Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

REACTIONS OF NITRILE OXIDES 63
several 5-substituted 3-(pyridin-2-yl)isoxazole derivatives 209 (R = C≡CSiMe3,
Ph, 2-thienyl, CH=CH2) were obtained by Pd-catalyzed coupling reactions. The
crystal structure of 2-(5-iodoisoxazol-3-yl)pyridine has been determined (368).
N
N
R = C C-SiMe3, Ph, 2-thienyl, vinyl
209
Arylethynyl(phenyl)iodonium salts, RC≡CI + Ph 4-MeC6H4SO3 − , react as
1,3-dipolarophiles with nitrile oxides R 1 CNO to afford phenyl(substituted isoxazolyl)iodonium
salts 210, which give iodoisoxazoles on reaction with nucleophiles.
The crystal structure of 210 (R = Ph, R 1 = mesityl) has been determined
(369).
R
O
N
R = Ph, 4-ClC6H4, 4-MeOC6H4; R 1 = mesityl, 2,6-Cl2C6H3
210
O
I + Ph
Syntheses of 1-aryl-3,3,3-trißuoro-1-propynes and their reactions with nitrile
oxides to give 3,5-diaryl-4-trißuoromethylisoxazoles have been carried out. In
particular, 1-(4-chlorophenyl)-3,3,3-trißuoro-1-propyne, on reaction with 4chlorobenzohydroximoyl
chloride in the presence of Et3N in PhMe, give a 45%
mixture of 211 (R = R 1 = 4-ClC6H4) and the regioisomer 212 in the ratio of 97:3,
respectively (370).
F 3C
R
O N
211
R 1
F 3C
R 1
R
Cl Cl
O N
212
The triethylamine-induced reaction of benzohydroximinoyl chlorides, precursors
of nitrile oxides, with β-trißuoromethyl-substituted acetylenic esters gives
rise to three products: 5-trißuoromethyl-4-isoxazolecarboxylates, 213 (R 1 = CF3,