Chemistry, biology, and medicinal potential of narciclasine and its congeners

Enamides and enesulfonamides as nucleophiles: formation of complex ring systems through a platinum(II)-catalyzed addition/Friedel-Crafts pathway

Cyclic enamine derivatives (enesulfonamides and enamides) tethered to an 1-arylalkynyl fragment undergo a platinum(II)-catalyzed tandem alkyne addition/Friedel-Crafts ring closure to form nitrogen-containing polycyclic structures. Regioselectivity in the initial addition of the enesulfonamide or enamide nucleophile to the platinum(II)-alkyne complex is important. Electron-rich arenes and heterocycles led to the formation of products resulting from an initial 6-endo cyclization. Twenty-three examples of this process are presented.

Synthesis of structurally simplified analogues of pancratistatin: truncation of the cyclitol ring

Pancratistatin is a phenanthridone-type natural product isolated from several plants of the Amaryllidaceae family. Its potent antiproliferative, antivascular, antiviral, and antiparasitic properties have attracted the attention of synthetic, biological, and medicinal chemists. Pancratistatin's low natural availability and complex structure have steered many of these research projects toward the preparation of its simplified synthetic analogues with useful levels of activity. In this work we have developed synthetic chemistry aimed at the preparation of pancratistatin analogues with a truncated cyclitol portion of the molecule. The described synthetic pathways are based on a highly anti-diastereoselective arylcuprate conjugate addition to gamma-alkoxy-alpha,beta-enoates and syn-selective azidation at the alpha-position of ester enolates. Analogues with the formally cleaved C3-C4 bond, and thus containing an open ring C, as well as a compound containing a truncated lactol moiety in lieu of the cyclitol, were prepared. Several of the analogues exhibited weak antiproliferative activity, with the highest potency observed in the case of the lactol analogue. From these results implications for the design of future pancratistatin analogues are discussed. Furthermore, the synthetic pathways can be used to construct pancratistatin-mimetic libraries, in which the cyclitol moiety is replaced by other cyclic motifs.

Lycorine, the main phenanthridine Amaryllidaceae alkaloid, exhibits significant antitumor activity in cancer cells that display resistance to proapoptotic stimuli: an investigation of structure-activity relationship and mechanistic insight

Twenty-two lycorine-related compounds were investigated for in vitro antitumor activity using four cancer cell lines displaying different levels of resistance to proapoptotic stimuli and two cancer cell lines sensitive to proapoptotic stimuli. Lycorine and six of its congeners exhibited potency in the single-digit micromolar range, while no compound appeared more active than lycorine. Lycorine also displayed the highest potential (in vitro) therapeutic ratio, being at least 15 times more active against cancer than normal cells. Our studies also showed that lycorine exerts its in vitro antitumor activity through cytostatic rather than cytotoxic effects. Furthermore, lycorine provided significant therapeutic benefit in mice bearing brain grafts of the B16F10 melanoma model at nontoxic doses. Thus, the results of the current study make lycorine an excellent lead for the generation of compounds able to combat cancers, which are naturally resistant to proapoptotic stimuli, such as glioblastoma, melanoma, non-small-cell-lung cancers, and metastatic cancers, among others.

Structure-activity studies on seco-pancratistatin analogs: potent inhibitors of human cytochrome P450 3A4

Two total syntheses of fully functionalized seco-analogs of the anticancer compound pancratistatin are reported. Structure-activity relationship (SAR) studies identified potent and selective inhibitors of human cytochrome P450 3A4 (CYP3A4) and revealed several core pharmacophoric elements. These studies identify potential roadblocks and will guide the further development of a viable selective clinical pancratistatin derivative.

Biological evaluation of ent-narciclasine, ent-lycoricidine, and certain enantiomerically-related congeners

The non-natural enantiomeric forms of narciclasine and lycoricidine ((-)-1 and (-)-2, respectively), as well as congeners 3-6 are available through chemoenzymatic synthesis. Accordingly, they have now been tested for their cytotoxic effects in a 13-member human cancer cell-line panel and found to be only weakly active. In contrast, an authentic sample of the natural enantiomeric form of narciclasine ((+)-1) was found to be highly active in the same screens.

Desymmetrization of hydrazinocyclohexadienes: a new approach for the synthesis of polyhydroxylated aminocyclohexanes

Hydrazinocyclohexadienes, easily prepared by an ene-reaction between commercially available azodicarboxylate reagents and cyclohexadiene, are interesting substrates for desymmetrization reactions. Under Sharpless asymmetric dihydroxylation conditions, they can lead efficiently to several chiral building blocks as well as advanced precursors of biologically active compounds.

Antineoplastic agents. 454. Synthesis of the strong cancer cell growth inhibitors trans-dihydronarciclasine and 7-deoxy-trans-dihydronarciclasine (1a)

To further pursue the antineoplastic leads offered by our isolation of trans-dihydronarciclasine (1a) and 7-deoxy-trans-dihydronarciclasine (1c) from two medicinal plant species of the Amaryllidaceae family, a practical palladium-catalyzed hydrogenation procedure was developed for the synthesis of these isocarbostyrils from narciclasine (2a) and 7-deoxynarciclasine (2c).

Stereocontrolled approach to phenyl cyclitols from (SR)-[(p-tolylsulfinyl)methyl]-p-quinol

Reactions of enantiopure (SR)-[(p-tolylsulfinyl)methyl]-p-quinol with ArAlMe(2) reagents allowed a highly diastereoselective 1,4-addition of the aryl group with an efficient desymmetrization of the prochiral cyclohexadienone moiety. The asymmetric synthesis of phenyl-substituted polyoxygenated cyclohexane derivatives was achieved by combining this reaction with a stereoselective reduction and elimination of the beta-hydroxysulfoxide, after oxidation to sulfone, to recover a carbonyl group, and a stereoselective epoxidation.

Structure-activity studies on the lycorine pharmacophore: A potent inducer of apoptosis in human leukemia cells

The direct chemoselective differential functionalization of the ring-C hydroxyl groups present in the Amaryllidaceae alkaloid lycorine is described allowing for selective manipulation of the 1,2-hydroxyl groups. A mini-library comprised of synthetic and natural lycorane alkaloids was prepared and their apoptosis-inducing activity investigated in human leukemia (Jurkat) cells. Further insights into the nature of this interesting apoptosis-inducing pharmacophore are described, including the requirement of both free hydroxyl groups in ring-C.

Selective cytochrome P450 3A4 inhibitory activity of Amaryllidaceae alkaloids

A library of natural and semi-synthetic Amaryllidaceae alkaloids was screened for cytochrome P450 3A4 (CYP3A4) inhibitory activity. Of the crinane, lycorane and galanthamine representatives examined two semi-synthetic silylated lycorane analogues, accessed via a chemoselective silylation strategy from lycorine, and the natural compound narciclasine exhibited low micromolar activities. Important pharmacological features uncovered include the lack of CYP3A4 inhibitory activity seen for galanthamine and the selective activity that is seen with narciclasine over pancratistatin.

Biological evaluation of structurally diverse amaryllidaceae alkaloids and their synthetic derivatives: discovery of novel leads for anticancer drug design

Twenty-nine Amaryllidaceae alkaloids and their derivatives belonging to the five most common groups, including lycorine, lycorenine, tazettine, crinine, and narciclasine types, were evaluated for antiproliferative, apoptosis-inducing, and anti-invasive activities in vitro. The antiproliferative properties of each test compound are in agreement with those reported in the literature, while the high potency of amarbellisine is reported for the first time. It was also found that with the exception of ungeremine, amarbellisine, and hippeastrine, the antiproliferative effect of the potent compounds is apoptosis mediated. Thus, apoptosis in Jurkat cells was triggered by narciclasine, narciclasine tetraacetate, C10b-R-hydroxypancratistatin, cis-dihydronarciclasine, trans-dihydronarciclasine, lycorine, 1-O-acetyllycorine, lycorine-2-one, pseudolycorine, and haemanthamine. With the exception of narciclasine, lycorine, and haemanthamine, the apoptosis-inducing properties of these compounds are reported for the first time. The collagen type I invasion assay revealed potent anti-invasive properties associated with N-methyllycorine iodide, hippeastrine, clivimine, buphanamine, and narciclasine tetraacetate, all of which were tested at non-toxic concentrations. The anti-invasive activity of buphanamine is particularly promising because this alkaloid is not toxic to cells even at much higher doses. This work has resulted in the identification of several novel leads for anticancer drug design.

Applications of multicomponent reactions to the synthesis of diverse heterocyclic scaffolds

The sequencing of multicomponent reactions (MCRs) and subsequent cyclization reactions is a powerful stratagem for the rapid synthesis of diverse heterocyclic scaffolds. The optimal MCR is sufficiently flexible that it can be employed to generate adducts bearing a variety of functional groups that may then be selectively paired to enable different cyclization manifolds, thereby leading to a diverse collection of products. The growing interest in diversity-oriented synthesis has led to increased attention to this paradigm for library synthesis, which has inspired many advances in the design and implementation of MCRs for the construction of diverse heterocyclic scaffolds.