Open Access

Further structure–activity relationships study of substituted dithiolethiones as glutathione-inducing neuroprotective agents

Chemistry Central Journal201610:64

DOI: 10.1186/s13065-016-0210-z

Received: 12 February 2016

Accepted: 5 October 2016

Published: 19 October 2016

Abstract

Background

Parkinson’s disease is a neurodegenerative disorder associated with oxidative stress and glutathione depletion. The induction of cellular glutathione levels by exogenous molecules is a promising neuroprotective approach to limit the oxidative damage that characterizes Parkinson’s disease pathophysiology. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, are known to increase cellular levels of glutathione; however, limited information is available regarding the influence of dithiolethione structure on activity. Herein, we report the design, synthesis, and pharmacological evaluation of a further series of dithiolethiones in the SH-SY5Y neuroblastoma cell line.

Results

Our structure–activity relationships data show that dithiolethione electronic properties, given as Hammett σp constants, influence glutathione induction activity and compound toxicity. The most active glutathione inducer identified, 6a, dose-dependently protected cells from 6-hydroxydopamine toxicity. Furthermore, the protective effects of 6a were abrogated by the inhibitor of glutathione synthesis, buthionine sulfoximine, confirming the importance of glutathione in the protective activities of 6a.

Conclusions

The results of this study further delineate the relationship between dithiolethione chemical structure and glutathione induction. The neuroprotective properties of analog 6a suggest a role for dithiolethiones as potential antiparkinsonian agents.

Keywords

Neuroprotection Parkinson’s disease Glutathione Dithiolethiones

Background

The incidences of neurodegenerative disorders are expected to greatly increase as the American population ages. Parkinson’s disease (PD), the second most common neurodegenerative disease, is a movement disorder characterized by the gradual disintegration of the nigrostriatal dopaminergic pathway. The resulting depletions of striatal dopamine (DA) give rise to the cardinal symptoms of the disease, including tremor, rigidity, bradykinesia, and postural instability. Additionally, cognitive issues, depression, and sleep disturbances are frequently observed non-motor symptoms. Although pharmacotherapeutic intervention is capable of providing symptomatic relief in PD, to date no therapy is able to arrest or reverse the progression of the disease.

The cause of PD is not currently fully understood; however, the etiology of sporadic PD, the most prevalent form of the disease, is probably multifactorial, involving a combination of genetic, environmental, and unknown factors. Increasingly, oxidative stress is emerging as a major player in neurodegenerative disorders such as PD. Analyses of the brains of PD patients have demonstrated extensive cellular damage caused by oxidative stress [1]. Neurons may be particularly prone to oxidative damage due to their high lipid content and oxygen consumption. Dopaminergic neurons experience an additional oxidative burden due to the autoxidation and metabolism of DA. These processes yield damaging electrophilic DA-quinones and reactive oxygen species (ROS). Additionally, many of the molecular hallmarks of PD, such as mitochondrial dysfunction, α-synuclein aggregation, neuroinflammation, increased monoamine oxidase B activity, and elevated levels of iron, are related to increased oxidative activity [27]. ROS cause lipid peroxidation, protein and DNA damage, and ultimately the demise of dopaminergic neurons [810] (Fig. 1).
Fig. 1

Sources of oxidative stress in PD

As reactive oxygen species occur naturally in all cells, various antioxidants and enzymes have been evolved to mitigate their harmful effects. Glutathione (GSH), a cysteine-containing tripeptide, is the most abundant non-protein antioxidant in the body, and plays a crucial role in the detoxification of ROS and dopamine metabolites [11]. GSH can detoxify ROS non-enzymatically, forming oxidized glutathione (GSSG). GSH also serves as a cosubstrate for several phase II enzymes. Glutathione S-transferase (GST) mediates the addition of GSH to electrophiles, such as dopamine o-quinone, and glutathione peroxidase (GPx) catalyzes the reduction of peroxides, including H2O2 [12, 13]. However, in PD, the oxidative load experienced by dopaminergic neurons overwhelms these endogenous cellular detoxification mechanisms. Indeed, postmortem analyses of the brains of PD patients have shown depleted levels of nigrostriatal GSH [14]. As such, increasing neuronal levels of GSH may provide therapeutic benefit against the damaging effects of oxidative stress in PD.

The rate-limiting step in the biosynthesis of GSH is mediated by glutamate cysteine ligase (GCL). Associated with the gene of this enzyme is the antioxidant response element (ARE), found in many genes that play a role in protecting cells from oxidative damage, including GCLC (the catalytic subunit of GCL), GST, GPx, NAD(P)H:quinone oxidoreductase (NQO1), superoxide dismutase, hemeoxygenase, catalase, and many others [15]. Stabilization and nuclear translocation of the transcription factor Nrf2 (nuclear factor-erythroid-2 related factor-2) enhances the transcription of ARE-associated genes [16]. Nrf2 is a short-lived protein, undergoing rapid ubiquitination and proteasomal degradation under basal conditions, mediated by its repressor Keap1 (Kelch-like ECH-associated protein-1) [1719]. Keap1 is a cysteine-rich protein that serves as a sensor of oxidative and electrophilic stress. The stabilization of Nrf2 is believed to involve modulation of some of the numerous cysteine residues of Keap1 by ROS and electrophiles, leading to enhanced Nrf2 stability and nuclear accumulation [2022].

Dithiolethiones (DTTs) are a class of sulfur-containing heterocycles (Fig. 2). DTTs have been shown to induce the expression of a variety of ARE-associated detoxification enzymes and molecules, including GCLC and GSH, in numerous cell and tissue types; however, limited information is available regarding the activities of these interesting molecules in the CNS [2325]. Our group is interested in exploring GSH induction as a potential neuroprotective strategy. In a previous report by our group, we described a preliminary SAR study of substituted DTTs as inducers of GSH in the SH-SY5Y neuroblastoma cell line (a dopaminergic cell line commonly employed in in vitro models of PD), with key findings that placement of electron withdrawing groups (EWGs) at the 4-position and electron donating groups (EDGs) at the 5-position induced the most glutathione [2628]. Additionally, three of these GSH inducers demonstrated neuroprotection in the in vitro 6-hydroxydopamine (6-OHDA) model of neurotoxicity. Based on these initial findings, we sought to better understand the influence of DTT substituents on GSH induction. In this report, we describe the synthesis and GSH induction activities of additional substituted DTTs. The relationship between DTT structure and pharmacological activity is discussed.
Fig. 2

Generalized structure of dithiolethiones

Chemistry

A series of 4-, 5-, and 4, 5-disubstituted DTTs was synthesized (Table 1) to determine the generality of the initial SAR findings previously communicated by us [26]. These molecules were designed to ensure that a diversity of electronic features were represented in the compounds evaluated, including various aryl, alkyl, and amino groups, with both electron donating and electron withdrawing properties. The syntheses of DTTs are shown in Scheme 1. Compounds 4ac, 5ad, and 6b, gi were synthesized from requisite β-keto esters by treatment with P4S10, S8, and (Me3Si)2O in refluxing toluene for 1–3 h in good to excellent yield [29]. Molecules 6de were synthesized from their corresponding nitriles via reaction with NaH, S8, and CS2 in DMF at 0 °C for 30 min, in excellent yield [30]. Compound 6c was synthesized by refluxing 6a in acetic anhydride for 30 min (Scheme 1). Molecules 6a and 6f were purchased commercially.
Table 1

Structures and Hammett sigma constants of DTTs

Entry

R1p) [31]

R2p) [31]

Entry

H

H

D3T

1a

4-NO2-C6H4 (0.26)

H (0)

4a

1b

Ethyl (−0.15)

H (0)

4b

1c

CO2Et (0.50)

H (0)

4c

2a

H (0)

Me (−0.17)

5a

2b

H (0)

4-F-C6H4 (0.06)

5b

2c

H (0)

4-pyridinyl (0.44)

5c

2d

H (0)

2-furanyl (0.02)

5d

3a

CO2Et (0.50)

NH2 (−0.66)

6a

3b

CO2Et (0.50)

Me (−0.17)

6b

3c

CO2Et (0.50)

NHC(O)Me (0.00)

6c

3e

4-Cl-C6H4 (0.12)

NH2 (−0.66)

6d

3d

SO2Ph (0.68)

NH2 (−0.66)

6e

3f

CN (0.66)

NH2 (−0.66)

6f

3g

Cl (0.23)

4-OMe-C6H4 (−0.08)

6g

3h

Cl (0.23)

C6H5 (−0.01)

6h

3i

Cl (0.23)

Ethyl (−0.15)

6i

Scheme 1

Synthesis of dithiolethiones

Results and discussion

DTTs were assayed for GSH induction. SH-SY5Y human neuroblastoma cells were treated with test compounds for 24 h at a concentration of 100 μM. The results are shown in Fig. 3 and are reported as a percentage of control. Among the four 5-substituted DTTs (5ad) evaluated, electron-donating 5-methyl substituted DTT 5a induced GSH to the highest extent (163 %) compared to the other 5-substituted DTTs evaluated. Compounds 5b, 5c, and 5d, each containing electron-withdrawing aromatic groups, induced a lesser amount of GSH (94, 114 and 130 %, respectively). These results are consistent with our previous findings that alkyl groups at this position are superior to aromatic groups.
Fig. 3

DTT-mediated GSH induction. SH-SY5Y cells were treated with test compounds (100 μM) for 24 h, at which time total cellular GSH was measured. Data shown are mean ± SEM of at least three different experiments. *P < 0.05

Next evaluated were three 4-substituted molecules, 4ac, containing p-nitrophenyl, ethyl, and ester groups, respectively. Interestingly, electronically-different 4a and 4b increased GSH levels by almost the same extent (156 % for 4a, and 149 % for 4b). The activity of 4b is unexpected, as our previous work suggested that EDGs at this position would induce less GSH as their electron-withdrawing counterparts. Surprisingly, when 4-ethyl ester substituted analog 4c was tested, significant toxicity was observed, and the GSH induction data for this compound was omitted (vide infra).

Next, we explored the effects on GSH induction of substituting both the 4- and 5-positions of the DTT core with a variety of functional groups (compounds 6ai). The most active molecule in this series was analog 6a (4-ethyl ester, 5-amino), which increased cellular GSH levels by 190 %. Interestingly, replacement of the primary amine of 6a with a methyl group, 6b, significantly reduced activity. Similarly, substitution of the ester of 6a with an aryl ring (6d) or chloro group (6gi), diminished activity, regardless of the nature of the 5-position. The SAR data from disubstituted DTTs suggest that GSH induction is highest when the 4- and 5-positions possess strongly electron withdrawing and strongly electron donating groups, respectively. Compounds 6e (4-phenylsulfonyl, 5-amino) and 6f (4-nitrile, 5- amino) exhibited toxicity when evaluated and the resulting GSH induction data were omitted (vide infra).

The above SAR data demonstrate that electronic parameters influence GSH induction activity. As such, we sought a method to quantitatively assess the electronic properties of substituted DTTs. We decided to explore Hammett’s σp constants (Table 1), which reflect the ability of substituted benzoic acids to stabilize a negatively charged carboxylate upon ionization of the corresponding acid. The constants given for these ionizations are an indication of the release (−σp) or withdrawal (+σp) of electrons by a substituent, and provide an indication of the combined contributions of both inductive and resonance effects. We plotted our GSH induction values for 4- and 5-substituted compounds from this and our previous study (structures shown in Table 2) against reported Hammett σp constants (Fig. 4) [31]. As EDGs at the 5-position were observed to be beneficial to activity, we chose to use +σp for these types of functional groups, and −σp for EWGs, which appeared to impair GSH induction. Analogously, as EWGs generally had a positive influence on activity at the 4-position, we used +σp; −σp were employed for the less active EDGs. As can be seen in Fig. 4a, a linear relationship was observed between DTT electronic properties and GSH induction, with only two molecules, 4b and 5c, laying outside of the curve (r2 = 0.7969 with 4b and 5c omitted). Interested in whether electronics similarly influence activity for the 4, 5-disubstituted molecules, we summed the σp constants of both substituents (using the same approach to the sign of σp described above) and plotted these values with the respective GSH activity. Again, a relationship was seen, supporting the influence of electronic properties on GSH induction (r2 = 0.5383, Fig. 4b).
Table 2

DTT structures from initial SAR study and corresponding Hammett sigma constants [26, 31]

R1p)

R2p)

Entry

4-OMe-C6H4 (−0.08)

H (0)

4d

C6H5 (−0.01)

H (0)

4e

CH2CF3 (0.09)

H (0)

4f

4-Cl-C6H4 (0.12)

H (0)

4g

H (0)

Ethyl (−0.15)

5e

H (0)

Cyclopropyl (−0.21)

5f

H (0)

4-Cl-C6H4 (0.12)

5g

H (0)

4-OMe-C6H4 (−0.08)

ADT

Fig. 4

GSH induction values of 4- and 5-substituted DTTs (a), and 4, 5-disubstituted DTTs (b) vs. Hammett σp constants

As previously mentioned, when DTTs 4c, 6e, and 6f were evaluated for GSH induction in SH-SY5Y cells, significant toxicity was observed, and the GSH induction data for these molecules was omitted from the study. Interestingly, analogs 6a and 6b, amino and methyl 5-substituted congeners of 4c, appeared to not be toxic to SH-SY5Y cells. Based on this observation, we began to suspect that DTT toxicity may be related to the value of σp at the 4-position. To test this hypothesis, we measured the viability of SH-SY5Y cells treated with our DTTs (100 µM, 24 h, Fig. 5). Molecules with 4-position σp constants ranging from −0.15 (4b) to 0.26 (4a) showed minimal toxicity to SH-SY5Y cells. However, when the σp constant was raised to 0.50 (4c), significant cell death was seen. Surprisingly, the addition of an amino or methyl substituent to the 5-position of 4c (compounds 6a and 6b, respectively) appeared to restore viability. To confirm the beneficial effects on toxicity of an electron-donating group at the 5-position, the amino group of 6a was acylated, yielding 6c. As the σp constant of the acetamide group is 0.0, electron donation should not take place, and 6c would be expected to be toxic. This was indeed observed as shown by the restoration of toxicity of 6c. The beneficial effects of placing electron-donating substituents at the 5-position appears to be limited, however. When the σp constant of the 4-position of 6a (ethyl ester, σp = 0.50) was increased to 0.66 (nitrile, compound 6f), or 0.68 (sulfone, compound 6e), cell viability was once again decreased.
Fig. 5

Toxicity of DTTs. SH-SY5Y cells were treated with the indicated molecules (100 μM) for 24 h, at which time viability was assessed. Data shown are mean ± SEM of at least three different experiments. *P < 0.05

The above observation that GSH induction is dependent on the magnitude of Hammett σp constants suggests that DTTs substituents influence the reactivity of the dithiolethione ring. Stabilization of Nrf2 by DTTs is believed to result from alteration of the interaction between Nrf2 and its repressor, Keap1. In the presence of oxygen and cellular thiols, the DTTs D3T, oltipraz, and ADT generate superoxide anion, O2, a progenitor to H2O2 [3234]. Either of these reactive oxygen species could oxidize the numerous sulfhydryl groups of Keap1, resulting in diminished ubiquitination and increased nuclear accumulation of Nrf2. The placement of substituents with larger σp constants on the dithiolethione ring may render the molecule more reactive to thiols, resulting in greater GSH induction. It is also likely that the toxicity observed by several of the evaluated DTTs may be a consequence of the above described mechanism of action. The DTTs that were observed to be toxic to SH-SY5Y cells (4c, 6c, 6e and 6f) would be expected to induce more GSH than other evaluated DTTs, based on extrapolation of our GSH induction vs. σp plots. Given the current evidence for the proposed mechanism of action of Nrf2 activation by DTTs, it is possible that toxicity results from an increased level of reactive oxygen species produced from DTTs with higher σp constants for the 4-position. Additional studies are currently planned to more clearly understand the nature of DTT toxicity.

The observed influence of DTT substituent σp constants on GSH induction and compound toxicity has important implications in the design and selection of future molecules as neuroprotective agents. 4-Monosubstituted congeners must possess substituents with σp constants that are less than 0.5 to avoid toxicity, thus limiting the extent of GSH induction possible. Their 5-monosubstituted counterparts must have strongly electron-donating groups to effect significant GSH induction; however, aliphatic groups, the most active function group at this position, were only able to increase GSH by a maximum of 165 % (compound 5a). Substitution of carbon-containing substituents at the 5-position with heteroatoms (O, N) would increase the electron donating effects at this site; however, efforts to synthesize such monosubstituted analogs proved to be problematic. Disubstituted DTT 6a appears to solve both of these issues: the strongly electron withdrawing ester at the 4-position, combined with the electron donating 5-amino group, provide the large values of σp needed for maximal GSH induction. Additionally, the 5-amino group mitigates the toxicity that is associated a large σp value for the 4-position. As the values of DTT substituents cannot be increased much more without causing toxicity, it is likely that the activity of analog 6a represents the upper limit of GSH induction for substituted DTTs.

Having identified a DTT that potently increases cellular GSH levels, we next evaluated the ability of 6a to protect against 6-OHDA induced toxicity, a commonly used neuroprotection model [3538]. SH-SY5Y cells were pretreated with 6a for 24 h at concentrations of 6.25, 12.5, 25, 50, and 100 μM, followed by concurrent exposure to 40 μM 6-OHDA for a further 24 h. Cell viability was then determined. As shown in Fig. 6 administration of 40 µM 6-OHDA reduced cellular viability to 22 %. Excitingly, pretreatment with 6a dose-dependently protected against the toxic effects of 6-OHDA. Protective effects were seen starting with a concentration of 12.5 µM (33 % viability), and plateaued with the doses of 50 and 100 µM; interestingly, these two doses were equally protective (56 and 58 %, respectively).
Fig. 6

Neuroprotection of 6a against 6-OH induced neurotoxicity. SH-SY5Y cells were treated for 24 h with various concentrations of 6a, followed by co-treatment with 6-OHDA (40 μM) for a further 24 h, at which time cellular viability was assed. Data shown are mean ± SEM of at least three different experiments. *P < 0.05

The mechanism of 6-OHDA toxicity involves the generation of ROS and electrophilic quinone metabolites [39]. The increase in cellular GSH levels mediated by 6a likely protects against the oxidative insult of 6-OHDA. To explore the role that GSH plays in this protection, SH-SY5Y cells were co-treated with 6a and buthionine sulfoximine (BSO), an inhibitor of GCLC [40]. As shown in Fig. 7, administration of BSO (25 µM) was able to inhibit the ability of 6a (100 µM) to induce GSH, demonstrating that GSH induction is mediated through actions of GCLC. Additionally, the abrogation of GSH induction by BSO was able to block the neuroprotective effects of 6a (Fig. 8), confirming the importance of GSH in neuroprotection. DTTs are known, via stabilization of Nrf2, to induce the expression of numerous cytoprotective phase II enzymes, and it is possible that the activity of these enzymes contribute to the protective effects of 6a. However, as the protective effects of 6a can be blocked by inhibition of GSH induction, the contribution to neuroprotection of other phase II enzymes in this model may be minimal.
Fig. 7

Suppression of GSH induction of 6a by BSO. SH-SY5Y cells were treated with 6a (100 μM) and/or BSO (25 μM) for 24 h, at which time total cellular GSH levels were assessed. Data shown are mean ± SEM of at least three different experiments. *P < 0.05

Fig. 8

Abrogation of protective neuroprotective effects of 6a by BSO. SH-SY5Y cells were treated with 6a (100 μM) and/or BSO (25 μM) for 24 h, at which time either 6-OHDA (40 μM) or DMSO was added. Cellular viability was measured 24 h later. Data shown are mean ± SEM of at least three different experiments. *P < 0.05

Many of the symptoms of PD arise as a result of depletion of nigrostriatal DA levels. As such, current antiparkinsonian pharmacotherapeutic approaches are DA focused. These treatments aim to replace DA (levodopa), slow its metabolism (inhibitors of monoamine oxidase B and catecholamine O-methyltransferase), or supplement its effects (dopamine agonists). While these agents are able to provide symptomatic relief in PD, they do little to halt or reverse the progression of the disorder since they do not address the underlying oxidative damage that is responsible for the loss of dopaminergic neurons. The results of this study, while preliminary, suggest that elevation of cellular levels of GSH may have promise as a potential antioxidant-based antiparkinsonian approach. Additional studies are currently planned to examine the neuroprotective potential of DTTs is additional cell lines and PD models.

Conclusions

In support of our effort to identify novel potential neuroprotective agents, a further series of substituted DTTs was synthesized and evaluated for GSH induction in the SH-SY5Y human neuroblastoma cell line. Our results showed that the extent of GSH induction is related to the electronic properties of DTTs. Plots of GSH induction vs. DTT substituent Hammett σp values demonstrated linear relationships for substituents of 4-, 5-, and 4, 5-disubstituted DTTs. It was also observed that the magnitude of σp at the 4-position influences DTT toxicity, which can be diminished by the presence of an EDG at the 5-position. The most potent inducer of GSH identified in this study, congener 6a, was minimally toxic to cells and was able to provide neuroprotection in the 6-OHDA model of neurotoxicity, suggesting GSH induction as a neuroprotective strategy. GSH induction was shown to be crucial to neuroprotection, as the protective effects of 6a were abrogated by treatment with the GCLC inhibitor, BSO. The data generated in this study suggest that dithiolethiones warrant additional exploration as potential neuroprotective, antiparkinsonian agents.

Experimental section

Chemistry methods

All solvents and reagents obtained from commercial sources were used without further purification, unless otherwise noted. Compounds 6a and 6f were purchased from Oakwood Chemical (West Columbia, SC) and purified prior to use. All reactions were carried out under an argon atmosphere unless otherwise noted. All final molecules were >95 % pure as judged by high-performance liquid chromatography (HLPC). HPLC analyses were performed on an Agilent 1220 Infinity system with an Agilent column (Poroshell 120 EC-C18, 4.6 × 150 mm, gradient of 0.1 % trifluoroacetic acid/acetonitrile). 1H and 13C NMR analyses were performed on a Varian Mercury 300 MHz spectrophotometer at 300 and 75 MHz, respectively. Chemical shifts are given in ppm in reference to tetramethylsilane (TMS) as an internal standard. Multiplicities are given as s (singlet), d (doublet), t (triplet), m (multiplet), and br s (broad signal). Low-resolution mass spectral data were obtained on an Agilent 1260 Infinity single quadrupole LCMS system. Melting points were taken on a Mel-Temp apparatus and are uncorrected. Thin layer chromatography (TLC) was performed on silica gel 60 F254-coated glass plates purchased from EMD Millipore, and visualized with UV light and/or basic KMnO4.

General procedure for the synthesis of dithiolethiones from β-keto esters, exemplified by 5-methyl-3H-1,2-dithiole-3-thione, 5a [41]

To a suspension of elemental sulfur (123 mg, 3.85 mmol), phosphorus pentoxide (1.03 g, 2.31 mmol), hexamethyldisiloxane (2.76 mL, 11.6 mmol), in toluene (10 mL) was added β-oxo ester 2a (500 mg, 3.85 mmol). The mixture was heated under reflux conditions until complete as judged by TLC (generally between 1 and 3 h), at which time the reaction mixture was cooled to 0 °C. Saturated aqueous K2CO3 was added (5 mL) to destroy any unreacted phosphorus pentoxide. The crude product was then extracted with ethyl acetate (10 mL × 3), dried (Na2SO4), filtered, concentrated, and purified by column chromatography (hexanes/ethyl acetate, 4:1) to give a low-melting red solid (521 mg, 91 %). R f  = 0.65 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 2.52 (d, J = 0.99 Hz, 3 H), 7.00–7.07 (m, 1 H). 13C NMR (75 MHz, CDCl3) δ: 18.43, 139.41, 172.22, 216.66. Calc. 148, found 149 [M+H]+.

4-(4-Nitrophenyl)-3H-1,2-dithiole-3-thione, 4a [42]

Prepared from 1a [43]. Red solid (92 %). Mp 152–154 °C. R f  = 0.37 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 7.89 (d, J = 8.73 Hz, 2 H), 8.30 (ds, J = 8.90 Hz, 2 H), 9.34 (s, 1 H). 13C NMR (75 MHz, CDCl3): δ = 128.67, 135.59, 145.44, 151.15, 152.50, 166.47, 218.57. Calc. 255, found 256 [M+H]+.

4-Ethyl-3H-1,2-dithiole-3-thione, 4b [44]

Prepared from 1b [45]. Yellow oil (81 %). R f  = 0.46 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 1.15 (t, J = 7.43 Hz, 3 H), 2.48–2.73 (m, 2 H), 8.86 (t, J = 0.82 Hz, 1 H). 13C NMR (75 MHz, CDCl3): δ 13.03, 23.52, 150.79, 155.45, 215.12. Calc. 162, found 163 [M+H]+.

Ethyl 3-thioxo-3H-1,2-dithiole-4-carboxylate, 4c [46]

Prepared from diethyl 2-(ethoxymethylene)malonate, 1c. Red solid (47 %). Mp 61–62 °C. R f  = 0.48 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 1.37 (t, J = 7.07 Hz, 3 H), 4.35 (q, J = 7.19 Hz, 2 H), 9.18 (s, 1 H). 13C NMR (75 MHz, CDCl3): δ 14.35, 62.12, 138.30, 160.81, 165.22, 211.31. Calc. 207, found 208 [M+H]+.

5-(4-Fluorophenyl)-3H-1,2-dithiole-3-thione, 5b [47]

Red solid (74 %). Mp 98–100 °C. R f  = 0.84 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 7.12–7.26 (m, 2 H) 7.39 (s, 1 H) 7.59–7.72 (m, 2 H). 13C NMR (75 MHz, CDCl3): δ 116.97/117.26 (CF, d, J = 22 Hz), 129.19, 129.31, 136.13, 163.45/166.83 (CF, d, J = 254 Hz), 171.62, 215.66. Calc. 228, found 229 [M+H]+.

5-(Pyridin-4-yl)-3H-1,2-dithiole-3-thione, 5c [48]

Red solid (34 %). Mp decomposed. R f  = 0.09 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 7.50 (s, 1 H) 7.52–7.59 (m, 2 H) 8.81 (d, J = 5.93 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ 121.02, 121.9, 145.67, 150.01, 175.25, 214.27. Calc. 211, found 212 [M+H]+.

5-(Furan-2-yl)-3H-1,2-dithiole-3-thione, 5d [49]

Red solid (63 %). Mp 97–100 °C. R f  = 0.71 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 6.61 (dd, J = 3.53, 1.72 Hz, 1 H), 6.95–7.02 (m, 1 H), 7.38 (s, 1 H), 7.64 (dd, J = 1.81, 0.54 Hz, 1 H). 13C NMR (75 MHz, CDCl3): δ 113.53, 113.59, 133.27, 146.60, 146.71, 160.27, 214.50. Calc. 200, found 201 [M+H]+.

Ethyl 5-methyl-3-thioxo-3H-1,2-dithiole-4-carboxylate, 6b [50]

Red solid (78 %). Mp 64–66 °C. R f  = 0.84 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 1.37 (t, J = 7.16 Hz, 3 H), 2.57 (s, 3 H), 4.39 (q, J = 7.07 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ 14.35, 19.11, 62.50, 140.80, 163.28, 174.05, 211.82. Calc. 220, found 221 [M+H]+.

4-Chloro-5-(4-methoxyphenyl)-3H-1,2-dithiole-3-thione, 6g [51]

Prepared from 3g [52]. Yellow solid (91 %). Mp 125–127 °C. R f  = 0.63 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 3.90 (s, 3 H), 7.07 (d, J = 9.06 Hz, 2 H), 7.67 (d, J = 9.06 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ 55.57, 114.78, 124.12, 130.39, 123.43, 162.45, 165.62, 206.59. Calc. 274, found 275 [M+H]+.

4-Chloro-5-phenyl-3H-1,2-dithiole-3-thione, 6h [51]

Prepared from 3h  [53]. Yellow solid (87 %). Mp 105–107 °C. R f  = 0.74 (2 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 7.49–7.73 (m, 5 H). 13C NMR (75 MHz, CDCl3): δ 127.07, 128.88, 129.49, 129.79, 131.91, 165.63, 206.88. Calc. 244, found 245 [M+H]+.

4-Chloro-5-ethyl-3H-1,2-dithiole-3-thione, 6i [54]

Prepared from 3i [55]. Yellow solid (59 %). Mp 83–84 °C. R f  = 0.71 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 1.40 (t, J = 7.52 Hz, 3 H), 2.98 (q, J = 7.61 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ 12.80, 27.99, 158.84, 171.46, 206.64. Calc. 196, found 197 [M+H]+.

Ethyl 5-acetamido-3-thioxo-3H-1,2-dithiole-4-carboxylate, 6c [56]

Compound 6a (100 mg, 0.452 mmol) was refluxed in acetic anhydride (5 mL) for 30 min. The solution was then cooled, concentrated to dryness, and the crude material purified by column chromatography (hexanes/ethyl acetate, 3:1) to give 6c as a red solid (104 mg, 88 %). Mp 156–157 °C. R f  = 0.39 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 1.43 (t, J = 7.16 Hz, 3 H), 2.40 (s, 3 H), 4.42 (q, J = 7.13 Hz, 2 H), 12.72 (br s, 1 H). 13C NMR (75 MHz, CDCl3): δ 14.15, 23.97, 62.68, 118.75, 166.36, 170.63, 174.56, 208.25. Calc. 263, found 264 [M+H]+.

General procedure for the syntheses of dithiolethiones from nitriles, exemplified by 5-amino-4-(4-chlorophenyl)-3H-1,2-dithiole-3-thione, 6d

To an ice-cooled suspension of NaH (263 mg, 6.58 mmol), carbon disulfide (220 μL, 3.62 mmol), and elemental sulfur (116 mg, 3.62 mmol) in DMF (5 mL) was added 3d (500 mg, 3.29 mmol) in DMF (1 mL). The mixture was allowed to stir at 0 °C for 30 min, at which time saturated Na2CO3 (10 mL) was added. The mixture was then extracted with ethyl acetate (10 mL × 3), washed with water (10 mL × 3), dried (Na2SO4), filtered, concentrated, and purified by column chromatography (hexanes/ethyl acetate 4:1) to yield 6d as a red solid (838 mg, 95 %). Mp 106–107 °C. R f  = 0.29 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 6.35 (br s, 2 H), 7.29 (d, J = 8.70 Hz, 2 H), 7.48 (d, J = 8.70 Hz, 1 H). 13C NMR (75 MHz, CDCl3): δ 130.00, 132.22, 132.27, 134.85, 151.04, 175.69, 234.84. Calc. 259, found 260 [M+H]+.

5-Amino-4-(phenylsulfonyl)-3H-1,2-dithiole-3-thione, 6e [30]

Red solid (69 %). Mp decomposed. R f  = 0.13 (20 % EtOAc/Hex). 1H NMR (300 MHz, CDCl3): δ 7.50–7.78 (m, 3 H), 7.91–8.05 (m, 2 H), 9.01 (bs 1 H), 10.09 (bs, 1 H). 13C NMR (75 MHz, CDCl3): δ 117.75, 127.39, 128.77, 133.74, 140.45, 180.23, 203.60. Calc. 289, found 290 [M+H]+.

Biological methods

Cell culture conditions

The SH-SY5Y human neuroblastoma cell line was obtained from the American Type Culture Collection (ATCC, Manassas, VA). Cells were grown in DMEM:F-12 media (1:1) supplemented with FBS (10 %) and 100 U/mL penicillin and 100 μg/mL streptomycin in 150 cm2 culture flasks in a humidified atmosphere of 5 % CO2. The media was replaced every 3–4 days, and cells were subcultured once a confluence of 70–80 % was reached. All test compounds were dissolved in DMSO and diluted in media (final DMSO concentration of 0.1 % v/v).

Measurement of intracellular GSH levels

SH-SY5Y cells were seeded in white 96-well plates and allowed to adhere overnight. Media was removed and replaced with media containing either test compounds (100 μM) or DMSO (0.1 %) for 24 h. Total glutathione levels (GSH + GSSG) were then measured using GSH/GSSG Glo© assay from Promega (Madison, WI). GSH levels were expressed as a percentage of control.

Neuroprotection assay

SH-SY5Y cells were seeded in white 96 well plates and allowed to attach overnight. Media was removed and replaced with media containing either test compounds (100 μM) or DMSO for 24 h. Next, 6-OHDA (Aldrich) in media (final concentration of 40 μM) of media was added and the cells were co-treated for 24 h. Cellular viability was assessed using the CellTiter Glo© assay from Promega (Madison, WI). Viability was expressed as a percentage of control.

Statistical analyses

One-way analysis of variance (ANOVA) was used to test for significant differences using GraphPad Prism software (La Jolla, CA). P values less than 0.05 were considered to be statistically significant. Results are expressed as mean ± SEM.

Abbreviations

6-OHDA: 

6-hydroxydopamine

ARE: 

antioxidant response element

BSO: 

buthionine sulfoximine

DA: 

dopamine

DTTs: 

dithiolethiones

EDGs: 

electron donating groups

EWGs: 

electron withdrawing groups

GCL: 

glutamate cysteine ligase

GPx: 

glutathione peroxidase

GSH: 

glutathione

GSSG: 

oxidized glutathione

GST: 

glutathione S-transferase

Keap1: 

Kelch-like ECH-associated protein-1

NQO1: 

NAD(P)H:quinone oxidoreductase

Nrf2: 

nuclear factor-erythroid-2 related factor-2

PD: 

Parkinson’s disease

ROS: 

reactive oxygen species

Declarations

Authors’ contributions

DB and HM synthesized target molecules; DB, SB, and PK performed the pharmacological characterization of molecules; DB, SB and JY provided guidance for the project; DB and JY wrote the paper. All authors read and approved the final manuscript.

Acknowledgements

The authors are grateful for Manchester University College of Pharmacy for funding this work.

Competing interests

The authors declare that they have no competing interests.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Department of Pharmaceutical Sciences, Manchester University College of Pharmacy
(2)
Department of Microbiology and Immunology, Indiana University School of Medicine

References

  1. Jenner P, Dexter DT, Sian J, Schapira AHV, Marsden CD (1992) Oxidative stress as a cause of nigral cell death in Parkinson’s disease and incidental Lewy body disease. Ann Neurol 32:S82–S87View ArticleGoogle Scholar
  2. Fahn S, Cohen G (1992) The oxidant stress hypothesis in Parkinson’s disease: evidence supporting it. Ann Neurol 32:804–812View ArticleGoogle Scholar
  3. Abou-Sleiman PM, Muqit MMK, Wood NW (2006) Expanding insights of mitochondrial dysfunction in Parkinson’s disease. Nat Rev Neurosci 7:207–219View ArticleGoogle Scholar
  4. Spillantini MG, Schmidt ML, Lee VMY, Trojanowski JQ, Jakes R, Goedert M (1997) α-Synuclein in Lewy bodies. Nature 388:839–840View ArticleGoogle Scholar
  5. Hirsch EC, Vyas S, Hunot S (2012) Neuroinflammation in Parkinson’s disease. Parkinsonism Relat Disord 18(Supplement 1):S210–S212View ArticleGoogle Scholar
  6. Ayton S, Lei P, Adlard P, Volitakis I, Cherny R, Bush A, Finkelstein D (2014) Iron accumulation confers neurotoxicity to a vulnerable population of nigral neurons: implications for Parkinson’s disease. Mol Neurodegener 9:27View ArticleGoogle Scholar
  7. Fowler CJ, Wiberg Å, Oreland L, Marcusson J, Winblad B (1980) The effect of age on the activity and molecular properties of human brain monoamine oxidase. J Neural Transm 49:1–20View ArticleGoogle Scholar
  8. Dexter DT, Carter CJ, Wells FR, Javoy-Agid F, Agid Y, Lees A, Jenner P, Marsden CD (1989) Basal lipid peroxidation in substantia nigra is increased in Parkinson’s disease. J Neurochem 52:381–389View ArticleGoogle Scholar
  9. Good PF, Hsu A, Werner P, Perl DP, Olanow CW (1998) Protein nitration in Parkinson’s disease. J Neuropathol Exp Neurol 57:338–342View ArticleGoogle Scholar
  10. Alam ZI, Jenner A, Daniel SE, Lees AJ, Cairns N, Marsden CD, Jenner P, Halliwell B (1997) Oxidative DNA damage in the parkinsonian brain: an apparent selective increase in 8-hydroxyguanine levels in substantia nigra. J Neurochem 69:1196–1203View ArticleGoogle Scholar
  11. Chakravarthi S, Jessop CE, Bulleid NJ (2006) The role of glutathione in disulphide bond formation and endoplasmic-reticulum-generated oxidative stress. EMBO Rep 7:271–275View ArticleGoogle Scholar
  12. Dagnino-Subiabre A, Cassels BK, Baez S, Johansson A-S, Mannervik B, Segura-Aguilar J (2000) Glutathione transferase m2-2 catalyzes conjugation of dopamine and dopa o-quinones. Biochem Biophys Res Commun 274:32–36View ArticleGoogle Scholar
  13. Rotruck JT, Pope AL, Ganther HE, Swanson AB, Hafeman DG, Hoekstra WG (1973) Selenium: biochemical role as a component of glutathione peroxidase. Science 179:588–590View ArticleGoogle Scholar
  14. Perry TL, Godin DV, Hansen S (1982) Parkinson’s disease: a disorder due to nigral glutathione deficiency? Neurosci Lett 33:305–310View ArticleGoogle Scholar
  15. Wasserman WW, Fahl WE (1997) Functional antioxidant responsive elements. Proc Natl Acad Sci USA 94:5361–5366View ArticleGoogle Scholar
  16. Friling RS, Bensimon A, Tichauer Y, Daniel V (1990) Xenobiotic-inducible expression of murine glutathione s-transferase γ subunit gene is controlled by an electrophile-responsive element. Proc Natl Acad Sci USA 87:6258–6262View ArticleGoogle Scholar
  17. McMahon M, Itoh K, Yamamoto M, Hayes JD (2003) Keap1-dependent proteasomal degradation of transcription factor Nrf2 contributes to the negative regulation of antioxidant response element-driven gene expression. J Biol Chem 278:21592–21600View ArticleGoogle Scholar
  18. Zhang DD, Hannink M (2003) Distinct cysteine residues in keap1 are required for Keap1-dependent ubiquitination of nrf2 and for stabilization of Nrf2 by chemopreventive agents and oxidative stress. Mol Cell Biol 23:8137–8151View ArticleGoogle Scholar
  19. Nguyen T, Sherratt PJ, Nioi P, Yang CS, Pickett CB (2005) Nrf2 controls constitutive and inducible expression of are-driven genes through a dynamic pathway involving nucleocytoplasmic shuttling by Keap1. J Biol Chem 280:32485–32492View ArticleGoogle Scholar
  20. Nguyen T, Sherratt PJ, Huang H-C, Yang CS, Pickett CB (2003) Increased protein stability as a mechanism that enhances Nrf2-mediated transcriptional activation of the antioxidant response element: degradation of Nrf2 by the 26 s proteasome. J Biol Chem 278:4536–4541View ArticleGoogle Scholar
  21. Wakabayashi N, Dinkova-Kostova AT, Holtzclaw WD, Kang M-I, Kobayashi A, Yamamoto M, Kensler TW, Talalay P (2004) Protection against electrophile and oxidant stress by induction of the phase 2 response: fate of cysteines of the Keap1 sensor modified by inducers. Proc Natl Acad Sci USA 101:2040–2045View ArticleGoogle Scholar
  22. Itoh K, Wakabayashi N, Katoh Y, Ishii T, Igarashi K, Engel JD, Yamamoto M (1999) Keap1 represses nuclear activation of antioxidant responsive elements by Nrf2 through binding to the amino-terminal Neh2 domain. Genes Dev 13:76–86View ArticleGoogle Scholar
  23. Yu Z, Shao W, Chiang Y, Foltz W, Zhang Z, Ling W, Fantus IG, Jin T (2011) Oltipraz upregulates the nuclear respiratory factor 2 α-subunit (Nrf2) antioxidant system and prevents insulin resistance and obesity induced by a high-fat diet in c57bl/6j mice. Diabetologia 54:922–934View ArticleGoogle Scholar
  24. Dong J, Yan D, S-y Chen (2011) Stabilization of Nrf2 protein by D3T provides protection against ethanol-induced apoptosis in PC12 cells. PLoS ONE 6:e16845View ArticleGoogle Scholar
  25. Munday R, Zhang Y, Paonessa JD, Munday CM, Wilkins AL, Babu J (2010) Synthesis, biological evaluation, and structure–activity relationships of dithiolethiones as inducers of cytoprotective phase 2 enzymes. J Med Chem 53:4761–4767View ArticleGoogle Scholar
  26. Brown DA, Betharia S, Yen J-H, Tran Q, Mistry H, Smith K (2014) Synthesis and structure–activity relationships study of dithiolethiones as inducers of glutathione in the SH-SY5Y neuroblastoma cell line. Bioorg Med Chem Lett 24:5829–5831View ArticleGoogle Scholar
  27. Jalewa J, Sharma MK, Hölscher C (2016) Novel incretin analogues improve autophagy and protect from mitochondrial stress induced by rotenone in SY-SY5Y cells. J Neurochem. doi:10.1111/jnc.13736 Google Scholar
  28. Zhao Q, Yang X, Cai D, Ye L, Hou Y, Zhang L, Cheng J, Shen Y, Wang K, Bai Y (2016) Echinacoside protects against MPP+-induced neuronal apoptosis via ros/atf3/chop pathway regulation. Neurosci Bull 32:1–14View ArticleGoogle Scholar
  29. Curphey TJ (2002) Thionation with the reagent combination of phosphorus pentasulfide and hexamethyldisiloxane. J Org Chem 67:6461–6473View ArticleGoogle Scholar
  30. Guzaev AP (2011) Reactivity of 3H-1,2,4-dithiazole-3-thiones and 3H-1,2-dithiole-3-thiones as sulfurizing agents for oligonucleotide synthesis. Tetrahedron Lett 52:434–437View ArticleGoogle Scholar
  31. Hansch C, Leo A, Taft RW (1991) A survey of Hammett substituent constants and resonance and field parameters. Chem Rev 91:165–195View ArticleGoogle Scholar
  32. Velayutham M, Villamena FA, Navamal M, Fishbein JC, Zweier JL (2005) Glutathione-mediated formation of oxygen free radicals by the major metabolite of oltipraz. Chem Res Toxicol 18:970–975View ArticleGoogle Scholar
  33. Jia Z, Zhu H, Trush M, Misra H, Li Y (2008) Generation of superoxide from reaction of 3 H-1,2-dithiole-3-thione with thiols: implications for dithiolethione chemoprotection. Mol Cell Biochem 307:185–191View ArticleGoogle Scholar
  34. Holland R, Navamal M, Velayutham M, Zweier JL, Kensler TW, Fishbein JC (2009) Hydrogen peroxide is a second messenger in phase 2 enzyme induction by cancer chemopreventive dithiolethiones. Chem Res Toxicol 22:1427–1434View ArticleGoogle Scholar
  35. Arodin L, Miranda-Vizuete A, Swoboda P, Fernandes AP (2014) Protective effects of the thioredoxin and glutaredoxin systems in dopamine-induced cell death. Free Radic Biol Med 73:328–336View ArticleGoogle Scholar
  36. Aureli C, Cassano T, Masci A, Francioso A, Martire S, Cocciolo A, Chichiarelli S, Romano A, Gaetani S, Mancini P, Fontana M, d’Erme M, Mosca L (2014) 5-S-cysteinyldopamine neurotoxicity: influence on the expression of α-synuclein and erp57 in cellular and animal models of Parkinson’s disease. J Neurosci Res 92:347–358View ArticleGoogle Scholar
  37. Kuang X-L, Liu F, Chen H, Li Y, Liu Y, Xiao J, Shan G, Li M, Snider BJ, Qu J, Barger SW, Wu S (2014) Reductions of the components of the calreticulin/calnexin quality-control system by proteasome inhibitors and their relevance in a rodent model of Parkinson’s disease. J Neurosci Res 92:1319–1329View ArticleGoogle Scholar
  38. Kwon S-H, Ma S-X, Hong S-I, Kim SY, Lee S-Y, Jang C-G (2014) Eucommia ulmoides oliv. bark. attenuates 6-hydroxydopamine-induced neuronal cell death through inhibition of oxidative stress in SH-SY5Y cells. J Ethnopharmacol 152:173–182View ArticleGoogle Scholar
  39. Soto-Otero R, Méndez-Álvarez E, Hermida-Ameijeiras Á, Muñoz-Patiño AM, Labandeira-Garcia JL (2000) Autoxidation and neurotoxicity of 6-hydroxydopamine in the presence of some antioxidants. J Neurochem 74:1605–1612View ArticleGoogle Scholar
  40. Drew R, Miners JO (1984) The effects of buthionine sulphoximine (BSO) on glutathione depletion and xenobiotic biotransformation. Biochem Pharmacol 33:2989–2994View ArticleGoogle Scholar
  41. Nishio T (1998) Sulfur-containing heterocycles derived by reaction of ω-keto amides with Lawesson’s reagent. Helv Chim Acta 81:1207–1214View ArticleGoogle Scholar
  42. Klingsberg E (1963) The 1,2-dithiolium cation. A new pseudoaromatic system. Iii. 1 conversion of dithiolium salts to quaternary pyrazolium salts and dithiolethiones. J Org Chem 28:529–530View ArticleGoogle Scholar
  43. Dudley ME, Morshed MM, Brennan CL, Islam MS, Ahmad MS, Atuu M-R, Branstetter B, Hossain MM (2004) Acid-catalyzed reactions of aromatic aldehydes with ethyl diazoacetate: an investigation on the synthesis of 3-hydroxy-2-arylacrylic acid ethyl esters. J Org Chem 69:7599–7608View ArticleGoogle Scholar
  44. Saquet M (1966) Organic sulfur compounds. Xi. Condensation of carbon disulfide with aldehydes. Synthesis of 4-aryl-1,2-dithiole-3thiones. Bull Soc Chim Fr 1582–1587Google Scholar
  45. Shimada N, Stewart C, Bow WF, Jolit A, Wong K, Zhou Z, Tius MA (2012) Neutral Nazarov-type cyclization catalyzed by palladium(0). Angew Chem Int Ed Engl 51:5727–5729View ArticleGoogle Scholar
  46. Curphey TJ (1993) Synthesis of 3H-1,2-dithiole-3-thiones by a novel oxidative cyclization. Tetrahedron Lett 34:7231–7239View ArticleGoogle Scholar
  47. Nuhrich A (1992) Synthesis and in vitro anti-hiv evaluation of disulfide linked derivatives of 1,2-dithiol-3-ylidene ketones containing a 2,3-dichloro-4-phenoxyacetic acid moiety. Eur J Med Chem 27:857–860View ArticleGoogle Scholar
  48. Legrand L (1955) Sulfuration of organic compounds. VII. 1,2-Dithiole-3-thiones with aliphatic or pyridine substituents. Bull Soc Chim Fr 79–83Google Scholar
  49. Zayed SE (2014) Oxoketene dithiols: synthesis of some heterocycles as antimicrobials utilizing shrimp chitin as a natural catalyst. Phosphorus, Sulfur Silicon Relat Elem 189:1682–1698View ArticleGoogle Scholar
  50. Thuillier A (1962) Organic sulfur compounds. V. Condensation of carbon disulfide with acetone. Bull Soc Chim Fr 2182–2186Google Scholar
  51. Quiniou H (1963) Heterocyclic sulfur compounds. X. The action of chlorine and bromine on several 5-aryl-1,2-dithiole-3-thiones. Bull Soc Chim Fr 1167–1171Google Scholar
  52. Jiang Y, Chen X, Zheng Y, Xue Z, Shu C, Yuan W, Zhang X (2011) Highly diastereoselective and enantioselective synthesis of α-hydroxy β-amino acid derivatives: Lewis base catalyzed hydrosilylation of α-acetoxy β-enamino esters. Angew Chem Int Ed Engl 50:7304–7307View ArticleGoogle Scholar
  53. Feske BD, Kaluzna IA, Stewart JD (2005) Enantiodivergent, biocatalytic routes to both taxol side chain antipodes. J Org Chem 70:9654–9657View ArticleGoogle Scholar
  54. Trebaul C (1969) Heterocyclic sulfur compounds. Iv. Reaction of phosphorus pentasulfide with β-oxo esters containing an α-chloro, α-cyano, or an α-ethoxycarbonyl group. Bull Soc Chim Fr 2456–2462Google Scholar
  55. Perrone MG, Santandrea E, Dell’Uomo N, Giannessi F, Milazzo FM, Sciarroni AF, Scilimati A, Tortorella V (2005) Synthesis and biological evaluation of new clofibrate analogues as potential PPARα agonists. Eur J Med Chem 40:143–154View ArticleGoogle Scholar
  56. Cmelik R (2003) Syntheses and structure study on 3,3a4,4-trithia-1-azapentalenes and their 3-oxa analogues. Collect Czech Chem Commun 68:1243–1263View ArticleGoogle Scholar

Copyright

© The Author(s) 2016