Copper‐Catalyzed Aerobic Oxidative Azo‐Ene Cyclization

One-Pot Synthesis of 1,3,4-Oxadiazines from Acylhydrazides and Allenoates

The framework of 1,3,4-oxadiazine is crucial for numerous bioactive molecules, but only a limited number of synthetic methods have been reported for its production. In 2015, Wang's group developed a 4-(dimethylamino)pyridine (DMAP)-catalyzed [2 + 4] cycloaddition of allenoates with N-acyldiazenes, which provided an atom-efficient route for 1,3,4-oxadiazines. However, the practicality of this method was limited by the instability of N-acyldiazenes as starting materials. Building upon our ongoing research about the aerobic oxidation of hydrazides and their synthetic applications, we hypothesized that aerobic oxidative cycloadditions using acylhydrazides instead of N-acyldiazenes may provide a more practical synthetic route for 1,3,4-oxadiazines. In this manuscript, we describe a one-pot synthetic protocol for 1,3,4-oxadiazines from acylhydrazides and allenoates. The developed one-pot protocol consists of aerobic oxidations of acylhydrazides into N-acyldiazenes using NaNO₂ and HNO₃, followed by the DMAP-catalyzed cycloaddition of allenoate with the generated N-acyldiazenes. A variety of 1,3,4-oxadiazines were produced in good to high yields. In addition, the practicality of the developed method was demonstrated by a gram-scale synthesis of 1,3,4-oxadiazine.

Synthesis of 2-Imino-1,3,4-oxadiazolines from Acylhydrazides and Isothiocyanates via Aerobic Oxidation and a DMAP-Mediated Annulation Sequence

In this work, an efficient synthesis of 2-imino-1,3,4-oxadiazolines from acylhydrazides and isothiocyanates is described. In the presence of 4-dimethylaminopyridine (DMAP) and molecular oxygen, various 2-imino-1,3,4-oxadiazolines were produced in good to high yields. The developed method showed a broad substrate scope and was effective on the gram scale. On the basis of the mechanistic studies and previous literature, it was proposed that the mechanism consists of an aerobic oxidation of acylhydrazides facilitated by DMAP and isothiocyanates, followed by a DMAP-mediated annulation of the in situ generated acyldiazenes with isothiocyanates.

Total Synthesis of (+)-Hinckdentine A: Harnessing Noncovalent Interactions for Organocatalytic Bromination

Hinckdentine A, a marine-sponge-derived tribrominated indole alkaloid bearing a unique indolo[1,2-c]quinazoline skeleton, was completed in 12 steps featuring the construction of the Nα-quaternary carbon center by asymmetric azo-ene cyclization. A novel organocatalyst was developed to promote high-yielding tribromination, which represents a challenging process encountered in previous syntheses. Density functional theory calculations scrutinized viable substrates and deciphered the origin of the enhancement of C8 electrophilic bromination with a bifunctional organocatalyst. Moreover, the application of organocatalyst-enabled bromination on various substrates was demonstrated to highlight future late functionalizations of biologically intriguing targets.