Gewald reaction

The Gewald reaction is an organic reaction involving the condensation of a ketone (or aldehyde when R2 = H) with a α-cyanoester in the presence of elemental sulfur and base to give a poly-substituted 2-amino-thiophene.



Reaction mechanism
The reaction mechanism of the Gewald reaction has only recently been elucidated. The first step is a Knoevenagel condensation between the ketone (1) and the α-cyanoester (2) to produce the stable intermediate 3. The mechanism of the addition of the elemental sulfur is unknown. It is postulated to proceed through intermediate 4. Cyclization and tautomerization will produce the desired product (6).



Variations
In one variation of the Gewald reaction a 3-Acetyl-2-aminothiophene is synthesized starting from a dithiane (an adduct of sulfur and acetone) and the sodium salt of cyanoacetone which in itself is very unstable :