to keep a possible low energy-favored structure. Therefore, bearing both substituents in two ortho positions of benzene ring would be
unfavorable for the stability of the product structure, so would be the [3+2] cycloaddition reactivity of the substrate (11). The unsuccessful
attempts of the corresponding experiments also reflected such deduction.
As shown in Table 1, most of the title compounds 12a−h exhibited apparent insecticidal activities against oriental armyworm at the
concentration of 200 μg/mL. In particular, compounds 12c and 12d possessed lethality rates of 80% and 50%, respectively. It was found
that compounds with R1 = CH3 and R2 = Cl or Br showed better insecticidal activity than others. From the fungicidal test results at 50
μg/mL shown in Table 1, we can see that most of compounds displayed weak fungicidal activity towards Pellicularia sasakii. Compound
12a held 69.2% growth inhibition against Sclerotinia sclerotiorum, better than that of chlorantraniliprole (53.0%) and triadimefon
(21.2%). Towards Botrytis cinerea, all of the title compounds possessed a higher inhibitory rate (29.2%−41.7%) than that of azoxystrobin
(12.5%), but lower than that of triadimefon (45.8%). All the title compounds had moderate activity against Fusarium oxysporum
(35.1%−45.9%), better than that of chlorantraniliprole (14.3%). Moreover, most of the compounds showed moderate to excellent
fungicidal activity towards Rhizoctonia cerealis with inhibitory rate of 43.3%−83.6%, among which 12c with 83.6% activity is more
effective than both chlorantraniliprole (77.2%) and azoxystrobin (79.1%). Noticeably, all the title compounds showed favorable or
excellent fungicidal activity against Physalospora piricola, especially 12a, 12b, 12e and 12h whose inhibition activities were 80.0%,
84.3%, 84.3% and 82.9% respectively, were more effective than azoxystrobin (77.1%) and comparable with triadimefon (84.3%). More
differently, the control chlorantraniliprole which has the same pyridylpyrazole heterocylic structural characteristic had almost no activity
towards Physalospora piricola. Through the structure-fungicidal activities analysis, compound 12a (R1 = H, R2 = H) whose fungicidal
activities against four fungi (Sclerotinia sclerotiorum, Botrytis cinerea, Physalospora piricola and Rhizoctonia cerealis) were
comparatively higher was found to have broader spectrum than others. Towards Physalospora piricola, when R1 is fixed as CH3, the
activity sequence corresponding to the R2 is H (12e) > Cl (12c) = CH3 (12f) > Br (12d), which indicates that a smaller group for R2 could
help to improve the fungicidal activity of the compound; when R2 = Cl, the R1 bearing an electron-withdrawing group (Cl (12b) or
C(=O)N(C2H5)2 (12h)) is better than the electron-donating group (CH3 (12c)). Interestingly, when R1 is fixed as CH3, the fungicidal
activity of the compounds against Rhizoctonia cerealis shows a trend of Cl (12c) > Br (12d) = CH3 (12f) > H (12e) for the substituent
R2, indicating an advantage of electron-withdrawing group; when R2 is fixed as Cl, R1 exhibits an opposite influence of electron effect
compared with the situation of that against Physalospora piricola, that is, the substituent R1 bearing an electron-donating group — CH3
(12c) is much better than electron-withdrawing groups — Cl (12b), C(=O)N(CH3)2 (12g) and C(=O)N(C2H5)2 (12h)). Among all of the
title compounds, the inhibitory rate of 12a (R1 = H, R2 = H), 12b (R1 = Cl, R2 = Cl), 12c (R1 = CH3, R2 = Cl) and 12h (R1 = C(=O)N(C2H5)2,
R2 = Cl) against the two fungi mentioned above are higher than 50%, which could be considered as fungicidal leading compounds.
Moreover, compounds 12c and 12d not only had better insecticidal activity, but also possessed relatively favourable fungicidal activities
could be made further structural optimizations for the research and development of new heterocyclic agrochemicals.
In summary, a series of novel N-pyridylpyrazole derivatives containing 1,2,4-oxadiazole moiety 12a−h were efficiently synthesized
by 1,3-dipolar cycloaddition. Their structures were identified by melting points, 1H NMR, 13C NMR and elemental analysis or HRMS.
The single-crystal structures of 12c and 12g were further analyzed, which revealed the stereochemical and substituent oriental
characteristics, and the relevance of the structure and the reaction activity of this type of compounds. The bioassay results showed that
several compounds held good insecticidal activity against oriental armyworm (Mythimna separata Walker) at 200 mg/L, particularly,
12c and 12d had a lethality rate of 80% and 50%, respectively. Some of the compounds exhibited favorable fungicidal activities at 50
μg/mL against Physalospora piricola, Rhizoctonia cereal, Sclerotinia sclerotiorum, etc. Especially, most of the compounds displayed
over 70% inhibition against Physalospora piricola. Among this 1,2,4-oxadiazole type of derivatives, 12a, 12b, 12c and 12h could be
considered as new fungicidal leading compounds for further structural optimization. These discoveries along with the structure-activity
relationship analysis in this paper will provide useful guidance for the innovative studies on new pyridylpyrazole derivatives and their
applications in agrochemical area.