Transition Metal-Free Cascade Cyclization of Epoxy-Ynamides: To Go for 1,3-Oxazines or 1,4-Oxazines?

An approach to both 1,3- and 1,4-oxazines is developed by the cascade cyclization of epoxy ynamides under transition metal-free conditions. Thus, while 1,3-oxazines are obtained in a regio- and stereoselective manner by using base, cyclization of epoxy ynamides with sodium azide as a nucleophile results in 1,4-oxazines. Deuterium-labeling experiment demonstrates the role of water as a proton source in the formation of 1,4-oxazines. Epoxy tethered alkyne precursors provide 1,4-oxazines that undergo click reaction.

Exploring the gold mine: [Au]-catalysed transformations of enynals, enynones and enynols.

Gold catalysed synthesis of carbo-/hetero-cyclic compounds is currently a topic of intense activity thanks to the unique properties bestowed upon gold by relativistic effects, the potential of which has been realised mainly during the past two decades. This review will dwell upon the reactions involving gold catalysis in which enynal, enynone or enynol is one of the reacting partners. Most often these reactions lead to fused hetero-/homo-cycles under mild conditions with environmentally benign byproducts. Thus an opportunity for the organic community to develop new methodologies, efficient asymmetric variants, and application in total synthesis by [Au]-catalysis has emerged.

Gold-Catalyzed Concomitant [3 + 3] Cycloaddition/Cascade Heterocyclization of Enynones/Enynals with Azides Leading to Furanotriazines.

[Au]-catalyzed [3 + 3] cycloaddition reaction of enynones/enynals with azides, which allows the efficient regioselective synthesis of highly fused furo[3,4-d][1,2,3]triazines in good-to-excellent yields under mild conditions, has been developed. The synthetic utility of furanotriazines was exploited by oxidation with cerium ammonium nitrate (CAN) affording highly functionalized dihydrotriazines. Both furo[3,4-d][1,2,3]triazines and dihydrotriazines exhibit good fluorescence activity.

Divergence in the reactivity between amine- and phosphine-catalyzed cycloaddition reactions of allenoates with enynals: one-pot gold-catalyzed synthesis of trisubstituted benzofurans from the [3 + 2] cycloadduct via 1,2-alkyl migration and dehydrogenation.

Regioselective synthesis of functionalized dihydropyran derivatives by DABCO-catalyzed [2 + 4] cycloaddition of allenoates with enynals or enynones has been developed. Phosphine-catalyzed [3 + 2] cycloaddition of allenoates with enynals provides 1,1-alkyne (aldehyde)-substituted cyclopentenes wherein enynals act as electrophiles. These alkyne-tethered cyclopentenes upon [Au]/[Ag] catalysis lead to substituted benzofurans via 1,2-alkyl migration and dehydrogenation (aromatization). One-pot reaction of allenoates with enynals using sequential phosphine and gold catalysis is also reported. The cyclopentene obtained from the PPh3-catalyzed reaction of allenoate H2C═C═CH(COO-t-Bu) with enynal undergoes decarboxylation under the [Au]/[Ag] catalysis and forms a carboxylate-free benzofuran. The structures of key products are confirmed by single-crystal X-ray analysis.

Heat stress induced apoptosis in BC-8 cells derived from AK-5 tumor involves downregulation of Bcl-2 and generation of reactive oxygen species.

BC-8, a rat histiocytoma undergoes apoptosis after heat shock, which is due to lack of an effective heat shock response. Heat shock induced generation of free radicals, which in turn are involved in the induction of apoptotic death in BC-8 cells. Treatment of BC-8 cells with N-acetylcysteine partially inhibited the heat induced apoptosis. Introduction of Bcl-2 gene in these cells did not protect them from apoptotic death, whereas transfection with hsp-70 gene did render these cells resistant to heat induced apoptosis transiently. Heat shock also downregulated the expression of Bcl-2 and p53 in these cells. These observations suggested that the heat shock induced apoptosis was mediated through reactive oxygen species and controlled upstream of Bcl-2 check point.

Role of STAT3 and NFkappaB signaling in the serum factor-induced apoptosis in AK-5 cells.

AK-5, a rat histiocytoma, is rejected in about 70% of the syngeneic animals when injected subcutaneously. The sera from the tumor rejecting animals possess a potent factor, referred to as serum factor (SF) that induces apoptosis in AK-5 tumor cells. In the present study, we show that treatment with SF or JAK/STAT inhibitors AG490 and Piceatannol induces apoptosis to a similar extent in BC-8 (a single cell clone of AK-5) cells. Our results demonstrate downregulation of a transcription factor, STAT3, as a critical regulator of SF-induced apoptosis in BC-8 cells. SF treatment enhanced the activity of NFkappaB, another transcription factor that regulates both pro- and antiapoptotic genes. The enhanced NFkappaB activity resulted in the elevation of TRAIL and its receptor DR4, both known to induce apoptosis. Activation of death receptors in turn enhances caspase-8 activity and stimulates the downstream pathways regulating BC-8 cell apoptosis. SF induced apoptosis in BC-8 cells mediated through downregulation of STAT3 and elevated NFkappaB activity is abrogated by treatment with MAPK inhibitors-PD98059 and SB203580. Our studies therefore indicate that modulation of MAPK activity plays a central role in SF-induced death signaling pathways in BC-8 cells.

Differential cytotoxic effects of Annona squamosa seed extracts on human tumour cell lines: role of reactive oxygen species and glutathione.

Annonaceous acetogenins are a new class of compounds that have been reported to have potent pesticidal, parasiticidal, anti-microbial, cell growth inhibitory activities. In this study, organic and aqueous extracts from the defatted seeds of Annona squamosa (custard apple) were tested on different human tumour cell lines for antitumoural activity. While organic and aqueous extracts induced apoptosis in MCF-7 and K-562 cells, they failed to do so in COLO-205 cells. Treatment of MCF-7 and K-562 cells with organic and aqueous extracts resulted in nuclear condensation, DNA fragmentation, induction of reactive oxygen species (ROS) generation and reduced intracellular glutathione levels. In addition downregulation of Bcl-2 and PS externalization by Annexin-V staining suggested induction of apoptosis in MCF-7 and K-562 cells by both the extracts through oxidative stress. On the contrary, COLO-205 cells showed only PS externalization but no change in ROS and glutathione levels. These observations suggest that the induction of apoptosis by A. squamosa extracts can be selective for certain types of cancerous cells.

Differential apoptotic and redox regulatory activities of curcumin and its derivatives.

We have synthesized different bioconjugates of curcumin, which were tested for their pro- and antioxidant properties. In the present study five representative derivatives of curcumin, i.e., 4,4'-di-(O-acetyl) curcumin, 4,4'-di-(O-glycinoyl) curcumin, 4,4'-di-(O-glycinoyl-di-N-piperoyl) curcumin, 4,4'-di-(O-piperoyl) curcumin, and 4,4'-(O,O-cystinoyl)-3,3'-dimethoxydiphenyl-1,6-heptadiene-3,5-dione, were used for testing their apoptotic potential on tumor cells. Dipiperoyl and diglycinoyl derivatives showed higher apoptotic activity at lower concentrations, whereas diacetyl curcumin had slightly lower apoptotic activity on tumor cells. On the other hand, diglycinoyl-dipiperoyl and cystinoyl heptadiene derivatives had lost their apoptotic potential significantly. The apoptotic activity of these derivatives correlated very well with the generation of ROS by the tumor cells, whereas GSH levels remained unaltered. Our studies also indicate downregulation of Bcl-2 and participation of caspase-3 in the apoptotic death of tumor cells.

Effect of curcumin on normal and tumor cells: role of glutathione and bcl-2.

Curcumin, a well-known dietary pigment derived from Curcuma longa, inhibited growth of several types of malignant cells both in vivo and in vitro. However, its mechanism of action still remains unclear. In this study, we have focused primarily on the cytotoxic effects of curcumin on three human tumor cell lines and rat primary hepatocytes. Curcumin induced apoptosis in MCF-7, MDAMB, and HepG2 cells in a dose-dependent and time-dependent manner. Apoptosis was mediated through the generation of reactive oxygen species. Attempts were made to establish the role played by endogenous glutathione on the apoptotic activity of curcumin. Depletion of glutathione by buthionine sulfoximine resulted in the increased generation of reactive oxygen species, thereby further sensitizing the cells to curcumin. Interestingly, curcumin had no effect on normal rat hepatocytes, which showed no superoxide generation and therefore no cell death. These observations suggest that curcumin, a molecule with varied actions, could be developed into an effective chemopreventive and chemotherapeutic agent.

Antitumour activity of Annona squamosa seed extracts is through the generation of free radicals and induction of apoptosis.

The plant, Annona squamosa traditionally known as custard apple possesses potent bioactive principles in all its parts. The effect of aqueous and organic extracts from defatted seeds of A. squamosa was studied on a rat histiocytic tumour cell line, AK-5. Both the extracts caused significant apoptotic tumour cell death with enhanced caspase-3 activity, down regulation of antiapoptotic genes Bcl-2 and Bcl(XL), and enhanced the generation of intracellular ROS, which correlated well with the decreased levels of intracellular GSH. In addition, DNA fragmentation and annexin-V staining confirmed that the extracts induced apoptosis in tumour cells through the oxidative stress. Aqueous extracts of A. squamosa seeds possessed significant antitumor activity in vivo against AK-5 tumor.

Phycocyanin-mediated apoptosis in AK-5 tumor cells involves down-regulation of Bcl-2 and generation of ROS.

C-phycocyanin, which is a major biliprotein of the blue-green algae, has been shown to possess cyclooxygenase-2 inhibitory activity. We have studied the effect of phycocyanin on a rat histiocytic tumor line. AK-5 cells are induced into apoptotic death program when treated with phycocyanin, which involves the activation of caspase-3. Phycocyanin-mediated apoptotic death is induced through the generation of reactive oxygen radicals. Free radical scavengers inhibited phycocyanin-induced apoptotic death in AK-5 cells. Bcl-2, an inhibitor of apoptosis, is shown to regulate ROS generation. Bcl-2 gene-transfected AK-5 cells are resistant to phycocyanin-induced death. Overexpression of Bcl-2 inhibited the production of ROS in phycocyanin-treated AK-5 cells. Thus, our observations demonstrate phycocyanin-induced apoptotic death in AK-5 cells, which is inhibited by Bcl-2 expression through the regulation of free radical generation. Phycocyanin, a natural product, could therefore be a possible chemotherapeutic agent through its apoptotic activity against tumor cells.

Protection conferred by Bcl-2 expression involves reduced oxidative stress and increased glutathione production during hypothermia-induced apoptosis in AK-5 tumor cells.

Hypothermia is known to retard mammalian cell growth, however, BC-8 cells, which have originated from AK-5 tumor after single cell cloning, were triggered into apoptotic pathway when grown at 30 degrees C. Cell death process showed typical apoptotic features like DNA fragmentation, cytochrome c release, etc. Introduction of Bcl-2 gene in BC-8 cells inhibited hypothermia-induced apoptotic process, which is ascribed to reduced ROS generation and higher glutathione production. Thus, Bcl-2 seems to control the apoptotic induction process at the level of redox regulation, in addition to its known effects at the mitochondrial dysregulation. These observations suggest that tumors, which are low in Bcl-2 expression, are sensitive to hypothermic shock and make hypothermia an interesting inducer of apoptosis in tumor cells.