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

In 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.

Automated summary

This study investigated the reactivity of 6-iminophosphoranepyrimidines with DMAD and ethyl propiolate as dienophiles. Pyrimidine derivatives with a viable 2-azadienic moiety can undergo [4+2]/retro-[4+2] and [2+2]/retro-[2+2] reactions, while those without this structure can only undergo [2+2]/retro-[2+2] reactions involving the phosphazene moiety.