Cyclopentannulation on 3-phospholenes: an expedient route to the 2-phosphabicyclo[3.3.0]octene ring system

Treatment of 1-phenyl-3-phospholene derivatives with 2 equiv. of LDA followed by quenching the metallated intermediates with 1,3-dihaloalkanes affords 2-phosphabicyclo[3.3.0]oct-3-ene derivatives in good yield. The annulation reactions are highly regio- and stereoselective and lead to the formation of exo-Ph-P substituted products exclusively. Reduction of the resulting bicyclic phosphine oxides by phenylsilane gives the corresponding phosphines with complete retention of configuration at P. Application of this annulation procedure to acyclic allylic substrates leads to the corresponding monocyclic annulation products.