Jeremy J. Hastings

Glycosidase-catalysed synthesis of glycosides by an improved procedure for reverse hydrolysis: application to the chemoenzymatic synthesis of galactopyranosyl-(1→4)-O-α-galactopyranoside derivatives

Abstract β-Galactosidase from Aspergillus oryzae, α-galactosidase from Aspergillus niger, β-mannosidase from Helix pomatia and β-glucosidase from almond were used to synthesise different glycosides by reverse hydrolysis: GlcβO(CH2)6OH 1 was obtained in 61% yield, β-D-Glc-O(CH2)3CH-CH2 2 in 50% yield, β-D-Glc-O(CH2)2Si(Me)3 3 in 11% yield, β-D-Gal-O(CH2)6OH 4 in 48% yield, β-D-Gal-O(CH2)3CH-CH2 5 in 22% yield, α-D-Gal-O(CH2)6OH 6 in 47% yield, α-D-GalO(CH2)3CH-CH2 7 in 37% yield and β-D-ManO(CH2)6OH 8 in 12% yield. Using the appropriate glycosides methyl O-(2,3,4,6-tetra-O-benzyl-α-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzoyl-1-O-α-D-galactopyranoside 11 and 6′-benzoylhexyl-O-(2,3,4,6-tetra-O-benzyl-α-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzoyl-1-O-β-D-galactopyranoside 15 were synthesised. This chemoenzymatic approach avoided at least four chemical steps that would have been necessary in a conventional synthesis.

Chemoenzymatic synthesis of ethyl 1-thio-(β-D-galactopyranosyl)-O-β-D-glycopyranosyl disaccharides using the β-galactosidase from Bacillus circulans

Abstract Different ethyl 1-thio-β-D-disaccharides have been synthesised by transgalactosylation using the β-galactosidase from Bacillus circulans as biocatalyst. This β-galactosidase shows mainly a β-1–4 specificity in the galactosyl transfer. Gal-β-(1–4)-O-β-D-GlcSEt 15 was obtained in 36% yield, Gal-β-(1ndash;4)-O-α-D-GlcSEt 19 in 30% yield, Gal-β-(1–4)-O-β-D-GalSEt 17 in 60% yield, Gal-β-(1–4)-O-β-D-GalNAcSEt 20 in 49% yield, Gal-β-(1–4)-O-β-D-Gal-β-(1–4)-O-β-D-GalNAcSEt 21 in 9% yield, Gal-β-(1–6)-O-β-D-GlcSEt 16 in 3% yield and Gal-β-(1–3)-O-β-D-XylSEt 18 in 25% yield.

A 183W, 1H and 17O nuclear magnetic resonance study of aqueous isopolytungstates

Isopolytungstates have been studied in aqueous solution between pH 8 and 1.5, using 183W, 17O and 1H NMR spectroscopy. The first polyanions to form upon acidification are the paratungstates A and B. Their resonances are largely assigned, and paratungstate B is shown to protonate with pKa= 4.59. Its protonated form has at least two isomers separated by a detectably slow proton-exchange process. On further acidification, paratungstate B loses one tungsten atom to give an anion with no symmetry, which is identified as the solution form of ψ-metatungstate. This in turn protonates with pKa= 2.65, before transmuting to the known species tungstate-Y. Six metatungstate species with Keggin structures are aslo observed at lower pH values. Five are metastable anions which slowly convert to the well known α-[H2W12O40]6– ion. Two have β-Keggin structures. In each case direct structural information is provided from the solution state. The rotational correlation times and average interproton distances of the α- and β-[H2W12O40]6– species are deduced from their 1H NMR relaxation data, and similar data are used to assist in identifying the three internally monoprotonated α- and β-Keggin anions.

Synthesis and characterisation of (η5-cyclopentadienyl)(η5-ring)titanium alkyl (ring = indenyl or C5H4But) complexes. Crystal and molecular structure of racemic [Ti(η5-C5H5)(η5-C9H7)(CH2SiMe3)Cl]

New, selective and high-yielding preparations of the mixed-ring complexes [Ti(η5-C5H5)(η5-C9H7)C2] and [Ti(η5-C5H5)(η5-C5H4But)Cl2] are reported. These have been used to prepare a range of mono- and di-substituted titanium(IV) alkyl and benzenethiolate complexes of the form [Ti(η5-C5H5)(η5-ring)(CH2SiMe3)Cl] and [Ti(η5-C5H5)(η5-ring)R2](ring = indenyl or C5H4But; R = Me, CH2Ph, CH2SiMe3 or SPh). While the indenyl ligand in the racemic, chiral-at-metal complex [Ti(η5-C5H5)(η5-C9H7)(CH2SiMe3)Cl] is bound in an η5 fashion, X-ray structural data clearly indicate that there is some ‘η3 ring-slip’ character to the bonding. The NMR and nuclear Overhauser effect experiments conducted on [Ti(η5-C5H5)(η5-C5H4But)(CH2SiMe3)Cl] demonstrate hindered rotation around the Ti–C5H4But bond and show the geometry to be fixed such that the But and SiMe3 groups are remote.

183W nuclear magnetic resonance of α-[H2W11VO40]7− and α-[H2W11MoO40]6−

Abstract The anions [H 2 W 11 VO 40 ] 7− and [H 2 W 11 MoO 40 ] 6− have been identified in aqueous solution by 183 W, 51 V and 1 H NMR. They almost certainly both have the α-Keggin structure.

Monotungstononavanadate and mer-tritungstotrivanadate

Abstract Two new tungstovanadates, [WV 9 O 28 ] 5− and mer -[W 3 V 3 O 19 ] 5− , have been identified in equilibrated aqueous solutions. 51 V- 51 V COSY spectra support structures analogous to known molybdovanadates, although the tungstovanadate species more readily support a higher anionic charge.

Oxygen and vanadium exchange processes in linear vanadate oligomers

The existence of the linear tri- and tetra-vanadate anions in aqueous solution has been confirmed by 51V and 17O NMR and potentiometry, yielding the formation constants. Their resonances are mostly broadened by an exchange process which is shown by the linewidths and by magnetisation-transfer experiments to be independent of the monomeric or the dimeric vanadates also present, and also of the solvent oxygens. The broadenings are not consistent with a simple process of exchange, but instead reveal the presence of an intermediate, probably cyclic, having a short but not insignificant lifetime. Also, simple proton transfer between the oxygens in the aqueous monoprotonated monovanadate anion is sufficiently slow to be detectable by NMR spectroscopy.

Aqueous tungstovanadate equilibria

A combined equilibrium study of tungstovanadates involving potentiometry and 51V, 183W and 17O NMR spectroscopy has been able to identify 11 main species, giving their formation constants and pKa values where relevant. Tentative identification was also made of two other non-equilibrium species at lower pH values, in addition to those with a V-centred Keggin structure. The 51V shifts of the unprotonated hexametalate anions fall into a rational pattern, and the formation constants are all higher than those of the corresponding molybdovanadate species, where comparisons are possible.