<p>The acid-induced hydrolysis of (<i>E</i>)-1-aryl-2-(methoxydiphenylmethyl)diazenes—N,O-ketals featuring an <i>N</i>-aryldiazenyl group—offers a convenient route for generating aryldiazenium ion intermediates at room temperature. These ions, bearing a positive charge, are highly reactive heterodienophiles. In the presence of 1,3-dienes acting as scavengers, in situ generated aryldiazenium ions are efficiently intercepted via a hetero Diels–Alder reaction, affording the corresponding [4 + 2⁺] cycloadducts—namely 1-aryl-1,2,3,6-tetrahydropyridazines—in good yields. Except for the 1-(4-nitrophenyl) derivatives, the resulting 1-aryl-1,2,3,6-tetrahydropyridazines are air-sensitive and have therefore been converted into more stable compounds: benzoylation yields 2-aryl-1-benzoyl-1,2,3,6-tetrahydropyridazines, whereas oxidation affords 1-aryl-1,6-dihydropyridazines. This communication also acknowledges pioneering reports from 1920 and 1925 on [4 + 2] cycloadditions. Notably, the 1925 paper, which employed a diazene derivative as the dienophile, is recognized as the earliest example of a hetero Diels–Alder reaction.</p> Graphical abstract <p></p>