Reactivity of pyrimidinylphosphazenes with acetylenic esters: Competitive [4 + 2] and [2 + 2] tandem cycloaddition or retro‐cycloaddition approaches

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AbstractIn the search for new intermediates for heterocyclic synthesis, the reactivity of 6‐iminophosphoranepyrimidines against dimethyl acetylenedicarboxylate (DMAD) and ethyl propiolate as dienophiles, was studied in this work. The presence of a viable 2‐azadienic moiety in pyrimidin‐4‐one rings (oxo derivatives) favored reaction of DMAD by [4 + 2]/retro‐[4 + 2] sequence, in addition to the expected [2 + 2] cycloaddition/retrocycloaddition involving phosphazene moiety. In contrast, pyrimidine derivatives lacking a viable 2‐azadienic residue reacted only through phosphazene group by the aforementioned [2 + 2]/retro‐[2 + 2] tandem process. Ethyl propiolate (non‐symmetric dienophile) proved less reactive in the study, giving rise to undesired side reactions.