A fast entry to furanoditerpenoid-based Hedgehog signaling inhibitors: identifying essential structural features

Synthesis of Neocaesalpin A, AA, and Nominal Neocaesalpin K

The first total synthesis of heavily oxidized cassane-type diterpenoid neocaesalpin A (1) is disclosed. At the heart of the synthesis lies an intermolecular Diels-Alder reaction that rapidly assembles the target framework from commercial materials. A carefully orchestrated sequence of oxidations secured the desired oxygenation pattern. Late-stage release of the characteristic butenolide occurred through a novel mercury(II)-mediated furan oxidation. Successful extension of the route allowed preparation of neocaesalpin AA (2) as well as nominal neocaesalpin K (3) and suggested structural revision of neocaesalpin K, leading to the hypothesis that the two are likely the same natural product with correct assignment as 2.

Enantioselective Total Syntheses of Cassane Furanoditerpenoids and Their Stimulation of Cellular Respiration in Brown Adipocytes

We report the first and enantioselective total syntheses of (+)-1-deacetylcaesalmin C, (+)-δ-caesalpin, (+)-norcaesalpinin MC, and (+)-norcaesalpinin P. Salient features of the synthetic strategy are an exo-selective intramolecular Diels-Alder reaction of a furanoquinone monoketal and subsequent chemoselective reduction of the resulting pentacyclic furfuryl ketal, furnishing a keystone intermediate. The latter enables access to the collection of natural products through implementation of stereoselective oxidations. Having accessed the cassane furanoditerpenoids, we unveil previously unknown bioactivity: (+)-1-deacetylcaesalmin C stimulates respiration in brown adipocytes, which has been suggested to play a central role in treatment of obesity.

Synthesis of Tricyclic Pterolobirin H Analogue: Evaluation of Anticancer and Anti-Inflammatory Activities and Molecular Docking Investigations

Pterolobirin H (3), a cassane diterpene isolated from the roots of Pterolobium macropterum, exhibits important anti-inflammatory and anticancer properties. However, its relatively complex tetracyclic structure makes it difficult to obtain by chemical synthesis, thus limiting the studies of its biological activities. Therefore, we present here a short route to obtain a rational simplification of pterolobirin H (3) and some intermediates. The anti-inflammatory activity of these compounds was assayed in LPS-stimulated RAW 264.7 macrophages. All compounds showed potent inhibition of NO production, with percentages between 54 to 100% at sub-cytotoxic concentrations. The highest anti-inflammatory effect was shown for compounds 15 and 16. The simplified analog 16 revealed potential NO inhibition properties, being 2.34 higher than that of natural cassane pterolobirin H (3). On the other hand, hydroxyphenol 15 was also demonstrated to be the strongest NO inhibitor in RAW 264.7 macrophages (IC50 NO = 0.62 ± 0.21 μg/mL), with an IC50NO value 28.3 times lower than that of pterolobirin H (3). Moreover, the anticancer potential of these compounds was evaluated in three cancer cell lines: HT29 colon cancer cells, Hep-G2 hepatoma cells, and B16-F10 murine melanoma cells. Intermediate 15 was the most active against all the selected tumor cell lines. Compound 15 revealed the highest cytotoxic effect with the lowest IC50 value (IC50 = 2.45 ± 0.29 μg/mL in HT29 cells) and displayed an important apoptotic effect through an extrinsic pathway, as evidenced in the flow cytometry analysis. Furthermore, the Hoechst staining assay showed that analog 15 triggered morphological changes, including nuclear fragmentation and chromatin condensation, in treated HT29 cells. Finally, the in silico studies demonstrated that cassane analogs exhibit promising binding affinities and docking performance with iNOS and caspase 8, which confirms the obtained experimental results.

Semisynthesis and Evaluation of Anti-Inflammatory Activity of the Cassane-Type Diterpenoid Taepeenin F and of Some Synthetic Intermediates

A new strategy for the semisynthesis of the aromatic cassane-type diterpene taepeenin F (6) is reported. The introduction of the methyl group at C-14, characteristic of the target compound, was achieved via dienone 13, easily prepared from abietic acid (10), the major compound in renewable rosin. Biological assays of selected compounds are reported. The antiproliferative activity against HT29, B16-F10, and HepG2 tumor cell lines has been investigated. Salicylaldehyde 21 was the most active compound (IC₅₀ = 7.72 μM). Products 16 and 21 displayed apoptotic effects in B16-F10 cells, with total apoptosis rates of 46 and 38.4%, respectively. This apoptotic process involves a significant arrest of the B16-F10 cell cycle, blocking the G0/G1 phase. Dienone 16 did not cause any loss of the mitochondrial membrane potential (MMP), while salicylaldehyde 21 caused a partial loss of the MMP. The anti-inflammatory activity of the selected compounds was investigated with the LPS-stimulated RAW 264.7 macrophages. All compounds showed potent NO inhibition, with percentages between 80 and 99% at subcytotoxic concentrations. Dienone 16 inhibited LPS-induced differentiation of RAW 264.7 cells, by increasing the proportion of cells in the S phase. In addition, salicylaldehyde 21 had effects on the cell cycle, recovering the cells from the G0/G1 full arrest produced in response to LPS action.

Synthesis and Biological Evaluation of Cassane Diterpene (5α)-Vuacapane-8(14), 9(11)-Diene and of Some Related Compounds

A set of thirteen cassane-type diterpenes was synthesized and an expedient synthetic route was used to evaluate 14-desmethyl analogs of the most active tested cassane. The anti-inflammatory activities of these 13 compounds were evaluated on a lipopolysaccharide (LPS)-activated RAW 264.7 cell line by inhibition of nitric oxide (NO) production, some of them reaching 100% NO inhibition after 72 h of treatment. The greatest anti-inflammatory effect was observed for compounds 16 and 20 with an IC_{50 NO} of 2.98 ± 0.04 μg/mL and 5.71 ± 0.14 μg/mL, respectively. Flow-cytometry analysis was used to determine the cell cycle distribution and showed that the inhibition in NO release was accompanied by a reversion of the differentiation processes. Moreover, the anti-cancer potential of these 13 compounds were evaluated in three tumor cell lines (B16-F10, HT29, and Hep G2). The strongest cytotoxic effect was achieved by salicylaldehyde 20, and pterolobirin G (6), with IC₅₀ values around 3 μg/mL in HT29 cells, with total apoptosis rates 80% at IC₈₀ concentrations, producing a significant cell-cycle arrest in the G0/G1 phase, and a possible activation of the extrinsic apoptotic pathway. Additionally, initial SAR data analysis showed that the methyl group at the C-14 positions of cassane diterpenoids is not always important for their cytotoxic and anti-inflammatory activities.

Acid-catalyzed transformation of cassane diterpenoids from Caesalpinia bonduc to aromatic derivatives

Transformations of cassane diterpenoids from Caesalpinia bonduc into aromatic derivatives, either in CDCl₃ or in CHCl₃ irradiated with UV light or catalyzed by AlCl₃, were described. Caesalmin C (2) was hydrolyzed with Na₂CO₃ upon refluxing in MeOH to yield compound 1. Dissolving compound 1 with CDCl₃ resulted in an unexpected aromatization process of a C ring to obtain 1a, and aromatic derivatives 6-acetoxy-3-deacetoxycaesaldekarin e (2a), caesall A (3a), caesaldekarin e (5a), caesalpinin MC (5b), 2-acetoxycaesaldekarin e (6a) and new compound 6b could be obtained from corresponding cassane diterpenoids (2–8) under the same conditions. Furthermore, the photochemical reactions of cassane diterpenoids 1–8 occurring in CHCl₃ also yielded aromatic derivatives 1a, 2a, 3a, 5a, 6a, new compounds 2b and 3b, and 17-norcassane diterpenoids norcaesalpinin MC (2c) and caesalmin J (3c). In addition, cassane diterpenoids 1–8, treated with AlCl₃ in CHCl₃ or CH₂Cl₂, gave the same results in CDCl₃ and with even shorter reaction time. The role of AlCl₃ in the aromatization of 1 has been explained by DFT calculations.

Transformations of cassane diterpenoids from Caesalpinia bonduc into aromatic derivatives, either in CDCl₃ or in CHCl₃ irradiated with UV light or catalyzed by AlCl₃, were described.

Synthesis and structure-activity relationships of novel abietane diterpenoids with activity against Staphylococcus aureus

Aim: To find alternative compounds against methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA), novel derivatives from dehydroabietic acid were synthesized. Methods & results: Compound 12 was the most effective against 15 MRSA and 11 MSSA with minimum inhibitory concentration values ranging from 3.9 to 15.6 μg/ml. Although less active than 12, compound 11, followed by 25 and 13, also exhibited anti-staphylococcal activity. Additional studies showed that compound 12 is devoid of toxic effect on non-target cells. A structure-activity relationship study revealed that an oxime at C-13 together with a hydroxyl at C-12 could play a key role in the activity. Conclusion: These structures, in particular compound 12, could arise as templates for the development of agents against MRSA and MSSA.

Design of Hedgehog pathway inhibitors for cancer treatment

Hedgehog (Hh) signaling is involved in the initiation and progression of various cancers and is essential for embryonic and postnatal development. This pathway remains in the quiescent state in adult tissues but gets activated upon inflammation and injuries. Inhibition of Hh signaling pathway using natural and synthetic compounds has provided an attractive approach for treating cancer and inflammatory diseases. While the majority of Hh pathway inhibitors target the transmembrane protein Smoothened (SMO), some small molecules that target the signaling cascade downstream of SMO are of particular interest. Substantial efforts are being made to develop new molecules targeting various components of the Hh signaling pathway. Here, we have discussed the discovery of small molecules as Hh inhibitors from the diverse chemical background. Also, some of the recently identified natural products have been included as a separate section. Extensive structure-activity relationship (SAR) of each chemical class is the focus of this review. Also, clinically advanced molecules are discussed from the last 5 to 7 years. Nanomedicine-based delivery approaches for Hh pathway inhibitors are also discussed concisely.

Synthesis of cassane-type diterpenes from abietane compounds: the first synthesis of taepeenin F

The first synthesis of taepeenin F from abietic acid is reported, utilizing as the key step the ipso-substitution of the isopropyl group of a dehydroabietane derivative by a formyl group, after treatment with Cl₂CHOMe and AlCl₃.

Diterpenoids of the Cassane Type from Caesalpinia decapetala

Eighteen compounds, including eight new cassane-type furanoditerpenoids, 3β-hydroxyphanginin H (1), 3β-acetoxyphanginin H (2), 7β-acetoxyphanginin H (3), 7β-hydroxyphanginin H (4), 4-epi-3β-hydroxycaesalpinilinn (5), 4-epi-3β-acetoxycaesalpinilinn (6), 20-acetoxytaepeenin D (7), and tomocin E (8), along with 10 known compounds (9-18) were isolated from the roots of Caesalpinia decapetala. Compounds 1-13 were isolated from C. decapetala for the first time. The new compounds with their absolute configurations were determined by extensive spectroscopic analysis, single-crystal X-ray diffraction, and electronic circular dichroism calculations. Compounds 1, 4, 5, 7, and 11 exhibited inhibitory activities against the SW1990 human pancreatic cancer cell line with IC₅₀ values ranging from 2.9 to 8.9 μM.

Identification of Gli-mediated transcription inhibitors through synthesis and evaluation of taepeenin D analogues

Abietic acid derivatives related to taepeenin D were identified as new Hh pathway inhibitors that operate downstream of Smo.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial as opposed to marine sources and includes, labdanes, clerodanes, pimaranes, abietanes, kauranes, gibberellins, cembranes and their cyclization products. The literature from January to December, 2014 is reviewed.