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

82 NITRILE OXIDES
are formed exclusively. When subjected to ßash-vacuum pyrolysis, the adducts
undergo thermal fragmentation, either by a retro-cleavage, or by loss of maleic
anhydride, to form products that are similar to those from reactions of acetylene
in the cycloaddition step. A concerted pathway is proposed for the pyrolytic
conversion into the “formal acetylene cycloadduct” rather than a stepwise radical
mechanism (432).
O
386
2-Thenoylcarbohydroxamoyl chloride, as precursor of 2-thienylglyoxylonitrile
oxide undergoes nucleophilic 1,3-addition with o-phenylenediamine, o-aminothiophenol
and methyl anthranilate to afford benzotriazine 387 (X = NH), benzothiadiazine
387 (X = S), and quinazoline 388, respectively. With acrylonitrile
and diethyl acetylenedicaboxylate, 1,3-dipolar cycloaddition proceeds to give
5-cyano-3-thenoylisoxazoline 389 and diethyl 3-thenoylisoxazole-4,5-dicarboxylate
390, respectively. However, nitroso derivatives of imidazo[1,2-a]pyridines
391 (X = X 1 = CH; R = H, 8-Me, 6-Cl), imidazo[1,2-a]pyrimidine 391 (X =
N, X 1 = CH, R = H), and imidazo[1,2-a]pyrazine 391 (X = CH, X 1 = N) have
been obtained in good yields by the action of 2-thenoylhydroxamoyl chloride to
2-aminopyridines, 2-aminopyrimidines and 2-aminopyrazines, respectively (433).
S
S
CO
H
N
N X
O
O
S
N
CO
HON
387 388
CO
N
O
CN
S
CO
N
O
389 390
R
ON
X 1
X
N N
391
S
O
CO2Et
CO 2Et