Nuclear magnetic resonance spectroscopy. Use of carbon-13 spectra to establish configurations of oximes

E/Z configurational determination of oximes and related derivatives through quantum mechanics NMR calculations: scope and limitations of the leading probabilistic methods

Oximes and related derivatives featuring a CN double bond are important in many areas of chemistry. Different methods for the determination of the E/Z configuration have been developed, each with its own scope and limitations. While some cannot be used when only one isomer is available, others require special NMR experiments. Here, three popular computational methodologies (DP4, DP4+, and ML-J-DP4) have been thoroughly studied using a challenging test set. Although DP4+ provides the best confidence, its computational cost might be high. On the other hand, ML-J-DP4 shows excellent performance in most cases in a fraction of CPU time. A detailed analysis of the structural factors affecting the NMR prediction and sense of the assignment is also provided.

2D-QSAR and CoMFA Models for Antitubercular Activity of Scalarane-Type Sesterterpenes

A series of scalarane sesterterpenes were prepared using heteronemin (1) as a primary precursor. Combined with the scalarane derivatives obtained from natural sources, a total of 22 antitubercular scalaranes were used to build QSAR models based in the 2D-QSAR and CoMFA approaches. Both models indicated the influences of substitutions in the vicinity of C-12 and C-16 of the scalaranes. A 2D-QSAR model suggested the necessity of hydrophilic functionalities on the peripherals with hydrophobic cores, and the lowering steric repulsion to improve the potential energy. This was complemented by the pictorial CoMFA model, which indicated the importance of the positive electrostatic with shortened steric extension crowning over C-12 and the lengthy negative functionalities extended from C-16.

Oxy-imino saccharidic derivatives as a new structural class of aldose reductase inhibitors endowed with anti-oxidant activity

Aldose reductase is a key enzyme in the development of long term diabetic complications and its inhibition represents a viable therapeutic solution for people affected by these pathologies. Therefore, the search for effective aldose reductase inhibitors is a timely and pressing challenge. Herein we describe the access to a novel class of oxyimino derivatives, obtained by reaction of a 1,5-dicarbonyl substrate with O-(arylmethyl)hydroxylamines. The synthesised compounds proved to be active against the target enzyme. The best performing inhibitor, compound (Z)-8, proved also to reduce both cell death and the apoptotic process when tested in an in vitro model of diabetic retinopathy made of photoreceptor-like 661w cell line exposed to high-glucose medium, counteracting oxidative stress triggered by hyperglycaemic conditions.

Discovery of Cryptic Largimycins in Streptomyces Reveals Novel Biosynthetic Avenues Enriching the Structural Diversity of the Leinamycin Family

Largimycins are hybrid nonribosomal peptide-polyketides that constitute a new group of metabolites in the leinamycin family of natural products displaying unique structural features such as containing an oxazole instead of a thiazole ring or being oxime ester macrocycles, unprecedented in nature, rather than macrolactams. Their discovery in Streptomyces argillaceus and Streptomyces canus has relied on the activation of two homologous silent gene clusters by overexpressing a transcriptional activator and cultivating in specific media. The proposed biosynthesis of largimycins includes the key action of the oxidoreductase LrgO, responsible for the formation of the oxime group involved in macrocyclization, and two putative cryptic biosynthetic steps consisting of chlorination of l-Thr by the NRPS loading module and incorporation of an olefinic exomethylene group by LrgJ PKS. The discovery of largimycins uncovers novel biosynthetic avenues employed in nature to enrich the structural diversity of leinamycins and provides tools for combinatorial biosynthesis.

Minor constituents of essential oils and aromatic extracts. Oximes derived from natural flavor and fragrance raw materials - Sensory evaluation, spectral and gas chromatographic characteristics

A small library of 57 low molecular weight oximes was prepared from fragrant aldehydes and ketones, and their olfactory profiles were determined. The most substantive and interesting in terms of the sensory impressions were (+)-isomenthone oxime (fresh, musty, green) and cyclocitral oxime (earthy with patchouli, moss and leather notes). The linear retention indices (LRI) were determined for DB-1, DB-5 and DB-WAX columns, and E/Z isomers of 22 out of 57 compounds were resolved on the DB-5 column. Attempts were made to resolve enantiomers of the chosen oximes on chiral GC columns. The best results were obtained by using a Cyclosil B column, on which the enantiomers of camphor, menthone, piperitone and carvone oximes were fully resolved. NMR and MS spectra were acquired to characterize the synthesized library. Gas chromatography-olfactometry was used to assess odoriferous properties of both isomers of oximes. In most cases both isomers possessed similar profile and intensity.

Tandem double acylation/[3,3]-rearrangement of aliphatic nitro compounds: a route to α-oxygenated oxime derivatives

A new tandem double acylation/rearrangement reaction of nitro compounds is described. It has a broad substrate scope allowing the mild and efficient synthesis of α-acyloxy oxime esters in high yields and regioselectivity. According to the obtained data, the mechanism for transformation was proposed. The utility of the obtained α-hydroxy oxime esters was demonstrated.

Synthesis and In Vitro Antitumor Activity of Naringenin Oxime and Oxime Ether Derivatives

Naringenin is one of the most abundant dietary flavonoids exerting several beneficial biological activities. Synthetic modification of naringenin is of continuous interest. During this study our aim was to synthesize a compound library of oxime and oxime ether derivatives of naringenin, and to investigate their biological activities. Two oximes and five oxime ether derivatives were prepared; their structure has been elucidated by NMR and high-resolution mass spectroscopy. The antiproliferative activity of the prepared compounds was evaluated by MTT assay against human leukemia (HL-60) and gynecological cancer cell lines isolated from cervical (HeLa, Siha) and breast (MCF-7, MDA-MB-231) cancers. Tert-butyl oxime ether derivative exerted the most potent cell growth inhibitory activity. Moreover, cell cycle analysis suggested that this derivative caused a significant increase in the hypodiploid (subG1) phase and induced apoptosis in Hela and Siha cells, and induced cell cycle arrest at G2/M phase in MCF-7 cells. The proapoptotic potential of the selected compound was confirmed by the activation of caspase-3. Antioxidant activities of the prepared molecules were also evaluated with xanthine oxidase, DPPH and ORAC assays, and the methyl substituted oxime ether exerted the most promising activity.

Addition reactions of Acetylenic Esters to 6,7-Dihydrobenzo[B]Furan-4(5H)-One, 6,7-Dihydroindol-4(5H)-One, 5,6-Dihydrobenzo[B]Furan-7(6H)-One and 5,6-Dihydroindol-7(6H)-One Ketoximes. Formation of Reduced Furo[G]- and Pyrrolo[G]-Indoles

Thermal rearrangement of 6,7-dihydrobenzo[ b]furan-4(5 H)-one and 4,5,6,7-tetrahydroindol-4-one 4(7)- O-( E)-(1,2-dimethoxycarbonylvinyl)ketoximes gave 4,5-dihydrofuro[2,3 g]- and 4,5-dihydropyrrolo[2,3 g]- and [3,2- g] indoles, three novel tricyclic systems

Assignment of oxime and hydrazone configuration using 1 H-15 N and 13 C-15 N coupling measurements at natural abundance

Measurement of ¹ H-¹⁵ N and ¹³ C-¹⁵ N coupling constants at natural abundance is demonstrated to be a reliable and generic method to determine the configuration of oximes, hydrazines, and related systems. Data on ¹ H-¹⁵ N and ¹³ C-¹⁵ N coupling constants on a variety of systems obtained at natural abundance confirm the geometric dependence of the measured ¹ H-¹⁵ N and ¹³ C-¹⁵ N coupling constants. In addition, we summarize a simple "decision-tree" for determining configuration based on practical considerations of sample quantity, solubility, and complexity. Copyright © 2016 John Wiley & Sons, Ltd.

Axially Chiral Enamides: Substituent Effects, Rotation Barriers, and Implications for their Cyclization Reactions

The barrier to rotation around the N-alkenyl bond of 38 N-alkenyl-N-alkylacetamide derivatives was measured (ΔG(⧧) rotation varied between <8.0 and 31.0 kcal mol(-1)). The most important factor in controlling the rate of rotation was the level of alkene substitution, followed by the size of the nitrogen substituent and, finally, the size of the acyl substituent. Tertiary enamides with four alkenyl substituents exhibited half-lives for rotation between 5.5 days and 99 years at 298 K, sufficient to isolate enantiomerically enriched atropisomers. The radical cyclizations of a subset of N-alkenyl-N-benzyl-α-haloacetamides exhibiting relatively high barriers to rotation round the N-alkenyl bond (ΔG(⧧) rotation >20 kcal mol(-1)) were studied to determine the regiochemistry of cyclization. Those with high barriers (>27 kcal mol(-1)) did not lead to cyclization, but those with lower values produced highly functionalized γ-lactams via a 5-endo-trig radical-polar crossover process that was terminated by reduction, an unusual cyclopropanation sequence, or trapping with H2O, depending upon the reaction conditions. Because elevated temperatures were necessary for cyclization, this precluded study of the asymmetric transfer in the reaction of individual atropisomers. However, enantiomerically enriched atropsiomeric enamides should be regarded as potential asymmetric building blocks for reactions that can be accomplished at room temperature.

Synthesis of 17-membered azalides from a 16-membered macrolide utilizing amide-selective silane reduction

Novel 17-membered azalides of N-methylated amine type were synthesized and their antibacterial activity was evaluated.

Chemical Constituents and Insecticidal Activities of the Essential Oil ofCinnamomum camphoraLeaves againstLasioderma serricorne

During our screening program for agrochemicals from Chinese medicinal herbs and wild plants, the essential oil ofCinnamomum camphoraleaves was found to possess strong fumigant and contact toxicity againstLasioderma serricorneadults with LC50/LD50values of 2.5 mg/L air and 21.25 μg/adult, respectively. The essential oil obtained by hydrodistillation was investigated by GC and GC-MS. The main components of the essential oil were identified to beD-camphor (40.54%), linalool (22.92%), cineole (11.26%), and 3,7,11-trimethyl-3-hydroxy-6,10-dodecadien-1-yl acetate (4.50%). Bioactivity-directed chromatographic separation on repeated silica gel columns led to the isolation ofD-camphor and linalool.D-camphor and linalool showed strong fumigant toxicity (LC50= 2.36 and 18.04 mg/L air, resp.) and contact toxicity (LD50= 13.44 and 12.74 μg/adult, resp.) againstL. serricorne. The results indicate that the essential oil ofC. camphoraand its active compounds had the potential to be developed as natural fumigants and insecticides for control ofL. serricorne.

Synthesis and conformational studies of newly synthesized cis-2r,6c-distyryltetrahydro thiopyran-4-one and its oxime: comparison of experimental and theoretical NMR spectral data

The cis-2r,6c-distyryltetrahydro thiopyran-4-one (4) was synthesized by the reaction of dicinnamylacetone with hydrogen sulphide. cis-2r,6c-distyryltetrahydro thiopyran-4-one oxime (5) was synthesized via oximination of 4. The synthesized compounds were characterized by IR, NMR spectral studies and elemental analysis. The proton and carbon chemical shift values were unambiguously assigned using two dimensional NMR (1H1H COSY, HSQC, HMBC, NOESY) spectra. 1H NMR and 13C NMR chemical shifts of 4 were also obtained by the density functional theory (DFT) using 6-311++G(d,p) basis sets and the theoretical values were compared with experimental values.

Stereospecificity of (1) H, (13) C and (15) N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on (1) H chemical shifts

In the (13) C NMR spectra of methylglyoxal bisdimethylhydrazone, the (13) C-5 signal is shifted to higher frequencies, while the (13) C-6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the (1) H-6 chemical shift and (1) J(C-6,H-6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the -CH═N- bond does not change. This paradox can be rationalized by the C-H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum-chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ((1) H-6) and (1) J(C-6,H-6) parameters. The effect of the C-H⋯N hydrogen bond on the (1) H shielding and one-bond (13) C-(1) H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The (1) H, (13) C and (15) N chemical shifts of the 2- and 8-(CH(3) )(2) N groups attached to the -C(CH(3) )═N- and -CH═N- moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8-(CH(3) )(2) N group conjugate effectively with the π-framework, and the 2-(CH(3) )(2) N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N-2- and N-8- nitrogen lone pairs to the π-framework varies, which affects the (1) H, (13) C and (15) N shieldings.

Synthesis and structure-activity relationships of novel 8a-aza-8a-homoerythromycin A ketolides

A series of novel 6-O-substituted 8a-aza-8a-homoerythromycin A ketolides was synthesized and evaluated for in vitro antibacterial activity. Key strategic elements of the synthesis include the base-induced E-Z isomerization of 3-O-descladinosyl-6-O-allylerythromycin A 9(E)-oxime followed by ring-expanding reaction of the resulting 9(Z)-oxime via Beckmann rearrangement. The ketolides showed potent activity against a variety of erythromycin-susceptible and macrolide-lincosamide-streptogramin B (MLS(B)) resistant Gram-positive and fastidious Gram-negative pathogens. The best compounds in this series overcome all types of resistance in relevant clinical Gram-positive pathogens and display in vitro activity comparable to telithromycin and cethromycin.

Pronounced stereospecificity of (1)H, (13)C, (15)N and (77)Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the (1)H and (13)C NMR spectra of 1-(2-selenophenyl)-1-alkanone oximes, the (1)H, the (13)C-3 and (13)C-5 signals of the selenophene ring are shifted by 0.1-0.4, 2.5-3.0 and 5.5-6.0 ppm, respectively, to higher frequencies, whereas those of the (13)C-1, (13)C-2 and (13)C-4 carbons are shifted by 4-5, approximately 11 and approximately 1.7 ppm to lower frequencies on going from the E to Z isomer. The (15)N chemical shift of the oximic nitrogen is larger by 13-16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the (77)Se resonance positions is revealed in the studied oxime isomers, the (77)Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the (1)H, (13)C, (15)N and (77)Se shielding constants in the energy-favorable conformation with the syn orientation of the-C=N-O-H group relative to the selenophene ring.

Comparative analysis of 13C shielding constants stereospecificity in the silicon and germanium derivatives of acetylenic aldehyde and ketone oximes based on the 13C NMR spectroscopy and GIAO calculations

In the acetylenic aldehyde oximes with substituents containing silicon and germanium, the (13)C NMR signal of the C-2 carbon of triple bond is shifted by 3.5 ppm to lower frequency and that of the C-3 carbon is moved by 7 ppm to higher frequency on going from E to Z isomer. A greater low-frequency effect of 5.5 ppm on the C-2 carbon signal and a greater high-frequency effect of 11 ppm on the C-3 carbon signal are observed in the analogous acetylenic ketone oximes. The carbon chemical shift of the C=N bond is larger by 4 ppm in E isomer relative to Z isomer for the aldehyde and ketone oximes. The (29)Si chemical shifts in the silicon containing acetylenic aldehyde and ketone oximes are almost the same for the diverse isomers. The trends in changes of the measured chemical shifts are well reproduced by the gauge-including atomic orbital (GIAO) calculations of the (13)C and (29)Si shielding constants.

Configurational Assignment of Long-Chain Alkylated Pyridinium Aldoxime Bromides by Noesy Experiments

A configurational study of long-chain N-alkylated pyridinium aldoxime derivatives and their positional isomers has been carried out by two-dimensional 1H-1H NOESY (nuclear Overhauser effect spectroscopy). Cross peak intensities were observed to be enhanced with increasing mixing time. Mixing times longer than 250 ms result in increasing contribution of spin diffusion that produces unrealistic hydrogen-hydrogen distances. The results of NOE measurements showed significant enhancement in the intensity of iminyl proton resonances on irradiation of hydroxyl proton resonances and vice versa. The chemical shift difference between hydroxyl proton resonances and iminyl proton resonances was found to be ~4 ppm for syn and ~5 ppm for anti configurations. The study reveals that these compounds exist in the E configuration i.e. the syn form in solution. The syn isomer predominates; the anti isomer amounts are 3% (2-oxime), 4% (3-oxime) and 6% (4-oxime).