The cycloadducts formed from the Diels−Alder reaction
of 3-amino-5-chloro-2(1H)-pyrazinones with
methyl acrylate in toluene are subject to two alternative modes of ring
transformation yielding
either methyl 6-cyano-1,2-dihydro-2-oxo-4-pyridinecarboxylates, e.g.
12a,b (loss of amine substituent), or the corresponding
3-amino-6-cyano-1,2,5,6-tetrahydro-2-oxo-4-pyridinecarboxylates,
e.g.
15a,b. From the latter compounds, 3-amino-2-pyridinones
can be generated through subsequent
loss of HCN. A mechanism accounting for the loss or retention of
the amine substituent is based
on (1) the effects observed for each reaction mode when varying the
substituents in both reaction
partners and (2) the exclusive retention of the amine substituent when
replacing toluene with the
basic solvent pyridine.