Thiadiazole dioxides, decomposition

In certain bicyclic and polycyclic thiadiazoles the ring can be hydrolytically cleaved to an o-diamine and sulfur dioxide. This process is a reversal of the reaction pathway involved in the synthesis of thiadiazoles from o -diamines and thionyl chloride or iV-sulfinylaniline under anhydrous conditions (see Section 4.26.5.1.1), and is analogous to, but much slower than, the hydrolysis of sulfurdiimides. In contrast to (1), which steam distilled without decomposition, steam distillation of [l,2,5]thiadiazolo[3,4-c][l,2,5]thiadiazole (21) produced only oxamide. Under milder conditions (75 °C, 12 h) a mixture of the diamine (22), oxamide and elemental sulfur was obtained (75JOC2749). Similar hydrolytic instability was observed in thiadiazole rings angularly joined to an anthraquinone (70RCR923) and ben-zofuroxan (78CPB3896). 

Scheme 17. Two possible ways of decomposition of the cycloadduct between acetylene and 1,3,4-thiadiazole 1,1-dioxide.

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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