Synthetic, mechanistic and photochemical studies of phosphate esters of substituted benzoins

Abstract

Synthesis of the phosphate esters of several benzoins, variously substituted with methoxy groups in one or both aromatic rings, proceeds via the corresponding cyclic ethylene ketals. These are phosphitylated with bis-(2-cyanoethyl) diisopropylphosphoramidite and the phosphites are oxidised to phosphates. Cleavage of the 2-cyanoethyl groups with methanolic alkali, followed by acid-catalysed hydrolysis of the ethylene ketal, yields the required phosphate monoesters. Application of the same reaction sequence to benzoin itself, not protected as the cyclic ketal, leads to at least three products, the phosphate monoester 5, the phosphate diester 7 and a substance not containing phosphorus, which appeared to be the ketal 8. The likely origin of these three products is discussed. The potential of these phosphate esters and their derivatives for study of rapid biochemical processes by fast photochemical cleavage of the benzoin group was examined. Best results were obtained with 3′,5′-dimethoxybenzoin phosphate 23b, where a single pulse of 347 nm light (100 mJ in 50 ns) resulted in 10% conversion into products, with a first-order rate constant > 10⁵ s^-1.