Divaccinosides A–D, four rare iridoid glucosidic truxillate esters from the leaves of Vaccinium bracteatum

Article abstract of DOI:10.1016/j.tetlet.2017.05.013

Divaccinosides A–D (1–4), four rare iridoid glucoside cyclodimers in the truxillate forms, were characterized from the leaves of Vaccinium bracteatum. They were presumably biosynthesized from two known iridoid glucoside monomers, vaccinoside (5) and 10-O-trans-p-coumaroyl-6α-hydroxyl-dihydromonotropein (6), via a [2+2] cycloaddition reaction. The structures of the new compounds were established by comprehensive spectroscopic measurements, combined with chemical conversions and single crystal X-ray crystallographic analyses.

Full text of DOI:10.1016/j.tetlet.2017.05.013

Tetrahedron Letters 58 (2017) 2385–2388
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Tetrahedron Letters
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Divaccinosides A–D, four rare iridoid glucosidic truxillate esters from
the leaves of Vaccinium bracteatum
Yong-Mei Ren a,b,c, Chang-Qiang Ke a, Chunping Tang a, Sheng Yao a, Yang Ye a,b,c,
⇑
a State Key Laboratory of Drug Research, and Natural Products Chemistry Department, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China
b School of Life Science and Technology, ShanghaiTech University, Shanghai 201203, China
c University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Divaccinosides A–D (1–4), four rare iridoid glucoside cyclodimers in the truxillate forms, were character-
ized from the leaves of Vaccinium bracteatum. They were presumably biosynthesized from two known iri-
Received 1 April 2017
Revised 4 May 2017
Accepted 5 May 2017
Available online 10 May 2017
doid
glucoside
monomers,
vaccinoside
(5)
and
10-O-trans-p-coumaroyl-6a-hydroxyl-
dihydromonotropein (6), via a [2+2] cycloaddition reaction. The structures of the new compounds were
established by comprehensive spectroscopic measurements, combined with chemical conversions and
single crystal X-ray crystallographic analyses.
Keywords:
Vaccinium bracteatum
Ericaceae
Ó 2017 Elsevier Ltd. All rights reserved.
Iridoid glucosides
Divaccinosides
[2+2] cycloaddition
Vaccinium bracteatum Thunb. (Ericaceae), known as ‘‘Nan Zhu”
in Chinese, is an evergreen shrubby tree distributed in mountain-
ous regions of southern China and recognized as both an edible
and a medicinal product in daily life. As a medicinal herb, V.
bracteatum was reported to possess significant health benefits,
such as antifatigue, antianemia, antioxidant, and immunomodulate
effects.1 An array of phytochemical investigations have revealed
the existence of fatty acids, flavonoids and triterpenes as the main
chemical components in this plant,2,3 as well as iridoid glucosides
as one type of minor constituents.4–6
Iridoids are a large group of secondary metabolites found both
in a variety of plants and selected animal species. In the past dec-
ades, considerable attention has been paid to naturally occurring
iridoids, since they display an interesting spectrum of biological
activity, such as cardiovascular,7 antihepatotoxic,8 anti-inflamma-
tory,9 and antiviral activities.10 Structurally, iridoids are cyclopen-
tanopyran monoterpenoids and usually exist in form of glycosides.
The most common structural feature of these compounds is the H-
5/H-9 b, b-cis-fused cyclopentanopyran ring system,11 and only
several enantiomer12 and H-5/H-9 trans-fused iridoids13–17 are
reported.
As part of systematical searching for iridoids with novel struc-
ture or bioactivity, the leaves of V. bracteatum were investigated,
which led to the isolation of four novel iridiod glucoside dimers
named divaccinoside A–D (1–4). Their structures could come pre-
sumably from the dimerization of the known compounds, vacci-
noside
(5)
and
10-O-trans-p-coumaroyl-6a-hydroxyl-
dihydromonotropein (6), through a [2+2] cycloaddition reaction.18
The absolute configurations of the cyclobutane nucleus of the new
compounds was finally determined by spectroscopic analysis,
chemical conversion, and single crystal X-ray crystallographic
diffraction as well. Herein, we describe the isolation and structural
elucidation of compounds 1–4 (Fig. 1).
Compound 1 was obtained as yellow amorphous powder. Its
molecular formula C50H56O26 with 23 double-bond equivalents
(DBEs) was determined by HRESIMS at m/z 1071.2986 [MꢀH]ꢀ
(calcd 1071.2987). The IR spectrum displayed absorption bands
for hydroxyl (3420 cmꢀ1), conjugated carbonyl (1718 cmꢀ1) and
aromatic functionalities (1642, 1518 cmꢀ1). In the 1H NMR spec-
trum of 1 (Table 1), signals at dH 7.42 (d, 1.2)/7.41 (d, 1.2), 6.17
(dd, 5.7, 2.6)/6.14 (dd, 5.7, 2.5), 5.66 (d, 2.3)/5.47 (d, 3.3), 5.37
(dd, 5.7, 1.9)/5.36 (dd, 5.7, 1.8), 3.47 (dd, 8.7, 0.8)/3.45 (dd, 8.6,
1.5), 2.40 (dd, 8.7, 2.3)/2.22 (dd, 8.6, 3.3), 3.97 (m)/3.96 (m), 3.40
(m)/3.58 (d, 10.9), 4.69 (d, 8.0)/4.67 (d, 7.9), 3.95 (dd, 12.0,
1.4)/3.88 (dd, 12.0, 2.0), 3.79 (m)/3.71 (dd, 12.0, 5.1), and 3.28–
3.40 (8H, m) were typical of two sets of monotropein moieties.9
In addition, proton signals at dH 7.18/7.20 (each 2H, d, J = 8.4),
6.79/6.76 (each 2H, d, J = 8.4), corresponding to carbons at dC
⇑
Corresponding author at: State Key Laboratory of Drug Research, and Natural
Products Chemistry Department, Shanghai Institute of Materia Medica, Chinese
Academy of Sciences, Shanghai 201203, China.
E-mail address: yye@mail.shcnc.ac.cn (Y. Ye).
http://dx.doi.org/10.1016/j.tetlet.2017.05.013
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.

2386
Y.-M. Ren et al. / Tetrahedron Letters 58 (2017) 2385–2388
at dH 4.32 (H-300, unit Ⅰ) to the carbons at dC 173.8 (C-100, unit Ⅰ),
173.7 (C-100, unit Ⅱ), and 129.9 (C-500, unit Ⅰ), and those from the
proton at dH 4.39 (H-300, unit Ⅱ) to the carbons at dC 173.8 (C-100,
unit Ⅰ), 173.7 (C-100, unit Ⅱ), and 130.0 (C-500, unit Ⅱ), along with
the 1H-1H COSY data aforementioned, suggested that the carbonyl
carbons and the p-hydroxyphenyl groups were connected to the
cyclobutane ring, which were arranged in the truxillic type
(head-to-tail). HMBC correlations from the proton signals at dH
3.40/3.97 (H-10, unit Ⅰ) to the carbonyl carbon at dC 173.8 (C-100,
unit Ⅰ) and those from dH 3.58/3.96 (H-10, unit Ⅱ) to dC 173.7 (C-
200, unit Ⅱ) implied that the cyclobutane ring was connected to C-
10 of the iridoid skeletons by forming two ester bonds. Accord-
ingly, compound 1 was established to be a truxillic type dimer of
vaccinoside containing a cyclobutane nucleus.
The ROESY correlations (Fig. 3) of H-200 (unit Ⅰ)/H-500 (unit Ⅰ) and
H-200 (unit Ⅱ)/H-500 (unit Ⅱ) established trans-configurations for H-
200 and H-300 of units Ⅰ and II, respectively. However, the relative
configuration of the cyclobutyl ring remained unclear. So com-
pound 1 was hydrolyzed with 1% potassium hydroxide to produce
a di-carboxyl derivative 1a (Scheme S1), which was then crystal-
ized from a mixture solvent of water and methanol. The whole
structure of 1a including the absolute configuration was confirmed
by the X-ray diffraction analysis (Fig. 4, CCDC 1534627). Thus, the
stereochemistry of the cyclobutane ring in 1 was determined. To
further confirm the absolute configuration of the iridoid glucoside
moieties, vaccinoside (5) was treated with acetic anhydride and
the reaction mixture was purified by preparative HPLC to yield
the acetylated derivative 5a (Scheme S2). The single crystal of 5a
was obtained from a mixture of water and acetonitrile. The
absolute configuration of 5a was confirmed firstly by the X-ray
diffraction experiment (Fig. 5, CCDC 1534622). Based on the unam-
biguous structures of these two crystals, the whole structure of
compound 1 was finally established, and named divaccinoside A.
Compound 2 was isolated as yellow amorphous powder and its
molecular formula was assigned to be C50H60O28 from the quasi-
molecular negative ion at m/z 1107.3212 [MꢀH]ꢀ (calcd
1107.3198) in the HRESIMS, requiring 21 degrees of unsaturation.
The IR spectrum displayed absorption bands for hydroxyl
Fig. 1. Structures of compounds 1–6.
129.9/130.0 and 116.5/116.2, revealed the existence of two aro-
matic rings with a typical AA0BB0 spin system, which were further
assigned as two p- hydroxyphenyl groups. Detailed analysis of all
the 1H and 13C NMR data revealed that these signals were similar
to those of the known compound vaccinoside (5), except for the
absence of the characteristic olefinic proton signals with a ca.
ꢁ16 Hz coupling constant for the trans-double bond. Instead, four
methine signals resonating at dH 3.91 (m)/3.93 (m), 4.32 (dd,
10.9, 7.4)/4.39 (dd, 10.9, 7.6) were observed. The 1H-1H COSY
experiment showed that these four methine protons were mutu-
ally coupled to one another in the sequence of d
3.91, 4.32,
H
(3430 cmꢀ1), conjugated carbonyl (1716 cmꢀ1
) and aromatic
3.93, and 4.39; the last one was found in turn to correlate with
the first one, indicative of a rare cyclobutane ring moiety. Such a
segment was further confirmed by the 1D TOCSY experiment
(Fig. S12). HMBC correlations (Fig. 2) from the methine protons
groups (1640, 1520 cmꢀ1). The 1H and 13C NMR data of 2 (Tables
1 and 2) showed high similarity to those of 1, except that a methy-
lene (dC 44.9/45.0) and an oxygenated methine (dC 76.5/77.1)
Table 1
1H NMR Data for Compounds 1–4 (600 MHz in CD3OD, d in ppm, J in Hz).
No.
1
2
3
4
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
1
3
5
6
7
5.66 (d, 2.3)
7.42 (d, 1.2)
5.47 (d, 3.3)
7.41 (d, 1.2)
5.60 (d, 3.2)
7.52 (d, 1.0)
5.41 (d, 4.9)
7.50 (d, 1.1)
5.48 (d, 3.3)
7.42 (d, 1.1)
5.60 (d, 3.3)
7.50 (br s)
5.40 (d, 3.3)
7.51 (br s)
5.66 (d, 1.8)
7.41 (d, 1.1)
3.47 (dd, 8.5, 1.4)
6.17 (br d, 5.6)
3.47 (dd, 8.7, 0.8) 3.45 (dd, 8.6, 1.5) 2.76 (dd, 9.2, 3.1) 2.65 (dd, 9.5, 6.3) 3.45 (dd, 8.5, 1.4) 2.76 (dd, 9.4, 3.8) 2.64 (br s)
6.17 (dd, 5.7, 2.6) 6.14 (dd, 5.7, 2.5) 4.26 (q, 4.9) 4.17 (q, 6.3) 6.15 (dd, 5.7, 2.5) 4.26 (q, 4.9) 4.16 (m)
5.37 (dd, 5.7, 1.9) 5.36 (dd, 5.7, 1.8) 1.81 (dd, 13.9, 5.5) 1.85 (dd, 13.5, 6.2) 5.38 (dd, 5.7, 2.0) 1.65 (dd, 13.9, 5.0) 1.67 (dd, 13.4, 7.1) 5.36 (d, 5.4)
1.65 (dd, 10.5, 5.0) 1.67 (dd, 13.1, 7.5) 1.81 (dd, 13.9, 5.6) 1.85 (dd, 13.4, 5.7)
2.40 (dd, 8.7, 2.3) 2.22 (dd, 8.6, 3.3) 2.38 (dd, 9.2, 3.2) 2.01 (dd, 9.5, 4.9) 2.22 (dd, 8.5, 3.3) 2.38 (dd, 9.4, 3.3) 2.00 (dd, 9.4, 3.3) 2.40 (br d, 8.5)
9
10
3.40 (m)
3.97 (m)
4.69 (d, 8.0)
3.30 (m)
3.40 (m)
3.31 (m)
3.32 (m)
3.79 (m)
3.58 (d, 10.9)
3.96 (m)
4.67 (d, 7.9)
3.28 (m)
3.36 (m)
3.31 (m)
3.50 (d, 10.9)
3.98 (m)
4.68 (d, 7.9)
3.61 (d, 10.8)
3.94 (m)
4.67 (d, 7.9)
3.58 (d, 10.9)
3.97 (m)
4.67 (d, 8.1)
3.50 (d, 10.9)
3.94 (m)
4.68 (d, 8.0)
3.60 (d, 10.9)
3.94 (m)
4.67 (d, 8.6)
3.37 (m)
3.98 (m)
4.69 (d, 8.3)
3.30 (m)
3.38 (m)
3.35 (m)
3.33 (m)
10
20
30
40
50
60
3.28 (dd, 9.0, 7.9) 3.26 (dd, 8.9, 7.9) 3.28 (dd, 8.9, 8.1) 3.28 (dd, 9.0, 8.0) 3.26 (t, 8.3)
3.38 (m)
3.34 (m)
3.32 (m)
3.37 (m)
3.34 (m)
3.32 (m)
3.39 (m)
3.34 (m)
3.38 (m)
3.39 (m)
3.34 (m)
3.38 (m)
3.38 (m)
3.35 (m)
3.33 (m)
3.32 (m)
3.71 (dd, 12.0, 5.1) 3.76 (dd, 11.9, 5.5) 3.68 (dd, 11.9, 4.7) 3.71 (dd, 12.1, 5.1) 3.76 (dd, 11.9, 5.8) 3.67 (dd, 12.0, 4.9) 3.80 (dd, 11.9, 4.5)
3.95 (dd,12.0, 1.4) 3.88 (dd, 12.0, 2.0) 3.94 (dd, 11.9, 2.1) 3.86 (br d, 11.7)
3.91 (m) 3.93 (m) 3.96 (m) 4.00 (m)
3.88 (dd, 12.1, 2.0) 3.94 (dd, 12.0, 2.2) 3.86 (br d, 11.8)
3.96 (m) 3.93 (m) 3.99 (m)
3.93 (m)
3.93 (m)
200
300
4.32 (dd, 10.9, 7.4) 4.39 (dd, 10.9, 7.6) 4.35 (dd, 10.8, 6.9) 4.44 (dd, 10.8, 7.1) 4.41 (dd, 10.8, 7.2) 4.33 (dd, 10.8, 6.8) 4.34 (dd, 10.8, 6.7) 4.43 (dd, 10.8, 7.3)
500/800 7.18 (d, 8.4)
600/900 6.79 (d, 8.4)
7.20 (d, 8.4)
6.76 (d, 8.4)
7.21 (d, 8.6)
6.77 (m)
7.24 (d, 8.6)
6.75 (m)
7.22 (d, 8.4)
6.75 (d, 8.4)
7.18 (d, 8.4)
6.79 (d, 8.4)
7.20 (d, 8.3)
6.77 (m)
7.22 (d, 8.3)
6.76 (m)

Y.-M. Ren et al. / Tetrahedron Letters 58 (2017) 2385–2388
2387
Fig. 2. 1H-1H COSY (H
compound 1.
H) and key HMBC (H
C) correlations of
Fig. 5. Structure and ORTEP drawing of compound 5a.
glycoside moieties. As mentioned above, the structure of 1 con-
tained two monotropein moieties while 2 contained two 6a-
hydroxyl-dihydromonotropein moieties. The 1H NMR spectrum of
3 (Table 1) showed signals at dH 6.15 (1H, dd, J = 5.7, 2.5 Hz, H-6)
and 5.38 (1H, dd, J = 5.7, 2.0 Hz, H-7), and signals at dH 4.26 (1H,
q, J = 4.9 Hz, H-6), 1.65 (1H, dd, J = 13.9, 5.0 Hz, H-7) and 1.81
(1H, dd, J = 13.9, 5.6 Hz, H-7), suggesting the presence of both a
Fig. 3. Key ROESY correlations of the cyclobutane nucleus of compounds 1–4.
monotropein and
a 6a-hydroxyl-dihydromonotropein moiety.
These signals were also found in the 1H NMR spectrum of 4. Fur-
ther analysis of 1D and 2D NMR spectra of 3 and 4 revealed that
they shared the same planar structure. The splitting patterns and
coupling constants, as well as the ROESY correlation patterns
(Fig. 3) of the cyclobutyl protons of 3 and 4 were exactly the same
with those of 1 and 2, suggesting that these two compounds pos-
sessed the same a-truxillic acid diester skeleton. Therefore, these
two compounds could only differ at the stereochemistry of the
cyclobutyl moiety. Given the cyclobutyl nucleus, compounds 3
and 4 were identified accordingly as a pair of enantiomers, and
named divaccinosides C and D, respectively.
Fig. 4. Structure and ORTEP drawing of compound 1a.
V. bracteatum has been proven as a rich resource of fatty acids,
flavonoids and triterpenes, however, it is the first time to report the
existence of iridoid glucoside cyclodimers regardless of four iridoid
glucoside monomers in previous literatures. To the best of our
knowledge, naturally occurring iridoid glucoside cyclodimers in
the truxillate form (head-to-tail arrangement, 2,4-diphenyl-1,
3-cyclobutanedicarboxylic acid derivative) or truxinate form
(head-to-head arrangement, 3,4-diphenyl-1,2-cyclobutane-dicar-
boxylic acid derivative) are quite rare. So far only 4,40-
dimethoxy-b-truxinic acid catalpol diester19 from the leaves of Pre-
rather than a double bond were present at C-6 and C-7 of the iri-
doid skeleton. These signals of the iridoid glycoside moieties were
similar to those reported for the known 10-O-trans-p-coumaroyl-
6a
-hydroxyl-dihydromonotropein (6).5 The signals at dH 3.96
(m)/4.00 (m) and 4.35 (dd, 10.8, 6.9)/4.44 (dd, 10.8, 7.1) in 2 indi-
cated the same splitting patterns and coupling constants of the
cyclobutane ring with those in 1. In addition, the cyclobutyl moiety
of 2 and 1 showed the same ROESY correlation patterns (Fig. 3).
These evidences supported that they had the same relative config-
uration. Thus, compound 2 was established and named divacci-
noside B.
Compounds 3 and 4, both isolated as yellow amorphous pow-
der, had the same molecular formula of C50H58O27 based on the
HRESIMS data. Detailed comparison of the NMR data of 3 and 4
(Tables 1 and 2) with those of 1 and 2 revealed that they were also
analogs with structural differences happening only in the iridoid
mna subscandens and coelobillardin20
Coelospermum billardieri have been reported. The findings of four
cyclobutyl dimers in the form of -truxillate skeleton in our study
(a-truxillate skeleton) from
a
significantly enriched the diversity of this kind of iridoid glucosidic
dimers. Considering that these compounds may be dimerized from
iridoid glucoside monomers via a [2+2] cycloaddition reaction,
other types of isomers, especially differing at the relative

2388
Y.-M. Ren et al. / Tetrahedron Letters 58 (2017) 2385–2388
Table 2
13C NMR Data for Compounds 1–4 (a150 MHz, b125 MHz, in CD3OD).
a
a
a
No.
1
2b
3
4
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
Unit Ⅰ
Unit Ⅱ
1
3
4
5
94.3
94.6
95.1
95.9
94.6
95.1
154.4
109.9
42.0
95.8
94.3
152.5
111.9
39.0
152.6
111.9
39.6
154.4
109.9
42.0
154.4
110.2
42.6
152.6
110.9
39.6
154.5
110.1
42.6
152.5
110.9
39.0
6
7
8
138.5
133.0
83.7
138.0
132.6
84.1
76.5
44.9
79.5
77.1
45.0
79.2
138.0
133.0
84.1
76.5
44.9
79.5
77.1
44.9
79.2
138.5
132.6
83.7
9
46.0
46.0
45.2
45.0
46.0
46.0
45.0
46.0
10
11
10
71.7
170.4
99.3
74.5
78.0
71.4
78.3
62.7
71.4
170.4
99.6
74.5
78.0
71.4
78.3
62.6
72.4
171.9
99.9
74.7
78.0
71.7
78.4
63.0
71.1
70.0
170.4
99.6
74.6
77.9
71.4
78.3
62.5
72.4
171.4
99.9
74.7
78.0
71.7
78.4
63.0
71.1
70.9
170.4
99.3
74.6
77.8
71.3
78.3
62.9
171.4
100.6
74.6
77.9
71.3
170.4
100.6
74.7
78.0
71.6
20
30
40
50
78.35
62.6
78.4
62.5
60
100
200
300
400
500/900
600/800
700
173.8
48.7
42.2
131.1
129.9
116.5
157.6
173.7
48.2
42.0
130.9
130.0
116.2
157.5
174.0
48.7
42.2
131.2
129.9
116.6
157.5
173.9
48.2
42.0
131.0
130.0
116.4
157.4
173.8
48.6
42.0
131.1
129.9
116.4
157.4
173.9
48.2
42.2
130.9
129.9
116.5
157.6
173.9
48.2
42.2
131.0
129.8
116.6
157.4
173.8
48.7
42.0
131.2
130.1
116.2
157.5
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These data include experimental procedures, physicochemiscal
data of compounds 1–4, and X-ray data for compounds 1a and 5a
(CIF). Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.05.
013.
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