Martin W. Bredenkamp

Templated polar order of a guest in a quasiracemic organic host

A quasiracemic mixture of Dianins compound and its thiol derivative enforces additional anisotropy of the guest-accessible space, thus facilitating a net polar arrangement of guest molecules; guest alignment is rationalized in terms of van der Waals volume considerations.

Tin-mediated equilibration of the benzoate esters of methyl 4,6-O- benzylidene-α-D-glucopyranoside

Abstract The selective dibutyltin oxide-mediated benzoylation of methyl 4,6- O -benzylidene-α- d -glucopyranoside at position 2 is not a post-acylation phenomenon as is the case with the equivalent benzoylation of phenylethyleneglycol. 1 H NMR temperature studies have shown that such equilibration occurs only above 85°C in toluene- d 8 for the glucopyranoside.

Breaking the trigonal host packing motif of Dianin's compound

Crystallisation of Dianins compound (DC) together with two organic amines (piperazine and piperidine) produces co-crystals of 2:1 DC–piperazine and 1:1 DC–piperidine, respectively. The structures of these adducts contrast sharply with that of the well-known clathrate structure of DC in which organic guests are generally included in ratios of 3:1 or 6:1 DC–guest.

Adsorbed surfactants for affinity chromatography: end-group modification of ethylene glycol polymers

The hydroxyl end-groups of Pluronic F108 [a tri-block copolymer surfactant of poly(ethylene glycol) and poly(propylene glycol) [PEG-PPG-PEG]] were converted into primary amine and quaternary ammonium equivalents for use in a new approach to affinity chromatography. The preparation of sulphonic acid end-groups was also attempted.

Crystal structures and conformational analysis of ochratoxin A and B: probing the chemical structure causing toxicity

Studies performed on two metabolites from Aspergillus ochraceus, ochratoxin A (1) and B (2), have yielded information related to the steric, conformational, and electronic considerations that could contribute to the different toxicities of these metabolites. The relative HCl-catalysed hydrolysis rates of the toxins are reversed compared with those observed in biological systems. This reversal of relative rate is ascribed to steric hindrance due to the chloro group in ochratoxin A (1). Single-crystal X-ray structural analyses, i.r., and high field n.m.r. analysis indicate no discernible difference in conformations.

Structure elucidation of the dibutylchlorostannyl intermediate during dibutyltin oxide-mediated acylation of sugars

Abstract 119 Sn NMR and APcI-MS indicate the presence of a dimeric species as the dibutylchlorostannyl intermediate after dibutyltin oxide-mediated bezoylation of methyl 4,6- O -benzylidene-α- d -glucopyranoside. The stannyl groups in the dimer dismutate to form a dialkoxystannyl species loosely complexed to dibutyltin dichloride. This species and its isomer were also prepared by treating the benzoylated sugar with bis(dibutylchlorotin) oxide. The dialkoxytin complex and isomer was also prepared by treating the benzoylated sugar with dibutyltin oxide.

NMR spectroscopy as basis for characterization of pluronic® F108 and its derivatives

The hydroxyl end groups of Pluronic®F108 {a triblock copolymer surfactant of poly(ethylene glycol) and poly(propylene glycol) [PEG-PPG-PEG]} were modified into primary amine, sulfonic acid, and quaternary ammonium equivalents for use in affinity chromatography. NMR was used for monitoring the efficacy of modifications on intermediaries and final products. The primary amine equivalents were prepared via conversion of the hydroxyl groups to a tosylate, its displacement with an azide, followed by reduction to the primary amine. The sulfonic acid equivalents were prepared via hydroxyl group tosylation, the displacement of tosylate with thiol, and its oxidation to sulfonic acid. The conversion to trimethyl ammonium was achieved via hydroxyl group tosylation, tosylate displacement by halide, and halide displacement with trimethylamine.

Resolution of (S,S)-4-(2,2,4-trimethylchroman-4-yl)phenyl camphanate and its 4-chromanyl epimer by crystallization.

Dianins compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) has been resolved by crystallization of the (S)-(-)-camphanic esters (S,S)- and (R,S)-4-(2,2,4-trimethylchroman-4-yl)phenyl 4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate, both C28H32O5, from 2-methoxyethanol, yielding the pure S,S diastereomer. The relative stereochemistry of both diastereomers has been determined by X-ray crystallography, from which the absolute stereochemistry could be deduced from the known configuration of the camphanate moiety. The crystallographic conformations have been analysed, including the 1:1 disorder of the R,S diastereomer.

A stereospecific phosphonium ylide-salt coupling reaction

Abstract The stereospecific formation of a phosphonium ylide-salt via the stereospecific coupling of an allylic phosphonium ylide with the corresponding phosphonium salt is described.

Polymorphism in bipodal O,O'-dimethyl N,N'-(m-phenylenedicarbonyl)bis(thiocarbamate).

The title compound, C12H12N2O4S2, crystallizes in white and yellow polymeric forms as a result of interesting anti-anti and syn-anti conformational isomerism of the thiocarbonyl and carbonyl moieties relative to one another. This work is the first reported X-ray crystallographic structure determination of isomers of this class of bipodal ligand. The white form, anti-anti, (I), crystallizes with the benzene ring lying about a twofold rotation axis, resulting in both of the thiocarbonyl and carbonyl moieties being anti relative to each other. The yellow modification crystallizes as syn-anti, (II), with one thiocarbonyl moiety syn and the other anti relative to the respective carbonyl groups. The individual molecules of both (I) and (II) are extensively linked through intermolecular hydrogen bonds. Intermolecular hydrogen bonding in (II) includes a network of bifurcated N-H...O and N-H...S hydrogen bonds, while molecules of (I) include bifurcated C-H...O hydrogen bonds.