A one-step preparation of 3,4-disubstituted β-lactones through Rh-catalyzed conjugate addition of aryl or alkenyl boronic acids to α-methylene-β-lactones is described. The operationally simple, stereoselective transformation provides a broad range of β-lactones from individual α-methylene-β-lactone templates. This methodology allowed for a direct, final-step C-3 diversification of nocardiolactone, an antimicrobial natural product.