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

Abstract

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.

A Minireview on the Scope of Cadogan Cyclization Reactions Leading to Diverse Azaheterocycles

“…In the same year, Benzi et al synthesised pyrrolo[3,2-c]carbazoles via classical Cadogan reaction by exploring the pyrrole-to-indole benzannulation method. [27] This method was possible only due to the easy reduction of the nitro group present in the substrate 27. Here, various 7-aryl-6-nitroindoles (27) were subjected to classical Cadogan conditions consisting of the use of triethyl phosphite under reflux to get pyrrolo[3,2-c]carbazoles 28 in good yield as shown in Scheme 5.…”

“…[27] This method was possible only due to the easy reduction of the nitro group present in the substrate 27. Here, various 7-aryl-6-nitroindoles (27) were subjected to classical Cadogan conditions consisting of the use of triethyl phosphite under reflux to get pyrrolo[3,2-c]carbazoles 28 in good yield as shown in Scheme 5. The authors suggested these molecules could help in the synthesis of polyheterocyclic systems with diverse applications.…”

A Nitrocarbazole as a New Microtubule-Targeting Agent in Breast Cancer Treatment

“…The synthesis and characterization of compounds 1-3 [21], 4 and 5 [22], and 6-8 [23] has already been reported; details are provided in Scheme 1 below. treated with the tested compound.…”

“…Although structurally different, compounds 1-8 share a synthetic protocol whereby a nitrobutadiene (12)(13)(14), deriving from the initial ring-opening of a suitably substituted 3-nitrothiophene ( 9) [33][34][35], acts as a benzannulating agent towards indole [21], pyrrole [22], or 2-aminopyridine [23] (Scheme 1). Scheme 1.…”

An Easy Access to Furan-Fused Polyheterocyclic Systems

“…As a matter of fact, starting from a pioneering report [ 32 ], during our previous studies, while exploring the reactivity of different nucleophiles towards nitrobutadienes generated by an initial ring-opening of nitrothiophenes [ 33 , 34 , 35 ], we obtained a great variety of nitrogen-containing heterocycles, such as methyleneazetidines [ 36 ], pyrroles [ 37 ], pyrazoles [ 38 ], carbazoles [ 39 ], imidazo[1,2- a ]pyridines [ 40 ], and nitroindoles [ 41 ]. However, the involvement of nucleophiles able to generate O-heterocycles, as, for instance, enols, has not been investigated so far.…”