Categories: C-O Bond Formation > Synthesis of alcohols (hydroxylation) >
Synthesis of protected enols
Recent Literature
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.