1. Introduction

Steroids are natural products that share a 17-carbon-atom skeleton and are composed of four fused rings: three cyclohexanes (A, B, and C rings) and one cyclopentane (D ring). These compounds vary on the attached functional groups, their position, and configuration ^[1]. In addition, steroids represent a unique class of chemical products, playing an important role in several biological processes, being the most important group of regulatory and signaling molecules ^[2][3]. Usually, steroids are lipophilic and readily enter cells, being able to interact with nuclear receptors as well as with membrane proteins. Therefore, they are associated with most physiological functions and pathological conditions. In addition, due to their low toxicity, less vulnerability to multidrug resistance, and high bioavailability ^{[4][5][6][7][8]}, steroid-based therapeutic drugs have called attention to the scientific academia and industry for a long time.

Steroids are natural products that share a 17-carbon-atom skeleton and are composed of four fused rings: three cyclohexanes (A, B, and C rings) and one cyclopentane (D ring). These compounds vary on the attached functional groups, their position, and configuration [1]. In addition, steroids represent a unique class of chemical products, playing an important role in several biological processes, being the most important group of regulatory and signaling molecules [2,3]. Usually, steroids are lipophilic and readily enter cells, being able to interact with nuclear receptors as well as with membrane proteins. Therefore, they are associated with most physiological functions and pathological conditions. In addition, due to their low toxicity, less vulnerability to multidrug resistance, and high bioavailability [4,5,6,7,8], steroid-based therapeutic drugs have called attention to the scientific academia and industry for a long time.

Due to their relevance, several modified steroids have been synthesized and biologically evaluated, being verified that their relevant pharmacological properties depend on the structural features of the steroidal four-ring skeleton and side-chain ^[5][9]. In fact, even a minor structural variation on the steroidal nucleus can lead to marked changes in their physiological activity ^[10]. Therefore, aiming to improve their pharmacological properties and/or develop compounds with different bioactivities, structural modifications of steroids have been an important focus of research over the last decades ^{[11][12][13][14][15]}.

Due to their relevance, several modified steroids have been synthesized and biologically evaluated, being verified that their relevant pharmacological properties depend on the structural features of the steroidal four-ring skeleton and side-chain [5,14]. In fact, even a minor structural variation on the steroidal nucleus can lead to marked changes in their physiological activity [15]. Therefore, aiming to improve their pharmacological properties and/or develop compounds with different bioactivities, structural modifications of steroids have been an important focus of research over the last decades [16,17,18,19,20].

2. Synthetic Approaches to Prepare Arylidene Steroidal Derivatives

Arylidenesteroids are usually obtained through an aldol condensation between a steroid and an aldehyde. In an aldol condensation, an enol or enolate reacts with a carbonyl in the presence of an acid or base catalyst to form a β-hydroxyaldehyde or a β-hydroxyketone, followed by dehydration to afford a conjugated enone ^[16]. In general, the reactions to prepare arylidenesteroids occur at room temperature (RT), under basic catalysis, and the solvent is, in most cases, methanol (MeOH) or ethanol (EtOH) (

Arylidenesteroids are usually obtained through an aldol condensation between a steroid and an aldehyde. In an aldol condensation, an enol or enolate reacts with a carbonyl in the presence of an acid or base catalyst to form a β-hydroxyaldehyde or a β-hydroxyketone, followed by dehydration to afford a conjugated enone [46]. In general, the reactions to prepare arylidenesteroids occur at room temperature (RT), under basic catalysis, and the solvent is, in most cases, methanol (MeOH) or ethanol (EtOH) (

Figure 1

).

Figure 1.

Claisen–Schmidt condensation to prepare 16

E

- and 21

E

-arylidenesteroids.

3. Bioactivity of 2- and 16E-arylideneandrostane Derivatives

The 16

E-arylideneandrostane steroidal skeleton has emerged as a relevant template to develop potential anticancer agents. In addition to cytotoxic effects, several studies reported other biological activities, such as aromatase inhibition, anti-inflammatory, neuroprotective and skeletal muscle relaxing activities and tissue-selective androgen receptor modulator effects ^[17][18][19].  A selection of the most relevant 16

-arylideneandrostane steroidal skeleton has emerged as a relevant template to develop potential anticancer agents. In addition to cytotoxic effects, several studies reported other biological activities, such as aromatase inhibition, anti-inflammatory, neuroprotective and skeletal muscle relaxing activities and tissue-selective androgen receptor modulator effects [38,57,58].  A selection of the most relevant 16

E

-arylideneandrostane derivatives is presented in

Table 1

. 

Table 1.

E

2

Antiproliferative activity (IC50 μM)

[11]

[16]

Cell line

2

5-FU (+)

HepG2

9.10

10.59

MCF-7

9.18

28.11

Cell cycle arrest at G2 phase in HepG2

3

Antiproliferative activity (IC

50

± SEM μM)

[20]

[60]

Cell line

3

Etoposide (+)

KB

0.6 ± 2.0

2.8 ± 16.8

T47-D

1.7 ± 14.8

1.2 ± 8

4

Antiproliferative activity (IC

50

μM)

[21]

[61]

Cell line

4

CCRF-CEM

3.94

K-562

2.61

RPMI-8226

6.90

SR

1.79

5

Antiproliferative activity (IC

50

μM ± SD μM)

[22]

[59]

Cell line

5

Cis (+)

HT-29

1.2 ± 0.4

66 ± 2

6

Cytotoxic activity–in vivo hollow fiber assay (score)

[18]

[57]

6

Taxol (+)

I.P.

2

Data not shown

S.C.

8

7

Cytotoxic activity–in vivo hollow fiber assay (score)

[23]

[62]

7

Taxol (+)

I.P.

12

Data not shown

S.C.

8

8

Aromatase inhibitory activity (IC

50

μM)

[24]

[70]

8

Aminoglutethimide (+)

5.2

28.5

9

Aromatase inhibitory activity (IC

50

μM)

[24]

[70]

9

Aminoglutethimide (+)

6.4

28.5

10

Aromatase inhibitory activity (IC

50

μM)

[25]

[71]

10

Aminoglutethimide (+)

4.4

28.5

11

Aromatase inhibitory activity (IC

50

μM)

[25]

[71]

11

Aminoglutethimide (+)

2.4

28.5

12

Anti-inflammatory activity TNF-α levels (pg.mg

−1

protein ± SD)

[26]

[72]

12

CEL (+)

DEX (+)

88.6 ± 1.8

68.2 ± 1.1

89.6 ± 2.0

13

Anti-inflammatory activity (IC

50

μM) (NO release of LPS-activated mouse microglial cell line BV2)

[27]

[73]

13

Minocycline (+)

2.69

5.97

14

Anti-inflammatory activity (IC

50

μM) (NO release of LPS-activated mouse microglial cell line BV2)

[27]

[73]

14

Minocycline (+)

3.28

5.97

4. Bioactivity of 21E-arylidenepregnene Derivatives

Arylidenesteroidal derivatives of progesterone and pregnenolone and other similar pregnanes constitute a smaller group than arylideneandrostanes, and they have mainly been studied as potential antitumoral agents. Of these, the most potent arylidenepregnene derivatives reported until now are presented in

Table 2

.

Table 2.

E

.

	15	Antiproliferative activity (IC			50			μM)			[28]	[69]
		Cell line	15
		HCT-15	0.81
	16	Antiproliferative activity (IC			50			μM)			[28]	[69]
		Cell line	16
		MCF-7	0.60
	17	Antiproliferative activity Growth inhibition (%)			[17]	[38]
		Cell line	17	Cis (+)
		HeLa	58	99
		MCF-7	64	88
	18	Antimicrobial activity Zone of inhibition (mm)			[29]	[76]
		Gram positive	18	AMP (+)
		Streptococcus pneumoniae	30	20
		Staphylococcus aureus	24	22

5. Bioactivity of 16E-arylidenoestrone and 16E-arylidenoestradiol Derivatives

It is well known that estrogenic hormones have an important contribution to estrogen-dependent diseases, being breast cancers primarily initiated and stimulated by estrogens the majority of these conditions ^[30]. Consequently, the structural modification of estrone and estradiol in different positions to prepare bioactive compounds in this context has been the focus of intensive research.

It is well known that estrogenic hormones have an important contribution to estrogen-dependent diseases, being breast cancers primarily initiated and stimulated by estrogens the majority of these conditions [77]. Consequently, the structural modification of estrone and estradiol in different positions to prepare bioactive compounds in this context has been the focus of intensive research.

Table 3.

E

	19	17β-HSD1 inhibitory activity (IC		50		μM)		[31]	[35]
		19	Estradiol (+)
		3.4	7.3
	20	17β-HSD inhibitory activity (% at 10 μM)		[32]	[83]
		20
		Type 1	Type 2
		72	13
	21	Tumor weight reduction (%) Human breast cancer (MCF-7) xenografts models		[33]	[85]
		21 (45 mg/kg/day)	2ME (+) (45 mg/kg/day)
		About 50% Significative reduction compared with negative control	About 50% Significative reduction compared with negative control

Of these prepared compounds, steroid

21

(

Table 3

) showed a potent antiangiogenic activity. Moreover, further studies suggested that this compound suppresses the tumor growth in about 50% in human breast cancer (MCF-7) xenograft models without relevant side effects. The action mechanism studies suggested that steroid

21 targeted the epithelial to mesenchymal transition process in MCF-7 cells and inhibited human umbilical vein endothelial cells (HUVEC) migration, contributing to angiogenesis interruption ^[33].

targeted the epithelial to mesenchymal transition process in MCF-7 cells and inhibited human umbilical vein endothelial cells (HUVEC) migration, contributing to angiogenesis interruption [85].

6. Importance of Steroidal Arylidene Derivatives as Synthetic Intermediates of Bioactive Molecules

In addition to their biological activity, steroidal arylidenes are also versatile synthetic intermediates in the preparation of other bioactive structures. In fact, these steroids have been used in the introduction of diverse chemical groups present in bioactive compounds, such as oximes, hydroxyl and hydrazones ^{[28][34][35][36][37]}, and are particularly useful in several heterocyclization reactions. In this context, over the years, a large number of bioactive heterocyclic steroidal derivatives have been synthesized, and some of them are already being clinically used ^[38][39]. Interestingly, diverse heterocyclic compounds, including arylpyrazolines and pyrazoles, arylpyrimidines, oxindoles, pyridones and pyridines as well as spiro-pyrrolidines were prepared from arylidenesteroids ^{[4][13][40][41][42][43][29][37][44][45][46][47][48][49][50][51][52][53][54]}. The most promising steroidal derivatives prepared from arylidenesteroids that have been reported until the moment are presented in

In addition to their biological activity, steroidal arylidenes are also versatile synthetic intermediates in the preparation of other bioactive structures. In fact, these steroids have been used in the introduction of diverse chemical groups present in bioactive compounds, such as oximes, hydroxyl and hydrazones [69,90,91,92,93], and are particularly useful in several heterocyclization reactions. In this context, over the years, a large number of bioactive heterocyclic steroidal derivatives have been synthesized, and some of them are already being clinically used [94,95]. Interestingly, diverse heterocyclic compounds, including arylpyrazolines and pyrazoles, arylpyrimidines, oxindoles, pyridones and pyridines as well as spiro-pyrrolidines were prepared from arylidenesteroids [4,18,42,49,53,56,76,93,96,97,98,99,100,101,102,103,104,105,106]. The most promising steroidal derivatives prepared from arylidenesteroids that have been reported until the moment are presented in

Table 4

. 

Table 4.

24

Antiproliferative activity (IC

50

μM)

[4]

Cell line

24

HT-29

0.24

HCT-15

0.25

25

Antiproliferative activity (IC

50

μM)

[41]

[49]

Cell line

25

5-FU (+)

HepG-2

5.41

>100

Huh-7

5.65

>95

SGC-790

10.64

>100

26

5AR-1 Inhibition (IC

50

± SEM μM)

[44]

[96]

26

Finasteride (+)

14.50 ± 0.48

21.6 ± 0.62

27

5AR-2 Inhibition (IC

50

± SEM μM)

[44]

[96]

27

Finasteride (+)

13.90 ± 0.75

15.4 ± 0.58

28

5AR-2 Inhibition (IC

50

± SEM μM)

[44]

[96]

28

Finasteride (+)

14.20 ± 0.75

15.4 ± 0.58

29

5AR-2 Inhibition (IC

50

± SEM nM)

[46]

[98]

29

Finasteride (+)

7.30 ± 0.62

2.4 ± 0.15

30

5AR-2 Inhibition (IC

50

± SEM nM)

[46]

[98]

30

Finasteride (+)

8.20 ± 0.55

2.4 ± 0.58

31

Antiproliferative activity (IC

50

μg.mL

−1

)

[13]

[18]

Cell line

31

5-FU (+)

Cis (+)

NCI-H460

10.30

2.48

0.699

HeLa

12.50

0.887

2.03

Cell cycle arrest at S phase in HeLa cells

32

Antiproliferative activity (IC

50

μM)

[43]

[56]

Cell line

32

5-FU (+)

SMMC-7721

4.30

9.78

MCF-7

2.06

7.54

33

Antiproliferative activity (IC

50

± SEM μM)

[43]

[56]

Cell line

33

5-FU (+)

SMMC-7721

6.05 ± 0.48

9.78 ± 0.99

MGC-803

5.79 ± 0.76

6.92 ± 0.35

Cell cycle arrest at G2/M phase in MGC cells

34

Antiproliferative activity (IC

50

± SEM μM)

[43]

[56]

Cell line

34

5-FU (+)

SMMC-7721

0.71 ± 0.11

9.78 ± 0.99

35

Antiproliferative activity (IC

50

μM)

[37]

[93]

Cell line

35

DOX (+)

MDA-MB 231

0.91

1.23

36

Osteoanabolic activity and tissue-selectivity (% of the effect of DHT)

[55]

[113]

OVX

36

DHT

BRF

120

100

ORX

VP

3

100

SV

21

100

37

Cytotoxic activity-in vivo hollow fiber assay (score)

[18]

[57]

37

Taxol (+)

I.P.

4

Data not shown

S.C.

6

38

Antiproliferative activity Cell growth (%)

[56]

[114]

Cell line

38

NCI-H460

−44

MFC-7

−44

SF-268

−79

39

Antiproliferative activity Cell growth (%)

[36]

[92]

Cell line

39

NCI-H460

−11

MCF-7

5

SF-268

−8

7. Summary

Steroids constitute an important group of structurally related natural, semi-synthetic, and synthetic compounds with remarkable functions, including regulatory and signaling activities. In the last three decades, steroidal arylidene derivatives have been prepared and screened for a range of biological activities and used as synthetic intermediates, with special attention to bioactive heterocyclic steroids. In conclusion, due to the straightforward synthesis of arylidenesteroids and their bioactivity, as well as the inherent chemical reactivity of α,β-unsaturated ketones, useful in the preparation of other derivatives, this class of compounds has been of high interest in the last years.