Piloty pyrrole synthesis

This reaction was first reported by Piloty in 1910. It is the thermal transformation of enolizable ketazines into pyrrole derivatives in the presence of a catalytic amount of Lewis acid (e.g., ZnCl2) or Brpnsted acid (e.g., HCl) and is known as the Piloty pyrrole synthesis,Piloty reaction, Piloty cyclization, or Piloty synthesis." In 1918, Robinson extensively studied this reaction again, especially for the mechanism, since then the Piloty pyrrole synthesis has also been referred to as the Piloty-Robinson pyrrole synthesis or simply the Piloty-Robinson synthesis or Piloty-Robinson reaction. It should be pointed out that for the thermal transformation of aliphatic ketazines, a competitive route also occurs to give pyrazoline as a by-product. ... 

Much less commonly found are a few other Fischer-type variations that will be briefly mentioned. The Piloty-Robinson indole synthesis is a simple variation of the Piloty pyrrole synthesis, where the [3,3] sigmatropic event is triggered by acetic anhydride or methyl iodide (Scheme 26) [142,143]. An example is shown in equation 2 [143]. 

The Piloty-Robinson pyrrole synthesis (74JOC2575,18JCS639) may be viewed as a monocyclic equivalent of the Fischer indole synthesis. The conversion of ketazines into pyrroles under strongly acidic conditions apparently proceeds through a [3,3] sigmatropic rearrange-... 

This reaction is related to the Bucherer Carbazole Synthesis, Fischer Indole Synthesis, Graebe-Ullmann Synthesis and Piloty-Robinson Pyrrole Synthesis. 

Other references related to the Piloty-Robinson pyrrole synthesis are cited in the literature. 

Scheme 14.14 Microwave-promoted Piloty-Rohinson pyrrole synthesis and application to the synthesis of porphyins. ...

Disubstituted pyrroles are important building blocks for porphyrin synthesis (cf p. 553). With two equal 3,4-substituents, they can be straightforwardly obtained by the Piloty-Robinson synthesis [118] ofhigher flexibility are isocyanide-based methods like the Barton-Zard synthesis [119] and the Van Leusen synthesis [120]. 

Parham cycloalkylation 343 Patemo-Btichi reaction 46 von Pechmann synthesis (coumarin) 322 Pellizari synthesis (1,2,4-triazole) 270 Perkin rearrangement 82 Pfitzinger synthesis (quinoline) 398 Pictet-Gams synthesis (isoquinoline) 414 Pictet-Spengler synthesis (isoquinoline) 415 Piloty-Robinson synthesis (pyrrole) 119 Pinner synthesis (pyrimidine) 467 Plancher rearrangement 128 Polonovski reaction 545 Pomeranz-Fritsch synthesis (isoquinoline) 415 Prileschajew reaction 23 Prins reaction 452 Pummerer rearrangement 26, 457... 

---