Routes to 1,3,4 - Oxadiazol-2(\u3cem\u3e3H\u3c/em\u3e)-ones

Various 1,3,4-oxadiazol-2(3H)-ones were synthesized by reaction of the bromocarbonylhydrazine salt(35) obtained from treatment of 4-bromo-3-phenylsydnone(34) with HCl with the corresponding acid chlorides in DME. The structures were confirmed by their physical, analytical and spectral data (IR, GC/MS, 1H and 13C NMR and CHN analysis). In general these compounds were synthesized in good to excellent yields and it is planned to continue exploration of their chemical behavior as a guide to their potential antimicrobial activity. As an initial approach to the synthesis of the desired 1,3,4-oxadiazol-2(3H)-ones, the starting material, 3-phenylsydnone, was prepared in 2 steps from commercially available N-phenylglycine in good overall yield. This sydnone was then converted into the versatile intermediate, 4-bromo-3-phenylsydnone. The latter was then converted by treatment with hydrochloric acid into the bromocarbonylhydrazine salt(35), which served as a key synthon for the preparation of a wide variety of 3,5-disubstituted oxadiazolinones. With respect to the cyclization step, the bromocarbonylhydrazine salt(35) was treated with an acid chloride (alkyl version) in DME and simple work-up with ice water / extraction resulted in a facile, high yield approach to the target oxadiazolinone. However, with aryl acid chlorides the use of a subsequent base treatment was needed to remove unreacted acid chloride and provide good yields of the corresponding oxadiazolinones. With a successful avenue in hand, attempts were made to optimize the yields of the desired 1,3,4-oxadiazol-2(3H)-ones by changing several parameters such as the amount of acid chloride, the reaction temperature and the reaction time. The optimized approach was then used for the successful preparation of a number of oxadiazolinones, including one new compound, viz. 5-(3-bromophenyl)-3-phenyl-1, 3, 4-oxadiazol-2(3H)-one (49), which was obtained in 60% yield