Phosphine-catalyzed Michael additions to α-methylene-γ-butyrolactones

The highly efficient addition of phosphorus and carbon pronucleophiles to α-methylene-γ-butyrolactones (tulipalin A and arglabin) under n -Bu 3 P catalysis is reported. Kinetic experiments indicate that the unprecedentedly high reactivity of α-methylene-γ-butyrolactones results from the rigid s- cis geometry of the 1-oxa-1,3-butadiene moiety that favors generation of zwitterionic intermediate stabilized by interaction between the phosphonium center and adjacent carbonyl oxygen. The presented strategy offers an economical and practical method for functionalization of natural biologically active α-methylene-γ-butyrolactones with high levels of chemo- and stereoselectivity. Increased reactivity of α-methylene-γ-butyrolactones towards tertiary phosphines is reported; this effect is used to improve the efficiency of phosphine-catalyzed reactions.