Acid-Free Aza Diels−Alder Reaction of Danishefsky's Diene with Imines

[reaction: see text] A highly efficient aza Diels-Alder reaction of Danishefsky's diene with imines was found to occur in methanol in the absence of any acids at room temperature to give corresponding 2-substituted dihydro-4-pyridone derivatives in high yields. This reaction can be also carried out in a three-component one-pot reaction manner. The reaction was found to proceed through a Mannich-type condensation mechanism.