α-Diazo phosphonates with a conjugated 1, 3-diene unit are synthesised by the Bamford- Stevens reaction (2→ 4→ 3). They undergo [4+ 2]-cycloadditions with the dienophile 5 to form the tetrahydrotriazolopyridazines 8, which possess an unchanged diazo group. In contrast, dimethyl acetylenedicarboxylate (9) reacts exclusively with the diazo dipole of 3 to yield the 3H-pyrazoles 10, which rearrange to 11 by sigmatropic PO-shifts and hydrolyse ...
