Hantzsch selenazole synthesis

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... 

The classical method of forming 1,3-selenazole derivatives involves a modification of the Hantzsch thiazole synthesis. For this purpose derivatives of selenocarboxamides, in place of thioamides, are allowed to react with ot-halocarbonyl compounds 16 to give the corresponding 1,3-selenazoles (17). - [Eq. (8)]. 

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 -ketoester seleno-semicarbazones were first converted to the corresponding 1-seleno-carbamoyl-3-aIkylpyrazol-5-one. These, by condensation with a-halo-genocarbonyl compounds according to the Hantzsch synthesis, formed the selenazole ring as a second step (17). 

A one pot procedure for the preparation of 1,3-sclenazoles has been reported. The method, a variation on the Hantzsch synthesis, involves the a-tosylation of ketones 90 with [hydroxy(tosyloxy)iodo]benzene followed by treatment with selenoamides to give 1,3-selenazoles 91 in moderate to high yields <00S1219>. 

Hofmann (1889LA(250)294) was the first to report the synthesis of selenazoles in a modified Hantzsch reaction in which derivatives of selenocarboxamides are allowed to react with a-chlorocarboxyl compounds. Other substituted selenazoles are prepared similary (Scheme 19). 

The modified Hantzsch synthesis first described by Hofmann <1889LA(250)294> has remained the general method for preparing 1,3-selenazoles. Depending on the substitution required in the selenazole the requisite selenocarboxamide and a-halocarbonyl derivatives are coupled (Scheme 7) <79HC(34)217>. Since a chlorooxirane is isomeric with an a-chlorocarbonyl, 2-aminoselenazoles are also formed with selenourea (Equation (6)) <81T2607>. The synthesis of a 5-quinonyl-l,3-selenazole... 

A number of selenazoles have been prepared by an extension of the Hantzsch synthesis. i8-2i7 jjj Hantzsch synthesis (see p. 357), intermediate A -selenazolines have been isolated. The first meso-ionic selenium compound has been prepared through conversion of A-phenylbenzimidoyl chloride, PhCCl=NPh, into selenobenzanilide, which, on condensation with a-bromo-phenylacetic acid followed by cyclization of the product, gives anhydro-2,3,5-triphenyl-4-hydroxyselenazolium hydroxide (120). ... 

Reference has already been made to recent reviews on selenazoles. Relatively little new work has appeared during the past two years. The Hantzsch synthesis has been adapted to the production of 2-thienyl (and furyl)selenazoles from the appropriate selenamides. - ... 

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