-Hs, and by
their HMBC spectra which show correlations between the
olefinic 7-H and 8-, 9- and 14-Cs. The structure of the D ring
was confirmed by their gCOSY spectra which showed
correlations of the olefinic 15-H with 16a- and 16b-Hs, and
that 7a and 7b are 10-epimers and the 19-CH is equatorial
3
and a-oriented in 7a, while is axial and b-oriented in 7b.
The comparatively downfield shift of the chemical shifts of
2b-H and 4b-H(d 1.74 and 2.55, respectively) in 7a
compared to those of 7b (d 1.51 and 2.04, respectively)
also indicated that the 10-OH is axial and b-oriented in 7a
and equatorial and a-oriented in 7b. Therefore, 7a and 7b
were assigned as (10R)-9,10-secocholesta-5,7,14-trien-
3b,10-diol and (10S)-9,10-secocholesta-5,7,14-trien-
3b,10-diol, respectively.
1
1
7-H with 16a-H, together with the HMBC correlation of
8-CH with 13- and 14-Cs. The structure of the seco-B ring
3
was confirmed by their HMBC spectra which showed
correlations between the olefinic 6-H and 5-, 7-, 8- and
1
4
0-Cs. The coupling constants of 3-H (8.0, 8.0, 4.4 and
.4 Hz for 7a, and 9.2, 9.2, 4.6 and 4.6 Hz for 7b,
respectively) suggest that the 3-H is axial and the A-ring
of 7a and 7b might be partitioned between a 30/70 and
The peak 8 with R of 10.1 min and the molecular ion peak
t
2
a chair with the 3b-OH equatorially oriented.
4/76 equilibrium mixture of chair conformers favoring an
9
of 401.3411 corresponds to a molecule with one more
oxygen than 5, same as 6 and 7 (C H O þH requires
,12
The NOE
enhancement was observed for the 7-H with 4a-H and 15-H,
2
7 44 2
401.3420). The UV absorption maximum of 206 nm
indicates the absence of conjugated double bonds in the
and for the 6-H with 9b-H and 19-CH in both of 7a and 7b,
3
1
13
indicating the triene configuration of the two compounds to
be (5E, 7E, 14E), which were also supported by the coupling
constant of the olefinic protons (J6,7¼12.0 Hz). The
difference between 7a and 7b were observed only in their
compound. The comparison of its H and C NMR spectra
1
0
(Table 1) with those of isotachysterol (5) showed
1
3
remarkable differences on C chemical shifts of 7- and
10-Cs, from olefinic carbons in 5 to oxygen-connecting

2884
X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
its H,H-COSY correlations of 9b-, 9a-, 11a-, 11b-, 12a-
and 12b-Hs, of 15a-, 15b-, 16a-, 16b- and 17a-Hs, and by
its HMBC correlations between 18-CH and 12-, 13- and
3
14-Cs. The connection between the tetrahydrofuran ring and
C-ring was supported by the HMBC correlations between
the oxygen-connecting 7-H and 6-, 8- and 14-Cs. The
NOESY 1D spectrum showed clear correlations between
19-CH and 1a-, 2a- and 4a-Hs, indicating that the 19-CH
3 3
is axial and the 3-H is equatorial which is consistent with the
coupling constant of 3-H (3.2, 3.2, 2.4 and 2.4 Hz). The
NOESY 1D spectrum also exhibited a correlation between
the 19-CH and 7-H, demonstrating that they are located on
3
the same side of the dihydrofuran ring. Thus 8 was assigned
as (7R,10R)-7,10-epoxy-9,10-secocholesta-5,8(14)-dien-
3
b-ol.
The peak 9 with R of 7.7 min and the molecular ion peak of
t
439.3211 corresponds to a molecule with two more oxygens
than 5 (C H O þNa requires 439.3188). The UV absorp-
2
7 44 3
tion maximum at 296 nm suggested the presence of an
extended conjugated system. However, this compound 9
was unstable and gradually converted to a new compound
1
0, i.e., peak 10 with R of 17.0 min and l
t
at 305 nm,
max
during the process of semipreparative HPLC separation of 9.
0 gave the molecular ion peak of 417.3373 corresponding
to a same molecular formula of 9 (C H O þH requires
1
Figure 2. Principal NOE correlations of 6a, 6b,7a,7b and 8 presented with
ball-and stick representations of the MM2-optimized structures.
2
7 44 3
417.3369). Its UV spectrum showed strong absorption
maximum at 305 nm, suggesting the existence of an
1
3
quaternary carbons in 8, suggesting that 8 is a 7,10 epoxide
of 5 containing a dihydofuran ring. The structure of 8 was
fully assigned by its 2D NMR spectroscopy. The structure
of the A-ring was confirmed by its gCOSY spectrum (Fig. 3)
in which the hydroxymethine proton 3a-H (d 4.06)
correlated with 2a-, 2b-, 4a- and 4b-Hs, and the 2b-H
correlated with 1a- and 1b-Hs, and by its gHMBC spectrum
extended conjugated system. The C NMR spectrum
revealed the presence of a –CvO, a –OCH– and a –O–
1
3
C–O– moieties. The comparison of its C NMR chemical
shifts with those of 5 demonstrated that, besides of the
A-ring carbons as well as 7- and 14-Cs, other chemical
shifts are almost identical. In the HMBC spectrum the
olefinic 6- and 7-Hs and the methylenic 4-Hs correlated with
the carbonyl carbon (d 198.7), indicating that the –CvO is
located at the 5-position, that also rationalizes the down-
field shift of 4-, 7- and 14-Cs in comparison with those of 5.
The chemical shift of 10-C (d 108.2) suggested it bonded to
two oxygens. Its HMBC showed correlations between the
(
Fig. 4) which shows correlations between 19-CH and 1-,
3
5
- and 10-Cs. The dihydrofuran ring was supported by the
HMBC correlations of the olefinic 6-H with 4-, 5-, 7- and
0-Cs. The structure of the C- and D-rings was confirmed by
1
10-C and 19-CH3 and 2-Hs, and between the oxygen-
connecting 3-C (d 77.5) with 2- and 4-Hs. Therefore, 10 was
assigned as 3,10-epoxy-5-oxo-5,10-seco-9,10-secocholesta-
6
,8(14)-dien-10-ol and 9 was assigned as 5,10-epidioxy-
1
13
isotachysterol. Total H and C NMR assignments are
listed in Table 1.
Figure 3. The gCOSY spectrum of 8.
Figure 4. The gHMBC spectrum of 8.

X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
2885
3. Discussion
has been reported previously that isotachysterol (5) could
1
isomerize to cis-isotachysterol (11) photochemically.
5a
1
3
Mordi and Walton have studied in detail the autoxidation
of b-carotene in the dark and proposed a self-initiated
autocatalytic mechanism for the formation of the 5,6-
epoxide of b-carotene and other oxidation products. Similar
mechanism might also be applicable to this autoxidation of
isotachysterol as shown in Scheme 1. That is, the all-trans-
triene structure in 5 isomerizes to the corresponding 6,7-cis-
isomer (11) via the singlet biradical transition state (12),
The present work demonstrates clearly that the trans/cis-
isomerization of isotachysterol can also take place thermally
because the singlet biradical 12 is thermodynamically
stabilized by delocalization of the two unpaired electrons
to the two allylic moieties. This thermal trans/cis-isomer-
ization of isotachysterol via the biradical (12) provides a
ready explanation for the lability of isotachysterol and the
self-initiated autoxidation of the substrate. That is, during
twisting of the central carbon–carbon bond of isotachy-
sterol the unpaired spin density would develop in each half
of the molecule, reaching a maximum (one free spin in each
half) in the perpendicular transition state (12). It is
reasonably to assume that the unpaired spin can be
‘captured’ by oxygen to produce a carbon-peroxyl triplet
biradical (13). Oxygen should preferably attack 10-C to
enable the extensive delocalization of another unpaired
electron. Being a triplet 13 would be relatively long-lived
1
3
similar to the case of b-carotene which has been proved by
Doering and co-workers to be able to take place at
temperatures ,40 8C. As a matter of fact, a small peak
close to the peak of 5 with absorption maximum at 253 nm
corresponding to 6,7-cis-isotachysterol (11) could be
observed if a hexane solution of isotachysterol (5) was put
in the dark and free of oxygen for 6 h as shown in Figure 5.
This demonstrated the unambiguous formation of 11, hence
the occurrence of such trans/cis-isomerization process. It
1
4
1
5
Scheme 1. Proposed mechanism for the autoxidation of isotachysterol.

2886
X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
Scheme 2. An alternative mechanism for the autoxidation of isotachysterol.
liable to autoxidation to form a variety of oxidation
products. The formation of these oxidation products is
interesting since they are formed in the dark and in the
absence of any other oxidants and/or initiators apart from
atmospheric oxygen. Other oxides of vitamin D derivatives
3
reported previously were all prepared by chemical and
4
–6
photochemical oxidations. Since isotachysterol is the
acid-catalyzed isomerization product of vitamin D and also
can be formed in the presence of acidic vitamins such as
ascorbic acid and folic acid, similar autoxidation reaction
might also take place in living systems and have biological
significance.
Figure 5. (A) HPLC diagram recorded from a hexane solution of
isotachysterol which was stored free from oxygen in the dark for 6 h with
a Zorbax Sil column (5 mm) eluted with hexane–AcOEt (85:15 v/v) at a
flow rate of 1 ml/min. The UV detector was put at 260 nm. The peak
numbers correspond to the numbers of the compounds. (B) The UV spectra
of 5 and 11 recorded from the same solution. The intensity of 11 was
magnified by 3 times.
3
1
8
and be able to add to a second molecule of 5 to form a new
biradical 14, again at 10-C. Obviously, 14 can subject to the
well-precedented intramolecular homolytic substitution
4. Experimental
4.1. General methods
1
6
(
S i) producing the 5,10-epoxides 6 and the 7,10-epoxide
H
8
by 5,10- and 7,10-ring closure, respectively, of the
HR-ESI-MS was determined on a Bruker APEX II FT-MS
13
1
intermediate alkoxyl biradical 15. Compound 7 was also
possibly derived from the alkoxyl biradical 15 by consecu-
tive 1,5-sigmatropic rearrangement of the allylic 15-H to
spectrometer. H, C and 2D NMR spectra were recorded
on a Bruker AM 400 NMR spectrometer in acetone-d with
6
TMS as the internal standard. IR spectra were taken on a
Nicolet 170SX IR spectrometer. UV spectra were recorded
with a Hitachi 557 spectrophotometer in methanol. Optical
rotation was measured on a Perkin–Elmer 341 polarimeter.
HPLC was carried out with a Hewlett Packard 1100 system
and a diode array detector. Best separations were achieved
6
-C and 1,4-sigmatropic rearrangement of the 6-H to 10-O.
On the other hand, the peroxyl biradical 13 may collapse to
the thermally unstable dioxetane 9 which would easily
subject to peroxide scission to produce the dicarbonyl
intermediate 16 followed by acetalation, yielding the cyclic
semiketal 10 (Scheme 1). Another possible initiation step
might be the direct hydrogen abstraction by oxygen from
allylic positions, preferably at C-19 to form the allylic
radical 17, which reacts with oxygen to form the peroxy
radical 18. Being similar to 13 it can follow similar follow-
up processes as mentioned above to give the products as
exemplified in Scheme 2.
2
with a 250£4.6 mm Phenomenex Nucleosil 5 mm C18 and
2
250£10 mm Whatman Partisil 10 mm ODS-3 columns.
1
7
Coupled LC–MS was carried out with the same HPLC
system and the Bruker APEX II FT-MS spectrometer in the
electrospray ionization mode.
4.1.1. Preparation of isotachysterol (5). To a solution of
vitamin D (1, 200 mg) in 30 ml methanol was added 0.1 ml
3
In conclusion, this work demonstrates that despite the
relative stability of vitamin D at ambient temperatures, its
acid-catalyzed isomerization product, isotachysterol, is
HCl and the solution was refluxed for 0.5 h. The reaction
mixture was neutralized with Na CO , extracted with
AcOEt and dried over anhydrous Na SO . After removing
2 4
3
2
3

X. Jin et al. / Tetrahedron 60 (2004) 2881–2888
2887
2
5
the solvent under reduced pressure using a rotavapor the
residue was column chromatographed on silica gel (20 g)
with AcOEt–PE (1:5) giving a pale yellow oil (150 mg,
401.3411 (C H O þH requires 401.3420); [a] ¼þ134
2
7
44
2
D
(c 0.3 in acetone); l
Table 1.
(MeOH, nm) 206. For NMR data see
max
7
5
5%) of isotachysterol (all-trans-9,10-secocholesta-
(10),6,8(14)-trien-3b-ol (5): HR-ESI-MS: 385.3463
4.3.6. 3,10-epoxy-5-oxo-5,10-seco-9,10-secocholesta-
6,8(14)-dien-10-ol (10). HR-ESI-MS (417.3378 for
2
5
(
C H OþH requires 385.3470); [a] ¼þ4 (c 0.3 in
(neat, cm ) 3403 (OH), 1671 and 1589
max
2
7
44
D
2
1
acetone); n
C H O þH, requires 417.3369); l (MeOH, nm) 305.
max
27 44
3
(
conjugated triene), 957 (trans-CHv); lmax (MeOH, nm)
For NMR data see Table 1.
288, indicative of an all-trans-triene system. For NMR data
see Table 1.
Acknowledgements
4
.2. Autoxidation of isotachysterol (5)
We thank the National Natural Science Foundation of China
Grant Nos. 20172025 and 20332020) for financial support.
We also thank the referee who suggested the alternative
mechanism as supplemented in Scheme 2.
The pale yellow oil of 5 (150 mg) was laid in a small beaker
at ambient temperature in the dark which was oxidized
rapidly to a very complex mixture as monitored by TLC,
and after 1–2 days little 5 was left. Oxidation by bubbling
air to a benzene solution of 5 at 40 8C for 4 h gave the same
result. The reaction mixture was separated by column
chromatography (silica gel, AcOEt–PE, 1:1 v/v) followed
by HPLC to give 6a (10.6 mg), 6b (6.3 mg), 7a (4.0 mg), 7b
(
References and notes
(
5.8 mg), 8 (8.0 mg) and 10 (3.2 mg) respectively. Com-
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1
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.3. Stereospecific epoxidation of isotachysterol (5)
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4
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3
4
.3.1. (5R)-5,10-epoxy-9,10-secocholesta-6,8(14)-dien-
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5
01.3413 (C H O þH requires 401.3420; [a] ¼þ25.8
2
7
44
2
D
2
1
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2
47. For NMR data see Table 1.
1
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4
3
4
.3.2. (5S)-5,10-epoxy-9,10-secocholesta-6,8(14)-dien-
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2
5
01.3422 (C H O þH requires 401.3420); [a] ¼þ29.1
2
7
44
2
D
2
1
(
c 1.0 in acetone); n
(neat, cm ) 3388 (OH), 1275, 874
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2
47. For NMR data see Table 1.
9
. Mizhiriskii, M. D.; Konstantinovskii, L. E.; Vishkautsan, R.
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4
.3.3. (10R)-9,10-secocholesta-5,7,14-triene-3b,10-diol
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7 44 2
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5
4
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D
max
1
973, 95, 6136–6137. (b) Sharpless, K. B.; Verhoeven, T. R.
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.3.4. (10S)-9,10-secocholesta-5,7,14-triene-3b,10-ol
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2
7 44 2
2
5
4
nm) 278. For NMR data see Table 1.
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D
max
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