Convenient Synthesis of Pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazoles via Intramolecular Nitrile Oxide Cycloaddition

A simple and efficient synthetic route to the novel 3a,4-dihydro-3H,7H- and 4H,7H-pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazole ring systems from 3-(prop-2-en-1-yloxy)- or 3-(prop-2-yn-1-yloxy)-1H-pyrazole-4-carbaldehyde oximes has been developed by employing the intramolecular nitrile oxide cycloaddition (INOC) reaction as the key step. The configuration of intermediate aldoximes was unambiguously determined using NOESY experimental data and comparison of the magnitudes of 1JCH coupling constants of the iminyl moiety, which were greater by approximately 13 Hz for the predominant syn isomer. The structures of the obtained heterocyclic products were confirmed by detailed 1H, 13C and 15N NMR spectroscopic experiments and HRMS measurements.

Poly[3-methyl-1,3-oxazolidin-2-iminium[µ3-cyanido-tri-µ2-cyanido-κ9C:N-tricuprate(I)]]

The unexpected formation of an oxazole ring has occurred during synthesis of a copper(I) cyanide network polymer as part of our ongoing studies of the structural chemistry of these networks. Crystals of the title compound were formed during the synthesis of a previously reported CuCN network solid containing protonated N-methylethanolamine and have been characterized by single crystal X-ray structure analysis. The structure shows well-defined oxazole-2-iminium cations sitting in continuous channels along the short a-axis of the crystal in a new three-dimensional copper(I) cyanide polymeric network. Evidently, a reaction has occurred between the cyanide ion and the protonated N-methylethanolamine base.

Structural study and Hirshfeld surface analysis of (Z)-4-(2-methoxybenzylidene)-3-phenylisoxazol-5(4H)-one

The title compound, C17H13NO3, adopts a Z configuration about the C=C bond. The isoxazole and methoxybenzylidene rings are almost coplanar with a dihedral angle of 9.63 (7)° between them. In contrast, the phenyl substituent is twisted significantly out of the plane of the oxazole ring, with the two rings inclined to each other by 46.22 (4)°. The crystal structure features C—H...O, C—H...N and C—H...π hydrogen bonds and π–π contacts. An analysis of the Hirshfeld surfaces points to the importance of H...H, H...C/C...H and H...O/O...H contacts. The included surface areas of the title compound were compared to those of the isomeric structure (Z)-4-(4-methoxybenzylidene)-3-phenylisoxazol-5(4H)-one [Zhang et al. (2015). CrystEngComm, 17, 7316–7322].

Crystal structure and Hirshfeld surface analysis of 4-(4-chlorophenyl)-5-methyl-3-{4-[(2-methylphenyl)methoxy]phenyl}-1,2-oxazole

In the title compound, C24H20ClNO2, the mean planes of 4-chlorophenyl, 2-methylphenyl and phenylene rings make dihedral angles of 62.8 (2), 65.1 (3) and 15.1 (2)°, respectively, with the 5-methyl-1,2-oxazole ring. In the crystal, molecules are linked by intermolecular C—H...N, C—H...Cl, C—H...π contacts and π–π stacking interactions between the phenylene groups. Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H...H (48.7%), H...C/C...H (22.2%), Cl...H/H...Cl (8.8%), H...O/O...H (8.2%) and H...N/N...H (5.1%) interactions.

Biosynthesis Gene Cluster and Oxazole Ring Formation Enzyme for Inthomycins in Streptomyces sp. Strain SYP-A7193

ABSTRACT Inthomycins belong to a growing family of oxazole-containing polyketides and exhibit a broad spectrum of anti-oomycete and herbicidal activities. In this study, we purified inthomycins A and B from the metabolites of Streptomyces sp. strain SYP-A7193 and determined their chemical structures. Genome sequencing, comparative genomic analysis, and gene disruption of Streptomyces sp. SYP-A7193 showed that the inthomycin biosynthetic gene cluster (itm) belonged to the hybrid polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) system. Functional domain comparison and disruption/complementation experiments of itm12 resulted in the complete loss of inthomycins A and B and the subsequent restoration of their production, confirming that itm12 encodes a discrete acyltransferase (AT), and hence, itm was considered to belong to the trans-AT type I PKS system. Moreover, the disruption/complementation experiments of itm15 also resulted in the loss and restoration of inthomycin A and B formation. Further gene cloning, expression, purification, and activity verification of itm15 revealed that Itm15 is a cyclodehydratase that catalyzes a straight-chain dehydration reaction to form an oxazole ring for the biosynthesis of inthomycins A and B. Thus, we discovered a novel enzyme that catalyzes oxazole ring formation and elucidated the complete biosynthetic pathway of inthomycins. IMPORTANCE Streptomyces species produce numerous secondary metabolites with diverse structures and pharmacological activities that are beneficial for human health and have several applications in agriculture. In this study, hybrid nonribosomal peptide synthetase/polyketide synthase metabolites inthomycins A and B were isolated from after fermenting Streptomyces sp. SYP-A7193. Genome sequencing, gene disruption, gene complementation, heterologous expression, and activity assay revealed that the biosynthesis gene assembly line of inthomycins A and B was a 95.3-kb trans-AT type I PKS system in the strain SYP-A7193. More importantly, Itm15, a cyclodehydratase, was identified to be an oxazole ring formation enzyme required for the biosynthesis of inthomycins A and B; it is significant to discover this catalyzation reaction in the PKS/NRPS system in the field of microbiology. Our findings could provide further insights into the diversity of trans-AT type I PKS systems and the mechanism of oxazole cyclization involved in the biosynthesis of natural products.

Evaluation of the alkali and oxidation resistance of PBO fibers

Poly( p-phenylene benzobisoxazole) (PBO) fiber, well-known for its super high strength, is a novel fiber with excellent heat resistance and flame retardancy. However, chemical stability appears to be one of its few weaknesses. In this study, PBO fibers were treated with sodium hydroxide (NaOH) and potassium permanganate (KMnO4) solutions under various conditions. Scanning electron microscopy, optical microscopy, tensile testing, Fourier-transform infrared spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis were employed to characterize the variations of its structure and properties. The results show that many longitudinal corrosion grooves appeared on the surface of PBO fibers treated with KMnO4, while only subtle microcracks occurred after treatment with NaOH. The breaking tenacity of the fiber decreased from 38.13 cN dtex−1 to 2.76 cN dtex−1 after treatment with KMnO4 for 6 h, while it remained at a higher level (27.67cN dtex−1) when treated with NaOH. After treatment with KMnO4 solution, a more obvious absorption peak appeared in the vicinity of 1448.3 cm−1, inferring an occurrence of chemical changes for oxazole ring. Moreover, the remaining mass and initial degradation temperature are significantly improved, also indicating that the cyclic or cross-linked structure is rebuilt. Furthermore, the cyclization or cross-linking of macromolecules destroyed the highly ordered structure of PBO fibers, demonstrated by acromion melting peaks at low temperature in the DSC curves. However, the aggregation and chemical structures of PBO fibers have no obvious changes after treatment with NaOH.

7a-Phenyltetrahydropyrrolo[2,1-b]oxazol-5(6H)-one

In the title compound, C12H13NO2, the pyrrolidinone moiety is almost flat while the oxazole ring adopts an envelope conformation with the carbon atom bearing the phenyl substituent as the flap: the angle between the mean planes of the fused heterocyclic rings is 45.47 (19)°. In the crystal, C—H...O and C—H...π contacts link the molecules into infinite [010] chains.