The synthesis of 9-substituted N-benzyl-8-azabicyclo[4.3.0]non-4-en-7-ones by the intramolecular Diels–Alder reaction

Abstract

The intramolecular thermal cyclisation of N-allyl-N-benzylpenta-2(E), 4-dienamide in refluxing NN-dimethylformamide gives N-benzyl-cis- and trans-8-azabicyclo[4.3.0]non-4-en-7-one; N-allyl-N-benzylhexa-2(E), 4(E)-dienamide gives (1RS, 3RS, 6SR)-3-methyl- and (1RS, 3RS, 6RS)-3-methyl-N-benzyl-8-azabicyclo[4.3.0]non-4-en-7-one. 9-Benzyl- and 9-isobutyl-N-benzyl-8-azabicyclo[4.3.0]-non-4-en-7-one are prepared by the analogous intramolecular thermal cyclisation of the appropriate α-substituted N-allylpenta-2 (E), 4-dienamides. The major products all have cis-fused rings, but all the possible configurations are formed in each case, including the products with trans-fused rings. The relative stereochemistries of the bicyclic products are assigned on the basis of their high-resolution ¹H n.m.r. spectra.