Piperazine-2,5-diones, 3,6-disubstituted

Some unusual zwitterionic pyrazines have been prepared by dehydrogenation of 1,4-disubstituted piperazine-2,6-diones. Honzl et al. (476a) prepared theanhydro-2,6-dihydroxy-l, 4-diphenyl-3,5-bis(phenylthio)pyrazinium dihydroxide [s/c] (92) (which yields adducts by dipolar cycloaddition of maleic anhydride or formaldehyde) by the reaction of 1,4-diphenylpiperazine-2,6-dione with benzenesulfonyl chloride in pyridine. Tanaka et al. (476b), from I,4-diphenylpiperazine-2,6-dione with benzoyl chloride, and tosyl chloride in pyridine at reflux, obtained the anhydro - 3 - benzoyl - 2,6 -dihydroxy -1,4 -diphenyl - 5 - (p - toly lthio)pyrazinium dihydroxide [src] (93), together with some of the S-p-tolyl analogue of (92). 

Scheme 23 Synthesis of 3,6-disubstituted-piperazine-2,5-diones promoted by complex 28...

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