Recent advances in the syntheses of pyrroles

Ammonium niobium oxalate (ANO) as an efficient catalyst in the Paal–Knorr synthesis of N-substituted pyrroles

“…Although improvements have been achieved in the last few years, many of the new protocols face some signicant drawbacks, such as the use of toxic and volatile solvents (DCM, THF, and MeOH), the need for long reaction times (up to 36 h) and high reaction temperatures (up to 160 °C), the use of strongly acidic conditions (H 2 SO 4 , HCl, H 3 PO 4 , and TsOH), specic reaction apparatus (MW or US reactors), and expensive reagents and catalysts (Scheme 1, typical drawbacks faced in Paal-Knorr reactions). [30][31][32][33][34][35][36][37][38][39][40] Niobium-based catalysts have been extensively applied in organic synthesis, being able to conduct several important transformations towards many classes of organic compounds. The two most widely studied Nb-based catalysts are niobium pentachloride (NbCl 5 ) and niobium pentoxide (Nb 2 O 5 ).…”

“…Although improvements have been achieved in the last few years, many of the new protocols face some significant drawbacks, such as the use of toxic and volatile solvents (DCM, THF, and MeOH), the need for long reaction times (up to 36 h) and high reaction temperatures (up to 160 °C), the use of strongly acidic conditions (H 2 SO 4 , HCl, H 3 PO 4 , and TsOH), specific reaction apparatus (MW or US reactors), and expensive reagents and catalysts (Scheme 1, typical drawbacks faced in Paal–Knorr reactions). 30–40…”

Vinylene Carbonate as Synthon in Transition Metal‐Catalyzed C−H Bond Activation/Annulation Reactions

“…Previously, unstable hydroxyacetylene was often used for the synthesis of phenols or aldehydes through cyclization processes involving this C2 synthon. [19,23,24] Until 2009, the discovery of vinylene carbonate provided an alternative reagent: it is indeed more stable, cheaper and more selective than hydroxyacetylene. In the following, the reactions are described according to the different metal catalysts used.…”

Base‐Promoted Synthesis of N−H Free Pyrroles via net [3+2]‐Cycloaddition

“…[8] In this regard, significant efforts have been dedicated to the synthesis of pyrroles. [9] Traditionally, access to pyrrole rings are based on Knorr, [10] Hantzsch, [11] and Paal-Knorr reactions. [12] Recently, many efficient methods, including transition metalcatalyzed processes [13] and multicomponent reactions, [14] among others, [15] have been developed to construct functionalized pyrroles.…”