Notiz zur Reaktion von 1‐Phenyl‐2‐propin‐1‐on mit Hydrazinderivaten

Regioselective Synthesis of 1,3,5-Substituted Pyrazoles from Acetylenic Ketones and Hydrazines

“…[13][14][15] However, the regiochemical outcome of this reaction has either not been detailed or it has been reported that mixtures of regioisomers are produced. [16][17][18][19][20][21] Herein we report our own investigations which have shown that substituted pyrazoles can be prepared from acetylenic ketones and substituted hydrazines with high and predictable regioselectivity.…”

Regioselective Microwave-Assisted Synthesis of Substituted Pyrazoles from Ethynyl Ketones

“…Although the cyclocondensation of hydrazine and ethynyl ketones proceeds efficiently under well-established conditions, 2 the regioselectivity of reactions involving substituted hydrazines can be poor [2][3][4][5] and is often contradictory in the literature. 3,25,26 We set out to discover a way to control the regioselectivity of this heteroannulation process, using either traditional methods or microwave irradiation, and to effect the transformation simply and rapidly, resolving any literature ambiguities.…”

“…According to Kirmse, the 1,3-disubstituted product 3 is obtained as a single regioisomer, albeit in only 15% yield, by stirring a solution of 1 and the hydrazine hydrochloride (2•HCl) in methanol overnight (Scheme 2, a), and is reported to proceed through conjugate addition of the anilino amine followed by cyclodehydration under the reaction conditions. 25 The related cyclocondensation of ethynyl ketone 5 and phenylhydrazine (2), studied by Baldwin as a route to non-proteinogenic pyrazole amino acids, liberated the free base of the hydrazine by the addition of Na 2 CO 3 and gave an inseparable mixture of 1,3-and 1,5-regioisomers, 6 and 7 respectively, in a ratio of 1:1 and 67% yield when stirred at reflux in ethanol (Scheme 2, b). 5 These reports can be contrasted with the findings of Miller and Reiser who observed that when a solution of an electron-rich arylpropynone, such as 8, and hydrazine derivative 9 was heated at reflux in methanolic hydrochloric acid for 12 hours, both pyrazole regioisomers were obtained in equal amounts in high yield and a different mechanism was suggested.…”