Tandem cyclization-[3+3] cycloaddition reactions of 2-alkynylbenzaldoxime: synthesis of fused 1,2-dihydroisoquinolines

A metal-free tandem dehydrogenative α-arylation reaction of propargylic alcohols with 2-alkynylbenzaldoximes toward the synthesis of α-(4-bromo-isoquinolin-1-yl)-propenone skeletons

A tandem reaction of 2-alkynylbenzaldoximes with propargylic alcohols has been developed for the synthesis of α-(4-bromo-isoquinolin-1-yl)-propenones. Employing 2-alkynylbenzaldoximes as a precursor in the presence of Br₂ generates 4-bromo-isoquinoline-N-oxides. Subsequently, dehydroxylation of propargylic alcohols gives carbocation intermediates, which are trapped using the N-oxides, affording aryl-substituted α-enones.

Synthesis of Indoles via Intermolecular and Intramolecular Cyclization by Using Palladium-Based Catalysts

As part of natural products or biologically active compounds, the synthesis of nitrogen-containing heterocycles is becoming incredibly valuable. Palladium is a transition metal that is widely utilized as a catalyst to facilitate carbon-carbon and carbon-heteroatom coupling; it is used in the synthesis of various heterocycles. This review includes the twelve years of successful indole synthesis using various palladium catalysts to establish carbon-carbon or carbon-nitrogen coupling, as well as the conditions that have been optimized.

Mechanistic Investigation of the Formation of Isoindole N-Oxides in the Electron Transfer-Mediated Oxidative Cyclization of 2'-Alkynylacetophenone Oximes

This paper describes a joint experiment-theory investigation of the formation and cyclization of 2'-alkynylacetophenone oxime radical cations using photoinduced electron transfer (PET) with DCA as the photosensitizer. Using a combination of experimental ¹H and ¹³C nuclear magnetic resonance (NMR) spectra, high-resolution mass spectrometry, and calculated NMR chemical shifts, we identified the products to be isoindole N-oxides. The reaction was found to be stereoselective; only one of the two possible stereoisomers is formed under these conditions. A detailed computational investigation of the cyclization reaction mechanism suggests facile C-N bond formation in the radical cation leading to a 5-exo intermediate. Back-electron transfer from the DCA radical anion followed by barrierless intramolecular proton transfer leads to the final product. We argue that the final proton transfer step in the mechanism is responsible for the stereoselectivity observed in experiment. As a whole, this work provides new insights into the formation of complex heterocycles through oxime and oxime ether radical cation intermediates produced via PET. Moreover, it represents the first reported formation of isoindole N-oxides.

Synthesis of functionalized 1,2-dihydroisoquinolines via one-pot reactions of isoquinoline, alkyl propiolate, and thiazolidin-2,4-dione

The reactive zwitterionic intermediates, generated from addition of isoquinoline to alkyl propiolates, react with thiazolidin-2,4-dione to afford 3-[1-(2,4-dioxothiazolidin-3-yl)isoquinolin-2(1 H)-yl]acrylates in good yields. Using (Z)-5-arylidenethiazolidine-2,4-diones as the NH-acidic component leads to 3-{1-[( Z)-5-benzylidene-2,4-dioxothiazolidin-3-yl] isoquinolin-2(1 H)-yl}acrylates in moderate to good yields in the absence of any catalysts under mild reaction conditions. The reaction between quinoline, ethyl propiolate, and thiazolidin-2,4-dione leads to ethyl 3-[2-(2,4-dioxothiazolidin-3-yl) quinolin-1(2 H)-yl]acrylate.

Synthesis of five-membered cyclic nitrones based on the Lewis acid-catalysed [3+2]-annulation reaction of donor–acceptor cyclopropanes with 1,4,2-dioxazoles

A Lewis acid-catalysed reaction of D–A cyclopropanes with 1,4,2-dioxazoles has been developed for the synthesis of five-membered cyclic nitrones.

Silver-catalysed reactions of alkynes: recent advances

Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon-carbon triple bond makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010-2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e., C-C, C-N, C-O, C-Halo, C-P and C-B). Compared with other coinage metals such as Au and Cu, silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions.

Transition-metal-free coupling reaction of vinylcyclopropanes with aldehydes catalyzed by tin hydride

Donor-acceptor cyclopropanes are useful building blocks for catalytic cycloaddition reactions with a range of electrophiles to give various cyclic products. In contrast, relatively few methods are available for the synthesis of homoallylic alcohols through coupling of vinylcyclopropanes (VCPs) with aldehydes, even with transition-metal catalysts. Here, we report that the hydrostannation of vinylcyclopropanes (VCPs) was effectively promoted by dibutyliodotin hydride (Bu2 SnIH). The resultant allylic tin compounds reacted easily with aldehydes. Furthermore, the use of Bu2 SnIH was effectively catalytic in the presence of hydrosilane as a hydride source, which established a coupling reaction of VCPs with aldehydes for the synthesis of homoallylic alcohols without the use of transition-metal catalysts. In contrast to conventional catalytic reactions of VCPs, the presented method allowed the use of several VCPs in addition to conventional donor-acceptor cyclopropanes.