Diastereomeric diterpenes from Coleus blumei.

The chloroform extract of the air-dried leaves of Coleus blumei afforded a mixture of diastereomers of a new abietane type diterpene whose structures were elucidated by extensive one and two dimensional (ID, 2D) NMR and mass spectrometry. Acetylation of the mixture afforded a single compound. Antimicrobial tests on the diterpene indicate that it is active against Bacillus subtilis, Pseudomonas aeruginosa and Candida albicans.     

Diastereomeric isomers of five-coordinate organoantimony compounds

A series of unsymmetrical diarylantimony trichlorides of the type PhArSbCl₃, where Ar is a substituted phenyl group have been prepared. An attempt was then made to convert these to the oxides (PhArSbCl₂)₂O. In addition to the known compound (Ph₂SbCl₂)₂O, only bis(phenyl-p-tolyldichloroantimony) oxide was obtained in pure form. An impure sample of bis(phenyl-p-fluorophenyldichloroantimony) oxide was also obtained. PMR spectra of the tolyl compound and ¹⁹F spectra of the fluoro compound revealed the presence of two diastereomeric forms for both compounds at low temperature.     

Diastereomeric Xe chemical shifts in tethered cryptophane cages

Cryptophane cages serve as host molecules to a Xe atom. Functionalization of cryptophane-A has permitted the development of Xe as a biosensor. Synthetic routes used to prepare cryptophanes result in racemic mixtures of the chiral cages. In the preparation of a tethered cryptophane-A cage for biosensor applications, some achiral and chiral substituents such as left-handed amino acids have been used. When the substituent is achiral, the NMR signal of the Xe atom in the functionalized cage in solution is a single isotropic peak, since the Xe shielding tensor components in the R and L cages differ by no more than the signs of the off-diagonal elements. Chiral substituents can split the cage-encapsulated Xe NMR signal into one or more sets of doublets, depending on the number of asymmetric centers in the substituent. We carry out quantum mechanical calculations of Xe nuclear magnetic shielding for the Xe atom at the same strategic position within an L cryptophane-A cage, under the influence of chiral potentials that represent r or l substituents outside the cage. Calculations of the Xe shielding response in the Lr and Ll diastereomeric pairs permit the prediction of the relative order of the Xe chemical shifts in solutions containing the Rl and Ll diastereomers. Where the substituent itself possesses two chiral centers, comparison of the calculated isotropic shielding responses in the Llr, Lrl, Rll, and Lrr systems, respectively, permits the prediction of the Xe spectrum of diastereomeric systems in solutions containing Llr, Rlr, Lll, and Rll systems. Assignment of the peaks observed in the experimental Xe NMR spectra is therefore possible, without having to undertake the difficult synthetic route that produces a single optically pure enantiomer.     

Synthesis of Diastereomeric 8-Fluoro-ABC-Steroid Building Blocks

An eight-step linear sequence for the preparation of two diastereomers of an 8-fluoro-ABC-steroid building block was developed. Key step was an intramolecular Diels–Alder reaction of an intermediate o-quinodimethane formed from a benzocyclobutene substituted with a 5-fluorohex-5-en-4-one chain. This side chain was prepared from 6-chlorohex-1-ene by bromofluorination, elimination of HBr, Finkelstein reaction and alkylation of a literature-known benzocyclobutene derivative with the thus-formed 6-iodo-2-fluorohex-1-ene. Allylic oxidation of side chain's fluorovinyl moiety to an α-fluoro-α,β-unsaturated ketone completed the preparation of the precursor for the [4+2]-cycloaddition.     

Practical Tactics in Resolution of Racemates via Diastereomeric Salt Formation

A practical tactics in the classical resolutions was illustrated for affording the desired isomers in maximum yields. According to the practical tactics resolution of racemic ethyl nipecotate, Corey lactone and huperzine A intermediate was successfully practiced. The basis of this practicability is the well-known Marckwald principle.     

Regioselective Diastereomeric Michael Adducts as Building Blocks in Heterocyclic Synthesis

The reaction of 4‐(4‐acetylamino/bromophenyl)‐4‐oxobut‐2‐enoic acids with carbon nucleophiles afforded Michael adducts depending on the type of nucleophilic reagents and medium (acidic or basic). The adducts 2 and 3 were used as key starting materials to synthesize some heterocyclic compounds, which include pyridazinone, furanone, 1,2‐oxazin‐5‐one, 1.2‐diazapine, pyrane, and hydroxyl pyridine derivatives. The Steric factor plays an important role in regioselectivity. The structure of newly synthesized compounds was elucidated by elemental analysis and spectroscopic data.     

Simple and rapid determination of mercury in urine and tissues by isotope exchange

The method eliminates oxidation of organic material. Tracer quantities of (203)Hg are added to the sample and equilibrated with the stable mercury. (200)Hg vapor in nitrogen is passed through the sample in the presence of cysteine and at pH 7.4, and exchanges with the (203)Hg. The vapor is collected in a tube of activated Hopcalite placed in a gamma-counter. The increasing activity is recorded and the half-time of exchange is directly proportional to the mercury concentration in the sample. The method is best suited for urine samples containing more than 10 mug Hg l .     

Diastereomeric Oxathia[n](1,1′)ferrocenophanes

Zusammenfassung Neue Oxathiaferrocenophane wurden durch Umsetzung von 1,1-Bis(hydroxymethyl)ferrocen mit Dithiolen dargestellt, welche Sauerstoff in den Alkylketten enthalten. Die Reaktion von 1,1-Bis(-hydroxyethyl)ferrocenen mit Dithiolen führte zu Mischungen von Diastereomeren, aus welchen reine Stereoisomere isoliert und charakterisiert wurden. Einige Aspekte des stereochemischen Verlaufes dieser Reaktionen werden diskutiert.

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Diastereomere oxathia[n](1,1)ferrocenophane

Novel oxathiaferrocenophanes have been synthesized by the reaction of 1,1-bis(hydroxymethyl)ferrocene with dithiols bearing oxygen in chains. The reactions of 1,1-bis(-hydroxyethyl)ferrocenes with dithiols afforded mixtures of diastereomeric products from which pure stereoisomers were isolated and characterized. Some aspects concerning a stereochemical course of the reactions described are discussed.

     

Diastereomeric P1-chiral diamidophosphites with terpene fragments in asymmetric catalysis

Diamidophosphite P¹-monodentate, ligands based on terpene alcohols and (S)- or (R)-(2-anilinomethyl)pyrrolidine, induce high enantioselectivities (ee’s up to 99%) in Pd-catalyzed allylic substitution reactions. In the Pd-catalyzed deracemization of ethyl (E)-1,3-diphenylallyl carbonate up to 92% enantioselectivity has been achieved. The Rh-catalyzed asymmetric hydrogenation of α-dehydrocarboxylic acid esters leads to a maximum of 56% ee with quantitative conversion. Diastereomeric diamidophosphites prepared from [(1S)-endo]-(−)-borneol were found to be the most efficient stereoselectors.     

Diastereomeric host-guest complex formation by an optically active paracyclophane in water

The first synthesis of a water-soluble cyclophane possessing a chiral hydrophobic cavity, and the formation of diastereomeric inclusion complexes with chiral hydrophobic guests in water are described.     

Diastereomeric recognition of chiral foldamer receptors for chiral glucoses

[reaction: see text] Three chiral aromatic hydrazide foldamers have been designed and synthesized, in which two R- or S-proline units were incorporated at the terminals of their backbones. The 1H NMR, circular dichroism (CD), and fluorescent experiments and molecular dynamics simulations revealed that the foldamers adopted a chiral helical conformation and complexed alkylated glucoses in chloroform with a good diastereomeric selectivity.