Synthesis of Various Schiff Bases Containing Isoxazole Ring and Their Applications with Thioglycollic Acid and Diverse Phosphorus Reagents

A series of Schiff bases bearing isoxazole and pyrazole rings were synthesized. Application of thioglycollic acid on two selective synthesized Schiff bases afforded the corresponding thiazolidin-4-one derivatives. On the other hand, following the multicomponents one-pot Kabachnik– Fields reaction, the Schiff base generated in situ from 4-chlorobenzaldehyde and 5-methyl isoxazol-3-amine was trapped by phosphorus reagents to produce the corresponding amino phosphonates in moderate yields. However, the latter products could also be obtained in better yields (≥78%) by directly applying the dialkylphosphites to a selective synthesized Schiff base. Similarly, a series of α-aminophosphonates could be obtained from 5-chloro-3-methyl-1H-pyrazol-4-carbaldehyde, 5-methylisoxazol-3-amine, and phosphorus reagents. Moreover, applying hexaalkyl triamido phosphites to the N-(4-chlorobenzylidene)-5-methylisoxazol-3-amine in ethanol afforded methylphosphonic diamide derivatives, whereas N-((5-chloro-3-methyl-1H-pyrazol-4-yl)methylene)-5-methylisoxazol-3-amine underwent dechlorination through reaction with hexaalkyl triamido phosphites to give the respective amine derivatives.