Synthesis, characterization and bioactivity of novel 5,6-dihydropyrrolo[3,4-c]pyrazol-4- (1H)one derivatives

Introduction

Some pyrrolopyrazole ketones containing a biheterocyclic ring system are bioactive [1–4]. The substituted 4,6-dihydropyrrolo[4,3-c]pyrazol-4-ones are useful herbicides, fungicides, insecticides or intermediate products in synthetic organic chemistry [5]. The hexahydropyrrolo[3,4-c]pyrazol-6-one derivatives can be effectively applied to plants or soils for plant disease control [6]. Another series of compounds, 4,5-dihydropyrrolo[3,4-c]pyrazol-6-(2H)ones, display activity as melanin concentrating hormone receptor-1 antagonists [7].

Tenuazonic acid [5-sec-butyl-3-(1-hydroxyethylidene)pyrrolidine-2,4-dione, TeA, 3 in Scheme 1] is a naturally occurring tetramic acid with a broad spectrum of bioactivity [8–11]. It has been used as a lead compound to design and synthesize new bioactivitive derivatives [12–14]. In this work, the total synthesis of TeA is described for the first step. And then this compound was used to synthesize a series of substituted 4,6-dihydropyrrolo[3,4-c]pyrazol-4-(1H)ones 5a–h. Their structures were characterized by ¹H NMR, FT-IR, EI-MS and elemental analysis. A single crystal X-ray diffraction analysis was conducted for the selected compound 5f. The fungicidal activity of the synthesized compounds were also evaluated.

Scheme 1

Synthetic route to title compounds 5a–h.

Results and discussion

Tenuazonic acid 3 was prepared by intramolecular Lacey-Dieckmann condensation [15, 16] of the β-keto amide 2 which, in turn, was obtained from L-isoleucine methyl ester hydrochloride 1 as the starting material according to the reported method [17, 18].

The treatment of TeA (3) with hydrazine gave compound 4a which underwent cyclization to diastereomers 5a under acidic conditions (Scheme 1). In a similar way, the treatment of 3 with phenylhydrazine and its substituted analogs furnished the corresponding compounds 4b–h that underwent cyclization to diastereomeric products 5a–h upon treatment with acid. The yields of 5a–h ranged from 53% to 78%.

The structures of the final products 5a–h were confirmed by spectroscopic techniques and elemental analysis. Compound 5f was additionaly characterized by single crystal X-ray analysis (Table 1). The crystal structure is shown in Figure 1, and selected bond lengths and bond angles are listed in Table 2. The crystallographic analysis reveals the presence of the S,S-isomer (molecule A) and R,S-isomer (molecule B) in the asymmetric unit. They are connected by intermolecular hydrogen bonds N1-H1A···O1a and N1a-H1aA···O1 to form a stable dimer. This result confirms tautomerism in compounds 5a–h. The bond length of C11-N3 (1.422(4) Å) is shorter than the normal length of C-N single bond (1.49 Å), suggesting the presence of an electron density delocalization between two aromatic rings of benzene and pyrazole to form a large conjugated system. There is a π···π stacking interaction between the parallel benzene rings (Figure 2). The centroid to centroid distance is 4.0625 Å, and vertical distance is 3.6035 Å. The angle between the centroid to centroid line and benzene plane is 27.541°. The intermolecular hydrogen bonds and π···π stacking interactions make the crystal packings form a 3-D supramolecular structure.

Figure 1

Molecular structure of compound 5f with thermal ellipsoids drawn at the 50% probability level.

Dashed lines show intramolecular hydrogen bonds.

Figure 2

Packing diagram of compound 5f.

Dashed lines show intermolecular hydrogenbonds and π···π interaction.

Biological evaluation

The products 5 were evaluated for the antifungal activities in vitro against Fusarium graminearum, Botrytis cinerea, Rhizoctoniacerealis and Colletotrichum capsici using a mycelia growth inhibition technique at the concentration of 100 μg/mL [19, 20]. The results are summarized in Table 3. The data indicate that almost all compounds exhibit certain inhibitory activities against four tested plant pathogenic fungi. Compounds 5d and 5f–h show inhibitory rates of 57.1–82.5% against F. graminearum. Compounds 5c–d and 5f–h display inhibitory rates of 58.2–81.8% against B. cinerea. Compounds 5d and 5f–g demonstrate inhibitory rates of 71.2–85.7% against R. cerealis. And compounds 5d and 5f–h give inhibitory rates of 61.0–70.6% against C. capsici. It can be seen that the compounds containing substituted phenyl moiety present better bioactivity.

Experimental

Melting points were determined on a WRS-1B digital melting-point apparatus and are uncorrected. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded on a Bruker-400 spectrometer in DMSO-d₆ solution for compound 5a and CDCl₃ solution for compounds 5b–h at room temperature. FT-IR spectra (4000–400 cm^-1) were recorded on a Bruker Tensor 27 FT-IR spectrometer in KBr disks. Mass spectra (electron impact) were recorded on a GC/MS-QP2010 spectrometer using a direct injection technique. Elemental analyses were performed on a Vario EL III elemental analyzer. The progress of the reactions was routinely monitored by thin layer chromatography (TLC) on silica gel GF254 and the products were visualized with an ultraviolet lamp (254 and 365 nm). All reagents and starting materials were obtained from commercial suppliers and were used without further purification.