Thiol-promoted intermolecular cyclization to synthesize 1,2,4-oxadiazoles including tioxazafen under transition metal-free conditions

A simple and efficient one-pot intermolecular annulation reaction for the synthesis of 1,2,4-oxadiazoles from amidoximes and benzyl thiols has been developed, in which benzyl thiols act as not only reactants but also organo-catalysts. The control experiments proved that thiol substrates could facilitate the dehydroaromatization step. High yield, functional group diversity and transition metal-free, extra oxidant-free, and mild conditions are the important practical features. Moreover, this protocol provides an effective alternative method for the synthesis of a commercially available broad-spectrum nematicide, tioxazafen.

Unusual Formation of 1,2,4-Oxadiazine Core in Reaction of Amidoximes with Maleic or Fumaric Esters

We have developed a simple and convenient method for the synthesis of 3-aryl- and 3-hetaryl-1,2,4-oxadiazin-5-ones bearing an easily functionalizable (methoxycarbonyl)methyl group at position 6 via the reaction of aryl or hetaryl amidoximes with maleates or fumarates. The conditions for this reaction were optimized. Different products can be synthesized selectively in good yields depending on the base used and the ratio of reactants: substituted (1,2,4-oxadiazin-6-yl)acetic acids, corresponding methyl esters, or hybrid 3-(aryl)-6-((3-(aryl)-1,2,4-oxadiazol-5-yl)methyl)-4H-1,2,4-oxadiazin-5(6H)-ones. The reaction is tolerant to substituents’ electronic and steric effects in amidoximes. As a result, a series of 2-(5-oxo-3-(p-tolyl)-5,6-dihydro-4H-1,2,4-oxadiazin-6-yl)acetic acids, their methyl esters, and 1,2,4-oxadiazoles based on them were prepared and characterized by HRMS, ¹H, and ¹³C NMR spectroscopy. The structures of three of them were elucidated with X-ray diffraction.