Beitrag zur Kenntnis der 1‐Alkyl‐3‐chlorpyrazole und der 1‐Alkyl‐3‐pyrazolone

Preparations ofC-Nitroso Compounds

“…Only electron-rich heterocyclic ring systems are subject to electrophilic attack by nitrosating agents: examples include pyrroles, , pyrazoles, antipyrine (2,3-dimethyl-1-phenylpyrazol-5-one), imidazoles, fused benzimidazoles, − 1 H -pyrrolo[1,2-a]imidazoles, , imidazo[2,1-b]thiazoles, − indolizines, , indoles, − 8-thia-1,4-diazacycl[3,3,2]azines, and thiazoles . Nitrosation of four 3,5-diamino-2 H -1,2,6-thiadiazine 1,1-dioxides with sodium nitrite and acetic acid in a DMF/water solution at 0−5 °C gives good yields of the corresponding 4-nitroso derivatives (eq 5; R = CH 2 Ph, CH 2 CH 2 Ph, Bu, Ph) …”

Preparation and Chemistry of 3/5-Halogenopyrazoles

“…Despite repeated attempts, treatment of 1,5-dimethyl-3-hydroxypyrazole with phosphorus oxybromide (Table , entry 17) failed to give the corresponding 3-bromopyrazole and between 60 and 80% of unreacted starting material was recovered . A trial with phosphorus oxychloride had to be made at 210 °C to provide an unspecified amount of the corresponding 3-chloropyrazole! The dehydrohalogenation results with 1-aryl-3-hydroxypyrazoles depicted in Table , entries 18 and 19, point out the need for high temperature or addition of diethylaniline and, in any case, led to average yields of the corresponding 3-bromopyrazoles.…”

“…In an attempt to complete early reports, ,,, Table lists examples of alkylation or arylation of nonsymmetric halogenated pyrazole substrates which often lead to a mixture of isomers. More general reviews reported the fact that the regioselectivity of alkylation or arylation of the pyrazole ring is impossible to predict. , Concerning the alkylation of 3/5-halogenated pyrazole anions, earlier work showed that the reaction had a tendency to take place on the most remote nitrogen with respect to the halogen atom .…”