A new metabolite of Paricalcitol: stereoselective synthesis of (22Z)-isomer of 1α,25-dihydroxy-19-norvitamin D2

Article abstract of DOI:10.1016/j.tetlet.2016.01.110

Stereoselective synthesis of (22Z)-isomer of Paricalcitol, an analog of 1,25-dihydroxyergocalciferol, an active form of vitamin D₂ (Ergocalciferol) has been described. The two key critical synthetic steps involved are Julia–Lythgoe's Wittig–Hor

Full text of DOI:10.1016/j.tetlet.2016.01.110

Tetrahedron Letters xxx (2016) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new metabolite of Paricalcitol: stereoselective synthesis
of (22Z)-isomer of 1
a
,25-dihydroxy-19-norvitamin D2
a
a
b
Ramakrishna Samala a, Somesh Sharma a, , Manas K. Basu , K. Mukkanti , Frank Porstmann
⇑
a Chemistry Services, GVK Biosciences Pvt. Ltd, Plot No. 5C, IDA, Uppal, Hyderabad 500 039, AP, India
b API R&D, AZAD Pharmaceutical Ingredients AG, 8200 Schaffhausen, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Stereoselective synthesis of (22Z)-isomer of Paricalcitol, an analog of 1,25-dihydroxyergocalciferol, an
active form of vitamin D2 (Ergocalciferol) has been described. The two key critical synthetic steps
involved are Julia–Lythgoe’s Wittig–Horner coupling of aldehyde functionality of CD-ring system with
Received 28 December 2015
Revised 25 January 2016
Accepted 30 January 2016
Available online xxxx
benzothiazolyl sulfone, and Horner–Wadsworth–Emmons reaction of phosphine oxide with
a
Windaus–Grundmann’s ketone to build a diene motif between the A and CD-ring system of Paricalcitol.
Ó 2016 Published by Elsevier Ltd.
Keywords:
Paricalcitol
19-Nor-1,25-(OH)2-vitamin D2
Vitamin D2 (Ergocalciferol)
D
-(À)-Quinic acid
Julia–Lythgoe’s olefination
Horner–Wadsworth–Emmons reaction
Introduction
However, these synthetic approaches focus on the synthesis of
the E-configuration at C-22. For verification of analytical methods
The natural hormones 1
,25-hydroxyvitamin D3 (2, Alfacalcidol), 1a
a
,25-dihydroxyvitamin D3 (1, Calcitriol),
for Paricalcitol it is necessary to provide reference material of the
corresponding Z-isomer. This requirement will become even more
important with the expected implementation of the new USP
method specifying this compound specifically.11
1a
,25-hydroxyvitamin
D2 (3, Doxercalciferol) (Fig. 1), are members of steroid/thyroid/
androgen nuclear receptor super-family, and act as endogenous
ligands for the nuclear vitamin D receptor (VDR), and show signif-
icant biological activities.1 Calcitriol (1) is well known as a primary
regulator for calcium and phosphate homeostasis,2,3 and plays a
critical role in regulation of the proliferation of malignant cells.4,5
In recent times, the non-natural vitamin D2 (6, Ergocalciferol)
has been administrated to humans and domestic animals6 in par-
allel to vitamin D to influence metabolism and biological activities.
There is also a strong evidence on vitamin D2 undergoing double
Various structural modifications (Fig. 1) explored on 1a,25-
dihydroxy 19-nor vitamin D are in side chain viz. reduced double
bond or substituents around double bond, surprisingly researchers
failed to evaluate the potential of isomerization at C-22, 23 posi-
tion. Herein, we report the first total synthesis of Z-stereoisomer
of Paricalcitol at C-22, 23 by maneuvering the coupling of A-ring
system with
a
Windaus–Grundmann’s keto-acetal under
Wittig–Horner condition,12–14 and subsequently Julia–Lythgoe15
olefination on ACD-ring system. (Strategy II, Fig. 2.)
hydroxylation7,8 to produce 1
a,25-dihydroxyvitamin D2.
Further, most of analogs examined so far belong to natural
vitamin D3 series. In comparison to vitamin D3 derivatives, vitamin
D2 derivatives are challenging to synthesize due to an additional
chiral center at C-24 and a double bond (E-geometry) at C-22.
Among these Paricalcitol (4) is being used to treat secondary
hyperparathyroidism (SHPT) in patients with chronic kidney dis-
ease (CKD).9 Several synthesis routes have been published for the
synthesis of Paricalcitol.10
Results and discussion
As a part of our quest on the stereoselective synthesis of
vitamin D2 metabolites, we approached our scientific efforts in
two ways: (i) synthesis of either precursor 7 (Strategy I) or 12
(Strategy II) (Fig. 2) and (ii) preparation of benzothiazolyl sulfone
8, a key intermediate for bringing side chain under Julia–Lythgoe
olefination condition.
Our studies began with the synthesis of intermediate 1316 from
⇑
Corresponding author. Tel.: +91 40 66281305; fax: +91 40 66929900.
E-mail address: somesh.sharma@gvkbio.com (S. Sharma).
D
-(À)-Quinic acid,17 and its conversion to phosphine oxide 11 in 10
http://dx.doi.org/10.1016/j.tetlet.2016.01.110
0040-4039/Ó 2016 Published by Elsevier Ltd.
Please cite this article in press as: Samala, R.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.01.110

2
R. Samala et al. / Tetrahedron Letters xxx (2016) xxx–xxx
22
22
23
23
OH
OH
OH
H
H
H
H
H
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
1 (Calcitriol)
2 (Alfacalcidol)
3 (Doxercalciferol)
4 ( -Paricalcitol)
E
5 ( -Paricalcitol)
Z
[Stereoisomer of Paricalcitol]
Figure 1. Structures of vitamin D receptor analogs.
H
S
N
O
S
O
O
O
O
H
strategy I
8
O
19
O
4 steps
3 steps
10
13 steps
H
H
O
OTBS
HO
7
9
Vitamin D2
OH
(6, Ergocalciferol)
H
H
O
HO
OH
Ph
P
O
Ph
O
O
(Z)-Paricalcitol (5)
OH
S
N
strategy II
10 steps
HO
H
S
O
O
O
HO
OH
8
3 steps
3 steps
OTBS TBSO
OH
D-(-)-Quinic acid
(10, IR,3R,4R,5R) (-)-Quinic acid
TBSO
OTBS
11
12
Figure 2. Retro synthetic approach of (E)-Paricalcitol and its stereoisomer (Z)-Paricalcitol.
steps (Scheme 1). The preparation of 11 was critical for the success
of this method, as this will participate in constructing A-ring
system18 in Paricalcitol (Fig. 2; Strategy II). Various attempts were
made to convert 15 to 11, either through the preparation of
tosylate of 15 under standard condition19 or chloride 16 with
NCS as reported by Uskokovic group,20 and subsequently their
displacement with lithium phenylphosphine. The preparation of
16 was challenging in the above condition, thereof, we shifted
our attention to triphosgene (in hexane) condition.21 Intermediate
16 was displaced with diphenyl phosphine oxide to get 11.
The main advantage of this reaction condition was the preparation
of air stable diphenylphosphine oxide from inexpensive
chlorodiphenylphosphine 17 and its usage. Another crucial inter-
mediate 7 was prepared from Ergocalciferol (6) in multiple steps
(Scheme 2) as per literature report.22
Julia–Lythgoe olefination of 7 and sulfone 8 in the presence of
NaHMDS/or LiHMDS in THF (À78 °C, 30 min) afforded prominently
trans-olefin 24 (E/Z-isomer ratio 82:18) in 59% yield. We further
explored the potential of other reagents like methyl 2-[bis(2,2,2-
trifluoroethoxy)phosphoryl]acetate and diphenoxy phosphonate
to obtain the desired result but were unsuccessful. The intermedi-
ate 24 was taken forward to synthesize Paricalcitol, thinking to get
separation of these isomers at some stage. However, our efforts
were not encouraging and isomers (E- and Z-isomer) were sepa-
rated only by preparative HPLC purification.
Unpromising results on first strategy (Scheme 2) impelled us to
look for alternative plan to achieve the desired results. The critical
intermediate 22 of CD-ring system was conveniently prepared by
ozonolytic cleavage of vitamin D2 (6) in methanol, using CHCl3 as
co-solvent (Scheme 3). The residual acid in CHCl3 was sufficient
OH
O
CO Et
2
O
OH
HO
HO
i
6 steps
75%
ii
50%
75%
OH
TBSO
OTBS
TBSO
OTBS
TBSO
OTBS
OH
13
14
15
D-(-)-Quinic acid (10)
O
Ph
P
O
Ph
P
Cl
Ph
Ph
H
18
O
iii
Cl
P
iv
v
P
Ph
Ph
H
Ph
Ph
95%
35%
70%
17
18
TBSO
OTBS
TBSO
OTBS
11
16
Scheme 1. Reagents and conditions: (i) (TMS)CH2CO2Et (1.5 equiv), LiHMDS (1 M in THF, 1.5 equiv), THF, 0 °C–rt, 15 min; then cooled to À78 °C, add 13, À78 °C, 3 h; (ii)
DIBAL-H (2.5 equiv); toluene, À78 °C to rt, 3 h; (iii) Triphosgene (0.5 equiv), pyridine (2 equiv), hexane, 0 °C–rt, 30 min; (iv) 18 (1 equiv), NaH (1 equiv), DMF, 0 °C–rt, 30 min;
then cooled to À60 °C, add 16 (1 equiv), À60 °C to 1 h; rt to 1 h; (v) 1 N aq HCl (2 vol), rt, 18 h.
Please cite this article in press as: Samala, R.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.01.110

R. Samala et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
H
S
O
S
O
O
O
O
O
O
N
i
O
O
O
O
8
ii
iii
59%
H
60%
H
H
H
70%
OTBS
OTBS
OH
25
O
7
26
24
Ph
P
O
Ph
O
OH
OH
O
H
H
H
v
vi
38%
TBSO
OTBS
11
iv
57%
HO
OH
TBSO
OTBS
TBSO
OTBS
72%
(E)-Paricalcitol (4)
E/ Z ratio: 82:18
27
28
Scheme 2. Reagents and conditions: (i) 8 (1.5 equiv), LiHMDS (1 M in THF; 1.5 equiv), THF, À78 °C to 15 min; then add 7 (1 equiv), À78 °C to rt, 5 h; (ii) TBAF (1 M in THF),
THF, 80 °C, 24 h; (iii) PDC (2.5 equiv), 4 A MS, CH2Cl2; 0 °C, 4 h; (iv) 11 (1 equiv), NaHMDS (1 M in THF, 1.1 equiv), THF, À78 °C, 10 min; then add 26 (1 equiv), À78 °C, 1 h; (v)
MeMgBr (3 M in Et2O, 5 equiv), THF, 0 °C, 4 h; (vi) NH4F (5 equiv), MeOH, reflux, 5 h.
H
OH
OTBS
OH
O
i
ii
iii
iv
H
60%
88%
85%
56%
H
H
H
H
OH
OTBS
OTBS
OTBS
HO
Vitamin D2
19
20
21
7
(6, Ergocalciferol)
H
O
O
Ph
O P Ph
v
60%
O
O
H
H
O
TBSO
OTBS
11
vi
vii
74%
H
TBSO
OTBS
O
TBSO
OTBS
48%
22
12
23
Scheme 3. Reagents and conditions: (i) (a) O3, NaHCO3, MeOH, 8 h; (b) NaBH4, rt, 18 h; (ii) TBSCl (3 equiv), Imidazole (4 equiv), DMAP (cat), DMF, rt, 18 h; (iii) TBAF (1 M in
THF), THF, 0 °C–rt, 18 h; (iv) PCC (2.2 equiv), Celite, CH2Cl2, 0 °C–rt, 18 h; (v) (a) O3, pyridine, MeOH, À78 °C, 8 h; Me2S (6 equiv), À78 °C to 1 h; rt to 1 h; (vi) 11 (1 equiv),
NaHMDS (1 M in THF), THF, À78 °C to 5 min, then add 22 (1 equiv), À78 °C to 15 min; (vii) CHCl3/H2O/TFA (4:2:1), 0 °C–rt, 1 h.
O
S
N
O
O
S
O
OH
OH
O
O
O
8
i
ii
iii
H
H
H
H
51%
50%
78%
TBSO
OTBS
HO
OH
TBSO
OTBS
TBSO
OTBS
(Z)-Paricalcitol (5)
12
29
30
Stereoisomer of Paricalcitol (4)
Scheme 4. Reagents and conditions: (i) 8 (1.1 equiv), LiHMDS (1 M in THF), THF, À78 °C to 15 min; then add 12 (1 equiv), À78 °C, 15 min; (ii) MeMgBr (3 M in Et2O, 3 equiv),
THF, 0 °C, 4 h; (iii) TBAF (1 M in THF, 5 equiv), THF, 60 °C, 2 h.
to catalyze acetalization of the aldehyde functionality to provide
keto-acetal 22 during a reductive workup with dimethyl sulfide
in 60% yield. Wittig–Horner coupling of keto-acetal 22 with phos-
phine oxide 11 (ring A synthon) afforded the diene keto-acetal 23,
which on deprotection provided aldehyde 12.23 However,
Julia–Lythgoe olefination on 12 with sulfone 8 in the presence of
LiHMDS in THF (À78 °C, 30 min) gave ester 29 in good yield
(78%) with desired stereoselectivity, a Z-isomer. Grignard reaction
on ester 29 with MeMgBr (3 M in Et2O) in ether (0 °C, 4 h), followed
by removal of silyl protecting group on 30 gave target Z-isomer of
Paricalcitol 5 in 51% yield (Scheme 4). The overall yield of this six-
teen step synthesis is 0.1% starting from Quinic acid (10).
The characteristic difference in E and Z-isomer of Paricalcitol
was in chemical shift of protons on C-22 and C-23 atoms. These
protons were observed as multiple peaks in the region 5.27–5.40
for E-isomer, however, in case of Z-isomers these protons were
well distinguished at 5.162–5.216 (m, 1H), 5.33–5.33 (m, 1H)
respectively by 1H NMR (Fig. 3). The other protons are in
Please cite this article in press as: Samala, R.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.01.110

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R. Samala et al. / Tetrahedron Letters xxx (2016) xxx–xxx
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Paricalcitol (5) in CDCl3 solvent (400 MHz).
agreement with reported value24 for E-isomer. These isomers are
separable on HPLC.
The solution form of Z-isomer of Paricalcitol 5 was found to be
stable under the thermal condition (60–180 °C) in various solvents
(viz. EtOH, acetonitrile, Dowtherm).
Conclusion
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In conclusion, we have succeeded in developing an efficient
synthetic approach for the synthesis of Z-isomer of Paricalcitol
from Quinic acid under Julia–Lythgoe’s olefination condition. The
key intermediates in this synthesis effort were phosphine oxide
11 and benzothiazolyl sulfone 8.
Acknowledgments
We sincerely thank GVK Biosciences Private Limited for finan-
cial support and facility for accomplishing this research work.
We are extremely thankful to Azad pharma for sharing research
challenge with us and supporting in our pursuit.
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.01.
110.
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