Theoretical study of 31P, 31P coupling constants in cyclotriphosphazenes.

Several scalar coupling constants (mainly 31P, 31P) were calculated for 10 cyclotriphosphazenes and compared with experimental results when available. Although the experimental values cannot be reproduced, the calculated values are proportional to the experimental values. Some difficult cases, such as 19F, 19F couplings, are discussed.

Hydrogen-bonding cooperativity: using an intramolecular hydrogen bond to design a carbohydrate derivative with a cooperative hydrogen-bond donor centre.

Neighbouring groups can be strategically located to polarise HO.OH intramolecular hydrogen bonds in an intended direction. A group with a unique hydrogen-bond donor or acceptor character, located at hydrogen-bonding distance to a particular OH group, has been used to initiate the hydrogen-bond network and to polarise a HO.OH hydrogen bond in a predicted direction. This enhanced the donor character of a particular OH group and made it a cooperative hydrogen-bond centre. We have proved that a five-membered-ring intramolecular hydrogen bond established between an amide NH group and a hydroxy group (1,2-e,a), which is additionally located in a 1,3-cis-diaxial relationship to a second hydroxy group, can be used to select a unique direction on the six-membered-ring intramolecular hydrogen bond between the two axial OH groups, so that one of them behaves as an efficient cooperative donor. Talose derivative 3 was designed and synthesised to prove this hydrogen-bonding network by NMR spectroscopy, and the mannopyranoside derivatives 1 and 2 were used as models to demonstrate the presence in solution of the 1,2-(e,a)/five-membered-ring intramolecular hydrogen bond. Once a well-defined hydrogen-bond is formed between the OH and the amido groups of a pyranose ring, these hydrogen-bonding groups no longer act as independent hydrogen-bonding centres, but as hydrogen-bonding arrays. This introduces a new perspective on the properties of carbohydrate OH groups and it is important for the de novo design of molecular recognition processes, at least in nonpolar media. Carbohydrates 1-3 have shown to be efficient phosphate binders in nonpolar solvents owing to the presence of cooperative hydroxy centres in the molecule.