Recent advances in 3-aminoindazoles as versatile synthons for the synthesis of nitrogen heterocycles

Nitrogen-based heterocycles are an important class of structural scaffolds distributed in biologically active natural products, medicinal chemistry, and agrochemicals. Hence, there is increasing interest in the development of novel synthetic strategies for the construction of these privileged structural motifs. Recently, 3-aminoindazoles have emerged as versatile synthons participating in a variety of condensation annulation, denitrogenative transannulation and rearrangement ring expansion reactions, which provide efficient synthetic routes for the formation of nitrogen heterocycles. This review systematically highlights for the first time the most recent advances in 3-aminoindazoles to provide a deep understanding of using 3-aminoindazoles as versatile synthons in organic transformations for synthetic and medicinal chemists.

Stereospecific Nitrogen Insertion Using Amino Diphenylphosphinates: An Aza-Baeyer-Villiger Rearrangement

Amino diphenylphosphinates, which are commercially available or easily prepared from hydroxylamine, undergo ring expansion of cyclobutanones toward γ-lactams under mild conditions. A reaction pathway profoundly different from the common Beckmann reaction is achieved through the ambivalent character of the aminating agent. Thus, rearrangement occurs from a Criegee-like intermediate prior to the formation of the oxime species, which is corroborated by mechanistic experiments. Based on this observation, the migrating aptitude of the adjacent groups is analyzed and found to be in line with the parent Baeyer-Villiger reaction rendering a regioselective (up to >99:1 rr), stereospecific (>99% enantiospecificity), and chemoselective (>99%) insertion process possible. The method thus qualifies for late-stage skeletal editing as showcased by the synthesis of Rolipram and its N-alkylated analogs.

Desymmetrization of Prochiral Cyclobutanones via Nitrogen Insertion: A Concise Route to Chiral γ-Lactams

Asymmetric access to γ-lactams is achieved via a cyclobutanone ring expansion using widely available (1S,2R)-1-amino-2-indanol for chiral induction. Mechanistic analysis of the key N,O-ketal rearrangement reveals a Curtin-Hammett scenario, which enables a downstream stereoinduction (up to 88:12 dr) and is corroborated by spectroscopic, crystallographic, and computational studies. In combination with an easy deprotection protocol, this operationally simple sequence allows the synthesis of a range of optically pure γ-lactams, including those bearing all-carbon quaternary stereocenters. In addition, the formal synthesis of drug molecules baclofen, brivaracetam, and pregabalin further demonstrates the synthetic utility and highlights the general applicability of the presented method.

Oxidative Rearrangement via in Situ Generated N-Chloroamine: Synthesis of Fused Tetrahydroisoquinolines

An oxidative rearrangement reaction of spiro tetrahydroisoquinolines has been developed for the synthesis of fused tetrahydroisoquinolines using in situ generated N-chloroamines. The reaction proceeds via initial chlorination of an amine, followed by a 1,2-carbon to nitrogen migration, and nucleophilic trapping of a ketiminium ion intermediate in a one-pot operation. The electrophilic nature of N-chloroamines allowed for the carbon-nitrogen bond formation in this reation.