PPh3-Mediated [4 + 2]- and [4 + 1]-Annulations of Maleimides with Azoalkenes: Access to Fused Tetrahydropyridazine/Pyrrolidinedione and Spiro-dihydropyrazole/Pyrrolidinedione Derivatives
摘要:
Unprecedented PPh3-mediated [4 + 2]- and [4 + 1]-annulation of maleimides with in situ formed azoalkenes have been successfully developed, affording fused tetrahy-dropyridazine/pyrrolidinedione and spiro-ihydropyrazole/pyrrolidinedione derivatives in good yields under mild reaction conditions. Maleimides serve as C2 synthons in the [4 + 2]-annulation using 1,2-dichloroethane as the solvent in the presence of 20 mol % of PPh3. With a stoichiometric amount of PPh3 in acetone, maleimides serve as Cl synthons, and the in situ formed phosphorus ylide is the key intermediate to realize this [4 + 1]-annulation.