Synthesis and Biological Activity of Enantiomers of Antitumor Irofulven

An overview of recent advances in duplex DNA recognition by small molecules

As the carrier of genetic information, the DNA double helix interacts with many natural ligands during the cell cycle, and is amenable to such intervention in diseases such as cancer biogenesis. Proteins bind DNA in a site-specific manner, not only distinguishing between the geometry of the major and minor grooves, but also by making close contacts with individual bases within the local helix architecture. Over the last four decades, much research has been reported on the development of small non-natural ligands as therapeutics to either block, or in some cases, mimic a DNA–protein interaction of interest. This review presents the latest findings in the pursuit of novel synthetic DNA binders. This article provides recent coverage of major strategies (such as groove recognition, intercalation and cross-linking) adopted in the duplex DNA recognition by small molecules, with an emphasis on major works of the past few years.

Strategies for the Optimization of Natural Leads to Anticancer Drugs or Drug Candidates

Natural products have made significant contribution to cancer chemotherapy over the past decades and remain an indispensable source of molecular and mechanistic diversity for anticancer drug discovery. More often than not, natural products may serve as leads for further drug development rather than as effective anticancer drugs by themselves. Generally, optimization of natural leads into anticancer drugs or drug candidates should not only address drug efficacy, but also improve absorption, distribution, metabolism, excretion, and toxicity (ADMET) profiles and chemical accessibility associated with the natural leads. Optimization strategies involve direct chemical manipulation of functional groups, structure-activity relationship directed optimization and pharmacophore-oriented molecular design based on the natural templates. Both fundamental medicinal chemistry principles (e.g., bioisosterism) and state-of-the-art computer-aided drug design techniques (e.g., structure-based design) can be applied to facilitate optimization efforts. In this review, the strategies to optimize natural leads to anticancer drugs or drug candidates are illustrated with examples and described according to their purposes. Furthermore, successful case studies on lead optimization of bioactive compounds performed in the Natural Products Research Laboratories at UNC are highlighted.

Total syntheses of natural products containing spirocarbocycles

The spiro motif is becoming an increasingly prevalent structure in medicinal and organic chemistry. The total syntheses of natural products containing all-carbon spirocycles is reviewed.

Catalytic enantioselective intermolecular cycloaddition of diazodiketoester-derived carbonyl ylides with indoles using chiral dirhodium(II) carboxylates

The first example of enantioselective intermolecular cycloaddition of carbonyl ylides with indoles is described. The cycloaddition of five- and six-membered carbonyl ylides derived from diazodiketoesters with N-methylindoles under catalysis by dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh(2)(S-TCPTTL)(4), gave cycloadducts in high yields and with high levels of enantioselectivity (up to 99% ee) as well as excellent exo diastereoselectivity.

Chemo-, regio-, and diastereoselectivity preferences in the reaction of a sulfur ylide with a dienal and an enone

Mechanistic insights into an interesting class of reaction between sulfur ylides with (i) a dienal, and (ii) an enone, obtained by using density functional theory, is reported. The kinetic and thermodynamic factors responsible for chemo-, regio-, and diastereoselectivities are established by identifying all key transition states and intermediates along the reaction pathway for 1,2-, 1,4-, and 1,6- modes of attack of dimethylsulfonium benzylide to 5-phenylpenta-2,4-dienal. The reaction profiles for 1,2- and 1,4- modes of addition are also evaluated for the reaction between dimethylsulfonium benzylide and pent-3-en-2-one. Our results show that the final outcome of the reaction with both these substrates would be decided by the interplay between kinetic and thermodynamic factors. It is found that the addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers to proceed through a 1,4- (conjugate) pathway under thermodynamic conditions, which is in accordance with the available experimental reports. However, the formation of epoxides via a 1,2- (direct) addition pathway is computed to be equally competitive, which could be the favored pathway under kinetic conditions. Even though the lower barrier for the initial addition step is kinetically advantageous for the direct (or 1,2-) addition pathway, the higher energy of the betaine intermediates--as well as the reversibility of the accompanying elementary step--may disfavor product formation in this route. Thus, high diastereoselectivity in favor of 2,3-trans cyclopropanecarbaldehyde is predicted in the case of the dienal, using the most favored conjugate addition (1,4-addition) pathway. Along similar lines, ylide addition to the enone is identified to exhibit a preference toward conjugate addition over direct (1,2-) addition. The importance of transition state analysis in delineating the controlling factors towards product distribution and diastereoselectivity is established.

Enantioselective total synthesis of (-)-acylfulvene and (-)-irofulven

We report our full account of the enantioselective total synthesis of (-)-acylfulvene (1) and (-)-irofulven (2), which features metathesis reactions for the rapid assembly of the molecular framework of these antitumor agents. We discuss (1) the application of an Evans Cu-catalyzed aldol addition reaction using a strained cyclopropyl ketenethioacetal, (2) an efficient enyne ring-closing metathesis cascade reaction in a challenging setting, (3) the reagent IPNBSH for a late-stage reductive allylic transposition reaction, and (4) the final RCM/dehydrogenation sequence for the formation of (-)-acylfulvene (1) and (-)-irofulven (2).

Observations in the Synthesis of the Core of the Antitumor Illudins via an Enyne Ring Closing Metathesis Cascade

Observations concerning the synthesis of the core spirocyclic AB-ring system of illudins using an enyne ring closing metathesis (EYRCM) cascade are discussed. Substituent effects, in addition to optimization of the reaction conditions and the olefin tether for the key EYRCM reaction, are examined.

Anomalous Reaction of Rh2(OAc)4-Generated Transient Carbonyl Ylides: Chemoselective Synthesis of Epoxy-Bridged Tetrahydropyranone, Oxepanone, Oxocinone, and Oxoninone Ring Systems

A series of symmetrical and unsymmetrical alpha,beta-unsaturated ketones such as arylmethylidenecycloalkanones (16, 19), bis(arylmethylidene)cycloalkanones (21, 23, 27), bisdiphenylnonatetraenones (30), and bis(phenylpropenylidene)cycloalkanones (33, 38, 42) were synthesized and subjected to the rhodium(II)-catalyzed tandem cyclization-cycloaddition reactions with various alpha-diazo ketones. These reactions afforded spiro epoxy-bridged tetrahydropyranone, spiro epoxy-bridged oxepanone (41, 43), epoxyoxocin-4(5H)-one (35, 37), and epoxyoxonin-5(6H)-one (40) frameworks starting from relatively simple precursors. The regio- and stereochemistry and solid-state architecture arrangements of several products were characterized by single-crystal X-ray structure analysis. The cycloaddition of carbonyl ylides with the compounds having both C=O and C=C groups was found to be chemo- and regioselective. Interestingly, an unusual ring enlargement of cycloadducts 34, 36, and 41 derived from C=O group addition was observed, affording epoxyoxocin-4(5H)-one and epoxyoxonin-5(6H)-one frameworks. Examples for the tandem cyclization-cycloaddition-ring enlargement reaction were also described. The rhodium(II)-generated carbonyl ylides behaved anomalously, furnishing the cycloadducts as a result of the cycloaddition of carbonyl ylides to C=O group despite the presence of C=C groups.

Density Functional Theory Investigations on Sulfur Ylide Promoted Cyclopropanation Reactions: Insights on Mechanism and Diastereoselection Issues

The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanation reactions have been studied using the density functional theory method. Addition of different substituted ylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHC=CH-COMe) has been investigated. The nature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode, leading to 1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplest and highly reactive model ylide (R=H) prefers a transoid addition mode. Diastereoselectivity is controlled by the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilic addition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivity toward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activation energies of diastereomeric torsional transition states. The energy differences between these transition states could be rationalized with the help of weak intramolecular as well as other stereoelectronic interactions.

Highly Regio- and Chemoselective Ring Opening of Oxa-Bridged Piperidinones toward Functionalized Furanones and Piperidines

Lewis acid mediated competitive ring-opening reactions of N-tosyl-substituted oxa-bridged piperidinone ring systems are demonstrated. A majority of the Lewis acids furnished the regio- and chemoselective reductive ring opening at the C(1)-N bond, whereas TiCl(4) furnished at the C(1)-O bond in the presence of triethylsilane, affording functionalized furanones and dihydroxypiperidines, respectively. [reaction: see text]

Pharmacokinetics, metabolism, and routes of excretion of intravenous irofulven in patients with advanced solid tumors

Irofulven is currently in Phase 2 clinical trials against a wide variety of solid tumors and has demonstrated activity in ovarian, prostate, gastrointestinal, and non-small cell lung cancer. The objectives of this study were to determine its pharmacokinetics and route of excretion and to characterize its metabolites in human plasma and urine samples after a 30-min i.v. infusion at a dose of 0.55 mg/kg in patients with advanced solid tumors. Three patients were administered i.v. 100 microCi of [14C]irofulven over a 30-min infusion on day 1 of cycle 1. Serial blood and plasma samples were drawn at 0 (before irofulven infusion) and up to 144 h after the start of infusion. Urine and fecal samples were collected for up to 144 h after the start of infusion. The mean urinary and fecal excretion of radioactivity up to 144 h were 71.2 and 2.9%, respectively, indicating renal excretion was the major route of elimination of [14C]irofulven. The C(max), AUC(0-infinity), and terminal half-life values for total radioactivity were 1130 ng-Eq/ml, 24,400 ng-Eq . h/ml, and 116.5 h, respectively, and the corresponding values for irofulven were 82.7 ng/ml, 65.5 ng . h/ml, and 0.3 h, respectively, suggesting that the total radioactivity in human plasma was a result of the metabolites. Twelve metabolites of irofulven were detected in human urine and plasma by electrospray ionization/tandem mass spectrometry. Among these metabolites, the cyclopropane ring-opened metabolite (M2) of irofulven was found, and seven others were proposed as glucuronide and glutathione conjugates.

Investigating the role of stereochemistry in the activity of anticancer acylfulvenes: synthesis, reductase-mediated bioactivation, and cellular toxicity

Acylfulvenes comprise a family of semisynthetic natural product derivatives with potent antitumor activities. Previous studies indicated that acylfulvenes are bioactivated by NADPH-dependent alkenal/one reductase (AOR), presumably generating intermediates with the capacity to alkylate cellular targets, such as DNA, proteins, and glutathione. This process is thought to induce apoptosis, and the chemical and biochemical pathways involved are topics of current investigation. In this study, four acylfulvene analogues were synthesized: (-)-acylfulvene, (+)-acylfulvene, (-)-(hydroxymethyl)acylfulvene, and (+)-(hydroxymethyl)acylfulvene. These compounds were synthesized by a chiral-resolution method, described for the first time in this report, and by asymmetric synthesis using a method formally demonstrated previously. Cell toxicity studies indicate a positive correlation between AOR level and acylfulvene sensitivity. The absolute configuration of acylfulvene analogues has a significant influence on cytotoxicity. (-)-(Hydroxymethyl)acylfulvene is 25 times more potent than (+)-(hydroxymethyl)acylfulvene in cells transfected with an AOR overexpression vector. Based on kinetic parameters, the rates of AOR-mediated activation are more strongly dependent on acylfulvene substitution than on absolute stereochemistry. These data support the role of AOR-mediated metabolism and indicate the involvement of other stereochemically dictated pathways, such as transport and biomolecule binding, in contributing to the cytotoxicity of acylfulvenes.

Medicinal mushroom modulators of molecular targets as cancer therapeutics

Empirical approaches to discover anticancer drugs and cancer treatments have made limited progress in the past several decades in finding a cure for cancer. The expanded knowledge of the molecular basis of tumorigenesis and metastasis, together with the inherently vast structural diversity of natural compounds found in mushrooms, provided unique opportunities for discovering new drugs that rationally target the abnormal molecular and biochemical signals leading to cancer. This review focuses on mushroom low-molecular-weight secondary metabolites targeting processes such as apoptosis, angiogenesis, metastasis, cell cycle regulation, and signal transduction cascades. Also discussed in this review are high-molecular-weight polysaccharides or polysaccharide-protein complexes from mushrooms that appear to enhance innate and cell-mediated immune responses, exhibit antitumor activities in animals and humans, and demonstrate the anticancer properties of selenium compounds accumulated in mushrooms.