Mild and selective deprotection method of acetylated steroids and diterpenes by dibutyltin oxide

Article abstract of DOI:10.1016/j.steroids.2006.10.004

Dibutyltin oxide (DBTO) was first utilized for the deacetylation of steroid and diterpene esters. The results showed the deprotection of acetylated steroids and diterpenes separately with moderate catalysis dibutyltin oxide in methanol selectively removed part acetyl groups of these substrates, whereas several functional groups of the steroids and diterpenes were retained and neither isomerization nor degradation of these substrates was observed. It seems that the acetyl groups with lower steric hindrance or near carbonyl, alkoxy, or hydroxyl groups can be cleaved by the reaction, whereas the acetyl groups with higher steric hindrance or without carbonyl, alkoxy, or hydroxyl groups neighboring were retained under the same conditions. One of the interesting results obtained was the selective hydrolysis of the 3β-O-acetyl group in the presence of the 6β group in 3β,6β-Di-O-acetyl-5α-hydroxypregn-16-en-20-one. This allows for subsequent introduction of one unit at C-3 and the other unit at C-6. This procedure is useful for the synthesis of a series of closely related isomers of 3β,5α,6β-trihydroxypregn-16-en-20-one and other widespread polyhydroxysteroids in marine organisms and some terrestrial species.

Full text of DOI:10.1016/j.steroids.2006.10.004

steroids 7 2 ( 2 0 0 7 ) 26–30
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/steroids
Mild and selective deprotection method of acetylated
steroids and diterpenes by dibutyltin oxide
Shao-Min Wang, Yan-Bing Zhang, Hong-Min Liu∗, Guo-Bin Yu, Ke-Rang Wang
Department of Chemistry, New Drug Research & Development Center, Zhengzhou University,
No. 75 Da Xue Road, Zhengzhou 450052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Dibutyltin oxide (DBTO) was first utilized for the deacetylation of steroid and diterpene
esters. The results showed the deprotection of acetylated steroids and diterpenes sepa-
rately with moderate catalysis dibutyltin oxide in methanol selectively removed part acetyl
groups of these substrates, whereas several functional groups of the steroids and diterpenes
were retained and neither isomerization nor degradation of these substrates was observed.
It seems that the acetyl groups with lower steric hindrance or near carbonyl, alkoxy, or
hydroxyl groups can be cleaved by the reaction, whereas the acetyl groups with higher steric
hindrance or without carbonyl, alkoxy, or hydroxyl groups neighboring were retained under
the same conditions. One of the interesting results obtained was the selective hydrolysis of
the 3␤-O-acetyl group in the presence of the 6␤ group in 3␤,6␤-Di-O-acetyl-5␣-hydroxypregn-
16-en-20-one. This allows for subsequent introduction of one unit at C-3 and the other
unit at C-6. This procedure is useful for the synthesis of a series of closely related iso-
mers of 3␤,5␣,6␤-trihydroxypregn-16-en-20-one and other widespread polyhydroxysteroids
in marine organisms and some terrestrial species.
Received 25 July 2006
Received in revised form
10 October 2006
Accepted 12 October 2006
Keywords:
Diterpene
Dibutyltin oxide
Deacetylation
Regioselective reaction
© 2006 Elsevier Inc. All rights reserved.
1.
Introduction
cation and selective O-acetylation of ribonucleosides and in
microwave-mediated N-acylation of 1,2- and 1,3-amino alco-
Selective protection and deprotection of functional groups are
of great importance in organic synthesis [1]. The pharma-
ceutical properties and high cost of steroids and diterpenes
make the research on these natural compounds even more
important. Selective O-deacylation of acetylated steroids and
diterpenes under mild conditions possesses great synthetic
value, as the polyfunctionalized rigid skeleton of these com-
pounds may have further synthetic utility as substrates.
Various methods and deacetylation reagents have been uti-
lized for this purpose [2,3]. Organotin compounds have been
introduced for selective acetylation, alkylation and oxidation
to prepare intermediates for organic synthesis [4–6]. Dibutyltin
oxide (DBTO) in methanol has been used in the transesterifi-
hols [7,8]. Furla´n et al. reported that the cleavage of carboxylic
esters by DBTO was inefficient compared with other organ-
otin compounds [9]. However, selective deacetylation of esters
using DBTO as the catalyst has been little studied from the
reverse viewpoint.
In earlier works, we developed a simple and efficient
method for the cleavage of O-acetyl groups with DBTO under
mild conditions in the synthesis of a telomerase inhibitor
and the regioselective deacetylation of some steroid glyco-
sides and diterpene glycosides [10,11]. The results showed
that DBTO is an active catalyst for the deacetylation of gly-
cosides under mild conditions, which allows high selectivities
between acetyl groups of the sugar moiety and the genin part,
∗
Corresponding author. Tel.: +86 371 67767200; fax: +86 371 67767200.
E-mail address: Liuhm@zzu.edu.cn (H.-M. Liu).
0039-128X/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.steroids.2006.10.004

steroids 7 2 ( 2 0 0 7 ) 26–30
27
Scheme 1 – Steroid and diterpene substrates.
and other acyl groups, including acetanilide and benzoyl ester,
were stable under these conditions. Why does the procedure
seem to be generally applicable for the cleavage of the acetyl
groups of the sugar units, while the acetyl groups of the steroid
and diterpene moieties were retained? Is it because of the
different steric hindrance between the acetyl groups of the
sugar units and the genin parts? Or does the reason lie in
some near functional groups, which would lead to the increase
or decrease in the activity of acetyl group? These questions
prompted us to investigate the applicability and limitations
of the cleavage of a broader spectrum of esters with DBTO. In
this report, we provide additional insight into the scope and
limitations of this method using steroid and diterpene esters
as substrates in the presence of a variety of functional groups
(see Scheme 1).
correlation spectra (HMBC) and DEPT were obtained in CDCl3
on a Bruker DPX-400 instrument with Me4Si as the internal
standard, and the chemical shifts were given in ı values. High-
resolution electronspray mass spectra (HR-ESI) were recorded
on a Waters Q-Tof MS/MS System in positive ion mod. All sol-
vents and reagents were of analytical grade.
Starting steroid esters 1a, 5a, and 6a were obtained com-
mercially and used without purification. Starting esters 2a
[12], 3a [13], 4a [14], 7a [15], 8a, 9a, 10a, 11a, and 12a were pre-
pared from the parent 2b, 3b, 4b, 7b, 8c [11], 9c, 11c, and 12c in
dry acetic anhydride by standard procedures and assayed by
TLC, 1H NMR analysis showed >98% purity. TLC and spectral
analyses (IR, 1H NMR, HRMS) indicated that all deacetylated
steroids product were identical with authentic material or in
agreement with those previously reported. Samples 2b, 3b, 9c
[15], 11c [15], and 12c [16] were isolated from plant materi-
als and crystallized before use. Samples 4b, 5b, and 7b were
obtained commercially. Samples 8c were synthesized by our
laboratories and crystallized before use.
2.
Experimental
2.1.
Analysis and materials
2.2.
General acetylation procedure
Reactions were monitored by TLC on precoated silica gel 60
GF254 (0.5 mm, E. Merck) in cyclohexane/ethyl acetate (2:1, v/v).
Components were visualized by UV light (254 nm), iodine or
by heating. Flash column chromatography (FCC) was carried
out on silica gel (100–200 mesh, Shanghai, China). Infrared
spectra were recorded on a Thermo Nicolet 200 spectrom-
eter using KBr disks in the 400–4000−1 cm region. Melting
points (Mps) were measured on a WC-1 melting-point appa-
ratus and are uncorrected. Survey spectra including 1H-NMR,
homonuclear correlation (COSY), heteronuclear single quan-
tum coherence spectra (HSQC), heteronuclear muliple boan
Acetic anhydride (3 ml) was added to a stirred solution of
steroid or diterpene (0.50 mmol) in absolute pyridine (6 ml)
containing catalytic amounts of DMAP. The resulting mix-
ture was stirred at room temperature for 0.5 h. To destroy
the excess acetic anhydride, saturated NaHCO3 solution was
dropped into the mixture and stirred under ice cooling until
no air bubble was generated. The solution was extracted with
ethyl acetate three times, and the organic layers were com-
bined, washed with water, dried over Na2SO4, filtered, and
evaporated to give a product.

28
steroids 7 2 ( 2 0 0 7 ) 26–30
11.36 Hz, 1␤-H), 5.47, 6.16 (each 1H, s, 17-H2), 5.51 (1H, s, 20-
H), 6.02 (1H, s, 6␣-H), 2.21, 2.14, 2.02 (each 3H, s, Ac × 3). HRMS
(ESI): C26H32O9Na [M + Na]+: calcd. 511.1944, found 511.1934.
2.2.1. 3ˇ,6ˇ-Di-O-acetyl-5˛-hydroxypregn-16-en-20-one
(8a)
Acetylated steroid 8a was obtained in 95% yield m.p.
242–244 ◦C. IR (KBr): 3418, 1736, 1649, 1585, 1373, 1248, 1206,
1031, 966. 1H NMR (400 MHz, CDC13): ı 0.91 (s, 3H, 18-Me),
1.13 (s, 3H, 19-Me), 2.03, 2.09 (s, each 3H, Ac × 2), 2.26 (s, 3H,
21-Me), 4.70 (brs, 1H, 6␣-H), 5.14 (m, 1H, 3␣-H), 6.69 (brs, 1H, 16-
H). HRMS (ESI): C25H36O6Na [M + Na]+: calcd. 455.2410, found
455.2414.
2.2.5. Andrographolide14,19-diacetate (12a)
Acetylated diterpene 12a was obtained in 92% yield m.p.
138–142 ◦C. IR(KBr): 3435, 1773, 1732, 1647, 1374, 1249, 1038,
980, 890.1H NMR (400 MHz, CDCl3): ı 0.78 (3H, s, 20-Me), 1.04
(3H, s, 18-Me), 2.05 (6H, s, Ac × 2), 2.06 (1H, m, 9-H), 3.50 (1H, s,
3␤-H), 4.11 (1H, d, J = 11.76 Hz, 19-H), 4.38 (1H, d, J = 11.72 Hz,
19-H), 4.45, 4.88 (each 1H, s, 17-H2), 4.61, 4.38 (each 1H, m,
15-H2), 6.17 (1H, brs, 14␤-H), 7.00 (1H, brs, 12-H). HRMS (ESI):
C24H34O7Na [M + Na]+: calcd. 457.2202, found 457.2211.
2.2.2. Oridonin 1,6,7,14-tetra-acetate (9a)
Acetylated diterpene 9a was obtained in 16% yield. IR(KBr):
1736, 1720, 1709, 1654, 1639, 1374, 1243, 1203, 1063, 909. 1H
NMR (400 MHz, CDC13): ı 1.l4 (6H, s, Me × 2), 1.98, 2.08, 2.10,
2.14 (each 3H, s, Ac × 4), 3.07 (1H, d, J = 10 Hz, 13-H), 4.20 (1H,
d, J = 10.7 Hz, 20-H), 4.34 (1H, d, J = 10.7 Hz, 20-H), 4.85 (1H, m,
1␤-H), 6.00 (1H, m, 6␣-H), 6.17, 5.88 (each 1H, s, 17-H2), 6.01 (1H,
s, 14␤-H). HRMS (ESI): C28H36O10Na [M + Na]+: calcd. 555.2206,
found 555.2216.
2.3.
General deacetylation procedure
An amount of steroid ester (100–300 mg) is heated under reflux
in dry MeOH or CH3Cl/MeOH (4:1, v/v) containing Bu2SnO (the
indicated mol equiv) for the indicated period (Table 1). The
product was afforded by flash chromatography using mixtures
of cyclohexane and ethyl acetate as eluting solvents.
2.2.3. Oridonin 1,14-diacetate (10a)
Acetylated diterpene 10a was obtained in 81% yield. m.p.
113–116 ◦C. IR(KBr): 3549, 1735, 1722, 1653, 1638, 1241, 1031,
910. 1H NMR (400 MHz, CDCl3): ı 1.l3 (6H, s, Me × 2), 2.02, 2.08
(each 3H, s, Ac × 2), 3.20 (1H, d, J = 10 Hz, 13-H), 3.80 (1H, m, 6␣-
H), 4.26 (1H, d, J = 0.8 Hz, 20-H), 4.28 (1H, d, J = 0.8 Hz, 20-H), 4.63
(1H, dd, J = 5.6 Hz, 10.8 Hz, 1␤-H), 5.52, 6.17 (each 1H, s, 17-H2),
5.80 (1H, s, 14␤-H). HRMS (ESI): C24H32O8Na [M + Na]+: calcd.
471.1995, found 471.1986.
2.3.1. 6ˇ-O-acetyl-3ˇ,5˛-di-hydroxypregn-16-en-20-one
(8b)
8b was obtained in 91% yield m.p. 111–113 ◦C. IR (KBr): 3419,
1736, 1648, 1585, 1372, 1249, 1206, 1030, 967. 1H NMR (400 MHz,
CDC13): ı 0.88 (s, 3H, 18-Me), 1.19 (s, 3H, 19-Me), 2.09 (s, each
3H, Ac), 2.20 (s, 3H, 21-Me), 4.01 (m, 1H, 3␣-H), 4.74 (brs, 1H,
6␣-H), 6.81 (brs, 1H, 16-H). HRMS (ESI): C23H34O5Na [M + Na]+:
calcd. 413.2304, found 413.2309.
2.2.4. Ponicidin 1,6,7-triacetate (11a)
Acetylated diterpene 11a was obtained in 30% yield. IR(KBr):
1775, 1735, 1638, 1374, 1233, 1140, 978, 876. 1H NMR (400 MHz,
CDC13): ı 0.92 (3H, s, 19-Me), 1.06 (3H, s, 18-Me), 3.21 (1H, m,
13-H), 4.49 (1H, d, J = 6.4 Hz, 14␤-H), 4.83 (1H, dd, J = 4.84 Hz,
2.3.2. Oridonin 1-acetate (9b)
9b was obtained in 93% yield m.p. 227–228 ◦C. IR(KBr): 3429,
1735, 1720, 1709, 1654, 1243, 1080, 1032, 908. 1H NMR (400 MHz,
CDCl3): ı 1.08, 1.12 (each 3H, s, CH3 × 2), 2.03 (3H, s, Ac), 3.05
Table 1 – Deprotection of O-acetyl groups of steroid and diterpene substrates with DBTOa
Entry
Substrate
Product Catalyst:substrate
(M/M)
Sv (v/v)
Temperature
(◦C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
18
19
1a
2a
3a
4a
5a
5a
5a
5a
6a
7a
8a
8a
8a
8a
9a
10a
11a
12a
1b
2b
3b
4b
5b
5b
5b
5b
6b
7b
8b
8b
8b
8b
9b
9b
11b
12b
1:2
MeOH:CHCl3 = 4:1
MeOH:CHCl3 = 4:1
MeOH:CHCl3 = 4:1
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Reflux
50
50
18
6
6
14
4
4
20
12
4
2.5
4
15
10
10
70
92
91
85
87
89
91
91
87
87
85
93
95
85
82
1:10
1:10
1:2
2.5:1
1:2
1:2
1:10
1:2
1:2
2.5:1
1:2
1:2
1:10
2:1
Reflux
Reflux
Reflux
50
Reflux
Reflux
Reflux
Reflux
Reflux
50
Reflux
Reflux
Reflux
Reflux
Reflux
4
22
12
5
5
18
4
2:1
2:1
2:1
a
The 1H NMR spectra of the isolated products were in accordance with those of authentic samples.
Isolated yields.
b

steroids 7 2 ( 2 0 0 7 ) 26–30
29
groups opens up the way to introduce one unit at C-3 and sub-
sequently, the other unit at C-6. These results have not been
previously reported.
(1H, d, J = 10 Hz, 13-H), 3.66 (1H, m, 6␣-H), 4.10 (1H, dd, J = 1.2,
10.4 Hz, 20-H), 4.28 (1H, dd, J = 1.2, 10.4 Hz, 20-H), 4.63 (1H, dd,
J = 5.2, 11.2 Hz, 1␤-H), 4.94 (1H, m, 14␤-H), 5.54, 6.03 (each 1H,
s, 17-H2). HRMS (ESI): C22H30O7Na [M + Na]+: calcd. 429.1889,
found 429.1879.
According to the deacetylation of compound 5a (entries
5–9), the higher reaction temperature caused the shorter reac-
tion time. Increasing the molar ratio of DBTO to compound 5a
from 1:10 to 1:2 did not improve the yield outstandingly in spite
of the shorter reaction time. Increasing the molar ratio from
1:2 to 2.5:1 improved the yield whereas the reaction time did
not change. These results were similar to those observed in the
deprotection of 8a (entries 12–15). Furthermore, we found that
this method was not suitable for cleaving the O-acetyl groups
of compounds 1a, 2a, and 3a, which are highly insoluble in
methanol.
2.3.3. Ponicidin 1,6-diacetate (11b)
11b was obtained in 85% yield m.p. 226–227 ◦C. IR(KBr): 3449,
1774, 1732, 1640, 1374, 1231, 1141, 878. 1H NMR (400 MHz,
CDCl3): ı 0.90 (3H, S, 19-Me), 0.94 (3H, S, 18-Me), 3.21 (1H,
m, 13-H), 4.53 (1H, d, J = 6.4 Hz, 14␤-H), 4.82 (1H, dd, J = 4.4 Hz,
11.2 Hz, 1␤-H), 4.85 (1H, s, 6␣-H), 5.43, 6.12 (each 1H, s, 17-H),
5.44 (1H, s, 20-H), 2.27, 2.11 (each 3H, s, Ac), 5.31 (1H, s, 7-
OH). HRMS (ESI): C24H30O8Na [M + Na]+: calcd. 469.1838, found
469.1835.
4.
Conclusion
2.3.4. Andrographolide 14-acetate (12b)
12b was obtained in 82% yield m.p. 127–129 ◦C. IR(KBr): 3435,
1772, 1735, 1646, 1374, 1247, 1037, 981, 893. 1H NMR (400 MHz,
CDCl3): ı 0.76 (3H, s, 20-Me), 1.04 (3H, s, 18-Me), 2.05 (3H,
s, Ac), 2.01 (1H, m, 9-H), 3.45 (1H, brs, 3␤-H), 3.46, 3.74
(each 1H, s, 19-H2), 4.62, 4.87 (each 1H, s, 17-H2), 4.59, 4.39
(each 1H, m, 15-H2), 5.48 (1H, brs, 14␤-H), 7.31 (1H, brs, 12-
H). HRMS (ESI): C22H32O6Na [M + Na]+: calcd. 415.2097, found
415.2086.
Removal of protective groups in steroids and diterpenes
requires controlled reaction conditions in order to avoid
the modifications of other functional groups present in the
molecule. The conventional base-catalyzed saponification
reaction is difficult and not always successful because the
basic medium may promote undesirable side reactions and
lack of selectivity.
Compared with the scope of this work with that of our prior
publications [10,11], it is obvious that the deacetylation proce-
dure was not limited only to the acetyl groups of the sugar
unit of a glycoside [10,11]. According to this report, DBTO is
an active catalyst for the deacetylation of steroid and diter-
pene esters under mild conditions that allows the survival
of common acid- or base-sensitive protecting groups as well
as a variety of unprotected functional groups, which led to a
mild and simple alternative to hydrolysis by acidic or basic
solutions. This deacetylation procedure also showed selectiv-
ity between acetyl groups of steroid and diterpene esters. It
seems that these acetyl groups with lower steric hindrance
or near carbonyl, alkoxy, or hydroxyl groups can be cleaved
by the reaction, whereas the acetyl groups with higher steric
hindrance or without carbonyl, alkoxy, or hydroxyl groups
neighboring were retained under the same conditions. Fur-
thermore, this method is not suitable for cleaving the O-acetyl
groups of the compounds 1a, 2a, and 3a that are highly insol-
uble in methanol.
One of the interesting results obtained was the selective
hydrolysis of the 3␤-acetyl group in the presence of the 6␤
group in 8a. This regioselectivity hydrolysis of the acetates
of the two steroidal hydroxyl groups opens a way to the syn-
thesis of a series of closely related isomers of 8a and other
widespread polyhydroxysteroids in marine organisms and
some terrestrial species.
It is also noteworthy from the cleavage of an O-acetyl group
of a chiral alcohol that the procedure is useful for the recovery
of chiral alcohols with complete retention of configuration.
DBTO is commercially available and inexpensive.
3.
Results and discussion
Several acetylated steroids and diterpenes were prepared
(Scheme 1) and then deacetylated separately with DBTO under
mild conditions (Table 1). The progress of the reaction was
monitored by TLC to check for the disappearance of the
starting material. As shown in Table 1, DBTO gave satisfac-
tory results in terms of conversion and selectivity. It can
catalyze the deprotection of acetyl groups located at differ-
ent rings of the steroid skeleton and the side chain (entries
1–15) affording the products in excellent yields. It also can
selectively catalyze the deprotention of acetyl groups at diter-
pene esters. Other functional groups of steroid or diterpene
esters are stable under these reaction conditions, and no
evidence was found for isomerization or degradation of the
substrates.
Under the same conditions, the acetyl group in the side
chain of compound 7a was cleaved in good yield (91%) in 2.5 h
(entry 11), whereas deprotection of other acetyl groups located
on the steroid skeleton required longer reaction times due to
steric hindrance around the carbinol atom.
One of the interesting results obtained was the selec-
tive hydrolysis of the 3␤-acetyl group in the presence of the
6␤ group in steroid 8a. Compound 8a was synthesized by
the acetylation of 8c, which was synthesized by chemical
or biological methods in our laboratory from 3␤-acetoxy-
5,16-pregnadien-20-one, an important intermediate of raw
medicines. Marine species, such as sponges, cnidaria, coe-
lenterates, molluscs, and echinoderms, are rich sources of
a large variety of polyhydroxysteroids [17–20]. The 3␤,5␣,6␤-
hydroxylation pattern is found in several polyhydroxysteroids
of marine origin and macro-fungi [21–24]. This regioselec-
tive hydrolysis of the acetates of these two steroidal hydroxyl
Acknowledgements
We thank the NNSF of PRC and Natural Science Foundation of
Henan Province for financial support of this work.

30
steroids 7 2 ( 2 0 0 7 ) 26–30
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