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Synthesis of protected enols

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Triflate salts of several transition metal catalysts greatly faciliate the conversion of alkynes to their corresponding vinyl triflates. Products are formed in high regioselectivity under mild conditions most especially using Zn(OTf)2. Internal alkynes bearing an aryl substituent afford vinyl triflates with a modest preference for the Z-isomer. A mechanism explains the unique role of silicon in this system.
M. H. Al-huniti, S. D. Lepore, Org. Lett., 2014, 16, 4154-4157.

Poly{[4-(hydroxy)(tosyloxy)iodo]styrene} was efficient in the halotosyloxylation reaction of alkynes with iodine or NBS or NCS. The polymer reagent could be regenerated and reused.
J.-M. Chen, X. Huang, Synthesis, 2004, 1557-1558.

A highly stereoselective Rh(I)-catalyzed 1,3-acetoxyl rearrangement of 1,2-allen-3-yl carboxylates leads to 2-acetoxy-1,3(E)-alkadienes. Features of the reaction are a high catalytic efficiency, broad scope and excellent E-selectivity.
X. Zhang, C. Fu, S. Ma, Org. Lett., 2011, 13, 1920-1923.

A rhodium-catalyzed, selective intermolecular anti-Markovnikov addition of carboxylic acids to terminal alkynes gives valuable Z-enol esters. The catalyst system is applicable to a broad substrate scope and displays a wide functional group tolerance.
A. Lumbroso, N. R. Vautravers, B. Breit, Org. Lett., 2010, 12, 5498-5501.

Ruthenium complexes were successfully applied in highly regioselective Markovnikov additions of carboxylic acids to terminal alkynes, yielding valuable enol esters. Selectivity and activity could be further improved by the addition of catalytic amounts of AgOTf. A broad range of simple as well as electronically or sterically challenging substrates could be isolated in good to excellent yields with high regioselectivity and under mild reaction conditions.
J. Jeschke, C. Gäbler, H. Lang, J. Org. Chem., 2016, 81, 476-484.

A cobalt-catalyzed highly regio- and stereoselective hydro-oxycarbonylation of terminal and internal alkynes with carboxylic acids provides enol esters in high yields. A catalyst in situ generated from Co(BF4)2, a tridentate phosphine ligand, and zinc exhibits a higher reactivity than the corresponding cobalt/diphosphine complex.
J.-F. Chen, C. Li, Org. Lett., 2018, 20, 6719-6724.

PPh3AuCl/AgPF6-catalyzed hydroacyloxylation of alkynes with carboxylic acids affords the Markonikov addition products, whereas PPh3AuCl/AgOTf catalyst gives the more stable isomerized products via the Markonikov products.
B. C. Chary, S. Kim, J. Org. Chem., 2010, 75, 7928-7931.

A silver-catalyzed trans addition of carboxylic acids to ynol ethers delivers a broad range of (Z)-α-alkoxy enol esters in good yields with excellent regio- and stereoselectivity. A subsequent Ni-catalyzed selective coupling of alkenyl C-OPiv bonds of (Z)-α-alkoxy enol esters with boronic acids enables a convenient route to (E)-enol ethers.
J. Yin, Y. Bai, M. Mao, G. Zhu, J. Org. Chem., 2014, 79, 9179-9185.

The hypervalent iodine reagent PIDA in combination with a nucleophilic amine (DABCO or pyridine) induces α-functionalization of enones via umpolung of Morita-Baylis-Hillman type intermediates. Depending on the presence of additional nucleophiles, α-chloro-enones, 1,2-diketones and α-tosyloxy-enones can be obtained.
S. Arava, S. K. Santra, G. K. Pathe, R. Kapanaiah, A. M. Szpilman, Angew. Chem. Int. Ed., 2000, 59, 15171-15175.