An Efficient Approach to Pyrroles and N-Bridgehead Pyrroles by Propargylation/Cycloamination of 4-Amino-1-azabutadiene Derivatives

The thermal cycloamination of several propargyl-substituted 4-amino-1-azabutadienes 2 leading to 3-functionalized pyrroles 4 − 6 is described. Derivatives of pyrrolizidines 9 and indolizidines and azaazulenes 11 − 13 are synthesized directly from cyclic imines 7 and 10, respectively, in a multistep process that involves metalation with LDA, addition of a nitrile, carbanion trapping with propargyl bromide, and cycloamination. In all cases the more substituted imine nitrogen is involved in the cyclization reaction; such an experimental finding is supported by theoretical calculations.