From an Acyclic, Polyunsaturated Precursor to the Polycyclic Taxane Ring System: The [4+2]/[2+2+2] and [2+2+2]/[4+2] Cyclization Strategies

Cycloparaphenylenes via [2+2+2] Cycloaddition

The [2+2+2] cycloaddition (CA) offers great potential as an atom economic method for the formation of substituted aromatic rings. In this article, we highlight the application of this versatile method...

Intermolecular cyclotrimerization of haloketoalkynes and internal alkynes: facile access to arenes and phthalides

A highly chemo- and regioselective cyclo(co)trimerization between 3-halopropiolamides and symmetrical internal alkynes is reported. The reaction is catalyzed by CpRuCl(COD) and proceeds under air at ambient temperature in ethanol with no additional precautions. Iodo-, bromo-, and chloropropiolamides, esters, and ketones are viable coupling partners and, in a 2 : 1 stoichiometry relative to internal alkyne, yield fully-substituted arenes in a single step. The highest regioselectivities (96% single isomer) were observed when employing 2° and 3°-halopropiolamides. A mechanistic hypothesis accounting for this selectivity is proposed. Notably, by using 1,4-butynediol as the internal alkyne, in situ lactonization following [2+2+2]-cycloaddition generates therapeutically-relevant phthalide pharmacophores directly.

Total Synthesis of the Schisandraceae Nortriterpenoid Rubriflordilactone A

Full details of the total synthesis of the Schisandraceae nortriterpenoid natural product rubriflordilactone A are reported. Palladium- and cobalt-catalyzed polycyclizations were employed as key strategies to construct the central pentasubstituted arene from bromoendiyne and triyne precursors. This required the independent assembly of two AB ring aldehydes for combination with a common diyne component. A number of model systems were explored to investigate these two methodologies, and also to establish routes for the installation of the challenging benzopyran and butenolide rings.

Total Synthesis of (+)-Rubriflordilactone A

Two enantioselective total syntheses of the nortriterpenoid natural product rubriflordilactone A are described, which use palladium- or cobalt-catalyzed cyclizations to form the CDE rings, and converge on a late-stage synthetic intermediate. These key processes are set up through the convergent coupling of a common diyne component with appropriate AB-ring aldehydes, a strategy that sets the stage for the synthetic exploration of other members of this family of natural products.

Selected synthetic strategies to cyclophanes

In this review we cover various approaches to meta- and paracyclophanes involving popular reactions. Generally, we have included a strategy where the reaction was used for assembling the cyclophane skeleton for further functionalization. In several instances, after the cyclophane is made several popular reactions are used and these are not covered here. We included various natural products related to cyclophanes. To keep the length of the review at a manageable level the literature related to orthocyclophanes was not included.

Homogeneous photocatalytic reactions with organometallic and coordination compounds--perspectives for sustainable chemistry

Since the time of Giacomo Ciamician at the beginning of the 20th century, photochemical transformations have been recognized as contributing to sustainable chemistry. Electronic excitation significantly changes the reactivity of chemical compounds. Thus, the application of activation reagents is frequently avoided and transformations can be performed under mild conditions. Catalysis plays a central role in sustainable chemistry. Stoichiometric amounts of activation reagents are often avoided. This fact and the milder catalytic reaction conditions diminish the formation of byproducts. In the case of homogeneous catalysis, organometallic compounds are often applied. The combination of both techniques develops synergistic effects in the sense of "Green Chemistry". Herein, metal carbonyl-mediated reactions are reported. These transformations are of considerable interest for the synthesis of complex polyfunctionalized compounds. Copper(I)-catalyzed [2+2] photocycloaddition gives access to a large variety of cyclobutane derivatives. Currently, a large number of publications deal with photochemical electron-transfer-induced reactions with organometallic and coordination compounds, particularly with ruthenium complexes. Several photochemically induced oxidations can easily be performed with air or molecular oxygen when they are catalyzed with organometallic complexes. Photochemical reaction conditions also play a certain role in C-H activation with organometallic catalysts, for instance, with alkanes, although such transformations are conveniently performed with a variety of other photochemical reactions.

[2+2+2] cycloaddition reactions of macrocyclic systems catalyzed by transition metals. A review

Polyalkyne and enediyne azamacrocycles are prepared from arenesulfonamides and various alkyne and alkene derivatives either under basic or neutral conditions. The new family of macrocyclic substrates is tested in the [2+2+2] cycloaddition reaction. Several catalysts are used for the cycloisomerization reaction, and their enantioinduction is evaluated as appropriate. The effect of the structural features of the macrocycles, namely the ring size, substituents in precise positions and the number and type of unsaturations, on the [2+2+2] cycloaddition reaction has also been studied.