Besides the mentioned processes, palladium-catalyzed carbonylation of alkynols represents another powerful methodology for the preparation of lactones. As early as in 1994, Gabriele and co-workers presented a PdI 2 /KI-catalyzed oxidative carbonylation of α-substituted hydroxyalkynes to four-membered lactones in good yields under mild conditions. Later on, they extended this methodology to but-3-yn-1-ols for the synthesis of five-membered lactones (Scheme 4a). 13 Scheme 1. Selected Examples of Heterocyclic Drugs Scheme 2. Palladium-Catalyzed Carbonylative Synthesis of Lactones Scheme 3. Palladium-Catalyzed Carbonylative Synthesis of Lactones from Alkenyloxiranes Scheme 4. Palladium-Catalyzed Carbonylative Synthesis of Lactones from Alkynols Chemical Reviews Review dx.