On the optical rotation of 1 (or 3)-stearoyl-sn-glycerol

Small specific rotation is one of the setbacks in the studies of 1 (or 3)-acyl-sn-glycerols, which makes assessment of the optical purity/establishment of the identity of new compounds difficult. Some unusually large data reported for 3-stearoyl-sn-glycerol in the literature suggested that use of low sample concentrations in recording optical rotations might alleviate the long-existing (though probably ‘hidden’ to those, who are not working in this area) problem. To verify the astonishing data and to explore the potential cause(s) for the unexplained phenomenon, a systematic study was performed. The results revealed that the ‘amplifying’ effect at low concentrations did not exist. And those seemingly ‘flawless’ literature data were proven to be artefacts. The sign of specific rotation for 3-stearoyl-sn-glycerol was shown to be changeable with dilution, providing a warning for the risk in assignment of absolute configurations of similar compounds on the basis of a single point optical rotation data.     

Biotransformation of (+)-(1R,2S)-fenchol by the larvae of common cutworm (Spodoptera litura)

Biotransformation of (+)-(1R,2S)-fenchol by the larvae of Spodoptera litura was carried out. Substrate was converted to three new terpenoids, (+)-(1R,2S)-10-hydroxyfenchol, (+)-(1R,2R,3S)-8-hydroxyfenchol and (−)-(1S,2S,6S)-6-exo-hydroxyfenchol, and one known terpenoid, (−)-(1R,2R,3R)-9-hydroxyfenchol. These structures were established by NMR, IR, specific rotation and mass spectral studies.     

Towards the development of oxadiazinanones as chiral auxiliaries: synthesis and application of N3-haloacetyloxadiazinanones

Oxadiazinanones derived from (1R,2S)-ephedrine and (1R,2S)-norephedrine were employed in the asymmetric α-halo aldol reaction. The optimized yields and diastereoselectivities for the ephedrine based oxadiazinanone aldol reaction ranged from fair to good. The ephedrine based aldol adducts were hydrolyzed to afford the α-bromo-β-hydroxycarboxylic acids. The absolute stereochemistry and enantiomeric purity of these products were determined by chiral HPLC and specific rotation measurements.     

Radioracemization and radiation-induced chiral amplification of chiral terpenes measured by optical rotatory dispersion (ORD) spectroscopy

For the first time the radioracemization of α(+)pinene and α(−)pinene, of turpentine and of R(−)-α-phellandrene has been studied by optical rotatory dispersion (ORD) spectroscopy. For all these compounds, the radioracemization implies a shift of the ORD curves toward lower levels of specific optical rotation. The radioracemization degree (R_R) has been defined and calculated for all the compounds studied. It has been found that for radiation dose of 1 MGy the radioracemization degree is about 4.5% for the compound with the highest optical purity and reaches 7–8% for the less optically pure compounds, demonstrating that impurities can affect greatly the radioracemization.In contrast with the general radioracemization effect exerted by high-energy radiation on chiral molecules, β(−)pinene, β(+)pinene when irradiated show an increment of their specific optical rotation. This fact has been measured for the first time by ORD spectroscopy and the amplification degree of chirality can reach 1000% in the near UV. This phenomenon is due to the formation of a chiral polymer, poly-β-pinene, which forms a solution with the monomer enhancing its optical activity. The implications for the theories of the origin of life of such unexpected phenomenon are discussed briefly.     

Chiral derivatives of titanium with terpene alcohols

A series of substituted alkoxy compounds of titanium were prepared by reactions of titanium isopropylate with 1-menthol, 1-borneol, (±)-isoborneol, and (-)-trans-3-hydroxymyrtanol. The specific rotation of the alcoholates was shown to be an additive function of the number of the terpene ligands. For titanium bornylates, the optical activity is a linear function of the degree of substitution. For trans-3-hydroxymyrtanol derivatives, an inversion of the optical rotation angle is observed along with an additive dependence of the optical activity on the degree of substitution.     

Asymmetric radical polymerization of butadiene 1-carboxylic acid

Optically active S(?)-α-phenethylammonium butadiene 1-carboxylate was prepared and polymerized in methanol, using azobisisobutyronitrile as initiator. The optical rotation, optical rotatory dispersion and circular dichroism spectra of the polymers, before and after removal of chiral amine, have demonstrated that the asymmetric induction occurred in the main chain. An asymmetric inductive polymerization mechanism is discussed.     

New chiral ligands and iron(III) complexes based on 2,6-bis(1-benzyl-4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-on-2-yl)pyridines

New ligands and their complexes with iron(III) chloride have been suggested and prepared: (R,S)-, (R,R)- and (S,S)-2,6-bis(1-benzyl-4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-on-2-yl) pyridines. Both the ligands and their complexes were characterised by ¹H and ¹³C NMR spectroscopy, optical rotation and X-ray diffraction.     

Orbital symmetry analysis of the reaction of silylenes with acetylenes and the dimerization of 1-silacyclopropenes

The addition of silylenes to acetylenes, and the dimerization of silacyclopropenes, are treated by Orbital Correspondence Analysis in Maximum Symmetry (OCAMS). The products observed in the latter reaction are consistent with an allowed pathway, a b_1g displacement involving rotation of the silacyclopropene molecules relative to one another in the transition state.     

A concise stereoselective synthesis of pterosin B

Pterosin B is a naturally occurring indanone found in bracken fern (Pteridium aquilinum) that displays a variety of interesting pharmacological properties, but for which few stereoselective syntheses exist. Herein we describe a 7-step stereoselective synthesis of (2R)-pterosin B via 6-bromo-5,7-dimethylindan-1-one whose structure was confirmed by NOE analysis and structure determination by X-ray crystallography. The hydroxyethyl chain was introduced via a Suzuki-Miyaura cross-coupling reaction. The 2-methyl group was introduced stereoselectively by methylation of a SAMP [(S)-1-amino-2-methoxymethyl)pyrrolidine] hydrazone and the chiral auxiliary was removed to produce (2R)-pterosin B. The structure of pterosin B was confirmed by specific rotation and structural determination by X-ray crystallography.     

The absolute configuration of peroxisomicines A1 and A2

Peroxisomicine A1 is a potentially antineoplastic compound isolated from the seeds of Karwinskia parvifolia. It is considered as a useful chemotype for the preparation of topoisomerase II targeted anticancer cells. Stereochemically, it is characterized by the presence of two stereocenters and a rotationally hindered and thus likewise stereogenic biaryl axis. In this contribution, the absolute configuration of peroxisomicine A1 and its epimer, peroxisomicine A2, was established by means of a five-step degradative procedure giving the respective R- and S-configured methyl 2-(2′-methyl-5′-oxotetrahydrofuryl)acetates. The configuration of the degradation product was obtained by means of optical rotation, ¹H NMR analysis using a chiral displacement reagent, and by experimental and quantum chemical circular dichroism (CD) investigations. Based on the results obtained here and considering our previous work on the relative configuration at centers versus axis of these compounds, peroxisomicine A1 resulted to be the P,3S,3′S-isomer and peroxisomicine A2 the P,3R,3′S-isomer.     

Conformational diastereoisomers of PPh3 coordinated to stereogenic metal centres as molecular optical switches

The specific rotation of (R_Fe,R,M)-4 switches sign upon epimerisation to (R_Fe,S,P)-5. X-Ray crystallographic studies suggest that inversion of the propeller configuration of the coordinated PPh₃ ligand is a major contributor to the switch of specific rotation. A simple model for predicting the conformational diastereoisomeric forms of PPh₃ is presented, suggesting future routes towards the design of molecular optical switching devices.     

Dynamic behaviour in (1′-t-butyl-2′,2′-dimethylpropyl)-π- (tricarbonylchromium)benzene

The temperature dependence of the ¹H NMR spectrum of the title compound was studied over the temperature range 0–115° C. Two conformers were detected. The conformational isomerism is caused by a restricted rotation about the bond between the alkyl group and the complexed aromatic ring. The gDG≠ value for the interconversion amounts to 16.9 kcal mol^?1, which is substantially smaller than the corresponding ΔG≠ value for the free ligand. This decrease is ascribed to additional steric strain between the alkyl and the tricarbonylchromium groups in the conformers of the complex.     

Molecular dynamics in polyethylene and ethylene-1-butene copolymer investigated by NMR methods

1H and 13C NMR spectra and 1H spin-lattice relaxation times T₁ and T_1ρ have been employed to study the structure and molecular dynamics in polyethylene and ethylene-1-butene copolymer in the temperature range from 100 to 370 K. Results are interpreted in terms of α, β and γ -relaxation, as well as methyl group rotation. The activation energies for all motions were established. The incorporation of 1-butene into ethylene chain leads to an increase of mobility in amorphous and crystalline phases as well as appearance the 13C resonance characteristic to the monoclinic structure in addition to the orthorhombic observed in both polymers. The crystallinity degree derived from T_1ρ in studied polymers is close to that determined using DSC method.     

Condensation of 1-alkoxy-1,3-butadienes with optically active glyoxylic acid esters

The condensation of 1-alkoxy-1,3-butadienes with optically active esters of glyoxylic acid leading to esters of 2-alkoxy-5,6-dihydro-2H-pyran-6-carboxylic acids was studied and the dependence of asymmetric induction on the following parameters examined: alkoxy group in dienes, dissymetric group in glyoxylates, solvent and the temperature of condensation. The optical yield and absolute configuration of the adducts were determined by optical rotation measurements of methyl malate obtained by degradation. The data are rationalized by postulating parallel formation of four active complexes corresponding to a transoid and cisoid conformation of the glyoxylate ester in the transition state.     

Rotation and inversion in nitrosamines

Geometry optimizations of the ground states as well as of the transition states for internal rotation and inversion have been performed by the semiempirical MNDO method for dimethyl nitrosamine (1), perfluordimethyl nitrosamine (2), N-nitroso aziridine (3), and N-nitroso azetidine (4). It was found that the potential barrier to internal rotation about the N-N bond is always of lower energy than that to inversion on the nitroso nitrogen.While the ground states tend to adopt structures which enable mesomerism, the lowest transition state is characterized by a pyramidal sp³-hybridized amino nitrogen. In accordance with experimental results the low barriers to rotation of 2 (7.96 kcal mol^?1), 3 (3.38 kcal mol^?1) and 4 (9.97 kcal mol^?1) in comparison with 1 (12.54 kcal mol^?1) indicate that in donor-acceptor molecules the transfer of charge can be limited by electronic and stereochemical effects. In particular, the equivalence of the α-methylene hydrogens which was observed in the NMR-spectrum of 3 is due to unhindered rotation and ring inveirsion.     

Assignment of the absolute configuration of (−)-linarinic acid by theoretical calculation and asymmetric total synthesis

The specific rotation of (−)-linarinic acid calculated using Hartree-Fock and density functional theory on 14 representative conformers of this molecule were all negative. The most stable conformer was found to be almost identical to the crystallographic structure. These results are consistent with the assumption that (−)-linarinic acid has a (1S)-configuration. An asymmetric total synthesis with starting products chosen on the basis of the theoretical calculations was found to corroborate the theoretical assignment. On the basis of the theoretical calculations of specific rotations and a total asymmetric synthesis, the absolute configuration of the natural product (−)-linarinic acid was determined to be 1S.     

Synthesis and stereochemical determination of an antifeeding bisabolanoid from Japanese cedar

The first enantioselective synthesis of (1S,3R,6R)-1-hydroxy-7(14),10-bisaboladien-4-one, a potent antifeedant isolated from the Japanese cedar, Cryptomeria japonica, was achieved starting from methyl (R)-4-hydroxy-3-methylbutanoate via a stereoselective carbonyl ene cyclization reaction as the key step. Comparison of the spectral data and specific rotation of the synthetic material with those of the natural product led to unambiguous stereochemical assignment of the antifeedant as 1S, 3R, and 6R.     

A theoretical analysis of the conformational space of tris(2-methylbenzimidazol-1-yl)methane

The conformation surface of tris(2-methylbenzimidazol-1-yl)methane has been explored locating four minima (uuu, uud, udd and ddd, each one corresponding to two enantiomers, the P and the M) and seven transition states. The known experimental barrier to racemization (119 kJ mol^?1) was calculated to be about 110 kJ mol^?1 (uuu or uud stereoisomers). GIAO calculations of absolute shieldings correlate very well with ¹H and ¹³C NMR chemical shifts. Finally, the specific rotation of the four minima was calculated allowing us to identify the absolute configuration of the first eluted enantiomer.     

A new coumarin from Murraya alata activates TRPV1 channel

Abstract

One new coumarin, muralatin R, was isolated from the leaves of Murraya alata Drake (Rutaceae). Its structure was elucidated by extensive analysis of the NMR and MS data, along with the specific rotation comparison. Muralatin R was found to be capable of activating the transient receptor potential vanilloid 1 (TRPV1) channel through desensitization mechanism. The results supply reference for clarification of the therapeutic basis and mechanism of action of Murraya plants for treating psychogenic pain or somatoform pain disorders.     

Evidence for diazenyl allyl diradicals in the thermolysis of 4-alkylidene-1-pyrazolines

Secondary deuterium kinetic isotope effects indicate that the CN bond anti to the Me group on 4-ethylidene-1-pyrazoline breaks preferentially to the syn bond. It is suggested that the product determining step involves an intramolecular radical displacement of nitrogen, as well as an electrocyclic rotation of the allylic methylene groups of the diazenyl allyl diradical, to generate the cyclopropane ring concerted with the loss of nitrogen. The position of deuterium in the methylmethylenecyclopropane can best be rationalized by this latter step.