Recent Updates on Azido‐reductive Cyclization Approaches: Syntheses of aza ‐Heterocyclic Frameworks

Synthesis of Azabicyclo[3.1.1]heptenes Enabled by Catalyst-Controlled Annulations of Bicyclo[1.1.0]butanes with Vinyl Azides

Bridged bicyclic scaffolds are emerging bioisosteres of planar aromatic rings under the concept of "escape from flatland". However, adopting this concept into the exploration of bioisosteres of pyridines remains elusive due to the challenge of incorporating a N atom into such bridged bicyclic structures. Herein, we report practical routes for the divergent synthesis of 2- and 3-azabicyclo[3.1.1]heptenes (aza-BCHepes) as potential bioisosteres of pyridines from the readily accessible vinyl azides and bicyclo[1.1.0]butanes (BCBs) via two distinct catalytic annulations. The reactivity of vinyl azides tailored with BCBs is the key to achieving divergent transformations. TiIII-catalyzed single-electron reductive generation of C-radicals from BCBs allows a concise (3 + 3) annulation with vinyl azides, affording novel 2-aza-BCHepe scaffolds. In contrast, scandium catalysis enables an efficient dipolar (3 + 2) annulation with vinyl azides to generate 2-azidobicyclo[2.1.1]hexanes, which subsequently undergo a chemoselective rearrangement to construct 3-aza-BCHepes. Both approaches efficiently deliver unique azabicyclo[3.1.1]heptene scaffolds with a high functional group tolerance. The synthetic utility has been further demonstrated by scale-up reactions and diverse postcatalytic transformations, providing valuable azabicyclics including 2- and 3-azabicyclo[3.1.1]heptanes and rigid bicyclic amino esters. In addition, the related sp2-hybridized nitrogen atom and the similar geometric property between pyridines and corresponding aza-BCHepes indicate that they are promising bioisosteres of pyridines.

Skeletal rearrangement through photocatalytic denitrogenation: access to C-3 aminoquinolin-2(1H)-ones

The addition of an amine group to a heteroaromatic system is a challenging synthetic process, yet it is an essential one in the development of many bioactive molecules. Here, we report an alternative method for the synthesis of 3-amino quinolin-2(1H)-one that overcomes the limitations of traditional methods by editing the molecular skeleton via a cascade C-N bond formation and denitrogenation process. We used TMSN3 as an aminating agent and a wide variety of 3-ylideneoxindoles as synthetic precursors for the quinolin-2(1H)-one backbone, which demonstrates remarkable tolerance of sensitive functional groups. The control experiments showed that the triazoline intermediate plays a significant role in the formation of the product. The spectroscopic investigation further defined the potential reaction pathways.

Visible-Light 2,4-Dinitrophenol-Mediated Photoannulation of α-Azidochalcones into 2,5-Diaryloxazoles

A novel and efficient visible-light 2,4-dinitrophenol (2,4-DNP) mediated photoannulation of α-azidochalcones into 2,5-diaryloxazoles was developed. The carbon-carbon double bond of α-azidochalcone was cleaved, leading to the formation of new C-O and C-N bonds in the photoannulation. Control experiments were carried out, and a plausible mechanism of the photoannulation was proposed. The scope of the reaction was studied by synthesizing a series of 2,5-diaryloxazoles including two naturally occurring oxazoles (Texamine and Balsoxin) in excellent yields.