Synthesis and biological activity of analogs of the antifungal antibiotic UK‐2A. II. Impact of modifications to the macrocycle benzyl position

Abstract

BACKGROUND: UK-2A is an antifungal antibiotic produced by Streptomyces sp. 517-02. Derivatization of its picolinamide OH to form the isobutyryl acetal led to the discovery of fenpicoxamid (InatreqTM active), which is currently under development as a fungicide by Dow AgroSciences LLC. This paper documents efforts to achieve additional efficacy enhancements through semi-synthetic modification of the benzyl substituent of the UK-2A macrocycle.RESULTS: Of 34 analogs prepared, the most active had mitochondrial electron transport IC 50 values 1.5-to 3.7-fold higher than UK-2A (IC 50 0.86 nM). The cyclohexyl analog (38, IC 50 1.23 nM) was the most intrinsically active derivative, and inhibited in vitro growth of Zymoseptoria tritici (EC 50 2.8 ppb) and Leptosphaeria nodorum (EC 50 6.2 ppb) more strongly than UK-2A (EC 50 5.3 and 11.3 ppb for Z. tritici and L. nodorum, respectively). Heterocyclic ring systems and polar linker functionalities resulted in substantial activity loss . Several analogs (20, 22, 23, 24, 36 and 38) translated Z. tritici in vitro growth inhibition activity to in planta disease control more effectively than did UK-2A, with log D being a key factor in this regard. CONCLUSIONS: UK-2A is amenable to further modification at the benzyl position on the macrocycle, which provides opportunities for manipulation of physical properties while retaining strong intrinsic and antifungal activity.

Synthesis and biological activity of analogs of the antifungal antibiotic UK‐2A. III. Impact of modifications to the macrocycle isobutyryl ester position

“…On this basis, we focused further SAR efforts on optimization of the more synthetically accessible macrocycle substituents, notably at the picolinamide, isobutyryl ester and benzyl substituents at positions 3, 7 and 8, respectively. The results of our SAR explorations at positions 3 and 8 were reported recently, substantially extending the preliminary efforts of Usuki et al . While the majority of the picolinic acid replacement analogs prepared were significantly less active than UK‐2A, two analogs (the 2‐OH phenyl & 3‐OH‐2‐pyridyl ring replacements) were more active than UK‐2A in controlling both Z. tritici and Puccinia triticina (wheat leaf rust) in 1‐day protectant greenhouse tests .…”

“…Also contained in Table are antifungal growth inhibition data, expressed as EC 50 values, for compounds 4–33 against the fungal pathogens Z. tritici and L. nodorum, while data for two additional pathogens, P. oryzae and U. maydis , is presented as Supplementary Information (Table 1(s)). As was the case for the previously reported SAR investigations focused on the picolinamide ring and the benzyl substituent at position 8 of the UK‐2A macrocycle, Z. tritici and L. nodorum are more sensitive to growth inhibition than are P. oryzae and U. maydis, mean EC 50 values across all analogs being 28.5 and 69.4 μg L −1 for Z. tritici and L. nodorum (Table ) compared to 190.3 and 228.2 μg L −1 for P. oryzae and U. maydis (Table 1(s)).…”

“…A valine residue (Val41) located in this region does not appear to participate in any interactions with the 7‐position substituent of any of compounds 5 – 33 , presumably due to the size and openness of this exterior pocket. We showed previously that Asp230 and Phe222 residues are involved in binding interactions with the substituted pyridine ring system of UK‐2A, while Tyr16 interacts with the exocyclic benzyl at position 8 of the macrocycle, and these are also indicated in Fig. .…”

“…The cyclopentane ether 3 was accessed via hydrogenation of the vinyl ether resulting from condensation of 1 with cyclopentanone dimethyl acetal (see Supplementary Information). Removal of the t ‐butyl carbamates and amide formation with 3‐hydroxy‐4‐methoxypicolinic acid proceeded as previously described to give final targets 5–18 and 33 .…”

“…At the 8 position, replacement of the phenyl substituent with cyclohexyl was particularly well tolerated, resulting in enhanced in vitro growth inhibition of Z. tritici , Leptosphaeria nodorum (glume blotch of wheat) and Pyricularia oryzae (rice blast) despite a very slight loss of intrinsic potency (MET IC 50 value = 1.23 nM, compared to 0.86 nM for UK‐2A) . In 1‐day preventative greenhouse tests v Z. tritici and P. triticina several phenyl replacements (notably 3‐CF 3 ‐phenyl, cyclohexyl, CH 2 ‐cyclohexyl, 1‐hexenyl, 4‐phenylphenyl and 3‐phenylphenyl) resulted in analogs providing excellent disease control, the 3‐phenylphenyl derivative matching the performance of epoxiconazole against both wheat diseases …”