4‐Acetylthio‐2,2‐dimethyl‐3‐oxobutyl Group as an Esterase‐ and Thermo‐Labile Protecting Group for Oligomeric Phosphodiesters

Abstract

Abstract(4‐Acetylthio‐2,2‐dimethyl‐3‐oxobutyl)‐protected oligomeric phosphodiesters 1 and 2 were synthesized and removal of the protecting groups in the presence and absence of hog liver esterase was followed at pH 7.5 and 37 °C. Phosphotriesters 1 and 2 were successfully converted into the desired fully deprotected phosphodiesters 3 and 4, respectively. Some cleavage of internucleosidic P–O bonds took place, which reduced the yield of 3 and 4. Non‐enzymatic removal of the protecting group was only modestly retarded by accumulation of negative charge on the molecule. With 1, the half‐lives for the departure of the first and second protecting groups were 7.8 and 10.7 h, respectively, and with 2, 6.2 and 7.2 h, respectively. After 4 d, 70 % of both starting materials 1 and 2 were converted into the unprotected phosphodiester. The presence of hog liver esterase (2.6 units mL–1) resulted in fast removal of the first protecting group (τ1/2 2.7 min and 36 min with 1 and 2, respectively), but the appearance of fully deprotected 3 and 4 was accelerated only by a factor of 2, consistent with dramatic retardation of the enzymatic reaction upon accumulation of the negative charge.