Alkynes selenium dioxide

The most useful application of 3 is its use in photochemical [2+2] cycloadditions with alkenes and alkynes at the carbon-carbon double bond to afford bicyclo[4.2.0]octane-2,5-diones and bicyclo[4.2.0]oct-7-ene-2,5-diones in good to excellent yield.6 Since selenium dioxide oxidation of the resulting adducts furnishes the corresponding 3-ene-2,5-diones, diketone 3 can be regarded as a 1,4-benzoquinone equivalent leading to [2+2] cycloadducts at the carbon-carbon double bond. 

Propargylic oxidation. o,a -Dioxygenation is the normal pattern observed in oxidation of alkynes with t-butyl hydroperoxide catalyzed by selenium dioxide. The reactivity sequence is CHj = CH > CHj. Alkynes with one CH2 and one CH group afford cnynones as the major product. ... 

I2 DMSO, " MeRe03/H202, " as well as by electrooxida-A ruthenium complex with a small amount of trifluoroacetic acid converts internal alkynes to the a-diketone. Ozone generally oxidizes triple-bond compounds to carboxyhc acids (19-9), but a-diketones are sometimes obtained instead. Selenium dioxide (Se02) with a small amount of H2SO4 oxidizes alkynes to a-diketones as well as arylacetylenes to a-keto acids (ArC=CH ArCOCOOH). " ... 

Other important cycloelimination procedures correspond to an elimination of H2O from cyclic ketones. Thus, the a-hydrogen atoms of semicarbazones of cyclic ketones are removed by oxidative cyclization with thionyl chloride or selenium dioxide (Scheme 8-7). The 1,2,3-thiadiazoles (71) or 1,2,3-selenadiazoles (72) which result from these reactions can be cleaved in a second step to yield cyclic alkynes (Scheme 8-8) [28]. Several fragmentation conditions are known, among them thermal decomposition and base-induced cleavage. The mechanism of these reactions has been studied in detaU [29]. It has been noted that the crucial step is the cleavage of the carbon-sulfur or carbon-selenium bond, as in this step the geometrical strain is introduced into the system. Clearly, due to the weakness of the C —Se... 

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