Reactions of 5-substituted 3-alkyl- and 3-aryl-isoxazoles with tetrasulfur tetranitride antimony pentachloride complex (S4N4•SbCl5):: complete regioselective formation of 4-substituted 3-acyl- and 3-aroyl-1,2,5-thiadiazoles and their mechanism of formation

The reactions of 3-alkyl-5a-f, 3-aryl-5g-m, 3-acylamido-5n,p, 3-benzamido-5o and 3-arylamino-5q-5-alkyl- and -5-aryl-isoxazoles with tetrasulfur tetranitride antimony pentachloride complex (S4N4. SbCl5) in toluene at 90 degrees C to reflux temperature give 3-acyl-6a-c, e, n-q and 3-aroyl-4-substituted-1,2,5-thiadiazoles 6d, f-m in 13 to 61% yields as single isomers, The same reactions of 3,4-dimethyl-5s, 5v, 4-ethyl-3-methyl-5t-5-alkyl- and/or 5-aryl-isoxazoles under the same conditions give 3-(1-acetyl-1-chloroethyl)-8a, 3-(1-benzoyl-1-chloropropyl)-8b, 3-(1-benzoyl-1-chioroethyl)-8d, or 3-(1-benzoyl-1-chloroethyl)-4-methyl-1,2,5-thiadiazoles 8c, which are a new type of 1,2,5-thiadiazole derivatives. In addition the reactions with 5-aryl-4-bromoisoxazoles (5,w,y,z) having an: electron-donating substituent such as methyl, 4-methylphenyl, and 4-methoxyphenyl groups at C3 under the same conditions afford 3-aroyl-4-substituted-1,2, 5-thiadiazoies 6d, 6j and 6k, whereas the starting isoxazoles are recovered from the reactions with 5-aryl-4-bromoisoxazoles 5x,z' having a phenyl or a 4-chlorophenyl group at C3, A plausible mechanism is proposed for the formation of the products.