[(1,3‐Dioxolan‐2‐ylidene)methyl]phosphonates and ‐phosphinates as [simple] Synthons in Heterocyclic SynthesisThe readily available [(1,3‐dioxolane‐2‐ylidene)methyl]phosphonates and ‐phosphinates2a–f(Scheme 1) can be transformed with amines to aliphatic ketene N,O‐and N,N‐acetales (seeScheme 2,2a→3–7). Alkanediamines yield with2a–fthe imidazolidines8a–fand the hexahydropyrimidines9a–d(Scheme 3). the oxazolidine derivatives10a–eand the thiazolidine11are accessible under special reaction conditions starting from2a, b(Scheme 4). Hydrazines react with the CN‐group‐containing ketene O,O‐acetals2a–cto the pyrazoles12a–g, whereof12a, d, ecan be cyclized to pyrazolo[1,5‐a]pyrimidines13a–d(Scheme 5). Amidines as starting materials transform2a–cin an analogous