3,5‐Disubstituted‐1,2,4‐oxadiazoles and 4,5‐dihydro‐3,5‐disubstituted‐1,2,4‐oxadiazoles

A series of 3,5‐disubstituted‐1,2,4‐oxadiazoles ( 2 ) were prepared from a mono‐ or dichlorophenyl‐substituted amidoxime and (i) an acid chloride, (ii) an isatoic anhydride, or (iii) a β‐keto ester. Although cyclizations of the same amidoximes with acetaldehyde gave 4,5‐dihydro‐5‐methyl‐substituted derivatives ( 5 ), that annulation procedure either failed or gave low yields with other aldehydes. A novel alternative method, the diborane reduction of 2 , has been found to be a generally applicable procedure for preparing 5 . The reduction is regioselective, i.e. , only the 4,5‐(C=N) linkage is reduced even when a large excess of diborane is present.