Hydrolysis of methylacetoin ethyl phosphate. competing pathways for carbonyl hydrate participation in a model for biotin carboxylation

Article abstract of DOI:10.1021/ja00056a007

Methylacetoin ethyl phosphate (1) is a phosphodiester with a carbonyl group β to phosphorus. The phosphate ester of a carbonyl hydrate, expected to be generated from an intramolecular reaction of 1 in base, is a model for an enzymic reaction intermediate which would form from the reaction of the tetrahedral addition production of biotin and bicarbonate with ATP. The hydrolysis of 1 is 10⁴-10⁵ times more rapid than that of dimethyl phosphate, extrapolated to common conditions (1 M NaOH, 55°C). Kinetic, product, and isotope labeling studies show that the hydrate of the carbonyl group of 1 serves as an intramolecular nucleophile toward phosphorus, forming the expected phosphate ester of the carbonyl hydrate through cyclic phosphorane intermediates. These adducts decompose by routes which parallel the proposed enzymic mechanism. In addition, solvent isotope incorporation reveals a route not found in reactions of carbonyl-substituted phosphotriesters: the conjugate base of the carbonyl hydrate acts an intramolecular nucleophile at the adjacent carbon atom, forming an unstable hydroxyoxirane (which becomes methylacetoin) and ethyl phosphate. An analogous reaction of ribonucleic acids (and RNAzymes) would have the 2′ hydroxyl group react at the 3′ carbon to form an oxyoxirane, cleaving the 3′ internucleotide bond.