Lalezari reaction

Selenadiazoles 176, which are readily formed by the action of selenium dioxide on semicarbazones of a-methylene ketones , decompose thermally to acetylenes (equation 25)106 the Lalezari reaction, is widely applicable. Recent uses are illustrated by the synthesis of four medium-sized cycloalkadiynes cyclonona-l,5-diyne (177), cyclodeca-1,5-diyne (178), cycloundeca-l,5-diyne (179) and cycloundeca-l,6-diyne (180) (Scheme 5) . Strained cycloalkynes with additional double bonds, e.g. the cyclododeca-3,7,11-triene-l-yne 181, have also been obtained by the Lalezari method. Compound 181 isomerizes spontaneously to the bicyclic hydrocarbon 182 . ... 

Application of the Lalezari reaction is highlighted in the synthesis of 1,2,3-selenodiazoles 134, which serve as the precursors of a series of novel 2-aminobenzo[ ]selenophenes (see Section 5.5.4.2) (130L1744). 

Applications of Lalezari and Hurd-Mori reactions are also highlighted in the synthesis of a new class of 1,2,3-selenadiazoles 243 and 1,2,3-thiadiazoles 244 <07JHC1165>. Reaction of sulfonylacetate 241 with semicarbazide 237 gives semicarbazone 242, and oxidative cyclization of 242 with selenium dioxide in acetic acid at 60-70 °C furnishes selenadiazole 243. Compound 242 also undergoes Hurd-Mori reaction with excess thionyl chloride to give thiadiazole 244. 

The reaction of 2-aminoselenazoles (94) with ethyl propiolate was studied by Shafiee and Lalezari. In this reaction, in addition to 7//-selenazolo-[3,2-a]pyrimidin-7-ones (95), adducts 96 and 97 were formed. Compound 95 is probably formed through ring closure of the cis-isomer of 96 [Eq. (25)]. 

Shafiee, Lalezari, and Savabi have shown that 2-thioxo-l,3-thiaselenoles (157) could be synthesized from the corresponding 4-substituted-1,2,3-selenadiazoles via reaction of the selenolate with carbon disulfide at room... 

The 1,2,3-selenadiazoles without fused rings were unknown until 1969 when Lalezari et a/.50,51 described the synthesis of 4-aryl-l,2,3-selenadiazoles (106) by the selenium dioxide oxidative ring closure of the semicarbazones of acetophenones. The mechanism proposed for this reaction is shown in Scheme 12. The mechanism was verified by the isolation of diphenylurea from the oxidation of the 4-phenylsemicarbazone derivative. The alternative mechanism proposed by Meier and Voigt does not account for this finding.52... 

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