Categories: Synthesis of N-Heterocycles, Synthesis of O-Heterocycles >
Synthesis of 1,3-oxazoles and benzoxazoles
Recent Literature
A modular and practical synthesis of highly substituted oxazoles consists of a
sequential copper-catalyzed amidation of vinyl halides followed by cyclization
promoted by iodine. A wide variety of functionalized oxazoles and polyazoles can
be obtained in a selective manner from simple and easily accessible precursors.
R. Martín, A. Cuenca, S. L. Buchwald, Org. Lett., 2007,
9, 5521-5524.
Zn(OTf)2 catalyzed the cyclization of propargyl alcohols with
anilines and phenols in toluene at 100°C without additive and gave various
indole and benzofuran products with different structures. The cyclization of
propargyl alcohols and amides gave oxazoles. Mechanisms for the different
substituation patterns are discussed.
M. P. Kumar, R.-S. Liu, J. Org. Chem., 2006, 71, 4951-4955.
The reaction of 1-(methylthio)acetone with different nitriles in the
presence of triflic anhydride gave 2-substituted
5-methyl-4-methylthio-1,3-oxazoles in good yield. The methylthio group at
the C4 position can easily be removed with Raney nickel. 4-Methylsulfonyl
derivatives were prepared by the oxidation of the MeS group with m-CPBA.
A. Herrera, R. Martinez-Alvarez, P. Ramiro, D. Molero, J. Almy, J. Org. Chem., 2006, 71, 3026-3032.
The reaction of aldehydes and ketones, including aliphatic and aromatic ones,
with amides of α-isocyano-β-phenylpropionic acid in toluene in the presence of
lithium bromide gives 2,4,5-trisubstituted oxazoles in good to excellent yield.
G. Cuny, R. Gamez-Montano, J. Zhu, Tetrahedron, 2004, 60,
4879-4885.
Lawesson’s reagent is an efficient promoter in the solvent-free
microwave-assisted synthesis of 2-substituted benzoxazoles and benzothiazoles
from carboxylic acids and 2-aminophenol or 2-aminothiophenol, respectively.
Various aromatic, heteroaromatic and aliphatic carboxylic acids react under the
conditions developed with good yields.
J. A. Seijas, M. P. Vázquez-Tato, M. R. Carballido-Reboredo, J.
Crecente-Campo, L. Romar-López, Synlett, 2007, 313-316.
A new method for a direct, copper-catalyzed arylation of heterocycle C-H bonds
by aryl iodides allows the conversion of electron-rich five-membered
heterocycles and electron-poor pyridine oxides. The best results are obtained by
using a combination of lithium tert-butoxide as base and copper iodide as
catalyst.
H.-Q. Do, O. Daugulis, J. Am. Chem. Soc.,
2007,
129, 12404-12405.
A versatile one-pot domino acylation annulation reaction of 2-bromoanilines with
acyl chlorides in the presence of Cs2CO3, catalytic CuI,
and 1,10-phenanthroline under microwave conditions was applied to the synthesis
of benzoxazoles. These copper-catalyzed approaches complement existing
strategies for benzoxazole synthesis, which typically utilize 2-aminopheonls as
precursors.
R. D. Viirre, G. Evindar, R. A. Batey, J. Org. Chem., 2008,
73, 3452-3459.
A general method for the formation of benzoxazoles and benzothiazoles via a copper-catalyzed
cyclization of ortho-haloanilides is reported. Optimal conditions for
cyclization used a catalyst combination of CuI and 1,10-phenanthroline as
ligand. The mechanism is believed to proceed via an
oxidative insertion/reductive elimination pathway through a Cu(I)/Cu(III)
manifold. The rate of reaction of ortho-haloanilides follows the order I >
Br > Cl, consistent with oxidative addition being the rate-determining step.
G. Evindar, R. A. Batey, J. Org. Chem., 2006, 71, 1802-1808.
