Novel Method for O-Acetylation of Cholesterol, Allobetulin, and Betulin

Article abstract of DOI:10.1007/s10600-019-02720-9

Primary and secondary triterpene alcohols were acetylated for the first time using tetraacetylglycoluril (TAGU). Cholesterol, allobetulin, and betulin acetates were obtained in high yields. The acetylation used p-TsOH or TFA in refluxing CHCl₃. TFA was found to be an effective acetylation catalyst.

Full text of DOI:10.1007/s10600-019-02720-9

DOI 10.1007/s10600-019-02720-9
Chemistry of Natural Compounds, Vol. 55, No. 3, May, 2019
NOVEL METHOD FOR O-ACETYLATION OF
CHOLESTEROL, ALLOBETULIN, AND BETULIN
Salah Arrous,* Imene Boudebouz, Abdigali Bakibaev,
Phuoc Hoang, and I. Parunov
Primary and secondary triterpene alcohols were acetylated for the first time using tetraacetylglycoluril
(TAGU). Cholesterol, allobetulin, and betulin acetates were obtained in high yields. The acetylation used
p-TsOH or TFA in refluxing CHCl . TFA was found to be an effective acetylation catalyst.
3
Keywords: betulin, allobetulin, cholesterol, acetylation, glycoluril.
Natural compounds that have been used for millennia to treat human diseases are now playing important roles in the
development of efficacious antitumor and anti-infectious drugs, among others [1, 2].
The yield of target product in multistep syntheses using compounds with several functional groups depends on the
protection and deprotection of them [3].
Acylation of natural compounds is a crucial problem because the variety of biologically active compounds could be
expanded by adding acyl groups [4–7].
Acetates were used effectively to protect alcohols because of their simplicity [8–10]. Usually, reagents such as acetic
anhydride or acetyl chloride in the presence of various acidic or basic catalysts are used for acetylation [11].
Many of the previously used O-acetylation methods have definite insufficiencies. The catalysts are expensive and
toxic and require difficultly accessible reagents.
Glycoluril reacted with acetic anhydride under acidic conditions to give tetraacetylglycoluril (2, TAGU,
2,4,5,8-tetraacetyl-2,4,5,8-tetraazabicyclo[3.3.0]octane-3,7-dione) [12], which was used as an N-acetylating agent for several
proteins [13] and amines [14]. However, TAGU has not previously been used as an acetylating agent for alcohols in classical
organic synthesis or during mechanochemical activation.
O
Ac
Ac
Ac
N
N
N
N
a
ROAc
ROH
+
Ac
1a-c
3a-c
O
2
O
H
H
OH
ROH =
HO
H
H
H
H
HO
HO
H
H
1a
1b
1c
a. CF COOH, CHCl , 70°C, 1–1.5 h
3
3
Chemistry Department, National Research Tomsk State University, 36 Prosp. Lenina, Tomsk, 634050,
e-mail: parroussalinkov@yahoo.com. Translated from Khimiya Prirodnykh Soedinenii, No. 3, May–June, 2019, pp. 412–414.
Original article submitted November 9, 2018.
©
482
0009-3130/19/5503-0482 2019 Springer Science+Business Media, LLC

TAGU was interesting because of its environmental safety and facile separation from the reaction mixture. Herein,
we report a new convenient and efficient method for acetylation of betulin, cholesterol, and allobetulin using TAGU as an
acetylating agent.
Previously, the reaction of betulin (1a) with TAGU (2) in the presence of p-TsOH gave betulin diacetate (3a) in good
yield after 2 h [15]. The yield of 3a was 98% after 1 h if TFA was used as the catalyst instead of p-TsOH. IR and NMR
spectroscopic data for 3a agreed with those in the literature [15].
Cholesterol (1b) and allobetulin (1c) under analogous conditions gave the target products in 92 and 95% yield,
respectively, after 1.5 h. The course of the reaction was monitored by TLC and IR spectroscopy. The synthesized products
were simple to prepare and purify. The acetylated products were identified by comparing their TLC, IR and NMR spectra, and
melting points with those of standards.
Alcohols 1a–c were treated with TAGU in CHCl at room temperature in the presence of TFA. The corresponding
3
acetyl esters 3a–c were not observed after 24 h. Conversely, the expected products were obtained after only 1.5 h at 70°C.
The same results were obtained if p-TsOH was used instead of TFA to convert 1b and 1c into 3b and 3c. However,
longer reaction times (> 6 h) were required and the yields were lower (by 20% vs. TFA, 1.5 h) if p-TsOH was used as the
catalyst for the conversion. The acetylation yields for converting 1b and 1c into 3b and 3c decreased whereas the reaction
times increased from 1.5 to 6 h if p-TsOH was used instead of TFA.
Cholesterol-acetate formation was confirmed by the appearance in the PMR spectrum of a singlet at δ 1.99 ppm for
13
the methyl and a CH multiplet shifted to 4.50 ppm because of deshielding by the acetyl. The C NMR spectrum was
3
consistent with a carbonyl resonance at δ 170.65 ppm whereas oxygenated C-3 resonated at 74.03 ppm. The IR spectrum
–1
–1
lacked O–H absorption in the range 3200–3600 cm and showed a new absorption band at 1726.6 cm that was characteristic
of ester C=O.
13
The structure of allobetulin acetate was confirmed by spectral data (IR, PMR, C NMR). The IR spectrum showed
a new sharp absorption band at 1726 cm , indicating that the C-3 OH of allobetulin was esterified. The shift of the H-3
–1
resonance from 3.45 to 4.38 ppm in the PMR confirmed that C-3 was esterified. The same effect was observed for C-3 in the
13
C NMR spectrum with the resonance shifting from 78.1 to 80.96 ppm.
EXPERIMENTAL
13
PMR and C NMR spectra were obtained with TMS internal standard (0 ppm) on a BrukerAvance III D spectrometer
at operating frequency 400 MHz. Coupling constants were given in Hz. The course of reactions was monitored using TLC on
Sorbfil plates and C H –CH Cl –MeOH (5:5:1, A). Spots were detected using aqueous phosphomolybdic acid (1%) followed
6
6
2
2
by heating to 110°C for 5 min. FTIR spectra were obtained using attenuated total internal reflection on a Bruker Tensor 27 FT-IR
–1
–1
spectrometer in the range 400–4000 cm with 4 cm resolution and 16 scans. Melting points (mp) were measured in open
capillaries on a Buchi apparatus.
Betulin 3,28-Di-O-acetate (3a). Betulin (0.3 g, 0.7 mmol) and TAGU (0.5 g, 1.6 mmol) were dissolved in CHCl
3
(25 mL), treated with TFA (10 mL), and stirred at 70°C for 1 h, becoming homogeneous after 15 min. When the reaction was
finished, the solvent was evaporated to dryness at reduced pressure. The obtained crude product was recrystallized from
–1
Me CO to afford 3a as a white powder (98% yield), R 0.58 (system A), mp 220°Ñ (lit. [15]: 220°C). IR spectrum (KBr, ν, cm ):
2
f
1
3070.90 (alkene C–H), 1739.93 (C=O), 1649.86 (C=C), 1246.14, 1085.78 (C–O–C). H NMR spectrum (400 MHz, CDCl , δ,
3
ppm, J/Hz): 0.78 (1H, m, H-5), 0.83 (3H, s, CH ), 0.85 (6H, s, CH ), 0.97 (3H, s, CH ), 1.03 (3H, s, CH ), 1.03–1.95 (10H, m,
3
3
3
3
CH ), 1.30 (1H, m, CH), 1.60 (1H, m, CH), 1.65 (1H, m, CH), 1.68 (3H, s, C H ), 2.04 (3H, s, CH CO), 2.07 (3H, s,
2
30
3
3
CH CO), 2.44 (1H, td, J = 10.7, 5.8, Í-19), 3.85 and 4.25 (1H each, d, J = 10.8, Í-28), 4.47 (1H, m, Í-3), 4.59 (1H, s, Í-9),
3
13
4.69 (1H, s, Í-29). C NMR spectrum (100 MHz, CDCl , δ, ppm): 14.7, 16.0, 16.2, 16.5 (CH ), 18.2 (C-6), 19.1 (C-30), 20.8
3
3
(CH ), 21.1 (CH CO), 21.4 (CH CO), 23.7 (C-2), 25.1 (CH ), 27.0 (CH ), 27.9 (CH ), 29.6 (C-21), 29.7, 34.1, 34.5 (CH ),
2
3
3
2
2
3
2
37.1, 37.5 (C-13), 37.8, 38.4 (CH ), 40.9, 42.7, 46.3, 47.7 (C-19), 48.7 (C-18), 50.3 (C-9), 55.3 (C-5), 62.8 (C-28), 80.9
2
(C-3), 109.9 (C-29), 150.1 (C-20), 170.0 (C=O), 171.6 (C=O).
Cholesteryl 3-O-Acetate (3b). Cholesterol (0.3 g, 0.8 mmol) and TAGU (0.5 g, 1.6 mmol) were dissolved in CHCl
3
(25 mL), stirred at room temperature for 5 min, treated with TFA (7 mL), and stirred at 70°C for 1.5 h. When the reaction was
finished, the solvent was evaporated to dryness at reduced pressure. The resulting crude product was dissolved in MeOH. The
solid was filtered off. The solvent was evaporated to produce a white solid (3b) (95% yield), R 0.56 (system A), mp 112°C
f
483

–1
(lit. [16]: 113–114°C). IR spectrum (KBr, ν, cm ): 2924–2855 (CH , CH ), 1726.6 (C=O), 1642.6 (C=C), 1247.3, 1031.6
3
2
1
(C–O–C). H NMR spectrum (400 ÌHz, CDCl , δ, ppm, J/Hz): 5.28 (1H, m, H-6), 4.50 (1H, m, Í-3), 1.99 (3H, s, ÑÎCH ),
3
3
0.92 (3H, s, CH -19), 0.82 (3H, d, J = 6.4, CH -30), 0.77 (3H, d, J = 5.2, CH -21), 0.84 (3H, t, J = 6.40, CH -28), 0.58 (3H, s,
3
3
3
3
13
CH -18). C NMR spectrum (100 MHz, CDCl , δ, ppm): 23.83 (C-1), 27.76 (C-2), 74.03 (C-3), 38.11 (C-4), 139.62 (C-5),
3
3
122.67 (C-6), 31.85 (C-7), 21.03 (C-8), 50.01 (C-9), 36.58 (C-10), 31.85 (C-11), 39.72 (C-12), 42.30 (C-13), 56.68 (C-14),
24.29 (C-15), 28.24 (C-16), 56.12 (C-17), 11.86 (C-18), 19.32 (C-19), 35.80 (C-20), 18.72 (C-21), 36.18 (C-22), 39.52
(C-23), 36.98 (C-24), 28.02 (C-25), 22.58 (C-26), 22.84 (C-27), 21.46 (CH COO), 170.65 (CH COO).
3
3
Allobetulin 3-O-Acetate (3c). TAGU (0.5 g, 1.6 mmol) was added to CHCl (25 mL) and allobetulin (0.3 g,
3
0.7 mmol), stirred for 5 min at room temperature, treated with TFA (7 mL), and stirred at 70°C for 1.5 h. The course of the
reaction was monitored by TLC. When the reaction was finished, the solvent was evaporated. The crude product was
recrystallized first from CHCl and then from Me CO to obtain acetylated product 3c as a white powder (92% yield), R 0.62
3
2
f
–1
(system A), mp 283°C (lit. [17]: 285–287°C). IR spectrum (KBr, ν, cm ): 2924.4–2856 (CH and CH ), 1726 (C=O), 1247.5
and 1023.3 (C–O–C). H NMR spectrum (400 MHz, CDCl , δ, ppm, J/Hz): 0.70 (3H, s, CH ), 0.74 (3H, s, CH ), 0.75 (3H, s,
3
2
1
3
3
3
CH ), 0.77 (3H, s, CH ), 0.83 (3H, s, CH ), 0.87 (3H, s, CH ), 0.94 (3H, s, CH ), 2.08 (3H, s, COCH ), 3.35 (1H, d, J = 7.6,
3
3
3
3
3
3
13
Í -28), 3.46 (1H, s, Í-19), 3.70 (1H, d, J = 7.6, Í -28), 4.38 (1H, m, Í-3α). C NMR spectrum (100 MHz, CDCl , δ, ppm):
à
b
3
15.7, 16.5, 16.5, 18.1, 21.0, 21.37 (CH COO), 23.7, 24.5, 26.2, 26.4 (C-2), 27.9, 28.8, 32.7, 33.8, 34.1, 36.2, 36.7, 37.1, 37.8,
3
38.6, 40.6, 40.7, 41.4, 46.8, 51.0, 55.56, 72.46, 80.96 (C-3), 88.04 (C-19), 171.15 (CH COO).
3
ACKNOWLEDGMENT
The work was sponsored by the Ministry of Education and Science of the Russian Federation (Grant No. 05-13108).
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