Recently, a significant progress in the transition metal-catalyzed tunable cascade reactions has been reported by the group of Prof. Pei-Nian Lin, in the title of “Tunable Cascade Reactions of Alkynols with Alkynes under Combined Sc(OTf)3 and Rhodium Catalysis”（Angew. Chem. Int. Ed. 2016, 55, 373-377; http://onlinelibrary.wiley.com/wol1/doi/10.1002/anie.201508914/abstract).

    Transition-metal-catalyzed cascade transformations of alkynols have attracted extensive interest because they enable the highly efficient synthesis of oxygen-containing heterocyclic compounds, including natural products and drugs. However, most of these reactions were catalyzed by the transition metal catalysts such as tungsten, gold, platinum, palladium, and copper catalysts and involve intramolecular cycloisomerization of alkynols together with diverse transformations, such as a Prins-type cyclization, Diels-Alder reaction, or Povarov reaction. This work successfully developed two tunable cascade reactions of alkynols with alkynes by merging Sc(OTf)3 and rhodium catalysis. In the absence of H2O, an endo-cycloisomerization/C-H activation cascade reaction provided 2,3-dihydronaphtho[1,2-b]furans in good to high yields. In the presence of H2O, the product of alkynol hydration underwent an addition/C-H activation cascade reaction with an alkyne, which led to the formation of 4,5-dihydro-3H-spiro[furan-2,1’-isochromene] derivatives in good yields under mild reaction conditions. Mechanistic studies of the cascade reactions indicated that the rate-determining step involves C-H bond cleavage and that the hydration of the alkynol plays a key role in switching between the two reaction pathways.
    This work was highlighted by Synfacts, 2016, 12, 130, in the title of “Rhodium-Catalyzed Oxidative Annulation Route to 1-Aminoindolenines”.