Imines and lactones derived from friedelanes and their cytotoxic activity

Article abstract of DOI:10.1080/14786419.2018.1508137

Friedelan-3-one (1) and friedelane-3,16-dione (2) isolated from leaves and branches of Maytenus robusta Reissek were subjected to structural modifications via nucleophilic addition to the carbonyl group and Baeyer-Villiger oxidation in order to synthesize potential cytotoxic compounds. The oximes friedelane-3-hydroxyimino (3) and 3-hydroxyiminofriedelan-16-one (4) together with the lactones friedelane-3,4-lactone (5) and 3,4-lactonefriedelan-16-one (6) were characterized by IR and NMR spectroscopic analyses. Compounds 4 and 6 are reported for the first time. Cytotoxic screening via MTT assay in human leukemia cell lines (THP-1 and K562) demonstrated no significant improvement of compounds 3-6 when compared to the starting materials. Only compounds 3 and 5 demonstrated an improvement against K562 cells. However, the same assay on ovarian and breast cancer cell lines (TOV-21G and MDA-MB-231) showed a reduction in the IC₅₀ for compounds 4-6, indicating that ring A modifications may enhance the biological potential.

Full text of DOI:10.1080/14786419.2018.1508137

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: http://www.tandfonline.com/loi/gnpl20
Imines and lactones derived from friedelanes and
their cytotoxic activity
Mariana G. Aguilar, Grasiely F. Sousa, Fernanda C. G. Evangelista, Adriano P.
Sabino, Sidney A. Vieira Filho & Lucienir P. Duarte
To cite this article: Mariana G. Aguilar, Grasiely F. Sousa, Fernanda C. G. Evangelista,
Adriano P. Sabino, Sidney A. Vieira Filho & Lucienir P. Duarte (2018): Imines and lactones
derived from friedelanes and their cytotoxic activity, Natural Product Research, DOI:
10.1080/14786419.2018.1508137
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2018.1508137
Imines and lactones derived from friedelanes and their
cytotoxic activity
a
a
b
Mariana G. Aguilar , Grasiely F. Sousa
, Fernanda C. G. Evangelista ,
b
c
a
Adriano P. Sabino , Sidney A. Vieira Filho and Lucienir P. Duarte
a
Departamento de Qu ꢀı mica, Universidade Federal de Minas Gerais, Belo Horizonte-MG, Brazil;
b
Faculdade de Farm ꢀa cia, Universidade Federal de Minas Gerais, Belo Horizonte-MG, Brazil;
Escola de Farm ꢀa cia, Universidade Federal de Ouro Preto, Ouro Preto-MG, Brazil
c
ABSTRACT
ARTICLE HISTORY
Friedelan-3-one (1) and friedelane-3,16-dione (2) isolated from
leaves and branches of Maytenus robusta Reissek were subjected to
structural modifications via nucleophilic addition to the carbonyl
group and Baeyer-Villiger oxidation in order to synthesize potential
cytotoxic compounds. The oximes friedelane-3-hydroxyimino (3)
and 3-hydroxyiminofriedelan-16-one (4) together with the lactones
friedelane-3,4-lactone (5) and 3,4-lactonefriedelan-16-one (6) were
characterized by IR and NMR spectroscopic analyses. Compounds 4
and 6 are reported for the first time. Cytotoxic screening via MTT
assay in human leukemia cell lines (THP-1 and K562) demonstrated
no significant improvement of compounds 3-6 when compared to
the starting materials. Only compounds 3 and 5 demonstrated an
improvement against K562 cells. However, the same assay on ovar-
ian and breast cancer cell lines (TOV-21G and MDA-MB-231) showed
a reduction in the IC50 for compounds 4-6, indicating that ring A
modifications may enhance the biological potential.
Received 30 March 2018
Accepted 27 July 2018
KEYWORDS
Pentacyclic triterpenes;
friedelane derivatives;
cytotoxicity property; THP-1
cell line; K562 cell line;
TOV-21G cell line and
MDA-MB-231 cell line
CONTACT Grasiely F. de Sousa
grasielysousa@ufmg.br
Departamento de Qu ꢀı mica, Universidade Federal de
Minas Gerais, Avenida Presidente Ant ^o nio Carlos, 6627, Pampulha, 31270-901, Belo Horizonte-MG, Brazil.
Supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2018.1508137.
ß 2018 Informa UK Limited, trading as Taylor & Francis Group

2
M. G. DE AGUILAR ET AL.
1. Introduction
Natural products are a rich source of bioactive compounds that can be employed in
obtaining substances with a great medicinal potential (Harvey 2008). In the cancer
treatment, the influence of natural products is quite marked. From the 1940s to the
end of 2014, 80% of the 246 anticancer worldwide approved drugs are related with
natural sources (Newman and Cragg 2016). Many pentacyclic triterpenoids have been
reported in the literature as possible antitumoral agents (Salvador et al. 2017; Shan
et al. 2013). The fact that these substances may act in multiple pathways displaying
antiproliferative, antiangiogenic, anti-inflammatory and pro-apoptotic activities (Safe
et al. 2012) place them as interesting compounds since multifunctional drugs are
promising in treating cancer (Petronelli et al. 2009; Sheng and Sun 2011).
Many species of the Celastraceae family are known for its several triterpenoids,
especially those of friedelane-type (Shan et al. 2013). Previous phytochemical studies
of Maytenus robusta Reissek, a Celastraceae member, led to the isolation of various
friedelanes, and some of them, such as friedelan-3-one (1) and friedelane-3,16-dione
(2), were found in appreciable quantities (De Andrade et al. 2008; Sousa et al. 2012a,
2012b, 2017). They showed in vivo anti-inflammatory activity (Antonisamy et al. 2011;
Sousa et al. 2017) and in vitro cytotoxic effect against 4T1 cells (Sousa et al. 2014).
Compound 1 also displayed in vitro cytotoxic activity against KS cells and proapoptotic
action (Martucciello et al. 2010).
Chemical modifications on ring A of pentacyclic triterpenoids may lead to
derivatives with cytotoxic activity (Salvador et al. 2017). For example, the semisynthetic
oleanolic acid derivative HIMOXOL, which presents an oxime group on ring A, is very
active in reducing cell viability in both HL-60 and drug resistant HL-60/AR leukemia
cells (Paszel-Jaworska et al. 2015). The present work aims to study the effects of ring
A modifications on the cytotoxic activity of compounds 1 and 2. Four derivatives
were obtained: friedelane-3-hydroxyimino (3), 3-hydroxyiminofriedelan-16-one (4),
friedelane-3,4-lactone (5) and 3,4-lactonefriedelan-16-one (6). Compounds 4 and 6 are
reported for the first time. All compounds were evaluated for their in vitro cytotoxicity
against human leukemia cell lines (THP-1 and K562) as well as ovarian and breast
cancer cell lines (TOV-21G and MDA-MB-231).
2. Results and discussion
Compound 1 (726.0 mg from leaves; 21.0 mg from branches) and 2 (676.8 mg from
13
branches) were isolated from M. robusta and characterized by comparison with
C
Scheme 1. Synthesis of friedelane derivatives 3–6.

NATURAL PRODUCT RESEARCH
3
NMR data in previous works (Sousa et al. 2012a, 2012b, 2014). In order to explore their
biological potential, these triterpenoids were transformed into oximes (3 and 4), via
nucleophilic addition to the carbonyl group, and lactones (5 and 6) via Baeyer-Villiger
oxidation (Scheme 1).
Oximes 3 and 4 were obtained as white powders with yields of 87 and 92%,
respectively. In the IR spectra, oxime hydroxyl and NO absorptions were observed for
ꢀ
1
ꢀ1
3
(at 3264 and 943 cm ) and 4 (at 3274 and 942 cm ). Compound 4 also displayed
ꢀ
1
a carbonyl absorption at 1690 cm , which suggested that the addition selectively
occurred at only one of the carbonyl groups, most probably the one at C-3. Methyl
groups at C-26, C-27 and C-28 in friedelane skeletons may create a steric hindrance on
13
the second C ¼ O at C-16, impeding the nucleophilic attack. The C NMR signals for 3
and 4 were similar to the ones of the respective starting materials with exceptions
of C-2, C-3 and C-4. These signals confirmed C-3, at dc 159.9 (3) and 159.7 (4), as the
reaction site because of the typical oxime chemical shifts. Also, compound 4 showed
a carbonyl signal at dc 218.6 (C-16), proving the selectivity of the reaction.
Based on the chemical shifts of C-2 and C-4 of compounds 3 and 4, the oximes
presented E configuration, with the OH anti to the tertiary C-4. These carbon atoms dis-
played higher-field shifts for 3 (dC-2 ¼ 24.6/dC-4 ¼ 51.1) and 4 (dC-2 ¼ 24.3/dC-4 ¼ 50.9)
when compared to the respective parent compounds 1 (dC-2 ¼ 41.5/dC-4 ¼ 58.2) and 2
(
dC-2 ¼ 41.4/dC-4 ¼ 58.1). The upfield effect was stronger in C-2, indicating the syn (C-2)
and anti (C-4) orientation with respect to the N-OH group (Hawkes et al. 1974; Shafikov
et al. 2009). All chemical shift assignments of compounds 3 and 4 were established by
detailed analysis of the H, C, HSQC, HMBC and NOESY spectra (Supplementary materials
S4-S18 and S20-S33).
1
13
Lactones 5 and 6 were obtained as white powders with yields of 79 and 74%,
respectively. In the IR spectra, absorption bands were discerned for lactone carbonyls
ꢀ
1
at 1734 and 1733 cm , O-C stretch coupled to C-C stretch (O-C-C) at 1073 and
ꢀ
1
ꢀ1
1
075 cm and O-CO stretch coupled to CO-C stretch (O-CO-C) at 1235 and 1236 cm
for compounds 5 and 6, respectively. Compound 6 also showed a carbonyl absorption
ꢀ
1
1
at 1690 cm related to the hindered C-16. In the H NMR spectra, the H-4 signals of
5
and 6 were detected at d 4.22 (q, 6.3, 1H), confirming the expected migration of
H
13
C-4 in the Baeyer-Villiger oxidation since it involved a tertiary alkyl group. The
C
Table 1. Cytotoxicity and selectivity of compounds 1-6 against two leukemic cell lines.
ꢁIC50 lM
Selectivity Index
a
b
Compounds
THP-1
K562
PBMCc
THP-1
K562
friedelan-3-one (1)
friedelane-3,16-dione (2)
friedelane-3-hydroxyimino (3)
44.85 ± 3,02
51.92 ± 2.81
46.80 ± 2.62
45.67 ± 2.63
47.95 ± 1.81
45.66 ± 1.81
20.75 ± 1.03
–
79.55 ± 4.24
39.20 ± 1.68
55.40 ± 2.98
54.63 ± 2.88
40.82 ± 3.23
38.50 ± 2.02
–
98.45 ± 3.90
87.94 ± 3.63
82.90 ± 4.12
73.97 ± 4.54
99.30 ± 3.01
82.70 ± 4.33
120.72 ± 6.34
109.34 ± 5.76
2.20
1.70
1.77
1.62
2.07
1.81
5.82
–
1.24
2.24
1.50
1.35
2.46
2.15
–
3
-hydroxyiminofriedelan-16-one (4)
friedelane-3,4-lactone (5)
,4-lactonefriedelan-16-one (6)
3
Cytarabine
Imatinib
24.58 ± 1,28
4.45
Values presented as average ± standard deviation.
a
THP-1, Cytarabine vs all compounds tested;
K562, Imatinib vs all compounds tested;
b
c
a,b,c
PBMC, Cytarabine and imatinib vs all compounds tested.
p value <0.05.

4
M. G. DE AGUILAR ET AL.
Table 2. Cytotoxicity and selectivity of compounds 1-6 against ovarian and breast cancer
cell lines.
Selectivity
Index
ꢁIC50 lM
a
b
c
Compounds
TOV-21G
MDA-MB-231
PBMC
TOV
MDA
friedelan-3-one (1)
friedelane-3,16-dione (2)
friedelane-3-hydroxyimino (3)
>100
>100
>100
35.67 ± 1.30
47.95 ± 1.81
65.60 ± 2.90
12.58 ± 0.98
>100
>100
>100
34.60 ± 1.70
40.82 ± 3.23
78.50 ± 3.10
14.58 ± 1.10
98.45 ± 3.90
87.94 ± 3.63
82.90 ± 4.12
73.97 ± 4.54
99.30 ± 3.01
82.70 ± 4.33
40.34 ± 3.80
–
–
–
2.07
2.07
1.26
3.21
–
–
–
2.14
2.46
1.05
2.77
3
-hydroxyiminofriedelan-16-one(4) (4)
friedelane-3,4-lactone (5)
,4-lactonefriedelan-16-one (6)
Etoposide
3
Values presented as average ± standard deviation.
a
TOV-21G, Cytarabine vs all compounds tested;
MDA-MB-231, Imatinib vs all compounds tested;
b
c
a,b,c
PBMC, Cytarabine and imatinib vs all compounds tested.
p value <0.05.
NMR spectra of 5 and 6 showed signals relative to lactone carbonyls at d 175.6 and
C
3
1
6
75.2 and sp -carbon bonded to oxygen at d 84.9 and 84.6, respectively. Compound
also displayed a carbonyl signal (d 218.7) regarding C-16. C NMR chemical shifts
C
C
13
of compound 5 were similar with the ones in the literature (Moiteiro et al. 2006).
1
13
The chemical shift assignments of compound 6 were established by H, C, HSQC,
HMBC and NOESY experiments. The NMR data of both compounds are available in the
supplementary material (S35-S37 and S39-S53).
The cytotoxic effect of compounds 1-6 was assessed against two human leukemia
cancer cell lines, THP-1 and K562, using cytarabine and imatinib as control drugs. Also,
assays were carried out against ovarian and breast cancer cell lines (TOV-21G and
MDA-MB-231), employing etoposide as positive control. Peripheral blood mononuclear
cells (PBMC) from healthy donors were used to establish the selectivity index of the
analysed substances (Tables 1 and 2).
For the human leukemia cell lines, no considerable improvement in the cytotoxicity
was observed for the modified compounds 4-6. All tested compounds showed activity
against THP-1 and K562 cells lines (IC50 <100 lM). Only compounds 3 (IC50
¼
5
5.40 lM) and 5 (IC50 ¼ 40.82 lM) exhibited a better activity against K562 cells than
the starting material 1 (IC50 ¼ 79.55 lM). In fact, compound 5 was the most selective
towards K562 (SI ¼2.46). The effects of 4 and 6 on K562 as well as 3-6 on THP-1cells
were comparable to the parent compounds. For the ovarian and breast cancer cell
lines (TOV-21G and MDA-MB-231), the modified compounds 4 (IC50 ¼ 35.67 lM), 5
(
IC50 ¼ 47.95 lM) and 6 (IC ¼ 65.60 lM) demonstrated a better activity than the
50
parent compounds 1 and 2 (IC50 > 100 lM). Indeed, compounds 4 and 5 were the
most selective towards TOV-21G and MDA-MB-231cells than PBMC cells. These results
suggest that ring A of this modified triterpenoids may be related to the activity
against solid tumour cells.
3. Experimental
Please refer to supplementary data file.

NATURAL PRODUCT RESEARCH
5
4. Conclusion
The triterpenoids 1 and 2 were subjected to structural modifications affording four
friedelane derivatives, two of them (4 and 6) reported for the first time in the litera-
ture. Cytotoxic screening via MTT assay in human leukemia (THP-1 and K562), ovarian
and breast (TOV-21G and MDA-MB-231) cancer cell lines showed that ring A modifica-
tions may improve the biological activity. Nucleophilic addition to the carbonyl group
furnished oximes 3, active against K562 cells, and 4, active against TOV-21G and
MDA-MB-231. Baeyer-Villiger oxidation produced the lactones 5, which demonstrated
an improvement in the cytotoxicity for all the tested cells except THP-1, and 6, active
against the solid tumour cells.
Acknowledgments
The authors thank the Conselho Nacional de Desenvolvimento Cient ꢀı fico e Tecnol oꢀ gico (CNPq),
the Funda c¸ ~a o Coordena c¸ ~a o de Aperfei c¸ oamento de Pessoal de N ꢀı vel Superior (CAPES), and the
Funda c¸ ~a o de Amparo ꢁa Pesquisa do Estado de Minas Gerais (FAPEMIG) for financial support. The
authors also thank Salom ~a o B. V. Rodrigues for revising the English and critical manuscript reviews.
Conflict of Interest
The authors declare that they have no conflict of interest.
ORCID
Grasiely F. Sousa
http://orcid.org/0000-0001-7425-2604
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