Synthesis of 4-Phosphinylpyrrolidin-3-ones via [3+2] Cycloaddition of Nitrones with Phosphinylallenes

A variety of 4-phosphinylpyrrolidin-3-ones was prepared via a [3+2] cycloaddition between aryl aldonitrones and phosphinylallenes. The products were isolated as unique 4,5-trans diastereomers, in yields between 47% and 80%, over 23 examples. In the case of chiral racemic allenes, a 2:1 to 4:1 moderate 2,5-diastereoselectivity was observed. Under the reaction conditions, the cycloadducts directly undergo a rearrangement to afford selectively the corresponding pyrrolidin-3-ones. DFT calculations provide some insights into the mechanism, involving the homolytic cleavage of the N-O bond of the cycloadduct.