N-Heterocyclic Carbenes in Late Transition Metal Catalysis

Throughout history, natural products have afforded a rich source of compounds that have found many applications in the fields of medicine, pharmacy and biology. Within the sphere of cancer, a number of important new commercialised drugs have been obtained from natural sources, by structural modification of natural compounds, or by the synthesis of new compounds, designed following a natural compound as model. The search for improved cytotoxic agents continues to be an important line in the discovery of modern anticancer drugs. The huge structural diversity of natural compounds and their bioactivity potential have meant that several products isolated from plants, marine flora and microorganisms can serve as “lead” compounds for improvement of their therapeutic potential by molecular modification. Additionally, semisynthesis processes of new compounds, obtained by molecular modification of the functional groups of lead compounds, are able to generate structural analogues with greater pharmacological activity and with fewer side effects. These processes, complemented with high-throughput screening protocols, combinatorial chemistry, computational chemistry and bioinformatics are able to afford compounds that are far more efficient than those currently used in clinical practice. Combinatorial biosynthesis is also applied for the modification of natural microbial products. Likewise, advances in genomics and the advent of biotechnology have improved both the discovery and production of new natural compounds.

Natural products as leads to anticancer drugs.

This Review describes the application of nitroxides to synthesis and polymer chemistry. The synthesis and physical properties of nitroxides are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis. The oxidation of alcohols and carbanions, as well as oxidative C-C bond-forming reactions are presented along with other typical oxidative transformations. A section is also dedicated to the extensive use of nitroxides as trapping reagents for C-centered radicals in radical chemistry. Alkoxyamines derived from nitroxides are shown to be highly useful precursors of C-centered radicals in synthesis and also in polymer chemistry. The last section discusses the basics of nitroxide-mediated radical polymerization (NMP) and also highlights new developments in the synthesis of complex polymer architectures.

Nitroxides: applications in synthesis and in polymer chemistry.

Cancer stem cells (CSCs) or tumor initiating cells (TICs) contribute to tumorigenesis, metastasis, recurrence and chemoresistance. CD133, a pentaspan membrane glycoprotein, has been used as a stem cell biomarker for isolation of stem-like cells from a variety of normal and pathological tissues as well as cell lines since its discovery in 1999. Recent studies are focusing on the functionality of CD133. In this review, we summarize new insights into CD133 regulation and the involvement of CD133 in cell self-renewal, tumorigenesis, metastasis, resistance, metabolism, differentiation, autophagy, apoptosis, and regeneration.

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CD133: a stem cell biomarker and beyond

Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

Activation and reactivity of (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes

Cancer stem cells (CSCs) play an important role in carcinogenesis, resistance to treatment and may lead to cancer recurrence and metastasis. However, the molecular mechanism of CSC involved in these events needs to be further elucidated. In this study, CD133(+) colon cancer cells were cultured, which showed CSC properties both in vitro and in vivo from metastatic tissue. Upstream molecules in Akt and mitogen-activated protein kinase (MAPK) pathways were preferentially expressed in these CD133(+) cells, as revealed by a global gene chip. The kinase activities of Akt and extracellular signal-regulated kinase (Erk)1/2 were also significantly upregulated in CD133(+) cells. In addition, the clonogenic growth of CD133(+) cell was reduced greatly by inhibiting the activity of Akt and Erk1/2. The results revealed the Akt and MAPK pathways were involved in the tumorigenesis of CD133(+) colon cancer cells, suggesting that molecules in these two pathways might be potential targets in the future therapy.

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Activation of Akt and MAPK pathways enhances the tumorigenicity of CD133+ primary colon cancer cells

TEMPO-linked metalloporphyrins as efficient catalysts for selective oxidation of alcohols and sulfides

A mild and efficient selective tetrahydropyranylation of primary alcohols and deprotection of THP ethers of phenols and alcohols using PdCl2(CH3CN)2 as catalyst

Soluble polymer-supported synthesis of isoxazoles

Chiral N-heterocyclic carbenes (NHC) were synthesized from naturally occurring podophyllotoxin. Their coordination with [(η3-allyl)Pd(Br)]2 afforded (NHC)Pd(allyl)Br complexes, whose structures were unambiguously established by X-ray single crystal analysis. These chiral NHC and NHC-Pd-allyl complexes were found to catalyze the substitution reaction of allylic compounds with high conversions and enantioselectivities (up to 87% ee).

Yan-Guang Wang

Papers

Activation of Akt and MAPK pathways enhances the tumorigenicity of CD133+ primary colon cancer cells

TEMPO-linked metalloporphyrins as efficient catalysts for selective oxidation of alcohols and sulfides

A mild and efficient selective tetrahydropyranylation of primary alcohols and deprotection of THP ethers of phenols and alcohols using PdCl2(CH3CN)2 as catalyst

Soluble polymer-supported synthesis of isoxazoles

Synthesis of new chiral N-heterocyclic carbenes from naturally occurring podophyllotoxin and their application to asymmetric allylic alkylation