Rapid and high-resolution analysis of geometric polyprenol homologues by connected octadecylsilylated monolithic silica columns in high-performance liquid chromatography

A Chromolith Performance octadecylsilyl (ODS) monolithic silica column (Merck) was compared with a conventional microparticulate ODS-bonded silica column in the high-performance liquid chromatography separation of natural polyprenols. A system comprising two connected monolithic columns afforded an equivalent separation at half the analysis time of the conventional method. Furthermore, ten connected columns achieved a tremendously high-resolution separation, in which the complicated series of homologous polyprenols with geometric isomerism were fully separated.     

Enantiodifferentiating Photodimerization of a 2,6‐Disubstituted Anthracene Assisted by Supramolecular Double‐Helix Formation with Chiral Amines

A novel 2,6‐anthrylene‐linked bis(m‐terphenylcarboxylic acid) strand ( 1 ) self‐associates into a racemic double‐helix. In the presence of chiral mono‐ and diamines, either a right‐ or left‐handed double‐helix was predominantly induced by chiral amines sandwiched between the carboxylic acid strands with accompanying stacking of the two prochiral anthracene linker units in an enantiotopic face‐selective way, as revealed by circular dichroism and NMR spectral analyses. The photoirradiation of the optically active double helices complexed with chiral amines proceeded in a diastereo‐ (anti or syn) and enantiodifferentiating way to afford the chiral anti‐photodimer with up to 98 % enantiomeric excess when (R)‐phenylethylamine was used as a chiral double‐helix inducer. The resulting optically active anti‐photodimer can recognize the chirality of amines and diastereoselectively complex with chiral amines.     

Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane system frameworks bearing multiple tuning factors: geometric and substituent effects on their Lewis acid properties

Boron complexes that contain new tridentate ligands, tris(o‐oxyaryl)methanes and ‐silanes, were prepared. These complexes had a cage‐shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open‐shaped boron compounds. The cage‐shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L?B(OC₆H₄)₃CH, L?B(OC₆H₄)₃SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage‐shaped borates had a large dihedral angle (C_ipso‐O‐B‐O) compared with open‐shaped borates. The geometric effect due to the dihedral angle means that compared with open‐shaped, the cage‐shaped borates have a greater Lewis acidity. The introduction of electron‐withdrawing groups on the aryl moieties in the cage‐shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C_ipso‐O‐B‐O. The ligand‐exchange rates of the para‐fluoro‐substituted compound B(OC₆H₃F)₃CH and the ortho‐phenyl‐substituted compound B(OC₆H₃Ph)₃CH were less than that of the unsubstituted borate B(OC₆H₄)₃CH. The ligand‐exchange rate of B(OC₆H₄)₃SiMe was much faster than that of B(OC₆H₄)₃CH. A hetero Diels–Alder reaction and Mukaiyama‐type aldol reactions were more effectively catalyzed by cage‐shaped borates than by the open‐shaped borate B(OPh)₃ or by the strong Lewis acid BF₃?OEt₂. The cage‐shaped borates with the bulky substituents at the ortho‐positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.     

NMR studies of phosphinenickel(0) complexes of ethyl methacrylate Ni(PR3)2(CH2=C(CH3)COOC2H5)

¹³C NMR data are given for a series of phosphinenickel(0) complexes of ethyl methacrylate (ema), Ni(PR₃)₂(CH₂=C(CH₃)COOC₂H₅) (PR₃ = PPh₃ (Ia), PEtPh₂ (Ib), PEt₂Ph (Ic), PMe₂Ph (Id), PEt₃ (Ie)). The olefinic carbon signals of ema shift upfield by 71.5–86.5 ppm on coordination, the magnitude of the upfield shift increasing with increase in the bacisity of the phosphine ligand. The effect of the basicity of PR₃ is discussed on the basis of the back-bonding from Ni to ema. Variable temperature¹H NMR studies reveal that the ema of Id, the complex having the least sterically demanding phosphine ligands, exchanges with free ema in toluene on the NMR time scale. The dependence of the rate of exchange on the concentration of ema shows that the exchange proceeds through anS_N2 mechanism. The activation parameters are: ΔH₂₇₃^≠ 2.75 kcal/mol, ΔG₂₇₃^≠ 12.7 kcal/mol, ΔS₂₇₃^≠ ?37 e.u. The³¹P NMR spectra of the complexes show two doublets when the exchange is frozen out, indicating the inequivalence of the two phosphine ligands in the ema-coordinated complex. The difference in the³¹P chemical shifts of the two coordinated tertiary phosphines increases with increase in the basicity of the PR₃ ligand.     

Small-angle x-ray scattering from solution of rodlike particles with length distribution

The scattering function of rods with a constant radius, 8 Å, and a length distribution of the Schulz–Zimm type was calculated on a computer as a model of helical synthetic polypeptide. The influence of length and length distribution on the scattered intensity of small-angle x-ray scattering was clarified. As length grows and length distribution broadens it is difficult to obtain reliable values of molecular weight and radius of gyration from a Zimm plot. The influence of length distribution on the mass per unit length, M_q, and the radius of gyration of the cross section, 〈S〉^1/2, from the Guinier plot of the cross-sectional factor decreases as the length increases, and reliable values of M_q and 〈S〉^1/2 can be obtained even for rods with wide distribution for rods more than 600 Å long. In particular, it is pointed out that the value of 〈S〉^1/2 is little influenced by length and length distribution.     

Rate of spontaneous deactivation of styryl and α-methylstyryl carbanions

The deactivation of α-methylstyryl and styryl carbanions followed by spectral change and titration was studied kinetically. First-order rate constants were obtained for the deactivation of oligo-α-methylstyryl lithium, disodium, and dipotassium in tetrahydrofuran and of polystyryl potassium in varying degrees of polymerization in benzene that contained a small amount of tetrahydrofuran and n-hexane. It was observed that the addition of cryptate [222] exerts a significant effect on the system of oligo-α-methylstyryl disodium in tetrahydrofuran. The effect of dilution, degree of polymerization of the polymer, and counterions on the rate of deactivation of carbanions was discussed.     

Synthesis and Characterization of Amphiphilic Biodegradable Copolymer,Poly(aspartic acid‐co‐lactic acid)

An amphiphilic biodegradable polymer, poly(aspartic acid‐co‐lactic acid) (PAL), was synthesized by simply heating a mixture of aspartic acid (Asp) and L ‐lactide without additional catalysts or solvents. The unique branched architecture comprising succinimide units and lactic acid units was confirmed by IR and NMR spectroscopy. A copolymer of sodium aspartate and lactic acid (PALNa) was prepared by reacting PAL with an aqueous sodium hydroxide solution. The PAL was soluble in many organic solvents, while the PALNa was soluble in methanol and water. The hydrolytic degradation behavior of PAL varied with the copolymer composition. A higher Asp content resulted in a faster molecular weight decrease, and introducing glycolic acid units accelerated the degradation rate.

Microphotograph of microsphere of PAL‐1/5.     

The synthesis of isoquinoline alkaloid and its related compounds using alanine derivatives as chiral auxiliaries

Chiral 1-substituted isoquinoline derivatives, which were obtained by the reaction using alanine derivatives as chiral auxiliaries, were transformed to (S)-2,3,9,10,11-pentamethoxyhomoprotoberberine (7) and a synthetic intermediate for O-methylkreysigine (9) in good yields and high stereoselectivity. The corresponding chiral allyl derivative of isoquinoline was transformed to a pyrrolidinoisoquinoline (16) in a highly enantioselective manner.     

A convenient stereoselective synthesis of β-lactams from β-hydroxy-α-thioalkylesters prepared from Michael/aldol tandem reaction or stereoselective addition of thiols to the Baylis-Hillman adducts

A convenient stereoselective synthesis of β-lactams from thio-Michael/aldol tandem adduct is described. syn-Selective tandem reaction followed by amidation and intramolecular S_N2 reaction provided β-lactams in diastereomerically pure form. The tandem reaction with aliphatic aldehydes, on the other hand, afforded a mixture of diastereomers of corresponding tandem adducts in about 3:1 ratio so that the conversion to β-lactams afforded a diastereomeric mixture. As an alternative approach to prepare the tandem adducts, the stereoselective Michael addition of aliphatic thiols to Baylis-Hillman adduct was developed. The stereoselectivity was sensitive to the protective group at the hydroxyl group and TBS protection brought the most successful syn-selective formation of the tandem adducts. The procedure could be applied to the ketone derivatives of the Baylis-Hillman adduct but no selectivity was observed for the nitrile-Baylis-Hillman adducts. A similar conversion of the adduct provided desired β-lactams stereoselectively.     

TYROSINE-SENSITIZED PHOTODIMERIZATION OF THYMINE IN AQUEOUS SOLUTION

Abstract. Photodimerization of thymine in aqueous solution in the presence of tyrosine was studied with monochromatic UV irradiation. The total dimer formation was sensitized in the presence of tyrosine. The action spectrum of sensitized total dimer formation has a peak near 280 nm corresponding to the absorption maximum of tyrosine. Triplet quenchers reduced the sensitization substantially. It seems probable that tyrosine-sensitized photodimerization of thymine occurred via triplet-triplet energy transfer from tyrosine to thymine.