Synthesis of Highly Oxygenated Carbocycles by Stereoselective Coupling of Alkynes to 1,3- and 1,4-Dicarbonyl Systems

Stereo-Controlled Synthesis of Vicinal Tertiary Carbinols: Application in the Synthesis of a Diol Substructure of Zaragozic Acid, Pactamycin and Ryanodol

A novel and flexible approach for the stereo-controlled synthesis of vicinal tertiary carbinols is reported. The developed strategy featured a highly diastereoselective singlet-oxygen (O2 1 ) [4+2] cycloaddition of rationally designed cyclohexadienones (derived from oxidative dearomatization of the corresponding carboxylic-acid appended phenol precursors), followed by programmed "O-O" and "C-C" bond cleavage. In doing so, a highly functionalized and versatile intermediate was identified and prepared in synthetically useful quantity as a plausible precursor to access a variety of designed and naturally occurring vicinal tertiary carbinol containing compounds. Most notably, the developed strategy was successfully applied in the stereo-controlled synthesis of advanced core structures of zaragozic acid, pactamycin and ryanodol.

Diastereoselective Synthesis of Bicyclo[3.3.0]octenones by Copper-Catalyzed Transannular Ring-Closing Reaction

A novel and efficient copper-catalyzed transannular ring-closing reaction of eight-membered rings has been developed that provides a straightforward way to synthesize bicyclo[3.3.0]octane derivatives in good yields. Mechanistic studies revealed that the reaction pathway might involve chlorination followed by the Kornblum reaction. Readily accessible starting materials and good functional group tolerance make this procedure attractive.

Inhibitory activity of flavonoids from Ephedrae Herba on hypoxia signaling in PANC-1 cells and the evaluation of their mechanisms

Pancreatic cancer is a lethal disease with a very poor prognosis. Recent reports indicate that hypoxia signaling mediated by hypoxia-inducible factor (HIF) contributes to the progression of pancreatic cancer. Therefore, elucidating the inhibitor of hypoxia signaling may lead to the development of a candidate for new anticancer agents. During our screening program for HIF inhibitor from crude drug extracts, new acylated kaempferol glycosides, kaempferol 3-O-[4″-(E)-p-coumaroyl-3″-O-dihydroxypalmityl] rhamnoside (1) and kaempferol 3-O-[4″-(E)-p-coumaroyl-2″-O-dihydroxypalmityl] rhamnoside (2), were isolated from an acetone extract of Ephedrae Herba, together with eight known flavonol glycosides (3-10). The structures of novel compounds 1 and 2 were elucidated based on spectroscopic and chemical analyses. Using a cell-based HRE-driven luciferase reporter assay in a PANC-1 pancreatic cancer cell line, we found that these compounds demonstrated potent inhibitory activity on hypoxia signaling with IC₅₀ values of 18.0 ± 0.6 and 13.3 ± 2.2 µM, respectively. Mechanistically, 2 reduced the amount of HIF-1α protein in the nuclear at 30 µM via the ubiquitin-proteasome pathway with no effect on the nuclear translocation of HIF proteins from cytosol and subsequently decreased Glut1 mRNA. These results indicate that 2 inhibits hypoxia signaling through a mechanism involving the reduction of HIF-1α protein levels and Glut1 mRNA and may have anti-pancreatic cancer effects.

Generation of Masked TiII Intermediates from TiIV Amides via β-H Abstraction or Alkyne Deprotonation: An Example of Ti-Catalyzed Nitrene-Coupled Transfer Hydrogenation

Simple Ti amide complexes are shown to act as sources for masked Ti^II intermediates via several pathways, as demonstrated through the investigation of a unique Ti-catalyzed nitrene-coupled transfer hydrogenation of 3-hexyne. This reaction proceeds through reduction of azobenzene by a masked Ti^II catalyst, wherein both amines and 3-hexyne can serve as the hydrogen source/reductant for Ti by forming putative titanaziridines via β-H abstraction or putative titanacyclopentynes via protonolysis, respectively.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Synthesis of Anhydroryanodol

A stereoselective entry to ryanoids is described that culminates in the synthesis of anhydroryanodol and thus the formal total synthesis of ryanodol. The pathway described features an annulation reaction conceived to address the uniquely complex and highly oxygenated polycyclic skeleton common to members of this natural product class. It is demonstrated that metallacycle-mediated intramolecular coupling of an alkyne and a 1,3-diketone can proceed with a highly functionalized enyne and with outstanding levels of stereoselection. Furthermore, the first application of this technology in natural product synthesis is demonstrated here. More broadly, the advances described demonstrate the value that programs in natural product total synthesis have in advancing organic chemistry, here through the design and realization of an annulation reaction that accomplishes what previously established reactions do not.

Studies Targeting Ryanodol Result in an Annulation Reaction for the Synthesis of a Variety of Fused Carbocycles

An annulation reaction is described to access a range of polycyclic and highly oxygenated carbocycles. First developed in an approach to the synthesis of ryanodol, metallacycle-mediated annulative diketone-alkyne coupling defines a framework for realization of new retrosynthetic relationships for complex molecule synthesis. In addition to demonstrating this reaction in the context of forging distinct carbocyclic systems, including those featuring a seven-membered ring, the choice of quenching reagent leads to unique reaction outcomes.

A Complementary Process to Pauson-Khand-Type Annulation Reactions for the Construction of Fully Substituted Cyclopentenones

A complementary process to the Pauson-Khand annulation is described that is well suited to forging densely substituted/oxygenated cyclopentenone products (including fully substituted variants). The reaction is thought to proceed through a sequence of metallacycle-mediated bond-forming events that engages an internal alkyne and a β-keto ester in an annulation process that forges two C-C bonds. A variant of this annulation process has also been established that delivers deoxygenated cyclopentenones that lack the allylic tertiary alcohol.

Kinetic resolution of 2,2-disubstituted-1,3-diketones via carbene catalysis

Organocatalytic kinetic resolution of 1,3-diketones with central quaternary stereocenters was achieved for the first time. The resolution proceeds via two basic modes, and the inherent principles between the different combinations of ketone groups and the resolution patterns were also disclosed.

Toward the Total Synthesis of Ryanodol via Oxidative Alkyne-1,3-Diketone Annulation: Construction of a Ryanoid Tetracycle

A synthetic strategy conceived with the intent of establishing a novel approach to the de novo construction of ryanoids is described that is based on a recently developed metallacycle-mediated intramolecular oxidative alkyne-1,3-diketone coupling reaction. In short, a one-pot annulation/oxidation sequence is shown to be capable of establishing a densely oxygenated polycyclic intermediate that could be converted to a composition of matter that contains the ABCD tetracyclic ring system present in the ryanoid family of natural products.