Synthesis of isoxazoles and benzisoxazoles
AuCl3-catalyzed cycloisomerization of α,β-acetylenic oximes leads to substituted isoxazoles in very good yields under moderate reaction conditions. The methodology is amenable for the selective synthesis of 3-substituted, 5-substituted or 3,5-disubstituted isoxazoles.
C. Praveen, A. Kalyanasundaram, P. T. Perumal, Synlett, 2010, 777-781.
Cycloadditions of copper(I) acetylides to azides and nitrile oxides provide ready access to 1,4-disubstituted 1,2,3-triazoles and 3,4-disubstituted isoxazoles, respectively. The process is highly reliable and exhibits an unusually wide scope with respect to both components. Computational studies revealed a nonconcerted mechanism involving unprecedented metallacycle intermediates.
F. Himo, T. Lovell, R. Hilgraf, V. V. Rostovtsev, L. Noodleman, K. B. Sharpless, V. V. Fokin, J. Am. Chem. Soc., 2005, 127, 210-216.
Various 3-substituted and 3,5-disubstituted isoxazoles have been efficiently synthesized in good yields by the reaction of N-hydroxyl-4-toluenesulfonamide with α,β-unsaturated carbonyl compounds. This strategy is associated with readily available starting materials, mild conditions, high regioselectivity, and wide scope.
S. Tang, J. He, Y. Sun, L. He, X. She, Org. Lett., 2009, 11, 3982-3985.
In a reliable procedure that can be used to access a wide range of 3-amino-5-alkyl and 5-amino-3-alkyl isoxazoles, the reaction temperature and pH were key factors that determined the regioselectivity.
L. Johnson, J. Powers, F. Ma, K. Jendza, B. Wang, E. Meredith, N. Mainolfi, Synthesis, 2013, 45, 171-173.
Enamine-triggered [3+2]-cycloaddition reactions of aldehydes and N-hydroximidoyl chlorides in the presence of triethylamine gives 3,4,5-trisubstituted 5-(pyrrolidinyl)-4,5-dihydroisoxazoles. Subsequent oxidation of the cycloadducts offers a high yielding, regiospecific and metal-free synthetic route for the synthesis of 3,4-disubstituted isoxazoles.
Q.-f. Jia, P. M. S. Benjamin, J. Huang, Z. Du, X. Zheng, K. Zhang, A. H. Conney, J. Wang, Synlett, 2013, 24, 79-84.
The sequential use of iron and palladium catalysts in an uninterrupted four-step sequence allows the synthesis of trisubstituted isoxazoles from readily available propargylic alcohols. The advantages of such a strategy are illustrated by the high overall yields and the time-saving procedure.
E. Gayon, O. Quinonero, S. Lemouzy, E. Vrancken, J.-M. Campagne, Org. Lett., 2011, 13, 6418-6421.
The direct regioselective synthesis of 3,5-disubstituted isoxazoles was achieved through a sequence involving a net bromination of an electron-deficient alkene, in situ generation of a nitrile oxide, 1,3-dipolar cycloaddition, and loss of HBr from an intermediate bromoisoxazoline. This one-pot process enables the direct synthesis of 3,5-disubstituted isoxazoles from electron-deficient alkenes.
J. Xu, A. T. Hamme II, Synlett, 2008, 919-923.
The consecutive Sonogashira coupling of acid chlorides with terminal alkynes, followed by 1,3-dipolar cycloaddition under dielectric heating of in situ generated nitrile oxides from hydroximinoyl chlorides furnishes isoxazoles in moderate to good yields in the sense of a one-pot three-component reaction.
B. Willy, F. Rominger, T. J. J. Müller, Synthesis, 2008, 293-303.
Alumino-heteroles are obtained from simple precursors in a fully chemo- and regioselective manner by a metalative cyclization. The carbon-aluminum bond is still able to react further with several electrophiles, without the need of transmetalation providing a straightforward access to 3,4,5-trisubstituted isoxazoles and 1,3,4,5-tetrasubstituted pyrazoles.
O. Jackowski, T. Lecourt, L. Micouin, Org. Lett., 2011, 13, 5664-5667.
A large number of functionally substituted 2-alkyn-1-one O-methyl oximes have been cyclized under mild reaction conditions in the presence of ICl to give the corresponding 4-iodoisoxazoles in moderate to excellent yields. The resulting 4-iodoisoxazoles undergo various palladium-catalyzed reactions to yield 3,4,5-trisubstituted isoxazoles.
J. P. Waldo, R. C. Larock, J. Org. Chem., 2007, 72, 9643-9647.
A thermally promoted cycloaddition between alkynyliodides and nitrile oxides offers excellent regioselectivity and a broad scope with respect to both starting materials. Further functionalization of the highly decorated iodoisoxazole motifs can be achieved via Suzuki cross-coupling.
J. A. Crossley, D. L. Browne, J. Org. Chem., 2010, 75, 5351-5354.
A 1,3-dipolar cycloaddition of phenyl vinylic selenide to nitrile oxides and subsequent oxidation-elimination furnished 3-substituted isoxazoles with good yields in a one-pot, two-step transformation.
S.-R. Sheng, X.-L. Liu, Q. Xu, C.-S. Song, Synthesis, 2003, 2763-2764.
The dehydration of primary nitro compounds can be performed by bases in the presence of dipolarophiles. Among the organic bases examined, DABCO gave the best results. The reaction is applicable to activated nitro compounds and to phenylnitromethane and affords isoxazoline derivatives in higher yields compared with those of other methods. The reaction, however, is not compatible with nitroalkanes.
L. Cecchi, F. De Sarlo, F. Machetti, Eur. J. Org. Chem., 2006, 4852-4860.
3,5-Disubstituted isoxazoles are regioselectively obtained in good yields by a mild and convenient one-pot, three-step procedure utilizing a copper(I)-catalyzed cycloaddition reaction between in situ generated nitrile oxides and terminal acetylenes.
T. V. Hansen, P. Wu, V. V. Fokin, J. Org. Chem., 2005, 70, 7761-7764.
Pyrazole or isoxazole derivatives are prepared by a palladium-catalyzed four-component coupling of a terminal alkyne, hydrazine (hydroxylamine), carbon monoxide under ambient pressure, and an aryl iodide.
M. S. M. Ahmed, K. Kobayashi, A. Mori, Org. Lett., 2005, 7, 4487-4489.
The reaction of various 2-alkyn-1-one O-methyl oximes with ICl, I2, Br2, or PhSeBr provided 3,5-disubstituted 4-halo(seleno)isoxazoles in good to excellent yields under mild reaction conditions.
J. P. Waldo, R. C. Larock, Org. Lett., 2005, 7, 5203-5205.
A series of 4-alkyl-5-aminoisoxazoles have been synthesized in high yield by nucleophilic addition of lithiated alkyl nitriles to (α)-chlorooximes.
M. P. Bourbeau, J. T. Rider, Org. Lett., 2006, 8, 3679-3680.
Various substituted benzisoxazoles have been prepared by a [3 + 2] cycloaddition of nitrile oxides and arynes. Both highly reactive intermediates, have been generated in situ by fluoride anion from readily prepared aryne precursors and chlorooximes. The reaction scope is quite general, affording a novel, direct route to functionalized benzisoxazoles under mild reaction conditions.
A. V. Dubrovskiy, R. C. Larock., 2010, 12, 1180-1183.
A divergent and regioselective synthesis of either 3-substituted benzisoxazoles or 2-substituted benzoxazoles from readily accessible ortho-hydroxyaryl N-H ketimines proceeds in two distinct pathways through a common N-Cl imine intermediate: (a) N-O bond formation to form benzisoxazole under anhydrous conditions and (b) NaOCl mediated Beckmann-type rearrangement to form benzoxazole, respectively.
C.-y Chen, T. Andreani, H. Li, Org. Lett., 2011, 13, 6300-6303.
Iron(II) bromide catalyzes the transformation of aryl and vinyl azides with ketone or methyl oxime substituents into 2,1-benzisoxazoles, indazoles, or pyrazoles through the formation of an N-O or N-N bond. This transformation tolerates various functional groups and facilitates access to a range of benzisoxazoles or indazoles.
B. J. Stokes, C. V. Vogel, L. K. Urnezis, M. Pan, T. G. Driver, Org. Lett., 2010, 12, 2884-2887.