Models for Enzyme-catalysed Phosphate Transfer: Comparisons of Reactivity towards a Neighbouring Hydroxy Group for Phosphodiester Anions and Acids. General Base Catalysis of the Cyclisation of a Hydroxyalkyl Phosphate Triester

Methylation of phenyl 1,2-isopropy~idene-~-~-xy~ofuranose 3'-phosphate increases the rate of intramolecular cyclisation by a factor of over lo5. This confirms previous estimates of the effect of protonation on the reactivity of phosphate diesters towards a neighbouring hydroxy group, which depend on the correct assignment of kinetically equivalent mechanisms, and makes available reliable data on the magnitude of the effect for reactions catalysed by a range of general acids and bases. General base catalysis is characterised for the intramolecular cyclisation of one diastereoisomer (7b) of methyl phenyl 1 2 isopropyl idene-P-~-xylofuranose 3'-phosphate triester: the Brsnsted P is 0.65 and catalysis is enhanced by the proximity of the positive centres of suitable diamine monocations.