**Ke-Jie Xiong and Feng-Pei Du \***

Department of Applied Chemistry, College of Science, China Agricultural University, Beijing 100193, China; kejieailan@163.com

**\*** Correspondence: dufp@cau.edu.cn; Tel.: +86-10-62732507

Received: 21 June 2020; Accepted: 4 July 2020; Published: 7 July 2020

**Abstract:** Two fenclorim derivatives (compounds **6** and **7**) were synthesized by linking active sub-structures using fenclorim as the lead compound. The chemical structures of the two compounds were confirmed by NMR spectroscopy, high resolution mass spectrometry, and X-ray diffraction analysis. Their fungicidal activity against six plant fungal strains was tested. Compounds **6** and **7** both crystallized in the monoclinic system, with a P21/c space group (a = 8.4842(6) Å, b = 24.457(2) Å, c = 8.9940(6) Å, V = 1855.0(2) Å<sup>3</sup> , Z = 4) and Cc space group (a = 10.2347(7) Å, b = 18.3224(10) Å, c = 7.2447(4) Å, V = 1357.50(14) Å<sup>3</sup> , Z = 4), respectively. The crystal structure of compound **6** was stabilized by C–H···N and C–H···O hydrogen bonding interactions and N–H···N hydrogen bonds linked the neighboring molecules of compound **7** to form a three-dimensional framework. Compound 6 displayed the most excellent activity, which is much better than that of pyrimethanil against *Botrytis cinerea* in vivo. Additionally, compound 6 exhibited greater in vitro activity against *Pseudoperonospora cubensis* compared to that of pyrimethanil. Moreover, compound **7** exhibited strong fungicidal activity against *Erysiphe cichoracearum* at 50 mg/L in vitro, while pyrimethanil did not. Compounds **6** and **7** could be used as new pyrimidine fungicides in the future.

**Keywords:** synthesis; crystal structure; fenclorim; antifungal activity; pyrimidine
