Remarkably large remote-stereocenter-induced chromatographic differences: azulenyl 1,5-diols

A series of four new azulene-1,5-diol diastereomers were prepared and found to exhibit large differences (ΔR_f 0.22–0.46) in silica TLC mobilities despite having stereocenters four bonds apart. The stereoisomers were identified by X-ray crystallography, and in all cases the RR/SS diastereomers were less strongly retained than the RS/SR diastereomers. The crystal structures suggested that this is due to a conformational preference for the CF₃ groups to be nearly perpendicular to the plane of the azulene ring, which caused the dihedral angle between the OH groups to be larger (in the RR/SS diastereomers) or smaller (in the RS/SR diastereomers). The smaller dihedral angles allow the RS/SR diastereomers to simultaneously bind to a silica surface and thus be more strongly retained. Two similar benzene derivatives and several cycloalkanediols with more proximate stereocenters showed little or no difference in mobilities between diastereomers, though the NMR differences were greater. Thus, the azulene ring is an important factor in enforcing the conformational preferences, either through steric interactions with the 4/8 substituents (H or methyl) or its significant dipole moment, or both.     

1-AMINO-1-ARYL- AND 1-AMINO-1-HETEROARYL-METHANEPHOSPHONIC ACIDS AND THEIR N-BENZHYDRYL–PROTECTED DIETHYL ESTERS: PREPARATION AND CHARACTERIZATION

n-Benzhydryl-protected diethyl[3.3pc] esters of 1-amino-1-aryl- (phenyl, cumyl, p-dimethylaminophenyl, piperonyl, 1′-naphthyl, 9′-anthryl, 1′-pyrenyl) and 1-amino-1-heteroaryl- (furyl, 2′-thienyl, 3′-thienyl, 2′-pyrrolyl)-methanephosphonic acids, prepared by the addition of diethyl phosphite to the corresponding benzhydryl imines, have been isolated, purified, and characterized. The presence of chiral α -carbon and prochiral phosphorus in these esters gives rise to complicated features in their NMR spectra, which are discussed. Hydrolysis of the crude 1-aryl compounds in situ gave modest yields of the corresponding aminophosphonic acids (except for 1′-pyrenyl). Of the 1-heteroaryl derivatives, only the 2′-thienyl compound gave the expected aminophosphonic acid; in other cases, alternative[-18pc] modes of decomposition may occur under hydrolytic conditions. NMR and mass spectral data are given for all products.     

THE CRUCIAL ROLE OF INORGANIC RING CHEMISTRY IN THE DEVELOPMENT OF NEW POLYMERS

The field of phosphazene high polymers has developed into a large area of more than 700 different types of macromolecules with novel combinations of properties and diverse applications. Small-molecule phosphazene rings have played a major role in these developments, first as starting materials for polymer synthesis, second as synthetic and structural models for the high polymers, and third as components of hybrid inorganic-organic macromolecules. These three aspects are reviewed, with examples taken from our recent work, together with some thoughts on the development of this and related fields in the future.     

ORGANOPHOSPHORUS COMPOUNDS AS POTENTIAL FUNGICIDES. PART I. N-(ω-GUANI-DINOALKYL)AMINOALKANEPHOSPHONIC ACIDS AND THEIR AMINOPHOSPHONIC PRECURSORS: PREPARATION,NMR SPECTROSCOPY,AND FAST ATOM BOMBARDMENT MASS SPECTROMETRY

Abstract

N-(ω-Aminoalkyl)- and N-(ω-guanidinoalkyl)-aminoalkanephosphonic acids have been prepared from α, ω-diaminoalkanes by reaction with chloromethanephosphonic acid (or an ester of a halogeno-alkanephosphonic acid), followed by treatment with S-methylisothiouronium chloride. Ethylene diamine yielded 1-phosphonomethyl-2-iminoimidazolidine. A number of 1:1 salts of the α, ω-diamines and chloromethanephosphonic acid are also reported. Doubly charged zwitterionic structures are assigned to both ω-amino and ω-guanidino compounds on the basis of ³¹P and ¹³C nmr data. Thus the addition of an excess of acid (D₂SO₄) causes the ³¹P chemical shift to move to higher field, from ca. 8 to 14 ppm, whilst ¹ Jpc increases from ca. 130 to 150 Hz. The ¹H and ¹³C chemical shifts of the terminal methylene groups in the polymethylene chain are unaffected by acidification.

Fast atom bombardment mass spectrometry gives rise to characteristic [M + H]⁺ ions, frequently as the base peak, and to fragmentations involving the loss of phosphorous acid, or the formation of ions resulting from carbon-nitrogen or carbon-carbon cleavage. The compounds show activity against a number of fungal pathogens and other microbial organisms.     

Dinuclear Nickel(II) and Zinc(II) Complexes of 2,6-[N,N′-bis(2-hydroxyphenylmethyl)-N,N′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol

Abstract

The symmetric 'end-off' compartmental proligand 2,6-[N,N′-bis(2-hydroxy-phenylmethyl)-N,N′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol (H₃L) has been used to generate homodinuclear nickel(II) and zinc(II) complexes. The crystal structures of the complexes reveal that the di-nickel(II) complex is donor asymmetric and the di-zinc(II) complex is coordination number asymmetric. In both complexes non-coordinated acetic acid molecules are tightly hydrogen-bonded to the pendant phenols of the ligand generating a double acid salt of the type[CH₃COO…H…L…H…OOCCH₃]^3? in the dinickel complex and a single acid salt of the type [CH₃COO…H…L]^3? in the dizinc complex. In both cases the ligand periphery has been extended to provide a supraligand in which the donor potential of the original ligand has been enhanced.     

The synthesis and crystal structure of a new diprotonated Schiff base cryptand

Abstract

The synthesis and crystal structure of the diprotonated cryptand derived from the reaction of tren and 2,6-diacetyl pyridine is reported. The imino nitrogens of the Schiff base linkages are directed so that the dicarbimine functions are in trans, trans geometry relative to the pyridine C-N bond. This configuration has not previously been reported in related pyridine-derived Schiff base macrocycles.     

Sodiation as a tool for enhancing the diagnostic value of MALDI‐TOF/TOF‐MS spectra of complex astaxanthin ester mixtures from Haematococcus pluvialis

The microalga Haematococcus pluvialis produces the pigment astaxanthin mainly in esterified form with a multitude of fatty acids, which results in a complex mixture of carotenol mono‐ and diesters. For rapid fingerprinting of these esters, matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI‐TOF/TOF‐MS) might be an alternative to traditional chromatographic separation combined with MS. Investigation of ionization and fragmentation of astaxanthin mono‐ and diester palmitate standards in MALDI‐TOF/TOF‐MS showed that sodium adduct parent masses [M + Na]⁺ gave much simpler MS² spectra than radical / protonated [M]^+● / [M + H]⁺ parents. [M + Na]⁺ fragments yielded diagnostic polyene‐specific eliminations and fatty acid neutral losses, whereas [M]^+● / [M + H]⁺ fragmentation resulted in a multitude of non‐diagnostic daughters. For diesters, a benzonium fragment, formed by polyene elimination, was required for identification of the second fatty acid attached to the astaxanthin backbone. Parents were forced into [M + Na]⁺ ionization by addition of sodium acetate, and best signal‐to‐noise ratios were obtained in the 0.1 to 1.0 mM range. This method was applied to fingerprinting astaxanthin esters in a crude H. pluvialis extract. Prior to MALDI‐TOF/TOF‐MS, the extract was fractionated by normal phase Flash chromatography to obtain fractions enriched in mono‐ and diesters and to remove pheophytin a, which compromised monoester signals. All 12 types of all‐trans esterified esters found in LC were identified with MALDI‐TOF/TOF‐MS, with the exception of two minor monoesters. Copyright © 2013 John Wiley & Sons, Ltd.     

Stabilization of Large Adsorbates by Rotational Entropy: A Time‐Resolved Variable‐Temperature STM Study

Investigating the dynamics in an adlayer of the oligopyridine derivative 2‐phenyl‐4,6‐bis(6‐(pyridine‐2‐yl)‐4‐(pyridine‐4‐yl)pyridine‐2‐yl)pyrimidine (2,4′‐BTP) on Ag(111) by fast scanning tunneling microscopy (video‐STM), we found that rotating 2,4′‐BTP adsorbates coexist in a two‐dimensional (2D) liquid phase (β‐phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α‐phase) and β‐phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature‐dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid‐size and large adsorbates.     

FeIII Quinolylsalicylaldimine Complexes: A Rare Mixed‐Spin‐State Complex and Abrupt Spin Crossover

A new synthesis of (8‐quinolyl)‐5‐methoxysalicylaldimine (Hqsal‐5‐OMe) is reported and its crystal structure is presented. Two Fe^III complexes, [Fe(qsal‐5‐OMe)₂]Cl ? solvent (solvent=2 MeOH ? 0.5 H₂O ( 1 ) and MeCN ? H₂O ( 2 )) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal‐5‐OMe)₂] Cl ? 2 MeOH ? 0.5 H₂O ( 1 ) exhibits rare crystallographically independent high‐spin and low‐spin Fe^III centres at 150 K, whereas [Fe(qsal‐5‐OMe)₂]Cl ? MeCN ? H₂O ( 2 ) is low spin at 100 K. In both structures there are extensive π–π and C? H???π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped‐spin crossover with T_1/2=245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half‐spin crossover also with a hysteresis of 10 K. The two compounds are the first Fe^III complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by ⁵⁷Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to Fe^II at ?0.18 V and irreversible oxidation of the coordinated qsal‐5‐OMe ligand at +1.10 V.     

A Resin‐Linker‐Vector Approach to Radiopharmaceuticals Containing 18F: Application in the Synthesis of O‐(2‐[18F]‐Fluoroethyl)‐L‐tyrosine

A Resin‐linker‐vector (RLV) strategy is described for the radiosynthesis of tracer molecules containing the radionuclide ¹⁸F, which releases the labelled vector into solution upon nucleophilic substitution of a polystyrene‐bound arylsulfonate linker with [¹⁸F]‐fluoride ion. Three model linker‐vector molecules 7 a – c containing different alkyl spacer groups were assembled in solution from (4‐chlorosulfonylphenyl)alkanoate esters, exploiting a lipase‐catalysed chemoselective carboxylic ester hydrolysis in the presence of the sulfonate ester as a key step. The linker‐vector systems were attached to aminomethyl polystyrene resin through amide bond formation to give RLVs 8 a – c with acetate, butyrate and hexanoate spacers, which were characterised by using magic‐angle spinning (MAS) NMR spectroscopy. On fluoridolysis, the RLVs 8 a , b containing the longer spacers were shown to be more effective in the release of the fluorinated model vector (4‐fluorobutyl)phenylcarbamic acid tert‐butyl ester ( 9 ) in NMR kinetic studies and gave superior radiochemical yields (RCY≈60 %) of the ¹⁸F‐labelled vector. The approach was applied to the synthesis of the radiopharmaceutical O‐(2‐[¹⁸F]‐fluoroethyl)‐L ‐tyrosine ([¹⁸F]‐FET), delivering protected [¹⁸F]‐FET in >90 % RCY. Acid deprotection gave [¹⁸F]‐FET in an overall RCY of 41 % from the RLV.