Science of Synthesis

Spitzner, D., Science of Synthesis, (2005) 15, 123.

Pyrimidine itself is not sufficiently electrodeficient to participate in inverse-electron-demand Diels–Alder reactions.[‌548‌,‌549‌] Pyrimidines bearing electron-withdrawing and electron-donating groups, however, do undergo [4+2] cycloadditions. The dienophiles include electron-rich alkenes (enamines, ketene acetals) and alkynes (ynamines, acetylenedicarboxylates).[‌550‌,‌551‌] The [4+2] cycloaddition with enamines or ynamines leads to intermediate aminodiazabicyclo[2.2.2]octadienes or -trienes which decompose by losing both a nitrile and an amine moiety to give the pyridine derivative. Attempts to isolate the bicyclic intermediates have failed. A mixture of pyridine ‌507‌ and the benzene byproduct ‌508‌ is formed from the reaction of pyrimidine-2,4,6-triamine ‌506‌ with dimethyl acetylenedicarboxylate (‌Scheme 165‌).[‌552‌] The mechanism for the pyrimidine to pyridine ring transformation is complicated by the nature of substituents in the pyrimidine ring (e.g., nitro). 1,1-Dimorpholinoethene adds across the N1 and C4 positions of 5-nitropyrimidine (‌509‌) to provide 3-nitropyridine (‌510‌) in 57% yield (‌Scheme 165‌).[‌551‌] In contrast, N,N-diethylprop-1-yn-1-amine (‌72‌) adds across the C2 and C5 positions of pyrimidines which leads to isomeric pyridines.[‌549‌,‌553‌]

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References

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• 8.Meeeee-Meee, (8888) M 8e, 888.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.
• 8.Meeeee-Meee, (8888) M 8e8, 888.