The reaction of diethyl diazomethanephosphonate (1) with cycloaliphatic thioketones (6) in THF at room temperature leads to the corresponding thiirane-2-phosphonates (7) in good yield. A reaction mechanism via 1,3-dipolar cycloaddition of the diazo compound with the C=S group to give the 2,5-dihydro-1,3,4-thiadiazole-2-phosphonate as an intermediate, which
spontaneously eliminates nitrogen is most likely. The resulting thiocarbonyl ylide undergoes a 1,3-dipolar electrocyclization to yield a thiirane. These products can be desulfurized smoothly by treatment with tris(diethylamino)phosphine to give alpha,beta-unsaturated phosphonates.