Stereospecific Conversion of Boronic Esters into Enones using Methoxyallene: Application in the Total Synthesis of 10-Deoxymethynolide

Enones are widely utilized linchpin functional groups in chemical synthesis and molecular biology. We herein report the direct conversion of boronic esters into enones using commercially available methoxyallene as a three-carbon building block. Following boronate complex formation by reaction of the boronic ester with lithiated-methoxyallene, protonation triggers a stereospecific 1,2-migration before oxidation generates the enone. The protocol shows broad substrate scope and complete enantiospecificity is observed with chiral migrating groups. In addition, various electrophiles could be used to induce 1,2-migration and give a much broader range of α-functionalized enones. Finally, the methodology was applied to a 14-step synthesis of the enone-containing polyketide 10-deoxymethynolide.     

Development and Validation of a GC–MS Method for the Determination of Methyl and Ethyl Camphorsulfonates in Esomeprazole Magnesium

A rapid gas chromatography–mass spectrometry method for the identification and determination of two carcinogenic impurities viz. methyl camphorsulfonate (MCS) and ethyl camphorsulfonate (ECS) in esomeprazole magnesium (EOM) is developed for the first time. The factors affecting method development and the fragmentation patterns of MCS and ECS based on the mass spectral analysis are described. The limit of detection and limit of quantitation values for both compounds were established as 3 and 10 ppm, respectively with respect to 50 mg mL^?1 of EOM. The method is linear within the range of 10–120 ppm and found to be precise, accurate, specific and robust.     

RP-LC Analysis and Hydrolytic Degradation Profile of Racecadotril

A sensitive, stability-indicating liquid-chromatographic method for analysis of racecadotril in the presence of its degradation products has been developed and validated. Efficient chromatographic separation was achieved on a C₁₈ column with a simple isocratic mobile phase—60:40 methanol–water. Quantification was by photo-diode array (PDA) detection at 220 nm. The linearity of the method was excellent over the range 1–32 μg mL^?1. The method was sensitive, with low limits of detection (20 ng mL^?1) and quantification (100 ng mL^?1). The recovery of the method was consistently good (98.7–100.9%), with low (<1%) intra-day and inter-day relative standard deviation. Robustness studies confirmed that peak area was unaffected by small changes in temperature, and mobile phase composition and flow rate. Both alkaline and acidic hydrolytic degradation were performed in methanolic solution. In alkaline medium the drug was degraded immediately; it was degraded within 90 min in acidic medium. The validated, stability-indicating, method was used for analysis of racecadotril in pharmaceutical dosage form and also to reveal the hydrolytic degradation profile of the racecadotril.     

Redox control of photoinduced electron transfer in axial terpyridoxy porphyrin complexes

The photophysical properties of axial-bonding types (terpyridoxy)aluminum(III) porphyrin (Al(PTP)), bis(terpyridoxy)tin(IV) porphyrin (Sn(PTP) 2), and bis(terpyridoxy)phosphorus(V) porphyrin ([P(PTP) 2] (+)) are reported. Compared with their hydroxy analogues, the fluorescence quantum yields and singlet-state lifetimes were found to be lower for Sn(PTP) 2 and [P(PTP) 2] (+), whereas no difference was observed for Al(PTP). At low temperature, all of the compounds show spin-polarized transient electron paramagnetic resonance (TREPR) spectra that are assigned to the lowest excited triplet state of the porphyrin populated by intersystem crossing. In contrast, at room temperature, a triplet radical-pair spectrum that decays to the porphyrin triplet state with a lifetime of 175 ns is observed for [P(PTP) 2] (+), whereas no spin-polarized TREPR spectrum is found for Sn(PTP) 2 and only the porphyrin triplet populated by intersystem crossing is seen for Al(PTP). These results clarify the role of the internal molecular structure and the reduction potential for electron transfer from the terpyridine ligand to the excited porphyrin. It is argued that the efficiency of this process is dependent on the oxidation state of the metal/metalloid present in the porphyrin and the reorganization energy of the solvent.     

Modulation of metal displacements in a saddle distorted macrocycle: synthesis, structure, and properties of high-spin Fe(III) porphyrins and implications for the hemoproteins

A rare family of five and six-coordinated high-spin Fe(III) porphyrins incorporating weak axial ligands are synthesized and structurally characterized which demonstrate, for the first time, stepwise metal displacements in a single distorted macrocyclic environment that has generally been seen in many biological systems. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin severely distorts the porphyrin geometry and provides an interesting modulation of the macrocycle properties which enables the facile isolation of "pure" high-spin Fe(III)(tn-OEP)Cl, Fe(III)(tn-OEP)(MeOH)Cl, and Fe(III)(tn-OEP)(H2O)2(+) in excellent yields in a saddle distorted macrocyclic environment that are known to stabilize intermediate spin states. The stepwise out-of-plane displacements of iron are as follows: 0.47 A for Fe(III)(tn-OEP)Cl; 0.09 A for Fe(III)(tn-OEP)(MeOH)Cl, and 0.01 A for Fe(III)(tn-OEP)(H2O)2(+) from the mean plane of the porphyrins. However, in both five and six-coordinated Fe(III) porphyrins, the Fe-Np distances are quite comparable while the porphyrin cores have expanded significantly, virtually to the same extent for the six-coordinate complexes reported here. The large size of the high-spin iron(III) atom in Fe(III)(tn-OEP)(H2O)2(+) is accommodated perfectly with no displacement of the metal. This expansion is accompanied by a significant decrease of the saddle distortion with a clear increase of the ruffling. Furthermore, the Fe atom in Fe(III)(tn-OEP)(MeOH)Cl is not out of plane because of the larger atom size; however, the displacement of the iron depends on both the relative strength of the axial ligands, as well as the nature and extent of the ring deformation. Our characterization demonstrates that increase in ruffling and/or decrease in macrocycle deformation brings the iron atom more into the plane in a distorted macrocyclic environment. Our observations thus suggest that the displacements of iron in proteins are the consequences of nonequivalent axial coordination, as well as protein induced deformations at the heme. The high-spin nature of the complexes reported here is believed to be due to the larger Fe-Np distances which then reduce substantially the interaction between iron d(x2)-y2 and porphyrin a(2u) orbital. The Fe(III)/Fe(II) reduction potential of Fe(III)(tn-OEP)Cl shows a reversible peak at large positive value (0.20 V), and no ring-centered oxidation was observed within the solvent limit (approximately 1.80 V). It is thus easier to reduce Fe(III)(tn-OEP)Cl by almost 700 mV compared to Fe(III)(OEP)Cl while oxidations are very difficult. Furthermore, the addition of 3-Cl-pyridine to Fe(III)(tn-OEP)Cl in air undergoes spontaneous auto reduction to produce the rare air-stable Fe(II)(tn-OEP)(3-Cl-py)2 that shows Fe(II)/Fe(III) oxidation peaks at high positive potential (0.79 V), which is approximately 600 mV more anodic compared to [Fe(II)(tn-OEP)Cl](-). This large anodic shift illustrates the effective removal of metal-centered electron density by the macrocycle when the metal is constrained to reside in the porphyrin plane.     

Understanding selenocysteine through conformational analysis,proton affinities,acidities and bond dissociation energies

Density functional methods have been employed to characterize the gas phase conformations of selenocysteine. The 33 stable conformers of selenocysteine have been located on the potential energy surface using density functional B3LYP/6‐31+G* method. The conformers are analyzed in terms of intramolecular hydrogen bonding interactions. The proton affinity, gas phase acidities, and bond dissociation energies have also been evaluated for different reactive sites of selenocysteine for the five lowest energy conformers at B3LYP/6‐311++G*//B3LYP/6‐31+G* level. Evaluation of these intrinsic properties reflects the antioxidant activity of selenium in selenocysteine. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Stereoselective total synthesis of (+)-cardiobutanolide

Stereoselective synthesis of styryllactone (+)-cardiobutanolide was accomplished in good overall yield from d-(-)-tartaric acid. Key features of the synthesis include the elaboration of a gamma-hydroxy butyramide obtained from the dimethylamide of tartaric acid, involving a combination of the addition of 1,3-dithian-2-yllithium and stereoselective reduction.     

Catalytic asymmetric syntheses of tyrosine surrogates

Amino acid esters 5-11 as tyrosine mimics have been synthesized in excellent enantioselectivity (up to 99.6% ee) and in good overall chemical yields. The key step in the sequence was the Burk's [Rh(COD)(2R,5R)-Et-DuPhos]BF4-catalyzed asymmetric hydrogenation of enamides with a variety of reactive functional groups.     

Stereoselective total synthesis of bioactive styryllactones (+)-goniofufurone, (+)7-epi-goniofufurone, (+)-goniopypyrone, (+)-goniotriol, (+)-altholactone, and (-)-etharvensin

Stereoselective total synthesis of biologically active styryllactones 7-epi-goniofufurone, goniofufurone, goniopypyrone, goniotriol, altholactone, and etharvensin was achieved in high overall yields from a common intermediate derived from d-(-)-tartaric acid. It is based on the utility of a masked tetrol, comprising an alkene tether and four contiguous hydroxy groups. The pivotal reaction sequence involves hydroxy-directed lactonization via the oxidation of alkene, and subsequent elaboration to styryllactones. The masked tetrol was prepared by the extension of gamma-phenyl-gamma-hydroxy butyramide, readily obtained from the bis-dimethylamide of tartaric acid, employing a combination of selective Grignard additions and a stereoselective reduction.