4-Phenyl-1,2,4-triazole-3,5-dione

1

4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) is an azodicarbonyl compound. PTAD is one of the strongest dienophiles and reacts rapidly with dienes in Diels-Alder reactions. The most prominent use of PTAD was the first synthesis of prismane in 1973.

Synthesis

The compound was first synthesized in 1894 by Johannes Thiele and Otto Stange. The oxidation of 4-Phenylurazole with lead tetroxide in sulfuric acid yielded small quantities of the substance. It took until 1971 until a practical synthesis was published. The synthesis starts by combining hydrazine and diethyl carbonate. The product of this step is reacted with phenyl isocyanate to form 4-Phenyl-1-carbethoxysemicarbazide (4), which is cyclized with base to form 4-Phenylurazole (5). Oxidation with tert-Butyl hypochlorite then yields PTAD (6).

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