Total synthesis of discodermolide: optimization of the effective synthetic route

Experimental and Theoretical Studies on the Reactions of Aliphatic Imines with Isocyanates

In the context of a project aiming at the replacement of the 3-substituted β-lactam ring in classical β-lactam antibiotics by an N(3)-acyl-1,3-diazetidinone moiety, we have investigated the reaction of isocyanates with imines derived from allyl glycinate and differently substituted propionaldehydes. Imines of aromatic aldehydes with anilines have been reported to react with acyl isocyanates to give 1,3-diazetidinones or 2,3-dihydro-4H-1,3,5-oxadiazin-4-ones, via [2+2] or [4+2] cycloaddition, respectively. However, neither of these products was formed with imines derived from allyl glycinate and 2-(mono)methyl propionaldehydes. α,α-Dimethylation of the imine enabled the [4+2] cycloaddition pathway, but the desired 1,3-diazetidinone products were not observed. Surprisingly, the imines obtained from thioesters of 2,2-dimethyl 3-oxo propionic acid reacted with aryl isocyanates or with benzyl isocyanate to give 5,5-dimethyl-2,4-dioxo-6-(aryl-/alkylthio)tetrahydropyrimidines, via thiol displacement and re-addition to a putative six-membered iminium intermediate. These experimental results obtained for the reactions could be rationalized by DFT calculations. In addition, we have shown that N(3)-acyl-1,3-diazetidinone and 2,3-dihydro-4H-1,3,5-oxadiazin-4-one products can be distinguished based on experimental IR data in combination with theoretical reference spectra employing the IR spectra alignment (IRSA) algorithm. This discrimination was not possible by means of 1 H, 13 C, or 15 N NMR spectroscopy.

An overview of microtubule targeting agents for cancer therapy

The entire world is looking for effective cancer therapies whose benefits would outweigh their toxicity. One way to reduce resistance to chemotherapy and its adverse effects is the so called targeted therapy, which targets specific molecules (“molecular targets”) that play a critical role in cancer growth, progression, and metastasis. One such specific target are microtubules. In this review we address the current knowledge about microtubule-targeting agents or drugs (MTAs/MTDs) used in cancer therapy from their synthesis to toxicities. Synthetic and natural MTAs exhibit antitumor activity, and preclinical and clinical studies have shown that their anticancer effectiveness is higher than that of traditional drug therapies. Furthermore, MTAs involve a lower risk of adverse effects such as neurotoxicity and haemotoxicity. Several new generation MTAs are currently being evaluated for clinical use. This review brings updated information on the benefits of MTAs, therapeutic approaches, advantages, and challenges in their research.

Advances in exploring the therapeutic potential of marine natural products

Marine natural products represent novel and diverse chemotypes that serve as templates for the discovery and development of therapeutic agents with distinct mechanisms of action. These genetically encoded compounds produced by an evolutionary optimized biosynthetic machinery are usually quite complex and can be difficult to recreate in the laboratory. The isolation from the source organism results in limited amount of material; however, the development of advanced NMR technologies and dereplication strategies has enabled the structure elucidation on small scale. In order to rigorously explore the therapeutic potential of marine natural products and advance them further, the biological characterization has to keep pace with the chemical characterization. The limited marine natural product supply has been a serious challenge for thorough investigation of the biological targets. Several marine drugs have reached the markets or are in clinical trials, where those challenges have been overcome, including through the development of scalable syntheses. However, the identification of mechanisms of action of marine natural products early in the discovery process is potentially game changing, since effectively linking marine natural products to potential therapeutic applications in turn triggers motivation to tackle challenging syntheses and solve the supply problem. An increasing number of sensitive technologies and methods have been developed in recent years, some of which have been successfully applied to marine natural products, increasing the value of these compounds with respect to their biomedical utility. In this review, we discuss advances in overcoming the bottlenecks in marine natural product research, emphasizing on the development and advances of diverse target identification technologies applicable for marine natural product research.

An intramolecular oxa-Michael reaction on α,β-unsaturated α-amino-δ-hydroxycarboxylic acid esters. Synthesis of functionalized 1,3-dioxanes

A highly diastereoselective intramolecular oxa-Michael reaction on α,β-unsaturated α-amino-δ-hydroxycarboxylic acid esters is presented; 1,3-dioxanes functionalized in positions 2,4 and 6 were obtained in good yields and with excellent selectivities; an experimental and computational study was carried out to understand the reaction course in terms of yields and selectivities. This reaction proceeds under mild reaction conditions using highly electrophilic aldehydes and ketones.

Diastereoselective synthesis of trifluoromethylated 1,3-dioxanes by intramolecular oxa-Michael reaction

A highly diastereoselective synthesis of trifluoromethylated 1,3-dioxanes is described. The reaction proceeds by an addition/oxa-Michael sequence and works efficiently under mild reaction conditions, with a good substrate scope and acceptable to good yields.

Dyotropic rearrangements in natural product total synthesis and biosynthesis

Some recent examples of dyotropic rearrangements involved in complex natural product total synthesis and biosynthesis are highlighted.

Dyotropic rearrangement of bridgehead substituents in closed dithienylethenes; conjugated verses non-conjugated analogues

Type I dyotropic rearrangement reactions of halogen and methyl substituents at the bridgehead position of diarylethenes and dihydroarylethenes have been studied through density functional theory at B3LYP/6-31+G(d) level. The calculations have been performed to explore the dyotropic rearrangement as a possible factor for the elusive nature of halogenated dithienylethenes (closed). The dyotropic rearrangement process in closed dithienylethenes is then compared with the dihydro analogues. Moreover, the effect of hetero atom and conjugation is also explored through quantum mechanical calculations.

A chiral phosphoric acid catalyst for asymmetric construction of 1,3-dioxanes

A chiral phosphoric acid catalyst enabled enantioselective 1,3-dioxane construction, useful for polyketide motif synthesis, via a hemiacetalization/intramolecular oxy-Michael addition cascade.

Directed studies towards the total synthesis of (+)-13-deoxytedanolide: simple and convenient synthesis of the C8-C16 fragment

A straightforward synthesis of the enantioenriched C8-C16 south part of (+)-13-deoxytedanolide has been reported. The strength of this approach relies on the preparation of similar functionalized fragments via the transformation of a unique dihydrofuran building block through a 1,2-metallate rearrangement.

Design and synthesis of (+)-discodermolide-paclitaxel hybrids leading to enhanced biological activity

Potential binding modes of (+)-discodermolide at the paclitaxel binding site of tubulin have been identified by computational studies based on earlier structural and SAR data. Examination of the prospective binding modes reveal that the aromatic pocket occupied by the paclitaxel side chain is unoccupied by (+)-discodermolide. Based on these findings, a small library of (+)-discodermolide-paclitaxel hybrids have been designed and synthesized. Biological evaluation reveals a two- to eight-fold increase in antiproliferative activity compared to the parent molecule using the A549 and MCF-7 cancer cell lines.

Regioselective rhodium-catalyzed hydroformylation of 1,3-dienes to highly enantioenriched β,γ-unsaturated aldehyes with diazaphospholane ligands

Regioselective and enantioselective rhodium-catalyzed hydroformylation of 1,3-dienes with chiral bisdiazaphospholane ligands yields β,γ-unsaturated aldehydes that retain a C═C functionality for further conversion. The reaction conditions are mild, featuring low catalyst loadings (0.5 mol %), pressures readily obtained in glass bottles, and convenient reaction times (1.5-12 h). Optimized reaction conditions produce high enantioselectivity (>90% ee), regioselectivity (88-99%), and conversion to β,γ-unsaturated aldehydes (99%) for ten 1,3-dienes encompassing a variety of substitution patterns.

Microtubule-binding agents: a dynamic field of cancer therapeutics

Microtubules are dynamic filamentous cytoskeletal proteins composed of tubulin and are an important therapeutic target in tumour cells. Agents that bind to microtubules have been part of the pharmacopoeia of anticancer therapy for decades and until the advent of targeted therapy, microtubules were the only alternative to DNA as a therapeutic target in cancer. The screening of a range of botanical species and marine organisms has yielded promising new antitubulin agents with novel properties. In the current search for novel microtubule-binding agents, enhanced tumour specificity, reduced neurotoxicity and insensitivity to chemoresistance mechanisms are the three main objectives.

Improved method for the synthesis of beta-carbonyl silyl-1,3-dithianes by the double conjugate addition of 1,3-dithiol to propargylic carbonyl compounds

Base-mediated double conjugate addition of 1,3-propane dithiol to various silylated propargylic aldehydes and ketones allows for an efficient and scalable synthesis of beta-carbonyl silyl-1,3-dithianes.

The discodermolide hairpin structure flows from conformationally stable modular motifs

(+)-Discodermolide (DDM), a polyketide macrolide from marine sponge, is a potent microtubule assembly promoter. Reported solid-state, solution, and protein-bound DDM conformations reveal the unusual result that a common hairpin conformational motif exists in all three microenvironments. No other flexible microtubule binding agent exhibits such constancy of conformation. In the present study, we combine force-field conformational searches with NMR deconvolution in different solvents to compare DDM conformers with those observed in other environments. While several conformational families are perceived, the hairpin form dominates. The stability of this motif is dictated primarily by steric factors arising from repeated modular segments in DDM composed of the C(Me)-CHX-C(Me) fragment. Furthermore, docking protocols were utilized to probe the DDM binding mode in beta-tubulin. A previously suggested pose is substantiated (Pose-1), while an alternative (Pose-2) has been identified. SAR analysis for DDM analogues differentiates the two poses and suggests that Pose-2 is better able to accommodate the biodata.

Functional group compatibility. Propargyl alcohol reduction in the presence of a vinyl iodide

Vinyl iodides are stable to the reduction of propargyl alcohols to cis allylic alcohols by hydrogen over Pd/CaCO(3) in hexane. They are also stable to the reduction of propargyl alcohols to saturated alcohols by hydrogen over Crabtree's iridium catalyst in CH(2)Cl(2).