A new access to phosphaindolizines by [3+2] cycloaddition of azomethine ylides onto phosphaalkynes

Abstract Azomethine ylides such as 1 and 4 in which the nitrogen atom is incorporated in a six-membered heterocyclic ring undergo regiospecific [3+2] cycloadditions with the phosphaalkynes 2a and b at 130–140 °C to furnish the phosphaindolizines 3 and 5a–c after elimination of ethyl formate or hydrogen cyanide, respectively. In contrast, dipoles of the type 6 react unspecifically with the phosphaalkyne 2a to yield the regioisomers 7 and 8 .