Intramolecular [4+2] cycloaddition in N-allyl- and N-propargyl-α-furyl lactams

Abstract

The size of nitrogen heterocycle in N-allyl- and N-propargyl-α-furyl lactams, as well as the nature of the unsaturated substituent linked to the nitrogen atom affected the possibility of thermal intramolecular [4+2] cycloaddition between multiple bond and the furan ring. N-Allyl-γ-(α-furyl)butyrolactam was shown to be unreactive at temperatures from 140 to 230°С. Substituted δ-valero- and e-caprolactams underwent partial Diels–Alder cyclization, forming tautomeric mixtures that contained both the initial open-chain form and the cyclic form (diastereomeric 3a,6-epoxyisoindoles fused with an aza ring) in ratios between 19:81 and 55:45. N-Propargyl-α-furyl lactams did not participate in thermal IMDAF reaction regardless of the ring size and the temperature of the synthesis.