Synthesis of quaternary carbon-centered indolo[1,2-a]quinazolinones and indazolo[1,2-a]indazolones via C-H functionalization

Catalytic System-Controlled [4 + 1] and [4 + 2] Annulation for the Construction of Pyrazolo[1,2-a]indazoles and Pyrazolo[1,2-a]cinnolines from Pyrazolidinones and Diazo Indandiones

Pyrazolo[1,2-a]indazoles and pyrazolo[1,2-a]cinnolines can be constructed selectively via catalytic system-controlled C(sp2)-H activation/[4 + 1] cyclization and C(sp2)-H activation/[4 + 2] cyclization reaction, utilizing pyrazolidinones and diazo indandiones. Employing two different Rh(III) catalysts, we successfully synthesized two distinct types of nitrogen-containing heterocyclic compounds, pyrazolo[1,2-a]indazoles and pyrazolo[1,2-a]cinnolines, respectively. Moreover, an inexpensive Ru(II) catalyst could also effectively generate pyrazolo[1,2-a]cinnolines with moderate to good yields. Overall, these methodologies offer advantages such as regioselectivity, broad substrate scope, operational simplicity, and product availability.

Nitrogen-bridgehead compounds: overview, synthesis, and outlook on applications

The nitrogen-bridgehead is a common structural motif present in a multitude of natural products. As many of these abundant compounds exhibit biological activities, e.g. against cancer or bacteria, these derivatives are of high interest. While natural products are often associated with problematic characteristics, such as elaborate separation processes, high molecular complexity and limited room for derivatization, purely synthetic approaches can overcome these challenges. Many synthetic procedures have been reported for preparation of artificial nitrogen bridgehead compounds, however, to our surprise only a fraction of these has been tested for their bioactivity. This review is therefore meant to give an overview of existing synthetic methods that provide scaffolds containing bridgehead nitrogen atoms, covering the period from 2000 to 2023. Reviews which cover subunits of this topic are referenced as well.

Synthesis of Indole-Fused Pyrazino[1,2-a]quinazolinones by Copper(I)-Catalyzed Selective Hydroamination-Cyclization of Alkynyl-tethered Quinazolinones

We described a copper(I)-catalyzed atom economic and selective hydroamination-cyclization of alkynyl-tethered quinazolinones to prepare a variety of indole-fused pyrazino[1,2-a]quinazolinones in good to excellent yields ranging from 39 %-99 % under mild reaction conditions. Control experiments revealed that coordination-directed method of quinazolinone moiety with copper(I) was important for the selective hydroamination-cyclization of alkynes at the N1-atom instead of N3-atom of quinazolinone. The reaction could be easily performed at gram scales and some prepared indole-fused pyrazino[1,2-a]quinazolinones with donating groups on the indole moiety showed a distinct fluorescence emission wavelength with blue shift under the acid conditions.

Rh(III)-catalyzed [4 + 1] annulation of 1-arylindazolones with alkynyl cyclobutanols: access to indazolo[1,2-a]indazolones

A convenient and efficient synthesis of structurally diverse indazolo[1,2-a]indazolones via a Rh(III)-catalyzed [4 + 1] annulation of 1-arylindazolones with alkynyl cyclobutanols has been achieved by combining C-H and C-C bond cleavage. This cascade reaction features readily available starting materials, good functional group tolerance, broad substrate scope, and excellent atom-economy.

Construction of Diversified Penta-Spiro-Heterocyclic and Fused-Heterocyclic Frameworks with Potent Antitumor Activity

Multiple-spiro/fused-heterocyclic frameworks containing indazolone are structurally unique and represent a class of potentially dominant skeletons. In this work, we successfully fulfilled Rh(III)-catalyst mediated substrate- and pH- controlled strategies to construct four novel types of complicated penta-spiro/fused-heterocyclic frameworks via C-H activation/[4+1] and [4+2] annulation cascades. This method had mild reaction conditions, a broad scope of substrates, moderate to good yields, and valuable applications, which could realize for the first time the generation of the novel di-spiro-heterocyclic and multiple fused-heterocyclic products with unique structures. More importantly, novel spiro[cyclohexane-indazolo[1,2-a]indazole] scaffold constructed by this method exhibited potent antitumor activity against a variety of refractory solid tumors and hematological malignancies in vitro. Overall, our work provided new insights into the construction of complex and diverse multiple spiro/fused-heterocyclic systems and offered novel valuable lead compounds for the discovery of antitumor drugs.

Rh(III)-catalyzed (5 + 2)-cycloaddition reactions of ortho-hydroxyethyl phenols with internal alkynes: efficient synthesis of benzoxepines

An unprecedented (5 + 2)-cycloaddition reaction of ortho-hydroxyethyl phenol and internal alkyne was developed. This Rh(III)-catalyzed reaction provided benzoxepine derivatives which have very high biological significance. A wide range of ortho-hydroxyethyl phenols and internal alkynes were studied to provide the benzoxepines in high yields.

Rh(III)-Catalyzed C-H Functionalization/Annulation of 1-Arylindazolones: Divergent Synthesis of Fused Indazolones and Allyl Indazolones

Rh(III)-catalyzed C-H/N-H annulation and C-H allylation of phenylindazolones have been realized by employing 5-methylene-1,3-dioxan-2-one and 4-vinyl-1,3-dioxolan-2-one as scalable cross-coupling partners, delivering functionalized indazolone fused heterocycles and branched and linear allyl indazolones respectively in moderate to high yield. These divergent synthesis protocols showcase mild conditions, broad substrate scope, and high functional-group compatibility. In addition, scale-up synthesis and preliminary mechanistic exploratory were also accomplished.

Tandem Transformation of Indazolones to Quinazolinones through Pd-Catalyzed Carbene Insertion into an N-N Bond

Serendipitous and expedite transformation of 1-aryl- and 2-aryl-1,2-dihydro-3H-indazol-3-ones to 1,2-di(hetero)aryl- and 2,3-di(hetero)aryl-2,3-dihydroquinazolin-4(1H)-ones, respectively, was achieved in high efficiency by reacting them with aldehydic N-tosylhydrazones. The protocol proceeded through a cascade process involving base-mediated Pd-carbenoid generation by the decomposition of N-tosylhydrazones, nucleophilic attack of indazolone on the Pd-carbenoid complex, and intramolecular ring expansion via N-N bond cleavage. The utility of the strategy is demonstrated toward the synthesis of bioactive NPS 53574, a calcium receptor antagonist.

Synthesis of Succinimide-Linked Indazol-3-ols Derived from Maleimides under Rh(III) Catalysis

The structural modification of N-aryl indazolols as tautomers of N-aryl indazolones has been established as a hot topic in pharmaceutics and medicinal chemistry. We herein disclose the rhodium(III)-catalyzed 1,4-addition reaction of maleimides with N-aryl indazol-3-ols, which provides the succinimide-bearing indazol-3-ol scaffolds with complete regioselectivity and a good functional group tolerance. Notably, the versatility of this protocol is demonstrated by the use of drug-molecule-linked and fluorescence-probe-linked maleimides.

A cascade indazolone-directed Ir(iii)- and Rh(iii)-catalyzed C(sp2)–H functionalization/[4 + 2] annulation of 1-arylindazolones with sulfoxonium ylides to access chemically divergent 8H-indazolo [1,2-a]cinnolines

An indazolone-directed chemoselective synthesis of 8H-indazolo [1,2-a]cinnolines has been realized via a cascade Cp*Ir(iii)- and Cp*Rh(iii)-catalyzed C–H activation/cyclization reaction of 1-arylindazolones with sulfoxonium ylides.

Synthesis of spirosuccinimides via annulative cyclization between N-aryl indazolols and maleimides under rhodium(III) catalysis

The rhodium(III)-catalyzed spiroannulation reaction between N-aryl indazol-3-ols and maleimides is described herein. The developed method is showcased by the construction of spirosuccinimides using bioactive molecule-linked and chemical probe-linked maleimides. Combined mechanistic investigations including the determination of an isolable rhodacycle complex aided the elucidation of a plausible reaction mechanism.

Ru(ii)-Catalyzed cascade decarbonylative annulation and dehydrogenative alkenylation reactions: synthesis of phthalides

A novel Ru(ii)-catalyzed decarbonylative annulation and direct C-H functionalization reaction of phthalic anhydrides with acrylates is reported. This reaction provides an efficient route for the synthesis of 3,7-disubstituted phthalides.