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 up to 80% over 23 examples. In the case chiral racemic allenes, 2:1 4:1 moderate 2,5-diastereoselectivity observed. Under reaction conditions, cycloadducts directly undergo rearrangement afford ...