Towards the total synthesis of amphidinolide E: an enantioselective synthesis of C12-C29 fragment

An enantioselective synthesis of the C12-C29 fragment of amphidinolide E is described. Key transformations include an intramolecular mercuriocyclization reaction, stereoselective introduction of methyl group at the C2 position, and Stille coupling for the introduction of the diene side chain.     

Self-assembly with degenerate prototropy

This work describes a rational approach for addressing the prototropy-related problems in heterocycle-based self-assembling systems by the use of degenerate prototropy. As a proof of principle, the utility of degenerate prototropy is demonstrated herein by developing heterocycle-based AADD-type self-assembling modules that exist as "single set of protameric pair (duplex)" in both solution and solid states. These self-assembling modules are quickly accessible in good yield by reacting 2-amino-5,5-disubstituted-1H-pyrimidine-4,6-diones, available in one step by the condensation of alpha,alpha-dialkyl malonates and free guanidine, with isocyanates. Evidence from NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray diffraction studies confirmed the formation of molecular duplexes. The effect of electronic repulsion in duplex formation is also investigated. Their ready synthetic accessibility, remarkably high propensity to crystal formation, and the novel property of degenerate prototropy would make these novel self-assembling molecules promising candidates for many proposed applications.     

Biodegradation of Phenol by the Yeast Candida tropicalis: An Investigation by NMR Spectroscopy

The process of phenol biodegradation by the yeast Candida tropicalis NCIM 3556 in aqueous medium was studied by ¹H, ¹³C, and DOSY NMR techniques. Samples at regular intervals were centrifuged to separate the cells, and ¹H spectral data were collected at 400 MHz. Though a gradual decrease in the concentration of phenol was observed, after an incubation period of ~8 h, formation of any intermediate products could not be detected. Experiments carried out with uniformly ¹³C-labeled phenol also failed to detect formation of any carboxylic acid intermediates during degradation. The studies indicated that the phenol was completely degraded to carbon dioxide and water in approximately 20 h. Self-diffusion coefficient measurements showed that the lifetime of phenol in the bound form is too small to impart any change in its diffusion behavior and the intermediates formed are converted to carbon dioxide and water at a very fast rate.     

Barium hexaaluminate nanowhiskers synthesized by novel sol-gel process in reverse micellar media

Barium hexaaluminate (BHA) synthesized by coupling of sol-gel process in reverse microemulsions shows a unique nanowhisker morphology and high surface area, which are retained after calcination at 1300 degrees C.     

Synthesis and RNA binding selectivity of oligonucleotides modified with five-atom thioacetamido nucleic acid backbone structures

Convenient chemical synthesis and incorporation of dithymidine and thymidine-cytidine dimer blocks connected with a five-atom amide linker N3'-CO-CH2-S-CH2 into oligonucleotides (ONs) are reported. The UV-Tm experiments for binding affinities of these mixed backbone ONs with complementary DNA and RNA sequences revealed important results such as significantly higher RNA-binding selectivity as compared with complementary DNA. NMR studies of the dimer blocks suggested a marginal increase in the N-type sugar conformations over that of the native DNA.     

Phytochemical Investigation of Artemisia pallens

Two new eudesmanolides, pallensin ( 1 ) and 4‐epipallensin ( 2 ), along with a known eudesmanolide, santonin, and a known flavone, 4‐epivalgarin, have been isolated from the aerial part of A. pallens. Their structures were established by detailed spectral studies (1D and 2D) and by comparison of their spectroscopic data with those of known compounds.     

GIAO/DFT studies on 1,2,4‐triazole‐5‐thiones and their propargyl derivatives

Density functional theory (DFT)/Becke–Lee–Yang–Parr (B3LYP) and gauge‐including atomic orbital (GIAO) calculations were performed on a number of 1,2,4‐triazole derivatives, and the optimized structural parameters were employed to ascertain the nature of their predominant tautomers. ¹³C and ¹⁵N NMR chemical shifts of 3‐substituted 1,2,4‐triazole‐5‐thiones and their propargylated derivatives were calculated via GIAO/DFT approach at the B3LYP level of theory with geometry optimization using a 6‐311++G** basis set. A good agreement between theoretical and experimental ¹³C and ¹⁵N NMR chemical shifts could be found for the systems investigated. The data generated were useful in predicting ¹⁵N chemical shifts of all the nitrogen atoms of the triazole ring, some of which could not be obtained in solution state ¹⁵N HMBC/HSQC NMR measurements. The energy profile computed for the dipropargylated derivatives was found to follow the product distribution profile of regioisomers formed during propargylation of 1,2,4‐triazole thiones. Copyright © 2013 John Wiley & Sons, Ltd.     

Concurrent display of both α- and β-turns in a model peptide

This article describes a model peptide that concurrently displays both α- and β-turns, as demonstrated by structural investigations using single crystal X-ray crystallography and solution-state NMR studies. The motif reported herein has the potential for the design of novel conformationally ordered synthetic oligomers with structural architectures distinct from those classically observed.     

Diastereospecific synthesis of novel [3.6.6.4.7]-fused pentacyclic β-lactams by 6-exo-trig, 7-endo-dig tandem radical cyclization

An efficient synthesis of pentacyclic β-lactams has been achieved in high yield via a novel 6-exo-trig, 7-endo-dig tandem radical cyclization.