Selenium has attracted great interest as an essential element, and certain diseases have been eradicated
by dietary supplementation of this element. Selenium is essential for cell metabolism as a component of
glutathione peroxidase and other enzyme systems. Current interest lies in the prevention of certain cancers by
supplementation with selenium [1–4]. One proposed mechanism for this activity is the cytotoxic effect of
selenium on tumor cells [5, 6]. Organoselenium compounds continue to attract the attention of many
investigators owing to a series of unique properties. There are data showing that organoselenium compounds are
capable of sensitizing processes in the living organism, and there are four known naturally occurring proteins
containing the selenium atom. Therefore, despite the high toxicity of selenium compounds [7–9], many of them
have been prepared as and anticancer agents [10–15] and for other medicinal effects like antiviral [16–18],
antimicrobial [19–22], and fungicidal [23–25] activities. Heterocyclic annelated pyridazines also attract
considerable attention, which arises mainly from the large variety of interesting pharmacological, herbicidal,
insecticidal, and fungicidal activities [26–30]. In the last two decades we have been involved in a program aimed
at developing new synthesis of heterocyclic systems of anticipated biological and/or pharmacological effects
[31–37]. In continuation of this program, some selenium-containing heterocyclic compounds were required for
evaluation as biodegradable agrochemicals. The starting compound 4-cyano- 5,6-diphenylpyridazine-3(2H)-
selenone 1, which was prepared as previously described [38] seemed suitable to fulfill this objective.