Synthesis and applications of fused N- and O-heterocyclic systems from nitrodienic building-blocks

N-containing and O-containing heterocycles are widely distributed in natural products essential for life and are often responsible for the pharmaceutical activity of a lot of drugs and lead-compounds. Exploiting the electrophilic nature of functionalized nitrodienic building-blocks, combined with an opportune nuclophile, new protocols for the synthesis of fused N- and O-heterocycles, has been developed. When using 2-aminopyridines or pyrrole as nucleophiles in the reaction with a suitable nitrodienic building block, it is possible to obtain imidazo[1,2-a]pyridines and 7-aryl-6-nitroindoles respectively. The particular substitution of these N-heterocycles was exploited to obtain fused heteropolycles systems: starting from imidazo[1,2-a]pyridines, through a photoinduced 6\u3c0-electrocyclization, new fused fluorescent polycycles were synthetized, and their fluorescent properties have been analysed, while performing a classical Cadogan reaction on 7-aryl-6-nitroindoles lead to functionalized pyrrolo[3,2-c]carbazoles. Pyrrolo[3,2-c]carbazoles can react with nitrosoalkenes, leading to the products of alkylation, and this reaction has been studied in collaboration with the Organic Chemistry Group of the University of Coimbra. Since the green approach in organic chemistry is gaining more and more importance, the possibility to use heterogeneous catalysis of Graphene Oxide (GO) in the synthesis of heterocycles, starting from nitrovinylic moieties, has also been investigated. Finally, from the reaction between some particular nitrostilbenes and 2-naphthtol or 8-hydroxyquinoline, it was possible to synthetize a small library of dihydronaphtho- and naphthofuran derivatives. Naphthofurans obtained shown a good fluorescence, and their nature of diarylethenes made them promising substrates for a 6\u3c0-electrocyclization, with further expansion of the aromatic system, and the formation of a new benzenic ring, leading to new fused O-heterocyclic systems

Sequential Annulations to Interesting Novel Pyrrolo[3,2-c]carbazoles

Herein we report a signi\ufb01cant, valuable extension of a recently implemented pyrrole benzannulation methodology that, employing versatile nitrodienes from our lab as useful C4 building blocks, led to indole derivatives characterized by unusual patterns of substitution. The 6-nitro-7-arylindoles resulting from suitably derivatized, non-symmetric dienes are of foreseeable synthetic interest in search for new polyheterocyclic systems. As an example, pyrrolocarbazoles with a rarely reported ring fusion were synthesized with the classical Cadogan protocol. Furthermore, the proven easy reducibility of the nitro group to amine will surely open the way to further interesting elaborations

An easy access to furan fused polyheterocyclic systems

Nitrostilbenes characterized by two different or differently substituted aryl moieties can be obtained from the initial ring-opening of 3-nitrobenzo[b]thiophene with amines. Such versatile building blocks couple the well-recognized double electrophilic reactivity of the nitrovinyl moiety (addition to the double bond, followed by, e.g., intramolecular replacement of the nitro group) with the possibility to exploit a conjugated system of double bonds within an electrocyclization process. Herein, nitrostilbenes are reacted with different aromatic enols provided by a double (carbon and oxygen) nucleophilicity, leading to novel, interesting naphthodihydrofurans. From these, as a viable application, aromatization and electrocyclization lead in turn to valuable polycondensed, fully aromatic O-heterocycles. © 2022 by the authors. Licensee MDPI, Basel, Switzerland

Sequential Annulations to Interesting Novel Pyrrolo[3,2-c]carbazoles

Herein we report a significant, valuable extension of a recently implemented pyrrole benzannulation methodology that, employing versatile nitrodienes from our lab as useful C4 building blocks, led to indole derivatives characterized by unusual patterns of substitution. The 6-nitro-7-arylindoles resulting from suitably derivatized, non-symmetric dienes are of foreseeable synthetic interest in search for new polyheterocyclic systems. As an example, pyrrolocarbazoles with a rarely reported ring fusion were synthesized with the classical Cadogan protocol. Furthermore, the proven easy reducibility of the nitro group to amine will surely open the way to further interesting elaborations

2\u2010Aryl\u20103\u2010Vinyl Substituted Imidazo[1,2\u2010a]pyridines and Fluorescent Electrocyclization Derivatives therefrom

A collection of 2\u2010aryl\u20103\u2010vinylimidazo[1,2\u2010a]pyridines, substituted on the pyridine ring and/or the aryl substituent, can be obtained by reaction of 2\u2010aminopyridines with properly substituted nitrobutadienes, originated by an initial ring opening of a nitrothiophenic precursor. Although the aza\u2010Michael addition of 2\u2010aminopyridine on a nitrovinyl moiety is the starting point of classic approaches to imidazopyridines, the final structures generated herein are the result of a cascade process made possible by the particular functionalization on the conjugated systems. Beside the general interest inherent to the pharmacologically versatile imidazo[1,2\u2010a]pyridine structural motif, the substituents present in the obtained compounds allow specific synthetic elaboration to e.\u2009g. polycyclic fluorescent heterocycles