1- -3-alkylpyrazol-5 -ones

Condensation of 2-hydrazinoseIenazoles with /3-keto esters (R,-CO-CHn-COOR) yields l- selenazol-2-yl)-3-alkylpyrazol-5-ones (Scheme 20) (34). 

MSeienazol-2-yl)-3-alkylpyrazol-5-ones. from 2-hydrazinoselenazoles, 235 table of products, 236 2- 2-Selenienyl)selenazoles by Hantzsch s synthesis, 237 nitration, 239... 

Hydrazines (of type 285 R =Me, Ph, orC6H4N02-m) are similarly converted by means of jS-keto-esters into l-(l,2,4-thiadiazol-5-yl)-3-alkylpyrazol-5-ones (286), and thence by p-nitrosodialkylanilines into the azomethine dyes (287), the absorption spectra of which have been recorded.176... 

Because of the ease of N-alkylation, there is but one report of C-alkyl-ation, namely the benzylation of N-alkylpyrazoles in the 4-position at high yield (60ZOB2942). 

The preparation of the selenazolyl-pyrazolones was then effected by a second method, in which first the pyrazolone ring and afterward the selenazole ring was formed. For this purpose /3-ketoester seleno-semicarbazones were first converted to the corresponding 1-seleno-carbamoyl-3-alkylpyrazol-5-one. These, by condensation with a-halo-genocarbonyl compounds according to the Hantzsch synthesis, formed the selenazole ring as a second step (17). 

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