MODELS FOR ENZYME-CATALYZED PHOSPHATE TRANSFER - COMPARISONS OF REACTIVITY TOWARDS A NEIGHBORING HYDROXY GROUP FOR PHOSPHODIESTER ANIONS AND ACIDS - GENERAL BASE CATALYSIS OF THE CYCLIZATION OF A HYDROXYALKYL PHOSPHATE TRIESTER

Methylation of phenyl 1,2- isopropylidene-beta-D-xylofuranose 3'-phosp hate increases the rate of intramolecular cyclisation by a factor of o ver 10(5). This confirms previous estimates of the effect of protonati on on the reactivity of phosphate diesters towards a neighbouring hydr oxy group, which depend on the correct assignment of kinetically equiv alent mechanisms, and makes available reliable data on the magnitude o f the effect for reactions catalysed by a range of general acids and b ases. General base catalysis is characterised for the intramolecular c yclisation of one diastereoisomer (7b) of methyl phenyl 1,2-isopropyli dene-beta-D-xylofuranose 3'-phosphate triester: the Bronsted beta is 0 .65 and catalysis is enhanced by the proximity of the positive centres of suitable diamine monocations.