Sugar migration induced by the Wagner-Meerwein rearrangement of 28-O-glycosyl-betulin derivatives

Article abstract of DOI:10.1016/j.tet.2017.02.024

Treatment of betulin-type saponins, in which the sugar moiety is connected at the 28-position, with TMSOTf affords 3-O-substituted allobetulin saponins. This unprecedented migration of the sugar part is driven by the Wagner-Meerwein rearrangement of the betulin core. According to our mechanistic studies an oxocarbenium ion released during the rearrangement reacts with the free 3-OH group of the triterpene affording the final allobetulin saponin. Migration rate depends on the size and configuration of the sugar moiety.

Full text of DOI:10.1016/j.tet.2017.02.024

Accepted Manuscript
Sugar migration induced by the Wagner-Meerwein rearrangement of 28-O-glycosyl-
betulin derivatives
Anna Korda, Zbigniew Pakulski, Piotr Cmoch, Katarzyna Gwardiak, Romuald
Karczewski
PII:
S0040-4020(17)30156-4
10.1016/j.tet.2017.02.024
TET 28465
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 October 2016
Revised Date: 3 February 2017
Accepted Date: 13 February 2017
Please cite this article as: Korda A, Pakulski Z, Cmoch P, Gwardiak K, Karczewski R, Sugar migration
induced by the Wagner-Meerwein rearrangement of 28-O-glycosyl-betulin derivatives, Tetrahedron
(2017), doi: 10.1016/j.tet.2017.02.024.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Sugar migration induced by the Wagner-Meerwein rearrangement
of 28-O-glycosyl-betulin derivatives
Anna Korda, Zbigniew Pakulski,* Piotr Cmoch, Katarzyna Gwardiak, Romuald Karczewski
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Author for correspondence: zbigniew.pakulski@icho.edu.pl
Abstract: Treatment of betulin-type saponins, in which the sugar moiety is connected at the
28-position, with TMSOTf affords 3-O-substituted allobetulin saponins. This unprecedented
migration of the sugar part is driven by the Wagner-Meerwein rearrangement of the betulin core.
According to our mechanistic studies an oxocarbenium ion released during the rearrangement reacts
with the free 3-OH group of the triterpene affording the final allobetulin saponin. Migration rate
depends on the size and configuration of the sugar moiety.
Keywords: betulin; allobetulin; saponins; the Wagner-Meerwein rearrangement; synthesis.
Introduction
Triterpene betulin (1, R = H) - isolated in large amounts from the outer bark of the birch tree
(Betula sp.) - and its derivatives are of continous interest to numerous research groups due to their
diversified pharmacological and biological properties.1-3 However, poor water solubility as well as
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other pharmacokinetic parameters limit their clinical applications. Inclusion of a sugar moiety to the
triterpene core can successfully combine good pharmacological properties of the aglycon, with
improved water solubility brought by the addition of a hydrophilic carbohydrate fragment to the
lupane core.4 Special attention is directed towards the synthesis of bidesmosidic saponins bearing
sugar moieties at both the C-3 and C-28 positions which are usually less haemolytic and less toxic
than the monodesmosides.5-7
Betulin saponins were usually prepared by the Koenigs-Knorr method utilizing glycosyl
bromides as donors and heavy metals (e.g. Ag2CO3) as promoters.8 Recently, much more efficient
glycosylation using glycosyl trichloroacetimidates (Schmidt donors)9 catalyzed by TMSOTf or
BF3.Et2O have been applied, enabling the preparation of the required lupane-type saponins in high
yield. However, the glycosylation of lupanes is often complicated by the Wagner-Meerwein
rearrangement leading to allobetulin 2 (Scheme 1). Such transformation of the E-ring of betulin or
betulinic acid is a well known process, generating numerous examples of allobetulin derivatives.10
Several allobetulin saponins were also synthesized and evaluated as potential drug candidates.11
Moreover, allobetulin saponins are often the major products in the TMSOTf catalyzed glycosylation
of some betulin or betulinic acid derivatives bearing hydroxyl or carbonyl groups at the C-28
position.12-14
Scheme 1. The Wagner-Meerwein rearrangement of betulin derivatives.
Results and Discussion
In order to circumvent these problems in the preparation of lupane-type bidesmosidic saponins,
starting materials possesing glycosylated 28-O position were used (7-10). According to the
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literature for such derivatives the Wagner-Meerwein rearrangement of the lupane E-ring was not
observed, hence, we expected that their glycosylation should provide the desired bidesmosides.
Monodesmosides 7-9 and 10,12 bearing a free hydroxyl group at the C-3 position, were prepared in
excellent yields (>90%, Scheme 2) by treatment of acetylated monodesmosides 3,15 4, 5,16 and 612
with dry hydrogen chloride in methanol. The next step of the synthesis required glycosylation of the
free hydroxyl group of monodesmosidic saponins 7-10. However, in most cases unprecedented
rearrangement of the starting materials to the allobetulin derivatives was observed. Obviously, the
glycosidic bond at the C-28 position, present in the starting materials, was cleaved under the
reaction conditions, and released the hydroxyl group which participated in forming the allobetulin
structure.
Scheme 2. Reagents and conditions. (i) HCl(g) in MeOH; (ii) TMSOTf, CH2Cl2, -40 °C for 4 h then
0 °C for 20 h.
To explain the above phenomenon, we exposed monodesmosides 7-10 to the reaction
conditions similar to glycosylation but without the addition of the Schmidt donor. In the presence of
TMSOTf, rearrangement of the E-ring to allobetulin was observed in most cases, whereas the sugar
fragment underwent migration from the 28-O to the 3-O position. The tendency of migration
depended on the size and the configuration of the carbohydrate fragment. For the smaller and less
hindered pentopyranoside 7, the process was quite efficient and allobetulin -arabinopyranoside 11
L
was isolated in 53% yield;
L-rhamnopyranoside 12 was obtained in a slightly lower yield (50%)
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while hexopyranosides 9 and 10 were less reactive. More sterically demanded
D-mannopyranoside
9 gave rearranged product 13 in only 14% yield together with allobetulin17 15 (16%) as an
inseparable mixture, while -glucopyranoside 10 did not react at all (Scheme 2).
D
A plausible mechanism for this transformation is given in Scheme 3. It is likely that in the first
step the isopropenyl side-chain on the C-19 position is protonated to form carbocation B which
undergoes rearrangement to a six membered ring providing carbocation C. Cleavage of the glycosyl
bond gives the oxocarbenium ion E and allobetulin D is formed simultaneously. Finally, ion E
reacts with the free 3-OH group of D to form allobetulin saponin F. The above process is somewhat
similar to glycosylation with pentenyl glycosides in which, after activation of the double bond, the
tetrahydrofurane derivative is formed and the oxocarbenium ion is released.18
Scheme 3. Proposed mechanism of sugar migration induced by the Wagner-Meerwein
rearrangement of lupane saponins.
To confirm the participation of oxocarbenium ion E in the proposed pathway, the reaction of
arabinoside 7 was conducted in the presence of methyl alcohol. Careful separation of the reaction
mixture by preparative TLC afforded the expected methyl 2,3,4-tri-O-benzoyl-α- -
L
arabinopyranoside (16,19 9%) with allobetulin (15, 43%), and the recovered starting material (7,
21%). In the absence of the competing free lupane 3-OH group, alkyl arabinopyranosides were
obtained in much higher yields. Treatment of acetylated saponin 3 with TMSOTf in the presence of
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-arabinopyranoside 16 (66%) and 3-O-acetyl-allobetulin17 (18, 42%).
methyl alcohol gave methyl
L
In a similar reaction of 3 with benzyl alcohol, benzyl
L-arabinopyranoside 17 (71%) and 3-O-acetyl-
allobetulin (18, 50%) were isolated (Scheme 4). As expected, due to the participation of the
neighboring benzoyl group, 1,2-trans glycosides were formed exclusively.
Scheme 4. Reagents and conditions. (i) TMSOTf, CH2Cl2, -40 °C for 4 h then 0 °C for 20 h.
Scheme 5. Reagents and conditions. (i) TMSOTf, CH2Cl2, -40 °C for 4 h then 0 °C for 20 h.
Possible transfer of the sugar to competing triterpene was also examined by the treatment of
saponin 7 with TMSOTf in the presence of lupeol (19). In this case both, lupenyl
arabinopyranoside (20)15 and allobetulinyl
L
-
L
-arabinopyranoside (11) were isolated in 19% and 14%,
respectively (Scheme 5). The above results supported the proposed mechanism for the
rearrangement in which the oxocarbenium ion E plays a crucial role.
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Conclusion
In summary, we have observed and examined the new TMSOTf catalyzed rearrangement of
lupane saponins bearing a sugar substituent at the O-28 position, in which the sugar substituent
migrated to the O-3 position, while rearrangement of the E-ring transforming the lupane core into
the allobetulin. In the presence of methanol, benzyl alcohol, or lupeol cross-migration of the sugar
moiety was observed. In many cases, the above process may prevent formation of bidesmosidic
lupane saponins and must be considered as an important factor strongly influencing the formation of
lupane bidesmosides.
Experimental
General procedures. Silica gel HF254 and Silica gel 230-400 mesh (E. Merck) were used for
1
13
TLC and column chromatography, respectively. H and C NMR spectra were recorded at 298 K
with Varian NMR-vnmrs600 or vnmrs500 spectrometers, using standard experimental conditions
and Varian software (ChemPack 4.1). Configurational assignments were based on the NMR
1
measurements, generated using two-dimensional techniques like COSY and H-13C gradient
1
selected HSQC (g-HSQC), as well as H-13C gradient selected HMBC (g-HMBC). Internal TMS
1
was used as the H and 13C NMR chemical shift standard. J values are given in Hertz. High-
resolution mass spectra (HRMS ESI) were acquired with Mariner and MaldiSYNAPT G2-S HDMS
(Waters) mass spectrometers. Optical rotations were measured with a Jasco P-2000 automatic
polarimeter.
3β
-O-Acetyl-28-O-(2,3,4-tri-O-benzoyl-
α
-L
-rhamnopyranosyl) betulin (4). A solution of 2,3,4-
tri-O-benzoyl-α,β-
L
-rhamnopyranosyl trichloroacetimidate20 (280 mg, 0.45 mmol) and 3-O-
acetylbetulin21 (200 mg, 0.41 mmol) in CH2Cl2 (30 mL) was stirred for 20 min. at rt over molecular
sieves (4 Å, 500 mg, finely ground), then cooled to –40 °C and TMSOTf (20 µL, 0.11 mmol) was
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added. After 30 min the reaction was quenched with Et3N (1 mL), and the solvents were evaporated
under diminished pressure. Column chromatography of the residue (hexane – ethyl acetate, 9:1 →
20
6:1) gave the title compound as a white foam (369 mg, 95%). [α]D 69.2 (c 0.4, chloroform). νmax
(film): 3068, 2944, 2871, 1731, 1452, 1278, 1262, 1248, 1108, 1068, 1028, 757, 711 cm-1. 1H NMR
(CDCl3) δ: 7.24-8.12 (m, 15 H, HAr), 5.80 (dd, 1 H, J3,2 3.4, J3,4 10.1 Hz, H-3'), 5.65-5.70 (m, 2 H,
H-2, H-4'), 4.97 (d, 1 H, J1,2 1.4 Hz, H-1'), 4.73 (br s, 1 H, H-29), 4.61 (br s, 1 H, H-29), 4.47 (dd, 1
H, J 5.4, 10.8 Hz, H-3), 4.12-4.18 (m, 1 H, H-5'), 3.57-3.63 (m, 2 H, H-28), 2.47-2.52 (m, 1 H),
1.96-2.10 (m, 3 H), 2.04 (s, 3 H, CH3), 0.79-1.70 (m, 21 H, lupane protons), 1.72 (s, 3 H, H-30),
1.39 (d, 3 H, J6,5 6.3 Hz, H-6'), 1.03 (s, 3 H, CH3), 1.00 (s, 3 H, CH3), 0.85 (s, 3 H, CH3), 0.85 (s, 3
H, CH3), 0.84 (s, 3 H, CH3). 13C NMR (CDCl3) δ: 171.0, 165.8, 165.6, 165.5, 150.4 (C-20), 133.4,
133.3, 133.0, 127.9-129.9 (CAr), 109.8 (C-29), 98.2 (C-1'), 80.9 (C-3), 71.8, 70.9, 70.2, 66.9 (CH2),
66.9, 55.4, 50.3, 48.8, 47.9, 47.1 (C), 42.8 (C), 40.9 (C), 38.4 (CH2), 37.8 (C), 37.6, 37.0 (C), 35.1
(CH2), 34.1 (CH2), 30.1 (CH2), 29.8 (CH2), 27.9, 27.2 (CH2), 25.2 (CH2), 23.7 (CH2), 21.3, 20.8
(CH2), 19.1, 18.2 (CH2), 17.8, 16.5, 16.1, 16.0, 14.7. HR-MS (ESI) calc. for C59H74NaO10 [M+Na]+:
965.5180. Found: 965.5169.
Deacetylation. General procedure. To methanol (6 mL) cooled in ice bath, acetyl chloride
(260 µL) was slowly added. Then, a solution of acetylated saponin (0.20 mmol) in CH2Cl2 (5 mL)
was added and stirred at 0 °C for 1h and overnight at room temperature. The reaction was quenched
with Et3N (1.0 mL) and the solvents were evaporated under reduced pressure. Column
chromatography of the residue (hexane – ethyl acetate, 9:1 → 2:1) gave the title compound.
28-O-(2,3,4-Tri-O-benzoyl-
α- -arabinopyranosyl) betulin (7). Starting from 3 (186 mg, 0.20
L
mmol) glycoside 7 (177 mg, quant.) was obtained as a white foam. [α]D20 107.7 (c 0.3, chloroform).
1
νmax (film): 3068, 2942, 2869, 1729, 1452, 1282, 1263, 1093, 1069, 1035, 757, 710 cm-1. H NMR
(CDCl3) δ: 7.31-8.07 (m, 15 H, HAr), 5.72 (dd, 1 H, J2,1 6.1, J2,3 8.4 Hz, H-2'), 5.68-5.68 (m, 1 H, H-
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4'), 5.62 (dd, 1 H, J3,2 8.4, J3,4 3.5 Hz, H-3'), 4.70 (d, 1 H, J1,2 6.1 Hz, H-1'), 4.66 (br s, 1 H, H-29),
4.56 (br s, 1 H, H-29), 4.32 (dd, 1 H, J5,4 4.1, J5,5’ 12.8 Hz, H-5'), 3.91 (dd, 1 H, J5,4 1.7, J5,5’ 12.8
Hz, H-5'), 3.67 (d, 1 H, J 9.0 Hz, H-28), 3.58 (d, 1 H, J 9.0 Hz, H-28), 3.17 (dd, 1 H, J 4.7, 11.3 Hz,
H-3), 2.33-2.38 (m, 1 H), 1.94-2.01 (m, 2 H), 1.82-1.84 (m, 1 H), 1.65 (s, 3 H, H-30), 0.63-1.62 (m,
22 H, lupane protons), 0.97 (s, 3 H, CH3), 0.91 (s, 3 H, CH3), 0.87 (s, 3 H, CH3), 0.80 (s, 3 H, CH3),
13
0.77 (s, 3 H, CH3). C NMR (CDCl3) δ: 165.8, 165.6, 165.1, 150.4 (C-20), 133.4, 133.3, 133.2,
128.3-129.9 (CAr), 109.6 (C-29), 101.7 (C-1'), 78.9 (C-3), 70.5, 70.0, 68.6 (CH2), 68.5, 62.5 (C-5),
55.3, 50.4, 48.7, 47.9, 47.0 (C), 42.6 (C), 40.8 (C), 38.8 (C), 38.7 (CH2), 37.7, 37.1 (C), 34.8 (CH2),
33.9 (CH2), 29.8 (CH2), 29.2 (CH2), 28.0, 27.4 (CH2), 27.0 (CH2), 23.1 (CH2), 20.8 (CH2), 19.0,
18.2 (CH2), 16.1, 15.9, 15.4, 14.8. HR-MS (ESI) calc. for C56H70NaO9 [M+Na]+: 909.4918. Found:
909.4911.
28-O-(2,3,4-Tri-O-benzoyl-
α
-L
-rhamnopyranosyl) betulin (8). Starting from 4 (190 mg, 0.20
20
mmol) glycoside 8 (167 mg, 92%) was obtained as a white foam. [α]D 68.8 (c 0.3, chloroform).
1
νmax (film): 3068, 2941, 2870, 1730, 1451, 1278, 1263, 1109, 1068, 1029, 757, 711 cm-1. H NMR
(CDCl3) δ: 7.24-8.11 (m, 15 H, HAr), 5.80 (dd, 1 H, J3,2 3.4, J3,4 10.1 Hz, H-3'), 5.70 (dd, 1 H, J2,1
1.4, J2,3 3.4 Hz, H-2'), 5.67 (t, 1 H, J4,3 = J4,5 = 10.1 Hz, H-4'), 4.97 (d, 1 H, J1,2 1.4 Hz, H-1'), 4.73
(br s, 1 H, H-29), 4.61 (br s, 1 H, H-29), 4.12-4.18 (m, 1 H, H-5'), 3.57-3.63 (m, 2 H, H-28), 3.19
(dd, 1 H, J 4.7, 11.4 Hz, H-3), 2.47-2.53 (m, 1 H), 1.98-2.10 (m, 2 H), 0.68-1.67 (m, 23 H, lupane
protons), 1.72 (s, 3 H, H-30), 1.39 (d, 3 H, J6,5 6.3 Hz, H-6'), 1.03 (s, 3 H, CH3), 1.00 (s, 3 H, CH3),
0.97 (s, 3 H, CH3), 0.82 (s, 3 H, CH3), 0.76 (s, 3 H, CH3). 13C NMR (CDCl3) δ: 165.8, 165.6, 165.5,
150.4 (C-20), 133.4, 133.3, 133.0, 128.2-129.9 (CAr), 109.8 (C-29), 98.2 (C-1'), 79.0 (C-3), 71.8,
70.9, 70.2, 66.9 (CH2), 66.9, 55.3, 50.4, 48.8, 47.9, 47.1 (C), 42.8 (C), 40.9 (C), 38.8 (C), 38.7
(CH2), 37.6, 37.1 (C), 35.1 (CH2), 34.2 (CH2), 30.1 (CH2), 29.8 (CH2), 28.0, 27.4 (CH2), 27.2
(CH2), 25.2 (CH2), 20.8 (CH2), 19.1, 18.3 (CH2), 17.8, 16.1, 16.0, 15.3, 14.8. HR-MS (ESI) calc.
for C57H72NaO9 [M+Na]+: 923.5074. Found: 923.5084.
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28-O-(2,3,4,6-Tetra-O-benzoyl-
α
-D
-mannopyranosyl) betulin (9). Starting from 5 (213 mg,
20
0.20 mmol) glycoside 9 (190 mg, 93%) was obtained as a white foam. [α]D -22.4 (c 0.3,
chloroform). νmax (film): 3068, 2941, 2869, 1731, 1451, 1266, 1108, 1068, 1027, 711 cm-1. 1H NMR
(CDCl3) δ: 7.25-8.12 (m, 20 H, HAr), 6.15 (t, 1 H, J4,3 = J4,5 = 10.2 Hz, H-4'), 5.89 (dd, 1 H, J3,2 3.2,
J3,4 10.2 Hz, H-3'), 5.73 (dd, 1 H, J2,1 1.5, J2,3 3.2 Hz, H-2'), 5.07 (d, 1 H, J1,2 1.5 Hz, H-1'), 4.71-
4.74 (m, 2 H, H-6', H-29), 4.60 (br s, 1 H, H-29), 4.49 (dd, 1 H, J6,5 3.7, J6,6’ 12.2 Hz, H-6'), 4.37-
4.39 (m, 1 H, H-5'), 4.06 (d, 1 H, J 9.4 Hz, H-28), 3.26 (d, 1 H, J 9.4 Hz, H-28), 3.18 (dd, 1 H, J
4.6, 11.4 Hz, H-3), 2.38-2.43 (m, 1 H), 1.97-2.13 (m, 3 H), 1.70 (s, 3 H, H-30), 0.66-1.75 (m, 22 H,
lupane protons), 1.01 (s, 3 H, CH3), 0.97 (s, 3 H, CH3), 0.96 (s, 3 H, CH3), 0.76 (s, 3 H, CH3), 0.74
13
(s, 3 H, CH3). C NMR (CDCl3) δ: 166.1, 165.5, 165.5, 165.4, 150.2 (C-20), 133.4, 133.4, 133.2,
133.0, 128.3-129.9 (CAr), 109.8 (C-29), 98.4 (C-1'), 78.9 (C-3), 70.5, 70.2, 69.2, 67.4 (CH2), 66.9,
62.8 (CH2), 55.3, 50.4, 48.8, 48.0, 47.0 (C), 42.7 (C), 40.8 (C), 38.8 (C), 38.7 (CH2), 37.6, 37.1 (C),
34.6 (CH2), 34.2 (CH2), 30.1 (CH2), 29.9 (CH2), 29.7 (CH2), 28.0, 27.4 (CH2), 27.2 (CH2), 25.1
(CH2), 20.8 (CH2), 19.1, 18.3 (CH2), 16.1, 16.0, 15.3, 14.8. HR-MS (ESI) calc. for C64H76NaO11
[M+Na]+: 1043.5285. Found: 1043.5287.
28-O-(2,3,4,6-Tetra-O-benzoyl-
β
-D-glucopyranosyl) betulin (10). Starting from 6 (213 mg,
20
0.20 mmol) glycoside 10 (184 mg, 90%) was obtained as a white foam. [α]D 27.6 (c 0.3,
20
chloroform), lit.12 [α]D 27.0 (c 0.5, chloroform). NMR spectroscopic data of 10 matched the
literature.12
Synthesis of allobetulin glycosides. General procedures. Method A – rearrangement of
betulin glycosides. A solution of saponin (7-10, 0.080 mmol) in DCM (7 mL) was stirred for 20
min. at room temperature over molecular sieves (4 Å, 100 mg, finely ground), then cooled to -40 °C
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and TMSOTf (20 µL, 0.11 mmol) was added. Stirring was continued at -40 °C for 4 h and at 0 °C
for 20 h. The reaction was quenched with Et3N (0.5 mL), and the solvents were removed under
diminished pressure. Product was isolated by preparative TLC (hexane – ethyl acetate, 5:1).
Method B – glycosylation of allobetulin. A solution of the glycosyl donor (0.25 mmol) and
allobetulin17 (15, 89 mg, 0.20 mmol) in CH2Cl2 (10 mL) was stirred for 20 min at room temp. over
molecular sieves (4 A, 250 mg, finely ground) and then cooled to –40 °C, and TMSOTf (20 µL, 0.1
mmol) was added. After 30 min the reaction was quenched with Et3N (1 mL), and the solvents were
removed under diminished pressure. Product was isolated by preparative TLC (hexane – ethyl
acetate, 5:1).
3β
-O-(2,3,4-Tri-O-benzoyl-
α- -arabinopyranosyl) allobetulin (11). Method A: Starting from 7
L
(71 mg, 0.080 mmol) saponin 11 (38 mg, 53%) was obtained as a white foam.
Method B: Glycosylation of allobetulin with 2,3,4-tri-O-benzoyl-α,β-
L-arabinopyranosyl
trichloroacetimidate20 afforded the title compound (121 mg, 68%) as a white foam. [α]D 62.9 (c
20
0.5, chloroform). νmax (film): 2944, 2928, 2865, 1728, 1451, 1282, 1262, 1093, 1069, 1029, 756,
1
710 cm-1. H NMR (CDCl3) δ: 7.30-8.07 (m, 15 H, HAr), 5.76 (dd, 1 H, J2,1 6.3, J2,3 8.7 Hz, H-2'),
5.66-5.68 (m, 1 H, H-4'), 5.60 (dd, 1 H, J3,2 8.7, J3,4 3.6 Hz, H-3'), 4.78 (d, 1 H, J1,2 6.3 Hz, H-1'),
4.33 (dd, 1 H, J5,4 4.0, J5,5’ 12.9 Hz, H-5'), 3.87 (dd, 1 H, J5,4 1.9, J5,5’ 12.9 Hz, H-5'), 3.76 (d, 1 H, J
7.8 Hz, H-28), 3.52 (s, 1 H, H-19), 3.43 (d, 1 H, J 7.8 Hz, H-28), 3.15 (dd, 1 H, J 4.7, 11.6 Hz, H-
3), 0.66-1.88 (m, 24H, lupane protons), 0.94 (s, 3 H, CH3), 0.93 (s, 3 H, CH3), 0.89 (s, 3 H, CH3),
0.83 (s, 3 H, CH3), 0.79 (s, 3 H, CH3), 0.78 (s, 3 H, CH3), 0.65 (s, 3 H, CH3). 13C NMR (CDCl3) δ:
165.8, 165.6, 165.2, 133.3, 133.3, 133.2, 128.3-129.9 (CAr), 103.0 (C-1'), 90.1 (C-3), 87.9 (C-19),
71.2 (C-28), 70.7 (C-3'), 70.2 (C-2'), 68.6 (C-4'), 62.5 (C-5'), 55.7 (C-5), 51.1 (C-9), 46.8 (C-18),
41.5 (C), 40.7 (C-14), 40.6 (C-8), 39.1 (C-4), 38.9 (C-1), 37.0 (C-10), 36.7 (CH2), 36.2 (C-20), 34.1
(C-13), 33.9 (C-7), 32.7 (C-21), 28.8 (C-30), 27.7 (C-23), 26.5 (CH2), 26.4 (C-15), 26.3 (CH2), 26.1
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(C-1), 24.5 (C-29), 21.0 (CH2), 18.1 (C-6), 16.4 (C-25), 16.1 (C-24), 15.7 (C-26), 13.5 (C-27). HR-
MS (ESI) calc. for C56H70NaO9 [M+Na]+: 909.4918. Found: 909.4916.
3β
-O-(2,3,4-Tri-O-benzoyl-
α- -rhamnopyranosyl) allobetulin (12). Method A: starting from 8
L
(72 mg, 0.080 mmol) saponin 12 (36 mg, 50%) was obtained as a white foam.
Method B: Glycosylation of allobetulin with 2,3,4-tri-O-benzoyl-α,β-
L-rhamnopyranosyl
20
trichloroacetimidate gave the title compound (146 mg, 81%) as a white foam. [α]D 96.0 (c 0.4,
chloroform). νmax (film): 3066, 3032, 2943, 2868, 1730, 1451, 1278, 1263, 1108, 1069, 1029, 756,
711 cm-1. 1H NMR (CDCl3) δ: 7.24-8.12 (m, 15 H, HAr), 5.83 (dd, 1 H, J3,2 3.3, J3,4 10.1 Hz, H-3'),
5.68 (t, 1 H, J4,3 = J4,5 = 10.1 Hz, H-4'), 5.65 (dd, 1 H, J2,1 1.5, J2,3 3.3 Hz, H-2'), 5.08 (d, 1 H, J1,2
1.5 Hz, H-1'), 4.29-4.34 (m, 1 H, H-5'), 3.78 (d, 1 H, J 7.8 Hz, H-28), 3.53 (s, 1 H, H-19), 3.45 (d, 1
H, J 7.8 Hz, H-28), 3.21-3.24 (m, 1 H, H-3), 0.74-1.85 (m, 24 H, lupane protons), 1.33 (d, 3 H, J
6.3 Hz, H-6'), 1.06 (s, 3 H, CH3), 1.00 (s, 3 H, CH3), 0.94 (s, 3 H, CH3), 0.93 (s, 3 H, CH3), 0.92 (s,
3 H, CH3), 0.92 (s, 3 H, CH3), 0.80 (s, 3 H, CH3). 13C NMR (CDCl3) δ: 165.8, 165.7, 165.6, 133.4,
133.2, 133.0, 128.2-129.9 (CAr), 99.7 (C-1'), 90.0 (C-3), 87.9 (C-19), 72.0 (C-4'), 71.3 (C-28), 71.2
(C-2'), 70.2 (C-3'), 66.8 (C-5'), 55.7, 51.1, 46.8, 41.5 (C), 40.7 (C), 40.6 (C), 39.2 (C), 38.9, 37.0
(C), 36.7, 36.3 (C), 34.2, 33.9 (CH2), 32.7 (CH2), 28.8, 28.2, 26.4 (CH2), 26.3 (CH2), 25.6 (CH2),
24.5, 21.0 (CH2), 18.2 (CH2), 17.6, 16.5, 16.4, 15.7, 13.5. HR-MS (ESI) calc. for C57H72NaO9
[M+Na]+: 923.5074. Found: 923.5057.
3β
-O-(2,3,4,6-Tetra-O-benzoyl-
α- -mannopyranosyl) allobetulin (13). Method A: starting from
D
9 (82 mg, 0.080 mmol) an inseparable mixture of required saponin 13 and allobetulin 15 (21 mg)
was obtained as a white foam. Approx. molar ratio 13 : 15 = 1 : 1.2. Calculated yield: 13 – 14%; 15
– 16%.
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Method B: Glycosylation of allobetulin with 2,3,4,6-tetra-O-benzoyl-α-
D-mannopyranosyl
20
trichloroacetimidate22 afforded the title compound 181 mg (89%) as a white foam. [α]D 16.8 (c
1.2, chloroform). νmax (film): 3063, 2946, 2867, 1730, 1451, 1266, 1108, 1068, 1027, 756, 711 cm-1.
1H NMR (CDCl3) δ: 7.25-8.11 (m, 20 H, HAr), 6.10 (t, 1 H, J4,3 = J4,5 = 10.2 Hz, H-4'), 5.91 (dd, 1
H, J3,2 3.3, J3,4 10.2 Hz, H-3'), 5.62 (dd, 1 H, J2,1 1.6, J2,3 3.3 Hz, H-2'), 5.29 (d, 1 H, J1,2 1.6 Hz, H-
1'), 4.68 (dd, 1 H, J6,5 2.3, J6,6’ 11.9 Hz, H-6'), 4.53-4.57 (m, 1 H, H-5'), 4.48 (dd, 1 H, J6,5 5.0, J6,6’
11.9 Hz, H-6'), 3.78 (d, 1 H, J 7.8 Hz, H-28), 3.53 (s, 1 H, H-19), 3.45 (d, 1 H, J 7.8 Hz, H-28),
3.36 (dd, 1 H, J 4.4, 11.7 Hz, H-3), 0.69-1.86 (m, 24 H, lupane protons), 1.11 (s, 3 H, CH3), 1.00 (s,
3 H, CH3), 0.95 (s, 3 H, CH3), 0.94 (s, 3 H, CH3), 0.93 (s, 3 H, CH3), 0.90 (s, 3 H, CH3), 0.81 (s, 3
H, CH3). 13C NMR (CDCl3) δ: 166.1, 165.6, 165.5, 133.4, 133.1, 133.0, 128.3-129.9 (CAr), 94.4 (C-
1'), 87.9 (C-19), 84.2 (C-3), 71.6 (C-2'), 71.3 (C-28), 70.2 (C-3'), 69.4 (C-5'), 67.0 (C-4'), 63.1 (C-
6'), 55.9 (C-5), 51.1, 46.8, 41.5 (C), 40.7 (C), 40.7 (C), 38.6 (C), 38.5 (CH2), 37.2 (C), 36.8 (CH2),
36.3 (C), 34.1, 33.9 (CH2), 32.7 (CH2), 29.7 (CH2), 28.8, 28.8, 26.4 (CH2), 26.3 (CH2), 24.5, 22.2
(CH2), 21.0 (CH2), 18.2 (CH2), 16.5, 16.5, 15.7, 13.5. HR-MS (ESI) calc. for C64H76NaO11
[M+Na]+: 1043.5285. Found: 1043.5284.
Benzyl 2,3,4-tri-O-benzoyl-
α- -arabinopyranoside (17). According to Method B. Starting from
L
2,3,4-tri-O-benzoyl-α,β- -arabinopyranosyl trichloroacetimidate (91 mg, 0.15 mmol) and benzyl
L
alcohol (23 µL, 0.23 mmol) the title compound was obtained (72 mg, 87%) as a foam. [α]D20 135.8
(c 0.3, chloroform). νmax (film): 1728, 1452, 1315, 1282, 1263, 1092, 1070, 1028, 754, 711 cm-1. 1H
NMR (CDCl3) δ: 7.23-8.04 (m, 20 H, ArH), 5.76 (dd, 1 H, J2,1 5.8, J2,3 8.1 Hz, H-2), 5.68-5.70 (m,
1 H, H-4), 5.60 (dd, 1 H, J3,2 8.1, J3,4 3.4 Hz, H-3), 4.93 (d, 1 H, J 12.1 Hz, PhCH2), 4.83 (d, 1 H,
J1,2 5.8 Hz, H-1), 4.68 (d, 1 H, J 12.1 Hz, PhCH2), 4.35 (dd, 1 H, J5,4 4.8, J5,5’ 12.6 Hz, H-5), 3.89
(dd, 1 H, J5,4 2.4, J5,5’ 12.6 Hz, H-5'). 13C NMR (CDCl3) δ: 165.7 (C=O), 165.5 (C=O), 165.2
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(C=O), 127.9-136.8 (aromatic carbons), 99.0 (C-1), 70.4 (PhCH2), 70.2, 69.9, 68.1, 62.0 (C-5). HR-
MS (ESI) calc. for C33H28NaO8 [M+Na]+: 575.1682. Found: 575.1680.
Rearrangement of betulin glycosides in the presence of methyl alcohol, benzyl alcohol and
lupeol. Reaction with methyl alcohol and benzyl alcohol. A mixture of arabinoside 3 or 7 (0.054
mmol) and methyl or benzyl alcohol (0.250 mmol) in DCM (5 mL) was stirred for 20 min. at room
temperature over molecular sieves (4 Å, 100 mg, finely ground), then cooled to -40 °C and
TMSOTf (20 µL, 0.110 mmol) was added. Stirring was continued at -40 °C for 4 h and at 0 °C for
20 h. The reaction was quenched with Et3N (0.5 mL), and the solvents were removed under
diminished pressure. Preparative TLC (hexane – ethyl acetate, 10:1) gave products as described in
the main text.
Reaction with lupeol. A mixture of arabinoside 7 (70 mg, 0.079 mmol) and lupeol (19, 70 mg,
0.160 mmol) in DCM (7 mL) was stirred for 20 min. at room temperature over molecular sieves (4
Å, 100 mg, finely ground), then cooled to -40 °C and TMSOTf (40 µL, 0.22 mmol) was added.
Stirring was continued at -40 °C for 4 h and at 0 °C for 20 h. The reaction was quenched with Et3N
(0.5 mL), and the solvents were removed under diminished pressure. Isolation by preparative TLC
(hexane – ethyl acetate, 10:1) afforded lupenyl arabinoside 20 (13 mg, 19%) and allobetulinyl
arabinoside 11 (10 mg, 14%). Both products were slightly contaminated by an unknown triterpene
component.
Acknowledgment. This work was financially supported by the National Science Centre,
Poland (grant No. 2012/07/B/ST5/00823).
Supporting Information. 1H and 13C NMR spectra of the new compounds.
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