A concise synthesis of 25-Hydroxycholesterol from hyodesoxycholic Acid

Article abstract of DOI:10.3184/174751918X15184426915885

A simple, efficient and economical method has been developed for the synthesis of 25-hydroxycholesterol in seven steps from hyodesoxycholic acid with an overall yield of 39%. The preparation of the 3β-tetrahydropyranyloxychol-5-en-24-al from 3β-tetrahydropyranyloxychol-5-en-24-oic acid methyl ester with di-isobutylaluminium hydride was achieved instead of using the conventional two-step reaction, thus avoiding the use of the toxic oxidant CrO₃. The terminal product was obtained by hydroxybromination of desmosterol with N-bromosuccinimide/H₂O, followed by reduction and deprotection of the halohydrins with LiAlH4. This simplified route gave an increased overall yield and used economical and environmentally benign reagents.

Full text of DOI:10.3184/174751918X15184426915885

96 RESEARCH PAPER
VOL. 42 FEBRUARY, 96–99
JOURNAL OF CHEMICAL RESEARCH 2018
A concise synthesis of 25-hydroxycholesterol from hyodesoxycholic acid
Can Jina, Yulei Wangb, Bin Suna and Weike Sua*
aCollaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310014, P.R. China
bKey Laboratory for Green Pharmaceutical Technologies and Related Equipment of Ministry of Education, College of Pharmaceutical Sciences, Zhejiang
University of Technology, Hangzhou 310014, P.R. China
A simple, efficient and economical method has been developed for the synthesis of 25-hydroxycholesterol in seven steps from hyodesoxycholic acid
with an overall yield of 39%. The preparation of the 3β-tetrahydropyranyloxychol-5-en-24-al from 3β-tetrahydropyranyloxychol-5-en-24-oic acid methyl
ester with di-isobutylaluminium hydride was achieved instead of using the conventional two-step reaction, thus avoiding the use of the toxic oxidant CrO .
The terminal product was obtained by hydroxybromination of desmosterol with N-bromosuccinimide/H2O, followed by reduction and deprotection of th3e
halohydrins with LiAlH4. This simplified route gave an increased overall yield and used economical and environmentally benign reagents.
Keywords: hyodesoxycholic acid, di-isobutylaluminium hydride, 3β-tetrahydropyranyloxychol-5-en-24-al, hydroxybromination,
25-hydroxycholesterol
Vitamin D3 has
a
series of metabolites, such as
carbonyl of compound 4 to an alcohol and then oxidising this
with CrO3. Just as with mercuric acetate, CrO3 is a poisonous
reagent and can seriously contaminate the desired products.
The long linear synthetic route and poisonous reagents have
prevented practical application of this method. Thus, there is
a need for a procedure that avoids these issues and provides an
efficient and environmentally benign method.
25-hydroxyvitamin D3 and 1α,25-dihydroxyvitamin D3, which
possess important biological activity. 25-Hydroxyvitamin D3 is
a significantly better anti-rachitic agent than the parent vitamin
D3. In addition, it is a more reasonable indicator of nutritional
vitamin D3 status. 25-Hydroxycholesterol is a crucial drug
intermediate for preparing these metabolites.1 Furthermore, it
also can prevent Zika virus (ZIKV) infection in human cortical
organs.2 Consequently, attention has been focused on the
synthesis of 25-hydroxycholesterol in the recent years.
A concise synthetic route for 25-hydroxycholesterol has
been developed from hyodesoxycholic acid, which is a more
economical precursor than the other previously reported
starting materials. The complete route, involving seven steps,
is more efficient and uses safe and economical reagents with
excellent yields. The reduction of compound 4 to aldehyde 5
(Scheme 1) immediately by di-isobutylaluminium hydride
(DIBAL-H) in only one step was carefully examined
in place of Khripach’s conventional two-step process.
Environmentally benign reagents were used in our chosen
route to avoid environmental contamination and increase the
isolated yield of the corresponding product. Then, the aldehyde
5 was transformed into desmosterol by a Wittig olefination.
In 1985, Zhong’s group3 reported
a
synthesis of
25-hydroxycholesterol from methyl hyodeoxycholate in nine
steps with an overall yield of 20.6%. C-25 was added to the
side chain by a Grignard reaction, and the introduction of the
25-hydroxy group was achieved by a mercuric acetate reaction.
However, mercuric acetate is a poisonous reagent that can
contaminate the products and pollute the environment, thus
making it difficult to use commercially. In 2001, Khripach’s
group4 reported another method for the preparation of
5
25-hydroxycholesterol from Δ -cholenic acid in nine steps.
5
24
Aldehyde 5 (Scheme 1) was prepared from Δ -cholenic acid
Halogenation of the Δ double bond is a superior method for the
methyl ester in two steps, which included reducing the ester
introduction of the 25-hydroxy group into steroids. Compared
Me
CO2
H
CO2
Me
CO2
H
2SO4
p-TsCl
CH3OH
DMAP
HO
HO
TsO
H
H
H
OH
OH
OTs
1
2
3
Me
CO2
O
1. AcOK, KOH, DMF
2. DHP
Ph3
P=CMe2
DIBAL-H
toluene
THF
THPO
THPO
4
5
Br
OH
OH
1. LiAlH4
NBS/H2O
THF
2.
CH3OH, HCl
HO
THPO
THPO
7
6
8
Scheme 1 Synthesis of 25-hydroxycholesterol 8 from the precursor hyodesoxycholic acid 1.
* Correspondent. E-mail: pharmlab@zjut.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2018 97
with other halogenation reagents, the N-halosuccinimide
(NBS5, NCS6) used in the reaction was more economical and
reacted under milder conditions. The hydroxybromination of
desmosterol proceeded smoothly with NBS/H2O to give the
halohydrin derivative 7. The reduction and deprotection of the
halohydrins was achieved smoothly with lithium aluminium
hydride (LiAlH4) in tetrahydrofuran (THF) (Scheme 1).5
increased as the amount of DIBAL-H was increased (up to 1.2
equiv.). However, the information from entries 8–10 indicated
that a further reduction by-product was gradually formed,
resulting in a lower yield when the amount of DIBAL-H was
greater than 1.2 equiv. In addition, the yield of compound 5 was
the highest when the reaction was carried out for 1 h (Table 1,
entries 11–12).
In the next stage of the reaction, the isopropenyl group
was introduced onto the side chain of aldehyde 5 by a Wittig
olefination with isopropyltriphenylphosphonium iodide, to give
Results and discussion
Hyodesoxycholic acid, a major component of pig bile, is
a suitable starting material for a concise route to prepare
25-hydroxycholesterol. Its structure permits the introduction of
the isopropenyl group of the side chain and the C-25 hydroxy
group. In the first stage, esterification of hyodesoxycholic
acid with methanol catalysed by concentrated sulfuric acid
gave the corresponding methyl ester 2 (Scheme 1).7 Then the
3,6-ditosylate 3, prepared from ester 2 with tosyl chloride, was
treated with boiling DMF/H2O in the presence of CH3COOK,
resulting in simultaneous inversion at the C-3 position
and elimination of the toluene-p-sulfonate at C-6 to give
3β-hydroxy-5-cholen-24-oate.8,9 The 3β-hydroxy group was
protected as the tetrahydropyranyl ether to afford compound 4
in 80% yield (Scheme 1).10
24
desmosterol (6) (Scheme 1). Subsequently, the Δ double bond of
6 was converted to the halohydrin derivative (Scheme 1). Different
halogenation reagents were tested, such as N-bromosuccinimide
(NBS5) and N-chlorosuccinimide (NCS6). NBS gave a better
result, and the desired product 7 was isolated in 72% yield
(Table 2, entries 1–2).12 With the optimised halogenation reagent
in hand, various solvent systems were investigated, including
EtOAc, THF, CH2Cl2 and toluene, all in combination with water
(Table 2, entries 2–5). Among them, the reaction in THF/H2O
(entry 2) gave the best result. When the THF/H2O ratio was
varied from 5:1 to 20:1 (volume ratio), the yield of product 7
decreased slightly (Table 2, entries 6–9). Therefore, a mixture
of THF/H2O (5:1 volume ratio) was chosen as the best solvent
system for this halogenation reaction.
A number of reducing agents were tried in order to synthesise
the aldehyde 5 from compound 4 in one step. These included
LiAlH4, LiBH4, NaBH4-AlCl3 and DIBAL-H.11 The desired
product was obtained when DIBAL-H (1.5 mol L−1 in toluene)
was used as reducing reagent (Scheme 1). Subsequently, the
reaction conditions were optimised for solvent, reaction time,
temperature and the amount of DIBAL-H (Table 1). The
reaction temperature was controlled at a relatively lower value,
which was varied from −30 °C to −70 °C. When the reaction was
initially carried out at −30 °C, the major product was shown to
be the further-reducing product 3β-tetrahydropyranyloxychol-
5-en-24-ol instead of the desired product 5. This was confirmed
by IR and NMR. We considered that a lower temperature would
lead to a better yield of product 5. This was confirmed by
comparative experiments (Table 1, entries 1–5). The reaction
proceeded satisfactorily at −70 °C, giving product 5 in 72%
yield (Table 1, entry 4). Toluene was selected as the solvent for
the reduction reaction because of the substrate’s better solubility
in it and its lower freezing point.
The reaction was carried out between −10 °C and 0 °C
using NBS as the halogenation reagent (Table 2, entries 9–11).
When the reaction temperature was above −5 °C, the yield of
the desired product decreased markedly and there was a lack
5
of selectivity between the Δ double bond and the side chain
24
Δ
double bond of compound 6. The reaction did not proceed
well at −10 °C. Finally, the reaction was carried out at −5 °C,
affording the product 7 in excellent yield (Table 2, entry 10).
Next, the amount of NBS was carefully varied from 1.0 to 1.8
equiv. for this bromohydrin reaction (Table 2, entries 12–15).
The results indicated that 1.5 equiv. was the best choice. Using
an amount of NBS above 1.5 equiv. resulted in the formation of
by-product (Table 2, entry 15). The yield of product 7 decreased
when the reaction time was extended to 3 h. Moreover, shortening
the reaction time to 1.5 h also led to a decrease in the yield.
Thus, the best overall result was obtained when the reaction was
performed in THF/H2O (5:1) in the presence of NBS (1.5 equiv.) at
−5 °C for 2 h, giving the corresponding product 7 in 90% isolated
yield (Table 2, entry 14). In the last step, dehalogenation of the
Next, the influence of different amounts of DIBAL-H was
investigated. Compound 4 was treated with different amounts
of DIBAL-H ranging from 1.0 equiv. to 1.5 equiv. in toluene
at −70 °C (Table 1, entries 6–10). It was found that the yield
Table 2 Further optimisation of the reaction conditionsa
Entry Reagent (equiv.) Solvent
Time (h) Temp. (oC) Yield (%)b
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
NCS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.0)
NBS (1.3)
NBS (1.5)
NBS (1.8)
NBS (1.5)
NBS (1.5)
THF:H2O (5:1)
THF:H2O (5:1)
EtOAc:H2O(5:1)
CH2Cl2:H2O(5:1)
Toluene:H2O(5:1)
THF:H2O (10:1)
THF:H2O (20:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
THF:H2O (5:1)
3.0
3.0
6.0
6.0
6.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.0
1.5
−10
−10
−10
−10
−10
−10
−10
−10
−5
0
−5
−5
−5
62
72
36
41
26
75
70
80
83
79
82
86
90
65
86
80
Table 1 Optimisation of the reaction conditionsa
Entry
1
2
3
4
5
6
7
8
DIBAL-H (equiv.)c
Time(h)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.5
Temp. (oC)
Yield (%)b
39
1.0
1.0
1.0
1.0
1.0
1.1
1.2
1.3
1.4
1.5
1.2
1.2
−30
−50
−60
−70
−80
−70
−70
−70
−70
−70
66
70
72
70
73
78
64
62
9
10
11
12
59
68
56
−5
−5
−5
−70
−70
aReagents and conditions: compound 4 (4 mmol); solvent: toluene.
bIsolated yield.
aReagents and conditions: compound 5 (2 mmol)
bIsolated yield.
cDIBAL-H (1.5 mol L−1 in toluene) was added dropwise to the reaction mixture.

98 JOURNAL OF CHEMICAL RESEARCH 2018
Na2SO4, and concentrated under reduced pressure to give an orange
oil. The residue was then dissolved in KOH/MeOH (2%, 50 mL) and
stirred for 2 h at room temperature. The solution was treated with 1 M
HCl (200 mL) and ice chips were added gradually to the mixture. The
precipitated solid was filtered off and washed with water. Purification
by silica gel chromatography eluting with n-hexane/ethyl acetate (8:1)
gave a white solid. In the next sequence of reactions, the solid (2.4 g,
6.2 mmol) was dissolved in CH2Cl2 (25 mL), dihydropyran (5.22g,
62 mmol) and pyridinium p-toluenesulfonate (0.312g, 12.4mmol) were
added to the solution, which was then stirred at room temperature under
a nitrogen atmosphere overnight. The solvent was removed in vacuo to
produce a white solid, which was extracted with EtOAc (3 × 15 mL),
then washed brine (2 × 30 mL), dried over anhydrous Na2SO4,
and concentrated under reduced pressure to give, without further
purification, compound 4: White crystals; 2.9 g (80%); m.p. 132–134 °C
(lit.10 133–135 °C); 1H NMR (600 MHz, CDCl3): δ 5.36 (s, 1H), 4.72 (s,
1H), 3.93 (m, 1H), 3.68 (s, 3H), 3.51 (m, 2H), 2.37–1.04 (m, 31H), 1.02 (s,
3H), 0.93 (d, J = 6.1 Hz, 3H), 0.69 (s, 3H); 13C NMR (150 MHz, CDCl3):
δ 177.0, 140.8, 121.3, 96.9, 75.9, 62.8, 56.2, 55.9, 51.9, 50.8, 42.7 , 39.8,
37.6, 35.1, 31.9, 31.8, 31.4, 30.9, 30.8, 29.7, 26.2, 25.6, 21.3, 20.6, 19.3,
17.0, 11.9; MS (ESI): C30H48NaO4 [M+Na]+; calculated: 495.7, found:
495.7.
halohydrin proceeded with LiAlH4 in THF at 70 °C. This was
followed by removal of the tetrahydropyranyl ether in CH3OH/
HCl, giving the final product, 25-hydroxycholesterol.
Conclusion
We have developed an efficient route for the synthesis of
25-hydroxycholesterol in seven steps from hyodesoxycholic
acid using economical and environmentally benign reagents.
Moreover, the synthetic procedures have been optimised
successfully and afford the product in good overall yield (39%).
Experimental
Melting points were determined on a digital melting point apparatus. 1H
and 13C NMR spectra were recorded on Bruker AV-600 spectrometer
at frequencies of 600 and 150 MHz, respectively, in CDCl3 using
tetramethylsilane as internal reference. The values of the chemical
shifts (δ, ppm) and coupling constants (J, Hz) are given below. Mass
spectra (MS) were obtained on a Bruker Daltonicsmicr OTOF-Q II
instrument. The starting material 1 was provided by Anhui Chem-Bright
Bioengineering Co., Ltd. THF and toluene were distilled from sodium-
benzophenone immediately prior to purification. Other reagents were
purchased commercially and were used without further purification. The
progress of the reaction was monitored by thin-layer chromatography.
Detection was performed by spraying with a molybdophosphoric acid–
ethanol solution (5%) at 80 °C. Column chromatography was performed
on silica gel (200–300 mesh) and the elution was performed with
n-hexane/ethyl acetate.
3β-Tetrahydropyranyloxychol-5-en-24-al (5)
Compound 4 (2.0 g, 4.23 mmol) was dissolved in toluene (10 mL) and
then the solution was cooled at −70 °C for 30 minutes. DIBAL-H (1.5
mol L−1 in toluene, 3.4 mL, 5.1 mmol) was then slowly added dropwise
for 40 minutes at −70 °C under a nitrogen atmosphere. The mixture
was stirred at −70 °C for 1 h. The reaction was quenched with saturated
Na2SO4 solution and extracted with EtOAc (3 × 15 mL). The suspension
was filtered off and the filtrate was then concentrated under reduced
pressure to give the product 5. Purification by silica gel chromatography
eluting with n-hexane/ethyl acetate (40:1) gave compound 5: White
solid; 1.46 g (78%); m.p. 124–126 °C (lit.11 122–124 °C); 1H NMR
(600 MHz, CDCl3): δ 9.76 (s, 1H), 5.34 (s, 1H), 4.71 (s, 1H), 3.91 (m,
1H), 3.51 (m, 2H), 2.37–1.04 (m, 31H), 1.00 (s, 3H), 0.93 (d, J = 6.7 Hz,
3H), 0.67 (s, 3H); 13C NMR (150 MHz, CDCl3): δ 203.5, 141.3, 121.5,
97.0, 75.7, 62.8, 56.9, 55.8, 50.1, 42.5, 41.1, 40.2, 39.7, 38.9, 37.5, 37.2,
36.7, 35.3, 31.8, 31.3, 29.7, 28.2, 27.9, 25.5, 24.2, 21.1, 20.1, 20.0, 19.3,
18.4, 11.8; MS (ESI): C29H46NaO3 [M+Na]+; calculated: 465.6, found:
465.7.
Synthesis of the intermediate compounds
Hyodeoxycholic acid methyl ester (2)
A solution of the hyodesoxycholic acid 1 (20 g, 51 mmol) in methanol
(100 mL) was treated with concentrated sulfuric acid (2.5 mL) slowly
added dropwise over 30 minutes at 0 °C under a nitrogen atmosphere.
The mixture was then stirred for 18 h at room temperature. Removal
of the solvent in vacuo produced an orange oil, which was extracted
with EtOAc (2 × 30 mL), then washed with NaHCO3 (2 × 30 mL) and
brine (2 × 30 mL). The organic extracts were concentrated in vacuo to
give compound 2: White solid; yield 21 g (100%); m.p. 66.8–67.6 °C
1
(lit.7 65–68 °C); H NMR (600 MHz, CDCl3): δ 4.05 (d, J = 4.7 Hz
1H), 3.65 (s, 3H), 3.49 (m, 1H), 2.42–1.04 (m, 26H), 1.02 (s, 3H), 0.94
(d, J = 6.1 Hz, 3H), 0.68 (s, 3H); 13C NMR (150 MHz, CDCl3): δ 174.1,
71.8, 68.2, 56.8, 56.0, 52.0, 50.1, 42.5, 40.0, 36.5, 35.8, 35.2, 34.3, 32.5,
31.4, 31.0, 30.9, 24.5, 24.1, 20.8, 19.3, 14.2, 13.9; MS (ESI): C25H42NaO4
[M+Na]+; calculated: 429.3, found: 429.3.
3β-Tetrahydropyranyloxycholesta-5,24-diene (6)
Isopropyltriphenylphosphonium iodide (0.95 g, 1.12 mmol) was
dissolved in THF (5 mL) and the solution was cooled to 0 °C under a
nitrogen atmosphere. After 30 minutes, n-butyllithium (2.5 mol L−1,
1.38 mL, 2.3 mmol) was slowly added dropwise. The mixture was stirred
for 30 minutes and then a solution of compound 5 (0.5 g, 1.12 mmol)
in THF was added to the resulting bright red mixture. This was stirred
at room temperature for 2 h. The suspension was filtered off, washed
with EtOAc and the filtrate concentrated under reduced pressure. The
residue was chromatographed by silica gel chromatography eluting with
n-hexane/ethyl acetate (100:1) to give compound 6: White solid; 0.42 g
(80%); m.p. 118–122 °C (lit.4 119–121 °C); 1H NMR (600 MHz, CDCl3):
δ 5.37 (s, 1H), 5.11 (br. t, 1H), 4.74 (br. s, 1H), 3.94 (m, 1H), 3.56 (m,
2H), 2.37–1.75 (m, 14H), 1.62 and 1.70 (2s, 2 × 3H), 1.58–1.07 (m, 17H),
1.02 (s, 3H), 0.96 (d, J = 6.7 Hz, 3H), 0.69 (s, 3H); 13C NMR (150 MHz,
CDCl3): δ 139.8, 131.3, 125.5, 122.8, 105.6, 75.7, 62.8, 56.9, 55.8,
50.1, 42.5, 41.1, 40.2, 39.7, 38.9, 37.5, 37.2, 36.7, 35.3, 31.8, 31.3, 29.7,
28.2, 27.9, 25.5, 24.2, 21.1, 20.1, 20.0, 19.3, 18.4, 17.9, 11.8; MS (ESI):
C32H52NaO2 [M+Na]+; calculated: 491.7, found: 491.7.
3α,6α-Ditosyloxy-5β-cholan-24-oate (3)
A solution of compound 2 (10.0 g, 25 mmol) in pyridine (50 mL)
was treated with
a solution of TsCl (14.1 g, 75 mmol) and
dimethylaminopyridine (0.342 g, 2.8 mmol) in pyridine (20 mL). The
mixture was left in a refrigerator for two days. The solution was then
poured into 1 M HCl (300 mL) and ice chips were added gradually to the
mixture. The precipitated solid was filtered off and washed with water
to give, without further purification, compound 3: White crystals; yield
17.8 g (100%); m.p. 166–167 °C (lit.8 165–167 °C); 1H NMR (600 MHz,
CDCl3): δ 7.79 (d, J = 8.1 Hz, 2H), 7.71 (d, J = 8.1 Hz, 2H), 7.35 (m, 4H),
4.79 (m, 1H), 4.30 (m, 1H), 3.65 (s, 3H), 2.84–0.92 (m, 26H), 2.4 and
2.47 (s, each 3H), 0.89 (d, J = 6.5 Hz, 3H), 0.80 (s, 3H), 0.60 (s, 3H);
13C NMR (150 MHz, CDCl3): δ 174.3, 144.6, 144.5, 134.0, 129.8, 129.7,
127.8, 127.7, 127.6, 81.7, 80.1, 55.7, 51.5, 46.3, 42.8, 39.8, 39.7, 36.1, 35.2,
34.8, 32.1, 30.8, 27.9, 27.4, 26.4, 23.9, 22.9, 21.7, 21.6, 20.5, 18.2, 11.9; MS
(ESI): C39H53O8S2 [M+Na]+; calculated: 737.3, found: 737.3.
25-Hydroxycholesterol (8)
3β-Tetrahydropyranyloxychol-5-en-24-oic acid methyl ester (4)
Compound 6 (200 mg, 0.430 mmol) was dissolved in THF/water (5:1)
(20 mL) and then the solution was cooled at −5 °C under a nitrogen
atmosphere. Then NBS (115.3 mg, 0.645 mmol) was slowly added in
four batches over 1 h. The mixture was stirred at −5 °C under a nitrogen
atmosphere. After 2 h, the reaction was quenched with saturated Na2S2O4
solution, washed with NaHCO3 (2 × 15 mL) and brine (2 × 15 mL),
A solution of compound 3 (10 g, 14 mmol) in DMF (80 mL) was treated
with a solution of AcOK (1.2 g, 12.2 mmol) in H2O (10 mL). The mixture
was refluxed at 105 °C under a nitrogen atmosphere overnight. When
the solution was cooled down to room temperature it was extracted with
EtOAc (100 mL), washed with brine (3 × 30 mL), dried over anhydrous

JOURNAL OF CHEMICAL RESEARCH 2018 99
dried over anhydrous Na2SO4, and concentrated under reduced pressure
to give the crude product. Purification by silica gel chromatography
eluting with n-hexane/ethyl acetate (10:1) gave a white solid 7 (218 mg,
90%). The solid (220 mg, 0.39 mmol) was dissolved in THF (12 mL) and
LiAlH4/THF (2.5 M, 0.624 mL, 1.6 mmol) was added dropwise at room
temperature under a nitrogen atmosphere. The reaction mixture was
refluxed at 70 °C for 3 h. The reaction was cooled to room temperature
and concentrated under reduced pressure to give a crude product. This
was dissolved in methanol/THF (2:1) (15 mL), HCl (36% wt, 0.65 mL)
was added dropwise, and the solution was then stirred for 1 h at room
temperature. The mixture was extracted with CH2Cl2 (3 × 15 mL),
washed with NaHCO3 (2 × 15 mL) and brine (2 × 15 mL), and dried over
anhydrous Na2SO4. The organic extract was concentrated under reduced
pressure to give the crude product 8. Recrystallisation of the crude
product from toluene gave the pure compound 8: White crystals; 140 mg
(80%); m.p. 191.5–193.4 °C (lit.3 185–187 °C); 1H NMR (600 MHz,
CDCl3): δ 5.37 (s, 1H), 3.56 (m, 1H), 2.30–1.24 (m, 22H), 1.23 (s, 6H),
1.21–1.05 (m, 5H), 1.02 (s, 3H), 0.96 (d, J = 6.5 Hz, 3H), 0.69 (s, 3H);
13C NMR (150 MHz, CDCl3): δ 140.8, 121.7, 77.2, 77.0, 76.8, 71.8, 71.14,
56.7, 56.1, 44.4, 42.3, 42.3, 39.8, 37.3, 36.5, 36.5, 35.7, 31.9, 31.6, 29.3,
29.2, 28.3, 24.3, 21.1, 20.8, 19.4, 18.7, 18.4, 11.9; MS (ESI): C27H46NaO2
[M+Na]+; calculated: 425.3, found: 425.3.
Received 18 December 2017; accepted 4 February 2018
Paper 1705150
https://doi.org/10.3184/174751918X15184426915885
Published online: 16 February 2018
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The authors are grateful for the support of the National Natural
Science Foundation of China (21406200).