Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

Reaction of 1,3,5-Triazinanes with Phosphoryl Diazomethanes: Access to 5-Phosphoryl-1,2,3,4-tetrahydropyrimidines

Conversion of 1,3,5-triazinanes into 5-phosporyl-1,2,3,4-tetrahydropyrimidines is achieved efficiently through the microwave-assisted reaction with phosphoryl diazomethanes. Both trialkyl and triaryl 1,3,5-triazinanes were converted by diazomethyldiarylphosphine oxides, dialkyl diazomethylphosphonates, and alkyl diazomethyl(aryl)phosphinates and functionalized simultaneously in good to excellent yields. The reaction is a sequence of 1,3,5-triazinane fragmentation, tandem nucleophilic addition of the generated formaldimines and phosphoryl diazomethanes, and final N,N-acetalization.

Bro̷nsted Acid-Catalyzed [5+1] and [4+1] Annulation of Cyclic Anhydrides with o-Alkynylanilines to Construct Fused-N-Heterocycles

An unprecedented p-TsOH and MsOH-catalyzed construction of valuable isoindolo/pyrido/pyrrolo-quinolinediones and isoindolo-indolones is demonstrated through annulation reactions of cyclic anhydrides or o-formylbenzoates with o-alkynylanilines. The metal-free Bro̷nsted acid-mediated new [5+1] and [4+1] fused-cyclization is an operationally simple, highly regioselective, atom economical, high yielding, sustainable, and catalytically efficient approach.

Formic Acid as Carbon Monoxide Source in the Palladium-Catalyzed N-Heterocyclization of o-Nitrostyrenes to Indoles

The reductive cyclization reaction of o-nitrostyrenes to generate indoles has been investigated for three decades using CO as a cheap reducing agent, but it remains an interesting area of research and improvements. However, using toxic CO gas has several drawbacks. As a result, it is highly preferable to use safe and efficient surrogates for in situ generation of CO from nontoxic and affordable sources. Among several CO sources that have been previously explored for the generation of gaseous CO, here we report the use of cheap and readily available formic acid as an effective reductant for the reductive cyclization of o-nitrostyrenes. The reaction is air and water tolerant and provides the desired indoles in yields up to 99%, at a low catalyst loading (0.5 mol %) and without generating toxic or difficult to separate byproducts. A cheap glass thick-walled "pressure tube" can be used instead of less available autoclaves, even on a 2 g scale, thus widening the applicability of our protocol.